Critical Role of Anions in Platinum(II) Precursors upon the Structural Motifs of Six-Membered Cycloplatinated N,N',N″-Triarylguanidines.

The reactions of cis-[Pt(OAc)2(DMSO)2] with 2 equiv of sym N,N',N″-triarylguanidines, [ArN=C(NHAr)2], in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(OAc){κ1 N(ArN=C(NHAr)2)}] [sym = symmetrical; Ar = 2-MeC6H4 (1) and 2,4-Me2C6H3 (2)], in 82 and 84% yields, respectively. The salt metathesis reaction of 1 with 1 equiv of AgTFA in CH2Cl2 at room temperature (RT) afforded [Pt{κ2(C,N)}(TFA){κ1 N(ArN=C(NHAr)2)}] (3) in 94% yield. The reaction of cis-[Pt(TFA)2(DMSO)2] with 1 equiv of [ArN=C(NHAr)2] in toluene under reflux condition for 8 h afforded six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(TFA)(DMSO)] [Ar = 2-MeC6H4 (4), 4-MeC6H4 (5), 2,4-Me2C6H3 (6), and 2-(MeO)C6H4 (7)], in ≥73% yields. The reaction of trans-[PtCl2(PhCN)2] with 2 equiv of [ArN=C(NHAr)2] in toluene under reflux condition for 48 h afforded trans-[PtCl2{ArN=C(NHAr)2}2] [Ar = 2-MeC6H4 (8) and 2,4-Me2C6H3 (9)] in 90 and 45% yields, respectively. Complexes 8 and 9 were separately refluxed in MeOH for 8 h to afford six-membered cycloplatinated guanidines, [Pt{κ2(C,N)}(μ-Cl)]2 (10 and 11), in 93 and 96% yields, respectively, with concomitant formation of the respective guanidinium salts, [(ArNH)3C]Cl, as the byproduct. Platinacycle 10 was treated with 2 equiv of AgTFA in CH2Cl2 at RT to afford six-membered cycloplatinated guanidine, [Pt{κ2(C,N)}(μ-TFA)]2 (12), in 94% yield. The new compounds were characterized by analytical techniques and multinuclear NMR (1H, 13C, and 195Pt) spectroscopy, and further, molecular structures of 10 compounds were determined by single-crystal X-ray diffraction. The structural motif in 1·1/2CH2Cl2 and 3 is novel in that it contains a planar six-membered [Pt{κ2(C,N)}] unit and a nonplanar eight-membered [Pt{κ2(N,O)}] ring, wherein OAc and the guanidine ligands are linked through a N-H···O hydrogen bond. The six-membered cycloplatinated structural motifs present in 10/11·C7H8 and 12·CH2Cl2 are also unprecedented in the literature. The number and nature of solution species of new complexes were unambiguously investigated by detailed NMR studies. The critical role of anions in Pt(II) precursors upon the course of cycloplatination and thus the motifs in the products were addressed. Plausible mechanisms of cycloplatination reactions are discussed.

Introduction

pan class="Chemical">Cycloplatinatedn> pan class="Chemical">imines and related compounds have been widely studied because of their relevance in C–H activation, organopan class="Chemical">metallic reactivity, and structural peculiarity and their utility as metallomesogens and luminescent materials.[1−10] Various Pt(II) precursors have been used to cycloplatinate N-donor ligands of which a mixture of cis-[PtCl2(DMSO)2] and NaOAc was shown to be the most convenient one for successful cycloplatination.[1−5,8−12] The OAc– in cycloplatination is believed to play a dual role as a nucleophile and as a base. Wu and co-workers hypothesized that cycloplatination of ferrocenylimine with cis-[PtCl2(DMSO)2] in the presence of NaOAc in toluene proceeds via a bicyclic transition state that contained a coordinated OAc.[7] The formation of an acetate-substituted product before cycloplatination of amino imines with a mixture of cis-[PtCl2(DMSO)2] and NaOAc was detected by 1H NMR spectroscopy,[5b] and such a product was isolated in a very minor amount in one instance but without the knowledge that this is one of the intermediates in OAc–-assisted cycloplatination.[13] The study aimed at understanding an intricate mechanism of cycloplatination of N-donor ligands in the absence of an external base is also being pursued.[14,15]

pan class="Chemical">Guanidinesn> and N-substituted pan class="Chemical">guanidines were shown to act as neutrpan class="Chemical">al and anionic N-donor ligands, respectively, exhibiting chelating and bridging coordination modes in structurally characterized Pt(II) complexes.[16−23] For the first time, sym N,N′,N″-triarylguanidines, [ArN=C(NHAr)2] (sym = symmetrical; Ar = Me-/OMe-substituted aryl rings), were cycloplatinated with a mixture of cis-[PtCl2(DMSO)2] and NaOAc, thereby affording six-membered cycloplatinated N,N′,N″-triarylguanidines, [Pt{κ2(C,N)}Cl(DMSO)] (A). During the course of syntheses of A, cis-/trans-[PtCl2(DMSO){ArN=C(NHAr)2}] (B) and acetate-substituted product, cis-[Pt(OAc)Cl(DMSO){ArN=C(NHAr)2}] (C), were also isolated, and the latter product was shown to be one of the intermediates for A.[24] The isolation of C partly corroborated the hypothesis put forward by Wu, Crespo, and co-workers for the mechanism of cycloplatination of N-donor ligands mediated by NaOAc.[5b,7] Recently, Vázquez-García, Vila, and co-workers have shown through density functional theory calculations that during the cycloplatination of hydrazine-based N-donor ligands with a mixture of cis-[PtCl2(DMSO)2] and NaOAc, indeed acetate enters into the coordination sphere of Pt(II) and forms a Wheland intermediate before forming terdentate-cycloplatinated hydrazine, [Pt{κ3(C,N,N)}Cl].[25]

pan class="Chemical">Carboxylaten>-assisted C–H activation of transition pan class="Chemical">metals has been intensively investigated through both experimentpan class="Chemical">al and computational techniques.[26] We cycloplatinated sym N,N′,N″-triarylguanidines with cis-[PtX2(DMSO)2] (X = OC(O)R; R = Me and CF3) with a view aimed at understanding the influence of carboxylates in these precursors upon the structural motifs of the resulting products. The difference in size of carboxylate moieties in the aforementioned precursors allowed us to isolate two distinct families of six-membered cycloplatinated guanidines, namely, 1–3 and 4–7; the motifs present in these platinacycles were determined by single-crystal X-ray diffraction (SCXRD) and are unprecedented in the literature. Further, 8 and 9 were successfully transformed to six-membered cycloplatinated guanidines 10 and 11, respectively, through a pathway, which is unprecedented in the literature.

Results and Discussion

Syntheses of Pt(II) Precursors

pan class="Chemical">trans-[PtCl2(PhCN)2]n> and cis-[pan class="Gene">PtX2(pan class="Chemical">DMSO)2] (X = Cl and OAc) were prepared in yields greater than the literature yields as outlined in the Supporting Information.[9,27−29] The metathesis reaction of cis-[PtCl2(DMSO)2] with 2 equiv of AgTFA in CH2Cl2 at room temperature (RT) for 24 h afforded cis-[Pt(TFA)2(DMSO)2] in 91% yield, as illustrated in Scheme .

Scheme 1

Syntheses of Cycloplatinated Guanidines and Plausible Mechanism

The reactions of pan class="Chemical">cis-[Pt(OAc)2(DMSO)2]n> with 2 equiv of [pan class="Chemical">ArN=pan class="Chemical">C(NHAr)2] (Ar = 2-MeC6H4 (LH22-tolyl) and 2,4-Me2C6H3 (LH22,4-xylyl) carried out separately in toluene under reflux condition for 8 h afforded 1 and 2 as colorless crystalline materials in 82 and 84% yields, respectively (see Scheme ). The aforementioned reaction carried out with [ArN=C(NHAr)2] (Ar = 2-(MeO)C6H4; LH22-anisyl) afforded only Pt black as a decomposition product, most likely due to greater basicity of the guanidine. The salt metathesis reaction of 1 with AgTFA in CH2Cl2 at RT for 24 h afforded 3 as a colorless crystalline material in 94% yield.

Scheme 2

The reactions of pan class="Chemical">cis-[Pt(TFA)2(DMSO)2]n> with 1 equiv of pan class="Chemical">LH22-tolyl and its pan class="Chemical">4-tolyl analogue, LH24-tolyl and LH22,4-xylyl, carried out separately in toluene under reflux condition for 8 h afforded 4–6 as colorless crystalline materials in 78, 73, and 83% yields, respectively (see Scheme ). The reaction of cis-[Pt(TFA)2(DMSO)2] with 1 equiv of LH22-anisyl in toluene under reflux condition for 8 h followed by slow cooling of the reaction mixture afforded [LH32-anisyl]TFA as a colorless crystalline material in low yield. Removal of volatiles from the reaction mixture and subsequent crystallization of the resulting solid in ethanol afforded 7 in 36% yield. The yields of 7 and [LH32-anisyl]TFA increased to 74 and 81%, respectively, when the aforementioned reaction was repeated with the Pt/guanidine ratio of 1:2 under otherwise identical condition. The precipitation of the guanidinium salt only in this reaction is probably ascribed to the greater basicity of LH22-anisyl. The reaction of cis-[Pt(OAc)2(DMSO)2] with 1 equiv of LH22-tolyl and the reaction of cis-[Pt(TFA)2(DMSO)2] with 2 equiv of the same guanidine under the condition identical to that mentioned in Schemes and 3 afforded 1 and 4 in 38 and 81% yields, respectively. This outcome suggests that it is the nature of Pt(II) precursors and not the Pt/guanidine ratio, which determines motifs in cycloplatinated guanidines.

Scheme 3

pan class="Chemical">trans-[PtCl2(PhCN)2]n> was treated with 2 equiv of pan class="Chemical">LH22-tolyl and pan class="Chemical">LH22,4-xylyl separately in toluene under reflux condition for 48 h to afford 8 and 9 in 90 and 45% yields, respectively, after column chromatographic work-up on basic alumina using CHCl3 as the eluent (see Scheme ). The aforementioned reaction carried out with cis-[PtCl2(DMSO)2] and LH22-tolyl under otherwise identical condition did not yield 8, presumably because of the difference in steric/electronic factors around Pt(II) in the precursor.

Scheme 4

The presence of vn class="Chemical">pan class="Chemical">ariable number of pn>an class="Chemical">guanidines in 1–2 and 4–7 stimulated us to isolate plausible intermediates such as D and E, respectively (see Figure ). The reaction of 8 with 2 equiv of AgOAc and the reaction of trans-[PtCl2(DMSO){ArN=C(NHAr)2}] (Ar = 2-MeC6H424) with 2 equiv of AgTFA in CH2Cl2 at RT for 24 and 3 h afforded 1 and 4 instead of D and E, respectively. The formation of 1 and 4 in the aforementioned reactions was confirmed by thin-layer chromatography (CHCl3/MeOH; v/v 60/40) and 1H NMR spectroscopy and through the isolation of 1 in 90% yield (see Method 1  under reactivity of 8 in Experimental Section). The reaction of 8 with 2 equiv of NaOAc in MeOH for 8 h under reflux condition also afforded 1 in 90% yield instead of D (see Method 2 ). The elusive nature of D from our attempts suggests that unless the N-donor ligand such as guanidine is sterically protected as illustrated through the isolation of trans-[Pd(OC(O)R)2L2] (R = Me, Ph, and Bu; L = [ArN=C(NHAr)2]; Ar = 2,5-Me2C6H3 (LH22,5-xylyl)),[30] cycloplatination would eventually occur even at RT because of the availability of both the acetate and the guanidine ligands in proximal position. The reaction of cis-[Pt(TFA)2(DMSO)2] with sym N,N′,N″-triarylguanidines either in 1:1 or 1:2 Pt/guanidine ratio can afford only the intermediate E and not the intermediate of type D because of steric reasons.

Figure 1

Proposed intermediates D and E formed during the course of formation of 1–2 and 4–7, respectively.

Pron class="Chemical">posed intermediates D and E formed during the course of formation of 1–2 and 4–7, respectively.

The mechanism of cyclopn class="Chemical">pan class="Chemical">alladation of pn>an class="Chemical">N,N-dimethylbenzylamine, L, with Pd(OAc)2 in CHCl3 was investigated by Ryabov and co-workers.[31] The initially formed 1:2 adduct [Pd(OAc)2L2] was shown to dissociate one of the Ls, thereby generating a 14-electron three-coordinate T-shaped intermediate, [Pd(OAc)2L], with trans geometry around Pd(II) initially and then to cis geometry. The latter species undergoes cyclopalladation in the rate-determining step to afford a five-membered cyclopalladated amine, [Pd{κ2(C,N)}(μ-OAc)]2. Accordingly, we hypothesize that the intermediate D transforms to three-coordinate intermediate F, which subsequently transforms to another intermediate G before finally forming platinacycles such as 1 and 2, as illustrated in Scheme . The intermediate G upon cycloplatination can liberate weakly acidic AcOH, which would fail to protonate the nonplatinated guanidine, and hence, the imine N atom of nonplatinated guanidine would recoordinate to Pt(II) to afford 1 rather than forming an admixture of six-membered cycloplatinated guanidine, [Pt{κ2(C,N)}(μ-OAc)]2, and the guanidinium acetate. The intermediate E upon losing dimethyl sulfoxide (DMSO) can give rise to three-coordinate intermediate H, which upon trans → cis isomerization can give another three-coordinate intermediate I. The intermediate I upon cycloplatination can give rise to 4–7 (see Scheme ).

Scheme 5

Scheme 6

Interestingly, the reaction of 8 with 1 equiv of pan class="Chemical">NaOAcn> in pan class="Chemical">MeOH under reflux condition for 8 h afforded a mixture of 1 and pan class="Chemical">[LH32-tolyl]Cl as colorless solids and 10 as pale green solid in low yields (see Scheme ). In this reaction, perhaps all NaOAc was consumed to form 1 from 50% of 8 and the remaining 50% could have undergone cycloplatination to afford 10 and the guanidinium salt. The observance of three products in the aforementioned reaction helped us to reoptimize synthetic routes for 10 and 11 as described below.

Scheme 7

Complexes 8 and 9 were sen class="Chemical">parately refluxed in n>an class="Chemical">methanol for 8 h to afford a mixture of 10 and [LH32-tolyl]Cl and a mixture of 11 and [LH32,4-xylyl]Cl, respectively (see Scheme ). Crystals of 10 and 11 were separated from the respective guanidinium salts by the hand-pick method and further purified by crystallization from the CH2Cl2/toluene mixture at RT. It is to be noted that heating trans-[PtCl2(APPY)2] (APPY = acetylmethylenetriphenylphosphorane, Ph3PCHC(O)Me) in MeCN for 44 h afforded five-membered cycloplatinated ylide, [Pt{κ2(C,C)}(μ-Cl)]2, and the salt, APPYHCl.[32]

It is to be noted that pan class="Chemical">trans-[PtCl2n>(pan class="Chemical">DMSO){pan class="Chemical">ArN=C(NHAr)2}] (Ar = 2-MeC6H4) resisted cycloplatination in MeOH under reflux condition, while 8 and 9 underwent cycloplatination under identical condition to afford 10 and 11, respectively, which suggests that the dissociated guanidine acts as a proton sponge, thereby driving the formation of respective platinacycles and the guanidinium salts instead of platinacycle J (see Figure ).

Figure 2

Putative cycloplatinated guanidines anticipated to form in Scheme .

Putative n class="Chemical">pan class="Chemical">cycloplatinated pn>an class="Chemical">guanidines anticipated to form in Scheme .

Adducts 8 and 9 upon reflux in n class="Chemical">pan class="Chemical">MeOH can give rise to three-coordinate intermediates K and L with trans and cis geometry, respn>ectively, pan class="Chemical">around the pan class="Chemical">Pt(II) atom (see Scheme ). The intermediate L upon cycloplatination could liberate strongly acidic HCl, which would protonate the imine N atom of nonplatinated guanidine to afford 10/11 and the respective guanidinium salts. The clean formation of 10/11 via this route is facilitated primarily because of the formation of stable guanidinium salts and their better solvation imparted by a polar protic solvent, MeOH. Thus, the structural motifs present in 1/2, 4–7, and 10/11 are influenced by a single or a combination of more than one of the following factors.

Scheme 8

The degree of steric encumpan class="Chemical">brn>ance pan class="Chemical">around the pan class="Chemical">Pt(II) atom in the precursors with sterically less hindered cis-[Pt(OAc)2(DMSO)2]/trans-[PtCl2(PhCN)2] permitting entry of two guanidines per Pt while sterically more hindered cis-[Pt(TFA)2(DMSO)2] permitting entry of only one guanidine per Pt.

The strength of acid liberated during the course of cycloplatination.

Influence of N–H···O pan class="Chemical">hydrogenn> bond present in intermediates F and G. Noncovn>an class="Chemical">alent-directed syntheses of coordination compounds pan class="Chemical">are an emerging area in synthetic inorganic/organometallic chemistry.[33]

pan class="Chemical">Cycloplatinatedn> pan class="Chemical">guanidine 10 was poorly soluble in organic solvents, and this precluded its solution chpan class="Chemical">aracterization by 195Pt and 13C NMR spectroscopy. Hence, 10 was derivatized with AgTFA in CH2Cl2 at RT for 24 h to afford more soluble 12 in 94% yield (see Scheme ).

Scheme 9

SCXRD Studies

Moleculpan class="Chemical">arn> structures of pan class="Chemical">cis-[Pt(TFA)2(DMSO)2], 1·pan class="Chemical">1/2CH2Cl2/3, 4/7, [LH32-anisyl]TFA, 8, 10/11·C7H8, and 12·CH2Cl2 have been determined by SCXRD and are shown in Figures –8. In cis-[Pt(TFA)2(DMSO)2], the Pt(II) atom revealed cis geometry as anticipated due to more effective (d–d) π bonding in this geometry.[34] The bond parameters around the Pt(II) atom in cis-[Pt(TFA)2(DMSO)2] are comparable with those found in the related compound cis-[Pt(TFA)2((M,S,S)-p-tolyl-binaso)].[35] Interestingly, the molecular structures of both cis- and trans-[Pd(TFA)2(S-DMSO)(O-DMSO)] have been reported, wherein DMSO is shown to reveal linkage isomerism.[36,37]

Figure 3

Molecular structure of cis-[Pt(TFA)2(DMSO)2] at 50% probability level. Hydrogen atoms have been removed for clarity.

Figure 8

Molecular structure of 12·CH2Cl2 at 50% probability level. The solvent molecule has been removed, and only hydrogen atoms of the amino moieties are shown for clarity.

Moleculpan class="Chemical">arn> structure of pan class="Chemical">cis-[Pt(TFA)2(DMSO)2] at 50% probability level. pan class="Chemical">Hydrogen atoms have been removed for clarity.

Moleculpan class="Chemical">arn> structures of 1·pan class="Chemical">1/2CH2Cl2 and 3 at 50% probability level. The solvent molecule in the case of 1·pan class="Chemical">1/2CH2Cl2 has been removed, and only hydrogen atoms of the amino moieties are shown for clarity.

Moleculpan class="Chemical">arn> structures of 4 and 7 at 50% probability level. Only n>an class="Chemical">hydrogen atoms of the amino moieties are shown for clpan class="Chemical">arity.

Moleculpan class="Chemical">arn> structures of pan class="Chemical">[LH32-anisyl]TFA and 8 at 50% probability level. Only relevant pan class="Chemical">hydrogen atoms of the guanidine and the guanidinium salt are shown for clarity.

Moleculpan class="Chemical">arn> structures of 10 and 11·pan class="Chemical">C7H8 at 50% probability level. pan class="Chemical">Toluene in 11·C7H8 has been removed, and only hydrogen atoms of the amino moieties are shown for clarity.

Moleculpan class="Chemical">arn> structure of 12·pan class="Chemical">CH2Cl2 at 50% probability level. The solvent molecule has been removed, and only pan class="Chemical">hydrogen atoms of the amino moieties are shown for clarity.

In 1·pan class="Chemical">1/2CH2Cl2n> and 3, the pan class="Chemical">Pt(II) atom is simultaneously a ppan class="Chemical">art of the six-membered [Pt{κ2(C,N)}] ring and the eight-membered [Pt{κ2(O,N)}] ring with the latter ring intramolecularly stabilized through >N–H···O=C< hydrogen bond involving one of the NH protons of the nonplatinated guanidine and carbonyl oxygen atom of OAc and TFA, respectively. The imine N atom of the nonplatinated guanidine is placed cis to the Pt–C bond of the C,N chelate, while OAc and TFA are placed cis to the Pt–N bond, and thus, the Pt(II) atom in both platinacycles revealed cisoid configuration.

The configuration of pan class="Chemical">Pd(II)n>/pan class="Chemical">Pt(II) atoms in cyclopan class="Chemical">metallated N-donor ligands was explained by invoking steric factor and antisymbiosis/transphobia of two mutually trans-oriented ligands and the influence of the latter factor was independently verified in one study by theoretical calculations.[4,24,38−40] The cisoid configuration of the Pt(II) atom in 1·1/2CH2Cl2 and 3 is unfavorable from the point of view of degree of transphobia, as hard imine N atom of both the guanidines lies trans to each other. In a hypothetical transoid configuration such as that illustrated in M, there would be greater steric repulsion between 2-tolyl substituents of the imine N atom of both platinated and nonplatinated guanidines (see Figure ). Thus, steric factor overrides the degree of transphobia in deciding the geometry of the Pt(II) atom in 1·1/2CH2Cl2 and 3.

Figure 9

Possible geometrical isomers of 1–3.

Possible geometricn class="Chemical">pan class="Chemical">al isomers of 1–3.

Intriguingly, the six-membered [pan class="Chemical">Ptn>{κ2pan class="Chemical">(C,N)}] ring is plapan class="Chemical">nar in both 1·1/2CH2Cl2 and 3 and is coplanar with the fused platinated 2-tolyl ring with the dihedral angle between two six-membered rings being 0.8(2)° (1·1/2CH2Cl2) and 3.1(1)° (3). The prevalence of a planar six-membered [Pd{κ2(C,N)}] ring in six-membered cyclopalladated N-donor ligands was indicated and in one instance such a species was identified by variable concentration 1H NMR spectroscopy.[38,41] The nonplatinated guanidine in 1·1/2CH2Cl2 and 3 revealed anti–anti conformation as reported for free LH22-tolyl.[42]

Unlike 1·pan class="Chemical">1/2CH2Cl2n> and 3, the six-membered [pan class="Chemical">Pt{κ2pan class="Chemical">(C,N)}] ring in 4 and 7 revealed a pseudo-boat conformation. The flagpole atoms Pt1 and N3 lie at 0.58(1) and 0.274(9) Å in 4 and at 0.62(1) and 0.26(1) Å in 7, respectively, above the basal plane constituted by N1, C1, C16, and C17. The bond parameters around the Pt(II) atom in 4 and 7 are comparable with the respective parameters reported for five-membered cycloplatinated N-donor ligands, [Pt{κ2(C,N)}(OC(O)R)(L)] (R/L = Me/DMSO[43a] and CF3/DMS[43b]).

The structure and bonding of the CN3 unit in cyclopan class="Chemical">metaln>lated sym N,N′,N″-tripan class="Chemical">arylpan class="Chemical">guanidines can be better understood from the value of ρ.[44] The ρ values for the structurally characterized compounds prepared in the present investigation are listed in Table . The lone pair of the amino N atoms can interact with the >C=N– π* orbital of the guanidine moiety through n−π conjugation, and for a maximum n−π conjugation, the ρ value should be 1.0 and any downward deviation from this value indicates hampered n−π conjugation. The ρ values for new compounds listed in Table are much greater than the ρ value known for the respective free guanidines.[42] The ρ value of 1.00 for [LH32-anisyl]TFA is as anticipated due to a maximum n−π conjugation, which arises because of the presence of a charge-delocalized Y-conjugated species (angle sum around CN3 carbon, ∑C ≈ 360°).[45] The amino N atoms of the guanidine moieties are planar (∑N ≈ 360°) in all compounds, further supporting enhanced n−π conjugation in all new compounds listed in Table . The greater ρ value for the C,N chelate in 3 than that observed for the same chelate in 1·1/2CH2Cl2 is attributed to the presence of more electronegative TFA in the former complex. The greater ρ value for the C,N chelate in 11·C7H8 than that observed for the same chelate in 10 is attributed to the stronger binding of the imine N atom of 2,4-xylyl guanidine moiety to the Pt(II) atom in the former. The cation and the anion in [LH32-anisyl]TFA are linked by a pair of N–H···O hydrogen bonds and thus form an eight-membered ring characteristic of this type of salts.[46]

Table 1

Comparison of ρ Values of 1·1/2CH2Cl2, 3, 4, 7, [LH32-anisyl]TFA, 8, 10, 11·C7H8, and 12·CH2Cl2a

complex	a (Å)	(b + c)/2 (Å)	ρ = 2a/(b + c)
1·1/2CH2Cls	1.309(5)/1.312(5)b	1.343(7)/1.352(7)b	0.97/0.97b
3	1.317(5)/1.314(5)b	1.324(7)/1.358(7)b	0.99/0.97b
4	1.305(8)	1.337(12)	0.98
7	1.318(8)	1.348(13)	0.98
[LH32-anisyl]TFA	1.337(2)	1.338(3)	1.00
8	1.295(9)b/1.303(9)b	1.356(13)b/1.340(10)b	0.97b/0.97b
10	1.303(8)	1.336(11)	0.97
11·C7H8	1.303(7)/1.322(7)	1.322(11)/1.338(10)	0.99/0.99
12·CH2Cl2	1.302(8)/1.301(8)	1.330(11)/1.334(13)	0.98/0.97

ρ = 0.93 (LH22-tolyl), 0.92 (LH22-anisyl) and 0.93 (LH22,4-xylyl).[42]

The values indicated are for the nonplatinated guanidine.

ρ = 0.93 (pan class="Chemical">LH22-tolyl)n>, 0.92 (pan class="Chemical">LH22-anisyl) and 0.93 (pan class="Chemical">LH22,4-xylyl).[42]

The vpan class="Chemical">aln>ues indicated pan class="Chemical">are for the nonplatinated pan class="Chemical">guanidine.

Four distinct conformations, namely, trans pan class="Gene">synn> pan class="Gene">syn–pan class="Gene">syn, trans syn syn–anti, trans syn anti–syn, and trans syn anti–anti, are possible for 8 and 9 depending upon (i) the disposition of the guanidine around the Pt(II) atom, (ii) the orientation of the >C=N– unit of one guanidine with respect to the orientation of the identical unit of the second guanidine, and (iii) the orientation of the aryl ring of the amino moieties with respect to the >C=N– unit of the guanidine (see Figure S1).[30] Complex 8 revealed trans syn anti–anti conformation as sterically hindered 2-tolyl substituent of the amino moieties is better accommodated in this conformation. The observed conformation is further stabilized by a pair of intramolecular N–H···Cl hydrogen bonds and three intramolecular C–H···Cl hydrogen bonds. The bond parameters around the Pt(II) atom in 8 are comparable with those parameters reported for the related trans-[PtCl2L2] (L = ferrocenyl-based amidine and guanidine).[23,28]

Cyclopan class="Chemical">metaln>lated five-membered N-pan class="Species">donor ligands, [M{κ2pan class="Chemical">(C,N)}(μ-X)]2 (M/X = Pd and Pt/Cl, Br, or I), are known to exist as either cisoid or transoid conformer in the solid state, but the former conformer is not possible for 10 and 11·C7H8 as there would be a repulsive interaction between two =NAr units of the guanidine ligands. However, 10 and 11·C7H8 can exist as five different conformers, namely, transoid syn planar, transoid anti planar, transoid syn puckered (or butterfly-like), transoid anti puckered (or bat-like), and transoid puckered, as illustrated in Figure . Transoid syn planar, transoid syn puckered, and transoid anti puckered possess a pseudo-C2 symmetry, which passes right across the center of the [Pt(μ-Cl)2Pt]2+ ring, while transoid anti planar possesses an inversion symmetry. Transoid puckered conformer does not possess any such symmetry.

Figure 10

Possible conformers of 10 and 11·C.

In the solid state, 10 revepan class="Chemical">aln>ed a transoid anti plan>an class="Chemical">nar conformation, while 11·C7H8 revepan class="Chemical">aled transoid anti puckered conformation. Thus, the four-membered [Pt(μ-Cl)2Pt]2+ ring in 10 is planar, while that in 11·C7H8 is puckered. The dihedral angle between two mean planes constituted by Pt1–Cl1–Cl1# and Pt1#–Cl1–Cl1# triangles in 10 is 0.0(1)°, and the corresponding dihedral angle in 11·C7H8 is 45.1(1)°. As a result, the nonbonded Pt···Pt distance, 3.6479(4) Å, in 10 is longer than the corresponding distance in 11·C7H8 (3.3869(3) Å). The Pt–Cl1# distance of 2.458(2) Å (10) and the corresponding distances of 2.458(1) Å, 2.449(1) Å (11·C7H8), both of which are trans to the Pt–C(aryl) bond, are longer than the second Pt–Cl distance(s) [2.325(2) Å (10) and 2.324(1) Å and 2.326(1) Å (11·C7H8)] because of greater trans influence of the platinated carbon of the aryl ring in the C,N chelate. It is to be noted that six-membered cyclopalladated 2-tolyl and 2,4-xylyl guanidines [Pd{κ2(C,N)}(μ-Br)]2 revealed transoid syn puckered and transoid anti puckered conformations, respectively.[39] The difference in the conformations of 10 and its Pd(II) counterpart probably arises because of the difference in size of the atoms in the [M(μ-X)2M]2+ (M/X = Pt/Cl and Pd/Br) ring and difference in the packing forces.

In princin class="Chemical">ple, 12·pan class="Chemical">CH2Cl2 can exist as three different conformers, namely, transoid in–in, transoid in–out, and transoid out–out, as we have shown for six-membered cyclopn>class="Chemical">n>an class="Chemical">alladated guanidines, [Pd{κ2(C,N)}(μ-OC(O)R)]2 (R = Me, CF3, and Bu).[38,39] In the solid state, 12·CH2Cl2 revealed transoid in–in conformation as analogously revealed by six-membered cyclopalladated 2-tolyl- and 2,4-xylyl guanidines, [Pd{κ2(C,N)}(μ-OAc)]2.[39]

Cyclopan class="Chemical">metaln>lated N-pan class="Species">donor ligands pan class="Chemical">are known since the pioneering work of Cope and Siekman who published their seminal paper in 1965.[47] A number of known structurally characterized five-membered cycloplatinated N-donor ligands of type I shown in Figure are greater than their six- and seven-membered counterparts.[1,9,12,15a,24,43,48] Further structurally characterized five-membered cycloplatinated N-donor ligands of types II and III are scarce,[2−4,49−52] while their six- and seven-membered counterparts are presently unknown. Thus, to the best of our knowledge, 10/11·C7H8 and 12·CH2Cl2 represent the first crystallographically characterized six-membered cycloplatinated N-donor ligands of types II and III to be reported in the literature. Further, 1·1/2CH2Cl2 and 3 represent the first structurally characterized planar six-membered cyclometallated N-donor ligands of this kind to be reported in the literature.

Figure 11

Structural motifs of known cycloplatinated N-donor ligands.

Structurpan class="Chemical">aln> motifs of known pan class="Chemical">cycloplatinated N-pan class="Species">donor ligands.

Spectroscopic Characterization

Attenuated Total Reflection IR (ATR–IR) Spectroscopy

pan class="Chemical">Pt(II) guanidinen> complexes prepn>n>an class="Chemical">ared in the present investigation were characterized by IR and multinuclear NMR (1H, pan class="Chemical">13C, 19F, and 195Pt) spectroscopy, elemental analyses, and mass spectrometry. IR spectroscopy is a powerful tool to unambiguously assign the coordination modes of the carboxylate moiety in metal carboxylate complexes.[53] The IR spectrum of cis-[Pt(TFA)2(DMSO)2] revealed two bands at 1722 and 1400 cm–1 assignable to νa(OCO) and νs(OCO), respectively. From the Δν = νa(OCO) – νs(OCO) value of 322 cm–1 and from the fact that this value is greater than the Δν value reported for NaTFA (223 cm–1),[53] the presence of a monodentate coordination mode for TFA in cis-[Pt(TFA)2(DMSO)2] is confirmed. The IR spectrum of cis-[Pt(TFA)2(DMSO)2] also revealed a band at 1144 cm–1 assignable to the ν(SO) stretch of S-bonded DMSO.[37,54]

The IR spectra of 1–3 reven class="Chemical">pan class="Chemical">aled a pn>air of bands at 1622 and 1386 cm–1 (1·pan class="Chemical">1/2CH2Cl2), 1624 and 1387 cm–1 (2), and 1676 and 1393 cm–1 (3) assignable to νa(OCO) and νs(OCO), respectively. The Δν vpan class="Chemical">alues of 236, 237, and 283 cm–1 indicated the presence of hydrogen-bonded monodentate OAc[55] in 1·1/2CH2Cl2 and 2 and hydrogen-bonded monodentate TFA in 3, respectively. The Δν values of 326, 315, 310, and 322 cm–1 for 4–7, respectively, indicated the presence of monodentately coordinated TFA, while the Δν value of 130 cm–1 for 12·CH2Cl2 indicated the presence of bridging bidentately coordinated TFA. The IR spectra of all other new Pt(II) complexes and guanidinium salts prepared in the present investigation revealed bands at anticipated positions assignable to the ν(NH), ν(C=N), and ν(SO) stretches, whichever is applicable.

NMR Spectroscopy

The Me/OMe substituents in pan class="Chemical">arn>yl rings of pan class="Chemical">guanidines in new complexes reported in this manupan class="Chemical">script are useful not only in modulating their solubility in low polar solvents but also in serving as the NMR (1H and 13C) spectroscopic handle to understand the number and nature of solution species. Further, different substitution patterns of aryl rings in guanidines afforded different information in solution, which enabled us to better interpret the number and nature of solution species.

195pan class="Chemical">Ptn> NMR spectroscopy is a powerful technique to understand the coordination environment around the pan class="Chemical">metal in platinum(II) complexes because of its favorable NMR properties.[56] Intramolecular cycloplatination of N-donor ligands results in upfield δ(195Pt) NMR shifts.[5b,12,24,56,57] Further, δ(195Pt) window is wider, covering about 6000 ppm interval, which permits one to distinguish even one set of geometrical isomers and conformers of [C,E] platinacycles (E = N and C) without any ambiguity.[24,40] Hence, the new complexes in solution were characterized by 195Pt NMR spectroscopy.

195pan class="Chemical">Ptn> NMR spectra of cis-[n>an class="Gene">PtX2(DMSO)2] (X = pan class="Chemical">OAc and TFA) revealed a singlet at δ(195Pt) −3086 and −3094 ppm, respectively, and these values are more downfield-shifted than that reported for cis-[PtCl2(DMSO)2] (δ(195Pt) −3450 ppm[29]) because of the presence of more electron-withdrawing carboxylate moieties in the former two complexes. 195Pt NMR spectra of 1–3 revealed a pair of signals for each platinacycle at δ(195Pt) −2784 and −2718 ppm (intensity ratio = 1.00:0.55), −2783 and −2724 ppm (intensity ratio = 1.00:0.62), and −2818 and −2879 ppm (intensity ratio = 1.00:0.75), respectively, revealing the presence of two solution species. 1H NMR spectra of 1 and 2 revealed the presence of two species in about 1.00:0.65 and 1.00:0.57 ratios, respectively, and the presence of two solution species is in agreement with the results obtained from 13C NMR spectroscopy (see the Supporting Information). Interestingly, the 1H NMR spectrum of 3 revealed the presence of four species in about 1.00:0.69:0.23:0.10 ratio, and the presence of four solution species was further supported by 13C and 19F NMR spectroscopy.

From the foregoing dipan class="Chemical">scn>ussion, we suggest the presence of a maximum of four solution spn>ecies for 1–3, and these four spn>ecies n>an class="Chemical">are assigned to four conformers, namely, syn–syn, syn–anti, anti–pan class="Gene">syn, and anti–anti, as illustrated in Figure . The nonplatinated guanidine in the solid-state isomer of 1 and 3 can decoordinate in solution to afford an acyclic intermediate N because of destabilizing transphobia effect (see Figure ). Subsequently, the nonplatinated guanidine in the intermediate N can undergo amine–imine tautomerization[58] to afford another intermediate O, which would recoordinate to the Pt(II) atom through the imine N atom to afford a mixture of conformers illustrated in Figure . The aryl substituent of the N atoms, which are not involved in intramolecular N–H···O hydrogen bond in intermediates N and O, is free to undergo incremental N–Caryl bond rotation, and hence, formation of three more conformers in addition to the solid-state conformer via intermediates N and O is likely possible.

Figure 12

Possible conformers of 1–3 in solution. The substituents on the aryl rings have been removed for clarity.

Figure 13

Interconversion of intermediates N and O via amine–imine tautomerization.

Possible conformers of 1–3 in solution. The substituents on the n class="Chemical">pan class="Chemical">aryl rings have been removed for clpn>an class="Chemical">arity.

Interconversion of intermediates N and O via pan class="Chemical">aminen>–pan class="Chemical">imine tautomerization.

To better understand the nature and number of solution species of 1–3, n class="Chemical">pan class="Chemical">platinacycle 13 was isolated (see Figure ). The pn>an class="Chemical">1H NMR spectrum of 13 is anticipated to reveal the presence of only one conformer in solution because of the presence of more symmetrical 4-tolyl rings. Thus, the 1H NMR spectrum of 13 revealed six singlets in a 1:1:1:1:2:1 ratio for CH3 protons of tolyl rings and the acetate moiety (see the Supporting Information). Further, a downfield-shifted signal was observed for the NH proton (δ = 11.42 ppm), which is involved in intramolecular N–H···O hydrogen bond. Thus, the 1H NMR spectrum of 13 clearly supported our hypothesis that more than one solution species of 1–3 arises from four distinct conformers illustrated in Figure .

Figure 14

Platinacycle 13 (Ar = 4-MeC6H4) which exists as a single isomer in solution.

pan class="Chemical">Platinacyclen> 13 (pan class="Chemical">Ar = pan class="Chemical">4-MeC6H4) which exists as a single isomer in solution.

195n class="Chemical">Ptn> NMR spectra of 4–7 reven>an class="Chemical">aled one peak each at δ(195Pt) −3650, −3674, −3655, and −3657 ppm, respectively, indicating the presence of a single solution species, and this conclusion was independently verified by 1H, 13C, and 19F NMR spectroscopy. As anticipated, the δ(195Pt) value of 7 is deshielded than that reported for its chloro analogue, [Pt{κ2(C,N)}Cl(DMSO)] (δ(195Pt) −3737 ppm[24]), because of the presence of more electron-withdrawing TFA in the former complex. The δ(195Pt) values reported for 4–7 are also upfield-shifted than that reported for five-membered cycloplatinated ferrocenylamine, [Pt{κ2(C,N)}(OAc)(DMSO)] (δ(195Pt) −2078 ppm[43a]) not only due to the difference in the [Pt{κ2(C,N)}] ring size but also due to the difference in the nature of ligands. The presence of one solution species for 4 contrasts with the presence of seven solution species reported for its chloro analogue, [Pt{κ2(C,N)}Cl(DMSO)] (δ(195Pt) −2645 (minor), −2710 (major), −2760 (minor), −2839 (minor), −2851 (minor), −2958 (major), and −2995 (minor) ppm[24]). This spectral difference in the number of solution species arises because of larger size of the TFA in platinacycle 4. As a result, only one conformer, α3, out of six possible conformers (α1, α2, α3, β1, β2, and β3[24]) is possible for 4 even in solution. The α and β nomenclature indicates conformers that arise due to three distinct orientations of DMSO with respect to the plane of the Pt(II) atom and two distinct orientations of the o-substituted aryl ring in the =NAr unit of the C,N chelate with respect to the basal plane of the boat, respectively.

The 195pan class="Chemical">Ptn> NMR spectrum of 8 reven>an class="Chemical">aled one peak at δ(195Pt) −1799 ppm, while pan class="Chemical">1H NMR spectra measured in both CDCl3 and C6D6 revealed the presence of two species in about 1.00:0.66 and 1.00:0.50 ratios, respectively. The presence of two solution species for 8 in CDCl3 was also independently verified by 13C NMR spectroscopy. Complex 9 also revealed the presence of two solution species as revealed by 195Pt, 1H, and 13C spectroscopy (see Experimental Section). The 195Pt NMR spectrum of 9 revealed two singlets at δ(195Pt) −1818 (minor) and −1804 (major) ppm, suggesting the presence of two solution species. The δ(195Pt) shifts observed for 8 and 9 are comparable with those shifts reported for the related 1:2 adducts (δ(195Pt) −1887, −2020, and −1872 ppm).[23,28,59] The major and minor solution species of 8 and 9 are assigned to trans syn anti–anti, the solid-state isomer, and trans syn anti–syn isomer with the latter isomer arising from the former via the restricted C–N(H)Ar single bond rotation, as illustrated in Scheme .

Scheme 10

Two hpan class="Chemical">aln>ves of the molecule in pan class="Chemical">all conformers excepan class="Chemical">pt transoid puckered for 10 and 11 are equivalent because of molecular symmetry (see Figure ). We would expect a maximum of three singlets for CH3 protons of the guanidine ligand for 10 and a maximum of six singlets for the same protons of the guanidine for 11 when the conformers possess either C2 or inversion symmetry for each isomer assuming that there are no overlapping CH3 signals. The transoid puckered conformer is expected to reveal a maximum of 6 singlets for CH3 protons of the guanidine ligand for 10 and a maximum of 12 singlets for the same protons of the guanidine for 11 assuming that there are no overlapping CH3 signals.

pan class="Chemical">Platinacycle 10n> is only sparingly soluble in common deuterated NMR solvents, which precluded its chpan class="Chemical">aracterization by 195Pt and 13C NMR spectroscopy. The 1H NMR spectrum of 10 in CDCl3 revealed the presence of three species in about 1.00:0.78:0.50 ratio as estimated from the integrals of CH3 protons of the guanidine ligand. The 1H NMR spectrum of 10 revealed three singlets for the first isomer, three singlets for the second isomer, and two singlets for the third isomer assignable to the CH3 protons, thus accounting for six Me groups of the guanidine ligand. Thus, the three isomers in solution possess either C2 or inversion symmetry. However, better solubility of 11 in common deuterated NMR solvents enabled us to characterize it by 1H, 195Pt, and 13C NMR spectroscopy. The 195Pt NMR spectrum of 11 revealed three peaks at δ(195Pt) −2863, −2854, and −2848, indicating the presence of three solution species. The three solution species observed for 10 and 11 contrasts with the single solution species reported for five-membered cycloplatinated imines, [Pt{κ2(C,N)}(μ-Cl)]2, which were shown to reveal a single δ(195Pt) shift (δ(195Pt) range: −3108 to −3165 ppm) in solution.[3b,3c]

The pan class="Chemical">1Hn> NMR spectrum of 11 in C6n>an class="Chemical">D6 revealed the presence of two species in about 1.00:0.96 ratio as estimated from the integrpan class="Chemical">als of CH3 protons of the guanidine ligand. Five singlets were observed for CH3 protons of the guanidine moiety in one isomer (11a), and nine singlets were observed for the same protons of the second isomer (11b), thus accounting for 12 Me groups of the guanidine ligand, which clearly revealed that isomer 11a possesses either C2 or inversion symmetry, while isomer 11b does not possess either of the two. The 1H NMR spectrum of 11 in CDCl3 revealed the presence of three isomers 11a, 11b, and 11c in about 1.00:1.13:0.91 ratio, respectively, as estimated from the integrals of CH3 protons of the ligand. The 1H NMR spectrum revealed four singlets for CH3 protons for both 11a and 11c while seven singlets for the same protons of 11b, thus accounting for 12 Me groups of the guanidine ligand for each species. Thus, it appears that both isomers 11a and 11c possess either C2 symmetry or inversion symmetry or one of these symmetries. Further, isomer 11b appears to possess neither C2 symmetry nor inversion symmetry, thereby referring to the transoid puckered conformer.

From the solution behavior of 11 in C6pan class="Chemical">D6n>, we suggest the presence of any two conformers, one originating from either transoid n>an class="Gene">syn puckered or transoid anti puckered and the other isomer arising from transoid puckered and the conformationpan class="Chemical">al equilibria in this case arise only due to the six-membered [Pt{κ2(C,N)}] ring inversion as C6D6 is noncoordinating. We suggest that the crystals of solid-state conformers upon dissolution in weakly coordinating CDCl3 undergo a bridge-splitting reaction to afford solvent-coordinated intermediates, P and Q, respectively, and these intermediates are free to undergo six-membered [Pt{κ2(C,N)}] ring inversion in solution before reforming any of the three conformers illustrated in Figure (see also Figure ). The conformational equilibria in solution are made possible for 10 and 11 because the Pt–Cl bond, which is trans to the Pt–C bond, is weaker because of greater trans influence of the platinated carbon. 1H, 195Pt, and 13C NMR spectra of 12 in solution indicated the presence of a single species. The δ(195Pt) −2555 ppm value observed for 12 is downfield-shifted when compared to those δ(195Pt) shifts of 11 because of the presence of more electron-withdrawing TFA bridges in the former.

Figure 15

Solvent-coordinated intermediates possibly formed in solution for 10 (P) and 11 (Q) in CDCl3 (Sol).

Solvent-coordinated intermediates possibly formed in solution for 10 (n class="Chemical">P) and 11 (Q) in pan class="Chemical">CDCl3 (Sol).

Conclusions

Cycloplatination of severn class="Chemical">pan class="Chemical">al pn>an class="Chemical">guanidines with Pt(II) carboxylate precursors was reported under facile reaction condition without recourse to the use of an external base to afford either 1 and 2 or 4–7 depending upon the carboxylates in the precursors. Thermolysis of 8 and 9 in MeOH under reflux condition afforded cycloplatinated 10 and 11, respectively. Complex 10 was further derivatized with AgTFA to afford 12. The yields of all complexes except one complex are good to very good. The new complexes were characterized by analytical, IR, and multinuclear NMR (1H, 13C, 19F, and 195Pt) spectroscopic techniques, and the structural motifs present in 10 complexes were unambiguously determined by SCXRD. To the best of our knowledge, the structural motifs present in some of the new complexes are unprecedented in the literature. The number and nature of solution species of new complexes were identified by multinuclear NMR spectroscopy. We have shown that the steric factor of anions in Pt(II) precursors is critical in determining the course of cycloplatination and hence structural motifs of the products.

Experimental Section

pan class="Chemical">Aln>l chpan class="Chemical">aracterizations of the new compounds were cpan class="Chemical">arried out with batches of single crystals.

cis-[Pt(TFA)2(DMSO)2]

pan class="Chemical">cis-[PtCl2(DMSO)2]n> (50.0 mg, 0.118 mmol) and pan class="Chemical">AgTFA (52.1 mg, 0.236 mmol) were taken in a 50 mL round-bottom (pan class="Chemical">RB) flask covered with an Al foil. To the flask was added 35 mL CH2Cl2, and the reaction mixture was stirred for 24 h. Subsequently, the reaction mixture was filtered through a 0.2 μM syringe filter. The volatiles from the filtrate were removed under vacuum to afford the solid. The solid was subsequently dissolved in acetone and layered with ethanol, and the resulting solution was stored at RT for 3 days to afford the title complex as colorless crystals. Yield: 91% (62.0 mg, 0.107 mmol). mp 173.4 °C. ATR–IR (cm–1): νa(OCO) 1722 (s); νs(OCO) 1400 (m); ν(S=O) 1144 (s). 1H NMR (CDCl3, 400 MHz): δ 3.45 (s, 4 × 3 H, (CH3)2S(O)). 1H NMR (acetone-d6, 400 MHz): δ 3.60 (s, JPtH = 9.4 Hz, 4 × 3 H, (CH3)2S(O)). 19F{1H} NMR (acetone-d6, 376.31 MHz): δ −75.65. 195Pt{1H} NMR (acetone-d6, 85.8 MHz): δ −3094. LRMS (EI) m/z [ion]: 736 [M + Na + NaTFA]+, 656 [M + DMSO + H]+, 578 [M + H]+, 464 [M – TFA]+. Anal. Calcd for C8H12O6F6S2Pt (Mw 577.375): C, 16.64; H, 2.09; S, 11.11. Found: C, 16.62; H, 1.95; S, 11.33.

[Pt{κ2(C,N)-C6H3Me-3(NHC(NHAr)(=NAr))-2}(OAc)(LH22-tolyl)] (Ar = 2-MeC6H4; 1)

pan class="Chemical">cis-[n class="Chemical">Pt(OAc)2(DMSO)2] (50.0 mg, 0.107 mmol) and class="Chemical">n>an class="Chemical">LH22-tolyl (71.2 mg, 0.216 mmol) were taken in a 25 mL RB flask, to which was added toluene (10 mL). The RB flask was fitted with a double surface condenser capped with a freshly prepared anhydrous CaCl2 guard tube. The reaction mixture was refluxed for 8 h, cooled, and filtered. The volume of the filtrate was reduced to about one-third and stored at RT for 1 week to afford 1 as colorless crystals. Crystals of 1·1/2CH2Cl2 suitable for SCXRD were grown from a CH2Cl2/Pr2O mixture at RT over a period of several days. Yield: 82% (80 mg, 0.088 mmol). Two additional procedures for the preparation of 1 are described later in this section.

mp 203.0 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3418 (w), 3392 (w); νa(OCO) 1622 (s); νpn>an class="Chemical">(C=N) 1591 (s), 1551 (s); νs(OCO) 1386 (s). The 1H NMR spectrum of 1 revealed the presence of two isomers, which are hereafter indicated as isomers 1a and 1b in about 1.00:0.65 ratio as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.41 (s, 3H, CH3, isomer 1a), 1.42, 1.63 (each s, 2 × 3H, CH3, isomer 1b), 1.66, 1.97 (each s, 2 × 3H, CH3, isomer 1a), 1.98 (s, 3H, CH3, isomer 1b), 2.08 (s, 3H, CH3, isomer 1a), 2.11 (s, 3H, CH3, isomer 1b), 2.17 (s, 3H, CH3, isomer 1a), 2.23, 2.34, 2.42 (each s, 3 × 3H, CH3, isomer 1b), 2.47, 2.53 (each s, 2 × 3H, CH3, isomer 1a), 5.53 (s, 1H, NH, isomer 1b), 5.66 (s, 1H, NH, isomer 1a), 5.70 (s, 2 × 1H, NH, isomers 1a & 1b), 6.47 (s, 1H, NH, isomer 1a), 6.53 (s, 1H, NH, isomer 1b), 6.57–7.04 (m, 2 × 12H, ArH, isomers 1a & 1b), 7.10–7.32 (m, 19H, ArH, isomer 1a (11H) & isomer 1b (8H)), 7.67 (dd, JHH = 6.9 Hz, 2H, ArH, isomer 1b), 7.87 (d, JHH = 8.4 Hz, 1H, ArH, isomer 1b), 10.54 (s, 1H, NH, isomer 1b), 10.71 (s, 1H, NH, isomer 1a). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2784 and −2718. LRMS (EI) m/z [ion]: 934 [M + Na]+. Anal. Calcd for C46H48N6O2Pt·H2O (Mw 930.004): C, 59.41; H, 5.42; N, 9.04. Found: C, 59.65; H, 5.18; N, 9.15.

[Pt{κ2(C,N)-C6H2Me2-3,5(NHC(NHAr)(=NAr))-2}(OAc)(LH22,4-xylyl)] (Ar = 2,4-Me2C6H3; 2)

Complex 2 was n class="Chemical">prepared from n>an class="Chemical">cis-[Pt(OAc)2(DMSO)2] (50.0 mg, 0.107 mmol) and LH22,4-xylyl (80.3 mg, 0.216 mmol) in toluene (10 mL) and purified as described previously for complex 1. Yield: 84% (90 mg, 0.090 mmol). mp 206.0 °C. ATR–IR (cm–1): ν(NH) 3411 (w), 3400 (sh); νa(OCO) 1624 (s); ν(C=N) 1603 (s), 1544 (m); νs(OCO) 1387 (m). The 1H NMR spectrum of 2 revealed the presence of two isomers, which are hereafter indicated as isomers 2a and 2b in about 1.00:0.57 ratio as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.377 (s, 3H, CH3, isomer 2a), 1.409, 1.606 (each s, 2 × 3H, CH3, isomer 2b), 1.632 (s, 3H, CH3, isomer 2a), 1.840 (s, 3H, CH3, isomer 2b), 1.948 (s, 3H, CH3, isomer 2a), 2.003 (s, 3H, CH3, isomer 2b), 2.037 (s, 3H, CH3, isomer 2a), 2.056 (s, 2 × 3H, CH3, isomers 2a & 2b), 2.063 (s, 3H, CH3, isomer 2a), 2.076 (s, 3H, CH3, isomer 2b), 2.099 (s, 3H, CH3, isomer 2a), 2.151 (s, 3H, CH3, isomer 2b), 2.171 (s, 3H, CH3, isomer 2a), 2.220, 2.253 (each s, 2 × 3H, CH3, isomer 2b), 2.278 (s, 3H, CH3, isomer 2a), 2.296 (s, 3 × 3H, CH3, isomer 2a (6H) & isomer 2b (3H)), 2.313, 2.348 (each s, 2 × 3H, CH3, isomer 2b), 2.388 (s, 3H, CH3, isomer 2a), 2.412 (s, 3H, CH3, isomer 2b), 2.477 (s, 3H, CH3, isomer 2a), 5.481 (s, 1H, NH, isomer 2b), 5.627 (s, 2 × 1H, NH, isomer 2a), 5.643 (s, 1H, NH, isomer 2b), 6.368–6.425 (m, 2H, ArH (isomer 2a, 1H) & NH (isomer 2b, 1H)), 6.524–6.644 (m, 8H, ArH (isomer 2a, 3H & isomer 2b, 4H) & NH (isomer 2a, 1H)), 6.721 (s, 1H, ArH, isomer 2a), 6.792–6.813 (m, 3H, ArH (isomer 2a, 2H & isomer 2b, 1H)), 6.923–7.016 (m, 12H, ArH (isomer 2a, 4H & isomer 2b, 8H)), 7.038 (s, 2H, ArH, isomer 2a), 7.081 (br, 2H, ArH, isomer 2b), 7.113 (s, 1H, ArH, isomer 2a), 7.136 (br, 1H, ArH, isomer 2a), 7.158 (s, 1H, ArH, isomer 2b), 7.442 (d, JHH = 9.6 Hz, 2H, ArH, isomer 2a), 7.686 (d, JHH = 8.4 Hz, 1H, ArH, isomer 2b), 10.237 (s, 1H, NH, isomer 2b), 10.481 (s, 1H, NH, isomer 2a). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2783 and −2724. LRMS (EI) m/z [ion]: 996 [M + H]+, 936 [M – OAc]+. Anal. Calcd for C52H60N6O2Pt (Mw 996.147): C, 62.70; H, 6.07; N, 8.44. Found: C, 62.65; H, 6.14; N, 8.50.

[Pt{κ2(C,N)-C6H3Me-3(NHC(NHAr)(=NAr))-2}(TFA)(LH22-tolyl)] (Ar = 2-MeC6H4; 3)

Complex 1 (100 mg, 0.110 mmol) and n class="Chemical">pan class="Chemical">AgTFA (24.3 mg, 0.110 mmol) were taken in a 50 mL class="Chemical">n>an class="Chemical">RB flask covered with an Al foil. To the flask was added CH2Cl2 (20 mL), and the mixture was stirred for 24 h. The reaction mixture was subsequently filtered through a 0.2 μM syringe filter. The volatiles from the filtrate were removed under vacuum to one-third of its original volume, layered with toluene (2 mL), and the resulting solution was stored at RT for a period of 1 week to afford 3 as colorless crystals. Yield: 94% (99.6 mg, 0.103 mmol). mp 240 °C. ATR–IR (cm–1): ν(NH) 3414 (w), 3395 (w); νa(OCO) 1676 (s); ν(C=N) 1603 (s), 1589 (m); νs(OCO) 1393 (m). The 1H NMR spectrum of 3 revealed the presence of four isomers, which are hereafter indicated as isomers 3a–d in about 1.00:0.69:0.23:0.10 ratio as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.621 (s, 3H, CH3, isomer 3d), 1.643 (s, 3H, CH3, isomer 3b), 1.670 (s, 3H, CH3, isomer 3c), 1.681 (s, 3H, CH3, isomer 3a), 1.962 (s, 2 × 3H, CH3, isomers 3a & 3c), 1.974 (s, 2 × 3H, CH3, isomers 3b & 3d), 2.056, 2.065 (each s, 2 × 3H, CH3, isomer 3c), 2.082 (s, 2 × 3H, CH3, isomers 3a & 3c), 2.107 (s, 3H, CH3, isomer 3b), 2.135 (s, 3H, CH3, isomer 3a), 2.182 (s, 3H, CH3, isomer 3b), 2.269, 2.367 (each s, 2 × 3H, CH3, isomer 3d), 2.400 (s, 3H, CH3, isomer 3a), 2.415 (s, 3H, CH3, isomer 3b), 2.422 (s, 2 × 3H, CH3, isomers 3b & 3d), 2.446 (s, 2 × 3H, CH3, isomers 3c & 3d), 2.475 (s, 3H, CH3, isomer 3a), 5.406 (s, 1H, NH, isomer 3c), 5.487 (s, 1H, NH, isomer 3d), 5.623 (s, 1H, NH, isomer 3b), 5.635 (s, 2 × 1H, NH, isomers 3a & 3d), 5.672 (s, 1H, NH, isomer 3b), 5.776 (s, 1H, NH, isomer 3a), 5.871, 6.392 (each s, 2 × 1H, NH, isomer 3c), 6.449 (s, 2 × 1H, NH, isomers 3a & 3d), 6.496 (s, 1H, NH, isomer 3b), 6.635–6.872 (m, 4 × 8H, ArH, isomers 3a–d), 6.911–7.280 (m, 56H, ArH, isomers 3a & 3b (2 × 13H) & isomers 3c & 3d (2 × 15H)), 7.593 (d, JHH = 7.2 Hz, 2H, ArH, isomer 3a), 7.649 (t, JHH = 8.6 Hz, 2H, ArH, isomer 3b), 9.307 (s, 1H, NH, isomer 3d), 9.484 (s, 1H, NH, isomer 3c), 9.683 (s, 1H, NH, isomer 3b), 9.839 (s, 1H, NH, isomer 3a). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −73.94, −73.88, −73.66, −73.54. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2818, −2879. LRMS (EI) m/z [ion]: 966 [M + H]+, 852 [M – TFA]+. Anal. Calcd for C46H45N6O2F3Pt (Mw 965.96): C, 57.20; H, 4.70; N, 8.70. Found: C, 57.49; H, 4.74; N, 8.72.

[Pt{κ2(C,N)-C6H3Me-3(NHC(NHAr)(=NAr))-2}(TFA)(DMSO)] (Ar = 2-MeC6H4) (4)

pan class="Chemical">cis-[Pt(TFA)2(DMSO)2]n> (50 mg, 0.087 mmol) and pan class="Chemical">LH22-tolyl (29 mg, 0.088 mmol) were taken in a 25 mL pan class="Chemical">RB flask, to which was added toluene (10 mL). The RB flask was fitted with a double surface condenser capped with a freshly prepared anhydrous CaCl2 guard tube. The reaction mixture was refluxed for 8 h, cooled, and filtered. The volume of the filtrate was reduced to about one-third of its original volume and stored at RT for 2 days to afford 4 as colorless crystals. Crystals suitable for SCXRD were grown from a mixture of CH2Cl2/cyclohexane at RT over a period of 3 days. Yield: 78% (48 mg, 0.068 mmol). mp 217.0 °C. ATR–IR (cm–1): ν(NH) 3391 (m); νa(OCO) 1682 (s); ν(C=N) 1610 (m); νs(OCO) 1358 (m); ν(S=O) 1120 (s). 1H NMR (CDCl3, 400 MHz): δ 1.66, 2.08, 2.33 (each s, 3 × 3 H, CH3), 3.13, 3.16 (each s, 2 × 3 H, (CH3)2S(O)), 6.21, 6.49 (each s, 2 × 1H, NH), 6.81 (d, JHH = 4.8 Hz, 2H, ArH), 7.18–7.24 (m, 4H, ArH), 7.28–7.36 (m, 4H, ArH), 7.79 (t, JHH = 4.6 Hz; JPtH = 29.2 Hz, 1H, ArH). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −74.43. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −3650. LRMS (EI) m/z [ion]: 601 [M – TFA]+. Anal. Calcd for C26H28N3O3F3SPt (Mw 714.656): C, 43.70; H, 3.95; N, 5.88; S, 4.49. Found: C, 43.66; H, 3.80; N, 5.69; S, 4.45.

[Pt{κ2(C,N)-C6H3Me-5(NHC(NHAr)(=NAr))-2}(TFA)(DMSO)] (Ar = 4-MeC6H4; 5)

Complex 5 was n class="Chemical">preppan class="Chemical">ared from n>an class="Chemical">cis-[Pt(TFA)2(DMSO)2] (50.0 mg, 0.087 mmol) and LH24-tolyl (29 mg, 0.088 mmol) in toluene (10 mL) and purified as described previously for complex 4. Yield: 73% (45 mg, 0.063 mmol). mp 166.0 °C. ATR–IR (cm–1): ν(NH) 3399 (w); νa(OCO) 1682 (s); ν(C=N) 1611 (s); νs(OCO) 1367 (m); ν(S=O) 1142 (s). 1H NMR (CDCl3, 400 MHz): δ 2.32, 2.33, 2.38 (each s, 3 × 3 H, CH3), 3.14 (s, 2 × 3 H, ((CH3)2S(O)), 6.36 (d, JHH = 8.0 Hz, 1H, ArH), 6.51, 6.52 (each s, 2 × 1H, NH), 6.82 (d, JHH = 7.6 Hz, 1H, ArH), 6.97 (d, JHH = 8.4 Hz, 2H, ArH), 7.12 (d, JHH = 8.4 Hz, 2H, ArH), 7.19 (d, JHH = 8.4 Hz, 2H, ArH), 7.24 (d, JHH = 8.4 Hz, 2H, ArH), 7.79 (s, JPtH = 32.4 Hz, 1H, ArH). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −74.69. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −3674. LRMS (EI) m/z [ion]: 601 [M – TFA]+. Anal. Calcd for C26H28N3O3F3SPt (Mw 714.656): C, 43.70; H, 3.95; N, 5.88; S, 4.49. Found: C, 43.57; H, 3.94; N, 5.81; S, 4.56.

[Pt{κ2(C,N)-C6H2Me2-3,5(NHC(NHAr)(=NAr))-2}(TFA)(DMSO)] (Ar = 2,4-Me2C6H3; 6)

Complex 6 was n class="Chemical">preppan class="Chemical">ared from n>an class="Chemical">cis-[Pt(TFA)2(DMSO)2] (50.0 mg, 0.087 mmol) and LH22,4-xylyl (33 mg, 0.088 mmol) in toluene (10 mL) and purified as described previously for complex 4. Yield: 83% (55 mg, 0.072 mmol). mp 156.0 °C. ATR–IR (cm–1): ν(NH) 3404 (w); νa(OCO) 1690 (m); ν(C=N) 1610 (m); νs(OCO) 1380 (w); ν(S=O) 1134 (s). 1H NMR (CDCl3, 400 MHz): δ 1.65, 2.03, 2.25, 2.28, 2.30, 2.34 (each s, 6 × 3 H, CH3), 3.12, 3.15 (each s, 2 × 3 H, (CH3)2S(O)), 6.13, 6.41 (each s, 2 × 1H, NH), 6.62 (s, 1H, ArH), 7.00–7.13 (m, 6H, ArH), 7.60 (s, JPtH = 31.6 Hz, 1H, ArH). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −74.53. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −3655. LRMS (EI) m/z [ion]: 757 [M + H]+, 779 [M + Na]+, 643 [M – TFA]+. Anal. Calcd for C29H34N3O3F3SPt·H2O (Mw 774.751): C, 44.96; H, 4.68; N, 5.42; S, 4.14. Found: C, 45.36; H, 4.56; N, 5.48; S, 4.20.

[Pt{κ2(C,N)-C6H3(OMe)-3(NHC(NHAr)(=NAr))-2}(TFA)(DMSO)] (Ar = 2-(MeO)C6H4; 7)

pan class="Chemical">cis-[Pt(TFA)2(DMSO)2]n> (50.0 mg, 0.087 mmol) and pan class="Chemical">LH22-anisyl (66.4 mg, 0.176 mmol) were taken in a 25 mL pan class="Chemical">RB flask, to which was added toluene (10 mL). The RB flask was fitted with a double surface condenser capped with a freshly prepared anhydrous CaCl2 guard tube. The reaction mixture was refluxed for 8 h and cooled. The reaction mixture was filtered and stored at RT for 5 h to afford colorless crystals of [LH32-anisyl]TFA on the walls of the container. Subsequently, these crystals were used for SCXRD and other characterization. The filtrate was further concentrated under vacuum to afford a white solid. The solid was dissolved in CH2Cl2 (2 mL), layered with ethanol (5 mL), and stored at RT for a period of 1 week to afford 7 as colorless crystals.

Complex 7

Yield: 74% (49 mg, 0.064 mmol). mp 208.6 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3370 (m); νa(OCO) 1693 (m); νpn>an class="Chemical">(C=N) 1611 (s); νs(OCO) 1371(m); ν(S=O) 1115 (s). 1H NMR (CDCl3, 400 MHz): δ 3.12, 3.18 (each br, 2 × 3 H, (CH3)2S(O)), 3.80, 3.87, 4.02 (each s, 3 × 3 H, OCH3), 6.69 (d, JHH = 7.2 Hz, 1H, ArH), 6.92–6.98 (m, 5H, ArH), 7.14 (t, JHH = 8.0 Hz, 1H, ArH), 7.19–7.24 (m, 2H, ArH), 7.29–7.31 (m, 1H, ArH), 7.61 (d, JHH = 8.4 Hz; JPtH = 28.6 Hz, 1H, ArH), 7.76, 7.99 (each s, 2 × 1H, NH). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −74.39. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −3657. LRMS (EI) m/z [ion]: 763 [M + H]+, 649 [M – TFA]+, 571 [M – DMSO – TFA]+. Anal. Calcd for C26H28N3O6F3SPt (Mw 762.654): C, 40.95; H, 3.70; N, 5.51; S, 4.20. Found: C, 41.17; H, 3.46; N, 5.50; S, 4.11.

[LH32-anisyl]TFA

Yield: 81% (35 mg, 0.070 mmol). mp 189 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): νa(OCO) 1679 (m); νpn>an class="Chemical">(C=N) 1639 (s); νs(OCO) 1498 (m). 1H NMR (CDCl3, 400 MHz): δ 3.84 (s, 3 × 3H, OCH3), 6.70–6.75 (m, 6H, ArH), 7.02 (t, JHH = 7.6 Hz, 3H, ArH), 7.10 (d, JHH = 7.6 Hz, 3H, ArH). The signal for NH protons was not observed. 19F{1H} NMR (CDCl3, 376.31 MHz): δ −75.29. HRMS (EI) m/z [ion]: 378 [M – TFA]+. Anal. Calcd for C24H24N3O5F3 (Mw 491.459): C, 58.65; H, 4.92; N, 8.55. Found: C, 58.61; H, 4.55; N, 8.27.

trans-[PtCl2{ArN=C(NHAr)2}2] (Ar = 2-MeC6H4; 8)

pan class="Chemical">trans-[PtCl2(PhCN)2]n> (50.0 mg, 0.106 mmol) and pan class="Chemical">LH22-tolyl (76.7 mg, 0.233 mmol) were taken in a 50 mL pan class="Chemical">RB flask, to which was added toluene (15 mL). The RB flask was fitted with a double surface condenser capped with a freshly prepared anhydrous CaCl2 guard tube. The reaction mixture was refluxed for 48 h and cooled. The volatiles from the reaction mixture were removed under vacuum to afford a dull yellow brown solid. The solid was subjected to column chromatography on basic alumina with CHCl3 as the eluent. The fraction containing the title complex was concentrated under vacuum to 2 mL and left at RT for 1 week to afford 8 as bright orange yellow crystals, which was subsequently used for all characterization including SCXRD. Yield: 90% (88 mg, 0.095 mmol). mp 243.0 °C. ATR–IR (cm–1): ν(NH) 3377 (w), 3275 (w); ν(C=N) 1622 (m). 1H NMR spectra of 8 were measured in CDCl3 and C6D6, which revealed the presence of two species, which are hereafter indicated as isomers 8a and 8b in about 1.00:0.66 and 1.00:0.50 ratios, respectively, as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.91 (s, 2 × 3H, CH3, isomer 8a), 1.93 (s, 4 × 3H, CH3, isomers 8a & 8b), 2.00 (s, 2 × 3H, CH3, isomer 8b), 2.48 (s, 2 × 3H, CH3, isomer 8a), 2.52 (s, 2 × 3H, CH3, isomer 8b), 5.55 (s, 2 × 2H, NH, isomers 8a & 8b), 6.71–6.90 (m, 2 × 12H, ArH, isomers 8a & 8b), 6.99–7.06 (m, 2 × 6H, ArH, isomers 8a & 8b), 7.12 (br, 2 × 4H, ArH, isomers 8a & 8b), 7.78 (t, JHH = 8.8 Hz, 2 × 2H, ArH, isomers 8a & 8b), 8.54 (s, 2 × 2H, NH, isomers 8a & 8b). 1H NMR (C6D6, 400 MHz): δ 1.41 (s, 2 × 3H, CH3, isomer 8a), 1.48 (s, 2 × 3H, CH3, isomer 8b), 1.98 (s, 2 × 3H, CH3, isomer 8a), 2.00 (s, 2 × 3H, CH3, isomer 8b), 2.70 (s, 2 × 3H, CH3, isomer 8a), 2.74 (s, 2 × 3H, CH3, isomer 8b), 5.53 (s, 2 × 1H, NH, isomer 8b), 5.57 (s, 2 × 1H, NH, isomer 8a), 6.39 (d, JHH = 7.6 Hz, 2 × 2H, ArH, isomers 8a & 8b), 6.45 (t, JHH = 7.4 Hz, 2 × 2H, ArH, isomers 8a & 8b), 6.56–6.59 (m, 2 × 6H, ArH, isomers 8a & 8b), 6.70–6.71 (br, 4H, ArH, isomers 8a & 8b), 6.79 (broad t, JHH = 6.4 Hz, 2 × 2H, ArH, isomers 8a & 8b), 6.86 (broad d, JHH = 7.6 Hz, 2 × 2H, ArH, isomers 8a & 8b), 6.95 (br, 2 × 2H, ArH, isomers 8a & 8b), 7.02 (d, JHH = 6.4 Hz, 2H, ArH, isomer 8b), 7.13–7.20 (m, 6H, ArH, 4H (isomer 8a) & 2H (isomer 8b)), 8.37 (d, JHH = 6.8 Hz, 2 × 2H, ArH, isomers 8a & 8b), 9.10 (s, 2 × 2H, NH, isomers 8a & 8b). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −1799. LRMS (ESI) m/z [ion]: 852 [{M – (2HCl)} + H]+, 930 [M + Li]+. Anal. Calcd for C44H46N6Cl2Pt (Mw, 924.8780): C, 57.14; H, 5.01; N, 9.09. Found: C, 57.25; H, 4.75; N, 9.01.

Reactivity of 8

Method 1

Complex 8 (100 mg, 0.108 mmol) and n class="Chemical">pan class="Chemical">AgOAc (37.0 mg, 0.220 mmol) were taken in a 50 mL pn>an class="Chemical">RB flask covered with an Al foil. To the flask was added CH2Cl2 (20 mL), and the mixture was stirred for 24 h. Subsequently, the reaction mixture was filtered through a 0.2 μM syringe filter. The volatiles from the filtrate were removed under vacuum to one-third of its original volume and layered with toluene (2 mL), and the resulting solution was stored at RT for a period of 1 week to afford 1 as colorless crystals. Yield: 90% (90 mg, 0.097 mmol).

Method 2

Complex 8 (100 mg, 0.108 mmol) and n class="Chemical">pan class="Chemical">NaOAc (19.5 mg, 0.238 mmol) were taken in a 50 mL pn>an class="Chemical">RB flask, to which was added methanol (20 mL), and the resulting mixture was refluxed for 8 h. The reaction mixture was cooled, and volatiles were removed under vacuum to afford a solid. The solid was dissolved in CH2Cl2 (20 mL), and the insoluble material was filtered off. The filtrate was further concentrated under vacuum to about 3 mL, layered with toluene (3 mL), and stored at RT for 1 week to afford 1 as colorless crystals. Yield: 90% (89 mg, 0.097 mmol).

trans-[PtCl2{ArN=C(NHAr)2}2] (Ar = 2,4-Me2C6H3; 9)

Complex 9 was n class="Chemical">prepared from the reaction of n>an class="Chemical">trans-[PtCl2(PhCN)2] (50.0 mg, 0.106 mmol) and LH22,4-xylyl (86.6 mg, 0.233 mmol) in toluene (15 mL) and purified by the procedure as described previously for 8. The reaction mixture was subjected to column chromatography on basic alumina and eluted with chloroform. The fraction containing the title complex was concentrated under vacuum to afford oil, which was dissolved in n-hexane (5 mL) and left at RT for 4 days to afford 9 as dull orange yellow crystals. Yield: 45% (48 mg, 0.048 mmol). mp 230.0 °C. ATR–IR (cm–1): ν(NH) 3391 (w), 3300 (w); ν(C=N) 1597 (s). 1H NMR spectra of 9 in CDCl3 and C6D6 revealed the presence of two isomers, which are hereafter indicated as isomers 9a and 9b in about 1.00:0.80 and 1.00:0.63 ratios, respectively, as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.86 (s, 4 × 3H, CH3, isomers 9a & 9b), 1.93 (s, 2 × 3H, CH3, isomer 9a), 1.98 (s, 2 × 3H, CH3, isomer 9b), 2.06, 2.14, 2.21 (each s, 12 × 3H, CH3, isomers 9a & 9b), 2.41 (s, 2 × 3H, CH3, isomer 9a), 2.44 (s, 2 × 3H, CH3, isomer 9b), 5.44 (s, 2 × 2H, NH, isomers 9a & 9b), 6.60 (d, JHH = 5.6 Hz, 2 × 6H, ArH, isomers 9a & 9b), 6.66 (d, JHH = 8.4 Hz, 2 × 2H, ArH, isomers 9a & 9b), 6.82–6.92 (m, 2 × 8H, ArH, isomers 9a & 9b), 7.61 (t, JHH = 8.4 Hz, 2 × 2H, ArH, isomers 9a & 9b), 8.36 (s, 2 × 2H, NH, isomers 9a & 9b). 1H NMR (C6D6, 400 MHz): δ 1.51 (s, 2 × 3H, CH3, isomer 9a), 1.57 (s, 2 × 3H, CH3, isomer 9b), 1.73, 1.88, 1.96 (each s, 12 × 3H, CH3, isomers 9a & 9b), 2.07 (s, 2 × 3H, CH3, isomer 9a), 2.09 (s, 2 × 3H, CH3, isomer 9b), 2.74 (s, 2 × 3H, CH3, isomer 9a), 2.76 (s, 2 × 3H, CH3, isomer 9b), 5.57 (s, 2 × 1H, NH, isomer 9b), 5.61 (s, 2 × 1H, NH, isomer 9a), 6.25 (s, 2 × 2H, ArH, isomers 9a & 9b), 6.43 (br, 2 × 4H, ArH, isomers 9a & 9b), 6.56 (br, 2 × 2H, ArH, isomers 9a & 9b), 6.69 (s, 2 × 2H, ArH, isomers 9a & 9b), 6.81 (d, JHH = 7.2 Hz, 2 × 2H, ArH, isomers 9a & 9b), 7.02–7.13 (m, 2 × 4H, ArH, isomers 9a & 9b), 8.31 (d, JHH = 8.0 Hz, 2 × 2H, ArH, isomers 9a & 9b), 9.06 (s, 2 × 2H, NH, isomers 9a & 9b). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −1818, −1804. LRMS (ESI) m/z [ion]: 936 [{M – (2HCl)} + H]+, 1008 [M + H]+. Anal. Calcd for C50H58N6Cl2Pt (Mw, 1009.017): C, 59.52; H, 5.79; N, 8.33. Found: C, 59.25; H, 5.73; N, 8.40.

[Pt{κ2(C,N)-C6H3Me-3(NHC(NHAr)(=NAr))-2}(μ-Cl)]2 (Ar = 2-MeC6H4; 10)

Complex 8 (100 mg, 0.108 mmol) was disn class="Chemical">persed in methanol (20 mL) in a 50 mL n>an class="Chemical">RB flask, and the resulting heterogeneous mixture was refluxed for 8 h and cooled. During the course of the reaction, yellow solid slowly disappeared with concomitant formation of pale green solid. The volatiles from the reaction mixture were evaporated under vacuum to afford a solid. The solid was dissolved in CH2Cl2 (20 mL), concentrated under vacuum to about 3 mL, layered with toluene (6 mL), and stored at RT for 1 week to afford 10 as light green crystals and [LH32-tolyl]Cl as white raisin-like solid. Compound 10 was separated from the mixture by the hand-pick method, and both 10 and the salt were recrystallized from CH2Cl2/toluene mixture at RT separately over a period of 1 week. These crystals were subsequently used for other characterization including SCXRD in the case of 10.

Complex 10

Yield: 93% (56 mg, 0.050 mmol). mp 295.0 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3402 (w), 3379 (w); νpn>an class="Chemical">(C=N) 1624 (s). The 1H NMR spectrum of 10 revealed the presence of three isomers, which are hereafter indicated as isomers 10a–c in about 1.00:0.78:0.50 ratio as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.62 (s, 4 × 3H, CH3, isomers 10b & 10c), 1.63 (s, 2 × 3H, CH3, isomer 10a), 2.12 (s, 6 × 3H, 2 × 3H (isomer 10a), 4 × 3H (isomer 10c), CH3), 2.32 (s, 2 × 3H, CH3, isomer 10a), 2.44, 2.45 (each s, 4 × 3H, CH3, isomer 10b), 5.67 (s, 2 × 1H, NH, isomer 10a), 5.72 (s, 2 × 1H, NH, isomer 10c), 5.73 (s, 2 × 1H, NH, isomer 10b), 6.37 (br, 3 × 2H, NH, isomers 10a–c), 6.58 (t, JHH = 7.4 Hz, 2 × 2H, ArH, isomers 10a & 10b), 6.65–6.70 (m, 11H, isomers 10a & 10b (2 × 3H) & isomer 10c (5H), ArH), 7.01–7.19 (m, 3 × 9H, ArH, isomers 10a–c), 7.22–7.32 (br, 22H, isomers 10a & 10c (2 × 8H) & isomer 10b (6H), ArH), 7.44 (d, JHH = 7.2 Hz, 2H, ArH, isomer 10b). LRMS (ESI) m/z [ion]: 1119 [M + H]+. Anal. Calcd for C44H44N6Cl2Pt2 (Mw 1117.921): C, 47.27; H, 3.97; N, 7.52. Found: C, 46.99; H, 3.81; N, 7.38.

[LH32-tolyl]Cl

Yield: 91% (36 mg, 0.098 mmol). mp 249.0 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3120 (w, sh), 3030 (pn>an class="Chemical">br, w); ν(C=N) 1620 (s). 1H NMR (CDCl3, 400 MHz): δ 2.35 (s, 3 × 3H, CH3), 7.00–7.06 (m, 12H, ArH), 11.18 (br, 3 × 1H, NH). LRMS (ESI) m/z [ion]: 330 [M – Cl]+. Anal. Calcd for C22H24N3Cl (Mw 365.898): C, 72.22; H, 6.61; N, 11.48. Found: C, 72.43; H, 6.41; N, 11.48.

[Pt{κ2(C,N)-C6H2Me2-3,5(NHC(NHAr)(=NAr))-2}(μ-Cl)]2 (Ar = 2,4-Me2C6H3; 11)

Compound 11 was n class="Chemical">prepared from 9 (100 mg, 0.100 mmol) and purified following the procedure repn>orted previously for 10. Single crystn>an class="Chemical">als of 11·C7H8 were grown from a mixture of pan class="Chemical">CH2Cl2/toluene at RT over a period of 4 days.

Complex 11

Yield: 96% (57 mg, 0.048 mmol). mp 277.0 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3408 (w), 3340 (sh); νpn>an class="Chemical">(C=N) 1626 (s). The 1H NMR spectrum of 11 in C6D6 revealed the presence of two isomers, which are hereafter indicated as isomers 11a and 11b in about 1.00:0.96 ratio as estimated from the integrals of CH3 protons. 1H NMR (C6D6, 400 MHz, 0.0011 M): δ 1.490 (s, 2 × 3H, CH3, isomer 11b), 1.509 (s, 2 × 3H, CH3, isomer 11a), 1.554 (s, 2 × 3H, CH3, isomer 11b), 1.575, 1.584 (each s, 2 × 3H, CH3, isomer 11b), 1.916 (s, 4 × 3H, CH3, isomer 11a), 2.096 (s, 2 × 3H, CH3, isomer 11b), 2.210, 2.357, 2.368 (each s, 6 × 3H, CH3, isomer 11a), 2.441, 2.447, 2.508, 2.537 (each s, 4 × 3H, CH3, isomer 11b), 5.321 (s, 2 × 1H, NH, isomer 11a), 5.348, 5.351 (each s, 2 × 1H, NH, isomer 11b), 6.159, 6.169 (each s, 2 × 1H, NH, isomer 11b), 6.194, 6.198 (each br, 2 × 1H, NH, isomer 11a), 6.366–6.420 (m, 3H, ArH, isomer 11a (2H) & isomer 11b (1H)), 6.481–6.583 (m, 2 × 5H, ArH, isomers 11a & 11b), 6.915–7.067 (m, 4H, ArH, isomer 11a (1H) & isomer 11b (3H)), 7.116–7.204 (m, 11H, ArH, isomer 11a (6H) & isomer 11b (5H)), 7.356 (br, 2H, ArH, isomers 11a & 11b), 7.705–7.722 (m, 1H, ArH, isomer 11a), 7.947 (br, 1H, ArH, isomer 11b).

The pan class="Chemical">1Hn> NMR spectrum of 11 in n>an class="Chemical">CDCl3 revealed the presence of three isomers, which pan class="Chemical">are hereafter indicated as isomers 11a–c in about 1.00:1.13:0.91 ratio, respectively, as estimated from the integrals of CH3 protons. 1H NMR (CDCl3, 400 MHz): δ 1.60 (s, 4 × 3H, CH3, isomer 11c), 1.61, 2.06 (each s, 4 × 3H, CH3, isomer 11b), 2.07, 2.10 (each s, 8 × 3H, CH3, isomer 11a), 2.17, 2.25, 2.26 (each s, 4 × 3H, CH3, isomer 11b), 2.31 (s, 4 × 3H (isomer 11c), 2 × 3H (isomer 11b), CH3), 2.32, 2.33 (each s, 4 × 3H, CH3, isomer 11a), 2.37 (s, 2 × 3H, CH3, isomer 11b), 2.40, 2.44 (each s, 4 × 3H, CH3, isomer 11c), 5.62 (s, 2 × 1H, NH, isomer 11b), 5.67 (s, 2 × 1H, NH, isomer 11c), 5.69 (s, 2 × 1H, NH, isomer 11a), 6.29 (d, JHH = 6.0 Hz, 3 × 2H, ArH, isomers 11a–c), 6.49 (d, JHH = 7.6 Hz, 3 × 2H, ArH, isomers 11a–c), 6.86 (d, JHH = 7.2 Hz, 3 × 1H, ArH, isomers 11a–c), 6.91–7.08 (m, 33H, ArH (3 × 9 H) & NH (3 × 2 H), isomers 11a–c), 7.15 (d, JHH = 8.0 Hz, 2 × 2H, ArH, isomers 11a & 11b), 7.22 (d, JHH = 4.8 Hz, 2H, ArH, isomer 11c). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2863, −2854, −2848. LRMS (ESI) m/z [ion]: 1766 [Pt{κ2(C,N)}(μ-Cl)]3 – Cl]+, 1239 [M + K]+, 1223 [M + Na]+, 1201 [M + H]+, 1165 [M – Cl]+, 623 [Pt{κ2(C,N)}(μ-Cl)]2/2 + Na]}+. Anal. Calcd for C50H56N6Cl2Pt2·1/4C7H8 (Mw 1225.114): C, 50.73; H, 4.77; N, 6.86. Found: C, 50.84; H, 5.06; N, 6.68.

[LH32,4-xylyl]Cl

Yield: 92% (37 mg, 0.092 mmol). mp 241.0 °C. ATR–IR n class="Chemical">pan class="Chemical">(cm–1): ν(NH) 3150 (pn>an class="Chemical">br, w); ν(C=N) 1624 (s). 1H NMR (CDCl3, 400 MHz): δ 2.19 (s, 3 × 3H, CH3), 2.29 (br, 3 × 3H, CH3), 6.76–6.87 (m, 3 × 3H, ArH), 11.01 (br, 3 × 1H, NH). LRMS (ESI), m/z [ion]: 372 [M – Cl]+. Anal. Calcd for C25H30N3Cl. (Mw 407.978): C, 73.60; H, 7.41; N, 10.30. Found: C, 74.03; H, 7.19; N, 9.99.

[Pt{κ2(C,N)-C6H3Me-3(NHC(NHAr)(=NAr))-2}(μ-TFA)]2 (Ar = 2-MeC6H4; 12)

Complex 10 (55 mg, 0.049 mmol) and n class="Chemical">pan class="Chemical">AgTFA (22 mg, 0.098 mmol) were taken in a 50 mL class="Chemical">n>an class="Chemical">RB flask covered with an Al foil. To the flask was added CH2Cl2 (20 mL), and the mixture was stirred at RT for 24 h. The reaction mixture was filtered through a 0.2 μM syringe filter. The volatiles from the filtrate were evaporated under vacuum to one-third its original volume, layered with toluene (2 mL), and the resulting solution was stored at RT for a period of 1 week to afford 12·CH2Cl2 as yellowish green crystals. Yield: 94% (59 mg, 0.046 mmol). mp 269 °C. ATR–IR (cm–1): ν(NH) 3420 (w) and 3381 (w); νa(OCO) 1670 (s); ν(C=N) 1628 (s), 1599 (m); νs(OCO) 1540 (m). 1H NMR (CDCl3, 400 MHz): δ 1.76, 1.90, 2.49 (each s, 6 × 3H, CH3), 5.41 (s, 2 × 1H, NH), 6.22 (d, JHH = 8.0 Hz, 2H, ArH), 6.32 (s, 2 × 1H, NH), 6.84 (d, JHH = 7.2 Hz, 2H, ArH), 6.90–6.94 (m, 6H, ArH), 7.06 (t, JHH = 7.4 Hz, 2H, ArH), 7.17–7.24 (m, 8H, ArH), 7.45–7.47 (m, 2H, ArH). 19F{1H} NMR (CDCl3, 376.31 MHz): δ −74.33. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2555. Anal. Calcd for C48H44N6O4F6Pt2·1.25H2O (Mw 1295.564): C, 44.50; H, 3.62; N, 6.49. Found: C, 44.87; H, 3.34; N, 6.11.