Models for Cooperative Catalysis: Oxidative Addition Reactions of Dimethylplatinum(II) Complexes with Ligands Having Both NH and OH Functionality.

The role of NH and OH groups in the oxidative addition reactions of the complexes [PtMe2(κ2-N,N'-L)], L = 2-C5H4NCH2NH-x-C6H4OH [3, x = 2, L = L1; 4, x = 3, L = L2; 5, x = 4, L = L3], has been investigated. Complex 3 is the most reactive. It reacts with CH2Cl2 to give a mixture of isomers of [PtMe2(CH2Cl)(κ3-N,N',O-(L1-H)], 6, and decomposes in acetone to give [PtMe3(κ3-N,N',O-(L1-H)], 7, both of which contain the fac tridentate deprotonated ligand. Complex 3 reacts with MeI to give complex 7, whereas 4 and 5 react to give [PtIMe3(κ2-N,N'-L2))], 8, or [PtIMe3(κ2-N,N'-L3)], 9, respectively. Each complex 3, 4, or 5 reacts with either dioxygen or hydrogen peroxide to give the corresponding complex [Pt(OH)2Me2(κ2-N,N'-L)], 10, L = L1; 11, L = L2; 12, L = L3. The ligand L3 in complexes 9 and 12 is easily oxidized to the corresponding imine ligand 2-C5H4NCH=N-4-C6H4OH, L4, in forming the complexes [PtIMe3(κ2-N,N'-L4)], 13, and [Pt(OH)2Me2(κ2-N,N'-L4)], 14, respectively. The NH and OH groups play a significant role in supramolecular polymer or sheet structures of the complexes, formed through intermolecular hydrogen bonding, and these structures indicate how either intramolecular or intermolecular hydrogen bonding may assist some oxidative addition reactions.

Introduction

There is great current interest in cooperative catalysis, which combines two or more forms of catalysis to enhance reactivity or selectivity or to enable tandem catalysis.[1] The combination of transition-metal catalysis and acid catalysis is particularly effective. For example, the oxidation of methane to methanol by platinum(II) complexes occurs most readily in protic solvents.[2] Oxygen is ideal as the oxidant for methanol production from methane if a good catalyst system can be found.[3] However, catalysts designed by chemists have not yet approached the efficiency and selectivity of enzymic methane monooxygenase catalysts, which are able to deliver, in the required order for catalysis, oxygen, methane, electrons, and protons to the active site.[4] For design of platinum complexes for methane oxidation, one valuable approach is to study the reactivity and mechanism in individual steps of a catalytic cycle, and the step requiring activation of dioxygen has been the focus of several recent studies.[2,3,5] Of particular interest is the observation that activation of dioxygen can occur in the absence of a protic solvent if the supporting ligand in a dimethylplatinum(II) complex carries an alcohol or a phenol substituent and that the reactivity is enhanced if the ligand also has a free pyridyl donor group (Scheme ).[5] The role of the phenol is to act as a proton donor to the incipient peroxide group in the reaction of A to give B, while concurrent coordination of the pyridyl group supports the electron transfer from the electron-rich dimethylplatinum(II) center to dioxygen. The proposed hydroperoxide intermediate reacted rapidly with a second equivalent of A to form two equivalents of C, and C finally isomerized to form the stable product D.[5,6]

Scheme 1

Proposed Mechanism of Reaction of Complex A with Dioxygen To Give Complex D

The organometallic chemistry of alkyl complexes with appended hydroxyl groups is limited because many metal–alkyl bonds undergo facile protonolysis.[7] However, alkylplatinum(II) complexes contain relatively nonpolar metal–carbon bonds which react only slowly with phenols, while alkylplatinum(IV) complexes are completely inert. As a result, there is now an extensive supramolecular chemistry of organoplatinum complexes with hydrogen bonds.[8] These intermolecular hydrogen bonds might also play an important role in catalysis or in stoichiometric bond activation.[9] The application of NH functionality in organometallic cooperative catalysis has also become important in catalyst design, by molecular recognition through hydrogen bonding to give catalyst–substrate or catalyst–catalyst interactions.[10a] Dimethylplatinum(II) complexes containing primary or secondary amine donors have not been studied systematically. These complexes, such as [PtMe2(H2NCH2CH2NH2)], are sometimes difficult to handle, and they react with oxygen in air only if an extra donor atom is present.[6,10b] This paper reports the reactivity toward oxidative addition of dimethylplatinum(II) complexes with ligands containing both O–H and N–H bond functionality and the rich supramolecular chemistry of the resulting organoplatinum(IV) complexes.

Results and Discussion

The aminopyridine ligands L1–L3, which contain phenol substituents at the amine donor, and their dimethylplatinum(II) complexes, which have been reported earlier, are shown in Scheme .[11,12] The complexes [PtMe2(L)], 3–5, are difficult to isolate from the equilibrium mixture formed by reaction between [Pt2Me4(μ-SMe2)2], 1,[13] and the ligands L1–L3 (Scheme ), and subsequent reactions were carried out from the complexes formed in situ.[12]

Scheme 2

Ligands Used in This Work and the Dimethylplatinum(II) Complexes 3–5

Oxidative Addition Chemistry of Complexes 3–5

Complexes 4 and 5 were stable in solutions in acetone, but complex 3 was less stable (see below). Complex 3 also reacted with solvent dichloromethane by oxidative addition and loss of HCl to give the isomeric complexes 6a and 6b in about 5:4 ratio (Scheme ). Because of the absence of symmetry, the 1H NMR spectrum of each isomer contained two methylplatinum resonances at δ = 0.76, 2J(PtH) = 71 Hz, and 0.96, 2J(PtH) = 69 Hz, for 6a and 1.03, 2J(PtH) = 70 Hz, and 1.09, 2J(PtH) = 71 Hz, for 6b, in the range expected for platinum(IV) complexes. The CH2N group for each isomer gave two resonances at δ 4.66 and 5.39 (6a) and at 4.64 and 4.82 (6b). Similarly, each PtCH2Cl group gave two resonances at δ 3.86, 2J(PtH) = 75 Hz, and 4.32, 2J(PtH) = 88 Hz, for 6a and 3.74, 2J(PtH) = 33 Hz, and 3.84, 2J(PtH) = 20 Hz, for 6b.[14a] The NMR data do not define the stereochemistry, but crystallization gave the complex [PtMe2(CH2Cl)(κ3-N,N′,O-L1-H)] in which 6a and 6b were present in about 70:30 disordered occupancy (Figure ). The structure determination reveals that the deprotonated ligand is present in the fac tridentate bonding mode, with the other three sites at the octahedral platinum(IV) center occupied by two methyl groups and a chloromethyl group. In 6a and 6b, the chloromethyl group is trans to pyridyl nitrogen and oxygen, respectively, and in each case, there is a hydrogen bond interaction NH···Cl, with the distance N···Cl = 3.23(1) Å in 6a. A third isomer is possible in which the chloromethyl group is trans to the amine nitrogen, but this was not observed, perhaps because no intramolecular NH···Cl hydrogen bond would be possible. The mechanism of the reaction with dichloromethane was not determined, but precedents usually invoke free-radical intermediates.[14b−14d]

Scheme 3

Formation of Complexes 6a and 6b by Reaction of 3 with Dichloromethane

Figure 1

Structures of complexes (above) 6a and (below) 6b, showing 30% probability ellipsoids. Selected bond parameters for 6a: Pt(1)C(1) 2.040(6), Pt(1)C(2) 2.047(14), Pt(1)C(3) 2.05(2), Pt(1)N(1) 2.148(5), Pt(1)N(2) 2.165(5), Pt(1)O(1) 2.115(4), C(3)Cl(1) 1.69(2), N(2)···Cl(1) 3.23(2) Å; N(1)Pt(1)N(2) 77.65(18), O(1)Pt(1)N(1) 88.0(2), O(1)Pt(1)N(2) 81.1(2)°.

The reactivity to oxidative addition was first tested by reaction of complexes 3–5 with methyl iodide (Scheme ).[15] Complex 3 reacted with loss of HI and coordination of the phenoxide group to give fac-[PtMe3{κ3-N,N′,O-(L1-H)}], 7, whereas 4 and 5 reacted by simple oxidative addition to give fac-[PtIMe3(κ2-N,N′-L2)], 8, and fac-[PtMe3I(κ2-N,N′-L3)], 9, respectively. Complex 7 has no symmetry, so it gave three methylplatinum resonances at δ 0.58, 2J(PtH) = 74 Hz, at δ 0.81, 2J(PtH) = 82 Hz, and at δ 1.02, 2J(PtH) = 69 Hz in the 1H NMR spectrum and at δ −6.8, −11.4 and −16.8 in the 13C NMR spectrum. The CH2 resonances of 7 were observed in the 1H NMR spectrum at δ = 4.61 and 4.76 as CHaCHb doublets with coupling constant 2J(HH) 16 Hz, and in the 13C NMR spectrum by a singlet at δ 62.9. Complex 7 crystallized as a hydrate 7·H2O and its molecular structure is shown in Figure . It confirms that the ligand L1 is deprotonated and coordinated in the fac-κ3-N,N′,O bonding mode, as observed for complex 6 (Figure ). The water molecule [O atom is O(1S) in Figure ] takes part in intermolecular hydrogen bonding.

Scheme 4

Reactions with Methyl Iodide

Figure 2

Structure of complex 7·H2O, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.039(2), Pt(1)C(2) 2.041(3), Pt(1)C(3) 2.025(2), Pt(1)N(1) 2.160(2), Pt(1)N(2) 2.162(2), Pt(1)O(1) 2.146(2), O(1)···O(1S) 2.86(1) Å; N(1)Pt(1)N(2) 77.91(7), O(1)Pt(1)N(1) 88.61(7), O(1)Pt(1)N(2) 81.55(7)°.

The complexes 8 and 9 exist in solution as a mixture of two isomers, arising from the chirality at the amine nitrogen (Scheme ). For example, in the 1H NMR spectrum of 9, there were three methylplatinum resonances at δ 0.57, 2J(PtH) = 73 Hz, 0.95, 2J(PtH) = 71 Hz, and 1.39, 2J(PtH) = 73 Hz, for the major isomer and three methylplatinum resonances at δ 0.80, 2J(PtH) = 71 Hz, 0.94, 2J(PtH) = 73 Hz, and 1.40, 2J(PtH) = 72 Hz, for the minor isomer. The complexes crystallized as the isomers 8a and 9a, with the N–H and Pt–I groups mutually syn, and the structures are shown in Figures and 4, respectively.

Figure 3

Structure of complex 8a, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.041(7), Pt(1)C(2) 2.240(5), Pt(1)C(3) 2.053(7), Pt(1)N(1) 2.169(6), Pt(1)N(2) 2.241(5) Å; N(1)Pt(1)N(2) 76.27(19)°.

Figure 4

Structure of complex 9a, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.037(7), Pt(1)C(2) 2.058(7), Pt(1)C(3) 2.048(6), Pt(1)N(1) 2.159(5), Pt(1)N(2) 2.263(6) Å; N(1)Pt(1)N(2) 77.7(2)°.

Under an inert atmosphere, complex 3, which was present as an equilibrium mixture with [PtMe2(SMe2)2], 2, free L1, and Me2S (Scheme ), decomposed slowly in acetone or acetonitrile to give a mixture of mainly two products, which were not easily separated. One of these products was identified as complex 7, which could be crystallized from the solution as either 7·H2O [the same product as obtained from 3 with methyl iodide (Figure )] or as a cocrystal with the ligand, 7·L1. However, the other product could not be crystallized and the structure was not defined by the NMR spectra. In acetone-d6, it gave a single methylplatinum resonance at δ 1.43, 2J(PtH) = 74 Hz, suggesting that the decomposition involves a disproportionation to give a mixture of PtMe3 and PtMe complexes, for which there are several precedents.[16]

Each of the complexes 3–5 in acetone solution reacted with hydrogen peroxide or with dioxygen to give the corresponding trans-dihydroxoplatinum(IV) complexes 10–12 (Scheme ) as colorless precipitates. The 1H NMR spectrum of each complex contained two methylplatinum resonances. For example, complex 10 gave methylplatinum resonances at δ 1.13, 2J(PtH) = 71 Hz, and 1.62, 2J(PtH) = 73 Hz, in the range expected for platinum(IV) complexes. Structure determinations were carried out for each complex as 10, 11·H2O, and 12·MeOH, and the molecular structures are shown in Figures –67, respectively. In each case, the platinum center has octahedral stereochemistry and the two oxygen donors are mutually trans. The complexes are drawn in Scheme as dihydroxo complexes with an appended phenol group, but, given the greater acidity of a phenol compared to a PtOH group, they might be more accurately represented as containing trans-[Pt(OH)(OH2)]+ groups with an appended phenolate group (Chart ). Under similar conditions, dimethylplatinum(II) complexes such as [PtMe2(2,2′-bipyridine)], which do not contain NH or OH functional groups, react similarly with hydrogen peroxide but not with dioxygen.[17] A hydroxylic solvent or a higher concentration of water is needed to enable the reaction of [PtMe2(2,2′-bipyridine)] with oxygen to occur, so the NH or OH group clearly plays a role in the reactions to give complexes 10–12 (Scheme ).[6,17] Complex 3 is the only precursor that is designed to form an intramolecular hydrogen bond to the forming peroxide group in an analogue of B (Scheme ), so an intermolecular stabilization is probable in the reactions of 4 and 5 with dioxygen.

Scheme 5

Formation of Complexes 10–12. In Each Case, the Reagent (i) Can Be Either H2O2 or O2/H2O

Figure 5

Structure of complex 10, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.037(9), Pt(1)C(2) 2.035(8), Pt(1)N(1) 2.176(7), Pt(1)N(2) 2.169(7), Pt(1)O(1) 2.018(5), Pt(1)O(2) 2.023(6) Å; N(1)Pt(1)N(2) 77.8(3)°; Pt(2)C(15) 2.026(8), Pt(2)C(16) 2.021(9), Pt(2)N(3) 2.155(7), Pt(2)N(4) 2.243(7), Pt(2)O(4) 2.011(5), Pt(2)O(5) 1.993(5) Å; N(3)Pt(2)N(4) 77.9(3)°.

Figure 6

Structure of complex 11·H2O, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.033(7), Pt(1)C(2) 2.027(6), Pt(1)N(1) 2.166(5), Pt(1)N(2) 2.211(5), Pt(1)O(1) 2.011(4), Pt(1)O(2) 2.023(4), O(1)···O(1S) 2.76(1) Å; N(1)Pt(1)N(2) 77.4(1)°.

Figure 7

Structure of complex 12·MeOH, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.034(5), Pt(1)C(2) 2.028(7), Pt(1)N(1) 2.175(6), Pt(1)N(2) 2.241(4), Pt(1)O(1) 1.995(4), Pt(1)O(2) 2.029(4), O(1)···O(1S) 2.62(1) Å; N(1)Pt(1)N(2) 75.0(1)°.

Chart 1

Intramolecular Hydrogen Bond in Complex 10 and Formulation as Pt(OH)2···Phenol, 10a, or [Pt (OH)(OH2)]+···Phenolate, 10b

In the structure of complex 10, there are two independent molecules, which have opposite chirality at both the amine nitrogen and platinum centers (Figure ). The hydrogen atoms were not directly located, so their placement in a complex hydrogen-bonded assembly is uncertain. There is a strong intramolecular hydrogen bond in both independent molecules, with O(2)···O(3) 2.49(1) and O(5)···O(6) 2.48(1) Å, and there are also weaker intermolecular hydrogen bonds (see later). The molecular structures of 11 and 12 are similar to that of 10 except that they lack the intramolecular hydrogen bond and instead have a hydrogen-bonded solvate molecule (H2O in 11, Figure , MeOH in 12, Figure ).

Complexes of Ligand L4

Two complexes of the imine ligand L4 were initially prepared unexpectedly by oxidation of the amine ligand L3 (Scheme ). The complex [PtIMe3(L4)], 13, was formed by reaction of complex 9 with Ag2O in an attempt to abstract the iodide ligand and deprotonate the phenol to form an oligomer or polymer [PtMe3(L3-H)}]. Instead, the silver oxide evidently oxidized the ligand L3. The complex [Pt(OH)2Me2(L4)], 14, was prepared during an attempt to recrystallize complex 12 in the presence of hydrogen peroxide, with oxidation of the ligand L3 by H2O2. The complexes 13 and 14 were then prepared independently by the oxidative addition of MeI or H2O2, respectively, to the platinum(II) complex [PtMe2(L4)], 15, which was prepared in situ by reaction of [Pt2Me4(μ-SMe2)2], 1, with the ligand L4. No oxidation of the ligands L1 or L2 was observed in similar reactions of complexes 3 or 4 with methyl iodide or hydrogen peroxide. In contrast to the complex 5, containing amine ligand L3, complex 15 did not react easily with dioxygen, indicating that the NH group in 5 plays a role in the oxygen activation step (Scheme ). The 1H NMR spectrum of 13 in CD3OD showed three methylplatinum resonances as expected at δ 1.46, 1.18, and 0.63 with values of 2J(PtH) in the range 72–73 Hz, and the resonance of the characteristic imine proton resonance at δ 9.02 with 3J(PtH) = 27 Hz. The imine resonance for 14 appeared at δ 9.15, 3J(PtH) = 16 Hz.

Scheme 6

Synthesis of Complexes 13 and 14, Containing Ligand L4. Reagents: (i) Ag2O and (ii) H2O2

The structure of complex 13 is shown in Figure . The main difference from complex 9 (Figure ) is the orientation of the phenol substituent and near planarity at the imine nitrogen atom N(2) of the ligand L4. There is an intermolecular OH···I hydrogen bonding interaction with distance O(1)···I(1A) 3.47(1) Å, which forms a racemic dimer with molecules related by an inversion center. The orientation of the ligand L4 in complex 14 is similar, as shown in Figure , and the hydroxo ligands are mutually trans, as in the complexes 10–12 (Figures –67).

Figure 8

Structure of complex 13, with 30% probability ellipsoids, showing the dimer formed by OH···I hydrogen bonding. Selected bond parameters: Pt(1)C(1) 2.057(5), Pt(1)C(2) 2.043(5), Pt(1)C(3) 2.049(5), Pt(1)N(1) 2.166(4), Pt(1)N(2) 2.227(5), Pt(1)I(1) 2.7851(15) Å; N(1)Pt(1)N(2) 76.2(2)°. Symmetry-related atoms: x, y, z; 2 – x, −y, 1 – z.

Figure 9

Structure of complex 14, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)C(1) 2.034(3), Pt(1)C(2) 2.043(3), Pt(1)N(1) 2.171(3), Pt(1)N(2) 2.212(3), Pt(1)O(1) 2.019(3), Pt(1)O(2) 2.005(3) Å; N(1)Pt(1)N(2) 76.4(1)°.

Supramolecular Structures of the Platinum(IV) Complexes

All of the complexes described above form hydrogen bonds. Complex 6 (Figure ) contains only an intramolecular hydrogen bond, and complex 13 (Figure ) forms only a head-to-tail dimer through intermolecular hydrogen bonding. However, the other platinum(IV) complexes form more complex supramolecular structures, ranging from 1D polymers to 2D sheets and these are discussed below.

The supramolecular structure of complex 7 as the 1:1 compound 7·L1 is shown in Figure . There are two independent molecules with platinum atoms Pt(1) and Pt(2). Each forms a supramolecular polymer by self-recognition between neighbors. For example, in the Pt(1) chain, each L1 molecule bridges between two Pt(1) molecules, having the same chirality, through PtO···HO and PtNH···N(py) hydrogen bonds with O(1)···O(2) 2.56 Å and N(2)···N(3) 3.03 Å. Each polymer chain is therefore isotactic. The lattice symmetry includes an inversion center, so there are equal numbers of polymer chains with C and A chirality of individual platinum centers. The NH groups of the free ligand molecules L1 do not take part in hydrogen bonding.

Figure 10

Supramolecular isotactic polymeric structure of complex 7·L1. Symmetry-related atoms in neighboring molecules: Pt(1) chain: x, y, z; 1–x, 1/2 + y, 1/2 – z; 1 – x, −1/2 + y, 1/2 – z; Pt(2) chain: x, y, z; −x, 1/2 + y, 1/2 – z; −x, −1/2 + y, 1/2 – z.

In contrast, the complex 7·H2O forms a hydrogen-bonded sheet structure illustrated in Figure . Each water molecule bridges between three molecules of 7 acting as the hydrogen bond donor to two, with O(1S)···O(1) 2.86 Å and O(1S)···O(1A) 2.90 Å, and acceptor to one, with O(1S)···N(2B) 2.94 Å. Propagation of this motif gives a racemic sheet structure.

Figure 11

Part of the racemic supramolecular sheet structure of complex 7·H2O. Symmetry-related atoms: x, y, z; A, 2 – x, 1 – y, 1 – z; B, 3/2 – x, −1/2 + y, 1/2 – z.

Complex 8 forms a more complex sheet structure shown in Figure . Syndiotactic polymer chains are formed by intermolecular OH···I hydrogen bonding [O(1)···I(1B) = O(1A)···I(1) = 3.55 Å], with self-discrimination (Figure a). A weaker intermolecular NH···I hydrogen bonding [N(2)···I(1B) = N(2A)···I(1) = 3.76 Å] gives isotactic polymer chains by self-recognition (Figure b). The combination of these two effects gives a corrugated sheet structure, in which parallel chains of the OH···I bonded polymers (shown end-on in Figure c) can be considered to be cross-linked by the interchain NH···I hydrogen bonds (Figure c).

Figure 12

Supramolecular corrugated sheet structure of complex 8, showing (a) the OH···I links, (b) the NH···I links, and (c) a side view of the resulting sheet structure.

Complex 9 also forms a sheet structure but with a significant difference (Figure ). In this case, isotactic polymer chains are formed by intermolecular OH···I hydrogen bonding [O(1)···I(1B) = O(1A)···I(1) = 3.51 Å], with self-recognition (Figure ). A weaker intermolecular NH···I hydrogen bonding [N(2)···I(1B) = N(2A)···I(1) = 3.77 Å] also gives isotactic polymer chains by self-recognition. The combination of these two effects gives a sheet structure, in a similar way as for complex 8, with the major difference that the sheets for 9 are chiral, with all platinum and nitrogen centers having the same chirality. The space group for 9 is P212121, which is common when resolution of enantiomers by direct crystallization is successful, including the classic case of Pasteur’s sodium ammonium tartrate.[18]

Figure 13

Part of the chiral sheet structure of complex 9, formed by OH···I and NH···I hydrogen bonding.

Complex 10 forms supramolecular polymeric chains by a combination of intermolecular OH···O and NH···O hydrogen bonding (Figure ), with O(1)O(5A) 2.68 Å, N(2)O(5A) 2.92 Å, O(2)O(4) 2.68 Å, and O(2)N(4) 2.88 Å. The independent Pt(1) and Pt(2) centers (Figure ) have opposite chirality but all equivalent atoms in each chain have the same chirality. The space group contains an inversion center, so there are equal numbers of chains in which the Pt(1)Pt(2)Pt(1)Pt(2) sequences have CACA and ACAC chirality.

Figure 14

Part of the polymeric chain structure of complex 10. Symmetry equivalent atoms: x, y, z; A, −1 + x, y, z; B, 1 + x, y, z.

Complex 11·H2O forms a supramolecular sheet structure (Figure ). The strongest hydrogen bonds involve the phenol (phenolate) groups which form head-to-tail dimer units (Figure b), with O(2)···O(3A) 2.60 Å. On the opposite side, there are head-to-tail dimers formed by NH···O hydrogen bonds (Figure a) with O(1)···N(2A) 2.87 Å. Individual molecules in each of these dimer units is related by an inversion center, and the combined effect is to form syndiotactic polymer chains by self-discrimination. The third hydrogen bonding interaction involves the solvent water molecules (Figure c, O(1)···O(1S) 2.76 Å, O(1)···O1(SA) 2.98 Å), which can be considered to cross-link the polymer chains to form the final sheet structure.

Figure 15

Intermolecular hydrogen bonding motifs in forming the racemic sheet structure of complex 11.

Part of the supramolecular sheet structure of complex 12·MeOH is shown in Figure . The strongest intermolecular hydrogen bond is O(2)···O(3A) = O(3)···O(2B) = 2.57 Å and this gives an isotactic polymeric unit (Figure a). These polymers are cross-linked to polymers of opposite chirality by multiple weaker hydrogen bonds involving bridging phenol(phenoxide) [O(3)···O(1B) 2.94 Å] and bridging methanol solvate molecules [O(1)···O(1S) 2.62 Å, O(1S)···N(2C) 2.89 Å] to give a complex racemic sheet structure (Figure b).

Figure 16

Intermolecular hydrogen bonds in the sheet structure of complex 12·MeOH.

The absence of an NH proton leads to a simpler supramolecular polymeric structure for complex 14·2MeOH (Figure ). Individual molecules in each chain lie on a twofold screw axis. They are connected through bridging phenol(phenoxide) groups [O(1)···O(3A) = O(3)···O(1B) = 2.51 Å, O(2)···O(3B) = O(3)···O(2A) = 2.96 Å] and methanol solvate molecules [O(1)···O(1S) = 2.86 Å, O(2)···O(1SC) = 2.62 Å]. The second methanol solvate is hydrogen-bonded only to O(2), with O(2)···O(2SC) = 2.76 Å.

Figure 17

Part of the supramolecular polymeric ribbon structure of complex 14·2MeOH.

Conclusions

The research was intended to discover how the presence of phenol OH and amine NH groups influence oxidative addition to dimethylplatinum(II) complexes, as a model for key steps in potential cooperative catalysis. Only the ligand L1 has the phenol OH group in a position to play a strong intramolecular role in supporting oxidative addition. It is interesting to note that the phenol group in [PtMe2(L1)] could provide anchimeric assistance as a donor or acceptor group. When the phenol is deprotonated with phenolate coordination during oxidative addition, the donor effect is more likely, and when coordination of phenolate does not occur, the acceptor role is more likely. These cases are represented by oxidative addition with methyl iodide (Scheme ) or hydrogen peroxide and O2/H2O (Scheme ) respectively, as illustrated by proposed mechanisms in Scheme . With methyl iodide, phenol coordination accompanies the oxidative addition (a new PtMe bond is formed on the face remote from the phenol) to give the cationic intermediate E and then product 7. In contrast, we suggest that hydrogen peroxide or dioxygen attacks at the face adjacent to the phenol substituent and in the case of H2O2 (Scheme ) serves to stabilize the leaving hydroxide group.

Scheme 7

Possible Roles of 2-Phenol Group in 3 as Electron Donor in Reaction with Methyl Iodide or Acceptor in Reaction with Hydrogen Peroxide

Each of the dimethylplatinum(II) complexes 3, 4, and 5 react with dioxygen, though only complex 3 contains the 2-phenol unit needed to form an intermediate analogous to B (Scheme ). It is likely that the reactions of 4 and 5 with dioxygen are instead aided by intermolecular hydrogen bonding to the forming peroxide group. It is possible that the NH group could also assist with the dioxygen activation through hydrogen bonding, but there is no direct evidence for this. Both the phenol OH and amine NH groups are important in the supramolecular chemistry of the complexes. The phenol groups form the shorter hydrogen bond distances and so probably have a dominant role in forming the preferred supramolecular structures. It is likely that the role in the oxidative addition reactions is also stronger for the phenol OH groups. Thus, the supramolecular structures observed for the platinum(IV) products can be considered to give a clue to the role of the NH and OH groups in the bond activation steps. For example, in the platinum(IV) complexes [Pt(OH)2Me2(L)], the phenol groups are involved in intramolecular hydrogen bonding when L = L1 (Figures and 14) but in intermolecular hydrogen bonding when L = L2, L3, or L4 (Figures –1617), and the same intramolecular versus intermolecular hydrogen bonding is proposed for the activation of dioxygen or hydrogen peroxide in forming these platinum(IV) products from complex 3 or 4, 5, and 15, respectively. Studies of both bond activation steps and supramolecular structures can provide insight into the ways that cooperative catalysis using electron-rich transition-metal complexes in combination with NH or OH functional groups might function.[1−4]

Experimental Section

NMR spectra were recorded using Bruker 400 NMR, Inova 400, and Inova 600 spectrometers. The NMR labeling is shown in Chart . The complex [Pt2Me4(μ-SMe2)2], dimethylplatinum(II) complexes 3–5, and ligands L1–L4 were synthesized according to literature procedures.[12,13,19,20]

Chart 2

NMR Labeling Scheme

Structure Determinations[21]

Single-crystal X-ray diffraction measurements were made using a Bruker APEX-II charge coupled device diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Single crystals of the complexes were immersed in paraffin oil and mounted on MiTeGen MicroMounts. The structures were solved using direct methods and refined by the full-matrix least-squares procedure of SHELXTL. The hydrogen atoms were introduced at idealized positions and were allowed to refine isotropically. Full crystallographic data are given in the CIF files (CCDC 1844768–1844777).

Many of the structures contain Pt–OH, C6H4OH, H2O, and MeOH groups, with extensive hydrogen bonding. The H-atom positions were usually determined by short H-bond distances, but in other cases, the positions are tentative. For complex 8, there was evidence for a small degree of disorder between the mutually trans methyl and iodo groups, which led to anomalous electron density around the methyl atoms and a long Pt–Me distance. Attempts to model the disorder were unsuccessful and some restraints were used to mitigate the problem.

[PtMe2(CH2Cl)(L1-H)], 6a and 6b

To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.166 g, 0.289 mmol) in dichloromethane (10 mL) was added a solution of ligand L1 (0.116 g, 0.578 mmol) in dichloromethane (5 mL). The solution was stirred for 12 h under nitrogen, then filtered to remove the insoluble material, and the filtrate was layered with pentane (60 mL) to give colorless plate crystals after several days. Yield: 0.055 g, 20%. NMR in CDCl3 (400 MHz, 25 °C): 6a, δ(1H) 8.62 (d, 1H, 3J(HH) = 5 Hz, H6a), 7.80 (dd, 1H, 3J(HH) = 9 Hz, 8 Hz, H4a), 7.42 (dd, 1H, 3J(HH) = 5 Hz, 9 Hz, H5a), 7.35 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.14 (d, 1H, 3J(HH) = 7 Hz, H3), 6.95 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H5), 6.74 (d, 1H, 3J(HH) = 8 Hz, H6), 6.46 (dd, 1H, 3J(HH) = 7 Hz, 7 Hz, H4), 5.39 (d, 1H, 2J(HH) = 15 Hz, 3J(HH) = 5, H7a), 4.87 (br, 1H, NH), 4.66 (d, 1H, 2J(HH) = 15 Hz, 3J(HH) = 5, H7b), 4.32 (d, 1H, 2J(HH) = 7 Hz, 2J(PtH) = 88 Hz, PtCHaHb), 3.86 (d, 1H, 2J(HH) = 7 Hz, 2J(PtH) = 75 Hz, PtCHaHb), 0.96 (s, 3H, 2J(PtH) = 69 Hz, PtMe); 0.76 (s, 3H, 2J(PtH) = 71 Hz, PtMe); 6b, δ(1H): 8.62 (d, 1H, 3J(HH) = 5 Hz, H6a), 7.83 (dd, 1H, 3J(HH) = 8 Hz, 9 Hz, H4a), 7.44 (dd, 1H, 3J(HH) = 5 Hz, 9 Hz, H5a), 7.40 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.12 (d, 1H, 3J(HH) = 7 Hz, H3), 6.94 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H5), 6.69 (d, 1H, 3J(HH) = 8 Hz, H6), 6.45 (t, 1H, 3J(HH) = 7 Hz, H4), 5.27 (br, 1H, NH), 4.82 (d, 1H, 2J(HH) = 14 Hz, 3J(HH) = 5, H7a), 4.64 (d, 2J(HH) = 14 Hz, 3J(HH) = 5, H7b), 3.84 (d, 1H, 2J(HH) = 7 Hz, 2J(PtH) = 20 Hz, PtCHaHb), 3.74 (d, 1H, 2J(HH) = 7 Hz, 2J(PtH) = 33 Hz, PtCHaHb), 1.09 (s, 3H, 2J(PtH) = 71 Hz, PtMe); 1.03 (s, 3H, 2J(PtH) = 70 Hz, PtMe).

[PtMe3(L1-H)], 7

To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.144 g, 0.251 mmol) in acetone (10 mL) was added a solution of ligand L1 (0.100 g, 0.502 mmol) in acetone (10 mL), followed by MeI (31 μL, 0.502 mmol). The solution was stirred for 1 h, then filtered and pentane (60 mL) was added to precipitate the product, which was washed with pentane (3 × 20 mL), and dried in vacuo. Yield: 0.088 g, 40%. Anal. Calcd for C15H20N2OPt: C, 41.00; H, 4.59; N, 6.38. Found: C, 40.81; H, 5.12; N, 6.33%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.62 (d, 1H, , 3J(PtH) = 16 Hz, 3J(HH) = 7 Hz, H6a), 7.81 (dd, 1H, 3J(HH) = 8 Hz, 9 Hz, H4a), 7.40 (m, 2H, H5a,H3a), 7.17 (d, 1H, 3J(HH) = 9 Hz, H3), 6.85 (dd, 1H, 3J(HH) = 8 Hz, 9 Hz, H5), 6.82 (br, 1H, NH), 6.56 (d, 1H, 3J(HH) = 9 Hz, H6), 6.46 (dd, 1H, 3J(HH) = 9 Hz, 8 Hz, H4), 4.76 (d, 1H, 2J(HH) = 16 Hz, 3J(HH) = 5 Hz, H7a), 4.61 (d, 2J(HH) = 16 Hz, 3J(HH) = 5 Hz, H7b), 1.02 (s, 3H, 3J(PtH) = 69 Hz, PtMe), 0.81 (s, 3H, 3J(PtH) = 82 Hz, PtMe), 0.58 (s, 3H, 3J(PtH) = 74 Hz, PtMe); δ(13C): 167.0, 160.7, 147.2, 139.4, 136.0, 129.8, 127.5, 125.4, 124.0, 121.4, 116.3, 62.9 (CH2), −6.8 (PtMe), −11.4 (PtMe), −16.8 (PtMe).

Complex 7·L1 was prepared as follows. To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.200 g, 0.350 mmol) in acetone (10 mL) was added a solution of ligand L1 (0.140 g, 0.700 mmol) in dry CH2Cl2 (10 mL). The solution was stirred for 12 h under air, then filtered to remove the insoluble material, and the filtrate was layered with pentane (75 mL) to give orange block crystals after several days. The single crystals were collected, washed with pentane, and dried in vacuo. Yield: 0.060 g, 20%.

[PtIMe3(L2)], 8

To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.063 g, 0.109 mmol) in acetone (10 mL) was added a solution of ligand L2 (0.044 g, 0.219 mmol) in acetone (10 mL), followed by MeI (7 μL, 0.109 mmol). The solution was stirred for 2 h. The mixture was filtered and layered with pentane (50 mL). After 6 days, the orange block crystals which formed were collected, washed with pentane, and dried in vacuo. Yield: 0.030 g, 24%. Anal. Calcd for C15H21IN2OPt: C, 31.76; H, 3.73; N, 4.94. Found: C, 31.57; H, 3.70; N, 4.97%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8a, 8.64 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.04 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4a), 7.76 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.55 (dd, 1H, 3J(HH) = 6 Hz, 7 Hz, H5a), 7.11 (dd, 1H, 3J(HH) = 8 Hz, 8 Hz, H5), 6.93 (s, 1H, H2), 6.52 (d, 1H, 3J(HH) = 8 Hz, H4), 6.48 (d, 1H, 3J(HH) = 8 Hz, H6), 5.18 (d, 1H, 2J(HH) = 16 Hz, H7a), 4.87 (d, 2J(HH) = 16 Hz, H7b), 1.42 (s, 3H, 2J(PtH) = 73 Hz, PtMe), 0.99 (s, 3H, 2J(PtH) = 71 Hz, PtMe), 0.51 (s, 3H, 2J(PtH) = 73 Hz, PtMe); δ(13C) 162.6, 159.4, 148.2, 147.8, 140.5, 130.9, 126.2, 124.1, 113.0, 109.2, 107.9, 58.5 (CH2), 10.0 (PtMe), −5.9 (PtMe), −7.4 (PtMe); 8b, δ(1H): 8.60 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.01 (dd, 1H, 3J(HH) = 8 Hz, 6 Hz, H4a), 7.70 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.55 (dd, 1H, 3J(HH) = 6 Hz, 6 Hz, H5a), 7.15 (dd, 1H, 3J(HH) = 8 Hz, 8 Hz, H5), 6.90 (s, 1H, H2), 6.58 (d, 1H, 3J(HH) = 8 Hz, H4), 6.55 (d, 1H, 3J(HH) = 8 Hz, H6), 5.63 (d, 1H, 2J(HH) = 14 Hz, H7a), 4.80 (d, 2J(HH) = 14 Hz, H7b), 1.41 (s, 3H, 2J(PtH) = 73 Hz, PtMe), 0.97 (s, 3H, 2J(PtH) = 73 Hz, PtMe), 0.78 (s, 3H, 2J(PtH) = 71 Hz, PtMe); δ(13C): 161.6, 159.2, 147.3, 147.1, 140.3, 130.7, 126.2, 124.3, 113.4, 113.1, 111.6, 61.6 (CH2), 8.1 (PtMe), −6.0 (PtMe), −6.1 (PtMe).

[PtIMe3(L3)], 9

This was prepared similarly but using ligand L3. Yield: 0.168 g, 61%. Anal. Calcd for C15H21IN2OPt: C, 31.76; H, 3.73; N, 4.94. Found: C, 32.02; H, 3.73; N, 4.92%. NMR in CD3OD (400 MHz, 25 °C): 9a, δ(1H) 8.68 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.09 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4a), 7.82 (d, 1H, 3J(HH) = 7 Hz, H3a), 7.60 (dd, 1H, 3J(HH) = 6 Hz, 8 Hz, H5a), 6.97 (d, 2H, 3J(HH) = 9 Hz, H2), 6.80 (d, 2H, 3J(HH) = 9 Hz, H3), 5.16 (d, 1H, 2J(HH) = 16 Hz, H7a), 4.95 (d, 2J(HH) = 16 Hz, H7b), 1.39 (s, 3H, 2J(PtH) = 73 Hz, PtMe); 0.95 (s, 3H, 2J(PtH) = 71 Hz, PtMe); 0.57 (s, 3H, 2J(PtH) = 73 Hz, PtMe); δ(13C): 161.7, 155.6, 148.1, 140.2, 137.5, 126.0, 124.0, 121.4, 116.3, 58.4 (CH2), 7.8 (PtMe), −6.6 (PtMe), −6.8 (PtMe). 9b, δ(1H): 8.62 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.05 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4a), 7.75 (d, 1H, 3J(HH) = 7 Hz, H3a), 7.60 (dd, 1H, 3J(HH) = 6 Hz, 8 Hz, H5a), 7.44 (d, 2H, 3J(HH) = 9 Hz, H2), 6.77 (d, 2H, 3J(HH) = 9 Hz, H3), 5.60 (d, 1H, 2J(HH) = 16 Hz, H7a), 4.86 (d, 2J(HH) = 16 Hz, H7b), 1.40 (s, 3H, 2J(PtH) = 72 Hz, PtMe); 0.94 (s, 3H, 2J(PtH) = 73 Hz, PtMe); 0.80 (s, 3H, 2J(PtH) = 71 Hz, PtMe); δ(13C): 161.2, 155.0, 147.0, 140.0, 137.9, 126.0, 124.0, 123.1, 116.2, 62.7 (CH2), 8.9 (Me), −6.6 (PtMe), −7.6 (PtMe).

[Pt(OH)2Me2(L1)], 10

To a mixture of [Pt2Me4(μ-SMe2)2] (0.127 g, 0.221 mmol) and ligand L1 (0.089 g, 0.443 mmol) under nitrogen was added a solution of H2O2 (30%, 89 μL, 0.886 mmol) in acetone (10 mL). The solution was stirred for 7 h. The product, which precipitated as an off-white solid, was separated, washed with acetone (2 × 30 mL) and pentane (2 × 30 mL), and dried in vacuum. The product was recrystallized from MeOH/CH2Cl2 by slow diffusion of pentane. Needle colorless crystals were obtained after 7 days. Yield: 0.110 g, 54%. Anal. Calcd for C14H20N2O3Pt: C, 36.60; H, 4.39; N, 6.10. Found: C, 36.43; H, 4.43; N, 5.83%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.77 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.09 (dd, 1H, 3J(HH) = 6 Hz, 8 Hz, H4a), 7.77 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.63 (dd, 1H, 3J(HH) = 6 Hz, 6 Hz, H5a), 7.16 (d, 1H, 3J(HH) = 8 Hz, H3), 7.10 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H5), 6.90 (d, 1H, 3J(HH) = 8 Hz, H6), 6.81 (dd, 1H, 3J(HH) = 8 Hz, 7 Hz, H4), 5.34 (d, 1H, 2J(HH) = 16 Hz, H7a), 4.67 (d, 1H, 2J(HH) = 16 Hz, H7b), 1.62 (s, 3H, 2J(PtH) = 73 Hz, PtMe), 1.13 (s, 3H, 2J(PtH) = 71 Hz, PtMe); δ(13C): 161.2, 153.1, 147.9, 140.9, 133.0, 128.5, 126.0, 124.7, 124.5, 121.1, 120.8, 56.6 (CH2), −4.5 (PtMe), −5.2 (PtMe).

Complex 10 was also prepared by reaction of [Pt2Me4(μ-SMe2)2] (0.044 g, 0.076 mmol) in acetone (2 mL) with ligand L1 (0.031 g, 0.153 mmol) in acetone (2 mL) under air. The mixture was stirred overnight to give the product as an off-white suspension. The 1H NMR spectrum was identical with that mentioned above.

[Pt(OH)2Me2(L2)], 11

This was prepared similarly but using ligand L2. Yield: 0.097 g, 47%. Anal. Calcd for C14H20N2O3Pt·H2O: C, 35.22; H, 4.64; N, 5.87. Found: C, 35.60; H, 4.59; N, 5.63%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.76 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.08 (dd, 1H, 3J(HH) = 9 Hz, 8 Hz, H4a), 7.79 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.62 (dd, 1H, 3J(HH) = 5 Hz, 9 Hz, H5a), 7.14 (dd, 1H, 3J(HH) = 8 Hz, 8 Hz, H5), 6.66 (s, 1H, H2), 6.62 (d, 1H, 3J(HH) = 8 Hz, H4), 6.57 (d, 1H, 3J(HH) = 8 Hz, H6), 5.10 (d, 1H, 2J(HH) = 15 Hz, H7a), 4.76 (d, 2J(HH) = 15 Hz, H7b), 1.62 (s, 2J(PtH) = 74 Hz, PtMe), 1.09 (s, 2J(PtH) = 71 Hz, PtMe); δ(13C): 161.8, 160.8, 148.1, 147.3, 140.9, 131.2, 126.1, 124.4, 113.9, 111.4, 108.6, 57.7 (CH2), −4.3 (PtMe), −4.9 (PtMe).

The same product was formed as a white precipitate by the reaction of [Pt2Me4(μ-SMe2)2] (0.040 g, 0.070 mmol) with ligand L2 (0.028 g, 0.140 mmol) in acetone (5 mL) under air for 1 day. The 1H NMR spectrum was identical with that for 11, reported above.

[Pt(OH)2Me2(L3)], 12

This was prepared similarly but using ligand L3. Yield: 0.123 g, 74%. Anal. Calcd for C14H20N2O3Pt·MeOH: C, 36.66; H, 4.92; N, 5.70. Found: C, 36.13; H, 4.93; N, 5.43%. NMR in CD3OD (600 MHz, 25 °C): δ(1H) 8.76 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.07 (dd, 1H, 3J(HH) = 8 Hz, 8 Hz, H4a), 7.77 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.62 (dd, 1H, 3J(HH) = 6 Hz, 8 Hz, H5a), 7.14 (d, 2H, 3J(HH) = 9 Hz, H2), 6.78 (d, 2H, 3J(HH) = 9 Hz, H3), 5.08 (d, 1H, 2J(HH) = 15 Hz, H7a), 4.73 (d, 2J(HH) = 15 Hz, H7b), 1.59 (s, 2J(PtH) = 69 Hz, PtMe), 1.05 (s, 2J(PtH) = 68 Hz, PtMe); δ(13C): 161.6, 156.2, 148.1, 140.9, 138.1, 126.1, 124.3, 122.7, 116.8, 59.0 (CH2), −4.7 (PtMe), −4.8 (PtMe).

[PtIMe3(L4)], 13

A solution of [Pt2Me4(μ-SMe2)2] (0.081 g, 0.141 mmol) and ligand L4 (0.056 g, 0.282 mmol) in MeCN (4 mL) was stirred for 1 h to give a dark red solution. The mixture was cooled to 0 °C, followed by addition of MeI (88 μL, 1.41 mmol). The solution was slowly warmed to room temperature and stirred for another 2 h. The solvent was removed under vaccum and the solid product was washed with ether (3 × 10 mL) and pentane (2 × 10 mL) and dried in vacuum. Yield: 0.096 g, 60%. Anal. Calcd for C15H19IN2OPt: C, 31.87; H, 3.39; N, 4.96. Found: C, 32.20; H, 4.00; N, 5.53%. NMR in CD3OD (600 MHz, 25 °C): δ(1H) 9.02 (s, 3J(PtH) = 27 Hz, N=CH), 8.98 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.25 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H4a), 8.24 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.40 (dd, 1H, 3J(HH) = 5 Hz, 7 Hz, H5a), 7.40 (d, 2H, 3J(HH) = 9 Hz, H2), 6.88 (d, 2H, 3J(HH) = 9 Hz, H3), 1.46 (s, 2J(PtH) = 72 Hz, PtMe), 1.18 (s, 2J(PtH) = 72 Hz, PtMe), 0.63 (s, 2J(PtH) = 73 Hz, PtMe); δ(13C): 165.0, 159.1, 155.6, 148.5, 140.6, 140.3, 130.5, 128.8, 125.3, 116.5, 8.15 (PtMe), −4.8 (PtMe), −5.5 (PtMe).

[Pt(OH)2Me2(L4)], 14

To a mixture of [Pt2Me4(μ-SMe2)2] (0.120 g, 0.208 mmol) and ligand L4 (0.083 g, 0.416 mmol) was added dichloromethane (5 mL) and methanol (1 mL). The mixture was stirred for 1 h and H2O2 (30%, 84 μL, 0.833 mmol) was added. The mixture was stirred for 1 h, the solvent was removed under vacuo, and the product was washed with CH2Cl2 (2 × 10 mL), ether (2 × 10 mL), and acetone (2 × 10 mL), and dried under vacuum. Yield: 0.070 g, 37%. Anal. Calcd for C14H18N2O3Pt: C, 36.76; H, 3.97; N, 6.12. Found: C, 37.06; H, 4.00; N, 5.15%. NMR in CD3OD (600 MHz, 25 °C): δ(1H) 9.15 (s, 3J(PtH) = 16 Hz, N=CH), 8.95 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.31 (d, 1H, 3J(HH) = 8 Hz, H3a), 8.30 (dd, 1H, 3J(HH) = 7 Hz, 8 Hz, H4a), 7.86 (dd, 1H, 3J(HH) = 5 Hz, 7 Hz, H5a), 7.36 (d, 2H, 3J(HH) = 9 Hz, H2), 6.90 (d, 2H, 3J(HH) = 9 Hz, H3), 1.81 (s, 2J(PtH) = 71 Hz, PtMe), 1.58 (s, 2J(PtH) = 70 Hz, PtMe).

[PtMe2(L4)], 15

Complex 15 was formed in situ, as a red solution, by reaction of [Pt2Me4(μ-SMe2)2] (0.081 g, 0.141 mmol) and ligand L4 (0.056 g, 0.282 mmol) in CD3CN. NMR (400 MHz, 25 °C): δ(1H) 9.35 (s, 3J(PtH) = 32 Hz, N=CH), 9.35 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.07 (dd, 1H, 3J(HH) = 8 Hz, 9 Hz, H4a), 7.77 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.62 (dd, 1H, 3J(HH) = 6 Hz, 9 Hz, H5a), 7.14 (d, 2H, 3J(HH) = 9 Hz, H2), 6.78 (d, 2H, 3J(HH) = 9 Hz, H3), 1.13 (s, 2J(PtH) = 87 Hz, PtMe), 0.79 (s, 2J(PtH) = 88 Hz, PtMe).