Tris(2-mercaptoimidazolyl)hydroborato Cadmium Thiolate Complexes, [TmBut]CdSAr: Thiolate Exchange at Cadmium in a Sulfur-Rich Coordination Environment.

A series of cadmium thiolate compounds that feature a sulfur-rich coordination environment, namely [TmBut]CdSAr, have been synthesized by the reactions of [TmBut]CdMe with ArSH (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, and C6H4-3-OMe). In addition, the pyridine-2-thiolate and pyridine-2-selenolate derivatives, [TmBut]CdSPy and [TmBut]CdSePy have been obtained via the respective reactions of [TmBut]CdMe with pyridine-2-thione and pyridine-2-selone. The molecular structures of [TmBut]CdSAr and [TmBut]CdEPy (E = S or Se) have been determined by X-ray diffraction and demonstrate that, in each case, the [CdS4] motif is distorted tetrahedral and approaches a trigonal monopyramidal geometry in which the thiolate ligand adopts an equatorial position; [TmBut]CdSPy and [TmBut]CdSePy, however, exhibit an additional long-range interaction with the pyridyl nitrogen atoms. The ability of the thiolate ligands to participate in exchange was probed by 1H and 19F nuclear magnetic resonance (NMR) spectroscopic studies of the reactions of [TmBut]CdSC6H4-4-F with ArSH (Ar = C6H4-4-But or C6H4-4-OMe), which demonstrate that (i) exchange is facile and (ii) coordination of thiolate to cadmium is most favored for the p-fluorophenyl derivative. Furthermore, a two-dimensional EXSY experiment involving [TmBut]CdSC6H4-4-F and 4-fluorothiophenol demonstrates that degenerate thiolate ligand exchange is also facile on the NMR time scale.

Introduction

class="Chemical">Thiolate ligaclass="Chemical">nds class="Chemical">n class="Chemical">are prevalent in the coordination chemistry of both transition and main group metals,[1] having found important applications in the fields of bioinorganic chemistry[2] and nanoscience.[3] For example, many enzymes feature metal coordination by the thiolate groups of cysteine residues,[4] as illustrated by a large variety of zinc enzymes, such as liver alcohol dehydrogenase, 5-aminolevulinate dehydratase, the Ada DNA repair protein, and zinc finger proteins.[5−7] Indeed, the first cadmium enzyme discovered likewise exhibits coordination by cysteine thiolate groups,[8] but it should be noted that such coordination is additionally associated with both (i) a mechanism of cadmium toxicity and (ii) the ability of metallothionein to protect against cadmium toxicity.[9−12] With respect to applications in nanoscience, cadmium–thiolate coordination has also been used as a means to cap cadmium chalcogenide nanoparticles.[3a,3b,13] Therefore, in view of the current relevance of cadmium–thiolate interactions, we report here an investigation of thiolate exchange at cadmium in a sulfur-rich coordination environment.

Results and Discussion

The class="Chemical">tris(2-mercaptoimidazolyl)hydroborato ligaclass="Chemical">nd sysclass="Chemical">n class="Chemical">tem, [TmR] (Figure ),[14−18] has been shown to be effective for providing an L2X[19] [S3] donor array for a variety of metal centers. For example, this class of ligands has been utilized for investigating zinc enzymes that have sulfur-rich active sites.[16,20−23] Cadmium[24−29] and mercury[28c,29a,30,31] counterparts have also been synthesized, with the latter providing a molecular model for mercury detoxification by organomercurial lyase (MerB).[30a] Thiolate exchange involving the {[TmR]Cd} platform, however, has received no attention, so here we report the synthesis and structures of [TmBu]CdSAr compounds and their exchange reactions with thiols.

Figure 1

[TmR] ligands in their κ3 coordination mode.

[n class="Chemical">TmR] ligaclass="Chemical">nds iclass="Chemical">n their κ3 class="Chemical">n class="Chemical">coordination mode.

Synthesis and Structural Characterization of [TmBu]CdSAr

class="Chemical">Cadmium thiolate class="Chemical">n class="Chemical">compounds of the class [TmR]CdSAr were first synthesized by Rabinovich via the reactions of [Tm]CdBr with the thallium(I) thiolate reagents, TlSAr.[28a,32] Subsequently, we demonstrated that [TmBu]CdSAr derivatives could also be obtained by treatment of [TmBu]CdMe with ArSH (Ar = C6H5 or C6H4-4-Me).[24] Since a variety of thiols are commercially available (in contrast to TlSAr), we have used the latter method to extend the series of [TmBu]CdSAr derivatives (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, or C6H4-3-OMe), as illustrated in Scheme . The molecular structures of all of the [TmBu]CdSAr derivatives have been determined by X-ray diffraction, as illustrated in Figures –5, and selected bond lengths and angles are listed in Table .

Scheme 1

Figure 2

Molecular structure of [TmBu]CdSC6H4-4-But.

Figure 5

Molecular structure of [TmBu]CdSC6H4-3-OMe.

Table 1

Selected Bond Lengths (angstroms) and Angles (degrees) for [TmBu]CdSAr

	C6H5a	C6H4-4-Mea	C6H4-4-But	C6H4-4-OMe	C6H4-3-OMe	C6H4-4-F
Cd–S(1)	2.5784(6)	2.5680(10)	2.5597(6)	2.5576(7)	2.5537(5)	2.5621(8)
Cd–S(2)	2.5537(6)	2.5622(10)	2.5648(6)	2.5672(7)	2.5579(5)	2.5560(7)
Cd–S(3)	2.5641(6)	2.5643(10)	2.5601(7)	2.5681(7)	2.5596(5)	2.5475(7)
Cd–S(4)	2.4595(7)	2.4648(10)	2.4419(7)	2.4308(7)	2.4462(5)	2.4565(7)
Cd–[TmBut]b	2.57[1]	2.565[3]	2.562[3]	2.564[6]	2.557[3]	2.555[7]
{Cd–[TmBut]b}–{Cd–S(4)}	0.11	0.100	0.120	0.133	0.111	0.099
S(1)–Cd–S(2)	97.68(2)	97.56(3)	95.82(2)	99.52(2)	97.879(16)	97.54(2)
S(1)–Cd–S(3)	102.78(2)	97.61(3)	102.443(19)	98.68(2)	98.899(15)	102.95(2)
S(1)–Cd–S(4)	122.39(2)	127.83(4)	122.81(2)	126.52(2)	134.208(16)	119.87(3)
S(2)–Cd–S(3)	100.458(19)	102.72(3)	103.09(5)	99.18(2)	103.193(16)	102.49(2)
S(2)–Cd–S(4)	122.00(2)	108.27(3)	126.10(2)	121.17(2)	113.083(16)	123.73(3)
S(3)–Cd–S(4)	108.12(2)	118.68(3)	103.152(19)	106.52(2)	105.421(17)	107.39(2)
Cd–S(4)–C(Ar)	105.14(9)	106.39(11)	105.87(7)	105.88(5)	104.02(6)	103.77(9)

Data taken from ref (24).

Average value of the Cd–S bond lengths involving the [TmBu] ligand.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSC6H4-4-But.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSC6H4-4-F.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSC6H4-4-OMe.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSC6H4-3-OMe.

Average value of the class="Chemical">Cd–S boclass="Chemical">nd leclass="Chemical">ngths iclass="Chemical">nvolviclass="Chemical">ng the [class="Chemical">n class="Chemical">TmBu] ligand.

The class="Chemical">coordiclass="Chemical">natioclass="Chemical">n geometry of the class="Chemical">n class="Chemical">cadmium center in each [TmBu]CdSAr derivative is distorted tetrahedral, as indicated by the deviation of the four-coordinate τ4 and τδ geometry indices[33] from the idealized value of 1.00 for a tetrahedral geometry (Table ). Specifically, the distortion is such that the structures approach a trigonal monopyramidal geometry (0.85) in which the thiolate ligand adopts an equatorial position. In this regard, the sum of the three bond angles (ΣS–Cd–S) that approximate the equatorial plane [333.7–347.2° (Table )] is greater than the idealized tetrahedral value (328.5°).

Table 3

Four-Coordinate τ4 and τδ Indices and ΣS–Cd–E Values for [TmBu]CdSAr and [TmBu]CdEPy (E = S or Se)

compound	τ4	τδ	ΣS–Cd–E (deg)b
[TmBut]CdSC6H5a	0.82	0.82	342.17
[TmBut]CdSC6H4-4-Mea	0.80	0.75	333.66
[TmBut]CdSC6H4-4-But	0.79	0.77	344.73
[TmBut]CdSC6H4-4-OMe	0.80	0.76	347.21
[TmBut]CdSC6H4-3-OMe	0.80	0.67	345.17
[TmBut]CdSC6H4-4-F	0.83	0.80	341.14
[TmBut]CdSPy	0.74c	0.72c	354.92
[TmBut]CdSePy	0.75c	0.75c	353.62

Data taken from ref (24).

Sum of the three angles for the atoms that approximate to trigonal planar.

Values assuming no Cd–N interaction.

Sum of the three angles for the atoms that approximan class="Chemical">te to trigoclass="Chemical">nal placlass="Chemical">nclass="Chemical">n class="Chemical">ar.

Values assuming no n class="Chemical">Cd–N iclass="Chemical">nclass="Chemical">n class="Chemical">teraction.

With respect to the class="Chemical">coordiclass="Chemical">natioclass="Chemical">n of the class="Chemical">n class="Chemical">thiolate ligands, the Cd–SAr bond lengths are ∼0.1 Å shorter than the average Cd–S bond lengths associated with the [TmBu] ligands (Table ), which is in accord with the latter involving a dative covalent component to the bonding interaction.[34] The Cd–S–Ar bond angles exhibit little variation [103.77(9)–106.39(11)°] and are comparable to the mean value of 106.5° for structurally characterized cadmium arylthiolate compounds listed in the Cambridge Structural Database (CSD).[35] Despite the similar Cd–SAr bond lengths and Cd–S–Ar bond angles, however, the Cd–S–C–C torsion angles (Figure ) vary significantly (Table ), with [TmBu]CdSC6H4-4-F having the smallest Cd–S–C–C torsion angle (2.09°) and [TmBu]CdSC6H4-4-OMe having the largest torsion angle (42.81°). Of note, [TmBu]CdSC6H4-4-OMe and [TmBu]CdSC6H4-3-OMe have similar torsion angles, which suggests that steric effects do not have much influence in this system. Since the distance between the ortho hydrogen and the cadmium varies with the torsion angle, it is appropriate to consider the possibility that the small torsion angle for [TmBu]CdSC6H4-4-F could reflect an agostic interaction.[36] The Cd···H distance (2.70 Å), however, is considerably longer than the sum of the covalent radii of Cd and H (1.75 Å)[37] and is also longer than the Cd···H–B distance in [κ2-TmBu]2Cd (2.49 Å).[24] As such, it is not reasonable to attribute the orientation of the aryl group of [TmBu]CdSC6H4-4-F to an agostic interaction, and crystal packing effects are more likely responsible for the variation of torsion angles.

Figure 6

Cd–S–C–C torsion angles in [TmBu]CdSAr.

Table 2

Bond Angles and Torsion Angles Pertaining to the Thiolate Ligands of [TmBu]CdSAr

compound	Cd–S–Cipso (deg)	Cd–S–Cipso–Cortho (deg)
[TmBut]CdSC6H5a	105.14(9)	15.25
[TmBut]CdSC6H4-4-Mea	106.39(11)	31.49
[TmBut]CdSC6H4-4-But	105.87(7)	19.56
[TmBut]CdSC6H4-4-OMe	105.88(5)	42.81
[TmBut]CdSC6H4-3-OMe	104.02(6)	38.91
[TmBut]CdSC6H4-4-F	103.77(9)	2.09

Data taken from ref (24).

class="Chemical">Cd–S–C–C torsioclass="Chemical">n aclass="Chemical">ngles iclass="Chemical">n [class="Chemical">n class="Chemical">TmBu]CdSAr.

Synthesis and Structural Characterization of [TmBu]CdSPy and [TmBu]CdSePy

In addition to class="Chemical">arylthiolate class="Chemical">n class="Chemical">compounds, [TmBu]CdSAr, we have also synthesized the pyridine-2-thiolate[38] counterpart, [TmBu]CdSPy, via the reaction of [TmBu]CdMe with pyridine-2-thione[39] (Scheme ). The molecular structure of [TmBu]CdSPy has been determined by X-ray diffraction (Figure ), which indicates that it exists as a discrete mononuclear compound. Although a variety of metal compounds derived from 2-mercaptopyridine have been reported,[40] the formation of [TmBu]CdSPy is noteworthy because there is only one pyridine-2-thiolate cadmium compound listed in the CSD,[35] namely Cd(SPy)2;[41,42] furthermore, Cd(SPy)2 is polymeric with each sulfur bridging two cadmium atoms.

Scheme 2

Figure 7

Molecular structure of [TmBu]CdSPy.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSPy.

class="Chemical">Selecclass="Chemical">n class="Chemical">ted bond lengths and angles for [TmBu]CdSPy are summarized in Table , indicating that the Cd–SPy bond length [2.4946(8) Å] is comparable to the Cd–SAr bond lengths in the aforementioned [TmBu]CdSAr complexes (Table ). Despite the similar Cd–S bond lengths, however, the bond angle at the thiolate sulfur [91.95(10)°] is much smaller than those of the arylthiolate compounds listed in Table [103.77(9)–106.39(11)°]. In addition to a small angle at sulfur, the Cd–S–C–N torsion angle is close to zero (0.35°), both of which indicate that the pyridine ring is oriented in a position that would maximize a Cd···N interaction. Of note, these structural features are not present in the pyridine-2-thione adduct, [Cd(SPyH)4]2+.[43] Specifically, the bond angles at the sulfur atoms of [Cd(SPyH)4]2+ (107.8° and 109.1°) are much larger than that for [TmBu]CdSPy, as are the torsion angles (121.0° and 175.0°).

Table 4

Selected Bond Lengths (angstroms) and Angles (degrees) for [TmBu]CdEPy (E = S or Se)

	[TmBut]CdSPy	[TmBut]CdSePy
Cd–S(1)	2.5509(7)	2.5513(6)
Cd–S(2)	2.5633(7)	2.5594(6)
Cd–S(3)	2.6438(7)	2.6361(5)
Cd–E	2.4946(8)	2.5709(4)
Cd···N(41)	2.766	3.000
S(1)–Cd–S(2)	99.78(2)	99.115(17)
S(1)–Cd–S(3)	99.47(2)	99.490(19)
S(2)–Cd–S(3)	97.43(2)	98.50(2)
S(1)–Cd–E	129.26(3)	127.095(17)
S(2)–Cd–E	125.88(3)	127.408(14)
S(3)–Cd–E	95.63(2)	97.125(17)
Cd–E–Cipso	91.95(10)	93.32(5)

However, despiclass="Chemical">te the favorable orieclass="Chemical">ntatioclass="Chemical">n of the class="Chemical">n class="Chemical">pyridine ring of [TmBu]CdSPy to participate in a Cd···N interaction, the Cd···N distance of 2.766 Å is distinctly longer than the average value of 2.350 Å for structurally characterized cadmium pyridine compounds listed in the CSD.[35,44] As such, it is evident that the Cd···N interaction in [TmBu]CdSPy cannot be regarded as strong. Pyridine-2-thiolate ligands are known to coordinate to a single metal center via three possible coordination modes (Figure ),[40,45] namely, κ1-S,[46] κ1-N,[47] and κ2-S,N,[46a,48] so it is apparent that [TmBu]CdSPy possesses a structure that lies on the border between κ1-S and κ2-S,N coordination modes.

Figure 8

Coordination modes for pyridine-2-thiolate ligands (only one resonance structure is shown in each case).

class="Chemical">Coordiclass="Chemical">natioclass="Chemical">n modes for class="Chemical">n class="Chemical">pyridine-2-thiolate ligands (only one resonance structure is shown in each case).

Inclass="Chemical">teresticlass="Chemical">ngly, eveclass="Chemical">n though the class="Chemical">n class="Chemical">Cd···N interaction is not strong, the presence of the nitrogen does, nevertheless, have an impact on the cadmium coordination geometry. For example, one of the Cd–S bonds involving the [TmBu] ligand is distinctly longer than the other two. Specifically, the sulfur that is approximately trans to the nitrogen atom [S(3)–Cd–N, 154.92°] has a Cd–S(3) bond length of 2.6438(7) Å, whereas the other two have bond lengths of 2.5509(7) and 2.5633(7) Å. For further comparison, the longest Cd–S bond length involving the [TmBu] ligand for the thiolate compounds listed in Table is 2.5784(6) Å. Neglecting the Cd···N interaction, the τ4 parameter (0.74) is smaller than the values for the other [TmBu]CdSAr compounds. As such, the cadmium center of the [CdS4] moiety is approaching a trigonal monopyramidal geometry in which the longest Cd–S bond occupies the axial position. In accord with the approximate trigonal monopyramidal description for the [CdS4] moiety, the PyS–Cd–S angle involving the axial sulfur of the [TmBu] ligand [95.63(2)°] is close to 90°, whereas the corresponding value for [TmBu]CdSPh [108.12(2)°] is close to the tetrahedral angle. Furthermore, the sum of the three bond angles (ΣS–Cd–S) that approximate the equatorial plane (354.9°) is very close to that required for a planar arrangement (360.0°). Thus, the structure of [TmBu]CdSPy may be considered to be intermediate between trigonal monopyramidal [CdS4] and distorted trigonal bipyramidal [CdS4N].

By class="Chemical">compclass="Chemical">n class="Chemical">arison to pyridine-2-thiolate compounds, their selenium counterparts have received comparatively little attention,[49−51] and there are only two structurally characterized cadmium pyridine-2-selenolate derivatives listed in the CSD, namely, Cd(SePy)2(tmeda)[51b] and Cd(SePy)2,[51a] of which the latter is polymeric. In this regard, we have extended this investigation to the synthesis of the selenium counterpart, [TmBu]CdSePy, as illustrated in Scheme . The molecular structure of [TmBu]CdSePy has been determined by X-ray diffraction (Figure ), thereby demonstrating that the pyridine-2-selenolate ligand coordinates in a predominantly κ1-Se manner, in contrast to the κ2-Se,N coordination mode observed for Cd(SePy)2(tmeda).[51b] Specifically, whereas the Cd–Se bond length of [TmBu]CdSePy [2.5709(4) Å] is shorter than that of Cd(SePy)2(tmeda) [2.734(3) and 2.735(3) Å],[52] the Cd···N distance of [TmBu]CdSePy (3.000 Å) is much longer than those for Cd(SePy)2(tmeda) [2.399(19) and 2.40(2) Å]. Furthermore, the Cd···N distance of [TmBu]CdSePy is also considerably longer than that for [TmBu]CdSPy (2.766 Å).[53] The Cd–Se–C–N torsion angle (0.95°) is, nevertheless, close to zero, so that it is appropriately located to participate in a potential Cd···N interaction. In this regard, the Cd–S bond [2.6361(5) Å] of the [TmBu] ligand that is approximately trans to the nitrogen is distinctly longer than the other two [2.5513(6) and 2.5594(6) Å], such that the structure approaches trigonal monopyramidal (τ4 = 0.75). Furthermore, the sum of the three bond angles (ΣS–Cd–E) that approximate the equatorial plane is 353.6°. Thus, even though the Cd···N distance is long, the presence of the nitrogen has an impact on the cadmium coordination geometry in a manner similar to that observed for [TmBu]CdSPy.

Figure 9

Molecular structure of [TmBu]CdSePy.

Moleculclass="Chemical">ar structure of [class="Chemical">n class="Chemical">TmBu]CdSePy.

Thiolate Exchange between [TmBu]CdSAr and Ar′SH

To evaluaclass="Chemical">te the factors that iclass="Chemical">nflueclass="Chemical">nce the class="Chemical">n class="Chemical">coordination of thiolate ligands to cadmium, we have investigated thiolate exchange reactions involving [TmBu]CdSAr and Ar′SH to determine which substituents promote thiolate coordination. For example, [TmBu]CdSC6H4-4-F reacts rapidly with Ar′SH (Ar = C6H4-4-But or C6H4-4-OMe) to yield an equilibrium mixture comprising [TmBu]CdSC6H4-4-F, [TmBu]CdSAr, and the respective thiols (Scheme ), as monitored by 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. The derived equilibrium constants are summarized in Table , which illustrates that coordination of thiolate is favored for the more electron-withdrawing fluoride substituent. This observation is in accord with our previous studies concerned with coordination of alkoxide to zinc, which shows that such coordination is also favored for electron-withdrawing substituents.[54] The thermodynamics of the cadmium thiolate exchange reactions are dictated by the differential effect of the substituent on the Cd–SAr and H–SAr bond energies. On the basis of the aforementioned zinc alkoxide study,[54] the observed thermodynamic trend can be rationalized by electron-withdrawing substituents increasing the Cd–SAr bond dissociation energies to a greater degree than the H–SAr bond dissociation energies.[55,56] Alternatively, in terms of arguments based on heterolytic bond dissociation energies, electron-withdrawing substituents weaken Cd–SAr bonds to a smaller degree than they do for H–SAr bonds.[57−59]

Scheme 3

Table 5

Equilibrium Constants (K) for the Reaction of [TmBu]CdSC6H4-4-F with ArSH

Ar	K
C6H4-4-F	1.00
C6H4-4-But	0.21
C6H4-4-OMe	0.19

While the equilibrium studies described above indicaclass="Chemical">te that class="Chemical">n class="Chemical">thiolate exchange is facile on the chemical time scale, two-dimensional EXSY[60] studies involving [TmBu]CdSC6H4-4-F and 4-fluorothiophenol indicate that degenerate thiolate ligand exchange is also facile on the magnetization transfer NMR time scale (Figure ).[61] Specifically, exchange is indicated by the observation of an off-diagonal cross peak between the 19F NMR spectroscopic signals for [TmBu]CdSC6H4-4-F and 4-fluorothiophenol. The observation of thiolate exchange between [TmBu]CdSAr and ArSH (Ar = C6H4-4-F) complements the observation that exchange of thiolate ligands between zinc and cadmium centers of [TmBu]ZnSCHC(O)N(H)Ph and [TmBu]CdSCHC(O)N(H)Ph is also facile on the NMR time scale.[20b,26,27]

Figure 10

19F two-dimensional EXSY experiment demonstrating exchange of the SAr groups between [TmBu]CdSAr and Ar′SH (Ar′ = C6H4-4-F).

19F two-dimensional EXSY experiment demonstrating exchange of the Sclass="Chemical">Ar groups betweeclass="Chemical">n [class="Chemical">n class="Chemical">TmBu]CdSAr and Ar′SH (Ar′ = C6H4-4-F).

Conclusions

A class="Chemical">series of class="Chemical">n class="Chemical">cadmium thiolate compounds that feature a sulfur-rich coordination environment, namely [TmBu]CdSAr, have been synthesized by the reactions of [TmBu]CdMe with ArSH (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, or C6H4-3-OMe). The molecular structures of the thiolate compounds have been determined by X-ray diffraction, which demonstrate that the coordination geometry is distorted tetrahedral and approaches a trigonal monopyramidal geometry in which the thiolate ligand adopts an equatorial position. The pyridine-2-thiolate and pyridine-2-selenolate derivatives, [TmBu]CdSPy and [TmBu]CdSePy, have also been obtained via the respective reactions of [TmBu]CdMe with pyridine-2-thione and pyridine-2-selone, and X-ray diffraction studies demonstrate that the nitrogen of the pyridine ring exhibits a long-range interaction with the cadmium. The ability of the thiolate ligands to participate in exchange was probed by 1H and 19F NMR spectroscopic studies of the reactions of [TmBu]CdSC6H4-4-F with ArSH (Ar = C6H4-4-But or C6H4-4-OMe), which demonstrate that (i) exchange is facile and (ii) coordination of thiolate to cadmium is most favored for the p-fluorophenyl derivative. Furthermore, a two-dimensional EXSY experiment involving [TmBu]CdSC6H4-4-F and 4-fluorothiophenol demonstrates that degenerate thiolate ligand exchange is also facile on the NMR time scale.

Experimental Section

General Considerations

All manipulations were performed by using a class="Chemical">combiclass="Chemical">natioclass="Chemical">n of glovebox, high-vacuum, aclass="Chemical">nd Schleclass="Chemical">nk class="Chemical">n class="Chemical">techniques under a nitrogen or argon atmosphere.[62] Solvents were purified and degassed by standard procedures. NMR spectra were recorded on Bruker 300 DRX, Bruker 300 DPX, Bruker 400 Avance III, Bruker 400 Cyber-enabled Avance III, and Bruker 500 DMX spectrometers. 1H NMR spectra are reported in parts per million relative to SiMe4 (δ 0) and were referenced internally with respect to the protio solvent impurity (δ 7.16 for C6D5H and δ 5.32 for CHDCl2).[63]13C NMR spectra are reported in parts per million relative to SiMe4 (δ 0) and were referenced internally with respect to the solvent (δ 128.06 for C6D6 and δ 53.84 for CD2Cl2).[63]19F NMR spectra are reported in parts per million relative to CFCl3 (δ 0) and were referenced internally with respect to C6F6 (δ −164.9).[64] Coupling constants are given in hertz. Infrared (IR) spectra were recorded on a PerkinElmer Spectrum Two spectrometer, and the data are reported in reciprocal centimeters. Mass spectra were recorded on a JEOL JMS-HX110HF tandem mass spectrometer using fast atom bombardment (FAB). 4-Fluorothiophenol (Aldrich), 4-tert-butylbenzenethiol (Acros), 4-methoxythiophenol (Aldrich), 3-methoxythiophenol (Aldrich), and pyridine-2-thione (Aldrich) were obtained commercially and used without further purification. [TmBu]CdMe[24,27] and pyridine-2-selone[65] were prepared by literature procedures.

X-ray Structure Determinations

X-ray diffraction data were class="Chemical">collecclass="Chemical">n class="Chemical">ted on a Bruker Apex II diffractometer, and crystal data, data collection, and refinement parameters are summarized in the Supporting Information. The structures were determined by using direct methods and standard difference map techniques and refined by full-matrix least-squares procedures on F2 with SHELXTL (version 2014/7).[66]

Synthesis of [TmBu]CdSC6H4-4-F

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (218 mg, 0.361 mmol) in C6H6 (∼9 mL) was treated with 4-fluorothiophenol (40.0 μL, 0.375 mmol), resulting in immediate effervescence. The mixture was stirred at room temperature, and the volatile components were removed in vacuo after a period of 40 min. The resulting powder was washed with Et2O (∼2 mL) to give [TmBu]CdSC6H4-4-F as a white solid (130 mg, 50%). Crystals of [TmBu]CdSC6H4-4-F suitable for X-ray diffraction were obtained via slow diffusion of pentane into a solution in benzene. Anal. Calcd for [TmBu]CdSC6H4-4-F: C, 45.2%; H, 5.3%; N, 11.7%. Found: C, 45.2%; H, 4.9%; N, 11.6%. 1H NMR (C6D6): δ 1.41 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 6.37 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.62 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.77 (m, 2H, CdSC6H4-4-F), 7.86 (m, 2H, CdSC6H4-4-F). 13C{1H} NMR (C6D6): δ 28.7 (9C, HB{C2N2H2[C(CH3)3]CS}3), 59.4 (3C, HB{C2N2H2[C(CH3)3]CS}3), 114.7 (d, 2JC–F = 21, 2C, CdSC6H4-4-F), 117.0 (3C, HB{C2N2H2[C(CH3)3]CS}3), 122.9 (3C, HB{C2N2H2[C(CH3)3]CS}3), 135.7 (d, 3JC–F = 7, 2C, CdSC6H4-4-F), 139.9 (d, 4JC–F = 3, 1C, CdSC6H4-4-F), 157.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 160.6 (d, 1JC–F = 240, 1C, CdSC6H4-4-F). 19F NMR (C6D6): δ −125.1. IR data for [TmBu]CdSC6H6-4-F (ATR, cm–1): 3182 (w), 2977 (w), 2926 (w), 2404 (w), 2290 (w), 2227 (w), 2162 (w), 2051 (w), 1980 (w), 1719 (w), 1585 (w), 1566 (w), 1483 (s), 1416 (m), 1397 (m), 1357 (vs), 1304 (m), 1254 (w), 1221 (m), 1192 (vs), 1171 (s), 1129 (m), 1088 (s), 1070 (m), 1061 (m), 1033 (w), 1014 (w), 984 (w), 929 (w), 819 (s), 773 (w), 757 (m), 732 (s), 688 (s), 626 (vs), 589 (m), 553 (m), 544 (w), 497 (m), 480 (w), 455 (w). FAB-MS: m/z 591.2 [M – SC6H4-4-F]+, M = [TmBu]CdSC6H4-4-F.

Synthesis of [TmBu]CdSC6H4-4-OMe

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (142 mg, 0.235 mmol) in C6H6 (∼5 mL) was treated with 4-methoxythiophenol (37.5 μL, 0.305 mmol), resulting in immediate effervescence. The mixture was stirred at room temperature for 45 min, after which period the volatile components were removed in vacuo. The resulting powder was washed with pentane (∼3 mL), yielding [TmBu]CdSC6H4-4-OMe as a white solid (107 mg, 63%). Crystals of [TmBu]CdSC6H4-4-OMe suitable for X-ray diffraction were obtained via slow diffusion of pentane into a solution in benzene. Anal. Calcd for [TmBu]CdSC6H4-4-OMe·C6H6: C, 50.6%; H, 5.9%; N, 10.4%. Found: C, 51.0%; H, 5.7%; N, 10.0%. 1H NMR (C6D6): δ 1.43 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 3.34 (s, 3H, CdSC6H4-4-OCH3), 6.37 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.63 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.73 (d, 3JH–H = 8, 2H, CdSC6H4-4-OMe), 7.94 (d, 3JH–H = 8, 2H, CdSC6H4-4-OMe). 13C{1H} NMR (CD2Cl2): δ 29.1 (9C, HB{C2N2H2[C(CH3)3]CS}3), 55.6 [1C, CdS(C6H4-4-OCH3)], 59.8 (3C, HB{C2N2H2[C(CH3)3]CS}3), 113.9 [2C, CdS(C6H4-4-OMe)], 117.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 123.4 (3C, HB{C2N2H2[C(CH3)3]CS}3), 128.7 [1C, CdS(C6H4-4-OMe)], 134.6 [2C, CdS(C6H4-4-OMe)], 156.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 156.4 (1C, CdSC6H4-4-OMe). IR data for [TmBu]CdSC6H6-4-OMe (ATR, cm–1): 3174 (w), 3130 (w), 3097 (w), 2971 (w), 2833 (w), 2458 (w), 1590 (m), 1565 (m), 1486 (s), 1464 (w), 1416 (m), 1398 (m), 1356 (vs), 1301 (m), 1279 (w), 1263 (w), 1229 (s), 1192 (s), 1173 (vs), 1132 (m), 1087 (w), 1071 (w), 1031 (m), 1018 (m), 927 (w), 822 (s), 773 (w), 756 (m), 744 (m), 730 (s), 684 (vs), 638 (m), 622 (m), 589 (m), 552 (m), 528 (m), 493 (m), 455 (m). FAB-MS: m/z 591.2 [M – SC6H4-4-OMe]+, M = [TmBu]CdSC6H4-4-OMe.

Synthesis of [TmBu]CdSC6H4-3-OMe

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (106 mg, 0.175 mmol) in C6H6 (∼10 mL) was treated with 3-methoxythiophenol (30.0 μL, 0.242 mmol), resulting in immediate effervescence. The mixture was stirred at room temperature for 1 h, after which period the volatile components were removed in vacuo. The resulting powder was washed with pentane (2 × 3 mL) and Et2O (∼3 mL), yielding [TmBu]CdSC6H4-3-OMe as a white solid (86 mg, 67%). Crystals of [TmBu]CdSC6H4-3-OMe suitable for X-ray diffraction were obtained via slow diffusion of pentane into a solution in benzene. Anal. Calcd for [TmBu]CdSC6H4-3-OMe·C6H6: C, 50.6%; H, 5.9%; N, 10.4%. Found: C, 51.3%; H, 5.6%; N, 9.9%. 1H NMR (CD2Cl2): δ 1.71 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 3.71 (s, 3H, CdSC6H4-4-OMe), 6.43 (d, 3JH–H = 8, 1H, CdSC6H4-4-OMe), 6.84 (m, 1H, CdSC6H4-4-OMe), 6.85 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.94 (m, 2H, CdSC6H4-4-OMe), 7.03 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3). 13C{1H} NMR (CD2Cl2): δ 29.1 (9C, HB{C2N2H2[C(CH3)3]CS}3), 55.4 (1C, CdSC6H4-3-OCH3), 59.8 (3C, HB{C2N2H2[C(CH3)3]CS}3), 109.2 (1C, CdSC6H4-3-OMe), 117.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 118.4 (1C, CdSC6H4-3-OMe), 123.5 (3C, HB{C2N2H2[C(CH3)3]CS}3), 126.4 (1C, CdSC6H4-3-OCH3), 128.6 (1C, CdSC6H4-3-OCH3), 156.0 (3C, HB{C2N2H2[C(CH3)3]CS}3), 159.5 (1C, CdSC6H4-3-OMe) (ipso and meta C of C6H4-3-OMe not observed). IR data for [TmBu]CdSC6H6-3-OMe (ATR, cm–1): 3190 (w), 3148 (w), 2974 (w), 2404 (w), 1585 (m), 1566 (m), 1481 (m), 1467 (m), 1417 (m), 1397 (w), 1357 (vs), 1303 (m), 1274 (m), 1222 (m), 1191 (s), 1172 (s), 1131 (m), 1097 (w), 1070 (m), 1041 (m), 930 (w), 854 (m), 844 (m), 820 (m), 757 (m), 733 (s), 686 (vs), 588 (m), 552 (m), 494 (m). FAB-MS: m/z 591.2 [M – SC6H4-3-OMe]+, M = [TmBu]CdSC6H4-3-OMe.

Synthesis of [TmBu]CdSC6H4-4-But

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (143 mg, 0.237 mmol) in C6H6 (∼5 mL) was treated with 4-tert-butylbenzenethiol (47.5 μL, 0.275 mmol), resulting in immediate effervescence. The mixture was stirred at room temperature for 45 min, after which period the volatile components were removed in vacuo. The resulting powder was washed with pentane (∼3 mL) to give [TmBu]CdSC6H4-4-But as a white solid (117 mg, 66%). Crystals of [TmBu]CdSC6H4-4-But suitable for X-ray diffraction were obtained via slow diffusion of pentane into a solution in benzene. Anal. Calcd for [TmBu]CdSC6H4-4-But·0.7C6H6: C, 52.2%; H, 6.4%; N, 10.4%. Found: C, 51.8%; H, 6.2%; N, 9.5%. 1H NMR (CD2Cl2): δ 1.24 [s, 9H, CdS(C6H4-4-Bu)], 1.71 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 6.85 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 6.97 (d, 3JH–H = 8, 2H, CdSC6H4-4-But), 7.03 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 7.25 (d, 3JH–H = 8, 2H, CdSC6H4-4-But). 13C{1H} NMR (C6D6): δ 29.1 (9C, HB{C2N2H2[C(CH3)3]CS}3), 31.6 {3C, CdS[C6H4-4-C(CH3)3]}, 34.3 {1C, CdS[C6H4-4-C(CH3)3]}, 59.8 (3C, HB{C2N2H2[C(CH3)3]CS}3), 117.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 123.4 (3C, HB{C2N2H2[C(CH3)3]CS}3), 125.1 (2C, CdSC6H4-4-But), 133.2 (2C, CdSC6H4-4-But), 139.4 (1C, CdSC6H4-4-But), 145.5 (1C, CdSC6H4-4-But), 156.1 (3C, HB{C2N2H2[C(CH3)3]CS}3).

Synthesis of [TmBu]CdSPy

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (83.3 mg, 0.138 mmol) in C6H6 (∼5 mL) was treated with pyridine-2-thione (20.2 mg, 0.182 mmol), and the mixture was stirred at room temperature for 4.5 h. After this period, the volatile components were removed in vacuo, yielding [TmBu]CdSPy as a yellow powder, and crystals suitable for X-ray diffraction were obtained by diffusion of pentane into a solution in benzene (60 mg, 62%). Anal. Calcd for [TmBu]CdSPy: C, 44.6%; H, 5.5%; N, 14.0%. Found: C, 44.9%; H, 5.5%; N, 13.7%. 1H NMR (CD2Cl2): δ 1.74 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 6.67 (m, 1H, CdSC5H4N), 6.86 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 7.04 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 7.23 (m, 1H, CdSC5H4N), 7.28 (m, 1H, CdSC5H4N), 7.76 (m, 1H, CdSC5H4N). 13C{1H} NMR (CD2Cl2): 29.2 (9C, HB{C2N2H2[C(CH3)3]CS}3), 59.6 (3C, HB{C2N2H2[C(CH3)3]CS}3), 116.0 (1C, CdSC5H4N), 117.0 (3C, HB{C2N2H2[C(CH3)3]CS}3), 123.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 125.1 (1C, CdSC5H4N), 136.1 (1C, CdSC5H4N), 147.3 (1C, CdSC5H4N), 156.9 (3C, HB{C2N2H2[C(CH3)3]CS}3), 171.9 (1C, CdSC5H4N). IR data for [TmBu]CdSPy (ATR, cm–1): 3173 (w), 3140 (w), 2977 (w), 2923 (w), 2404 (w), 2228 (w), 1686 (w), 1567 (m), 1545 (w), 1479 (w), 1445 (m), 1411 (m), 1396 (m), 1355 (vs), 1303 (m), 1265 (w), 1227 (m), 1191 (s), 1172 (s), 1130 (s), 1071 (m), 1044 (w), 1029 (w), 986 (w), 928 (w), 862 (w), 820 (m), 774 (w), 758 (m), 749 (m), 731 (vs), 688 (s), 627 (w), 589 (m), 553 (m), 545 (m), 494 (m), 487 (m). FAB-MS: m/z 589.1 [M – SPy]+, M = [TmBu]CdSPy.

Synthesis of [TmBu]CdSePy

A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdMe (86.5 mg, 0.143 mmol) in C6H6 (∼5 mL) was treated with pyridine-2-selone (30.3 mg, 0.192 mmol), and the mixture was stirred at room temperature for 4.5 h. After this period, the volatile components were removed in vacuo, yielding [TmBu]CdSePy as a dark yellow-orange powder, and crystals suitable for X-ray diffraction were obtained by diffusion of pentane into a solution in benzene (63 mg, 59%). Anal. Calcd for [TmBu]CdSePy: C, 41.8%; H, 5.1%; N, 13.1%. Found: C, 41.0%; H, 4.8%; N, 12.7%. 1H NMR (CD2Cl2): δ 1.74 (s, 27H, HB{C2N2H2[C(CH3)3]CS}3), 6.78 (m, 1H, CdSeC5H4N), 6.86 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 7.04 (d, 3JH–H = 2, 3H, HB{C2N2H2[C(CH3)3]CS}3), 7.17 (m, 1H, CdSeC5H4N), 7.48 (m, 1H, CdSeC5H4N), 7.82 (m, 1H, CdSeC5H4N). 13C{1H} NMR (CD2Cl2): δ 29.2 (9C, HB{C2N2H2[C(CH3)3]CS}3), 59.6 (3C, HB{C2N2H2[C(CH3)3]CS}3), 117.0 (3C, HB{C2N2H2[C(CH3)3]CS}3), 117.4 (1C, CdSeC5H4N), 123.2 (3C, HB{C2N2H2[C(CH3)3]CS}3), 128.8 (1C, CdSeC5H4N), 135.4 (1C, CdSeC5H4N), 148.6 (1C, CdSeC5H4N), 156.9 (3C, HB{C2N2H2[C(CH3)3]CS}3), 163.7 (1C, CdSeC5H4N). IR data for [TmBu]CdSePy (ATR, cm–1): 3185 (w), 2975 (w), 2924 (w), 2404 (w), 2227 (w), 1686 (w), 1574 (m), 1547 (w), 1480 (w), 1445 (w), 1409 (m), 1396 (w), 1356 (vs), 1303 (m), 1267 (w), 1227 (m), 1191 (s), 1172 (s), 1129 (m), 1113 (s), 1081 (w), 1070 (w), 1045 (w), 1030 (w), 983 (w), 929 (w), 864 (w), 821 (m), 774 (w), 757 (m), 750 (m), 731 (vs), 699 (m), 689 (s), 621 (w), 589 (m), 553 (m), 494 (m), 471 (m), 454 (w). FAB-MS: m/z 591.1 [M – SePy]+, M = [TmBu]CdSePy.

Thiolate Exchange between [TmBu]CdSAr and Ar′SH

(a) A solution of [class="Chemical">TmBu]class="Chemical">n class="Chemical">CdSC6H4-4-F in C6D6 (0.7 mL) was treated with ArSH (Ar = C6H4-4-But or C6H4-4-OMe, 1 equiv), and the sample was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of an equilibrium mixture (Table ). As previously noted,[56c] hydrogen bonding is not considered to perturb the equilibrium constant significantly. (b) A solution of [TmBu]CdSC6H4-4-F in C6D6 (0.7 mL) was treated with 4-fluorothiophenol, and exchange at room temperature was demonstrated by a 19F two-dimensional EXSY experiment.