Thioxoethenylidene (CCS) as a Bridging Ligand - American Chemical

Dec 19, 2014 - directly obtained from [Mo( CBr)(CO)2(Tp*)] and Li2S. The reaction presumably proceeds via the intermediacy of the bis(alkylidynyl)thio...
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Thioxoethenylidene (CCS) as a Bridging Ligand Lorraine M. Caldwell, Anthony F. Hill,* Robert Stranger, Richard N. L. Terrett, Kassetra M. von Nessi, Jas S. Ward, and Anthony C. Willis Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia S Supporting Information *

ABSTRACT: The reaction of [Mo(CBr)(CO)2(Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) with [Fe2(μ-SLi)2(CO)6] affords, inter alia, the unsymmetrical binuclear thioxoethenylidene complex [Mo2(μ,σ(C):η2(C′S)-CCS)(CO)4(Tp*)2], which may be more directly obtained from [Mo(CBr)(CO)2(Tp*)] and Li2S. The reaction presumably proceeds via the intermediacy of the bis(alkylidynyl)thioether complex S{CMo(CO)2(Tp*)}2, which was, however, not directly observed but explored computationally and found to lie 78.6 kJ mol−1 higher in energy than the final thioxoethenylidene product. Computational interrogation of the molecules [M2(μ-C2S)(CO)2(Tp*)2] (M = Mo, W, Re, Os) reveals three plausible coordination modes for a thioxoethenylidene bridge which involve a progressive strengthening of the C−C bond and weakening of the M−C and M−S bonds, as might be expected from simple effective atomic number considerations.

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INTRODUCTION Wilkinson’s discovery of the first coordination complex of carbon monosulfide, [RhCl(CS)(PPh3)2],1,2 predated the observation of this diatomic species in interstellar space,3 where high dilution ensures its longevity. Back on earth, carbon disulfide exposed to electric discharge allows the generation and matrix entrapment of CS at −190 °C.4 Its tendency to polymerize explosively above such temperatures, however, somewhat curtails its synthetic utility, a feature that has inspired the study of its more easily tamed coordination complexes.2 Somewhat later, Saito also observed thioxoethenylidene, CCS, in interstellar clouds5 and, similarly, irradiation (248 nm) of propadienedithione (SC3S) entrapped in an argon matrix (10 K) has allowed the terrestrial observation of transient, triplet CCS (ν1 1666.6 cm−1).6 In contrast to the rich organometallic chemistry of carbon monosulfide, no complex of coordinated C2S has been reported, though the ketenylidene ligand (CCO) has often been observed in cluster chemistry7 and a terminal mononuclear ketenylidene ligand has been implicated in the cleavage of carbon suboxide by [WCl2(PMePh2)4].8 Thioketenyl and selenoketenyl complexes LnM(η2C,C′-ACCR) (M = Mo, W; A = S, Se) have, however, been reported to arise from the reactions of ketenyl complexes with the Lawesson and Woollins reagents, respectively.9 Herein we report both serendipitous and strategic syntheses of the first example of a complex of thioxoethenylidene, viz. [Mo2(μ,σ(C):η2(C′S)-CCS)(CO)4(Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) and an exploration of its bonding as a function of the number of available metal valence electrons.

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from either the insertion of zerovalent platinum into the C−Se bond of alkynylselenolatocarbynes10a or reactions of the selenocarbonylate [Mo(CSe)(CO)2(Tp*)]− anion11 with metal halide complexes.10b,c This latter approach builds upon the early demonstration by Angelici that thiocarbonyl ligands are more prone to attack at sulfur by electrophiles (including transition-metal centers) than are carbonyl ligands.12 Notably, however, Angelici’s studies of the reactions of [W(CS)(CO)2(Tp)]− (Tp = hydrotris(pyrazol-1-yl)borate) with metal electrophiles afforded bimetallic complexes in which the thiocarbonyl adopted either a semibridging (twoelectron)13 or σ−π (four-electron) bridging14 mode, rather than an isothiocarbonyl linkage. The reactions of [Fe2(μ-S2)(CO)6]15 with alkynyl nucleophiles result in alkynethiolato-bridged diiron complexes, subsequent electrophilic quenching of which may occur at the μ-sulfido or either carbon atom of the alkynethiolato bridge (Scheme 1).16 In search of alternative methods of assembling isothiocarbonyl linkages, we therefore considered whether the nucleophilic carbido complex [Mo(CLi)(CO)2(Tp*)] (1)11 might similarly cleave the disulfido bridge of [Fe2(μ-S2)(CO)6] to generate an intermediate (Li[A]; Scheme 1) akin to that in Scheme 1. Although this type of bridging mode is unprecedented for CS, we have recently observed it for CSe in the tetranuclear complex [Mo2Rh2(μ-CSe)2(CO)4(η4cod)2(Tp*)2] (cod = 1,5-cyclooctadiene).10f We therefore anticipated that trapping the intermediate with 1 equiv of [Mo(CBr)(CO)2(Tp*)] (2) might afford the analogous complex [Mo2Fe2(μ3-CS) 2(CO)10(Tp*)2] (B, Scheme 2). Alternatively, and more conveniently, we considered whether the same species [A]− and B might be

RESULTS AND DISCUSSION

We have previously reported the synthesis of polymetallic assemblies bridged by isoselenocarbonyl ligands.10 These arise © 2014 American Chemical Society

Received: November 10, 2014 Published: December 19, 2014 328

dx.doi.org/10.1021/om5011319 | Organometallics 2015, 34, 328−334

Organometallics

Article

accounts for the reaction of Li[Mo(CS)(CO)2(Tp*)] with another 1 equiv of 2 to afford 3 (Scheme 2). Complex 3 may be thought of as a hybrid of an alkylidyne complex and a thioacyl complex, with each component having precedent. The field of carbyne complexes ligated by poly(pyrazolyl)borate ligands is diverse and has been reviewed.17 Mononuclear chalcoacyl complexes of the group 6 metals18 are more scarce, being limited to those arising from the reactions of alkylidyne complexes with either propylene sulfide18a or mesityl isoselenocyanate.18c A small number of thiocarboxamide, thioaryloxycarbonyl, and dithioaryloxycarbonyl derivatives are known, however, in which the organyl substituent is replaced by amino, alkoxy, or thiolato substituents.19 Binuclear chalcoacyl complexes, however, all adopt a dimetallachalcotetrahedrane geometry in which the CE (E = S, Se, Te) group transversely bridges a metal−metal bond (Chart 1),20 with the exception of [MRu(μ-SCC6H4Me4)(CO)4(η5-C2B9H9Me2)(Tp)] (Chart 1e; Tp = hydrotris(pyrazol-1-yl)borate).21

Scheme 1. Generation and Electrophilic Quenching of a Binuclear Alkynethiolato Complex16

Scheme 2. Alternative Syntheses of a Thioxoethenylidene Complex

Chart 1. Coordination Modes for Chalcoacyl and Related Ligands: (a) η1; (b) η2; (c) Parallel Bridging; (d) Transverse Bridging; (e) σ−π Bridging

The spectroscopic data for 3, while indicating a binuclear complex of low symmetry, did not allow for unequivocal identification, not least because the small range of 13C NMR data reported for thioacyl groups18 overlaps somewhat with the wide range observed for alkyidyne ligands (vide infra; Table 118,19,22−28).17 The formulation was confirmed, however, by a crystallographic analysis, the results of which are summarized in Figure 1. Although, as noted, group 6 thioacyl complexes have been reported previously,18 this was without recourse to structural characterization. Accordingly, the previously described complex [Mo(η2-SCC4H3S-2)(CO)2(Tp)] (5)18a was characterized crystallographically to provide a benchmark for more conventional thioacyl metrics (Figure 2). Metrical parameters resulting from geometrical optimization at the BP/TZP level of theory (vide infra) are collected with those derived experimentally for 3 and 5 in Table 2, from which it is clear that the coordination of the CCS bridge to Mo1 is indeed best described as a thioacyl/molybdathiirene composite. Thus, the Mo1−C1 bond length of 2.018(3) Å points toward the retention of considerable multiple bond (metallathiirene) character, being shorter than typically observed for molybdenum bound to η1 sp2-C vinyl, acyl, and N-heterocyclic carbene ligands (2.15−2.30 Å) and falling between the ranges typical of Fischer-type (>2.0 Å) and Schrock-type carbene ( 2σ(I). Structural Analysis of [Mo(η2-SCC4H3S-2)(CO)2(Tp)] (5). The complex [Mo(η2-SCC4H3S-2)(CO)2(Tp)] (5) was prepared as described previously.18b Crystals of the solvate 5·CH2Cl2 were obtained by slow diffusion of hexane into a dichloromethane solution of 5. Crystal data: C16H13BMoN6O2S·CH2Cl2, Mr = 577.13, T = 200(2) K, monoclinic, space group P21/n, a = 13.5866(2) Å, b = 8.37410(10) Å, c = 21.0109(3) Å, β = 107.1596(6)°, V = 2284.12(5) Å3, Z = 4, Dcalcd = 1.678 Mg m−3, μ(Mo Kα) 1.018 mm−1, black block, 0.12 × 0.15 × 0.37 mm, 32944 measured reflections with 2θmax = 54.9°, 5233 independent, adsorption-corrected reflections used in F2 refinement, 296 parameters, 18 restraints, R1 = 0.0237, wR2 = 0.0254 for 3110 reflections with I > 3σ(I).

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Article

S Supporting Information *

Text, figures, and CIF and xyz files giving crystallographic data for 3 and 4 and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail for A.F.H.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council (DP130102598 and DP110101611). REFERENCES

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