Evidence for the Existence of a Late-Metal Terminal Sulfido Complex

The transition-metal-sulfur bond is an important linkage in both biological systems1 and industrial catalysts.2 Nickel sulfides, in particular, are key components of natural hydrogenases3 and are active promoters in current hydrotreating catalysts.2 Understanding the nature of the metal-sulfur linkages at active sites can offer insight on ways to improve catalysis and provide a better understanding of cluster formation and cluster interconversion reactions in general. Scheme 1 shows three possible binding modes of a sulfur atom to supported nickel. While numerous examples of binding modes A4 and B5 can be found throughout the literature, there exists no sound evidence for a terminal sulfido complex such as C in the solution phase.6 Additionally, there have been only a few cases for the group 9 and 10 metals where the intermediacy of a terminal sulfido complex has been postulated,7 and there has been no experimental evidence to support the existence of such a species. Terminal sulfido complexes of the earlier transition-metals are well-known, and display a wide range of reactivity.8 Late-metal analogues might then be anticipated to afford similarly rich chemistry. Raney nickel, and homogeneous nickel complexes,9 have been found to desulfurize a variety of organosulfur substrates. The fate of the sulfur atom in these reactions is usually nickel sulfide, thereby preventing catalysis under traditional laboratory conditions. A number of recent reports from our group,10 however, indicate that sulfur can be extracted from organic compounds without the formation of nickel sulfide. These results prompted us to further explore the mechanism of these unique desulfurizations, and to systematically investigate the intermediacy of the three possible binding modes of a sulfur atom to nickel (Scheme 1). In these efforts we have found kinetic and structural evidence that support the chemical feasibility of binding mode C, namely a terminal sulfido complex at nickel. Initial studies were directed toward finding an efficient way to prepare a nickel sulfido complex. Bergman has reported routes to transient [Cp2TidS] and [Cp2ZrdS] using Cp2Ti(SH)H and Cp2Zr(SH)I, respectively. The synthetic strategy that we found to be successful with nickel was based on two very different but related findings. It was discovered by Bergman and co-workers that thermolysis of Cp*2Zr(OH)(Ph) led to the loss of benzene and formation of the terminal oxo complex, which was subsequently trapped by a variety of substrates.12,13 Additionally, Osakada and co-workers found that thermolysis of trans-Ni(Ar)(SH)(PEt3)2 (Ar ) aryl ligand) led to decomposition of the metal with formation of Ar-H and SdPEt3. Curiously, no Ar-SH was formed.14 We wondered whether the loss of arene in this instance was concomitant with the formation of a transient terminal sulfido complex, mimicking the zirconium-oxo chemistry. We therefore prepared a nickel complex containing chelating phosphines and cis-(Ar)(SH) groups to examine if elimination of arene would readily occur. Chelating phosphines were used since recent reports have shown that [L2M(μ-S)]2 complexes (where L2 is a chelating ligand) are thermally quite stable.10b,15 Li2Ni(SH)(Ph) (1a and 1b) [L2 ) dippe (1,2-bis(diisopropylphosphino)ethane) (1a) and dcpe (1,2-bis(dicyclohexylphosphino)ethane) (1b)] were prepared and their thermolysis behavior was studied in solution. Nickel thiols of type 1 were synthesized by the addition of NaSH to L2Ni(Cl)(Ph). It was found that mild heating of 1 in THF solutions led to loss of benzene (1 equiv observed by 1H NMR spectroscopy) and production of the bridged sulfido dimers 2 in quantitative yields by NMR spectroscopy (eq 1). Complex