The reactivity and coordination chemistry of nickel with sulfur containing ligands such as SR − , SH − , and S x n− has been the subject of considerable research efforts. 1 For example, among its many industrial applications, nickel can function as a key activator in hydrodesulfurization of fossil fuel sources. 2 Jones and co-workers have made considerable progress in deducing the mechanistic details of desulfurization in homogeneous model systems by uncovering a wealth of variable structure types including an intriguing terminal nickel sulfide, the existence of which is inferred from reaction kinetics and chemical trapping with nitroxide reagents. 3 To access new nickel-sulfur structural motifs that can be kinetically trapped and examined in detail, we have investigated an alternative preparative pathway, namely, the reaction of nickel(I) precursors with elemental sulfur. 4 An inherent difficulty in the study of such synthetic systems is the tendency of nickel to form binary sulfides. Nonetheless, several dinickel complexes with bridging sulfide ligands have been prepared. 5 To date, the three crystallographically characterized disulfido. (S 2 2− ) dinickel complexes contain an η 2 :η 2 or "side-on" Ni 2 S 2 core structure. 4,6 Complementary to these studies, we report herein the preparation and characterization of the first example of a bridging "end-on" disulfide motif of nickel. This complex provides an excellent platform for the spectroscopic evaluation of this core structure type, reactivity investigations aimed at obtaining fundamental insights relevant to small molecule activation, and the development of stoichiometric and catalytic reagents.To access new nickel-sulfur structure types, we have employed analogous monovalent nickel precursors as in our recent investigations into dioxygen activation. 7 Specifically, the addition of elemental sulfur to the Ni 1+ complex [Ni(tmc)](OTf) (1, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and 444 nm (ε = 2020, 1420, 670 M −1 cm −1 , THF, 195 K), which are typical for transition metal sulfur adducts. 8,9 Optical titration experiments indicate maximum intensity at 647 nm is achieved at a 1:1 Ni:S stoichiometry ( Figure 1). Further addition of another equivalent of sulfur results in a decrease of the band at 647 nm and concomitant growth of the two higher energy features at 529 and 444 nm, which we therefore attribute to a distinct, as yet unidentified species, 3 (Supporting Information (SI)). Room temperature decomposition of 2 is facile yielding an intractable brown solid, presumably nickel sulfide.Resonance Raman spectra obtained with laser excitation into the most intense absorption feature of 2 (excitation λ= 647 nm) exhibit a dominant feature at 474 cm −1 , which downshifts to 462 cm −1 (Δ = −12 cm −1 ) in samples prepared with 34 S 8 . On the basis of reduced mass calculations, this feature is assigned as a v(S-S) stretching mode (for an isolated harmonic oscillator: Δ = −14 cm −1 ). The first overtone of this mode is present at 943 cm −1 ...
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