Cooperative Sulfur Transformations at a Dinickel Site: A Metal Bridging Sulfur Radical and Its H-Atom Abstraction Thermochemistry.

Starting from the dinickel(II) dihydride complex [ML(Ni-H)(2)] (1(M)), where L(3-) is a bis(tridentate) pyrazolate-bridged bis(β-diketiminato) ligand and M(+) is Na(+) or K(+), a series of complexes [KLNi(2)(S(2))] (2(K)), [MLNi(2)S] (3(M)), [LNi(2)(SMe)] (4), and [LNi(2)(SH)] (5) has been prepared. The μ-sulfido complexes 3(M) can be reversibly oxidized at E(1/2) = -1.17 V (in THF; vs Fc(+)/Fc) to give [LNi(2)(S(•))] (6) featuring a bridging S-radical. 6 has been comprehensively characterized, including by X-ray diffraction, SQUID magnetometry, EPR and XAS/XES spectroscopies, and DFT calculations. The pK(a) of the μ-hydrosulfido complex 5 in THF is 30.8 ± 0.4, which defines a S-H bond dissociation free energy (BDFE) of 75.1 ± 1.0 kcal mol(-1). 6 reacts with H atom donors such as TEMPO-H and xanthene to give 5, while 5 reacts with 2,4,6-tri(tert-butyl)phenoxy radical in a reverse H atom transfer to generate 6. These findings provide the first full characterization of a genuine M-(μ-S(•-))-M complex and provide insights into its proton-coupled electron transfer (PCET) reactivity, which is of interest in view of the prominence of M-(μ-SH/μ-S)-M units in biological systems and heterogeneous catalysis.