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)] (), where L is a bis(tridentate) pyrazolate-bridged bis(β-diketiminato) ligand and M is Na or K, a series of complexes [KLNi(S)] (), [MLNiS] (), [LNi(SMe)] (), and [LNi(SH)] () has been prepared. The μ-sulfido complexes can be reversibly oxidized at = -1.17 V (in THF; vs Fc/Fc) to give [LNi(S)] () featuring a bridging S-radical. has been comprehensively characterized, including by X-ray diffraction, SQUID magnetometry, EPR and XAS/XES spectroscopies, and DFT calculations. The p of the μ-hydrosulfido complex in THF is 30.8 ± 0.4, which defines a S-H bond dissociation free energy (BDFE) of 75.1 ± 1.0 kcal mol. reacts with H atom donors such as TEMPO-H and xanthene to give , while reacts with 2,4,6-tri(-butyl)phenoxy radical in a reverse H atom transfer to generate . 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.