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 p K 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.