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

Abstract

Starting from the dinickel­(II) dihydride complex [ML­(Ni–H)2] (1 M ), where L3– is a bis­(tridentate) pyrazolate-bridged bis­(β-diketiminato) ligand and M+ is Na+ or K+, a series of complexes [KLNi 2 (S2)] (2 K ), [MLNi2S] (3 M ), [LNi2(SMe)] (4), and [LNi2(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 [LNi2(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.