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.