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] (1M), where L3- is a bis(tridentate) pyrazolate-bridged bis(ß-diketiminato) ligand and M+ is Na+ or K+, a series of complexes [KLNi2(S2)] (2K), [MLNi2S] (3M), [LNi2(SMe)] (4), and [LNi2(SH)] (5) has been prepared. The µ-sulfido complexes 3M can be reversibly oxidized at E1/2 = -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 pKa 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.

Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO4} Motif.

Most 3d metal-based single-molecule magnets (SMMs) use N-ligands or ligands with even softer donors to impart a particular coordination geometry and increase the zero-field splitting parameter |D|, while complexes with hard O-donor ligands showing slow magnetization relaxation are rare. Here, we report that a diamagnetic NiII complex of a tetradentate ligand featuring two N-heterocyclic carbene and two alkoxide-O donors, [LO,ONi], can serve as a {O,O'}-chelating metalloligand to give a trinuclear complex [(LO,ONi)Co(LO,ONi)](OTf)2 (2) with an elongated tetrahedral {CoIIO4} core, D = -74.3 cm-1, and a spin reversal barrier Ueff = 86.9 cm-1 in the absence of an external dc field. The influence of diamagnetic NiII on the electronic structure of the {CoO4} unit in comparison to [Co(OPh)4]2- (A) has been probed with multireference ab initio calculations. These reveal a contrapolarizing effect of the NiII, which forms stronger metal-alkoxide bonds than the central CoII, inducing a change in ligand field splitting and a 5-fold increase in the magnetic anisotropy in 2 compared to A, with an easy magnetization axis along the Ni-Co-Ni vector. This demonstrates a strategy to enhance the SMM properties of 3d metal complexes with hard O-donors by modulating the ligand field character via the coordination of diamagnetic ions and the benefit of robust metalloligands in that regard.