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A Series of Dimeric Cobalt Complexes Bridged by N‑Heterocyclic Phosphido Ligands

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journal contribution
posted on 18.03.2020, 13:35 by Andrew M. Poitras, Mark W. Bezpalko, Curtis E. Moore, Diane A. Dickie, Bruce M. Foxman, Christine M. Thomas
A tridentate [PPP] ligand has been used to construct a series of dimeric cobalt complexes and explore cooperative multielectron redox processes that are both metal- and ligand-centered. Reduction of (PPClP)­CoCl2 (1) with excess magnesium affords the CoICoI N-heterocyclic phosphido (NHP)-bridged symmetric dimer [(μ-PPP)­Co]2 (2). Two-electron oxidation of 2 with FcPF6 generates an asymmetrically bridged dication [(μ-PPP)­Co]2[PF6]2 (3) in which the oxidation has occurred in a delocalized fashion throughout the Co2P2 core. In contrast, [(μ-PPP)­Co]2+ (5), which can be generated either by one-electron oxidation of 2 with FcPF6 or comportionation of 2 and 3, features an asymmetric geometry and localized mixed valence. Treatment of 1 with the milder reductants CoCp2 and KBEt3H does not lead to formation of 2, 3, or 5 but instead generates dimeric species [(PPP)­CoCl]2 (6) and [(PPP)­CoH]2 (7). Unlike 25, where the phosphine side arms of the tridentate [PPP] ligand span the two Co centers, complex 6 and 7 are connected solely by NHP ligands that bridge the two (PPP)Co fragments.