Redox Behavior of Cobalt–Phosphine Complexes vs Their Catalytic Activity in Organozinc Compound Formation: Background for Mechanistic Investigations

Catalytic activity in arylzinc compound formation was studied for eight Co complexes with phosphines along with their redox properties for implementing the idea of rational design. It was found that Co(XantPhos)Cl₂ and Co(N-XantPhos)Cl₂ demonstrated distinct reversible Co^II/Co^I redox processes and acted as efficient catalysts of arylzinc compound formation. Meanwhile, for Co(DPEphos)Cl₂, Co(dppf)Cl₂, Co(dppb)Cl₂, Co(PPh₃)₂Cl₂, and Co(XantPhos)(Piv)₂ (the latter one without the addition of LiCl), reversible redox processes were not observed. These catalysts did not act efficiently for the model process of organozinc compound formation. Co₄(dppe)₅Cl₈ was the only exception, explained by a completely different structure (CoP₄Cl and CoPCl₃) of donor sets instead of CoP₂X₂ (X = Cl or O). The stability of complexes in tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) solutions was studied by UV–vis spectroscopy. Previously unknown X-ray structures for Co(XantPhos)(Piv)₂, Co(N-XantPhos)Cl₂, and {Co(DMF)₆}{(CoCl₃)₂(dppb)} were determined. The use of pivalate counterions instead of chloride for Co(XantPhos)²⁺ led to a significant (ca. 20 times) increase of the kinetic solubility in THF compared to Co(XantPhos)Cl₂, preserving high catalytic productivity upon the addition of LiCl. This allowed the latter to be efficiently used in combination with LiCl as the catalyst for arylzinc compound formation on a 2 g scale. The data obtained in this work can be regarded as experimental confirmation of the first and last stages of the plausible reaction pathway of arylzinc compound formation, involving Co^II → Co^I and Co^I → Co^II transformations, which could be a significant framework for further mechanistic investigations.