In recent years, more and more reports appeared on the use of Co-based catalysts in organic chemistry. It was supposed that a large number of Co-based catalytic processes involved Co(I), for example catalytic formation of arylzinc derivatives from aryl halides. Nevertheless, Co(I) species are not stable to oxidation and have a tendency to disproportionation, therefore Co(I) requires special ligands for stabilization. Phosphines are among the most promising ligands of this type, but despite numerous reports on the use of cobalt complexes with phosphines in catalysis, information on the redox properties of these complexes is scarce. The aim of our work was to study the redox properties of cobalt complexes with phosphines by cyclic voltammetry (CV) and to compare such properties with the catalytic productivity of these compounds in the formation of arylzinc compounds. For this purpose, 8 cobalt complexes with phosphines were obtained (Fig. 1); Co(N-XantPhos)Cl2 and Co(XantPhos)(OPiv)2 were synthesized and characterized for the first time. It was found that in the case of PPh3, dppf, dppb, and DPEphos ligands there was no effective stabilization of Co(I) in conditions of CV on criteria of redox potential and reversibility. In contrast, in the case of Co(XantPhos)Cl2 and Co(N-XantPhos)Cl2 quasi-reversible Co2+/Co+ electrochemical processes were found. The best stabilization of Co(I) was observed in the case of dppe (Figure 1). When the chloride anion in Co(XantPhos)Cl2 was replaced by a pivalate, the quasi-reversible redox process disappeared, indicating the role of chloride in the stabilization of Co(I). The optimization of anion is also made. The catalytic productivity of complexes was evaluated in forming a 4-anisole zinc derivative. The catalytic properties of the compounds obtained we detected and will be discussed.

Redox properties of cobalt-phosphine complexes vs their catalytic productivity in the formation of arylzinc compounds

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