Structural and Electrochemical Study of a 2,2′-Biphosphinine

An improved synthesis of 4,4′,5,5′-tetramethyl-2,2′,-biphosphinine from 2-bromo-4,5-dimethylphosphinine using (Ph₃P)₂CoCl₂ as the coupling reagent is described. The coupling can also be effected in the coordination sphere of molybdenum tetracarbonyl via a bromine to lithium exchange followed by C-C bond formation with (DME)NiBr₂. The recovery of the free 2,2′-biphosphinine from the Mo(CO)₄ chelate thus obtained appears to be difficult. The X-ray crystal structure analysis of the 2,2′-biphosphinine at −100 °C suggests that the interaction between the two rings and the rotation barrier around the C-C bridge are weak. The complexation with Cr(CO)₄ does not alter the geometry of the biphosphinine, and no electronic delocalization seems to be operative within the five-membered metallacycle. The electrochemical reduction of the tetramethylbiphosphinine is easier than that of 4,4′-dimethyl-2,2′-bipyridine. The first monoelectronic wave occurs at −1.85₂ V vs SCE in DMF for the phosphorus ligand as compared to −2.20₁ V vs SCE for the nitrogen ligand. As a consequence, the 2,2′-biphosphinine is a better ligand for electron-rich metals and is able to displace 2,2′-bipyridine from its Cr(CO)₄ chelate.