A mechanistic study on complexes of type mer-[Cr(CO)3(η2-L–L)(σ-L–L)](where L–L = Ph2PCH2PPh2, Ph2PNHPPh2, or Ph2PNMePPh2) using spectroscopic and convolutive electrochemical techniques

Abstract

The new complexes mer-[Cr(CO)₃(η²-L–L)(σ-L–L)][where L–L = Ph₂PCH₂PPh₂(dppm), Ph₂PNHPPh₂(dppa) or Ph₂PNMePPh₂(dppma)] were synthesized by treating [Cr(CO)₃(C₇H₈)](C₇H₈= cyclohepta-1,3,5-triene) with the appropriate diphosphine. The complexes were characterized by i.r. and by ³¹P-{¹H} and ¹H-{³¹P} n.m.r. spectroscopy. The electrochemical and chemical oxidation of these complexes was investigated and the products identified by electrochemical techniques and by e.s.r. and i.r. spectroscopy. The one-electron oxidation of mer-[Cr(CO)₃(η²-L–L)(σ-L–L)] gives the 17-electron chromium(I) complex mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]⁺. These chromium(I) complexes have only limited stability and have been shown to follow two reaction pathways, disproportionation (electron transfer) as in equation (i) and intramolecular displacement of CO as in equation (ii). The rates of these reactions increase markedly from L–L = dppm to dppa or dppma. A second one-electron oxidation process [equation (iii)] has been observed but the dications are very unstable and decompose to give solvated Cr²⁺, free diphosphine, and CO gas. 2 mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]⁺→mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]⁰+mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]²⁺(i), mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]⁺→trans-[Cr(CO)₂(η²-L–L)₂]⁺+ CO (ii), mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]⁺→mer-[Cr(CO)₃(η²-L–L)(σ-L–L)]²⁺+ e (iii)