Selective CC Coupling of Ir–Ethene and Ir–Carbenoid Radicals

Abstract

The reactivity of the paramagnetic iridium(II) complex [Ir^II(ethene)(Me₃tpa)]²⁺ (1) (Me₃tpa=N,N,N-tris(6-methyl-2-pyridylmethyl) amine) towards the diazo compounds ethyl diazoacetate (EDA) and trimethylsilyldiazomethane (TMSDM) was investigated. The reaction with EDA gave rise to selective CC bond formation, most likely through radical coupling of the Ir–carbenoid radical species [Ir^III{CH^.(COOEt)}(MeCN)(Me₃tpa)]²⁺ (7) and (the MeCN adduct of) 1, to give the tetracationic dinuclear complex [(MeCN)(Me₃tpa)Ir^III{CH(COOEt)CH₂CH₂}Ir^III(MeCN)(Me₃tpa)]²⁺ (4). The analogous reaction with TMSDM leads to the mononuclear dicationic species [Ir^III{CH₂(SiMe₃)}(MeCN)(Me₃tpa)]²⁺ (11). This reaction probably involves a hydrogen-atom abstraction from TMSDM by the intermediate Ir–carbenoid radical species [Ir^III{CH^.(SiMe₃)}(MeCN)(Me₃tpa)]²⁺ (10). DFT calculations support pathways proceeding via these Ir–carbenoid radicals. The carbenoid–radical species are actually carbon-centered ligand radicals, with an electronic structure best described as one-electron-reduced Fischer-type carbenes. To our knowledge, this paper represents the first reactivity study of a mononuclear Ir^II species towards diazo compounds.