Binuclear iridium complexes in catalysis

Abstract

In this chapter, recent progresses in the use of binuclear iridium complexes as catalysts for the preparation of value-added organic molecules and the catalytic cycles involved in these reactions are presented. The reactivity of these complexes toward a variety of substrates and the intermetallic cooperation mechanisms that differentiate binuclear entities from their mononuclear counterparts are reviewed and analyzed in detail. Oxidative addition and reductive elimination reactions usually occur at one of the iridium centers followed by ligand migration to the vicinal iridium atom or to bridging positions, although cooperative activation of various substrates has been proposed in the literature. The close proximity of two metal centers in binuclear complexes, and their ability to cooperate, brings about new reactivity patterns that very often differ from those expected for related monometallic systems. Especially noteworthy is the transmission of ligands trans effects (or influences) via bridging ligands or intermetallic bonds, together with the facile migration of hydrides between metals and the interaction between iridium centers in Ir2 I,I dimers, which seem to govern the chemistry of diiridium complexes. In this regard, it is worth noting that single-site activation does not exclude a cooperative bimetallic cycle. Most of the reported catalytic cycles based on bimetallic iridium complexes follow inner sphere mechanisms, but the presence of outer sphere pathways cannot be excluded as exemplified by recent reports on ionic or dimetal–ligand bifunctional mechanisms; therefore, bimetallic iridium catalysis may show one or more cooperation mechanism.