Homo- and Heterobimetallic Complexes in Catalysis

A dirhodium hydrido-carbonyl catalyst system based on a binuclating tetraphosphine ligand is discussed. Spectroscopic and DFT computational studies support the formulation of the key catalyst complex in acetone solvent as [Rh₂(μ-H)₂(CO)₂(rac-P4)]²⁺, which is highly active and regioselective for producing linear aldehydes under mild conditions. This dicationic catalyst suffers from facile fragmentation reactions in acetone that lead to inactive monometallic and bimetallic complexes. The addition of water to the acetone solvent leads to deprotonation from the dicationic catalyst to form monocationic dirhodium catalyst species that are far less susceptible to deactivation. Spectroscopic and DFT computational studies indicate that the key monocationic catalyst is [Rh₂(μ-H)(CO)₃(rac-P4)]⁺. Although the monocationic bimetallic catalyst is less active on a per molecule basis relative to the dicationic catalyst, there is a higher concentration present producing better overall catalyst rates and selectivity.

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.

The reactivity and catalysis at axial sites of [Ru–Ru] bonded compounds are described. Effect of axial donor at axial site of a [Ru–Ru] single bond, having electronic configuration σ²π⁴δ²δ*²π*⁴, is examined. It is shown that the stronger donor leads to longer metal–metal distances. The C–H bond activation and C–C bond formation are studied at axial site of a [Ru₂(CO)₄]²⁺ core. Metal–metal and metal–ligand cooperation is exploited for catalytic alcohol dehydrogenation to aldehyde and subsequent coupling with amine to access imine selectively. Catalytic carbene transfer reactions are discussed for a wide range of diruthenium(I,I) compounds. Catalytic utility of metal–metal multiply bonded diruthenium(II,III) compounds for C–H amination reaction is also discussed.

Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond.

By combining an ever-increasing number of catalysts or catalytic functions, cooperative catalysis is a research area that grows fast. In the field, “early–late” heterobimetallic complexes are rather old objects but they still continue to fascinate chemists because of their latent reactivity. After a brief and concise overview of cooperative catalysis, this review focuses on “early–late” heterobimetallic complexes that were used in catalysis over the last decades. Examples of dual catalysis using early and late metal partners are also described. This chapter ends with an opening towards therapeutic applications of “early–late” heterobimetallic complexes.

In the context of metal-mediated organic synthesis, cooperativity and synergism are rather broad terms which are often used to denote systems where unusual rate or selectivity effects are observed. These effects can be exhibited by monometallic, heterobimetallic and even multimetallic systems. The present contribution looks exclusively at one of the simplest cases, namely, systems possessing simultaneously both mononuclear and dinuclear complexes (hence both monometallic and heterobimetallic are included, but multimetallic systems are excluded). In Sect. 1, a brief introduction to the general area and a working definition for catalytic binuclear elimination reaction (CBER) is provided. In Sect. 2, we step back and classify the broad range of systems under consideration in order to enumerate the host of reaction networks considered, the potential for non-linear kinetic effects and how this relates to concepts of synthetic efficiency. In Sect. 3, we return to specific examples of CBER, how they fit into the overall context of the systems classification and how they can be identified in an unambiguous manner using in situ spectroscopic techniques. Indeed, tests can be constructed which permit the experimentalist to check crucial features and characteristics consistent with CBER. The present contribution focuses on the subarea in which CBER systems exist and hence CBER’s scope for organic syntheses.

The active sites of several bioenergetically important metalloenzymes that perform multielectron redox reactions feature heterobimetallic complexes. Herein, we review recent understanding of the structure and mechanisms of hydrogenases, formate dehydrogenases, and carbon monoxide dehydrogenases. Then we evaluate progress toward creating functional, small-molecule complexes that reproduce the activities of these active sites. Particular emphasis is placed on comparing catalytic properties including turnover number, turnover frequency, required overpotential, and catalyst stability. Opportunities and challenges for future work are also considered.