Late Transition Metal Hydroxide in Homogeneous Catalysis

EPSRC Grant YEP138: Late Transition Metal Hydroxide in Homogeneous Catalysis

The development of new (late) transition metal (LTM) catalysed transformations continues to drive advances in the fields of organic synthesis and small-molecule activation. Research in our group is predominantly concerned with the exploration of well-defined LTM-complexes and their utility in homogeneous catalysis. In particular, we are interested in the use of nitrogen containing heterocyclic carbenes (NHC’s) and understanding the properties that they confer on metal centres. Of particular interest to us is the stabilizing effect that NHCs can have on metal-ligand interactions. For example, we have investigated the preparation of LTM-hydroxides for fundamental organometallic chemistry and homogeneous catalysis.
Our interest in LTM-hydroxides is two-fold. Firstly, hydroxides are hard bases, while a low oxidation state metal behaves as a soft acid. This hard base/soft acid mismatch leads in most cases to weak metal-hydroxide bonds and hence, unstable metal complexes, but when stabilised, the M-OH moieties can display fascinating reactivity. Secondly, various LTM-hydroxides have been identified (or proposed) as important intermediates in a number of catalytic transformations. Preparing well-defined LTM-hydroxides enables us to study such mechanims in detail. By coordination of the metal centre to electron-rich NHC ligands, we are able to stabilise the metal-hydroxide bond and provide isolable complexes.
Recently, we have reported the preparation of hydroxide complexes of copper, gold, ruthenium, palladium and rhodium metals. Extensive studies on these metal complexes have shown them to possess unprecedented reactivity as bond activation agents and versatile synthons in organometallic chemistry. For example, gold-hydroxides were shown to be potent transmetallation agents, reacting with boronic acids to give aryl-gold complexes under very mild conditions compared to established methods. Digold-hydroxide complexes were also developed and the incorporation of two gold centres on a single hydroxide moiety provided a very active catalyst for water-inclusive organic transformations such as nitrile hydration and Meyer−Schuster rearrangement.
Investigations into the use of NHC-metal complexes of the group 9 and 10 metals has borne fruit in the form of novel palladium complexes, which proved very useful in C-H bond activation of sulphur- and nitrogen-containing heterocycles. Furthermore, studies with rhodium complexes allowed the catalysis of tandem reactions to concomitantly incorporate silicon and deuterium elements onto organic compounds under very mild conditions.
Our investigations into the preparation of novel late transition metal complexes is on-going, and mainly focussed on the interaction between metals and different ligands. Some of the more uncharacteristic interactions, such as the interaction with hard bases have led to very significant findings that continue to spur our efforts in organometallic synthesis and homogeneous catalysis.