N-heterocyclic carbenes have emerged as interesting replacements for phosphines as ligands on transition metals. Our group was one of the first to document the use of Rh-N-heterocyclic carbenes as catalysts for the important hydroformylation reaction. Since then, we have demonstrated that highly unusual bonding modes can be observed in rhodium complexes of dioxygen, where, for example, O2 binds to the metal without oxidizing it. When this reaction was done in the solid state, a crystal-to-crystal transformation was observed.  Even more impressively, a third transformation could also be carried out without any loss of crystallinity by exposing the O2 complex to carbon monoxide.   Thus this results in a crystal-to-crystal-to-crystal transformation![1]

crystal to crystal

In terms of catalysis, we have recently developed a highly unsaturated Rh–NHC complex that, by virtue of the loss of ethylene and cleavage of the dimer, is essentially equivalent to a 12 electron complex. [2] The high reactivity of this compound is illustrated in its facile reaction with bipy derivatives, under conditions where the corresponding COD complex is completely unreactive.  This reactivity could then be used to make a variety of complexes that have remarkably different  UV/Vis profiles. [3]

COD image

In recent work, we have shown that this compound has higher reactivity for the C–H borylation of phenyl pyridine than the best homogeneous compounds for this reaction.[4] In future work, we are looking at further testing what this class of catalyst can do!

catalysis

[1]O.V. Zenkina, E.C. Keske, R. Wang, C.M. Crudden*, “Double Single-Crystal to Single-Crystal Transformations and Small Molecule Activation in Rhodium NHC Complexes” Angewandte Chemie International Edition 2011, 35, 8100-8104.

[2]O.V. Zenkina, E. C. Keske, R. Wang, C.M. Crudden*, “Dimeric Rhodium-ethylene NHC complexes as Reactive Intermediates for the Preparation of Tetra-heteroleptic NHC Complexes” Organometallics, 2011, 30, 6423-6432.

[3]O.V. Zenkina,E.C. Keske, G.S. Kochar,R.Wang,and C.M. Crudden*, “Novel Heteroleptic NHC Rhodium Complexes with Pyridine-Derived Ligands: Synthetic Accessibility and Reactivity Towards Oxygen.” Dalton Transactions, 2013, 42, 2282-2293.

[4]E.C. Keske, B.D. Moore, O.V. Zenkina, R. Wang, G. Schatte and C.M. Crudden, “Highly Selective Directed Arylation Reactions via Back-to-Back Dehydrogenative C-H Borylation/Arylation Reactions”, Chemical Communications, 2014, 9883-9886.