Rhodium(I) complexes containing a bulky pyridinyl N-heterocyclic carbene ligand: Preparation and reactivity

Coordination chemistry of a new pyridine imidazole-2-ylidene ligand (pyN∧C) system with sterically hindered substituents toward rhodium(I) metal ions has been investigated. The rhodium complex [(pyN∧C)RhCl(COD)] (COD = 1,5-cyclooctadiene) was prepared via the transmetallation from the silver complex [(C-pyN∧C)2Ag]AgI2. Upon the abstraction of chloride, the pyridinyl nitrogen coordinated to the metal center and formed [(C,N-pyN∧C)Rh(COD)]BF4 with the chelation of pyN∧C. The pyridinyl nitrogen donor was found to be labile and could be replaced by various donors such as phosphine, azide and halides. Substitution of COD by various donors does not proceed except strong π-acid ligands such as CO and P(OCH3)3. However, the chelation of pyN∧C was replaced by the bisphosphine (P∼P) to form [(P∼P)2Rh]BF4, which was subsequently oxidized to yield [(P∼P)2Rh(O2)]BF4.