Mono- vs bis(carbene) complexes: A detailed study on platinum(II)-benzimidazolin-2-ylidenes

The reaction of PtBr2 with NaOAc and 1,3-diisopropylbenzimidazolium bromide (A) in DMSO afforded the mixed monocarbene-DMSO complex cis-[PtBr2(DMSO)(Pr-i(2)-bimy)] (cis-1) and the bis(carbene) complex trans-[PtBr2(Pr-i(2)-bimy)(2)] (trans-2). The DMSO ligand in cis-1 can be easily replaced by stronger donors such as triphenylphosphine and pyridine to give novel benzannulated monocarbene complexes trans-[PtBr2(Pr-i(2)-bimy)(PPh3)] (trans-3), cis-[PtBr2(Pr-i(2)-bimy)(PPh3)] (cis-3), and trans-[PtBr2(Pr-i(2)-bimy)(Pyridine)] (trans-4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometry (ESI, FAB). X-ray diffraction studies on single crystals of cis-1, trans-2, cis-3, and trans-4 revealed a square-planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H protons pointing to the metal center to maximize interesting and rare C-H center dot center dot center dot Pt preagostic interactions. These interactions are also retained in solution, as indicated by the large downfield shift of the isopropyl C-H protons in the H-1 NMR spectrum compared to that in the precursor salt A.