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)(iPr2-bimy)] (cis-1) and the bis(carbene)
complex trans-[PtBr2(iPr2-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(iPr2-bimy)(PPh3)] (trans-3), cis-[PtBr2(iPr2-bimy)(PPh3)] (cis-3), and trans-[PtBr2(iPr2-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···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 1H NMR spectrum compared to that in the precursor
salt A.