Electronic Properties of N-Heterocyclic Carbene (NHC) Ligands:Synthetic,Structural,and Spectroscopic Studies of (NHC)Platinum(II) Complexes

摘要

N-Heterocyclic carbene complexes of platinum(II) have been synthesized,notably monocarbene complexes cis-[(IPr)Pt(dmso)(Cl)2],6,cis-[(IMes)Pt(dmso)(Cl)2],7,cis-[(SIPr)Pt(dmso)(Cl)2],8,cis-[(STMes)Pt(dmso)(Cl)2],9,and cis-[(TTP)Pt(dmso)(Cl)2],10.All complexes have been fully characterized by multinuclear NMR spectroscopy.Complex 7,9,and 10 have been characterized by X-ray crystallography.The data obtained have allowed for the differentiation between electronic contributions (sigma and pi) present in the Pt-NHC bond.Supported by computational analyses,the percentage of pi back-donation from the metal to the NHC is found to be on the order of 10%.More interestingly,we find that saturated NHC (SIPr and SEVIes) are more efficient pi back-acceptors than their unsaturated NHC congeners (IPr and IMes).The synergistic effect between pi back-donation and a donation present in the saturated NHC systems results in increased electron density at the platinum center compared to the bonding situation in the unsaturated NHC examples.