From NHC to Imidazolyl Ligand: Synthesis of Platinum and Palladium Complexes d¹⁰-[M(NHC)₂] (M = Pd, Pt) of the NHC 1,3-Diisopropylimidazolin-2-ylidene

The widely held belief that N-heterocyclic carbenes (NHCs) act only as innocent spectator ligands is not always accurate, even in the context of well-explored reactions. Ligand exchange in the conversion of [Pt­(PPh3)2(η2-C2H4)] (3) to [Pt­(iPr2Im)2] (2) depends critically on the particular reaction conditions employed, with slight changes leading to vastly different outcomes. In addition to [Pt­(iPr2Im)2] (2), complexes [Pt­(iPr2Im)­(PPh3)­(η2-C2H4)] (5) and trans-[Pt­(iPr2Im)2(iPr-Im*)­(H)] (6) were isolated and in the case of 6 fully characterized. Complex 5 represents the first mixed-olefin complex in transition metal chemistry containing both an NHC and a phosphine ligand. Chemical degradation of the NHC was shown to yield the new imidazole-2-yl iPr-Im* in 6. Therefore, the synthesis of [Pt­(iPr2Im)2] (2) via metallic reduction of the ionic precursor [Pt­(iPr2Im)3(Cl)]+Cl– (9) is favorable, a procedure adaptable to analogous palladium compounds. While [Pd­(iPr2Im)3(Cl)]+Cl– (8) is the only product obtained from the reaction of iPr2Im and PdCl2, neutral [Pt­(iPr2Im)2(Cl)2] (10), formed as a mixture of its two stereoisomers cis-10 and trans-10, is available through precise control of the stoichiometry in the reaction of PtCl2 and exactly 2 equiv of iPr2Im.