Coordinatively Unsaturated T-Shaped Platinum(II) Complexes Stabilized by Small N-Heterocyclic Carbene Ligands. Synthesis and Cyclometalation

Abstract

The reaction of the N-heterocyclic carbenes (NHCs) IiPr2Me2 and IiPr2 with [PtMe3I]4 does not lead to the clean formation of complexes trans-[PtMeI(NHC)2], as previously found for bulkier NHCs. However, these species can be prepared in high yields by an alternative procedure using the dimethyl species [PtMe2(NHC)2] and iodomethane. The halogenated complexes trans-[PtMeI(NHC)2] have been used as precursors for the formation of the coordinatively unsaturated Pt(II) derivatives [PtMe(NHC)2][SbF6] using AgSbF6 as a halogen abstractor, whereas NaBArF is not reactive enough to completely remove the iodide ligand, leading instead to the dinuclear species [Pt2(μ-I)(Me)2(NHC)4][BArF]. Alternatively, low-coordinate T-shaped complexes [PtMe(NHC)2][BArF] can be prepared by protonation of the dimethyl species [PtMe2(NHC)2] with [H(OEt2)2][BArF]. The complex [PtMe(IiPr2Me2)2][BArF] undergoes a cyclometalation process under mild heating, leading to the derivative [Pt(IiPr2Me2′)(IiPr2Me2)][BArF] (where the prime denotes the cyclometalated ligand). This result is in contrast with the fast cyclometalation observed for the related Pt(II) complex bearing tert-butyl-substituted NHC (ItBu). Different cyclometalation reaction mechanisms have been computationally addressed. DFT calculations indicate that the cyclometalation involving complexes with not very bulky NHCs ligands occurs via cis intermediates. Cyclometalation products bearing tert-butyl-substituted NHCs are kinetically and thermodynamically favored over those involving isopropyl groups.