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Cationic five-coordinate Pt(IV) alkyl complexes stabilized by bulky N-heterocyclic carbenes have been isolated and fully characterized. Related species have been postulated as key intermediates in carbon-heteroatom coupling reactions and most particularly in Shilov-type chemistry. The alkyl groups exhibit a pronounced electrophilic character and can undergo nucleophilic addition of pyridine, bromide, or iodide to form new carbon-heteroatom bonds. Nevertheless, direct reductive coupling to form C-X bonds can be operative in the absence of an external nucleophile source

The reaction of the N-heterocyclic carbenes (NHCs) IiPr 2Me2 and IiPr2 with [PtMe 3I]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(¿-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(IiPr2Me 2)2][BArF] undergoes a cyclometalation process under mild heating, leading to the derivative [Pt(IiPr 2Me2¿)(IiPr2Me 2)][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. © 2014 American Chemical Society.

The aromatization–dearomatization reaction of pincer-type complexes prompted by protonation–deprotonation of the pincer “arm” is a key step in bond activation chemistry and atom-economic catalytic ...