Changing the properties of monodentate and P,C-chelating ferrocenyl-substituted 1,2,3-triazol-5-ylidene ligands through an inserted carbonyl moiety

Abstract

Triazolylidenes derived from readily accessible triazoles are useful ligands for coordination chemistry and catalysis. This work describes the synthesis of Group 11 metal complexes of new ferrocenyl-substituted triazolylidene ligands in which the ferrocene and triazolylidene moieties are separated by a carbonyl linker. In particular, complexes of types [MCl(FcC(O){CCN(Mes)NN(Me)}-κC⁵)] (M = Cu or Au; Fc = ferrocenyl) and [M(FcC(O){CCN(Mes)NN(Me)}-κC⁵)₂][BF₄] (M = Cu, Ag, or Au) were prepared from FcC(O)CCH and characterised by spectroscopic methods, X-ray diffraction analysis and cyclic voltammetry. Using a similar strategy, the Pd(II) complex trans-[PdCl₂(Ph₂PfcC(O){CCN(Mes)NN(Me)}-κ²P,C⁵)] (fc = ferrocene-1,1′-diyl) was synthesised and analogously characterised. The phosphinocarbene ligand in this compound coordinates as a trans P,C-chelating ligand, unlike its analogues that lack the CO spacer and similar compounds that combine the phosphine and carbene donor groups on the ferrocene scaffold. The influence of the carbonyl spacer was evaluated in a pair of Pd(II) bis-carbene complexes, [PdBr₂({C(Fc)CN(Mes)NN(Me)}-κC⁵)(iPr₂-bimy)] and [PdBr₂(FcC(O){CCN(Mes)NN(Me)}-κC⁵)(iPr₂-bimy)] (iPr₂-bimy = 1,3-diisopropyl-1,3-dihydro-2H-benzimidazol-2-ylidene), by Huynh's electronic parameters and the Fe^II/Fe^III redox potential from cyclic voltammetry, which suggested an electron density decrease at the ferrocenyl group and decreased σ donor ability of the triazolylidene moiety upon introduction of the CO linker. The Group 11 metal complexes were tested as catalysts for the metal-catalysed cyclisation of N-propargylbenzamide into 2-phenyl-5-methylene-4,5-dihydrooxazole. Among them, the chlorogold complex activated with a silver salt achieved the best results.