Synthesis and Structures of Tungsten (1-Pyridinio)imido Complexes and Their Facile N−N Bond Cleavage1

Abstract

The tungsten (1-pyridinio)imido complexes [WX2(NNC5H4OMe)(L)(PMe2Ph)2]+, [WX2(NNC5H2Me3)(L)(PMe2Ph)2]+, and [WCl2{NNC5H2(COOEt)2Ph}(L)(PMe2Ph)2]+ (X = Cl, Br; L = CO, C2H4, PMe2Ph) were synthesized by the reaction of the salts of pyrylium cations with the hydrazido complexes [WX2(NNH2)(L)(PMe2Ph)2], which were derived from the dinitrogen complex cis-[W(N2)2(PMe2Ph)4]. The complexes obtained were characterized by spectroscopic measurements as well as crystallographic studies of cis,mer-[WBr2(NNC5H4OMe-4)(PMe2Ph)3][PF6], cis,trans-[WCl2(NNC5H4OMe-4)(CO)(PMe2Ph)2][ClO4], cis,trans-[WCl2(NNC5H2Me3-2,4,6)(CO)(PMe2Ph)2][OTf] (OTf = OSO2CF3), and cis,trans-[WCl2(NNC5H4OMe-4)(C2H4)(PMe2Ph)2][ClO4], which revealed that the N−N bond distances of these (1-pyridinio)imido complexes are shorter than those of the trialkyl- or unsubstituted hydrazidium complexes so far reported. The (1-pyridinio)imido complexes with a π-acidic ligand (CO or C2H4) underwent smooth N−N bond cleavage on reaction with cobaltocene under ambient conditions to liberate the pyridines in moderate to high yields. In some reactions that were conducted in the presence of HNEt3Cl, the corresponding imido (NH) complexes were recovered as the metal products. The (1-pyridinio)imido complexes also reacted with KOH in MeOH to give the pyridines and ammonia in high yields. These reactions provide the first examples of N−N bond cleavage of well-defined hydrazidium complexes and accomplish the synthesis of pyridines from molecular nitrogen under mild conditions.