Synthesis and characterization of [W(NC4Me4)2Cl2] and [W(NC4Me4)2(CH3)2], the first azametallocene tungsten complexes with pyrrolyl ligands. Electronic structure and bonding of tungsten bispyrrolyl complexes

The synthesis of the first tungsten azametallocene complexes with pyrrolyl ligands, [W(NC4Me4)2L2], is reported. The dichloro complex (L=Cl) was obtained from the reaction of WCl4DME (DME=1,2-dimethoxyethane) with Li(NC4Me4) in poor yield (6.5%). A small amount (ca. 0.063 g, yield: 15.8%) of the dimethyl complex, [W(NC4Me4)2(CH3)2], resulted from the reaction of the dichloro species (L=Cl) with LiCH3. Both complexes were characterised by 1H, 13C{1H} NMR and mass spectroscopy. Molecular orbital studies based on B3LYP/DFT calculations were performed in order to study the complexes electronic structure, focusing the pyrrolyl coordination mode. Equivalent bond strengths were found for the two extreme coordination modes of that ligand: π coordination (η5) through the pyrrolyl π system, and N-σ coordination established by the nitrogen lone pair. The relative stability of the isomers originated by the different pyrrolyl coordination modes, results from a delicate balance between electronic and stereochemical factors, on each case. The calculations suggest the existence of a fluxional process in solution, involving the π/π and the σ/π isomers of [W(NC4Me4)2(CH3)2].