Complexes of Niobium(V) and Tantalum(V) Halides with Ligands that contain N–C=O and P=O Functionalities: a Synthetic and Structural Study

The reactions of niobium and tantalum pentahalides, MX5, with dialkylureas (tetramethyl- and tetraethylurea, TMU and TEU, respectively), amides (N,N,dimethylformamide, DMF, N,N-diethylformamide, DEF, N-phenylacetamide, PhA), and P=O-containing compounds (triphenylphosphane oxide, TPPO, and trimethyl phosphate, TMP) are not dependent on the reactant to metal molar ratio, and afford hexacoordinate complexes of the general formula MX5(L). The only exception is given by the reaction of TaBr5 with TMU, which proceeds by bromide migration from one metal center to another, and provides the ionic adduct [TaBr4(TMU)2][TaBr6]. Hydrolysis of TaCl5(TMP), due to adventitious water, has yielded the dinuclear species Ta2(μ-O)Cl8(TMP)2. All compounds have been fully characterized in solution, and X-ray crystal structures have been determined for NbCl5(TEU), NbCl5(DMF), TaCl5(DMF), TaCl5(PhA), TaBr5(PhA), TaCl5(TPPO), [TaBr4(TMU)2][TaBr6], and Ta2(μ-O)Cl8(TMP)2. No evidence for formation of M=O bonds due to oxygen atom transfer from the ligand to the metal has been observed in any case.