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, MX 5 , 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 MX 5 (L). The only exception is given by the reaction of TaBr 5 with TMU, which proceeds by bromide migration from one metal center to another, and provides the ionic adduct [TaBr 4 (TMU) 2 ][TaBr 6 ]. Hydrolysis of TaCl 5 (TMP), due to adventitious water, has yielded the dinuclear species Ta 2 (μ‐O)Cl 8 (TMP) 2 . All compounds have been fully characterized in solution, and X‐ray crystal structures have been determined for NbCl 5 (TEU), NbCl 5 (DMF), TaCl 5 (DMF), TaCl 5 (PhA), TaBr 5 (PhA), TaCl 5 (TPPO), [TaBr 4 (TMU) 2 ][TaBr 6 ], and Ta 2 (μ‐O)Cl 8 (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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)