[PNP]-Stabilized Niobium(IV) and Tantalum(IV) Complexes: Synthesis and Characterization of an Open-Shell Tantalum Alkylidene

Abstract

A flexible [PNP] ligand (forming six-membered chelates upon coordination) was employed for the synthesis of niobium(IV) and tantalum(IV) complexes. Starting from the protioligand (H[PNP]) and the tetrachlorides NbCl4(thf)2 or TaCl4, the trichloro complexes [PNP]MCl3 (1-M, M = Nb, Ta) were prepared via in situ deprotonation using LiN(SiMe3)2. Attempts to alkylate complexes 1-M (M = Nb, Ta), however, were impeded due to their limited solubility in nonchlorinated solvents. Hence, the lithiated ligand was reacted with (thf)2Cl3M=CHCMe2Ph (M = Nb, Ta) to afford the d 0-configured alkylidene complexes 2-Nb and 2-Ta, respectively. These meridionally coordinated and freely soluble complexes were found to be stable in nonchlorinated and chlorinated solvents, even at elevated temperature. Reduction of compounds 2-M with magnesium anthracene (2-Nb and 2-Ta) or sodium naphthalide (2-Ta) afforded the corresponding d 1-configured alkylidenes (3-Nb and 3-Ta), although the reduced niobium derivative was found to decompose rapidly. A 10-line and an 8-line EPR pattern was detected for 3-Nb (I(93Nb) = 9/2) and 3-Ta (I(181Ta) = 7/2), respectively, confirming the metalloradical character of each complex. For 3-Ta, the molecular structure was elucidated by single crystal X-ray diffraction. With the molecular structure of this d 1-configured (open-shell) tantalum alkylidene ascertained, its electronic structure was examined by DFT and CASSCF calculations.