Preparation and Reactivity of Mono- and Dinuclear Derivatives of Niobium and Tantalum Pentahalides with Alkyl-Aryl-Ethers

The niobium and tantalum pentahalides MX5 (X = F, Cl) react with a series of bifunctional alkyl aryl ethers in a 2:1 ratio, resulting in formation of the dinuclear species (MX5)2[μ-κ2- (O–O)] [2a–h, 3a–d; O–O = 1,4-(OMe)2C6H4, 1,4-(OMe)2-2,5- C6H2F2, 1,3-(OMe)2C6H4, PhO(CH2)2OPh]. The mononuclear complexes MX5(L) [L = κ1-1,4-(OMe)2C6H4, X = F, M = Nb, 4a; L = κ2-1,3-(OMe)2C6H4, X = Cl, M= Ta, 4c; L = MeOC6H5, X = F, M = Nb, 4d; L = MeOC6H5, X = Cl, M = Nb, 4e] have been prepared by 1:1 molar reactions of MX5 with the appropriate reactant. Alternatively, NbCl5[κ1-1,4-(OMe)2C6H4] (4b) has been obtained upon addition of acetone to (NbCl5)2- [μ-κ2-1,4-(OMe)2C6H4] (2b). The aryloxide complexes TaBr4[κ1-OC6H4(4-OMe)] (5a), NbCl4[κ1-OC6H3(Me)(4-OEt)] Introduction The use in organic synthesis of niobium and tantalum pentahalides MX5 (1)[1] has seen significant progress in the last years due to the unique behaviour shown by these complexes compared to other early transition-metal halides.[2] In particular, the strong oxophilicity of 1 has been exploited to perform a variety of reactions requiring a C–O bond cleavage step.[3] In spite of the recent development of MX5-catalyzed reactions, the coordination chemistry of 1, especially with oxygen-containing molecules, has been scarcely investigated, and the little information available pertains mainly to chlorides.[4] In order to expand the knowledge about the products that form from the direct interaction of O-donor species with the pentahalides of the heavier group 5 metals, we recently started a research project on the stoichiometric reactions of 1 with a variety of potential oxygen donor ligands. Hence, a family of coordination adducts of 1 with monofunctional O ligands has been reported.[5] [a] Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy Fax: +39-050-221-246 E-mail: pampa@dcci.unipi.it [b] Dipartimento di Chimica Fisica e Inorganica, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy [‡] Born in Bologna (Italy) in 1974 Eur. J. Inorg. Chem. 2010, 767–774 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 767 (5b) and TaX4[κ2-OC6H4(2-OMe)] (X = Cl, 7a; X = Br, 7b) form an admixture with equimolar amounts of alkyl halide by addition of dialkoxyarenes to MX5 (X = Cl, Br), as result of room-temperature C–O bond cleavage. Differently, the ionic species [MF4(κ2-O–O)2][M2F11] [O–O = 1,2-(OMe)2C6H4, M = Nb, 6a; O–O = 1,2-(OMe)2C6H4, M = Ta, 6b; O–O = 1,2,4- (OMe)3C6H3, M = Nb, 6c] are produced cleanly by treating ortho-dimethoxyarenes with MF5 in the opportune stoichiometry. All the products described herein were obtained selectively in good to excellent yields and were characterized by spectroscopic and analytical techniques. Moreover, the molecular structure of 2b was elucidated by an X-ray diffraction study.