Synthesis and reactivity of oxametallacyclic niobium compounds by using α,ß-unsaturated carbonyl ligands

Abstract

Reduction of mono(cyclopentadienyl)niobium complexes [NbCpRCl4] [CpR = C5Me4H (1), C5H4SiMe2Cl (2), C5H4SiMe3 (3)] with Na/Hg in the presence of methyl methacrylate [MMA, CH2=C(Me)C(O)OMe (a)], methyl acrylate [MA, CH2=CHC(O)OMe (b)] and mesityl oxide [MO, CMe2=CHC(O)Me (c)] afforded the corresponding derivatives [NbCpRCl2(LL)] [CpR = C5Me4H, LL = MMA (1a); CpR = C5H4SiMe2Cl, LL = MMA (2a), MA (2b), MO (2c); CpR = C5H4SiMe3, LL = MMA (3a), MA (3b)] in variable yields depending on both the cyclopentadienyl and the ¿,ß-unsaturated carbonyl compounds. The reactivity of these complexes was studied toward protic and Lewis acids. Addition of triflic acid TfOH (Tf = CF3SO2) to 3b gave the triflate complex [NbCpRCl2{(CH2)2C(O)OMe}(OTf)] [CpR = C5H4SiMe3 (4)]. The Lewis acids E(C6F5)3 (E = B, Al) reacted with complexes Introduction The bonding interaction of a butadiene ligand with a metal center is clearly dependent on the metal atom and may be formulated as a system containing a dianionic ligand for early high-valent transition metals or as a neutral ligand for low-valent late transition metals. However, the stability of the formal oxidation state of the metal atom and the nature of the ancillary substituents of both the complex and the diene ligand could play an important role in defining this interaction. Hence, complexes with this type of ligand are better represented with contributions from two main canonical forms, ¿2,¿-metallacyclic or ¿2-butadiene (Figure 1).[1¿5] [a] Departamento de Química Inorgánica, Universidad de Alcalá, Campus Universitario, 28871 Alcalá de Henares (Madrid), Spain Fax: 34-91-885-4683 E-mail: pascual.royo@uah.es [¿] X-ray diffraction studies Supporting information for this article is available on the WWW under http://www.eurjic.org/ or from the author. Eur. J. Inorg. Chem. 2008, 2313¿2320 © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2313 2b and 3b to give the three-membered metallacyclic (or ¿2- enone) compounds [NbCpRCl2{¿2-CH2=CHC(OMe){O· E(C6F5)3}}] [CpR = C5H4SiMe2Cl, E = B (5), Al (6); CpR = C5H4SiMe3, E = B (7), Al (8)], which decomposed to the corresponding adducts MA·E(C6F5)3. The same reaction with the 2a and 3a derivatives only allowed the observation of the corresponding adducts MMA·E(C6F5)3. Complexes 2a,b and 3a,b reacted with CO with elimination of the acrylate compounds, MA and MMA, respectively, to give the carbonylniobium( III) compounds [NbCpRCl2(CO)2]2 [CpR = C5H4Si- Me2Cl (9), C5H4SiMe3 (10)]. Analogous reactions with CNAr showed the elimination of the free MA and MMA compounds.