Reactivity studies on a trihydride niobocene complex towards α,β-Unsaturated carboxylic acids

Abstract

The reactions of the trihydride niobocene complex [NbCp’2H3] (Cp’ = η5-C5H4SiMe3) (1) with different α,β-unsaturated carboxylic acids have been studied. The reactions with fumaric and maleic acids (A and B) gave a new dinuclear carboxylato-containing niobocene derivative [(NbCp’2)2(μ-κ2O,O-OOC-CH2-CH2-COO-κ2O,O)] (2), due to the occurrence of two processes, dihydrogen elimination and hydrogenation of the CC bond of the α,β-unsaturated carboxylic acid. Likewise, the use of an α,β-unsaturated carboxylic acid derivative such as mono-methyl fumarate (ester) (C) gave the mononuclear [NbCp’2(OOC-CH2-CH2-COOMe-κ2O,O)] (3). When other α,β-unsaturated carboxylic acids with less activated alkenes were used, such as trans-cinnamic acid (F), (E)-2-methyl-3-phenylacrylic acid (G), cyclohex-1-ene-1-carboxylic acid (H), 3-methylbut-2-enoic acid (I), (E)-2-methylbut-2-enoic acid (J) or (E)-pent-2-enoic acid (K), only the complexes [NbCp’2(OOCR-κ2O,O)], (R = trans-CH=CHPh (4), trans-CMe = CHPh (5), 1-cyclohexenyl (6), (CH=CMe2) (7), (cis-CMe = CHMe) (8), (trans-C(H) = CH-Et) (9)), were isolated, respectively, resulting from a process with dihydrogen elimination and the hydrogenation of the CC bond do not takes place.Treatment of 2–9 with CO (3 atm) promote that the coordination mode of the carboxylate ligand changes from (κ2O,O-OOC-R) to (κ1O-OOC-R) and yielding new carbonyl derivatives [(NbCp’2)2(μ-κ1O-OOC-CH2-CH2-COO-κ1O) (CO)2] (10); and [NbCp’2(OOCR-κ1O) (CO)] (R = CH2-CH2-COOMe (11), trans-CH=CHPh (12), trans-CMe = CHPh (13), 1-cyclohexenyl (14), CH=CMe2 (15), cis-CMe = CHMe (16) and R = trans-CHCH-Et) (17)).Reaction of 2 with O2 afford a new diperoxo compound [{NbCp’2(O2-κ2O,O)}2 (μ−κ1O-OOC-CH2-CH2-COO-κ1O)](19), through a monoperoxo derivative [NbCp’2(O2-κ2O,O) (μ-κ1O-OOC-CH2-CH2-COO-κ2O,O)NbCp’2] (18). All the new complexes were characterized by usual IR and NMR spectroscopic methods and the crystal structure of 16 and 18 were determined by X-ray diffraction studies.