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

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 C C 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 C C 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-CH CH-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.