Fragmentation of oxygen-containing molecules via C-O bond cleavage promoted by coordination to niobium and tantalum pentahalides

The novel mu-oxo complexes NbOX3[kappa(2)-O(Me)CH2CO2Me]NbX5 (X = Cl, 3a; X = Br, 3b), NbOCl3[kappa(2)-(MeO2C)CH=CH(CO2Me)]NbCl5 (7) and NbOCl3[kappa(2)-CH2(CO2Me)(2)]NbCl5 (8) have been prepared in good yields by 1 : 1 molar ratio reactions of the halides NbX5 (X = Cl, Br) with methoxymethyl acetate [MeOCH2CO2Me], dimethyl maleate [(MeO2C)CH=CH(CO2Me)] and dimethyl malonate [CH2(CO2Me)(2)] respectively, in different experimental conditions. NMR studies have indicated that the oxo unit of 3, 7 and 8 forms via selective fragmentation (C-O bonds cleavage) of half equivalent of the organic material. The ionic complexes [NbX4{O-O}(2)][NbX6][X = Cl, O-O = MeOCH2CO2Me, 4; X = Cl, O-O = (MeO2C)CH=CH(CO2Me), 5b; X = Cl, O-O = CH2(CO2Me)(2), 5e] have been identified as intermediates of the reactions affording 3a, 7 and 8. The stable complexes [NbX4{O-O}(2)][NbX6] [X = F, O-O = (MeO2C)CH=CH2(CO2Me), 5a; X = Cl, O-O = (EtO2C)CH=CH2(CO2Et), 5c; X = F, O-O = CH2(CO2Me)(2), 5d] and [NbCl4{(EtO2C)CH= CH(CO2Et)}][NbCl6] (6) have been prepared by reacting NbX5 (X = F, Cl) with variable amounts of the appropriate diester. Alternatively, NbX5 (X = F, Cl) react with diethyl fumarate in 2 : 1 molar ratio affording the dinuclear [NbX5](2){mu-kappa(2)-(EtO2C)CH=CH(CO2Et)} (X = F, 9a; X = Cl, 9b), in high yields. The neutral, monomeric, compounds MX5(L) [L = HCO2Me, Et2NCO2Me, OMe2, OEt2, OMeCH2Cl, OMeCH2CH2Cl, OMeCH2CH2Br, O(CH2CH2Cl)(2), 1,4-dioxane] have been obtained upon addition of the appropriate O-donor to MX5. Complexes NbX5(OMeCH2CH2Y) (X = Br, Y = Cl, 10i; X = Y = Br, 10j; X = I, Y = Br, 10k) slowly convert into the corresponding alkoxides [NbX4(OCH2CH2Y)](2) (X = Br, Y = Cl, 11a; X = Y = Br, 11b; X = I, Y = Br, 11c) in CDCl3. The further conversion of 11b to NbOBr3, MeBr and Br(CH2)(2)Br takes place at 60 degrees C. N2CHCO2Et behaves as a ligand with respect to NbF5, whereas it undergoes fragmentation and halogenation when contacted with NbCl5. The new compounds have been characterised by NMR and IR spectroscopies, elemental analyses and solution electrical conductivity measurements. The latter have proved to be efficient for discriminating between the neutral or the cationic nature of the products. Moreover, the solid state structures of 3a, b, 7 and 8 have been ascertained by X-ray diffraction studies.