C-C and C-O bond formation reactions promoted by coordination of O-donor ligands to Ta(V) chloride

The coordination chemistry of oxygen donor ligands to Group 5 metal halides MX5 (M = Nb, Ta) has been scarcely developed,1 apart from alcohols and diols:2 significantly, only two compounds of general formula [MX5(O-ligand)] have been characterized by X-ray diffraction hitherto.3 We report herein that the reactions of TaCl5 (1) in CH2Cl2 suspension with a series of ketones RR’C=O, in 1:2 ketone to metal molar ratio, afford cleanly the octahedral complexes [TaCl5(OCRR’)] (R = R’ = Me, 2a; R = R’ = Ph, 2b; R = Me, R’ = Ph, 2c), in up to 90 % yield. Complex 2a is still reactive towards the addition of further ketone, giving the aldolate species [TaCl4{2(O)-OC(Me)CH2C(R)(R’)O}] (R = R’ = Me, 3a; R = R’ = Ph, 3b; R = Me, R’ = Ph, 3c). The addition of THF to TaCl5, in 1:2 molar ratio, generates the adduct [TaCl5(THF)] (4); however, when this reaction is performed by using a slight excess of THF, the ionic species [TaCl4(THF)O(CH2)4O(CH2)3CH2][TaCl6] (5) has been isolated. Compound 5 is believed to represent an intermediate species during the course of THF polymerization catalyzed by TaCl5. The X ray molecular structures of 2b, 3a, 4 and 5 will be discussed