Activation of bidentate oxygen donor ligands by coordination to early-transition metal halides in high oxidation state

The coordination chemistry of group 5 metal pentahalides, MX5 (M = Nb, Ta, X = F, Cl, Br, I), with oxygen donor ligands was scarcely developed in the past. Recently, we have reported the X ray structures of some coordination adducts of MX5 with monodentate O-donors, for which C-H and C-O bonds activation takes place in some cases [1]. Here, we describe the reactions of MX5 with bidentate oxygen donor ligands O-O [O-O = MeO(CH2)2OMe (dme), CH2(CO2Me)2, cis-(MeO2C)CH=CH(CO2Me) (dmm), etc], leading to different outcomes depending on the strength of the M–X bond. Thus, ionic derivatives of the type [MF4(O-O)2][MF6] have been prepared by mixing MF5 with the appropriate organic material [2]. Alternatively, the reactions of NbX5 (X = Cl, Br) with O-O in 1:1 molar ratio proceed with fragmentation of half organic material, providing a oxo-moiety. More in particular, the -oxo complexes NbOX3[2-O-O]NbX5 (X = Cl, Br) are obtained in moderate to very good yields [2]. Interestingly, the 1:2 ratio reaction of MCl5 (M = Nb, Ta) with dme gives MOCl3(dme), MeCl and dioxane, as result of unprecedented multiple C–O bonds cleavage and coupling process [3]. The fragmentation pathways operative in the distinct cases will be presented in details, moreover a comparison with the chemistry exhibited by groups 4,6 halides in high oxidation states towards bidentate oxygen donors will be discussed.