Isolobal Zwitterionic Niobium and Tantalum Imido and Zirconium Monocyclopentadienyl Complexes: Theoretical and Methyl Methacrylate Polymerization Studies

The trichloro derivatives [MCl3(NR)(py)(2)] reacted with LiOAr to give the imido aryloxo complexes [MCl2(NR)(OAr)(py)(2)] (M = Nb, R = tBu (1a), Ar (1b); M = Ta, R = tBu (2a); Ar 2,6-iPr(2)C(6)H(3)) and were alkylated with [BzMgCl], affording the tribenzyl derivatives [NbBz(3)(NR)] (R tBu (3a), Ar (3b)). Similar alkylations of the aryloxo derivatives 1 and 2 gave the dibenzyl complexes [MBz(2)(NR)(OAr)(THF)] (M = Nb, R = tBu (5a); M = Ta, R = tBu (6a), Ar (6b)). The zwitterionic imido complexes [MBz(2)(NtBu){eta(6)-C6H5CH2E(C6F5)(3)}] (M = Nb, E B (7a-B), Al (7a-Al); M = Ta, E = Al (8a-Al)) were obtained upon addition of 1 equiv of E(C6F5)(3) (E B, Al) to the tribenzyl derivatives [MBz(3)(NtBu)] (M = Nb (3a), Ta (4a)). The aryloxo compound 6a reacted with 2 equiv of the Lewis acids E(C6F5)3 (E = B, Al) to give the corresponding zwitterionic imido complexes [TaBz(NtBu)(OAr){eta(6)-C6H5CH2E(C6F5)(3)}] (E = B (11a-B), Al (11a-Al)) and the adduct (THF)center dot E(C6F5)(3), through two different intermediates, the ionic [TaBz(NtBu)(OAr)(THF)][BzB(C6F5)(3)] (9a) for E = B and the neutral [TaBz(2)(NtBu)(OAr)] (10a) for E = Al. The related monocyclopentadienyl complex [ZrCpBz(2){eta(6)-C6H5CH2Al(C6F5)(3)}] (12-Al) was also isolated by reaction of [ZrCpBz(3)] with Al(C6F5)(3). DFT calculations were carried out to further understand this type of zwitterionic imido derivative in comparison with the isolobal zirconium cyclopentadienyl compound [ZrCpBz(2){eta(6)-C6H5CH2B(C6F5)(3)}]. MMA polymerization was investigated for the [MBz(2)X(NtBu)]/E(C6F5)(3) (X Bz, OAr) and [ZrCpBz(3)]/E(C6F5)(3) (E = B, Al) systems.