NCN-Pincer Metal Complexes (Ti, Cr, V, Zr, Hf, and Nb) of the Phebox Ligand (S,S)-2,6-Bis(4′-isopropyl-2′-oxazolinyl)phenyl

Abstract

Reaction of (S,S)-2,6-bis(4′-isopropyl-2′-oxazolinyl)phenyllithium (i-Pr-Phebox-Li) (2a) with 4,4′-bis[P-(chlorogold(I))diphenylphosphino]biphenyl [(dppbp)(AuCl)2] (5) afforded the new, bimetallic gold complex 4,4′-bis[P-(η1-C-i-Pr-Phebox-gold)diphenylphosphino]biphenyl [(P-Au(η1-C-i-Pr-Phebox))2(dppbp)] (6). Transmetalation of 6 with 2 equiv of Cl3MX (MX = TiOi-Pr, VCl, CrPy, ZrCl, HfCl, NbO) afforded the corresponding monopincer compounds [MCl2X(i-Pr-Phebox)] (M = Ti, V, Cr) (7) and [MCl2X(i-Pr-Phebox)]2 (M = Zr, Hf, Nb) (8) in high yields and the gold starting material 5, which could be quantitatively recovered and reused. The structures in the solid state of the digold compound 6, the monopincer compounds R-PheboxAu(PPh3) (R = i-Pr, t-Bu), and a number of Phebox-ETM complexes (7a, 7b, 7c, 8a, 8b, and 8c) were obtained. In each of these structures the i-Pr-Phebox monoanion is mer-N,C,N-tridentate bonded. The monopincer Phebox-Zr and -Hf compounds are dimeric because of two bridging chlorides, while the corresponding Nb compound has bridging oxygen atoms. Reaction of 6 with iron(III) chloride resulted in the formation of a N4(FeCl2)2 complex, characterized by X-ray crystal structure determination, comprising a bridging, tetradentate N4 ligand formed by C–C coupling of two Phebox anions to a biphenyl species. The new Phebox complexes 7 and 8 have been tested as olefin polymerization precatalysts. Rapid catalyst deactivation was observed in ethene polymerization under homogeneous conditions, whereas stable activity was obtained after immobilization on MgCl2-based supports. From preliminary investigations on the reaction of 8a with either methylmagnesium chloride or methyllithium we assume that the low reactivity in homogeneous polymerization is due to the alkylation of the Phebox ligand.