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The novel bulky Schiff base chelate ligand [(4,5-diisopropyl-1H-pyrrole-2-yl)methylene]-4-(tert-butyl)aniline ( i Pr2 HL) bearing two isopropyl groups close to the pyrrole nitrogen atom reacts with MoCl2(dme)O2 (dme = 1,2-dimethoxyethane) to give the sterically congested complex MoVI( i Pr2 L)2O2 ( i Pr2 1; OC-6–4–4 configuration). In spite of the increased steric shielding of the [MoO2] unit i Pr2 1 is active in oxygen-atom transfer to PMe3 and PPh3 to give OPMe3 and OPPh3, respectively. Because of the increased steric bulk of the chelate ligand, formation of dinuclear complexes [MoV( i Pr2 L)2O]2(μ-O) ( i Pr2 3) by comportionation is effectively prevented in contrast to the highly favored formation of [MoV( H2 L)2O]2(μ-O) ( H2 3) with the less bulky ligand H2 HL. Instead, the smaller PMe3 ligand coordinates to the resulting pentacoordinate intermediate MoIV( i Pr2 L)2O ( i Pr2 5), giving the hexacoordinate complex MoIV( i Pr2 L)2O(PMe3) ( i Pr2 2) with OC-6–3–3 configuration. The larger potential ligands PPh3 and OPPh3 are only able to weakly coordinate to i Pr2 5, giving labile and sensitive MoIV( i Pr2 L)2O(L) complexes ( i Pr2 6, L = PPh3; i Pr2 7, L = OPPh3). Traces of water and dioxygen in solutions of i Pr2 6/ i Pr2 7 yield the di(μ-oxido) complex [MoV( i Pr2 L)O]2(μ-O)2 ( i Pr2 4) with reduced steric congestion due to dissociation of the bulky chelate ligands. According to electron paramagnetic resonance studies, the much more strongly bound small PMe3 ligand in i Pr2 2 can be slowly liberated by one-electron oxidation to MoV, with Ag+ leaving a free coordination site at MoV. Hence, essentially pentacoordinate MoIV and MoV complexes are accessible as a result of the increased steric bulk.