Synthesis and Reactivity of Ene-Diamido and Ene-Diolato [(Trimethylsilyl)cyclopentadienyl]niobium(V) Complexes and a Comparative DFT Study of the Bonding Capabilities of Diazabutadiene and Butadiene Ligands

The reactions of [NbCp′Cl2(Me3SiCCSiMe3)] (Cp′ = η5-C5H4SiMe3) with 2,3-dimethyl-1,4-diphenyl-1,4-diazabuta-1,3-diene (Me,Ph-DAD) and diketones R−CO−CO−R (R = Me, Ph) gave ene-diamido complexes [NbCp′Cl2(Me,Ph-DAD)] (1) and ene-diolato complexes [NbCp′Cl2{O(R)CC(R)O}] (R = Me, 2; Ph, 3), respectively, with simultaneous elimination of bis(trimethylsilyl)acetylene. The NMR spectroscopic data of these compounds are in agreement with a supine conformation, with respect to the Cp′ ring, of the ene-diamido and ene-diolato ligands. In order to gain further information concerning the bonding capabilities of the ene-diamido ligand, a DFT study of the model complex [NbCpCl2(HNCHCHNH)] (I) was performed. For comparison, a DFT analysis of the model compound [NbCpCl2(η4-CH2CHCHCH2)] (II) was also carried out. Differences in the bonding description of the diazabutadiene and butadiene ligands bonded to a common [NbCpCl2] moiety are discussed. A detailed analysis of the fragment orbital interactions in I provides no evidence for any positive overlap between the Nb centre and the inner carbon atoms of the DAD ligand. Dialkylation of the dichloro complex 1 gave dialkyl derivatives [NbCp′R2(Me,Ph-DAD)] (R = Me, 4; CH2SiMe3, 5; CH2C6H5, 6), for which the NMR spectroscopic data suggests a prone conformation of the DAD ligand. Treatment of the dimethyl derivative 4 with carbon monoxide and 2,6-dimethylphenyl isocyanide led to the formation of the oxa- and azaniobacyclopropane complexes [NbCp′{η2-(E)CMe2}(Me,Ph-DAD)] [E = O, 7; N(2,6-Me2C6H3), 8], respectively. Subsequent insertion of isocyanide into the Nb−C bond of 7 proceeds with formation of derivatives [NbCp′{η2-(O)CMe2-(CNAr)x}(Me,Ph-DAD)], (Ar = 2,6-Me2C6H3; x = 1, 9; 2, 10). All the complexes isolated were characterized by IR and NMR (1H, 13C{1H}) spectroscopy. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)