Synthesis, characterisation and catalytic behaviour of a novel class of chromium(III) and vanadium(III) complexes containing bi- and tri-dentate imidazole chelating ligands:: a comparative study

The syntheses and characterisation of tri- and bi-dentate coordinated chromium(III) and vanadium(III) complexes of the general composition [MCl3(N<^>D<^>N)], [MCl3(N<^>N)](2) {M = Cr, (N<^>D<^>N) = (mim)(3)COCH3 2a [mim = 1-methylimidazolyl], (mim)(2)CHCH2PPh2 2b, (mim)(2) CHCH2C(O)But 2c, (tBupim)(3)P 2d [tBupim = 1-isopropyl-4-tert-butylimidazolyl], (N<^>N) = (mim)(2)CH2 3e, (mim)(2)CH2PPh2 3f, (mim)(2)CO 3g, (Bzmim)(2)CO 3h [Bzmim = 1-methybenzimidazolyl], (tBupim)(2)CO 3i, (mim)(2)C=NPh 3j; M = V: (N<^>D<^>N) = (mim)(3)COCH3 5a; (N<^>N) = (mim)(2)CO 5g}, [CrCl3(mim)(3)] 4k, and [CrCl3(MeTAM)](6) (MeTAM = 1-methyltriacetylmethane) is described. Crystallisation of 3g from CH3CN/Et2O gave the mononuclear complex [CrCl3{(mim)(2)CO}(CH3CN)] 3g'. The molecular structure of 3g' shows the chromium atom is quasi-octahedral, six-coordinate, the three coordinated chlorine atoms disposed mer in the coordination sphere. The electronic spectra of the chromium complexes exhibit d-d transitions typical of a pseudo-octahedral coordinated d(3) ion, falling into the region nu(1) 4A(2g) --> T-4(2g) 600-700 nm and nu(2) (4)A(2g) --> T-4(1g) (F) 430-470 nm, and 10Dq values between 14 400 and 16 700 cm(-1). In the presence of MMAO the complexes give active catalyst systems for the conversion of ethylene into 1-alkenes or polymers, with activities and selectivities depending on the electronic and steric factors of the ligand system and the metal centre, respectively.