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)3COCH32a [mim = 1-methylimidazolyl], (mim)2CHCH2PPh22b, (mim)2CHCH2C(O)But2c, (tBupim)3P 2d [tBupim = 1-isopropyl-4-tert-butylimidazolyl], (N⁁N) = (mim)2CH23e, (mim)2CH2PPh23f, (mim)2CO 3g, (Bzmim)2CO 3h [Bzmim = 1-methybenzimidazolyl], (tBupim)2CO 3i, (mim)2CNPh 3j; M = V: (N⁁D⁁N) = (mim)3COCH35a; (N⁁N) = (mim)2CO 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)2CO}(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 d3 ion, falling into the region ν14A2g → 4T2g 600–700 nm and ν24A2g → 4T1g(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.