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

Abstract

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 ν₁⁴A_2g → ⁴T_2g 600–700 nm and ν₂⁴A_2g → ⁴T_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.