Vanadium complexes derived from O,N,O-tridentate 6-bis(o-hydroxyalkyl/aryl)pyridines: structural studies and use in the ring-opening polymerization of ε-caprolactone and ethylene polymerization

Interaction of [VO(OiPr)₃] with 6-bis(o-hydroxyaryl)pyridine, 2,6-{HOC(Ph)₂CH₂}₂(NC₅H₃), LH₂, afforded [VO(OiPr)L] (1) in good yield. The reaction of LNa₂, generated in-situ from LH₂ and NaH, with [VCl₃(THF)₃] led to the isolation of [VL₂] (2) in which the pyridyl nitrogen atoms are cis; a regioisomer 3∙2THF, in which the pyridyl nitrogen atoms are trans, was isolated when using [VCl₂(TMEDA)₂]. The reaction of the 2,6-bis(o-hydroxyalkyl)pyridine {HOC(iPr)₂CH₂}₂(NC₅H₃), L¹H₂, with [VO(OR)₃] (R = nPr, iPr) led, following work-up, to [VO(OR)L¹] (R = nPr (4), iPr (5)). Use of the bis(methylpyridine)-substituted alcohol (tBu)C(OH)[CH₂(C₅H₃Me-5)]₂, L²H, with [VO(OR)₃] (R = Et, iPr) led to the isolation of [VO(μ-O)(L²)]₂ (6). Complexes 1 to 6 have been screened for their ability to act as pre-catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL), and rac-lactide (r-LA) and compared against the known catalyst [Ti(OiPr)₂L] (I). Complexes 1, 4–6 were also screened as catalysts for the polymerization of ethylene (in the presence of dimethylaluminium chloride/ethyltrichloroacetate). For the ROP of ε-CL, in toluene solution, conversions were low to moderate, affording low molecular weight products, whilst as melts, the systems were more active and afforded higher molecular weight polymers. For δ-VL, the systems run as melts afforded good conversions, but in the case of r-LA, all systems as melts exhibited low conversions (<10%) except for 6 (<54%) and I (<39%). In the case of ethylene polymerization, the highest activity (8600 Kg·mol·V⁻¹bar⁻¹h⁻¹) was exhibited by 1 in dichloromethane, affording high molecular weight, linear polyethylene at 70 °C. In the case of 4 and 5, which contain the propyl-bearing chelates, the activities were somewhat lower (≤1500 Kg·mol·V⁻¹bar⁻¹h⁻¹), whilst 6 was found to be inactive.