Bi-Dentate Pyridyl Alkoxide Complexes of Aluminium and Vanadium: Synthesis, Structure and ROP Capability

The reaction of the pyridylalcohol Ph₂C(OH)CH₂-2-py-6-Me (IH) with Me₃Al in refluxing toluene led to the isolation of the dimer [AlMe₂(μ-OC(Me)Ph₂)]₂ (1), whilst at ambient temperature the complex [(I)AlMe₂]·MeCN (2·MeCN) was isolated. Complex 1 is also readily available via the interaction of diphenylethanol and Me₃Al. Similar treatment of iPr₂C(OH)CH₂-2-py-6-Me (IIH) at ambient temperature afforded [(II)AlMe₂] (3). Treatment of IH and IIH with [VO(OiPr)₃] led to oxo-bridged complexes of the type [(VO)(μ₂-O)(I/II)]₂ (I (4·0.67MeCN), II (5)). The molecular structures of 1–5 are reported. These complexes have been employed as catalysts for the ring-opening polymerization (ROP) of the cyclic esters ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL). For aluminium, complex 1/BnOH produced medium- to high-molecular-weight (M_n) PCL at 20 to 110 °C in solution, though some bi-/multi-modal behaviour was observed; for melts the M_n values were toward the lower end. For complexes 2 and 3, far lower M_n values for PCL were observed at 20 °C in solution and as melts, whilst in solution at 110 °C higher M_n values were achieved, though with less control. In general, M_n values for the PCL obtained using the vanadium complexes were low (≤8560 Da for 4, ≤2920 Da for 5). In the case of PVL, 1/BnOH in solution exhibited higher M_n values at lower temperatures with good control, and when employed as a melt, the M_n was toward the higher end (30,830 Da) observed. For 2/BnOH, much lower M_n values (≤2740 Da) were recorded both in solution and as a melt, whilst for 3, high M_n values were only observed in the absence of BnOH. Low M_n values (≤2920 Da) were also observed for the vanadium complexes 4 and 5. Kinetic results (both ε-CL and δ-VL) revealed that the vanadium complexes, particularly 4, outperformed the aluminium complexes. MALDI-ToF spectra revealed the formation of linear PCL polymers with BnO/H end groups for the aluminium/BnOH complexes in solution, and cyclic polymers when employed as melts. For vanadium, cyclic PCL polymers were the major family present. In the case of PVL, linear (BnO/H end groups) and cyclic polymers were observed when employing the Al/BnOH systems, whilst cyclic polymers were observed for vanadium. Copolymerization of ε-CL and δ-VL using 4/BnOH at 110 °C over 24 h led to incomplete conversion and formation of a random-type copolymer.