Lewis Acid-Base Chemistry in the Group 4: Complexation and Disproportionation of Metal Alkoxy Halides

Lewis acid-base adducts are a cornerstone of chemistry. Group 4 Lewis acids are well-known catalysts and precursors for (non-aqueous) sol-gel chemistry. Here, we investigate the interaction of Lewis bases with metal tetrahalides (MX4, M = Ti, Zr, Hf and X = Cl, Br) and metal alkoxy halides (MXx(OR)4-x, x = 1-3, R = OiPr, OtBu). Complexation with Lewis base ligands yields the simple Lewis acid-base adduct MX4L2 (L = tetrahydrofuran or triphenylphosphine oxide). Analysis via single-crystal XRD shows that the cis or trans configuration of the Lewis bases is determined by their steric bulk and the nature of the metal halide. The interaction with mixed alkoxy halides is more complex. 31P NMR spectroscopy reveals that excess of phosphine oxide yields predominantly the complexation product, while a substoichiometric amount of phosphine oxide causes disproportionation of the MXx(OR)4-x species into MXx+1(OR)3-x and MXx-1(OR)5-x. The combination of complexation and disproportionation yields an atypical Job plot. In the case of zirconium isopropoxy chlorides, we fitted the concentration of all observed species and extracted thermodynamic descriptors from the Job plot. The complexation equilibrium constant decreases in the series; ZrCl3(OiPr) > ZrCl2(OiPr)2 > ZrCl(OiPr)3, while the disproportionation equilibrium constant follows the opposite trend. Using calculations at the DFT level of theory, we show that disproportionation is driven by the more energetically favourable Lewis acid-base complex formed with the more acidic species. The disproportionation reaction turns out to be a general phenomenon, for titanium, zirconium and hafnium, for chlorides and bromides and for iso-propoxides and tert-butoxides.