Complexation and disproportionation of group 4 metal (alkoxy) halides with phosphine oxides

Abstract

Group 4 Lewis acids are well-known catalysts and precursors for (non-aqueous) sol–gel chemistry. Titanium, zirconium and hafnium halides, and alkoxy halides are precursors for the controlled synthesis of nanocrystals, often in the presence of Lewis base. Here, we investigate the interaction of Lewis bases with the tetrahalides (MX₄, X = Cl, Br) and metal alkoxy halides (MX_x(OR)_4−x, x = 1–3, R = OⁱPr, O^tBu). The tetrahalides yield the expected Lewis acid–base adducts MX₄L₂ (L = tetrahydrofuran or phosphine oxide). The mixed alkoxy halides react with Lewis bases in a more complex way. ³¹P NMR spectroscopy reveals that excess of phosphine oxide yields predominantly the complexation product, while a (sub)stoichiometric amount of phosphine oxide causes disproportionation of the MX_x(OR)_4−x species into MX_x+1(OR)_3−x and MX_x−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 favorable Lewis acid–base complex formed with the more acidic species. We also gain more insight into the isomerism of the complexes. The disproportionation reaction turns out to be a general phenomenon, for titanium, zirconium and hafnium, for chlorides and bromides, and for isopropoxides and tert-butoxides.