Synthesis of Zirconium Pinacolate and Mechanism of Its Thermal Transformation to ZrO₂:  Impact of a Vicinal Diol Ligand

Reaction of Zr₂(OⁱPr)₈(HOⁱPr)₂ with a slight excess of pinacol (HOCMe₂CMe₂OH) yields Zr₂(OCMe₂CMe₂O)₂(OCMe₂CMe₂OH)₄, as confirmed by X-ray crystallography. Inter- and intramolecular hydrogen bonding results in the polymeric solid-state structure and the subsequent nonvolatility and reduced solubility of the species. The thermal transformation of zirconium pinacolate occurs in two distinct steps, as seen by the thermal gravimetric analysis (TGA), and occurs at <300 °C, yielding ZrO₂, as determined by X-ray powder diffraction. GC−MS analysis of the thermolysis byproducts reveals six species:  pinacol, pinacolone, 2,3-dimethyl-2-butene (tetramethylethylene) (trace amounts), 2,3-dimethylbutadiene (DMBD), 2,3-dimethyl-3-buten-2-ol (DMB-ol), 2,2,5,6,6-pentamethyl-4-hepten-3-one (PMH), and 1,3,5-tri-tert-butylbenzene (TTB). Real-time monitoring of the thermal transformation is achieved by carrying out the experiment inside a mass spectrometer. This novel experiment confirms that while only pinacol is lost in step I, step II consists of two phases. In step IIA, pinacolone, DMB-ol, PMH, and TTB are evolved, while in step IIB, only pinacolone and DMBD are formed. The observations support a primary mechanism for the formation of all C₆ products except DMBD, which is a dehydration product of pinacolone. Mechanistic studies involving isotopic labeling experiments suggest that the C₁₂ and C₁₈ products are formed by a secondary process involving the interaction of evolved pinacolone with the metal oxide species. Crystallographic data (P1̄/−169 °C) for Zr₂(OCMe₂CMe₂O)₂(OCMe₂CMe₂OH)₄ are as follows:  a = 21.2305(5) Å, b = 22.905(5) Å, c = 10.508(3) Å, β = 104.01(1)°, with Z = 2.