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

Abstract

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.