Volatile Metal Alkoxides according to the Concept of Donor Functionalization

Since the fundamental works of D. C. Bradley and R. C. Mehrotra, metal alkoxides have attracted attention because of the diversity of their low- and high-molecular-weight structures; they are also generating increasing interest as precursor compounds for solving technical materials problems. The understanding of the hydrolytic nucleation behavior is a prerequisite for the optimization of materials from sol–gel processes. For metal alkoxides to be precursors in chemical vapor deposition (CVD) processes in the preparation of inorganic oxidic materials, they should be sufficiently volatile, and sublimation should occur without decomposition at as low a temperature as possible (< 150 °C). Only recently, using the “donor functionalization” concept, was a ligand type systematically developed that unifies the advantages of both steric demands and σ-donor stabilization and so stabilizes low-molecularweight metal alkoxides. Even large metal ions of low charge (for example Ba²⁺) can thus form volatile alkoxides. O- and N-donor functions in bidentate and multidentate alkoxo ligands are particularly advantageous; hence, for example, the vanadium derivative [V(OCMe₂CH₂OMe)₃] is one of the most volatile metal alkoxides known to date. The first alkoxides of the alkaline earth metals calcium, strontium, and barium, which sublime without decomposition, have the formula [M₂{OC(CH₂OiPr)₂tBu}₄]. This article presents a critical inventory of the metal alkoxides with particular regard to the aspect of volatility. It also describes successes of the donor functionalization concept and shows—in prespective—how alkoxo ligands can be “tailor made” for metals according to their charge-to-radius ratio by further development of the concept.