Chemistry of 2,2,6,6,-Tetramethyl-3,5-heptanedione (Hthd) Modification of Zirconium and Hafnium Propoxide Precursors

The modification of different zirconium propoxide and hafnium propoxide precursors with 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) was investigated by characterization of the isolated modified species. The complexes
[Zr(OnPr)3(thd)]2, [Zr(OnPr)(OiPr)2(thd)]2, Zr(OiPr)(thd)3, [Hf(OnPr)3(thd)]2, and Hf(OiPr)(thd)3 were isolated and
characterized. The structure of the n-propoxide analogue of Zr(OiPr)(thd)3 could not be refined, but its existence
was clearly demonstrated by XRD and 1H NMR. The modification of the propoxide precursors involves mono- and
trisubstituted intermediate compounds and does not involve a disubstituted compound; thus, the commercial product
that is claimed to be “Zr(OiPr)2(thd)2” and is most commonly used for the MOCVD preparation of ZrO2 does not
exist. No evidence was found for the presence of such a compound in either zirconium- or hafnium-based systems.
Formation of the dimeric hydroxo-di-thd-substituted complex, [Hf(OH)(OiPr)(thd)2]2, which could be isolated only for
hafnium-based systems, occurs on microhydrolysis. All heteroleptic intermediates are eventually transformed to the
thermodynamically stable Zr(thd)4 or Hf(thd)4. The compounds obtained from isopropoxide precursors showed a
higher stability than those with n-propoxide ligands or a combination of both types. In addition, it is important to
note that residual alcohol facilitates the transformation and strongly enhances its rate. The unusually low solubility
and volatility of MIV(thd)4 has been shown to be due to close packing and strong van der Waals interactions in the
crystal structures of these compounds.