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.

本研究通过对分离得到的改性物种进行表征，探究了2,2,6,6-四甲基-3,5-庚二酮（2,2,6,6-tetramethyl-3,5-heptanedione，简称Hthd）对不同丙氧基锆与丙氧基铪前驱体的改性过程。成功分离并表征了配合物[Zr(OnPr)₃(thd)]₂、[Zr(OnPr)(OiPr)₂(thd)]₂、Zr(OiPr)(thd)₃、[Hf(OnPr)₃(thd)]₂与Hf(OiPr)(thd)₃。其中，Zr(OiPr)(thd)₃的正丙氧基类似物的晶体结构未能完成精修，但通过X射线衍射（X-ray Diffraction，XRD）与核磁共振氢谱（¹H NMR）明确证实了其存在。该丙氧基前驱体的改性过程仅涉及单取代与三取代中间产物，并未生成二取代化合物；因此，市面宣称用于二氧化锆（ZrO₂）金属有机化学气相沉积（Metal-Organic Chemical Vapor Deposition，MOCVD）制备、且最常被使用的“Zr(OiPr)₂(thd)₂”商业产物实际并不存在。在锆基与铪基体系中，均未检测到该类化合物存在的证据。仅在铪基体系中可分离得到的二聚羟基二(thd)取代配合物[Hf(OH)(OiPr)(thd)₂]₂，是通过微量水解反应生成的。所有杂配体中间产物最终都会转化为热力学稳定的Zr(thd)₄或Hf(thd)₄。由异丙氧基前驱体制得的化合物，其稳定性高于以正丙氧基配体或混合正丙/异丙氧基配体制得的产物。此外，体系中残留的醇类可促进该转化过程，并显著提升反应速率。研究证实，四价金属四(thd)配合物（M⁴⁺(thd)₄）异常低下的溶解度与挥发性，源于其晶体结构中的紧密堆积效应与强范德华相互作用。