Stern Rebecca D, Kingsbury Ryan S, Persson Kristin A
Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States.
Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
Inorg Chem. 2021 Oct 18;60(20):15456-15466. doi: 10.1021/acs.inorgchem.1c02078. Epub 2021 Oct 7.
Framework materials constitute a broad family of solids that range from zeolites and metal-organic frameworks (MOFs) to coordination polymers. The synthesis of such network structures typically rely on precursor molecular building blocks. As an example, the UiO-66 MOF series is constructed of hexanuclear [ZrO(OH)(CO)] cluster nodes and linear carboxylate linkers. Unfortunately, these Zr MOF cluster nodes cannot currently be manufactured in a sustainable way, motivating a search for "green" alternative synthesis methods. Stabilizing the hexanuclear Zr(IV) cluster (i.e., the hexamer, {Zr}) without the use of organic ligation would enable the use of environmentally friendly solvents such as water. The Zr(IV) tetranuclear cluster (i.e., the tetramer, {Zr}) can be stabilized in solution with or without organic ligands, yet the hexamer has yet to be synthesized without supporting ligands. The reasons why certain zirconium clusters are favored in aqueous solution over others are not well understood. This study reports the relative thermodynamic instability of the hypothetical hexamer {Zr} compared to the ubiquitous {Zr} tetramer. Density functional theory calculations were performed to obtain the hydrolysis Gibbs free energy of these species and used to construct Zr Pourbaix diagrams that illustrate the effects of electrochemical potential, pH, and Zr(IV) concentration. It was found that the aqueous {Zr} hexamer is ∼17.8 kcal/mol less stable than the aqueous {Zr} tetramer at pH = 0, V = 0, and [Zr(IV)] = 1 M, which is an energy difference on the order of counterion interactions. Electronic structure analyses were used to explore trends in the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, frontier molecular orbitals, and electrostatic potential distribution of these clusters. The evidence suggests that the aqueous {Zr} hexamer may be promoted with more strategic syntheses incorporating minimal ligands and counterions.
框架材料构成了一个广泛的固体家族，其范围从沸石、金属有机框架（MOF）到配位聚合物。这类网络结构的合成通常依赖于前体分子构建单元。例如，UiO - 66 MOF系列由六核[ZrO(OH)(CO)]簇节点和线性羧酸盐连接体构成。不幸的是，目前这些锆MOF簇节点无法以可持续的方式制造，这促使人们寻找“绿色”替代合成方法。在不使用有机连接的情况下稳定六核Zr(IV)簇（即六聚体，{Zr}）将能够使用诸如水等环境友好型溶剂。Zr(IV)四核簇（即四聚体，{Zr}）无论有无有机配体都能在溶液中稳定存在，但尚未在没有支撑配体的情况下合成出六聚体。某些锆簇在水溶液中比其他锆簇更受青睐的原因尚不清楚。本研究报告了假设的六聚体{Zr}与普遍存在的{Zr}四聚体相比的相对热力学不稳定性。进行了密度泛函理论计算以获得这些物种的水解吉布斯自由能，并用于构建Zr Pourbaix图，该图说明了电化学势、pH值和Zr(IV)浓度的影响。研究发现，在pH = 0、V = 0和[Zr(IV)] = 1 M时，水溶液中的{Zr}六聚体比{Zr}四聚体稳定性低约17.8 kcal/mol，这是一个与抗衡离子相互作用量级相当的能量差。利用电子结构分析来探究这些簇的最高占据分子轨道 - 最低未占据分子轨道能隙、前沿分子轨道和静电势分布的趋势。证据表明，通过采用含有最少配体和抗衡离子的更具策略性合成方法，可能会促进水溶液中的{Zr}六聚体的形成。
