Bara Dominic, Wilson Claire, Mörtel Max, Khusniyarov Marat M, Ling Sanliang, Slater Ben, Sproules Stephen, Forgan Ross S
WestCHEM School of Chemistry , University of Glasgow , Joseph Black Building, University Avenue , Glasgow G12 8QQ , U.K.
Department of Chemistry and Pharmacy , Friedrich-Alexander University Erlangen-Nürnberg , Egerlandstrasse 1 , 91058 Erlangen , Germany.
J Am Chem Soc. 2019 May 22;141(20):8346-8357. doi: 10.1021/jacs.9b03269. Epub 2019 May 8.
Phase control in the self-assembly of metal-organic frameworks (MOFs) is often a case of trial and error; judicious control over a number of synthetic variables is required to select the desired topology and control features such as interpenetration and defectivity. Herein, we present a comprehensive investigation of self-assembly in the Fe-biphenyl-4,4'-dicarboxylate system, demonstrating that coordination modulation can reliably tune between the kinetic product, noninterpenetrated MIL-88D(Fe), and the thermodynamic product, two-fold interpenetrated MIL-126(Fe). Density functional theory simulations reveal that correlated disorder of the terminal anions on the metal clusters results in hydrogen bonding between adjacent nets in the interpenetrated phase and this is the thermodynamic driving force for its formation. Coordination modulation slows self-assembly and therefore selects the thermodynamic product MIL-126(Fe), while offering fine control over defectivity, inducing mesoporosity, but electron microscopy shows MIL-88D(Fe) persists in many samples despite not being evident by diffraction. Interpenetration control is also demonstrated using the 2,2'-bipyridine-5,5'-dicarboxylate linker; it is energetically prohibitive for it to adopt the twisted conformation required to form the interpenetrated phase, although multiple alternative phases are identified due to additional coordination of Fe cations to its N donors. Finally, we introduce oxidation modulation-the use of metal precursors in different oxidation states from that found in the final MOF-to kinetically control self-assembly. Combining coordination and oxidation modulation allows the synthesis of pristine MIL-126(Fe) with BET surface areas close to the predicted maximum for the first time, suggesting that combining the two may be a powerful methodology for the controlled self-assembly of high-valent MOFs.
金属有机框架（MOF）自组装过程中的相控制通常需要反复试验；需要对多个合成变量进行明智的控制，以选择所需的拓扑结构并控制诸如互穿和缺陷率等特征。在此，我们对铁-联苯-4,4'-二羧酸酯体系中的自组装进行了全面研究，结果表明配位调制能够可靠地在动力学产物、非互穿的MIL-88D(Fe)和热力学产物、双重互穿的MIL-126(Fe)之间进行调节。密度泛函理论模拟表明，金属簇上末端阴离子的相关无序导致互穿相中相邻网络之间形成氢键，这是其形成的热力学驱动力。配位调制减缓了自组装过程，因此选择了热力学产物MIL-126(Fe)，同时对缺陷率进行了精细控制，诱导了介孔率，但电子显微镜显示，尽管在衍射中不明显，但MIL-88D(Fe)仍存在于许多样品中。使用2,2'-联吡啶-5,5'-二羧酸酯连接体也证明了对互穿的控制；尽管由于铁阳离子与其氮供体的额外配位而确定了多个替代相，但它形成互穿相所需的扭曲构象在能量上是不利的。最后，我们引入了氧化调制——使用与最终MOF中不同氧化态的金属前驱体——来动力学控制自组装。结合配位调制和氧化调制首次实现了具有接近预测最大值的BET表面积的原始MIL-126(Fe)的合成，这表明将两者结合可能是一种用于高价MOF可控自组装的强大方法。
