Heterometallic Titanium–Organic Frameworks by Metal-Induced Dynamic Topological Transformations

Reticular chemistry has boosted the design of thousands of metal and covalent organic frameworks for unlimited chemical compositions, structures, and sizable porosities. The ability to generate porous materials at will on the basis of geometrical design concepts is responsible for the rapid growth of the field and the increasing number of applications derived. Despite their promising features, the synthesis of targeted homo- and heterometallic titanium–organic frameworks amenable to these principles is relentlessly limited by the high reactivity of this metal in solution that impedes the controlled assembly of titanium molecular clusters. We describe an unprecedented methodology for the synthesis of heterometallic titanium frameworks by metal-exchange reactions of MOF crystals at temperatures below those conventionally used in solvothermal synthesis. The combination of hard (titanium) and soft (calcium) metals in the heterometallic nodes of MUV-10­(Ca) enables controlled metal exchange in soft positions for the generation of heterometallic secondary building units (SBUs) with variable nuclearity, controlled by the metal incorporated. The structural information encoded in the newly formed SBUs drives an MOF-to-MOF conversion into bipartite nets compatible with the connectivity of the organic linker originally present in the crystal. Our simulations show that this transformation has a thermodynamic origin and is controlled by the terminations of the (111) surfaces of the crystal. The reaction of MUV-10­(Ca) with first-row transition metals permits the production of crystals of MUV-101­(Fe,Co,Ni,Zn) and MUV-102­(Cu), heterometallic titanium MOFs isostructural with archetypical solids such as MIL-100 and HKUST. In comparison to de novo synthesis, this metal-induced topological transformation provides control over the formation of hierarchical micro-/mesopore structures at different reaction times and enables the formation of heterometallic titanium MOFs not accessible under solvothermal conditions at high temperature, thus opening the door for the isolation of additional titanium heterometallic phases not linked exclusively to trimesate linkers.网状化学（Reticular chemistry）极大推动了数千种金属有机框架（Metal-Organic Framework，MOF）与共价有机框架的研发，这类材料可实现化学组成、结构与孔隙率的无限调控。依托几何设计理念按需制备多孔材料的能力，驱动了该领域的快速发展以及衍生应用的持续增长。尽管这类材料具备诸多应用前景，但遵循上述原则合成目标同金属与异金属钛基MOF却始终面临局限：溶液中钛金属的高反应性会阻碍钛分子簇的可控组装。我们报道了一种全新的合成策略，可通过在低于常规溶剂热合成温度的条件下对MOF晶体开展金属交换反应，制备异金属钛基框架材料。在MUV-10(Ca)的异金属节点中，硬酸金属钛与软酸金属钙的协同作用，使得其软配位位点可实现可控金属交换，进而生成核数可调的异金属次级结构单元（Secondary Building Unit，SBU），核数由引入的金属种类决定。新形成的SBU所承载的结构信息，驱动了MOF到MOF的拓扑转变，最终得到与晶体中原生有机连接体连接模式兼容的二分网格结构。我们的模拟研究表明，该转变由热力学主导，并受晶体(111)晶面的表面终止状态调控。将MUV-10(Ca)与第一过渡系金属进行反应，可制备得到MUV-101(Fe,Co,Ni,Zn)与MUV-102(Cu)晶体，这类异金属钛基MOF与MIL-100、HKUST等典型固体材料同构。相较于从头合成路径，这种金属诱导的拓扑转变可在不同反应时长下调控分级微/介孔结构的形成，且能够获得高温溶剂热条件下无法合成的异金属钛基MOF，从而为分离出不限于均苯三甲酸连接体的其他钛基异金属相提供了可行途径。