Cooperative Metal Ion Combinations in Ti-Based Multivariate Metal–Organic Frameworks

Multivariate metal-organic frameworks (MTV-MOFs), which incorporate multiple metal ions or organic linkers within a single framework, provide a powerful platform for investigating structure-composition-property relationships, which in turn enable the rational tuning of material performance. To establish such a correlation, it is critical to achieve precise compositional control while maintaining high crystallinity, thereby minimizing variability in crystallinity and disorder as confounding factors. We report a family of titanium-based multivariate metal-organic frameworks (MTV-MOFs), Ti₂M₄(μ₃-O)₂pbpta₃ (designated DGIST-14; M = Ni²⁺, Co²⁺, Mn²⁺), in which various combinations of transition metal ions are integrated into heterometallic clusters within highly crystalline soc-topology frameworks. Based on the hypothesis that Ti⁴⁺, a hard acid, can direct the formation of robust frameworks through strong interactions with hard basic carboxylate linkers, we aim to incorporate soft M²⁺ without compromising structural integrity. This approach facilitates the integration of diverse transition metal ions with precise control over metal ratios. The synergy among the metal centers leads to substantial improvements in structural stability, surface area (up to ∼4600 m²/g), and gas uptake (up to ∼1677 cm³/g). Notably, the controlled incorporation of Ni²⁺ and Mn²⁺ ions enables the selective generation of singlet oxygen under visible-light via a metal-composition-dependent pathway. This work establishes a structure-composition-function relationship in Ti-based MTV-MOFs and demonstrates a design strategy that leverages cooperative metal-ion combinations to simultaneously optimize framework stability and photoactivity.