Heterometallic Titanium-Organic Frameworks as Dual-Metal Catalysts for Synergistic Non-buffered Hydrolysis of Nerve Agent Simulants

Abstract

Heterometallic or mixed-metal Metal-Organic Frameworks (MOFs), incorporating two or more metal ions to the inorganic node of the frameworks, are increasingly gaining importance as a route to produce materials with increasing chemical and functional complexity. Heterometallic MOFs can offer important advantages over their homometallic counterparts to enable targeted modification of the adsorption properties, structural response, electronic structure or chemical reactivity of the framework. This field is still in its infancy likely due to the difficulties of controlling the formation of heterometallic nodes by direct synthesis. This restriction is even more acute in the case of titanium frameworks for which their challenging chemistry renders post-synthetic doping of preformed materials as the only route available. However, this often results in partial or non-homogeneous metal substitution in detriment of the potential benefits of controlling metal distribution at an atomic level toward performance improvement. We report the first family of heterometallic titanium frameworks that can be prepared by direct synthesis from metal precursors and trimesic acid. MUV-101 frameworks [TiM2(µ3-O)(O2CR)6X3] (M = Mg, Fe, Co, Ni; X = H2O, OH -, O 2-) combine mesoporosity with good chemical stability. We use these materials to exemplify the advantages of controlling metal distribution across the framework in heterogeneous catalysis by exploring their activity toward the degradation of a nerve agent simulant of Sarin gas. MUV-101(Fe) is the only pristine MOF capable of catalytic degradation of (diisopropyl-fluorophosphate) DIFP in non-buffered aqueous media without the presence of a basic/nucleophilic co-catalyst. Compared to MUV-101(Fe), other titanium heterometallic and homometallic MOFs as MUV-101(Mg, Co and Ni), MUV-10(Mn), MIL-100(Ti and Fe) or UiO-66(Zr), all display a poorer performance or are poisoned by the degradation products. The catalytic activity of MUV-101(Fe) cannot be explained only by the association of Ti(IV) and Fe(III) but to their synergistic cooperation. Our simulations suggest that the combination of Ti(IV) Lewis acid and Fe(III)-OH Brönsted base sites in this dual metal catalyst leads to a much lower energy barrier for more efficient degradation of DIFP in absence of a base. Overall, this mechanism resembles the activity of the metalloenzyme purple acid phosphatase that displays also bimetallic active sites.

Selective Implantation of Diamines for Cooperative Catalysis in Isoreticular Heterometallic Titanium–Organic Frameworks

“…This selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of functional moieties in MOFs. We are confident the combined use of the empty space offered by these porous frameworks, alongside the different chemistries intrinsic to metal clusters and organic linkers, offers a promising playground for producing chemically complex environments by implantation of multiple components for unprecedented opportunities in synergistic phenomena [33] …”

“…We have introduced here the first family of isoreticular titanium frameworks,MUV-10 and MUV-12, to demonstrate the ability of heterometallic Ti 2 Ca 2 clusters to direct the selective implantation of diamines to the soft Ca(II) sites in the inorganic node.T his results in the combination of Lewis acid Ti(IV) centers and available -NH 2 sites.C hemical modification of MUV-12-DAP with CH 3 Ie nables quantitative cycloaddition of CO 2 to propylene oxide at room temperature and atmospheric pressure for complete conversion in absence of additional co-catalysts and with excellent cyclability.T his selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of functional moieties in MOFs.W eare confident the combined use of the empty space offered by these porous frameworks,a longside the different chemistries intrinsic to metal clusters and organic linkers,o ffers ap romising playground for producing chemically complex environments by implantation of multiple components for unprecedented opportunities in synergistic phenomena. [33] Angewandte Chemie Forschungsartikel…”

Selective Implantation of Diamines for Cooperative Catalysis in Isoreticular Heterometallic Titanium–Organic Frameworks

“…We have introduced here the first family of isoreticular titanium frameworks,MUV-10 and MUV-12, to demonstrate the ability of heterometallic Ti 2 Ca 2 clusters to direct the selective implantation of diamines to the soft Ca(II) sites in the inorganic node.T his results in the combination of Lewis acid Ti(IV) centers and available -NH 2 sites.C hemical modification of MUV-12-DAP with CH 3 Ie nables quantitative cycloaddition of CO 2 to propylene oxide at room temperature and atmospheric pressure for complete conversion in absence of additional co-catalysts and with excellent cyclability.T his selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of functional moieties in MOFs.W eare confident the combined use of the empty space offered by these porous frameworks,a longside the different chemistries intrinsic to metal clusters and organic linkers,o ffers ap romising playground for producing chemically complex environments by implantation of multiple components for unprecedented opportunities in synergistic phenomena. [33]…”

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

“…56 While attempts to incorporate Mg 2+ to complete the series of alkaline Earth metals did not yield a crystalline product, a recent study demonstrated that the inclusion of Mg 2+ results in a separate MOF isostructural to the MIL-100 family. 57 We suspect that ionic radius is a critical determinant of isostructural MOF syntheses, given that the ionic radius for Mg 2+ in a six-coordinate geometry is 0.72 compared to Ca 2+ = 1.0, Mn 2+ = 0.83, Ba 2+ = 1.35, Sr 2+ = 1.18, and Cd 2+ = 0.95. 58 Interestingly, computational methods in the seminal report on MUV-10(Ca, Mn) predicted that an MUV-10(Cd) variant would be the least thermodynamically favorable, and yet we demonstrate it can be prepared following similar procedures.…”

“…The energetic degeneracy of the loose geometries could create a high density of mid-gap orbitals that would produce low-energy absorption, akin to Urbach tailing. 74,75 Although typically assumed to be crystalline, defects in MOFs, along with amorphous, liquid, and phase-change MOFs have attracted intense attention recently, 57,[76][77][78] and we propose that disorder near MOF nodes must be considered as well, especially to understand optical behavior.…”