Stabilizing large pores in a flexible metal-organic framework via chemical cross-linking.

A barrier to the isoreticular expansion of flexible metal-organic frameworks is their complex breathing behavior, which can lead to pore closure upon solvent exchange and removal. Here we show that chemical cross-linking stabilizes the open form of a flexible aluminum framework with large 17 Å pores.

Thermolabile Cross-Linkers for Templating Precise Multicomponent Metal-Organic Framework Pores.

While a number of approaches toward multicomponent metal-organic frameworks have been reported, new strategies affording greater structural versatility and molecular precision are needed to replicate the sophisticated active sites found in enzymes. Here, we outline a general method for templating functional groups within framework pores using thermolabile ligand cross-linkers. We show that tertiary ester-based cross-linkers can be used to install well-defined carboxylic acid pairs at precise relative distances and orientations. The tertiary ester linkages remain intact during framework formation but are readily cleaved to reveal free carboxylic acids upon microwave heating. Successful cross-linker synthesis, framework incorporation, and thermolysis is demonstrated using the mesoporous, terphenyl expanded analogues of MOF-74. When short cross-linkers are used, modeling studies show that the carboxylic acids are installed in a single configuration down the pore channels, spaced ∼7 Šapart. These precisely positioned acid pairs can be used as synthetic handles to build up more complex cooperative active sites.

The role of leached Zr in the photocatalytic reduction of CO2 to formate by derivatives of UiO-66 metal organic frameworks.

This work reports the photocatalytic reduction of CO2 to formate that is mediated by UiO-66 Zr MOF derivatives. Amino-substituted UiO-66 is a known photocatalyst for this transformation, and herein we identify that this catalysis is due to leached Zr, not the MOF itself. No correlation between catalytic activity and crystal size is observed for UiO-66-NH2. Recycling studies along with SEM images of the crystals prior to and after catalysis support our conclusion that the catalytic performance can be described by the amount of leached Zr. Moreover, when the effect of the linker on the catalytic reaction is probed, all MOFs that facilitate the reduction of CO2 are found to leach Zr into solution. Correlation of the MOF (or linker) band gap energies to formate production indicates that this is an important parameter to the leached species. Combined with a linker exchange study, this indicates that the leached Zr still coordinates the linker. These results indicate that the UiO-66 Zr MOFs are not stable under typical photochemical conditions, and emphasize the importance of considering the role that leached metals play in catalysis.

Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition.

The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equilibrium is approximately temperature-independent; relative to H2, it is more favored at elevated temperatures. We also measure the hydricity of (PNP)Ru-H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.