Formate ligands: Informative probes of metal oxide cluster nodes of metal–organic frameworks

Small-molecule probes (e.g., CO, methanol) have been used extensively with spectroscopic methods to elucidate metal oxide surface chemistry. This approach has barely been applied to the metal oxide cluster nodes of metal−organic frameworks (MOFs)—and deserves greater attention. This review is a summary showing how understanding of MOF nodes (e.g., Zr₆O₈, Al₃O) has emerged from probing their surfaces with formate formed from formic acid. We posit that formate is an optimal probe of MOF nodes, mimicking the bonding of common linkers (carboxylates). Spectra of node formates determine the following, for example: formate bonds in both mono- and bi-dentate modes, differentiating MOFs with isolated and paired nodes. Formate's tendency to displace node ligands such as Cl⁻ and extraframework linkers indicates similarities between node defect sites and structural vacancy sites. Counts of formate ligands installed on nodes by ligand exchange quantify (approximately) the numbers of accessible node sites. The reactions of node formate ligands, including decomposition; conversions with alcohols to form esters; and interactions with water that modify formate–node bonding, demonstrate distinct reactivities of formate on various types of node sites, providing evidence that neighboring node sites work in concert to affect reactivities. Such insights help elucidate reactive sites for selective adsorption and catalysis on MOFs.