Creating Order in Ultrastable Phosphonate Metal–Organic Frameworks via Isolable Hydrogen-Bonded Intermediates
Racheal P. S. Huynh, David R. Evans, Jian Xiang Lian, Denis M. Spasyuk, Samira Siahrostami, George K. H. Shimizu
Abstract
The stability presented by trivalent metal–organic frameworks (MOFs) makes them an attractive class of materials. With phosphonate-based ligands, crystallization is a challenge, as there are significantly more binding motifs that can be adopted due to the extra oxygen tether compared to carboxylate counterparts and the self-assembly processes are less reversible. Despite this, we have reported charge-assisted hydrogen-bonded metal–organic frameworks (HMOFs) consisting of [Cr(H 2 O) 6 ] 3+ and phosphonate ligands, which were crystallographically characterized. We sought to use these HMOFs as a crystalline intermediate to synthesize ordered Cr(III)-phosphonate MOFs. This can be done by dehydrating the HMOF to remove the aquo ligands around the Cr(III) center, forcing metal–phosphonate coordination. Herein, a new porous HMOF, H-CALF-50, is synthesized and then dehydrated to yield the MOF CALF-50. CALF-50 is ordered, although it is not single crystalline. It does, however, have exceptional stability, maintaining crystallinity and surface area after boiling in water for 3 weeks and soaking in 14.5 M H 3 PO 4 for 24 h and 9 M HCl for 72 h. Computational methods are used to study the HMOF to MOF transformation and give insight into the nature of the structure and the degree of heterogeneity.