Accessing 14-Connected Nets: Continuous Breathing, Hydrophobic Rare-Earth Metal Organic Frameworks Based on 14-c Hexanuclear Clusters with High Affinity for Non-Polar Vapors.

Abstract

Highly connected metal organic frameworks (MOFs) in which at least one building block has connectivity higher than twelve are very rare and much desirable. We report here the first examples of isostructural 14-connected MOFs, RE-frt-MOF-1, constructed from the assembly of 14-c hexanuclear rare-earth clusters, [RE6(μ3-X)8(COO)12]2- (RE: Y3+, Tb3+, Dy3+, Ho3+, Er3+, Yb3+ and X: OH-/F-) with a tritopic carboxylate-based organic linker. This linker serves as a 3-c and 4-c organic node resulting in the formation of a unique, trinodal (3,4,14)-c framework. RE-frt-MOF-1 are stable in air and alkaline aqueous solutions and show an intriguingly continuous, reversible breathing behavior, between a wide and a narrow-pore phase, upon guest removal. Crystallinity is retained during breathing, and single-crystal X-ray diffraction shed light into the associated structural transformation. Vapor sorption studies performed on Y-frt-MOF-1 revealed a high affinity for non-polar vapors such as n-hexane, cyclohexane, and benzene, displaying type I isotherms with high uptake at low relative pressures (<10-3 p/p0), associated with the hydrophobic nature of the 1D channels and also with their rhombic shape. In contrast, polar vapors such as acetonitrile and ethanol show type V isotherms due to favorable vapor-vapor interactions. Notably these vapors, except cyclohexane, trigger the transition from the narrow to the wide pore phase, accompanied by a remarkable increase in uptake, reaching 70.6, 109, 100.4, and 87.7% for n-hexane, benzene, acetonitrile, and ethanol, respectively.