Exceptional Thermal Stability in a Supramolecular Organic Framework: Porosity and Gas Storage

Reaction of beta-amino-beta-(pyrid-4-yl)acrylonitrile with the aromatic dicarboxaldehydes 9,10-bis(4formylphenyl)anthracene and terephthalaldehyde affords the dihydropyridyl products 9,10-bis(4-((3,5dicyano-2,6-dipyridyl)dihydropyridyl)phenyl)anthracene (L-1) and 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (L-2), respectively. In the solid state [L-1]center dot 2.5DMF center dot 3MeOH (SOF-1) crystallizes in the monoclinic space group P2(1)/C and forms a 3D stable supramolecular organic framework via strong N-H center dot center dot center dot N-py hydrogen bonds and pi-pi interactions. The material incorporates pyridyl-decorated channels and shows permanent porosity in the solid state. The pore volumes of the desolvated framework SOF-1a calculated from the N-2 isotherm at 125 K and the CO2 isotherm at 195 K are 0.227 and 0.244 cm(3)g(-1) respectively. The N-2 absorption capacity of SOF-1a at 77 K is very low, with an uptake of 0.63 mmol g(-1) at 1 bar, although saturation N-2 adsorption at 125 K is 6.55 mmol g(-1) (or 143 cm(3) g(-1)). At ambient temperature, SOF-1 a shows significant CO2 adsorption with approximately 3 mol of CO2 absorbed per mole of host at 16 bar and 298 K, corresponding to 69 cm(3) g(-1) at STP. SOF-1a also adsorbs significant amounts of C2H2, with an uptake of 124 cm3 (STP) g(-1) (5.52 mmol g(-1)) at 1 bar at 195 K. Methane uptake at 195 K and 1 bar is 69 cm3 (STP) g(-1). Overall, gas adsorption measurements on desolvated framework SOF-1a reveal not only high capacity uptakes for C2H2 and CO2, compared to other crystalline molecular organic solids, but also an adsorption selectivity in the order C2H2 > CO2 > CH4 > N-2. Overall, C2H2(270 K)/CH4(273 K) selectivity is 33.7 based on Henry's Law constant, while the C2H2 (270 K)/CO2(273 K) ratio of uptake at 1 bar is 2.05. The less bulky analogue L2 crystallizes in the triclinic space group PT as two different solvates [L-2]center dot 2DMF center dot 5C(6)H(6) (S2A) and [L-2]center dot 2DMF center dot 4MeOH (S2B) as pale yellow tablets and blocks, respectively. Each L-2 molecule in S2A participates in two N-H center dot center dot center dot O hydrogen bonds between dihydropyridyl rings and solvent DMF molecules. Packing of these layers generates 1D nanochannels along the crystallographic a and b axes which host DMF and benzene molecules. In S2B, each L-2 ligand participates in hydrogen bonding via an N-H center dot center dot center dot O interaction between the N-H of the dihydropyridyl ring and the O of a MeOH and also via an N-H center dot center dot center dot O interaction between the N center of a pyridine ring and the H-O of a second MeOH molecule. The presence of the L-2-HO Me hydrogen bonds prevents ligand-ligand hydrogen bonding. As a result, S2B crystallizes as one-dimensional chains rather than as an extended 3D network. Thermal removal of solvents from S2A results in conversion to denser phase S2C which shows no effective permanent porosity.