Reprogramming kinetic phase control and tailoring pore environments in Coᴵᴵ and Znᴵᴵ metal-organic frameworks

Abstract

Metal−organic frameworks (MOFs) 1-Co and 1-Zn ([M3(L)(H2L)(DMF)(DABCO)], where M = Co and Zn), which are based on trimeric nodal clusters (MTdMOhMTd), have been synthesized from the ligands 2,2′- dihydroxy-1,1′-biphenyl-4,4′-dicarboxylic acid (H4L) and 1,4- diazabicyclo[2.2.2]octane (DABCO). High temperature synthesis (150 °C) led to the formation of 1-Co, but an identical reaction mixture gave exclusively 2-Co ([Co(H2L)(DMF)2]) when reacted at 65 °C. Reactions at intermediate temperatures gave a mixture of products confirming that 1-Co is the thermodynamic product and 2-Co is the kinetic product. Conditions used to form 2-Co at 65 °C were “reprogrammed” by doping the reaction solution with ZnII to generate the thermodynamically favored phase (1-M) with a mixed CoII/ZnII composition, 1-CoZn. Heterometallic mixtures of ZnII/CoII were explored for a range of starting metal ratios, showing preferential incorporation of CoII over ZnII at 150 °C. Furthermore, coordination of CoII ions to the free diol moieties in 1-Zn was achieved by post-synthetic doping of 1-Zn with Co(NO3)2 in MeOH, generating Co@1-Zn. On the basis of pore size distributions and fluorescence emission spectroscopy, CoII was shown to bind to the diol moieties for all CoII-containing forms of 1 during MOF synthesis but this does not occur for excess ZnII in 1-Zn. These synthetic conditions allow precise control over both the internal pore dimensions and pore environment for variants of 1, leading to demonstrable improvements in the enthalpy of CO2 adsorption.