Highly selective CO₂ uptake in uninodal 6-connected "mmo" nets based upon MO₄^2- (M = Cr, Mo) pillars

A novel 4⁸.6⁷ topology metal-organic material (MOM) platform of formula [M(bpe)₂(M′O₄)] (M = Co or Ni; bpe = 1,2-bis(4-pyridyl)ethene; M′ = Mo or Cr) has been synthesized and evaluated in the context of gas sorption. These MOMs have been assigned RCSR code mmo and are uninodal 6-connected nets. [Ni(bpe)₂(MoO ₄)], MOOFOUR-1-Ni, and its chromate analogue, CROFOUR-1-Ni, exhibit high CO₂ affinity and selectivity, especially at low loading. This behavior can be attributed to exceptionally high isosteric heats of adsorption (Q_st) of CO₂ in MOOFOUR-1-Ni and CROFOUR-1-Ni of ∼56 and ∼50 kJ/mol, respectively, at zero loading. These results were validated by molecular simulations which indicate that the electrostatics of these inorganic anions affords attractions toward CO₂ that are comparable to those of unsaturated metal centers.