Gas separations with porous materials are economically important and provide a unique challenge to fundamental materials design, as adsorbent properties can be altered to achieve selective gas adsorption. Metal−organic frameworks represent a rapidly expanding new class of porous adsorbents with a large range of possibilities for designing materials with desired functionalities. Given the large number of possible framework structures, quantum mechanical computations can provide useful guidance in prioritizing the synthesis of the most useful materials for a given application. Here, we show that such calculations can predict a new metal− organic framework of potential utility for separation of dinitrogen from methane, a particularly challenging separation of critical value for utilizing natural gas. An open V(II) site incorporated into a metal−organic framework can provide a material with a considerably higher enthalpy of adsorption for dinitrogen than for methane, based on strong selective back bonding with the former but not the latter.
■ INTRODUCTIONCoordination of dinitrogen to transition-metal cations is important both fundamentally and industrially. Dinitrogen is highly inert and generally considered to be a poor ligand. In 1965, however, it was shown that a simple coordination complex, [Ru(NH 3 ) 5 ] 2+ , could reversibly bind N 2 . 1 In subsequent years, a number of dinitrogen−transition-metal complexes have been isolated for metals in varying oxidation states with various coordination numbers. 2,3 These complexes typically feature low-valent, relatively reducing metal cations coordinated to dinitrogen in an end-on binding mode. Activating dinitrogen at a metal center to promote its reduction by hydrogen to ammonia under moderate conditions remains a critical goal for homogeneous catalysis. Somewhat weaker metal−dinitrogen binding, however, may be useful for adsorptive separation of gas mixtures. An example is provided by the need to remove dinitrogen (an omnipresent but noncombustible contaminant) from natural gas or other methane-rich gases. This is an extraordinarily difficult separation based on physical properties alone, as both gases lack a permanent dipole and have similar polarizabilities, boiling points, and kinetic diameters. Although cryogenic distillation is currently utilized for separation of these gases, the cost-and capital-intensive nature of this separation has led to development of a number of competing processes, such as membraneor kinetics-based separations, which generally suffer from low selectivities. 4