Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although nature's non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.

We discuss some of the challenges facing density functional theory (DFT) and recent progress in DFT for both ground and excited electronic states. We discuss key aspects of the results we have been able to obtain with the strategy of designing density functionals to have various ingredients and functional forms that are then optimized to accurately predict various types of properties and systems with as much universality as possible. Finally, we make specific recommendations of approximate density functionals that are well suited for particular kinds of applications. The Advancement of DFT by Adding IngredientsRecent advances in the development of Kohn-Sham DFT (KS-DFT) (see Glossary) have focused on obtaining more accurate and universal density functionals. KS-DFT has been successful in describing numerous properties of atoms, molecules, and solids, and very large systems can be treated accurately at an affordable computational expense. Nevertheless, KS-DFT suffers from limitations. In practical applications of the original theory, these may all be considered to arise from the need to approximate the exchange-correlation functional. Most approximate functionals suffer from self-interaction error, delocalization error, or both and their task is made particularly difficult by the need to make up for the representation of the density as a single-configuration reference wave function that corresponds to noninteracting electrons.

We review state-of-the-art electronic structure methods based both on wave function theory (WFT) and density functional theory (DFT). Strengths and limitations of both the wave function and density functional based approaches are discussed, and modern attempts to combine these two methods are presented. The challenges in modeling excited-state chemistry using both single-reference and multireference methods are described. Topics covered include background, combining density functional theory with single-configuration wave function theory, generalized Kohn-Sham (KS) theory, global hybrids, range-separated hybrids, local hybrids, using KS orbitals in many-body theory (including calculations of the self-energy and the GW approximation), Bethe-Salpeter equation, algorithms to accelerate GW calculations, combining DFT with multiconfigurational WFT, orbital-dependent correlation functionals based on multiconfigurational WFT, building multiconfigurational wave functions from KS configurations, adding correlation functionals to multiconfiguration self-consistent-field (MCSCF) energies, combining DFT with configuration-interaction singles by means of time-dependent DFT, using range separation to combine DFT with MCSCF, embedding multiconfigurational WFT in DFT, and multiconfiguration pair-density functional theory.

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