The Interconversion of Ammonia with its Elements by Molybdenum Complexes: Fundamental Investigations

Abstract

Ammonia (NH3) is essential for sustaining life on Earth. In addition to its role as the principal component of most modern fertilizers, ammonia is also a hydrogen-rich small molecule that is a transportable, carbon-neutral fuel alternative. The energy-efficient interconversion of ammonia with N2 and H2 is desirable yet presents a complex challenge owing to the movement of six electrons and six protons as well as the activation of the N≡N triple bond, one of the strongest in chemistry. At the core of energy-efficient NH3 synthesis and oxidation is understanding the fundamental thermochemistry associated with the formation and cleavage of N–H bonds especially in cases where the nitrogen-containing molecule is bound to a transition metal catalyst. The research presented in this dissertation systematically establishes N–H bond dissociation free energies (BDFEs) for various classes of nitrogen ligands (ammine, amide, imide, diazenide, hydrazide) in complexes of molybdenum relevant to catalytic ammonia synthesis and oxidation cycles.

A host of molybdenum complexes supported by bis(diphosphine), terpyridine/bis(phosphine) and pyridine(diimine) ligands were synthesized, structurally characterized and the corresponding N–H BDFEs were determined experimentally and supported by density functional theory (DFT) calculations. A blueprint was established for various complexes on how coordination environment, metal oxidation state, overall charge affects N–H BDFEs and consequently, N–H bond formation/cleavage reactivity by proton-coupled electron transfer (PCET). The phenomenon of “coordination-induced bond weakening” was demonstrated in cationic terpyridine bis(phosphine) complexes of molybdenum for X–H (X = N, O) bonds in small molecules such as ammonia, hydrazine and water, including the isolation of a nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+; (PhTpy = 4′-Ph-2,2′,6′,2′′-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) that was shown to be thermodynamically unstable with respect to H2 evolution owing to an exceptionally low ammine N–H BDFE. Accordingly, the complex [1-NH3]+ undergoes H2 loss upon mild thermolysis and can serve as a thermodynamically potent source of H-atoms in PCET reactions. A related nonclassical pyrrolidine complex of molybdenum was synthesized and applied as a mechanistic probe that provides key insight into H2 evolution from coordinated amines by deuterium labeling experiments. The applications of these fundamental studies were extended to demonstrate the interconversion of imido/amido as well as olefin/alkyl ligands by PCET in the coordination sphere of molybdenum. Additionally, a catalytic partial hydrogenation of a pyridine(diimine) chelate in a molybdenum nitrido ethylene complex was demonstrated and the thermodynamics of a plausible PCET pathway were examined by DFT.