DFT Study on N2 Activation by a Hydride-Bridged Diniobium Complex. N≡N Bond Cleavage Accompanied by H2 Evolution

Theoretical mechanistic insights into dinitrogen cleavage by a dititanium hydride complex bearing PNP-pincer ligands

The mechanism of dinitrogen cleavage by a PNP-coordinated dititanium polyhydride complex has been computationally investigated. A "multi-state reactivity" scenario has been disclosed for the whole process of N₂ coordination and activation. Remarkably, the H₂ elimination prior to the N-N cleavage is accomplished by the coupling of two terminal hydrides, and planar PNP-pincer ligand could stabilize the corresponding transition state. Besides, the tetrahydrofuran (THF) solvent could also promote the H₂ elimination due to the similar polarity of the corresponding intermediates or transition states to THF molecule.

Dinitrogen functionalization at a ditantalum center. Balancing N2 displacement and N2 functionalization in the reaction of coordinated N2 with CS2

The reaction of carbon disulfide (CS2) with the side-on end-on dinitrogen complex ([NPNSi]Ta)2(μ-η1:η2-N2)(μ-H)2 (1) (where [NPNSi] = [PhP(CH2SiMe2NPh)2]) has been studied and shown to generate two products, the ratio of which depends on the concentration of added carbon disulfide. At high concentrations of CS2, N-N bond cleavage and functionalization occur to generate a ditantalum complex with an isothiocyanato ligand N-bound to Ta along with bridging sulfido and nitrido moieties. At lower concentrations of CS2, less dinitrogen functionalization is observed; instead, N2 is displaced and the CS2 molecule is completely disassembled to generate a ditantalum derivative with bridging methylene and two sulfide moieties. Kinetic and DFT studies have been performed and provide clues about the product ratio and mechanistic information and shed light on why N2 functionalization is challenging.

Redox-Initiated Reactivity of Dinuclear β-Diketiminatoniobium Imido Complexes

High-valent dichloride and dimethylniobium complexes 1 and 2 bearing tert-butylimido and N,N'-bis(2,4,6-trimethylphenyl)-β-diketiminate (BDI^Ar) ligands were prepared. The dimethyl complex reacted with dihydrogen to release methane and generate the hydride-bridged diniobium(IV) complex 3 in high yield. One-electron oxidation of 3 with silver salts resulted in the release of dihydrogen and conversion to a mixed-valent Nb^III-Nb^IV complex, 4, that displayed a frozen-solution X-band electron paramagnetic resonance signal consistent with a slight dissymmetry between the two Nb centers. Spectroscopic and computational analysis supported the presence of Nb-Nb σ-bonding interactions in both 3 and 4. Finally, one-electron reduction of 4 resulted in conversion to the highly dissymmetric Nb^V-Nb^V dimer 5 that formed from the reductive C-N bond cleavage of one of the BDI^Ar supporting ligands.