Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound

Abstract

Construction of nitrogen−nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy.Here, we report that room temperature photolysis of Ru 2 (chp) 4 N 3 (chp − = 2-chloro-6hydroxypyridinate) in CH 2 Cl 2 produces N 2 via reductive coupling of Ru 2 (chp) 4 N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane "zigzag" geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor−acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru−N π* orbital and (2) Ru−N π to Ru−N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N−N σ bond, while radical coupling interactions dominate the formation of N−N π bonds. Comparison is made to isoelectronic Ru 2 -oxo compounds. Formation of an O−O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, ∼−500 kJ/mol, for N−N coupling vs a more modest driving force for O−O coupling, −40 to −140 kJ/mol.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

“…A smaller increase in nitride character in the π* orbitals is observed for [Cr V NSal NMe2 ] •+ in agreement with ligand-based oxidation, and a substantially slower rate of decay. Interestingly, a larger increase in the nitride character in the π* orbitals is calculated for Mn VI in comparison to Cr VI (Table S3), which matches the observation of facile homocoupling for the oxidized Mn VI complexes. ,, It is possible that excited states with nitride radical character are energetically accessible for the oxidized Mn VI complexes or that radical character develops along the homocoupling reaction coordinate, accelerating the homocoupling process. , While outside the scope of the current work, further insight could be gained by reaction profile and complete-active space self-consistent-field calculations (CASSCF). ,, …”

“…63,84 While outside the scope of the current work, further insight could be gained by reaction profile and complete-active space self-consistentfield calculations (CASSCF). 6,34,84 The relatively slow homocoupling rate for the oxidized Cr complexes herein provided an opportunity to probe nitride reactivity differences based on metal ([Cr VI NSal R ] + ; R = CF 3 , tBu) or ligand-based oxidation ([Cr V NSal NMe2 ] •+ ). Previous studies by Brown and Mayer have detailed the increased electrophilicity of charged metal oxo complexes in comparison to their neutral complexes, 85−87 and Holland et al have recently reported a switch from nucleophilic to electrophilic nitride reactivity upon both metal and ligand oxidation (amide to nitroxide) of a Re V nitride complex with a PNP pincer ligand.…”

“…40,47,48 It is possible that excited states with nitride radical character are energetically accessible for the oxidized Mn VI complexes or that radical character develops along the homocoupling reaction coordinate, accelerating the homocoupling process. 63,84 While outside the scope of the current work, further insight could be gained by reaction profile and complete-active space self-consistentfield calculations (CASSCF). 6,34,84 The relatively slow homocoupling rate for the oxidized Cr complexes herein provided an opportunity to probe nitride reactivity differences based on metal ([Cr VI NSal R ] + ; R = CF 3 , tBu) or ligand-based oxidation ([Cr V NSal NMe2 ] •+ ).…”

Screening Carbon‐Boron Frustrated Lewis Pairs for Small‐Molecule Activation including N2, O2, CO, CO2, CS2, H2O and CH4: A Computational Study

“…[2] Therefore, N 2 activation under mild conditions has become a hot research topic. Metal complexes including Mo, [3][4][5][6] Zr, [7] Fe, [8][9][10][11] Ti, [12] W, [13][14] Ru [15] and so on are commonly used for N 2 activation. The activation mechanism involves filled d orbitals for donating electron into π* orbitals of N 2 as well as empty metal d orbitals for accepting electrons from N 2 .…”

Structure and Reactivity of One- and Two-Electron Oxidized Manganese(V) Nitrido Complexes Bearing a Bulky Corrole Ligand

“…Metal oxo (M=O) and nitrido (M≡N) species are important classes of high-valent transition metal complexes . On one hand, metal oxo species have long been used as oxidants in chemical and biological systems. − On the other hand, the oxidation chemistry of M≡N is much less explored, although metal nitrido species are potentially useful nitrogenation reagents. − Attempts to design highly oxidizing/electrophilic metal nitrido species, such as the use of weakly coordinating ancilliary ligands or to increase the oxidation state of the metal centers, are hampered by the relative ease of the nitrido ligands to undergo bimolecular coupling to give dinitrogen. − …”