On the modulation of N2 activation from molecular orbitals viewpoint: Triamidoamine-Mo complexes as case of study

Simplified models (model A: [(PhNCH2CH2)3N]Mo; model B: [(PhNCH2CH2)3N]Mo–N2) based on triamidoamine complexes were proposed to evaluate the electrostatic/orbital effects of the phenyl rings on complexation and reduction of N2. The effect of systematic synchronized rotations (from 0° to 90°) of phenyl dihedral angles on the charge and/or orbital contribution of molybdenum to complex N2 was studied by chemical descriptors like electrophilicity index (ω) and Mulliken charges. According with the results, there is a favorable modulation of the energy gap of interaction between HOMO/SOMOmodelA and LUMO/LUMO+1N2 corresponding to π-back-donation interaction. It is confirmed that phenyl rings not only work as steric walls but also affect the orbital interactions. In accordance with ω values, Model A behaves as nucleophile with respect to N2. This observation is in agreement with the suggestion that back-donation governs the complex-N2 interaction. The rotation of dihedral angles of model B also has an effect on the electrophilicity index, allowing modulation in reductive processes. Inductive effects were also assessed by the substituents CN, Br and NH2 in para position on the phenyl rings. The best scenario for N2 complexation and activation was obtained with [(p-NH2PhenylNCH2CH2)3N]Mo. The Density Functional Theory was used as theoretical framework.

Graphical abstractThe understanding of what molecular orbitals are involved in the interaction Mo-catalyst-N2 makes possible the modulation of such interaction by systematic rotations of the dihedral angles Mo–N–Caryl–Caryl to find an optimal geometry that weaken the N–N bond. The optimal geometry would be fixed by rigid dendritic scaffolds to construct a nitrogen-reducer macromolecular catalyst. DFT framework was used in the present study.Figure optionsDownload full-size imageDownload as PowerPoint slide