Stability and bonding analysis of metal-dinitrogen bond - a deeper insight Sai Manoj Gorantla and Dr. Kartik Chandra Mondal Indian Institute of Technology, India The MoFe 7 S 9 C 1 − unit of the nitrogenase cofactor (FeMoco) is in the prime focus of researchers for its remarkable ability to bind and catalytically convert aerial dinitrogen (N 2 ) into ammonia (NH 3 ). Several model complexes that are structurally relevant to nitrogenase co-factor (MoFe 7 S 9 C), have been synthesized, characterized and studied theoretically. 1 The binding of the dinitrogen molecule to the metal center is the first and very crucial step toward dinitrogen activation. However, the mode of N 2 binding and its reaction pathways are yet not clear. We have employed density functional theory (DFT) calculations coupled with state-of-the-art computational methods like natural bond orbital (NBO), atoms-in-molecules (AIM) and energy decomposition analyses (EDA) in combination with natural orbital for chemical valence (NOCV) to unravel the nature of the M-N 2 bond (M = Fe, Ni) in previously reported dinitrogen complexes besides proposing new hypothetical molecules. 2 NBO analysis reveals a reduction in the Wiberg bond indices (2.4-2.6) relative to that of free N 2 (3.03), indicating the weakening of the N-N bond after binding to metal. The deviation of ellipticity values from zero (0.1-0.3) for the M-N bond from AIM analysis suggests a π contribution from the metal. The EDA-NOCV analysis illustrates one σ-electron donation (M←N 2 ) from 3σ g + of N 2 into vacant d-orbitals of the metal and two π-backdonations (M→N 2 ) from hybrid d z 2 -d x 2 y 2 orbital of metal into vacant degenerate π* orbitals (1π g , 1π g ’) of N 2 . Furthermore, the pairwise contributions from the EDA-NOCV calculations suggest stronger M→N 2 π-backdonations (60-85%), which correlate well with the AIM analysis.
Figure 1. Optimized structures of Ni and Fe dinitrogen complexes (above) and deformation density pictures illustrating σ and π interactions (below). References 1. G. Ung and J. C. Peters, Chem., Int. Ed ., 2014, 53 , 1; 2. I. Čorić, B. Q. Mercado, E. Bill, D. J. Vinyard and P. L. Holland, Nature 2015, 526 , 96; 3. S. F. McWilliams, D. L. J. Broere, C. J. V. Halliday, S. M. Bhutto, B. Q. Mercado and P. L. Holland, Nature 2020, 584 , 221. 4. S. M. N. V. T. Gorantla and K. C. Mondal, ACS Omega 2021, 49 , 33389; 5. S. M. N. V. T. Gorantla and K. C. Mondal, ACS Omega 2021, 49 , 33932; 6. S. M. N. V. T. Gorantla and K. C. Mondal, RSC Adv ., 2022, 12 , 3465; 7. S. M. N. V. T. Gorantla, H. S. Karnamkkott, S. Arumugam, S. Mondal and K. C. Mondal , J Comput Chem , 2023, 44 , 43.
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