Due to their potential future applications in high-density data storage devices, single-ion magnets (SIMs) have become one of the most exciting classes of materials for research at present. Vanadium complexes, with their unique multiple oxidation states and coordination geometries, are excellent candidates for investigating such properties. In the present study, we have explored the SIM properties of two mixed-valent organo-polyoxovanadyl complexes, viz. (NH 4 ) 4 [H 6 , each of which contains four vanadium(V) atoms and one vanadium(IV) atom. One unpaired electron on the central Kramers vanadium(IV) (S = 1/2) atom gives the molecule its magnetic moment, which is responsible for the reversal of its magnetization/spin at low temperatures. As such, the investigation of these complexes has involved a combination of experimental techniques, including superconducting quantum interference device (SQUID) magnetometry, electron paramagnetic resonance (EPR) spectroscopy, and a computational technique that used the CASSCF-based wave function theory and included relativistic effects by considering NEVPT2 for more accurate results. AC magnetic susceptibility measurements have revealed the single ion magnet (SIM) behaviors of both the complexes under the application of an external DC magnetic field, which were characterized by maxima in the plots of the "out-of-phase" magnetic susceptibility against the AC frequency (χ″ vs ν) at different temperatures. The spin relaxation time (τ) has been determined to be in the range of 2−10 K. From the fitting of the plot of relaxation time (τ) versus temperature to different models, we have tried to understand the type of slow relaxation process present in the system under a particular applied DC magnetic field. Finally, the ab initio method, viz. the CASSCF-based computational methods, has been employed to justify/ rationalize and correlate the experimental results.

Dinitrogen (N2) binding and its activation have been achieved by non-metal compounds like intermediate cAACborylene with the general formula of (cAAC)2(B-Dur)2(N2) [cAAC = cyclic alkyl(amino)carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = aryl-borylene].

The FeVco cofactor of nitrogenase (VFe 7 S 8 (CO 3 )C) is an alternative in the molybdenum (Mo)-deficient free soil living azotobacter vinelandii. The rate of N 2 reduction to NH 3 by FeVco is a few times higher than that by FeMoco (MoFe 7 S 9 C) at low temperature. It provides a N source in the form of ammonium ions to the soil. This biochemical NH 3 synthesis is an alternative to the industrial energy-demanding production of NH 3 by the Haber–Bosch process. The role of vanadium has not been clearly understood yet, which has led chemists to come up with several stable V–N 2 complexes which have been isolated and characterized in the laboratory over the past three decades. Herein, we report the EDA–NOCV analyses of dinitrogen-bonded stable complexes V(III/I)–N 2 ( 1–4 ) to provide deeper insights into the fundamental bonding aspects of V–N 2 bond, showing the interacting orbitals and corresponding pairwise orbital interaction energies (Δ E orb( n ) ). The computed intrinsic interaction energy (Δ E int ) of V–N 2 –V bonds is significantly higher than those of the previously reported Fe–N 2 –Fe bonds. Covalent interaction energy (Δ E orb ) is more than double the electrostatic interaction energy (Δ E elstat ) of V–N 2 –V bonds. Δ E int values of V–N 2 –V bonds are in the range of −172 to −204 kcal/mol. The V → N 2 ← V π-backdonation is four times stronger than V ← N 2 → V σ-donation. V–N 2 bonds are much more covalent in nature than Fe–N 2 bonds.

The factors/structural features which are responsible for the binding, activation and reduction of N 2 to NH 3 by FeMoco of nitrogenase have not been completely understood well. Several relevant model complexes by Holland et al. and Peters et al. have been synthesized, characterized and studied by theoretical calculations. For a matter of fact, those complexes are much different than real active N 2 -binding Fe-sites of FeMoco, which possesses a central C(4-) ion having an eight valence electrons as an μ 6 -bridge. Here, a series of [(S 3 C(0))Fe(II/I/0)-N 2 ] ncomplexes in different charged/ spin states containing a coordinated σand π-donor C(0)-atom which possesses eight outer shell electrons [carbone, (Ph 3 P) 2 C(0); Ph 3 P!C(0) PPh 3 ] and three S-donor sites (i.e. -S-Ar), have been studied by DFT, QTAIM, and EDA-NOCV calculations. The effect of the weak field ligand on Fe-centres and the subsequent N 2 -binding has been studied by EDA-NOCV analysis. The role of the oxidation state of Fe and N 2binding in different charged and spin states of the complex have been investigated by EDA-NOCV analyses. The intrinsic interaction energies of the FeÀN 2 bond are in the range from À42/À35 to À67 kcal/mol in their corresponding ground states. The S 3 C(0) donor set is argued here to be closer to the actual coordination environment of one of the six Fe-centres of nitrogenase. In comparison, the captivating model complexes reported by Holland et al. and Peter et al. possess a stronger π-acceptor C-ring (S 2 C ring donor, π-C donor) and stronger donor set like CP 3 (σ-C donor) ligands, respectively.

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