Kartik Chandra Mondal

Dinitrogen (N2) binding and electron transfer reduction of N2 to ammonia (NH3) by the FeMoco cofactor of the nitrogenase enzyme are captivating. They are a part of the textbook for general chemistry. The nature of N2 bonding by reduced FeMoco is speculated based on the experimental evidence. The inorganic core MoFe7S9C1− possesses a Fe6(μ6-C4−) unit. The mode of N2-binding at one of the Fe-centers of the elusive Fe6(μ6-C4−) unit and the role of light element C4− is intriguing. In the past, the mode of N2-binding and the kinetics of N2 reduction have been studied by spectroscopic and other tools. Herein, we report on the energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) calculations/analyses to shed light on the deeper insight of the N2 binding and especially on the influence of the C-atom of previously reported Fe-complexes with an EP3 donor set (E=C, Si). The role of the C-atom in the iron-carbon site has been studied by elaboration with deformation electron densities. The intrinsic interaction energy of the bond between Fe and N2 and pairwise orbital interactions between them have been quantitatively estimated. The influence of σ-donation of three phosphine ligands and their effects on the Fe−N2 bond have been thoroughly studied.

Metal ions-based inorganic-organic hybrid composites are often reported acting as good to excellent catalysts with various substrate scopes under milder reaction conditions. The active catalyst of a catalytic cycle is sometimes proposed to be a short-lived reactive intermediate species. A three coordinate (L−Me)Ni(II) intermediate species [L−Me=O2N donor dianionic ligand] can bind with short-lived carbene-ester ligands to produce four coordinate Ni(II) species which can act as carbene transfer intermediate under suitable reaction conditions for C−H functionalization and/or cyclopropanation reactions. The dissociation of phosphine (PPh3) from the Ni(II) centre of (L−Me)Ni(II)(PPh3) (1 a) and binding of short lived carbene esters (:CR1−CO2R2; R1=H, Ph; R2=aliphatic group; 2–4 and other carbenes; 5–10) to Ni(II) rationalize the phenomenon in solution. Air stable Ni(0)-olefin complexes/intermediates (12–18) have recently been shown to mediate a variety of organic transformations. This analysis will further help organic/organometallic chemists to rationalize the design and synthesis of future catalysts for organic transformation. EDA-NOCV calculations have been performed to shed light on the stability and bonding of those species. Additionally, our analysis provides a proper reason why the analogous (L−Me)Pd−PPh3 complex (1 b) does not dissociate in solution and hence, a similar catalytic product has not been isolated from identical reaction conditions. The stability and the labile nature of Ni(II/0) complexes have been investigated by state-of-the-art EDA-NOCV analyses.

Arenes [C6H3R(TMS)(OTf); also called benzyne/aryne precursors] containing inter-related leaving groups Me3Si (TMS) and CF3-SO3-(OTf) on the adjacent positions (1,2-position) are generally converted to their corresponding aryne-intermediates via the addition of fluoride anion (F−) and subsequent elimination of TMS and OTf groups. This reaction is believed to proceed via the formation of an anionic intermediate [C6H4(TMS-F)(OTf)]−. The EDA-NOCV analysis (EDA-NOCV = energy decomposition analysis-natural orbital for chemical valence) of over 35 such precursors of varied types have been reported to reveal bonding and stability of CAr-Si and C-OTf bonds. EDA-NOCV showed that the nature of the CAr-Si bond of C6H3R(TMS)(OTf) can be expressed as both dative and electron sharing [CAr-Si, CAr→Si]. The CAr-OTf bond, on the other hand, can be described explicitly as dative [CAr←OTf]. The nature of CAr-Si bond of [C6H4(TMS-F)(OTf)]− exclusively changes to covalent dative σ-bond CAr→S(Me)3F on the attachment of F− to the TMS group of C6H4(TMS)(OTf). Introduction of σ-electron withdrawing group (like OMe, NMe2, and NO2) to the ortho-position of the TMS group of functionalized arynes C6H3R(TMS)(OTf) prefer to have a covalent dative σ-bond (CAr→Si) over an electron-sharing covalent σ-bond (CAr-Si). If this σ-electron withdrawing group is shifted from ortho-position to meta- and para-positions, then the preference for a dative bond decreases significantly, implying that the electronic effect on the nature of chemical bonds affects through bond paths. This effect dies with distance, similar to the well-known inductive effect.

The FeVco cofactor of nitrogenase (VFe7S8(CO3)C) is an alternative in the molybdenum (Mo)-deficient free soil living azotobacter vinelandii. The rate of N2 reduction to NH3 by FeVco is a few times higher than that by FeMoco (MoFe7S9C) at low temperature. It provides a N source in the form of ammonium ions to the soil. This biochemical NH3 synthesis is an alternative to the industrial energy-demanding production of NH3 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-N2 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)-N2 (1-4) to provide deeper insights into the fundamental bonding aspects of V-N2 bond, showing the interacting orbitals and corresponding pairwise orbital interaction energies (ΔEorb(n)). The computed intrinsic interaction energy (ΔEint) of V-N2-V bonds is significantly higher than those of the previously reported Fe-N2-Fe bonds. Covalent interaction energy (ΔEorb) is more than double the electrostatic interaction energy (ΔEelstat) of V-N2-V bonds. ΔEint values of V-N2-V bonds are in the range of-172 to-204 kcal/mol. The V → N2 ↠V π-backdonation is four times stronger than V ↠N2 → V σ-donation. V-N2 bonds are much more covalent in nature than Fe-N2 bonds.

(Tip)2SbCl (1, Tip = 2,4,6-triisopropylphenyl) has been utilized as a precursor for the synthesis of the distibane (Tip)4Sb2 (4) via one-electron reduction using KC8. The two-electron reduction of 1 and 4 afforded the novel trinuclear antimonide cluster [K3((Tip)2Sb)3(THF)5] (6). Changing the reducing agent from KC8 to a different alkali metal resulted in the solid-state isolation of corresponding stable dimeric alkali metal antimonides with the general formula [M2((Tip)2Sb)2(THF)p-x(tol)x] (M = Li (14), Na (15), Cs (16)). In this report, different aspects of the various reducing agents [K metal, KC8, and [K2(Naph)2(THF)]] used have been studied, correlating the experimental observations with previous reports. Additional reactivity studies involving 1 and AgNTf2 (Tf = trifluoromethanesulfonyl) afforded the corresponding antimony cation (Tip)2Sb+NTf2- (19). The Lewis acidic character of 19 has been unambiguously proved via treatment with Lewis bases to produce the corresponding adducts 20 and 21. Interestingly, the precursors 1 and 4 have been observed to be highly luminescent, emitting green light under short-wavelength UV radiation. All the reported compounds have been characterized via NMR, UV-vis, mass spectrometry, and single-crystal X-ray diffraction analysis. Cyclic voltammetry (CV) studies of 1 in THF showed possible two electron reduction, suggesting the in situ generation of the corresponding radical-anion intermediate 1- and its subsequent conversion to the monomeric intermediate (Tip)2Sb- (5) upon further reduction. 5 undergoes oligomerization in the solid state to produce 6. The existence of 1- was proved using electron paramagnetic resonance (EPR) spectroscopy in solution. CV studies of 6 suggested its potential application as a reducing agent, which was further proved via the conversion of Tip-PCl2 to trimeric (Tip)3P3 (17), and cAACP-Cl (cAAC = cyclic alkyl(amino)carbene) to (cAAC)2P2 (18) and 4, utilizing 6 as a stoichiometric reducing agent.

A Schiff-base nickel(II)-phosphene-catalyzed chemodivergent C-H functionalization and cyclopropanation of aromatic heterocycles is reported in moderate to excellent yields and very good regioselectivity and diastereoselectivity. The weak, noncovalent interaction between the phosphene ligand and Ni center facilitates the ligand dissociation, generating the electronically and coordinatively unsaturated active catalyst. The proposed mechanisms for the reported reactions are in good accord with the experimental results and theoretical calculations, providing a suitable model of stereocontrol for the cyclopropanation reaction.

The donor ligand bonded singlet (L)2Si2C containing a bent Si2C unit in the middle has been studied by theoretical quantum mechanical calculations (NBO, QTAIM, EDA-NOCV analyses) [L = cAAC, NHC, Me3P]. EDA-NOCV analysis suggests that this Si2C is possible to stabilize by a pair of donor base ligands. The bond dissociation energy of the Si2C fragment is endothermic (85-45 kcal/mol) with a sufficiently high intrinsic interaction energy (ΔEint = −89 to −48 kcal/mol). Fifty percent of the total stabilization energy arises from electrostatic interactions, and nearly 45% is contributed by covalent orbital interaction between Si2C and (L)2 fragments in their singlet states. 75-80% of the orbital interaction energy is contributed by two sets of σ-donation L → SiCSi ← L. The π-back-donation is only 15-10%. The dispersion energy is not negligible (3-5%). The interaction energy is highest for 1 (L = cAAC) among three compounds. Additionally, (cAAC)2Si2C-Ni(CO)3 (4) has been studied. The interaction energy between 1 and Ni(CO)3 is nearly 61 kcal/mol with the major contribution coming from donation of electron cloud from electron rich Si2C backbone to empty hybrid orbital of Ni(CO)3 fragment. A sufficiently strong π-back-donation from (OC)3Ni to Si2C has also been identified.

Binding of dinitrogen (N2) to a transition metal center (M) and followed by its activation under milder conditions is no longer impossible; rather, it is routinely studied in laboratories by transition metal complexes. In contrast, binding of N2 by main group elements has been a challenge for decades, until very recently, an exotic cAAC-borylene (cAAC = cyclic alkyl(amino) carbene) species showed similar binding affinity to kinetically inert and non-polar dinitrogen (N2) gas under ambient conditions. Since then, N2 binding by short lived borylene species has made a captivating news in different journals for its unusual features and future prospects. Herein, we carried out different types of DFT calculations, including EDA-NOCV analysis of the relevant cAAC-boron-dinitrogen complexes and their precursors, to shed light on the deeper insight of the bonding secret (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence). The hidden bonding aspects have been uncovered and are presented in details. Additionally, similar calculations have been carried out in comparison with a selected stable dinitrogen bridged-diiron(I) complex. Singlet cAAC ligand is known to be an exotic stable species which, combined with the B-Ar group, produces an intermediate singlet electron-deficient (cAAC)(B-Ar) species possessing a high lying HOMO suitable for overlapping with the high lying π*-orbital of N2 via effective π-backdonation. The B-N2 interaction energy has been compared with that of the Fe-N2 bond. Our thorough bonding analysis might answer the unasked questions of experimental chemists about how boron compounds could mimic the transition metal of dinitrogen binding and activation, uncovering hidden bonding aspects. Importantly, Pauling repulsion energy also plays a crucial role and decides the binding efficiency in terms of intrinsic interaction energy between the boron-center and the N2 ligand.

A series of possible precursors for generating C2 with the general formula Me3E-C2-I(Ph)FBF3 [E = C (1), Si (2), and Ge (3)] has been theoretically investigated using quantum chemical calculations. The equilibrium geometries of all species show a linear E-C2-I+ backbone. The inspection of the electronic structure of the Me3E-C2 bond by energy decomposition analysis coupled with the natural orbital for chemical valence (EDA-NOCV) method suggests a combination of electron sharing C-C σ-bond and v weak π-dative bond between Me3C and C2 fragments in the doublet state for species 1 (E = C). For species 2 (Si) and 3 (Ge), the analysis reveals σ-dative Me3E-C2 bonds (E = Si, Ge; Me3EC2) resulting from the interaction of singly charged (Me3E)+ and (C2-IPh(BF4))- fragments in their singlet states. The C2-I bond is diagnosed as an electron sharing σ-bond in all three species, 1, 2 and 3.

The binding of the dinitrogen molecule to the metal center is the first and crucial step toward dinitrogen activation. Favorable interaction energies are desired by chemists and biochemists to study model complexes in the laboratory. An electrochemically reduced form of a previously isolated sulfur-bridged Ni3S8 complex is inferred to bind N2 at multiple Ni centers, and this bonded N2 undergoes reductive protonation to produce hydrazine (N2H4) as the product in the presence of a proton donor. Density functional theory (DFT) calculations and quantum theory of atoms in molecules (QTAIM) analysis have been carried out to shed light on the nature of N2 binding to an anionic trinuclear Ni3S8 complex. Additionally, energy decomposition analysis with the combination of natural orbital for chemical valence (EDA-NOCV) analysis has been performed to estimate the pairwise interaction energies between the Ni center and the N2 molecule under experimental conditions.