Kartik Chandra Mondal

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.

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.

The factors/structural features which are responsible for the binding, activation and reduction of N2 to NH3 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 N2-binding Fe-sites of FeMoco, which possesses a central C(4-) ion having an eight valence electrons as an μ6-bridge. Here, a series of [(S3C(0))Fe(II/I/0)-N2]n- complexes in different charged/spin states containing a coordinated σ- and π-donor C(0)-atom which possesses eight outer shell electrons [carbone, (Ph3P)2C(0); Ph3P→C(0)←PPh3] 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 N2-binding has been studied by EDA-NOCV analysis. The role of the oxidation state of Fe and N2-binding in different charged and spin states of the complex have been investigated by EDA-NOCV analyses. The intrinsic interaction energies of the Fe−N2 bond are in the range from −42/−35 to −67 kcal/mol in their corresponding ground states. The S3C(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 (S2Cring donor, π-C donor) and stronger donor set like CP3 (σ-C donor) ligands, respectively.

Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent–Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the C-C bond of the cyclopropane (splitting two bonds at a time out of three C-C bonds) ring is preferred to form two dative covalent C-C bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3-rings. These corresponding QTAIM parameters of C3-rings are quite different for C-C single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal–olefin systems.