Nitrogen Lone Pair Research Articles

Lewis or Brønsted? A Rectification of the Acidic and Aromatic Nature of Boranol-Containing Naphthoid Heterocycles.

Boron-containing heterocycles are important in a variety of applications from drug discovery to materials science; therefore a clear understanding of their structure and reactivity is desirable to optimize these functions. Although the boranol (B-OH) unit of boronic acids behaves as a Lewis acid to form a tetravalent trihydroxyborate conjugate base, it has been proposed that pseudoaromatic hemiboronic acids may possess sufficient aromatic character to act as Brønsted acids and form a boron oxy conjugate base, thereby avoiding the disruption of ring aromaticity that would occur with a tetravalent boronate anion. Until now no firm evidence existed to ascertain the structure of the conjugate base and the aromatic character of the boron-containing ring of hemiboronic "naphthoid" isosteres. Here, these questions are addressed with a combination of experimental, spectroscopic, X-ray crystallographic, and computational studies of a series of model benzoxazaborine and benzodiazaborine naphthoids. Although these hemiboronic heterocycles are unambiguously shown to behave as Lewis acids in aqueous solutions, boraza derivatives possess partial aromaticity provided their nitrogen lone electron pair is sufficiently available to participate in extended delocalization. As demonstrated by dynamic exchange and crossover experiments, these heterocycles are stable in neutral aqueous medium, and their measured pKa values are consistent with the ability of the endocyclic heteroatom substituent to stabilize a partial negative charge in the conjugate base. Altogether, this study corrects previous inaccuracies and provides conclusions regarding the properties of these compounds that are important toward the methodical application of hemiboronic and other boron heterocycles in catalysis, bioconjugation, and medicinal chemistry.

Synthesis and Structure-Photophysics Evaluation of 2-N-Amino-quinazolines: Small Molecule Fluorophores for Solution and Solid State.

2-N-aminoquinazolines were prepared by consecutive SN Ar functionalization. X-ray structures display the nitrogen lone pair of the 2-N-morpholino group in conjugation with the electron deficient quinazoline core and thus representing electronic push-pull systems. 2-N-aminoquinazolines show a positive solvatochromism and are fluorescent in solution and in solid state with quantum yields up to 0.73. Increase in electron donor strength of the 2-amino substituent causes a red-shift of the intramolecular charge transfer (ICT) band (300-400 nm); whereas the photoluminescence emission maxima (350-450 nm) is also red-shifted significantly along with an enhancement in photoluminescence efficiency. HOMO-LUMO energies were estimated by a combination of electrochemical and photophysical methods and correlate well to those obtained by computational methods. ICT properties are theoretically attributed to an excitation to Rydberg-MO in SAC-CI method, which can be interpreted as n-π* excitation. 7-Amino-2-N-morpholino-4-methoxyquinazoline responds to acidic conditions with significant increases in photoluminescence intensity revealing a new turn-on/off fluorescence probe.

Excited States of Bromopyrimidines Probed by VUV Photoabsorption Spectroscopy and Theoretical Calculations.

We report absolute photoabsorption cross sections for gas-phase 2- and 5-bromopyrimidine in the 3.7–10.8 eV energy range, in a joint theoretical and experimental study. The measurements were carried out using high-resolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with time-dependent density functional theory, along with additional Franck–Condon Herzberg–Teller calculations for the first absorption band (3.7–4.6 eV). The cross sections of both bromopyrimidines are very similar below 7.3 eV, deviating more substantially from each other at higher energies. In the 7.3–9.0 eV range where the maximum cross-section is found, a single and broad band is observed for 5-bromopyrimidine, while more discernible features appear in the case of 2-bromopyrimidine. Several π* ← π transitions account for the most intense bands, while weaker ones are assigned to transitions involving the nitrogen and bromine lone pairs, the antibonding σ*Br orbital, and the lower-lying Rydberg states. A detailed comparison with the available photo-absorption data of bromobenzene is also reported. We have found significant differences regarding the main absorption band, which is more peaked in bromobenzene, becoming broader and shifting to higher energies in both bromopyrimidines. In addition, there is a significant suppression of vibrational structures and of Rydberg states in the pair of isomers, most noticeably for 2-bromopyrimidine.

Comparative study of oxazolidine and imidazolidine compounds as inhibitors of SAE 1020 steel corrosion in aqueous HCl solution

In many industries, safe organic inhibitors that are soluble in corrosive media, environmentally friendly, and effective need to be developed for protection against corrosion during descaling and steel pickling in dilute acids. In this work we describe, for the first time, the effects of 2-(nitromethylene) oxazolidine (OXA) and 2-(nitromethylene) imidazolidine (IMD) as inhibitors of the corrosion of SAE 1020 steel in 0.1 mol L−1 HCl aqueous solution. Electrochemical analyses showed that the IMD inhibitor was most effective, acting as a mixed type inhibitor. SEM analysis confirmed the electrochemical results, clearly showing the excellent corrosion inhibition effect of IMD. The corrosion inhibition was related to the protective film deposited on the surface, forming a layer that reduced the corrosion reactions. DFT calculations showed that donor and acceptor interactions between the heterocyclic nitrogen lone pair electrons and the vacant d orbitals of the metal surface atoms could explain the better corrosion inhibition values obtained for IMD. Finally, in addition to its efficiency, in silico toxicity predictions indicated that IMD could be considered environmentally friendly.

Can Counter-Intuitive Halogen Bonding Be Coulombic?

We use the term "counter-intuitive" to describe an intermolecular interaction in which the electrostatic potentials of the interacting regions of the ground-state molecules have the same sign, both positive or both negative. In the present work, we consider counter-intuitive halogen bonding with nitrogen bases, in which both the halogen σ-hole and the nitrogen lone pair have negative potentials on their molecular surfaces. We show that these interactions can be treated as Coulombic despite the apparent repulsion between the ground-state molecules, provided that both electrostatics and polarization are explicitly taken into account. We demonstrate first that the energies of 20 counter-intuitive interactions with four nitrogen bases can be expressed very well in terms of just two molecular properties: the electrostatic potential of the halogen σ-hole and the average polarizability of the nitrogen base. Then we show that the same two properties can also represent the energies of an expanded data base that includes the 20 counter-intuitive plus an additional 20 weak and moderately-strong intuitive halogen bonding interactions (in which the σ-hole potentials are now positive).

Nitrogen overgrowth as a catalytic mechanism during diamond chemical vapour deposition

Nitrogen is frequently included in chemical vapour deposition feed gases to accelerate diamond growth. While there is no consensus for an atomistic mechanism of this effect, existing studies have largely focused on the role of sub-surface nitrogen and nitrogen-based adsorbates. In this work, we demonstrate the catalytic effect of surface-embedded nitrogen in nucleating new layers of (100) diamond. To do so we develop a model of nitrogen overgrowth using density functional theory. Nucleation of new layers occurs through C insertion into a C–C surface dimer. However, we find that C insertion into a C–N dimer has substantially reduced energy requirements. In particular, the rate of the key dimer ring-opening and closing mechanism is increased 400-fold in the presence of nitrogen. Full incorporation of the substitutional nitrogen defect is then facilitated through charge transfer of an electron from the nitrogen lone pair to charge acceptors on the surface. This work provides a compelling mechanism for the role of surface-embedded nitrogen in enhancing (100) diamond growth through the nucleation of new layers. Furthermore, it demonstrates a pathway for substitutional nitrogen formation during chemical vapour deposition which can be extended to study the creation of technologically relevant nitrogen-based defects.

Synthesis, spectral characterization and conformational assignment of N-formyl-2,7-diaryl-1,4-diazepan-5-ones as potent antibacterial agents and type I DHQase inhibitors

• Four N -formyldiazepan-5-ones are synthesized and characterized by IR, mass and NMR spectra. • The conformational assignment is made by NMR spectral studies. • All the formamides prefer to adopt flattened boat conformations. • Antibacterial efficacy for 9-12 have been evaluated by resazurin reduction assay. • Molecular docking is performed for 9-12 with type 1 3-DHQase protein target. Four new N -formyl-1,4-diazepan-5-ones 9-12 have been synthesized and their spectral characterization is made using IR, mass and NMR spectra. In NMR spectra, the compounds 9-12 show doubling of NMR spectral signals due to the delocalization of nitrogen lone pair into the carbonyl П cloud and thereby creating a partial double character at N C(= O ) bond which results in an equilibrium between syn and anti rotamers. The preferred conformations of N -formyl-1,4-diazepan-5-ones 9-12 are arrived from the NMR spectral data, dihedral angles (°) and destabilizing strain factors. The syn and anti rotamers of these compounds prefer to adopt equilibrium between flattened boat conformations with quasi-axial orientations of aryl groups at C-2 and C-7. Furthermore, all the compounds are active against all the tested bacterial strains. The compounds 9 & 10 against E. coli strain, compound 11 against B. cereus strain and compounds 11 & 12 against S. typhi strain show equal potency as that of standard drug Streptomycin. Hence, the formamides 9-12 are believed to be good antibacterial agents. The synthesised molecules are subjected to molecular docking studies with 3-dehydroquinate dehydratase (DHQase) protein. The molecular modelling studies exhibit high inhibition and binding energy are conforming to antibacterial activity. The compounds 9-12 exhibit better docking score and glide energy than that of DHQase co-crystallized ligand.

Synthesis, crystal structure, Hirshfeld surface analyses, antimicrobial activity, and thermal behavior of some novel nanostructure hexa‐coordinated Cd(II) complexes: Precursors for CdO nanostructure

A new tetradentate Schiff base ligand entitled as (L = (N2E, N2′E)‐N1, N1′‐(ethane‐1,2‐diyl)bis(N2‐((E)‐3‐(2‐nitrophenyl)allylidene)ethane‐1,2‐diamine) and its six cadmium(II) complexes formulated as CdLX2 in which X is Cl, Br, I, NO3, NCS, or N3 were synthesized under ultrasound irradiation and characterized by various physical and spectral techniques. Moreover, some complexes were also prepared in nanostructured dimensions confirmed by scanning electron microscopy (SEM), energy dispersive X‐ray analysis (EDXA), and X‐ray powder diffraction (XRPD) techniques and then were used as precursor for preparation of CdO nanoparticles via direct calcination at 600°C. As typical, the crystal structure of cadmium(II) nitrate complex was determined using single crystal X‐ray diffraction. Accordingly, it was found that [Cd(L)(NO3)2] complex crystallizes in the monoclinic space group P21/n with two independent molecules in the asymmetric unit with two coordination modes of nitrate anions. This complex has zero‐dimensional (0D) coordination supramolecular structure. Hirshfeld surface analyses suggested that H … O (49.5%), H … H (30.3%), and H … C (11.8%) intermolecular interactions are most considerable in the crystal packing. Computational studies of ligand structure leading to electrostatic potentials of the solid state geometry and some occupied molecular orbitals (HOMO, HOMO‐1, HOMO‐2, and HOMO‐5) prevailing the contribution of nitrogen lone pairs are also reported. Thermal behavior studies of the compounds under nitrogen atmosphere from room temperature to 900°C revealed that the ligand is completely decomposed without any residual whereas the complexes are decomposed via two to four thermal steps with trace amount of metallic cadmium as residue. Moreover, antibacterial and antifungal screening well showed that the complexes inhibit from the growth of the various bacteria and fungi more efficient than free ligand especially against Gram‐positive bacteria. DNA cleavage potentials of the compounds may be one of the reasons for their antimicrobial activity.

Localized orbital locator as a descriptor for quantification and digital presentation of lone pairs: benchmark calculations of 4-substituted pyridines.

The parameters of the (3,-3) critical point in the localized orbital locator topology near a heteroatom have been found to reflect the changes in the size, density and electron energy of the lone pair and correlate with the donor ability of the lone pair carrying heteroatom.

Tuning the electronic effects in designing ligands for the inhibition of rotamase activity of FK506 binding protein

FK506-binding protein (FKBP) is a cytosolic enzyme, which catalyzes the cis–trans isomerization of prolyl peptides. The twisted amide geometries assist the isomerization process and inhibit the rotamase activity of FKBP. This work reveals the role of electronic effects in ligands to facilitate the twisted amide geometries for efficient inhibition of the rotamase activity of FKBP12. The azirine ligand (5A) lowered the rotation barrier of –C–N amide bond by ~ 14.0 kcal/mol compared to the reference ligand molecule (1A) using M062X/cc-pVDZ//M062X/6-31G(d) level of theory. A significant lowering in rotation barrier has been achieved in (5A) due to the ring's anti-aromatic nature. The ring strain in the 1H-azirine ring pyramidalizes the amide bond's nitrogen center by 78° in the ground state compared to 1A. The natural bond orbital (NBO) analysis confirmed the lowering in the delocalization of electron density from nitrogen lone pair to π*C = O orbital in amide bonds. The designed models (1A-5A) were developed into ligands to mimic the binding domain of FK506 (1B-5B). The binding of these ligands (1B-5B) was examined with the quantum mechanics and molecular mechanics (QM/MM) ONIOM approach in the presence of FKBP12 binding site residues. The calculated results reveal the preferential binding of twisted amide forms with FKBP12 over their planar analogous. The hydrophobic interactions augment the binding affinity of the twisted forms with the FKBP12 binding site in agreement with the experimental observations. These studied ligands could act as potential inhibitors of FKBP12 for their therapeutic application.

Mechanistic Vistas of Trivalent Nitrogen Compound Reduction by Samarium Diiodide

AbstractIn trivalent nitrogen compounds, the nitrogen lone pair is highly amenable to bind to samarium iodide prior to electron transfer. As SmI2 is more azaphilic than oxophilic, its affinity to the nitrogen lone pair is more pronounced in aza substrates than in the analogous carbonyl compounds. The impact of this binding is very large in terms of reaction rate and at the same time gives rise to unique mechanistic phenomena, which include unusual kinetic rate orders, catalysis by quantum dots, and remote functionalization. Even reduction of nitriles was found to be affected by this coordination to SmI2. This review focuses on the reduction mechanisms of three groups of compounds: imines, azobenzene, and nitriles.

Hydrogen‐Bonding as a Factor to Determine the Regioselectivity for Pd‐mediated C−H Activation of Pyridine

AbstractDirect and regioselective functionalization of pyridine is a topic of high scientific and technological importance. In spite of extensive efforts, the regioselective functionalization of pyridine still remains a significant challenge due to their low reactivity and presence of Lewis‐basic sp2 nitrogen. Here, we studied the effect of hydrogen bonding interactions on the regiochemical outcome of Pd‐mediated C−H activation of pyridine by utilizing DFT calculations. We demonstrated that hydrogen bonding can act as a second independent factor to override the inherent regioselectivity of pyridine. This novel approach complements previously reported strategies, such as: (a) coordination of pyridine to transition metal center via its N‐center, (b) installation of directing group (DG) and then coordination of pyridine to the transition metal center via this DG (i. e. chelation assistant strategy), (c) protection of its nitrogen lone pair with N‐oxide or N‐imino groups or with Lewis acids, (d) the inherent positional reactivity of C−H bonds based on the electronic or steric properties of the substituents, and (e) by the identity of the oxidant used. We have also demonstrated that the oxidation state of the Pd catalyst has impact on the regiochemical outcome of the C−H activation step in pyridine. The implications of our study for regioselective C−H functionalization catalyst design of heteroarenes are twofold: It demonstrates (1) hydrogen bonding as a viable design principle, and (2) Pd(IV) as a catalyst for C−H functionalization.

N-derivatives of formaldimines: the reason for the high nitrogen inversion barriers in N-methyl- and N-chloroimines

The energy and electronic parameters of the nitrogen inversion in imines Н 2 С=NХН n (ХН n = СН 3 , NH 2 , OH, F, SiH 3 , PH 2 , SH, Cl) have been calculated with the DFT method (B3LYP 6-311+G(d,p)) in terms of natural bond orbital. It has been established that the interactions of the nitrogen lone pair (LP) with the bond orbitals at the imino carbon atom are practically independent of the X atom and contribute to the decrease of the inversion barriers (Δ Е і ≠ ). While nN®σ * X–H , nN«σ X–H and nN«nX interactions substantially depend on the heteroatom type and promote the increase in the Δ Е і ≠ values with the rise in electronegativity of the X atom. The contribution of the interactions of the nitrogen LP with the Rydberg orbitals of the C=N–X group atoms is small and they cannot be the main reason of the decrease in the Δ Е і ≠ values when X atoms of the second period are replaced by atoms of the third period of the same group. The interactions of the LP of the X atoms and the X–H bond orbitals with the C=N bond orbitals have the main influence on the inversion barriers. The contribution of nX®π * C=N interactions to the Δ Е і ≠ values is dominant. The main reason of the “anomalous” inversion barriers of N-methyl- and N-chloroformaldimines is the destabilization of inversion transition states because of the reduction in the energies of σ X–H ®π * C=N and nX→π * C=N interactions and the rise in the energies of nN«nСl interactions. The contributions of electronegativity of ХН n substituents and energies of intramolecular interactions to the Δ Е і ≠ values have been determined.

Thermal unimolecular reactivity pathways in dehydro‐diazines radicals

AbstractIn this work, we investigated the detailed reactivity of dehydro‐diazine radicals through unimolecular dissociation pathways. Aromatic six‐membered rings with two nitrogens as heteroatoms are classified as diazines, which include pyrimidine 1, pyridazine 2, and pyrazine 3. The C–H bond scission in pyrimidine, pyridazine, and pyrazine leads to three (1a–1c), two (2a and 2b), and one (3a) radical isomers, respectively. Through quantum chemical calculations, reactivity aspects of all the six radical isomers have been investigated by considering different modes of unimolecular reactions. On the basis of the computations at (U)B3LYP/cc‐pVTZ, (U)M06‐2X/cc‐pVTZ, and (U)CCSD(T)/cc‐pVTZ//(U)B3LYP/cc‐pVTZ levels of theory and analysis, we found out that ring‐opening pathways were kinetically more favorable compared with isomerization (1,2‐H shift) pathways or C–H bond dissociation leading to biradicals. In all the cases, ring‐opening via C–N bond cleavage has a lower energy barrier compared with C–C bond cleavage owing to the stabilizing interaction between radical electron and the nitrogen lone pair. Moreover, the unimolecular decomposition channels can lead to several fragmentation products, which are interesting in the astrochemical context. On the basis of these investigations and also the impact of three centered–five electrons (3c–5e) interactions on the reactivity aspects, we determined the kinetic stability order among the six isomeric dehydrodiazine radicals.

Analysis of supramolecular interactions directing crystal packing of a trans,trans,trans-[diaquabis(4-quinolin-3-yl)-4H-1,2,4-triazole)bis(tricyanomethanide)iron(II)] complex: A combination of XRD, MEP, NBO, QTAIM, and NCI analyses

The synthesis and structural characterization of a hydrated high-spin iron(II) complex [Fe(4-qtrz)2(tcm)2(H2O)2] are reported where 4-qtrz = 4-quinolin-3-yl-4H-1,2,4-triazole and tcm = tricyanomethide. The complex was prepared solvothermally and crystallizes in the triclinic space group P1¯ with Z = 1, a = 8.5221(3) Å, b = 8.9343(5) Å, c = 10.0081(5) Å, α = 85.147(2)°, β = 77.166(2)°, γ = 83.784(2)°. The complex is centrosymmetric, with mutually trans pairs of water molecules, of tcm, and monodentate 4-qtrz coordinated via the triazole unit and a combination of O–H⋅⋅⋅N and C–H⋅⋅⋅N hydrogen bonds, forming a three-dimensional framework structure in which the shortest Fe⋅⋅⋅Fe distance is 8.5221(3) Å. An analysis of non-covalent interactions was conducted through reduced density gradient, quantum theory of atoms in molecules and natural bond orbitals. Accordingly, the important contributions of several intra- and inter-molecular hydrogen bonds stabilize the supramolecular structure. The hydrogen bonds occur by electron transfer from the tricyanomethanide nitrogen lone pairs to σ* orbitals in the triazole, quinoline and water moieties. Other hydrogen bonds are attributed to π(CN) → σ*, in triazole and quinoline, transfer. Additionally, a set of π⋅⋅⋅π* interactions between cyano groups (CN⋅⋅⋅CN), π(phenyl of quinoline)⋅⋅⋅π*(CN), and π[CC of C(CN)3] to π*(phenyl of quinoline) were also observed.

Ultrafast Dynamics of the Two Lowest Bright Excited States of Cytosine and 1-Methylcytosine: A Quantum Dynamical Study.

The nonadiabatic quantum dynamics (QD) of cytosine and 1-methylcytosine in the gas phase is simulated for 250 fs after a photoexcitation to one of the first two bright states. The nuclear wavepacket is propagated on the coupled diabatic potential energy surfaces of the lowest seven excited states, including ππ*, nπ*, and Rydberg states along all the vibrational degrees of freedom. We focus in particular on the interplay between the bright and the dark nπ* states, not considering the decay to the ground electronic state. To run these simulations, we implemented an automatic general procedure to parametrize linear vibronic coupling (LVC) models with time-dependent density functional theory (DFT) computations and interfaced it with Gaussian package. The wavepacket was propagated with the multilayer version of the multiconfigurational time dependent Hartree method. Two different density functionals, PBE0 and CAM-B3LYP, which provide a different description of the relative stability of the lowest energy dark states, were used to parametrize the LVC Hamiltonian. Part of the photoexcited population on lowest HOMO-LUMO transition (πHπL*) decays within less than 100 fs to a nπ* state which mainly involves a promotion of an electron from the oxygen lone pair to the LUMO (nOπL*). The population of the second ππ* state decays almost completely, in <100 fs, not only to πHπL* and to nOπL* states but also to another nπL* state involving the nitrogen lone pair. The efficiency of the adopted protocol allowed us to check the accuracy of the predictions by repeating the QD simulations with different LVC Hamiltonians parametrized either at the ground-state minimum or at stationary structures of different relevant excited states.

Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials.

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

An energy decomposition analysis approach to the rotational barriers of amides and thioamides

The most popular interpretation for the origin of rotational barrier in amides is the electron delocalization of the nitrogen lone pair over the NCO π system. Many attempts have been made to quantify the extent of delocalization in amide and amide like systems. An alternative approach to understand rotational barriers is the energy decomposition analysis (EDA) class of methods. In this work, the rotational barrier of a series of amides is investigated using the Canonical Molecular Orbital Energy Decomposition Analysis (CMOEDA) method. Based on CMOEDA, it was possible to explain i) the origin of the rotational barriers, ii) the energy difference between the two transition states (TS1/TS2) and iii) the variation in the rotational barrier as structural modifications are introduced. Regarding the origin of the rotational barrier, for instance, it was verified that electrostatic and polarization interactions are responsible for building the barrier, while exchange-repulsion contributes to decrease its value.

Thermodynamic studies of carbon nanotube interaction with Gemcitabine anticancer drug: DFT calculations

One of the main causes worldwide is cancer. One of the important approaches to cancer treatment is the use of chemotherapy drugs but nowadays, the use of smart drugs by researchers is being used to treat cancer. The most important carrier of drug delivery to cells is the use of carbon nanotubes. In this research, the possibility of formation of interactions between carbon nanotube and Gemcitabine anticancer drug is investigated with M06/6-311G level. The results obtained by M06/6-311G level indicate adsorption of the drug into nanotubes is physical. The obtained interaction energy and obtained bonding energy values with standard method were corrected by basis set superposition error (BSSE) on the same level of theory. The binding energy in the solvent phase is lower than the gas phase. Hence, the stability of the studied reaction increases in solvent phase with regard to the gas phase. The NBO analysis shows which the transfer electron can occur from the lone pair of Nitrogen (donor atom) in the Gemcitabine to the $$ \sigma^{ * } $$ orbital of the carbon atoms (acceptor atoms) in single-walled carbon nanotube. As well as the data of the atoms in a molecule (AIM) theory showed which the N17–C61 bond is a partial covalent bond.

A hemilabile diphosphine pyridine pincer ligand: σ- and π-binding in molybdenum coordination complexes

A series of molybdenum compounds supported by a hemilabile pyridine diphosphine pincer ligand have been synthesized. The ligand demonstrates variable binding modes, adapting to the electronic and geometric requirements of the metal center. In Mo0 and MoII polycarbonyl complexes, coordination through the σ-donating pyridine nitrogen lone pair is observed. Upon oxidative group transfer with anionic azide, a MoIV nitride compound is formed, accompanied by a change in the ancillary ligand binding mode to the pyridine π-system. The σ-coordination was restored by subsequent functionalization of the nitride moiety with either silyl electrophiles or protons. Protonation results in redox disproportionation with concomitant nitride functionalization. Characterization by single crystal X-ray diffraction, nuclear magnetic resonance, and infrared spectroscopy is discussed.