Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N−H⋅⋅⋅O and C−H⋅⋅⋅O Hydrogen Bond Networks**

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C₁₂H₃₀N₈)]³⁺ ⋅ 3 NO₃⁻ is reported to undergo a disorder-order phase transition at T_c1 ≈133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T_c1. The modulation wave vector varies as function of temperature and locks in at T_c3≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T_c1<T<T_c3. Distinctive levels of competitions: trade-off between longer N−H⋅⋅⋅O and shorter C−H⋅⋅⋅O hydrogen bonds; steric constraints to dense C−H⋅⋅⋅O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock-in phase.     

Spectroscopic and Crystallographic Characterization of the R3N+−C−H⋅⋅⋅X Interaction

As appreciation for nonclassical hydrogen bonds has progressively increased, so have efforts to characterize these interesting interactions. Whereas several kinds of C−H hydrogen bonds have been well-studied, much less is known about the R₃N⁺−C−H⋅⋅⋅X variety. Herein, we present crystallographic and spectroscopic evidence for the existence of these interactions, with special relevance to Selectfluor chemistry. Of particular note is the propensity for Lewis bases to engage in nonclassical hydrogen bonding over halogen bonding with the electrophilic F atom of Selectfluor. Further, the first examples of ¹H NMR experiments detailing R₃N⁺−C−H⋅⋅⋅X (X=O, N) hydrogen bonds are described.     

N−H Bond Formation at a Diiron Bridging Nitride

Despite their connection to ammonia synthesis, little is known about the ability of iron-bound, bridging nitrides to form N−H bonds. Herein we report a linear diiron bridging nitride complex supported by a redox-active macrocycle. The unique ability of the ligand scaffold to adapt to the geometric preference of the bridging species was found to facilitate the formation of N−H bonds via proton-coupled electron transfer to generate a μ-amide product. The structurally analogous μ-silyl- and μ-borylamide complexes were shown to form from the net insertion of the nitride into the E−H bonds (E=B, Si). Protonation of the parent bridging amide produced ammonia in high yield, and treatment of the nitride with PhSH was found to liberate NH₃ in high yield through a reaction that engages the redox-activity of the ligand during PCET.     

Intramolecular N−H⋅⋅⋅F Hydrogen Bonding Interaction in a Series of 4-Anilino-5-Fluoroquinazolines: Experimental and Theoretical Characterization of Electronic and Conformational Effects

The 4-anilino-6,7-ethylenedioxy-5-fluoroquinazoline scaffold is presented as a novel model system for the characterization of the weak NH⋅⋅⋅F hydrogen bonding (HB) interaction. In this scaffold, the aniline NH proton is forced into close proximity with the nearby fluorine (d_H,F∼2.0 Å, ∠∼138°), and a through-space interaction is observed by NMR spectroscopy with couplings (^1hJ_NH,F) of 19±1 Hz. A combination of experimental (NMR spectroscopy and X-ray crystallography) and theoretical methods (DFT calculations) were used for the characterization of this weak interaction. In particular, the effects of conformational rigidity and steric compression on coupling were investigated. This scaffold was used for the direct comparison of fluoride with methoxy as HB acceptors, and the susceptibility of the NH⋅⋅⋅F interaction to changes in electron distribution and resonance was probed by preparing a series of molecules with different electron-donating or -withdrawing groups in the positions para to the NH and F. The results support the idea that fluorine can act as a weak HB acceptor, and the HB strength can be modulated through additive and linear electronic substituent effects.     

New Type of Aromatic π-Systems for Anion Recognition: Strong Anion-π and C−H⋅⋅⋅Anion Interactions Between Halides and Aromatic Ligands in Half-Sandwich Compounds

Half-sandwich compounds of benzene, cyclopentadienyl, pentamethylcyclopentadienyl, and indenyl were studied as a new type of aromatic π-systems for interactions with halide anions. Although uncoordinated benzene forms only C−H⋅⋅⋅anion interactions, and hexafluorobenzene forms only anion-π interactions, aromatic ligands in half-sandwich compounds can form both types of interactions, because their entire electrostatic potential surface is positive. These aromatic ligands can form stronger anion-π interactions than organic aromatic molecules, as a consequence of more pronounced dispersion and induction energy components. Moreover, C−H⋅⋅⋅anion interactions of aromatic ligands are stronger than anion-π interactions, and significantly stronger than C−H⋅⋅⋅anion interactions of benzene. Our study shows that transition-metal coordination can make aromatic moieties suitable for strong interactions with anions, and gives insight into the design of new anion receptors.     

Can the substituent in the para position of anilide ion influence the N−···H–F → N–H···F− switching: a quantum chemical study

The effect of substituents in the para position of anilide ion (An) on the N^?···H–F → N–H···F^? switching in X–An–HF (X = H, Me, CHO, CN, NO, F, NO₂, OH and OMe) complexes was investigated by means of B3LYP and MP2 quantum chemical methods. To delve into the mechanistic details of the proton transfer process, potential energy curve and further geometrical parameters involved in H-bonding during the course of the proton transfer process were evaluated at the MP2/6-311++G(2d,2p) level of theory. The changes in H-bond strength because of variation of substituents were well accompanied by changes in formation energy of complexes, structural parameter, electron density, natural charge and charge transfer between subunits. For X = H, Me, CHO, CN, NO, F and NO₂ substituents, our results at MP2/6-311++G(2d,2p) level showed that the minimum energy structures correspond to the N···HF H-bonded complexes without proton transfer occurring. On the other hand, for electron-donating substituents OH and OMe, proton is transferred from HF to anilide ion and the minimum energy structures are HNH···F^? H-bonded complexes. The nature of HN^?···HF and HN–H···F^? interactions in complexes was characterized by means of atoms in molecules and natural bond orbital analyses.     

Dynamics of lysine side-chain amino groups in a protein studied by heteronuclear 1H−15N NMR spectroscopy

Despite their importance in macromolecular interactions and functions, the dynamics of lysine side-chain amino groups in proteins are not well understood. In this study, we have developed the methodology for the investigations of the dynamics of lysine NH3(+) groups by NMR spectroscopy and computation. By using 1H?15N heteronuclear correlation experiments optimized for 15NH3(+) moieties, we have analyzed the dynamic behavior of individual lysine NH3(+) groups in human ubiquitin at 2 °C and pH 5. We modified the theoretical framework developed previously for CH3 groups and used it to analyze 15N relaxation data for the NH3(+) groups. For six lysine NH3(+) groups out of seven in ubiquitin, we have determined model-free order parameters, correlation times for bond rotation, and reorientation of the symmetry axis occurring on a pico- to nanosecond time scale. From CPMG relaxation dispersion experiment for lysine NH3(+) groups, slower dynamics occurring on a millisecond time scale have also been detected for Lys27. The NH3(+) groups of Lys48, which plays a key role as the linkage site in ubiquitination for proteasomal degradation, was found to be highly mobile with the lowest order parameter among the six NH3(+) groups analyzed by NMR. We compared the experimental order parameters for the lysine NH3(+) groups with those from a 1 μs molecular dynamics simulation in explicit solvent and found good agreement between the two. Furthermore, both the computer simulation and the experimental correlation times for the bond rotations of NH3(+) groups suggest that their hydrogen bonding is highly dynamic with a subnanosecond lifetime. This study demonstrates the utility of combining NMR experiment and simulation for an in-depth characterization of the dynamics of these functionally most important side-chains of ubiquitin.     

A Spiro Phosphamide Catalyzed Enantioselective Proton Transfer of Ylides in a Free Carbene Insertion into N−H Bonds

Free carbene readily causes multiple side reactions due to its high energy, thus its asymmetric transformation is very difficult. We present here our findings of high-pK_a Brønsted acid catalysts that enable free carbene insertion into N−H bonds of amines to prepare chiral α-amino acid derivatives with high enantioselectivity. Under irradiation with visible light, diazo compounds produce high-energy free carbenes that are captured by amines to form free ylide intermediates, and then the newly designed high-pK_a Brønsted acids, chiral spiro phosphamides, promote the proton transfer of ylides to afford the products. Computational and kinetic studies uncover the principle for the rational design of proton-transfer catalysts and explain how the catalysts accelerate this transformation and provide stereocontrol.     

Low-Temperature Isolation of a Labile Silylated Hydrazinium-yl Radical Cation, [(Me3Si)2N−N(H)SiMe3].+

The oxidation of silylated hydrazine, (Me₃Si)₂N−N(H)SiMe₃, with silver salts led to the formation of a highly labile hydrazinium-yl radical cation, [(Me₃Si)₂N−N(H)SiMe₃]^.+, at very low temperatures (decomposition > −40 °C). EPR, NMR, DFT and Raman studies revealed the formation of a nitrogen-centered radical cation along the N−N unit of the hydrazine. In the presence of the weakly coordinating anion [Al{OCH(CF₃)₂}₄]⁻, crystallization and structural characterization in the solid state were achieved. The hydrazinium-yl radical cation has a significantly shortened N−N bond and a nearly planar N₂Si₃ framework, in contrast to the starting material. According to DFT calculations, the shortened N−N bond has a total bond order of 1.5 with a π-bond order of 0.5. The π bond can be regarded as a three-π-electron, two-center bond.     

Frustrated Lewis Pair Chemistry Enables N2 Borylation by Formal 1,3-Addition of a B−H Bond in the Coordination Sphere of Tungsten

The first example of a formal 1,3-B−H bond addition across the M−N≡N unit of an end-on dinitrogen complex has been achieved. The use of Piers’ borane HB(C₆F₅)₂ was essential to observe this reactivity and it plays a triple role in this transformation: 1) electrophilic N₂-borylation agent, 2) Lewis acid in a frustrated Lewis pair-type B−H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid-state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2-bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N₂ occurred without hydride transfer.     

Change of intensity of a1Δg → X3Σ g − transition in oxygen molecule upon interaction with H2, N2, and CS2

The semiempirical MINDO/3 CI method has been used to calculate collision complexes of the oxygen molecule with certain diamagnetic molecules, simulating the effect of solvents in increasing the intensity of the a¹_g X³ _g ^– transition in the oxygen molecule.Cherkassy Engineering Technology Institute, 460 Shevchenko Street, Cherkassy 257006, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 143–146, May–June, 1996. Original article submitted July 12, 1995.     

The thioantimonate anion [Sb4S8]4− acting as a tetradentate ligand: Solvothermal synthesis,crystal structure and properties of {[In(C6H14N2)2]2Sb4S8}Cl2 exhibiting unusual uniaxial negative and biaxial positive thermal expansion

The new compound {[In(C₆H₁₄N₂)₂]₂Sb₄S₈}Cl₂ was prepared under solvothermal conditions reacting InCl₃, Sb and S using 1,2-trans-diaminocyclohexane as solvent and structure directing molecule. The compound crystallizes in the monoclinic space group C2/c with a = 29.0259(12), b = 6.7896(2), c = 24.2023(12) Å, β = 99.524(4)°, V = 4703.9(3) ų. The central structural motif is the thioantimonate(III) anion [Sb₄S₈]^4? acting as a tetradentate ligand thus joining two symmetry related In³⁺ centered complexes. This binding mode was never observed before for the [Sb₄S₈]^4? anion. The optical band gap was determined as 2.03 eV in agreement with the red color of the compound. The thermal decomposition was monitored with in-situ X-ray diffraction experiments. After the emission of the amine molecules an amorphous intermediate is formed followed by the crystallization of InSbS₃ which is stable up to about 590 °C. On further heating, InSbS₃ is destroyed and reflections of γ-In₂S₃ appear being contaminated with some elemental Sb. Temperature dependent in-situ X-ray powder diffractometry performed between 30 and 220 °C reveals an unusual reversible negative and positive thermal expansion. The decrease of the a-axis in the temperature range is about 0.74 Å and the increase of the c-axis ca. 0.54 Å. Interestingly, the b-axis exhibits also a thermal expansion, i.e., a biaxial positive and an uniaxial negative thermal expansion coexist which is very unusual. The relative negative expansion coefficients for the a-axis of ?194 × 10^?6K^?1 (30–120 °C) and ?82 × 10^?6K^?1 (120–220 °C) are in the region of so-called colossal thermal expansion.