The Importance of 1,5-Oxygen⋅⋅⋅Chalcogen Interactions in Enantioselective Isochalcogenourea Catalysis

The importance of 1,5-O⋅⋅⋅chalcogen (Ch) interactions in isochalcogenourea catalysis (Ch=O, S, Se) is investigated. Conformational analyses of N-acyl isochalcogenouronium species and comparison with kinetic data demonstrate the significance of 1,5-O⋅⋅⋅Ch interactions in enantioselective catalysis. Importantly, the selenium analogue demonstrates enhanced rate and selectivity profiles across a range of reaction processes including nitronate conjugate addition and formal [4+2] cycloadditions. A gram-scale synthesis of the most active selenium analogue was developed using a previously unreported seleno-Hugerschoff reaction, allowing the challenging kinetic resolutions of tertiary alcohols to be performed at 500 ppm catalyst loading. Density functional theory (DFT) and natural bond orbital (NBO) calculations support the role of orbital delocalization (occurring by intramolecular chalcogen bonding) in determining the conformation, equilibrium population, and reactivity of N-acylated intermediates.     

Multivalent C−H⋅⋅⋅Cl/Br−C Interactions Directing the Resolution of Dynamic and Twisted Capsules

In this work, we report a mechanism by which stereoisomeric and twisted capsules P/M- 1 direct their dynamic chirality in the presence of haloalkane guests. The capsule comprises a static, but twisted, cage that is linked to a dynamic tris(2-pyridylmethyl)amine (TPA) lid at its top. From the results of experimental (NMR spectroscopy and X-ray crystallography) and computational (DFT) studies, the TPA lid was shown to assume clockwise (+) and counterclockwise (−) folds with diastereomeric (but racemic) capsules M- 1 (+) and M- 1 (−) interconverting at a rapid rate (ΔG^≠_189K=9.1 kcal mol⁻¹). The relative stability of the capsules was found to be a function of guest(s) residing in their interior (243/262 ų) with small CH₂Cl₂ (61 ų) yielding roughly equal population of diastereomeric inclusion complexes. Larger guests, such as CCl₄ (89 ų) and CBr₄ (108 ų), however, formed M- 1 (−)⊂CX₄ at the expense of M- 1 (+)⊂CX₄ in circa 3:1 ratio. To account for the observation, theory (DFT:M06-2X/6–31+G*) and experiments (¹H NMR spectroscopy) were used to deduce that CX₄ guests become localized inside the twisted cage of the capsule by forming a C−X⋅⋅⋅π halogen bond [N_c=d/(r_H+r_X)=0.91–0.92] with the benzene “floor” while encountering electrostatic repulsions with closer naphthalimide boundaries. At last, the TPA lid used its central methylene hydrogens to establish, within the M- 1 (−)⊂CX₄, three stabilizing C−H⋅⋅⋅X−C interactions with the guest. The same C−H⋅⋅⋅X−C interactions, however, became weaker (or possibly vanished) after the conformational reorganization of the lid and the formation of less stable M- 1 (+)⊂CX₄ complex. On individual basis, C−H⋅⋅⋅X−C intermolecular contacts are weak and hardly detectable in the solution phase. In the case of capsule P/M- 1 , however, these contacts were multivalent and altogether strong enough to direct the host's dynamic chirality.     

C−H and C−M Activation,Aromaticity Tuning,and Co⋅⋅⋅Ru Interactions Confined in the Azuliporphyrin Framework

Taking advantage of the specific properties of azuliporphyrin and the reactivity of cobalt(II), activation of an azulene C(sp²)−H bond occurred and organometallic complexes with Co−C bonding were formed. The system allowed for macrocyclic aromaticity tuning through metal coordination and oxidation. Thanks to the Co^II−C and parallel tested Cu^II−C reactivity and the affinity of metal centers to dioxygen, oxygen atom insertion into the M−C bond could be investigated. Insertion starts with an oxygen molecule coordination and leads to monomeric and dimeric complexes of specific electronic structures. Formation of unique paramagnetic σ/π-hybrid bimetallic complexes enabled spectroscopic and theoretical investigations of peculiar Co^II⋅⋅⋅Ru⁰ interactions.     

Observation of CH⋅⋅⋅π Interactions between Methyl and Carbonyl Groups in Proteins

Protein structure and function is dependent on myriad noncovalent interactions. Direct detection and characterization of these weak interactions in large biomolecules, such as proteins, is experimentally challenging. Herein, we report the first observation and measurement of long-range “through-space” scalar couplings between methyl and backbone carbonyl groups in proteins. These J couplings are indicative of the presence of noncovalent C−H⋅⋅⋅π hydrogen-bond-like interactions involving the amide π network. Experimentally detected scalar couplings were corroborated by a natural bond orbital analysis, which revealed the orbital nature of the interaction and the origins of the through-space J couplings. The experimental observation of this type of CH⋅⋅⋅π interaction adds a new dimension to the study of protein structure, function, and dynamics by NMR spectroscopy.     

Nature and Strength of Weak O⋅⋅⋅O Interactions in Nitryl Halide Dimers

The use of real space functions and molecular graphs has pushed some chemists to wonder: Are interactions between negatively charged oxygen atoms possible? In this contribution we analyze whether there is a real interaction between oxygen atoms in nitryl halide dimers (XNO₂)₂ (X=F, Cl, Br and I) and in tetranitromethane and derivatives. Based on ab-initio and density functional theories (DFT) methods, we show these complexes are weakly stabilized. Energy decomposition analyses based on local molecular orbitals (LMOEDA) and interacting quantum atoms (IQA) reveal both dispersion and exchange play a crucial role in the stabilization of these complexes. Electron charge density and IQA analyses indicate that the oxygen atoms are connected by privileged exchange channels. In addition, electrostatic interactions between O and N atoms are also vital for the stabilization of the complexes. Finally, a reasonable explanation is given for the dynamic behavior of nitryl groups in tetranitromethane and derivatives.     

Anharmonic C?H and C=O Interactions in Peptide and Sugar

C?H and C=O stretching modes are two among many structural and dynamic probes of proteins and peptides in condensed phases. Anharmonic properties of these two modes in peptide and sugar have been examined using a second-order perturbative vibrational approach. High order force constants were obtained and examined to ˉnd how crucial they are in determining the degree of mode localization and the nature of mode anharmonicity of the two stretching modes. It is found that the C?H mode is highly localized,and its diagonal anharmonicity is mainly determined by the mode itself. However, the C=O mode is largely delocalized, and the diagonal anharmonicity involves contributions from other modes. The o?-diagonal anharmonicity between C?D and C=O modes is found to be negative in deuterated species, di?ering from those of the non-deuterated ones. It is also found that inter-mode interaction between each of the two modes with low-frequency modes contribute signiˉcantly to the o?-diagonal anharmonicity. These low-frequency modes give rise to a network of energy relaxation or intramolecular vibrational energy redistribution pathways which can be used to examine temporal behavior of intramolecular vibration energy °ow, provided a femtosecond broadband two-dimensional infrared spectroscopy is available.     

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.     

On the Relative Importance of O–H/π Interactions in 7-Norbornenols

The geometries of the two H–O–C rotamers of synand two of anti-7-norbornenol have been optimized at the ab initio HF/6-31G** and B3LYP hybrid HF-DFT levels of theory by using a 6-31G** basis set. Contrary to an earlier report, we find that the (nongeometry constrained) anti-trans isomer (1d) is predicted to be more stable than the corresponding syn-cis form (1a). The increased stability of 1d vis-à-vis 1a can be accounted for in terms of relative H(1)–C(2)–O(3)–H(4) torsion energy effects. The computational results indicate that the hydroxyl proton in 1a enters into intramolecular hydrogen bonding with the proximate C=C bond. Supporting evidence for this conclusion resides in the fact that the 1a is predicted to possess the lowest O–H stretching frequency, a result that can be attributed to -hydrogen bonding.     

Monohydrates of cuprous chloride and argentous chloride: H2O⋅⋅⋅CuCl and H2O⋅⋅⋅AgCl characterized by rotational spectroscopy and ab initio calculations

Pure rotational spectra of the ground vibrational states of ten isotopologues of each of H(2)O???CuCl and H(2)O???AgCl have been measured and analyzed to determine rotational constants and hyperfine coupling constants for each molecule. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the CCSD(T) level. Both experiment and theory are consistent with structures that are nonplanar at equilibrium. The heavy atoms are collinear while the local C(2) axis of the water molecule intersects the axis defined by the heavy atoms at an angle, φ = 40.9(13)° for Cu and φ = 37.4(16)° for Ag. In the zero-point state, each molecule is effectively planar, undergoing rapid inversion between two equivalent structures where φ has equal magnitude but opposite sign. The equilibrium geometry has C(s) symmetry, however. The ab initio calculations confirm that the timescale of this inversion is at least an order of magnitude faster than that of rotation of the molecule in the lowest rotational energy levels. The molecular geometries are rationalized using simple rules that invoke the electrostatic interactions within the complexes. Centrifugal distortion constants, Δ(J) and Δ(JK), nuclear quadrupole coupling constants, χ(aa)(Cu), χ(aa)(Cl), (χ(bb) - χ(cc))(Cu), and (χ(bb) - χ(cc))(Cl), and the nuclear spin-rotation constant of the copper atom, C(bb)(Cu)+C(cc)(Cu), are also presented.     

Anion⋅⋅⋅Anion Interactions Involving σ-Holes of Perrhenate,Pertechnetate and Permanganate Anions

In this communication experimental and theoretical results are reported affording strong evidence that interactions between electron rich atoms and the metal of tetroxide anions of group 7 elements are a new case of attractive and σ-hole interactions. Single crystal X-ray analyses, molecular electrostatic potentials, quantum theory of atoms-in-molecules, and noncovalent interaction plot analyses show that in crystalline permanganate and perrhenate salts the metal in Mn/ReO₄⁻ anion can act as electron acceptors, the oxygen of another Mn/ReO₄⁻ anion can act as the donor and supramolecular anionic dimers or polymers are formed. The name matere bond (MaB) is proposed to categorize these noncovalent interactions and to differentiate them from the classical metal-ligand coordination bond.     

Mapping C−H⋅⋅⋅M Interactions in Confined Spaces: (α-ICyDMe)Au,Ag, Cu Complexes Reveal “Contra-electrostatic H Bonds” Masquerading as Anagostic Interactions**

What happens when a C−H bond is forced to interact with unpaired pairs of electrons at a positively charged metal? Such interactions can be considered as “contra-electrostatic” H-bonds, which combine the familiar orbital interaction pattern characteristic for the covalent contribution to the conventional H-bonding with an unusual contra-electrostatic component. While electrostatics is strongly stabilizing component in the conventional C−H⋅⋅⋅X bonds where X is an electronegative main group element, it is destabilizing in the C−H⋅⋅⋅M contacts when M is Au(I), Ag(I), or Cu(I) of NHC−M−Cl systems. Such remarkable C−H⋅⋅⋅M interaction became experimentally accessible within (α-ICyD^Me)MCl, NHC-Metal complexes embedded into cyclodextrins. Computational analysis of the model systems suggests that the overall interaction energies are relatively insensitive to moderate variations in the directionality of interaction between a C−H bond and the metal center, indicating stereoelectronic promiscuity of fully filled set of d-orbitals. A combination of experimental and computational data demonstrates that metal encapsulation inside the cyclodextrin cavity forces the C−H bond to point toward the metal, and reveals a still attractive “contra-electrostatic” H-bonding interaction.     

Through-Space Spin Coupling in a Silver(II) Porphyrin Dimer upon Stepwise Oxidations: AgII⋅⋅⋅AgII,AgII⋅⋅⋅AgIII,and AgIII⋅⋅⋅AgIII Metallophilic Interactions

Metallophilic interactions between closed-shell metal ions are becoming a popular tool for a variety of applications related to high-end materials. Heavier d⁸ transition-metal ions are also considered to have a closed shell and can be involved in such interactions. There is no systematic investigation so far to estimate the structure and energy characteristics of metallophilic interactions in Ag^II/Ag^II (d⁹/d⁹), Ag^III/Ag^III (d⁸/d⁸), and mixed-valent Ag^II/Ag^III (d⁹/d⁸) complexes, which have been demonstrated in the present study. Both interporphyrinic and intermetallic interactions were investigated on stepwise oxidation by using a rigid ethene-bridged cis silver(II) porphyrin dimer and the results compared with those for highly flexible ethane-bridged analogues. By controlling the nature of chemical oxidants and their stoichiometry, both 1e and 2e oxidations were done stepwise to generate Ag^II/Ag^III mixed-valent and Ag^III/Ag^III porphyrin dimers, respectively. Unlike all other ethene-bridged metalloporphyrin dimers reported earlier, in which 2e oxidation stabilizes only the trans form, such an oxidation of silver(II) porphyrin dimer stabilizes only the cis form because of the metallophilic interaction. Besides silver(II) ⋅⋅⋅ silver(II) interactions in cis silver(II) porphyrin dimer, stepwise oxidations also enabled us to achieve various hitherto-unknown silver(II) ⋅⋅⋅ silver(III) and silver(III) ⋅⋅⋅ silver(III) interactions, which thereby allow significant modulation of their structure and properties. The strength of Ag ⋅⋅⋅ Ag interaction follows the order Ag^II/Ag^II (d⁹/d⁹)<Ag^II/Ag^III (d⁹/d⁸)<Ag^III/Ag^III (d⁸/d⁸). Single-crystal XRD, X-ray photoelectron spectroscopy (XPS), ¹H NMR and EPR spectroscopy, and variable-temperature magnetic investigations revealed various oxidation states of silver and metallophilic interactions, which are also well supported by computational analysis.     

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.     

Noncovalent Axial I⋅⋅⋅Pt⋅⋅⋅I Interactions in Platinum(II) Complexes Strengthen in the Excited State

Coordination compounds of platinum(II) participate in various noncovalent axial interactions involving metal center. Weakly bound axial ligands can be electrophilic or nucleophilic; however, interactions with nucleophiles are compromised by electron density clashing. Consequently, simultaneous axial interaction of platinum(II) with two nucleophilic ligands is almost unprecedented. Herein, we report structural and computational study of a platinum(II) complex possessing such intramolecular noncovalent I⋅⋅⋅Pt⋅⋅⋅I interactions. Structural analysis indicates that the two iodine atoms approach the platinum(II) center in a “side-on” fashion and act as nucleophilic ligands. According to computational studies, the interactions are dispersive, weak and anti-cooperative in the ground electronic state, but strengthen substantially and become partially covalent and cooperative in the lowest excited state. Strengthening of I⋅⋅⋅Pt⋅⋅⋅I contacts in the excited state is also predicted for the sole previously reported complex with analogous axial interactions.     

Exploring the C–H…O Interactions in Glycoproteins

Glycoproteins are an important class of proteins that play a significant role in many cellular events. In the present study, we analyze the influence of C–H…O interactions in relation to other environmental preferences in glycoproteins. CH…O interactions are now accepted as a genuine hydrogen bond. Main chain–main chain interactions are predominant. Proline residues stabilize strands by C–H…O interactions in glycoproteins. Majority of the C–H…O interacting residues were conserved and had one or more stabilization centers. CH…O interactions might be responsible for the global conformational stability, since long-range CH…O contacts were predominant. The results presented in this study might be useful for structural stability studies in glycoproteins.     

Weak Interactions Initiate C−H and C−C Bond Dissociation of Ethane on Nbn+ Clusters

The conversion of ethane into value-added chemicals under ambient conditions has attracted much attention but the mechanisms remain not fully understood. Here we report a study on the reaction of ethane with thermalized Nb_n⁺ clusters based on a multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). It is found that ethane reacts with Nb_n⁺ clusters to form both products of dehydrogenation and methane-removal (odd-carbon products). Combined with density functional theory (DFT) calculations, we studied the reaction mechanisms of the C−C bond activation and C−H bond cleavage on the Nb_n⁺ clusters. It is unveiled that hydrogen atom transfer (HAT) initiates the reaction process, giving rise to the formation of Nb−C bonds and an elongated C−C distance in the HNb_n⁺CH₂CH₃ motif. Subsequent reactions allow for C−C bond activation and a competitive HAT process which is associated with CH₄ removal or H₂ release, resulting in the production of the observed carbides.     

Proving and Probing the Presence of the Elusive C−H⋅⋅⋅O Hydrogen Bond in Liquid Solutions at Room Temperature

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C−H group and an electron-rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C−H group and an amide carbonyl oxygen atom in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6–20 kJ mol⁻¹. Ab-initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen-bond making and breaking.     

Balancing Donor-Acceptor and Dispersion Effects in Heavy Main Group Element π Interactions: Effect of Substituents on the Pnictogen⋅⋅⋅π Arene Interaction

High-level ab initio calculations using the DLPNO-CCSD(T) method in conjunction with the local energy decomposition (LED) were performed to investigate the nature of the intermolecular interaction in bismuth trichloride adducts with π arene systems. Special emphasis was put on the effect of substituents in the aromatic ring. For this purpose, benzene derivatives with one or three substituents (R=NO₂, CF₃, OCHO, OH, and NH₂) were chosen and their influence on donor-acceptor interaction as well as on the overall interaction strength was examined. Local energy decomposition was performed to gain deeper insight into the composition of the interaction. Additionally, the study was extended to the intermolecular adducts of arsenic and antimony trichloride with benzene derivatives having one substituent (R=NO₂ and NH₂) in order to rationalize trends in the periodic table. The analysis of natural charges and frontier molecular orbitals shows that donor-acceptor interactions are of π→σ* type and that their strength correlates with charge transfer and orbital energy differences. An analysis of different bonding motifs (Bi⋅⋅⋅π arene, Bi⋅⋅⋅R, and Cl⋅⋅⋅π arene) shows that if dispersion and donor-acceptor interaction coincide as the donor highest occupied molecular orbital (HOMO) of the arene is delocalized over the π system, the M⋅⋅⋅π arene motif is preferred. If the donor HOMO is localized on the substituent, R⋅⋅⋅π arene bonding motifs are preferred. The Cl⋅⋅⋅π arene bonding motif is the least favorable with the lowest overall interaction energy.     

Importance of aliphatic C–H hydrogen bonding on anion recognition

We have designed and synthesised new anion receptors 1 and 2, both of their C–H groups were at the α positions to carbonyl groups and further polarised by the attached polarising substituents. This enabled us to study hydrogen bonding donor ability of C–H bonds. The polarising substituents are electron withdrawing cyano group for host 1, while charged pyridinium group for host 2. As expected from charge effects, host 2 shows roughly an order of magnitude higher binding constants against various anion guests than those of receptor 1. Since the magnitude of polarisation change should be greatest for C–H group among various hydrogen bonding groups, this indicates the importance of C–H hydrogen bonding. In contrast, the relative order of binding constants was the same for both host 1 and 2. The order of association constants was found to be (CH₃)₂POO^? > CH₃COO^? > C₆H₅COO^? > Cl^? > Br^?. DFT calculation results were in good agreement with experimental binding constants and confirmed the importance of charged group substitution. In addition, receptor 1 showed the highest association constant for dimethyl phosphinate, which is implicated in many metabolic diseases.     

Syntheses and Structures of the Hexacoordinated Antimony Complexes: Ph3(C2H4O2)Sb⋅⋅⋅DMSO, (3-FC6H4)3(C2H4O2)Sb⋅⋅⋅DMSO,and (4-MeC6H4)3(C6H4O2)Sb⋅⋅⋅DMSO

Russian Journal of Coordination Chemistry - Complexes Ph3(C2H4O2)Sb???DMSO (I), (3-FC6H4)3(C2H4O2)Sb???DMSO (II), and...