Dihydrogen bonding: structures, energetics, and dynamics

Photoelectron spectroscopy and computational investigations of the electronic structures and noncovalent interactions of cyclodextrin-closo-dodecaborate anion complexes χ-CD·B12X122− (χ = α, β, γ; X = H, F)

We report a joint negative ion photoelectron spectroscopy and computational study on the electronic structures and noncovalent interactions of a series of cyclodextrin-closo-dodecaborate dianion complexes, χ-CD·B₁₂X₁₂²⁻ (χ = α, β, γ; X = H, F).

Prototypical cyclohexane dimers: spectroscopic evidence for σ stacking at low temperatures

In this work, the first unambiguous spectroscopic evidence for the existence of σ stacking interactions in cyclohexane dimers has been provided using matrix isolation infrared spectroscopy.

Chemical modification of neuropeptide Y for human Y1 receptor targeting in health and disease

As a very abundant neuropeptide in the brain and widely distributed peptide hormone in the periphery, neuropeptide Y (NPY) appears to be a multisignaling key peptide. Together with peptide YY, pancreatic polypeptide and the four human G protein-coupled receptor subtypes hY1R, hY2R, hY4R and hY5R it forms the NPY/hYR multiligand/multireceptor system, which is involved in essential physiological processes as well as in human diseases. In particular, NPY-induced hY1R signaling plays a central role in the regulation of food intake and stress response as well as in obesity, mood disorders and cancer. Thus, several hY1R-preferring NPY analogs have been developed as versatile tools to unravel the complex NPY/hY1R signaling in health and disease. Further, these peptides provide basic lead structures for the development of innovative drugs. Here, the current research is summarized focusing on the development of differently sized hY1R-preferring NPY analogs as well as their advances with respect to hY1R profiling, potential therapeutic applications and targeted cancer imaging and therapy. Finally, major limitations and innovative strategies for next generation hY1R-preferring NPY analogs are addressed.

Bio-mimetic self-assembled computationally designed catalysts of Mo and W for hydrogenation of CO2/dehydrogenation of HCOOH inspired by the active site of formate dehydrogenase

Density functional theory modelling has been used to design Mo and W-based catalysts MoIII(tBu)(CO) and WIII(tBu)(CO) for CO2 hydrogenation and HCOOH dehydrogenation, which are bio-mimics of the active site of formate dehydrogenase. Based on DFT calculations, the molybdenum and tungsten based complexes are good catalysts in the +3 oxidation state for CO2 hydrogenation with free energies of 24.03 and 21.31 kcal mol-1, respectively. Such a low barrier indicates that our newly designed Mo and W-based complexes are very efficient for CO2 hydrogenation or HCOOH dehydrogenation catalysis. Overall, our computational results provide in depth insights that can serve as a great tool for the design and development of new and efficient molybdenum and tungsten based catalysts for CO2 hydrogenation or HCOOH dehydrogenation.

Metallacarboranes as a tool for enhancing the activity of therapeutic peptides

Metallacarboranes are anionic boron clusters with high affinity to serum albumin, ability to cross biological membranes, and no apparent toxicity in vitro and in vivo. Thus, conjugation with cobalt bis(1,2-dicarbollide), [COSAN]^- , ([3,3'-Co(1,2-C₂ B₉ H₁₁ )₂ ]^- ) may improve the properties of therapeutic peptides or proteins at both molecular and systemic levels. Here, we conjugated [COSAN]^- with the therapeutic peptide thymosin β4 (Tβ4), which has a pleiotropic activity that results in enhanced healing and regeneration of injured tissues. Using fluorescence quenching of human serum albumin and surface plasmon resonance techniques, we showed that the conjugates have a high affinity to human serum albumin. Using an in vitro wound closure assay, we showed that conjugation with [COSAN]^- enhances the activity of Tβ4 toward fibroblasts (MSU1.1 cell line). These results indicate an application of metallacarboranes in the development of analogs of various therapeutic peptides/proteins with superior pharmacological properties.

Dihydrogen Bonds in Salts of Boron Cluster Anions [BnHn]2− with Protonated Heterocyclic Organic Bases

Dihydrogen bonds attract much attention as unconventional hydrogen bonds between strong donors of H-bonding and polyhedral (car)borane cages with delocalized charge density. Salts of closo-borate anions [B10H10]2− and [B12H12]2− with protonated organic ligands 2,2’-dipyridylamine (BPA), 1,10-phenanthroline (Phen), and rhodamine 6G (Rh6G) were selectively synthesized to investigate N−H...H−B intermolecular bonding. It was found that the salts contain monoprotonated and/or diprotonated N-containing cations at different ratios. Protonation of the ligands can be implemented in an acidic medium or in water because of hydrolysis of metal cations resulting in the release of H3O+ cations into the reaction solution. Six novel compounds were characterized by X-ray diffraction and FT-IR spectroscopy. It was found that strong dihydrogen bonds manifest themselves in FT-IR spectra that allows one to use this technique even in the absence of crystallographic data.

Borohydride-containing coordination polymers: synthesis, air stability and dehydrogenation

Control of the reactivity of hydride (H-) in crystal structures has been a challenge because of its strong electron-donating ability and reactivity with protic species. For metal borohydrides, the dehydrogenation activity and air stability are in a trade-off, and control of the reactivity of BH4 - has been demanded. For this purpose, we synthesize a series of BH4 --based coordination polymers/metal-organic frameworks. The reactivity of BH4 - in the structures is regulated by coordination geometry and neighboring ligands, and one of the compounds [Zn(BH4)2(dipyridylpropane)] exhibits both high dehydrogenation reactivity (1.4 wt% at 179 °C) and high air stability (50 RH% at 25 °C, 7 days). Single crystal X-ray diffraction analysis reveals that H δ+···H δ- dihydrogen interactions and close packing of hydrophobic ligands are the key for the reactivity and stability. The dehydrogenation mechanism is investigated by temperature-programmed desorption, in situ synchrotron PXRD and solid-state NMR.

Computational prediction of pentadentate iron and cobalt complexes as a mimic of mono-iron hydrogenase for the hydrogenation of carbon dioxide to methanol

A series of amidate-ligated pentadentate iron and cobalt complexes with N-heterocyclic pyridinol groups were proposed and computationally screened as potential catalysts for CO₂ reduction. Density functional theory calculations reveal a ligand assisted heterolytic H₂ cleavage mechanism with a total free energy barrier of 23.3 kcal mol^-1 for the hydrogenation of CO₂ to methanol catalysed by a pentadentate Co complex with a 2-[bis(pyridine-2-ylmethyl)]amino-N-3,9-purin-2-one ligand.

Formation of an unexpected 3,3-diphenyl-3H-indazole through a facile intramolecular [2 + 3] cycloaddition of the diazo intermediate

The one-pot reaction of 2,6-bis(diphenylmethyl)-4-methoxyaniline with tert-butylnitrite, BTEAC and DABSO in the presence of CuCl2 provided an unexpected 3H-indazole product 8. The structure of the compound was determined by HRMS, IR, NMR and further confirmed by single crystal X-ray crystallography. The compound crystallises in the triclinic P-1 space group, with unit cell parameters a = 9.2107 (4), b = 10.0413 (5), c = 14.4363 (6) Å, α = 78.183 (2), β = 87.625 (2), γ = 71.975 (2)°. The formation of 8 proceeded through a facile intramolecular [2 + 3] cycloaddition of the diazo intermediate 9. The molecules of 8 are organised by edge-face Ar-H···π, face-face π···π, and bifurcated OCH2-H···N interactions. In addition to these, there are Ar-H···H-Ar close contacts, (edge-edge and surrounding inversion centres) arranged as infinite tapes along the a direction.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Operando spectroscopic analyses for the ammonia absorption process of sodium borohydride

The ammonia absorption process of sodium borohydride for ammonia storage has been studied by using the operando NMR and FT-IR measurements under various ammonia pressures. As a result, the characteristic variation in the chemical states of both materials due to the liquefaction has been clarified.

Anion control of tautomeric equilibria: Fe-H vs. N-H influenced by NH···F hydrogen bonding

Addition of excess BF₄^– to the iron hydride [Fe(P^EtN^MeP^Et)(CO)₃H]⁺[B(C₆F₅)₄]^– leads to the NH tautomer, due to NH···F hydrogen bonding.

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(P^EtN^MeP^Et)(CO)₃ (P^EtN^MeP^Et = (Et₂PCH₂)₂NMe) at Fe or N. Protonation of Fe(P^EtN^MeP^Et)(CO)₃ by [(Et₂O)₂H]⁺[B(C₆F₅)₄]^– occurs at the metal to give the iron hydride [Fe(P^EtN^MeP^Et)(CO)₃H]⁺[B(C₆F₅)₄]^–. In contrast, treatment with HBF₄·OEt₂ gives protonation at the iron and at the pendant amine. Both the FeH and NH tautomers were characterized by single crystal X-ray diffraction. Addition of excess BF₄^– to the equilibrium mixture leads to the NH tautomer being exclusively observed, due to NH···F hydrogen bonding. A quantum chemical analysis of the bonding properties of these systems provided a quantification of hydrogen bonding of the NH to BF₄^– and to OTf^–. Treatment of Fe(P^EtN^MeP^Et)(CO)₃ with excess HOTf gives a dicationic complex where both the iron and nitrogen are protonated. Isomerization of the dicationic complex was studied by NOESY NMR spectroscopy.

Biomimetic heterobimetallic architecture of Ni(ii) and Fe(ii) for CO2 hydrogenation in aqueous media. A DFT study

In this work, density functional theory has been employed to design a heterobimetallic catalyst of Ni(ii) and Fe(ii) for the effective CO₂ hydrogenation to HCOOH. Based on computational results, our newly designed catalyst is found to be effective for such conversion reactions with free energy as low as 14.13 kcal mol⁻¹ for the rate determining step. Such a low value of free energy indicates that the NiFe heterobimetallic catalyst can prove to be very efficient for the above said conversion. Moreover, the effects of ligand substitutions at the active metal center and the effects due to various spin states are also explored, and can serve as a great tool for the rational design of NiFe catalyst for CO₂ hydrogenation.

The hydrogenation of CO₂ by our newly designed [NiFe] heterobimetallic catalyst inspired by the active site of [NiFe] hydrogenase.

Noncovalent Interactions between Molecular Hydrogen and the Alkali Fluorides: H-H···F-M (M = Li, Na, K, Rb, Cs). High Level Theoretical Predictions and SAPT Analysis

Various types of hydrogen bonds have been recognized during the past century. In this research, a new type of noncovalent interaction, the dipole-induced hydrogen bond formed between a hydrogen molecule and an alkali halide, H-H···F-M, is studied. Proposed by Zhang and co-workers ( Phys. Chem. Chem. Phys. 2015, 17, 20361), these systems are extensively investigated initially using the "gold standard" CCSD(T) method in conjunction with augmented correlation-consistent polarized core-valence basis sets up to quadruple-ζ. The full triple excitations CCSDT method has been used to further refine the energies. Several properties including geometries, bond energies, vibrarional frequencies, charge distributions, and dipole moments have been reported. The earlier Zhang research considered only the linear H-H···F-M structures. However, we find these linear stationary points to be separated by very small barriers from the much lower lying bent C _s structures. The CCSDT/aug-cc-pCVQZ(-PP) method predicts the dissociation energies for bent H-H···F-M (M = Li, Na, K, Rb, Cs) are 2.76, 2.96, 3.00, 2.89, and 2.49 kcal mol^-1, respectively, suggesting that the H···F hydrogen bond becomes gradually stronger when alkali metal M goes down the periodic table from Li to K but becomes slightly weaker for Rb and even more for Cs. This Li < Na < K > Rb > Cs order is consistent with that for the dipole moments for the isolated MF (M = Li, Na, K, Rb, Cs) diatomics. Symmetry adapted perturbation theory (SAPT) is used to understand these unusual noncovalent interactions.

Molecular Dynamics Simulation of Cyclooxygenase-2 Complexes with Indomethacin closo-Carborane Analogs

Molecular dynamics simulation of carborane-containing ligands in complex with target enzymes is a challenging task due to the unique structure and properties of the carborane substituents and relative lack of appropriate experimental data to help assess the quality of carborane force field parameters. Here, we report results from energy minimization calculations for a series of carborane-amino acid complexes using carborane force field parameters published previously in the literature and adapted for use with the AMBER ff99SB and ff14SB potential functions. These molecular mechanics results agree well with quantum mechanical geometry optimization calculations obtained using dispersion-corrected density functional theory methods, suggesting that the carborane force field parameters should be suitable for more detailed calculations. We then performed molecular dynamics simulations for the 1,2-, 1,7-, and 1,12-dicarba- closo-dodecaborane(12) derivatives of indomethacin methyl ester bound with cyclooxygenase-2. The simulation results suggest that only the ortho-carborane derivative forms a stable complex, in agreement with experimental findings, and provide insight into the possible molecular basis for isomer binding selectivity.

Reversible Coordination of H2 by a Distannyne

The terphenyl tin(II) hydride [Ar^iPr₄Sn(μ-H)]₂ (1) (Ar^iPr₄ = C₆H₃-2,6(C₆H₃-2,6-ⁱPr₂)₂) was shown to form an equilibrium with the distannyne Ar^iPr₄SnSnAr^iPr₄ (2) and H₂ in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H₂ are 2.23 × 10^-4 ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by ¹H NMR spectroscopy and 2.33 × 10^-4 ± 6.2% and 5.86 kcal/mol ± 0.73%, respectively, by UV-vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating of 2 at ca. 100 °C afforded the known pentagonal-bipyramidal Sn₇ cluster Sn₅(SnAr^iPr₄)₂ (3). Mechanistic studies show that 3 is formed from distannyne 2, which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero, and the rate constant is 1.77 × 10^-5 M s^-1 at 100 °C. Hydride 1 was further characterized by cyclic voltammetry, and its pK_a was found to be 18.8(2) via titration with 1,8-diazabicyclo[5.4.0]undec-7-ene. The bond dissociation free energy was estimated to be 51.1 kcal/mol ± 3.4% on the basis of its pK_a and reduction potential. Studies with deuterium indicate ready exchange of D₂ with the hydrides in 1.

Dihydrogen vs. hydrogen bonding in the solvation of ammonia borane by tetrahydrofuran and liquid ammonia

The solvation structures of two systems rich in hydrogen and dihydrogen bonding interactions have been studied in detail experimentally through neutron diffraction with hydrogen/deuterium isotopic substitution.

Microhydration of BH4-: Dihydrogen Bonds, Structure, Stability, and Raman Spectra

Hydridic-to-protonic interactions in unconventional dihydrogen bonding influence the structure, reactivity, and selectivity in solution and in the solid state. In this study, the structure, stability, and Raman spectra of BH₄^- hydrated clusters, [BH₄(H₂O)_n]^- (n = 1-8, 10, 12, 14, 16) are systematically investigated using density functional theory (DFT) at the wB97XD/6-311++g(3df,3pd) basis set level. The successive microhydration process is described to illustrate in detail the changes in dihydrogen bonding with increasing hydration cluster size. The results of DFT calculations indicate that seven or eight water molecules hydrate BH₄^- with a total of 12 dihydrogen bonds in the tetrahedral edge or tetrahedral corner forms, and a maximum of six water molecules in the tetrahedral-edge form. Raman spectra of [BH₄(H₂O)_n]^- show a blue shift in the B-H stretching band due to hydration. Car-Parrinello molecular dynamics simulations verify strong BH₄^- water interactions. The hydration number of BH₄^- is 6.7, with a hydration B-O(W) distance of 3.40 Å, and each hydrogen in BH₄^- bonds with 2.66 hydrogen atoms from water.

A Critical Check for the Role of Resonance in Intramolecular Hydrogen Bonding

Although resonance-assisted H-bonds (RAHBs) are well recognized, the role of π resonance in RAHBs is controversial, as the seemingly enhanced H-bonds in unsaturated compounds may result from the constraints imposed by the σ skeleton. Herein the block-localized wave function (BLW) method, which can derive optimal yet resonance-quenched structures with related physiochemical properties, was employed to examine the correlation between π resonance and the strength of intramolecular RAHBs. Examination of a series of paradigmatic molecules with RAHBs and their saturated analogues showed that it is inappropriate to compare a conjugated system with its saturated counterpart, as they may have quite different σ frameworks. Nevertheless, comparison between a conjugated system and its resonance-quenched (i.e., electron-localized) state, which have identical σ skeletons, shows that in all studied cases, π resonance unanimously reduces the bonding distance by 0.111-0.477 Å, strengthens the bonding by 40-56 %, and redshifts the D-H vibrational frequency by 104-628 cm^-1 . Furthermore, there is an excellent correlation between hydrogen-bonding strength and the classical Coulomb attraction between the hydrogen-bond donor and the acceptor, which suggests that the dominant role of the electrostatic interaction in H-bonds and RAHBs originates from the charge flow from H-bond donors to acceptors through π conjugation.

Attractive PHHP interactions revealed by state-of-the-art ab initio calculations

We report in this work a combined structural and state-of-the-art computational study of homopolar P-HH-P intermolecular contacts. Database surveys have shown the abundance of such surprisingly unexplored contacts, which are usually accompanied by other weak interactions in the solid state. By means of a detailed theoretical study utilizing SAPT(DFT), MP2, SCS-MP2, MP2C and CCSD(T) methods and both aug-cc-pVXZ and aug-cc-pCVXZ (X = D, T, Q, 5) basis sets as well as extrapolation to the CBS limit, we have shown that P-HH-P contacts are indeed attractive and considerably strong. SAPT(DFT) calculations have revealed the dispersive nature of the P-HH-P interaction with only minor contribution of the inductive term, whereas the first-order electrostatic term is clearly overbalanced by the first-order exchange energy. In general the computed interaction energies follow the trend: E ≈ E < E < E. Our results have also shown that the aug-cc-pVDZ (or aug-cc-pCVDZ) basis set is not yet well balanced and that the second-order dispersion energy term is the slowest converging among all SAPT(DFT) energy components. Compared to aug-cc-pVXZ basis sets, their core-correlation counterparts have a modest influence on all supermolecular interaction energies and a negligible influence on both the SAPT(DFT) interaction energy and its components.

Frustration across the periodic table: heterolytic cleavage of dihydrogen by metal complexes

This perspective examines frustrated Lewis pairs (FLPs) in the context of heterolytic cleavage of H2 by transition metal complexes, with an emphasis on molecular complexes bearing an intramolecular Lewis base. FLPs have traditionally been associated with main group compounds, yet many reactions of transition metal complexes support a broader classification of FLPs that includes certain types of transition metal complexes with reactivity resembling main group-based FLPs. This article surveys transition metal complexes that heterolytically cleave H2, which vary in the degree that the Lewis pairs within these systems interact. Many of the examples include complexes bearing a pendant amine functioning as the base with the metal functioning as the hydride acceptor. Consideration of transition metal compounds in the context of FLPs can inspire new innovations and improvements in transition metal catalysis.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Gold(I) Hydrides as Proton Acceptors in Dihydrogen Bond Formation

Wavefunction and DFT calculations indicate that anionic dihydride complexes of Au^I form strong to moderate directed Au-H⋅⋅⋅H bonds with one or two HF, H₂ O and NH₃ prototype proton donor molecules. The largely electrostatic interaction is influenced by relativistic effects which, however, do not increase the binding energy. Very weak Au⋅⋅⋅H associations-exhibiting a corresponding bond path-occur between neutral AuH and HF units, although ultimately F becomes the preferred donor atom in the most stable structure. Increasing the hydridicity of AuH by attachment of an electron donating NHC ligand effects Au-H⋅⋅⋅H bonding of moderate strength only with HF, whereas competing Au⋅⋅⋅H interactions dominate for H₂ O and NH₃ . Rare η² coordinated and HX (X=F or OH) associated H₂ complexes are produced during interaction with a single ion of stronger acidity, H₂ F⁺ or H₃ O⁺ . Theoretically, reaction of excess [AuH₂ ]^- as proton acceptor with H₃ O⁺ or NH₄⁺ in 3:1 or 4:1 ionic ratios, respectively, affords H⋅⋅⋅H bonded analogues of Eigen-type adducts. Outstanding analytical relationships between selected bonding parameters support the integrity of the results.

Anion-Redox Mechanism of MoO(S2)2(2,2'-bipyridine) for Electrocatalytic Hydrogen Production

Redox processes of molybdenum-sulfide (Mo-S) compounds are important in the function of materials for various applications from electrocatalysts for the hydrogen evolution reaction (HER) to cathode materials for batteries. Our group has recently described a series of Mo-S molecular HER catalysts based on a MoO(S₂)₂L₂ structural motif. Herein, reductive pathways of MoO(S₂)₂bpy (Mo-bpy) (bpy = 2,2'-bipyridine) are presented from both experimental and theoretical studies. We tracked chemical reduction of Mo-bpy with UV-vis spectroscopy using sodium napthalenide (NaNpth) as the reducing agent and found that Mo-bpy undergoes anionic persulfide reduction to form the tetragonal Mo(VI) complex [MoOS₃]^2-. We also identified silver mercury amalgam as an inert working electrode (WE) for spectroectrochemical (SEC) studies. UV-vis spectra in the presence of trifluoroacetic acid with an applied potential confirmed that Mo-bpy maintains its structure during catalytic cycling. Finally, theoretical catalytic reaction pathways were explored, revealing that Mo=O may function as a proton relay. This finding together with the observed anion reduction as the redox center is of broad interest for amorphous Mo-S (a-MoS_x) electrocatalytic materials and anion-redox chalcogel battery materials.