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Quantum computers are special purpose machines that are expected to be particularly useful in simulating strongly correlated chemical systems. The quantum computer excels at treating a moderate number of orbitals within an active space in a fully quantum mechanical manner. We present a quantum phase estimation calculation on F$_2$ in a (2,2) active space on Rigetti's Aspen-11 QPU. While this is a promising start, it also underlines the need for carefully selecting the orbital spaces treated by the quantum computer. In this work, a scheme for selecting such an active space automatically is described and simulated results obtained using both the quantum phase estimation (QPE) and variational quantum eigensolver (VQE) algorithms are presented and combined with a subtractive method to enable accurate description of the environment. The active occupied space is selected from orbitals localized on the chemically relevant fragment of the molecule, while the corresponding virtual space is chosen based on the magnitude of interactions with the occupied space calculated from perturbation theory. This protocol is then applied to two chemical systems of pharmaceutical relevance: the enzyme [Fe] hydrogenase and the photosenzitizer temoporfin. While the sizes of the active spaces currently amenable to a quantum computational treatment are not enough to demonstrate quantum advantage, the procedure outlined here is applicable to any active space size, including those that are outside the reach of classical computation., Comment: Submitted to Journal of Computational Chemistry

Computational chemistry is an essential tool in the pharmaceutical industry. Quantum computing is a fast evolving technology that promises to completely shift the computational capabilities in many areas of chemical research by bringing into reach currently impossible calculations. This perspective illustrates the near-future applicability of quantum computation of molecules to pharmaceutical problems. We briefly summarize and compare the scaling properties of state-of-the-art quantum algorithms and provide novel estimates of the quantum computational cost of simulating progressively larger embedding regions of a pharmaceutically relevant covalent protein-drug complex involving the drug Ibrutinib. Carrying out these calculations requires an error-corrected quantum architecture that we describe. Our estimates showcase that recent developments on quantum phase estimation algorithms have dramatically reduced the quantum resources needed to run fully quantum calculations in active spaces of around 50 orbitals and electrons, from estimated over 1000 years using the Trotterization approach to just a few days with sparse qubitization, painting a picture of fast and exciting progress in this nascent field.

In this work, we present a quantum algorithm for ground-state energy calculations of periodic solids on error-corrected quantum computers. The algorithm is based on the sparse qubitization approach in second quantization and developed for Bloch and Wannier basis sets. We show that Wannier functions require less computational resources with respect to Bloch functions because: (i) the L$_1$ norm of the Hamiltonian is considerably lower and (ii) the translational symmetry of Wannier functions can be exploited in order to reduce the amount of classical data that must be loaded into the quantum computer. The resource requirements of the quantum algorithm are estimated for periodic solids such as NiO and PdO. These transition metal oxides are industrially relevant for their catalytic properties. We find that ground-state energy estimation of Hamiltonians approximated using 200--900 spin orbitals requires {\it ca.}~$10{}^{10}$--$10^{12}$ T gates and up to $3\cdot10^8$ physical qubits for a physical error rate of $0.1\%$., Published in Physical Review Research 23 March 2023

Engineering of enzymes on the basis of protein structures are rational and efficient approaches to acquire biocatalysts of desired performances. In this study, we focused on a special mono- and diacylglycerol lipase (MDGL) isolated from the lipolytic enzyme-enriched fungus Aspergillus oryzae and discovered improved variants based on its crystal structure. We first solved the crystal structure of Aspergillus oryzae lipase (AOL) at 1.7 A resolution. Structure analysis and sequence alignment of AOL and other MDGLs revealed that the residue V269 is of vital importance for catalysis. Replacement of the V269 in AOL with the corresponding residues in other MDGLs has led to noticeable changes in hydrolysis without sacrificing the thermostability and substrate specificity. Among the investigated variants, V269D exhibited about a six-fold higher hydrolysis activity compared to the wild type. Molecular dynamics simulations and protein-ligand interaction frequency analyses revealed that the Asp substitution enhanced the substrate affinity of AOL. Our work sheds light on understanding the catalytic process of AOL and helps tailoring MDGLs with desired catalytic performance to fulfill the demand for biotechnological applications.

Neutron scattering combined with ab initio calculations provides a powerful tool for studying metal complexes in different solvents and, particularly, in water. The majority of traditional characterization techniques in catalysis provide only limited information on homogeneous catalytic processes. Neutron scattering, on the other hand, thanks to its sensitivity to hydrogen atoms, and therefore water molecules, can be used to build detailed models of reaction paths and to observe, at a molecular level, the influence of solvent molecules on a catalytic process. In this Mini-Review we describe several examples on how neutron scattering combined with ab initio calculations can be used to examine the nature of the interaction of water molecules with catalytically active metal complexes in solution.

We question the statement of a recent publication ( Organometallics 2020, 39, 132, DOI: 10.1021/acs.organomet.9b00699) that the M → CO π backdonation in the alkaline earth carbonyl complexes M(CO)8...

Alkalides, the alkali metals in their −1 oxidation state, represent some of the largest and most polarizable atomic species in condensed phases. This study determines the solvation environment around the sodide anion, Na–, in a system of co-solvated Li+. We present isotopically varied total neutron scattering experiments alongside empirical potential structure refinement and ab initio molecular dynamics simulations for the alkali–alkalide system, LiNa·10MeNH2. Both local coordination modes and the intermediate range liquid structure are determined, which demonstrate that distinct structural correlations between cation and anion in the liquid phase extend beyond 8.6 Å. Indeed, the local solvation around Na– is surprisingly well defined with strong solvent orientational order, in contrast to the classical description of alkalide anions not interacting with their environment. The ion-paired Li(MeNH2)4+·Na– species appears to be the dominant alkali–alkalide environment in these liquids, whereby Li+ and Na– share a MeNH2 molecule through the amine group in their primary solvation spheres.

Engineering of enzymes on the basis of protein structures are rational and efficient approaches to acquire biocatalysts of desired performances. In this study, we focused on a special mono- and diacylglycerol lipase (MDGL) isolated from the lipolytic enzyme-enriched fungus

Quantum chemical calculations of the alkaline‐earth oxides, imides and dihydrides of the alkaline‐earth atoms (Ae=Be, Mg, Ca, Sr, Ba) and the calcium cluster Ca6H9[N(SiMe3)2]3(pmdta)3 (pmdta=N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine) have been carried out by using density functional theory. Analysis of the electronic structures by charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and (n−1)d orbitals. The alkaline‐earth metals Be and Mg build covalent bonds like typical main‐group elements, whereas Ca, Sr and Ba covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline‐earth metals, but are also very important for understanding and designing experimental studies., Involve‐d! The valence orbitals of the heavier alkaline‐earth atoms Ca, Sr and Ba comprise the (n)s and (n−1)d orbitals (see figure), which sharply contrasts the lighter atoms Be and Mg. Thus, the alkaline‐earth metals Be and Mg build covalent bonds like typical main‐group elements, whereas Ca, Sr and Ba covalently bind like transition metals.

Double deprotonation of the diamine 1,1′‐(tBuCH2NH)‐ferrocene (1‐H2) by alkaline‐earth (Ae) or EuII metal reagents gave the complexes 1‐Ae (Ae=Mg, Ca, Sr, Ba) and 1‐Eu. 1‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1‐Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1‐Ca, 1‐Sr, and 1‐Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1‐Ae complexes shows that the heavier species 1‐Ca, 1‐Sr, and 1‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed., To d or not to d: Ferrocene complexes of Ca, Sr, and Ba show clear evidence for Fe⋅⋅⋅metal bonding while no such interaction is present in the Mg complex. Analysis with the QTAIM and EDA‐NOCV methods points to Fe→Ae bonds that involve vacant d‐orbitals of alkaline‐earth metals.

The isomerization of 1-penten-3-ol into 3-pentanone catalyzed by [RuCp(H2O-κO)(PTA)2](CF3SO3) (1CF3SO3) (PTA = 1,3,5-triaza-7-phosphaadamantane) was studied and two water soluble ruthenium catalyst reaction intermediates were characterized. The main intermediate, the complex [RuCp(exo-2-1-penten-3-ol)(PTA)2](CF3SO3)·2H2O (exo- 2CF3SO3·2H2O) was isolated and characterized by NMR in solution and by single-crystal X-ray diffraction in solid state, constituting the first example of a fully characterized complex containing a coordinated 2-allylic alcohol and the first crystal structure for a water-soluble metal complex containing a 2-allyl ligand. NMR and Eyring analysis show the crucial involvement of water molecules both in the transformation of allylic alcohol into a ketone as well as in the concomitant isomerization of the exo-coordinated substrate into the endo conformer. DFT structure and bonding analyses are used to assess the relative stabilities of the isomers and how the metal drives the electronic distribution on the substrate.

The hepatitis C virus (HCV) viroporin p7 oligomerizes to form ion channels, which are required for the assembly and secretion of infectious viruses. The 63-amino acid p7 monomer has two putative transmembrane domains connected by a cytosolic loop, and has both N- and C- termini exposed to the endoplasmic reticulum (ER) lumen. NMR studies have indicated differences between p7 structures of distantly related HCV genotypes. A critical question is whether these differences arise from the high sequence variation between the different isolates and if so, how the divergent structures can support similar biological functions. Here, we present a side-by-side characterization of p7 derived from genotype 1b (isolate J4) in the detergent 6-cyclohexyl-1-hexylphosphocholine (Cyclofos-6) and p7 derived from genotype 5a (isolate EUH1480) in n-dodecylphosphocholine (DPC). The 5a isolate p7 in conditions previously associated with a disputed oligomeric form exhibits secondary structure, dynamics, and solvent accessibility broadly like those of the monomeric 1b isolate p7. The largest differences occur at the start of the second transmembrane domain, which is destabilized in the 5a isolate. The results show a broad consensus among the p7 variants that have been studied under a range of different conditions and indicate that distantly related HCVs preserve key features of structure and dynamics.

The progress in experimental and theoretical research in the area of main group compounds that possess dative bonds, which was made in the last decade, is summarized. The focus lies on the information about the bonding situation that is available from the analysis of the electronic structure using modern methods of quantum chemistry and the implication on experimental work. The account describes complexes EL2 and E2L2 of group 13–15 atoms E where a single- or diatomic centre is stabilized by donor ligands L. The work is complemented by didactical excursions which demonstrate the strength of the dichotomous interpretation of chemical bonding in terms of dative or electron-sharing interactions. It is important to distinguish between the description of a bonding situation and the explanation of the molecular electronic structure, which provides the fundament for the pictorial representation of the bonds.

A biologically relevant reactive sulfur species (RSS), the hypothiocyanite radical OSCN, is generated in the gas phase through flash vacuum pyrolysis (FVP) of trifluoromethyl sulfinyl cyanide CF3S(O)CN at ca. 1000 K. Upon UV light irradiation (365 nm), OSCN rearranges to novel isomers OSNC and SOCN, and further visible light irradiation (400 ± 20 nm) leads to reverse isomerization. The identification of OSCN, OSNC, and SOCN in cryogenic matrices (Ar and N2, 2.8 K) with IR spectroscopy is supported by quantum chemical calculations up to the CCSD(T)-F12/VTZ-F12 level. The potential energy surface for the interconversion of OSCN isomers and their bonding properties are computationally explored by using the CCSD(T)-F12/VTZ-F12 and EDA-NOCV methods, respectively.

C.J. and A.S. gratefully acknowledge financial support from the Australian Research Council, while C.J. thanks the U.S. Air Force Asian Office of Aerospace Research and Development (FA2386-14-1-4043) for funding. G.F. acknowledges financial support from the Deutsche Forschungsgemeinschaft. The first example of a well-defined binary, low-oxidation-state aluminum hydride species that is stable at ambient temperature, namely the dianion in [{(DepNacnac)Mg}2(μ-H)]2[H3Al-AlH3] (DepNacnac=[(DepNCMe)2CH]−, Dep=2,6-diethylphenyl), has been prepared via a magnesium(I) reduction of the alanate complex, (DepNacnac)Mg(μ-H)3AlH(NEt3). An X-ray crystallographic analysis has shown the compound to be a contact ion complex, which computational studies have revealed to be the source of the stability of the aluminum(II) dianion. Postprint

Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds EL2 with atoms E = C–Pb and the ligands L = PPh3, N-heterocyclic carbene (NHC), bicyclic NHC, and cyclic alkyl-amino carbene (cAAC). The equilibrium structures are reported and the bonding situation was analyzed with a variety of charge- and energy decomposition methods. Some of the molecules are experimentally known, but most molecules have not yet been prepared. The bonding analysis suggests that all but one molecule should be considered as ylidones that possess dative bonds L→E←L. The sole exception is C(cAAC)2, which has a linear structure and classical double bonds (cAAC)=C=(cAAC). The ylidones EL2 have two lone-pair orbitals at atom E with σ and π symmetry. The π lone pair is somewhat delocalized due to L←E→L π-backdonation. The contribution of L←E→L π-backdonation relative to L→E←L σ-donation increases for the heavier elements. The compounds have rather large first and second proton affinities, which is a characteristic feature of ylidones. They are thus double Lewis bases that could be utilized in chemical reactions.

The focus of this review is the presentation of the most important aspects of chemical bonding in molecules of the main group atoms according to the current state of knowledge. Special attention is given to the difference between the physical mechanism of covalent bond formation and its description with chemical bonding models, which are often confused. This is partly due to historical reasons, since until the development of quantum theory there was no physical basis for understanding the chemical bond. In the absence of such a basis, chemists developed heuristic models that proved extremely valuable for understanding and predicting experimental studies. The great success of these simple models and the associated rules led to the fact that the model conceptions were regarded as real images of physical reality. The complicated world of quantum theory, which eludes human imagination, made it difficult to link heuristic models of chemical bonding with quantum chemical knowledge. In the early days of quantum chemistry, some suggestions were made which have since proved untenable. In recent decades, there has been a stormy development of quantum chemical methods, which are not limited to the quantitative accuracy of the calculated properties. Also, methods have been developed where the experimentally developed models can be quantitatively expressed and visually represented using mathematically well-defined terms that are derived from quantum chemical calculations. The calculated numbers may however not be measurable values. Nevertheless, as orientation data for the interpretation and classification of experimental findings as well as a guideline for new experiments, they form a coordinate system that defines the multidimensional world of chemistry, which corresponds to the Hilbert space formalism of physics. The nonmeasurability of model values is not a weakness of chemistry but a characteristic by which the infinite complexity of the material world becomes scientifically accessible and very useful for chemical research. This review examines the basis of the commonly used quantum chemical methods for calculating molecules and for analyzing their electronic structure. The bonding situation in selected representative molecules of main-group atoms is discussed. The results are compared with textbook knowledge of common chemistry.

The viroporin p7 of the hepatitis C virus forms multimeric channels eligible for ion transport across the endoplasmic reticulum membrane. Currently the subject of many studies and discussion, the molecular assembly of the ion channel and the structural characteristics of the p7 monomer are not yet fully understood. Structural investigation of p7 has been carried out only in detergent environments, making the interpretation of the experimental results somewhat questionable. Here, we analyze by means of molecular dynamics simulations the structure of the p7 monomer as a function of its sequence, initial conformation and environment. We investigate the conductance properties of three models of a hexameric p7 ion channel by examining ion translocation in a pure lipid bilayer. It is noteworthy that although none of the models reflects the experimentally observed trend to conduct preferentially cations, we were able to identify the position and orientation of titratable acidic or basic residues playing a crucial role in ion selectivity and in the overall conductance of the channel. In addition, too compact a packing of the monomers leads to channel collapse rather than formation of a reasonable pore, amenable to ion translocation. The present findings are envisioned to help assess the physiological relevance of p7 ion channel models consisting of multimeric structures obtained in non-native environments.

Fifteen cyclic and acylic carbenes have been calculated with density functional theory at the BP86/def2-TZVPP level. The strength of the internal X→p(π) π-donation of heteroatoms and carbon which are bonded to the C(II) atom is estimated with the help of NBO calculations and with an energy decomposition analysis. The investigated molecules include N-heterocyclic carbenes (NHCs), the cyclic alkyl(amino)carbene (cAAC), mesoionic carbenes and ylide-stabilized carbenes. The bonding analysis suggests that the carbene centre in cAAC and in diamidocarbene have the weakest X→p(π) π-donation while mesoionic carbenes possess the strongest π-donation.

Glycine is the smallest and simplest amino acid with several applications in the chemical and pharmaceutical industries. To date, a wide conflicting literature about the hydration and aggregation of glycine is available. The aim of the present work is to study the hydration and the aggregation properties of glycine in aqueous solution as a function of glycine concentration and temperature. These features are determined at atomistic level, by neutron diffraction with isotopic H/D substitution, augmented by a Monte Carlo simulation. Concerning the glycine-glycine aggregation, the coexistence of direct and water-mediated interactions is demonstrated. In particular, chains or cyclic dimers are preferred, depending on concentration and temperature. Finally, the perturbation of the water network, induced by glycine, is reported and described in detail. Present results give relevant information about formation of polymers and crystallization seeds in glycine solutions.

The photoisomerisation of non-toxic trans-combretastatin CA4 to its cytotoxic cis isomer demonstrates the high potential of this and similar compounds for localised cancer therapy. The introduction of intramolecular charge-transfer character by altering the substituents of combretastatin systems opens up possibilities to tailor these stilbene derivatives to the special demands of anticancer drugs. In this TDDFT study we explore how absorption wavelengths for both the trans and cis isomers can be red shifted to enable deeper light penetration into tissue and how the trans → cis and cis → trans isomerisations are affected by charge transfer effects to different degrees.

The Hepatitis C virus (HCV) has developed a small membrane protein, p7, which remarkably can self-assemble into a large channel complex that selectively conducts cations1-4. We are curious as to what structural solution has the viroporin adopted to afford selective cation conduction because p7 has no homology with any of the known prokaryotic or eukaryotic channel proteins. The p7 activity can be inhibited by amantadine and rimantadine2,5, which also happen to be potent blockers of the influenza M2 channel6 and licensed drugs against influenza infections7. The adamantane derivatives were subjects of HCV clinical trials8, but large variation in drug efficacy among the various HCV genotypes has been difficult to explain without detailed molecular structures. Here, we determined the structures of this HCV viroporin as well as its drug-binding site using the latest nuclear magnetic resonance (NMR) technologies. The structure exhibits an unusual mode of hexameric assembly, where the individual p7 monomers, i, not only interact with their immediate neighbors, but also reach farther to associate with the i+2 and i+3 monomers, forming a sophisticated, funnel-like architecture. The structure also alludes to a mechanism of cation selection: an asparagine/histidine ring that constricts the narrow end of the funnel serves as a broad cation selectivity filter while an arginine/lysine ring that defines the wide end of the funnel may selectively allow cation diffusion into the channel. Our functional investigation using whole-cell channel recording showed that these residues are indeed critical for channel activity. NMR measurements of the channel-drug complex revealed six equivalent hydrophobic pockets between the peripheral and pore-forming helices to which amantadine or rimantadine binds, and compound binding specifically to this position may allosterically inhibit cation conduction by preventing the channel from opening. Our data provide molecular explanation for p7-mediated cation conductance and its inhibition by adamantane derivatives.

Boron hydride clusters are an extremely diverse compound class, which are of enormous importance to many areas of chemistry. Despite this, stable aluminium hydride analogues of these species have remained staunchly elusive to synthetic chemists. Here, we report that reductions of an amidinato-aluminium(III) hydride complex with magnesium(I) dimers lead to unprecedented examples of stable aluminium(I) hydride complexes, [(ArNacnac)Mg]2[Al6H6(Fiso)2] (ArNacnac = [HC(MeCNAr)2]−, Ar = C6H2Me3-2,4,6 Mes; C6H3Et2-2,6 Dep or C6H3Me2-2,6 Xyl; Fiso = [HC(NDip)2]−, Dip = C6H3Pri2-2,6), which crystallographic and computational studies show to possess near neutral, octahedral hypercloso-hexaalane, Al6H6, cluster cores. The electronically delocalised skeletal bonding in these species is compared to that in the classical borane, [B6H6]2−. Thus, the chemistry of classical polyhedral boranes is extended to stable aluminium hydride clusters for the first time., While polyhedral boron hydride complexes have found application in a number of diverse fields, the isolation of stable aluminium analogues remains highly challenging. Here, Jones and colleagues demonstrate that reduction of an amidinato-aluminum(III) hydride complex with magnesium(I) dimers affords stable aluminium(I) hydride compounds.

We study the intermediate of the reaction between a ruthenium complex and 1-propen-3-ol in water using an atomistic approach for gaining information about the conformation and dynamics of complex molecules in aqueous solution, which combines density-functional-theory-based ab initio molecular dynamics and neutron scattering data based on empirical potential structure refinement simulations. We apply our method to the study of the water-soluble η2-allylic complex [RuCp(exo-η2- CH2 CH−CH2−OH)(PTA)2]+ (2) (PTA = 1,3,5-triaza-7- phosphaadamantane), an important intermediate in the isomerization of 1-propen-3-ol into propanal catalyzed by {RuCp(H2O- κO)(PTA)2}+. We identify the factors responsible for the stabilization of a specific conformer of 2 in water solution and we examine the involvement of water molecules in the formation of this species. In particular, we show that long-lived (ca. 10 ps) bonded chains of water molecules play a crucial role in influencing the conformation and, potentially, the chemical reactivity of 2.

The attempted synthesis of NHC-stabilized dicarbon (NHC=C=C=NHC) through deprotonation of a doubly protonated precursor ([NHC-CH=CH-NHC](2+) ) is reported. Rather than deprotonation, a clean reduction to NHC=CH-CH=NHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→CC←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAAC=C=C=cAAC), which has a large HOMO-LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor-acceptor interactions L→C2 ←L (L=NHC, cAAC).

Quantum chemical calculations using density functional theory at the BP86 level in conjunction with triple-zeta polarized basis sets have been carried out for the title compounds. The nature of the bonding between the diatomic fragment and the NHC ligands is investigated with an energy decomposition analysis. The chemical bonds in the [(NHCMe)2(E2)] complexes can be discussed in terms of donor-acceptor interactions which consist of two NHCMe→E2←NHCMe donor components and two weaker components of the NHCMe←E2→NHCMe π backdonation. The out-of-phase (+)=(-) contribution of the s donation is always stronger than the in-phase (+)=(+) contribution. The electronic reference state of N2 in the dinitrogen complex [(NHCMe)2(N2)] is the highly excited 11Γg state which explains the anti-periplanar arrangement of the ligands. The gauche arrangement of the ligands in the heavier homologues [(NHCMe)2(E2)] (E = P-Bi) may be discussed using either the excited 11Γg state or the X1Σg + ground state of E2 as reference states for the donor-acceptor bonds. The EDA-NOCV calculations suggest that the latter bonding model is better suited for the complexes where E = As-Bi while the phosphorus complex is a borderline case.

A 1,4-diamino-2,3-disila-1,3-butadiene derivative of composition (Me2-cAAC)2(Si2Cl2) (Me2-cAAC = :C(CMe2)2(CH2)N-2,6-iPr2C6H3) was synthesized by reduction of the Me2-cAAC:SiCl4 adduct with KC8. This compound is stable at 0 °C for 3 months in an inert atmosphere. Theoretical studies reveal that the silicon atoms exhibit pyramidal coordination, where the Cl-Si-Si-Cl dihedral angle is twisted by 43.3° (calcd 45.9°). The two silicon-carbon bonds are intermediates between single and double Si-C bonds due to twisting of the C-Si-Si-C dihedral angle (163.6°).

A summary of theoretical and experimental work in the area of low-coordinated compounds of boron and group-14 atoms C-Sn in the last decade is presented. The focus of the account lies on molecules EL2, E2L2 and E3L3, which possess dative bonds between one, two or three atoms E and σ-donor ligands L that stabilize the atoms E through L→E donor-acceptor interactions. The interplay between theory and experiment provides detailed insight into the bonding situation of the molecules, which serves as guideline for the synthesis of molecules that possess unusual bonding motifs.

A summary of theoretical and experimental work in the area of low-coordinated compounds of boron and group-14 atoms C-Sn in the last decade is presented. The focus of the account lies on molecules EL2, E2L2 and E3L3, which possess dative bonds between one, two or three atoms E and σ-donor ligands L that stabilize the atoms E through L→E donor-acceptor interactions. The interplay between theory and experiment provides detailed insight into the bonding situation of the molecules, which serves as guideline for the synthesis of molecules that possess unusual bonding motifs.

Quantum chemical calculations using density functional theory at the BP86/TZVPP level and ab initio calculations at the SCS-MP2/TZVPP level have been carried out for the group 13 complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX(3) and the bidentate Lewis acids E(2) X(n) bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E(2)X(n), which may vary for different atoms E and X. All complexes [(NHC)(2)(E(2)X(4))] possess C(s) symmetry in which the NHC ligands bind in a trans conformation to the group 13 atoms E. The complexes [(NHC)(2)(E(2)H(2))] with E=B, Al, Ga have also C(s) symmetry with a trans arrangement of the NHC ligands and a planar CE(H)E(H)C moiety that has a E=E π bond. In contrast, the indium complex [(NHC)(2)(In(2) H(2))] has C(i) symmetry with pyramidal-coordinated In atoms in which the hydrogen atoms are twisted above and below the CInInC plane. The latter C(i) form is calculated for all chloride systems [(NHC)(2)(E(2)Cl(2))], but the boron complex [(NHC)(2)(B(2)Cl(2))] deviates only slightly from C(s) symmetry. The B(2) fragment in the linear coordinated complex [(NHC)(2)(B(2))] has a highly excited (3)(1)Σ(g)(-) reference state, which gives an effective B≡B triple bond with a very short interatomic distance. The heavier homologues [(NHC)(2)(E(2))] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E(2) fragments have a (1)(1) Δ(g) reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH(3))] and [(NHC)(2)(E(2)H(n))] becomes energetically more favourable when atom E becomes heavier. The indium complexes should therefore be the best candidates of the investigated series for hydrogen-storage systems that could potentially deliver dihydrogen at close to ambient temperature. The hydrogenation reaction of the dimeric magnesium(I) compound [LMgMgL] (L=β-diketiminate) with [(NHC)(EH(3))] becomes increasingly exothermic with the trend BAlGaIn.

The use of 2-Br-fluorobenzene as the solvent permitted the isolation of the addition compound [Cl2Be(C{PPh3}2)] (5) from the reaction of the carbodiphosphorane 1 (carbon) with BeCl2 featuring a three coordinate beryllium atom. In other solvents such as THF, DCM, etc. deprotonation occurs with formation of the cations (HC{PPh3}2)+ or (H2C{PPh3}2)2+. Compound 5 was characterized by an X-ray diffraction analysis. The analysis of the bonding situation in Cl2Be1 reveals that the orbital donation comes mainly from the σ lone-pair orbital of 1, whereas the π donation is rather weak. However, the Cl2Be1 π donation appears strong enough to make the donation of a second donor species 1 unfavorable. The calculation of 1Cl2Be1 shows that the latter complex is higher in energy than 1Cl2Be and free 1. The carbodiphosphorane may bind, albeit weakly, a second BeCl2 species in the complex Cl2Be1BeCl2. The bond dissociation energies for the first and second BeCl2 fragments are De = 31.8 kcal·mol–1 and 7.0 kcal·mol–1. The dimerization of Cl2Be1 is energetically slightly endoenergetic.

A synthetic route to the new amidine (DipNH)(DipN)C(C(6)H(4)Bu(t)-4) (ButisoH; Dip = C(6)H(3)Pr(i)(2)-2,6) has been developed. Its deprotonation with either LiBu(n) or KN(SiMe(3))(2) yields the amidinate complexes [M(Butiso)] (M = Li or K). Their reactions with group 14 element halides/pseudohalides afford the heteroleptic group 14 complexes [(Butiso)SiCl(3)], [(Butiso)ECl] (E = Ge or Sn), and [{(Butiso)Pb(μ-O(3)SCF(3))(THF)}(∞)], all of which have been crystallographically characterized. In addition, the synthesis and spectroscopic characterization of the homoleptic complex [Pb(Butiso)(2)] is reported. Reductions of the heteroleptic complexes with a soluble magnesium(I) dimer, [{((Mes)Nacnac)Mg}(2)] ((Mes)Nacnac = [(MesNCMe)(2)CH](-); Mes = mesityl), have given moderate-to-high yields of the group 14 element(I) dimers [{(Butiso)E}(2)] (E = Si, Ge, or Sn), the X-ray crystallographic studies of which reveal trans-bent structures. The corresponding lead(I) complex could not be prepared. Comprehensive spectroscopic and theoretical analyses of [{(Butiso)E}(2)] have allowed their properties to be compared. All complexes possess E-E single bonds and can be considered as intramolecularly base-stabilized examples of ditetrelynes, REER. Taken as a whole, this study highlights the synthetic utility of soluble and easy to prepare magnesium(I) dimers as valuable alternatives to the harsh, and often insoluble, alkali-metal reducing agents that are currently widely employed in the synthesis of low-oxidation-state organometallic/inorganic complexes.

The desorption of dihydrogen from magnesium(II) hydride, MgH2 (containing 7.6 wt% H), is reversible. MgH2 therefore holds promise as a hydrogen storage material in devices powered by fuel cells. We believed that dimeric magnesium(I) dimers (LMgMgL, L=β-diketiminate) could find use as soluble models to aid the study of the mechanisms and/or kinetics of the hydrogenation of magnesium and its alloys. Here, we show that LMgMgL can be readily hydrogenated to yield LMg(µ-H)2MgL by treatment with aluminium(III) hydride complexes. In one case, hydrogenation was reversed by treating LMg(µ-H)2MgL with potassium metal. The hydrogenation by-products are the first thermally stable, neutral aluminium(II) hydride complexes to be produced, one of which, [{(IPr)(H)2Al}2] (IPr=:C[{(C6H3-i-Pr(2)-2,6)NCH}2]), is an N-heterocyclic carbene adduct of the elusive parent dialane4 (Al2H4). A computational analysis of this compound is presented.

The donor-acceptor complex [BeCl2(Ph2PCH2PPh2)2] (1·2CH2Cl2) was prepared from beryllium dichloride and bis(diphenylphosphanyl)methane in dichloromethane suspension to give colorless single crystals, which were characterized by X-ray diffraction, IR spectroscopy, and DFT calculations. Compound 1·2CH2Cl2 crystallizes monoclinically in space group C2/c with four formula units per unit cell. Lattice dimensions at 193(2) K: a = 2339.5(2), b = 916.7(1), c = 2415.4(2) pm, β = 97.15(1)°, R1 = 0.0386. The structure of 1 consists of distorted tetrahedral molecules with site symmetry C2 and bond lengths Be–Cl of 196.8(2) pm and Be–P of 220.6(3) pm. Quantum chemical calculations at the RI-BP86/SV(P) level give a structure of 1, which is in very good agreement with the experimental one. The calculations predict that the bond dissociation energies of the first and second Ph2PCH2PPh2 ligand from 1 are De = 16.8 kcal·mol–1 and 24.3 kcal·mol–1, respectively.

Electronic structure methods are used to explore the origin of selectivity in a number of organic, organometallic and bioorganic processes. Diastereosellectivity in reactions of sulfur and phosphorus ylides is shown to arise during a variety of different elementary steps, and is due to steric and electronic effects. Unusual rearranged products from Heck reaction of o-bromo biphenyl derivatives are shown to result from an unusual electrophilic addition step. Predicting selectivity in oxidation of aromatic substrates by cytochrome P450 isoforms is a challenging problem, which can be tackled using hybrid QM/MM methods. Differences in the electronic structure of the Compound I active intermediates of different cytochrome P450 isoforms do not appear to be large enough to explain the different selectivity of these different isoforms. Copyright (c) 2006 John Wiley & Sons, Ltd.

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

Quantum chemical calculations at the BP86/def2-TZVPP level have been carried out for the donor-acceptor complexes [(PMe3)2(E2Hn)] for n = 4, 2, 0. The focus of this works lies on the E–E bonding situation. The electronic structure of the molecules was analyzed with the EDA-NOCV meth-od and with NBO calculations. The EDA-NOCV analysis of the E–E interactions in [(PMe3)2(E2Hn)] (n = 4, 2, 0) provide deep insights into the nature and the strength of the bonds. The calculated intrinsic interactions ΔEint suggest that the trend for the bond strength of the E–E single bond [(PMe3)(H)2E–E(H)2(PMe3)] has the order B > Ga > Al > In. The orbital interactions ΔEorb which exhibit the same trend as ΔEint have one dominant contribution which comes from the coupling of the singly occupied orbitals in the (PMe3)(H)2E fragments. A slightly different trend B > Ga ~ In > Al is found for the in-teraction energy ΔEint of the E–E bonds in [(PMe3)(H)E–E(H)(PMe3)]. The orbital term ΔEorb which has the order B > Ga > In > Al has one major and one minor component which in case of the boron compound may be identified with a σ and a π bond. The heavier homologues [(PMe3)(H)E–E(H)(PMe3)] (E = Al – In) have pyramidally coordinated atoms E. The dominant orbital interactions in the latter species come from the formation of a “slipped” π bond while the minor component comes from the formation of the σ bond. This can be explained with the change in the hybridization of the orbitals at atom E along the formation of the E–E bond. The compounds [(PMe3)E–E(PMe3)] exhibit three different types of bonding situations depending on atoms E. The boron system [(PMe3)B≡B(PMe3)] has a classical triple bond which consist of a σ bond that provides 56 % to the orbital interactions and two degenerate π bonds which contribute 40 % to the covalent bonding. The aluminium and gallium complexes [(PMe3)E–E(PMe3)] (E = Al, Ga) are also triply bonded species where the covalent bonding has one strong and two weaker components. The strong component comes from the “slipped” π bond while the minor components come from the formation of the σ and π bonds. The indium complex [(PMe3)In–In(PMe3)] has only an In–In single bond and two electron lone pairs at the indium atoms. The charge donation Me3P → E2(Hn) ← PMe3 has for all atoms E the trend for n 4 > 2 > 0.

Quantum chemical calculations using density functional theory at the BP86/TZ2P level have been carried out to determine the geometries and stabilities of Group 13 adducts [(PMe3)(EH3)] and [(PMe3)2(E2H(n))] (E = B-In; n = 4, 2, 0). The optimized geometries exhibit, in most cases, similar features to those of related adducts [(NHC(Me))(EH3)] and [(NHC(Me))2(E2H(n))] with a few exceptions that can be explained by the different donor strengths of the ligands. The calculations show that the carbene ligand L = NHC(Me) (:C(NMeCH)2) is a significantly stronger donor than L = PMe3. The equilibrium geometries of [L(EH3)] possess, in all cases, a pyramidal structure, whereas the complexes [L2(E2H4)] always have an antiperiplanar arrangement of the ligands L. The phosphine ligands in [(PMe3)2(B2H2)], which has Cs symmetry, are in the same plane as the B2H2 moiety, whereas the heavier homologues [(PMe3)2(E2H2)] (E = Al, Ga, In) have Ci symmetry in which the ligands bind side-on to the E2H2 acceptor. This is in contrast to the [(NHC(Me))2(E2H2)] adducts for which the NHC(Me) donor always binds in the same plane as E2H2 except for the indium complex [(NHC(Me))2(In2H2)], which exhibits side-on bonding. The boron complexes [L2(B2)] (L = PMe3 and NHC(Me)) possess a linear arrangement of the LBBL moiety, which has a B≡B triple bond. The heavier homologues [L2(E2)] have antiperiplanar arrangements of the LEEL moieties, except for [(PMe3)2(In2)], which has a twisted structure in which the PInInP torsion angle is 123.0°. The structural features of the complexes [L(EH3)] and [L2(E2H(n))] can be explained in terms of donor-acceptor interactions between the donors L and the acceptors EH3 and E2H(n), which have been analyzed quantitatively by using the energy decomposition analysis (EDA) method. The calculations predict that the hydrogenation reaction of the dimeric magnesium(I) compound L'MgMgL' with the complexes [L(EH3)] is energetically more favorable for L = PMe3 than for NHC(Me).