Theoretical Studies of Inorganic Compounds. 27 Quantum Chemical Investigations of Transition Metal Nitrido Complexes with a TM‐N‐E Linkage (E = Main Group Element)

It is reported about quantum chemical DFT calculations of various transition metal (TM) nitrido complexes which contain a TM‐N‐E linkage. The goal is to elucidate the nature of the TM‐N‐E bonding situation with modern quantum chemical tools. Five comparative investigations have been carried out. (a) Comparison of the N‐donor ability in the nitrido complexes Cl₃W‐N‐ECl_n where ECl_n = NaCl, MgCl₂, AlCl₃. (b) Comparative analysis of the bonding situation in Cl₄W‐N‐X where X = Na, MgCl, AlCl₂, SiCl₃, PCl₂, SCl, Cl. (c) Comparison of the structure and bonding in Cl₅W‐NPH₃, Cl₅W‐OPH₃⁺, Cl₄W‐(NPH₃)(OPH₃)⁺. (d) Comparative analysis of the bonding situation in Cl₅Ta‐OPH₃, Cl₅W‐NPH₃, Cl₅Re‐CPH₃. (e) Energy decomposition analysis of the bonding of the isolobal ligands NPH₃ and Cp with WCl₅.     

QTAIM and stress tensor interpretation of the (H2O)5 potential energy surface

Using the quantum theory of atoms in molecules a near complete combined directed spanning quantum topology phase diagram (QTPD) was constructed from the nine (H₂O)₅ reaction‐pathways and five unique Poincaré–Hopf solutions that were found after an extensive search of the MP2 potential energy surface. Two new energy minima that were predicted from earlier work are found and include the first (H₂O)₅ conformer with a 3‐D_QT quantum topology. The stress tensor Poincaré–Hopf relation indicated a preference for 2‐D_QT (H₂O)₅ topologies as well as the presence of coupling between shared‐shell O? H BCPs to the hydrogen‐bond BCPs that share an H NCP. The complexity of the near complete combined QTPD was explained in terms of the O…O bonding interactions that were found in six of the nine (H₂O)₅ reaction‐pathways and for all points of the combined QTPD. The stabilizing role of the O…O bonding interactions from the values of the total local energy density was explored. © 2016 Wiley Periodicals, Inc.     

Beyond the Halogen Bond: Examining the Limits of Extended Polybromide Networks through Quantum‐Chemical Investigations

The bonding environments of some polybromide monoanions and networks were examined by quantum‐chemical methods to investigate electronic interactions between dibromine–dibromine contacts. Examination of thermodynamic parameters and a bond critical point analysis give strong evidence for such bonding modes, which have been previously treated disparately in the literature. The thermodynamic stability of large polybromides up to [Br₃₇]^? was also predicted by these methods.     

Oligonuclear Molecular Models of Intermetallic Phases: A Case Study on [Pd2Zn6Ga2(Cp*)5(CH3)3]

The synthesis, characterization, and theoretical investigation by means of quantum‐chemical calculations of an oligonuclear metal‐rich compound are presented. The reaction of homoleptic dinuclear palladium compound [Pd₂(μ‐GaCp*)₃(GaCp*)₂] with ZnMe₂ resulted in the formation of unprecedented ternary Pd/Ga/Zn compound [Pd₂Zn₆Ga₂(Cp*)₅(CH₃)₃] ( 1 ), which was analyzed by ¹H and ¹³C NMR spectroscopy, MS, elemental analysis, and single‐crystal X‐ray diffraction. Compound 1 consisted of two C_s‐symmetric molecular isomers, as revealed by NMR spectroscopy, at which distinct site‐preferences related to the Ga and Zn positions were observed by quantum‐chemical calculations. Structural characterization of compound 1 showed significantly different coordination environments for both palladium centers. Whilst one Pd atom sat in the central of a bi‐capped trigonal prism, thereby resulting in a formal 18‐valence electron fragment, {Pd(ZnMe)₂(ZnCp*)₄(GaMe)}, the other Pd atom occupied one capping unit, thereby resulting in a highly unsaturated 12‐valence electron fragment, {Pd(GaCp*)}. The bonding situation, as determined by atoms‐in‐molecules analysis (AIM), NBO partial charges, and molecular orbital (MO) analysis, pointed out that significant Pd? Pd interactions had a large stake in the stabilization of this unusual molecule. The characterization and quantum‐chemical calculations of compound 1 revealed distinct similarities to related M/Zn/Ga Hume–Rothery intermetallic solid‐state compounds, such as Ga/Zn‐exchange reactions, the site‐preferences of the Zn/Ga positions, and direct M? M bonding, which contributes to the overall stability of the metal‐rich compound.     

Energy Dissipation Processes of singlet‐excited 1‐Hydroxyfluorenone and its Hydrogen‐bonded Complex with N‐methylimidazole¶

Effects of solvent, pH and hydrogen bonding with N‐methylimidazole (MIm) on the photophysical properties of 1‐hydroxyfluorenone (1HOF) have been studied. Fluorescence lifetime, fluorescence quantum yield and triplet yield measurements demonstrated that intersystem crossing was the dominant process in apolar media and its rate constant significantly diminished with increasing solvent polarity. The acceleration of internal conversion in alcohols paralleled the strength of intermolecular hydrogen bonding. The faster energy dissipation from the singlet‐excited state in cyclohexane was attributed to intramolecular hydrogen bonding. The pK_a of 1HOF decreased from 10.06 to 5.0 on light absorption, and H₃O⁺ quenched the singletexcited molecules in a practically diffusion‐controlled reaction. On addition of MIm in toluene, dual fluorescence was observed, which was attributed to reversible formation of excited hydrogen‐bonded ion pair. Rate constants for the various deactivation pathways were derived from the combined analysis of the steady‐state and the time‐resolved fluorescence results.     

Synthesis,Structure, and Properties of SrC(NH)3, a Nitrogen‐Based Carbonate Analogue with the Trinacria Motif

Strontium guanidinate, SrC(NH)₃, the first compound with a doubly deprotonated guanidine unit, was synthesized from strontium and guanidine in liquid ammonia and characterized by X‐ray and neutron diffraction, IR spectroscopy, and density‐functional theory including harmonic phonon calculations. The compound crystallizes in the hexagonal space group P6₃/m, constitutes the nitrogen analogue of strontium carbonate, SrCO₃, and its structure follows a layered motif between Sr²⁺ ions and complex anions of the type C(NH)₃^2?; the anions adopt the peculiar trinacria shape. A comparison of theoretical phonons with experimental IR bands as well as quantum‐chemical bonding analyses yield a first insight into bonding and packing of the formerly unknown anion in the crystal.     

Formation and Characterization of the Boron Dicarbonyl Complex [B(CO)2]−

We report the synthesis and spectroscopic characterization of the boron dicarbonyl complex [B(CO)₂]^?. The bonding situation is analyzed and compared with the aluminum homologue [Al(CO)₂]^? using state‐of‐the‐art quantum chemical methods.     

Comparative Analysis of Electron‐Density and Electron‐Localization Function for Dinuclear Manganese Complexes with Bridging Boron‐ and Carbon‐Centered Ligands

Bonding in borylene‐, carbene‐, and vinylidene‐bridged dinuclear manganese complexes [MnCp(CO)₂]₂X (X=B‐tBu, B=NMe₂, CH₂, C?CH₂) has been compared by analyses based on quantum theory of atoms in molecules (QTAIM), on the electron‐localization function (ELF), and by natural‐population analyses. All of the density functional theory based analyses agree on the absence of a significant direct Mn? Mn bond in these complexes and confirm a dominance of delocalized bonding via the bridging ligand. Interestingly, however, the topology of both charge density and ELF related to the Mn‐bridge‐Mn bonding depend qualitatively on the chosen density functional (except for the methylene‐bridged complex, which exhibits only one three‐center‐bonding attractor both in ??²ρ and in ELF). While gradient‐corrected functionals provide a picture with localized two‐center X? Mn bonding, increasing exact‐exchange admixture in hybrid functionals concentrates charge below the bridging atom and suggests a three‐center bonding situation. For example, the bridging boron ligands may be described either as substituted boranes (e.g., at BLYP or BP86 levels) or as true bridging borylenes (e.g., at BHLYP level). This dependence on the theoretical level appears to derive from a bifurcation between two different bonding situations and is discussed in terms of charge transfer between X and Mn, and in the context of self‐interaction errors exhibited by popular functionals.     

Experimental and theoretical study of the intramolecular C–H···N and C–H···S hydrogen bonding effects in the 1H and 13C NMR spectra of the 2‐(alkylsulfanyl)‐5‐amino‐1‐vinylpyrroles: a particular state of amine nitrogen

In the ¹H NMR spectra of the 1‐vinylpyrroles with amino‐ and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the H_A and H_B terminal methylene protons of the vinyl group is discovered. Also, the one‐bond ¹J(C_β,H_B) coupling constant is surprisingly greater than the ¹J(C_β,H_A) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π‐system of the pyrrole ring so that it is directed toward the H_B hydrogen. These factors are favorable to the emergence of the intramolecular C–H_B???N hydrogen bonding in the s‐cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the H_B hydrogen provides an opportunity of the intramolecular C–H_B???S hydrogen bonding in the s‐cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C–H???N and C–H???S hydrogen bonding. Therefore, an unusual high‐frequency shift of the H_B signal and the increase in the ¹J(C_β,H_B) coupling constant can be explained by the effects of hydrogen bonding. Copyright © 2013 John Wiley & Sons, Ltd.     

The Solid Solution Sr1−xBaxGa2: Substitutional Disorder and Chemical Bonding Visited by NMR Spectroscopy and Quantum Mechanical Calculations

Complete miscibility of the intermetallic phases (IPs) SrGa₂ and BaGa₂ forming the solid solution Sr_1?xBa_xGa₂ is shown by means of X‐ray diffraction, thermoanalytical and metallographic studies. Regarding the distances of Sr/Ba sites versus substitution degree, a model of isolated substitution centres (ISC) for up to 10 % cation substitution is explored to study the influence on the Ga bonding situation. A combined application of NMR spectroscopy and quantum mechanical (QM) calculations proves the electric field gradient (EFG) to be a sensitive measure of different bonding situations. The experimental resolution is boosted by orientation‐dependent NMR on magnetically aligned powder samples, revealing in first approximation two different Ga species in the ISC regimes. EFG calculations using superlattice structures within periodic boundary conditions are in fair agreement with the NMR spectroscopy data and are discussed in detail regarding their application on disordered IPs.     

Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Lewis Acid/Base Reactions with Triorganylboranes

Reaction of the donor‐stabilized silylene 1 (which is three‐coordinate in the solid state and four‐coordinate in solution) with BEt₃ and BPh₃ leads to the formation of the Lewis acid/base complexes 2 and 3 , respectively, which are the first five‐coordinate silicon compounds with an Si?B bond. These compounds were structurally characterized by crystal structure analyses and by multinuclear NMR spectroscopic studies in the solid state and in solution. Additionally, the bonding situation in 2 and 3 was analyzed by quantum chemical studies.     

Bonding in Diborane–Metal Complexes: A Quantum‐Chemical and Experimental Study of Complexes Featuring Early and Late Transition Metals

The coordination chemistry of the doubly base‐stabilised diborane(4), [HB(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido‐[1,2‐a]pyrimidinate), was extended by the synthesis of new late transition‐metal complexes containing Cu^I and Rh^I fragments. A detailed experimental study was conducted and quantum‐chemical calculations on the metal–ligand bonding interactions for [HB(hpp)]₂ complexes of Group 6, 9, 11 and 12 metals revealed the dominant B? H? M interactions in the case of early transition‐metal fragments, whereas the B? B? M bonding prevails in the case of the late d‐block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]₂ and model reactions between [B₂H₄ ? 2NMe₃] and [Rh(μ‐Cl)(C₂H₄)₂] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the B? B bond. However, the formation of σ‐complexes is thermodynamically favoured. The results point to the selective B? H or B? B bond‐activation of diborane compounds by complexation, depending on the chosen transition‐metal fragment.     

The Role of Molecular Electrostatic Potentials in the Formation of a Halogen Bond in Furan⋅⋅⋅XY and Thiophene⋅⋅⋅XY Complexes

The halogen bonding of furan???XY and thiophene???XY (X=Cl, Br; Y=F, Cl, Br), involving σ‐ and π‐type interactions, was studied by using MP2 calculations and quantum theory of “atoms in molecules” (QTAIM) studies. The negative electrostatic potentials of furan and thiophene, as well as the most positive electrostatic potential (V_S,max) on the surface of the interacting X atom determined the geometries of the complexes. Linear relationships were found between interaction energy and V_S,max of the X atom, indicating that electrostatic interactions play an important role in these halogen‐bonding interactions. The halogen‐bonding interactions in furan???XY and thiophene???XY are weak, “closed‐shell” noncovalent interactions. The linear relationship of topological properties, energy properties, and the integration of interatomic surfaces versus V_S,max of atom X demonstrate the importance of the positive σ hole, as reflected by the computed V_S,max of atom X, in determining the topological properties of the halogen bonds.     

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃⁻, SbFe(CO)₃⁻, and BiFe(CO)₃⁻ were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃⁻. Chemical bonding analyses on the whole series of AFe(CO)₃⁻ (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃⁻ (A=As, Sb, Bi) complexes.     

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃^?, SbFe(CO)₃^?, and BiFe(CO)₃^? were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃^?. Chemical bonding analyses on the whole series of AFe(CO)₃^? (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃^? (A=As, Sb, Bi) complexes.     

SiP Double Bonds: Experimental and Theoretical Study of an NHC‐Stabilized Phosphasilenylidene

An experimental and theoretical study of the first compound featuring a Si?P bond to a two‐coordinate silicon atom is reported. The NHC‐stabilized phosphasilenylidene (IDipp)Si?PMes* (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene, Mes*=2,4,6‐tBu₃C₆H₂) was prepared by SiMe₃Cl elimination from SiCl₂(IDipp) and LiP(Mes*)SiMe₃ and characterized by X‐ray crystallography, NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroscopy. It has a planar trans‐bent geometry with a short Si? P distance of 2.1188(7) Å and acute bonding angles at Si (96.90(6)°) and P (95.38(6)°). The bonding parameters indicate the presence of a Si?P bond with a lone electron pair of high s‐character at Si and P, in agreement with natural bond orbital (NBO) analysis. Comparative cyclic voltammetric and UV/Vis spectroscopic experiments of this compound, the disilicon(0) compound (IDipp)Si?Si(IDipp), and the diphosphene Mes*P?PMes* reveal, in combination with quantum chemical calculations, the isolobal relationship of the three double‐bond systems.     

Density functional theory and topological analysis on the hydrogen bonding interactions in cysteine‐thymine complexes

Hydrogen bonding interactions between amino acids and nucleic acid bases constitute the most important interactions responsible for the specificity of protein binding. In this study, complexes formed by hydrogen bonding interactions between cysteine and thymine have been studied by density functional theory. The relevant geometries, energies, and IR characteristics of hydrogen bonds (H‐bonds) have been systematically investigated. The quantum theory of atoms in molecule and natural bond orbital analysis have also been applied to understand the nature of the hydrogen bonding interactions in complexes. More than 10 kinds of H‐bonds including intra‐ and intermolecular H‐bonds have been found in complexes. Most of intermolecular H‐bonds involve O (or N) atom as H‐acceptor, whereas the H‐bonds involving C or S atom usually are weaker than other ones. Both the strength of H‐bonds and the structural deformation are responsible for the stability of complexes. Because of the serious deformation, the complex involving the strongest H‐bond is not the most stable structures. Relationships between H‐bond length (ΔR_X‐H), frequency shifts (Δv), and the electron density (ρ_b) and its Laplace (?²ρ_b) at bond critical points have also been investigated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011