Electronic coupling in covalently-linked MM quadruple bonds: M2δ to ligand π conjugation

Dimetal tetracarboxylates, M₂(O₂CR)₄, where R=alkyl and M=Mo or W, can be linked in a perpendicular or parallel manner through the agency of a variety of ligands to give ‘dimers of dimers’ [M₂(O₂CR)₃]₂(bridge), or extended chains. A ring of formula [M₂(O₂CR)₂(bridge)]₄ is an alternative to a chain [M₂(O₂CR)₃]₂(bridge)₂]_∞ and ‘molecular squares’ of formula [(RNCHNR)₂M₂(bridge)]₄ have recently been reported by Cotton and Murillo, where M=Mo and Rh and R=p-MeOC₆H₄. The electronic coupling between the M₂ units manifests itself in electrochemical data and in the electronic spectra of the compounds. The electronic coupling occurs by M₂δ to ligand π-conjugation and specific examples are illustrated for the bridging ligands oxalate, perfluoroteraphthalate, 1,8-anthracenedicarboxylate and 2,7-dioxynaphthyridine. Correlations of a variety of spectroscopic data and computations employing density functional theory are presented.     

Construction of M–M bonds in late transition metal complexes

The 16-electron Co, Rh and Ir half-sandwich complexes of Cp^#M[E₂C₂(B₁₀H₁₀)] and Cp^#M(E₂S₂C₆H₄) (M = Co, Rh, Ir, Ru; E = S, Se) containing chelating 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolato ligands and benzenedithiolato ligands are promising precursors to build multimetallic clusters by reactions with low oxidation state late transition metal reagents. Such reactions lead to successful constructions of M–M bonds between iridium, rhodium, cobalt, ruthenium, and other late transition metals. Most of these complexes have been characterized by X-ray single crystal determinations and some have been studied by computational methods. Such theoretical studies reveal the covalent bonding nature of those multinuclear complexes. Some of these clusters have been found to have interesting nonlinear optical properties.     

Theoretical study of bifurcated bent blue-shifted hydrogen bonds CH_2…Y

Ab initio quantum chemistry methods were applied to study the bifurcated bent hydrogen bonds Y… H2CZ (Z = O, S, Se) and Y…H2CZ2 (Z = F, Cl, Br) (Y = Cl-, Br-) at the MP2/6-311 G(d,p) and MP2/6-311 G(2df,2p) levels. The results show that in each complex there are two equivalent blue-shifted H-bonds Y…H—C, and that the interaction energies and blue shifts are large, the energy of each Y…H—C H-bond is 15-27 kJ/mol, and Δr(CH) = -0.1 - -0.5 pm and Δv(CH) = 30 - 80 cm-1. The natural bond orbital analysis shows that these blue-shifted H-bonds are caused by three factors: large rehybridization; small direct intermolecular hyperconjugation and larger indirect intermolecular hy- perconjugation; large decrease of intramolecular hyperconjugation. The topological analysis of elec- tron density shows that in each complex there are three intermolecular critical points: there is one bond critical point between the acceptor atom Y and each hydrogen, and there is a ring critical point inside the tetragon YHCH, so these interactions are exactly H-bonding.     

The interaction strength of intermolecular systems formed by NaH⋯2(HF) and NaH⋯4(HF): Hydrogen bonds,dihydrogen bonds and halogen–hydride bonds

In this paper, a theoretical study of the molecular properties of NaH⋯2(HF) and NaH⋯4(HF) complexes is reported. Based on MP2/6-311++G(d,p) calculations, the dihydrogen bonds (H⋯H), hydrogen bonds (F⋯H) and halogen-hydride bonds (F⋯Na) of these intermolecular systems were fully characterized. The characterization involved the following procedures: the examination of structural parameters, analysis of vibration modes such as frequencies shifted to red or blue in the infrared spectrum, modeling of the electronic topology, quantification of the cooperative energy followed by charge transfer and, finally, natural bond orbital analysis. The results show short intermolecular distances with high electronic density, while the stretch frequencies of the proton donors and acceptors are unusually shifted, and some values reach 1000 cm⁻¹. When all subunits of the complexes are taken into account, in this case the NaH and HF molecules, the high value for the strength of the H⋯H dihydrogen bond in NaH⋯2(HF) suggests the formation of an additional subpart, i.e., the H₂ molecule.     

Theory of chemical bonds in metalloenzymes VI: Manganese–oxo bonds in the photosynthesis II system

Electronic and spin structures of high-valent manganese–oxo bonds in the photosynthesis II system (oxygen evolving center, OEC) are investigated by the use of spin polarized hybrid DFT (HDFT) method. Theoretical calculations of a high-valent manganese–oxo porphyrin complex are also performed to elucidate common characteristic of the active MnO bonds in both native OEC and artificial systems. The oxygen site of the high-valent MnO is found to be electrophilic in nature, in accord with our previous work, where the SE₂, ¹O- and ³O-models have been presented for theoretical understanding of complex behaviors of oxygenation reactions by metal–oxo species. The ¹O- and ³O-models are applicable to model complexes examined here, since the manganese–oxo bonds exhibit strong biradical character. Possibility of the SE₂-like transition structure model for OEC is also discussed on both the theoretical and experimental grounds. Implications of present computational results are discussed in relation to hydroxylation reaction by MMO and P450.     

Molecular dynamics simulations of the hydration of poly(vinyl methyl ether):Hydrogen bonds and quasi-hydrogen bonds

Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.     

Recent strategic advances for the activation of benzylic C–H bonds for the formation of C–C bonds

Alkylarenes, obtained from abundant hydrocarbon feedstock sources, are an attractive starting material for the formation of complex molecular architectures. Conventional activation strategies of the relatively inert sp³-hybridized benzylic C–H bonds usually require relatively harsh conditions and are difficult to apply to the synthesis of fine chemicals. The present review describes recent strategic advances for the activation of benzylic C–H bonds for the catalytic formation of C–C bonds. In particular, two activation methods, i.e., strategies that generate benzylic radicals or benzyl anions, are discussed.     

M2TNR(M=AG, CHZ)的制备和晶体结构

通过三硝基间苯二酚(TNR,斯蒂酚酸)和氨基胍(AG)重碳酸盐、碳酰肼(CHZ)反应,制备斯蒂酚酸氨基胍和斯蒂酚酸碳酰肼。并用X-射线衍射分析、元素分析、差示扫描量热法和红外光谱对其进行了结构表征。它们的分子式分别为(NH2NHCNHNHNH2)2^+[C6H(NO3)3O2]^2.0.5H2O,(NH2NHCONHNH3)2^+[C6H(NO3)3O2]^2^-,二者都属于单斜晶系,C2/c空间群。     

Study of series of the M1,M2‖F,Cl,Br quaternary reciprocal systems (M1 and M2 are group 1 s-block elements)

Quaternary reciprocal systems of alkali metal fluorides, chlorides, and bromides were partitioned into simplexes by a geometric method and a graph method. Phase transformations and chemical reactions in the bordering ternary reciprocal systems were described. Conversion lines were experimentally studied, and information on crystallizing phases within the composition prisms of the M₁,M₂‖F,Cl,Br systems (M₁ and M₂ are group 1 s-block elements) was obtained.     

Intramolecular σ-bond metathesis between carbon-carbon and silicon-silicon bonds

An intramolecular σ-bond metathesis between carbon-carbon and silicon-silicon bonds took place on treatment of a disilane tethered to a cyclobutanone with a palladium(0) catalyst, furnishing a silaindane skeleton as well as an acylsilane functionality at once.     

The open-close mechanism of M2 channel protein in influenza A virus:A computational study on the hydrogen bonds and cation-π interactions among His37 and Trp41

The M2 protein from influenza A virus is a tetrameric ion channel. It was reported that the permeation of the ion channel is correlated with the hydrogen bond network among His37 residues and the cation-π interactions between His37 and Trp41. In the present study,the hydrogen bonding network of 4-methyl-imidazoles was built to mimic the hydrogen bonds between His37 residues,and the cation-π interactions between 4-methyl-imidazolium and indole systems were selected to represent the interac-tions between His37 and Trp41. Then,quantum chemistry calculations at the MP2/6-311G level were carried out to explore the properties of the hydrogen bonds and the cation-π interactions. The calcula-tion results indicate that the binding strength of the N-H···N hydrogen bond between imidazole rings is up to -6.22 kcal·mol-1,and the binding strength of the strongest cation-π interaction is up to -18.8 kcal·mol-1(T-shaped interaction) or -12.3 kcal·mol-1(parallel stacking interaction). Thus,the calcu-lated binding energies indicate that it is possible to control the permeation of the M2 ion channel through the hydrogen bond network and the cation-π interactions by altering the pH values.     

Intramolecular O-H··N hydrogen bonds in 2-hydroxybenzaldimine compounds: Spectroscopic and quantum chemical investigations

Summary Spectroscopic and quantum chemical data that characterize the intramolecular hydrogen bonds in a series of 2-hydroxybenzaldimine compounds (Ph(OH)(CH=N-R);R=-NH(Ph), -OH,-OCH₃, -NH₂, -Ph, -CHO, -H, -CH₃, -(CO)(CH₃) are reported. Optimized geometries and vibrational spectra were calculated at HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels, NMR spectra at GIAO-BLYP/6-311++G(d,p)//B3LYP/6-31G(d,p) level of theory. The sequences of calculated frequencies (v(OH) andv(CN)) and chemical shifts ((OH)) agree reasonably well with the corresponding experimental data. The consistency of the calculated data is demonstrated by exploiting several correlations between bond distances, vibrational frequencies, chemical shifts, and hydrogen bond energies. In particular, there exists an almost perfect relation between the hydrogen bond strengths, as measured byr(O-H) distances, and the hydrogen bond distancesr(H··N) andr(O··N) It is shown that electrostic potentials and several kinds of partial charges (Mulliken, CHELPG, MK, and NPA) of the nitrogen atoms, to a first approximation, may serve as a means for characterizing the proton acceptor capabilities of the different imino groups.

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Intramolekulare O-H··N-Wasserstoffbrückenbindungen in 2-Hydroxybenzaldiminverbindungen: Spektroskopische und quantenchemische Untersuchungen

Zusammenfassung Intramolekulare Wasserstoffbrückenbindungen in einer Reihe von 2-Hydroxybenzaldiminverbindungen (Ph(OH)(CH=N-R);R=-NH(Ph), -OH, -OCH₃, -NH₂, -Ph, -CHO,-H, -CH₃, -(CO)CH₃) werden mit Hilfe von spektroskopischen und quantenchemischen Daten charakterisiert und untersucht. Optimierte Geometrien und Schwingungsspektren wurden mittelsab initio (HF/6-31G(d,p)) und DFT Methoden (B3LYP/6-31G(d,p)) berechnet, NMR-Spektren mittels GIAO-BLYP/6-311++G(d,p)//B3LYP/6-31G(d,p). Die berechneten Frequenzen (v(OH) undv(CN)) sowie die chemischen Verschiebungen ((OH)) stimmen mit den entsprechenden experimentellen Daten gut überein. Die innere Konsistenz der Rechnungen wird durch verschiedene Korrelationen zwischen Bindungsabständen, Schwingungsfrequenzen, chemischen Verschiebungen und Wasserstoffbrückenbindungsenergien demonstriert. Insbesondere findet man eine nahezu perfekte Relation zwischen den Bindungsabständenr(O-H), die als Maß für die Stärken der Wasserstoffbrückenbindungen dienen, und den Wasserstoffbrückenbindungsabständenr(H··N) undr(O··N). Es wird gezeigt, daß die Protonenakzeptorfähigkeiten der unterschiedlichen Iminogruppen in erster Näherung durch elektrostatische Potentiale und verschiedene Partialladungen (Mulliken, CHELPG, MK und NPA) der Stickstoffatome charakterisiert werden können.

     

Reactivity of the tethered alkyl uranium bonds of (η5:κ1-C5Me4SiMe2CH2)2U

Uranium-carbon bond reactivity has been investigated with the bis(tethered silylalkyl) uranium metallocene (η⁵:κ¹-C₅Me₄SiMe₂CH₂)₂U, 1. Tert-butyl nitrile, ^tBuCN, inserts into both of the tethered U-C bonds to produce the bis(tethered ketimide) complex [η⁵:κ¹-C₅Me₄SiMe₂CH₂C(^tBu)N]₂U, 2, which has unusually bent U-N-C bond angles. Carbon dioxide also inserts into both U-C bonds of 1 yielding the bis(tethered carboxylate) (C₅Me₄SiMe₂CH₂CO₂)₂U, 3. Neither PhCCPh nor PhCCH insert into the U-C bonds, but PhCCH cleaves the silylalkyl tethers in 1 to generate (C₅Me₄SiMe₃)^1? ligands in the complex (C₅Me₄SiMe₃)₂U(CCPh)₂, 4.     

SiO_2负载的Pt－M、Rh－M异双核络合物催化剂催化CO_2氢化反应

考察了ＳｉＯ２负载的（ＰＰｈ３）２ＨＰｔ（μ－ＣＯ）（μ－ＰＰｈ２）Ｍ（ＣＯ）４和（ｄｐｐｅ）Ｒｈ（μ－ＣＯ）２Ｍ（ＣＯ）３（Ｍ＝Ｃｒ、Ｍｏ、Ｗ；ｄｐｐｅ＝Ｐｈ２Ｐ（ＣＨ２）２ＰＰｈ２）异双核络合物催化剂催化ＣＯ２氢化反应的活性和选择性．两者皆表现出较高的催化活性和含氧化合物选择性，而前者比后者更好．     

Pt2M2异金属四核原子簇化合物成键规律探讨

对十二个Ｐｔ２Ｍ２四核异金属簇化合物进行了量子化学ＤＶ－Ｘα方法的计算与讨论，探讨了这些原子簇的成键规律，它们大致可分为三类：Ｐｔ与Ⅷ族金属形成的四核簇，Ｐｔ尽量保持自己原有的价电荷；Ｐｔ与ⅥＢ族金属形成的原子簇，ⅥＢ族元素是电荷的主要提供者，Ｐｔ与ＩＢ族金属形成的原子族，体系主要由配体ＰＰｈ３提供电荷，并提出，簇合时，三苯基膦酸体起电子调节作用，环戊烯基配体起推电子作用，从能带分析，ⅥＢ族或ＩＢ     

Redox-active metal–metal bonds between lanthanides in dimetallofullerenes

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