Theoretical study of the Si–H group as potential hydrogen bond donor

The ability of the Si–H group as hydrogen bond (HB) donor has been studied theoretically. Most of the selected molecules include the Si–H group in a polar environment that could produce an electron deficiency on the hydrogen atom. In addition, analogous derivatives where the silicon atom has been replaced by a carbon atom have been considered. In all cases, ammonia has been used as HB acceptor. The calculations have been carried out at the MP2/6‐311++G** computational level. The electron density of the complexes has been characterized within the atoms in molecules (AIM) framework. A search in the Cambridge Structural Database (CSD) has been carried out to verify the existence of this kind of interactions in solid phase. The results of the theoretical study on these HB complexes between ammonia and the silicon derivatives provides long HB distances (2.4 to 3.2 Å) and small interaction energies (?2.4 to ?0.2 kcal/mol). In all cases, the HBs of the corresponding carbon analogs show shorter interaction distances corresponding to stronger complexes. The CSD search provides a small number of short interactions between Si and other heavy atoms in agreement with the small stabilizing energy of the Si–H?N HB and the lack of SiH bond in polar environment within the database. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

C-H bond activation of arenes by a transient eta2-cyclopropene niobium complex

The intermolecular C-H bond activation of benzene occurs under very mild conditions (room temperature) via a rare stereospecific 1,3-H addition on an unsaturated eta2-cyclopropene intermediate generated by a beta-H abstraction of CH4 from TpMe2NbMe(c-C3H5)(MeCCMe) to give TpMe2NbPh(c-C3H5)(MeCCMe).     

C-H Group as proton donor by formation of a weak hydrogen bond

A systematic CNDO/2 study has been carried out on the weak C-H...F, C-H...O, and C-H...N hydrogen bonds. An H-bond formation has been revealed for proton donors with negative excess charge on the hydrogen. The concept of the positive character of the hydrogen as well as some essential features of the hydrogen bond have been discussed. Some useful correlations have been found and the importance of the charge shifts has been emphasized.     

Silanediols: a new class of hydrogen bond donor catalysts

Silanediols are introduced as a new class of hydrogen bond donor catalysts for the activation of nitroalkenes toward nucleophilic attack. Excellent yields of product are obtained for the conjugate addition of indole to β-nitrostyrene catalyzed with a stable, storable dinaphthyl-derived silanediol. The preparation and structural characterization of a C(2)-symmetric chiral silanediol is also reported along with its ability to catalyze the conjugate addition reaction.     

A non-classical hydrogen bond in the molybdenum arene complex [eta 6-C6H5C6H3(Ph)OH]Mo(PMe3)3: evidence that hydrogen bonding facilitates oxidative addition of the O-H bond

Mo(PMe3)6 reacts with 2,6-Ph2C6H3OH to give the eta 6-arene complex [eta 6-C6H5C6H3(Ph)OH]Mo(PMe3)3 which exhibits a non-classical Mo...H-OAr hydrogen bond; DFT calculations indicate that the hydrogen bonding interaction facilitates oxidative addition of the O-H bond to give [eta 6,eta 1-C6H5C6H3(Ph)O]Mo(PMe3)2H.     

Oxo-tethered ruthenium(II) complex as a bifunctional catalyst for asymmetric transfer hydrogenation and H2 hydrogenation

Newly developed oxo-tethered Ru amido complexes (R,R)-1 and their HCl adducts (R,R)-2 exhibited excellent catalytic performance for both asymmetric transfer hydrogenation and the hydrogenation of ketonic substrates under neutral conditions without any cocatalysts to give chiral secondary alcohols with high levels of enantioselectivity.     

The boron-boron single bond in diborane(4) as a non-classical electron donor for hydrogen bonding

An ab initio study of an isomer of diborane(4) [B(2)H(4)] has been carried out at MP2/aug-cc-pVTZ to investigate the ground-state properties of this unusual molecule, a derivative of which has been described in the recent literature. The geometric, electronic and orbital characteristics of B(2)H(4)(4) have been analyzed using AIM, NBO, and ELF methodologies. A region with a high concentration of electron density is located near and along the B-B bond, on the opposite side of this bond relative to the bridging H atoms. This site serves as an electron-donor site to electrophiles, resulting in hydrogen-bonded complexes of B(2)H(4) with proton donors HF, HNC, HCl, HCN, and HCCH, and a van der Waals complex with H(2). These complexes have C(2v) symmetry and stabilization energies that vary from 2 to 27 kJ mol(-1). The SAPT2 energy decomposition analysis shows that the relative importance of the various terms that contribute to the interaction energy depends on the strength of the interaction.     

Molecular chemistry in a zeolite: genesis of a zeolite Y-supported ruthenium complex catalyst

Dealuminated zeolite Y was used as a crystalline support for a mononuclear ruthenium complex synthesized from cis-Ru(acac)2(C2H4)2. Infrared (IR) and extended X-ray absorption fine structure spectra indicated that the surface species were mononuclear ruthenium complexes, Ru(acac)(C2H4)2(2+), tightly bonded to the surface by two Ru-O bonds at Al(3+) sites of the zeolite. The maximum loading of the anchored ruthenium complexes was one complex per two Al(3+) sites; at higher loadings, some of the cis-Ru(acac)2(C2H4)2 was physisorbed. In the presence of ethylene and H2, the surface-bound species entered into a catalytic cycle for ethylene dimerization and operated stably. IR data showed that at the start of the catalytic reaction, the acac ligand of the Ru(acac)(C2H4)2(2+) species was dissociated and captured by an Al(3+) site. Ethylene dimerization proceeded approximately 600 times faster with a cofeed of ethylene and H2 than without H2. These results provide evidence of the importance of the cooperation of the Al(3+) sites in the zeolite and the H2 in the feed for the genesis of the catalytically active species. The results presented here demonstrate the usefulness of dealuminated zeolite Y as a nearly uniform support that allows precise synthesis of supported catalysts and detailed elucidation of their structures.     

1H- and 2H-T1 relaxation behavior of the rhodium dihydrogen complex [(triphos)Rh(eta 2-H2)H2]+

Protonation of the classical trihydride [(triphos)RhH3] (2) at 210 K in either THF or CH2Cl2 by either HBF4.OMe2 or CF3SO2OH gives the nonclassical eta 2-H2 complex [(triphos)Rh(eta 2-H2)H2]+ (1) [triphos = MeC(CH2PPh2)3]. Complex 1 is thermally unstable and highly fluxional in solution. In THF above 230 K, 1 transforms into the solvento dihydride complex [(triphos)Rh(eta 1-THF-d8)H2]+ (5) that, at room temperature, quickly converts to the stable dimer trans-[[(triphos)RhH]2(mu-H)2]2+ (trans-6). In CH2Cl2, 1 is stable up to 240 K. Above this temperature, the eta 2-H2 complex begins to convert into a mixture of trans- and cis-6, which, in turn, transform into the bridging-chloride dimers trans- and cis-[[(triphos)RhH]2(mu-Cl)2]2+ at room temperature. Complex 1 contains a fast-spinning H2 ligand with a T1min of 38.9 ms in CD2Cl2 (220 K, 400 MHz). An NMR analysis of the bis-deuterated isotopomer [(triphos)RhH2D2]+ (1-d2) did not provide a J(HD) value. At 190 K, the perdeuterated isotopomers [(triphos)RhD3] (2-d3) and 1-d4 show T1min values of 16.5 and 32.6 ms (76.753 MHz), respectively, for the rapidly exchanging deuterides. An analogous 2-fold elongation of T1min is also observed on going from [(triphos)IrD3] to [(triphos)Ir(eta 2-D2)D2]+. A rationale for the elongation of T1min in nonclassical polyhydrides is proposed on the basis of both the results obtained and recent literature reports.     

Interactions of polar hydrogen bond donor solvents with ions: a theoretical study

Structural Chemistry - Intermolecular interactions between molecules of protic solvents (water, methanol, formic acid, formamide, methylamine and ammonia) and monatomic ions (Li+, Na+, K+,...     

Synthesis and characterization of a cationic ruthenium complex featuring an unusual Bis(eta2-BH) monoborane ligand

The reaction of Cp*Ru(P ( i )Pr 3)Cl with MesBH 2 (Mes = 2,4,6-trimethylphenyl), followed by chloride abstraction with LiB(C 6F 5) 4.2.5OEt 2 (LiBF 20), afforded the crystallographically characterized complex [Cp*Ru(P ( i )Pr 3)(BH 2Mes)] (+)B(C 6F 5) 4 (-); notably, this represents the first reported cationic complex to feature an eta (2)-BH monoborane ligand, as well as a rare example of bis(eta (2)-BH) ligation.     

Communication: Frequency shifts of an intramolecular hydrogen bond as a measure of intermolecular hydrogen bond strengths

Infrared-ultraviolet double resonance spectroscopy has been applied to study the infrared spectra of the supersonically cooled gas phase complexes of formic acid, acetic acid, propionic acid, formamide, and water with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. In these complexes each binding partner to 9HFCA can function as both proton donor and acceptor. Relative to its frequency in free 9HFCA, the 9-hydroxy (9OH) stretch is blue shifted in complexes with formic, acetic, and propionic acids, but is red shifted in the complexes with formamide and water. Density functional calculations on complexes of 9HFCA to a variety of H bonding partners with differing proton donor and acceptor abilities reveal that the quantitative frequency shift of the 9OH can be attributed to the balance struck between two competing intermolecular H bonds. More extensive calculations on complexes of glycolic acid show excellent consistency with the experimental frequency shifts.     

Synthesis,characterization, and catalytic activity of a ruthenium carbene complex coordinated with bidentate 2-pyridine-carboxylato ligands

The halide and phosphine free complex [(sIMes)(C₅H₄N-2-CO₂)₂RuCHPh] (7) (sIMes = 1,3-dimesitylimidazolidin-2-ylidene) bearing two bidentate 2-pyridinecarboxylato ligands was synthesized from the carbene complex [(sIMes)(PCy₃)(Cl)₂RuCHPh] (4) and the silver 2-pyridine-carboxylate (8). The molecular structure of the octahedral complex 7 reveals that the two carboxylato functions are coordinated in cis geometry to the ruthenium center. Catalyst 7 exhibits activity in ring-closing metathesis (RCM) reactions after addition of a cocatalyst (HCl) in dichloromethane as well as in methanol solution.     

Detection of hydrogen bonding in solution: a 2H nuclear magnetic resonance method based on rotational motion of a donor/acceptor complex

The effect of hydrogen bonding on the rotational correlation time of an H-bond acceptor, pyridine N-oxide-d(5), in various solvents was investigated using the (2)H spin-lattice relaxation time (T(1)). The results demonstrate a linear relationship between viscosity and measured rotational correlation times, an example of Stokes-Einstein-Debye behavior. The results also clearly demonstrate reduced rotational rates for the probe in hydrogen bonding solvents in comparison to solvents incapable of forming hydrogen bonds with the probe. The utility of this observation was exploited to estimate the association constant (K(a)) through an NMR titration procedure. These results are presented as a new technique that can be applied to the characterization of hydrogen bonding in similar systems.     

Spectroscopic evidence for a dinitrogen complex of gallium and estimation of the Ga-N2 bond strength

Matrix-isolation experiments were performed to study the interaction between Ga atoms and N2 by using Raman and UV/Vis spectroscopies for detection and analysis. It was revealed that a weak complex is formed, for which resonance Raman spectra were obtained. Several overtones were sighted, allowing a rough estimate of the Ga-N2 fragmentation energy to be made (approximately 19 kJ mol(-1)). The excitation profile obtained from the spectra at different laser wavelengths agrees with the UV/Vis spectrum and shows that the complex exhibits an electronic transition at around 410 nm. At the Ga atom, this transition can be described as a 2S<--2P or 2D<--2P excitation, which is red-shifted from its position for free Ga atoms (approximately 340 nm and 270 nm for 2S<--2P and 2D<--2P, respectively) as a result of N2 complexation. The effect of complexation involves, therefore, only slight stabilization of the 2P ground state but relatively strong stabilization of the excited (2)S state. Accordingly, for the Ga atom in its excited 2S state, the Ga-N2 bond energy can be estimated to be around 79 kJ mol(-1).     

Coordination chemistry of a kinetically stabilized germabenzene: syntheses and properties of stable eta6-germabenzene complexes coordinated to transition metals

The first stable eta6-germabenzene complexes, that is, [M(CO)3(eta6-C5H5GeTbt)] {M=Cr (2), Mo (3), and W (4); Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}, have been synthesized by ligand-exchange reactions between [M(CO)3(CH3CN)3] (M=Cr, Mo, and W) and the kinetically stabilized germabenzene 1 and characterized by 1H and 13C NMR, IR, and UV/Vis spectroscopy. In the 1H and 13C NMR spectra of 2-4, all of the signals for the germabenzene rings were shifted upfield relative to their counterparts in the free germabenzene 1. X-ray crystallographic analysis of 2 and 4 revealed that the germabenzene ligand was nearly planar and was coordinated to the M(CO)3 group (M=Cr, W) in an eta6 fashion. The formation of complexes 2-4 from germabenzene 1 should be noted as the application of germaaromatics as 6pi-electron ligands toward complexation with Group 6 metals. On the other hand, treatment of 1 with [{RuCp*Cl}4] (Cp*=C5Me5) in THF afforded a novel eta5-germacyclohexadienido complex of ruthenium-[RuCp*{eta5-C5H5GeTbt(Cl)}] (9)-instead of the expected eta6-germabenzene-ruthenium cationic complex [RuCp*{eta6-C5H5GeTbt}]Cl (10). Crystallographic structural analysis of 9 showed that the five carbon atoms of the germacyclohexadienido ligand of 9 were coordinated to the Ru center in an eta5 fashion.     

Carboxylate coordination chemistry of a mononuclear Ni(II) center in a hydrophobic or hydrogen bond donor secondary environment: relevance to acireductone dioxygenase

A series of Ni(II) carboxylate complexes, supported by a chelate ligand having either secondary hydrophobic phenyl groups (6-Ph2TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) or hydrogen bond donors (bnpapa, N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been prepared and characterized. X-ray crystallographic studies of [(6-Ph2TPA)Ni(O2C(CH2)2SCH3)]ClO4.CH2Cl2 (4.CH2Cl2) and [(6-Ph2TPA)Ni(O2CCH2SCH3)]ClO(4).1.5CH2Cl2 (5.1.5CH2Cl2) revealed that each complex contains a distorted octahedral Ni(II) center and a bidentate carboxylate ligand. A previously described benzoate complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3)) has similar structural characteristics. Recrystallization of dry powdered samples of 3, 4.0.5CH2Cl2, and 5 from wet organic solvents yielded a second series of crystalline Ni(II) carboxylate complexes having a coordinated monodentate carboxylate ligand ([(6-Ph2TPA)Ni(H2O)(O2CPh)]ClO4 (6), [(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4.0.2CH2Cl2 (7.0.2CH2Cl2), [(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]ClO4 (8)) which is stabilized by a hydrogen-bonding interaction with a Ni(II)-bound water molecule. In the cationic portions of 7.0.2CH2Cl2 and 8, weak CH/pi interactions are also present between the methylene units of the carboxylate ligands and the phenyl appendages of the 6-Ph2TPA ligands. A formate complex of the formulation [(6-Ph2TPA)Ni(H2O)(O2CH)]ClO4 (9) was isolated and characterized. The mononuclear Ni(II) carboxylate complexes [(bnpapa)Ni(O2CPh)]ClO4 (10), [(bnpapa)Ni(O2C(CH2)2SCH3)]ClO4 (11), [(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12), and [(bnpapa)Ni(O2CH)]ClO4 (13) were isolated and characterized. Two crystalline solvate forms of 10 (10.CH3CN and 10.CH2Cl2) were examined by X-ray crystallography. In both, the distorted octahedral Ni(II) center is ligated by a bidentate benzoate ligand, one Ni(II)-bound oxygen atom of which accepts two hydrogen bonds from the supporting bnpapa chelate ligand. Spectroscopic studies of 10(-13) suggest that all contain a bidentate carboxylate ligand, even after exposure to water. The combined results of this work enable the formulation of a proposed pathway for carboxylate product release from the active site Ni(II) center in acireductone dioxygenase.     

The visible absorbance maximum of 2-hydroxy-1,4-naphthoquinone as a novel probe for the hydrogen bond donor abilities of solvents and solvent mixtures

Summary The electronic absorption spectra of 2-hydroxy-1,4-naphthoquinone (HNQ) in one- and two-component solvents are discussed. The visible absorption of this reagent has been shown to arise from a charge-transfer transition of the zwitterionic tautomer stabilized by a hydrogen bond donating solvent. The formation constant of the 1:1 hydrogen bonded complex with methanol, ethanol, andn-propanol has been determined at 25 °C from the spectral behaviour in mixed solvents. The tautomerization equilibrium ofHNQ in aqueous ethanol is demonstrated and characterized. The dependence of the absorbance maximum ofHNQ on the hydrogen bond donor ability of the solvent in the visible spectrum, as measured by theTaft-Kamlet parameter, has been established. Experimentally derived evidence is supplied to justify the recommendation ofHNQ as a novel probe for hydrogen bond donor ability in pure solvents and in mixed aqueous solvents.

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Das Absorptionsmaximum von 2-Hydroxy-1,4-naphthochinon im sichtbaren Bereich als neues Maß für die Wasserstoffbrückenbindungsbildungsfähigkeit von Lösungsmitteln und Lösungsmittelgemischen

Zusammenfassung Die Absorptionsspektren von 2-Hydroxy-1,4-naphthochion (HNQ) in Ein- und Zweikomponentensystemen werden diskutiert. Die Absorption im sichtbaren Bereich stammt von einemcharge-transfer — Übergang des zwitterionischen Tautomers, das über Wasserstoffbrückenbindungen zum Lösungsmittel stabilisiert wird. Die Bildungskonstanten der binären Komplexe mit Methanol, Ethanol undn-Propanol wurden aus spektroskopischen Daten in Lösungsmittelgemischen bei 25 °C emittelt. Das Tautomeriegleichgewicht vonHNQ in wäßrigem Ethanol wird iskutiert. Die Abhängigkeit des Absorptionsmaximums vonHNQ vond der Wasserstoffbrückenbindungsbildungsfähigkeit des Lösungsmittels wird mittels desT aft-Kamlet — Parameters beschrieben. Experimentelle Ergebnisse ermutigen zur Verwendung vonHNQ als neue Testsubstanzfür die Wasserstoffbrückenbindungsbildungsfähigkeit von reinen und gemischten Lösungsmitteln.

     

Unusual properties of the first copper complex containing a pi(eta 2)-coordinated phosphorus-carbon double bond moiety

The first stable phosphaarene pi-complex of copper displays unusual 31P NMR data that suggest a novel interpretation of 31P coordination shifts in pi-complexes of phosphorus containing multiple bonds.