Π-acceptor Ability Research Articles

Rhodium-Catalyzed Asymmetric Cycloisomerization of 1,6-Enynes

1,6-Enynes can be converted into bicyclo[4.1.0]heptene derivatives via 6-endo-dig cycloisomerization catalyzed by π-acidic metals. Despite the high synthetic utility of this transformation, there have been only few asymmetric variants reported so far. In this paper the authors describe a rhodium-catalyzed cycloisomerization of oxygen- and nitrogen-bridged 1,6-enynes proceeding with high yields (71-94%) and enantio­selectivities (68-99% ee). Substituted tetrafluoro­benzobarrelene (TFB, 1) was found to be the optimum ligand showing high catalytic activity presumably due to its high π-accepting ability. It can be expected that the unique catalyst design will be suitable for other applications of transition-metal catalysis.

The roles of ligands proton affinity, π-back donation and metal fragment hardness on the Au–N bond in N-donor heterocycles gold(III) complexes

A DFT study on the Au–N interaction for some groups of N-donor heterocycles L {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AuX 3L complexes {AuX 3 = AuBr 3, AuCl 3, trans-AuCl(CN) 2, Au(CN) 3} is reported. Linear relationships between the AuX 3 Mulliken charge in AuX 3L and the computed proton affinity (PA) of the heterocycle were found for all the considered ligands. The different slopes found on changing the N-donor species represent a measure of the π-acidity of these nitrogen ligands once coordinated to the metal centre, by the consequence a π-acceptor ability scale has been derived. The π-acceptor ability of the 5-membered N-donor ligands resulted in all the cases greater than that of the 6-membered N-heterocycles. The proton affinity average value corresponding to a zero charge of the AuX 3 and L fragments in the AuX 3L species has been estimated. This parameter represents the minimum PA value for the formation of a bond between the N-heterocycles and gold(III) and it does not depend on the electronic features of the coordinated ligands. The sensitivity of the AuX 3 fragments towards ligands PA variations follows the order Au(CN) 3 < trans-AuCl(CN) 2 < AuCl 3 < AuBr 3 and this last result has been explained on the basis of the metal fragments relative hardness.

Synthesis and Photophysical Properties of Ruthenium(II) Isocyanide Complexes Containing 8-Quinolinolate Ligands

A series of ruthenium(II) bis(8-quinolinolato) complexes bearing isocyanide ligands (RNC) have been synthesized by the reaction of [RuQ3] (Q = 8-quinolinolate) with RNC in the presence of Zn/Hg. These complexes have the general formula [RuQ2(RNC)2] (1, R = tert-butyl; 2, R = 4-MeOPh; 3, R = 4-ClPh; 4, R = 2,4,6-Br3Ph). Both the yellow cis,cis,trans (a) and orange-red trans,trans,trans (b) isomers have been isolated for complexes 1−4. trans,trans,trans-[Ru(Tol-Q)2(tBuNC)2] (6, HTol-Q = 8-hydroxyl-5-tolylquinoline) has also been prepared from [Ru(PPh3)2Cl2]. The structures of 2a, 3a, and 4b have been determined by X-ray crystallography. These complexes exhibit an intense absorption band in the UV region (λmax = 320−390 nm) with molar extinction coefficients (ε) on the order of 104 dm3 mol−1 cm−1 and a moderately intense absorption with ε on the order of 103 dm3 mol−1 cm−1 at 400−492 nm. The intense absorption at 320−390 nm is assigned to the ligand-centered π→π* transitions of the quinolinolate ligands, probably mixed with the π→π* transitions of the isocyanide ligands. The lower energy absorptions at 400−492 nm are assigned to Ru(dπ)→π*(Q) metal-to-ligand charge transfer (MLCT) transitions. Upon excitation at λ > 350 nm, 1a−3a in dichloromethane solution exhibit orange-red luminescence (645−680 nm). In 77 K EtOH/MeOH glass, complexes 1−4 and 6 give intense structured emission spectra (593−638 nm). The cyclic voltammograms (CV) of 1a−4a generally exhibit an irreversible or quasi-reversible RuIII/II couple at the potential range of 0.02−0.38 V vs Fc+/Fc, except in the case of 1a, where a reversible RuIII/II and a quasi-reversible RuIV/III (0.68 V vs Fc+/Fc) couple are observed. The potential for the RuIII/II couple increases with the π-accepting ability of the isocyanide ligands. In the CV of 1b−4b, a reversible RuIII/II (−0.16 to 0.065 V) and quasi-reversible RuIV/III (0.70−0.85 V) couple are observed.

Synthesis and characterization of trans-[Ru(NO)Cl(L)4](PF6)2 (L = isonicotinamide; 4-acetylpyridine) and related species

The syntheses and characterization of trans -[RuCl 2 (L) 4 ], trans -[RuCl(NO)(L) 4 ](PF 6 ) 2 (L = isonicotoninamide; 4-acetylpýridine) and the water soluble trans -[Ru(OH)(NO)(pyridine) 4 ]Cl 2 are described. All nitrosyl complexes show the infrared ν NO band consistent with a nitrosonium character of the NO ligand and undergo successive reductions, passing from {RuNO} 6 to {RuNO} 7 to {RuNO} 8 in acetonitrile. The trans -[RuCl 2 (L) 4 ], trans -[Ru(NO)Cl (L) 4 ](PF 6 ) 2 (L = isonicotinamide and 4-acetylpyridine) and trans -[Ru(NO)(OH)(py) 4 ]Cl 2 (py = pyridine) complexes have been prepared and characterized by elemental analysis, UV–visible, infrared, and 1 H NMR spectroscopies, and cyclic voltammetry. The MLCT band energies of trans -[RuCl 2 (L) 4 ] increase in the order 4-acpy < isn < py. The reduction potentials of trans -[RuCl 2 (L) 4 ] and trans -[Ru(NO)Cl(L) 4 ] 2+ increase in the order py < isn < 4-acpy. The stretching band frequency, ν NO , of the nitrosyl complexes ranges from 1913 to 1852 cm −1 indicating a nitrosonium character for the NO ligand. Due to the large π-acceptor ability of the equatorial ligands, the coordinated water is much more acidic in the water soluble trans -[Ru(NO)(H 2 O)(py) 4 ] 3+ than in trans -[Ru(NO)(H 2 O)(NH 3 ) 4 ] 3+ .

A new π-acidity scale for several N-donor heterocycles as ligands in neutral gold(III) complexes

A computational DFT approach for the comparison of the π-acceptor character of some N-donor heterocycles L {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AuCl 3L complexes is reported. The electronic properties of these ligands have been tuned by adding methyl and/or trifluoromethyl groups in various positions. Linear relationships between the Mulliken charge of the AuCl 3 fragment in AuCl 3L and the computed proton affinity (PA) of the heterocycle were obtained for all the considered ligands. The different slopes found on changing the N-donor type represent a measure of the π-acidity of these nitrogen ligands once coordinated to AuCl 3, and as a consequence the π-acceptor ability scale pyridines ≈ pyrimidines < imidazoles < pyrazoles ≈ isoxazoles has been derived. Moreover, on the basis of the metal fragment charge variation, a minimum proton affinity value for a meaningful interaction between the ligands and the AuCl 3 fragment has been estimated.

Dehydrative Allylation of Alcohols and Deallylation of Allyl Ethers Catalyzed by [CpRu(CH3CN)3]PF6 and 2-Pyridinecarboxylic Acid Derivatives. Effect of π-Accepting Ability and COOH Acidity of Ligand on Reactivity

Abstract 2-Quinolinecarboxylic acid efficiently promotes both the dehydrative allylation of alcohols and the deallylation of allyl ethers and esters in the presence of [CpRu(CH3CN)3]PF6. Comparison of the relative reactivity of various 4-substituted 2-pyridinecarboxylic acids has revealed two linear relations with different ρ values in the Hammett plots. These phenomena can be rationalized by the balance between the π-accepting ability of the pyridine moiety and the acidity of the carboxylic acid of the 2-pyridinecarboxylic acid derivative.

Comparative properties of fluorous phosphine ligand complexes: W(CO) 5L F. Crystal structure of W(CO) 5P(C 6H 4-4-CH 2CH 2(CF 2) 7CF 3) 3

The compounds W(CO) 5P(C 6H 4-4-CH 2CH 2(CF 2) 7CF 3) 3 ( 1) and W(CO) 5P(CH 2CH 2(CF 2) 5CF 3) 3 ( 2) were synthesized in order to probe the electronic and physical effects of ligation by perfluorocarbon substituted tertiary phosphine ligands in a W(CO) 5L complex. The π-accepting ability of the fluorous phosphines was found to rank with non-fluorous comparators as P(CH 2CH 2(CF 2) 5CF 3) 3 > P(C 6H 4-4-CH 2CH 2(CF 2) 7CF 3) 3 > PPh 3 > P( p-tolyl) 3 > P( n-octyl) 3. The X-ray crystal structure of W(CO) 5P(C 6H 4-4-CH 2CH 2(CF 2) 7CF 3) 3 shows strong intermolecular association of fluorous components but confirms that the para fluorocarbon subtituents have an insignificant effect on the tungsten coordination environment. Partition coefficients (toluene/perfluoromethylcyclohexane) were measured for compounds 1 and 2.

A density functional investigation of the Bursten ligand additivity model for the d 6 octahedral complexes [Mn(CO) n(CNMe) (6−n)] + (M = Mn(I), Cr(0))

The Bursten ligand additivity model provides a description of orbital energies of complexes of the type [M(L) n (L′) 6− n ] x +, where L and L′ are ligands of different π-acceptor abilities. The model allows for the prediction molecular orbital energies, oxidation potentials and valence photoelectron spectra of octahedral complexes. The model was derived by Bursten from the results of Fenske–Hall approximate molecular orbital calculations. In this study non-local density functional calculations are used to determine optimized geometries, molecular orbital energies and charge distributions for the complexes [M(CO) n (CNMe) (6− n )] + (M = Mn(I), Cr(0)). The calculated DFT molecular orbital energies are used to derive ligand additivity relationships. Variations in C–O and M–C bond lengths and charge distributions are used to study the effect of ligand substitution on the degree of π-back-bonding in each complex. It is found that the results of DFT calculations show excellent agreement with the Bursten ligand additivity model, despite the considerable difference in computational methods and structural changes that occur on ligand substitution.

Synthesis and Reactions of Diphosphinidenecyclobutene Ruthenium Complexes Relevant to Catalytic Hydrosilylation of Terminal Alkynes

The complex [RuCl(μ-Cl)(CO)(DPCB-OMe)]2 (1a), bearing a low-coordinated phosphorus ligand (DPCB-OMe = 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is readily reduced to [RuH(μ-Cl)(CO)(DPCB-OMe)]2 (2a) by the reaction with water and HSiMe2Ph. The reaction proceeds via a [RuCl2(CO)(H2O)(DPCB-OMe)] intermediate, which is characterized by X-ray diffraction analysis. Complexes 1a and 2a serve as highly efficient catalysts for Z-selective hydrosilylation of phenylacetylene. The reason for the high catalyst efficiency of DPCB-OMe complexes has been investigated by reaction and structure analysis of the presumed intermediate [Ru(CH═CHPh)Cl(CO)(DPCB-OMe)] (3a). It has been found that 3a has ample space to associate with hydrosilane and, therefore, readily undergoes metathesis between Ru−C and H−Si bonds. This structural feature in conjunction with the strong π-accepting ability of the DPCB-OMe ligand leads to highly efficient catalysis for Z-selective hydrosilylation of ...

Rhodium and Iridium Complexes of Abnormal N-Heterocyclic Carbenes Derived from Imidazo[1,2-a]pyridine

Rhodium and iridium complexes of a new type of abnormal N-heterocyclic carbenes (NHCs) derived from imidazo[1,2-a]pyridiniums have been prepared via silver transmetalation, where metalation can be directed to either the C-2 or the C-3 position of imidazo[1,2-a]pyridine ring provided that the other position is appropriately blocked. The donating abilities of these new NHC ligands have been assessed from the average CO stretching frequencies of their corresponding iridium dicarbonyl complexes. By varying the N-alkyl groups or fusing the pyridine moiety with an aromatic ring, electronic effects of abnormal NHCs of this type can be readily tuned to match the extremely donating imidazole-derived abnormal NHCs or the relatively less donating normal NHCs (imidazolin-2-ylidenes). Both VT NMR studies and CO stretching frequency values strongly support the π-accepting ability of an abnormal NHC derived from imidazo[1,2-a]quinoline.

Theoretical investigation of the bonding properties of N-heterocyclic carbenes coordinated to electron-rich d 8 metal centers

Density functional theory (DFT) at the generalized gradient approximation (GGA) level has been applied for the analysis of the bond between group 10 metals and N-heterocyclic carbene (NHC) in complexes [MX 3(NHC)] − (M = Ni, Pd, Pt, X = H, Cl, I). For comparative purposes, similar calculations have been performed for analogous pyridine complexes [MX 3(py)] − (py = pyridine). Full geometry optimizations have been performed for all complexes. The role of the M–L π interaction was investigated by the aid of respectively, energy decomposition analysis, Hirshfeld atomic charge variation, molecular orbital considerations and bond order decomposition analysis. The π-bonding contribution increases in the order I < Cl < H, and Pt < Pd < Ni. Most significantly, the absolute π-acceptor ability of the NHC in these complexes is larger than that of pyridine. However, due to the dominant σ donor interactions, the relative contribution, that is the π/σ ratio, is predicted to be smaller.

Application of palladium(II) complex with bidentate phosphine sulfide ligands to palladium-catalyzed C–C coupling reaction

A phosphine sulfide Pd(II) complex, [Pd(p 2S 2) 2](BF 4) 2 ( 1) (p 2S 2 = 1,2-bis(diphenylphosphino)ethane disulfide), was synthesized and characterized by an X-ray crystal structure analysis and 31P NMR spectroscopy. The p 2S 2 ligand exchange rate of 1 with free p 2S 2 in chloroform was revealed to be comparable to the general solvent exchange rate on Pd(II). The catalytic activity of 1 was evaluated by carrying out the Heck reaction. The diminishing of the induction period and acceleration of the reaction were observed for 1 by comparing the phosphine Pd(II) complexes with a leaving chloro ligand, [PdCl(p 3)]Cl (p 3 = bis[2-(diphenylphosphino)ethyl]phenylphosphine) and [PdCl(pp 3)]Cl (pp 3 = tris[2-(diphenylphosphino)ethyl]phosphine), and the catalytic activity was comparable to that of the phosphine Pd(0) complex, [Pd(PPh 3) 4]. Such a high catalytic activity of 1 is attributed to the π-accepting ability of the phosphine sulfide S atom which stabilizes the catalytically active Pd(0) species electronically and weak σ-donation of the S atom which does not block the formation and a subsequent reaction of the Pd(II) substrate adduct in the catalytic cycle.

Rapid geometrical equilibrium of palladium(II) complexes with tris[2-(diphenylphosphino)ethyl]phosphine disulfide and diselenide and their catalytic activity for Suzuki coupling reaction

Palladium(II) complexes with a tetradentate pseudo-tripodal ligand having two phosphino groups and two phosphine sulfide or selenide groups, pp 3X 2 (pp 3 = tris[2-(diphenylphosphino)ethyl]phosphine, X = S ( 1) or Se ( 2)), were prepared from [PdCl(pp 3)]Cl. Both of these phosphine chalcogenide complexes 1 and 2 showed rapid equilibrium between the five-coordinate [PdCl(pp 3X 2)]Cl with two bound phosphine chalcogenide groups and four-coordinate [PdCl 2(pp 3X 2)] with two dissociated pendant ones in chloroform. The thermodynamic parameters for the reaction, [PdCl(pp 3X 2)] + + Cl −⇄[PdCl 2(pp 3X 2)], were obtained by low-temperature 31P NMR as follows: K 298 = 3.7 × 10 3 and 5.4 × 10 2 mol −1, Δ H ∘ = 11.3 ± 0.3 and 13.4 ± 0.4 kJ mol −1, and Δ S ∘ = 106 ± 2 and 97 ± 2 J mol −1 K −1 for 1 and 2, respectively. The rate for the geometrical change at 246.7 K for 1 was appreciably faster than that for 2. These thermodynamic and kinetic results indicate that the phosphine selenide Se atoms can stabilize the five-coordinate structure by effective π-back donation from Pd(II) compared with the phosphine sulfide S atoms. Difference in retention of the catalytic activity for Suzuki coupling, 2 > 1 > [PdCl(pp 3 or p 3)]Cl, was explained by difference in the π-accepting ability that stabilizes the catalytically active Pd(0) species. Considering the rapid dissociation-coordination equilibrium of the phosphine chalcogenide groups on Pd(II), it is probable that the oxidative addition and the subsequent transmetallation of the Pd(II) species are hardly blocked by the phosphine chalcogenide groups.

Multistep Electrochemical Reduction Path of Clusters [Os3(CO)10(α-diimine)]: Comparison of Electrochemical and Photochemical Os-Os(α-diimine) Bond Cleavage

Electrochemical reduction of the triangular clusters [Os3(CO)10(α-diimine)] (α-diimine = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym)) and [Os3(CO)10(μ-bpym)ReBr(CO)3] produces primarily the corresponding radical anions. Their stability is strongly determined by the π-acceptor ability of the reducible α-diimine ligand, which decreases in the order μ-bpym &gt; bpym &gt;&gt; bpy. Along this series, increasing delocalisation of the odd electron density in the radical anion over the Os(α-diimine) chelate ring causes weakening of the axial (CO)4Os-Os(CO)2(α-diimine) bond and its facile cleavage for α-diimine = bpy. In contrast, the cluster radical anion is inherently stable for the bridging bpym ligand, the strongest π-acceptor in the studied series. In the absence of the partial delocalisation of the unpaired electron over the Re(bpym) chelate bond, the Os3-core of the radical anion remains intact only at low temperatures. Subsequent one-electron reduction of [Os3(CO)10(bpym)]•- at T = 223 K gives the open-triosmium core (= Os3*) dianion, [Os3*(CO)10(bpym)]2-. Its oxidation leads to the recovery of parent [Os3(CO)10(bpym)]. At room temperature, [Os3*(CO)10(bpym)]2- is formed along a two-electron (ECE) reduction path. The chemical step (C) results in the formation of an open-core radical anion that is directly reducible at the cathodic potential of the parent cluster in the second electrochemical (E) step. In weakly coordinating tetrahydrofuran, [Os3*(CO)10(bpym)]2- rapidly attacks yet non-reduced parent cluster molecules, producing the relatively stable open-core dimer [Os3*(CO)10(bpym)]22- featuring two open-triangle cluster moieties connected with an (bpym)Os-Os(bpym) bond. In butyronitrile, [Os3*(CO)10(bpym)]2- is stabilised by the solvent and the dimer [Os3*(CO)10(bpym)]22- is then mainly formed by reoxidation of the dianion on reverse potential scan. The more reactive cluster [Os3(CO)10(bpy)] follows the same reduction path, as supported by spectroelectrochemical results and additional valuable evidence obtained from cyclic voltammetric scans. The ultimate process in the reduction mechanism is fragmentation of the cluster core triggered by the reduction of the dimer [Os3*(CO)10(α-diimine)]22-. The products formed are [Os2(CO)8]2- and {Os(CO)2(α-diimine)}2. The latter dinuclear fragments constitute a linear polymeric chain [Os(CO)2(α-diimine)]n that is further reducible at the α-diimine ligands. For α-diimine = bpy, the charged polymer is capable of reducing carbon dioxide. The electrochemical opening of the triosmium core in the [Os3(CO)10(α-diimine)] clusters exhibits several common features with their photochemistry. The same Os-α-diimine bond dissociates in both cases but the intimate mechanisms are different.

Pentacarbonyl(2-ferrocenylpyridine)metal(0) complexes of Group 6 elements. Synthesis and characterization

2-Ferrocenylpyridine (2-Fcpy) was prepared according to the literature procedure. 1,1′-Dipyridylferrocene was also obtained as a minor product and characterized by 1H- and 13C-n.m.r. spectroscopy. Prolonged irradiation of Cr(CO)6 in the presence of 2-Fcpy in n-hexane gave Cr(CO)5(2-Fcpy) which could not be isolated due to its instability even at low temperature, but was detected in solution by i.r. spectroscopy. The preparation of Mo(CO)6(2-Fcpy) from direct photolysis of Mo(CO)5 with 2-Fcpy could not be achieved. However, the reaction of Mo(CO)5 (THF) with 2-Fcpy gave Mo(CO)5 (2-Fcpy) which was isolated and characterized by i.r., 1H- and 13C-n.m.r. spectroscopic techniques. W(CO)5 (2-Fcpy) was prepared by irradiation of W(CO)6 in the presence of 2-Fcpy in n-hexane. The complex was isolated and characterized by i.r., 1H-, 13C-n.m.r. spectroscopic techniques. W(CO)5 (2-Fcpy) thus appears to be more stable than the Mo and Cr analogues. The main reason for the general instability of the M(CO)5 (2-Fcpy) complexes is assigned to the weak π-accepting ability of 2-ferrocenylpyridine.

Why is the reaction of ethyl (2-cyanoacetyl)carbamate with ethyl orthoformate highly stereoselective?

The reaction of ethyl (2-cyanoacetyl)carbamate (1) with ethyl orthoformate in the presence of acetic anhydride is highly stereoselective and only E-ethyl (2-cyano-3-ethoxyacryloyl)carbamate (E-2) is isolated. The reaction is thermodynamically controlled and the product distribution depends on the relative stability between E-2 and Z-2. Both the resonance stabilization of 1.47 kcal mol−1 and the steric hindrance of 2.28 kcal mol−1 in favour of E-2 contribute to the relative stability (3.75 kcal mol−1) between Z-2 and E-2, which is calculated from four isodesmic reactions, and this is the reason why the reaction of compound 1 with ethyl orthoformate is highly stereoselective. The electron-withdrawing ability of some substituents was evaluated. The sequence of π-accepting ability is C(O)NHC(O)OEt > C(O)NH2 > CN and the sequence of σ-accepting ability is CN > C(O)NHC(O)OEt > C(O)NH2. Copyright © 2005 John Wiley & Sons, Ltd.

Synthetic, structural and spectroscopic studies of (pseudo)halo(1,3-di-tert-butylimidazol-2-ylidine)gold complexes

A series of (pseudo)halo(1,3-di-tert-butylimidazol-2-ylidine)gold complexes [(But2Im)AuX](X = Cl, Br, I, CN, N3, NCO, SCN, SeCN, ONO2, OCOCH3, CH3) have been synthesized and characterised spectroscopically and structurally. 13C NMR chemical shifts for the carbene carbon vary widely with differing ancillary anion, correlating well with the sigma-donor ability of the latter and with the M-C(carbene) bond distance. These results reinforce the notion that N-heterocyclic carbene ligands are primarily sigma-donor ligands with little pi-acceptor ability.

Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac) 2 (PR 3) (R = iPr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Reaction of cis-[Ru(acac) 2(η 2-C 8H 14) 2] ( 1) (acac = acetylacetonato) with two equivalents of P i Pr 3 in THF at −25 °C gives trans-[Ru(acac) 2(P i Pr 3) 2], trans- 3, which rapidly isomerizes to cis- 3 at room temperature. The poorly soluble complex [Ru(acac) 2(PCy 3) 2] ( 4), which is isolated similarly from cis-[Ru(acac) 2(η 2-C 2H 4) 2] ( 2) and PCy 3, appears to exist in the cis-configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans- 4. In benzene or toluene 2 reacts with P i Pr 3 or PCy 3 to give exclusively cis-[Ru(acac) 2(η 2-C 2H 4)(L)] [L = P i Pr 3 ( 5), PCy 3 ( 6)], whereas in THF species believed to be either square pyramidal [Ru(acac) 2L], with apical L, or the corresponding THF adducts, can be detected by 31P NMR spectroscopy. Complexes 3– 6 react with CO (1 bar) giving trans-[Ru(acac) 2(CO)(L)] [L = P i Pr 3 ( trans- 8), PCy 3 ( trans- 9)], which are converted irreversibly into the cis-isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis-[{Ru(acac) 2(P i Pr 3)} 2(μ-N 2)] ( 10). The structures of cis- 3, trans- 4, 5, 6 and 10 · C 6H 14 have been determined by single-crystal X-ray diffraction. Complexes trans- and cis- 3, 5, 6, cis- 8, and trans- and cis- 9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH 2Cl 2 at −50 °C with E 1/2(Ru 3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac) 2 (C CHR)(P i Pr 3)] [R = SiMe 3 ( 11), Ph ( 12)] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: P i Pr 3, PCy 3 <C 2H 4 < C CHR < CO. The UV–Vis spectrum of the thermally unstable, electrogenerated Ru III–ethene cation 6 + has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH 2Cl 2 at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10 + (Ru II,III) and 10 2+ (Ru III,III). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm −1 ( w 1/2 ca. 3700 cm −1) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10 + 10 2+ ⇌ 2 10 + at 223 K is estimated as 10 13.6.

Does type of phosphine affect rotational barrier of vinylidene in the complexes OsHCl(C [dbnd]CH 2)(L) 2 (L = phosphine)?

Density functional molecular orbital calculations at the BP86 level were performed to investigate rotational barrier of vinylidene ligand in the model complexes OsHCl(C CH 2)(L) 2 (L = PMe 3, PH 3, PF 3). Results of calculations indicate that the rotational barrier increases with the increasing π-accepting ability of phosphine. This result has been explained on the basis of the extent of π-interaction between metal and vinylidene ligand in the complexes.

Evidence for pi-pi electron donor-acceptor interactions between pi-donor aromatic compounds and pi-acceptor sites in soil organic matter through pH effects on sorption.

Elucidation of molecular-level interactions controlling the sorption of organic compounds in soils is of major theoretical and practical interest. Sorption of pi-electron donor compounds, pentamethylbenzene (PMB), naphthalene (NAPH), and phenanthrene (PHEN), in a number of soils was found to increase with decreasing pH in the range of approximately pH 2.5-7. This behavior could not be attributed to pH-dependent alteration of the hydrophobic character of humic substances, pi-H-bonding, interaction with mineral surfaces, interaction with black carbons, solute coplanarity, or pH effects on solute activity coefficient. No significant effect of pH was observed for non-pi-donor hydrophobic compounds, whether planar or not: trans-1,2-dichlorocyclohexane (DCCH), hexachloro-1,3-butadiene (HCBD), 1,2,4-trichlorobenzene (TCB), 2,2',5,5'-tetrachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl. The opposite pH effect was observed for 2-nonanol and 2-nonanone, which are non-pi-donors, but capable of H-bonding. Also, no pH-dependent sorption was observed between the pi-donor PHEN and alumina, a model inorganic surface. We propose that the pi-donor solutes interact with pi-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple carboxyl groups, aromatic amines, or heteroaromatic amines. The pi-acceptor ability of such aromatic moieties would increase with protonation. pi-pi Interactions between PMB, NAPH, and PHEN, and model SOM acceptors, 1,3,5-benzenetricarboxylic acid (BTA), 1,4,5,8-naphthalenetetracarboxylic acid (NTA), and pyridine (PY) in methanol and methanol-water, were verified by the appearance of pH-dependent upfield 1H NMR chemical shifts induced by ring current effects. UV/vis spectra showed pH-dependent charge-transfer bands for various donors with NTA. No NMR shifts or charge-transfer bands were found for nondonor compounds paired with the model acceptors.