Synthesis and structural characterization of acetatobis(triphenylphosphine)dicarbonylrhenium(I). new car☐ylato derivatives of rhenium(I)

Acetatobis(triphenylphosphine)dicarbonylrhenium (I), (PPh₃)₂(CO)₂Re(O₂CCH₃), has been prepared in a novel way by treating (PPh₃)₂(CO)₂Re(NHCOR) (R = C₆H₅, p-MeC₆H₄) with triethylamine and water in the presence of air. Oxidation of the ethyl group of the tertiary amine is presumably involved in the formation of the acetate ligand. Three-dimensional single-crystal X-ray diffraction analysis shows that the complex is octahedral with the phosphines in trans positions and the acetate ion acting as a chelating ligand. The complex crystallizes in theP2₁/c space group with cell dimensions a = 17.63(2), b = 9.72(1), c = 20.95(2)Å, β = 104°38'(6'), Z = 4. The mean values of bond lengths observed are Re-P 2.415, Re-O 2.21 and Re-(CO) 1.85Å. The same acetate derivative and a series of car?ylato complexes (PPh₃)₂(CO)₂Re(O₂CR') have been obtained from reactions of Re(CO)₂(PPh₃)₃H with car?ylic acids R'COOH (R' = H, CH₃, CH₂Cl, CH₂CH₃, C₆H₅). When trifluoroacetic acid is used, a product of formula (PPh₃)₂(CO)₃Re(OC(O)CF₃) is isolated. The action of neutral ligands L on some of these products gave rise to derivatives of formula (PPh₃)₂(CO)₂(L)Re(OC(O)R') (L = CO, R' = H, CH₃, C₆H₅; L = pMeC₆H₄NC, R' = CH₃), having monodentate car?ylato moieties.     

Metal carbonyl catalysis of carbon monoxide and formate reactions

The water gas shift reaction (CO + H₂O = CO₂+ H₂) is catalyzed by aqueous metal carbonyl systems derived from simple mononuclear carbonyls such as Fe(CO)₅ and M(CO)₆ (M = Cr, Mo, and W) and bases in the 140–200 °C temperature range. The water gas shift reaction in a basic methanol-water solution containing Fe(CO)₅ is first order in [Fe(CO)₅], zero order in [CO], and essentially independent of base concentration and appears to involve an associative mechanism with a metallocarboxylate intermediate [(CO)₄Fe-CO₂H]^–. The water gas shift reactions using M(CO)₆ as catalyst precursors are first order in [M(CO)₆], inverse first order in [CO], and first order in [HCO₂ ^–] and appear to involve a dissociative mechanism with formatometallate intermediates [(CO)₅M-OCHO]^–.The Reppe hydroformylation of ethylene to produce propionaldehyde and 1-propanol in basic solutions containing Fe(CO)₅ occurs at 110–140 °C. This reaction is second order in [Fe(CO)₅], first order in [C₂H₄] up to a saturation pressure >1.5 MPa, and inhibited by [CO]. These experimental results suggest a mechanism where the rate-determining step involves a binuclear iron carbonyl intermediate. The substitution of Et₃N for NaOH as the base facilitates the reduction of propionaldehyde to 1-propanol but results in a slower rate for the overall reaction.The homogeneous photocatalytic decomposition of the formate ion to H₂ and CO₂ in the presence of Cr(CO)₆ appears to be closely related to the water gas shift reaction. The rate of H₂ production from the formate ion exhibits saturation kinetics in the formate ion and is inhibited by added pyridine. The infrared spectra of the catalyst solutions indicate an LCr(CO)₅ intermediate. Photolysis of the Cr(CO)₆/formate system in aqueous methanol in the presence of an aldehyde RCHO (R =n-heptyl,p-tolyl, andp-anisyl) results in catalytic hydrogenation of the aldehyde to the corresponding alcohol RCH₂OH by the formate ion. Detailed kinetic studies onp-tolualdehyde hydrogenation by this method indicates saturation kinetics in formate ion, autoinhibition by thep-tolualdehyde, and a threshold effect for Cr(CO)₆ at concentrations >0.004 mol L^–1. The presence of an aldehyde can interrupt the water gas shift catalytic cycle by interception of an HCr(CO)₅ ^– intermediate by the aldehyde.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1533–1539, September, 1994.     

An unusual reaction of A π-complexed lithio-arene: the characterization and structure of μ-(2,2'-difluoro-5,5'-dimethylbenzophenone)bis(tricarbonylchromium(0))

(π-Lithioarene)tricarbonychromium(0) complexes are suggested to be in equilibrium with carbene-bridged dimers at low temperature; the dimers can be trapped with acid chlorides or chloroformate esters yielding bis[(arene)tricarbonylchromium(0)]-carbinols or -ketones, respectively. Thus, the lithiation of (p-fluorotoluene)tricarbonylchromium(0) followed by addition of methyl chloroformate yields μ(2,2'-difluoro-5,5'-dimethylbenzophenone)bis(tricarbonylchromium(0)); crystals of C₂₁H₁₂Cr₂F₂O₇ are monoclinic space group P2₁/_c, with a = 7.170(2), b = 21.832(5), c = 13.428(2) Å, β = 98.91(2)°. The structure has been determined by the analysis of 1959 observed reflections recorded on a Syntex P2₁ automatic diffractometer and refined by full-matrix least-squares to R = 0.054, R_w = 0.045. The two Cr(CO)₃ tripods are oriented differently with respect to the bound arene rings. This shows that the orientation of the Cr(CO)₃ moiety is not governed primarily by the electronic properties of the ring substituents.     

Calculations of pKa values of carboxylic acids in aqueous solution using density functional theory

With the specific aim of calculating the acidity equilibrium constant (K_a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK_a (−lg K_a) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK_as of 66 carboxylic acids in aqueous with the average error of 0.5 in pK_a units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK_a value higher than 3.     

Substituent effects in π-(tricarbonylchromium)arenes: V. Thermodynamic dissociation constants of some substituted π-(tricarbonylchromium)benzoic acids

The thermodynamic dissociation constants of a series of 38 substituted π-(tricarbonylchromium)benzoic acids in 50% aqueous ethanol at 25°C have been determined. The results require revision of some literature values.The pK_a^*-values of the π-(tricarbonylchromium)benzoic acids were correlated with the electronic substituent parameters in terms of the Yukawa-Tsuno equation. The reaction constant (ρ) decreases from 1.4 for the benzoic acids to 0.8 for the π-(tricarbonylchromium)benzoic acids, reflecting the decreased ability of the complexed aromatic system to transmit electronic substituent effects. For the alkylsubstituted π-(tricarbonylchromium)benzoic acids, conformational effects of the Cr(CO)₃ group can account for some of the anomalies observed. The substituent parameters, σ_meta and σ_para, of the π-(Cr(CO)₃)phenyl group as a substituent were derived from the dissociation constants of the complexed phenylbenzoic acids.     

Spectkophotometric determination of the dissociation constants of n,n'-bis(3-hydroxypropyl)dithiooxamide and n,n'-bis(2-hydroxypropyl)dithiooxamide

The dissociation constants of N,N'-bis(3-hydroxypropyl)dithiooxamide and N,N'-bis(2-hydroxypropyl)dithiooxamide were determined by a spectrophotometric method using a weighted least squares technique for the calculations. For N,N'-bis(3-hydroxypropyl) dithiooxamide a thermodynamic constant pK₁^T of 11.37 was found. At ionic strength μ = 1, pK₁ = 11.27 and pK₂ = 14.29. For N,N'-bis-(2-hydroxypropyl) dithiooxamide, these values were respectively: pK₁^T = 11.11; pK₁ = 10.99 and pK₂ = 13.75.     

Acid-Base Properties and Prototropic Tautomerism of Isomeric 1,2,4-Triazin-3- and -5-ones

The acidity and basicity constants of isomeric phenyl(aryl)-1,2,4-triazin-3- and -5-ones in aqueous solution were determined by spectrophotometry: pK _a = 7.3–6.2; pK _BH+= 0.1 to –2.2. 1,2,4-Triazin-3-ones are weaker bases than the corresponding 1,2,4-triazin-5-ones. According to the AM1 calculations, the most thermodynamically favorable tautomer in the gas phase is the oxo form: namely, 2H-tautomers of the neutral bases and 2,4-H,H ⁺-tautomers of the conjugate acids.     

Analytical applications of the reaction of thorium with benzenephosphonic acid

Benzenephosphonic acid quantitatively precipitates thorium as Th(C₆H₅PO₃)₂·3H₂O at pH values as low as 0.5. The compound may be dried at 140° to 180° C and weighed, as a gravimetric means of determining thorium. On ignition, Th (C₂H₅PO₃)₂ 3 H₂O undergoes decomposition at 240° to 300° C to form Th(C₆H₅PO₃)₂·2H₂O, at 450° to 650° C to form Th(HPO₄)₂·2H₂O and finally at 800° to 1000° C to form Th(HPO₄)₂. The latter compound is stable to 1200° C.Potentiometrically (pK₁' = 0.91, pK₂' = 6.41) and spectrophotometrically (pK₁' = 0.96, pK₂' = 6.51) determined pK' values are reported. Absorption spectra of C₆H₅PO₃H₂, C₆H₅PO₃H^- and C₆H₅PO₃^-2 are reported. The solubility of Th (C₆H₅PO₃)₂·3H₂O was studied as a function of pH and the average value of the solubility product (K_sp = 4s³) was found to be 3.24·10^-31.     

Amidines-XVIII: Tautomeric equilibria and pka values of N,N'-disubstituted amidines. Substituent effects

The influence of substitution of amidine group on tautomeric equilibria constants and basicities is discussed. Equations based on correlation analysis methods are derived enabling predictions of both, microscopic pK_a, values of individual tautomers, measured macroscopic pK_a values of the tautomeric mixture, as well as the tautomeric equilibrium constant (as pK_T). It is shown that pK_arn values of unsymmetrically N,N'-disubstituted amidines should obey a non-linear relation with σ° constants, and only for symmetrically N,N'-disubstituted amidines obey the linear Hammett equation. Tautomeric equilibrium constants of N,N'-disubstituted amidines correlate withσ° substituent constants. The methods of prediction of pK_a value of both tautomers and pK_T value are proposed.Derived relations are applied to the series of N,N'-diphenylacetamidines and benzamidines.     

Influence of medium acidity upon the luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole

The luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole in aqueous solutions of sulfuric acid (pH 7 to H_o − 10) were studied. The spectral parameters are essentially acidity dependent, which is due to the acid-base equilibria of these heterocycles both in the ground and in the first excited singlet states. The difference between these two compounds is governed by their dissimilar solvation. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole in the S₀ state (pK_BH+ = − 2.49) and 2,5-diphenyl-1,3-oxazole in the S₀ and S₁ states (pK_BH+ = −1.93, pK_BH+^* = 1.95) were experimentally obtained. The enthalpies of formation, electron charge density, and geometry of the bases and corresponding conjugate acids in the S₀ and S₁ states were calculated by the MNDO method.     

Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)₃) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)₃ (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)₃ were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)₃ complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)₃ were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)₃. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol⁻¹. Notably, PBE0/BS1 describes the (CO)₃ rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)₃ complexes (except for (TMCOT)W(CO)₃), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)₃ complex (computed with the PBE0 functional) has a ΔG^‡ of 12.6, 12.8, and 13.2 kcal mol⁻¹ for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed ¹H-NMR and ¹³C-NMR chemical shifts for (TMCOT)Cr(CO)₃ and (TMCOT)Mo(CO)₃ at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures.     

NMR study of (C6H5)3-nPXn (X = Cl,Br, I; n = 0-3) and (C6H6)3-n PXnCr(CO)5 compounds

The ³¹P chemical shift of the (C₆H₅)_3-nPX_n ligands (X = Cl, Br, I; n = 0–3) is dominated by the electronegativity of the substituents. π bonding is only important for derivatives with three strongly electronegative substituents. The ³¹P chemical shift of the corresponding complexes (C₆H₅)_3-nPX_nCr(CO)₅ is governed by the simultaneous effects of the electronegativity, steric hindrance and π bonding. The resonance parameter, δ', indicates an increasing (p_ring → d_p)π and (d_cr→ d_p)π electron delocalization with halogen substitution.     

Kinetische und mechanistische untersuchungen von übergangsmetallkomplex-reaktionen : VI. Kinetik und mechanismus der einschiebung von dimethylcyanamid in die metall—carbenkohlenstoff-bindung von pentacarbonyl(arylphenylcarben)wolfram

Pentacarbonyl(arylphenylcarbene)tungsten complexes, (CO)₅W[C(p-C₆H₄R)C₆H₅] (Ia, R = OCH₃; Ib, R = CH₃; Ic, R = H; Id, R = Br; Ie, R = CF₃) react with dimethylcyanamide (II) via insertion of the CN group into the metalcarbene bond. The formation of pentacarbonyl[dimethylamino(imino)carbene]tungsten(0) (IIIa–IIIe) follows a second-order rate law: d[III]/dt = k[I][II]. Replacement of R = H by electron-withdrawing substituents (Br, CF₃) results in an increase, by electron-donating groups (CH₃, OCH₃) in a decrease of the reaction rate. The rate constants correlate well with Hammett's σ-constants. The activation enthalpies ΔH^≠ are low (37.3–41.6 kJ mol^?1), the activation entropies ΔS^≠ strongly negative (?119 to ?133 J mol^?1 K^?1). The results are discussed on the basis of an associative stepwise mechanism with a nucleophilic attack of the CN group of II at the carbene carbon in the first reaction step.     

Rates of base-catalysed cleavage of pyridyl-, quinolyl-, picolyl- and (quinolylmethyl)-trimethylsilanes

Rates of cleavage of some picoyl- and (quinolylmethyl)-trimethylsilanes (RSiMe₃, where R = PyCH₂ or QnCH₂SiMe₃) have been measured in “90%” aqueous methanolic sodium methoxide at 50°C. Relative reactivities are: 2-PyCH₂, 1.0; 3-PyCH₂, 0.030; 4-PyCH₂, 8.9; 2-QnCH₂, 41; 3-QnCH₂, 0.161; 4-QnCH₂, 37. The rates correlate well with those for base-catalysed hydrogen-exchange in the parent carbon acids RH. Approximate pK_a's (based on the scale of ion-pair acidities in CsNHC₆H₁₁H₂NC₆H₁₁, with pK_a of 9-phenylfluorene = 18.6) for the carbon acids, RH, can be derived as follows: 2-PyCH₃, 29.5; 3-PyCH₃, 34; 4-PyCH₃, 27; 2-QnCH₃, 25; 3-QnCH₃, 32; 4-QnCH₃, 25.Rates of cleavage of pyridyl- and quinolyl-trimethylsilanes (PySiMe₃ and QnSiMe₃) by sodium hydroxide in 4 : 1 v/v Me₂SO/H₂O at 50°C have also been measured; and the relative reactivities are: 2-Py, 1.0; 3-Py, 2.9; 4-Py, 8.4; 2-Qn, 15.9; 3-Qn, 12.7; 4-Qn, 184. The sequence of reactivity differes from that for base-catalysed hydrogen-exchange at the relevant positions of pyridine and quinoline, indicating that the reactivities are not determined in both cases (if in either) solely by the stabilities of the corresponding carbanions.     

The infrared spectra of the hydrogen halide-olefin complexes in argon matrices

On the basis of Mulliken's charge-transfer theory the frequency shifts of vibration of the hydrogen halides in its complexes with olefins have been discussed. The relations between the a/b coefficient and the ionization potential of olefin, i_d, or pK_a of acid component has been found.     

Some physico-chemical properties and sorption ability of benzylated polyethyleneimines in aqueous solutions

An investigation has been made on the change in the physico-chemical properties of aqueous solutions of branched and linear polyethyleneimine (a high mol. wt catalyst for hydrolysis of nitrophenyl esters) occurring on its being alkylated by benzyl chloride. The first result of alkylation is benzyldiethylamine fragments in PEI. Provided that the degree of conversion is below 0–3, the reaction is second order. Then the rate constant decreases, probably due to a decrease in the steric factor. Alkylation diminishes pK_a's of nitrogen atoms of PEI. Accumulation of benzyl groups in PEI results in a marked contraction of its macromolecules in aqueous solutions and enhances the sorption ability with respect to n-nitroanilides. The free energy of sorption is ?4 · 1 to ? 5·5 kcal/mole.     

Thermodynamic cycle for the calculation of ab initio pKa values for hydroxamic acids

A thermodynamic cycle to calculate pK_a values (Minus log of acid dissociation constants) of hydroxamic acids is presented. Hydroxamic acids exist mainly as amide isomers in the aqueous medium. The amide form of hydroxamic acids has two deprotonation sites and may yield either an N-ion or an O-ion upon deprotonation. The thermodynamic cycle proposed includes the gas-phase N–H deprotonation of the hydroxamic acid, the solvent phase transformation of the N-ion to the O-ion and the solvation of the hydroxamic acid molecule and the O-ion in water. The CBS-QB3 method was employed to obtain gas-phase free energy differences between 12 hydroxamic acids and their respective anions. The aqueous solvation Gibbs free energy changes were calculated at the HF/6-31G(d)/CPCM and HF/6-31+G(d)/CPCM levels of theory using HF/6-31+G(d)/CPCM geometries. For the proton, literature values of the gas-phase free energy of formation and the solvation free energy change were used. The free energy change for the transformation of the N-ion to O-ion in the aqueous medium was calculated by employing CBS-QB3/CPCM in the aqueous medium. For this, the hydroxamic acids were divided in two classes according to the substituent at the carbonyl carbon. A common transformation free energy difference for aliphatic substituted hydroxamic acids and a separate common transformation free energy difference for aromatic substituted hydroxamic acids were obtained. The pK_a calculation yielded a root mean square error of 0.32 pK_a units.     

Umlagerung und Spaltung von α, β-Dioxopropionsäureestern in alkalischem Milieu. 28. Mitt. über Reduktone und Tricarbonylverbindungen [1]

The behaviour of α,β-dioxopropionic acid derivatives of the type R? CO? CO? COX (R = phenyl, p-substituted phenyls, CF₃, mesityl; X = OC₂H₅, NH₂) was investigated under benzilic acid rearrangement conditions. Nearly all compounds were cleaved by alkali to give the corresponding acids R? COOH and glyoxylic acid. Only the sterically hindered ethyl β-mesityl-α,β-dioxopropionate underwent rearrangement (after hydrolysis of the ester group); it was shown by ¹⁴C-labelling that the carboxylate group migrates to the β-carbonyl group.     

Iodomethane oxidative addition and CO migratory insertion in monocarbonylphosphine complexes of the type [Rh((C6H5)COCHCO((CH2)nCH3))(CO)(PPh3)]: Steric and electronic effects

The chemical kinetics, studied by UV/Vis, IR and NMR, of the oxidative addition of iodomethane to [Rh((C₆H₅)COCHCOR)(CO)(PPh₃)], with R = (CH₂)_nCH₃, n = 1-3, consists of three consecutive reaction steps that involves isomers of two distinctly different classes of Rh^III-alkyl and two distinctly different classes of Rh^III-acyl species. Kinetic studies on the first oxidative addition step of [Rh((C₆H₅)COCHCOR)(CO)(PPh₃)] + CH₃I to form [Rh((C₆H₅)COCHCOR)(CH₃)(CO)(PPh₃)(I)] revealed a second order oxidative addition rate constant approximately 500-600 times faster than that observed for the Monsanto catalyst [Rh(CO)₂I₂]⁻. The reaction rate of the first oxidative addition step in chloroform was not influenced by the increasing alkyl chain length of the R group on the β-diketonato ligand: k₁ = 0.0333 ([Rh((C₆H₅)COCHCO(CH₂CH₃))(CO)(PPh₃)]), 0.0437 ([Rh((C₆H₅)COCHCO(CH₂CH₂CH₃))(CO)(PPh₃)]) and 0.0354 dm³ mol⁻¹ s⁻¹ ([Rh((C₆H₅)COCHCO(CH₂CH₂CH₂CH₃))(CO)(PPh₃)]). The pK_a^′ and keto-enol equilibrium constant, K_c, of the β-diketones (C₆H₅)COCH₂COR, along with apparent group electronegativities, χ_R of the R group of the β-diketones (C₆H₅)COCH₂COR, give a measurement of the electron donating character of the coordinating β-diketonato ligand: (R, pK_a^′, K_c, χ_R) = (CH₃, 8.70, 12.1, 2.34), (CH₂CH₃, 9.33, 8.2, 2.31), (CH₂CH₂CH₃, 9.23, 11.5, 2.41) and (CH₂CH₂CH₂CH₃, 9.33, 11.6, 2.22).     

Use of Cr(phen)33+ as luminescence probe to study the binding constants of phenols to micelles of SDS

Steady-state and time resolved luminescence quenching measurements of (²T/²E)Cr(phen)₃³⁺ were used to investigate the association of phenols to sodium dodecyl sulfate (SDS) micelles. Steady-state results show the quenching process occurs in the micellar pseudo phase. Scatchard plots indicate that the process is a partition between aqueous and micelles. The k₊ and k₋ rate constant have been evaluated from time resolved data and the binding constants were obtained. The trend found in the K's were 4-H-Ph < 2,6-diMe-Ph < 4-Br-Ph. We concluded that it is possible to use *Cr(phen)₃³⁺ as a luminescent probe to determine association parameters for quenchers to micelles of SDS.