P-benzoquinone Complex Research Articles

Origin of attraction in p-benzoquinone complexes with benzene and p-hydroquinone.

The origin of the attraction in charge-transfer complexes (a p-hydroquinone-p-benzoquinone complex and benzene complexes with benzoquinone, tetracyanoethylene and Br2) was analyzed using distributed multipole analysis and symmetry-adapted perturbation theory. Both methods show that the dispersion interactions are the primary source of the attraction in these charge-transfer complexes followed by the electrostatic interactions. The natures of the intermolecular interactions in these complexes are close to the π/π interactions of neutral aromatic molecules. The electrostatic interactions play important roles in determining the magnitude of the attraction. The contribution of charge-transfer interactions to the attraction is not large compared with the dispersion interactions in these complexes.

Photochemistry of transition metal p-benzoquinone complexes: Photoreduction of the quinone induced by CT excitation

Complexes of Pd(II), Rh(I) and Ir(I) with p-quinone ligands are characterized by metal or ligand (iodide,azide) to quinone CT transitions. CT excitation yields hydroquinones as reduction products.

CpCo-Mediated Reactions of Cyclopropenones: A DFT Study

Quantum chemical calculations using density functional theory at the B3LYP/6-311G(d)/Wachters+f level have been carried out to model possible pathways for the reaction of CpCo(CO)2 (15) with cyclopropenone to yield CpCo-capped p-benzoquinone complexes. For a number of intermediates, including all relevant 16-electron species, energies and geometries of the triplet electronic state were also obtained, which permitted us to locate four crossings of the singlet and triplet potential surfaces. It was found that first a metallacyclobutenone complex (21) is formed. Our calculations indicate a dissociative mechanism; the previously observed associative mechanism in ligand substitution reactions of 15 is shown to be a consequence of lower reaction temperatures. The addition of a second cyclopropenone unit to 21 reveals a higher energy path (path I) than does the addition of CO to 21, followed by acetylene addition (path II). The latter path is favored, which has already been shown by labeling and trapping experiments. Despite favorable energetics, the paths involving the triplet surface have not been observed experimentally; we tentatively attribute this to the very short lifetime of the initially formed triplet.

Symmetry-Adapted Perturbation-Theory Interaction-Energy Decomposition for Hydrogen-Bonded and Stacking Structures.

This letter reports the computational ab initio studies on the stacked and hydrogen-bonded geometries of the uracil dimer and pyrimidine···p-benzoquinone complex with a special regard to the ratios of different interaction-energy terms calculated by means of the symmetry-adapted perturbation theory (SAPT). In the hydrogen-bonded systems the absolute value of the dispersion term constitutes approximately half of the absolute value of the total SAPT0 interaction energy, while in the stacking complexes the ratio of the dispersion to the total interaction energy is much larger, ca. 1.2-2.0. Our SAPT results are compared with the DFT-SAPT results published recently by the Hobza group (J. Chem. Phys. 2007, 127, 075104), and the role of the dispersion contribution in stacking and hydrogen-bonded arrangements is discussed. The methodological part of this letter presents the influence of counterpoise corrections in the optimization procedure on the geometries of the systems and the calculated SAPT contributions.

Synthesis, structure, and dynamic behaviour of (p-quinone)copper(I) complexes. Crystal structure determination of a copper p-benzoquinone complex stabilized by a tridentate oxygen ligand

The quinonecopper(I) complexes [Cu(olefin)L][olefin =p-benzoquinone (bq), methyl-p-benzoquinone, or 1,4-naphthoquinone] of the tridentate oxygen ligand L–=[(C5H5)Co{P(O)(OCH3)2}3]– have been prepared by the reaction of NaL and the appropriate quinone with [Cu(CH3CN)4]PF6 and by carbon monoxide substitution from [Cu(CO)L]. The complex [Cu(bq)L] crystallizes in space group P(Z= 2) with a= 11.358(6), b= 12.138(7), c= 12.465(6)A, α= 92.18(2), β= 134.24(2), γ= 99.49(4)°, and its X-ray crystal structure was refined to R= 0.056 for 4 140 reflections [|Fo| > 2.5σ(Fo)]. The co-ordination about the copper atom is distorted tetrahedral with one long [2.184(2)A] and two short [1.966(2), 2.001 (2)A] copper–oxygen contacts. p-Benzoquinone co-ordinates through one olefinic double bond to the copper atom. The complex dismutates in solution according to the equilibrium 2 [Cu(bq)L]⇌[LCu(µ-bq)CuL]+ bq. In addition, it exhibits several other dynamic processes, the lowest-energy of which is olefin rotation. The intramolecular exchange of the free and the co-ordinated double bond gives rise to a coalescence temperature of about 305 K in the 1H n.m.r. spectrum, corresponding to an activation barrier of ca. 60 kJ mol–1. Intermolecular exchange of co-ordinated and non-co-ordinated bq becomes rapid on the n.m.r. time-scale above ca. 370 K.

Concentration and solvent effects on proline— p-benzoquinone complexes

Spectrophotometric studies have been used to show that the concentration of proline has a marked effect on its interaction with p-benzoquinone in pH 7 buffer. As the concentration is decreased, both the back reaction rate constant and the equilibrium constant increase whereas the entropy change decreases very rapidly. Similar effects are observed in a constant proline concentration but adding an increasing amount of dimethyl sulphoxide (DMSO). The extinction coefficient for the proline- p-benzoquinone complex is lower in the presence of DMSO. Extinction coefficients and equilibrium constants obtained by both Benesi—Hildebrand and Rose—Drago methods are identical. The interaction is shown to be first order with respect to each component and second order overall. The reaction is both temperature and dilution reversible. Rate constants and thermodynamic parameters have been calculated over a wide range of conditions.

The effects of bulky substituents in the reactions of alkynes with (η-C5H5)M(CO)2, M = Co or Rh

The reactions between (η-C5H5)Co(CO)2 and MeC≡CR (R = Bu, Pri) at 120° gave the cyclopentadienone complexes (η-C5H5)Co[C4Me2R2CO] (2) and small amounts of the p-benzoquinone complexes(η-C5H5)Co[C4Me2R2(CO)2] (1); all possible isomers of both complexes were obtained. For R = But, the only products isolated were two isomers of the cyclopentadienone complex (2). The reactions between (η-C5H5)Rh(CO)2 and MeC=CR at 160° gave the cyclopentadienone complexes(η-C5H5)Rh[C4Me2R2CO] (2), and small amounts of the p-benzoquinone complexes (η-C5H5)Rh-[C4Me2R2(CO)2] (1) and of the binuclear complexes (η-C5H5)2Rh2(MeC2R)2CO (3). With R = But, three isomers of (2), two isomers of (1) and three isomers of (3) were identified. The products obtained with R = Pri were three isomers of (2) and three isomers of (3). With R = But, three isomers of(2) and a very small amount of one of the isomers of (3) were separated. Comparisons of these reactions have shown that increasing the bulk of R results in a decrease in the total yields of products, a decrease in the range of products, and a change in the distribution of the isomers (M = Co only).

Kinetic spectrophotometric study of effect of triazolopyrazines on p-benzoquinone complexes.

The effect of adding triazolopyrazines to a p-benzoquinone-proline system and a p-benzoquinone-histamine system was studied by observing the kinetics of the spectral changes and analyzing the results using digital simulation methods. Below 35 degrees, the major effect was that the triazolopyrazine competed with the electron donors for the acceptors to form complexes. Above 35 degrees, there was predominantly a chemical interaction between the triazolopyrazine and the acceptors. A separate experiment showed that the triazolopyrazine interacted with p-benzoquinone to form a complex initially but that a chemical interaction took place later.

The reactions of some diynes with (C5H5)Co(CO)2 and Fe(CO)5: Inter- or Intra-molecular cyclizations?

The reactions of several diynes MeC≡C(CH2)nC≡CMe, n = 2-4, with (η- C5H5)Co(CO)2 and Fe(CO)5 have been compared. A p-benzoquinone complex, (η-C5H5)Co[(C8H10)2(CO)2], and three isomers of the cyclopentadienone complex, (η-C5H5)Co[(C8H10)2CO], are formed in the reaction of (η- C5H5)Co(CO)2 with octa-2,6-diyne. In each complex, the ligand is formed by intermolecular cyclization of two diynes with inclusion of CO. The corresponding reactions with nona-2,7-diyne and deca-2,8-diyne give complexes (η-C5H5)Co[(C9H12)CO] and (C5H5)Co[(C10H14)CO] in which a bicyclic cyclopentadienone has been formed by intramolecular condensation of the diyne. Intramolecular cyclization is also achieved when all three diynes are treated with Fe(CO)5. The reactions with octa- 2,6-diyne and deca-2,8-diyne give metallodiene complexes, Fe2(CO)6(diyne), in which the ferracyclopentadiene ring is fused to either a 4- or 6-membered ring. With nona-2,7-diyne, a cyclopentadienone complex of formula Fe(CO)3[(C10H14)CO] is isolated. In the reactions with Fe(CO)5,organic products of the type (diyne)2, (diyne)3 and [(diyne)CO]2 are also isolated.

Reactions of Hex-3-yne with Dicarbonyl(η-cyclopentadienyl)-cobalt and -rhodium

The reaction of hex-3-yne with (η-C5H5)Co(C0)2 gives the p-benzoquinone complex (η-C5H5)Co-[(EtC2Et)2(C0)2] and the cyclopentadienone complex (q-C5H5)Co[(EtC2Et)2CO]. The corresponding reaction with (q-C2H2)Rh(CO)2 gives the cyclopentadienone complex (η-C5H5)Rh[(EtC2Et)2C0] and a binuclear 4,6-diethylnonadien-5-one complex, (η-C5H5)2Rh2(EtC2Et)2Co. The 1H N.M.R. spectra of all complexes have been analysed, in some instances with the aid of partial decoupling experiments. An analysis of the 13C N.M.R. spectra of the two cyclopentadienone complexes is also given.

Photochemical Reactions of p-Benzoquinone Complexes with Aromatic Molecules

Abstract Electronic absorption spectra and ESR spectra have been measured for the photolyzed charge transfer complexes between p-benzoquinone and aromatic donors in low temperature matrices. When the p-benzoquinone complex with toluene or p-xylene was irradiated at 77°K, an electronic absorption spectrum attributable to the semiquinone radical was obtained together with that of the benzyl-type radical. The spectral shape and position of the former are somewhat different from those of the semiquinone radicals obtained by the photolysis of p-benzoquinone or hydroquinone. These differences indicate the presence of interaction between semiquinone and benzyl-type radicals. The ESR spectra obtained from these systems show that these radicals form pairs in the triplet (S=1) state. It is concluded that the radical pairs are produced as a result of intermolecular hydrogen atom transfer from the aromatic donor molecule to p-benzoquinone in the excited states of the CT complexes formed between them. The irradiations of various p-benzoquinone complexes were carried out in the wavelength regions corresponding to the lowest CT absorption bands at 77°K, and the reactivities were discussed in relation to the electronic structures and geometrical configurations of the CT complexes in the excited states. From the analysis of ESR spectra, the fine structure constant was evaluated for each of the radical pairs, from which the average distance between interacting unpaired electrons was calculated.

Vibrational structure of molecular charge transfer bands, part. I. hydrocarbon-tetrahalo p-benzoquinone complexes in solution

Under high resolution the electronic charge transfer band of hydrocarbon-tetrahalo p-benzoquinone complexes in chloroform have been found to show vibrational structure of very low frequency of 150-250 cm-1. This has been associated with the change in donor-acceptor bond. The force constant of the intermolecular bond lies in the range