Multistep π Dimerization of Tetrakis(n‐decyl)heptathienoacene Radical Cations: A Combined Experimental and Theoretical Study

Radical cations of a heptathienoacene α,β‐substituted with four n‐decyl side groups (D4T7 ^{. +}) form exceptionally stable π‐dimer dications already at ambient temperature (Chem. Comm. 2011 , 47, 12622). This extraordinary π‐dimerization process is investigated here with a focus on the ultimate [D4T7 ^{. +}]₂ π‐dimer dication and yet‐unreported transitory species formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the π‐dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one‐electron oxidation: 1) a different transient conformation of the ultimate [D4T7 ^{. +}]₂ π‐dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]₂ ^{. +} π‐dimer radical cations formed prior to the fully oxidized [D4T7]₂ ^{. +} π‐dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems.     

Organometallic compound with Lewis base—IV : Dimerization of methyl crotonate by aluminum alkyl-tertiary amine complex

Aluminum alkyl-tertiary amine complex was found to induce the catalytic dimerization of methyl crotonate (MCr) to dimethyl 2-methylpent-4-ene-1,3-dicarboxylate (1) and dimethyl 2-methylpent-cis-3-ene-1,3-dicarboxylate (2). The of the γ-hydrogen of the MCr molecule. Dimer 2 is formed through the isomerization of dimer 1. The complex of AlR₃ with a bidentate ligand, sparteine, produces dimer 1, selectively. The complex of AlR₃ with monodentate ligand NEt₃, on the other hand, induces the isomerization of dimer 1 to the cis-form of dimer 2. The coordination number of aluminum alkyl-tertiary amine complex seems to control the dimerization mechanism of MCr.     

A theoretical study of the reactions of carbonyl oxide with water in atmosphere: the role of water dimer

Carbonyl oxide is a well-known intermediate formed in gas-phase reactions of ozone with alkenes. Secondary reactions of carbonyl oxide are suggested to lead to the formation of HO, H₂O₂ and organic peroxides in the atmosphere. We performed a theoretical study of reactions of carbonyl oxide with water and a water dimer. Using CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311+G(2d,2p) calculations we found that the most energetically favourable channel is the formation of hydroxymethyl hydroperoxide (HMHP) as the result of reactions of carbonyl oxide with the water dimer. The potential importance of water dimer reactions in the chemistry of the troposphere is discussed herein.     

Binding of singlet oxygen with a stacked guanine dimer

Geometry optimization calculations were performed using the B3LYP/6‐31+G* method on the complexes of ¹O₂ and ³O₂ molecules with a stacked dimer of planar guanine, varying the distance (D) between the planes of the guanine molecules. In this process, geometries of the guanine molecules were held fixed, D was fixed at different values, while the bond lengths of ¹O₂ and ³O₂ as well as their orientations with respect to the guanine molecules were optimized for each value of D. The complexes in their most stable geometries were solvated in water using the integral equation formalism of the polarized continuum model of the self‐consistent reaction field theory. In gas phase, the most stable complex between ¹O₂ and the guanine dimer (2G.¹O₂) is formed when D is about 6 Å, while the most stable complex between ³O₂ and the guanine dimer (2G.³O₂) is formed when D is about 3.75 Å. In the minimum total energy geometry of 2G.¹O₂, ¹O₂ is located between the guanine molecules, above the imidazole ring of one of them. However, in the minimum total energy geometry of 2G.³O₂, ³O₂ is located outside the stack of guanine molecules, near the amino group of one of them. The solvation calculations showed that in aqueous media, ¹O₂ would bind with the stacked guanine dimer more strongly than in gas phase, while ³O₂ would not bind with the same. The mode of binding of ¹O₂ with the stacked guanine dimer is such that it seems that ¹O₂ would replace one basepair in DNA, as happens in the intercalative mode of binding of drugs and other molecules, and it can lead to the formation of 8‐oxoguanine that has a mutagenic nature. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Activation of Molecular Hydrogen by Bis(η5,η1‐pentafulvene)‐titanium Complexes – Efficient Formation of Titanium(III)hydrides

The synthesis and crystal structures of two dinuclear titanocene hydride complexes are reported. Both complexes, namely bis(η⁵‐(di‐para‐tolylmethyl)cyclopentadienyl)titanium hydride dimer, [(η⁵‐C₂₀H₁₉)₂Ti(μ‐H)]₂ ( 2a ), and bis(η⁵‐2‐adamantylcyclopentadienyl)‐titanium hydride dimer, [(η⁵‐C₁₅H₁₉)₂Ti(μ‐H)]₂ ( 2b ), are formed via activation of molecular hydrogen by the corresponding bis(η⁵,η¹‐pentafulvene)titanium complexes 1a and 1b at ambient temperatures and pressures in high yields. The hydride complexes 2a and 2b exhibit planar [Ti₂H₂] cores and, as a result of the heterolytic cleavage of molecular hydrogen, substituted Cp Ligands were formed during the reaction.     

Preparation,molecular-crystal structure,and chemical peculiarities of the potassium salt of 3,3,6,6-tetramethyl-1,8-dioxo-1,2, 3,4,5,6,7,8,9,10-decahydroacridine-9-carboxylic acid

It was established by x-ray diffraction analysis that the crystalline title compound exists as a dimer formed by two paired potassium ions bonded to the oxygen atoms of the carboxy group and the coordinated oxygen atoms of the carbonyl groups. The solvate of the dimer with the composition 2C₁₈H₂₂NO₄K·3H₂O·CH₃COCH₃ is the crystallochemically independent structural unit. A shift of the electron density toward the carbonyl groups and the formation of a strong hydrogen bond between NH and the oxygen atom of the carboxylate group are observed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 666–672, May, 1980.     

An unexpected ethyl transfer reaction between Et2Zn and DI(t-Butyl)-glyoxaldiimine (t-BuDAB). Studies of the persistent [EtZn(t-BuDAB)] radical which is in equilibrium with its C---C coupled dimer

Whereas p-Tol₂Zn reacts with t-BuN=CHCH=N-t-Bu (t-BuDAB) to give a stable complex [p-Tol₂Zn(t-BuDAB)], Et₂Zn gives EtZ -(t-Bu) via intramolecular ethyl transfer in the unstable Et₂Zn(t-BuDAB) complex. In solution the stable persistent organozinc radical EtZn(t-BuDAB), which is formed in trace amounts in the Et₂Zn/t-BuDAB reaction, is in equilibrium with its stable C---C coupled dimer [EtZn-t-BuN=CH(t-BuN)CH]₂. The dimer can be prepared in quantitative yield by the reaction of (EtZnCl)₄ with K(t-BuDAB).     

Cyclic Phosphorus(V) Compounds: Phosphazanes and Anhydrides

Abstract

³¹P n.m.r. has been extensively used to study the preparation and reactions of cyclic phosphorus-(V) compounds containing P-N-P and P-O-P linkages. Michaelis “Oxyphosphazobenzolchlorid”,[C₆H₅NPOCl]_n has been shown by ³¹P n.m.r. and mass spectroscopy to be the trimer (n=3), not the dimer as proposed by Michaelis. The competing reactions of aniline hydrochloride and POCl₃ to form the dimer (two isomers cis- and trans-) and the trimer have been elucidated. With PSCl₃ only the dimer [C₆H₅NPSCl]₂ is formed (both isomers).     

Molecular orbital theory of the hydrogen bond. 24. Ground-state water–uracil complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen-bonding sites in the uracil molecular plane. Hydrogen-bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N₁? H and O₂, N₃? H and O₂, and N₃? H and O₄. Open structures form at O₄, N₁? H, and N₃? H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N₁? H and the cyclic one at N₁? H and O₂. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N₃? H and a cyclic structure at N₃? H and O₄. The third cyclic structure at N₃? H and O₂ collapses to the “wobble” dimer at N₃? H and O₄. The two “wobble” dimers are significantly more stable than the open dimer formed at O₄, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N₁? H than N₃? H, owing to a more favorable molecular dipole moment alignment when association occurs through H₁. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen-bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen-bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen-bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.     

Dimers of 3,7-dehydrotropones (bicyclo[3.2.0]hepta-1(7),2,4-trien-6-ones)

2-(tert-Butyl)-3,7-dehydrotropone (7-(tert-butyl)bicyclo[3.2.0]hepta-1(7),2,4-trien-6-one; 1 ) was found to dimerize reversibly to 2A by [2 + 4]-cycloaddition/cycloreversion reaction. The equilibrium lies on the side of the highly strained dimer 2A in the solid state, and on the side of the monomer 1 in solution. The [2 + 4]-reaction is fully perisite-, regio- and stereoselective. Above room temperature, 1 irreversibly formed a decarbonylated dimer 6 , probably via the intermediate 9A or 9B , which resulted either from a dimerisation of 1 by [4 + 6]-cycloaddition or from a sigmatropic rearrangement of the originally formed dimer 2A or 2B . Similary, the 6-bromo derivative 14 afforded the corresponding decarbonylated dimer 15 . Should the formation of 6 and 15 be due to a primary cycloaddition then that reaction is fully peri-, site- and regioselective. Mild LiAlH₄-reduction of 6 and subsequent acetylation yielded the acetate 11 , the structure of which was established by an X-ray analysis. More vigorous LiAlH₄-treatment also reduced the terminal fulvenoid double bond of 6 and acetylation of the crude product led to the acetated 12 and 13 .     

Novel di-μ-chloro-bis[chloro(4,7-dimethyl-1,10-phenanthroline)cadmium(II)] dimer complex: synthesis, spectral, thermal, and crystal structure studies

A novel di-μ-chloro-bis[chloro(4,7-dimethyl-1,10-phenanthroline)cadmium(II)] dimer complex has been prepared by reacting CdCl₂·2.5H₂O with 4,7-dimethyl-1,10-phenanthroline (dmphen) ligand. The complex was characterized on the basis of elemental analysis, FAB-MS, IR, UV–visible, ¹H, and ¹³C NMR spectroscopy, TG/DTA, and X-ray single-crystal diffraction studies. The Cd(II) ions in [CdCl₂(C₁₄H₁₂N₂)]₂ are coordinated to three Cl atoms with the centrosymmetric dimer bridged through the Cl atoms and two N atoms in a slightly distorted square-pyramidal disposition. Several hydrogen bonds formed between the terminal Cl atoms and H-Me/H-Ph groups may stabilize the structure in the dimer form.     

Electrochemical study of heteronuclear complex [Cp(CO)2MnCu(μ-C=CHPh)(μ-Cl)]2

The electrochemical behavior of the heteronuclear dimeric vinylidene complex [Cp(CO)₂MnCu(μ-C=CHPh)(μ-Cl)]₂ (I) is studied using polarography, cyclic voltammetry, and controlled-potential electrolysis on mercury and platinum electrodes in acetonitrile. The dimer is reduced in two two-electron stages: at the first stage, metal-halogen bonds are broken and binuclear monomers [Cp(CO)₂MnCu(μ-C=CHPh)(Cl)]^? are formed; at the second stage, mononuclear manganese complexes Cp(CO)₂Mn=C=CHPh, metallic copper, and chloride ions are formed. The two-electron oxidation of dimer [Cp(CO)₂MnCu(μ-C=CHPh)(μ-Cl)]₂ also results in the formation of binuclear monomers, which decompose to complex Cp(CO)₂Mn=C=CHPh, Cu²⁺ ions, and Cl^? ions.     

Bis(dimethyl sulfoxide‐S)tetrakis(μ‐p‐hydroxybenzoato‐O:O′)dirhodium(II)–tetrakis(μ‐butyrato‐O:O′)bis(dimethyl sulfoxide‐S)dirhodium(II) cocrystal ethanol disolvate

The title structure, [Rh₂(C₇H₅O₃)₄(C₂H₆OS)₂]·[Rh₂(C₄H₇­O₂)₄(C₂H₆OS)₂]·2C₂H₆O, contains two discrete neutral Rh–Rh dimers cocrystallized as the ethanol disolvate. Each dimer is situated on an inversion center. The butyrate chain displays disorder in one C‐atom position. In each dimer, the di­methyl sulfoxide ligand (dmso) is bound via S, as expected. The ethanol is a hydrogen‐bond acceptor for one p‐hydroxy­benzoate hydroxyl group and acts as a hydrogen‐bond donor to the dmso O atom of a neighboring p‐hydroxy­benzoate dirhodium complex. A third hydrogen bond is formed from the other p‐hydroxy­benzoate hydroxyl group to the dmso O atom of a butyrate–dirhodium complex.     

Cationic oligomerization of anethole: Selective synthesis of dimers and trimers

A linear unsaturated dimer of anethole [II, (E)-1,3-bis(p-methoxyphenyl)-2-methylpentene-1], was prepared in 98% yield with an acetyl perchlorate (AcClO₄) catalyst in a nonpolar solvent (C₆H₆) at a high temperature (70°C). At 0°C a linear unsaturated trimer was formed in high yield with dimer II. The geometric structure of the linear unsaturated dimers was determined by analysis of the nuclear Overhauser effect (NOE) on their ¹H-NMR spectra. NOE analysis showed that at 0°C with AcClO₄, trans-anethole gives the (E)-form (II), whereas cis-anethole yields the (Z)-form. On the other hand, with a metal-halide catalyst (BF₃OEt₂) a cyclic dimer and a cyclic trimer were produced in high yields in a polar solvent [(CH₂Cl₂)] at 70°C; higher oligomers (≥ tetramer) were scarcely formed. The effect of catalysts on the structure of oligomers was explained in terms of the interaction between a growing carbocation and a counterion.     

Coordination‐Driven Dimerization of Zinc Chlorophyll Derivatives Possessing a Dialkylamino Group

Zinc chlorophyll derivatives Zn‐1 – 3 possessing a tertiary amino group at the C3¹ position have been synthesized through reductive amination of methyl pyropheophorbide‐d obtained from naturally occurring chlorophyll‐a . In a dilute CH₂Cl₂ solution as well as in a dilute 10 %(v/v) CH₂Cl₂/hexane solution, Zn‐1 possessing a dimethylamino group at the C3¹ position showed red‐shifted UV/Vis absorption and intensified exciton‐coupling circular dichroism (CD) spectra at room temperature owing to its dimer formation via coordination to the central zinc by the 3¹‐N atom of the dimethylamino group. However, Zn‐2/3 bearing 3¹‐ethylmethylamino/diethylamino groups did not. The difference was dependent on the steric factor of the substituents in the tertiary amino group, where an increase of the carbon numbers on the N atom reduced the intermolecular N⋅⋅⋅Zn coordination. UV/Vis, CD, and ¹H NMR spectroscopic analyses including DOSY measurements revealed that Zn‐1 formed closed‐type dimers via an opened dimer by single‐to‐double axial coordination with an increase in concentration and a temperature decrease in CH₂Cl₂, while Zn‐2/3 gave open and flexible dimers in a concentrated CH₂Cl₂ solution at low temperature. The supramolecular closed dimer structures of Zn‐1 were estimated by molecular modelling calculations, which showed these structures were promising models for the chlorophyll dimer in a photosynthetic reaction center.     

Two-phase reactions of tin(IV)–tetraphenylporphine complexes as carrier of anion selective electrodes

Two-phase reactions of [Sn(OH)₂(tpp)] with lipophilic anions and inorganic acids were studied and compared with those of [Zr(OH)₂(tpp)] having the same oxidation state of the central metal ion but different structural characteristics. The reaction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate formed a 2:1 cationic dimer and 1:1 and 1:2 ion-pairs, while the reaction with dodecyl sulfate formed 1:1 and 1:2 ion-pairs. The 1:1 and 1:2 species formed concomitantly in the reaction with HCl and CH₃COOH having higher coordination abilities, while forming in stepwise manner in the reaction with HClO₄ and HNO₃ having lower coordination abilities. The 2:1 cationic dimer of Sn(IV) had significantly lower stability than that of Zr(IV). The affinity of Sn(IV) for Cl⁻ relative to OH⁻ was much higher, compared with Zr(IV). Except in extremely acidic media, [SnCl₂(tpp)] working as a carrier in PVC membranes hydrolyzed to give [Sn(OH)Cl(tpp)] or [Sn(OH)₂(tpp)], which showed stronger but less-selective potential responses at lower pH and weaker but more-selective responses to salicylate at higher pH. The affinities and responses of Sn(IV) complexes to oxygen-containing anions were weaker than those of Zr(IV) complexes.     

Diverse Modes of Guest Inclusion in a Cyclodextrin: X-ray Structural and Thermal Characterization of a 4:3 β-cyclodextrin—Cyclizine Complex

Abstract

1-Diphenylmethyl-4-methylpiperazine (cyclizine) is an antiemetic drug which forms an inclusion complex with β-cyclodextrin of formula (β-cyclodextrin)₄ · (cyclizine)₃ · 50H₂O. This species crystallizes in the monoclinic space group P2₁ with a = 15.246(1), b = 65.075(5), c = 15.609(1) Å, β = 102.62(1)° and Z = 2 formula units. Complex water content and the host:drug stoichiometric ratio were determined by thermogravimetry and UV spectrophotometry respectively. Differential scanning calorimetry showed that the crystals dehydrate in at least two stages and begin to decompose from approximately 250°C. The crystal structure was solved by a combination of Patterson search and direct methods. Isotropic refinement converged at R = 0.094 for 8806 reflections with I > 2σ(I). The unusual stoichiometry is accounted for as follows: the four β-cyclodextrin molecules comprising the asymmetric unit occur as two independent head-to-head dimers, each formed by O—H…O hydrogen bonding across the macro-cyclic secondary surfaces. One dimer contains two cyclizine guest molecules in head-to-tail orientation, thus accounting for two distinct modes of drug inclusion. In the second dimer, only one β-cyclodextrin molecule is significantly occupied by a cyclizine molecule (in a mode analogous to one of those in the first dimer), the other half of the dimer being largely devoid of guest. A possible mechanism for the formation of this unusual structure is proposed and the crystal packing arrangement is shown to be based on a novel disrupted tetrameric channel motif.     

Alternating cooligomerization of isoprene and propylene with vanadium catalyst

Alternating cooligomerization of isoprene with propylene has been investigated between ?30 and 0°C, VO(acac)₂–Et₃Al–Et₂AlCl being used as catalyst. In the presence of an excess of propylene, 2,4,7-trimethyl-1,4-octadiene and 2,4-dimethyl-1,4-nonadiene are selectively formed. The formation is explained by the alternating coordination of isoprene and propylene to the vanadium. When triphenylphosphine or pyridine is added to the catalyst, the cooligomerization is suppressed while the formation of the dimer and trimer of isoprene is high.     

On the transitional properties of rigid oligomers. The orientational order of the monomer 4-n-pentyl-4′-cyanobiphenyl and its platinum dichloride linked dimer

Abstract

We have studied the orientational order of the monomer 4-n-pentyl-4′-cyanobiphenyl (5CB) and of the dimer, [PtCl₂(5CB)₂], formed by linking two cyanobiphenyl units via a platinum dichloride bridge, dissolved in a common nematic solvent, using deuterium NMR spectroscopy. Analysis of the second rank order parameters, obtained from these experiments, in terms of a molecular field theory yields the anisotropic solute-solvent strength parameter responsible for the solute alignment. In the limit of low solvent order the strength parameters for the monomer and dimer differ significantly, in accord with the differing anisotropies of these two solutes. However, as the solvent order increases, so the relative difference in the strength parameters decreases, tending to zero. A possible explanation for this intriguing behaviour is proposed.