The interplay of inverted redox potentials and aromaticity in the oxidized states of new pi-electron donors: 9-(1,3-dithiol-2-ylidene)fluorene and 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives

Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single-electron oxidations (E(1)ox < E(1)ox) were observed for 9-(1,3-dithiol-2-ylidene)fluorene systems (9). In contrast, derivatives of 9-(1,3-dithiol-2-ylidene)thioxanthene (10) display the unusual phenomenon of inverted potentials (E(1)ox > E(1)ox) resulting in a single, two-electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (10(2+)) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two-electron oxidation wave in the thioxanthene derivatives is split into two separate one-electron waves in the corresponding sulfoxide and sulfone derivatives 27-29 owing to destabilisation of the dication state.     

Solvent effects on the redox properties of tris-(1,10-phenanthroline)iron(II/III) complexes

The redox properties of the system Fe(tmphen)₃(II/III) (tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) have been studied in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate, dimethylformamide, dimethylacetamide, dimethylsulfoxide and of the systems Fe(phen)₃(II/III) (phen=1,10-phenanthroline) and Fe(niphen)₃(II/III) (niphen=5-nitro-1,10-phenanthroline) in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate and acetone. The redox potentials of Fe(tmphen)₃(II/III) are nearly independent of the solvent suggesting that the system might be used as a reference redox couple similar to the systems ferrocene/ferricinium or bisbiphenylchromium(0/I). In contrast the redox potentials of Fe(niphen)₃(II/III) show a significant decrease with increasing donor number of the solvent which can be explained by nucleophilic attack of solvent molecules at the iron. It is shown that such a mechanism is consistent with the known solvent and salt effects on the kinetics of dissociation of ferroin and ferriin type complexes.     

One-electron redox reactions involving chromium(0) and molybdenum(0) bis-1,3,5-trimethylbenzene derivatives

The reaction of M(η⁶-1,3,5-Me₃C₆H₃)₂, M = Cr, Mo, with the tetrahalides of Groups 4 and 5 elements proceeds with the monoelectronic oxidation of the metal bis-arene to the [M(η⁶-Me₃C₆H₃)₂]⁺ cation. In the case of MX₄, M = Ti, X = Cl, Br, M = V, X = Cl, and of Nb₂Cl₁₀ the reduction products are the titanium(III), vanadium(III) halides and the niobium(IV) chloride, isolated as the solvate anions [MCl₄(THF)₂]⁻ and [NbCl₄(CH₃CN)]⁻. The reaction of the tetrachloro complexes MCl₄(THF)₂, M = Zr, Hf, with Cr(η⁶-1,3,5-Me₃C₆H₃)₂ in THF produces the ionic [Cr(η⁶-1,3,5-Me₃C₆H₃)₂][MCl₅(THF)], which has been characterized by single-crystal X-ray diffraction in the case of hafnium.     

Solvent and phosphine dependency in the reaction of cis-RuCl2(P-P)2 (P-P = dppm or dppe) with terminal alkynes

Reaction of cis-[RuCl₂(dppm)₂] (dppm = 1,2-bis(diphenylphosphino)methane) with PhCCH and NaPF₆ utilising methanol as solvent results in the formation of the η³-butenynyl complex [Ru(η³-PhCCCCHPh)(dppm)₂][PF₆] in good yield. Similar reactions with Bu^tCCH and PrⁿCCH resulted in the corresponding alkyl-substituted complexes and all three of these compounds have been characterised by NMR spectroscopy and X-ray crystallography. The mechanism of this reaction has been probed by employing labelling experiments with both PhCCD and PhC¹³CH allowing the identity of possible intermediates in the reaction to be determined. Furthermore, [Ru(η³-PhCCCCHPh)(dppm)₂][PF₆] has been shown to be an effective regio- and stereo-selective catalyst for the dimerisation of PhCCH to Z-PhCCCHCHPh in the absence of solvent. In contrast, no evidence for the formation of alkyne coupling was obtained from the reaction of cis-[RuCl₂(dppe)₂] (dppe = 1,2-bis(diphenylphosphino)ethane) with PhCCH and NaPF₆.     

Solvent and salt effects on the redox behaviour of trisacetylacetonato manganese(III)

The polarographic and voltammetric behaviour of trisacetylacetonato manganese(III) [Mn(acac)₃] has been studied in methanol, ethanol, tetrahydrofurane, butyrolactone, propylenecarbonate, N,N-dimethylformamide, acetonitrile, nitromethane, N-methylpyrrolidone(2), 1,2-dichloroethane, dichloromethane, dimethylsulfoxide and acetic acid, Mn(acac)₃ was found to undergo a reversible one-electron reduction to Mn(acac)₃^? in most of the solvents mentioned. A further reduction at very negative potentials has been also observed in several solvents. The oxidation of Mn(acac)₃ to Mn(acac)₃⁺ has been studied by cyclovoltammetry in dichloromethane, nitromethane, acetonitrile, propylenecarbonate, N-methylpyrrolidinone(2), N,N-dimethylformamide and dimethylsulfoxide. The polarographic behaviour of NaMn(acac)₃ and Mn(acac)₂ has been investigated in the seven solvents listed above as well as in methanol. The half-wave potentials and the peak potentials referred to bisbiphenylchromium(I)/bisbiphenylchromium(0) as a reference redox system were found to vary with the nature of the solvent. Conductivity studies of Mn(acac)₃ and NaMn(acac)₃ have been carried out in these solvents. U.v.-visible and near i.r. spectra have been recorded of Mn(acac)₃, NaMn(acac)₃, Mn(acac)₂ and Na(acac) in the solvents mentioned. It has further been observed that the half-wave potentials for the polarographic reduction of Mn(acac)₃ shifted to more positive values by the addition of alkali metal ions and to more negative values by the addition of halide ions. The interactions of the solvent with Mn(acac)₃ and the variation of redox potentials with both the solvent and the added electrolytes will be discussed.     

Solvent effects on the redox potentials of tetraethylammonium hexacyanomanganate(III) and hexacyanoferrate(III)

Tetraethylammonium hexacyanomanganate(III) was studied in formamide, N-methylformamide, methanol, propylenecarbonate, N,N-dimethylthioformamide, N-methylthiopyrrolidinone(2), butyrolactone, acetonitrile, dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidinone(2), nitromethane and tetramethylenesulfone employing polarographic and voltammetric techniques. Reversible or nearly reversible behaviour for the reaction Mn(CN)₆^3?/Mn(CN)₆^2? was observed in most solvents on the stationary platinum electrode. The reaction Mn(CN)₆^3?/Mn(CN)₆^4? was studied on both the dropping mercury electrode and the stationary platinum electrode. Besides the reaction Mn(CN)₆^3?/Mn(CN)₆^4? several anodic waves due to successive reactions of mercury with the cyano-ligand of the complex occurred at the dropping mercury electrode. No redox reaction for (et₄N)₃Mn(CN)₆ was found in nitromethane. The polarographic behaviour of tetraethylammonium hexacyanoferrate(III) was studied in formamide, N-methylformamide, N-methylpyrrolidinone(2) and butyrolactone. The variation of E_1/2 and 1/2 (E_pa+E_pc) values versus bisphenylchromium(I)/bisbiphenylchromium(0) as reference redox system of the processes Mn(CN)₆^3?/Mn(CN)₆^2?, Mn(CN)₆^3?/Mn(CN)₆^4? and Fe(CN)₆^3?/Fe(CN)₆^4? with the nature of the solvent is discussed within the donor-acceptor concept. Correlations between the E_1/2 and 1/2(E_pa+E_pc) values and the acceptor properties of the solvent have been observed. The preparation of tetraethyl- and tetrabutylammonium hexacyanomanganate(III) is described.     

Solvent effect on the redox potential of quinone-semiquinone systems

Polarographic reduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone and 9,10-anthraquinone was studied in pyridine, acetone, hexamethylphosphoramide, dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide and propylene carbonate. The variation of the E_1/2-values proper for the first electron transfer step (measured against oxidation potential of ferrocene) with the properties of solvent is analyzed in terms of the donor-acceptor concept. Linear correlations of E_1/2 vs. acceptor number are observed; the slopes of corresponding lines correlate with the spin densities at oxygen atoms of semiquinones.     

1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): Electronic effects on cross-coupling reactions and thermal decompositions

The synthesis of 1,3-diarylimidazolidin-2-ylidene (NHC) precursor, 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, (3b) has been extended to the electronically and sterically modified NHC precursors 3a (X = H), 3c (X = Br) and 3e (X = Cl) in order to investigate the electronic effect of a p-substituent (X) on cross-coupling catalysts. Complexes of the type PdCl₂(NHC)₂ (5), PdCl₂(NHC)(PPh₃) (6) and [RhCl(NHC)(cod)] (7) were prepared from 3 or 4d (1,3-bis(2,4-dimethylphenyl)-2-trichloromethylimidazolidin). Initial decomposition temperatures of the complexes 5 and 6 were determined by TGA. In situ formed complexes from Pd(OAc)₂ and 3 as well as the preformed complexes 5 and 6 have been tested as catalysts in coupling of phenylboronic acid with 4-haloacetophenones. The electron donating ability of NHCs derived from 3 was assessed by measuring C-O frequencies in the respective [RhCl(NHC)(CO)₂] complex 8 which was prepared by replacement of cod ligand of 7 with CO. An interesting correlation between the electron-donating nature of the aryl substituent and catalytic activity and also initial decomposition temperature of the complexes 5 and 6 was observed.     

Solvent effects on the redox properties of thioethers

The one-electron reduction potential of the radical cations of thioanisole (1), benzyl methyl sulfide (2) and 2-hydroxyethyl benzyl sulfide (3) in water, formamide, acetonitrile, acetone, 1,1,1,3,3,3-hexafluoropropan-2-ol, methanol and 2-propanol was investigated by cyclic voltammetry. For comparison the one-electron reduction potentials in water were also measured using pulse radiolysis. The redox potential is strongly influenced by the nature of the solvent and the solvent sensitivity increases with charge localization. The present results have been used to evaluate solvent effects in view of the Kamlet-Taft relationship. The Kamlet-Taft expression quantitatively describes the solvent effects on the redox properties of 1-3 and gives the relative importance of the different solvent properties. The dominating contribution to the solvent effects is the solvent dipolarity/polarizability pi*, whereas alpha appears to be of minor importance. Furthermore, the relationship between the pi* and reduction potential of radical cations of 1-3 appear to be linear. It was also possible to find the same trend between the solvent dipole moment and peak potential of 1-3. These facts indicate that the nature of solvation is mainly nonspecific.     

Synthesis of 2-substituted 5-(1,3-dithiolan-2-ylidene)-1,3-dioxane-4,6-dione derivatives

Some 2-substituted 5-(1,3-dithiolan-2-ylidene)-1,3-dioxane-4,6-dione derivatives 3 were prepared from the condensation reaction of 1,3-dithiolan-2-ylidenemalonic acid ( 2 ) with carbonyl compounds in 40–93% yields.     

1,3-丙二硫醇替代试剂1-（1,3-二噻-2-亚基）丙酮的缩硫醛/酮化反应

通过易制备的3-(1,3-二噻-2-亚基)-2,4-戊二酮(1b)在酸性条件下的脱乙酰化反应制得1-(1,3-二噻-2-亚基)丙酮(1f),产率为86%. 在MeCOCl/MeOH体系中,常温或回流条件下,化合物1f能作为有效的1,3-丙二硫醇替代试剂与醛和酮进行缩硫醛/酮化反应,高产率(86%～99%)合成1,3-二噻烷衍生物. 与已报道的替代试剂2-(2,4-二氯-1,4-戊二烯-3-亚基)-1,3-二噻烷(1a)、3-(1,3-二噻-2-亚基)-2,4-戊二酮(1b)、2-(1,3-二噻-2-亚基)-3-羰基丁酸甲酯(1c)、2-(1,3-二噻-2-亚基)-3-羰基丁酰胺(1d)和2-(1,3-二噻-2-亚基)-3-羰基丁酸(1e)相比,化合物1f是活性最好的1,3-丙二硫醇替代试剂.     

Solvent effect in cis-1,4 polymerization of 1,3-butadiene by a catalyst based on neodymium

In this work a laboratory polymerization scale process was studied for the production of polybutadiene with high content of cis-1,4 repeating units. A Ziegler-Natta catalytic system based on neodymium versatate (catalyst), diisobutylaluminium hydride (cocatalyst) and tert-butyl chloride (chlorinating agent) was used. The influence of solvent nature (pure grade) and possible contaminants (electron donors) in a recovered solvent from a butadiene-styrene anionic polymerization industrial plant on the stereoselectivity and catalytic activity, molecular weight and molecular weight distribution of the resultant polybutadienes was studied. The polymers were characterized by infrared spectroscopy and size exclusion chromatography. Polybutadienes with cis-1,4 units content in the range of 99-98% were produced. The polymers weight-average molecular weight, , varied from 2.23 × 10⁵ to 4.47 × 10⁵ and the molecular weight distribution, MWD, from 3.1 to 5.1.     

A stable carbonyl-pyrazine-metal(0) complex: Synthesis and characterization of cis-tetracarbonylpyrazinetrimethylphosphitetungsten(0)

Pentacarbonylpyrazinetungsten(0), (CO)₅W(pyz), is not stable in solution in polar solvents such as acetone or dichloromethane and undergoes conversion to a bimetallic complex, (CO)₅W(pyz)W(CO)₅ plus free pyrazine. These three species exist at equilibrium. Using the quantitative ¹H NMR spectroscopy, the equilibrium constant could be determined to be K_eq = (5.9 ± 0.8) × 10⁻² at 25 °C. Introducing a second pyrazine ligand into the molecule does not stabilize the complex, as cis-W(CO)₄(pyz)₂ was found to be less stable than W(CO)₅(pyz) and, therefore, could not be isolated. However, introducing trimethylphosphite as a donor ligand into the complex leads to the stabilization of the carbonyl-pyrazine-metal(0) complexes, as shown by the synthesis of cis-W(CO)₄[P(OCH₃)₃](pyz). This complex could be isolated from the reaction of the photogenerated W(CO)₄[P(OCH₃)₃](tetrahydrofuran) with trimethylphosphite upon mixing for 2 h at 10 °C in tetrahydrofuran and characterized by elemental analysis, IR, MS, ¹H, ¹³C, and ³¹P NMR spectroscopy.     

Reaction of (1,3-dioxo-2,3-dihydro-1H-inden-2-ylidene)propanedinitrile with N-arylisoindolines

In a multistep reaction, 3,3′-(2-aryl-2H-isoindol-1,3-ylene)-di-(1,4-naphthoquinone-2-carbonitriles) 13a-f have been formed in 25-61% yield from a series of N-arylisoindolines 8a-f with (1,3-dioxo-2,3-dihydro-1H-inden-2-ylidene)propanedinitrile (1) in aerated pyridine. The structure of one of these products (13f) has been unambiguously confirmed by a single crystal X-ray structure analysis. Under otherwise the same conditions, 2-(3-methoxyphenyl)-isoindoline (8g) and 1 gave 38% of [4-(2,3-dihydro-1H-isoindol-2-yl)-2-methoxyphenyl]-1,3-dioxoindan-2-ylidene)acetonitrile (15). Rationales for these conversions involving the known rearrangement of the radical anion of 1 into the radical anion of 1,4-naphthoquinone-2,3-dicarbonitrile (3) are presented.     

2-(1,3-Dithiolan-2-ylidene)-5-(1,3-dithian-2-ylidene)-1,3,4,6- tetrathiapentalene(DHDA-TTP), a hybrid of BDH-TTP and BDA-TTP,and its metallic cation-radical salts

The synthesis and electrochemical properties of the DHDA-TTP donor, a hybrid of 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (BDH-TTP) and 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), has been investigated, and its ability to form metallic cation-radical salts is elucidated.     

Novel Bis-Fused pi-Electron Donors for Organic Metals: 2-(1,3-Dithiol-2-ylidene)-5-(thiopyran-4-ylidene)-1,3,4,6-tetrathiapentalene

Bis-fused pi-electron donors composed of tetrathiafulvalene (TTF) and 2-(thiopyran-4-ylidene)-1,3-dithiole (TPDT), 2-(1,3-dithiol-2-ylidene)-5-(thiopyran-4-ylidene)-1,3,4,6-tetrathiapentalene (1a, TPDT-TTP), and its derivatives (1b-d, 2a-d) have been synthesized as donor components for organic conductors. An X-ray structure analysis of bis(methylthio)-1 (1c) revealed that the TPDT-TTP skeleton is almost planar except for the outer 1,3-dithiole ring, and that the crystal has a two-dimensional "theta-type" arrangement of molecules. The cyclic voltammograms of TPDT-TTPs exhibit four pairs of single-electron redox waves. The first oxidation potential (E(1)) of 1a (+0.37 V vs SCE, in PhCN) is comparable to that of TTF (+0.35 V) and is higher by 0.1 V than that of TPDT (+0.27 V). The observed substituent effect on E(1) values suggests that the first one-electron oxidation mainly occurs in the 2-(thiopyran-4-ylidene)-1,3-dithiole (TPDT) moiety. On the other hand, on-site coulombic repulsion estimated from the E(2) - E(1) value is lower than in TTF and TPDT. MNDO MO calculations reveal that all the sulfur atoms in the 1,3-dithiole rings have the same phase in the HOMO, a condition necessary for realization of effective transverse intermolecular interaction. The present donors have produced many charge-transfer complexes and cation radical salts showing relatively high conductivity (sigma(rt) = 10(-)(1)-10(1) S cm(-)(1)), several of which display metallic temperature dependence.     

cis,cis,cis,cis-1,2,3,4,5-Pentakis(hydroxymethyl)cyclopentane

All-cis pentamethanolcyclopentane has been obtained in six steps by Diels–Alder condensation of maleic anhydride with (benzyloxymethyl)cyclopenta-2,4-diene, reduction of the anhydride to a diol that was protected as the acetonide. Then, ozonolysis of the double bond, followed by reduction led to a cis-diol. Then successive deprotections of the three other methanol groups gave the cis,cis,cis,cis-1,2,3,4,5-pentakis(hydroxymethyl)cyclopentane.