Non-donor Solvents Research Articles

THE DIRHENIUM(III) COMPLEX COMPOUNDS WITH IMIDAZOLE AND BENZIMIDAZOLE

Imidazole and it’s derivatives are structural fragments of certain enzymes, aminoacids, alkaloids and drugs. To date, the participation of imidazole in human metabolism, especially in the biosynthesis of natural purine bases of RNA and DNA, has been proven. Benzimidazole is one of the most used compound in medical chemistry. The various derivatives of substances, that exhibit analgesic, antidiabetic, anti-inflammatory, antitumor and antiviral activities were synthesized based on benzimidazole. This fact paves the way for the synthesis of antimetabolites, which can delay biosynthesis during biotransformation and inhibit the growth of malignant tumors. Thus, in the present work, the interaction of complex compounds with a single cluster fragment of Re 2 6+ (NBu 4 ) 2 Re 2 Cl 8 and trans-tetrachlorodi-μ-alkylcarboxylates of dirhenium(III) with representatives of the azole class – imidazole and benzimidazole was studied under various conditions. As a result of the experiments, methods for the synthesis of cis-[Re 2 (X) 4 Cl 4 (CH 3 CN) 2 ]Cl 2 (X is imidazole, benzimidazole) in an electron-donating organic solvent – CH3CN under heating in an inert atmosphere were developed. The starting materials were (NBu4)2Re2Cl8 and the selected azoles at a molar ratio of 1:10. The yield of the target compounds was 67-75%. We also investigated the reaction between trans-tetrachloridi-μ-alkylcarboxylates of dirhenium(III) and indicated azoles in a non-donor solvent (1,2-dichloroethane) in inert atmosphere at a molar ratio of reactants of 1:20. As a result of the interaction, complex compounds of the general formula (XH) 2 Re 2 Cl 8 (X – imidazole, benzimidazole) are formed, in which azoles are cations. The yield of reaction products was 83-87%. All target substances are synthesized, isolated in individual state, their spectral properties are investigated. The composition and structure of the obtained complex compounds were confirmed by IR- and NMR- spectroscopy on 1H and 13C, conductometry, and electronic absorption spectroscopy.

Syntheses and Structures of Transition Metal Complexes with Phosphanylphosphinidene Chalcogenide Ligands.

The reactivity of the phosphanylphosphinidene complex [(DippN)2W(Cl)(η2-P-P tBu2)]- (1) toward chalcogens (Ch = Se, S) was studied. Reactions of stoichiometric amounts of 1 with chalcogens in DME yielded monomeric tungsten complexes with phosphanylphosphinidene chalcogenide ligands of the formula tBu2P-P-Ch (Ch = Se (in 2) and S (in 5)), which can be regarded as products of the addition of a chalcogen atom to a P═W bond in starting complex 1. The dissolution of selenophosphinidene complex 2 in nondonor solvents led to the formation of a dinuclear complex of tungsten (3) bearing a tBu2P(Se)-P ligand together with [ tBuSe2Li(dme)2]2 and polyphosphorus species. Under the same reaction conditions, thiophosphinidene complex 5 dimerized via the formation of transient complex 7, possessing a thiotetraphosphane-diido moiety tBu2P(S)-P-P-P tBu2. The elimination of the tBu2PS group from 7 yielded stable dinuclear tungsten complex 8 with an unusual phosphinidene tBu2P-P-P ligand. The reaction of 1 with excess chalcogen led to the cleavage of the P-P bond in the tBu2P-P ligand and the formation of [(DippN)2W(PCh4)]22- and [ tBuCh2Li(dme)2]2. The isolated compounds were characterized by NMR spectroscopy and X-ray crystallography. Furthermore, the calculated geometries of the free selenophosphinidenes, tBu2P-P-Se and tBu2P(Se)-P, were compared with their geometries when serving as ligands in complexes 2 and 3.

Designing an Effective Metal‐Free Lewis Acid Catalyst for Ammonia‐Borane Dehydrogenation: A DFT Investigation on Triarylboranes

AbstractThe catalytic dehydrogenation of ammonia‐borane (NH3BH3) is dominated largely by transition‐metal catalysts. Metal‐free catalysis for NH3BH3 dehydrogenation is a rarity. It is well known that mono‐boron‐based Lewis acids are largely ineffective to facilitate the catalytic dehydrogenation of NH3BH3. Herein, through theoretical investigations, we have identified the routes with catalytic potential for B(C6F5)3 and its congeners and also the factors that are likely to prevent effective catalysis for these systems. Our findings reveal for triarylboranes that potential catalytic dehydrogenation routes comprise of two main events: ion pair formation from NH3BH3 in the presence of a catalyst assisted by a nucleophile and subsequent H2 release from the ion pair. Donor solvents and the B−H hydride of NH3BH3 act as a nucleophile to facilitate ion‐pair formation from NH3BH3 and the Lewis acid catalyst in donor and nondonor solvents, respectively. A good nucleophilic solvent decreases the activation barrier of ion‐pair formation but it increases the activation barrier associated with the subsequent H2 release process. The reverse is true for nondonor solvents, in which case NH3BH3 acts as a nucleophile. Our studies reveal that by the careful tuning of the hydride affinity of the Lewis acid catalyst in combination with nondonor solvents, rate‐limiting barriers for dehydrogenation can be reduced to approximately 19–20 kcal mol−1, which would enable catalytic turnovers at room temperature.

Octakis(2-pyridyl)porphyrazine and Its Neutral Metal Derivatives: UV-Visible Spectral, Electrochemical, and Photoactivity Studies.

Tetrapyrrolic porphyrazine macrocycles with externally appended 2-pyridyl rings were synthesized and characterized as to their spectroscopic, electrochemical, and photophysical properties. The investigated compounds are represented as [Py8PzH2], the unmetalated octakis(2-pyridyl)porphyrazine, and the metal complexes [Py8PzM], where M is MgII(H2O), CuII, ZnII, or CoII. The spectroscopic properties and the electrochemical behavior of these compounds were examined in solution of polar (pyridine, dimethyl sulfoxide, and dimethylformamide) and nondonor solvents (CHCl3, CH2Cl2), and the data were compared with those obtained from earlier studies on the related tetrapyrazinoporphyrazines, [Py8TPyzPzM], and the tetraquinoxalinoporphyrazines, [Py8QxPzM], also bearing externally 2-pyridyl rings, and characterized by a more extended central π-conjugated macrocyclic framework. The newly synthesized porphyrazines possess good solubility and are present in their monomeric form in all the examined solvents as shown by UV-visible spectra. The unmetalated species and metal derivatives undergo multiple one-electron reductions within the potential range of the nonaqueous examined solvents. The derivatives with nonredox active metal centers could accept four electrons on the conjugated macrocycle, while the CoII complex was characterized by a single one-electron oxidation and five reductions in DMSO. The photosensitizer activity for the generation of singlet oxygen was also examined for the MgII(H2O) and ZnII complexes in DMF, with measured ΦΔ values being, respectively, 0.42 and 0.64, this latter value indicating the ZnII species as being a promising material for use as anticancer agent in photodynamic therapy.

Interaction between Hydrogen-Donor and Nondonor Solvents in Direct Liquefaction of Bulianta Coal

Solvent plays two very important roles in coal liquefaction: (1) Physically, it serves as a medium for coal transport and heat transfer, and (2) chemically, it serves as an important source of transferable hydrogen (hydrogen donor) and hydrogen shuttle between hydrogen and coal. In this work, eight types of non-hydrogen-donor solvents, namely, decalin, 1-methylnaphthalene, naphthalene, fluorene, anthracene, phenanthrene, pyrene, and fluoranthene, were examined in combination with the hydrogen-donor solvent tetralin in the liquefaction of Bulianta coal to clarify the interactions between the hydrogen-donor and non-hydrogen-donor solvents and the mechanism of hydrogen transfer. In the absence of a catalyst, the mixed solvents of tetralin with phenanthrene, pyrene, and fluoranthene showed favorable effects on coal conversion and oil yield compared with pure tetralin, regardless of whether the reactions were conducted in a H2 or N2 atmosphere. The reactions of non-hydrogen-donor solvents with tetralin showed ...

Dispersion‐Force‐Assisted Disproportionation: A Stable Two‐Coordinate Copper(II) Complex

The synthesis of the first linear coordinated Cu(II) complex Cu{N(SiMe3 )Dipp}2 (1 Dipp=C6 H5 -2,6Pr(i) 2 ) and its Cu(I) counterpart [Cu{N(SiMe3 )Dipp}2 ](-) (2) is described. The formation of 1 proceeds through a dispersion force-driven disproportionation, and is the reaction product of a Cu(I) halide and LiN(SiMe3 )Dipp in a non-donor solvent. The synthesis of 2 is accomplished by preventing the disproportionation into 1 by using the complexing agent 15-crown-5. EPR spectroscopy of 1 provides the first detailed study of a two-coordinate transition-metal complex indicating strong covalency in the Cu-N bonds.

Divalent Tetra- and Penta-phenylcyclopentadienyl Europium and Samarium Sandwich and Half-Sandwich Complexes: Synthesis, Characterization, and Remarkable Luminescence Properties

The synthesis of the bulky divalent (polyphenylcyclopentadienyl)lanthanoid sandwich complexes [Ln(C5Ph5)2] (Ln = Sm, Eu) and [Ln(C5Ph4H)2(solv)] (Ln = Sm, solv = thf; Ln = Eu, solv = dme)], from redox-transmetalation/protolysis (RTP) reactions, has been achieved. An analogous reaction with Yb afforded the solvent-separated ion pair [Yb(dme)4][C5Ph4H]2. In addition, rare divalent samarium halide complexes [Sm(C5Ph5)(μ-Br)(thf)2]2 and [Sm(C5Ph4H)I(thf)3], were also prepared, either by RTP or ligand rearrangement. X-ray studies showed that the [Ln(C5Ph5)2] complexes adopt highly symmetrical sandwich structures, whereas the [Ln(C5Ph4H)2(solv)] complexes have open sandwich structures. The unexpected, but limited, solubility of the [Ln(C5Ph5)2] complexes allowed for variable-temperature NMR spectra of [Sm(C5Ph5)2] to be obtained. Detailed 1D and 2D NMR studies were conducted on [Sm(C5Ph4H)2(thf)] to ascertain its structure in donor and nondonor solvents. During the course of these studies, the mixed tetraarylcy...

Probing the limits of alkaline earth-transition metal bonding: an experimental and computational study.

Reduction of Fp2 (Fp = CpFe(CO)2) or [Co(CO)3(PCy3)]2 (15) with Mg-mercury amalgam gave [Mg{TM(L)}2(THF)]2 (TM(L) = Fp or Co(CO)3(PCy3) (19)) in which the TM is bonded to two Mg atoms. Reduction of 15 with Ca-, Sr-, Ba-, Yb-, Eu- and Sm-mercury amalgam gave a series of compounds "M{Co(CO)3(PCy3)}2(THF)n" (M = Ae or Ln) in which the M-Co bonding varies with the charge-to-size ratio of M. For M = Ca or Yb (24), each metal forms one M-Co bond and one M(μ-OC)Co η(1)-isocarbonyl linkage. With M = Sr (21) or Eu (25), a switch from M-Co bonding to side-on (η(2)) CO ligand coordination is found. Sm(II){Co(CO)3(PCy3)}2(THF)3 disproportionates in pentane to form Sm(III){Co(CO)3(PCy3)}3(THF)3 containing two Sm(III)-Co bonds, in contrast with 25, showing the importance of the Ln charge on Ln-TM bonding. Diffusion NMR spectroscopy found that in solution, 21 and 24 are dimeric compounds [M{Co(CO)3(PCy3)}2(THF)3]2 that, according to DFT calculations, contain either one (Ae = Ca) or two (Ae = Sr) Ae-Co bonds per Co atom. DFT calculations in combination with Ziegler Rauk energy decomposition and atoms in molecules analysis were used to assess the nature and energy of Ae-Co bonding in a series of model compounds. The Ae-Co interaction energies decrease from Be to Sr, and toward the bottom of the group, side-on (η(2)) CO ligand coordination competes with Ae-Co bonding. The PCy3 ligand plays a pivotal role by increasing solubility in nondonor solvents and the Ae-Co interaction energy.

Chemical structure and reactivity alterations of brown coals during thermal treatment with aromatic solvents

Thermal treatments of two brown coals (Chinese and Australian brown coals) -slurries with four different aromatic solvents (hydrogen donor and non-donor, polar and non-polar solvents) were conducted with bench-scale apparatus, and the treated samples (TSs) were separated and characterized in detail to investigate the structural alterations of the coals, as well as the interactions between the solvents and the coals. It is observed that the thermal treatments in aromatic solvents are effective in upgrading the brown coals, but the reactivity of brown coals towards pyrolysis is reduced due to the extraction of small molecules and the enhancement of cross-linking reactions. The solvents are tightly combined with the brown coals owning to the formation of new chemical forces (such as hydrogen bonds or π–π conjugations) as well as the physical disruptions such as the collapse of pores during slurry-heating. Different solvents behave variously with their unique properties, especially tetrahydroquinoline (THQ), which is more tightly combined with the brown coals than the other solvents, including tetralin (THN), 1-methylnaphthalene (1-MN), recycle solvent (RS) due to the electron-affluent nitrogen atom on its aromatic ring. Hydrogen-donor solvent is superior to non-donor solvent for thermal treatment of brown coals.

Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: solving some stereochemical mysteries.

Compounds 1 a-f-H2 form "monomeric" triscatecholate titanium(IV) complexes [Ti(1 a-f)3](2-), which in the presence of Li cations are in equilibrium with the triple lithium-bridged "dimers" [Li3(Ti(1 a-f)3)2](-). The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is "locked". The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti(1 c-e)3](2-) or [Li3(Ti(1 c-e)3)2](-). A surprising exception was found for the dimer [Li3(Ti(1 f)3)2](-). Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li3(Ti(1 f)3)2](-) the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal-complex units of [Li3(Ti(1 c-e)3)2](-) or [Li3(Ti(1 f)3)2](-).

Phosphine and Diphosphine Complexes of Silicon(IV) Halides

The reaction of SiX4 (X = Cl or Br) with PMe3 in anhydrous CH2Cl2 forms trans-[SiX4(PMe3)2], while the diphosphines, Me2P(CH2)2PMe2, Et2P(CH2)2PEt2, and o-C6H4(PMe2)2 form cis-[SiX4(diphosphine)], all containing six-coordinate silicon centers. With Me2PCH2PMe2 the product was trans-[SiCl4(κ(1)-Me2PCH2PMe2)2]. The complexes have been characterized by X-ray crystallography, microanalysis, IR, and multinuclear ((1)H, (13)C{(1)H}, and (31)P{(1)H}) NMR spectroscopies. The complexes are stable solids and not significantly dissociated in nondonor solvents, although they are very moisture and oxygen sensitive. This stability conflicts with the predictions of recent density functional theory (DFT) calculations (Wilson et al. Inorg. Chem. 2012, 51, 7657-7668) which suggested six-coordinate silicon phosphines would be unstable, and also contrasts with the failure to isolate complexes with SiF4 (George et al. Dalton Trans. 2011, 40, 1584-1593). No reaction occurred between phosphines and SiI4, or with SiX4 and arsine ligands including AsMe3 and o-C6H4(AsMe2)2. Attempts to make five-coordinate [SiX4(PR3)] using the sterically bulky phosphines, P(t)Bu3, P(i)Pr3, or PCy3 failed, with no apparent reaction occurring, consistent with predictions (Wilson et al. Inorg. Chem. 2012, 51, 7657-7668) that such compounds would be very endothermic, while the large cone angles of the phosphines presumably preclude formation of six-coordination at the small silicon center. The reaction of Si2Cl6 with PMe3 or the diphosphines in CH2Cl2 results in instant disproportionation to the SiCl4 adducts and polychlorosilanes, but from hexane solution very unstable white [Si2Cl6(PMe3)2] and [Si2Cl6(diphosphine)] (diphosphine = Me2P(CH2)2PMe2 or o-C6H4(PMe2)2) precipitate. The reactions of SiHCl3 with PMe3 and Me2P(CH2)2PMe2 also produce the SiCl4 adducts, but using Et2P(CH2)2PEt2, colorless [SiHCl3{Et2P(CH2)2PEt2}] was isolated, which was characterized by an X-ray structure which showed a pseudo-octahedral complex with the Si-H trans to P. Attempts to reduce the silicon(IV) phosphine complexes to silicon(II) were unsuccessful, contrasting with the isolation of stable N-heterocyclic carbene adducts of Si(II).

Synthesis and properties of a heteroleptic µ-nitrido tetrapyrrolic complex

Multi-component organic chromophores having intense visible absorption characteristics covering large part of solar radiation are currently an object of much attention in view of their exploitation as artificial light-harvesting antenna. In particular those containing the phthalocyanine (Pc) and porphyrin (P) macrocyclic ligands have been recently studied being interesting for the peculiar photoinduced energy transfer processes[1] and being also attractive for their potential use in the field of molecular photovoltaics, artificial photosynthesis and molecular electronic applications.[2-4] Several strategies can be used to link each other the two different subunits to obtain a hetero phthalocyanine-porphyrin dyads, resulting in different geometries and properties. It is possible for example to link the different moieties through a conjugated or not spacer between the peripheral positions of one macrocycle[5] toward either the periphery or the central metal of the other macrocycle.[1] Moreover single atom (O, N, C) bridged systems as μ-oxo, μ-nitrido and μ-carbido can also allow a combination, via metal centres, leading to homo/heterometallic and homo/heteroleptic systems.[6] To the aim recalled above, it should be important that the orbital interactions could promote an electronic communication along the stack thereby optical absorption will change and, in the case of complementary chromophores, extend. As other analogues mixed-ligand porphyrin/phthalocyanine bridged systems are expected to mutate the electronic configuration and to extend the visible absorption to the near IR wavelength. Among the μ-nitrido species, the mixed-ligand tetraphenylporphyrin-phthalocyanine μ-nitrido complex of iron was synthesised and fully characterised as solid material by Ercolani et al.;[7,8] it showed a good thermal stability (> 300°C) and an interesting electronic and magnetic exchange between the iron centres trough the linear Fe-N-Fe bond system. Unfortunately its optical (UV-vis) spectrum was not reported likely because of its very scarce solubility in non-donor solvent. Being this feature indispensable for the processability of these material in view of technological applications, we attempted to increase the solubility of such species and we present here preliminary investigations on the substituted analogous (μ-nitrido)[((tetraphenylporphyrinato) iron)-(tetra-tert-butylphthalocyaninato)iron], I.§

Germanium(II) and Tin(II) Complexes of a Sterically Demanding Phosphanide Ligand

The reaction between PhPCl(2) and 1 equiv of RLi, followed by in situ reduction with LiAlH(4) and an aqueous workup yields the secondary phosphane PhRPH [R = (Me(3)Si)(2)CH]. Treatment of PhRPH with n-BuLi in diethyl ether generates the lithium phosphanide (RPhP)Li(Et(2)O)(n) [15(Et(2)O)], which may be crystallized as the tetrahydrofuran (THF) adduct (RPhP)Li(THF)(3) [15(THF)]. Compound 15(Et(2)O) reacts with 1 equiv of either NaO-tBu or KO-tBu to give the corresponding sodium and potassium phosphanides (RPhP)Na(Et(2)O)(n) (16) and (RPhP)K(Et(2)O)(n) (17), which may be crystallized as the amine adducts [(RPhP)Na(tmeda)](2) [16(tmeda)] and [(RPhP)K(pmdeta)](2) [17(pmdeta)], respectively. The reaction between 2 equiv of 17 and GeCl(2)(1,4-dioxane) gives the dimeric compound [(RPhP)(2)Ge](2).Et(2)O (18.Et(2)O). In contrast, the reaction between 2 equiv of 15 and SnCl(2) preferentially gives the ate complex (RPhP)(3)SnLi(THF) (19) in low yield; 19 is obtained in quantitative yield from the reaction between SnCl(2) and 3 equiv of 15. Crystallization of 19 from n-hexane/THF yields the separated ion pair complex [(RPhP)(3)Sn][Li(THF)(4)] (19a); exposure of 19a to vacuum for short periods leads to complete conversion to 19. Treatment of GeCl(2)(1,4-dioxane) with 3 equiv of 15 yields the contact ion pair (RPhP)(3)GeLi(THF) (20), after crystallization from n-hexane/THF. Compounds 15(THF), 16(tmeda), 17(pmdeta), 18.Et(2)O, 19a, and 20 have been characterized by elemental analyses, multielement NMR spectroscopy, and X-ray crystallography. While 15(THF) is monomeric, both 16(tmeda) and 17(pmdeta) are dimeric in the solid state. The diphosphagermylene 18.Et(2)O adopts a dimeric structure in the solid state with a syn,syn-arrangement of the phosphanide ligands, and this structure appears to be preserved in solution. The ate complex 19a crystallizes as a separated ion pair, whereas the analogous ate complex 20 crystallizes as a discrete molecular species. The structures of 19 and 20 are retained in non-donor solvents, while dissolution in THF yields the separated ion pairs 19a and [(RPhP)(3)Ge][Li(THF)(4)] (20a).

Tetrakis(thiadiazole)porphyrazines: 7. Synthesis and structure of μ-oxo-bis[tetrakis(thiadiazole)porphyrazinato-aluminum(III)

The complex μ-oxo-bis[tetrakis(thiadiazole)porphyrazinato-aluminum(III)], [( TTDPzAl )2 O ], has been obtained from the corresponding mononuclear Al(III) hydroxo-complex, [ TTDPzAlOH ], by refluxing a solution of this latter in α-chloronaphthalene or by heating the solid hydroxo species (250–300 °C) under vacuum (10-2 mmHg). The complex, which appears as a stable species in air, shows a typical IR absorption at 983 cm-1, assigned as ν as (Al-O-Al) . Contact with water determines disappearance of this absorption consequent to the formation of the related hydroxide. [( TTDPzAl )2 O ], insoluble in most organic non-donor solvents, and very scarcely soluble in dimethylsulfoxide and dimethylformamide, shows some instability in pyridine, with probable breaking of the μ-oxo bridge and formation of the mononuclear hydroxide. Single crystals suitable for X-ray analysis were obtained by subliming [( TTDPzAl )2 O ] under vacuum at 500 °C under a continuous N 2 flow. Crystal data for [( TTDPzAl )2 O ], C 32 Al 2 N 32 OS 8: a = 7.613(2) Å, b = 11.280(2) Å, c = 23.363(5) Å, β = 96.869(3)°, space group P21/c ( n . 14), U = 1991.9(7) Å3, Z = 2, R(F)a = 0.0492, wR(F2) = 0.1024. The μ-oxo bridged complex has a central linear Al-O-Al bond system, with the Al(III) atoms displaced out of the respective N 4 square planar coordination sites by 0.441 Å and the two TTDPz planes lying in an eclipsed position, the overall structure of the complex approaching closely that of the related phthalocyaninato complex [( PcAl )2 O ]. Intermolecular contacts, instead, are definitely different for the two species, the type found present in [( TTDPzAl )2 O ] highly reminiscent of those of other previously reported "thiadiazoleporphyrazine" macrocycles, with the observed short inter-unit contacts between adjacent TTDPz macrocycles again determined by the presence in the TTDPz framework of the annulated peripheral (S, N) -containing heterocycles.

One-Electron Oxidation of Ruthenocene: Reactions of the Ruthenocenium Ion in Gentle Electrolyte Media

The electrochemical oxidation of ruthenocene, RuCp(2) (Cp = eta(5)-C(5)H(5)), 1, has been studied in dichloromethane using a supporting electrolyte containing either the [B(C(6)F(5))(4)](-) (TFAB) or the [B(C(6)H(3)(CF(3))(2))(4)](-) (BArF(24)) counteranion. A quasi-Nernstian process was observed in both cases, with E(1/2) values of 0.41 and 0.57 V vs FeCp(2) in the respective electrolyte media. The ruthenocenium ion 1(+) equilibrates with a metal-metal bonded dimer [Ru(2)Cp(4)](2+), 2(2+), that is increasingly preferred at low temperatures. Dimerization equilibrium constants determined by digital simulation of cyclic voltammetry (CV) curves were in the range of 10(2)-10(4) M(-1) at temperatures of 256 to 298 K. Near room temperature, and particularly when BArF(24) is the counteranion, the dinuclear species [Ru(2)Cp(2)(sigma:eta(5)-C(5)H(4))(2)] (2+), 3(2+), in which each metal is sigma-bonded to a cyclopentadienyl ring, was the preferred electrolytic oxidation product. Cathodic reduction of 3(2+) regenerated ruthenocene. The two dinuclear products, 2(2+) and 3(2+), were characterized by (1)H NMR spectroscopy on anodically electrolyzed solutions of 1 at low temperatures in CD(2)Cl(2)/[NBu(4)][BArF(24)]. The variable temperature NMR behavior of these solutions showed that 3(2+) and 2(2+) take part in a thermal equilibrium, the latter being dominant at the lowest temperatures. Ruthenocene hydride, [1-H](+), was also identified as being present in the electrolysis solutions. The oxidation of ruthenocene is shown to be an inherent one-electron process, giving a ruthenocenium ion which is highly susceptible to reactions that allow it to regain an 18-electron configuration. In a dry non-donor solvent, and in the absence of nucleophiles, this electronic configuration is attained by self-reactions involving formation of Ru-Ru or Ru-C bonds. The present data offer a mechanistic explanation for the previously described results on the chemical oxidation of osmocene (Droege, M.W.; Harman, W.D.; Taube, H. Inorg. Chem. 1987, 26, 1309) and are relevant to the manner in which sigma:eta(5)-C(5)H(4)-complexes of other second and third-row metals are formed.

The Trinuclear Gallium‐Bridged Ferrocenophane [{Fe(η5‐C5H4)2}3Ga2]: Synthesis, Bonding, Structure, and Coordination Chemistry

The trinuclear ferrocenophane [{Fe(eta(5)-C(5)H(4))(3)}(2)Ga(2)] (3) featuring two sp(2)-hybridized gallium atoms in bridging positions between three ferrocene-1,1'-diyl units represents a novel type of ferrocene derivative. Compound 3 is obtained by thermal treatment of 1,1'-bis(dimethylgallyl)ferrocene (1) in nondonor solvents or in diethyl ether as solvent and subsequent thermal decomplexation. The [1.1]ferrocenophane [{Fe(eta(5)-C(5)H(4))(2)}(2){GaMe}(2)] (2) is an intermediate in the formation of 3. The reaction of 3 with an excess of trimethylgallium leads back to 1 and proves the reversibility of the multistep reaction sequence. Theoretical calculations reveal a carousel-type D(3h) structure for 3. The compound can best be described as being composed of three only weakly interacting ferrocenediyl units covalently connected by gallium atoms without any pi-bond contribution in the Ga--C bonds. Owing to steric constraints 3 cannot be reduced to the dianion 3(2-), which would feature a Ga--Ga bond. Compound 3 represents a stereochemically rigid difunctional Lewis acid allowing the formation of the adducts 3 a-3 d possessing linear donor-aceptor-aceptor-donor arrangements. Crystal structure data for 3 a-3 d show a symmetry-reduced chiral ferrocenophane core (D(3h)-->D(3)). A polymeric rodlike structure is observed for 3 b and 3 d caused by pi-stacking effects (3 b) or by a difunctional donor-acceptor interaction (3 d). In solution, the chirality of the adducts is lost by rapid interconversion of the enantiomers. A cyclic voltammogram of 3 b in pyridine reveals three quasi-reversible oxidation steps at -356, -154, and 8 mV, indicating only weak electron delocalization in the cationic species. The redox potentials of the pyridine adduct 3 b are compared with those of other pyridine-stabilized gallyl-sustituted ferrocene derivatives and with ferrocene itself.

Modelling the impact of geometric parameters on the redox potential of blue copper proteins

The synthesis and structure of a homologous series of cationic N 2S 2 copper(I) Schiff base complexes constructed using o- tert-butylthiobenzaldehyde and a series of terminal diamines (ethane, propane, butane) are reported. The complexes differ only in the length of the methylene chain between the imine groups. This simple modification forces the copper centre to shift geometry from a planar (1,2-diaminoethane) to a more distorted tetrahedral motif (1,4-diaminobutane). The redox potentials of the three cations were measured using cyclic voltammetry in donor (acetonitrile) and non-donor solvents (dichloromethane). The S–Cu–N angles for each complex are correlated against the respective redox potential allowing an analysis of the geometric impact on the redox potential in soft copper centres. The redox potential is observed to increase as the metal centre moves from a planar towards a tetrahedral motif. Comparing this data with the reported structures of the blue copper proteins (rusticyanin and plastocyanin) allows an assessment of the contribution of the geometry of the metal binding site to the operating potential of these proteins to be made.

Barriers to Racemization in C3-Symmetric Complexes Containing the Hydrotris(2-mercapto-1-ethylimidazolyl)borate (TmEt) Ligand

The tripodal ligands hydrotris(N-ethyl-2-mercaptoimidazol-1-yl)borate (NaTm(Et)) (1) and hydrotris(N-benzyl-2-mercaptoimidazol-1-yl)borate (NaTm(Bn)) (2), analogues of the hydrotris(N-methyl-2-mercaptoimidazol-1-yl)borate ligand (Tm) containing alternative nitrogen substituents, have been employed to examine the racemization of their C3-symmetric complexes with both four- and six-coordinate metals. The ligands react at room temperature with metal halides to provide C3-symmetric metal complexes. The syntheses of the four-coordinate complexes [Tm(Et)ZnCl] (3), [Tm(Et)CdBr] (4), [Tm(Et)HgCl] (5), [Tm(Et)CuPPh3] (6), [Tm(Et)AgPPh3] (7), and [Tm(Bn)ZnCl] (8) are reported. The six-coordinate complexes [Tm(Et)Ru(p-cymene)]Cl (9), [Tm(Et)Ru(p-cymene)]PF(6) (10), and [Tm(Et)Mn(CO)3] (11) were also synthesized. The X-ray crystal structures of 3, 4, 6, and 9 are reported. The diastereotopic nature of the ethyl and benzyl hydrogen atoms in the ligands allows the enantiomeric forms of these complexes to be distinguished by 1H NMR spectroscopy. Variable-temperature (VT) 1H NMR spectra have thus been used to investigate the energies of the racemization processes occurring in these chiral complexes. In solvents the activation energies to racemization for the four-coordinate complexes lay in the range of 53-77 kJ mol(-1). In non-donor solvents the energies are reduced and a dissociative mechanism is therefore implicated. No interconversion could be observed by VT NMR for the six-coordinate complexes in any solvent. To further explore the racemization mechanisms ab initio density functional theory calculations have been conducted on the ground- and transition-state structures of representative six-coordinate [Mn(I)] and four-coordinate [Zn(II)] complexes following a proposed nondissociative mechanism of racemization. The calculated energy barriers to racemization are 163 and 121 kJ mol(-1), respectively. It is concluded that the low-energy racemization of substitution-labile four-coordinate complexes occurs via a dissociative mechanism, while substitution-inert six-coordinate complexes experience a significantly higher barrier to racemization. Whether this is due to the operation of a dissociative mechanism with a higher activation barrier or to a nondissociative mechanism remains unknown.

Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 1. Tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine: A New Macrocycle with Remarkable Electron-Deficient Properties

A new pyrazinoporphyrazine macrocycle carrying externally appended pyridine rings, tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine (hydrated), [Py(8)TPyzPzH(2)].2H(2)O, was prepared in high yield by direct cyclotetramerization of the precursor, 2,3-dicyano-5,6-di(2-pyridyl)-1,4-pyrazine, [(CN)(2)Py(2)Pyz], in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The single-crystal X-ray structure of [(CN)(2)Py(2)Pyz] shows a noncoplanar positioning of the pyrazine and pyridine rings in the two slightly different independent molecular units present in the crystal. UV-vis spectra of [Py(8)TPyzPzH(2)] were measured in two nondonor solvents (CHCl(3), CH(2)Cl(2)), a slightly basic solvent (pyridine), and an acidic solvent (CH(3)COOH). In all cases, the spectral changes are consistent with the occurrence of molecular aggregation and colloidal dispersions which break up with time to give clear solutions containing exclusively the monomeric form of the macrocycle, either neutral [Py(8)TPyzPzH(2)] (in CHCl(3), CH(2)Cl(2), and CH(3)COOH) or dianionic [Py(8)TPyzPz](2)(-) (in pyridine). A spectrally monitored titration of [Py(8)TPyzPzH(2)] in CH(2)Cl(2) with TBA(OH) shows the loss of two protons from the macrocyclic core and quantitative conversion of [Py(8)TPyzPzH(2)] to [Py(8)TPyzPz](2)(-). Cyclic voltammetry and thin-layer spectroelectrochemical measurements show that [Py(8)TPyzPzH(2)] is present in CH(2)Cl(2) while [Py(8)TPyzPz](2)(-) is present in pyridine, but both forms of the compound exhibit identical electrochemical behavior, consistent with a conversion of the dianion to the neutral porphyrazine in pyridine prior to electroreduction via four reversible one-electron transfer steps. No oxidations of the macrocycle are observed in either solvent containing 0.1 M tetrabuthylammonium perchlorate (TBAP). A comparison of the electrochemical behavior for [Py(8)TPyzPzH(2)] with what is reported for related phthalocyanine and porphyrazine analogues highlights the remarkable electron-accepting properties of the presently investigated free-base macrocycle.

Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 2. Metal Complexes of Tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine: Linear and Nonlinear Optical Properties and Electrochemical Behavior

A series of metal complexes of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine, [Py(8)TPyzPzH(2)], having the general formula [Py(8)TPyzPzM].xH(2)O (M = Mg(II)(H(2)O), Mn(II), Co(II), Cu(II), Zn(II); x = 3-8) were synthesized by reaction of the free-base macrocycle with the appropriate metal acetate in pyridine or dimethyl sulfoxide under mild conditions. Clathrated water and retained pyridine molecules for the Mn(II) and Co(II) species are easily eliminated by heating under vacuum, the water molecules being recovered by exposure of the unsolvated macrocycles to air. Magnetic susceptibility measurements and EPR spectra of the materials in the solid state provide basic information on the spin state of the Cu(II), Co(II), and Mn(II) species. Colloidal solutions caused by molecular aggregation are formed in nondonor solvents (CH(2)Cl(2), CHCl(3)), a moderately basic solvent (pyridine), and an acidic solvent (CH(3)COOH), with the extent of aggregation depending on the specific solvent and the central metal ion. UV-vis spectral monitoring of the solutions after preparation indicates that disaggregation systematically occurs as a function of time leading ultimately to the formation of clear solutions containing the monomeric form of the porphyrazine. Cyclic voltammetry and thin-layer spectroelectrochemistry show that each compound with an electroinactive metal ion undergoes four reversible one-electron reductions, leading to formation of the negatively charged species [Py(8)TPyzPzM](n-) (n = 1 - 4). The stepwise uptake of four electrons is consistent with a ring-centered reduction, but in the case of the cobalt complex a metal-centered (Co(II) --> Co(I)) reduction occurs in the first process and only three additional reductions are observed. No oxidations are observed in pyridine or CH(2)Cl(2) containing 0.1 M tetrabuthylammonium perchlorate (TBAP). The nonlinear optical properties (NLO) of the species [Py(8)TPyzPzM] (M = 2H(I), Cu(II), Zn(II), Mg(II)(H(2)O)) have also been examined with nanosecond pulses at 532 nm in dimethyl sulfoxide solution. Reverse saturable absorption is shown by all of the [Py(8)TPyzPzM] species, which exhibit distinct behavior depending on the nature of M and extent of aggregation.