Photolysis of the ion pair [Pt(NH3)5Cl]S2O8: reduction of platinum(IV) induced by outer-sphere charge transfer excitation

The ion pair [Pt^IV(NH₃)₅Cl]³⁺S₂O₈²⁻ shows a S₂O₈²⁻ → [Pt(NH₃)₅Cl]³⁺ outer-sphere charge transfer (OSCT) absorption at λ_max=267 nm. OSCT excitation leads to the reduction of Pt(IV) by S₂O₈²⁻ to Pt(II) with φ=3×10⁻³ at λ_irr=280 nm.     

Strain in organometallics II: controlling the properties of tetra-coordinated iridium complexes using diastereomers of a bis(tropp) ligand system

The tetra-chelating ligands 1,2-bis[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphanyl]-ethane, bis(tropp^Ph)ethane, and 1,3-bis[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphanyl]-propane, bis(tropp^Ph)propane, were synthesised. For the binding of transition metals, these ligands offer two olefin moieties and two phosphorus centres and form mixtures of diastereomers with a R,S-configuration at the phosphorus centres (meso), or a R,R(S,S)-configuration (rac), respectively. meso/rac-bis(tropp^Ph)ethane was separated by fractional crystallisation and reacted with [Ir(cod)₂]OTf (cod=cylcooctadiene, OTf⁻=CF₃SO₃ ⁻) to give the penta-coordinated complex-cations meso/rac-[Ir(bis(tropp^Ph)ethane)(cod)]⁺, where the bis(tropp^Ph)ethane serves as tridentate ligand merely. One olefin unit remains non-bonded, however, a slow intra-molecular exchange between this olefin and the coordinated olefin unit was established (meso-[Ir(bis(tropp^Ph)ethane)(cod)]⁺: k<0.5 s⁻¹; rac-[Ir(bis(tropp^Ph)ethane)(cod)]⁺: k≈35 s⁻¹). The ligand meso/rac-bis(tropp^Ph)propane reacts with [Ir(cod)₂]OTf to give the corresponding complexes containing the tetra-coordinated 16-electron complex-cations meso/rac-[Ir(bis(tropp^Ph)propane)]⁺. The diastereomers were separated by fractional crystallisation. The complex rac-[Ir(bis(tropp^Ph)propane)]⁺ is reduced at relatively low potentials (E¹_1/2=−0.95 V, E²_1/2=−1.33 V versus Ag/AgCl) to give the neutral 17-electron complex [Ir(bis(tropp^Ph)propane)]⁰ and the 18-electron anionic iridate [Ir(bis(tropp^Ph)propane)]⁻, respectively. With acetonitrile, [Ir(bis(tropp^Ph)propane)]⁺ reacts to give the penta-coordinated complex rac-[Ir(MeCN)(bis(tropp^Ph)propane)]⁺ (K=45 M⁻¹, k_f=6×10³ M⁻¹ s⁻¹, k_d=1×10² s⁻¹) and with chloride to yield the relatively stable complex rac-[Ir(Cl)(bis(tropp^Ph)propane)] (k_d<0.5 s⁻¹). Compared to the rac-isomer, the meso-[Ir(bis(tropp^Ph)propane)]⁺ shows significantly cathodically shifted reduction potentials (E¹_1/2=−1.25 V, E²_1/2=−1.64 V versus Ag/AgCl), an acetonitrile complex could not be detected, and the chloro-complex, meso-[Ir(Cl)(bis(tropp^Ph)propane)], is much more labile (k_d≈20′000 s⁻¹). meso-[Ir(bis(tropp^Ph)propane)]⁺ reacts with one equivalent H₂ to give the trans-dihydride complex-cation, meso-[Ir(H)₂(bis(tropp^Ph)propane)]⁺, while the rac-isomer, rac-[Ir(bis(tropp^Ph)propane)]+, reacts with two equivalents H₂ to give rac-{Ir(H)₂(OTf)[(tropp^Ph)(H₂tropp^Ph)propane]}, a cis-dihydride complex containing a hydrogenated 10,11-dihydro-5H-dibenzo[a,d]cycloheptene unit, H₂tropp^Ph. The triflate anion in this complex is rather firmly bound and dissociates only slowly (k=29 s⁻¹). All differences between the different stereoisomers are attributed to the fact that the ligand backbone in the meso-isomer, meso-[Ir(bis(tropp^Ph)propane)]⁺, enforces a planar coordination sphere at the metal. On the contrary, already in the tetra-coordinated rac-[Ir(bis(tropp^Ph)propane)]⁺, the metal has a tetrahedrally distorted coordination sphere which does not impede the reduction to the d⁹-Ir(0) and d¹⁰-Ir(−1) complexes and allows more easily a distortion towards a trigonal bipyramidal (tbp) or octahedral structure for penta- or hexa-coordinated complexes, respectively. A comparison of the NMR data for iridium bonded olefins in equatorial or axial positions in tbp structures shows that the latter experience only modest metal-to-ligand back-donation, while the olefins in the equatorial positions have a high degree of metallacyclopropane character.     

The synthesis of triethylphosphine gold(I) 4-nitrobenzenethiolate and solvent dependent visible absorption spectra of 4-nitrobenzenethiolate

The synthesis of triethylphosphine gold(I) 4-nitrobenzenethiolate, Et₃PAu(SC₆H₄NO₂-4), is reported. Et₃PAu(SC₆H₄NO₂-4) displays a low energy visible electronic absorption band which is solvent dependent: EtOH (λ_max = 385 nm), acetonitrile (λ_max = 391 nm), THF (λ_max = 395 nm), and DMSO (λ_max = 402 nm). The corresponding low energy visible electronic absorption band of 4-nitrobenzenethiolate, 4-NO₂C₆H₄S⁻ also shows solvent dependency: acetonitrile, (λ_max = 484 nm), DMSO (λ_max = 502 nm), dimethylformamide (λ_max = 505 nm). The positive solvatochromic shifts for Et₃PAu(SC₆H₄NO₂-4) and 4-NO₂C₆H₄S⁻ are consistent with an intraligand (IL) charge transfer transition, i.e. π(S) → ∗π (C₆H₄NO₂-4) or n(S) → ∗π (C₆H₄NO₂-4). Assignment of 4-NO₂C₆H₄S⁻ was aided by a DFT calculation.     

Radiative and non-radiative charge recombination pathways in Photosystem II studied by thermoluminescence and chlorophyll fluorescence in the cyanobacterium Synechocystis 6803

The mechanism of charge recombination was studied in Photosystem II by using flash induced chlorophyll fluorescence and thermoluminescence measurements. The experiments were performed in intact cells of the cyanobacterium Synechocystis 6803 in which the redox properties of the primary pheophytin electron acceptor, Phe, the primary electron donor, P₆₈₀, and the first quinone electron acceptor, Q_A, were modified. In the D1Gln130Glu or D1His198Ala mutants, which shift the free energy of the primary radical pair to more positive values, charge recombination from the S₂Q_A⁻ and S₂Q_B⁻ states was accelerated relative to the wild type as shown by the faster decay of chlorophyll fluorescence yield, and the downshifted peak temperature of the thermoluminescence Q and B bands. The opposite effect, i.e. strong stabilization of charge recombination from both the S₂Q_A⁻ and S₂Q_B⁻ states was observed in the D1Gln130Leu or D1His198Lys mutants, which shift the free energy level of the primary radical pair to more negative values, as shown by the retarded decay of flash induced chlorophyll fluorescence and upshifted thermoluminescence peak temperatures. Importantly, these mutations caused a drastic change in the intensity of thermoluminescence, manifested by 8- and 22-fold increase in the D1Gln130Leu and D1His198Lys mutants, respectively, as well as by a 4- and 2.5-fold decrease in the D1Gln130Glu and D1His198Ala mutants, relative to the wild type, respectively. In the presence of the electron transport inhibitor bromoxynil, which decreases the redox potential of Q_A/Q_A⁻ relative to that observed in the presence of DCMU, charge recombination from the S₂Q_A⁻ state was accelerated in the wild type and all mutant strains. Our data confirm that in PSII the dominant pathway of charge recombination goes through the P₆₈₀⁺Phe⁻ radical pair. This indirect recombination is branched into radiative and non-radiative pathways, which proceed via repopulation of P₆₈₀^* from ¹[P₆₈₀⁺Ph⁻] and direct recombination of the ³[P₆₈₀⁺Ph⁻] and ¹[P₆₈₀⁺Ph⁻] radical states, respectively. An additional non-radiative pathway involves direct recombination of P₆₈₀⁺Q_A⁻. The yield of these charge recombination pathways is affected by the free energy gaps between the Photosystem II electron transfer components in a complex way: Increase of ΔG(P₆₈₀^* ↔ P₆₈₀⁺Phe⁻) decreases the yield of the indirect radiative pathway (in the 22-0.2% range). On the other hand, increase of ΔG(P₆₈₀⁺Phe⁻ ↔ P₆₈₀⁺Q_A⁻) increases the yield of the direct pathway (in the 2-50% range) and decreases the yield of the indirect non-radiative pathway (in the 97-37% range).     

Photoredox reactivity of copper(II)-3,5-diisopropylsalicylate induced by ligand-to-metal charge transfer excitation of the copper-phenolate chromophore

The electronic spectrum of Cu^II(dps)₂ in CH₃CN with dps=3,5-diisopropylsalicylate shows a ligand field absorption at λ_max=711 nm (ε=140 M⁻¹ cm⁻¹), and a phenolate to Cu(II) ligand-to-metal charge transfer (LMCT) band at λ_max=428 nm (ε=950). LMCT excitation of Cu^II(dps)₂ leads to the reduction of Cu(II) to Cu(I). Copper(II) disappears with φ=2.8×10⁻³ at λ_irr=436 nm.     

Cationic induced assembly of two 2D zinc-terephthalate polymeric networks

Based on templates of [Ph₃PCH₂Ph]Cl and [Ph₄P]Cl ([Ph₃PCH₂Ph] = benzyltriphenylphosphonium, [Ph₄] = tetraphenylphosphonium), the hydrothermal reactions of zinc acetate dihydrate, H₂tp (tp = terephthalate) and water give rise to two new zinc-terephthalate coordination polymers, [Ph₃PCH₂Ph][Zn(tp)Cl] (1) and [Ph₄P][Zn(tp)(H₂O)₂·0.5tp] (2). X-ray single-crystal structural analysis reveals that both 1 (C₃₃H₂₆ClO₄PZn) and 2 (C₃₆H₃₀O₈PZn) crystallize in the 2D non-interpenetrating layered supramolecular networks with guest organophosphonium cations. Due to template effect of different guest cations, 1 presents an interesting 2D smectite-like lamellar framework that formed by the 4-linked (4,4) anionic zinc-terephthalate polymeric network and the interlayer [Ph₃PCH₂Ph]⁺ exchangeable cations, while 2 shows a 2D 3-linked (6,3) H-bonded anionic zinc-terephthalate polymeric brickwall network with encapsulated guest [Ph₄P]⁺ cations. Both compounds are stable up to about 300 °C, and exhibit intense fluorescent emission band at 446 nm (λ_exc = 328 nm) for 1 and 420 nm (λ_exc = 340 nm) for 2 in the solid state at room temperature.     

Optical properties of Ag(tripod)X with tripod = 1,1,1-tris(diphenyl-phosphinomethyl)ethane and X = Cl and I: Intraligand and ligand-to-ligand charge transfer

The complexes Ag^I(tripod)X with tripod = 1,1,1-tris(diphenylphosphinomethyl)ethane and X⁻ = Cl⁻ and I⁻ are luminescent in solution at r.t. It is suggested that the emission is a phosphorescence which originates from a tripod intraligand state for X = Cl (λ_max = 464 nm) and a X⁻ → tripod ligand-to-ligand charge transfer state for X = I (λ_max = 482 nm).     

Optical properties of silver(I) perrhenate: luminescence from a metal-to-metal charge transfer state

The salt Ag⁺ReO₄ ⁻ shows an orange luminescence (λ_max=580 nm), which originates from a Ag^I → Re^VII MMCT triplet.     

Electrochemical behavior of S,S-bridged adducts of square planar metalladithiolene complexes [M(S2C2Ph2)2](M=Ni, Pd, and Pt)

The electrochemical behavior of the S,S^′-bridged adducts of square planar metalladithiolene complexes was investigated by using cyclic voltammetry and electrochemical spectroscopies (visible, near-IR, and ESR). The norbornene-bridged S,S^′-adduct [Ni(S₂C₂Ph₂)₂(C₇H₈)] (2a; C₇H₈=norbornene) formed by [Ni(S₂C₂Ph₂)₂] (1a) and quadricyclane (Q) was dissociated by an electrochemical reduction, and anion 1a⁻ and norbornadiene (NBD) were formed. Q was isomerized to NBD in the overall reaction. The o-xylyl-bridged S,S^′-adduct [Ni(S₂C₂Ph₂)₂(CH₂)₂(C₆H₄)] (3a; (CH₂)₂(C₆H₄)=o-xylyl) was also dissociated by an electrochemical reduction, and this reaction gave the o-xylyl radical (o-quinodimethane). The reduction of complex 3a in the presence of excess o-xylylene dibromide underwent the catalytic formation of o-quinodimethane. The butylene-bridged S,S^′-adduct [Ni(S₂C₂Ph₂)₂(CH₂)₄] (4a; (CH₂)₄=butylene) was stable on an electrochemical reduction. The lifetimes of reduced species of these adducts 2a-4a were influenced by the stability of the eliminated group (stability: NBD > o-xylyl radical (o-quinodimethane) > butylene radical). Therefore, the reduced species are stable in the sequence 4a⁻ > 3a⁻ > 2a⁻. Although the palladium complex [Pd(S₂C₂Ph₂)₂] (1b) was easier to reduce than the nickel complex 1a or the platinum complex [Pt(S₂C₂Ph₂)₂] (1c), their S,S^′-adducts were easier to reduce in the order of Ni adduct > Pd adduct > Pt adduct.     

A ferromagnetically coupled copper(II) trinuclear secondary building unit as precursor for the preparation of molecule-based magnets

The compounds 3D-{(Ph₄P)₂[ZnCu₃(Hmesox)₃Cl]·2.5H₂O} (1) and 3D-{(Ph₄P)₂[NiCu₃(Hmesox)₃Cl]·2.5H₂O} (2) have been prepared and their structure and magnetic properties investigated (H₄mesox = mesoxalic acid, 2-dihydroxymalonic acid). The compounds are obtained by means of the same procedure followed to prepare 3D-{(Ph₄P)₂[MnCu₃(Hmesox)₃Cl]·3.5H₂O}and 3D-{(Ph₄P)₂[CoCu₃(Hmesox)₃Cl]}, to which 1 and 2 are isostructural as deducted from the X-ray powder diffraction patterns and IR spectra. During the synthesis the {Cu₃(Hmesox)₃Cl}⁴⁻ unit is generated which acts as ferromagnetically coupled secondary building unit (SBU) to give 3D chiral networks with (10,3)-a topology, the [Zn{Cu₃(C₃HO₆)₃Cl}]²ⁿ⁻ and [Ni{Cu₃(C₃HO₆)₃Cl}]²ⁿ⁻, in 1 and 2, respectively. The tetraphenylphosphonium cations are located in the voids of the 3D anion framework giving a supramolecular 3D cation net with the same (10,3)-a topology as the anion framework and both can be thought of forming interpenetrating supramolecular and covalent (10,3)-a nets. Because of the different nature of the interpenetrated 3D (10,3)-a frameworks a single crystal of 1 and 2 is chiral and enantiopure. The analysis of the magnetic properties in 1 by means of the isotropic spin Hamiltonian, H = −J(S₁S₂ + S₂S₃ + S₁S₃), reveal a ferromagnetic coupling with J = +11.2 cm⁻¹ in the copper(II) trinuclear unit of 1. Compound 2 exhibits long-range magnetic ordering with a T_c of 7.0 K due to a ferrimagnetic coupling between the ferromagnetically coupled copper(II) ions of the trinuclear unit, antiferromagnetically coupled to the Ni(II) ions.     

An unexpected destabilisation of copper(II) phenoxyl radical species by steric protection

The complexes [CuL(Tp^Ph)] (HL = 5-tertbutylsalicylaldehyde, 5-tertbutyl-3-methylsulfanylsalicylaldehyde or 5-tertbutyl-3-phenylsulfanylsalicylaldehyde; [Tp^Ph]⁻ = tris-{3-phenylpyrazolyl}hydridoborate) have been prepared, and adopt square-pyramidal coordination geometries. Each compound exhibits a ligand-based oxidation in CH₂Cl₂ that is chemically reversible by voltammetry. However, Coulometric determinations showed that the resultant phenoxyl radical products decomposed rapidly at low temperatures in bulk solution. This instability may reflect intramolecular steric repulsions between the phenoxide tertbutyl substituents, and a pyrazolylborate phenyl group. These results contrast with a previously reported analogous compound, bearing a 5-methyl-3-methylsulfanylsalicylaldehydato ligand, which yields a phenoxyl radical oxidation product that is stable for hours under the same conditions.     

Mixed-valency with cyanides as terminal ligands: Diruthenium(III,II) complexes with the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine bridge and variable co-ligands (CN vs. bpy or NH3)

New diruthenium complexes (PPN)₄[(NC)₄Ru(μ-bptz)Ru(CN)₄], (PPN)₄1, and [(bpy)₂Ru(μ-bptz)Ru(CN)₄], 2, (PPN⁺ = bis(triphenylphospine)iminium; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy = 2,2′-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant K_c = 10^7.0 of the mixed-valent species [(NC)₄Ru(μ-bptz)Ru(CN)₄]³⁻ as obtained by oxidation of 1⁴⁻ in CH₃CN is much lower than the K_c = 10^15.0 previously detected for [(H₃N)₄Ru(bptz)Ru(NH₃)₄]⁵⁺, reflecting the competition between CN⁻ and bptz for the π-electron density of the metals. Comparison with several other bptz-bridged diruthenium(II,III) complexes reveals an approximate correlation between K_c and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ_max = 624 nm, the main IVCT (intervalence charge transfer) band of 1³⁻ was detected by spectroelectrochemistry at λ_max = 1695 nm (in CH₃CN; ε = 3200 M⁻¹ cm⁻¹). The experimental band width at half-height, Δν_1/2 = 2700 cm⁻¹, is slightly smaller than the theoretical value Δν_1/2 = 3660 cm⁻¹, calculated from the Hush approximation for Class II mixed-valent species. In agreement with comparatively moderate metal-metal coupling, the mixed-valent intermediate 1³⁻ was found to be EPR silent even at 4 K. The unsymmetrical mixed-valent complex [(bpy)₂Ru^II(μ-bptz)Ru^III(CN)₄]⁺, obtained in situ by bromine oxidation of 2 in CH₃CN/H₂O, displays a broad NIR absorption originating from an IVCT transition at λ_max = 1075 nm (ε ≈ 1000 M⁻¹ cm⁻¹, Δν_1/2 ≈ 4000 cm⁻¹). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)₄]²⁻ was measured in H₂O by laser flash photolysis; the obtained value of τ = 19.6 ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz = 2,2′-bipyrazine) in analogous tetracyanoruthenium complexes.     

[{Cu(pz)(Opo)}2(μ-Cl)2]: A new dinuclear copper(II) complex with a chloride bridge and mixed blocking ligands

A new copper(II) complex with chloride bridges and mixed blocking ligands, [{Cu(pz^Ph)(Opo)}₂(μ-Cl)₂] (1), whereby pz^Ph = 3-phenyl-pyrazolyl, and OpoH = 2-hydroxypyridine-N-oxide, has been obtained by degrading tris(pyrazolyl)borate in the presence of hydrated CuCl₂ salt and the ligand Opo⁻. The molecular structure of 1 was solved by X-ray diffraction. The two Cu(II) atoms of the dinuclear complex have a pyramidal arrangement in which the two pyramids share one base-to-apex edge with parallel basal planes. Magnetic susceptibility measurements revealed ferromagnetic coupling between the Cu atoms, with J = +8.72 cm⁻¹. X-band EPR spectra of CH₂Cl₂ solutions of 1 were recorded at different temperatures.     

Use of a cubane-type Mo3CoS4 molecular cluster as paramagnetic unit in the synthesis of hybrid charge-transfer salts

Selective substitution of the chlorine atom coordinated to cobalt in the paramagnetic Mo₃(CoCl)S₄(dmpe)₃Cl₃ (dmpe = 1,2-bis(dimethylphosphanyl) ethane) complex with a S = 1/2 ground state has been achieved by iodine oxidation to afford the also paramagnetic [Mo₃(CoI)S₄(dmpe)₃Cl₃]I ([1]I) salt with a S = 1 ground state in almost quantitative yield. Replacement of chorine by iodine has no significant effect on the structural and electrochemical properties of the Mo₃CoS₄ system. Metathesis of the [1]I salt with the paramagnetic nickel anionic dithiolate [Ni(mnt)₂]⁻ (mnt = maleonitrilodithiolate) affords [1]₂[Ni(mnt)₂]. The stoichiometry evidenced by X-ray analysis reveals that reduction of the [Ni(mnt)₂]⁻ radical to the corresponding diamagnetic closed shell [Ni(mnt)₂]²⁻ dianion, presumably via dismutation, has occurred during the metathesis process. The crystal structure of [1]₂[Ni(mnt)₂] consists of [Ni(mnt)₂]²⁻ dianions sandwiched by two cluster 1⁺ cations which yield {1⁺·[Ni(mnt)₂]²⁻·1⁺} subunits arranged along the crystallographic c axis. Magnetic susceptibility measurements for [1]₂[Ni(mnt)₂] show a χT product of 0.99 emu K/mol largely unchanged in the 10-300 K range. This behavior agrees with the presence of an S = 1 cluster 1⁺ cation while the Ni(mnt)₂ moiety does not contribute to the paramagnetism of the sample.     

Diastereomeric preference in 1,4,7-tris((S)-2-hydroxy-3-phenylpropyl)-1,4,7-triazacyclononanelithium(I) and its sodium(I) analogue

In N,N-dimethylformamide (DMF), 1,4,7-tris((S)-2-hydroxy-3-phenylpropyl)-1,4,7-triazacyclononane forms metal complexes, [M(S-thppc9)]⁺, for which log K (dm³ mol⁻¹)=3.01, 2.65, 2.66, 2.65, 2.42 and 7.59 (all±0.05) where M⁺=Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺, respectively. Variable temperature ¹³C{¹H} NMR spectroscopy shows that the interchange between equivalent forms of a single diastereomer occurs for [Li(S-thppc9)]⁺ and [Na(S-thppc9)]⁺ characterised by: k=43±5 and 2900±100 s⁻¹, at 298.2 K, ΔH^‡=22.5±1.6 and 33.8±1.6 kJ mol⁻¹, and ΔS^‡=−133±5 and −59±6 J K⁻¹ mol⁻¹, respectively. Gas phase ab initio modelling shows these complexes and their K⁺ analogue to preferentially form distorted trigonal prismatic Λ, Δ, and Λ diastereomers, respectively.     

Structure and magnetic properties of a tetranuclear Cu4O4 open-cubane in [Cu(L)]4·4H2O with L = (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide

The in-situ formed hydrazone Schiff base ligand (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide (L²⁻) reacts with copper(II) acetate to a tetranuclear open cubane [Cu(L)]₄ complex which crystallizes as two symmetry-independent (Z′ = 2) S₄-symmetrical molecules in different twofold special positions with a homodromic water tetramer. The two independent (A and B) open- or pseudo-cubanes with Cu₄O₄ cores of 4 + 2 class (Ruiz classification) each have three different magnetic exchange pathways leading to an overall antiferromagnetic coupling with J_1B = J_2B = −17.2 cm⁻¹, J_1A = −36.7 cm⁻¹, J_2A = −159 cm⁻¹, J_3A = J_3B = 33.5 cm⁻¹, g = 2.40 and ρ = 0.0687. The magnetic properties have been analysed using the H = −Σ_i,jJ_ij(S_iS_j) spin Hamiltonian.     

Structural, spectroscopic and redox studies of a new ruthenium(III) complex with an imidazole-rich tripodal ligand

The present paper describes a new tripodal ligand containing imidazole and pyridine arms and its first cis-[Ru^III(L)(Cl)₂]ClO₄ complex (1). The crystal structure of 1 shows Ru^III in a distorted octahedral geometry, in which two chloride ions, cis-positioned to each other, are coordinated besides the four nitrogen atoms from the tetradentate ligand L. The cyclic voltammogram of 1 exhibits three redox processes at −67, +73 and +200 mV versus SCE, which are attributed to the Ru^III/Ru^II couple in the cis-[Ru^III(L)(Cl)₂]⁺, cis-[Ru^II(L)(H₂O)(Cl)]⁺ and cis-[Ru^II(L)(H₂O)₂]²⁺, respectively. After chemical reduction (Zn(Hg) or Eu^II) only the cis-[Ru^II(L)(H₂O)₂]²⁺ species is observed in the cyclic voltammetry. Complex 1 absorbs at 470 nm (ε=1.4×10³ mol⁻¹ L cm⁻¹), 335 nm (ε=7.9×10³ mol⁻¹ L cm⁻¹), 301 nm (ε=6.7×10³ mol⁻¹ L cm⁻¹) and 264 nm (ε=9.9×10³ mol⁻¹ L cm⁻¹), in water solution (CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ with CF₃COONa). Spectroelectrochemical experiments show a decrease of the bands at 335 and 301 nm, which are attributed to LMCT transitions from the chloride to the Ru^III center and the appearance of a broad band at 402 nm ascribed to MLCT transition from the Ru^II center to the pyridine ligand. The lability of the water ligands in the cis-[Ru^II(L)(H₂O)₂]²⁺ species has been investigated using the auxiliary ligand pyrazine. Reactions in the presence of stoichiometric and excess of pyrazine yield the same species, cis-[Ru^II(L)(H₂O)(pz)]²⁺, which exhibits a reversible redox process at 493 mV versus SCE and absorbs at 438 nm (ε=5.1×10³ mol⁻¹ L cm⁻¹) and 394 nm (ε=4.2×10³ mol⁻¹ L cm⁻¹). Experiments performed with a large excess of pyrazine gave a specific rate constant k₁=(2.8±0.5)×10⁻² M⁻¹ s⁻¹, at 25 °C, in CF₃COOH, 0.01 mol L⁻¹, μ=0.1 mol L⁻¹ (with CF₃COONa).     

Nickel(II) complexes of di- and tri-substituted thiourea mono- and di-anions

Reaction of nickel(II) acetate, 1,2-bis(diphenylphosphino)ethane (dppe), and a di- or tri-substituted thiourea R¹NHC(S)R²R³ (R³ = H or alkyl) with trimethylamine in hot methanol gave cationic nickel(II) complexes containing N,S-chelated thiourea monoanion ligands [Ni{SC(NR²R³)NR¹}(dppe)]⁺, which can be readily isolated as their BPh₄⁻ salts. The X-ray crystal structure of [Ni{SC(NMe₂)NPh}(dppe)]⁺BPh₄⁻ is reported.     

Is o-carborane photoluminescent?

At 77 K in the solid state and in ethanol glasses, o-carborane (1,2-C₂B₁₀H₁₂) shows a relatively intense (? ∼ 10⁻³ at λ_exc = 260 nm) structured photoluminescence (λ_max = 441 nm).In agreement with the rather slow decay of this emission (τ ∼ 4 s) it is suggested to be a phosphorescence. While it appears to be a genuine property of o-carborane an unknown impurity as origin of this luminescence is not completely excluded.     

Multiple cytochromes b in Mycobacterium phlei

Electron transport particles from $\text{M. phlei}$ contain at least 3 different active forms of cytochrome b, one reduced by NADH, with a λ_max at 563 nm (b_N563), and the other two reduced by either succinate or NADH, with λ_max at 559 and 563 nm (b_S559) and (b_S563). Low temperature λ_max for cytochrome b reduction with NADH or succinate are described. During steady state only b_S563 was observed with succinate. In the presence of ATP, succinate reduced an increased amount of a b563. A branching of the NAD⁺-linked pathway and a convergence at the level of cytochrome c is suggested, with only one branch accessible to succinate.