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[FeFe]-Ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H2 studied, including 1,1′-bis(diphenylphosphino)ferrocene (dppf) complexes Fe2(CO)4(μ-dppf)(μ-S(CH2)nS) (n = 2, edt; n = 3, pdt) and Fe2(CO)4(μ-dppf)(μ-SAr)2 (Ar = Ph, p-tolyl, p-C6H4NH2), together with the more electron-rich 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf) complex Fe2(CO)4(μ-dcpf)(μ-pdt). Crystallographic characterisation of four of these show similar overall structures, the diphosphine spanning an elongated Fe–Fe bond (ca. 2.6 Å), lying trans to one sulfur and cis to the second. In solution the diphosphine is flexible, as shown by VT NMR studies, suggesting that Fe2⋯Fe distances of ca. 4.5–4.7 Å in the solid state vary in solution. Cyclic voltammetry, IR spectroelectrochemistry and DFT calculations have been used to develop a detailed picture of electronic and structural changes occurring upon oxidation. In MeCN, Fe2(CO)4(μ-dppf)(μ-pdt) shows two chemically reversible one-electron oxidations occurring sequentially at Fe2 and Fc sites respectively. For other dppf complexes, reversibility of the first oxidation is poor, consistent with an irreversible structural change upon removal of an electron from the Fe2 centre. In CH2Cl2, Fe2(CO)4(μ-dcpf)(μ-pdt) shows a quasi-reversible first oxidation together with subsequent oxidations suggesting that the generated cation has some stability but slowly rearranges. Both pdt complexes readily protonate upon addition of HBF4·Et2O to afford bridging-hydride cations, [Fe2(CO)4(μ-H)(μ-dcpf)(μ-pdt)]+, species which catalytically reduce protons to generate H2. In the presence of pyridine, [Fe2(CO)4(μ-dppf)(μ-pdt)]2+ catalytically oxidises H2 but none of the other complexes do this, probably resulting from the irreversible nature of their first oxidation. Mechanistic details of both proton reduction and H2 oxidation have been studied by DFT allowing speculative reaction schemes to be developed. [ABSTRACT FROM AUTHOR]

This work describes the synthesis and properties of a dicyanomethylene‐substituted indolo[3,2‐b]carbazole diradical ICz‐CN. This quinoidal system dimerises almost completely to (ICz‐CN)2, which contains two long C(sp3)−C(sp3) σ‐bonds between the dicyanomethylene units. The minor open‐shell ICz‐CN component in the solid‐state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV–visible spectroelectrochemical data, as well as comparison with reference monomer ICz‐Br reveal that the nature of the one‐electron oxidation of (ICz‐CN)2 at ambient temperature and ICz‐CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co‐existence of (ICz‐CN)2 and ICz‐CN. The involvement of the dicyanomethylene groups stabilizes the close‐lying LUMO and LUMO+1 of (ICz‐CN)2 and especially ICz‐CN compared to ICz‐Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz‐CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(–NIR) electronic transitions in parent (ICz‐CN)2 and ICz‐CN and their different redox forms have been assigned convincingly with the aid of TD‐DFT calculations. The σ‐bond in neutral (ICz‐CN)2 is cleaved in solution and in the solid‐state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue–green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa). [ABSTRACT FROM AUTHOR]

Four cyclometalated diiridium complexes, with IrCp*Cl (Cp*=η5‐C5Me5−) termini bridged by 1,4‐ and 1,3‐bis(p‐tolyliminoethyl)benzene (1, 2), or 1,4‐ and 1,3‐bis(2‐pyridyl)benzene (3, 4), were prepared and characterized by nuclear magnetic resonance (NMR) spectroscopy and single‐crystal X‐ray diffraction (complexes 1, 2, and 4). The two iridium centers in complexes 1 and 3 are thus bound at the central benzene ring in the para‐position (trans‐Ir2), whereas those in complexes 2 and 4 are in the meta‐position (cis‐Ir2). Cyclic voltammograms of all four complexes show two consecutive one‐electron oxidations. The potential difference between the two anodic steps in 1 and 3 is distinctly larger than that for 2 and 4. The visible–near‐infrared (NIR)–short‐wave infrared (SWIR) absorption spectra of trans‐Ir2 monocations 1+ and 3+ are markedly different from those of cis‐Ir2 monocations 2+ and 4+. Notably, strong near‐infrared electronic absorption appears only in the spectra of 1+ and 3+ whereas 2+ and 4+ absorb only weakly in the NIR‐SWIR region. Combined DFT and TD‐DFT calculations have revealed that (a) 1+ and 3+ (the diiridium‐benzene trans‐isomers) display the highest occupied spin‐orbitals (HOSO) and the lowest unoccupied spin‐orbital (LUSO) evenly delocalized over both molecule halves, and (b) their electronic absorptions in the NIR‐SWIR region are attributed to mixed metal‐to‐ligand and ligand‐to‐ligand charge transfers (MLCT and LLCT). In contrast, cis‐isomers 2+ and 4+ do not feature this stabilizing π‐delocalization but a localized mixed‐valence state showing a weak intervalence charge‐transfer (IVCT) absorption in the SWIR region. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The photochemical reactivity, photophysical properties and redox behavior of the complexes trans,cis-[Ru(X)(X′)(CO)2(α-diimine)] and their derivatives are strongly dependent on the complex geometry, the nature and electronic properties of the α-diimine ligand and, most importantly, on the axial ligands X and X′ (alkyl, halide, phosphine, donor solvent, etc.). This paper deals mainly with comparison of reduction pathways for several different types of the trans,cis-[Ru(X)(X′)(CO)2(α-diimine)] complexes, also presenting some new results in this field. An equally important goal has been the comparison and discussion of the photo- and redox reactivity of these complexes from the viewpoint of the frontier orbitals involved and character of the Ru&z.sbnd;X/X′ bonding. [Copyright &y& Elsevier]

Abstract: Peculiar reduction pathways of the complexes fac-[Re(imH)(CO)3(phen)]+ and fac-[Re(imCH3)(CO)3(phen)]+ (imH=imidazole, imCH3 = N-methylimidazole and phen=1,10-phenanthroline) have been unravelled by performing combined cyclic voltammetric and in situ IR spectroelectrochemical experiments. In the temperature range of 293–233K, the initial reduction of the phen ligand in [Re(imH)(CO)3(phen)]+ results in irreversible conversion of the imidazole ligand to 3-imidazolate by a rapid phen•− →imH intramolecular electron transfer coupled with N H bond cleavage. This process is followed by second phen-localized 1e− reduction producing [ReI(3-im−)(CO)3(phen•−)]−, similar to the analogous 2,2′-bipyridine complex. In contrast to the bpy analogue, the stability of the phen•−-containing complexes is significantly affected by lowering the temperature. At 233K, a secondary reaction occurs in both [Re(3-im−)(CO)3(phen•−)]− and [Re(imCH3)(CO)3(phen•−)]. The resulting products exhibit ν(CO) wavenumbers indistinguishable from those of the parent phen•− complexes; however, their oxidation occurs at a considerably more positive electrode potential. It is proposed that these species are produced by a new C C bond formation between the C(2) site of 3-im− or imCH3 and the C(2) site of the phen•− ligand. [Copyright &y& Elsevier]

Thedissymmetrical naphthalene-bridged complexes [Cp′Fe(μ-C10H8)FeCp*] (3; Cp* = η5-C5Me5, Cp′ = η5-C5H2-1,2,4-tBu3) and[Cp′Fe(μ-C10H8)RuCp*] (4) were synthesized via a one-pot procedure from FeCl2(thf)1.5, Cp′K, KC10H8, and [Cp*FeCl(tmeda)](tmeda = N,N,N′,N′-tetramethylethylenediamine) or [Cp*RuCl]4, respectively. The symmetrically substituted iron ruthenium complex[Cp*Fe(μ-C10H8)RuCp*] (5)bearing two Cp* ligands was prepared as a reference compound. Compounds 3–5are diamagnetic and display similarmolecular structures, where the metal atoms are coordinated to oppositesides of the bridging naphthalene molecule. Cyclic voltammetry andUV/vis spectroelectrochemistry studies revealed that neutral 3–5can be oxidized to monocations 3+–5+and dications 32+–52+. The chemicaloxidation of 3and 4with [Cp2Fe]PF6afforded the paramagnetic hexafluorophosphate salts[Cp′Fe(μ-C10H8)FeCp*]PF6([3]PF6) and [Cp′Fe(μ-C10H8)RuCp*]PF6([4]PF6), which were characterized by various spectroscopic techniques,including EPR and 57Fe Mössbauer spectroscopy. Themolecular structure of [4]PF6was determinedby X-ray crystallography. DFT calculations support the structuraland spectroscopic data and determine the compositions of frontiermolecular orbitals in the investigated complexes. The effects of substitutingCp* with Cp′ and Fe with Ru on the electronic structures andthe structural and spectroscopic properties are analyzed. [ABSTRACT FROM AUTHOR]