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In recent years, the field of organometallic chemistry has made a great progress and diverse types of metallaaromatics have successively been reported. In those studies, incorporation of ligated osmium centers into metallaaromatic systems played a prominent role. The reviewed literature documents that certain metallaaromatics with unconventional photophysical properties, redox and electronic transport properties and magnetism, have potential to be widely used in diverse practical applications, with selected examples of amino acid and fluoride anion identification, photothermal effects, functional materials, photodynamic therapy (PDT) in biomedicine, single‐molecule junction conductors, and electron‐transport layer materials (ETLs) in solar cells. [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]

Electrochemical and photochemical reduction of CO2 are both well‐established, independent catalytic routes toward producing added‐value chemicals. The potential for any cross‐reactivity has, however, hardly been explored so far. In this report, we assess a system primarily using spectroelectrochemical monitoring, where photochemistry assists the cathodic activation of precursor complexes fac‐[Mn(CO)3(2,2′‐bipyridine)Br] and [Mo(CO)4(6,6′‐dimethyl‐2,2′‐bipyridine)] to lower the catalytic overpotential needed to trigger the electrocatalytic reduction of CO2 to CO. Following the complete initial 1e− reduction of the parent complexes, the key photochemical cleavage of the Mn−Mn and Mo−CO bonds in the reduction products, [Mn(CO)3(2,2′‐bipyridine)]2 and [Mo(CO)4(6,6′‐dimethyl‐2,2′‐bipyridine)].−, respectively, generates the 2e−‐reduced, 5‐coordinate catalysts, [Mn(CO)3(2,2′‐bipyridine)]− and [Mo(CO)3(6,6′‐dimethyl‐2,2′‐bipyridine)]2− appreciably closer to the initial cathodic wave R1. Experiments under CO2 confirm the activity of both electrocatalysts under the photoirradiation with 405 nm and 365 nm light, respectively. This remarkable achievement corresponds to a ca. 500 mV positive shift of the catalytic onset compared to the exclusive standard electrocatalytic activation. [ABSTRACT FROM AUTHOR]

Diarylamine‐substituted osmanaphthalyne complexes that feature two redox centers linked by the rigid skeleton of the metallacycle (C^C+), specifically, [OsCl2(PPh3)2{(C^C+)NAr2}][BF4−] (Ar=Ph (1 a), p‐MeOPh (1 b)) and their open‐ring precursors [OsHCl2(PPh3)2{(≡C−C(PPh3+)=CHPh)NR2}][BF4−] (Ar=Ph (2 a), p‐MeOPh (2 b)), were successfully synthesized and characterized by 1H, 13C, and 31P NMR spectroscopy, ESI‐MS, and elemental analysis. The solid‐state molecular structures of complexes 1 a and 2 a were ascertained by single‐crystal X‐ray diffraction. The Os≡C bond length in both complexes 1 a and 2 a fell within the range reported for similar osmanaphthalynes and osmium carbyne complexes, respectively. The structural parameters determined for complex 1 a, which were successfully reproduced by theoretical calculations, point to a π‐delocalized metallacycle structure. The purple color of compounds 1 a and b was explained by the diarylamine→Os(metallacycle) charge‐transfer absorption in the visible region. The neutral, one‐electron‐oxidized and one‐electron‐reduced states of compounds 1 a, b, and a reference complex that lacked the diarylamine substituent, [OsCl2(PPh3)2{(C^C+)}][BF4−] (1′), were investigated by cyclic and square‐wave voltammetry, UV/Vis/NIR spectroelectrochemistry, and DFT calculations. The spin density in singly oxidized complexes [1 a]+ and [1 b]+ predominantly resided on the aminyl segment, with osmium involvement controlled by the diphenylamine substitution. Spin density in stable, singly‐reduced [1′]− was distributed mainly over the osmanaphthalyne metallacycle. Electronic communication in metallocycles: Two intriguing diphenylamine (DPA)‐substituted heterocyclic osmanaphthalyne complexes are investigated. Electronic communication between the donor DPA and acceptor metallacycle redox centers is observed. The studies also reveal that the spin density in the singly oxidized osmanaphtalynes predominantly resides on the aminyl segment. In contrast, for the reference unsubstituted osmanaphthalyne, both one‐electron oxidation and reduction are localized on the metallacycle. [ABSTRACT FROM AUTHOR]

A series of molybdenum tetracarbonyl complexes with dimethyl‐substituted 2,2′‐bipyridine (dmbipy) ligands were investigated by cyclic voltammetry (CV) combined with infrared spectroelectrochemistry (IR‐SEC) in tetrahydrofuran (THF) and N‐methyl‐2‐pyrrolidone (NMP) to explore their potential in a reduced state to trigger electrocatalytic CO2 reduction to CO. Addressed is their ability to take advantage of a low‐energy, CO‐dissociation two‐electron ECE pathway available only at an Au cathode. A comparison is made with the reference complex bearing unsubstituted 2,2′‐bipyridine (bipy). The methyl substitution in the 6,6′ position has a large positive impact on the catalytic efficiency. This behavior is ascribed to the advantageous positioning of the steric bulk of the methyl groups, which further facilitate CO dissociation from the one‐electron‐reduced parent radical anion. On the contrary, the substitution in the 4,4′ position appears to have a negative impact on the catalytic performance, exerting a strong stabilizing effect on the π‐accepting CO ligands and, in THF, preventing exploitation of the low‐energy dissociative pathway. Better together: [Mo(CO)4(x,x′‐dmbipy)] (x=4–6) complexes all demonstrate efficient electrocatalytic reduction of CO2 to CO. The position of the methyl substituents either direct the catalyst toward a high‐energy EEC or a low‐energy ECE pathway; the latter is also promoted by the synergy of the gold cathodic surface and the N‐methyl‐2‐pyrrolidone solvent, as compared to Pt and tetrahydrofuran. [ABSTRACT FROM AUTHOR]

This work reports on a quinodimethane-type molecule, 2,7-dicyanomethylene-9-(2-ethylhexyl)carbazole ( 1), one of the shortest π-conjugated biradicaloids reported to be stable in solution under ambient conditions. This carbazole-based quinoidal precursor is able to form a macrocyclic σ-bonded tetramer ( 2). The resolved single-crystal X-ray structure of tetramer 2 shows that four molecules of 1 are linked together through four long (CN)2C−C(CN)2 bonds (1.631 Å) resulting from coupling of the unpaired electrons in biradicaloid 1. Dynamic interconversion between monomer 1 and cyclophane tetramer 2 is achieved by reversible cleavage and recovery of the four (CN)2C−C(CN)2 bonds upon soft external stimuli (light absorption, temperature and pressure), which is accompanied by significant color changes. These novel photo-, thermo-, and mechanochromic properties expand the versatility of π-conjugated biradicaloid compounds as novel functional materials that, in combination with spin chemistry and dynamic covalent chemistry, can be relevant in molecular machines, sensors, and switches. [ABSTRACT FROM AUTHOR]

A new method to detect the vibrational circular dichroism (VCD) of a localized part of a chiral molecular system is reported. A local VCD amplifier was implemented, and the distance dependence of the amplification was investigated in a series of peptides. The results indicate a characteristic distance of 2.0±0.3 bonds, which suggests that the amplification is a localized phenomenon. The amplifier can be covalently coupled to a specific part of a molecule, and can be switched ON and OFF electrochemically. By subtracting the VCD spectra obtained when the amplifier is in the ON and OFF states, the VCD of the local environment of the amplifier can be separated from the total VCD spectrum. Switchable local VCD amplification thus makes it possible to 'zoom in' on a specific part of a chiral molecule. [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 electronic properties of four divinylanthracene-bridged diruthenium carbonyl complexes [{RuCl(CO)(PMe3)3}2(μCHCHArCHCH)] (Ar=9,10-anthracene ( 1), 1,5-anthracene ( 2), 2,6-anthracene ( 3), 1,8-anthracene ( 4)) obtained by molecular spectroscopic methods (IR, UV/Vis/near-IR, and EPR spectroscopy) and DFT calculations are reported. IR spectroelectrochemical studies have revealed that these complexes are first oxidized at the noninnocent bridging ligand, which is in line with the very small ν(CO) wavenumber shift that accompanies this process and also supported by DFT calculations. Because of poor conjugation in complex 1, except oxidized 1+, the electronic absorption spectra of complexes 2+, 3+, and 4+ all display the characteristic near-IR band envelopes that have been deconvoluted into three Gaussian sub-bands. Two of the sub-bands belong mainly to metal-to-ligand charge-transfer (MLCT) transitions according to results from time-dependent DFT calculations. EPR spectroscopy of chemically generated 1+- 4+ proves largely ligand-centered spin density, again in accordance with IR spectra and DFT calculations results. [ABSTRACT FROM AUTHOR]

The new compounds [Ru(R-DAB)(acac)2] (R-DAB = 1,4-diorganyl-1,4-diazabuta-1,3-diene; R = tert-butyl, 4-methoxyphenyl, 2,6-dimethylphenyl; acac- = 2,4-pentanedionate) exhibit intrachelate ring bond lengths 1.297 < d(CN) < 1.344 Å and 1.382 < d(CC) < 1.425 Å, which suggest a RuIII(R-DAB ·-) oxidation state formulation. This notion is confirmed by the negligible solvatochromism of the intense ( ϵ ≈ 104 M-1 cm-1) charge-transfer absorption band in the visible region and by DFT calculations. Oxidation of the compounds occurs mainly at the R-DAB ·- radical ligand to produce UV/Vis/NIR and electron paramagnetic resonance (EPR) spectroelectrochemically detectable RuIII species, whereas the reduction proceeds less reversibly and yields predominantly (R-DAB)-ligand-based spin for the 4-methoxyphenyl derivative, measured at low temperature. The results are discussed with respect to metal-to-ligand charge-transfer (MLCT) excited states of conventional (α-diimine)ruthenium(II) complexes and in view of other (α-diimine)metal complexes with ambiguous oxidation-state assignments. [ABSTRACT FROM AUTHOR]

Novel bis(azidophenyl)phosphole sulfide building block 8 has been developed to give access to a plethora of phosphole-containing π-conjugated systems in a simple synthetic step. This was explored for the reaction of the two azido moieties with phenyl-, pyridyl- and thienylacetylenes, to give bis(aryltriazolyl)-extended π-systems, having either the phosphole sulfide ( 9) or the phosphole ( 10) group as central ring. These conjugated frameworks exhibit intriguing photophysical and electrochemical properties that vary with the nature of the aromatic end-group. The λ3-phospholes 10 display blue fluorescence ( λem = 460-469 nm) with high quantum yield ( ΦF = 0.134-0.309). The radical anion of pyridyl-substituted phosphole sulfide 9b was observed with UV/Vis spectroscopy. TDDFT calculations on the extended π-systems showed some variation in the shape of the HOMOs, which was found to have an effect on the extent of charge transfer, depending on the aromatic end-group. Some fine-tuning of the emission maxima was observed, albeit subtle, showing a decrease in conjugation in the order thienyl < phenyl < pyridyl. These results show that variations in the distal ends of such π-systems have a subtle but significant effect on photophysical properties. [ABSTRACT FROM AUTHOR]

Schwingungscirculardichroismus (VCD) wird zur Bestimmung der absoluten Konfiguration und Konformation von chiralen Molekülen in Lösung verwendet, aber niedrige Signalintensitäten und spektrale Überlappung verhindern oft die Untersuchung der lokalen Struktur. In der Zuschrift auf S. 14266 ff. beschreiben F. Hartl, W. J. Buma, S. Woutersen et al. die Anbringung eines Ferrocen ‐ basierten elektrochemisch schaltbaren Verstärkers an ein Molekül, wodurch die lokale Verstärkung des VCD ‐ Signals ermöglicht wird. [ABSTRACT FROM AUTHOR]

Vibration circular dichroism (VCD) spectroscopy is used to determine the absolute configuration and conformation of chiral molecules in solution but low signal intensities and spectral congestion often prohibit the investigation of local structure. In their Communication on page 14042 ff., F. Hartl, W. J. Buma, S. Woutersen et al. describe the attachment of a ferrocene‐based electrochemically switchable amplifier to a molecular system, which enables the local amplification of the VCD signal. [ABSTRACT FROM AUTHOR]