We report the synthesis and structural characterisation of new Mn19 and Mn18M analogues, Mn12IIIMn7II(μ4-O)8(μ3-OCH3)2(μ3-Br)6(HLMe)12(MeOH)6Br2 (2) and Mn12IIIMn6IISr(μ4-O8(μ3-Cl)8(HLMe)12(MeCN)6Cl2 ...cluster (3), where H3LMe is 2,6-bis(hydroxymethyl)-p-cresol. The electrochemistry of 2 and 3 has been investigated and their activity as catalysts in the oxidation of benzyl alcohol has been evaluated. Selective oxidation of benzyl alcohol to benzaldehyde by O2 was achieved using 1 mol% of catalyst with conversions of 74% (2) and 60% (3) at 140 °C using TEMPO as a co-catalyst. No partial conversion of benzaldehyde to benzoic acid was observed. The results obtained revealed that different operative parameters – such as catalyst loading, temperature, time, solvent and the presence of molecular oxygen – played an important role in the selective oxidation of benzyl alcohol.
Tri‐organyl and tricoordinate N‐heterocyclic carbene (NHC) Zn–NHC alkyl cations (nNHC)2Zn‐Me+ (nNHC=C2‐bonded‐IMes/‐IDipp; 3+ and 4+; IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene, ...IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were first synthesized and structurally characterized by ionization of the corresponding neutral precursors (nNHC)ZnMe2 with Ph3CB(C6F5)4 in the presence of one equivalent of free NHC. Whereas cation (nIMes)2Zn‐Me+ (3+) is stable, its sterically congested analogue (nIDipp)2Zn‐Me+ (4+) readily undergoes an nNHC‐to‐aNHC isomerization in the presence of THF or IDipp to afford the more thermodynamically stable (aIDipp)(nIDipp)Zn‐Me+ (aIDipp=C4‐bonded IDipp, 5+), reflecting the adaptable‐to‐sterics coordination chemistry of these cations for improved stability. Cations 3+–5+ are the first Zn cations of the type Zn(C)(C′)(C′′)+ (C, C′, C′′=σ‐donor carbyl ligand). Kinetic studies combined with DFT calculations agree with an nNHC‐to‐aNHC process proceeding through the initial deprotonation of 4+ (at a Zn‐bonded C4‐IDipp moiety) by IDipp. Unlike 3+ and 4+, the rearranged cation 5+ reacts with CO2 through insertion into the Zn–Me bond yielding the corresponding Zn(κ2‐OAc)+ cation 6+. Both cations 5+ and 6+ were successfully used in CO2 hydrosilylation catalysis for silylformate formation.
Set in its ways: Tri‐organyl ZnII cations, (NHC)2Zn‐Me+, were synthesized and structurally characterized to unveil a coordination chemistry strongly dependent on sterics. Severe steric hindrance may induce a normal‐to‐abnormal N‐heterocyclic carbene (NHC) isomerization. Cation (aIDipp)(nIDipp)Zn‐Me+ (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) inserts CO2 into its Zn–alkyl bond at room temperature and catalyzes CO2 hydrosilylation (see scheme).
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Electrochemically active corannulene derivatives with various numbers of electron-donating 4-(N,N-dimethylamino)phenylethynyl (1–4) or electron-withdrawing cyanobutadienyl peripheral substitutents ...(5–8) were prepared. The latter derivatives resulted from formal 2 + 2 cycloaddition of cyanoolefins to 1–4 followed by retro-electrocyclization. Conformational properties were examined by variable-temperature NMR and X-ray diffraction and opto-electronic properties by electronic absorption/emission spectra and electrochemical measurements; these analyses were corroborated by dispersion-corrected density functional calculations at the level of B97-D/def2-TZVPP. In CH2Cl2, 1–4 exhibit intramolecular charge-transfer (ICT) absorptions at 350–550 nm and green (λem ∼ 540 nm) or orange (600 nm) fluorescence with high quantum yields (56–98%) and are more readily reduced than corannulene by up to 490 mV. The variation of optical gap and redox potentials of 1–4 does not correlate with the number of substituents. Cyanobutadienyl corannulenes 5–8 show red-shifted ICT absorptions with end-absorptions approaching 800 nm. Intersubstituent interactions lead to distortions of the corannulene core and lower the molecular symmetry. NMR, X-ray, and computational studies on 5 and 8 with one cyanobutadienyl substituent suggested the formation of intermolecular corannulene dimers. Bowl-inversion barriers around ΔG ⧧ = 10–11 kcal/mol were determined for these two molecules.
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Nonplanar push–pull chromophores with tetrathiafulvalene and ferrocene donors (see picture) were prepared by formal 2+2 cycloadditions between electron‐rich alkynes and tetracyanoethylene (TCNE) or ...7,7,8,8‐tetracyanoquinodimethane (TCNQ). X‐ray crystallographic studies, complemented by UV/Vis and electrochemical measurements, provide clear insight into the remarkably strong intramolecular charge‐transfer interactions in these chromophores, despite their pronounced nonplanarity.
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We report on a series of electron donor–acceptor conjugates incorporating a ZnII–porphyrin‐based electron donor and a variety of non‐conjugated rigid linkers connecting to push–pull chromophores as ...electron acceptors. The electron acceptors comprize multicyanobutadienes or extended tetracyanoquinodimethane analogues with first reduction potentials ranging from −1.67 to −0.23 V vs. Fc+/Fc in CH2Cl2, which are accessible through a final‐step cycloaddition–retroelectrocyclization (CA‐RE) reaction. Characterization of the conjugates includes electrochemistry, spectroelectrochemistry, DFT calculations, and photophysical measurements in a range of solvents. The collected data allows for the construction of multiple Marcus curves that consider electron‐acceptor strength, linker length, and solvent, with data points extending well into the inverted region. The enhancement of electron–vibration couplings, resulting from the rigid spacers and, in particular, multicyano‐groups in the conformationally highly fixed push–pull acceptor chromophores affects the charge‐recombination kinetics in the inverted region drastically.
Molecular photovoltaics: A series of electron donor–acceptor conjugates incorporating a ZnII–porphyrin‐based electron donor and multicyanobutadienes or extended tetracyanoquinodimethane analogues as electron acceptors with first reduction potentials ranging from −1.67 to −0.23 V vs. Fc+/Fc. As such, the rich redox chemistry, broad absorption in the visible and near infrared region, and electron‐transfer behavior upon photoexcitation with charge recombination kinetics in the inverted region render these systems promising materials for applications in the field of molecular photovoltaics.
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The design and synthesis of Aviram–Ratner‐type molecular rectifiers, featuring an anilino‐substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the σ‐spacer ...bicyclo2.2.2octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X‐ray analysis. The photophysical properties of the TTF‐BCO‐exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir–Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current–voltage (I–V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I–V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor.
Aviram–Ratner mechanism of rectification: A molecular dyad, closely resembling the original design by Aviram and Ratner, featuring a strong donor (TTF), separated by a rigid insulating σ‐spacer from a strong extended TCNQ acceptor (exTCNQ), has been designed, synthesized, and fully characterized. Langmuir–Blodgett films were prepared and showed the asymmetric current–voltage (I–V) curve characteristic for rectification, whereas control compounds containing the TTF but not the exTCNQ unit or featuring the exTCNQ acceptor moiety but lacking the TTF donor showed symmetric I–V curves.
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A versatile, two‐step synthesis of highly substituted, cyano‐functionalized diaryltetracenes has been developed, starting from easily accessible tetraaryl3cumulenes. This unprecedented transformation ...is initiated by 2+2 cycloaddition of tetracyanoethylene (TCNE) to the proacetylenic central double bond of the cumulenes to give an intermediate zwitterion, which after an electrocyclization cascade and dehydrogenation yields 5,5,11,11‐tetracyano‐5,11‐dihydrotetracenes in a one‐pot procedure. A subsequent copper‐assisted decyanation/aromatization provided the target 5,11‐dicyano‐6,12‐diaryltetracene derivatives. All of the postulated structures were confirmed by X‐ray crystallography. The new chromophores are thermally highly stable and feature promising fluorescence properties for potential use in optoelectronic devices. They are selective chemosensors for CuI ions, which coordinate to one of the CN substituents and form a 1:1 complex with an association constant of Ka=1.5×105 L mol−1 at 298 K.
Condensed sensors: The title reaction starts with the cycloaddition of tetracyanoethylenene to the central CC bond of cumulenes, followed by a multi‐reaction cascade to yield tetracyanodihydrotetracenes. Elimination of (CN)2 provides tetracenes with a rubrene‐like substitution pattern as highly fluorescent chromophores that can be used as molecular chemosensors for Cu+ and Ag+ ions.
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A carbopalladation cascade reaction of easily accessible gem-dibromoolefins and alkynes furnishes monobenzo- and mononaphthopentalenes. The new chromophores accessed by this short route exhibit small ...HOMO–LUMO gaps and redox amphoteric behavior with tunable redox potentials.
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We report the reactions of electron‐deficient alkenes, tetrasubstituted by carboxylic ester and cyano groups, with electron‐rich (dimethylamino)phenyl‐substituted alkynes. Mono‐ or ...diester‐substituted alkenes exclusively undergo the 2+2 cycloaddition–retroelectrocyclization (CA–RE) reaction, well established for multicyanated ethenes, whereas tri‐ and tetraester‐substituted alkenes also undergo a 4+2 hetero‐Diels–Alder (HDA) reaction with a third product being formed, presumably by a 3+2 cycloaddition reaction followed by rearrangement. Electrochemical studies revealed cathodic shifts of the first reduction potential of the buta‐1,3‐dienes obtained from the CA–RE reaction as cyano groups are substituted for ester moieties. Post‐CA–RE functionalization of the ester‐substituted buta‐1,3‐dienes by transesterification, diazonium chemistry, and cross‐coupling is described. The formation of a pharmacologically interesting pyrazolopyran illustrates the synthetic utility of ester‐substituted CA–RE products.
Tetrasubstituted electron‐deficient alkenes with varying numbers of cyano and ester groups show different reactivities. Electron‐rich aminophenylalkynes undergo cycloaddition–retroelectrocyclization, hetero‐Diels–Alder reactions, and a novel 3+2 cycloaddition reaction followed by rearrangement. The optoelectronic properties of the products have been investigated and their reactivities explored.
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We report the synthesis and physical study of a series of 1,1‐dicyano‐4‐4‐(diethylamino)phenylbuta‐1,3‐dienes in which the number and position of additional CN substituents along the ...1,1‐dicyanobuta‐1,3‐dienyl fragment is systematically varied. While X‐ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2‐ and C4‐positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge‐transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push‐pull chromophores as third‐order nonlinear‐optical molecular materials.
Increasingly cyanated 1,1‐dicyano‐4‐(N,N‐diethylanilino)‐substituted buta‐1,3‐dienes were synthesized by unusual routes and characterized by X‐ray analyses. Their opto‐electronic properties strongly depend on both the number and position of the additional CN groups. Substitution in positions 2 and 4 affects the intramolecular charge‐transfer bands more strongly than substitution in position 3, which is rationalized by theoretical calculations.
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