Quasi-reversible One-electron Oxidation Research Articles

Ditopic ligand effects on solution structure and redox chemistry in discrete [Cu12S6] clusters with labile Cu-S bonds.

Copper chalcogenide nanoclusters (Cu-S/Se/Te NCs) are a broad and diverse class of atomically precise nanomaterials that have historically been studied for potential applications in luminescent devices and sensors, and for their beautiful, mineral-like crystal structures. By the "cluster-surface" analogy, Cu-S/Se NCs are prime candidates for the development of nanoscale multimetallic catalysts with atomic precision. However, the majority of studies conducted to date have focused exclusively on their solid-state structures and physical properties, leaving open questions as to their solution stability, dynamics, and reactivity. Herein, we report the first detailed interrogation of solution structure, dynamics, electrochemistry, and decomposition of Cu-S NCs. Specifically, we report the detailed NMR spectroscopy, diffusion-ordered spectroscopy, MALDI mass spectrometry, electrochemical and stoichiometric redox reactivity studies, and DFT studies of a series of [Cu12S6] clusters with labile Cu-S bonds supported by monodentate phosphines and ditopic bis(diphenylphosphino)alkane ligands PPh2R (R = Et, -(CH2)5-, -(CH2)8-). We find that the ligand binding topology dictates the extent of speciation in solution, with complete stability being afforded by the longer octane chelate in dppo (1,8-bis(diphenylphosphino)octane) according to 1H and DOSY NMR and MALDI-MS studies. Furthermore, a combined electrochemical and computational investigation of [Cu12S6(dppo)4] reveals that the intact [Cu12S6] core undergoes a quasireversible one-electron oxidation at mild applied potentials ([Cu12S6]0/+: -0.50 V vs. Fc0/+). In contrast, prolonged air exposure or treatment with chemical oxidants results in cluster degradation with S atom extrusion as phosphine sulfide byproducts. This work adds critical new dimensions to the stabilization and study of atomically precise metal chalcogenide NCs with labile M-S/Se bonds, and demonstrates both progress and challenges in controlling the solution behaviour and redox chemistry of phosphine-supported copper chalcogenide nanoclusters.

Primary amines as ligands and linkers in complexes of tripyrrindione radicals

Biopyrrin pigments, which result from the degradation of heme in biological settings, feature three or two pyrrole rings and characteristic pyrrolin-2-one termini. These scaffolds serve as redox-active ligands and electron reservoirs in coordination compounds. Tripyrrin-1,14-dione coordinates divalent transition metals as a dianionic ligand hosting a delocalized radical. Herein, we report the synthesis and characterization of palladium(II) and platinum(II) tripyrrindione complexes featuring a primary amine (i.e., aniline, tert-butylamine, 1,2-ethylenediamine) at the fourth coordination site within square planar geometries. Interligand hydrogen-bonding interactions are observed between the coordinated amine and the carbonyl groups on the tripyrrindione scaffold. Notably, 1,2-ethylenediamine is employed to link two Pt(II) tripyrrindione complexes. As revealed by optical absorption and electron paramagnetic resonance (EPR) spectroscopy, all resulting complexes present ligand-based radicals that are stable at room temperature and when exposed to air. Spin pairing through multicenter interactions leads to [Formula: see text]-dimerization of the tripyrrindione radicals and a decrease in the EPR signal at low temperatures. Electrochemical measurements indicate that the ligand system undergoes quasi-reversible one-electron oxidation and reduction, thus confirming the ability of tripyrrindione to form square planar complexes in three different redox states.

Biomimics of [FeFe]-hydrogenases with a pendant amine: Diphosphine complexes [Fe2(CO)4{µ-S(CH2)nS}{κ2-(Ph2PCH2)2NR}] (n = 2, 3; R = Me, Bn) towards H2 oxidation catalysts

We report the synthesis and molecular structures of [FeFe]-ase biomimics [Fe2(CO)4{µ-S(CH2)nS}{κ2-(Ph2PCH2)2NR}] (1–4) (n = 2, 3; R = Me, Bn) and a comparative study of their protonation and redox chemistry, with the aim of assessing their activity as catalysts for H2 oxidation. They are prepared in good yields upon heating the hexacarbonyls and PCNCP ligands in toluene, a minor product of one reaction (n = 3, R = Bn) being pentacarbonyl [Fe2(CO)5(µ-pdt){Ph2PCH2N(H)Bn}] (5). Crystal structures show short Fe-Fe bonds (ca. 2.54 Å) with the diphosphine occupying basal-apical sites. Each undergoes a quasi-reversible one-electron oxidation and IR-SEC shows that this results in formation of a semi-bridging carbonyl. As has previously been observed, protonation products are solvent dependent, nitrogen being the favoured site of protonation site upon addition of one equivalent of HBF4.Et2O in d6-acetone, while hydride formation is favoured in CD2Cl2. However, the rate of N to Fe2 proton-transfer varies greatly with the nature of both the dithiolate-bridge and amine-substituent. Thus with NMe complexes (1–2) N-protonation is favoured in acetone affording a mixture of endo and exo isomers, while for NBn complexes (3–4) proton-transfer to afford the corresponding μ-hydride occurs in part (for 3 edt) or exclusively (for 4 pdt). In acetone, addition of a further equivalent of HBF4.Et2O generally does not lead to hydride formation, but in CD2Cl2 dications [Fe2(CO)4{µ-S(CH2)nS}(μ-H){κ2-(Ph2PCH2)2NHR}]2+ result, in which the diphosphine can adopt either dibasal or basal-apical positions. Proton-transfer from Fe2 to N has been previously identified as a required transformation for H2 oxidation, as has the accessibility of the all-terminal carbonyl isomer of cations [Fe2(CO)4{µ-S(CH2)nS}{κ2-(Ph2PCH2)2NR}]+. We have carried out a preliminary H2 oxidation study of 3, oxidation by Fc[BF4] in the presence of excess P(o-tolyl)3 affording [HP(o-tol)3][BF4], with a turnover of ca. 2.3 ± 0.1 mol of H2 consumed per mole of 3

Access to and reactions of P-functional 1,4-dihydro-1,4-diphosphinines fused to two TTF units.

P-Functional phosphanylated tetrathiafulvalenes 3a-f were synthesised via stepwise lithiation and phosphanylation of TTF derivatives, and then reacted with PCl3 to form the related P-chloro compounds 4a-f. Reactions of 4c-f with LDA resulted in the formation of the corresponding 1,4-dihydro-1,4-diphosphinines 5c-f. As a case in point, P-oxidation reactions of 5d,f with elemental chalcogens were performed, and the former were also converted into 1,4-dichloro-1,4-dihydro-1,4-diphosphinine 9f. The latter was reduced to form the related 1,4-diphosphinine 10f which could not be isolated but formed the corresponding 1,4-diphosphabarrelene 11f in a [4 + 2]-cycloaddition with 1-hexene. All compounds were characterised by multinuclear NMR spectroscopy and mass spectrometry and also by single crystal X-ray diffraction studies in some cases. Intensive cyclic voltammetry studies were performed for all isolated compounds with the special focus on using TTF units as sensors to study the substituent effects on oxidation potentials and, hence, the degree of electronic communication between redox active moieties in the bis-TTF species. E.g., 5d possesses four quasi-reversible one-electron oxidation steps thus forming a tetracation species at highest potential (+0.54 V vs. Fc+/0). Additionally, high level DFT calculations were undertaken to get a deeper understanding of various aspects of this novel combination of phosphorus and TTF chemistry.

Spectroscopic, structural and computational studies of thiophenolato bridged dirhenium(III,III) complexes

Thiophenol (HLH), 4-methylthiophenol (HLMe) and 4-chlorothiophenol (HLCl) react with dirhenium(III,II) complex [Re2(µ-O2CCH3)Cl4(µ-dppm)2] (1) (dppm = Ph2PCH2PPh2) in refluxing dry toluene to afford the diamagnetic Re2 6+ complexes of the type [Re2(μ-LR)2Cl4(μ-dppm)2] (2(LR)) [LR = C6H5S–, p-MeC6H4S– and p-ClC6H4S–]. These are the first examples of thiophenolato bridged dirhenium complexes with a Re2 6+ core containing dppm ligand. The spectral properties of the complexes are reported. In the electrochemical experiment, the type 2(LR) complexes show one quasi-reversible one-electron oxidation and one-electron reduction waves. The identity of 2(LMe) has been established by single-crystal X-ray structure determination (Re–Re distance = 2.6178(5) Å) and is shown to have an edge-shared bi-octahedral structure. DFT analysis shows that the highest occupied molecular orbitals are mainly composed of metal δ-based orbitals whereas the lowest unoccupied molecular orbitals are metal π*-based orbitals and the electronic ground state for the 2(LR) complexes is σ2π2δ*2δ2. Time-dependent DFT (TD-DFT) analysis shows that the sharp peak at 455 nm in the experimental UV-vis spectrum is mainly due to the [Cl(π)]→[Re2(π*)] transition.

Diketonato based ferrocene appended cyclometalated iridium(III) complexes: Anti-microbial and anti-cancer studies

Three newly synthesized ferrocene appended cyclometalated iridium(III) complexes (1, 2 and 3) were characterized by FT-IR, 1H and 13C NMR, LCMS spectroscopies and elemental analysis. The electrochemical studies suggested that all of the ferrocene appended iridium complexes exhibited two quasi-reversible one-electron oxidations. The antimicrobial activities of samples were evaluated against Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli) and a fungal strain (Candida albicans). The complex 1 and 2 showed a significant zone of inhibition with potent bactericidal and fungicidal activities. The cytotoxic effects of the synthesized complexes (1-3) were studied by the MTT method against two different human cell lines (normal kidney cell line HEK293 and liver cancer cell line HepG2). The complex 1 showed the best cytotoxicity against HepG2 cancer cells among all the synthesized complexes.

Highly conjugated donor-acceptor dyad based on monotetrathiafulvalene covalently attached to a magnesium norphthalocyanine unit

A novel highly conjugated donor–acceptor dyad composed of a tetrathiafulvalene (TTF) moiety covalently attached to a magnesium norphthalocyanine (NPc) unit (1) has been synthesized and full characterized. Ultraviolet–visible spectroscopy and electron paramagnetic resonance data reveal that 1 forms an electron transfer complex with 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ). Detailed electrochemical investigations show one irreversible one-electron oxidation wave, two quasi-reversible one-electron oxidation waves, and two irreversible one-electron reduction waves, indicating that 1 is a good π-electron donor. Compound 1 shows clear intramolecular charge transfer (ICT) interactions from the tetrathiafulvalene fragments to the norphthalocyanine core. This phenomenon can be explained by density functional theory (DFT).

Synthesis and Electrochemical Estimation of DNA-Binding Capacity of Novel Ferrocene-Containing Pyrrolidines

The design, synthesis, spectral and electrochemical characterization of a series of novel pyrrolidine derivatives have been described. The synthesis was achieved by 1,3-dipolar cycloaddition of azomethine ylides and ferrocene-substituted chalcones, while detailed characterization of the compounds was performed by IR, NMR and elemental analyses, followed by X-ray diffraction experiment for one representative. The electrochemical properties of obtained ferrocene-containing heterocycles were examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These techniques revealed the quasi-reversible one-electron oxidation process. DNA-binding capacity of all products was studied using CV and DPV, highlighting a derivative with five times better binding affinity than the others. DFT calculations and molecular docking studies were carried out to gain more exhaustive insight into the type of interactions of the distinguished compound and the nucleic acid.

Electrochemical characterization and estimation of DNA-binding capacity of a series of novel ferrocene derivatives

The synthesis of a series of methyl 2-alkyl-5-aryl-4-ferrocenoylpyrrolidine-2-carboxylates has been achieved by [3 + 2] dipolar cycloaddition of azomethine ylides to acryloylferrocene. The electrochemical properties of novel products were examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). These techniques revealed the quasi-reversible one-electron oxidation process. The DNA-binding capacity of all the products was also studied using CV and DPV, and significant interactions between synthesized compounds and nucleic acid, mostly of the electrostatic type, were disclosed. DFT calculations and molecular docking tests were carried out to gain a more exhaustive insight into the interactions of the obtained products with nucleic acid. A detailed characterization of the new compounds was performed by IR, NMR and elemental analyses, followed by single-crystal X-ray diffraction experiments for two representatives.

Octahedral {Ta6I12} Clusters.

Ta powder reacts with I2 at 650 °C with the formation of Ta6I14, which belongs to the family of {M6(μ-X)12} clusters. It undergoes aquation with the formation of the intensely colored [Ta6I12(H2O)6]2+. The crystal structure was determined for [Ta6I12(H2O)6](BPh4)2·xH2O (Ta-Ta 2.9322(6) Å, Ta-I 2.8104(7) Å, Ta-O 2.3430(5) Å). With DMF, [Ta6I12(DMF)6]I2·xDMF was isolated (Ta-Ta 2.9500(2) Å, Ta-I 2.8310(4) Å, Ta-O 2.2880(7) Å). Cyclic voltammetry of [Ta6I12(H2O)6]2+ shows two consecutive quasi-reversible one-electron oxidations (E1/2 0.61 and 0.92 V vs Ag/AgCl). Reaction of Ta6I14 with Bu4NCN yields (Bu4N)4[Ta6I12(CN)6]·xCH3CN (Ta-Ta 2.9777(4) Å, Ta-I 2.8165(6) Å, Ta-C 2.2730(7) Å). Quantum chemical calculations reproduce well the experimental geometry of the aqua complex and show the essentially Ta-centered nature of both the HOMO and LUMO. The long-term stability of [Ta6I12(H2O)6]2+ solutions can be greatly enhanced in the presence of polystyrenesulfonate (PSS), which forms nanoparticle associates with the aqua complex in water (ca. 1 cluster per 3 PSS monomeric units).

X-ray crystal structural and spectral studies of copper(II) and nickel(II) complexes of functionalized bis(thiosemicarbazone) ligands and investigation of their electrochemical behavior

Neutral bivalent copper and nickel bis(thiosemicarbazone) complexes bearing the methoxy and bromine substituents on the dialdehyde-backbone of the ligand have been synthesized. All of them have been characterized by spectroscopic methods and in two cases by X-ray crystallography. Upon formation of these complexes, the each two bis(thiosemicarbazone) arms is deprotonated, acting as dianionic, tetradentate N2S2 ligands. The geometrical distortion from square planar in the copper complex is very greater than the nickel complex. Cyclic voltammetry studies of Cu(II) and Ni(II) complexes in DMF show that all complexes are able to stabilize low oxidation states of Cu(I) and Ni(I). The introduction of different substitutions on the aromatic ring attached to the imine carbon led to two consequences. First, The E1/2 values for the CuI/CuII and NiI/NiII redox couples were cathodically shifted in comparison with the unsubstituted analog. Second, the copper complexes underwent a quasi-reversible one-electron oxidation (CuII/CuIII).

Comparing Isomeric Tridentate Carbazole-Based Click Ligands: Metal Complexes and Redox Chemistry.

Two novel bis(triazolyl)carbazole ligands Hbtc1 (3,6-di(tert-butyl)-1,8-bis[(1-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-4-yl)]-9H-carbazole) and Hbtc2 (3,6-di(tert-butyl)-1,8-bis[(4-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-1-yl)]-9H-carbazole), differing in the regiochemistry of triazole attachment, have been synthesized by Cu-catalyzed azide-alkyne cycloaddition, the so-called "click-reactions". Metalation with Ru, Zn, and Ni precursors led to the formation of M(btc)2 complexes (M=Ru, Zn, Ni), with two deprotonated ligands coordinating to the metal center in tridentate fashion, forming almost perfectly octahedral coordination spheres. The redox properties of M(btc)2 complexes have been investigated by cyclic voltammetry, UV/Vis spectroscopy, spectroelectrochemistry, and chemically. The CV of the ruthenium complexes revealed three quasi-reversible one-electron oxidation processes, one assigned as the RuII/III couple and two originating from ligand-based oxidations. The CVs of both Zn and Ni complexes contained only two oxidation waves corresponding to the oxidation of the two ligands. The oxidation potentials of complexes derived from Hbtc1 ligands were found to be 300-400 mV lower than those of the corresponding complexes derived from Hbtc2, reflecting the significant difference in donation through the N(2) or N(3) atom of the triazole moiety.

Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts

Summary One-electron oxidation of known ( t Bu 3 P) 2 M (1, M = Pd; 2, M = Pt) with [Ph 3 C][HCB 11 Cl 11 ] leads to two-coordinate, monovalent cations of the formula [( t Bu 3 P) 2 M][HCB 11 Cl 11 ] (3, M = Pd; 4, M = Pt), which also possess linear geometry but with elongated M–P bonds. Spectroscopic and computational studies consistently show that the unpaired electron of the d 9 configuration of 3 and 4 belongs to largely non-bonding orbitals: the s/d z2 hybrid for Pd and the degenerate d x2-y2 /d xy pair for Pt. We show that molecular-orbital-based arguments alone are incapable of predicting or rationalizing the observed M–P bond lengthening on oxidation; correct prediction and rationalization are achieved only by inclusion of electrostatic and Pauli effects. This emphasizes the dangers of interpreting any perturbative changes in bond metrics solely on the basis of energies and occupancies of molecular orbitals; the inclusion of electrostatic and Pauli components is essential to providing a more complete picture.

Synthesis and electron-donating properties of novel norphthalocyanines containing thiacrown ether-linked tetrathiafulvalene moieties

A novel magnesium-base and metal-free norphthalocyanine (6 and 7) containing peripheral thiacrown ether-linked tetrathiafulvalene moieties have been synthesized and fully characterized. Electrochemical investigations showed two quasi-reversible one-electron oxidation waves, one irreversible one-electron oxidation wave and two irreversible one-electron reduction waves, indicating that these triads are good π-electron donors. Triad 6 reacted with 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ) to form a charge transfer complex that exhibited absorption bands in the vicinity of 750 and 960nm. Compound 6 showed evident intramolecular charge transfer interactions in ground states and these may be explained on the basis of the density functional theory.

Synthesis of mixed-ligand complexes of VO2+ and VO3+ incorporating hydrazone, 1,10-phenanthroline and 8-hydroxyquinoline

2-Aminobenzoylhydrazide (abh) reacts with equimolar amounts of either [VIVO(acac)2] or [VIVO(bzac)2] (where acac− and bzac− are the monoanionic forms of acetylacetone (Hacac) and benzoylacetone (Hbzac), respectively) in the presence of equimolar amounts of 1,10-phenanthroline (phen) to form the octahedral mixed-ligand complexes [VIVO(L1)(phen)] (1) and [VIVO(L2)(phen)] (2), where (L1)2− and (L2)2− are the dianionic forms of the 2-aminobenzoylhydrazone of acetylacetone (H2L1) and benzoylacetone (H2L2). Upon substituting phen by 8-hydroxyquinoline (Hhq), pentavalent [VVO(L1)(hq)] (3) and [VVO(L2)(hq)] (4) complexes were instead obtained. In the crystal structures of 3 and 4, the hydrazone ligands coordinate to the vanadium center through the enolic-O, one imine-N and amide-O in a mer geometry. The amine and the second imine nitrogen form intramolecular hydrogen bonds. Complexes 1 and 2 display quasi-reversible one-electron oxidation peaks near +0.60 V, while the pentavalent 3 and 4 exhibit quasi-reversible one-electron reduction peaks near −0.18 V versus Ag/AgCl in CH2Cl2 solution. EPR spectroscopic studies on 1 and 2 suggest that the unpaired electron is present in the dxy orbital. DFT studies for 3 indicate that the dxy orbital of vanadium is the main contributor to the LUMO.

Synthesis and Characterization of Rare Earth Corrole-Phthalocyanine Heteroleptic Triple-Decker Complexes.

We recently reported the first example of a europium triple-decker tetrapyrrole with mixed corrole and phthalocyanine macrocycles and have now extended the synthetic method to prepare a series of rare earth corrole-phthalocyanine heteroleptic triple-decker complexes, which are characterized by spectroscopic and electrochemical methods. The examined complexes are represented as M2[Pc(OC4H9)8]2[Cor(ClPh)3], where Pc = phthalocyanine, Cor = corrole, and M is Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), or Tb(III). The Y(III) derivative with OC4H9 Pc substituents was obtained in too low a yield to characterize, but for the purpose of comparison, Y2[Pc(OC5H11)8]2[Cor(ClPh)3] was synthesized and characterized in a similar manner. The molecular structure of Eu2[Pc(OC4H9)8]2[Cor(ClPh)3] was determined by single-crystal X-ray diffraction and showed the corrole to be the central macrocycle of the triple-decker unit with a phthalocyanine on each end. Each triple-decker complex undergoes up to eight reversible or quasireversible one-electron oxidations and reductions with E1/2 values being linearly related to the ionic radius of the central ions. The energy (E) of the main Q-band is also linearly related to the radius of the metal. Comparisons are made between the physicochemical properties of the newly synthesized mixed corrole-phthalocyanine complexes and previously characterized double- and triple-decker derivatives with phthalocyanine and/or porphyrin macrocycles.

Synthesis and photovoltaic response of a solution-processable dithienyldiketopyrrolopyrrole-based molecular semiconductor with thienylvinylthienyl endgroups

We describe the synthesis and preliminary photovoltaic performance of a solution-processable organic small-molecule electron donor DT-DPP(TVT)2 that consists of dithienyldiketopyrrolopyrrole (DT-DPP) as the core and thienylvinylthiophene (TVT) as the endgroups. The new compound is a crystalline solid with a Tm of approximately 216°C. Cyclic voltammetry indicates that DT-DPP(TVT)2 exhibits two quasi-reversible one-electron oxidation waves at ca. 0.68 and 0.90 V versus an Ag/AgCl reference electrode, respectively, leading to an estimated highest occupied molecular orbital (HOMO) level of about −5.08 eV. Introducing the branched 2-hexyldecyl side chain provides DT-DPP(TVT)2 with a high solubility in chloroform up to ca. 36 mg mL−1 at room temperature. Thermal annealing increases the crystallinity of the as-cast film from chloroform solution, thereby rendering slightly red-shifted charge-transfer absorption maxima. Fitting the space-charge-limited current characteristics of the thermally annealed thin film yields an improved hole mobility of ∼2.14×10−4 cm2 V−1 s−1 at low voltages versus ∼1.46×10−4 cm2 V−1 s−1 of the as-cast film. A first characterization of the solar cell [ITO/PEDOT: PSS/DT-DPP(TVT)2: PC61BM/Al] produces a power conversion efficiency of ∼3% with VOC≈0.78 V, JSC≈7.91 mA cm−2, and FF≈48.7%, under simulated AM1.5G with an illumination intensity of 100 mW cm−2. It should be noted that the thermal effect on the thin film absorption of DT-DPP(TVT)2 does not seem to be completely similar to the molecular donor DPP(TFNa)2 reported earlier, which bears 6-fluoronaphthyl endgroups.

Highly Soluble Monoamino-Substituted Perylene Tetracarboxylic Dianhydrides: Synthesis, Optical and Electrochemical Properties.

Three dialkylamino-substituted perylene tetracarboxylic dianhydrides with different n-alkyl chain lengths (n = 6, 12 or 18), 1a–1c, were synthesized under mild conditions in high yields and were characterized by 1H NMR, 13C NMR and high resolution mass spectroscopy. Their optical and electrochemical properties were measured using UV-Vis and emission spectroscopic techniques, as well as cyclic voltammetry (CV). This is the first time that the structures and the properties of monoamino-substituted perylene tetracarboxylic dianhydrides have been reported. These molecules show a deep green color in both solution and the solid state and are soluble in most organic solvents. They all show a unique charge transfer emission in the near-infrared region, and the associated peaks exhibit solvatochromism. The dipole moments of the compounds have been estimated using the Lippert-Mataga equation, and upon excitation, they show slightly larger dipole moment changes than those of corresponding perylene diimides, 2a–2c. Additionally, Compounds 1a–1c undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Complementary density functional theory calculations performed on these chromophores are reported in order to gain more insight into their molecular structures and optical properties.

Homoleptic nickel(II) complexes of redox-tunable pincer-type ligands.

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

Green Perylene Bisimide Dyes: Synthesis, Photophysical and Electrochemical Properties.

Three asymmetric amino-substituted perylene bisimide dyes with different n-alkyl chain lengths (n = 6, 12, or 18), 1-(N,N-dialkylamino)perylene bisimides (1a–1c), were synthesized under mild condition in high yields and were characterized by 1H NMR, 13C NMR (nuclear magnetic resonance), HRMS (High Resolution Mass Spectrometer), UV-Vis and fluorescence spectra, as well as cyclic voltammetry (CV). These molecules show intense green color in both solution and solid state and are highly soluble in dichloromethane and even in nonpolar solvents, such as hexane. The shapes of the absorption spectra of 1a–1c in solid state and in solution were found to be virtually the same, indicating that the long alkyl chains could efficiently prevent aggregation. They exhibit a unique charge transfer emission in the near-infrared region, of which the peak wavelengths show strong solvatochromism. The dipole moments of the compounds have been estimated using the Lippert-Mataga equation, and upon excitation, they show larger dipole moment changes than that of 1-aminoperylene bisimide (2). Furthermore, all of the compounds exhibit two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Complementary density functional theory (DFT) calculations performed on these dyes are reported in order to rationalize their molecular structures and electronic properties.