Ion-pair Charge-transfer Complexes Research Articles

Thiol-Mediated α-Amino Radical Formation via Visible-Light-Activated Ion-Pair Charge-Transfer Complexes.

Visible-light-activated electron donor-acceptor complexes offer distinct reaction pathways for the synthesis of complex molecules under mild conditions. Herein, we report a method for the reductive generation of α-amino radicals via the reaction of a visible-light-activated ion-pair charge-transfer complex formed between an in situ-generated alkyl-iminium ion and a thiophenolate. This distinct activation mode is demonstrated through the development of a multicomponent coupling reaction to form substituted aminomethyl-cyclopentanes from secondary amines, cyclopropyl aldehydes, and alkenes. The operationally straightforward transformation displays broad scope and provides a means to generate cyclic amine-containing scaffolds from readily available feedstocks.

Visible-Light-Induced Oxazoline Formations from N-Vinyl Amides Catalyzed by an Ion-Pair Charge-Transfer Complex

Visible-light photoredox catalysis plays an important role in various reactions which are inaccessible under typical thermal conditions. Distinctly different from common visible-light photoredox catalysis which often involves transition-metal complexes, conjugated organic dyes, or electron donor–acceptor complexes, herein, the use of ion-pair charge-transfer (IPCT) complex-induced visible-light photoredox catalytic reactions are described, wherein the cyclization–methoxylation of N-vinyl amides in methanol was achieved under irradiation with blue LEDs. The reaction employs a heteroarenium iodide as the photocatalyst and can be extended to cyclization–alkoxylation, −acyloxylation, and −hydroxylation. This protocol provides an eco-friendly synthetic route to a wide range of oxazoline derivatives. Mechanistic investigations with UV–visible spectroscopy and control experiments confirm the existence of the IPCT absorption band in the visible region for the heteroarenium iodide, which is responsible for the observed reactivity.

Role of Pyridinium Groups and Iodide Ions in Photoelectrochromism in Viologen-Based Ion-Pair Charge-Transfer Complexes: Molecular Orbital Analysis

Quantum chemistry calculations were performed to examine the fundamentals of photoinduced electron transfer (ET) in viologen (V)-based ion-pair charge-transfer complexes and the resulting photoelectrochromism with respect to photoswitching applications. For the purpose, the photoinduced ET from counter anions to viologen or biphenyl (BP) derivatives was modeled, and its relationship with their structures was analyzed. Our results showed that the electron reduction of V2+, assuming photoinduced ET, that is, V2+ → V+ → V0, changes the conformational preference from a twisted to a planar structure because of the lowest unoccupied molecular orbital (LUMO) in V2+ showing a planar tendency. A similar feature appears in the reduction of neutral BP, that is, BP0 → BP– → BP2–, leading to a twisted → planar preference change because of the LUMO in BP0. The similarity between V2+ and BP0 can be explained by their similar MO shape and their identical number of electrons. The time-dependent density functional theory (...

Syntheses, crystal structures, electronic spectra and magnetic properties of two ion-pair charge transfer complexes based on [Ni(mnt)2

Two new bimetallic ion-pair complexes 1 and 2 with general formula [M(phen)3][Ni(mnt)2]2 (phen = 1, 10-phenanthroline, mnt2− = maleonitriledithiolate; M = Ni2+, Fe2+ for 1 and 2), have been prepared and characterized by IR, elemental analysis, single crystal X-ray diffraction, UV-vis-NIR spectra and magnetic measurements. The structural determination reveals that the crystals of two ion-pair complexes, with monoclinic space group C2/c, have similar cell parameters and the [M(phen)3]2+ cations and the [Ni(mnt)2]- anions are packed by forming alternate layers. Thermogravimetric (TG) analyses revealed that 1 and 2 are thermally stable up to ∼170 °C. UV-vis-NIR spectra discloses that two complexes exhibit sizable absorption in near-IR region because of ion-pair charge transfer (IPCT) transitions in 1 and 2. Investigation of the magnetic properties 1 shows Curie-Weiss-type paramagnetic behavior in the temperature range 2–400 K. For 2, the paramagnetic behavior above ∼40 K indicated the [Fe(phen)3]2+ ion has a low-spin state and the weak Curie-Weiss type tail below 40 K results from a trace amount of paramagnetic [Ni(mnt)2]- species.

Fluorescence properties of ion pair charge transfer complex with bithiophene group

The fluorescence properties of ion pair charge transfer (IPCT) complex with bithiophene group were studied by steady-state and time-resolved measurements in various solvents. The photophysical properties of 5,5′-di(4-pyridyl)-2,2′-bithiophene (DPBT) were identical to the unsubstituted bithiophene except for the peak wavelength. The absorption of polymeric 5,5′-di(4-pyridiniumyl)-2,2′-bithiophene (PBT 2+) salts showed both the locally excited state (LE) and intramolecular charge transfer (ICT) bands in all solvents studied. In less-polar solvents such as cyclohexane and toluene, absorption spectra showed an IPCT band in addition to the LE and ICT bands. The fluorescence spectra of polymeric PBT 2+ salts showed dual emission which originated from the LE and ICT states in polar solvents. The ICT fluorescence lifetime strongly depends on the solvent polarity. The fluorescence from the PBT 2+ salts was effectively quenched by their counter ion in less-polar solvent. These results indicate that the counter halide anion affects on the excited-state relaxation process.

Ion-pair charge transfer complexes with M…H-bonding interactions: Syntheses, crystal structures, electronic spectra and DFT calculations

Three new ion-pair complexes, consisting of 1-benzyl-4-aminopyridinium with [M(mnt) 2] 2− where mnt 2− = maleonitriledithiolate and M = Ni 2+, Pd 2+ or Pt 2+ for complexes 1– 3, have been synthesized and structurally characterized. X-ray single crystal studies revealed that three complexes are isostructural and crystallize in monoclinic space group C2 /c, and there exist weak H-bonding interactions between –NH 2 groups of cation and –CN group of anion as well as weak M…H-bonding interactions between anions and cations. The calculations of charge density distribution disclosed that the M…H-bonding interaction in crystals of 1– 3 is mainly attributed to static electric interactions. Two types of H-bonding interactions lead to the formation of two-dimensional sheet structure which is parallel to the crystallographic bc-plane in the crystals of 1– 3. A near-IR absorbance appears in the UV–vis-NIR spectra of 1 and 3, respectively, while does not come out in the spectra of 2.

Ion-pair charge transfer complexes with intense near IR absorption: Syntheses, crystal structures, electronic spectra and DFT calculations

Five ion-pair complexes, consisting of R-benzylidene-1-aminopyridinium derivatives and [Ni(mnt) 2] 2− (R = p-nitro ( 1), p-methyl ( 2), p-bromo ( 3), p-chloro ( 4) and m-nitro ( 5); mnt 2− = maleonitriledithiolate), were synthesized and structurally characterized. As for 1, it is interesting to observe a large deviation from square-planar coordination geometry for the Ni atom, while no deviation is observed in the other four complexes. In the solid state, UV–Vis–NIR spectra of 2– 5 show similar properties with intense absorption in the 200–750 nm and moderate near IR absorption in the 750–1000 nm region, whereas 1 exhibits an intense absorption from UV/Visible to near-IR region (200–1100 nm). This unique spectral feature of 1 is attributed to its distinctive structural differences from 2 to 5, namely the strong intermolecular packing interactions between anions and cations, as well as a significant deviation from the planarity of the anion. Based on DFT and TDDFT calculations, near-IR absorbance bands in 1– 5 were assigned to combined transitions of d–d, MLCT and π–π ∗ in the [Ni(mnt) 2] 2− anion as well as the ion-pair charge transfer (IPCT) from the anionic HOMO to the cationic LUMO. The IPCT band position in acetonitrile is independent of the substituent group feature in benzene ring of cations for 1– 5, which could be interpreted that the substituent group in benzene ring only has a minor contribution to the cationic LUMO.

Synergistic effects in multicomponent electrocatalysts: the Pb-Ir-O system.

The ionic interactions were studied in aqueous solutions of Na(3)IrCl(6) + Pb(NO(3))(2) in order to develop a facilitated electrosynthesis of iridium-based catalytic surfaces. Spectroscopic studies indicated that ion pair charge-transfer complexes [IrCl(6)(3-)]-Pb(II) (K = 6 x 10(3)) and [Ir(H(2)O)Cl(5)(2-)]-Pb(II) (K = 2 x 10(3)) were formed in fresh and aged solutions, respectively. Electrochemical studies showed that interactions between the Ir(H(2)O)Cl(5)(2-) and Pb(II) species lead to synergistic lowering of the overpotential that was necessary for nucleation and growth of mixed metal oxide PbIrOx on the surface of glassy carbon electrodes. The Ir:Pb stoichiometry of the PbIrOx surface films was the same (1:1) as that of the high-temperature phase of Pb-Ir-O pyrochlore. Compared to IrOx, the PbIrOx films displayed enhanced catalytic activity toward the electrooxidation of carbohydrates. This was ascribed to synergism that involved retention of carbohydrate molecules at the Pb(II) sites of a PbIrOx film and oxidation at the adjacent Ir(IV) sites. The synergistic electroplating utilizing interactions between the partially aquated transition metal complex and posttransition metal ion represents a new synthetic route to highly homogeneous and reactive films of mixed metal oxides.

Ion pair charge-transfer complexes between anionic and cationic metal-dithiolenes [M(II) = Pd, Pt

New [M(R(2)pipdt)(2)](BF(4))(2) salts [R(2)pipdt = N,N'-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II), R = Me, Et, Pr(i)] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These cations react with the redox-active [M(mnt)(2)](2-) [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that [M(Me(2)pipdt)(2)][M(mnt)(2)] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): triclinic, Ponemacr; (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) A, b = 8.460(6) A, c = 13.510(5) A, alpha = 100.63(2) degrees, beta = 104.04(2) degrees, gamma = 96.90(2) degrees; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately square-planar [Pt(Me(2)pipdt)(2)] dications and regular square-planar [Pt(mnt)(2)] dianions form an infinite anion-cation one-dimensional stack along axis a with a Pt...Pt a/2 distance of 3.392 A and a Pt...Pt...Pt angle of 180 degrees. Anions and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong absorptions in the visible-near-infrared region assigned to ion pair charge-transfer transitions. A relation between the optical and thermal electron transfer in the solid state is obtained using a "Marcus-Hush model", and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement with the existence of charge-transfer interactions between the cationic and anionic components of the salts.

Study on transition metal–4,5-dimercapto-1,3-dithiole-2-one complexes and related compounds: toward structure–property correlation

Electrical and electrochemical properties of a series of metal bisdithiolene ion-pair charge transfer complexes of 1,3-dithiole-2-one-4,5-dithiolate, 1,3-dithiole-2-thione-4,5-dithiolate, 1,3-dithiole-2-selone-4,5-dithiolate, and 5,6-dihydro-1,4-dithiin-2,3-dithiolate ligands with general formula [C] x [ML2] (C=counter cation, such as tetrabutylammonium, tetraethylammonium, and tetramethylammonium; M=Ni, Cu, Zn, Pd, or Pt; x=1, 2 or non-integer; L=ligand) have been studied and compared to each other. The effects of the central chelating metal ion, counter cation, stoichiometry, and the ligand on the electrical and electrochemical behaviour are discussed. A crystal structure of bis(tetrabutylammonium) bis(1,3-dithione-2-one-4,5-dithiolato)zincate(II) is determined and compared with its analog compounds. Structure–property relationships are discussed.

Ultrafast Absorption Changes Both in the Visible and Near- Infrared Regions Due to Photoinduced Electron Transfer Reactions of Dicyanovinylstyrylpyridinium Tetraphenylborate

Ultrafast dynamics of photoinduced electron transfer reaction and dimer radical cation formation of ion pair charge transfer complexes between dicyanovinylstyrylpyridinium cation and tetraphenylborate anion were studied at room temperature in less polar solvents by femtosecond laser flash photolysis. Transient absorptions were observed both in the visible and near-infrared regions. They showed very rapid rise in less than one picosecond, which was limited by the time resolution of our flash photolysis system.

C60 as Photosensitizing Electron-Transfer Mediator for Ion-Pair Charge-Transfer Complexes between Borate Anions and Methyl Viologen Dication

Photochemical reactions between the excited triplet state of C60 and the ion-pair charge-transfer complex which consists of electron-acceptor cation (methyl viologen (MV2+)) and electron-donor anion (organo borates (-BPh3R, where R = Ph or Bu)) have been investigated by both steady-state and laser-flash photolysis. By photoirradiation of C60 in the presence of MV2+(-BPh3R)2 in organic solvents, the amount of MV+• increases with irradiation time, and persists for a long time even in the dark. From the laser-flash photolysis, it is found that electron transfer proceeds via 3C60* from -BPh3R in the complex, yielding •BPh3R and C60-•. Although generated C60-• does not decay in the absence of MV2+ because of the rapid dissociation of •BPh3R, in the presence of MV2+, C60-• decays quickly by transferring an electron to MV2+ yielding MV+•. Thus, it is proved that C60 acts as a photosensitizer for pumping up an electron from -BPh3R as well as an electron mediator to MV2+. In these systems, MV+• persists even in air-saturated solution for more than an hour, suggesting that the electron transfer from MV+• to O2 is retarded.

Structure and properties of the ion pair charge-transfer complex of octacyanotungstate(IV) with the 2,2′-bipyridinium dication

The X-ray crystal structure of (bpyH2)2[W(CN)8]·4H2 O (bpyH2=2,2′-bipyridinium) is described. The [W(CN)8]4− anion has an approximately square antiprismatic (D4d) conformation, seemingly imposed by strong anion–water–cation hydrogen-bonding interactions. Bond distances in the anion are: WC 2.150(5), 2.163(5); CN 1.128(6), 1.145(6)A and the angles WCN are 177.6(5), 178.3(5)°. The dihydrate and the anhydrous salt are both intensely black solids, exhibiting ion-pair charge-transfer interaction between cation and anion. E.s.r. spectra indicate that 30% of the tungsten is present as WV in the solid state, but that in solution only the WIV complex is present. The electron withdrawing effect of the cation is discussed and compared with that in a series of salts with different bipyridinium cations.

Ion-Pair Charge-Transfer Complexes Based on (o-Phenylenebis(oxamato))cuprate(II) and Cyclic Diquaternary Cations of 1,10-Phenanthroline and 2,2'-Bipyridine: Synthesis, Crystal Structure, and Physical Properties.

The ion pair charge-transfer (IPCT) complexes (C(14)H(12)N(2))[Cu(opba)].3H(2)O (1) and Na(2)(C(12)H(12)N(2))[Cu(opba)](2).4H(2)O (2), opba = o-phenylenebis(oxamate), have been isolated. Both compounds crystallize in the triclinic space group P&onemacr;. Crystal data for the compounds 1, a = 7.3216(13) Å, b = 10.1756(9) Å, c = 16.4890(16) Å, alpha = 107.786(8) degrees, beta = 94.79(1) degrees, gamma = 104.16(1) degrees, V = 1117.4(3) Å(3), Z = 2, R = 0.033 for 5332 observed reflections with I > 3sigma(I); 2, a = 6.732(3) Å, b = 11.169(3) Å, c = 12.126(2) Å, alpha = 111.01(2) degrees, beta = 102.71(3) degrees, gamma = 93.57(3) degrees, V = 820.1(5) Å(3), Z = 2, R = 0.065 for 3354 observed reflections with I > 3sigma(I). The crystal structures consist of alternated anion complex and diquaternary cation layers pi-pi interacting through the corresponding aromatic rings. The metallic layers contain the [Cu(opba)](2)(4)(-) dimeric group where the copper(II) ions are in square-pyramidal environment. For both compounds an ion-pair charge-transfer (IPCT) band is observed at 517 nm. Their thermal behavior has also been explained on the basis of the different crystal arrangements. The dimeric nature of compound 1 is clearly confirmed by the observation of temperature-dependent ESR transitions between its singlet and triplet states. The |J| value, determined from the positions of these transitions, is 0.085 cm(-)(1) at room temperature and increases notably with decreasing temperature being 0.14 cm(-)(1) at 4.2 K. Comparison of these exchange constants with those observed for other carboxylate-bridged copper compounds allow us to deduce that the temperature dependence of J is due to lattice shrinkage leading to appreciable change of the Cu-O(ap) distance. The ESR spectrum of compound 2 is characteristic of an axial g-tensor with g( parallel) = 2.178 and g( perpendicular) = 2.049, showing a weak half-field DeltaM(s) = 2 transition in agreement with the dimeric nature of the complex. The thermal evolution of the magnetic susceptibility corresponds to a compound with weak antiferromagnetic interactions. The best fit of the magnetic data to a dimer S = (1)/(2) model gives J = -0.8 cm(-)(1), in good agreement with the topology of the bridging unit with the magnetic orbitals in parallel planes and a Cu-O-Cu angle of 95.4 degrees.

Ultrafast dynamics of photogenerated styrylpyridinyl radical and its dimer radical cation with styrylpyridinium cation studied by femtosecond laser flash photolysis

Ultrafast photoinduced electron transfer reaction and dimer radical cation formation have been studied at room temperature in less polar solvents by femtosecond laser flash photolysis of ion-pair charge-transfer complexes between a styrylpyridinium cation and a tetraphenylborate anion. In addition to the visible absorption due to the styrylpyridinyl radicals, the charge resonance absorption band (CR band) due to the electronic interaction between the photogenerated styrylpyridinyl radical and the styrylpyridinium cation was observed in the near infrared region. Transient absorptions of both radicals and the CR band showed very rapid rise in less than 1 ps, which was limited by the time resolution of the flash photolysis system. No interaction of styrylpyridinium cations was observed before photoexcitation. These results strongly suggested that the electronic interaction between the styrylpyridinium ion and its reduced form occurred without a diffusion process immediately after photoinduced electron transfer. From the decay dynamics, the dimer radical cation was found to show a faster back electron transfer than the radical itself.

Metal-organic ion pairs with dithiooxalate d 8 metal donors and viologens. Spectroscopic, structural and electronic characterization

Ion-pair charge-transfer complexes of the type {A 2+[M(dto) 2] 2−}, where A 2+ represents bipyridinium and phenanthrolinium compounds of different redox potential, M=Ni, Pd, Pt, and dto=1,2-dithiooxalate, were synthesized and characterized with respect to their spectroscopic, structural and electronic properties. A weak absorption band at the red-tail of the low-energy two-band system of the complex donor component can be attributed to the ion-pair charge-transfer (IPCT) transition type. As expected from electrochemical data, the IPCT band of these systems is markedly hypsochromically shifted compared with ion pairs containing dithiolene donors. The different nature of the highest occupied MO may offer an explanation for the distinct electro- and photochemical properties. Applying the Hush model reorganization energies are estimated in the range 107–125 kJ mol −1 for the thermal electron transfer from [Ni(dto) 2] 2− to the acceptor site. The amount of charge that is transferred from the donor to the acceptor ( α 2) is calculated as 10 −5–10 −6, lowered by 2–3 orders of magnitude compared with the metal dithiolenes. As shown by X-ray analysis of HEM[Ni(dto) 2] (HEM= N-2-hydroxyethyl- N′-methyl-4,4′-bipyridinium cation), the molecular structure consists of alternating donor–acceptor stacks with a typical plane-to-plane distance of 345 pm. In addition to supramolecular interactions both components are bonded by a hydrogen bridge between the OH group of the cation and the electron-rich oxygen of the dto ligand.

Evaluation of Anisotropic Photoresponses in Ultrathin Organic films by a Highly Sensitive Polarized Optical Waveguide Method

Highly sensitive polarized absorption measurement by the optical waveguide (OWG) detection system was made to analyze the orientation of photogenerated radicals in Langmuir-Blodgett (LB) films. The extinction coefficient changes upon photoexcitation of specific ion-pair charge-transfer complexes between two kinds of amphiphilic 4,4′-bipyridinium and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate were estimated by this system. Amphiphilic 4,4′-bipyridinium ions in LB films deposited directly on an OWG showed different anisotropy from those deposited on an OWG covered with three monolayers of cadmium arachidate. The structure of substituents also affected the orientation.

Ion Pair Charge-Transfer Complexes of Dithiolatocobaltates with Cationic Copper or Nickel Macrocycles: Crystal Structure and Magnetic Properties

Abstract The complexes {[M(tim)]2+[Co(mnt)2]2−}, (M = Ni (NiCo), Cu (CuCo), tim = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene, mnt2− = cis-1,2-dicyano-1,2-ethylenedithiolato) have been isolated. NiCo and CuCo have isomorphous crystal structures (P1-), the basic feature of which is the presence of mixed donor–acceptor chains induced by short intrachain distances of 3.15 and 3.03 Å, respectively. For both NiCo and CuCo, an ion pair charge-transfer band is observed at 707 and 843 nm; application of the Hush model affords reorganization energies for electron transfer from the dianion to the dication of 89 and 104 kJ mol−1, respectively. The magnetic properties of the powder samples have been studied in the temperature range of 2 K to 300 K. In NiCo the diamagnetic [Ni(tim)]2+ acceptor induces antiferromagnetic coupling of the spin a half system [Co(mnt)2]2− along the a-axis over a distance of 6.3 Å. From the exchange coupling constant of J = −37 cm−1 a Weiss temperature of 26 K is calculated. Contrary to that, the magnetic orbitals of [Cu(tim)]2+ and [Co(mnt)2]2− remain uncoupled in CuCo.

Ion-pair charge-transfer complexes of a dithiooxalate zinc donor component with viologens. Synthesis, structural and electronic characterization

Bipyridinium and phenanthrolinium acceptors of different reduction potentials form with zinc 1,2-dithiooxalates (dto) ion-pair charge-transfer complexes of the general formula {A 2+[Zn(dto) 2] 2−}. The contact ion pairs exhibit absorptions in the range 390–490 nm which can be attributed to the ion-pair charge-transfer (IPCT) type. On the base of spectroscopic, electrochemical and quantum-chemical investigations the relation between optical and thermal electron transfer within the ion pair applying the Hush theory is discussed. The mean reorganization energy of 12 complexes is 180 U mol −1 and exceeds the values found for the dithiolene systems. Due to the diminished donor ability of the dithiooxalate unit, a hypsochromic shift of the position of the IPCT band, compared with the metal dithiolenes, results. The extent of electron delocalization from [Zn(dto) 2] 2− to pQ 2+, as described by the parameter α 2, is calculated as 4.1 × 10 −6. X-ray analyses of BQ[Zn(dto) 2] and DP[Zn(dto) 2] reveal that the solid-state structure is largely determined by the geometry of the acceptor component.

Ultrafast photon-mode recording and switching by photoinduced electron transfer

Abstract