Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

The metal-to-metal charge transfer (MMCT) transitions of a series of Class II mixed valence dinuclear complexes bearing cyano bridging ligands may be varied systematically by variations to either the hexacyanometallate(II) donor or Co(III) acceptor moieties. Specifically, the new dinuclear species trans-[L(14S)CoNCFe(CN)(5)](-)(L(14S)= 6-methyl-1,11-diaza-4,8-dithia-cyclotetradecane-6-amine) and trans-[L(14)CoNCRu(CN)(5)](-)(L(14)= 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) have been prepared and their spectroscopic and electrochemical properties are compared with the relative trans-[L(14)CoNCFe(CN)(5)](-). The crystal structures of Na(trans-[L(14S)CoNCFe(CN)(5)]).51/2H(2)O.1/2EtOH, Na(trans-[L(14)CoNCRu(CN)(5)]).3H(2)O and Na(trans-[L(14)CoNCRu(CN)(5)]).8H(2)O are also reported. The ensuing changes to the MMCT energy have been examined within the framework of Hush theory, and it was found that the free energy change between the redox isomers was the dominant effect in altering the energy of the MMCT transition.     

Time-resolved spectroscopy of the metal-to-metal charge transfer excited state in dinuclear cyano-bridged mixed-valence complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCo(III)NCM(II)(CN)(5)](-), where L is a pentadentate macrocyclic pentaamine (L(14)) or triamine-dithiaether (L(14S)) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L(14)Co(III)NCRu(II)(CN)(5)](-) was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at room temperature enabled the lifetimes of their Co(II)-Fe(III) MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.     

Pressure and temperature effects on metal-to-metal charge transfer in cyano-bridged CoIII-FeII complexes

The effects of pressure and temperature on the energy (Eop) of the metal-to-metal charge transfer (MMCT, FeII-->CoIII) transition of the cyano-bridged complexes trans-[L14CoNCFe(CN)5]- and cis-[L14CoNCFe(CN)5]- (where L14=6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) were examined. The changes in the redox potentials of the cobalt and iron metal centres with pressure and temperature were also examined and the results interpreted with Marcus-Hush theory. The observed redox reaction volumes can mainly be accounted for in terms of localised electrostriction effects. The shifts in Eop due to both pressure and temperature were found to be less than the shifts in the energy difference (DeltaE degrees]) between the CoIII-FeII and CoII-FeIII redox isomers. The pressure and temperature dependence of the reorganisational energy, as well as contributions arising from the different spin states of CoII, are discussed in order to account for this trend. To study the effect of pressure on CoIII electronic absorption bands, a new cyano-bridged complex, trans-[L14CoNCCo(CN)5], was prepared and characterised spectroscopically and structurally. X-Ray crystallography revealed this complex to be isostructural with trans-[L14CoNCFe(CN)5].5H2O.     

Influence of anions on intervalence charge transfer (IVCT) in mixed-valence dinuclear complexes

Spectroelectrochemical studies of the intervalence charge transfer (IVCT) characteristics of both diastereoisomeric forms of the dinuclear complex [{Ru(bpy)2}2(mu-dpi-)]n+ [bpy=2,2'-bipyridine; dpi-=4,5-di(2-pyridyl)imidazolate] showed that the degree of inter-metal electronic coupling (or valence delocalization) is dependent on stereochemical identity. Increasing the relative concentration of the strongly associating anion toluene-4-sulfonate in acetonitrile/[(n-C4H9)4N]{B(C6F5)4} solution differentially decreased the level of delocalization for the two diastereoisomers. In a comparative investigation of electrochemical and spectroelectrochemical techniques of the anion-induced electron localization in [{Ru(bpy)2}2(mu-dpo)]5+ [dpo=3,4-di(2-pyridyl)-1,2,5-oxadiazole], differences were observed between the two methods in the order and extent of effects induced by a number of inorganic anions (PF6-, BF4-, ClO4-). It was determined that the measure of coupling derived from electrochemical methods was less reliable than that obtained from spectral methods. Comparative electrochemical studies were undertaken on [{M(bpy)2}2(mu-BL)]n+ {M=Ru, Os; BL=dpo, dpi-), which revealed substantial differences in DeltaEox (the separation between the redox potentials for the MII-MII/MIII-MII and MII-MIII/MIII-MIII couples) for the two metal centers and therefore the comproportionation constant Kc, dependent on the neutral or anionic nature of the bridging ligand.     

Solvent adiabaticity effects on ultrafast electron transfer in viologen charge transfer complexes

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.     

Substituent effects on the intramolecular charge transfer and fluorescence of bimetallic platinum complexes

An investigation of a series of platinum-containing organometallic complexes for the study of fluorescence phenomena in organometallic chromophores controlled by the intramolecular charge transfer (ICT) is presented in this work. We report steady-state and time-resolved spectroscopic experiments as well as quantum chemistry calculations to investigate the substituent effects on the ICT and fluorescence emission. We demonstrate that the fluorescence maximum and lifetimes greatly depend on different substituents and the presence of bimetallic platinum donor. This work paves the way for an understanding of the fluorescence phenomena controlled by molecular ICT characters of these kinds of platinum-containing organometallic complexes.     

A new method for recycling asymmetric catalysts via formation of charge transfer complexes

[reaction: see text]. A new concept for recycling asymmetric bis(oxazoline)-type catalysts is reported. The formation of charge-transfer complexes between the chiral ligand and trinitrofluorenone and their subsequent precipitation and reuse by addition of new substrate solutions is described. The efficiency of this procedure is demonstrated in a Diels-Alder reaction to reach the expected endo product as major isomer (up to 97% de and 94% ee): the catalyst was used up to 12 times without loss of either activity or selectivity.     

有机酸取代基和溶剂效应的静电模型

本文根据气相实验结果和以往的静电理论, 建立了一个描述有机酸电离取代基和溶剂效应的静电模型。该模型不仅满足气相边界条件, 而且其参数B具有较明确的物理意义, 即与酸官能团的溶剂化半径以及取代基与溶剂分子之间特殊相互作用有关, 因而是描述溶质-溶剂相互作用的重要参数。     

Theoretical description of solvent effects on fluorescence spectra of bulky charge transfer compound DMA-DMPP

Several theoretical models are compared to reproduce the spectroscopic fluorescence shift of 4-(4′-N,N-dimethylaminophenyl)-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′e]-pyridine (DMA-DMPP) in different solvents. DMA-DMPP is used as a model compound because it shows a large shift in emission energy for solvents of various polarities and dual fluorescence in polar protic solvents. Although the simple Onsager model is not able to reproduce the experimental results, the self-consistent reaction field (SCRF) model with extension to excited states based on the AM1 Hamiltonian yields excellent agreement. According to the latter model, the red-shifted emission band can be related to a highly polar charge transfer state without geometrical rearrangements, whereas the normal (short wavelength) emission is attributed to emission from an excited state with increased conjugation in a flattened geometry. A supramolecular approach with six molecules of water surrounding the solute can explain satisfactorily the two distinct fluorescence bands. In protic solvents, the emitting CT state shows additional stabilization of the locally excited state with a planar conformation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1584–1595, 1998     

Modulation of water exchange in europium(III) DOTA-tetraamide complexes via electronic substituent effects

The chemical exchange saturation transfer (CEST) efficiency for a series Eu3+-based tetraamide complexes bearing p-substituents on a single coordinating pendant arm is highly sensitive to water exchange rates. The CEST effect increases in the order Me < MeO < F approximately CO2tBu < CN < H. These results show that CEST contrast can be modulated by changes in electron density at a single ligating atom, and this forms the basis of creating imaging agents that respond to chemical oxidation and reduction.     

Binuclear ruthenium and osmium mixed-valence complexes containing fused and flexible polypyridyl bridging ligands

{Os(bpy)₂}²⁺ and {Ru(CN)₄}²⁻ mononuclear and binuclear complexes with ligands 2,3-di-(2-pyridyl)quinoxaline (dpq) and dipyrido[2,3-a:3′,2′-c]phenazine (ppb) have been prepared. For the binuclear complexes a splitting in oxidation potentials is observed consistent with the formation of mixed-valence species with comproportionation constants (K_com) ranging from 2.5 × 10⁴ to 1.8 × 10⁶. The electronic absorption spectra of the mixed-valence species reveal IVCT transitions in the near infrared region. The absorption maximum for the IVCT band ranges from 5800 to 9980 cm⁻¹ and the extinction coefficients from 80 to 6300 M⁻¹ cm⁻¹. In general the {Os(bpy)₂}²⁺ complexes show larger K_com values and more intense IVCT bands than the corresponding {Ru(CN)₄}²⁻ complexes.     

Calculations of substituent and solvent effects on the kinetic isotope effects of Menshutkin reactions

Nitrogen, deuterium, halogen, and carbon kinetic isotope effects have been modeled for the Menshutkin reaction between methyl halides and substituted N,N-dimethylaniline at the HF/6-31G(d) level of theory augmented by the C-PCM continuum solvent model for several solvents. Systematic changes in geometries of the transition states and Gibbs free energies of activation have been found with phenyl ring substituents, solvent, and the leaving group. Kinetic isotope effects also change systematically; however, these changes are predicted to be small, inside the usual precision of the experimental measurements. On the contrary, no correlation has been found between the kinetic isotope effects and the Hammett constants for para substituents. Thus opposite to previous assumptions, our results indicate that kinetic isotope effects on the Menshutkin reaction cannot be used to predict the position of the transition state on the reaction coordinate.     

N‐Inversion in N‐phenyloxaziridine: substituent and solvent effects via density functional theory

In this work, using density functional theory, the kinetic effects of the substitution of a t‐butyl group andor the incorporation of an oxygen atom, and both, at the aziridine ring moiety were investigated for N‐inversion in N‐phenylaziridine. Then, for N‐inversion in 3‐t‐butyl‐N‐phenyloxaziridine, the kinetic Hammett substituent effects were studied using the different para‐substituted groups on the N‐phenyl ring moiety. The natural bond orbital (NBO) study was the last case in this work. The calculations were performed in the gas phase and solution (in carbon tetrachloride and dichloromethane). The incorporation of an oxygen atom in the aziridine ring strongly weakens the N‐inversion process. In addition, while both t‐butyl substituent and solvent slightly reinforce the N‐inversion of N‐phenyloxaziridine, in N‐phenylaziridine, they decrease the N‐inversion rate to some extent. In both phases, more pronounced in solution and especially in dichloromethane, and in agreement with the NBO results, the electron‐withdrawing groups on para position of the N‐phenyl ring strongly increase the rate of N‐inversion of 3‐t‐butyl‐N‐phenyloxaziridine molecule.     

Electrical properties of polyvinylcarbazole-tetracyanoquinodimethane charge transfer complexes

Electrical properties of polyvinylcarbazole-tetracyanoquinodimethane (PVK:TCNQ) charge transfer complexes were investigated as a function of the composition of the mixture, temperature, pressure and the method of sample preparation. It was shown that the electrical conductivity of the PVK:TCNQ complex (for TCNQ content <10% by weight) increased only slowly with increasing concentration of TCNQ and was only slightly higher than the conductivity of pure PVK. At higher TCNQ concentrations however an abrupt increase of the complex conductivity was observed. This effect may be attributed to the formation of semi-conductive tracks composed of uncomplexed neutral TCNQ molecules. The electrical conductivity of PVK:TCNQ complex was lower than the conductivity of TCNQ alone. The activation energy for electrical conductivity in the complex decreased with increasing TCNQ concentration from 0.99 eV for pure PVK to 0.37 eV for PVK:TCNQ (6:1) complex. The high field conductivity of the PVK:TCNQ complex could be explained by using the Poole-Frenkel model.     

Intramolecular diels-alder reactions of the furan diene: substituent and solvent effects

An internal Diels-Alder reaction, using furan as the diene component, formed a highly functionalized 5-membered carbocyclic ring in excellent yield. Solvent and substituent effects of this reaction were examined.