Charge-transfer (CT) complexes formed between an NADH model compound, l-benzyl-l,4-dihydronicotinamide (BNAH), and a series of p-benzoquinone derivatives Q were isolated from benzene solutions of these reactants. Some isolated CT complexes exhibited long-wavelength absorption maxima in the range 670-735 nm, depending on the electron-acceptor ability of the quinone derivatives. Transient CT bands equivalent to the CT bands of the isolated complexes were observed also in the course of the hydride-transfer reactions from BNAH to Q in acetonitrile, suggesting that the CT complexes are intermediates for the hydride-transfer reactions. The rate constants k for the hydride-transfer reactions vary significantly with the redox potentials £°(Q/Q-•) of pbenzoquinone derivatives and span a range of more than 10u. The primary kinetic isotope effects kH/kD also show a large variation in the range 1.5-6.2, and a bell-shaped dependence of the kK/kO values on the £°(Q/Q"•) values has been obtained with a clear "Westheimer maximum". Quantitative analyses for these correlations of the rate constants and the isotope effects with the redox potentials of p-benzoquinone derivatives have been presented on the basis of a sequential electron-proton-electron transfer mechanism where the radical ion pair [BNAH+.Q"•] formed by the first electron transfer from BNAH to Q in the CT complex [BNAH-Q] is considered to be closer to a "transition state" than an "intermediate" for most p-benzoquinone derivatives used in this study.Although oxidation-reduction reactions of models for dihydronicotinamide coenzymes have generally been considered to involve one-step hydride transfer,1 the presence of intermediates such as a charge-transfer (CT) complex and a radical ion pair formed by electron transfer from an NADH model compound to a substrate has often been suggested.2•3 In general, CT complexes are formed when compounds which have low redox potentials, acting as electron donors, are combined with compounds with high reduction potentials as electron acceptors.4 However, very

"Pickering-type" emulsions were prepared using polydopamine (PDA) particles as a particulate emulsifier and n-dodecane, methyl myristate, toluene or dichloromethane as an oil phase. All the emulsions prepared were oil-in-water type and an increase of PDA particle concentration decreased oil droplet diameter. The PDA particles adsorbed to oil-water interface can be crosslinked using poly(ethylene imine) as a crosslinker, and the PDA particle-based colloidosomes were successfully fabricated. Scanning electron microscopy studies of the colloidosomes after removal of inner oil phase revealed a capsule morphology, which is strong evidence for the attachment of PDA particles at the oil-water interface thereby stabilizing the emulsion. The colloidosomes after removal of inner oil phase could retain their capsule morphology, even after sonication. On the other hand, the residues obtained after oil phase removal from the PDA particle-stabilized emulsion prepared in the absence of any crosslinker were broken into small fragments of PDA particle flocs after sonication.

The effects of Mg2+ ion on hydride-transfer reactions from an NADH model compound, 1 -benzyl-l,4dihydronicotinamide (BNAH), t o a series of p-benzoquinone derivatives (Q) as well as on the redox potentials of BNAH and Q in acetonitrile have been examined. The Mg2+ ion shows both accelerating and retarding effects on the hydride-transfer reactions depending on the p -benzoquinone derivative and the Mg2+ concentration. Such dual effects of the Mg2+ ion have been well correlated with the change of the redox potentials of BNAH and Q in the presence of Mg2+ ion since it has been found that there is a simple correlation between the logarithm of the rate constant and the difference of the redox potentials between BNAH and Q in the absence and presence of Mg2+ ion. It is shown that a proposed reaction mechanism involving electron transfer from BNAH to Q followed by proton transfer from BNAH +' t o Q-' in the rate-determining step of the hydride-equivalent transfer provides a quantitative evaluation of the single and unified correlation between the logarithm of the rate constant and the difference between the redox potentials of BNAH and Q in the absence and presence of Mg2+ ion. The effect of Mg2+ ion on the primary kinetic isotope effects on hydride transfer from BNAH t o Q is also shown to be consistent with the proposed reaction mechanism. Moreover, a ternary complex involving BNAH, Mg2+ ion, and Q as a reaction intermediate prior t o electron transfer from BNAH to Q has been detected for the first time.

A new methodology for experimental determinations of redox potentials of oxidants or reductants in both reversible and irreversible systems is presented. The Rehm-Weller free energy relationship between the activation free energy ΔG− and the free energy change ΔG for the electron transfer quenching of the excited states of oxidants or reductants by a series of reductants or oxidants is converted to a linear correlation between ΔG\eweq−ΔG and (ΔG\eweq)−1, which can be utilized in convenient determinations of redox potentials of oxidants or reductants. This methodology is established in various reversible systems where the redox potentials of oxidants or reductants determined based on the Rehm-Weller free energy relationship agree well with those determined independently by electrochemical methods. By using this method, one-electron oxidation potentials of two quite different compounds, one is an NADH model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the other [Rh2(dicp)4]2+ (dicp=1,3-diisocyanopropane), in MeCN have been determined as 0.60±0.10 and 0.38±0.10 V vs. SCE, respectively. The relation between the oxidation potential and the oxidation peak potential of BNAH measured by the cyclic voltammetry is discussed in the context of free energy relationships.

The kinetics of the oxidative addition of iodine to the following isocyaniderhodium(I) monomers and dimers in MeCN has been examined; [Rh(RNC)4]+, [Rh(RNC)2(PPh3)2]+, [Rh2(dppm)2(RNC)4]2+, and [Rh2(dicp)4]2+ (R=alkyl, aryl; dppm=bis(diphenylphosphino)methane; dicp=1,3-diisocyanopropane). The reaction proceeds via two consecutive steps which consist of the formation of the initial adduct followed by the intramolecular isomerization to yield the final trans-adduct. The kinetic results for the first step of the oxidative addition reactions suggest that there exist three kinds of intermediates formed between the Rh(I) complex and iodine with 1:1, 1:2, and 2:1 stoichiometry prior to the formation of the initial adduct. This mechanism coincides with that for the electron transfer reactions between the Rh(I) complexes and inorganic oxidants such as [Fe(N-N)3]3+ (N-N=1,10-phenanthroline and 2,2′-bipyridine). The values of the rate constants for the first step of the oxidative addition of iodine to dppm- or dicp-bridged Rh(I) dimers are found to agree with those for the electron transfer reactions between the same reactants by using linear free energy relationships. The possibility of electron transfer activation as the rate determining step in the oxidative addition to Rh(I) complexes is discussed.