Search

Two novel ruthenium complexes, [Ru(2,2′-bipyridine-6,6′-diphosphonato)(pic)2] (2) (pic=4-picoline) and [Ru(6,6′-diisopropyl-2,2′-bipyridine-6,6′-diphosphonato)(pic)2] (3) bearing phosphonate groups have been synthesized and characterized by NMR spectroscopy, elemental analyses, X-ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. Both complexes show catalytic water oxidation activity by electrochemistry. At pH 7, the RuII/III redox couple of 2 is observed at a lower potential than that of 3, yet significantly, 3 oxidizes water at a lower onset potential. At pH 1 however, 2 and 3 have comparable catalytic reactivity using sacrificial oxidant CeIV. We propose that water oxidation activities of 2 and 3 are influenced by overall charges. For example, in oxidizing from RuII to RuIII at pH 7, 2 acquires a −1 overall charge whereas 3 acquires a +1 charge. Charge-dictated electrostatic effects may govern binding of a water molecule to the metal site.

Cyclic voltammetric and spectroelectrochemical evidence is presented showing that the H-bonded dimer formed from a ureidopyrimidone derivative containing a phenylenediamine redox couple can be reversibly broken apart at mM concentrations in CH2Cl2 by an electrochemically induced proton-coupled electron transfer reaction.

The voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine, H2PD, has been studied and compared to that of its isomer N,N,N’N’-tetramethyl-p-phenylenediamine, Me2PD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1e- oxidation to the radical cations, Me2PD+ and H2PD+, and a second 1e- oxidation at more positive potentials to the quinonediimine dications, Me2PD2+ and H2PD2+. While the voltammetry of Me2PD agrees with this simple mechanism, that of H2PD does not. The second voltammetric wave is too small. UV/Vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of H2PD+ to H2PD2+ in solution. The fact that the second wave is not present at all at the lowest concentrations (5 µM), and that it increases at longer times and higher concentrations, indicates that H2PD+ is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one H2PD in the the full reduced, fully protonated state, H4PD2+, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other H2PD’s, to give 2 H2PD+, which is the thermodynamically favored species in solution.

Cyclic voltammetric studies of 1-phenyl-3-(4-N,N-dimethylamino)phenyl urea (UH) in CH2Cl2 in the presence of several pyridine derivatives are described. By itself, UH undergoes a reversible one electron oxidation to the radical cation, UH+, and a second less reversible oxidation at more positive potentials. Addition of ten equivalents of 1,8-naphthyridine or pyridine causes the second oxidation to shift 700 mV negative, resulting in a quasi-reversible two electron oxidation at the potential of the UH/UH+ wave. Possible explanations for the large shift are proton transfer between UH2+ and the guest or very strong hydrogen bonding between the two. Computer simulation studies and comparison to known pKa values suggest that neither by themselves are adequate to explain the observed voltammetry. A mechanism involving both hydrogen bonding and proton transfer is more likely.

Electrodes modified with Nafion films containing 2,7-dimethyldiazapyrenium (DAP2+) were prepared and characterized with voltammetry by themselves and in the presence of organic substrates. The large, planar, electron-poor aromatic surface in DAP2+ facilitates pi-stacking interactions with other planar aromatic molecules, particularly those that are negatively charged or electron-rich. Previous studies showed that the reduction of DAP2+ decreases the strength of these interactions, making the binding redox-dependent, and resulting in negative shifts in the E(1/2) of DAP2+/+. This study shows that the redox-dependent binding ability of DAP2+ is retained in Nafion, but the selectivity is considerably different. Most significantly, the electron-rich, neutral aromatic compounds that produced small shifts in the E(1/2) of DAP2+/+ in solution cause much larger shifts, up to -110 mV, with the modified electrodes. With indole as a substrate, Nernstian behavior is observed (-60 mV shift per log[indole]) between 10 and 0.5 mM.

This paper describes the electrochemical characterization of electrode-confined siloxane polymers that contain both naphthoquinone (NQ) and benzylviologen (BV2+) subunits. These “homopolymers,” abb...

The electrochemistry of several p-phenylenediamine derivatives, in which one of the amino groups is part of an urea functional group, has been investigated in methylene chloride and acetonitrile. The ureas are abbreviated U(R)R', where R' indicates the substituent on the N that is part of the phenylenediamine redox couple and R indicates the substituent on the other urea N. Cyclic voltammetry and UV-vis spectroelectrochemical studies indicate that U(Me)H and U(H)H undergo an apparent 1e(-) oxidation that actually corresponds to 2e(-) oxidation of half the ureas to a quinoidal-diimine cation, U(R)(+). This is accompanied by proton transfer to the other half of the ureas to make the electroinactive cation HU(R)H(+). This explains the observed irreversibility of the oxidation of U(Me)H in both solvents and U(H)H in acetonitrile. However, the oxidation of U(H)H in methylene chloride is reversible at higher concentrations and slower scan rates. Several lines of evidence suggest that the most likely reason for this is the accessibility of a H-bond complex between U(H)(+) and HU(H)H(+) in methylene chloride. Reduction of the H-bond complex occurs at a less negative potential than that of U(H)(+), leading to reversible behavior. This conclusion is strongly supported by the appearance of a more negative reduction peak at lower concentrations and faster scan rates, conditions in which the H-bond complex is less favored. The overall reaction mechanism is conveniently described by a "wedge scheme", which is a more general version of the square scheme typically used to describe redox processes in which proton transfer accompanies electron transfer.

Studies are presented showing that N,N′-dimethyl-2,7-diazapyrenium, DAP 2+ , acts as a redox-dependent receptor for aromatic carboxylates in aqueous solution. Cyclic voltammetric studies of DAP 2+ by itself indicate that its one-electron reduction to DAP + is only quasi-reversible in both aqueous and acetonitrile solutions due to the slow oxidation of DAP + back to DAP 2+ . In aqueous solution, the oxidation is very sensitive to other conditions such as pH and the presence of small amounts of iodide. However, in pH 7 phosphate buffer, fairly reversible voltammetry is observed at slow scan rates. Under these conditions, addition of excess aromatic carboxylate results in large negative shifts in the E 1/2 of DAP 2+/+ . Similar changes are observed in the 1 H NMR spectrum of DAP 2+ in the presence of the different carboxylates, indicating that the potential shift is due to strong interactions between the carboxylates and DAP 2+ . Upon reduction, the favorable coulombic interactions between the anionic carboxylates and the cationic pyrenium are weakened, resulting in the observed negative shifts in E 1/2 . Although the perturbation of electrostatic interactions causes the potential shifts, the magnitude of the shifts correlates more with the strength of π-stacking interactions between the carboxylates and DAP 2+ . Those carboxylates whose π system can come into maximum close contact with the DAP 2+ π system give the largest potential shifts.

Cyclic voltammetry of a viologen-based cyclophane receptor, VH, in water, 10% acetonitrile/water, and acetonitrile is described. Minimal electrolyte was necessary in order to prevent precipitation of the reduced VH in aqueous solutions. Shifts in the half-wave potential of the first viologen reduction are observed in the presence of various substituted benzenes. The direction and magnitude of the shifts are explained in terms of two competing factors : (i) the change in the electron donor-acceptor character of the host that occurs upon reduction and (ii) the change in the hydrophobic/hydrophilic character of the host upon reduction. The first factor promotes binding of acceptor-substituted guests to the reduced host and donorsubstituted guests to the oxidized host The second factor promotes binding of all the guests to the reduced host in aqueous solution. Large potential shifts are observed when both factors work in the same direction

Cyclic voltammetry of the viologen-based cyclophane host (1) in acetonitrile by itself and in the presence of various parasubstituted benzenes is described. Changes in peak separation and shifts in half-wave potentials of the viologen reduction waves are observed in the presence of excess benzene derivative. This behavior is shown to be due to preferential binding of the benzene guests in one oxidation state of 1. The oxidation state preferred depends on guest structure and can be predicted on the basis of donor-acceptor considerations and ion-dipole interactions. The strongest preference is observed with an ethoxy ether-substituted benzene that interacts strongly with positively charged sites on 1. Reduction of 1 leads to a >4000-fold decrease in binding strength with this guest

Cyclic voltammetry studies are reported for two representative quinones, benzoquinone and 2-anthraquinonesulfonate, in buffered and unbuffered aqueous solution at different pH's. While the redox reaction of quinones in buffered water is well described as an overall 2 e-, 2 H+ reduction to make the hydroquinone, a much better description of the overall reaction in unbuffered water is as a 2 e- reduction to make the strongly hydrogen-bonded quinone dianion, which will exist in water as an equilibrium mixture of protonation states. This description helps to unify quinone electrochemistry by bridging the apparent gap between the redox chemistry of quinones in water and that in aprotic organic solvents, where quinones undergo two sequential 1 e- reductions to form the quinone dianion.

[reaction: see text] The electrochemistry of 1,2-dinitrobenzene (1,2-DNB), 1,3-dinitrobenzene (1,3-DNB), and 1,4-dinitrobenzene (1,4-DNB) is strongly affected by the presence of 1,3-diphenylurea. In DMF, the second reduction potential of all three DNBs shifts substantially positive in the presence of the urea, indicating very strong hydrogen bonding to the dianions. With 1,2- and 1,3-DNB, the hydrogen bonding leads to irreversible chemistry, likely due to proton transfer from the urea to the dianions. No such irreversible behavior is observed with 1,4-DNB. Instead, the second reduction shifts into the first reduction, producing a single, reversible, two-electron cyclic voltammetric wave at high urea concentrations. Computer simulations show that the changes in wave shape accompanying this process are well accounted for by the stepwise formation of a 1:1 and 2:1 1,3-diphenylurea/DNB2- complex, with sequential binding constants of approximately 5.5 x 10(4) M(-1) and approximately 4.0 x 10(3) M(-1) in DMF.

Reduction of nitrobenzene derivatives in the presence of arylureas in aprotic solvents results in large positive shifts in potential of the nitrobenzene(0/)(-) cyclic voltammetry wave with little change in wave shape. This behavior is indicative of reversible hydrogen bonding between nitrobenzene radical anions and arylureas. Computer fitting of the cyclic voltammetry of 4-nitroaniline, NA, plus 1,3-diphenylurea in DMF shows essentially no binding between urea and NA in the oxidized state (K(ox) < 1 M(-)(1)), but very strong binding in the reduced state (K(red) = 8 x 10(4) M(-)(1)), along with very rapid rates of hydrogen bond formation (k(f)'s approximately 10(8)-10(10) M(-)(1) s(-)(1)), making this system a fast on/off redox switch.

9,10-Phenanthrenequinone and acenaphthenequinone are shown to act as simple redox-dependent receptors toward aromatic ureas in CH(2)Cl(2) and DMF. Reduction of the o-quinones to their radical anions greatly increases the strength of hydrogen bonding between the quinone carbonyl oxygens and the urea N-hydrogens. This is detected by large positive shifts in the redox potential of the quinones with no change in electrochemical reversibility upon addition of urea guests. Cyclic voltammetric studies with a variety of possible guests show that the effect is quite selective. Only guests with two strong hydrogen donors, such as O-H bonds or amide N-H bonds, that are capable of simultaneously interacting with both carbonyl oxygens give large shifts in the redox potential of the quinones. The electronic character and conformational preference of the guest are also shown to significantly affect the magnitude of the observed potential shift. In the presence of strong proton donors the electrochemistry of the quinone becomes irreversible indicating that proton transfer has taken place. Experiments with compounds of different acidity show that the pK(a) of the protonated quinone radical is about 15 on the DMSO scale, >4 pK(a) units smaller than that of 1,3-diphenylurea. This is further proof that hydrogen bonding and not proton transfer is responsible for the large potential shifts observed with this and similar guests.

The study of phenanthrenequinone(PQ)-modified electrodes, prepared by electropolymerization of a phenanthrenequinone-pyrrole derivative, is described. The surface-confined PQ is shown to behave similarly to PQ in solution, acting as a redox-dependent receptor toward aromatic ureas in aprotic solvents. Large, positive shifts in the E1/2 of the PQ0/- redox couple are observed in the presence of these ureas, due to a strong hydrogen bonding interaction between the PQ radical anion and urea. The effect is fully reversible. Of the substrates examined, only aromatic ureas produce a significant shift. Nonaromatic ureas or other HN and HO containing compounds have little effect on the E1/2. The magnitude of the shift is also independent of electrode coverage, allowing reproducible measurements to be made despite significant loss in material from the surface.

The authors report results concerning use of the surface-derivatizing reagent I that demonstrates a kind of rectification that is important in systems such as the photosynthetic apparatus. The important finding is that an electrode-bound redox polymer formed via hydrolysis of the Si-OMe bonds if O, (BV-Q-BV/sup 6 +/)/sub n/, undergoes a 4e/sup -//2H/sup +/ reduction, but below pH 5 only 2e/sup -/ can be electrochemically withdrawn from the reduced material. Reduction of quinone centers, Q, in the (BV-Q-BV/sup 6 +/)/sub n/ polymer to their dihydroxy form, QH/sub 2/, is mediated by the viologen, BV/sup 2+/+/, redox system, but the process cannot be reversed at low pH because the QH/sub 2/ is thermodynamically incapable of delivering charge back to the BV/sup 2 +/ centers. Direct equilibration of the electrode with the Q/QH/sub 2/ centers in the (BV-Q-BV/sup 6 +/)/sub n/ polymer does not occur. Relevance of the new finding to systems like the photosynthetic apparatus rests in the fact that charge separation in the photosynthetic system is achieved by a kind of rectification: the charge transport process involves a series of effectively unidirectional e/sup -/ transfer events.