Slow hydrogen atom transfer reactions of oxo- and hydroxo-vanadium compounds: the importance of intrinsic barriers

Tuning of the thermochemical and kinetic properties of ascorbate by its local environment: solution chemistry and biochemical implications

Ascorbate (vitamin C) is a ubiquitous biological cofactor. While its aqueous solution chemistry has long been studied, many in vivo reactions of ascorbate occur in enzyme active sites or at membrane interfaces, which have varying local environments. This report shows that the rate and driving force of oxidations of two ascorbate derivatives by the TEMPO radical (2,2',6,6'-tetramethylpiperidin-1-oxyl) in acetonitrile are very sensitive to the presence of various additives. These reactions proceed by the transfer of a proton and an electron (a hydrogen atom), as is typical of biological ascorbate reactions. The measured rate and equilibrium constants vary substantially with added water or other polar solutes in acetonitrile solutions, indicating large shifts in the reducing power of ascorbate. The correlation of rate and equilibrium constants indicates that this effect has a thermochemical origin rather than being a purely kinetic effect. This contrasts with previous examples of solvent effects on hydrogen atom transfer reactions. Potential biological implications of this apparently unique effect are discussed.

Oxygen activation with transition-metal complexes in aqueous solution

Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO(*), ROOH, and RO(*)). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples Cr(aq)OO(2+)/Cr(aq)OOH(2+) and L(1)(H(2)O)RhOO(2+)/L(1)(H(2)O)RhOOH(2+) was estimated to be 10(1+/-1) M(-1) s(-1). The use of this value in the simplified Marcus equation for the Cr(aq)O(2+)/Cr(aq)OOH(2+) cross reaction provided an upper limit k(CrO,CrOH) <or= 10((-2+/-1)) M(-1) s(-1) for Cr(aq)O(2+)/Cr(aq)OH(2+) self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O(2) with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.

The importance of precursor and successor complex formation in a bimolecular proton-electron transfer reaction

The transfer of a proton and an electron from the hydroxylamine 1-hydroxyl-2,2,6,6-tetramethylpiperidine (TEMPOH) to [Co(III)(Hbim)(H(2)bim)(2)](2+) (H(2)bim = 2,2'-biimidazoline) has an overall driving force of DeltaG degrees = -3.0 +/- 0.4 kcal mol(-1) and an activation barrier of DeltaG(degrees) = 21.9 +/- 0.2 kcal mol(-1). Kinetic studies implicate a hydrogen-bonded "precursor complex" at high [TEMPOH], prior to proton-electron (hydrogen-atom) transfer. In the reverse direction, [Co(II)(H(2)bim)(3)](2+) + TEMPO, a similar "successor complex" was not observed, but upper and lower limits on its formation have been estimated. The energetics of formation of these encounter complexes are the dominant contributors to the overall energetics in this system: DeltaG degrees ' for the proton-electron transfer step is only -0.3 +/- 0.9 kcal mol(-1). Thus, formation of the precursor and successor complexes can be a significant component of the thermochemistry for intermolecular proton-electron transfer, particularly in the low-driving-force regime, and should be included in quantitative analyses.

Synthesis and reactivity of oxo-peroxo-vanadium(V) bipyridine compounds

The vanadium(IV) compound [V(IV)O(OH)((t)Bu(2)bpy)(2)]BF(4) (V(IV)O(OH)) ((t)Bu(2)bpy = 4,4'-di-tert-butylbipyridine) is slowly oxidized by O(2) in ethereal solvents to give the oxo-peroxo compound [V(V)O(O(2))((t)Bu(2)bpy)(2)]BF(4) (V(V)O(O(2))) in excellent yield. This and related compounds were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an X-ray crystal structure of the 4,4'-dimethylbipyridine analog, [V(V)O(O(2))(Me(2)bpy)(2)]BF(4). Monitoring the reaction of V(IV)O(OH) with O(2) in THF/acetonitrile mixtures by (1)H NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compound [V(V)(O)(2)((t)Bu(2)bpy)(2)]BF(4) (V(V)O(2)), which then converts to V(V)O(O(2)). Reaction of V(IV)O(OH) with (18)O(2) gives ca. 60% triply (18)O labeled V(V)O(O(2)). The mechanism of formation of V(V)O(O(2)) is complex and may occur via initial reduction of O(2) at vanadium(IV) to give a superoxo-vanadium(V) intermediate, autoxidation of the THF solvent, or both. That V(V)O(2) is generated first appears to be due to the ability of V(IV)O(OH) to act as a hydrogen atom donor. For instance, V(IV)O(OH) reacts with V(V)O(O(2)) to give V(V)O(2). V(V)O(O(2)) is also slowly reduced to V(IV)O(OH) by the organic hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by triphenylphosphine. Notably, the peroxo complex V(V)O(O(2)) is much less reactive with these substrates than the analogous dioxo compound V(V)O(2).

Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO(*)) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2'-6,6'-tetramethyl-piperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and alpha-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.

Proton-coupled electron transfer of ruthenium(III)-pterin complexes: a mechanistic insight

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.

Concerted proton-electron transfer in a ruthenium terpyridyl-benzoate system with a large separation between the redox and basic sites

To understand how the separation between the electron and proton-accepting sites affects proton-coupled electron transfer (PCET) reactivity, we have prepared ruthenium complexes with 4'-(4-carboxyphenyl)terpyridine ligands, and studied reactivity with hydrogen atom donors (H-X). Ru(II)(pydic)(tpy-PhCOOH) (Ru(II)PhCOOH), was synthesized in one pot from [(p-cymene)RuCl(2)](2), sodium 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine ([Na(+)]tpy-PhCOO(-)), and disodium pyridine-2,6-dicarboxylate (Na(2)pydic). Ru(II)PhCOOH plus (n)Bu(4)NOH in DMF yields the deprotonated Ru(II) complex, (n)Bu(4)N[Ru(II)(pydic)(tpy-PhCOO)] (Ru(II)PhCOO(-)). The Ru(III) complex (Ru(III)PhCOO) has been isolated by one-electron oxidation of Ru(II)PhCOO(-) with triarylaminium radical cations (NAr(3)(*+)). The bond dissociation free energy (BDFE) of the O-H bond in Ru(II)PhCOOH is calculated from pK(a) and E(1/2) measurements as 87 kcal mol(-1), making Ru(III)PhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 A separating the metal from the carboxylate basic site in Ru(III)PhCOO. Even with this separation, Ru(III)PhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol(-1), DeltaG(o)(rxn) = -21 kcal mol(-1)) by removal of an electron and a proton to form Ru(II)PhCOOH and TEMPO radical in a concerted proton-electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 +/- 0.1) x 10(5) M(-1) s(-1) with k(H)/k(D) = 2.1 +/- 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, Ru(III)(pydic)(tpy-COO(-)) (Ru(III)COO). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [(t)Bu(2)(OMe)ArOH] to Ru(III)PhCOO is several orders of magnitude slower than the analogous reaction with Ru(III)COO.

Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer

Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO(*) (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO(*) occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO(*) with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.