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. Author manuscript; available in PMC: 2009 Jun 11.
Published in final edited form as: J Am Chem Soc. 2008 May 14;130(23):7210–7211. doi: 10.1021/ja801672w

Facile Concerted Proton-Electron Transfers in a Ruthenium Terpyridine-4′-Carboxylate Complex with a Long Distance Between the Redox and Basic Sites

Virginia W Manner †, Antonio G DiPasquale §, James M Mayer †,*
PMCID: PMC2562563  NIHMSID: NIHMS60250  PMID: 18479096

Abstract

We have designed and prepared ruthenium complexes with terpyridine-4′-carboxylate (tpyCOO) ligands, in which there are six bonds between the redox-active Ru and the basic carboxylate. The protonated Ru(II) complex, RuII(dipic)(tpyCOOH) (RuIICOOH), is prepared in one-pot from [(p-cymene)RuCl2]2, tpyCOONa, and then sodium pyridine-2,6-dicarboxylate [Na(dipic)]. A crystal structure of the deprotonated Ru(II) complex, RuIICOO−, shows a distance of 6.9Å between the metal and basic sites. The Ru(III) complex (RuIIICOO) has been isolated by one-electron oxidation of RuIICOO− with triarylaminium radical cations (NAr3·+). RuIIICOO has a bond dissociation free energy (BDFE) of 81 ± 1 kcal mol−1, from pKa and E1/2 measurements. It oxidizes 2,4,6-tri-tert-butylphenol (BDFE = 77 ± 1 kcal mol−1) by removal of e− and H+ (H·) to form 2,4,6-tri-tert-butylphenoxyl radical and RuIICOOH, with a second order rate constant of (2.3 ± 0.2) × 104 M−1 s−1 and a kH/kD of 7.7 ± 1.2. Thermochemical analysis suggests a concerted proton-electron transfer (CPET) mechanism for this reaction, despite the 6.9 Å distance between the redox-active Ru and the H+-accepting oxygen. RuIIICOO also oxidizes the hydroxylamine TEMPOH to the stable free radical TEMPO, and xanthene to bixanthyl. These reactions appear to be similar to processes that have been previously termed hydrogen atom transfer.

Reactions that involve transfer of both a proton and an electron are important in a wide range of chemical and biochemical processes.1 When the two particles transfer in a single step from a donor to an acceptor, XH + Y → X + YH, such reactions are termed concerted proton-electron transfer (CPET) or in certain cases, hydrogen atom transfer (HAT); the exact definition of these terms is a matter of continuing discussion.2 This reactivity has been observed even when the proton and electron-accepting (or donating) sites are separated, as in reactions of phenols, ascorbate, and many transition metal species.1 Within this framework, a CPET process has four relevant distances: the distances traveled by the electron and by the proton, and the separations of the H+ and e− in the donor and in the acceptor. CPET reactions in which at least one of these distances is long have been implicated in photosystem II and class 1 ribonucleotide reductases,3 and the distance dependence of pure electron transfer has long been studied.4 We have reported HAT and CPET reactions of iron and ruthenium complexes with imidazole or related ligands, in which H+ transfers to or from a nitrogen that s 3 bonds and ~4 Å distant from the metal center where the redox change primarily occurs.5–7 Even with this separation, the rate constants typically correlate well with those for related organic processes. 1c,e This report describes studies of ruthenium terpyridine-4′-carboxylate complexes in which the basic site is ~6.9 Å removed from the metal center, to probe how this distance affects CPET reactivity.8

The protonated ruthenium(n) complex, RuII(pydic)(tpyCOOH) (RuIICOOH, Scheme 1), is prepared as a dark purple solid from [(η.6-cymene)RuCl(μ-Cl)]2, sodium 2,2′:6′,2″-terpyridine-4′-carboxylate (tpyCOONa), followed by disodium pyridine-2,6-dicarboxylate (Na2pydic) and then aqueous HC1, in a modification of a related procedure.9,10 Treating a DMF solution of RuIICOOH with nBu4NOH (1M in MeOH) gives the deprotonated Ru(II) complex, nBu4N[RuII(pydic)(tpyCOO)] (RuIICOO−). RuIICOO− and RuIICOOH have been characterized by 1H NMR, ESI/MS, UV-vis spectroscopy, elemental analyses, and an X-ray crystal structure of RuIICOO− (Scheme 1, nBu4N+ not shown).10 The structure shows an anion with the expected distorted octahedral geometry, containing two planar, meridonal tridentate ligands. The carboxylate oxygen atoms (the site of proton binding) are 6.9 Å from the ruthenium(II) center and the carboxylate (CO2) plane is rotated 53° from its connected pyridine ring.

Scheme 1.

Scheme 1

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Cyclic voltammograms of RuIICOO− in MeCN show a chemically reversible oxidation at E1/2 = 0.047 ± 0.02 V vs. FeCp2+/0. Chemical oxidation of RuIICOO− (λmax 520 nm, ε 9400 ± 400 M−1 cm−1) with [(p-BrC6H4)3]N·+][B(C6F5)4−]10 yields zwitterionic RuIII(pydic)(tpyCOO) (RuIIICOO [the carboxylate is anionic but the overall complex is neutral]) (λmax 435 nm, ε 3400 ± 700 M−1 cm−1). Addition of decamethylferrocene as a reductant regenerates RuIICOO− in 77% yield, along with 23% RuIICOOH by UV-visible spectroscopy. RuIIICOO has been isolated from RuIICOO− plus [(p-tol)3N·+]PF6− and has been fully characterized. Its 1H NMR spectrum in CD3CN shows 7 paramagnetically shifted resonances (2:1:2:2:2:2:2) from δ15 to −38 ppm.10 A typical in situ generated solution of RuIIICOO decomposes ~16% over 5 hours under an N2 atmosphere.

RuIICOOH and RuIIICOO differ by H+ + e− (a hydrogen atom). The O-H bond dissociation free energy (BDFE) for RuIICOOH in MeCN is 81 ± 1 kcal mol−1, using the thermochemical cycle in Scheme 1 (BDFE = 23.1E1/2 + 1.37pKa + CG5d). The pKa of RuII-COOH in MeCN is 18.5 ± 0.1, determined by titrating RuIICOO− with benzoic acid (pKa = 20.711). This is a large BDFE, corresponding to a BDE of 86 kcal mol−1 (ignoring any entropic effects5d), making RuIIICOO among the thermodynamically strongest hydrogen atom acceptors that have been isolated.12

graphic file with name nihms60250e1.jpg (1)

RuIIICOO reacts rapidly with excess 2,4,6-tri-t-butylphenol (tBu3ArOH) in MeCN to form RuIICOOH and tBu3ArO·, both in 77 ± 10% yield based on UV-Visible spectra (eq 1).10 This reaction is a net transfer of H·. Based on the BDFEs of RuIICOOH and tBu3ArOH (77 ± 1 kcal mol−1),13 eq 1 has ΔG°1H = −4 kcalmol−1. The reaction has been monitored under pseudo 1st-order conditions, following the growth of the strong absorbance of RuIICOOH (λmax 527 nm). Stopped-flow rapid-scanning UV-vis spectrophotometry gives a derived second-order rate constant of k1H = (2.3 ± 0.2) × 104 M−1 s−1 (Figure 1; ΔG‡1H = 11.5 ± 0.1 kcal mol−1).10,13 Eyring analysis (288–323 K) gives ΔH‡1H = 3.5 ± 1.4 kcal mol−1 and ΔS‡1H = −27 ± 5 cal mol−1 K−1 at 298 K. The analogous reaction with tBu3ArOD is considerably slower, k1D= (3.0 ± 0.4) × 103 M−1 S−1 (Figure 1), indicating k1H/k1D= 7.7 ± 1.2.14

Figure 1.

Figure 1

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Kinetic data for RuIIICOO + tBu3ArOH in MeCN (eq 1): (a) Optical spectra over 1 sec showing the appearance of RuIICOOH; (b) Plot of 1st-order kobs vs. [tBu3ArOH] (♦, k1H and vs. [tBu3ArOD] (•, k1D).

There are three possible mechanistic pathways for reaction (1). The H+ and e− could transfer from tBu3ArOH to RuIIICOO in a single kinetic step (CPET) or by pathways with two separate kinetic steps. Initial electron transfer (to form the intermediates RuIICOO− + tBu3ArOH+) and initial proton transfer (to form RuIICOOH+ + tBu3ArO−) are ruled out on thermochemical grounds: the ground state free energy changes of the initial steps, ΔG°ET = +26.1 ± 0.7 kcal mol−1 and ΔG°PT = +16.1 ± 0.8 kcal mol− 1,10,13 are larger than the observed free energy barrier, ΔG‡1H = 11.5 ± 0.1 kcal mol−1 (and ΔG† ≥ ΔG°). Thus reaction 1 proceeds via concerted transfer of H+ to the carboxylate and e− to the ruthenium, in a single kinetic step. The primary k1H/k1D of 7.7 is also most consistent with such a CPET pathway, as kH/kD would likely be close to 1 for ET or for PT between O atoms.

RuIIICOO also abstracts a hydrogen atom from the hydroxylamine TEMPOH (BDFE = 66.5 ± 0.5 kcal mol−1)5d,6,7 to form the stable nitroxyl radical TEMPOH· and RuIICOOH, as determined by NMR and UV-visible spectroscopies. Stopped-flow kinetics experiments, as above, yielded kTEMPOH = (2.0 ± 0.6) × 105 M−1 s−1 (ΔG+TEMPOH = 10.2 ± 0.2 kcal mol−1). Thermochemical arguments analogous to those given above indicate a similar concerted pathway for this reaction.10 Preliminary experiments suggest that RuIIICOO also can remove H· from the weak C-H bond in xanthene (BDE = 75.5 ± 2 kcal mol−1),15 to form RuIICOOH and bixanthyl (by UV-vis and GC/MS).

The thermochemical affinity of RuIIICOO for H· will only lead to CPET reactivity if there is communication between the redox Ru and the basic oxygens 6.9 Å away. One crude measure of this communication or coupling is the thermochemical interaction between these sites, as indicated by the difference between the E1/2 values for RuIICOOH vs deprotonated RuIICOO−: ΔE1/2 = 0.13 V in DMF (due to low solubility in MeCN).16 Analogous ΔE1/2 values range from 0.3 to 0.5 V for Fe, Co, and Ru imidazole and biimidazoline complexes, which each have three bonds and n-conjugation between the metal and basic site.17 Additionally, there are Ru oxo/hydroxo complexes with ΔE1/2 > 1.1 V.1d,12a The small ΔE1/2 for RuCOO(H) could be a result of the long distance between the metal and carboxyl site, or to decreased resonance stabilization of the carboxylate anion in RuIIICOO relative to, for instance, an imidazolate ligand. Still, even though the thermochemical data imply less communication between the redox and basic sites, CPET reactivity is still facile.

In conclusion, we have designed and isolated three new complexes in a system with six bonds and a distance of 6.9 Å between the redox-active and basic sites. The small shift of the redox potential on protonation (ΔE1/2) indicates that communication between the Ru and the carboxylate group is decreased relative to other systems. Despite the separation and decreased communication, RuIIICOO readily removes a hydrogen atom from tBu3ArOH and TEMPOH. Kinetic and thermochemical data indicate that this reaction proceeds by concerted transfer of H+ and e− (CPET). These reactions appear to be similar to other H+/e− transfers to metal complexes, from cytochrome P450 compound I to iron biimidazolines, that have traditionally been termed hydrogen atom transfer (HAT).1d,110,18 This study is the first to show that the separation in the acceptor can be as large as 6.9 Å without preventing such reactivity. Modifications of the terpyridine ligand are currently underway to increase the distance between the Ru and the basic site still farther, as further probes of the effect of redox center/basic site communication on CPET (HAT) rate constants.

Supplementary Material

SI1

Supporting Information Available: Details for the syntheses, kinetic measurements, and crystal structure, and additional analysis. This material is available free of charge via http://pubs.acs.org.

SI2

Acknowledgments

We gratefully acknowledge support from the U.S. National Institutes of Health (GM50422) and the University of Washington.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

SI1

Supporting Information Available: Details for the syntheses, kinetic measurements, and crystal structure, and additional analysis. This material is available free of charge via http://pubs.acs.org.

NIHMS60250-supplement-SI1.cif (24.3KB, cif)
SI2
NIHMS60250-supplement-SI2.pdf (1MB, pdf)

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