Facile concerted proton-electron transfers in a ruthenium terpyridine-4′-carboxylate complex with a long distance between the redox and basic sites

Article abstract of DOI:10.1021/ja801672w

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, Ru^II(dipic)(tpyCOOH) (Ru^IICOOH), 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, Ru^IICOO^-, shows a distance of 6.9 A between the metal and basic sites. The Ru(III) complex (Ru^IIICOO) has been isolated by one-electron oxidation of Ru^IICOO^- with triarylaminium radical cations (NAr₃·⁺). Ru^IIICOO has a bond dissociation free energy (BDFE) of 81 ± 1 kcal mol^-1, from pK_a 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 Ru^IICOOH, with a second-order rate constant of (2.3 ± 0.2) × 10⁴ M^-1 s^-1 and a k_H/k_D of 7.7 ± 1.2. Thermochemical analysis suggests a concerted proton-electron transfer (CPET) mechanism for this reaction, despite the 6.9 A distance between the redox-active Ru and the H⁺-accepting oxygen. Ru^IIICOO 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. Copyright

Full text of DOI:10.1021/ja801672w

Published on Web 05/14/2008
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,‡ and James M. Mayer*,†
Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700, and Department of Chemistry
and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093
Received March 7, 2008; E-mail: mayer@chem.washington.edu
Scheme 1
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 f 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 is three 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
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 seven
paramagnetically shifted resonances (2:1:2:2:2:2:2) from δ 15 to
-38 ppm.10 A typical in situ generated solution of RuIIICOO
decomposes ∼16% over 5 h under a 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 thermo-
chemical cycle in Scheme 1 (BDFE ) 23.1E1/2 + 1.37pKa + CG5d).
The pKa of RuIICOOH 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 bond dissociation enthalpy of 86
kcal mol-1 (ignoring any entropic effects),5d making RuIIICOO
among the thermodynamically strongest hydrogen atom acceptors
that have been isolated.10,12
The protonated ruthenium(II) complex, RuII(pydic)(tpyCOOH)
(RuIICOOH, Scheme 1), is prepared as a dark purple solid from
[(η6-cymene)RuCl(µ-Cl)]2, sodium 2,2′:6′,2′′-terpyridine-4′-car-
boxylate (tpyCOONa), followed by disodium pyridine-2,6-dicar-
boxylate (Na2pydic) and then aqueous HCl, in a modification of a
related procedure.9,10 Treating a DMF solution of RuIICOOH with
nBu4NOH (1 M in MeOH) gives the deprotonated Ru(II) complex,
nBu4N[RuII(pydic)(tpyCOO)] (RuIICOO-). RuIICOO- and RuII
-
RuIIICOO reacts rapidly with excess 2,4,6-tri-tert-butylphenol
(tBu3ArOH) in MeCN to form RuIICOOH and tBu3ArO•, both in
77 ( 10% yield based on UV-visible spectra (eq 1).10 This reaction
1
COOH have been characterized by H 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.
Cyclic voltammograms of RuIICOO- in MeCN show a chemi-
is a net transfer of H•. On the basis of the BDFEs of RuIICOOH
and tBu3ArOH (77 ( 1 kcal mol-1),13 eq 1 has ∆G°1H ) -4 kcal
mol-1. The reaction has been monitored under pseudo first-order
conditions, following the growth of the strong absorbance of
RuIICOOH (λmax ) 527 nm). Stopped-flow rapid-scanning UV-vis
spectrophotometry gives a second-order rate constant of k1H ) (2.3
+/0
cally reversible oxidation at E1/2 ) 0.047 ( 0.02 V vs FeCp2
.
Chemical oxidation of RuIICOO- (λmax ) 520 nm, ε ) 9400 (
400 M-1 cm-1) with [(p-BrC6H4)3N•+][B(C6F5)4- 10 yields zwit-
]
terionic RuIII(pydic)(tpyCOO) (RuIIICOO [the carboxylate is
† University of Washington.
‡ University of California, San Diego.
( 0.2) × 104 M-1 s-1 (Figure 1; ∆Gq ) 11.5 ( 0.1 kcal
1H
9
7210 J. AM. CHEM. SOC. 2008, 130, 7210–7211
10.1021/ja801672w CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
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, including cytochrome P450 compound I and iron biimidazo-
lines, that have traditionally been termed hydrogen atom transfer
(HAT).1d,5–7,10,18 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, to further probe the effect
of redox center/basic site communication on CPET (HAT) rate constants.
t
Figure 1. Kinetic data for RuIIICOO + Bu3ArOH in MeCN (eq 1): (a)
optical spectra over 1 s showing the appearance of RuIICOOH; (b) plot of
first-order kobs versus [tBu3ArOH] ((, k1H) and versus [tBu3ArOD] (b, k1D).
mol-1).10,13 Eyring analysis (288-323 K) gives ∆Hq ) 3.5 (
1H
Acknowledgment. We gratefully acknowledge support from the
U.S. National Institutes of Health (GM50422) and the University of
Washington.
1.4 kcal mol-1 and ∆Sq ) -27 ( 5 cal mol-1 K-1 at 298 K.
1H
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
Supporting Information Available: Details for the syntheses, kinetic
measurements, and crystal structure, and additional analysis. This material
is available free of charge via the Internet at http://pubs.acs.org.
There are three possible mechanistic pathways for reaction 1.
The H+ and e- could transfer from Bu3ArOH to RuIIICOO in a
t
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 RuIIICOOH+
+ tBu3ArO-) are ruled out on thermochemical grounds: the ground-
state free energy changes of the initial steps, ∆G°ET ) +26.1 (
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,
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1H
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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 hydroxyl-
amine TEMPOH (BDFE ) 66.5 ( 0.5 kcal mol-1 5d,6,7
to form
)
the stable nitroxyl radical TEMPO• and RuIICOOH, as determined
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TEMPOH
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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).
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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 versus deprotonated RuIICOO-: ∆E1/2
)
0.13 V in DMF (used because of low solubility in MeCN).16
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Ru imidazole and biimidazoline complexes, which each have three
bonds and π-conjugation between the metal and basic site.17
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>
1.1 V.1d,12a The small ∆E1/2 for RuCOO(H) could be a result of
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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
(∆E1/2) indicates that communication between the Ru and the carboxylate
(16) This ∆E1/2 implies that the pKa(RuII)-pKa(RuIII) ) 2.2 units.1c
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