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

Article abstract of DOI:10.1021/ja902942g

(Figure Presented) 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

Full text of DOI:10.1021/ja902942g

Published on Web 07/01/2009
Concerted Proton-Electron Transfer in a Ruthenium Terpyridyl-Benzoate
System with a Large Separation between the Redox and Basic Sites
Virginia W. Manner and James M. Mayer*
Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700
Received April 13, 2009; E-mail: mayer@chem.washington.edu
Scheme 1
Coupling electron transfer to proton transfer is key to a wide
range of chemical and biochemical processes, such as converting
solar energy to chemical fuels.1 While many of the fundamentals
of electron transfer (ET) are well understood, the principles of
proton-coupled electron transfer (PCET) are still being developed.
The effects of increasing the distance between reaction centers has
been much studied for ET,2 and we have started to explore PCET
systems in which the electron- and proton-accepting sites are
increasingly separated.3 PCET processes with large separations
appear to be important in a number of biological systems, such as
ribonucleotide reductases and photosystem II.4 They may also be
involved in charge injection into oxide semiconductors from
ruthenium polypyridyl-carboxylate complexes.5 In our previously
reported ruthenium terpyridine-4′-carboxylate complex RuIIICOO
(Scheme 1), the Ru is six bonds and 6.9 Å removed from the basic
carboxylate oxygen atoms.3 Despite this separation, the reported
reactions occur with H+ and e- transferring in the same kinetic
step, by concerted proton-electron transfer (CPET).1,3,6-8 In this
report, the distance between the metal and basic sites is extended
further, by inserting a phenyl spacer between the terpyridine and
the carboxylate. Reactivity is contrasted between the two systems,
which are the first studies of long, well-defined separations.
The new protonated Ru(II) complex, RuII(pydic)(tpy-PhCOOH)
(RuIIPhCOOH), was prepared from [(η6-cymene)RuCl(µ-Cl)]2 and
the known ligands, 4′-(4-carboxyphenyl)-2,2′:6′,2′′-terpyridine
([Na+]tpy-PhCOO-) and pyridine-2,6-dicarboxylate (Na2pydic).3,9
Deprotonation of this complex with nBu4NOH in DMF yields
nBu4N[RuII(pydic)(tpy-PhCOO)] (RuIIPhCOO-). Both compounds
have been characterized by 1H NMR and UV-visible spec-
troscopies, electrochemistry, ESI-MS, and elemental analyses. Based
on the structure of the related homoleptic complex RuII(tpy-
PhCOO-)2, the distance between the Ru and the carboxylate
oxygens is 11.2 ( 0.1 Å.10 The optical spectra of RuIIPhCOOH
and RuIIPhCOO- in the visible region are very similar, as shown
in Figure 1a.9 The differences in the spectra are small, but they are
consistent and reversible upon addition of acid and base.9 Titration
of RuIIPhCOO- with benzoic acid (pKa ) 20.7 ( 0.111) in MeCN
gives a pKa for RuIIPhCOOH of 20.5 ( 0.2. Cyclic voltammo-
grams of RuIIPhCOOH and RuIIPhCOO- in DMF show almost
identical chemically reversible oxidations, with E1/2 ) 0.081 (
0.006 V and 0.083 ( 0.019 V vs FeCp2+/0, respectively.
A formal O-H bond dissociation free energy (BDFE) can be
defined for RuIIPhCOOH, despite the 11.2 Å distance between
Ru and O. ∆G° for RuIIPhCOOH f RuIIIPhCOO + H• in MeCN
13
) 23.1E1/2 + 1.37pKa + CG ) 87 ( 1 kcal mol-1, using the pKa
given above and E1/2 ) 0.17 ( 0.03 V vs FeCp2 for RuIIPh-
+/0
COO- in 90/10 MeCN/DMF.9 The BDFE is 6 kcal mol-1 higher
than that found for the complex without the phenyl spacer,
RuIICOOH (81 ( 1 kcal mol-1). Thus, RuIIIPhCOO is a strong
hydrogen atom acceptor,3 which may partly explain its instability.
RuIIIPhCOO reacts with the hydroxylamine TEMPOH within
seconds to form the nitroxyl radical TEMPO and predominantly
RuIIPhCOOH, (eq 1), as indicated by H NMR and UV-visible
1
spectra.9 The protonated RuIIPhCOOH product is implicated by
the peak at 400 nm in the optical spectrum, which shifts to 394 nm
upon addition of base (Figure 1a).9,14 The reaction is quite downhill,
∆G°1 ) -21 kcal mol-1, based on BDFE(TEMPOH) ) 66.5 kcal
15
mol-1
.
Stopped flow rapid-scanning spectrophotometry under
pseudo first-order conditions of excess TEMPOH shows a ca. 80%
Oxidation of RuIIPhCOO- by [(p-tol)3N•+]PF6- in MeCN gives
the neutral, deprotonated Ru(III) carboxylate complex, RuIII(py-
dic)(tpy-PhCOO) (RuIIIPhCOO, Scheme 1). This zwitterionic
complex can be precipitated as a brown solid with CH2Cl2 but it is
difficult to handle without some decomposition in solution,9 so it
is more conveniently generated in situ from RuIIPhCOO- plus [(p-
BrC6H4)3N•+][B(C6F5)4-] in MeCN. The 1H NMR spectrum of
RuIIIPhCOO shows all nine paramagnetic peaks, and the optical
spectrum has a shoulder at 435 nm (ε ≈ 9000 M-1 cm-1). Reduction
of in situ generated RuIIIPhCOO with (C5Me5)2Fe rapidly regener-
ates Ru(II), with a yield of ∼95% based on the absorption at 531
nm, along with other product(s). Addition of base to this solution
causes a shifting of the 400 nm peak, suggesting that the product
mixture is ca. 70/30 RuIIPhCOO-/RuIIPhCOOH.9 The data
indicate that RuIIIPhCOO is predominantly deprotonated in solution
but may contain some RuIIIPhCOOH+.12
Figure 1. (a) UV-vis spectra [λ/nm (ꢀ/M-1 cm-1)] for RuIIPhCOOH
[400 (20 000), 533 (15 000)] and RuIIPhCOO- [394 (20 000), 531
(15 000)], each 0.030 mM in MeCN; ꢀ’s all (1000.9 (b) Spectra for
RuIIIPhCOO (0.026 mM) + TEMPOH (0.20 mM) in MeCN over 0.5 s,
showing the growth of RuIIPhCOOH.
9
9874 J. AM. CHEM. SOC. 2009, 131, 9874–9875
10.1021/ja902942g CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Rate Constant ka and ∆G°b for RuIII(Ph)COO + X-H
In conclusion, we have designed and prepared a system with a
well-defined separation of 10 bonds and 11.2 Å between the metal
(Ru) and basic (carboxylate) sites. At this distance, there is almost
no interaction between the redox and basic sites, as indicated by
thermodynamic and spectroscopic measurements. Despite this lack
of communication, the reaction of RuIIIPhCOO with TEMPOH
(eq 1) occurs by concerted transfer of H+ and e- (CPET). However,
the more exoergic reactions of RuIIIPhCOO proceed more slowly
than those of RuIIICOO. Thus the increased distance and decreased
communication do appear to affect the reaction rates, as will be
discussed in a future report focused on the dependence of CPET
rate constants on driving force20 and on the position and interaction
of redox and acid/base sites.
RuIIIPhCOO + X-H
RuIIICOO + X-H
X-H
∆G°
-21
k
∆G°
-15
k
c
(2.0 ( 0.6) × 105
TEMPOH
ArOH
(1.1 ( 0.1) × 105
d
e
e
(1.0 ( 0.1) × 103
(1.5 ( 0.6) × 106
-13
-7
a In M-1 s-1
.
b In kcal mol-1
.
c Reference 3. 2,6-tBu2(4-MeO)ArOH.
d
e Reference 19.
yield of RuIIPhCOOH (Figure 1b). It is possible that the low yield
is due to the presence of protonated Ru(III) in solution, which reacts
more slowly with TEMPOH.14 The pseudo-first-order rate constants
vary linearly with [TEMPOH] (Figure S109) indicating a simple
second-order rate law, with k1H ) (1.1 ( 0.1) × 105 M-1 s-1 and
9
∆G1 ) 10.6 ( 0.1 kcal mol-1. Using rate data from 17-52 °C,
‡
Acknowledgment. We gratefully acknowledge support from the
U.S. National Institutes of Health (GM050422) and the University
of Washington.
the activation parameters are ∆H1 ) 6.8 ( 1.1 kcal mol-1 and
‡
∆S1‡ ) -13 ( 4 cal mol-1 K-1. The reaction with TEMPOD gives
k1D ) (5.6 ( 0.3) × 104 M-1 s-1. The small primary isotope effect,
k1H/k1D ) 2.1 ( 0.2, indicates that the proton is transferred in the
rate limiting step.16
Supporting Information Available: Details for the synthesis, kinetic
measurements, and additional analysis. This material is available free
of charge via the Internet at http://pubs.acs.org.
Reaction 1 could occur by (i) initial proton transfer (PT) to form
RuIIIPhCOOH+ and TEMPO- followed by electron transfer (ET),
(ii) ET followed by PT, or (iii) CPET with no intermediates. The
References
pKa’s of RuIIIPhCOOH+ and TEMPOH are 20.5 and 39, respec-
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9
tively, so the initial step in path (i) has ∆G°PT ) 25.3 kcal mol-1
.
)
Similarly, using the reduction potentials of the reactants, ∆G°ET
12.5 kcal mol-1. Since these are the minimum barriers for initial
PT and ET (∆G‡ > ∆G°), and they are larger than the observed
∆G1 (10.6 ( 0.1 kcal mol-1), neither of the stepwise pathways
‡
can be occurring. Thus even with the large separation between Ru
and COO-, the reaction still occurs by CPET. This is also indicated
by the primary kinetic isotope effect (KIE) of 2.1.
(4) (a) Meyer, T. J.; Huynh, M. H. V.; Thorp, H. Angew. Chem., Int. Ed. 2007,
46, 5284–5304. (b) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y.
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An important issue in a PCET reagent is the amount of interaction
or communication between the redox and basic sites.3 One measure
of this is the thermodynamic coupling, in this system how much
the pKa shifts depending on the Ru oxidation state (∆pKa), and
equivalently1d how much the E° shifts with the protonation state
(∆E1/2). Cyclic voltammograms of RuIIPhCOO-and RuIIPhCOOH
in DMF are the same within error (∆E1/2 ) 2 ( 20 mV), indicating
that there is essentially no communication between the redox and
basic sites. This is also indicated by the close similarity of pKa’s
of RuIIPhCOOH and benzoic acid. For comparison, ∆E1/2 for
RuIICOO(H) (with no Ph spacer) is 0.13 V (other systems have
larger values3). CPET may be the favored mechanism for reaction
1 because of the large ∆E1/2 for TEMPO(H), ca. 2.6 V.9 In a related
intermetal PCET system without this large ∆E1/2, concerted transfer
was not observed.17
RuIIIPhCOO also rapidly oxidizes 2,6-di-tert-butyl-4-methoxy-
phenol (ArOH) to give the aryloxyl radical ArO•18 and RuIIPh-
COOH.9 Pseudo first-order kinetic studies give kArOH ) (1.0 (
0.1) × 103 M-1 s-1 and kH/kD ) 2.6 ( 0.4.9,16 In this case,
thermochemical analyses do not rule out initial PT or ET mecha-
nisms since ∆G°PT and ∆G°ET (10.8 ( 0.5 and 11.8 ( 0.8 kcal
mol-1)9 are both lower than the observed ∆G‡ ) 13.4 ( 0.1 kcal
mol-1. Therefore kArOH is an upper limit for the CPET rate constant.
The mechanism is still likely to be CPET because ∆G°PT and ∆G°ET
are much larger than ∆G°CPET and because the KIE of 2.6 is larger
than would be expected for ET or for PT between oxygen atoms.
Table 1 compares the rate constants and ∆G° values for the
reactions of RuIIIPhCOO vs RuIIICOO. For the reactions with
TEMPOH, the rate constants are within a factor of 2, even though
CPET to RuIIIPhCOO is 6 kcal mol-1 more exoergic.3 For
oxidation of ArOH, the RuIIIPhCOO rate constant is a thousand
times slower despite the 6 kcal mol-1 larger driving force. Thus
the larger driving force for the reactions of RuIIIPhCOO is offset
by the decreased communication and longer distance.
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(6) The term CPET was coined by: Costentin, C.; Evans, D. H.; Robert, M.;
Save´ant, J.-M.; Singh, P. S. J. Am. Chem. Soc. 2005, 127, 12490–12491.
(7) Reactions of the form XH + Y f X + YH are also sometimes called
hydrogen atom transfer (HAT); see ref 3 and references therein.
(8) Another type of PCET is ET modulated by a hydrogen-bonded interface,
e.g., with 4-carboxybipyridine-Ru complexes: (a) Kirby, J. P.; Roberts, J. A.;
Nocera, D. G. J. Am. Chem. Soc. 1997, 119, 9230–9236. (b) Reference 5a.
(9) Full details are given in the Supporting Information.
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Schaffner, S.; Schaper, F.; Batten, S. R. Dalton Trans. 2007, 4323–32. (b)
Ru · · · O distances in both RuIICOO- 3 and RuII(tpy-COO-)210a are 6.9 Å.
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(12) In our experience, it is difficult to avoid the formation of trace acid when
working with strong oxidants in organic solvents.
(13) CG ) 54.9 kcal mol-1: (a) Tilset, M. In Electron Transfer in Chemistry 2;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; pp 677-713. (b) Mader,
E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem. Soc. 2007, 129, 5153–
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(14) Quantitative determination of the protonation state of the product is difficult
(Figure 1) particularly since Ru(II) is formed in only 80% yield. The
remaining 20% is likely to be RuIIIPhCOOH+ which may be present in
the reactant (see above). Independent experiments show that in situ
generated RuIIIPhCOOH+ reacts with TEMPOH more slowly, to form a
product other than RuIIPhCOOH.9
(15) (a) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J. A.;
Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 4335–4345. (b) Reference 13b.
(16) Carpenter, B. K. Determination of Organic Reaction Mechanisms; John
Wiley & Sons, Inc.: New York, 1984.
(17) Lebeau, E. L.; Binstead, R. A.; Meyer, T. J. J. Am. Chem. Soc. 2001, 123,
10535–10544.
(18) (a) Altwicker, E. R. Chem. ReV. 1967, 67, 475–531. (b) Waidmann, C. R.;
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(19) (a) BDFE(ArOH) ) 73.7 kcal mol-1 = BDE18b- T∆S°(H•).13b (b) Lucarini,
M.; Pedrielli, P.; Pedulli, G. F. J. Org. Chem. 1996, 61, 9259–9263.
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