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from the corresponding Pb2-macrocycle. After controlled
potential electrolysis, both the one-electron and two-electron
reduced complexes were isolated, and structurally
(ESI, Fig. S8 and S9w) and spectroscopically characterized.
The electronic absorption spectrum of [LH*CoIICoI]3+ exhibits a
at low reduction potentials because the management of
protons and electrons is a critical step that needs to be
addressed if one wants to later redirect such reactants toward
other substrates. The macrocycle complexes were also found
to accommodate five redox states, including unusual mixed-
valence complexes of CoIICoI and CoIICoIII. Multi-metallic
scaffolds such as these are attractive complexes to explore
within the context of directing cooperative substrate binding
and subsequent multi-electron transfer reactions. Their ability
to support four reversible one-electron redox events across two
nearby metal centers underscores this latter point.
broad, low-energy band at 975 nm (e970 = 2704 L molÀ1cmÀ1
;
FWHM = 2196 cmÀ1), which is consistent with a Class II–III
system as for [LMeCoIICoI]+. The EPR spectrum of
[LH*CoIICoI]3+ in DMF glass at 77 K affords a similar
axial pattern.
Chemical reactivity studies showed that in the presence of a
proton source such as HBF4, the fully reduced complex
[LH*CoICoI]2+ first reacts to form the mixed-valence species,
[LH*CoIICoI]3+, which is further oxidized to form the parent
[LH*Co2]4+ (ESI, Fig. S10 and S11w) on a slower time scale
than the first oxidation, with B95% conversion and without
the appearance of any additional intermediates, even at À40 1C.
The step-wise oxidation would appear to disfavor an H2-
forming mechanism, where two Co–H centers react intra-
molecularly to release H2 and the corresponding [LH*Co2]4+
product. Intrigued by this result, we sought to probe the
reactivity of [LRCo2]n+ with protons by electrochemical methods.
Upon titration with a strong acid such as 2,6-dichloro-
This work was generously supported by BP and an NSF
Center for Chemical Innovation grant (Grant CHE-0802907).
We thank Dr Yunho Lee and Dr Sebastian Stoian for EPR
assistance, and Dr Bruce Brunschwig for helpful discussions.
Notes and references
1 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,
2006, 103, 15729.
2 For lead reviews see: D. L. DuBois and M. Rakowski DuBois, Acc.
Chem. Res., 2009, DOI: 10.1021/ar900110c; E. E. Benson,
C. P. Kubiak, A. J. Sathrum and J. M. Smieja, Chem. Soc. Rev.,
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2005, 249, 1518; J. C. Peters and M. P. Mehn, in Activation of
Small Molecules, Wiley-VCH, Weinheim, 2006, ch. 3.
3 X. Hu, B. M. Cossairt, B. S. Brunschwig, N. S. Lewis and
J. C. Peters, Chem. Commun., 2005, 4723; X. L. Hu,
B. S. Brunschwig and J. C. Peters, J. Am. Chem. Soc., 2007, 129,
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4 M. Razavet, V. Artero and M. Fontecave, Inorg. Chem., 2005, 44,
4786; C. Baffert, V. Artero and M. Fontecave, Inorg. Chem., 2007,
46, 1817.
5 S. Brooker, R. J. Kelly and P. G. Plieger, Chem. Commun., 1998,
1079.
6 These processes were assigned as electrochemically reversible based
on the invariance of the peak position with scan rate, as well as the
linear relationship between the peak current and the square root of
the scan rate over three orders of magnitude.
7 These processes were assigned as quasi-reversible based on the
variance of the peak position with scan rate.
8 C. Creutz, Prog. Inorg. Chem., 1983, 30, 1.
9 Spectra acquired at 4 K were similar in appearance, and feature a
small shift for the g tensors (2.29 and 2.04).
10 A. Bakac, M. E. Brynildson and J. H. Espenson, Inorg. Chem.,
1986, 25, 4108.
anilinium tetrafluoroborate,15
a cathodic shift in the
CoIICoI/CoICoI redox couple was observed along with an
increase in the peak current (Fig. 4; ESI Fig. S22 and S23w).
This behavior is consistent with a catalytic process in which an
electroactive species reacts at the electrode at a faster rate than
the scan rate of the electrochemical experiment. The intensity
of the catalytic wave for [LMeCo2]2+ increases dramatically
upon the addition of acid equivalents, and the current
response maintains a peak shape throughout the additions,
which suggests that the electrocatalytic reduction of protons is
limited by diffusion to the electrode surface.16 More detailed
studies are in progress.
In summary, a new series of bimetallic macrocycles has been
prepared that incorporate zwitterionic difluoroboryl linkages
into a pyridazine-templated construct. These were initially
targeted within the context of electrocatalytic proton reduction
11 F. Arena, C. Floriani and P. F. Zanazzi, J. Chem. Soc., Chem.
Commun., 1987, 183; T. L. Schull, L. Henley, J. R. Deschamps,
R. J. Butcher, D. P. Maher, C. A. Klug, K. Swider-Lyons,
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12 N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391; B. S. Brunschwig,
C. Creutz and N. Sutin, Chem. Soc. Rev., 2002, 31, 168.
13 M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 1968, 10, 247.
14 An alternative two-step protocol to generate diformyl pyridazine in
moderate yields was developed in which diformyl pyridazine is an
intermediate en route to the dioxime (see the ESIw).
15 pKa = 5.06 in MeCN, corresponding to a thermodynamic poten-
tial for the H+/H2 couple at À0.06 V. For pKa values, see:
I. Kosuke, in Acid–Base Dissociation Constants in Dipolar Aprotic
Solvents, Blackwell Scientific Publications, Oxford, 1990;
Fig. 4 (a) Cyclic voltammograms of [LHCo2]2+ (0.2 mM) in the
presence of increasing amounts of 2,6-dichloroanilinium tetrafluoro-
borate (bottom to top; the lowest fuchsia trace is the control with acid
only) in 0.1 M [nBu4N][ClO4] MeCN (0.2 mM): 0, 1.0, 2.5, 4.5, 7.0 and
9.9 mM, and (b) the same experiment with [LMeCo2]2+ (0.2 mM)
(bottom to top; the lowest fuchsia trace is the control with acid only):
0, 1.0, 2.5, 4.5, 7.0, 9.9 and 13.7 mM 2,6-dichloroanilinium tetrafluoro-
L. Soovali, I. Kaljurand, A. Kutt and I. Leito, Anal. Chim. Acta,
¨
2006, 566, 290; for a discussion of thermodynamic potentials of
¨
H
+/H2 redox couples, see: G. A. N. Felton, R. S. Glass,
D. L. Lichtenberger and D. H. Evans, Inorg. Chem., 2006, 45,
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16 J. M. Save
´
ant and K. B. Su, J. Electroanal. Chem., 1984, 171, 341;
C. P. Andrieux, C. Blocman, J. M. Dumas-Bouchiat, F. M’Halla
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´
borate. Scan rate = 100 mV sÀ1
.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6729–6731 | 6731