Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism

Article abstract of DOI:10.1002/chem.201701783

The activity of the water reduction catalyst [Co^III(L¹)(pyr)₂]PF₆ (1), where (L¹)^2? is a bis-amido pyridine ligand and pyr is pyrrolidine, is investigated. Catalyst 1 has an overpotential o

Full text of DOI:10.1002/chem.201701783

A Journal of
Accepted Article
Title: Deactivation of a Cobalt Catalyst for Water Reduction via
Valence Tautomerism
Authors: Claudio N. Verani, Habib Baydoun, Shivnath Mazumder, and
H. Bernhard Schlegel
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Deactivation of a Cobalt Catalyst for Water Reduction via Valence
Tautomerism
Habib Baydoun, Shivnath Mazumder, H. Bernhard Schlegel and Cláudio N. Verani*
Abstract: The activity of the water reduction catalyst
[CoIII(L1)(pyr)2]PF6 (1), where (L1)-2 is a bis-amido pyridine ligand
and pyr is pyrrolidine, is investigated. Catalyst 1 has an
overpotential of 0.54 V and a high observed TOF of 23 min-1,
albeit for a relatively short time. Considering the significant
activity of 1 and aiming to improve catalyst design, a detailed
structural and electronic study is performed to understand the
mechanisms of deactivation. Experimental and theoretical
evidence support that the metal-reduced [CoI(L1)]– is in
tautomeric equilibrium with the ligand-reduced [CoII(L1●)]–
species. While [CoI(L1)]– favors formation of a CoIII–H– relevant
for catalysis, the [CoII(L1●)]– species leads to ligand protonation,
structural distortions and, ultimatley, catalyst deactivation.
deactivation.
In order to evaluate this hypothesis we examined the
electronic and redox structure of the pseudo-octahedral
[CoIIIL1(pyr)2]PF6 (1) complex, where (L1)2- is the doubly
deprotonated form of a bis-amido pyridine ligand and pyr
denotes axially coordinated pyrrolidines, as shown in Scheme 1.
Complex 1 was synthesized by adapting reported procedures,[6]
where the ligand was treated under aerobic conditions with 1
equiv. of Co(OAc)2•4H2O in presence of pyrrolidine using
methanol as the solvent. The formation of a microcrystalline
precipitate was induced by the addition of NH4PF6. Complex 1
1
was thoroughly characterized using H–NMR, FTIR, ESI-MS(+),
and elemental analysis (See Experimental Section for details),
as well as X-ray crystallography (vide infra). As it will be
discussed, this species is capable of robust water reduction
followed by rapid deactivation.
Substantial efforts have been directed towards the
development of molecular catalysts for water reduction based on
abundant and affordable 3d transition metals.[1] Such catalysts
must withstand drastic electronic and structural changes from
high to low redox states required for hydride formation that
precedes H2 evolution. To this end, cobalt complexes have been
extensively studied because of the energetically affordable
Scheme 1: Synthetic scheme of catalyst 1.
stepwise conversions from 3d6 CoIII to 3d8 CoI and back to CoIII–
[1b, 1c, 1g, 1j, 1k, 2]
H– and CoII–H– hydride species.
As such,
mechanistic understanding of catalytic pathways —including
those of deactivation— becomes a necessary condition to the
development of robust catalysts.
The cyclic voltammogram (CV) of 1 was taken in CH3CN
and shows five independent redox processes (Figure 1). The
processes at E1/2 = 1.34 VNHE (ΔE = 0.10 V, |Ipa/Ipc| = 1.08) and
Epa = 1.94 VNHE are assigned as amido to amidyl radical
oxidations[7] (for potentials vs. Ag/AgCl and Fc+/Fc see Table
S1).[8] The process at Epc = -0.32 VNHE is assigned to the
CoIII/CoII couple.[6b, 9] The process at -1.08 VNHE (ΔEp = 0.16 V,
|Ipa/Ipc| = 0.84) is tentatively assigned to a CoII/CoI couple, while
the third process at -1.79 VNHE (ΔE = 0.11 V) is attributed to a
pyridine-based reduction.
The CV of 1 in phosphate buffer (1 mol/L, pH 7, Figure 2)
shows a catalytic wave at -0.95 VNHE in presence of 1 with
concurent evolution of gas at the surface of the electrode. This
corresponds to an overpotential of 0.54 V. Moreover, the onset
potential of -0.95 VNHE closely resembles that of the CoII/CoI
Our group has studied the mechanisms of several proton
and water reduction cobalt catalysts, including some phenolate-
rich CoIII [N2O3] catalysts that served as the stepping stone to
much improved pyridine-rich CoII/III [N2Npy3] catalysts for water
reduction that display TON > 7000 mol-1.[1c, 3] We have gathered
evidence that some molecular catalysts such as cobalt oximes
[2d]
are converted into nanoparticulates through ligand hydrolysis
triggered by radical-based mechanisms.[1b] Therefore, although
the involvement of ligands in the catalytic cycle has been
reported,[4] we conclude that radical formation may have
deleterious effects on H2 production.[5] Here we examine this
issue in detail and suggest that formation of energetically
equivalent valence tautomers, viz. [CoI(L)]– ↔ [CoII(L•)]– offers
additional conversion pathways that lead to catalyst
[]
Mr. H. Baydoun, Dr. S. Mazumder, Prof. H. B. Schlegel, Prof. C. N.
Verani
Department of Chemistry, Wayne State University
5101 Cass Ave, Detroit, MI 48202 (USA)
E-mail: cnverani@chem.wayne.edu
Homepage: http://chem.wayne.edu/veranigroup/
Dr. S. Mazumder
Current address: Department of Chemistry, Hofstra University,
Berliner Hall, Hempstead, NY 11549 (USA)
Figure 1: The CV of 1 (1 mM) in ACN. Glassy carbon, Ag/AgCl, Pt
Supporting information for this article is given via a link at the end of
the document.
wire, TBAPF6 (0.1 M). Ferrocene is used as an internal standard.
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couple observed at E1/2 = -1.08 VNHE obtained in acetonitrile. This
observation confirms that the active species in catalysis is the
CoI complex in accordance with the accepted mechanisms for
proton reduction using cobalt metal complexes; catalysis is
initiated by the reaction of CoI with a proton to form a CoIII–H–
hydride intermediate.[1b, 1c, 1g, 1j, 1k, 2]
The identity of the evolved gas was determined as H2 by
means of gas chromatography following a bulk electrolysis
experiment that was performed in an air-tight H-type cell (See SI
for details).
properties of the complex in distinct reduced oxidation states. To
this end we used experimental observations along with DFT
calculations. The structural information was obtained using
potassium graphite (KC8) as a stoichiometric reducing agent in
order to isolate chemically the CoII and CoI reduced forms of our
catalyst. Starting from 1 we were able to isolate the singly
reduced CoII analogue [CoIIL1(pyr)]o (2) and the doubly reduced
CoI analogue [CoIL1]K (3). We were able to grow X-ray quality
crystals for 1, 2, and 3 (Figure 3). The structure of the CoIII
species 1 (Figure 3a) shows the expected pseudo-octahedral
geometry, with the ligand (L)-2 occupying the equatorial plane
and the two pyrrolidines binding to the axial positions. Excellent
agreement was observed between the obtained bond lengths
and angles and that of structurally related complexes with a
trivalent cobalt ion.[6a, 6b]
The structure of the CoII species 2 shown in Figure 3b, on
the other hand, displays a square pyramidal geometry in which τ
= 0.018.[11] This decrease from six- to five-dentate coordination
upon reduction from CoIII to CoII agrees with similar results from
our group observed in oxime environments.[2d] Similarly, it is
interesting to note that when compared to 1, minor changes
occur in the Co-L bond lengths, while a considerable 0.135 Å
elongation takes place along the Co-N5 bond. The maintenance
of the bond lengths within the equatorial plane suggests that
upon metal-centered reduction from CoIII to CoII the incoming
electron is transferred to the unoccupied dz2 orbital, while the
electrons in the dxy, dxz, and dyz orbitals remain largely
unaffected, as previously proposed by our group.[2d] The dx2-y2
orbital remains unoccupied. Moreover, the EPR spectrum of 2
(Figure 4a) shows a signal with a g value of 2.018 which is
consistent with the presence of one unpaired electron. Hence,
the CoII ion is found in a doublet LS3d7 configuration. This
proposition was further examined by DFT calculations that
showed excellent agreement between the crystal structure of 2
and the optimized structure of a doublet LS3d7 CoII ion (Figure
4b, 4c, and S2).
Figure 2: Polarization curve for 1 in phosphate buffer (1 mol/L, pH 7).
Inset: Charge consumption over time for 1 (8 µmol/L) at -1.16 VNHE. The
dotted line represents an idealized charge consumption. Electrodes: Hg-
pool (w), Pt (aux), Ag/AgCl.
The catalyst showed significant initial activity yielding a
TON of 675 ± 30 after 30 min of electrolysis (TOF = 23 min-1)
with Faradaic efficiency of 97 ± 3%. However, this high activity
persisted only for a short period of time. After ca. 30 minutes of
catalysis, considerable decrease in charge consumption was
observed (Figure 2 inset). The observed TON is only a lower
limit of the maximum value as it was measured following
significant deacivation. Similar catalytic behavior has been
observed for certain polypyridine frameworks.[10] Moreover, the
solution changes color from green to colorless (Figure S1a
inset). Compared to the UV-visible spectrum of the solution prior
to catalysis, the post-catalytic spectrum shows the
disappearance of charge transfer (CT) processes at ca. 413 nm
associated with an Namido  CoIII ligand-to-metal CT. However,
the peaks associated with intraligand CT and observed below
300 nm persist (Figure S1). This suggests that the complex is
undergoing demetallation. These observations prompted us to
investigate the mechanism by which catalyst degradation takes
place with the aim of providing guiding principles for future
catalyst design.
Figure 3: Crystal structures of 1 (a, CCDC 1533010), 2 (b, CCDC
1533009) and 3 (c, CCDC 1533008). Hydrogen atoms, solvents, and
counter ions removed for clarity. Ellipsoids shown at 50% probability.
Selected bond lengths for 1: Co1-N1 1.9861(18), Co1-N4 1.9998(17),
Co1-N2 1.8887(18), Co1-N3 1.8887(18), Co1-N5 2.0154(18), Co1-N6
2.0047(18). For 2: Co1-N1 1.971(2), Co1-N4 1.993(2), Co1-N2
1.882(2), Co1-N3 1.882(2), Co1-N5 2.139(2). For 3: Co1-N1 1.889(4),
Co1-N4 1.892(4), Co1-N2 1.874(4), Co1-N3 1.865(4).
Because the active form of the catalyst must contain CoI,
the elucidation of the deactivation pathways requires the
investigation of the structural and electronic
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Figure 4: (a) EPR spectrum of 2 taken at 110 K in MeCN. Comparison of
the crystal structure with the optimized structure of the calculated LS (b),
and HS (c) structures.
The structure of 3 warrants some detailed discussion; unlike its
5-coordinate CoI oxime congener,[2d] this doubly reduced
derivative of 1 is composed of a tetracoordinate cobalt complex
in a distorted square planar geometry, where the maximal
distortion between any two opposing planes among the Co-N
bonds deviates by 9.5o from the idealized 0o (Figure 3c).[12]
Compared to the previous two structures, the Co-Namide bond
lengths remain unchanged while its Co-Npyridine bond lengths are
elongated by ca. 0.1 Å. Moreover, other bond lengths on the
ligand framework remain largely unchanged (Figure S3). This
observation implies that occupation of the dz2 orbital is favored in
the solid state, and that the structure contains a bona fide 3d8
CoI ion. Interestingly, DFT results indicate that two lowest lying
isoenergetic states are possible for the nominal “CoI species”: (i)
a metal-centered singlet species [3d8CoIL1]– akin to the crystal
structure, or (ii) a ligand-reduced and radical-containing triplet
Figure 5: Normalized UV-Visible-NIR spectra of 1 (gray trace) and 3
(black trace) in MeCN. Inset: Calculated natural transition orbitals (NTOs
at isovalue = 0.05 au) showing π-π* ILCT transition at 1023 nm for 3 in
acetonitrile solvent.
solution the species described as [CoII(L1●)]K is accesible. This
conclusion receives further support from time-dependent-DFT
calculations shown in Figures S11 and S12 where the simulated
UV-Visible-NIR spectrum of [CoIL1]– lacks significant absorption
processes above 800 nm, while the simulated spectrum for
[CoII(L1●)]– shows absorption peaks at 1023 and 1205 nm of the
NIR region. These transitions are mainly due to intraligand π-π*
charge transfers centered on the amidopyridine moiety shown in
Figures 5 inset and S13, respectively.
The small calculated energy difference of 2.7 kcal/mol
between the two species is well within the limit of the DFT
method, and coupled with the detection of [CoIL1]K in the solid
state and [CoII(L1●)]K in acetonitrile, suggests that there is an
equilibrium between the two states. Ergo, it is conceivable that
both species will coexist under catalytic aqueous conditions.
Such an equilibrium is consistent with the formation of valence
tautomers.[14] As such, we propose that the difference in
reactivity of the two tautomers with protons can be used to
explain the deactivation of the catalyst. In presence of protons,
the [CoIL1]– tautomer significantly favors the formation of a CoIII–
H– species (-22 kcal/mol), which is the first step in the catalytic
cycle for H2 production (Figure 6 pathway a and Figure S14).
Conversely, concomitant formation of the [CoII(L1●)]– tautomer
favors ligand protonation (Figure 6 pathways b-d). Upon such
protonation of the ligand framework the structure deviates
significantly from planarity (pathways b and c) with pathway b
leading to a tridentate metal complex and from there to
demetalation. These observations lead us to conclude that
[CoIL1]K is active in catalysis and leads to the formation of H2,
while [CoII(L1●)]K is protonated, and eventually leads to the
deactivation of the catalyst. This observation prompted us to
hypothesize that running catalysis at a lower pH would favor
protonation of the ligand and lead to deactivation. Indeed,
catalytic runs at pH ≤ 6 led to a faster decay in the consumption
of charge over time associated with an overall decrease in
activity (Figure S15-S16).
[
3d7CoII(L1●)]– species with the unpaired electron centered on the
amido-pyridine moiety. These two states display a calculated
energy difference of ca. 3 kcal/mol, thus within the limits of the
method (Figure S4). Furthermore, both the 3d8 [CoIL1]– and the
coupled (3d7 – ½) [CoII(L1●)]– species yield spin integers that are
NMR active (Figure S5). However, comparison of the Co-N
bond lengths, including those in the ligand framework, show
better agreement with the metal-centered [CoIL1]– than with the
ligand-reduced [CoII(L1●)]– species (Figures S6-S8).
Pivotal information necessary to probe the electronic
nature of the reduced species comes from the UV-Visible-NIR
spectra of 1, 2, and 3. Figure 5 displays the spectra for 1 and 3,
while the spectrum of 2 is shown in Figure S9. As previously
discussed, the spectrum of 1 shows a strong LMCT absorption
at 413 nm. The spectrum of 2, on the other hand, is
characterized by an absorpion at 336 nm with a shoulder at 452
nm assigned to a CoII  Namidopyridine MLCT transition that
confirms metal reduction. However, the spectrum of 3 also
shows strong absorptions in the NIR region at 1028 and 1160
nm unquestionably attributed to ligand-stabilized radicals.[13]
Complex 3 was independently generated via electrochemical
reduction, and a spectrum with identical features was obtained
(Figure S10) indicating that the same species can be
conveniently obtained chemically or electrochemically.
Therefore, analysis of these results suggest that in the solid
state the [CoIL1]K species prevails for 3, while in an acetonitrile
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Synthesis of [CoIIL1(pyr)]o (2). [CoIIL1(pyr)]o was isolated using standard
glovebox techniques. A sample of 1 (108 mg, 0.16 mmol) was dissolved in THF and
added into a vial containing of KC8 (22 mg; 0.16 mmol). The solution immediately
changed from green to red and was allowed to stir for 2h. The sample was filtered
and a solution was obtained that yielded crude 2. X-ray quality crystals were
obtained by recrystallization in acetonitrile. Anal. Calc. for C22H21CoN5O2: 59.20; H:
4.74; N: 15.69; Found: C: 57.27; H: 4.45; N: 14.50.
Synthesis of [CoIL1]K. (3). [CoIL1]K was isolated in a similar way as for 2
using 44 mg of KC8 (2 equiv. ; 0.32 mmol). The solution changed color from green to
dark blue. X-ray quality crystals of
3 were obtained after filtration via slow
evaporation from the THF solution. 1H NMR, ppm (CD3CN, 600 MHZ, Figure S5): δ
9.36 (2H), δ 8.37 (2H), δ 8.26 (t, 2H), δ 6.72 (2H), δ 6.53 (2H), δ 6.46 (2H).
Water Reduction Experiments: Turnover numbers were determined using
a custom built H-type bulk electrolysis setup. The cell consisted of two airtight
compartments separated by a fine frit. One compartment was used to house the
auxiliary electrode (Pt coil) while the other compartment was used to house the
reference electrode (Ag/AgCl) and the working electrode (mercury pool). Before the
application of a potential the headspace was thoroughly purged with nitrogen gas.
The amount of H2 gas produced was determined by gas chromatography. In a
typical experiment 100 µL of headspace were injected into the GC to determine the
total amount of H2. Turnover numbers were determined by dividing the total number
of moles of hydrogen produced by the number of moles of catalyst used. While the
faradaic efficiency was determined by dividing the actual number of moles of
hydrogen produced by the number of moles of hydrogen that should have been
produced based on the charge consumed.
Figure 6: Energetics of protonation of the [CoIL1]– and [CoII(L1●)]– at
different sites in water solvent. Free energies are reported in kcal/mol.
Computational Methods: Electronic structure calculations were carried out
In summary, this work reports on a novel cobalt complex
capable of performing water reduction at an overpotential of 0.54
V with TOF of 23 min-1 following 30 min of electrolysis with
Faradaic efficiency of ca. 97 %. This initial catalytic activity
decreases significantly after 30 minutes, and structural and
electronic evaluation revealed that valence tautomerization is
possible. The “CoI state” can afford either [CoIL1]– or [CoII(L1●)]–
within less than 3 kcal/mol. While the [CoIL1] – species supports
the formation of a catalytically active CoIII–H– species required
for H2 formation, the tautomer [CoII(L1●)]– favors ligand
protonation accompanied by significant structural distortion that
ultimatly leads to catalyst deactivation associated with
demetallation. These results allow us to postulate that efficient
catalytic water reduction based on square planar ligands must
proceed exclusively by means of the metal center while carefully
avoiding ligand protonation. Current work in our labs builds on
these results for the design of systems where the [CoIL1]–
tautomer is energetically separated from its [CoII(L1●)]– tautomer.
using the B3LYP* functional[15] as implemented in
a development version of
Gaussian.[16] The SDD basis set and effective core potential[17] were used for the Co
atom and the 6-31G(d,p) basis set[18] was used for the other atoms. Solvation effects
in acetonitrile and water were incorporated using the implicit SMD solvation model[19]
and were included during structure optimization. All of the optimized structures were
confirmed as minima by harmonic vibrational frequency calculations and the
converged wave functions were tested for the SCF stability. The zero-point energy
and thermal corrections were included for the calculation of the free energies. The
standard states of 1 M concentration were considered for all the reactants and
products for calculating the free energies of reactions. The literature value of –270.3
kcal/mol is used for the free energy of proton in water.[20] The spin density plots
(isovalue
=
0.004 au) were visualized using GaussView.[21] Vertical electronic
excitation energies and intensities were evaluated using time-dependent DFT (TD-
DFT)[22] and the orbital transitions of each excited state were characterized using the
natural transition orbital (NTO) method.[23]
Acknowledgements
Experimental Section
This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under award number DE-SC0001907 to CNV and HBS,
Synthesis of [CoIIIL1(pyr)2]PF6 (1).
A
MeOH solution of 1.6
g of
Co(OAc)2.4H2O (6.2 mmol) was added dropwise to a MeOH solution containing 2.0
g of H2L1 (6.2 mmol) To this mixture an excess of 10 mL of pyrrolidine was added.
The solution was allowed to stir overnight at room temperature. Then oxygen was
bubbled into the solution for 5 minutes. The reaction mixture was filtered and an
excess of 1.5 g of NH4PF6 (9.2 mmol) were added to precipitate 1. X-ray quality
crystals were grown via diethylether vapor diffusion into an acetonitrile solution of 1.
Yield: 86%. ESI (m/z+) = 517 for [CoIIIL1(pyrrolidine)2]+. IR (KBr, cm-1) 3167 (νN-H),
1626 (νC=O), 1599 and 1572 (νC=N, and νC=C), 844 (νPF6). 1H NMR, ppm (CD3CN, 400
MHZ): δ 9.45 (d, 2H), δ 8.90 (m, 2H), δ 8.41 (t, 2H), δ 8.29 (d, 2H), δ 8.00 (t, 2H), δ
7.12 (m, 2H), δ 3.22 (2H), 2.11 (4H), δ 1.53 (4H), δ 1.29 (8H). Anal. Calc. for
C26H30CoN6O2PF6 : 47.14; H: 4.56; N: 12.69; Found: C: 47.05; H: 4.44; N: 12.49.
including financial support for HB (synthesis, catalysis
&
crystallography) and SM (calculations). The authors
acknowledge Dr. Oleg Poluektov at Argonne National Labs for
helpful EPR discussions. HB also acknowledeges WSU-
Chemistry for a Thomas C. Rumble Graduate Fellowship.
Keywords: Cobalt complex ● valence tautomerism ● amido-
pyridine ligands ● water reduction ● catalysis
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[1]
a) Y.-J. Yuan, J.-R. Tu, H.-W. Lu, Z.-T. Yu, X.-X. Fan, Z.-G. Zou,
[9]
R. Shakya, S. S. Hindo, L. Wu, M. M. Allard, M. J. Heeg, H. P.
Dalton Trans. 2016, 45, 1359-1363; b) D. Basu, S. Mazumder, X. T. Shi,
R. J. Staples, H. B. Schlegel, C. N. Verani, Angew. Chem. Int. Edit. 2015,
54, 7139-7143; c) D. Basu, S. Mazumder, X. Shi, H. Baydoun, J. Niklas,
O. Poluektov, H. B. Schlegel, C. N. Verani, Angew. Chem., Int. Ed. 2015,
54, 2105-2110; d) P. Zhang, M. Wang, Y. Yang, D. Zheng, K. Han, L.
Sun, Chem. Commun. 2014, 50, 14153-14156; e) P. Zhang, M. Wang, Y.
Yang, T. Yao, L. Sun, Angew. Chem., Int. Ed. 2014, 53, 13803-13807; f)
K. Kawano, K. Yamauchi, K. Sakai, Chem. Commun. 2014, 50, 9872-
9875; g) Z. Han, R. Eisenberg, Acc. Chem. Res. 2014, 47, 2537-2544; h)
A. Call, Z. Codolà, F. Acuña-Parés, J. Lloret-Fillol, Chem. Eur. J. 2014,
20, 6171-6183; i) P. Zhang, M. Wang, F. Gloaguen, L. Chen, F. Quentel,
L. Sun, Chem. Commun. 2013, 49, 9455-9457; j) V. S. Thoi, Y. Sun, J. R.
Long, C. J. Chang, Chem. Soc. Rev. 2013, 42, 2388-2400; k) M. Wang, L.
Chen, L. Sun, Energy & Environmental Science 2012, 5, 6763-6778; l) P.
H. A. Kankanamalage, S. Mazumder, V. Tiwari, K. K. Kpogo, H.
Bernhard Schlegel, C. N. Verani, Chem. Commun. 2016, 52, 13357-
13360; m) S. Mandal, S. Shikano, Y. Yamada, Y.-M. Lee, W. Nam, A.
Llobet, S. Fukuzumi, J. Am. Chem. Soc. 2013, 135, 15294-15297.
a) J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, H. B. Gray, Acc.
Chem. Res. 2009, 42, 1995-2004; b) J. L. Dempsey, J. R. Winkler, H. B.
Gray, J. Am. Chem. Soc. 2010, 132, 16774-16776; c) J. L. Dempsey, J. R.
Winkler, H. B. Gray, J. Am. Chem. Soc. 2010, 132, 1060-1065; d) D.
Basu, S. Mazumder, J. Niklas, H. Baydoun, D. Wanniarachchi, X. Shi, R.
J. Staples, O. Poluektov, H. B. Schlegel, C. N. Verani, Chem. Sci. 2016, 7,
3264-3278.
Hratchian, B. R. McGarvey, S. R. P. Da Rocha, C. N. Verani, Inorg.
Chem. 2007, 46, 9808-9818.
[10]
a) W. M. Singh, T. Baine, S. Kudo, S. Tian, X. A. N. Ma, H. Zhou, N. J.
DeYonker, T. C. Pham, J. C. Bollinger, D. L. Baker, B. Yan, C. E.
Webster, X. Zhao, Angew. Chem., Int. Ed. 2012, 51, 5941-5944; b) M.
Vennampalli, G. Liang, L. Katta, C. E. Webster, X. Zhao, Inorg. Chem.
2014, 53, 10094-10100.
[11]
[12]
A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn, G. C. Verschoor, J.
Chem. Soc., Dalton Trans. 1984, 1349-1356.
An idealized square planar structure displays 0o between opposing angles,
whereas an idealized tetrahedral structure displays those planes at 90o.
See e.g. S. Blanchard, F. Neese, E. Bothe, E. Bill, T. Weyhermüller, K.
Wieghardt, Inorg. Chem. 2005, 44, 3636-3656.
[13]
a) A. Kochem, H. Kanso, B. Baptiste, H. Arora, C. Philouze, O. Jarjayes,
H. Vezin, D. Luneau, M. Orio, F. Thomas, Inorg. Chem. 2012, 51,
10557-10571; b) M. M. Allard, J. A. Sonk, M. J. Heeg, B. R. McGarvey,
H. B. Schlegel, C. N. Verani, Angew. Chem. Int. Edit. 2012, 51, 3178-
3182; c) C. C. Scarborough, S. Sproules, T. Weyhermüller, S. DeBeer, K.
Wieghardt, Inorg. Chem. 2011, 50, 12446-12462; d) T. Storr, P. Verma,
Y. Shimazaki, E. C. Wasinger, T. D. P. Stack, Chem. Eur. J. 2010, 16,
8980-8983; e) W. Kaim, Coord. Chem. Rev. 2011, 255, 2503-2513.
a) D. A. Shultz, in Magnetism: Molecules to Materials, Wiley-VCH
Verlag GmbH & Co. KGaA, 2003, pp. 281-306; b) R. M. Buchanan, C. G.
Pierpont, J. Am. Chem. Soc. 1980, 102, 4951-4957; c) R. D. Schmidt, D.
A. Shultz, J. D. Martin, P. D. Boyle, J. Am. Chem. Soc. 2010, 132, 6261-
6273.
[2]
[14]
[3]
[4]
D. Basu, M. M. Allard, F. R. Xavier, M. J. Heeg, H. B. Schlegel, C. N.
Verani, Dalton Trans. 2015, 44, 3454-3466.
[15]
[16]
M. Reiher, O. Salomon, B. Artur Hess, Theor. Chem. Acc. 2001, 107, 48-
55.
a) A. Zarkadoulas, M. J. Field, C. Papatriantafyllopoulou, J. Fize, V.
Artero, C. A. Mitsopoulou, Inorg. Chem. 2016, 55, 432-444; b) C. S.
Letko, J. A. Panetier, M. Head-Gordon, T. D. Tilley, J. Am. Chem. Soc.
2014, 136, 9364-9376; c) L. M. A. Quintana, S. I. Johnson, S. L. Corona,
W. Villatoro, W. A. Goddard, M. K. Takase, D. G. VanderVelde, J. R.
Winkler, H. B. Gray, J. D. Blakemore, Proc. Natl. Acad. Sci. USA 2016,
113, 6409-6414.
a) N. Kaeffer, A. Morozan, J. Fize, E. Martinez, L. Guetaz, V. Artero,
ACS Catalysis 2016, 6, 3727-3737; b) B. D. McCarthy, C. L. Donley, J. L.
Dempsey, Chem. Sci. 2015, 6, 2827-2834; c) E. Anxolabéhère-Mallart, C.
Costentin, M. Fournier, S. Nowak, M. Robert, J.-M. Savéant, J. Am.
Chem. Soc. 2012, 134, 6104-6107.
a) M. Amirnasr, K. J. Schenk, S. Meghdadi, Inorg. Chim. Acta 2002, 338,
19-26; b) S. Meghdadi, M. Amirnasr, M. H. Habibi, A. Amiri, V. Ghodsi,
A. Rohani, R. W. Harrington, W. Clegg, Polyhedron 2008, 27, 2771-
2778; c) D. J. Barnes, R. L. Chapman, R. S. Vagg, E. C. Watton, J. Chem.
Eng. Data 1978, 23, 349-350.
a) S. K. Dutta, U. Beckmann, E. Bill, T. Weyhermüller, K. Wieghardt,
Inorg. Chem. 2000, 39, 3355-3364; b) U. Beckmann, E. Bill, T.
Weyhermüller, K. Wieghardt, Inorg. Chem. 2003, 42, 1045-1056.
R. R. Gagne, C. A. Koval, G. C. Lisensky, Inorg. Chem. 1980, 19, 2854-
2855.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson,
et_al, Revision H.35 ed., Gaussian, Inc., Wallingford, CT, 2014.
M. Dolg, U. Wedig, H. Stoll, H. Preuss, J. Chem. Phys. 1987, 86, 866-
872.
a) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta 1973, 28, 213-222; b)
M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. J.
DeFrees, J. A. Pople, J. Chem. Phys. 1982, 77, 3654-3665.
A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113,
6378-6396.
C. P. Kelly, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2006, 110,
16066-16081.
R. K. Pennington, T.; Millam, J. M., Semichem, Inc., Shawnee Mission,
KS 2009.
a) L. Petit, P. Maldivi, C. Adamo, J. Chem. Theory Comput. 2005, 1,
953-962; b) E. Runge, E. K. U. Gross, Phys. Rev. Lett. 1984, 52, 997-
1000; c) R. E. Stratmann, G. E. Scuseria, M. J. Frisch, J. Chem. Phys.
1998, 109, 8218-8224.
[17]
[18]
[5]
[6]
[19]
[20]
[21]
[22]
[7]
[8]
[23]
R. L. Martin, J. Chem. Phys. 2003, 118, 4775-4777.
This article is protected by copyright. All rights reserved.

10.1002/chem.201701783
Chemistry - A European Journal
COMMUNICATION
Layout 2:
COMMUNICATION
Habib Baydoun, Shivnath Mazumder, H.
Bernhard Schlegel and Cláudio N.
Verani*
Page No. – Page No.
Deactivation of a Cobalt Catalyst for
Water Reduction via Valence
Tautomerism
A new Co-based water reduction catalyst displays a high TOF of 23 min-1, albeit for
a relatively short time. We perform a detailed structural and electronic study to
understand the mechanisms of deactivation and conclude that tautomeric
equilibrium between [CoI(L1)]– and [CoII(L1●)]– takes place. While the [CoI(L1)]– state
supports catalysis via CoIII–H– formation, the [CoII(L1●)]– state leads to catalyst
deactivation. This knowledge is instrumental for the future design of robust
catalysts.
This article is protected by copyright. All rights reserved.