Ligand transformations and efficient proton/water reduction with cobalt catalysts based on pentadentate pyridine-rich environments

Article abstract of DOI:10.1002/anie.201409813

A series of cobalt complexes with pentadentate pyridine-rich ligands is studied. An initial Co^II amine complex 1 is prone to aerial oxidation yielding a Co^III imine complex 2 that is further converted into an amide complex 4 in presence of adventitious water. Introduction of an N-methyl protecting group to the ligand inhibits this oxidation and gives rise to the Co^II species 5. Both the Co^III 4 and Co^II 5 show electrocatalytic H₂ generation in weakly acidic media as well as in water. Mechanisms of catalysis seem to involve the protonation of a Co^II-H species generated in situ.

Full text of DOI:10.1002/anie.201409813

Angewandte
Chemie
DOI: 10.1002/anie.201409813
Water Reduction
Ligand Transformations and Efficient Proton/Water Reduction with
Cobalt Catalysts Based on Pentadentate Pyridine-Rich
Environments**
Debashis Basu, Shivnath Mazumder, Xuetao Shi, Habib Baydoun, Jens Niklas, Oleg Poluektov,
H. Bernhard Schlegel,* and Clꢀudio N. Verani*
Abstract: A series of cobalt complexes with pentadentate
pyridine-rich ligands is studied. An initial CoII amine complex
1 is prone to aerial oxidation yielding a CoIII imine complex 2
that is further converted into an amide complex 4 in presence of
adventitious water. Introduction of an N-methyl protecting
group to the ligand inhibits this oxidation and gives rise to the
CoII species 5. Both the CoIII 4 and CoII 5 show electrocatalytic
H2 generation in weakly acidic media as well as in water.
gated.[6] Several of these systems are also water-soluble, and
therefore relevant for direct water reduction.
Our groups have a long-standing interest in redox-active
phenylenediamine-bridged pentadentate [N2O3] ligands capa-
ble of forming stable first-row transition metal complexes.[7]
Thus far we emphasized phenolate-rich environments, and
observed a facile conversion from the secondary amine to
imine. While FeIII species have been used in current rectifying
devices,[7e,f] CoIII complexes are electrocatalysts for proton
reduction in trifluoroacetic acid/MeCN.[7g] However, these
species have intrinsic negative overpotentials, and we
hypothesized that similar [N2Npy3] pentadentate pyridine-
rich environments would yield affordable catalysis and water
solubility. Such systems allow the proton to bind to the sixth
position of the metal in a framework that is potentially redox-
active and a p-acceptor, thus contributing to the stabilization
of the CoI state.
Mechanisms of catalysis seem to involve the protonation of
II
ꢀ
a Co H species generated in situ.
T
he ever-increasing demand for alternative energy sources
along with the continuous decline of fossil fuel reserves have
driven extensive research on water splitting aiming to
generate dihydrogen.[1] Earth-abundant transition metals
like cobalt, nickel, and iron are of immense interest due to
their ability to electrocatalyze the formation of dihydrogen
from acidic solutions.[2] Cobalt is chief among these metals
due to its energetically affordable conversions from 3d6 CoIII
to 3d7 CoII to nucleophilic 3d8 CoI species, and has been
extensively used.[3] Cobalt oximes figure among the most
studied examples used for proton reduction.[4] The p-accept-
ing nature of these ligands stabilizes the CoI state that takes
The [N2Npy3] ligand L1H was obtained by treatment of
phenylenediamine with picolyl chloride in water and in the
presence of sodium hydroxide and hexadecyl trimethyl
ammonium chloride.[8] The purified ligand was treated with
CoCl2·6H2O in methanol under aerobic conditions for 3 h and
followed by counterion exchange with NaClO4. An initial
pink solution containing [CoII(L1H)Cl]2+ turned greenish
up a proton to generate cobalt/hydride species CoIII
ꢀ
ꢀ
H
II
ꢀ
ꢀ
amenable to reduction to Co H . The latter species
subsequently reacts with a second proton to generate H2.[5]
Recent results point to the importance of pyridine-containing
ligands in proton reduction, for which complexes of imino-,
di-, tetra-, and pentapyridine ligands have been investi-
within minutes and yielded a crystalline mixture of an
=
orange [CoIII(L1C N)Cl](ClO4)2 (2) and
a
green [CoIII-
(L1OMe)Cl]ClO4 (3) species after two days (Scheme 1). When
the mother liquor was allowed to stand for 5–7 days, light-
=
orange crystals of [CoIII(L1C O)Cl]ClO4 (4) were obtained.
Identical results were obtained by recrystallizing the mixture
of 2 and 3 from either MeCN/diethyl ether (1:1) or ethanol/
acetone (1:1). Furthermore, species 4 can be generated
directly upon complexation of L1H and CoCl2·6H2O in
acetone/water (1:1) at room temperature after 5–7 days
under aerobic conditions.
[*] D. Basu, Dr. S. Mazumder, X. Shi, H. Baydoun, 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
Complexes 2–4 have been characterized spectroscopically
1
Homepage: http://chem.wayne.edu/veranigroup/
by FTIR, and H NMR methods (Figure S2), as well as by
Dr. J. Niklas, Dr. O. Poluektov
Argonne National Laboratory, Chemistry Division, E117
9700 South Cass Ave. Argonne, IL 60439 (USA)
electrospray ionization (ESI) mass spectrometry (Figure S3)
and elemental analyses.
1
Well-defined and sharp peaks in the H NMR spectra in
CD3CN confirm the diamagnetic nature (3d6 LSCoIII) of these
complexes. Figure 1 shows the ORTEP representations of
crystals for 2, 3, and 4, and relevant bond lengths and
crystallographic parameters are summarized in Tables T1 and
T2 (Supporting Information, SI). These complexes are
pseudo-octahedral in which two Namine, one Npy, and one Cl
define the basal plane and two Npy atoms occupy the axial
[**] 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 C.N.V. and H.B.S., including
financial support for D.B. (synthesis and catalysis), H.B. (crystal-
lography), S.M. and X.S. (calculations). Prof. John F. Endicott is
acknowledged for critical discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409813.
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Figure 2. Reaction energy profile for the hydroxy to amide conversion
in MeCN. The transition state * is not explicitly located.
Scheme 1. Synthesis of CoIII complexes 2, 3, and 4.
documented.[9]
A
detailed density functional theory
(DFT)[10] study was also performed to evaluate details of
the hydroxy to amide conversion mechanism (Figure 2).
Calculations indicate that the transformation requires atmos-
3
ꢀ
pheric O2 to react with the C H function of the intermediate
ꢀ
hydroxy complex. The C H hydrogen abstraction event is
rate-limiting and nearly isoenergetic as the resulting inter-
mediate I is about 2 kcalmolꢀ1 higher than the starting
hydroxy complex. In species I the hydroperoxo radical
(.OOH) is weakly bonded to the hydroxy and the unpaired
electron generated on the ligand is transferred to the metal
center reducing it to CoII. Thus, the metal center helps
ꢀ
stabilize the radical intermediate I and makes the C H
Figure 1. ORTEP representations of 2–4 at 50% probability.
hydrogen removal event nearly isoenergetic.[9] An intersys-
tem crossing (triplet to singlet surface) from species I
followed by the removal of the hydroxy hydrogen by the
hydroperoxo radical gives rise to the amide complex 4 and the
overall process is favored by about 38 kcalmolꢀ1. Geometry
optimization of intermediate I on the singlet surface results in
the transfer of the hydroxy hydrogen from the metal complex
to the hydroperoxo radical, giving rise to 4.
positions. The amine moiety of the ligand framework is
oxidized to imine in 2, as observed by a shorter 1.28 ꢀ C(5)–
ꢀ
N(3) bond than a regular C N bond length (~ 1.5 ꢀ). Species
3 is obtained through addition of a methoxide to C(26) from
the solvent methanol. The C(26)–N(5) bond length increases
from 1.28 ꢀ in 2 to 1.42 ꢀ and N(5) becomes formally
negative. Both the imine and methoxy functionalities in 2 and
3 are converted into an amide moiety in 4. The N(3) amide
atom is also negatively charged and coordinated to CoIII at
Other mechanistic pathways probed for the formation of 4
can be found in Figure S4a–c. Experimental results showed
that the conversion of 2 and 3 to the amide 4 does not take
place in the absence of oxygen (Figure S5). Inspection of the
optimized geometry of 3 shows that a direct SN2 attack on the
carbon atom bearing the methoxy function by a water
molecule is also not feasible because of steric crowding
imposed by one of the pyridine rings (Figure S6). As a result,
conversion of 3 to 4 must proceed through the regeneration of
2 that then undergoes water addition followed by oxidation
with O2 (Figure S6).
ꢀ
1.88 ꢀ; this bond is the strongest among all the Co N bonds
=
of the species. The C(9) O(1) moiety is 1.23 ꢀ long and the
ꢀ
C(9)–N(3) distance of 1.35 ꢀ is shorter than a regular C N
bond, suggestive of significant p-conjugation in the ligand
framework. Isolation of the methoxy species 3 demonstrates
the susceptibility of the imine function in 2 to undergo facile
nucleophilic attack from the solvent. Similarly, to explain the
amide conversion from 2 to 4, a hydroxy intermediate must be
invoked by addition of adventitious water present in the
solvent to the imine moiety of 2. This step has been
To prevent the initial oxidation of L1H into its imine
counterpart, an N-methylamine derivative ligand L2 was
2
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designed. Synthesis of the ligand was performed by the
reaction of N-methyl phenylenediamine with picolyl chloride
in the presence of NaOH and hexadecyl trimethyl ammonium
chloride (Figure S7). This new ligand allowed for the com-
plexation of a 3d7 CoII from CoCl2·6H2O in methanol under
aerobic conditions without the previously observed oxidation
to imine. The pinkish colored [CoII(L2)Cl]ClO4 (5) was
isolated by counterion exchange with NaClO4 after 2 h
lengths between 0.03–0.08 ꢀ, whereas the same deviation for
an LS species varies from 0.03–0.34 ꢀ (Figure S9).
Inspection of the geometries of the CoIII 2 and the CoII 5
complexes shows a change in the conformation of the ligand
framework. This reversal of the conformational stability is
imposed by the change of the ligand from imine to N-
methylamine, rather than by the change of the oxidation state
from CoIII to CoII (Figures S10 and S11). The UV/Vis spectra
of the CoIII complexes 2–4 in MeCN are shown in Figure S12
and Table T5 (SI). They are dominated by intense intraligand
UV processes along with faint visible processes associated
with d–d transitions. The imine 2 and amide 4 show
absorptions at 490 nm (20400 cmꢀ1, e = 240mꢀ1 cmꢀ1) and
ꢀ
(Figure 3). The X-ray structure of 5 shows the Co Cl bond
488 nm (20500 cmꢀ1, e = 320mꢀ1 cmꢀ1), respectively, associ-
!
1
1
ated with the T2g(I) A1g(D) transition of an ideal octahe-
dral field.[11] These species absorb in the blue region and are
characterized by a faint orange color. The methoxy species 3
shows a lower energy absorption at 667 nm (15000 cmꢀ1, e =
!
1
410mꢀ1 cmꢀ1) associated with T2g(I) A1g(D). It absorbs in
the red region and is green colored. On the other hand, the
pinkish color observed for the HSCoII species 5 is the result of
1
Figure 3. Synthesis of CoII complex 5 and its ORTEP representation at
50% ellipsoid probability.
an absorption in the green region at 504 nm (19800 cmꢀ1, e =
!
4
4
at 2.33 ꢀ, thus longer than those found in 2–4 (Tables T1, T2,
T3, and T4, SI). The complex was characterized by FTIR,
ESI-MS (Figure S8), and elemental analysis. DFT calcula-
tions predict that the high-spin (S = 3/2) configuration is more
stable than the low-spin (S = 1/2) configuration by 13.7 kcal
molꢀ1. Indeed, the 1H NMR spectrum yielded broad and
shifted peaks indicating a paramagnetic nature of the com-
plex.
46mꢀ1 cmꢀ1) associated with an idealized T1g(P) T1g(F)
process.[11b]
The cyclic voltammetry (CV) profile for 2 in dry MeCN
shows two reversible and two quasi-reversible processes at
ꢀ0.27, ꢀ1.16, ꢀ2.01, and ꢀ2.12 VFc/Fc+ (Figure 5). Complex 4
shows two reversible and one irreversible process at ꢀ0.69,
ꢀ1.99, and ꢀ2.41 VFc/Fc+, whereas two reversible and one
quasi-reversible process were found at ꢀ0.02, ꢀ1.92, and
ꢀ2.39 VFc/Fc+ for species 5 (Figure 5). CV profiles of 1 and 3
are shown in Figure S13.
The electron paramagnetic resonance (EPR) spectrum of
5 in MeCN was taken at 4 K and confirms the species as HSCoII
in
a
pseudo-octahedral environment (Figure 4a). The
The reduced species generated at affordable potentials for
the chemically robust species 4 and 5 are of immense interest
to proton and water reduction, and a detailed electronic
observed peaks are interpreted as transitions between
Dms =ꢁ 1/2 sublevels and can be simulated with the param-
eters g1 = 6.34, g2 = 3.47, g3 = 1.96, and A1 = 474, A2 = 130, and
A3 = 170 MHz. As a rule, the EPR lines for Dms =ꢁ 3/2 are
not observed in noncrystalline 3d7 systems due to their strong
orientation dependence. Comparison of the crystal structure
with the calculated high-spin (Figure 4b) and low-spin (Fig-
ure 4c) configurations also find a much better agreement with
the HS species, showing deviations in the metal–ligand bond
Figure 4. a) EPR spectrum of HS5 (S=3/2) in MeCN. Comparison of
the crystal structure with the calculated HS (b) and LS (c) species.
Figure 5. Cyclic voltammetry profiles of 2, 4, and 5 in dry MeCN.
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characterization is offered here. Important CV parameters for
the reduced analogues of 2–5 are presented in Table T6 (SI)
and electronic and energetic analyses are shown in Fig-
ure S14. Mulliken spin density analysis finds the first reduc-
tion event associated to the CoIII/CoII couple for 4 yielding
a 3d7 HSCoII center. Occupation of the antibonding metal-
based and idealized eg* molecular orbitals (MOs) increases
the metal–ligand bond length in the CoII species and leads to
a rapid loss of the chloro ligand in the newly formed six-
coordinate CoII complex. This loss is energetically favorable
by 5.5 kcalmolꢀ1 and results in a 3d five-coordinate HSCoII
species. Because of the presence of a planar and conjugated
amide framework, some degree of redox non-innocence
behavior is shown by the ligand during the second reduction
event (Figure S14). Complex 5 undergoes oxidation at
ꢀ0.02 V (CoII to CoIII), whereas the first reduction event
happens at ꢀ1.92 V. Calculations suggest that this reduction
event is metal-based and results in a 3d8 CoI center. Loss of
the chloro ligand from CoI complex is favored by 5.4 kcal
molꢀ1. On the other hand, chloride elimination from the six-
coordinate CoII analogue is unfavorable by 4.9 kcalmolꢀ1.
The electrocatalytic behavior was investigated with com-
plexes 4 and 5 due to their stability and robust nature. The
weak organic acids, acetic acid (HOAc; Figure 6a) and
triethyl ammonium chloride (Et3NHCl; Figure S15) were
used as proton sources. A catalytic peak was found near the
second reduction process for 4 upon addition of various
equivalents of HOAc in MeCN (Figure 6a). An observed
catalytic current of 200 mA was measured after addition of
10 equiv of HOAc and the overpotential was 0.740 V when
the homoconjugation effect of the acid was considered.[12]
The rate of H2 generation (kobs) was 7.39 sꢀ1 and the
identity of dihydrogen was confirmed by bulk electrolysis at
ꢀ1.7 VAg/AgCl. A turnover number (TON) of 15.44 after 3 h
was calculated with 100 equiv of acid. Faradaic efficiency was
measured to be approximately 90%. Control experiments
with HOAc in MeCN in absence of 4 exhibited catalytic peaks
at significantly more negative potentials (Figure S16), thus
validating the role of that species as a catalyst. Comparison of
the charge (Q) versus time (t) plots in the presence and
absence of 4 confirms the catalytic activity of the complex
(inset: Figure 6a). Electrocatalytic measurements with com-
plex 5 found a catalytic peak close to the second redox couple
(Figure 6b) as observed with 4. The catalytic current and kobs
are 150 mA and 4.29 sꢀ1, respectively, in the presence of
10 equiv of HOAc. An overpotential of 0.69 V was found and
H2 evolution was confirmed by electrolysis at ꢀ1.7 VAg/AgCl. A
TON of 14.35 was observed after 3 h with 100 equiv of acid
and Faradaic efficiency was 75%. Control experiments (Fig-
ure S16) and Q versus t plots (inset: Figure 6b) confirm the
catalytic behavior of 5. Comparison of the electrocatalytic
behavior of 4 and 5 with that of a blank solution is shown in
Figure S17.
Figure 6. CV experiments of H2 generation by 4 (a) and 5 (b) in
MeCN. The numbers 0–10 indicate the HOAc equiv used in compar-
ison to the complex. Insets: Charge versus time plots during the bulk
electrolysis of 4 and 5 (applied potential: ꢀ1.7 VAg/AgCl). Complex:
4 mmol; HOAc: 0.4 mmol during electrolysis.
without obvious signs of catalyst decomposition, as suggested
by the straight Q vs. t plots shown as insets in Figure 7 and in
Figure S18. The catalytic peak for 5 appears at the slightly
more positive onset overpotential of 0.70 V, and electrolysis at
ꢀ1.7 VAg/AgCl confirmed dihydrogen generation. The TON and
Faradaic efficiency were calculated to be 1400 and 95%,
respectively, over 3 h. No obvious decomposition of the
catalyst 5 was observed after 18 h, when the TON reaches
over 6000 and a linear Q versus t plot persisted (Figure S19).
Control experiments generate some dihydrogen at more
negative potentials than those with 4 and 5. Charge con-
sumption over time is much higher with both the complexes
than that with the blank solution. Comparison of the electro-
catalytic behavior of 4 and 5 in water is shown in Figure S20.
Relevant catalytic parameters are reported in Table T7 (SI).
Another control experiment was performed to further probe
the catalytic nature of species 4 and 5; the catalytic activity of
those species was compared with that of a cathodically
polarized working glassy carbon electrode after catalysis,
rinsed and dipped in fresh electrolyte. The results, shown in
Figures S21 and S22, confirm the need for the catalysts to
decrease the overpotential for H2 formation. The robustness
of the catalysts was also investigated by spectroscopic and
spectrometric means (Figures S23 and S24). Both species
The ultimate goal of electrocatalytic H2 generation from
neutral water (pH 7) in phosphate buffer (1m) was attained
with 4 and 5 (Figure 7a and 7b). The catalytic peak appears at
an onset overpotential of 0.55 V for 4. Electrolysis at
ꢀ1.7 VAg/AgCl yielded a TON of 1,615 over 3 h with Faradaic
efficiency of 95%. The TON reaches over 7000 after 18 h
4
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Figure 8. Catalytic mechanism of H2 generation by 4 in MeCN. It
II
ꢀ
involves a Co
The calculated energetics for all events is reported as free energy
changes in kcalmolꢀ1
H
ꢀ species that undergoes protonation to generate H2.
.
(Figure 8), regenerating the five-coordinate CoII complex
to restart the catalytic cycle. The reaction of a proton with
II
[5c]
ꢀ
Co H is expected to be activationless.
The homolytic
II
ꢀ
pathway by the combination of two Co H complexes is
significantly less exothermic compared to the heterolytic
mechanism. A proton-coupled electron transfer (PCET)
event is not invoked for the CoII/CoIII
H
transformation,
ꢀ
ꢀ
because no anodic shift was found in the experimental
electrocatalytic measurement of 4 upon decrease of pH.
Complex 5 follows a similar catalytic mechanism as described
Figure 7. CV experiments of H2 generation by 4 (a) and 5 (b) from
water (pH 7 with 1m phosphate buffer). Insets: charge versus time
plots during the bulk electrolysis of 4 and 5 (applied potential:
ꢀ1.7 VAg/AgCl). Complex: 0.2 mmol during electrolysis.
I
II
ꢀ
in Figure S25; however, the involvement of a Co /Co
H
PCET mechanism may be relevant. The generation of H2 in
both mechanisms favors a heterolytic pathway with another
proton, rather than a bimolecular mechanism through the
II
ꢀ
ꢀ
combination of two Co H complexes.
show similar aerobic UV/Vis spectra before and after bulk
electrolysis, and the presence of ESI(+)-MS peaks with m/z =
488 and 489 associated with the cations [4]+ and [5]+,
respectively. Interestingly, species 4 displays a blue-shifted
spectrum after anaerobic bulk electrolysis that is converted to
the original spectrum in the presence of air. This observation
suggests that the CoIII species 4 is a procatalyst in good
agreement with the calculated catalytic cycles discussed in
Figures 8 and S25.
In conclusion, we have described the syntheses and
characterization of a series of pentadentate, pyridine-rich
Co complexes 2–5 with imine, methoxy, amide, and N-
methylamine functionalities. The methoxy and amide species
arise from the imine complex 2 through reaction with
methanol and water solvent, respectively. The CoIII imine 2
arises from aerial oxidation of the corresponding CoII amine
species 1. Introduction of an N-methyl function in 1 inhibits
this oxidation and yields the HSCoII N-methylamine 5.
Electrocatalytic experiments with 4 and 5 generate H2 with
TONs of 15.44 and 14.35, respectively, after 3 h in the
presence of acetic acid in MeCN solutions. Remarkably, 4 and
5 are excellent water reduction catalysts with TONs of 7000
and 6000, respectively, after 18 h. The present TONs insert
these two species among a select group of top catalysts for
Mechanisms of electrocatalytic H2 evolution by 4 and 5
have been evaluated by DFT calculations. Figure 8 describes
the catalytic pathway for complex 4 in MeCN. The five-
coordinate CoII species, generated after dissociation of the
chloro ligand, undergoes reduction to the corresponding
CoI complex. The reduction potential is calculated as
ꢀ1.83 VFc/Fc+. Uptake of a proton by the CoI complex is
favorable by 22.5 kcalmolꢀ1 and results in the six-coordinate
water reduction. Catalytic mechanisms obtained by DFT
ꢀ
calculations involve the formation of a CoIII H species that
ꢀ
ꢀ
CoIII
Co
H
ꢀ
complex, which gets reduced to the more reactive
species. The latter is high-spin in nature and
undergoes further reduction followed by protonation of the
hydride on the cobalt center. Current efforts in our groups
aim to enable the use of these species for photocatalysis.
ꢀ
HS
II
ꢀ
H
occupation of the idealized eg* MOs weakens the metal–
ꢀ
ligand interactions. The Co H bond elongates from 1.49 ꢀ in
the LSCoIII complex to 1.73 ꢀ in the HSCoII species. As a result,
II
ꢀ
ꢀ
the hydride in the Co H moiety is susceptible to heterolytic
attack by an external proton. Uptake of a proton and
generation of H2 by this complex is favored by 54.0 kcalmolꢀ1
Received: October 6, 2014
Revised: November 25, 2014
Published online: && &&, &&&&
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Keywords: cobalt complexes · Co H species ·
.
proton reduction · pyridine ligands · water reduction
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Water Reduction
Pyridine and cobalt: A series of cobalt
complexes with pentadentate pyridine-
rich ligands is studied both for their
ligand transformations as well as for their
catalytic activity toward proton and water
reduction. Turnover numbers of up to
7000 after 18 h were observed.
D. Basu, S. Mazumder, X. Shi,
H. Baydoun, J. Niklas, O. Poluektov,
H. B. Schlegel,*
C. N. Verani*
&&&& — &&&&
Ligand Transformations and Efficient
Proton/Water Reduction with Cobalt
Catalysts Based on Pentadentate
Pyridine-Rich Environments
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!