Evaluation of the coordination preferences and catalytic pathways of heteroaxial cobalt oximes towards hydrogen generation

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Evaluation of the coordination preferences and

catalytic pathways of heteroaxial cobalt oximes

towards hydrogen generation†

Cite this: DOI: 10.1039/c5sc04214c

Debashis Basu,‡^aShivnath Mazumder,‡^aJens Niklas,^bHabib Baydoun,^a

Dakshika Wanniarachchi,^aXuetao Shi,^aRichard J. Staples,^cOleg Poluektov,^b

a

*

H. Bernhard Schlegel and Claudio N. Verani

´

Three new heteroaxial cobalt oxime catalysts, namely [Co^III(prdioxH)(^4tBupy)(Cl)]PF₆(1),

[Co^III(prdioxH)(^4Pyrpy)(Cl)]PF₆(2), and [Co^III(prdioxH)(^4Bzpy)(Cl)]PF₆(3) have been studied. These species

contain chloro and substituted tert-butyl/pyrrolidine/benzoyl-pyridino ligands axially coordinated to

a

trivalent cobalt ion bound to the N₄-oxime macrocycle (2E,2⁰E,3E,3⁰E)-3,3⁰-(propane-1,3-

diylbis(azanylylidene))bis(butan-2-one)dioxime, abbreviated (prdioxH)^ꢀin its monoprotonated form.

Emphasis was given to the spectroscopic investigation of the coordination preferences and spin

conﬁgurations among the diﬀerent 3d⁶Co^III, 3d⁷Co^II, and 3d⁸Co^Ioxidation states of the metal, and to

the catalytic proton reduction with an evaluation of the pathways for the generation of H₂via Co^III–H^ꢀ

or Co^II–H^ꢀintermediates by mono and bimetallic routes. The strong ﬁeld imposed by the (prdioxH)^ꢀ

ligand precludes the existence of high-spin conﬁgurations, and 6-coordinate geometry is favored by the

^LSCo^IIIspecies. Species 1 and 3 show a split Co^III/Co^IIelectrochemical wave associated with partial

chemical conversion to a [Co^III(prdioxH)Cl₂] species, whereas 2 shows a single event. The reduction of

these Co^IIIcomplexes yields ^LSCo^IIand ^LSCo^Ispecies in which the pyridine acts as the dominant axial

ligand. In the presence of protons, the catalytically active Co^Ispecies generates a Co^III–H^ꢀhydride

species that reacts heterolytically with another proton to generate dihydrogen. The intermediacy of

a triﬂuoroacetate-bound Co^III/Co^IIcouple in the catalytic mechanism is proposed. These results allow

for a generalization of the behavior of heteroaxial cobalt macrocycles and serve as guidelines for the

development of new catalysts based on macrocyclic frameworks.

Received 5th November 2015

Accepted 30th January 2016

DOI: 10.1039/c5sc04214c

www.rsc.org/chemicalscience

systems and have been extensively scrutinized. In spite of this

interest and of available information for individual cobalt

oximes,⁶comprehensive spectroscopic studies that interrogate

Introduction

Research in molecular catalysts for proton and water reduction

has been intensive and in recent years special attention was

paid to the use of Earth-abundant metals.^1–4Cobalt ions, with

their ability to span redox states from 3+ (3d⁶) to 1+ (3d⁸) have

been emphasized, and eﬀective catalysts have been designed for

both proton and water reduction with oximes.³Starting with the

adventitious observation by Espenson & Connolly⁵that biomi-

metic cobalt oximes are capable of proton reduction in acidic

organic media, these catalysts gure among the rst observed

the transformation of oxidation states along the reductive

process that leads to the catalytically active Co^Ispecies are rare.

Furthermore, the vast majority of studied catalytic systems

contain equivalent axial ligands, hereaer called homoaxial,

and added complexity is observed when non-equivalent axial

ligands are present. These heteroaxial cobalt oximes display two

Co^III/Co^IIredox processes associated with the formation of

a new Co^IIIspecies in solution. These distinct species yield

a monovalent catalyst. A previous study to rationalize this

behavior was performed by Gerli and Marzilli⁷where an exclu-

sively electrochemical pathway was proposed, leading to

a square planar Co^IIspecies that yields a catalytic 4-coordinate

Co^Iintermediate. However, both experimental^8,9and theoret-

ical^6gevidence suggest that this intermediate requires 5-coor-

dination with N-containing donors.

^aDepartment of Chemistry, Wayne State University, Detroit, MI 48202, USA. E-mail:

cnverani@chem.wayne.edu

^bChemical Sciences and Engineering Division, Argonne National Laboratory, Argonne,

IL 60439, USA

^cDepartment of Chemistry, Michigan State University, Lansing, MI 48824, USA

† Electronic supplementary information (ESI) available. CCDC 1434959, 1434960,

1448834 and 1449138. For ESI and crystallographic data in CIF or other electronic

format see DOI: 10.1039/c5sc04214c

Thus, we propose that a unifying investigation based on UV-

visible, nuclear magnetic and electron paramagnetic (NMR &

EPR) resonance spectroscopies allied with DFT calculations will

‡ D. B. and S. M. contributed equally to this work.

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ꢀ

allow for the understanding of the coordination preferences of broad peak around 850 cm^ꢀ1indicative of the presence of PF₆

heteroaxial cobalt oxime systems and, therefore, of the behavior counterions. High-resolution ESI mass spectra (Fig. S2†) display

of the trivalent, divalent, and monovalent species crucial for the species [M]⁺at 468.1573, 481.1563 and 516.1207 respectively

catalytic H₂generation. This knowledge will enable us to chart for 1, 2, and 3. Experimental and simulated isotopic distribu-

the transformations that occur during the catalytic cycle tions are in agreement with the proposed molecular composi-

1

addressing the nature of formed hydrides, the viability of tion. H-NMR spectra were taken in CD₃CN and the spectrum

proton-coupled electron transfer, the homo or heterolytic for 1 is shown in Fig. 2, and the narrow lines conrm the

nature of hydrogen formation, and the regeneration of the diamagnetic nature of these species, as associated with a 3d⁶

catalyst.

Continuing

^LSCo^IIIion.

a

long-standing collaboration on cobalt

The spectra show appropriate line-splitting patterns for 1

systems,¹⁰we have recently examined the catalytic activity of with four aromatic protons between 6 and 8 ppm originating

cobalt species associated with pendant phenolates^10eand pyr- from the pyridine moiety, one OH proton at 18.85 ppm, six

idines.^10fWe have also studied in detail a new pentadentate methylene protons between 2.0 and 4.5 ppm and 12 methyl

cobalt oxime system^10gdesigned to stabilize the above- protons between 2.4 and 2.6 ppm. The intense peaks found

mentioned preferential 5-coordination favored by Co^Iin cata- around 1.3 ppm are from the protons (9H) of the t-butyl

lysts used for proton/water reduction. We observed that upon substituent. Complexes 1 and 3 yielded diﬀraction-quality

metal coordination this new oxime incorporates a molecule of crystals whose ORTEP plots are shown in Fig. 3. In both cases

water and enables H⁺reduction in presence of weak acids in a pseudo-octahedral geometry of the cobalt center was found

acetonitrile (CH₃CN).

with the oxime ligand occupying the equatorial plane, while

In this paper we present three new heteroaxial catalysts, chloride and ^4tBupyridine/^4Bzpyridine ligands occupy the axial

namely [Co^III(prdioxH)(^4tBupy)(Cl)]PF₆(1), [Co^III(prdioxH)(^4Pyrpy)- positions. One of the oxime-oxygen atoms remains protonated

(Cl)]PF₆(2), and [Co^III(prdioxH)(^4Bzpy)(Cl)]PF₆(3), Fig. 1, in forming an O–H/O hydrogen-bonded macrocyclic framework.

which the axial pyridine displays both electron-donating and The Co–N^oximebond lengths (1.88–1.93 A) are in excellent

electron-withdrawing functionalities. We interrogate by agreement with other oxime complexes in the literature.^3c,7

˚

concerted experimental and theoretical methods the trans- Comparison of the Co–Cl bond length between 1 and 3 reveals

˚

formations in nature and coordination preferences associated a slightly longer bond in 1 (2.2376 A) than in 3 (2.2336 A). On the

with the 3+, 2+, and 1+ oxidation states of the metal, and propose other hand, the bond length between Co and the N^Pyatom

˚

a viable catalytic pathway for hydrogen formation. These results (Co1–N5) is shorter in 1 (1.975 A) than in 3 (1.990 A). These

are relevant to the development of strategies for the improve- results suggest that incorporation of the electron-withdrawing

ment of new catalysts for proton and water reduction based on benzoyl substituent on the pyridine ring of 3 weakens the Co–

heteroaxial cobalt macrocycles.

N5 (pyridine) bond and strengthens the Co–Cl bond.

Coordination preferences of diﬀerent oxidation states in the

proton reduction pathway

Results and discussion

Synthesis and characterization of 1, 2, and 3

The 1e^ꢀand 2e^ꢀreduced analogs of 1–3 generated in the elec-

trochemical pathway towards catalysis determine the viability of

proton reduction. Therefore, a detailed evaluation of their

coordination preferences, spin states, and electronic structures

is of the utmost importance in the understanding of catalytic

pathways. We probed each of these species by cyclic

The tetradentate ligand prdioxH₂was obtained by literature

methods.^11,12Complexes 1, 2, and 3 were generated by stepwise

reaction of CoCl₂$6H₂O with the ligand prdioxH₂to obtain the

dichloro complex which was further treated with 4-substituted

pyridines and KPF₆to yield 1–3 (Fig. S1†). The FTIR spectra of

the complexes reveal a C]N stretching vibration mode associ-

ated with the oxime ligand at ca. 1600 cm^ꢀ1and a strong and

Fig. 1 The mononuclear complexes [Co^III(prdioxH)(^4tBupy)(Cl)]PF₆(1),

[Co^III(prdioxH)(^4Pyrpy)(Cl)]PF₆(2), and [Co^III(prdioxH)(^4Bzpy)(Cl)]PF₆(3).

Fig. 2 ¹H-NMR spectra of 1 in CD₃CN.

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In order to check for potential coordination of the solvent

CH₃CN, thus generating a new species [Co^III(prdioxH)(CH₃-

CN)₂]²⁺associated with the second Co^III/Co^IIredox event

observed in 1 and 3, the CV of 1 was repeated in non-coordi-

nating CH₂Cl₂(Fig. S3†). Both Co^III/Co^IIevents persisted, thus

ruling out the formation of bis-acetonitrile species. The origin

of this second Co^III/Co^IIprocess will be discussed later.

Electronic properties. Complexes 1–3 are nearly transparent

in the visible region (Fig. S4†). A detailed study was conducted

with the parent Co^IIIspecies and Co^IIand Co^Ireduced coun-

terparts of 1, isolated in CH₃CN by bulk electrolysis at ꢀ0.46

and ꢀ0.95 V_Ag/AgCl, respectively (Fig. 5). The spectrum of 1

displays a light yellow color and is marked by the absence of

charge-transfer (CT) bands characteristic of a 3d^{6 LS}Co^IIIcenter

bound to a strong-eld p-acceptor ligand framework.¹²The

orange Co^IIspecies shows a distinct peak at 480 nm (3 ¼ 2288

M^ꢀ1cm^ꢀ1) attributed to a metal-to-ligand charge transfer

(MLCT) transition. The optical spectrum of the Co^Ispecies is

tentatively associated with both intraligand charge transfer

(ILCT) and MLCT bands at 572 and 685 nm (3 ¼ 1738 and 1570

M^ꢀ1cm^ꢀ1), respectively, accounting for the intense dark blue

color of the complex. As suggested by Espenson et al.⁸the higher

orbital energy in the Co^Istate compared with Co^II, shis the

lower energy transition to the region between 500 and 700 nm.

Similar observations were made by Peters et al.⁹when

studying hydrogen oxidation reactions with Co^IIhomoaxial

oxime complexes. The parent yellow solution of Co^IIspecies

(l_max¼ 440 nm) turns to Co^Iupon hydrogen oxidation. The Co^I

product exhibited a dark blue color and showed two strong

absorption bands between 500 and 700 nm.⁹Eisenberg et al.^3h

also detected similar Co^Ispectra (l_max¼ 550, 650 nm) for Co^II

oxime complexes with homoaxial water ligands and with Pt-

chromophore and TEOA as the sacricial donor in the presence

of visible light. Thus, the parent compounds 1–3 seem to allow

for reduction to Co^Iat aﬀordable potentials. Knowledge on the

nature of this species is relevant for the understanding of

competing mechanisms that precede catalytic proton reduction

in acidic organic media.

Fig. 3 ORTEP representation of the crystal structures for [Co^III(pr-

dioxH)(^Xpy)(Cl)]⁺cations in 1 (CCDC 1434960) and 3 (CCDC 1434959)

at 50% ellipsoid probability. For 1: Co(1)–N(1) ¼ 1.903(3); Co(1)–N(2) ¼

1.917(2); Co(1)–N(3) ¼ 1.914(2); Co(1)–N(4): 1.898(2) ¼ Co(1)–N(5) ¼

1.975(3); Co(1)–Cl(1) ¼ 2.237(8) A˚. For 3: Co(1)–N(1) ¼ 1.891(3); Co(1)–

N(2) ¼ 1.934(3); Co(1)–N(3) ¼ 1.923(3); Co(1)–N(4) ¼ 1.893(3); Co(1)–

N(5) ¼ 1.990(3); Co(1)–Cl(1) ¼ 2.2336(11) A˚.

voltammetry (CV), ¹H-NMR, UV-visible, EPR, and DFT methods

aiming to characterize comprehensively the nature of these

species.

Redox properties. Complexes 1–3 were investigated by CV in

CH₃CN with the resulting traces shown in Fig. 4 and relevant

potentials summarized in Table 1. Species 1 revealed two events

associated with the Co^III/Co^IIcouple at ꢀ0.49 V (E_p,c) and ꢀ0.70

V (E_1/2), similar to the values for a structurally related species,⁷

whereas the Co^II/Co^Iprocess appears at ꢀ1.09 V (E_1/2); all

potentials are given versus the ferrocene/ferrocenium (Fc/Fc⁺)

couple. One additional reduction process was found at ꢀ2.11

+

V_Fc/Fcand may be attributed to the imine/oxime ligand. A

similar prole was observed for 3, while a slightly diﬀerent

behavior was observed for 2 with seemingly only one Co^III/Co^II

event.

The coordination environment of the Co^IIIspecies. Crystal

1

structures combined with H-NMR spectroscopy provided the

strongest evidence to elucidate the coordination environment

for Co^IIIin the parent state. The crystal structure of 1 (Fig. 3)

conrmed the expected hexacoordination for this 3d⁶ion with

the oxime as the planar ligand, and chloride and 4-substituted

pyridines as axial ligands in the solid state. The ¹H-NMR spectra

(Fig. 2) also conrmed the presence of 4-substituted pyridines

in the Co^IIIstate in solution even in the presence of a coordi-

nating solvent such as CD₃CN. In the process of elucidating the

nature of the second Co^IIIspecies observed for 1 during the CV

experiment we considered the formation of the homoaxial

species [Co^III(prdioxH)(Cl)₂] and/or [Co^III(prdioxH)(^4tBupy)₂]-

(PF₆)₂. These species were independently synthesized and their

redox behavior was compared, as shown in Fig. 6.

III

II

+

A potential of ꢀ0.70 V_Fc/Fcobserved for the second Co /Co

process of 1 matched the equivalent process in the dichloro

analog, thus suggesting its formation during the CV experi-

ment. A similar result was obtained for 3. On the other hand,

Fig. 4 Cyclic voltammograms of 1, 2, and 3 in CH₃CN (supporting

electrolyte: TBAPF₆; scan rate: 100 mV s^ꢀ1; glassy C: working elec-

trode; Pt-wire: auxiliary electrode; Ag/AgCl: reference electrode).

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Table 1 Cyclic voltammetric parameters for 1–3 in CH₃CN. Potentials reported vs. Fc/Fc⁺

Co^III/Co^II

E_1/2, [E_p,c; E_p,a], V

Co^II/Co^I

E_1/2, V

C]N/Cc–N^ꢀ

E_1/2, V

Other L-based events

E_1/2, V

(DE, V) |i_pc/i_pa

|

(DE, V) |i_pc/i_pa

|

(DE, V) |i_pc/i_pa

|

(DE, V) |i_pc/i_pa|

1

E_p,c: ꢀ0.49; E_p,a: ꢀ0.19

ꢀ0.70 (0.10) |0.91|

ꢀ0.59 (0.12) |0.45|

E_p,c: ꢀ0.43; E_p,a: ꢀ0.13

ꢀ0.68 (0.11) |1.41|

ꢀ1.09 (0.11) |0.99|

ꢀ2.11 (0.23) |2.23|

2

3

ꢀ1.07 (0.08) |1.09|

ꢀ1.09 (0.08) |1.19|

ꢀ2.07 (0.23) |N/A|

ꢀ2.22 (0.12) |5.99|

ꢀ1.79 (0.08) |N/A|

ꢀ1.91 (0.03) |1.03|

there was considerable mismatch between the cathodic poten-

III

II

+

tial for 1 at ꢀ0.49 V_Fc/Fcand the cathodic Co /Co potential of

+

the bis-pyridine species at ꢀ0.37 V_Fc/Fc. These combined results

are suggestive that the uncoordinated chloride ion is capable of

displacing the pyridine coordinated to the Co ion in 1. However

formation of a homoaxial bis-pyridine adduct is highly unlikely.

In order to determine whether this substitution is driven by

electrochemical reduction⁷or by chemical lability of the axial

ligands in the trivalent state of 1, we performed ¹H-NMR

experiments in which external sources of chloride or ^4tBupyr-

idine were added. In independent experiments we added 1

equiv. ^4tBupyridine, tetraethylammonium chloride (Et₄NCl) or

triethylamine (Et₃N) to the CD₃CN solution of 1 and compared

their ¹H-NMR spectra with those of 1 and free ^4tBupyridine

(Fig. 7a). Note that in these experiments the valence of the Co^III

ion remained unchanged. Complete spectra are shown in

Fig. S13.†

Fig. 5 UV-visible absorption spectra of 1 (Co^III) and its reduced Co^II

analogue with l ¼ 480 nm (3 ¼ 2288 M^ꢀ1cm^ꢀ1) and Co^Ianalogue with

l ¼ 572 nm (3 ¼ 1738 M^ꢀ1cm^ꢀ1) and l ¼ 685 nm (3 ¼ 1570 M^ꢀ1cm^ꢀ1)].

Uncoordinated ^4tBupyridine exhibited two sets of ¹H-NMR

peaks at 7.36 (H_B) and 8.50 (H⁰_A) ppm whereas the coordinated

^4tBupyridine in 1 displayed peaks at 7.35 (H_B) and 7.52 (H_A) ppm.

The H_Bprotons retain the same chemical shi but the H⁰_A

protons shied from 8.50 to 7.52 ppm on coordination of the

^4tBupyridine to the metal center. Aer 1 equiv. of ^4tBupyridine

was added to the CD₃CN solution of 1, the appearance of signals

from the uncoordinated ^4tBupyridine protons at 8.55 ppm (2H)

was observed and an increase of the intensity of the peak at 7.37

ppm (2H + 2H) was found whereas the proton count at 7.48 ppm

of the coordinated ^4tBupyridine did not change in intensity

suggesting no coordination of the external ^4tBupyridine ligand

and therefore no replacement of the chloride on cobalt. On the

contrary, when 1 equiv. of Et₄NCl (the external chloride source)

was added to 1, peaks at 7.39 (2H) and 8.50 ppm (2H) were

found which are characteristic of the uncoordinated ^4tBupyr-

idine ligand. Therefore we conclude that ^4tBupyridine in 1 can be

replaced by chloride. This substitution is chemical in nature

and the Co ion remains trivalent. The opposite, i.e. replacement

of chloride by pyridine, is not observed. This substitution

pattern was corroborated by DFT calculations¹³via comparison

of the energetics of the diﬀerent chemical events. The calcula-

tions, reported as free energy changes in kcal mol^ꢀ1, found the

low spin states to be energetically favorable (Table S1†). The low

spin state was also conrmed by EPR spectroscopy. The

substitution of the ^4tBupyridine ligand by chloride is favorable

by ca. 10 kcal mol^ꢀ1, while replacement of chloride by external

Fig. 6 Comparison of CVs for the heteroaxial 1 (bottom), homoaxial

[Co^III(prdioxH)(Cl)₂] (top), and homoaxial [Co^III(prdioxH)(^4tBupy)₂]PF₆

(middle), in CH₃CN. The second Co^III/Co^IIevent appears at similar

potentials for 1 and its dichloro analogue, suggesting the formation of

the latter species during the reduction of 1.

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Table 2 Energetics of pyridine substitution by DFT calculations

X

A^a

B^a

(A + B)^a

^4tBupy

^4Pyrpy

^4Bzpy

13.5

20.0

14.1

ꢀ23.2

ꢀ9.7

ꢀ3.7

ꢀ9.1

Values in kcal mol^ꢀ1

.

a

contrary, when 1 equiv. of Et₃N was added to 1 in CD₃CN, we

observed near complete disappearance of the oxime-OH peak

intensity and slight shi of the position of the signal from the

OH suggesting that Et₃N can deprotonate the ligand. In

summary, these results allow us to conclude that the second

Co^III/Co^IIredox event observed for 1 and 3 is associated with the

chemical formation of [Co^III(prdioxH)(Cl)₂]. The chemical,

rather than electrochemical basis for this process is supported

by the fact that external ^4tBupyridine is unable to replace the

chloride in the trivalent Co ion nor to deprotonate the oxime

–O–H/O– moiety. On the other hand, facile replacement of

^4tBupyridine by chloride on Co(III) was observed. This observa-

tion of a chemical, rather than electrochemical process diverges

from the mechanism proposed by Gerli and Marzilli.⁷Further-

more, the presence of an electron-donating moiety attached to

the pyridine strengthens the Py–Co bond and prevents substi-

tution in 2.

Fig. 7 (a) ¹H-NMR experiments of 1 in CD₃CN upon addition of 1

equiv. of ^4tBupyridine, Et₄NCl, or Et₃N. (b) Energetics of substitution of

the axial ligands in 1 as obtained by DFT calculations (energies given in

kcal mol^ꢀ1). As demonstrated by the ¹H-NMR spectra and DFT

calculations, chloride can replace the ^4tBupyridine ligand in 1 but the

^4tBupyridine ligand cannot substitute the chloride.

The coordination environment of the Co^IIspecies. The

nature and spin states of the Co^IIspecies generated aer the

rst and second Co^IIIreduction events were interrogated by EPR

spectroscopy. Bulk electrolyzed samples of 1 were obtained by

the application of appropriate potentials at ꢀ0.20 V_Ag/AgCl

(ꢀ0.63 V_Fc/Fc) for the rst or ꢀ0.46 V_Ag/AgClfor the second Co^III/

+

Co^IIcouple. The third reduction event at ꢀ0.66 V_Ag/AgClis

attributed to formation of a Co^Ispecies. The CW X-band EPR

spectra are shown in Fig. 8. The parent complex is EPR silent (S

¼ 0), as expected for a low spin Co^IIIspecies (^LS3d⁶; Fig. 8a). The

species resulting from the rst redox event (ꢀ0.20 V_Ag/AgCl) is

paramagnetic and generates a spectrum (Fig. 8(b1)) with rela-

tively low g-tensor anisotropy and small ⁵⁹Co hyperne splitting

(I(⁵⁹Co) ¼ 7/2), and resolved superhyperne structure in the

high-eld region. The magnetic resonance parameters obtained

by simulation of the spectrum (Fig. 8(b2)) are as follows: g ¼

2.207, 2.176, 2.015; A(⁵⁹Co) ¼ 65, 80, 260 MHz; A(¹⁴N) ¼ 36, 39,

48 (both ¹⁴N nuclei). This type of spectrum and these magnetic

resonance parameters are in good agreement with previous EPR

studies of cobalt oximes (^LS3d⁷, S ¼ 1/2) axially coordinated by

two nitrogen atoms, which are responsible for the super-

hyperne structure.¹⁴Note, that superhyperne structure from

equatorial nitrogen atoms is here not observed in the EPR

spectra, which is typical for cobalt oximes or related complexes,

since their hyperne couplings are much smaller than those of

^4tBupyridine is unfavorable by ca. 12 kcal mol^ꢀ1(Fig. 7b). The

energetics of pyridine substitution are shown in Table 2 for 1–3

and the proposed mechanism involves an unsaturated ve-

coordinate intermediate. This event requires ca. 14 kcal mol^ꢀ1

,

is energetically unfavorable and the limiting step for 1 and 3.

Chloride addition to the ve-coordinate intermediate is favored

by 23 kcal mol^ꢀ1and drives the overall (A + B) substitution

process forward by 9–10 kcal mol^ꢀ1

.

On the other hand, complex 2 with an electron-donating

pyrrolidine ligand has the most energy-demanding rst step (20

kcal mol^ꢀ1) and shows little preference for pyridine substitution

with a total energy of the substitution process at ꢀ3.2 kcal

mol^ꢀ1. This is in agreement with absence of a second Co^III/Co^II

process during the CV experiment. We also probed the acidic

nature of the oxime-OH hydrogen as it can contribute to the CV

response and found out that no deprotonation, i.e. no change of

the proton count or shi of the oxime-OH hydrogen at 18.84

ppm was observed upon addition of ^4tBupyridine. On the

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the following magnetic resonance parameters: g ¼ 2.320, 2.200,

2.017; A(⁵⁹Co) ¼ 120, 95, 180 MHz. These results suggest that

axial coordination by nitrogen atoms is unlikely.^14aSimilarly,

the magnetic resonance parameters, especially the hyperne

splitting in the high eld region due to ⁵⁹Co, do not support

coordination by oxygen, e.g., from adventitious H₂O. We tenta-

tively assign this spectrum to a Co-species with one or two axial

chloride ions. Titration experiments were carried out with 1 to

evaluate the eﬀects of adding chloride and ^4tBupyridine ligands

during the electrochemical experiment (Fig. 10a).

We observed that in presence of increasing amounts of

Et₄NCl, the rst Co^III/Co^IIprocess decreases while the second

process increases in intensity. While the rst process dis-

appeared aer ca. 0.6 equivalents of Et₄NCl, addition of

^4tBupyridine showed only minor cathodic shis for both the

Co^III/Co^IIprocesses. The CV (Fig. S5†) of a fresh solution

prepared with 1 : 1 Et₄NCl and 1 conrmed the disappearance

of the rst Co^III/Co^IIprocess and an increase in intensity of the

second process. As expected, both the Co^III/Co^IIprocesses

retained their intensities in a similarly prepared solution of 1 : 1

^4tBupyridine and 1 (Fig. S5†). These results conrm the CV

experiments in Fig. 10a and suggest that the chloride ion is

involved in both the Co^III/Co^IIreduction events of complex 1. On

the other hand, ^4tBupyridine has little or no eﬀect on the Co^III/

Co^IIreduction. DFT calculations support that loss of chloride

aer the rst Co^III/Co^IIreduction of 1 is thermodynamically

favorable by 6.8 kcal mol^ꢀ1and yields the ve-coordinate

[Co^II(prdioxH)(^4tBupy)]⁺. Alternatively, dissociation of the

^4tBupyridine ligand is calculated to be lower in energy by 10.2

kcal mol^ꢀ1. These results demonstrate that there is a greater

thermodynamic preference of [Co^II(prdioxH)(^4tBupy)(Cl)] to

release ^4tBupyridine rather than chloride giving rise to [Co^II(pr-

dioxH)(Cl)]. However, calculations of the transition states of the

axial ligand dissociation nd that loss of chloride is associated

with a smaller kinetic barrier than that of the dissociation of

^4tBupyridine (Fig. 10b). The diﬀerence between the activation

Fig. 8 CW X-band EPR experiments to probe the spin-states and

coordination environments of the complexes (Co^III, Co^II, and Co^I)

generated during the electrochemistry of 1 in CH₃CN. Spectra (b1) and

(c1) were obtained after bulk electrolysis of 1 at ꢀ0.20 V_Ag/AgCland

ꢀ0.46 V_Ag/AgCl, respectively.

axially coordinated nitrogen atoms.^14,15These results suggest

axial ligation of the central cobalt ion by two nitrogen (I(¹⁴N) ¼

1) atoms implying a six-coordinate cobalt center, either with two

^4tBupyridines, one ^4tBupyridine and one CH₃CN, or two CH₃CN

moieties.

Based on the EPR data alone it is not possible to pinpoint the

nature of these axial ligands, however we rule out the presence

of two ^4tBupyridines based on the mismatch of observed redox

potentials between 1 and [Co^III(prdioxH)(^4tBupy)₂]²⁺in Fig. 6.

Furthermore, DFT calculations suggest that formation of

a [Co^II(prdioxH)(CH₃CN)₂]⁺from [Co^II(prdioxH)(CH₃CN)]⁺is

unfavorable by about 9 kcal mol^ꢀ1in good agreement with

results reported by Artero et al.^6mTherefore, we propose that

once the chloride is lost, a ve-coordinate species [Co^II(pr-

dioxH)(^4tBupy)]⁺is formed as an intermediate. Loss of chloride

following the reduction of 1 leads to minor conformational

energies is 1.2 kcal mol^ꢀ1. The Arrhenius model, k ¼ A e^(ꢀE/RT)

,

estimates that the rate constant k of a chemical reaction

increases by 10 times with a decrease of the activation energy E

by 1.36 kcal mol^ꢀ1at 298 K. Therefore, we propose that the

dissociation of the axial ligands from [Co^II(prdioxH)(^4tBupy)(Cl)]

is kinetically controlled favoring the formation of

[Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺which is conrmed by EPR as

well (Fig. 8). A comparison of X-ray crystal and DFT-calculated

changes, where the Co^IIion is displaced by 0.15 A out of the

˚

plane of the macrocyclic ligand and towards the remaining

^4tBupyridine (Fig. 9).

This change likely sterically precludes coordination of

a second ^4tBupyridine, but may allow for coordination of a linear

CH₃CN molecule. This coordination to [Co^II(prdioxH)(^4tBupy)]⁺

species is, in principle, thermodynamically unfavorable in good

agreement with the literature.^6mHowever it may take place

considering the large excess of CH₃CN as solvent. The Co^II-

species resulting from the second redox event at ꢀ0.46 V_Ag/AgCl

equally supports a ^LS3d⁷center with S ¼ 1/2, but with a diﬀerent

g-tensor, hyperne structure, and no superhyperne splitting

(Fig. 8(c1)). The simulation of this spectrum (Fig. 8(c2)) yields

Fig.

9

DFT-optimized geometries of [Co^III(prdioxH)(^4tBupy)(Cl)]⁺,

[Co^II(prdioxH)(^4tBupy)(Cl)], and [Co^II(prdioxH)(^4tBupy)]⁺. The metal

center is displaced by 0.15 A˚ oﬀ of the plane of the macrocyclic oxime

ligand in [Co^II(prdioxH)(^4tBupy)]⁺.

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reduction of 1. We conclude that a six-coordinate geometry with

^4tBupyridine as one axial ligand and a CH₃CN ligand as second is

favored by the Co^IIspecies formed aer the rst reduction.

A ve-coordinate geometry with ^4tBupyridine as the axial

ligand maybe present in a certain admixture. Even though the

binding of CH₃CN is – according to DFT calculations – ener-

getically not favorable, the large excess of CH₃CN as solvent

shis the equilibrium towards the six-coordinate species. The

3d⁷Co^IIspecies are low-spin. The second reduction involves

conversion of [Co^III(prdioxH)(Cl)₂] to [Co^II(prdioxH)(Cl)₂]^ꢀthat

yields [Co^II(prdioxH)(Cl)] and possibly [Co^II(prdioxH)(CH₃-

CN)(Cl)]. In the divalent state chloride is amenable to substitu-

tion by ^4tBupyridine ligand thus forming [Co^II(prdioxH)(^4tBupy)]⁺

and possibly [Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺. Therefore, prior to

the reduction from Co^II

/

Co^I

a

single species

[Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺is present in solution, as evi-

denced by the presence of a single wave in Fig. 4. Like for the rst

reduction, the 3d⁷Co^IIspecies are low-spin.

The coordination environment of Co^Ispecies. The resulting

EPR spectrum of the fraction obtained aer bulk electrolysis of

1 at ꢀ0.95 V_Ag/AgClin CH₃CN showed only a very weak signal

around g ¼ 2.01 (Fig. 8d). We propose that this Co^I(^LS3d⁸)

species is ve-coordinate with ^4tBupyridine occupying the axial

position of the metal center. This species was also indepen-

dently obtained by reduction of 1 in dry THF in presence of 2

equiv. of decamethylcobaltocene under an argon atmosphere.¹³

The ¹H-NMR spectrum of the resulting dark-blue Co^Ispecies in

CD₃CN is shown in Fig. 11.

Well-dened peaks between 1 and 9 ppm conrmed the low

spin nature of the species. Three peaks associated with the

aromatic protons of the ^4tBupyridine ligand appear between 7

and 9 ppm. The peaks observed at 7.70 and 7.00 ppm are

associated with the H_Aand H_Bprotons of the coordinated

^4tBupyridine, as discussed in Fig. 7. This species is described as

[Co^I(prdioxH)(^4tBupy)] (4). Five-coordinate Co^Icomplexes

similar to 4 have been reported.^8,9,16

While running the previous NMR experiments we were able

to grow crystals from the deuterated solvent and solve the

structure of a dimeric product in Fig. 12. The structure conrms

the presence of an apical pyridine on the catalytically active Co^I

species thus validating the proposed mechanism beyond

reasonable doubt. In the time scale of the cyclic voltammetric

and electrocatalytic experiment, this species is clearly mono-

meric, suggesting that the observed dimer is the result of

a kinetically sluggish reaction. Dimer formation resulting from

reduction with cobaltocene derivatives has been observed in

related oxime species¹⁷and might point out to possible deac-

tivation pathways for the catalyst.

In light of the comprehensive UV-visible, EPR, NMR, and

DFT data accumulated in our study, we propose a viable

chemical and electrochemical pathway starting from the parent

1 and reducing to the Co^IIcounterpart and to the catalytically

active monovalent species (Scheme 1). The rst Co^III/Co^II

reduction shown in step 1 is followed by loss of a chloride to

yield the ve-coordinate Co^IIspecies [Co^II(prdioxH)(^4tBupy)]⁺.

The latter species incorporates a CH₃CN molecule to form the

six-coordinate Co^IIcomplex [Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺

Fig. 10 (a) Changes of the CVs of 1 upon addition of Et₄NCl or

^4tBupyridine in CH₃CN. The ﬁrst Co^III/Co^IIevent gradually disappears

with increasing Cl^ꢀ, whereas ^4tBuPy has no eﬀect. (b) Calculated free

energies and activation barriers for 1. Dissociation of Cl^ꢀis more

favorable than that of ^4tBuPy yielding [Co^III(prdioxH)(^4tBupy)]⁺.

metal–ligand bond distances for 1 is shown in Table 3 along

with the DFT bond distances for the 1e^ꢀreduced analog of 1.

The Co^IIIreduction is associated with lling of the d_zorbital

2

which is antibonding in nature. This process increases the bond

distance along the z axis of the Co^IIcomplex facilitating disso-

ciation of the axial ligands. The Co–Cl bond elongates by 0.441

˚

A, thus considerably more than the 0.300 A observed for the Co–

N5 (^4tBupy) bond and becomes more labile so that loss of chlo-

ride is invoked upon reduction of 1. The dichloro analog

[Co^II(prdioxH)(Cl)₂]^ꢀ, obtained from the reduction of the in situ

generated [Co^III(prdioxH)(Cl)₂], is negatively charged and

chloride loss is expected to be even more favorable. This

ligand loss is calculated at 9.7 kcal mol^ꢀ1and yields the ve-

coordinate neutral [Co^II(prdioxH)(Cl)] species. Uptake of

a CH₃CN molecule by the latter species followed by substitu-

tion of the remaining chloride by ^4tBupyridine yields

[Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺. These events are calculated at

10.0 and 3.6 kcal mol^ꢀ1, respectively. These substitution

processes are thermodynamically less favorable and may be

slow, as suggested by the absence of hyperne structure in the

EPR spectrum of the Co^II-species obtained aer the second Co^III

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Table 3 Co–ligand bond lengths^aof 1 from X-ray crystal and DFT-optimized structures. Metal–ligand bond distances^aof the one-electron

reduced (Co^II) analog of 1 are also reported

Co–N₁

Co–N₂

Co–N₃

Co–N₄

Co–N₅

Co–Cl

Experiment

Theory

[Co^III(prdioxH)(^4tBupy)(Cl)]⁺

[Co^II(prdioxH)(^4tBupy)(Cl)]

1.903

1.907

1.904

1.917

1.925

1.927

1.914

1.928

1.898

1.905

1.904

1.975

1.988

2.288

2.237

2.304

2.745

a

˚

Values are in A.

remaining chloride with one ^4tBupyridine present in solution to

give rise to the species [Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺in step

6. This species is then reduced and transforms into the ve-

coordinate and catalytically active [Co^I(prdioxH)(^4tBupy)] with

release of the CH₃CN molecule in step 7. It is important to

mention that Marzilli⁷proposed generation of a Co^II–Cl species

already in step 1. It was further proposed that an outer sphere

electron transfer mechanism involving the newly generated

Co^II–Cl species and the parent Co^IIIcomplex, and chloride

transfer from one metal center to another could yield a Co^III

dichloro species. The chloride transfer event is calculated to be

favorable by 6.7 kcal mol^ꢀ1. The present study oﬀers an alter-

native pathway where the Co^IIIdichloro species is generated via

substitution of ^4tBupy already on the parent Co^IIIcomplex by an

external chloride, this process being favorable by 9.7 kcal

mol^ꢀ1

.

Fig. 11 ¹H-NMR spectra of [Co^I(prdioxH)(^4tBupy)], in CD₃CN. The

asterisk denotes the presence of adventitious toluene. See ESI† for the

complete spectra.

Catalytic activity and proposed mechanisms

The optimization of proton-reducing catalysts requires

a detailed study of catalytic parameters such as overpotentials

(h), turnover numbers (TON), and faradaic eﬃciencies (% FE)

allied to an understanding of the plausible catalytic mecha-

nisms. In the next paragraphs we discuss the experimental and

theoretical results observed for complexes 1–3.

Catalytic proles and yields. We investigated the electro-

catalytic behavior of 1 in the presence of triuoroacetic acid

(HTFA, pK_a¼ 12.7 in CH₃CN), and the CV data revealed a cata-

lytic peak appearing near the reduction potential of the Co^II/Co^I

couple upon incremental addition of 0–10 equiv. of HTFA

(Fig. 13). The results conrm the Co^Ispecies as catalytically

active, and generation of H₂happens at the low overpotential of

0.35 V measured including the homoconjugation eﬀect of the

acid.¹⁸The CV prole of the Co^III/Co^IIcouple did not change

with increasing amounts of acid, ruling out the possibility of the

involvement of this couple towards proton reduction. We also

observed a new process appearing at a slightly more negative

potential than that of the catalytic peak in the presence of $5

equiv. of HTFA and tentatively attributed to the Co^III–H^ꢀ/Co^II–

H^ꢀcouple. This attribution is supported by DFT calculations.

The reduced analog for 1 was generated via controlled potential

electrolysis at ꢀ1.0 V_Ag/AgClin the presence of HTFA and TONs

were determined by gas-chromatography. A TON of 18.7 (ca.

40% conversion ratio) was observed aer 3 h in CH₃CN in

presence of 4.0 ꢁ 10^ꢀ5mol of 1 and 100 equiv. of HTFA with %

FE z 80%. This solution displayed a color change from light

yellow, typical of the Co^IIIparent species, to dark green during

Fig. 12 ORTEP representation of the crystal structure for the neutral

dimer of 4 (CCDC 1448834) at 50% ellipsoid probability. Hydrogen

atoms omitted for clarity. Co(1)–N(1)

¼

1.997(4); Co(1)–N(2)

¼

1.902(4); Co(1)–N(3) ¼ 1.866(4); Co(1)–N(4): 1.850(4); Co(1)–N(5) ¼

1.890(4); C(4)–C(8) ¼ 1.592(6).

described in step 2. The released chloride in step 1 replaces the

^4tBupyridine in the remaining [Co^III(prdioxH)(^4tBupy)(Cl)]⁺(1) as

observed generating a second six-coordinate Co^IIIspecies with

two chlorides occupying the axial positions, step 3, along with

uncoordinated ^4tBupyridine. This is a chemical, rather than

electrochemical step. The second Co^III/Co^IIreduction, shown in

step 4, yields the six-coordinate [Co^II(prdioxH)(Cl)₂]^ꢀthat is

subsequently converted into the ve-coordinate [Co^II(prdioxH)

Cl]. Uptake of a solvent CH₃CN molecule by the latter species

(step 5) may take place and is followed by replacement of the

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Scheme 1 Proposed chemical and electrochemical pathway from 1 the catalytically active Co^Ispecies.

[Co^I(prdioxH)(^4tBupy)] and [Co^II(prdioxHc)(^4tBupy)]. However the

3d⁷-Lc species would be strongly antiferromagnetically coupled

yielding an overall singlet (S ¼ 0) state not observable by EPR

spectroscopy.

Catalytic pathways for H₂evolution. Previous studie-

s^{6a,c,d,f,g,i,k,m,10g}on hydrogen evolution by cobalt oximes propose

the formation of a Co^III–H^ꢀhydride intermediate, either by

heterolytic [LCo^II–H^ꢀ] + H⁺/ [LCo^II] + H₂or by bimolecular

homolytic 2[LCo^II–H^ꢀ] / 2[LCo^I] + H₂pathways. The chosen

pathway depends on the strength of the acid used as proton

source in the organic medium.^3eProton-coupled electron

transfer (PCET) may become relevant in suﬃciently acidic

systems. Theoretical results^6i,f,gsupport the formation of a Co^III–

H^ꢀintermediate from protonation of the Co^Icomplex as the

most likely mechanism. A 1e^ꢀreduction of the Co^III–H^ꢀspecies

generates the Co^II–H^ꢀintermediate which reacts heterolytically

Fig. 13 CV experiments showing the electrocatalytic behavior of 1

towards H₂generation in the presence of HTFA in CH₃CN solution.

The numbers 0–10 indicate the number of equiv. of HTFA. (Inset) Plot

of charge consumed against time during the bulk-electrolysis exper-

iment for dihydrogen generation by 1 after 3 h.

^III–H^ꢀ/Co^II–H^ꢀ)

with a proton to produce H₂. However if the E_(Co

potential for Co^II–H^ꢀgeneration is more negative than E_{(Co /Co )}

,

II

I

the controlled potential electrolysis experiment. Complex 1

consumed ca. 200 C of charge over 3 h (Fig. 13, inset) validating

its catalytic activity. Electrocatalytic H₂generation was also

conrmed for 2 and 3 (Fig. S6 and S7†) following similar

methodology. Overpotentials for 1–3 were calculated from CV

experiments whereas TONs and % FEs were measured from

controlled potential electrolysis and are reported in Table 4. All

species exhibited low overpotential, higher charge consump-

tion, and catalytic current with respect to the blank (Fig. S8–

S10†). These results suggest that although variation of the

substituents on the pyridine ligand has a minor eﬀect on the

overpotential of 1–3, the TONs showed considerable diﬀer-

ences. Complex 1 with an electron-donating group displayed the

highest catalytic activity. The % FE for proton reduction was

calculated to be ca. 80%, thus suggesting the possible conver-

sion of the catalyst into non-catalytic species. Indeed, calcula-

tions found some degree of redox non-innocence in the ligand

during the reduction event (Fig. S11, Table S2†) suggesting

intermediacy of a Co^II–H^ꢀspecies is unlikely and H₂evolution

will likely be realized from protonation of the Co^III–H^ꢀspecies

in a heterolytic manner. Support for a bimolecular homolytic

mechanism has been reported,⁹albeit without strong kinetic

evidence. The formation of a transient Co^III–H^ꢀcomplex has

been reported^4bfrom the protonation of a [Co^I(triphos)(CH₃-

CN)]PF₆(triphos

¼

1,1,1-tris(diphenylphosphinomethyl)

ethane) species and both the heterolytic and homolytic path-

ways were invoked for H₂generation. Protonation at the oxime

bridge (O–H/O) of the Co^IIspecies is only possible in presence

of a suﬃciently strong acid with a pK_a# 10.7 in CH₃CN.^6f

In presence of p-cyanoanilinium (pK_a7.6 in CH₃CN), the

electrocatalytic wave of a cobalt oxime complex was observed at

a potential of 320 mV more positive than that required for the

Co^II/Co^Ireduction^6mand a pathway involving the protonation of

the oxime bridge was proposed to explain the observed poten-

tial shi. Although the thermodynamics of these pathways have

been studied in reasonable detail, the free energy barriers for

the proton transfer and H₂production steps are considerably

less understood.

the possibility of

a

3d⁸/3d⁷-Lc equilibrium between

Recent studies consider the protonation of the glyoxime

bridge by a Co^III–H^ꢀmoiety¹⁹or even the lack of formation of

Co^III–H^ꢀ.¹⁷Although potentially relevant—especially to evaluate

catalyst decomposition mechanisms—these pathways will

require further validation and for the purposes of this study we

will assume the need for formation of the Co^III–H^ꢀhydride

intermediate. The protonation of the oxime bridge in

Table 4 Catalytic parameters for 1–3 in CH₃CN in the presence of

HTFA at a catalyst-to-acid ratio of 1 : 100

Catalytic parameters

1

2

3

E(H⁺/H₂) (V) vs. Fc/Fc⁺

Overpotential (V) (10 eq. HTFA)

TON/3 h (4.0 ꢁ 10^ꢀ5mol)

Faradaic eﬃciency (%)

ꢀ1.03

0.35

18.7

77

ꢀ1.04

0.36

13.8

84

ꢀ1.03

0.35

13.8

75

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[Co^II(prdioxH)(^4tBupy)]⁺species seems energetically unfavorable protonation of the latter Co^III–H^ꢀspecies by HTFA leads to the

by 19 kcal mol^ꢀ1. This is consistent with a previous study where generation of H₂. This event is calculated to be slightly uphill by

it was proposed that the weak acid HTFA cannot protonate the 2.2 kcal mol^ꢀ1and is facilitated by the excess of acid present.

oxime moiety.^6f

The Co^IIIcomplex obtained aer the evolution of H₂contains

Complexes 1–3 displayed electrocatalytic waves in the pres- a triuoroacetate ligand bound to the metal center.^6lThe

+

ence of 10 equiv. HTFA at potentials close to the reduction of reduction of this species is calculated at ꢀ0.01 V_Fc/Fcand yields

(H⁺/H₂)

Co^II/Co^I. The respective E_1/2

are ꢀ1.03, ꢀ1.04 and ꢀ1.03 the corresponding Co^IIcomplex, [Co^II(prdioxH)(^4tBupy)(TFA)],

V_Fc/Fcand the Co^II/Co^Ireduction potentials are ꢀ1.09, ꢀ1.07 which can exchange the triuoroacetate ligand by a CH₃CN

+

and ꢀ1.09 V_Fc/Fcfor 1, 2, and 3, respectively. The catalytic molecule, reentering the catalytic cycle. This ligand exchange

pathway calculated for 1 is shown in Fig. 14. The calculated event is nearly isoenergetic at about 1 kcal mol^ꢀ1and is ex-

potential for the Co^II/Co^Ireduction of 1 is ꢀ0.93 V_Fc/Fcand the pected to be favored by the large concentration of the solvent.

+

species obtained aer reduction, [Co^I(prdioxH)(^4tBupy)], is

The bimolecular mechanism involving two [Co^III(H^ꢀ)(pr-

neutral and can easily take up a proton upon reaction with dioxH)(^4tBupy)]⁺complexes can generate H₂favorably at ꢀ28.3

HTFA. This cobalt protonation is favorable by 8.4 kcal mol^ꢀ1kcal mol^ꢀ1. However, this pathway may have a moderate kinetic

and yields the cobalt-hydride species described as barrier arising from the electrostatic repulsion between two

[Co^III(H^ꢀ)(prdioxH)(^4tBupy)]⁺. This is expected because Co^Iis cationic species and is likely plausible at lower concentrations

a closed-shell 3d⁸center and can behave as a good Lewis base. of acid. It is also noteworthy that the complex [Co^I(pr-

The reduction potential for the generation of a Co^II–H^ꢀfrom dioxH)(^4tBupy)]

obtained

aer

the

reduction

of

Co^III–H^ꢀis ꢀ1.43 V_Fc/Fcand it is in agreement with the [Co^II(prdioxH)(^4tBupy)(CH₃CN)]⁺can undergo an intramolecular

+

appearance of a shoulder at about ꢀ1.40 V_Fc/Fcat higher electron transfer from metal to the oxime ligand framework

concentrations of acid in Fig. 13. This potential is more negative giving rise to a radical-containing [Co^II(prdioxHc)(^4tBupy)].

than that of the Co^II/Co^Icouple of ꢀ0.93 V_Fc/Fc. Similarly, Similar to other pathways^17,19where the ligand is involved, the

+

a PCET event for conversion of the Co^Icomplex into the Co^II–H^ꢀlatter species may contribute to catalyst decomposition. A

+

species is calculated to be ꢀ1.39 V_Fc/Fc. Because electrocatalysis detailed study about the role of this species is currently

II

I

occurs near the measured value of E_{(Co /Co )}(Fig. 13), the underway.

involvement of a Co^II–H^ꢀspecies is not invoked in the catalytic

mechanism. The electrocatalytic wave did not show an anodic

Conclusions

shi with the decrease of pH, when compared to the reduction

potential of the Co^II/Co^Icouple (Fig. 13) and therefore, a PCET A series of heteroaxial complexes in a trivalent cobalt/oxime

mechanism from the Co^IIcomplex, [Co^II(prdioxH)(^4tBupy)(CH₃- macrocyclic framework was synthesized and characterized by

CN)]⁺, to the Co^III–H^ꢀspecies is not considered. The multiple physico-chemical techniques. The electrochemistry of

these species exhibited complex behavior with two Co^III/Co^II

and one Co^II/Co^Iprocesses; the origin of which has been iden-

tied with multiple redox and spectroscopic methods like CV,

¹H-NMR, UV-vis, EPR, and DFT calculations. A detailed under-

standing of the chemical and electrochemical pathways with

distinct coordination preferences was revealed: (i) the rst Co^III/

Co^IIreduction involves chloride loss followed by incorporation

of CH₃CN to form a six-coordinate Co^IIcomplex; (ii) the free

chloride replaces ^4tBupyridine in the remaining unreduced 1,

and forms a second Co^IIIspecies with chlorides occupying the

axial positions; (iii) this complex is then reduced and yields

a ve-coordinate Co^IIspecies that accommodates a solvent

molecule at its sixth axial position; (iv) available ^4tBupyridine

replaces the remaining chloride; (v) the resulting species is then

reduced, transforming into the ve-coordinate and catalytically

active [Co^I(prdioxH)(^4tBupy)]. The complexes exhibited catalytic

properties towards proton reduction in HTFA/CH₃CN with low

overpotentials and good turnover numbers. Variation of the

substituent groups in the pyridine ligand aﬀected the nal

TONs; however overpotentials remained relatively unchanged. A

Fig. 14 The catalytic pathways calculated for H₂evolution by 1 in the

presence of HTFA in CH₃CN (ACN). The formation of a Co^III–H^ꢀdetailed catalytic mechanism favors a heterolytic pathway for

hydrogen generation, and involves: (i) a favorable Co^Iproton-

species is invoked. This complex can react either heterolytically with

a proton or homolytically with another Co^III–H^ꢀcomplex to yield H₂.

ation that yields the required Co^III–H^ꢀspecies; (ii) this complex

The heterolytic pathway involves an acetate-bound Co complex. The

reacting heterolytically with an available proton from HTFA,

homolytic mechanism is more likely at low concentrations of acid.

generating H₂, and therefore regenerating Co^III; (iii) this Co^III

Energies given in kcal mol^ꢀ1as calculated at the B3PW91//TZVP/6-

311++G(d,p) level of theory.

species rst incorporating TFA and undergoing reduction

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followed by CH₃CN exchange prior to reentering the catalytic CCD (charge coupled device) based diﬀractometer equipped

cycle. Bimolecular mechanisms seem favorable only at lower with an Oxford Cryostream low-temperature apparatus oper-

acid concentrations, and both the Co^II/Co^III–H^ꢀand Co^I/Co^II– ating at 173 K. Data were measured using omega and phi scans

H^ꢀPCET events seem unlikely. These results allow for some of 0.5^ꢂper frame for 30 s. The total number of images was based

generalizations on the behavior of heteroaxial cobalt macro- on results from the program COSMO V1.58 (ref. 21a) where

˚

cycles and serve as guidelines for the development of new redundancy was expected to be 4.0 and completeness to 0.83 A

catalysts based on macrocyclic frameworks.

to 100%. Cell parameters were retrieved using APEX II so-

ware^21band rened using the SAINT soware^21cfor all observed

reections. Data reduction was performed using SAINT which

corrects for Lp. Scaling and absorption corrections were applied

using the SADABS^21dmulti-scan technique, supplied by George

Sheldrick. The structure was solved by the direct method using

the SHELXS-97 program and rened by the least squares

method on F², SHELXL-97, which is incorporated in SHELXTL-

Materials, methods and synthetic

procedures

General

Reagents and solvents were used as received from commercial

sources. Infrared spectra were recorded from 4000 to 650 cm^ꢀ1

as KBr pellets on a Bruker Tensor 27 FTIR spectrophotometer.

¹H-NMR spectroscopy was carried out with a Mercury FT-NMR

400 MHz setup using CD₃CN as solvent at 25 ^ꢂC. Chemical shis

are plotted vs. TMS. ESI-(+) mass spectrometry was measured in

a triple quadrupole Micromass Quattro LC equipment where

experimental mass patterns were tted with the theoretical

isotopic distribution. Elemental analysis (C, H, and N) was

performed using an Exeter analytical CHN analyzer by Midwest

Microlab, an independent lab located in Indianapolis, Indiana,

USA. UV-visible spectra were obtained using quartz cells at

room temperature in a Varian Cary 50 spectrophotometer

operating in the range of 200 to 1000 nm. Values of 3 are given in

M^ꢀ1cm^ꢀ1. HPLC-grade CH₃CN solutions were used for cyclic

voltammetry and UV-absorption spectroscopy studies.

PC V 6.10.^21eThe structure was solved in the space group I4 (#

ꢀ

82). All the non-hydrogen atoms are rened anisotropically.

Hydrogens were calculated by geometrical methods and rened

as a riding model. Solvent molecules were found to reside in the

lattice void, which is presumably octane resulting from crys-

tallization. Attempts to model these solvents and/or to redene

as CH₂Cl₂failed to generate a chemically sensible model. The

SQUEEZE (P. V. D. Sluis & Spek, 1990)^21ffunction of PLATON

(Spek, 2003)^21gwas used to eliminate the contribution of the

electron density in the void from the intensity data. The total

3

˚

solvent area volume was found to be 625 A , with electron count

of about 80 electrons. This corresponds to approximately two

molecules of CH₂Cl₂residing in the cell. The calculated F(000)

and density were obtained for the cell containing two molecules

of CH₂Cl₂per cell or one per molecule of interest. The rene-

ment was carried out on the new reection le generated by

PLATON. The crystal used for the diﬀraction study showed no

decomposition during data collection. All the drawings are

done at 50% ellipsoids. In the case of complex 3, a yellow plate-

shaped crystal with dimensions 0.16 ꢁ 0.10 ꢁ 0.04 mm was

mounted on a Nylon loop using a small amount of paratone oil.

Data were collected using a Bruker CCD (charge coupled device)

based diﬀractometer equipped with an Oxford Cryostream low-

temperature apparatus operating at 173 K. Data were measured

using omega and phi scans of 0.5^ꢂper frame for 30 s. The total

number of images was based on results from the program

COSMO V1.61 (ref. 21h) where redundancy was expected to be

Electrochemistry and bulk electrolysis

The electrochemical behavior of complexes 1–3 was investigated

with a BAS 50W potentiostat/galvanostat. Cyclic voltammo-

grams were obtained at room temperature in CH₃CN solutions

containing 0.1 M of n-Bu₄NPF₆as the supporting electrolyte

under an argon atmosphere. The electrochemical cell was

comprised of three electrodes: glassy-carbon (working), plat-

inum wire (auxiliary) and Ag/AgCl (reference). The ferrocene/

ferrocenium redox couple Fc/Fc⁺(E^o¼ 400 mV vs. NHE)²⁰was

used as the internal standard. Peak to peak potential separation

(DE_p¼ |E_p,cꢀ E_p,a|) and |i_pa/i_pc| values were measured to

evaluate the reversibility of the redox processes. Bulk electrol-

ysis was performed in a custom-made air-tight H-type cell with

vitreous carbon as the working electrode, Ag/AgCl as the refer-

ence electrode placed in the same compartment and a Pt-coil

used as the auxiliary electrode placed in another compartment

separated by a frit. Controlled potential electrolysis of the

complex has been done in 20 mL CH₃CN with tetrabutyl

ammonium hexauorophosphate (TBAPF₆) as supporting elec-

trolyte until the calculated nal charge is attained. Potentials

were applied to the cell by a BAS 50W potentiostat/galvanostat

and measured against the Ag/AgCl reference electrode.

˚

4.0 and completeness to 0.83 A to 100%. Cell parameters were

retrieved using APEX II soware²¹ⁱand rened using SAINT for

all observed reections. Data reduction was performed using

the SAINT soware.^21jScaling and absorption corrections were

applied using the SADABS^21dmulti-scan technique. The struc-

ture was solved by the direct method using the SHELXS-97

program and rened by the least squares method on F²,

SHELXL-97,^21ewhich is incorporated in OLEX2.^21kThe structure

was solved in the space group P2₁/c (# 14). All the non-hydrogen

atoms are rened anisotropically. Hydrogens were calculated by

geometrical methods and rened as a riding model. All the

drawings are done at 50% ellipsoids.

X-ray structural determination

In the case of complex 4, blue needle-shaped crystals were

For complex 1 an orange needle-shaped crystal with dimensions grown via slow evaporation from deuterated acetonitrile. A

0.40 ꢁ 0.16 ꢁ 0.11 mm was mounted on a Nylon loop using single weakly diﬀracting crystal with dimensions 0.05 ꢁ 0.08 ꢁ

a small amount of paratone oil. Data were collected in a Bruker 0.13 mm was mounted on a Nylon loop using a small amount of

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paratone oil. Data were collected using a Bruker CCD (charge functions of the optimized structures were tested for SCF sta-

coupled device) based diﬀractometer equipped with an Oxford bility.^22m–oIsosurface plots of orbitals and spin densities were

Cryostream low-temperature apparatus operating at 100 K. Data visualized using GaussView.^22pThe calculation of the reduction

were measured using omega and phi scans of 0.5^ꢂper frame for potentials of the complexes included zero-point energy and

90 s. Cell parameters were retrieved using APEX II soware²¹ⁱthermal corrections and standard thermodynamic equation DG

and rened using SAINT for all observed reections. Data ¼ ꢀnFE was used. The calculated potentials were referenced to

reduction was performed using the SAINT soware.^21jScaling a value of E_1/2¼ 4.678 V for the ferrocene/ferrocenium couple

and absorption corrections were applied using SADABS21d calculated under our level of theory.

multi-scan technique. The structure was solved by the direct

method using the SHELXS-97 program and rened by the least

squares method on F², SHELXL-97,^21ewhich is incorporated in

OLEX2.^21kThe structure was solved in the space group C2/c. All

the non-hydrogen atoms are rened anisotropically. Hydrogens

were calculated by geometrical methods and rened as a riding

model. All the drawings are done at 50% ellipsoids. We present

this structure as a dimer molecule with a C8–C4 bond distance

of 1.592 through symmetry of the unit cell. Although we are

condent that our interpretation is correct, analysis of the raw

data suggests that the crystal chosen is of a multiple nature with

more than 3 domains, which may indeed mean that we have not

correctly identied the correct unit cell. Use of diﬀerent index-

ing programs, and thresholding levels along with viewing the

data with a reciprocal viewer program provides us with what we

present here as the most reasonable unit cell.

Catalytic activity

Proton reduction electrocatalysis was evaluated for 1–3 by cyclic

voltammetry in the presence of triuoroacetic acid (HTFA)

using a glassy carbon as working electrode, platinum wire as

auxiliary and Ag/AgCl as reference electrodes with tetra-butyl

ammonium tetrauoroborate (TBABF₄) as supporting electro-

lyte. Overpotential (h), i_c/i_p, and kinetic rate constant, k_obs, have

been calculated from the observed changes in the voltammo-

gram. To determine the amount of dihydrogen release, bulk

electrolysis has been performed in a custom-made air-tight H-

type cell in the presence of a mercury-pool as working electrode,

Ag/AgCl as reference electrode placed in the same compartment

whereas Pt-coil used as the auxiliary electrode placed in the

other compartment separated by a frit. TBABF₄was used as the

supporting electrolyte. The main chamber was lled with an

electrolyte solution and proton source (TBABF₄: 1.317 g; TFA:

0.456 g [4 mmol], 20 mL CH₃CN) and the glass-tted chamber

EPR spectroscopy

Continuous wave (CW) X-band (9–10 GHz) EPR experiments was lled with another electrolyte solution (TBABF₄: 0.329 g; 5

were carried out with a Bruker ELEXSYS E580 EPR spectrometer mL CH₃CN). In a standard experiment, the cell was purged with

(Bruker Biospin, Rheinstetten, Germany), equipped with a TE₁₀₂N₂gas for 20 minutes followed by sample head space gas (100

rectangular EPR resonator (Bruker ER 4102st) and a helium gas- mL) to ensure O₂-free environment in the gas chromatograph

ow cryostat (ICE Oxford, UK). An intelligent temperature (GC). The solution was electrolyzed in the absence of the cata-

controller (ITC503) from Oxford Instruments, UK, was used. lyst for 180 minutes at ꢀ1.0 V vs. Ag/AgCl. The head space gas

Data processing was done using Xepr (Bruker BioSpin, Rhein- was again injected into the GC and the amount of dihydrogen

stetten) and Matlab 7.11.1 (the MathWorks, Inc., Natick) envi- was recorded. Then, the cell was purged with N₂gas for another

ronment. Simulations were performed using the EasySpin 20 minutes followed by injection of the catalyst (0.04 mmol)

soware package (version 4.5.5) (Stoll & Schweiger, J. Magn. dissolved in CH₃CN. Bulk electrolysis was conducted for

Reson., 2006).

another 180 minutes and the head space gas (100 mL) was

injected into the GC to record the amount of dihydrogen

produced. Aer the background subtraction, the turnover

number was calculated as the ratio of the moles of dihydrogen

produced over the moles of catalyst used. Faradaic eﬃciency

was calculated from the gas chromatography measurements

Computational methods

Calculations were performed with a development version of the

Gaussian suite of programs,^22ausing B3PW91 (ref. 22b–d)

functional with double-zeta SDD basis set on cobalt and D95

(ref. 22e and f) basis on the other atoms. All optimized struc-

tures were conrmed as minima by analyzing the harmonic

vibrational frequencies. Solvation eﬀects in CH₃CN were esti-

mated using the IEF polarizable continuum model (PCM)^22g–j

and were included during structure optimization. Single-point

energies were reevaluated with triple-zeta TZVP basis^22kon the

metal atom and 6-311++G(d,p) basis^22lon the other atoms in

presence of the continuum solvation model. The free energies

were calculated using the triple-zeta SCF energy while the zero-

using the equation % FE ¼ [(n_H)/(Q/2)] ꢁ 100, where Q is in

2

Faraday unit. The GC is a Gow-Mac 400 instrument equipped

with a thermal conductivity detector. An 8⁰ꢁ 1/8⁰⁰long 5 A

˚

ꢂ

molecular sieve column operating at 60 C was used with the

carrier gas N₂. The calibration was carried out with dihydrogen

gas (dihydrogen GC grade 99.99%, Scotty analyzed gases, Sigma

Aldrich).

Synthetic procedures

point energy and thermal corrections were included from the The synthesis of the oxime ligand, prdioxH₂,¹¹and the precursor

double-zeta calculations. The standard states of 1 M concen- [Co^III(prdioxH)(Cl)₂],¹²followed the literature procedure. The

tration were considered for all the reactants and products for complex [Co^III(prdioxH)(^4tBupy)₂](PF₆)₂used for electrochemical

calculating the free energies of reactions. Low-spin congura- comparisons was obtained by modications to the literature

tions yielded lower energies for all the species. The wave procedure⁷and described in Scheme S1.† The synthetic

Chem. Sci.

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procedure and characterization of [Co^III(prdioxH)(Cl)₂] are

described in Scheme S2.†

Acknowledgements

[Co^III(prdioxH)(^4tBupy)(Cl)]PF₆(1). [Co^III(prdioxH)(Cl)₂] (1

mmol, 0.369 g) was dissolved in 20 mL methanol. A solution of

KPF₆(1.7 mmol, 0.3128 g) in 5 mL water was added to the

solution followed by ^4tBupyridine (1 mmol, 0.135 g) in 5 mL of

methanol. The reaction mixture was stirred for 2 h and then

rotary-evaporated to 10 mL. This solution was kept in storage for

3 days when brownish-yellow crystals were obtained. X-ray

quality crystals were obtained aer recrystallization from

acetone/water (1 : 1) mixture. Yield: 75%. IR (KBr, cm^ꢀ1) 3645

(w) (OH); 3234 (w) (aromatic-CH); 2969 (s), 2907 (m), 2871 (m)

(aliphatic CH); 1620 (s) (C]N); 1432 (m) (C]C); 847 (s) (PF₆^ꢀ).

¹H-NMR [400 MHz, CD₃CN, 300 K] d/ppm ¼ 1.249 [s, 9H (t-

butyl)]; 2.534 [s, 6H (CH₃)]; 2.631 [s, 6H (CH₃)]; 4.173 [m, 4H

(CH₂)]; 7.358 [d, 2H (aryl)]; 7.526 [d, 2H (aryl)]. ESI pos. in

MeOH: m/z ¼ 468.1573 for [Co^III(prdioxH)(^4tBupy)(Cl)]⁺. Anal.

calcd for C₂₀H₃₂ClCoF₆N₅O₂P: C: 39.13; H: 5.25; N: 11.41; found:

C: 39.05; H: 5.33; N: 11.37.

This research was made possible by the Division of Chemical

Sciences, Geosciences, and Biosciences, Oﬃce of Basic Energy

Sciences of the U.S. Department of Energy through the Single-

Investigator and Small-Group Research (SISGR) – Solar Energy

program grants DE-SC0001907 and DE-FG02-09ER16120 to C.

N. V. and H. B. S., including nancial support to D. B., S. M., H.

B., D. W., and X. S. Work at the Argonne National Laboratory (J.

N. and O. G. P.) was supported by the U.S. Department of

Energy, Oﬃce of Science, Oﬃce of Basic Energy Sciences, Divi-

sion of Chemical Sciences, Geosciences, and Biosciences, under

contract number DE-AC02-06CH11357. D. B. and S. M.

contributed equally to this work. We thank Prof. J. Endicott,

WSU, for insightful discussions and suggestions. We also

acknowledge the WSU-Computing Grid for high-level DFT

calculations.

References

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KPF₆(1.7 mmol, 0.3128 g) in 5 mL water was added to the

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Evaluation of the coordination preferences and catalytic pathways of heteroaxial cobalt oximes towards hydrogen generation

DOI: 10.1039/c5sc04214c

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