Combined X-ray Absorption Near-Edge Structure and X-ray

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Combined X-ray Absorption Near Edge Structure and X-ray Photoelectron Study of the Electrocatalytically Active Cobalt(I) Cage Complexes and the Clathrochelate Cobalt(II)- and Cobalt(III)-Containing Precursors and Analogs Dmitry I. Kochubey, Vasily V. Kaichev, Andrey A. Saraev, Stefania V. Tomyn, Alexander S. Belov, and Yan Z Voloshin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3085606 • Publication Date (Web): 15 Jan 2013 Downloaded from http://pubs.acs.org on January 18, 2013

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The Journal of Physical Chemistry

Combined X-ray Absorption Near Edge Structure and X-ray Photoelectron Study of the Electrocatalytically Active Cobalt(I) Cage Complexes and the Clathrochelate Cobalt(II)- and Cobalt(III)-Containing Precursors and Analogs Dmitry Kochubey,† Vasily Kaichev,† Andrey Saraev,† Stefania Tomyn,‡ Alexander Belov,§ Yan Voloshin*,§ †

Boreskov Institute of Catalysis SB RAS, Novosibirsk, 630090 Russia ‡

§

Kyiv National Taras Shevchenko University, Kyiv, 01601 Ukraine

Nesmeyanov Institute of Organoelement Compounds RAS, Moscow, 119991 Russia. E-mail: [email protected]. Tel.: +7(499)135-9344. Fax: +7(499)135-5085

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ABSTRACT: As the catalytic cycle for electrochemical hydrogen generation includes cobalt(I, II, and III)-containing clathrochelate species, we performed a detailed study of their electronic structure. The Co K-edge spectra demonstrated a lowering of the Co1s ionization potentials from cobalt(III) complexes to their cobalt(II)-containing analogs and then to the cobalt(I) clathrochelates. The absence of pre-edge structure and specific peculiarities suggested a high symmetry of the N6coordination polyhedra of an encapsulated cobalt(I) ion. The Co2p core-level spectra contained very weak shake-up satellites, suggesting a hybridization of the cobalt-localized 3d-orbitals and the valent orbitals of their encapsulating ligands, while the binding energy Co2p3/2 increased with a formal oxidation state of an encapsulated cobalt ion(I, II or III) from 780.5 – 780.8 eV and 780.9 – 781.2 eV to 781.8 – 782.2 eV. The Cl2p, C1s, N1s, O1s, B1s, and Co2p core-level spectra and data of X-ray absorption near edge structure (XANES) proved that both the electronic and spatial structures of the highly conjugated polyene macrobicyclic ligands are affected by the metallocalized redox processes. The nature of these encapsulating ligands influenced the redox characteristics of the caged metallocenters, allowing them to adopt unusual catalytically active oxidation states.

Keywords: Electrocatalysis; Hydrogen production; Macrocyclic compounds; Electronic structure.


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ig.


INTRODUCTION Great attention is now paid to the evolution of traditional methods of generating heat and electricity into modern hydrogen energetics. One of the most rapidly developing areas is the search for highly efficient catalytic systems that can generate molecular hydrogen from aqueous solutions. Cheap and synthetically available cobalt clathrochelates and their electrodeposited nanosized derivatives1–4 have been used as the electrocatalysts for hydrogen production. These compounds demonstrate unusually high stability over time and, in many cases, may easily be functionalized with up to eight substituents.6,7 Faradaic efficiency of the electrocatalytic process 2H+/H2 strongly depends on pK of the corresponding acid as a source of H+ ions as well as on the Co2+ / + redox potential due to an acid – base equilibrium between the electrochemically generated encapsulated cobalt(I) ion as a base and this acid. One of the possible ways to decrease overpotential of this reduction and, hence, to make the process more thermodynamically favorable, is to vary the macrobicyclic ligands. In particular, the halogen-containing cobalt cage complexes showed high catalytic activity in the electrocatalytic H2 generation without an over-potential2, while their thiolterminated sulfide analogs have been immobilized4 on a gold electrode surface with a formation of the self-assembled monolayers (Figure 1). The catalytic cycle for hydrogen generation from acidic solutions1–4 (Scheme 1) includes fast transfer of H+ ion to the basic encapsulated cobalt(I)-containing metallocenter resulting from the Co2+ / + reduction6 with a formation of the corresponding Co – H hydride complex as the most probable intermediate of this electrocatalytic process. This hydride clathrochelate then easily undergoes protonation followed with a fast intramolecular formation of the H – H bond and an evolution of H2 molecule. The resulting cobalt(III) cage complex undergoes Co3+ / 2+ reduction to the initial cobalt(II) clathrochelate. Hence, the cobalt(I, II, and III)-containing clathrochelate species are involved in this cycle, and the detailed study of their electronic structure by modern physical methods seems to be of great importance.



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X-ray absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) have been successfully used for studying various (electro)catalytical materials and processes.6–11 In this paper, we present XPS and X-ray absorption near edge structure (XANES) spectral data for a wide range of the electrocatalytically active cobalt(I) clathrochelates and their macrobicyclic precursors and analogs.

EXPERIMENTAL Analytically pure samples of the cobalt cage complexes (Scheme 2) were obtained, thoroughly purified and characterized as described elsewhere.6,7,14–16 For the XANES experiments, the calculated amount of each sample under study was ground, mixed with crystalline cellulose, pressed into a 13-mm-in-diameter pellet, set on a capton tape and mounted on a sample holder. X-ray absorption spectra at the Co Kα-edge were obtained on a beamline A1 of HASYLAB (DESY, Hamburg). These spectra were collected in the conventional transmission mode using ionization chambers. XANES spectra were recorded using a Si(111) double-crystal as a monochromator. The spectra were recorded at 300K in the photon energy range 7630 – 7800 eV. The energy scale was calibrated with a cobalt metal foil and its resolution was estimated to be 1.0 eV. The absorption gap was normalized in the range 0 – 1 on the absorption maximum using a XANDA program.17 XPS measurements were performed on a SPECS’s photoelectron spectrometer (Germany) equipped with a hemispherical electron energy analyzer PHOIBOS-150-MDC-9 and X-ray source XR-50 with a twin Al/Mg anode. All the spectra were obtained in ultrahigh vacuum using a nonmonochromatic Mg Kα radiation (hν = 1253.6 eV) and with the fixed analyzer pass energy equal to 20 eV. The powdered samples were pressed onto conductive double-side adhesive type and fixed on a sample holder. The binding energy was calibrated by the internal standard method using the N1s peak at 401.0 eV from the oxime nitrogen atoms of the complexes under study. For detailed


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analysis, these spectra after the Shirley-type background subtraction were deconvoluted into individual components using the CasaXPS software.18 RESULTS AND DISCUSSION XANES spectra were obtained for ten cobalt clathrochelates (Figures 2 – 5) with an encapsulated metal ion in three different oxidation states (+1, +2, and +3); their macrobicyclic encapsulating ligands were apically and ribbed functionalized with various substituents. The XANES study of the cage complexes of the same macrobicyclic ligand with a cobalt ion in different oxidation states and the clathrochelates with different apical and ribbed substituents but with the encapsulated cobalt ion in the given oxidation state, allowed analyzing the influence of these factors on the electronic structure of the electrocatalytically active cobalt(I)-containing clathrochelate intermediates and their macrobicyclic cobalt(II)- and cobalt(III)-containing precursors and analogs. The use of XANES for determination of an oxidation state of the element under study is based on a shift of the absorption edge with a formal charge of its ion, as this shift is caused by a change of the ionization potential for the given electronic level. In the case of the cobalt complexes, this ionization corresponds to the 1s1/2 → np3/2 transitions. The spectra of the complexes [((CH3)2N)4P][Co(Cl2Gm)3(BC6H5)2],

Co(Cl2Gm)3(BC6H5)2,

and

[Co((n-

C4H9NH)2Gm)3(BC6H5)2](ClO4) (Scheme 2) with a similar ligand arrangement but different oxidation states of an encapsulated cobalt ion are shown in Figure 2. An increase in a formal oxidation state of an encapsulated cobalt ion (and, thus, its effective positive charge) leads to the shift of the Kα-absorption edge to the higher energies; the maximal occupation of the valent levels of this ion is observed in the case of the cobalt(I) clathrochelate. The shape of the Co K-edge XANES spectra for the cobalt cage complexes has some peculiarities. Usually, XANES spectra contain three main ranges: a pre-edge range A corresponding to the forbidden s → d transitions, a range of the absorption edge B, and a post-edge range C. The maximum in the range C of the spectrum of the cobalt(III) clathrochelate [Co((nC4H9NH)2Gm)3(BC6H5)2](ClO4) is determined by the characteristic interactions of an encapsulated 5 ACS Paragon Plus Environment


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cobalt ion with the hexamine macrobicyclic ligand bearing six electron-donor ribbed substituents. In the case of the cobalt(II) clathrochelates with various apical and ribbed substituents (Figure 3), such maximum is observed in the spectra of the complexes CoNx3(Bn-C4H9)2, CoBd3(Bn-C4H9)2, and CoBd2(Cl2Gm)(BF)2 with only aliphatic or aromatic ribbed substituents or with a predominant electronic amount of such substituents. At the same time, in the spectra of the hexachlorine- and hexasulfide-containing

cobalt(I)

and

cobalt(II)

clathrochelates

[((CH3)2N)4P][Co(Cl2Gm)3(BC6H5)2], Co(Cl2Gm)3(Bn-C4H9)2, Co(Cl2Gm)3(BC6H5)2, and Co(S2Nx)3(BC6H5)2 with electron-withdrawing ribbed substituents, this maximum is absent. An opposite effect is observed in the range B of these spectra: a feature on the absorption edge is observed in the spectra of the macrobicycles Co(Cl2Gm)3(Bn-C4H9)2, Co(Cl2Gm)3(BC6H5)2, and Co(S2Nx)3(BC6H5)2, which don’t have the feature in the range C (Figure 4). It should be noted that in the spectrum of the cobalt(II) complex Co(Cl2Gm)3(BC6H5)2 such feature is observed, whereas it is absent in the spectrum of the cobalt(I) complex [((CH3)2N)4P][Co(Cl2Gm)3(BC6H5)2] with the same encapsulating ligand. This result may be explained by a difference in their electronic configurations (d7 and d8, respectively). Similar to the maximum in the range C, the presence of the feature in the XANES spectra of the cobalt(II) clathrochelates depends on the electromeric characteristics of their ribbed substituents (first of all, the Hammett σpara constants). The range A that is determined by both the structural changes and the forbidden transitions typically is very informative, but the accuracy of our experiments is not high enough to discuss it in detail. It is clear, however, that in the XANES spectrum of the cobalt(I) complex [((CH3)2N)4P][Co(Cl2Gm)3(BC6H5)2] such transitions are not observed (Figure 5). The Cl2p, C1s, N1s, O1s, B1s, and Co2p core-level spectra were obtained for ten cobalt complexes under study (Figures 6 – 8). As these clathrochelates have the nitrogen atoms of the donor oxime groups in a similar arrangement, the corresponding N1s peaks with the binding energy equal to 401.0 eV were used as a reference. The Cl2p3/2, N1s, O1s, B1s, and Co2p3/2 binding energies are listed in Table 1. 6 ACS Paragon Plus Environment

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The Co2p3/2 binding energy increases with the increase in the formal oxidation state of the cobalt ion: for the metallic cobalt the binding energy is equal to 778.4 eV, whereas for the cage cobalt(I) complexes the Co2p3/2 binding energy is in the range 780.5 – 780.8 eV. Going to the cobalt(II) and cobalt(III) clathrochelates increases it to 780.9 – 781.2 eV and 781.8 – 782.2 eV, respectively. As in the case of XANES spectra (vide supra), this shift in the Co2p3/2 binding energy is caused by the increase in effective charge of the cobalt metallocenter. It should be noted that the spectra of all the XPS-studied cobalt clathrochelates contain weak peaks of the so-called “shake up” satellites (Sat1 and Sat2). This complex structure of the Co2p spectra is caused by a charge transfer effect: the electrons of the ligands can transfer to the 3d-metal ion under study (its final electronic configuration resulted after an X-ray photoemission) and, thus, to shield a hole that is formed in its 2p-level.19–24 The most intensive shake-up satellites are usually observed in the spectra of the highspin (and thus paramagnetic) cobalt(II) compounds like CoO, CoO(OH)2, etc.25 In the spectra of the cobalt oxides like Co3O4 and NiCo2O4 with the low-spin cobalt(III) ions in an octahedral O6coordination environment (the electronic configuration d6 with three fully occupied bonding t2glevels and two anti-bonding vacant eg*-levels22–24), the main Co2p3/2 peak corresponds to the resulting electronic configuration 3d5, whereas the small shake up satellite corresponds to the 3d6L electronic state (where L is a hole localized on the ligand orbitals). In the spectrum of the double oxide SrCoO3–x, the intense shake up satellite has been assigned to the charge transfer from the O2p σ-orbital to the vacant eg* orbitals of the Co3+ and Co4+ ions25. Thereby observation of the shake up satellites in the spectra of the cobalt cage complexes (Figures 7 and 8) suggests a hybridization of the partially occupied 3d-orbitals of the encapsulated cobalt ion and of the occupied π-orbitals of their conjugated polyene macrobicyclic ligands. Certainly, quantum chemistry calculations are necessary to elucidate this process in detail. Values of the spin-orbital splitting (i.e., the difference in the binding energies for the Co2p1/2 and Co2p3/2 levels) only slightly depend on a formal oxidation state of an encapsulated cobalt ion: for the cobalt(I) cage complexes those are approximately 15.20 – 15.25 eV, while for the cobalt(III) clathrochelate they vary in the range 7 ACS Paragon Plus Environment


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14.81 – 14.94 eV. The significant variation of the spin-orbital splitting values is observed in the case of the cobalt(II) complexes (from 15.04 to 15.36 eV). It should be noted that these values for the above high-spin cobalt(II) inorganic compounds are in the range 15.5 – 16 eV, whereas for the low-spin cobalt(III) compound like LiCoO2 it is equal to 15.0 eV.12,13 The B1s binding energy for the boron-containing cobalt tris-dioximates studied is strongly affected by the local tetrahedral coordination environment of their capping boron atoms: in the case of the fluoroboron-capped clathrochelates [(n-C4H9)4N][Co(Cl2Gm)3(BF)2], Co(Cl2Gm)3(BF)2, [CoDm3(BF)2](BF4), and CoDm3(BF)2, the B1s binding energy is equal to 193.0 ± 0.1 eV, and is in the range 191.0 – 191.9 eV for the cobalt complexes with cross-linking O3BС apical fragments, depending on both the formal oxidation state of an encapsulated cobalt ion and the inductive effects of the apical substituents (i.e., their Tafts σI constants). The Cl2p3/2 binding energy for the hexachlorine-containing cobalt clathrochelates with six strong electron-withdrawing chlorine atoms in their α-dioximate chelate ribbed fragments is in the range 200.7 – 201.0 eV,

and

is

equal

to

208.6 eV

for

the

cobalt(III)

complex

[Co((n-

C4H9NH)2Gm)3(BC6H5)2](ClO4) with ClO4– counter-ion containing a chlorine atom in a formal oxidation state +7. The same effect is observed in the case of the O1s binding energy: its values are in the range 532.5 – 533.1 eV for the cobalt(I) and cobalt(II) clathrochelates (Table 1) and reach 533.2 eV

in

the

spectrum

of

the

cobalt(III)-containing

ionic

associate

[Co((n-

C4H9NH)2Gm)3(BC6H5)2](ClO4) with the oxygen-containing ClO4– counter-ion. The ionic associate of the cobalt(I) cage complexes [((CH3)2N)4P][Co(Cl2Gm)3(Bn-C4H9)2] and [(n-C4H9)4N][Co(Cl2Gm)3(BF)2] contain two types of the nitrogen atoms: those of the donor oxime groups of the macrobicyclic ligand and those of the amino groups of their organic cations. Usually, XPS does not allow differentiating the amine and oxime nitrogen atoms, as their chemical shifts in the N1s spectra are very close; however, in the spectra of the above cobalt(I) clathrochelates, besides the main peak at approximately 400 eV, the additional peaks in the range 399.4 – 399.6 eV +

+

assigned to the counter-ions ((CH3)2N)4P and (n-C4H9)4N are observed. 8 ACS Paragon Plus Environment

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CONCLUSIONS The Cl2p, C1s, N1s, O1s, B1s, and Co2p core-level and Co K-edge XANES spectra proved that both the electronic and spatial structures of the highly conjugated polyene macrobicyclic ligands are affected by the metal-localized redox processes. The nature of these encapsulating ligands influenced the redox characteristics of these encapsulated metallocenters, allowing them to adopt unusual catalytically active oxidation states.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGEMENT The XANES experiments were carried out on a synchrotron irradiation source DORIS III (beamline A1) at DESY, a member of the Helmholtz Association (HGF). We thank Dr. E. Welter for assistance in these experiments. We also kindly acknowledge Prof. I. Fritsky (Kyiv National Taras Shevchenko University) for the fruitful discussion of the experimental results. The authors gratefully acknowledge financial support of RFBR (grants 10-03-00613, 11-03-00986, and 12-0300961), RAS (programs 7 and 18) and Ministry of Education and Science of the Russian Federation (contract 14.518.11.7022).



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(23) Pettito, S. C.; Langell, M. A. Surface Composition and Structure of Co3O4(110) and the Effect of Impurity Segregation. J. Vac. Sci. Technol. A. 2004, 22, 1690–1696. (24) McIntyre, N. S.; Cook, M. G. X-ray Photoelectron Studies on Some Oxides and Hydroxides of Cobalt, Nickel, and Copper. Anal. Chem. 1975, 47, 2208–2213. (25) Oda, H.; Yamamoto, H.; Watanabe, H. On the Shake-up Satellite of Co2p in SrCoO3-x. J. Phys. Soc. Jpn. 1978, 44, 1391–1392.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

Table 1. B1s, Cl2p3/2, N1s, O1s, and Co2p3/2 binding energies, spin-orbital splitting of Co2p as well as shift of the S1 and S2 shake-up satellite relative to Co2p3/2 (eV) for the cage cobalt complexes Compound

∆(Co2p3/2)a

∆(Co2p)b

B1s

Cl2p3/2

N1s

O1s

Co2p3/2

S1

S2

[(n-C4H9)4N][Co(Cl2Gm)3(BC6H5)2]

191.10

200.88

399.36 401.00

532.50

780.72

4.73

7.69

15.20

[((CH3)2N)4P][Co(Cl2Gm)3(Bn-C4H9)2]

191.19

780.77

4.58

7.90

15.25

192.98

399.56 401.00 399.36 401.00

532.54

[(n-C4H9)4N][Co(Cl2Gm)3(BF)2]

197.59 200.79 197.58 200.66

532.69

780.48

4.91

7.63

15.22

200.92

401.00

532.88

780.90

4.81

401.00

533.05

780.99

4.24

9.09

15.09

401.00

532.82

781.07

5.01

8.67

15.20

401.00

532.99

780.92

3.91

9.01

15.04

401.00

532.91

781.17

5.10

8.46

15.23

401.00

532.86

781.81

3.79

401.00

533.21

782.22

5.17

Cobalt(I) clathrochelates

Cobalt(II) clathrochelates Co(Cl2Gm)3(BC6H5)2

191.57

Co((n-C4H9S)2Gm)3(BC6H5)2

191.69

Co(Cl2Gm)3(Bn-C4H9)2

191.02

CoDm3(BF)2

193.13

Co(Cl2Gm)3(BF)2

192.90

201.00

200.77

15.36

Cobalt(III) clathrochelates [CoDm3(BF)2](BF4)

193.11

[Co((n-C4H9NH)2Gm)3 (BC6H5)2](ClO4) 191.93

208.62

8.54

14.81 14.94

Peak plasmon loss Metallic cobaltc a

778.37

a shift of the S1 and S2 shake-up satellite relative to Co2p3/2;

b

3.24

14.99

a spin-orbital splitting

c

Co2p3/2 – Co2p1/2 ; the spectrum of metallic cobalt was calibrated on the Fermi level (EF = 0.0 eV)

ACS Paragon Plus Environment

14

Page 15 of 30

Normalized XANES absorption

Figure 1. Self-assembled clathrochelate monolayers on a gold surface.

C 0.8

B

0.4

(((CH3)2N)4P)[Co(Cl2Gm)3(BC6H5)2]

Co(Cl2Gm)3(BC6H5)2

A

[Co((n-C4H9NH)2Gm)3(BC6H5)2](ClO4)

0.0 7.68

7.76

7.72

7.80

E, keV

Figure 2. Co K-edge XANES spectra of the cobalt(I, II, and III)-containing clathrochelate


analogs.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a 0.8

0.4

Co(Cl2Gm)3(BC6H5)2 Co(Cl2Gm)3(Bn-C4H9)2 CoBd2(Cl2Gm)(BF)2

0.0

7.71

7.74

7.77

7.80

b 0.8

CoNx3(Bn-C4H9)2

0.4

CoBd3(Bn-C4H9)2 Co(S2-Nx)3(BC6H5)2 0.0

7.71

7.74

E, keV

7.77

7.80

E, keV

Figure 3. Co K-edge XANES spectra of the chlorine-containing (a), aromatic and aliphatic (b) clathrochelate cobalt(II) complexes.


15


a 0.8

0.4

Co(Cl2Gm)3(BC6H5)2 Co(Cl2Gm)3(Bn-C4H9)2 CoBd2(Cl2Gm)(BF)2

0.0

7.710

Page 16 of 30

b 0.8

CoNx3(Bn-C4H9)2

0.4

CoBd3(Bn-C4H9)2 Co(S2-Nx)3(BC6H5)2 0.0

7.740

7.730

7.720

7.720

7.710

7.730

7.740

E, keV

E, keV

Figure 4. Ranges B of the Co K-edge XANES spectra of the chlorine-containing (a), aromatic and aliphatic (b) cobalt(II) clathrochelates.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.20

0.15

0.10

(((CH3)2N)4P)[Co(Cl2Gm)3(BC6H5)2]

Co(Cl2Gm)3(BC6H5)2 [Co((n-C4H9NH)2Gm)3(BC6H5)2](ClO4)

0.05

A

0.00

– 0.05 7.700

7.704

7.708

7.712

7.716

7.720

E, keV

Figure 5. Pre-edge ranges A of the Co K-edge XANES spectra of the cobalt(I, II, and III) clathrochelate analogs.


16

Page 17 of 30

((n-C4H9)4N)[Co(Cl2Gm)3(BF)2]

Co2p3/2

Co2p1/2 Sat1 Sat2

XPS intensity, arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(((CH3)2N)4P)[Co(Cl2Gm)3(Bn-C4H9)2]

((n-C4H9)4N)[Co(Cl2Gm)3(BC6H5)2]

775

780

785

790

795

800

805

810

Binding energy, eV

Figure 6. Core-level Co2p spectra of the cobalt(I) clathrochelates.


17


Co(Cl Gm) (BF)

2 3 2 Co2p3/2 Co2p Sat1 1/2 Sat2

XPS intensity, arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

CoDm3(BF)2


Co((n-C4H9S)2Gm)3(BC6H5)2

Co(Cl2Gm)3(BC6H5)2

775

780

785

790

795

800

805

810

Binding energy, eV

Figure 7. Core-level Co2p spectra of the cobalt(II) clathrochelates.


18

Page 19 of 30


Co2p3/2 Co2p1/2 Sat1 XPS intensity, arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[CoDm3(BF)2](BF4)

metallic cobalt

775

780

785

790

795

800

805

810

Binding energy, eV

Figure 8. Core-level Co2p spectra of the cobalt(III) clathrochelates and those of the metallic cobalt as a standard.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

+e

Initial Co(II) clathrochelate

Page 20 of 30

Co(I)-containing electrocatalyst + H+

+e

fast

Co(III) macrobicycle

+ H+ – H2

Hydrid Co – H intermediate

fast

Scheme 1.


20

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O– R

R

N

Cl

N

Cl

O– substituted glyoxime dianion (Gm2–)

O–

O–

O–

N

N

S

N

N

S

O–

O–

B Cl

N

O Cl

((n-C4H9)4N)+ Cl

{(((CH3)2N)4P)+}

N

Cl

N

Cl

N

Cl Cl

O

B Cl Cl

N N

Cl

Cl

B O Cl

Cl

N

N

Cl

Cl

N

N O

Co(Cl2Gm)3(BC6H5)2

S

Co 2+ N S N N O O O B

Cl

Cl

N

Cl

N N Co+

Cl

–

N

Cl

N O

Cl

O Cl

N

((n-C4H9)4N)+ Cl

N

Cl

Cl

N

Cl

N O

S

H 3C

S

H 3C

Cl Cl

N N

Cl

Cl

Cl

N

N

Cl

N O

N O

O


Co(Cl2Gm)3(BF)2

N N O

H 3C

O

CoNx3(Bn-C4H9)2

F O

N N Co2+

CH3

N

CH3

N O

Cl

N O

B O O N N 2+ Co N N O O B

O

N N Co2+

B F

O H 3C

N

((n-C4H9)4N)[Co(Cl2Gm)3(BF)2]

O

O

O

O

Cl

B F

F B Cl

O

N N Co +

O

O

O

O

N N Co2+

F B

O N N O

O

B

CoDm3(BF)2

B

O

O

O

N N Co2+

N

N N Co2+

Cl

N

N O

N

Cl

O

B F

Co(S2-Nx)3(BC6H5)2

α-benzyldioxime dianion (Bd2–)

O

B

O

O S

O

O

B

B O O S N N N

N O–

dimethylglyoxime dianion (Dm2–)

(((CH3)2N)4P)[Co(Cl2Gm)3(Bn-C4H9)2]

O

O

N O–

B

N N Co2+

O

N

O

((n-C4H9)4N)[Co(Cl2Gm)3(BC6H5)2] (((CH3)2N)4P)[Co(Cl2Gm)3(BC6H5)2]

O

N

O

O

(((CH3)2N)4P)+

B

O

O–

N

F B

B O Cl

O

H3C

O–

–

O

N N Co+

O

N

α,α'-dithio cyclohexanedion1,2-dioxime dianion (S2-Nx2–)

–

O

H3C

O–

cyclohexanedion1,2-dioxime dianion (S2-Nx2–)

dichloroglyoxime dianion (Cl2Gm2–)

N

N O O B

CoBd3(Bn-C4H9)2

O

N O O B F

CoBd2(Cl2Gm)(BF)2

+ S O S S

O

N N O S

B

O

N N Co 2+

S

N

S

N OO B

Co((C4H9S)2Gm)3(BC6H5)2

H N N H

NH B O O O N N N 3+ Co N N N O O O NH B

F B O H N

H 3C

N

H 3C

N

O H3C

(ClO4)–

N H


O

H3C

+ O

N N Co3+

CH3

N

CH3

N O

(BF4)–

O

B F

[CoDm3(BF)2](BF4)

Scheme 2.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

Table of Contents Image Initial Co(II) clathrochelate

+e

Co(I)-containing electrocatalyst + H+

+e

Co(III) macrobicycle

fast + H+ – H2

Hydrid Co – H intermediate

fast


22

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Self-assembled clathrochelate monolayers on gold surface. 169x76mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Co K-edge XANES spectra of the cobalt(I, II, and III)-containing clathrochelate analogs 247x177mm (300 x 300 DPI)


Page 24 of 30

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co K-edge XANES spectra of the chlorine-containing (a), aromatic and aliphatic (b) clathrochelate cobalt(II) complexes. 171x62mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ranges B of the Co K-edge XANES spectra of the chlorine-containing (a), aromatic and aliphatic (b) cobalt(II) clathrochelates 203x73mm (300 x 300 DPI)


Page 26 of 30

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pre-edge ranges A of the Co K-edge XANES spectra of the cobalt(I, II, and III) clathrochelate analogs 249x181mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Core-level Co2p spectra of the cobalt(I) clathrochelates 199x260mm (300 x 300 DPI)


Page 28 of 30

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Core-level Co2p spectra of the cobalt(II) clathrochelates 199x260mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Core-level Co2p spectra of the cobalt(III) clathrochelates and those of the metallic cobalt as a standard 228x304mm (300 x 300 DPI)


Page 30 of 30

Combined X-ray Absorption Near-Edge Structure and X-ray

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