Cobalt corroles with phosphonic acid pendants as catalysts for oxygen and hydrogen evolution from neutral aqueous solution

Article abstract of DOI:10.1039/c7cc02400b

Cobalt corroles with different acid/base pendants, L^Br-Co, L^COOH-Co, L^PO(OH)₂-Co, and LCH₂PO^(OH)₂-Co (L = 5,15-bis-(pentafluorophenyl)-10-(4-dibenzofuran)corrole), were synthesized and examined as catalysts for oxygen and hydrogen evolution from neutral aqueous solutions. Co corroles with phosphonic acid pendants showed improved activities in both processes, highlighting the importance of the secondary coordination sphere in catalyst design.

Full text of DOI:10.1039/c7cc02400b

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Cobalt corroles with phosphonic acid pendants as catalysts for
oxygen and hydrogen evolution from neutral aqueous solution
Huiling Sun,‡a Yongzhen Han,‡a Haitao Lei,a Mingxing Chen,a and Rui Cao*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Cobalt corroles with different acid/base pendants, LBr-Co, LCOOH
-
Co, LPO(OH)2-Co, and LCH PO(OH)2-Co (L = 5,15-bis-(pentafluorophenyl)-
10-(4-dibenzofuran)corrole), were synthesized and examined as
catalysts for oxygen and hydrogen evolution from neutral aqueous
solutions. Co corroles with phosphonic acid pendants showed
improved activities in both processes, highlighting the importance
of the secondary coordination sphere in catalyst design.
2
Scheme 1 Molecular structures of Co corroles. Pyridine groups
bound to the axial positions of Co are omitted for clarity.
An attractive way to meet the growing global energy demand
is to convert solar energy to chemical fuels.1,2 Water splitting is
an ideal protocol for this solar-to-fuel strategy.3,4 However, its
two half reactions, oxygen evolution reaction (OER) and
hydrogen evolution reaction (HER), are kinetically slow.3,4
Therefore, designing cheap and efficient OER4,5 and HER6,7
catalysts has attracted extensive interests. A variety of
molecular catalysts of Mn,8-10 Fe,11-13 Co,14-26 Ni,27-31 and Cu32-35
have been identified as OER and/or HER catalysts. Particular
emphases have been placed on providing new insights into
catalyst design to reveal the structure-function relationship.
Metallocorroles have been shown to be highly active for
OER3,9,14,15,17 and HER.3,22,23,32 Trianionic corrole ligands can
afford a stable square-planar coordination mode and are very
effective in stabilizing high-valent metal ions, which are usually
involved in OER.3 Consequently, corrole ligands can offer low-
valent metal ions large reducing powers. All these features
make metal corroles attractive for OER and HER catalysis.
In addition, the second coordination sphere of metal ions
has a substantial effect on catalysis.36 For example, DuBois and
co-workers demonstrated that pendant amines proximate to
the metal ion can significantly improve the catalytic efficiency
of Ni-based HER catalysts.29,30 Protonated amine groups can
function as intramolecular proton relays to facilitate proton
transfer processes. Nocera and co-workers designed hangman
metalloporphyrins as HER catalysts and showed that the
hanging carboxyl acid group is able to assist HER by mediating
proton-coupled electron transfer (PCET) steps.11,21 These
authors further revealed that the rate of HER catalysis is
affected by the proton-donating ability of the hanging group.
For OER, Nocera and co-workers showed that the hanging
carboxyl group can increase the catalytic activity of Co
corroles.17 Theoretical studies by Cramer37 and by us38
suggested that the carboxyl group in the second coordination
sphere of the Co ion can act as an intramolecular base to assist
the rate-limiting O−O bond formation through a concerted
oxygen atom-proton transfer mechanism.33,39 However, due to
synthetic challenges to introduce functional groups to the
second coordination sphere of metal ions, few examples have
been reported to compare the effects of different acid/base
pendants on OER and HER catalysis.
It is known that the increase of the proton-accepting ability
of the base will increase the O−O bond formation rate.3 We
are thus interested in attaching phosphonic acid groups to the
second coordination sphere of Co corroles. As the pKa2 of
phosphonic acid groups is larger than the pKa of carboxyl acid
groups, Co corroles with phosphonic acid pendants are
considered to have improved OER activities as compared to
analogues without such groups. In addition, because of the
different proton-donating abilities of phosphonic and carboxyl
acid groups, it is intriguing to study the HER activity of Co
corroles with phosphonic acid pendants. Herein we report the
syntheses and characterizations of four Co corroles appended
with different acid/base groups. Complexes LBr-Co, LCOOH-Co,
a. Department of Chemistry, Renmin University of China, Beijing 100872 China.
ruicao@ruc.edu.cn.
b. School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119 China.
‡ These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Details for the syntheses and
characterizations of all complexes. X-ray data as CIF files (CCDC 1526192-1526195).
Schemes S1-S4. Tables S1-S3. Figures S1-S76. See DOI: 10.1039/x0xx00000x
LPO(OH)2-Co, and LCH PO(OH)2-Co all have an active Co corrole site
2
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Scheme 2 Synthetic routes of LCH PO(OH)2-Co (route A), LPO(OH)2-Co (route B), LCOOH-Co (route C), and LBr-Co (route D).
2
attached to the 4-position of the dibenzofuran unit and have distances are indicative of a d6 CoIII electronic configuration,15
different pendants at its 6-position (Scheme 1). All these which is consistent with the diamagnetism of LCH PO(OEt)2-Co as
2
1
CH PO(OH)
2 2
complexes are active for OER and HER from neutral aqueous established by H NMR. Because LCH PO(OEt)2-Co and L
-
2
solutions. Our results show that appended groups at the 6- Co should have very similar structural aspects except that the
position of dibenzofuran play crucial roles in both processes. −CH2PO(OEt)2 moiety is converted to −CH2PO(OH)2 in the latter
Complex LCH PO(OH)2-Co outperforms others in both OER and complex, the place and orientation and also the distance of the
2
HER. This work therefore highlights the importance of the −CH2PO(OH)2 unit to Co can be estimated based on the X-ray
structure of LCH PO(OEt)2-Co. Structural analysis showed that the
2
secondary coordination sphere in catalyst design.
The synthetic routes of the four Co corroles are depicted in distance of the phosphonic oxygen atoms to Co is about 6.6 Å,
Scheme 2. For LCH PO(OH)2-Co, the introduction of two formyl indicating that they are close enough to Co to potentially act as
2
groups at 4- and 6-position of dibenzofuran was realized by an intramolecular base to facilitate the nucleophilic attack of a
treating it with n-BuLi followed by reaction with water molecule to Co-oxo unit for O−O bond formation.33,39
dimethylformamide (DMF, route A). Details of synthesis and
characterization are in electronic supplementary information.
One of the formyl group was converted to −CH2PO(OEt)2, and
the other one was converted to a corrole unit containing two
pentafluorophenyl units at its 5- and 15-position. Subsequent
hydrolysis and reaction with Co(OAc)2 gave LCH PO(OH)2-Co. Its
2
identity and purity were confirmed by NMR spectroscopy (Fig.
S46 and Fig. S47) and High-resolution mass spectrometry
(HRMS, Fig. S48). Attempts to grow crystals of LCH PO(OH)2-Co for
2
X-ray analysis were not successful. Instead, we synthesized its
1
ester form LCH PO(OEt)2-Co for structural studies ( H NMR, Fig.
2
S49; HRMS, Fig. S50).
Complex LCH PO(OEt)2-Co crystallized in the orthorhombic
2
space group P21212 (cell parameters in Table S1). The Co ion is
coordinated by the four N atoms of the corrole unit, which
define an equatorial plane (Fig. 1a). Two additional pyridine Fig. 1 Thermal ellipsoid plot (50% probability) of the X-ray
groups are bound to the axial positions of Co. The resulted structures of (a) LCH PO(OEt)2-Co, (b) LPO(OEt)2, (c) LCOOMe, and (d)
2
octahedral coordination environment and short Co−N bond LBr-Co-PPh3.
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Co corrole LPO(OH)2-Co was synthesized following route B
(Scheme 2). Complex 4,6-dibromodibenzofuran was treated
with one equiv. of PhLi followed by reaction with DMF to give
4-formyl-6-bromodibenzofuran. Subsequent conversion of −Br
to −PO(OEt)2 and conversion of −CHO to corrole were realized.
The resulted LPO(OEt) was structurally characterized (cell Fig. 2 (a) Linear sweep voltammetry (LSV) of OER using blank
parameters in Table S1), showing an appended −PO(OEt)2 and catalyst-coated (20 nmol cm−2) FTO electrodes in 0.1 M pH
group proximate to corrole unit (Fig. 1b). The corresponding 7 phosphate buffers. (b) The plot of potential at 0.52 mA cm−2
LPO(OH)2-Co was characterized by NMR (Fig. S31 and S32) and vs pKa of appended groups. (c) LSV of HER using blank and
HRMS (Fig. S33). Starting from 4-formyl-6-bromodibenzofuran, catalyst-coated (70 nmol cm−2) GC electrodes in 0.1 M pH 7
−Br could be converted to −COOMe (route C). LCOOMe was phosphate buffers.
2
structurally characterized (Fig. 1c, cell parameters in Table S1).
The corresponding LCOOH-Co was also carefully characterized able to act as an intramolecular base to assist the O−O bond
(Fig. S19 and S20). Additionally, LBr-Co was synthesized as an formation through a concerted oxygen atom-proton transfer
2−
analogue lacking acid/base pendants (route D, 1H NMR, Fig. S7; mechanism. Because ArCH2PO3 (pKa2 of ArCH2PO3H2 ca. 8.4)
HRMS, Fig. S8). In order to grow crystals of LBr-Co, one equiv. is more basic than others in this series (pKa2 of ArPO3H2 ca. 7.1,
of PPh3 was added. The resulted LBr-Co-PPh3 crystallized in the pKa of ArCOOH ca. 4.2), it is more effective in facilitating the
orthorhombic space group Pbca (Fig. 1d, cell parameters in O−O bond formation. Fig. 2b shows the plot of potential
Table S1). The Co ion has a square pyramidal coordination (measured at 0.52 mA cm−2) vs pKa of appended groups (for
environment with the four N atoms of corrole defining the −PO(OH)2 and −CH2PO(OH)2, pKa2 are used). The slope is −55
equatorial plane and PPh3 occupying the axial position.
mV per pKa unit. Similar observation was reported by Groves
and Wang: plotting the potential at a fixed current vs pKa of
PO(OEt)
One the basis of structural studies of LCH PO(OEt)2-Co, L
,
2
2
LCOOMe, and LBr-Co-PPh3, we can conclude that four complexes the buffer anion gave a slope of –54 mV per pKa.16 These
contain different acid/base pendants proximate to corrole results indicate that the OER mechanism involves a rate-
units. The dibenzofuran unit is therefore valuable to make determining PCET step (i.e., O−O bond formation), in which the
functional groups at the second coordination sphere of metal electron transfer process becomes more favorable because of
centers. It is worth noting that the two meso-C6F5 substituents the improved proton transfer process as facilitated by a
have strong electron-withdrawing features. The use of meso- stronger basic group.16,17,39 Controlled potential electrolysis
C6F5 substituents is aimed to (1) decrease the electron density (CPE) studies were then carried out to examine the stability of
of the corrole ring and thus increase its stability during OER these Co corroles during OER (Fig. S59-S62). During 5-h CPE,
and (2) shift the redox events of corroles to the anodic currents remained relatively stable. The Faradaic efficiencies of
direction to reduce the overpotential for HER.
all four Co corroles for O2 evolution were measured to be
Cyclic voltammograms (CVs) of Co corroles were recorded more than 90% (Fig. S69-S72). In addition, the washed FTO
in acetonitrile (0.1 M Bu4NPF6). Due to the very low solubility electrodes after electrolysis exhibited no catalytic OER
of LPO(OH)2-Co and LCH PO(OH)2-Co, their ester forms were used for activities, and no heterogeneous particles were formed on the
2
CV measurements. In general, these Co corroles each showed washed electrodes as examined by scanning electron
four redox events (Fig. S55-S58, data summarized in Table S3). microscope (SEM) and energy dispersive X-ray spectroscopy
As can be seen from these data, the redox potentials of these (EDX) (Fig. S67). All these results suggest the molecular nature
Co corroles are almost identical. These results demonstrate of these Co corroles for OER catalysis.
that pendants at the second coordination sphere of Co ions
Catalytic HER with these Co corroles was also examined
have small influence on the redox potentials. It is worth noting using catalyst-coated glassy carbon (GC) electrodes in 0.1 M
that the second reduction wave of LCOOH-Co is irreversible. This pH 7 phosphate buffers. The amount of catalysts loaded on GC
result indicates the reduction of the acid protons. Similar is intended to be 70 nmol cm−2. Our results give an activity
observation was reported by Nocera and co-workers on order of LCH PO(OH)2-Co > LPO(OH)2-Co > LCOOH-Co > LBr-Co (Fig. 2c).
2
hangman metalloporphyrins.21
This trend is likely due to the higher protonation level of
2−
Catalytic OER was performed in 0.1 M pH 7 phosphate −CH2PO3 in neutral aqueous solutions, which leads to an
buffers using catalyst-coated fluorine-doped tin oxide (FTO) effectively higher local proton concentration.11 Similarly, their
working electrodes. The amount of catalysts loaded on FTO is stabilities for HER were examined by CPE studies, which gave
intended to be 20 nmol cm−2. Fig. 2a shows that the OER almost stable current profiles (Fig. S63-S66). The Faradaic
activity has the order of LCH PO(OH)2-Co > LPO(OH)2-Co > LCOOH-Co > efficiencies for H2 evolution were determined to be larger than
2
LBr-Co. The onset potential of the catalytic wave with 80% (i.e., for LCH PO(OH)2-Co, it is 94%, Fig. S73-S76). The washed
2
LCH PO(OH)2-Co is at 1.27 V vs normal hydrogen electrode (NHE, GC electrodes after electrolysis were not active for HER and
2
all potentials reported are vs NHE unless otherwise noted), showed no heterogeneous phases in SEM and EDX analyses
corresponding to an onset overpotential of 450 mV. This value (Fig. S68). All these results suggest that these Co corroles are
is smaller than other Co corrole OER catalysts reported in the real catalysts for HER. It is necessary to note that Co corroles
literature.15,17 The appended base group is considered to be
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Acknowledgements
We are grateful for the support from the “Thousand Talents
Program” of China, the National Natural Science Foundation of
China under Grants 21101170 and 21573139, the Fundamental
Research Funds for the Central Universities, and the Research
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