Inclusion of Peripheral Basic Groups Activates Dormant Cobalt-Based Molecular Complexes for Catalytic H2 Evolution in Water

Article abstract of DOI:10.1021/acscatal.8b04640

The protein scaffold plays a key role during the enzymatic catalysis for metalloenzymes. Here we have rationally designed an enzyme-inspired outer coordination sphere in the form of protic functionalities, such as natural amino acid derived carboxylic acid and phenolic -OH groups, on the fringe of the cobalt-salen like complexes. This inclusion has enabled electrocatalytic H₂ evolution for an otherwise inactive cobalt-salen like core. The complexes containing peripheral carboxylic acid groups exhibited unique pH-switchable catalytic H₂ production that is connected with the pK_a of the carboxylic acid group (～4.0), suggesting the crucial involvement of the carboxylate group during the catalytic activity. The one- and two-dimensional NMR results of the complexes have indicated the presence of a possible hydrogen bonding network, generated by those protic groups in aqueous solution. These results highlight that an inactive metal complex can be activated for specific small molecule activation via rational inclusion of outer coordination sphere functionalities.

Full text of DOI:10.1021/acscatal.8b04640

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Article
Inclusion of peripheral basic groups activates dormant cobalt-
2
based molecular complex for catalytic H evolution in water
Shikha Khandelwal, Afridi Zamader, Vivek Nagayach, Dependu Dolui, Ab Qayoom Mir, and Arnab Dutta
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04640 • Publication Date (Web): 31 Jan 2019
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Inclusion of peripheral basic groups activates
dormant cobalt-based molecular complex for
catalytic H2 evolution in water
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Shikha Khandelwala, Afridi Zamadera, Vivek Nagayacha, Dependu Doluia, Ab Qayoom Mira,
Arnab Duttaa,b*
[a] Chemistry Discipline, Indian Institute of Technology Gandhinagar, Palaj, Gujarat 382355
[b] Center for Sustainable Development, Indian Institute of Technology Gandhinagar, Palaj,
Gujarat 382355
ABSTRACT
The protein scaffold plays a key role during the enzymatic catalysis for metalloenzymes. Here
we have rationally designed an enzyme-inspired outer coordination sphere in the form of protic
functionalities, such as natural amino acid derived carboxylic acid and phenolic-OH groups, on
the fringe of the cobalt-salen like complexes. This inclusion has enabled electrocatalytic H2
evolution for an otherwise inactive cobalt-salen like core. The complexes containing peripheral
carboxylic acid groups exhibited unique pH-switchable catalytic H2 production that is connected
with the pKa of the carboxylic acid group (~ 4.0), suggesting the crucial involvement of the
carboxylate group during the catalytic activity. The one- and two-dimensional NMR results of
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the complexes have indicated the presence of possible hydrogen bonding network, generated by
those protic groups in aqueous solution. These results highlight that an inactive metal complex
can be activated for specific small molecule activation via rational inclusion of outer
coordination sphere functionalities.
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KEYWORDS
Electrocatalytic H2 Production • Cobalt-Salen like complexes • pH-switchable outer coordination
sphere • Enzyme-inspired catalyst design• Water soluble H2 production catalysts
Introduction
Hydrogen is considered as one of the top candidates for energy carrier for the efficient use of
renewable energy resources.1–6 In recent years, substantial research has been done in the quest
for an efficient and sustainable catalytic system for H2 production from a widely abundant source
like water.7,8 Hitherto, platinum and hydrogenase enzymes have emerged as the best catalysts for
efficient H2 production.9 However, the low abundance of the precious platinum and the chemical
sensitivity of the hydrogenases have restricted their large-scale usage.10–13 Despite the
shortcomings, hydrogenase enzyme active site has remained as the inspiration for developing
biomimetic H2 production catalysts.14–21 However, most of those structural models were inactive
or poorly active for H2 production despite closely imitating the enzyme active site. The absence
of the crucial protein environment around the metallocenter is believed to the prime reason
behind this catalytic inactivity.22–24 In recent years, different research groups have incorporated
proteins and smaller peptides around synthetic metallocenter to induce active catalysis on the
metal center highlighting the importance of the outer coordination sphere during the design of
synthetic hydrogenase models.25–31 However, the fragile nature of the proteins has raised stability
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issues for long-term usage of synthetic metallopeptides in acidic conditions. In a different
approach, some research groups have developed rather robust catalysts by introducing only the
minimal and essential feature of the protein scaffold around metalloporphyrin, and nickel
bis(diphosphine) core.32–36 Despite the improved reactivity of both set of complexes upon outer
coordination sphere feature inclusion, their long-term stability in acidic water, especially in
aerobic condition, is still uncertain.
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The cobalt-salen (Co-salen) complexes are stable under broad pH conditions in aqueous
oxygenic solutions, though, they are primarily known for O2 binding and ring opening reactions
for epoxides.37–39 Recently, research groups of Du and Wang have reported photocatalytic H2
production by Co-salen complex; however, the detailed electrochemical behavior of the complex
and the possible nature of H2 production catalysis (homogeneous or heterogeneous) have not
been probed yet.40,41 In this work, we have developed four Co-salen like complexes varying
peripheral protic functionalities (Scheme 1). Natural amino acid phenylalanine was embedded in
one of the complexes to position carboxylic acid (-COOH) groups in the periphery. The
Inclusion of tyrosine ensured the presence of a phenolic-OH group along with the -COOH as
Scheme 1. Reaction scheme for the salen ligand (Ln) and Co-Salen like complex (Cn)
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outer coordination sphere feature in another complex. Two control complexes were prepared for
each of those complexes, containing peripheral benzylamine and tyramine, respectively, where,
the -COOH groups are absent. The experimental data suggested that the Co-salen like core gets
activated for H2 evolution in aqueous solution only in the presence of protic functionalities such
as –COOH and phenolic-OH. The order of H2 production was as following: tyrosine ≈
phenylalanine > tyramine. The –COOH group binds to the metal center in its deprotonated form,
and that leads to unique pH-activated H2 production by the Co-salen like complexes only below
pH 4.0, which is the pKa of the peripheral –COOH group. The NMR data have also indicated the
possible formation of hydrogen bonding network by the fringe protic functionalities around the
complexes in water, which is vital for triggering catalytic H2 production by the otherwise
inactive Co-salen like core. This project has illustrated that the incorporation of interactive
peripheral basic functionalities can induce electrocatalytic H2 production in water for otherwise
inactive Co-salen like complexes. Thus, the inclusion of basic functionalities in outer
coordination sphere can be used as a common strategy, which can not only improve the moderate
catalysts but also spark life into dormant metal complexes, especially for small molecule
activating reactions that involve exchange of protons.
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Experimental Section
Salicylaldehyde (98%), MES hydrate (≥99.5%) and L-tyrosine (≥98%) were purchased from
Sigma-Aldrich. Benzylamine (98%), Tyramine hydrochloride (98%) and L-phenylalanine (98%)
were purchased from Avra Synthesis Private Ltd, India. Hydrochloric acid and sulphuric acid
were purchased from SD fine chemicals limited India. Sodium Hydroxide (AR grade), Sodium
Hydroxide (AR grade) and Sodium sulphate anhydrous (AR grade), N, N-Dimethylformamide
(HPLC grade) was purchased from Finar Chemicals Private Ltd, India. Tetrabutylammonium
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tetrafluoroborate was purchased from TCI Chemicals India Private Ltd. All the chemicals were
used as received without further purification. All solvents used in this project were distilled
before their use. Millipore ultrapure water (resistivity of 18.2 MΩ.cm at 25°C) was used for
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preparing all the aqueous solutions in this project. H NMR spectra were recorded on Bruker
Avance III Ascent FT 500 MHz Spectrometer. Tetramethylsilane (TMS) was used as an internal
standard in CDCl3 while the solvent signal was used for referencing in (CD3)2SO (d6-DMSO).
UV-Visible spectra were recorded on Jasco V-750 spectrophotometer using quartz cuvette with
1cm path length. Infrared spectra were recorded on Perkin Elmer Spectrum Two spectrometer.
The Mass data were recorded on Waters Synapt-G2S ESI-Q-TOF mass spectrometer.
Electrochemical experiments were performed at room temperature under N2 atmosphere using
Metrohm Autolab PGSTAT 101 potentiostat along with 1 mm glassy carbon disc working
electrode, Pt-wire counter electrode and Ag/AgCl (in saturated KCl) as a reference electrode. All
the cyclic voltammetry data reported in this work was internally referenced against
[Fe(CN)6]3‒/4‒ couple [E0
= +0.360 V vs. standard Hydrogen Electrode (SHE)] in
[Fe(CN)6]3‒/4‒
aqueous medium. All the potential values reported in this work are referenced against standard
Hydrogen Electrode (SHE) unless otherwise mentioned. Buffer solutions of pH 3-6 were
prepared by using 2-(N-Morpholino)ethanesulfonic acid hydrate (MES hydrate) buffer. pH 2 and
pH 1 solutions were prepared by adding an adequate amount of 18 M sulphuric acid in water.
Tetrabutylammonium tetrafluoroborate and anhydrous sodium sulphate were used as a
supporting electrolyte in organic solvents and buffer solutions, respectively. Before each run
glassy carbon working electrode was polished using a slurry of 0.05 m alumina power in Milli-
Q water. Circular dichroism (CD) studies were carried out using Jasco J-815 CD spectrometer
using quartz cuvette with 1mm path length. Data showed in this article is average of 5 runs. CD
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studies for C2 and C4 were performed in buffered solution of different pH.
Spectroelectrochemistry was performed at room temperature in buffered solution using Avantes
starline Avaspec-2048 spectrophotometer. The sample was placed in a quartz cuvette (3.5 mL
volume, 1cm path length) placed in an external sample holder connected to the light source and
detector via optical fibres. The system was further connected to the Metrohm Autolab PGSTAT
101 using a 3 mm glassy carbon rod as working electrode, Pt wire as counter electrode, and
Ag/AgCl (in saturated KCl) as a reference electrode. Bulk electrolysis was also performed using
Metrohm Autolab PGSTAT 101. The bulk electrolysis setup includes a four-neck glass vessel
(Volume 150 ml including the headspace), coiled 23 cm Pt wire as counter electrode, Ag/AgCl
(in saturated KCl) as a reference electrode, and reticulated vitreous carbon or plastic-chip
electrode prepared from graphite powder- and poly(methyl methacrylate) as the working electrode.
75 ml of 0.5 mM C4 and 0.25 mM of C2 in aqueous buffer solutions were used, respectively,
during their chrono-coulometric experiments. The pH titrations of Co-salen like complexes were
performed using Thermo Scientific Orion Star A111 pH meter in aqueous media.
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Synthesis of ligands:
Salen-benzylamine ligand (L1): Ligand L1 was synthesized as per the reported procedure.42
Methanolic solution of salicylaldehyde (0.01 mol, 1.2 ml) was added dropwise to a solution of
benzylamine (0.01 mol, 1.3 ml) with continuous stirring. The color of the solution turned yellow
after the addition and the mixture solution was allowed to stir for 24 hours at room temperature.
The progress of the reaction was monitored using thin layer chromatography using 30 %
ethylacetate and hexane mixture as a mobile phase. After 24 hours, the solvent was evaporated
under reduced pressure and washed subsequently with hexane (3 times) and chilled ether (2
times) and dried under vacuum to obtain yellow colored solid product.
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Yield: 55% (1.46 g). Mass Data: [M]+: Calculated: 212.1; Experimental: 211.9; NMR data H
NMR (500 MHz, CDCl3)  (ppm) = 4.82 (s, 2H, -CH2), 6.87-6.98 (m, 2H, Ar-H), 7.26-7.36 (m,
7H, Ar-H),8.45 (s, 1H, -CH=N), 13.40 (s, 1H, -OH).
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Salen-tyramine ligand (L3): Ligand L3 was synthesized via a modified version of the reported
procedure.43 L3 ligand was prepared by adding (0.01 mol, 106.4 L) salicylaldehyde dropwise to
a solution of (0.01 mol, 173.6 mg) tyramine hydrochloride in methanol containing (0.01 mol, 40
mg) sodium hydroxide with continuous stirring. The color of the solution turned colorless to
yellow following the addition. The reaction mixture was allowed to reflux at 70°C for about four
hours where the reaction progress was monitored by thin layer chromatography. After
completion of the reaction the solvent was removed under reduced pressure and the solid residue
was washed with ethyl acetate and hexane (8:2) mixture. Then, the product was dried under
vacuum and a light yellow colored powdered product was obtained.
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Yield: 56% (137 mg). Mass Data: [M]+: Calculated: 241.4; Experimental: 241.8; NMR data H
NMR (500 MHz, CDCl3)  (ppm) = 2.93 (t, J=7Hz, 2H, -CH2), 3.80 (t, J=6.9Hz, 2H, -CH2),
6.75-7.31 (m, 8H, Ar-H), 8.19 (s, 1H, -CH=N).
Salen-phenylalanine and Salen-tyrosine ligand (L2 and L4): For the preparation of amino acid
containing ligands L2 and L4, first sodium hydroxide (0.01 mol, 40 mg) was dissolved in
methanol, along with one equivalent amount of L-amino acids (tyrosine or phenylalanine, 0.01
mol, 165.2 mg phenylalanine and 122.2 mg tyrosine) with continuous stirring until the amino
acids were dissolved completely. Then a methanolic solution of salicylaldehyde (0.01 mol, 106.4
L) was added to the reaction mixture drop wise and the color of the solution changed from
colorless to yellow. Then the reaction mixture was refluxed at 70°C four hours. Then the reaction
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mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure
using a rotatory evaporator. A yellow colored solid product was obtained which was further
washed with chilled ethanol and dried under vacuum.
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Yield: L2 : 44 % (120 mg); Mass Data: [M]+: Calculated: 269.8; Experimental: 269.8; NMR
data 1H NMR (500 MHz, CDCl3)  (ppm) = 2.95 (dd, J=13.8, 9.5Hz,diastereotopic proton 1H, -
CH2), 3.30 (dd, J= 13.8, 4.1Hz, diastereotopic proton 1H, -CH2), 3.88 (dd, J= 9.4, 4.1Hz, 1H, -
CH), 6.68-7.24 (m, 9H, Ar-H), 8.12 (s, 1H, -CH=N).
Yield: L4: 49 % (140 mg); Mass Data: [M]+: Calculated: 285.8; Experimental: 285.8; NMR data
1H NMR (500 MHz, CDCl3)  (ppm) = 2.79 (dd, J= 13.8, 9.5 Hz, diastereotopic proton 1H, -
CH2), 3.14 (dd, J= 13.8, 4Hz, diastereotopic proton 1H, -CH2), 3.73 (dd, J= 9.3, 4.1Hz, 1H, -
CH), 6.13-7.18 (m, 8H, Ar-H), 7.94 (s, 1H, -CH=N).
Synthesis of Complexes: The following common procedure was used to synthesize the cobalt
complexes (C1-C4) with prepared ligands (L1-L4), which is a modified version of the reported
procedure for the preparation of analogous cobalt-salen complexes.44 A methanolic solution of
Cobalt (II) chloride hexahydrate (0.5 equivalents) was slowly added into the ligand (L1 or L2 or
L3 or L4) solution in methanol in 1:1 ratio in the presence of air. After the addition of cobalt, the
color of the solution turned green [in case of benzylamine (C1) and tyramine (C3)] or dark
brown (in case of phenylalanine (C2) and tyrosine (C4)) from yellow colored ligand solution.
The reaction mixture was stirred at room temperature for 4 hours. Then the solvent was removed
under reduced pressure using a rotatory evaporator. The product was washed with cold ether and
dried under vacuum to obtained powdered product. Yield and characterization results for the
complexes were as following:
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C1: 49 %, Mass Data: [M-2Cl+H]+: Calculated: 478.8; Experimental: 478.1; NMR data H
NMR (500 MHz, CDCl3)  (ppm)= 4.0-5.0 (m, 4H, -CH2), 6.9-7.6 (m, 20 H, Ar-H including
CH=N);
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C2: 31 %, Mass Data: [M-2Cl+CH3OH]+: Calculated: 625.1; Experimental: 624.7; NMR data
1H NMR (500 MHz, CDCl3)  (ppm)= 4.5 (br, 4H, -CH2), 5.4 ppm (br, 2H, -CH), 6.5-8.4 (m, br,
20H, Ar-H and -CH=N);
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C3: 51%; Mass Data: [M-2Cl+H]+: Calculated: 539.1; Experimental: 538.9; NMR data H
NMR (500 MHz, CDCl3)  (ppm)= 2.7-3.0 (br, 8H, -CH2), 6.3-7.3 (m, br, 16H, Ar-H), 7.74, 8.0,
9.15, 9.33 ppm (s, 2H, phenolic -OH), and 7.9, 8.0, 9.0, 9.1(s, 1H,-CH=N) [Four different imine
and phenolic-OH possibly due to both cis- and trans- isomer formation].
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C4: 35%, Mass Data: [M-2Cl]-: Calculated: 625.4; Experimental: 625.3; NMR data: H NMR
of C4 (500 MHz, d6-DMSO)  (ppm)= 4.1-4.7 (br, 6H, -CH2 and -CH), 6.2-7.3 (m, br, 16H, Ar-
H), 9.3 and 10.8 ppm (s, 2H, phenolic -OH), and 10.29 (s, 1H,-CH=N).
pH titration experiment: For the pH titration experiment for C2 and C4, an aqueous solution of
the complexes were prepared (0.25 mM for C2 and 0.5 mM for C4). Then, sufficient amount of
0.1 M NaOH solution was added to the solution to make the pH of the solution ~13. The pH of
the solution was measured by a pH meter (Thermo Scientific Orion Star A111). Then, 2 L of
0.1 M HCl was added to the solution with stirring and the pH of the solution was measured until
it was stabilized (Figure S18). This last step was repeated until the pH of the solution stopped
changing significantly even after the HCl addition.
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Bulk electrolysis experiment: The bulk electrolysis experiments of complexes C2, C3 and C4
were performed in a four-neck glass vessel (Volume 150 ml including the headspace) where
three of the necks were occupied with a coiled 23 cm Pt wire as counter electrode, Ag/AgCl (in
saturated KCl) as a reference electrode, and reticulated vitreous carbon or plastic-chip electrode
prepared from graphite powder- and poly(methyl methacrylate)45 as working electrode,
respectively. Last of the necks was closed with a 24/40 rubber septum, which was used for N2
purging before the experiments and for headspace gas collection (via gas tight leur-lock
Hamilton syringe). During the experiment, 75 mL of complexes (0.25 mM C2 or 0.5 mM C4)
were added in the vessel, all the electrodes (along with a magnetic bead) were inserted along
with rubber septum cap (in gas-tight manner) and the solution was purged with N2 for 30
minutes. Then, the purging was stopped and the chrono-coulometric experiment was started at
corresponding catalytic potentials (-0.8 V vs. SHE for C2 and -1.0 V vs. SHE for C4). The
solution was continuously stirred with a magnetic stirrer during the experiment. The headspace
gas was collected by a gas-tight leur-lock Hamilton syringe after 4200 s (for C2) and 5500 s (for
C4), respectively, and they were analyzed via gas chromatography (GC) instrument (Figure S7
and S10). The GC was calibrated with a control gas mixture (49.5%H2 in N2). Calculation of the
current efficiency of C2 and C4 complex is shown in the SI.
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Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS)
analysis: Scanning electron microscopy (SEM) images were obtained using JEOL JSM7600F
instrument. The images were obtained at an energy of 5 KeV at a working distance of 8 mm. The
plastic-chip electrode prepared from graphite powder- and poly(methyl methacrylate) electrode
was used for this experiment prior and after the bulk electrolysis experiment. Energy-dispersive X-
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ray spectroscopy (EDS) spectra is obtained in SEM mode with working distance of 15mm to align the
EDS detector, at 15 KeV.
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Spectroelectrochemistry of C4 in aqueous media: The change in the oxidation states of C4 in
aqueous media was investigated via spectroelectrochemical technique. C4 exhibited two distinct
reduction signals around -0.09 V and -0.30 V, respectively, at pH4. Chronocoulometric
experiments were performed on compound C4 at pH4 buffered solution, where it was reduced
electrochemically at -0.25 V and -0.50 V, respectively, over time and the gradual change in its
optical spectra was monitored. The electrochemical reductions were performed at a slightly
cathodic direction compared to the reduction peak maxima to ensure the complete reduction
during chronocoulometry. The same experiment was performed for C4 at pH 2.0 at -0.4 V.
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Rinse Test: The rinse test of the complex C2 and C4 were performed in acidic aqueous solution
(pH 2.0) to figure out the nature of the catalytic H2 production reaction; i.e. whether it is
heterogeneous or homogeneous. For this purpose, we have performed three consecutive runs for
each sample. In the first run, a complete scan was recorded for the complex in the pH 2.0
solution, including the cathodic scan, where catalytic H2 production was observed (Figure S8).
Then, the 1 mm diameter glassy carbon disc electrode (working electrode in this case) was
thoroughly rinsed with water and polished with 0.25 micron alumina powder to remove any
possible heterogeneous species formed on the electrode surface. Afterwards, a second run was
performed in the same solution containing the complexes with the cleaned electrode. Here, this
second run was stopped at a negative potential, close to the potential where the maximum
catalytic H2 production was observed (Figure S13). Then, the working electrode was only rinsed
with water (no polishing with alumina) and third voltammogram was recorded in a fresh acidic
aqueous solution (pH 2.0), which did not contain any metal complex. Here the scan was started
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at a negative potential, close to the potential where the maximum catalytic H2 production was
observed (Figure S13). As the third run did not exhibit any significant catalytic H2 production,
compared to the first two runs, it can be concluded that the catalytic H2 production is generated
due to homogeneous catalytic pathway.
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Results
Optical and CD spectroscopy: The salen ligands L1-L4 were prepared by reacting
salicylaldehyde with benzylamine, L-phenylalanine, tyramine, or L-tyrosine, respectively. The
corresponding Co(III)-Salen like complexes (C1-C4) were prepared by separately adding the
Co(II) ion to these ligands in the presence of aerial oxygen. All the complexes are soluble in
organic solvent N,N’-dimethylformamide (DMF) but only C2 and C4 are soluble in water. The
optical spectrum for all the complexes, recorded in DMF, commonly demonstrated three distinct
electronic transitions. The strongest absorbance was observed around 270 nm region for the
complexes ( > 10000 M-1cm-1). This can be attributed to * transition from the aromatic
moiety (Figure 1, Table S1), which was also observed for the free ligands (Figure S1).46
Additionally, a broad feature was noticed for all the complexes in 300-450 nm region with
moderate molar extinction coefficient values ( ~ 4000 M-1cm-1) (Figure 1A, Table S1). This
band is possibly due to the ligand (N and O lone pairs of the salen) to metal (vacant d orbitals of
low spin cobalt (III) (d6) center) charge transfer (LMCT).46 All these complexes also displayed a
weak transition around 600-700 nm region ( < 500 M-1cm-1), which are characteristic of the
metal-based d-d transitions (Figure 1A (inset), Table S1).47 These characteristic d-d transition
bands were blue shifted for C2 and C4 compared to C1 and C3, possibly due to the influence of
the COOH groups present in C2 and C4.
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Optical spectra of C2 and C4 were also recorded in aqueous solution with varying pH. Figure
1B exhibits the optical spectra for C4 that was recorded in aqueous solution at pH 2-6 (Table
S2). No evident changes were observed in the optical signatures for , LMCT (Figure 1B)
and d-d transitions (Figure S2) for C4 when the acidity of the solution was gradually changed
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Figure 1. (A) Optical spectra of complexes C1 (black trace), C2 (red trace), C3 (blue trace),
and C4 (green trace) recorded in DMF highlighting the * and LMCT transitions in the
UV-visible region. The d-d transitions of the complexes are shown in the inset. (B)
Comparative optical spectra of C4 in aqueous solution at pH 2 (black trace), pH 3 (red
trace), pH 4 (blue trace), pH 5 (green trace), and pH 6 (violet trace). (C) The CD spectra of
the analogous solutions of C4 exhibiting difference in the negative LMCT band above 300
nm region.
from pH 6 to pH 2. Circular dichroism (CD) spectroscopy was utilized to further probe any pH -
dependent spectral variations of C4 in aqueous solution. CD is a common technique for
investigating the structural dynamics of optically active molecules.48,49 In this case, the presence
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of optically active L-tyrosine in the coordination sphere of C4 induces chirality into an otherwise
achiral Co-salen like core. The CD spectrum of C4 at pH 6 solution displayed a positive band at
305 nm and a broad negative band at 360 nm (Figure 1C), which are presumably originated from
the LMCT band, as suggested by the comparative optical spectra (Figure 1B&C). This CD
activity confirmed that the Co-salen like core is connected to chiral L-Tyrosine possibly via the
amine (via the Schiff base imine) and carboxylate groups. As the pH of the solution was
gradually changed to pH 2, the negative CD band at 360 nm blue shifted significantly by 20 nm.
C2 also demonstrated analogous alterations in the CD signal originated from the LMCT band
around 300-350 nm region below pH 3 (Figure S3). On the other hand, C1 and C3 did not exhibit
any detectable signal in CD spectroscopy due to the absence of any chiral component in the
ligand structure (Figure S4). This data insinuates a peripheral amino acid induced structural
change in the complex C2 and C4 beyond pH 3.
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Electrochemical properties: Complexes C2 and C4 have exhibited appreciable aqueous
solubility for exploring their electrochemical behavior in variable pH conditions in water. As
shown in Figure 2A (inset), C4 exhibited two distinct irreversible reduction peaks centered at
0.09 V and -0.37 V at pH 6.0. These peaks can be recognized as stoichiometric Co(III/II) and
Co(II/I) reductions as observed for analogous Co-diimine complexes such as
Co(dimethylglyoxime)2.50 These stoichiometric metal reduction signals are irreversible in nature
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Figure 2.(A) Cyclic voltammograms of C4 recorded in aqueous solution at pH 1 (violet trace),
pH 2 (orange trace), pH 3 (green trace), pH 4 (magenta trace), pH 5 (blue trace), and pH 6 (red
trace). A control blank was recorded at pH 2 (black trace). The inset figure highlights the
changes in the stoichiometric signals for C4 with pH. A 1 mm glassy carbon disc electrode, a Pt
wire and Ag/AgCl in saturated KCl were used as working, counter, and reference electrodes,
respectively, during this experiment. Scan rate during all the CVs were 0.1 V/s.
in aqueous solution. Similar results were also obtained by research groups of Coutsolelos and
Delgadillo for cobalt imine complexes in water.51,52 The Co(II/I) stoichiometric peak started
shifting significantly to anodic direction with increasing acidity of the solution, while the
position of the Co(III/II) peak fluctuated slightly towards the cathodic direction (Figure 2A inset,
Table S3). At pH 2, the Co(III/II) and Co(II/I) peaks merged completely to a single two-electron
reduction signal (Figure 2A), and it remained the same at pH 1. The generation of a single step
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two electron transfer was also supported by the spectroelectrochemical data. At pH 4, when C4
was reduced at -0.25V (at a potential in between the two stoichiometric reduction signals for C4
at the same condition), it induced a gradual decrease (by ~25%) in the LMCT band at 370 nm,
while the band around 300 nm remained the same (Figure S5, Table S2). While, the reduction of
C4 at -0.5 V induced further depletion of the optical band at 370 nm (overall ~ 66%) along with
a decrease of 300 nm band at the same solution of pH 4 (Figure S5). This data suggests a
possible two-step reduction of the Co-N2O2 salen like core in C4 complex, which can be
attributed to Co(III/II) and Co(II/I) oxidation state changes. However, the same compound C4
displayed a single reduction peak at -0.2 V in pH 2 solution (Figure S5). The chronocoulometric
reduction of the compound at -0.4 V exhibited concurrent decrease in both the 300 nm and 370
nm optical bands (Figure S5). The resultant optical spectrum observed after this single reduction
step for C4 at pH 2 solution is analogous to the spectrum recorded for the same compound after
two subsequent reduction steps at pH 4. Thus, this data implies a solo-step two electron
reduction at pH 2 for C4 complex, which can be attributed to possibly Co(III/I) oxidation state
change. The Co(II/I) peak shifted ~30 mV/pH unit in the pH range of 3-6, which plausibly
signifies the presence of a proton-coupled electron transfer (PCET) here before the merging it
with Co(III/II) signal below pH 3 (Figure S6). Similar pH dependent unification of metal-based
stoichiometric signals were observed earlier for Ni(P2N2)2 complexes containing similar
peripheral COOH groups.35 C4 did not exhibit any significant reductive catalytic response that
can be attributed to homogeneous H2 production in aqueous solutions at pH > 3. However, it
started displaying catalytic reduction current in water at pH 3 or below (Figure 2A). The onset
potential and the potential at which maximum current (Ecat) was observed are mentioned in
Table S3. The bulk electrolysis data along with complementary gas chromatography (GC) also
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suggested that H2 gas is produced due to this catalytic activity by C4 with > 90% current
efficiency (Figure S7-S8, SI). Thus this reductive catalytic response can be attributed to catalytic
H2 production by C4 in acidic aqueous solution. The catalytic H2 production for C4 improved
slightly as the pH of the solution was decreased gradually to pH 1 (Figure 2). An additional
catalytic feature was noticed in the CV for C4 around -1.05 V at pH 1.0. This trait can be
attributed to H2 evolution via a different mechanism during the catalytic cycle. Analogous
features were observed earlier for Ni-bis-(diphosphine) catalysts.53 C2 also exhibited analogous
shift of Co(II/I) signal ( ~30 mV/pH unit) and ultimate merging of stoichiometric Co(III/II) and
Co(II/I) peaks in water with decreasing pH of the solution (Figure S9A-B, Table S4). The
catalytic H2 production signal by C2 was again observed only below pH 3 and the catalytic
response was improved appreciably as the acidity of the solution is increased to pH 1 (Figure
S9A, Table S4). Corresponding chrono-coulometric and GC experiments corroborated
production of H2 gas during the catalytic reduction with > 90% current efficiency (Figure S8,
S10).
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The electrochemical study of complexes C1 and C3 were performed in a DMF/water mixture
solvent due to their aqueous insolubility. 1% and 2.5% DMF in water solutions were used for the
study of C1 and C3, respectively. C1 exhibited only stoichiometric Co(III/II) and Co(II/I) signals
without any significant catalytic response for homogeneous H2 production (Figure S11) even at
acidic conditions (pH < 3). The stoichiometric signals for cobalt based reductions for C1 at pH
2.0 remained distinctly separated unlike to the merged cobalt signals for C2 and C4 (Figure S11).
This feature possibly highlights the absence of PCET mechanism during electron transfer for C1.
However, the stoichiometric Co(III/II) and Co(II/I) signals for C3 were merged at all conditions
below pH 5.0 and the stoichiometric peak shifted to cathodic direction by ~65 mV/pH unit
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(Figure S12). These observations again suggested that PCET mechanism is involved during the
reduction of cobalt center, possibly via the phenolic OH group present in tyramine. C3 also
showed significant catalytic response in the cathodic direction beyond -0.9 V at all aqueous
solution below pH 6.0 (Figure S12A).The bulk electrolysis study of complex C3 demonstrated
reductive charge accumulation with the following increasing order: pH 5 < pH 4 < pH 3 < pH 2
(Figure S8C). The corresponding GC study supported the formation of H2 gas during the
cathodic catalytic step for each of those solutions with > 90% current efficiency. The origin of
this H2 production was probed carefully to exclude the possibility of heterogeneous H2
production, which was observed earlier for analogous cobalt complexes.54 Here, the rinse test
was performed in pH 2.0 aqueous solution, which exhibited homogeneous catalytic H2
production for all the complexes (Figure S13, SI). Additionally, scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed on the plastic chip
electrode before and after the bulk electrolysis experiment for C4 at pH 1.0. The SEM and EDS
data clearly demonstrated no deposition of cobalt based materials on the electrode following the
electrolysis, supporting the homogeneous nature of the first catalytic H2 production step (Figure
S14). The stability of all the complexes during the catalytic process was also supported by the
almost identical comparative optical spectra for the complexes C2-C4 before and after the
chrono-coulometric experiments (Figure S15). Another catalytic response was observed for C2-
C4 at further cathodic direction following this homogeneous H2 production step. As illustrated in
Figure S14, for C4 exhibited the additional catalytic response at -1.30 V beyond the
homogeneous response at -1.0 V. To explore the nature of the additional catalytic feature,
analogous rinse test was performed by extending the partial cathodic scan till -1.30 V. Rinse test
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of the unpolished electrode in a blank solution following this run confirmed the heterogeneous
nature of this additional catalytic process at -1.30 V(Figure S16).
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Calculating H2 production turn over frequency (TOF) and overpotential: The catalytic rate
(kobs or TOF) for the complexes were calculated by using the following equation (1):
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1/2
푖푐푎푡
푖푝
푅푇푘표푏푠
)
퐹휐
푛
(Equation 1)55–57
=
(
0.4463
Here icat and ip are catalytic and stoichiometric current, respectively, n= number of electrons
involved in this process, R = universal gas constant, T = temperature in degree K, F = 1 Faraday,
and  = scan rate. On the other hand, the overpotential requirement of the catalyst was calculated
as the difference between the potential of half maxima catalytic response (Ecat/2) and equilibrium
potential for H+/0.5 H2 couple (EH+/0.5H2) at a particular pH condition vs. SHE. The potential at
which the catalytic H2 evolution reaction (HER) wave started to shift significantly from the
background current was determined as the onset potential. The calculation showed that C4
produces H2 in aqueous solution at a rate of 178 s-1 with an overpotential requirement of 780 mV
at pH 3.0 (Table 1). The rate improved further in acidic solutions to 190 s-1 (pH 2.0) and 277 s-1
(pH 1.0), respectively, while the overpotential requirement gradually increased to 840 mV at pH
1.0 (Table 1). Analogously, C2 also displayed catalytic H2 production activity only below pH
4.0. The catalytic rates for H2 production for C2 were similar to C4; however, the overpotential
requirement was reduced significantly (by ~ 200 mV) (Table 1). However, the complex was
unstable at pH 1.0, and the maximum catalytic response for C3 was recorded at pH 2.0, where it
produces H2 at a rate of ~ 160 s-1 with an overpotential requirement of ~780 mV (Table 1).
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Table 1. Overpotential requirement (OP) and catalytic HER turnover frequencies (TOFs) of
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complexes C2, C3, and C4 in aqueous medium. All the TOFs are calculated at 1V/s.
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C2
C3a
TOF (s-1)
C4
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pH
TOF (s-1)
OP (mV)
OP (mV)
TOF(s-1)
OP (mV)
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670
(± 5)
277
(±23.5)
840
(± 5)
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unstable
unstable
(±11.01)b
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565
(± 5)
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(±16.5)
780
(± 5)
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775
(± 5)
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(± 20)
580
(± 5)
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(± 5)
720
(± 5)
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(±11.9)
780
(± 5)
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(± 5.3)
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(± 5)
n.d.
n.d.
n.d.c
n.d.
n.d.
n.d.
110
(± 5.6)
660
(± 5)
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n.d.
aRecorded in 2.5% DMF in aqueous buffer mixture
bThe error range is calculated from 3 independent runs for each sample
cn.d.: not detected
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NMR Spectroscopy: Detailed 1H NMR and two dimensional NMR spectroscopy was performed
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on the ligands (L3-L4) and complexes (C3-C4) to explore the role of the peripheral basic
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functionalities on the overall proton trafficking around the complex. The comparative H NMR
data (recorded in d6-DMSO containing minute amount of water) of L4 and C4, clearly exhibited
a significant change of phenolic –OH proton upon cobalt binding (Figure S17A). In the ligand
stage, the phenolic –OH was broad in nature ( ~ 14.5 ppm), which was transformed into two
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Figure 3. (A) The NOESY spectra of C4 and (B) C3 in d6-DMSO at room temperature, recorded
with 0.3 s mixing time. The cross peaks are connected by the dotted black line.
sharp signals (at  ~ 11.0 and  ~ 9.3 ppm) in the complex C4 (Figure S17A). The sharpening of
the exchangeable phenolic –OH group in C4 suggests the possible involvement of the tyrosine
side chain in robust hydrogen bonding network once the cobalt compound is formed. Similar
changes in phenolic-OH proton NMR signals were also observed by Gerothanassis et al.58 The
two signals of phenolic –OH in C4 can be presumably attributed to the formation of trans-
1
structure of C4. Analogous changes in phenolic –OH H NMR signal was also observed for the
tyramine containing L3 and C3 compounds (Figure S17B). The two dimensional Nuclear
Overhauser Effect Spectroscopy (NOESY) was also performed for complexes C3 and C4 in the
same d6-DMSO to further explore the exchange properties of the phenolic –OH groups at room
temperature. The NOESY data for C4 clearly demonstrated exchange of protons between the
phenolic –OH groups and solvent water molecules. The two different phenolic –OH groups also
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swapped protons among themselves that was evident from the appearance of cross peaks at 11.0
and 9.3 ppm (Figure 3A). Analogous cross peaks were observed for C3 at 9.5, 8.0, and 3.3 ppm,
which signify the exchange of protons between the variable phenolic-OH groups and solvent
water, respectively (Figure 3B). These results highlight the crucial role of phenolic –OH groups
in the proton exchange phenomenon and possible formation of hydrogen bonding network
around the complexes in presence of solvent water molecules.
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Discussion
Here in this work, we have developed a series of complexes containing identical Co-salen like
core with varying peripheral functionalities. The electrochemical experiments showed that
incorporation of peripheral basic functionalities such as –COOH and/or phenolic –OH groups
can induce catalytic H2 production reactivity for the otherwise inactive Co-salen like core
complexes in acidic aqueous solution. Complexes C2 and C4 exhibited catalytic H2 production
only below pH 4.0. This observation indicates to the plausible vital role of the -COOH group in
the catalysis as the pKa of carboxylic acid groups are generally documented in that range under
analogous conditions.59 The pH titration experiment with both C2 and C4 in aqueous solution
revealed that both these complexes indeed have a pKa ~ 4, possibly due to the -COOH
functionality existing in the periphery (Figure S18). C4 also exhibited an additional pKa ~ 11.2,
presumably due to the presence of phenolic –OH functionality.59 Interestingly, both these
complexes showed an additional pKa ~ 7, which is characteristic to the metal bound water
molecule (Figure S18).60 Thus, the pH switch around 4.0 can be attributed to the altered
interaction between the cobalt core and the ligand moiety connected via –COOH group. This
change in metal coordination was also monitored through the CD spectroscopy for C2 and C4.
Both these complexes contained chiral amino acid based ligands that can induce chirality in the
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metal center and subsequently in the LMCT bands. The pH-regulated variations in the CD
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spectra (Figure 1C and S3B) demonstrated that the stark change in the metal-ligand interaction
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occurs only around pH 3, suggesting the crucial role of only carboxylic acid moiety in this pH
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switch. The -COOH group coordination to metal is expected to be different in its protonated (-
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COOH) vs. deprotonated (COO-) versions. The CD data also suggested that the deprotonated
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