Efficient photocatalytic water reduction by a cobalt(II) tripodal iminopyridine complex

Abstract

The visible light-driven catalytic water reduction by a cobalt(II) tripodal iminopyridine complex [Co(tachpy₃)](ClO₄)₂ (1) (tachpy₃ = cis,cis-1,3,5-tris(pyridine-2-carboxaldimino)-cyclohexane) has been investigated in aqueous acetonitrile. The catalysis is found to be homogeneous and an impressive H₂ TON of 16 440 has been achieved, which is far better than that by a structurally related complex [Co(trenpy₃)](ClO₄)₂ (2) (trenpy₃ = tris-[4-(2-pyridyl)-3-azabut-3-enyl]amine) bearing identical donor groups. A comparison between the two complexes reveals that the catalytic properties are sensitive to the change in ligand rigidity and thus the coordination geometry, as reflected by their respective reaction potentials and H₂ TONs. The catalysis of 1 proceeds via a penta-coordinate species formed by the protonation of the corresponding Co^I state. The results in spectroscopic and electrochemical studies are supplemented by DFT computation.

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Introduction

In the recent decade, the development of functional materials for tapping solar energy by the light-driven conversion of earth abundant materials, e.g. water and carbon dioxide, into storable solar fuels has gained increasing momentum.^1–7 In particular, the catalytic generation of hydrogen by the reduction of water has been widely investigated, as H₂ is an energy vector used in fuel cells. For economic reasons, the development of molecular water reduction catalysts (WRCs) based on first-row transition metals (e.g. Co, Cu, Fe and Ni) has received much attention.^8–39 Co WRCs are commonly studied, due to their desirable catalytic properties. For example, Co oxime catalysts were found to function at low overpotentials.¹⁵ Recently, enhanced H₂-evolution performance was achieved using a Co–NHC catalyst,⁴⁹ while Co polypyridyl catalysts exhibit higher stability.^{17–19,25,50,51} In contrast, photo-driven H₂ generation with Co catalysts is, however, usually less efficient compared to that with other metal catalysts.^16,24,28,36

There have been only a few studies on Co iminopyridine WRCs despite their fast catalytic rates.^20–23 We report herein the photocatalytic water reduction by a cobalt(II) tripodal iminopyridine complex [Co(tachpy₃)](ClO₄)₂ (1) (tachpy₃ = cis,cis-1,3,5-tris(pyridine-2-carboxaldimino)-cyclohexane) in aqueous acetonitrile (Scheme 1).⁴⁵1 is found to be an active molecular WRC. To shed light on the factors affecting the reactivity, a structurally related complex [Co(trenpy₃)](ClO₄)₂ (2) (trenpy₃ = tris-[4-(2-pyridyl)-3-azabut-3-enyl]amine)⁴⁶ which bears the same metal centre and donor groups is also investigated. The catalytic properties, e.g. stability and reaction potential, are compared using photo- and electrochemical methods supplemented with DFT computation. The coordination geometry and the more rigid ligand structure in 1 are suggested to contribute to its enhanced WRC reactivity.

Scheme 1 Structures of 1, 2 and IrPS⁺.

Results and discussion

The H₂ content in the headspace of a sealed reaction tube containing an acetonitrile solution (5 mL) of catalyst 1 (1 μM), cyclometallated Ir photosensitizer IrPS⁺ (0.2 mM), 0.2 M triethylamine (TEA) and water (10% v/v) was determined by GC-TCD. H₂ is produced upon irradiation with visible light (λ > 420 nm), but the yield of H₂ levels off after 8 h of irradiation (10 μmol). The control experiment in the absence of the catalyst generates less H₂ (4 μmol).

No H₂ could be detected in the absence of IrPS⁺, H₂O or TEA (Fig. 1). Both the production rate and yield of H₂ decrease, when the amount of H₂O is lowered from 10% to 2.5% (Fig. S2†); increasing [H₂O] to 15% results in no further improvement in the H₂ yield. Cyclometallated Ir photosensitizers are known to decompose via the dissociation of the diimine ligand from the unstable reduced state.^47,48 In an attempt to restore the H₂ production, a fresh solution of IrPS⁺ (0.2 mM) was added to the reaction solutions after the first 16 h of photolysis. The activity of 1 was restored and a comparable amount of H₂ (12.3 μmol) was produced after another 16 h of irradiation, indicating that the decomposition of IrPS⁺ is the major cause for the loss of activity. H₂ production was maintained and a total H₂ yield of 109 μmol was attained after 5 cycles of IrPS⁺ addition (overall 80 h of irradiation), whereas only 27.4 μmol H₂ was recorded in the control experiment without the catalyst, leading to an effective TON of 16 440 (vs. catalyst). The performance of several reported Co WRCs is shown in Table 1. Co catalysts bearing pentadentate polypyridyl ligands are, so far, the most efficient among all Co WRCs reported.^50–52 A H₂ TON of 11 000 was reported for [Co^II(aPPy)(Br)]Br (aPPy = bis-2,2′′-bipyridine-6-yl(pyridine-2-yl)methanol), using a Re^I photosensitizer and ascorbic acid/ascorbate as the sacrificial donor under aqueous conditions,⁵⁰ while a higher TON (30 000) has been achieved when a secondary sacrificial electron donor was provided.⁵¹ A H₂ TON of 20 000 has recently been reported also in acetonitrile for Co(TMPA)Cl₂ (TMPA = 1-(6-substituted-pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methane amine).⁵² The H₂ TON for 1 is therefore amongst the highest reported for Co WRCs without the use of a secondary sacrificial donor. A similar yield (9.2 μmol) and response to H₂O concentration have been observed for 2 in the first catalytic cycle (16 h) under the same conditions (Fig. 1 and S2†). However, the yield of H₂ in the subsequent cycles is similar to that of the control experiment when additional IrPS⁺ was provided, reflecting that the catalyst has been deactivated after the first cycle. The change in the H₂ yield during the first hour of photolysis at varied concentrations of 1 has been recorded (Fig. 2). At the selected concentrations, there was no observable lag phase and H₂ yield increases linearly with time, indicating that the catalytic reaction is homogeneous.⁴⁰ The first order rate constant (k_initial), obtained as the initial rate of H₂ production in the first hour, exhibits linear dependence on [1]. A second order rate constant k₂ of 0.25 h⁻¹ was obtained accordingly.

Fig. 1 Hydrogen evolution of 1 and 2 (1 μM) in aqueous (10% v/v) acetonitrile at 25 °C. [IrPS⁺] = 0.2 mM and [TEA] = 0.2 M upon photoexcitation.

Table 1 Photocatalytic H₂ production of some reported Co WRCs

Catalyst	TON per catalyst	Reaction medium	Ref.
Co^II(dmgH)2(py)Cl	127	1 : 1 CH3CN/H2O	16
[Co^II(aPPy)(Br)]Br	11 000	pH 4.1 ascorbate buffer	50
Co^II(TMPA)Cl2	20 000	9 : 1 CH3CN/H2O	52
1	16 440	10% H2O in CH3CN

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Fig. 2 a) Hydrogen yields during the first hour of photolysis at varied concentrations of [1] in acetonitrile containing 10% water (v/v), 0.2 mM IrPS⁺ and 0.2 M TEA at 28 °C; b) plot of k_initial against [1]; slope = 0.2521 ± 0.0043, y-intercept = (−4.474 ± 0.47) × 10⁻⁷, R² = 0.999.

Metal-based nanoparticles generated by electro- or photochemical deposition are found to be real active species for some noble and non-noble metal WRCs.^40–44 To verify whether the observed catalytic activity is caused by the colloidal particle formation rather than by 1 itself, the photolysis was performed in the presence of Hg⁰ (Fig. S3†). The activity of 1 does not change significantly (<20%) in the presence of Hg⁰ (0.5 mL). The temporal change of the particle size during the photoreactions (first 8 h) has also been monitored using dynamic light scattering (DLS) measurement (Fig. S4†). It falls in line with the above result; particle formation and significant change in particle size were not observed during the photoreactions in comparison with a fresh reaction solution, indicating that the catalysis is homogeneous. Similar results are also observed for 2.

The change of the absorption spectrum of acetonitrile solution containing IrPS⁺ (0.25 mM) and TEA (6.25 mM) upon irradiation with visible light (λ > 420 nm) is shown in Fig. S5.† New absorption bands are observed at 463, 497 and 532 nm after 120 s, which are consistent with those reported for electrochemically generated IrPS⁰.⁴⁴ The quenching rate constants (k_q) for IrPS⁺ with TEA, 1 and 2 have been determined with Stern–Volmer plots (Fig. S6†) at varied concentrations of the quenchers (k_q = 1.2 × 10⁹, 4.8 × 10⁸ and 1.3 × 10⁸, respectively). The measured k_q for IrPS⁺ with TEA is comparable to the recently reported value (1 × 10⁹).⁴⁴ Thus, the photoreaction is believed to proceed predominantly by the reductive quenching of photoexcited IrPS⁺ to produce IrPS⁰, which then reduces the Co catalysts (Co^II to Co^I) to mediate proton reduction. The quantum yields of H₂ generation over the first 2 h for 1 and 2 are similar (φ = 0.56), as determined using a chemical actinometer.

To reveal the nature of active species and redox processes involved, cyclic voltammetry (CV) of 1 and 2 was performed in acetonitrile (Fig. 3 and Table 2). In the CV of 1 (Fig. 3a), quasi-reversible and reversible couples are observed at 0.47 and −0.68 V vs. SCE, tentatively assigned to the Co^III/II and Co^II/I processes. The quasi-reversible couples at −1.50 and −1.80 V are assigned to the ligand-centred L^0/˙⁻ and Co^I/0 processes. The CV of 2 exhibits similar Co^III/II and Co^II/I couples at 0.45 V and −0.98 V vs. SCE (Fig. 3b, red line). An irreversible L^0/˙⁻ couple (E_p,c = −1.61 V vs. SCE) is observed on further cathodic scanning (Fig. 3b, black line). In the subsequent anodic scan, it is noted that Co^III/II and Co^II/I couples became less reversible. The irreversible L^0/˙⁻ process is probably associated with the deactivation of 2 during the photocatalysis. A comparison between the redox potentials of the two complexes reveals a substantial difference of E(Co^II/I) by 0.30 V, while E(Co^III/II) and E(L^0/˙⁻) are less sensitive to the change in the coordination sphere. Thus, tachpy₃ is suggested to better stabilize both the coordination environment and the Co^I state than trenpy₃.

Fig. 3 CVs of 1 mM (a) 1 and (b) 2 in 0.1 M ⁿBu₄NPF₆ acetonitrile solution under an Ar atmosphere. Scan rate = 100 mV s⁻¹.

Table 2 Electrochemical data of 1 and 2 in 0.1 M ⁿBu₄NPF₆ acetonitrile solution

	E°, V vs. SCE (ΔE, mV)
	Co^III/II	Co^II/I	L^0/˙^−	Co^I/0
a E p,c, irreversible couple.
1	0.47 (81)	−0.68 (66)	−1.50 (132)	−1.80 (117)
2	0.45 (92)	−0.98 (66)	−1.61^a	—

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On addition of one mole equivalent of 4-bromoanilinium triflate (BAN), a new peak is observed in the CV of 1 at a slightly positive potential of the Co^II/I couple (onset potential = −0.55 V). The peak current increases at higher BAN concentrations (1–6 mM), suggesting the catalytic reduction of protons via the Co^I state (Fig. 4). A similar catalytic wave is also observed for 2 at a slightly positive potential of the corresponding Co^II/I couple (onset potential = −0.76 V) in the presence of BAN. As [BAN] increases, the peak current of the wave increases, while the onset potential is anodically shifted. When aqueous citric acid (CA) was added instead of BAN, similar catalytic waves are also observed in the CVs at the same potentials (Fig. S7†). In the absence of the catalyst, both acids give rise to negligible reduction currents (Fig. S8†).

Fig. 4 CVs of 1 mM (a) 1 and (b) 2 at varied concentrations of 4-bromoanilinium triflate ([BAN] = 1, 2, 3, 4, 5 and 6 mM) in 0.1 M ⁿBu₄NPF₆ acetonitrile solution under an Ar atmosphere. Scan rate = 100 mV s⁻¹.

To characterize the intermediate involved in the protonation of the Co^I state, controlled-potential electrolysis was performed (−1.0 V vs. SCE) with 0.1 M ⁿBu₄NPF₆ acetonitrile solution containing 1 (1 mM). The solution was then diluted (×10) and analysed with ESI-MS (Fig. S9 and S10†). The observed single peak at m/z = 629.5 in the MS is assigned to the N₂-adduct [Co(L) + H + PF₆ + N₂]⁺ (supported by the isotopic pattern) and suggests the formation of dicationic [Co^I(η⁵-HL)(N₂)]²⁺, which bears in the presence of solvent moisture a protonated tachpy₃ ligand. Such a N₂ adduct is not observed in the MS of the corresponding [Co^II(L)]²⁺ (Fig. S10†). Under photochemical conditions, the more negative reductive IrPS⁰ state (−1.36 V vs. SCE)⁴⁴ should in principle be sufficiently reducing to generate, in the presence of a proton source, similar five-coordinate [Co^I(η⁵-HL)]²⁺. The formation of [Co^I(η⁵-HL)]²⁺ upon protonation of [Co^I(L)]⁺ is consistent with the observed new peak near the Co^II/I couple and also with the DFT calculation (Fig. 6). Co^III hydride [Co^III(H)(η⁵-L)]²⁺ may then be formed via an intramolecular PCET process and further react to produce H₂.¹⁵ The presence of a Co^III–H intermediate is supported by the CV of 1 (Fig. 4 and S7†). A new quasi-reversible couple is observed at −0.85 V vs. SCE, upon addition of 1 mole equivalent of the acids, and assigned to the [Co^III/II(H)(η⁵-L)]^2+/+ process. A similar Co^III/II(H) couple was reported for a Co oxime WRC.¹⁵

X-ray crystal data obtained in this (Fig. S1, Table S1 and S2†) and previous work⁴⁵ show that the Co^II complexes 1 and 2 exhibit a trigonal prismatic and a distorted octahedral geometry, respectively. The energies of the possible spin states of the two complexes in acetonitrile were calculated by DFT methods.^61–69 The high-spin states of 1 and 2 (with 3 unpaired d electrons) are predicted to be more stable than the low-spin states by 42.2 and 48.7 kJ mol⁻¹, respectively. It is expected that only the high-spin state would be observed at room temperature. The measured effective magnetic moments (μ_eff) of 1 and 2 at room temperature are 5.11 and 4.54 BM, respectively, which are typical for a high-spin d⁷ configuration. The high-spin state nature of the two complexes at room temperature is also revealed in the UV-vis absorption spectra. By comparison of the absorption features, the experimental absorption spectra match with the calculated spectra of the high-spin states of the complexes, rather than the low-spin ones. The difference of the transition energies of the absorption features of the two complexes is reproduced by the calculation, but the energies are overestimated by 0.2–0.4 eV (Fig. S11†). Overestimation of absorption energy is commonly observed for other Co complexes.^70,71

The optimized structures of 1 at the high-spin Co⁰, Co^I and Co^II states (Fig. 5 and S12, Tables S3 and S4†) exhibit a trigonal prismatic geometry. In contrast, an octahedral geometry is energetically favored for the low-spin Co^II state. All optimized structures of 2 adopt a distorted octahedral geometry. The optimized structures for both complexes in the Co^II state are therefore consistent with the experimental results from X-ray crystallography and spectrophotometry. A close look at the optimized structures of 1 and 2 gives possible explanations for the different coordination geometries. When comparing the degrees of freedom of the three imine N atoms, the tris(imino)cyclohexane group is less flexible compared to the tris(2-iminoethyl)amine moiety. Also, due to the partially filled Co d_σ* orbitals, which are anti-bonding (Co–N) in nature, in the high spin state of 1 (in the oxidation state of Co⁰, Co^I and Co^II), their structures would have much longer Co–N bond lengths than the corresponding optimized structures in the low spin state. Thus, with longer Co–N in the high-spin state of 1, the C–N bonds between the imine and the cyclohexyl group will be strained if the complex adopted an (distorted) octahedral geometry. As a result, the distorted octahedral structure in the high spin state is much more unstable with respect to that in the trigonal prismatic geometry.

Fig. 5 Optimized structures of 1 in the a) Co^II and b) Co^I oxidation states.

The reduction potentials of the two Co^II/I redox couples were calculated using the differences of the Gibbs free energy of the optimized structures with Co^II and Co^I metal centers. The values of the calculated Co(II/I) reduction potentials for 1 and 2 are close to the experimental values and the large difference in reduction potentials observed in the experiment can be reproduced (Table 3). The calculated Co^I/0 reduction potential for 1 is also consistent with the measured value. Given the fundamental difference in the coordination geometry for both the measured and optimized structures of the two complexes, the stabilized E(Co^II/I) in 1 is attributed to its trigonal prismatic geometry.

Table 3 Calculated reduction potentials of 1 and 2

	Calculated E°(Co^II/I)/V vs. SCE	Calculated E°(Co^I/0)/V vs. SCE
1	−0.67	−1.83
2	−0.89	—

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When the Co^I state of 1 is optimized by the DFT method with an added proton, it is shown that the pyridine moieties of tachpy₃ are more likely to be protonated. The protonation causes the pyridine moiety to dissociate from the Co^I center, while the other five nitrogen-donor atoms are only slightly affected (Table S5†). Subsequently, a penta-coordinate structure [Co^I(η⁵-HL)]²⁺ is obtained, in line with our experimental data (Fig. 6 and S10†). On the other hand, further reduction of the singly positively charged Co^I state of 1 leads to an optimized structure which reveals the neutral doubly reduced [Co^I(L˙⁻)]⁰ state and supports the assignment of ligand-centred reduction. The extra spin densities corresponding to the added electron would be mainly residing on the –N=C(H)–C moiety of the two iminopyridine functions closer to the Co^I centre, with a smaller amount also on the aromatic rings, (Fig. S13†), while much lower spin density would be located on the one further away.

Fig. 6 Optimized structures of 1 in the Co^I state in the presence of one mole equivalent of proton.

The above results demonstrate that replacing the tris(2-iminoethyl)amine moiety in 2 with tris(imino)cyclohexane in 1 has significantly enhanced the catalyst stability and lowered the thermodynamic barrier for proton reduction. 1 is found to be an active homogeneous WRC with impressive catalytic performance. The Co–N bond in the singly reduced [Co^I(L)]⁺ state of 1 is destabilized in the presence of a proton source to give the five-coordinate [Co^I(η⁵-HL)]²⁺, which may then give [Co^III(H)(η⁵-L)]²⁺ to produce H₂. Further investigation on the reaction mechanism is now in progress and will be published in due course.

Experimental

Materials and methods

Compounds 1 and 2 were prepared according to literature procedures.^45–47IrPS⁺ was synthesized using a modified literature method using 4,4′-dimethyl-2,2′-dipyridyl.⁵⁵ 4-Bromoanilinium triflate (BAN) was prepared by the addition of trifluoromethane sulfonic acids into a stirred solution of 4-bromoaniline in diethyl ether followed by recrystallization from acetonitrile/diethyl ether. 0.7 M aqueous citric acid (CA) was prepared using Mini-Q water. All other chemicals (Sigma Aldrich) were obtained commercially and used without further purification.

Crystallography

Single crystals of 2 were obtained by recrystallization from acetonitrile/diethyl ether. Crystal data and experimental details are given in Table S1.† X-ray diffraction data were collected at 293 K on an Oxford CCD diffractometer using graphite-monochromated Cu KR radiation (λ = 1.54178 Å) in the ω-scan mode. Absorption corrections were performed by the multi-scan method. The structure was resolved by the heavy-atom Patterson method or direct methods, refined by full-matrix least-squares using SHELX-97 and expanded using Fourier techniques.⁵⁶ All non-hydrogen atoms were refined anisotropically. H atoms were generated using the program SHELXL-97. The positions of H atoms were calculated on the basis of riding mode with thermal parameters equal to 1.2 times that of the associated C atoms and participated in the calculation of final R-indices. Selected bond lengths and bond angles are given in Table S2.†

Electrochemical measurement

Cyclic voltammetry (CV) was carried out on a CH Instrument Electrochemical Station Model 660C, using a glassy-carbon working electrode, platinum-wire counter electrode and saturated calomel electrode (SCE), with 0.1 M ⁿBu₄NPF₆ in acetonitrile (HPLC grade, Anaqua Chemical Supply) as the supporting electrolyte. In the CV experiments, BAN (1–6 mM) and aqueous citric acid (1–7 mM) were added. Anilinium was commonly used for studying electrocatalytic proton reduction in organic solutions.²⁶ Aqueous citric acid (CA) was added instead of BAN as an aqueous proton source to mimic the photocatalytic conditions. Their low concentrations and weak acidity (pK_a = 9.6 in CH₃CN for BAN⁵³ and pK_a > 5 for CA; pK_a of CA = 5.0 in 30% v/v H₂O/CH₃CN (ref. 54)) in acetonitrile should maintain the pH close to neutral and be relatively comparable to the photochemical conditions. Controlled-potential electrolysis was conducted using an ITO-plate working electrode, platinum-wire counter electrode and saturated calomel electrode (SCE). ⁿBu₄NPF₆ was recrystallized in ethanol three times and dried in vacuo at 120 °C overnight before use. Ferrocene (Fc) was used as the internal reference in all measurements.

Photocatalytic hydrogen generation

Photolysis was performed using a 10 mL Quickfit test tube (Pyrex) sealed with a serrated rubber septum. In general, a degassed acetonitrile solution (5 mL) containing the catalyst (1 or 2), IrPS⁺, TEA and H₂O was irradiated using a white LED lamp (>420 nm) obtained from PRIME LED. The photocatalytic H₂ generation was performed in unbuffered aqueous acetonitrile to avoid the potential influence of the protonated sacrificial donor (TEAH⁺) on the reductive quenching in the catalytic process.⁵⁷ The content of the headspace was then analyzed by a gas chromatograph equipped with a thermoconductivity detector (GC-TCD) and a Chrompack 5A° molecular sieve column (25 m × 0.32 mm) at 45 °C. The H₂ concentration was determined with reference to a calibration curve of a hydrogen standard prepared at one atmospheric pressure using a graduated syringe fitted with a three-way valve and a rubber septum.

The quantum yield for hydrogen generation was determined as follows. A reaction solution (2 mL) in a 1 cm quartz cuvette sealed with a rubber septum was degassed by bubbling argon for 20 min and then irradiated at 430 nm for 2 h using a 500 W mercury–xenon lamp equipped with a Newport monochromator (Model 77200). The hydrogen content in the headspace of the cuvette was then determined by GC-TCD. The incident light intensity at 430 nm was taken from the average values measured just before and after the photolysis experiment using ferrioxalate as a chemical actinometer and was corrected for the absorbance of the actinometer solution at the same wavelength.⁵⁸ The quantum yield of hydrogen generation (Φ_H2) of the catalytic system is defined by eqn (1) and calculated on the basis of the total amount of hydrogen evolved after 2 h of irradiation:^59,60

Φ_H2 = 2 × H₂ produced (mole)/photon absorbed (Einstein) (1)

Luminescence quenching measurement

All luminescence quenching experiments were carried out by adding known amounts of quencher to 0.05 mM IrPS⁺ solution at room temperature. The solutions were then rigorously degassed on a high-vacuum line in a two-compartment cell for at least four successive freeze-pump-thaw cycles. Steady-state luminescence spectra of each sample were then recorded on a SPEX FluoroLog 3-TCSPC spectrofluorometer. The relative emission intensity or lifetime of the characteristic emission of IrPS⁺ in the presence of the different concentrations of the quencher was used to calculate the quenching rate constant (k_q) using the Stern–Volmer equation (eqn (2)).⁶⁰where I₀ and I are the emission intensities in the absence and presence of the quencher, respectively; τ₀ and τ are the emission lifetimes in the absence and presence of the quencher and [Q] is the concentration of the quencher.

Dynamic light scattering

The light source for the scattering experiments was a He–Ne laser emitting vertically polarized light at a wavelength of 638 nm and operating at 4 mW. Data were collected at 20 °C with the scattering angle set to 173 degrees (non-invasive back-scatter), using an Avalanche photodiode detector connected to a correlator (Zetasizer nanoparticle analyser series, Nano ZS, ZEN 3600, Malvern Instruments).

Magnetic susceptibility measurement

The molar susceptibility (X_M) of 1 and 2 was determined using a Sherwood magnetic balance. The effective magnetic moment (μ_eff) was calculated using eqn (3), where T = temperature:

DFT calculations

Density functional theory (DFT) calculations on complexes 1 and 2 were carried out using the GAUSSIAN 09 program.⁶¹ The hybrid meta-GGA M06 functional and a mixed basis set of 6-31G(d) (for non-metals) and LANL2DZ effective core potential (for Co atom) were employed in the calculations.^61–64 Frequency calculations were performed on the optimized structures and no imaginary frequencies were found. The solvent effect of acetonitrile was taken into account using the polarized continuum model (PCM).⁶⁵ The electronic transition energies of the complexes were calculated by TD-DFT methods.^66,67 The simulated UV-vis absorption spectra were generated^68,69 by the combination of individual bands of Gaussian peak shapes with a full width at half maximum (FWHM) of 0.275 eV.

Conclusions

Our study reveals that the Co tripodal iminopyridine complex [Co(tachpy₃)](ClO₄)₂ (1) is an active homogeneous WRC and mediates H₂ photogeneration at an impressive TON of 16 440 which is amongst the highest in all Co WRCs reported in the absence of a secondary sacrificial donor. The catalysis by 1 is suggested to proceed via penta-coordinate [Co^I(η⁵-HL)]²⁺ formed upon the protonation of the pyridine in [Co^I(L)]⁺. A comparison with the structural analogue [Co^II(trenpy₃)](ClO₄)₂ (2), bearing the same donor groups, reveals that the difference in ligand rigidity and thus the coordination geometry results in significant change in catalytic properties. Complex 1 exhibits not only more anodic reaction potential but also much higher stability. Consistency between the electrochemical and computation data reveals that the coordination geometry above the metal active site, as imposed by the ligands, has significant influence on these catalytic properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is financially supported by the HK Research Grants Council (project number 28300014 and 18300715) and EdUHK (04024:ECR16, 04255:SFRS8 and R3736:RG95). The Croucher Foundation is also acknowledged for the Visitorship for PRC Scholars (J. Xiang, 2016/17) in EdUHK.

Notes and references

1. K. Yamauchi, S. Masaoka and K. Sakai, J. Am. Chem. Soc., 2009, 131, 8404–8406.
2. H. Ozawa and K. Sakai, Chem. Commun., 2011, 47, 2227–2242.
3. E. D. Cline, S. E. Adamson and S. Bernhard, Inorg. Chem., 2008, 47, 10378–10388.
4. J. Xie, C. Li, Q. Zhou, W. Wang, Y. Hou, B. Zhang and X. Wang, Inorg. Chem., 2012, 51, 6376–6384.
5. A. M. Appel, D. L. DuBois and M. R. DuBois, J. Am. Chem. Soc., 2005, 127, 12717–12726.
6. H. I. Karunadasa, C. J. Chang and J. R. Long, Nature, 2010, 464, 1329–1333.
7. H. I. Karunadasa, E. Montalvo, Y. J. Sun, M. Majda, J. R. Long and C. J. Chang, Science, 2012, 335, 698–702.
8. I. Bhugun, D. Lexa and J.-M. Saveánt, J. Am. Chem. Soc., 1996, 118, 3982–3983.
9. S. Ott, M. Kritikos, B. Akermark, L. C. Sun and R. Lomoth, Angew. Chem., Int. Ed., 2004, 43, 1006–1009.
10. F. Gartner, B. Sundararaju, A. E. Surkus, A. Boddien, B. Loges, H. Junge, P. H. Dixneuf and M. Beller, Angew. Chem., Int. Ed., 2009, 48, 9962–9965.
11. S. Kaur-Ghumaan, L. Schwartz, R. Lomoth, M. Stein and S. Ott, Angew. Chem., Int. Ed., 2010, 49, 8033–8036.
12. F. Wang, W.-J. Liang, J.-X. Jian, C.-B. Li, B. Chen, C.-H. Tung and L.-Z. Wu, Angew. Chem., Int. Ed., 2013, 52, 8134–8138.
13. T. Yu, Y. Zeng, J. Chen, Y.-Y. Li, G. Yang and Y. Li, Angew. Chem., Int. Ed., 2013, 52, 5631–5635.
14. M. J. Rose, H. B. Gray and J. R. Winkler, J. Am. Chem. Soc., 2012, 134, 8310–8313.
15. X. L. Hu, B. S. Brunschwig and J. C. Peters, J. Am. Chem. Soc., 2007, 129, 8988–8998.
16. T. Lazarides, T. McCormick, P. Du, G. Luo, B. Lindley and R. Eisenberg, J. Am. Chem. Soc., 2009, 131, 9192–9194.
17. J. R. Long, C. J. Chang and Y. Sun, J. Am Chem. Soc., 2013, 2, 9212–9215.
18. A. Rodenberg, M. Orazietti, B. Probst, C. Bachmann, R. Alberto, K. K. Baldridge and P. Hamm, Inorg. Chem., 2015, 54, 646–657.
19. B. Shan, T. Baine, X. Zhao and R. H. Schmehl, Inorg. Chem., 2013, 20, 3164–3170.
20. B. D. Stubbert, J. C. Peters and H. B. Gray, J. Am. Chem. Soc., 2011, 133, 18070–18073.
21. C. C. L. McCrory, C. Uyeda and J. C. Peters, J. Am. Chem. Soc., 2012, 134, 3164–3170.
22. C.-F. Leung, Y.-Z. Chen, H.-Q. Yu, S.-M. Yiu, C.-C. Ko and T.-C. Lau, Int. J. Hydrogen Energy, 2011, 36, 11640–11645.
23. S. Varma, C. E. Castillo, T. Stoll, J. Fortage, A. G. Blackman, F. Molton, A. Deronzier and M.-N. Collomb, Phys. Chem. Chem. Phys., 2013, 15, 17544–17552.
24. W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel, P. L. Holland and R. Eisenberg, J. Am. Chem. Soc., 2011, 133, 15368–15371.
25. W. M. Singh, T. Baine, S. Kudo, S. L. Tian, X. A. N. Ma, H. Y. Zhou, N. J. DeYonker, T. C. Pham, J. C. Bollinger, D. L. Baker, B. Yan, C. E. Webster and X. Zhao, Angew. Chem., Int. Ed., 2012, 51, 5941–5944.
26. P.-A. Jacques, V. Artero, J. Pécaut and M. Fontecave, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 20627–20632.
27. L. L. Efros, H. H. Thorp, G. W. Brudvig and R. H. Crabtree, Inorg. Chem., 1992, 31, 1722–1724.
28. M. P. McLaughlin, T. M. McCormick, R. Eisenberg and P. L. Holland, Chem. Commun., 2011, 47, 7989–7991.
29. M. L. Helm, M. P. Stewart, R. M. Bullock, M. R. DuBois and D. L. DuBois, Science, 2011, 333, 863–866.
30. M. P. Stewart, M. H. Ho, S. Wiese, M. L. Lindstrom, C. E. Thogerson, S. Raugei, R. M. Bullock and M. L. Helm, J. Am. Chem. Soc., 2013, 135, 6033–6046.
31. M. A. Gross, A. Reynal, J. R. Durrant and E. Reisner, J. Am. Chem. Soc., 2013, 136, 356–366.
32. W. Zhang, J. Hong, J. Zheng, Z. Huang, J. Zhou and R. Xu, J. Am. Chem. Soc., 2011, 133, 20680–20683.
33. J. Han, W. Zhang, T. Zhou, X. Wang and R. Xu, RSC Adv., 2012, 2, 8293–8296.
34. H. Cui, J. Wang, M. Hu, C. Ma, H. Wen, X. Song and C. Chen, Dalton Trans., 2013, 42, 8684–8691.
35. H. N. Kagalwala, E. Gottlieb, G. Li, T. Li, R. Jin and S. Bernhard, Inorg. Chem., 2013, 52, 9094–9101.
36. Z. Han, L. Shen, W. W. Brennessel, P. L. Holland and R. Eisenberg, J. Am. Chem. Soc., 2013, 135, 14659–14669.
37. J. Dong, M. Wang, X. Li, L. Chen, Y. He and L. Sun, ChemSusChem, 2012, 5, 2133–2138.
38. P. Zhang, M. Wang, Y. Yang, T. Yao and L. Sun, Angew. Chem., Int. Ed., 2014, 53, 13803–13807.
39. H. Junge, Z. Codolà, A. Kammer, N. Rockstroh, M. Karnahl, S. P. Luo, M. M. Pohl, J. Radnik, S. Gatla and S. Wohlrab, J. Mol. Catal. A: Chem., 2014, 395, 449–456.
40. V. Artero and M. Fontecave, Chem. Soc. Rev., 2013, 42, 2338–2356.
41. E. Anxolabéhère-Mallart, C. Costentin, M. Fournier, S. Nowak and M. Robert, J. Am. Chem. Soc., 2012, 134, 6104–6107.
42. L. A. Berben and J. C. Peters, Chem. Commun., 2010, 46, 398–400.
43. S. Cobo, J. Heidkamp, P.-A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave and V. Artero, Nat. Mater., 2012, 11, 802–807.
44. L. Chen, G. Chen, C. F. Leung, S. M. Yiu, C. C. Ko, E. Anxolabéhère-Mallart, M. Robert and T. C. Lau, ACS Catal., 2015, 5, 356–364.
45. R. A. D. Wentworth, P. S. Dahl, C. J. Huffman, W. O. Gillium, W. E. Streib and J. C. Huffman, Inorg. Chem., 1982, 21, 3060–3063.
46. E. Larsen, G. N. La Mer, B. E. Wagner, J. E. Parks and R. H. Holm, Inorg. Chem., 1972, 11, 2652–2667.
47. L. L. Tinker, N. D. McDaniel, P. N. Curtin, C. K. Smith, M. J. Ireland and S. Bernhard, Chem. – Eur. J., 2007, 13, 8726–8732.
48. Y.-J. Yuan, Z.-T. Yu, J.-G. Cai, C. Zheng, W. Huang and Z.-G. Zou, ChemSusChem, 2013, 6, 1357–1365.
49. K. Kawano, K. Yamauchi and K. Sakai, Chem. Commun., 2014, 50, 9872–9875.
50. C. Bachmann, M. Guttentag, B. Spingler and R. Alberto, Inorg. Chem., 2013, 52, 6055–6061.
51. C. Bachmann, B. Probst, M. Guttentag and R. Alberto, Chem. Commun., 2014, 50, 6737–6739.
52. J. Wang, C. Li, Q. Zhou, W. Wang, Y. Hou, B. Zhang and X. Wang, Catal. Sci. Technol., 2016, 6, 8482–8489.
53. R. T. Edidin, J. M. Sullivan and J. R. Norton, J. Am. Chem. Soc., 1987, 109, 3945–3953.
54. J. Barbosa, J. L. Beltrán and V. Sanz-Nebot, Anal. Chim. Acta, 1994, 288, 271–278.
55. M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, G. G. Malliaras and S. Bernhard, Chem. Mater., 2005, 17, 5712–5719.
56. G. M. Sheldrick, SHELX-97, University of Göttingen, Göttingen, Germany, 1998.
57. Y. Pellegrin and F. Odobel, C. R. Chim., 2017, 20, 283.
58. S. L. Murov, I. Carmicheal and G. L. Hug, Handbook of photochemistry, Marcel Dekker Inc., New York, 2nd edn, 1993.
59. C. G. Hatchard and C. A. Parker, Proc. R. Soc. London, Ser. A, 1956, 235, 518.
60. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Press, New York, 2006.
61. M. J. Frisch, G. W. Trucks and H. B. Schlegel, et al., Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2009.
62. Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241.
63. A. Schäfer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992, 97, 2571–2577.
64. W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284–298.
65. G. Scalmani and M. J. Frisch, J. Chem. Phys., 2010, 132, 114110.
66. R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996, 256, 454–464.
67. G. Scalmani, M. J. Frisch, B. Mennucci, J. Tomasi, R. Cammi and V. Barone, J. Chem. Phys., 2006, 124, 094107.
68. Creating UV/Visible Plots from the Results of Excited States Calculations, http://gaussian.com/uvvisplot/, (accessed September 2017).
69. Convoluting UV-Vis spectra using oscillator strengths, http://gausssum.sourceforge.net/GaussSum_UVVis_Convolution.pdf, (accessed September 2017).
70. G. Cioncoloni, H. M. Senn, S. Sproules, C. Wilson and M. D. Symes, Dalton Trans., 2016, 45, 15575–15585.
71. M. Rudolph, T. Ziegler and J. Autschbach, Chem. Phys., 2011, 391, 92–100.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1565200 contains the supplementary crystallographic data for this paper. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cy01524k

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