Introduction

The exponential global economic and population growth has resulted in a rapid depletion of fossil fuel resources and in an increase of environmental concerns that accompany the combustion of these energy supplies.[1] Therefore exploring renewable, sustainable and clean alternative energy sources is fundamental to address these pressing issues.[1] Hydrogen gas produced through hydrogen evolution reactions (HERs) has been considered as a promising alternative fuel due to its high energy density, and its clean and carbon-free nature.[2-4] However, the main challenges of HERs are their high overpotential and sluggish kinetics.[5,6] Thus, to facilitate these reactions, efficient and robust catalysts are required.[7] Although platinum metal has been well demonstrated as the best HER electrocatalyst showing zero overpotential and large current density, its use is highly restricted by its high cost and scarcity.[8] In order to achieve more economically and environmentally viable hydrogen production, significant efforts have been made to explore earth-abundant materials based catalytic systems.[9,10]

Current research on HERs mainly focus on transition metal (Fe, Co, Ni, Mn) and metal-free based electrocatalysts.[9,11-17] Molecular HER electrocatalysts have received substantial interest due to their ease of rational tuning of chemical properties and redox potentials to control their catalytic performance and selectivity.[18-20] In contrast to their bulk heterogeneous counterparts, homogeneous HER molecular catalysts allow for detailed mechanistic study of hydrogen generation by utilizing spectroscopic measurements, among other techniques.[21,22] One of the most frequently studied groups of molecular electrocatalysts are cobalt-nitrogen coordinated complexes such as cobalt diglyoximes, cobalt porphyrins, cobalt salens, cobalt corroles and cobalt chlorins, etc.[23-29] Most of the cobalt complexes studied for hydrogen evolution are supported by electron-withdrawing ligands that tune the redox profile of the metal center by bringing the reduction potential to more anodic ranges, or by redox non-innocent ligands that can participate as electron reservoirs.[30-31] By contrast, cobalt-nitrogen coordinated complexes based HER electrocatalysts with electron-donating ligands are rare. Yet, the use of electron-donating ligands can potentially increase the electron-richness of the cobalt centers and, thus, enhance the reactivity at the catalytic center. Thus, further exploration of cobalt-nitrogen coordinated complexes bearing electron-donating ligands can provide new insight into the development of new ligand architectures that can improve the performance of homogeneous HER electrocatalysts.

Herein, we report the synthesis of a novel cobalt complex (1) where a single metal center is supported by two electron rich nitrogen-based DippF ligands (DippF = N,N’-bis[2,6-diisopropylphenyl]-formamidinate). We characterized 1 by spectroscopic means including UV-vis, nuclear magnetic resonance (NMR) spectroscopies and X-ray crystallography. Complex 1 was assessed as an electrocatalyst for hydrogen gas evolution by electrochemical measurements in MeCN with three different organic acids, namely p-toluenesulfonic (tosic) acid, benzoic acid and phenol. HER kinetic studies were also performed by foot-of-the-wave analysis (FOWA) in order to provide insights of the relationship between rate constant and acid concentration.

Experimental

General considerations

Synthesis of 1 was accomplished using standard Schlenk techniques under N₂ atmosphere and a dry N₂-filled glovebox. All solvents used were dried using a Pure Process Technology solvent purification system.

Materials

All materials used were purchased from commercially available sources. Tetrabutylammonium hexafluorophosphate (TBAPF₆), p-toluenesulfonic (tosic) acid, benzoic acid, phenol and CoCl₂·6H₂O were dried under vacuum at 60 °C prior to use. The remaining chemicals were used as received. HDippF was synthesized according to a previously reported method.[32]

Physical measurements

¹H NMR spectroscopy was recorded on a Bruker 400 MHz NMR spectrometer, using deuterated benzene (C₆D₆) as the solvent. The residual protic signal of C₆D₆ was used as the internal standard (δ=7.16 ppm). UV-vis spectra were obtained using a SEC2000 instrument with a VISUAL SPECTRA 2.1 software.

Information of X-ray crystal structure of 1

A specimen of 1 was used for the X-ray crystallographic analysis. The X-ray intensity data were measured on a Bruker SMART APEX CCD system equipped with a graphite monochromator and a MoKα fine-focus tube (λ = 0.71073 Å).The total exposure time was 4.00 hours. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using an orthorhombic unit cell yielded a total of 22716 reflections to a maximum θ angle of 26.00° (0.81 Å resolution), of which 4567 were independent (average redundancy 4.974, completeness = 100.0%, Rint = 3.81%, Rsig = 3.65%) and 4362 (95.51%) were greater than 2σ(F2). The final cell constants of a = 14.4164(7) Å, b = 20.1178(10) Å, c = 15.9876(8) Å, volume = 4636.8(4) Å3, are based upon the refinement of the XYZ-centroids of 9910 reflections above 20 σ(I) with 4.784° < 2θ < 57.66°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.889.

The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group C 2 2 21, with Z = 4 for the formula unit, C₅₀H₆₈CoN4. The final anisotropic full-matrix least-squares refinement on F2 with 278 variables converged at R1 = 5.46%, for the observed data and wR2 = 15.32% for all data. The goodness-of-fit was 1.088. The largest peak in the final difference electron density synthesis was 0.486 e-/Å3 and the largest hole was -0.612 e-/Å3 with an RMS deviation of 0.079 e-/Å3. On the basis of the final model, the calculated density was 1.123 g/cm3 and F(000), 1692 e-.

Evans method

The magnetic susceptibility was measured through ¹H NMR spectroscopy, which was carried out with a sealed capillary tube containing pure C₆D₆ placed inside an NMR tube already loaded with a solution of 1 (0.0160 g) in C₆D₆ (1.5 mL) in order to measure the chemical shift difference of the solvent caused by the different electronic environments. The number of unpaired electrons (n) was calculated as shown below:[33,34]

where, is the mass susceptibility, is the frequency separation, is the NMR frequency being studied (400*10⁶ Hz), m is the mass concentration (g/cm³) of 1, is the mass susceptibility of the solvent (-8.8*10^-9 m³/kg for benzene), d_o is the density of the solvent and d_s is the density of the solution.

The molar susceptibility () is given by:

where, is the Avogadro’s number, is the vacuum permeability (4π*10^-7 NA^-2), is magnetizability, is the diamagnetic contribution and is the paramagnetic contribution. The effective paramagnetic moment is expressed as:

The number of unpaired electrons (n) can be finally calculated through:

Synthesis of 1

Synthesis of 1 was performed in a dried N₂-filled glovebox. Anhydrous CoCl₂ (0.500 g, 3.85 mmol) was dissolved in 20 mL THF using a 50 mL round-bottom flask. Then, HDippF (2.80 g, 7.70 mmol) was added to the solution. Subsequently, methyllithium (4.81 mL, 7.70 mmol) was added using a glass syringe. The solution changed from a sky blue to a dark aqua-green color. The resulting solution was allowed to vigorously stir under room temperature for three hours. Afterwards, diethyl ether (Et₂O) was added to precipitate the LiCl byproduct. The resulting suspension was filtered using a medium coarse glass frit packed with Celite. The filtrate was dried under reduced pressure to obtain 1.058 g of the final green product. The product was crystallized using of a concentrated solution of Et₂O. Yield 35.05%. UV-vis: λ max: 564, 628, and 674 nm.

Electrochemical measurements

Electrochemical measurements were recorded on a CHI760D potentiostat using a three-electrode electrochemical cell in a dry N₂-filled glovebox, with glassy carbon (4 mm diameter) as the working electrode, platinum wire and Ag/Ag⁺ as the counter and reference electrode, respectively. Ferrocene (Fc) was added subsequently after each measurement as an internal reference and all potentials displayed are referred to ferrocene/ferrocenium (Fc/Fc⁺) redox couple.

Hydrogen gas detection

Hydrogen gas was confirmed and quantified using a Gas Chromatograph (GC) after performing bulk electrolysis. Bulk electrolysis was operated using a sealed custom-built three-electrode electrochemical cell with an H shape. The working electrode (1 cm² carbon rod), reference electrode (Ag/Ag⁺) were separated with the counter electrode (Pt mesh) through a glass frit. Solutions containing 50 mL of 1 mM of 1, 0.1 M of TBAPF₆ and 10 mM of the respective acid studied (tosic acid, benzoic acid, phenol) in MeCN were used for bulk electrolysis at a constant potential of -2.0 V vs. Fc/Fc⁺ for an hour. An aliquot of 300 µL of the gas in the headspace in the side of the cell containing the working electrode was injected to the GC for analysis.

Calculation of 𝑘_𝑜𝑏s and 𝑘 _cat from foot-of-the-wave analysis (FOWA)

The HER kinetics can be measured by the foot-of-the-wave analysis (FOWA), which has been developed by Savéant and coworkers.[35] For an ECEC or ECCE mechanistic process,[35-36] the current is described by:

where, F is the Faraday constant (96485 C/mol), A is the surface area of the working electrode (cm²), is the concentration of the catalyst (mol/L), D is the diffusion coefficient (cm²/s) and 𝑘_𝑜𝑏s is the observed rate constant.

According to the Randles-Sevcik equation, the peak current is:[37]

where, ν is the scan rate.

By diving Eq.7 with Eq.8, we can get:

After plotting versus from the linear region we can obtain a slope of m, which

Thus can be obtained from

Then the catalytic rate constant (𝑘 _cat ) is calculated from:

Calculation of structural geometry index for a mononuclear 4-coordinate complex

To disambiguate whether 1 bears closer structural resemblance to a square planar or tetrahedral complex, the model proposed by Yang et al. (Eq. 13) was used.³⁸ In this equation, where is the structural index, β and α are the two largest valence angles in a 4-coordinate molecule respectively, and θ is 109.5^◦. For a 4-coordinate species, a of 1 represents a tetrahedral molecule while a of 0 indicates a square planar molecule.

Results and discussion

The solid-state structure of 1 was confirmed by X-ray crystallography (Fig. 1) and the detailed crystallographic data are summarized in Table 2. The bond distances of Co(1)-N(1) and Co(1)-N(2) are 1.998(3) Å and 1.999(3) Å, respectively, which are longer than those of the reported Co diglyoxime complex (1.886(5) Å).[23] The N2-Co1-N2, N1-Co1-N1, N2-Co1-N1 bond angles are found to be 155.0º (2), 151.0º (2), and 119.66º (14) respectively. Based on the bond angles, the calculated geometry index is 0.38. Thus, 1 is assigned as a distorted square planar complex.

Characterization of 1 by ¹H NMR proved challenging due to the intense paramagnetic broadening of the observed signals, reflecting the paramagnetic nature of the complex, and consistent with previous reports of 4-coordinate d⁷ Co(II) complexes.[39,40] The magnetic susceptibility of 1 was probed through the Evans method. The ¹H NMR signal for benzene is shifted by +0.51 ppm in the presence of 1, resulting in a μ _eff of 4.13 corresponding to three unpaired electrons (vide supra). This is consistent with other 4-coordinate cobalt complexes bearing nitrogen-donor ligands.[41] The higher μ _eff value compared to the predicted spin-only μ _eff of 3.87 is attributed to the typical spin-orbit coupling experienced by cobalt.[42,43]

Cyclic voltammograms (CVs) of 1 are presented in Figures 2-4. Compound 1 shows one nonreversible one-electron reduction wave at E_p = -1.59 V vs. Fc/Fc⁺, resulting from the Co^II/Co^I reduction couple. The nonreversibility of this event can be attributed to the electron-rich nature of 1. Upon addition of tosic acid (pK_a= 8.5 in MeCN), a catalytic wave is observed with an onset potential of -1.2 V vs. Fc/Fc⁺, accompanying with another reduction wave at E_p = -1.73 V vs. Fc/Fc⁺. Another two significantly less acidic proton sources, namely benzoic acid (pK_a= 21.51 in MeCN) and phenol (pK_a= 29.14 in MeCN), were also introduced to evaluate the electrocatalytic activity of 1. The CVs of 1 upon addition of both benzoic acid and phenol show considerable increase of cathodic catalytic current for proton reduction, with an onset potential of -1.45 V and -1.89 V vs. Fc/Fc⁺ for benzoic acid and phenol, respectively Figures (3-4). The ability of 1 to catalyze proton reduction in weak acidic media, which is noteworthy for this type of study, can be attributed to the enhanced electro-activity of the cobalt center due to the electron-donating formamidinate ligand. Hydrogen gas detection and quantification were carried out after performing bulk electrolysis for an hour at a constant potential of -2.0 V vs. Fc/Fc⁺ using 1 mM of 1 and 10 mM of the chosen acids. The accumulated charge over time (Fig. 5) shows 36.16, 14.26, 8.08 C of the passed charge for an hour of electrolysis when using tosic acid, benzoic acid and phenol, respectively, corresponding to Faradaic efficiencies of 91.4%, 85.0% and 100% for hydrogen production.

The reaction kinetics of HER using 1 with tosic acid was studied through the use of foot-of-the-wave analysis (FOWA). The FOWA plot is presented in Fig. 6 (a), which is derived from the linear sweep voltammograms shown in Fig. 6 (b). The observed rate constants (𝑘_𝑜𝑏s) and catalytic rate constants (𝑘_cat) at different concentrations of acid addition are summarized in Table 3 (see experimental section for detailed calculation). The relationship of 𝑘_𝑜𝑏s with acid concentration is linear (Fig. 7), suggesting there is a first order dependence on acid concentration.

Conclusion

In conclusion, we have successfully synthesized a cobalt complex anchored by formamidinate ligand. Structural analysis by X-ray crystallography shows that 1 has a distorted square planar chemical structure and magnetic susceptibility measurements show a μ _eff of 4.13 B.M. at room temperature, indicative of an electronic structure bearing three unpaired electrons. This molecule is found to be active for electrochemical hydrogen generation with both strong acid and weak acid substrates in MeCN. The electrochemical onset potentials for hydrogen generation range from -1.2 V to -1.89 V vs. Fc/Fc⁺. This work shows the potential of using electron rich ligand architectures for tuning the redox activity of cobalt HER electrocatalysts.

Acknowledgment

This work was financially supported by NSF under award number CHE-1305124. LMAQ thanks COURI-Meritus at UTEP and CCI-Solar for research finance. IBA thanks MARC for an undergraduate scholarship

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