Angewandte Research Articles Chemie www.angewandte.org How to cite: Angew. Chem. Int. Ed. 2022, 61, e202209075 Hydrogen Evolution Very Important Paper International Edition: doi.org/10.1002/anie.202209075 German Edition: doi.org/10.1002/ange.202209075 Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst Bing Jiang, Marcos Gil-Sepulcre,* Pablo Garrido-Barros, Carolina Gimbert-Suriñach, Jia-Wei Wang, Jordi Garcia-Anton, Pau Nolis, Jordi Benet-Buchholz, Nuria Romero,* Xavier Sala,* and Antoni Llobet Abstract: A cobalt complex bearing a k-N3P2 ligand is presented (1 + or CoI(L), where L is (1E,1’E)-1,1’-(pyridine-2,6- diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1 + is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1 + reveals a two-electron CoI/CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset = 1.6 V vs Fc/Fc +. In the presence of 1 equiv of bis(trifluoromethane)- sulfonimide, 1 + forms the cobalt hydride derivative CoIII(L)-H (22 +), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII(L)-H (2 +) species, which rapidly forms hydrogen and regenerates the initial CoI(L) (1 +). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1 +. Introduction has experienced a significant progress in the last fifteen years giving rise to a broad family of highly active Hydrogen fuel production via proton reduction (2 H + + compounds that take advantage of the versatility of ligand 2 e !H2) is considered a promising method to generate a design to tune the electronic, steric and second coordination renewable and highly energetic fuel in a sustainable manner. effects in the catalytic process.[1–8] One of the key aspects Homogeneous molecular catalysts have been actively inves- that has enabled the improvement of the catalytic perform- tigated, largely due to the wealth of mechanistic information ance is the possibility to study the mechanism of the reaction that these well-defined systems can offer. The use of cobalt through static and time-resolved spectroscopic techniques complexes to mediate this two electron reduction process that allow to follow the kinetics of the process as well as the detection and characterization of intermediate species.[9–22] [*] Dr. B. Jiang, Dr. C. Gimbert-Suriñach, Dr. J. Garcia-Anton, Theoretical calculations have also contributed in the under- Dr. N. Romero, Dr. X. Sala, Prof. A. Llobet standing and supporting of the experimental findings.[23–25] Departament de Química, Universitat Autònoma de Barcelona From these studies, it derives that a reduced formal CoI Cerdanyola del Valles, 08193 Barcelona (Spain) species is required to trigger the catalytic process. The latter E-mail: xavier.sala@uab.cat can react with a proton to generate a cobalt (III) hydride Dr. M. Gil-Sepulcre, Dr. P. Garrido-Barros, Dr. C. Gimbert-Suriñach, species (CoIII-H) that is an important intermediate in the Dr. J.-W. Wang, Dr. J. Benet-Buchholz, Prof. A. Llobet catalytic cycle from which hydrogen can evolve. Alterna- Institute of Chemical Research of Catalonia (ICIQ), tively, the CoIII-H can be reduced to its CoII-H derivative Barcelona Institute of Science and Technology (BIST) Av. Països Catalans 16, 43007 Tarragona (Spain) that can also lead to hydrogen generation. For many highly E-mail: mgil@iciq.es active catalysts, these hydride species remain elusive Dr. P. Nolis because of their high reactivity. Only a few examples Servei de Ressonància Magnètica Nuclear, reported in the literature give experimental evidence for the Universitat Autònoma de Barcelona formation of such cobalt-hydride[3, 13, 18, 22] and other metal- 08193 Bellaterra, Barcelona, Catalonia (Spain) hydride[26–31] species as actual hydrogen evolving intermedi- Dr. N. Romero ates. Of special interest is also the elucidation of the Laboratoire de Chimie de Coordination (LCC)—UPR 8241 mechanism by which those metal hydrides operate towards 205 Route de Narbonne, BP44099, formation of an H H bond, predominantly involving path- 31077 Toulouse Cedex 4 (France) ways first order in either CoIII or CoII hydride species. E-mail: nuria.romero@lcc-toulouse.fr Examples of bimolecular HER from first-row transition © 2022 The Authors. Angewandte Chemie International Edition metal hydrides are still limited.[32–41] published by Wiley-VCH GmbH. This is an open access article under In this work we isolate and characterize a CoIII(L)-H the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any med- compound derived from a CoI(L) complex, which is a key ium, provided the original work is properly cited, the use is non- intermediate towards the hydrogen evolution reaction. The commercial and no modifications or adaptations are made. cobalt complexes have a neutral pentadentate ligand (L) Angew. Chem. Int. Ed. 2022, 61, e202209075 (1 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie based on the redox active 2,6-bis(imino)pyridine scaffold with two phosphine arms (L = (1E,1’E)-1,1’-(pyridine-2,6- diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). The former has shown the capability of storing multiple electrons,[42–48] assisting reductive catalysis, while the π- acceptor nature of the phosphine groups in L lower the electron density on the metal potentially reducing reduction potentials for the HER. Results and Discussion Synthesis and Structural Characterization of 1 + Complex [CoI(L)] + (1 +) was prepared by mixing the N3P2 ligand (1E,1’E)-1,1’-(pyridine-2,6-diyl)bis(N-(3-(diphenyl- phosphanyl)propyl)ethan-1-imine) [ 4 9 , 5 0 ] with Co I I - (BF4)2·6 H2O in tetrahydrofuran at room temperature. After Figure 1. ORTEP drawing of 1 + with 50 % probability thermal ellipsoids. stirring for 1 day, the dark black-green solution was dried Hydrogen atoms and counter ions have been omitted for clarity. Color under vacuum and the resulting powder re-dissolved in a code: Carbon (grey), Nitrogen (Blue), Phosphorous (orange) and dichloromethane/methanol mixture. Layering with pentane Cobalt (purple). afforded black crystals of 1BF4 resulting in a 26 % of total yield (see Experimental section in the Supporting Information).[51] Complex 1 + is stable under air in the solid distances, a moderate elongation (1.3263(9) and 1.3256(9) Å state in contrast to related CoI complexes bearing the 2,6- compared to unreduced bis(imino)pyridine ligands 1.28 Å) bis(imino)pyridine ligand,[15, 20, 21, 48] highlighting the π-accept- is observed in both cases. Moreover, the Cim Cpy(o) bond ing character of the two extra phosphine moieties, which lengths (1.4449(10) and 1.4405(10) Å), that is C(2) C(3) and stabilize the low-valent metal center. Compound 1 + has C(8) C(7) in Figure 1, are obviously contracted relative to been fully characterized by ESI mass spectrometry, single neutral chelates (1.50 Å).[42, 47] Even if these metrical param- crystal X-ray diffraction, NMR and UV/Vis spectroscopies eters may imply a reduced bis(imino)pyridine chelate as as well as cyclic voltammetry (CV). The ESI-MS spectrum monoanion, we cannot determine this conclusion without confirmed the presence of the cation 1 + at m/z = 672.1 and further evidence. For instance, similar bond elongation and the double charged [CoII(L)-MeOH]2 + species at m/z = contraction observed in a manganese complex with the 352.1, formed under the ESI conditions (Figure S17). formula [MnI(PDI)(CO)] + (PDI is pyridyl(diimine)) has The crystal structure of 1 + belongs to a P2(1)/n been attributed to Mn-to-PDI, back-bonding rather than to monoclinic space group. The cobalt center is five coordi- the reduction on redox-active ligand through Density Func- nated with three N atoms and two P atoms from the tional Theory (DFT) calculations.[46, 50] Likewise, in this case, pentadentate ligand (Figure 1), where the metal is closer to DFT calculations resulted in the most stable singlet config- a distorted trigonal-bipyramidal environment, with the uration corresponding to a CoI complex with a consistent pyridine nitrogen atom and two phosphorus atoms forming trigonal-bipyramidal coordination environment, although the equatorial plane and two imine nitrogen atoms occupy- with less than 1 kcal mol 1 difference from the corresponding ing the axial positions. The angles of the trigonal plane, triplet configuration revealing the close proximity of both P(1) Co(1) P(2) = 111.27°, N(2) Co(1) P(2) = 123.53° and electronic structures. Similar to the [MnI(PDI)(CO)] + sys- N(2) Co(1) P(1) = 125.19°, deviate somewhat from the tem, analysis of the HOMO, which is mainly centered in the ideal trigonal angle of 120°. However the sum of these metal d orbitals, evidences partial contribution of the π- angles is 360°, confirming the planarity of the trigonal plane orbitals from the bis(imino) ligand that can also be that is composed of N(2), P(1) and P(2) atoms. The solid- explained as strong back-bonding interactions (Figure S24). state structure determined by X-ray crystallography displays Compound 1 + is a diamagnetic compound and has been a monocationic complex with the presence of only one BF4 fully characterized by 1D and 2D NMR spectroscopy in as counterion. This is in agreement with the resulting CD2Cl2 (Figure S1–S6). These analyses indicate the symmet- elemental analyses and suggests that a disproportion of the ric character of 1 + which belongs to the C2 point group in corresponding CoII species, formed from the CoII- solution, with the rotational axis passing through C(5), the (BF4)2·6 H2O precursor, takes place during the synthetic N(2) atom of the pyridine ring, and the central Co atom. A procedure (see electrochemistry below). Taking into account singlet at 43.2 ppm appears in the 31P{1H} NMR spectrum, the redox non-innocent character of the bis(imino)pyridine typical chemical shift range for coordinated phosphorus chelate, the electronic configuration of 1 + can be described donor atoms.[49] The 1H NMR spectrum shows six methylene as a low valent CoI ion stabilized by the neutral ligand, or resonances, which contrasts to the three resonances dis- can be regarded as a CoII ion antiferromagnetically coupled played by the free ligand. In addition to the propyl-bridged to a ligand radical anion. Inspecting the imine C=N chelate arms, long range 31P-coupling to the bis(imino)- Angew. Chem. Int. Ed. 2022, 61, e202209075 (2 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie pyridine backbone methyl (H5, t, 6H) and p-pyridine (H1, tt, 1H) resonances at 2.27 and 8.08 ppm, respectively, were also observed (Figure S1, insets). 1 + also differs from the parent free ligand in the splitting of the resonances corresponding to the phenyl substituents on the P atoms, showing six signals in the 7.55 to 6.12 ppm range instead of three, due to the loss of symmetry of the phenyl moieties upon coordination. The electronic absorption spectrum of 1 + in acetonitrile shows two intense characteristic absorption bands in the UV region at λ = 260 nm (ɛ = 33 000 M 1 cm 1) and λ = 317 nm (ɛ = 11 000 M 1 cm 1), which are assigned to ligand-based π!π* transitions (Figure S18). In the visible region, 1 + displays a band of medium intensity at λmax = 432 nm (ɛ � 16 000 M 1 cm 1), attributed to metal to ligand charge transfer (MLCT). In addition, two broad absorption bands with relatively low intensity (ɛ � 3000 M 1 cm 1) are observed at 498–611 nm and 611–800 nm, more likely resulting from d–d transitions. Similar broad bands in the same spectral region have been reported for related CoI compounds.[10, 15, 52] Electrochemical Analysis of 1 + The redox properties of 1 + were studied by cyclic voltamme- try. As shown in Figure 2a, the CV of 1 + in dry DMF (0.1 M nBu4NPF6 solution) features two reversible waves at E11/2 = 2.02 V (ΔEp = 79 mV) and E21/2 = 0.24 V (ΔEp = 112 mV), all potentials reported in this manuscript have been meas- ured using a Ag/AgNO3 (0.01 M AgNO3 in acetonitrile containing 0.1 M nBu4NPF6) reference electrode and have been converted to Fc/Fc + by adding ferrocene as internal standard at the end of the experiments. The first wave at E1 is tentatively assigned to the ligand based L0/ couple, thus generating CoI(L ) species (1) upon * * one-electron reduction. This is also supported by DFT Figure 2. a) Cyclic voltammograms of 1 mM 1 + in 0.1 M nBu4NPF6 DMF in the absence (black line) and presence (red line) of 5 mM p- calculations showing the LUMO of the 1 + complex being TsOH at scan rate of 100 mV s 1. b) Simulated cyclic voltammogram of centered in the bis(imino)pyridine ligand (Figure S24). 1 mM solution of 1 + in presence of 5 mM p-TsOH in 0.1 M nBu4NPF6 Consistently, the optimized one-electron reduced species DMF at scan rate of 100 mV s 1. CV obtained by means of DigiSim features a doublet electronic configuration with a SOMO 2.0 CV simulator software using the proposed mechanistic model in corresponding to ligand π-orbitals, with a calculated redox Scheme 2. potential of 1.92 V that is in good agreement with the observed experimental value (Figures S25 and S26). The wave observed at E21/2 = 0.24 V integrates twice as slightly endothermic according to the standard free energy, compared to E1 (80.4 vs 39.7 μC) and is attributed to the ΔG° = 5.9 kcal mol 1, suggesting its plausible coordination two-electron oxidation of 1 + to form the corresponding using DMF as the solvent. When the corresponding oxidized CoIII(L)-S complex (33 +, where S is DMF or MeCN depend- CoIII(L) complex is considered, coordination of a DMF ing on the utilized solvent), due to a disproportionation of molecule at the vacant position is now thermodynamically the transient intermediate CoII(L)-S species. This oxidative favorable according to ΔG°, leading to an octahedral event was also studied by DFT in order to elucidate the coordination environment forming the complex CoIII(L)-S nature of the two-electron oxidation and results are (33 + ) (Figure S28). The structure rearrangement and the summarized in Figures S27–S29. Computationally, one-elec- coordination event trigger the disproportionation reaction tron oxidation of the CoI(L) complex leads to a CoII(L) according to the calculated ΔG° = 1.9 kcal mol 1 for the species with a distorted square pyramidal geometry and a overall process (Figure S29).[53] This reactivity also explains high spin, quartet configuration, which is close in energy to the observed disproportionation process during the synthetic the low-spin, doublet configuration. Interestingly, the large procedure, where either the water molecules in the starting geometric rearrangement upon oxidation exposes a vacant material, Co(BF4)2·6 H2O, or the methanol solvent from site in the CoII(L) metal center that is now accessible. recrystallization might promote coordination and oxidation Coordination of a solvent molecule in this oxidation state is to the corresponding CoIII complex. Angew. Chem. Int. Ed. 2022, 61, e202209075 (3 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie The oxidation of 1 + was studied chemically by employ- contributes with two electrons to form the Co H bond. ing stoichiometric amounts of an oxidizing reagent, ferroce- Interestingly, the oxidation signature corresponding to the nium tetrafluoroborate (FcBF4), causing a color change CoIII(L)-S/CoI(L) couple is unaltered, suggesting that the from dark green-brown to orange immediately. The reaction new species converts to the original complex 1 + after was monitored by UV/Vis and 1H NMR spectroscopy scanning through the catalytic process, which remains as the (Figure S19 and Figure S7, respectively). With regards to resting state. The electrocatalytic formation of hydrogen was UV/Vis spectroscopy, a new band at 379 nm appeared while confirmed by a controlled potential electrolysis (CPE) the bands at 434 nm, 498–611 nm and 611–800 nm vanished experiment of a solution of 1 + at Eapp = 2.0 V carried out upon adding aliquots of FcBF4 showing isosbestic points at in a gas-tight two-compartment electrochemical cell 283 nm and 411 nm. The 1H NMR spectrum of the resulting equipped with a Clark-type electrode to monitor hydrogen species (Figure S7) shows a new diamagnetic species evolution in the headspace. Thereby, a fairly stable current attributed to CoIII(L)-S (33 +). This species displays signifi- profile (i vs t) was observed during 1 hour of experiment cantly altered chemical shifts with regard to its parent 1 + achieving a TON of 8.7 and a Faradaic efficiency of 84 % derivative, such as the representative m-pyridine doublet (Figure S32). resonance at 8.37 ppm in 1 + shifted to higher field The electrochemical scenario in the absence and in the (7.88 ppm) or the p-pyridine triplet of triplets at 8.07 ppm presence of proton source was investigated by simulating the moved to lower field (8.17 ppm). In addition, the resonances experimental CV recorded at 100 mV s 1 by an iterative of phenyl protons shift to lower fields and to a narrower mathematical process using the Digisim 2.0 CV simulator chemical shift range (6.76–7.79 ppm). software to get insights into kinetic (k) and thermodynamic (K) constants of the processes. Figure 2b shows the resulting simulated CV, which matches with the electrochemical Reactivity of 1 + under Electrocatalytic Proton Reduction response of 1 + in the presence of p-TsOH in DMF (see also Conditions Figures S33 and S34). As discussed above, the shift of the onset of first reduction wave observed in the presence of a As shown in the cyclic voltammogram of Figure 2a, when p- proton source suggests a first non-faradaic intramolecular toluenesulfonic acid (p-TsOH, pKa = 2.6 in DMF)[54] is added redox event that involves the formation of CoIII(L)-H to a DMF solution of 1 +, a significantly different profile is species, 22 +, coexisting with 1 + in solution (A in Scheme 1, I observed in its reduction chemistry. When scanning to the in Scheme 2). This equilibrium was calculated and resulted cathodic direction, an irreversible wave starting at Eonset = to be shifted towards the formation of hydride species 1.6 V is observed, followed by a sharp current increase (Keq = 1800) with a kinetic constant of k1 = 2.5 × 10 2 M 1 s 1. assigned to the electrocatalytic hydrogen evolution reaction. Afterwards, an electron transfer process takes place to The shift of the first reduction accounts for more than CoIII(L)-H, 22 +, with the SOMO being partially delocalized 200 mV when compared to pure complex 1 + and indicates over the bis(imino)pyridine backbone, the metal center and that a different species is responsible for this redox process. the hydride to generate a formal CoII(L)-H (2 +) as Additionally, the presence of the CoI(L)/CoI(L ) redox * supported by DFT calculations (see below) and triggering feature at E1ap = 2.04 V also in the presence of a proton the hydrogen evolution reaction (D in Scheme 1 and II in source suggest the existence of an equilibrium between Scheme 2). Interestingly, CV simulations match with the initial species in solution, 1 +, and the new formed species formation of hydrogen through a homolytic mechanism, that we attribute to the hydride derivative CoIII(L)-H, 22 + which involves the interaction of two CoII(L)-H units to (process A in Scheme 1). This species can be formed after form the H H bond (F in Scheme 1 and III in Scheme 2). A the reaction of 1 + with a H +, where the CoI center second-order kinetic constant of kcat = 7 × 103 M 1s 1 was obtained for the catalytic process, which perfectly reprodu- ces the experimentally observed cathodic potential and Scheme 2. Proposed scenario for electrochemical reduction and oxida- Scheme 1. Mechanistic hydrogen evolution pathways from the CoI(L) tion reactions of complex 1 + in presence of a proton source in DMF. complex 1 +. The mechanistic pathway followed by 1 + is highlighted The scheme shows the potentials (Eo), kinetic (k) and thermodynamic (red arrows), the highlighted squares indicate the isolated and (K) constants utilized for simulating the cyclic voltammogram shown characterized compounds in this work. in Figure 2 by DigiSim 2.0 CV. Angew. Chem. Int. Ed. 2022, 61, e202209075 (4 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie current (Ecat = 1.98 V and Icat = 72.6 μA, respectively). isosbestic points at 357 nm and 397 nm, which indicate a Simultaneously, non-reacted CoI(L), 1 +, can be reduced to clean transformation between the initial and final species. CoI(L ), 1, at a more negative potential (E1’ = 2.04 V, * The 1H NMR spectrum in CD3CN shows the gradual process II’). Finally, the CoIII(L)-S/CoI(L) feature was growing of a triplet at 9.43 ppm (with JH-P = 57 Hz) upon simulated in the subsequent anodic scan as a two-electron addition of the acid, indicating that a diamagnetic cobalt wave resulting from the disproportion of the formed CoII- hydride is generated in the presence of p-TsOH (Figure 3a). (L)-S species (IV and V in Scheme 2). Consistent with the UV/Vis and electrochemical experi- CVs of 1 + performed in the presence of a stronger acid ments discussed above, the NMR analysis confirms the slow trifluoromethanesulfonimide (TFSI, pKa = 11.9 in 1,2- formation of CoIII(L)-H, 22 +, with only partial conversion of dichloroethane),[55] show a sharp increase of the current at the initial CoI(L) complex, 1 +. A careful inspection on the Eonset = 1.6 V, matching with the catalytic process observed aryl region of the spectrum reveals eight new generated with p-TsOH (Figure S36). Interestingly, the precatalytic resonances (highlighted in Figure 3a), which coincides with process observed at Eonset = 1.4 V is not present, and the number of resonances of the parent complex 1 +. The 31 therefore, it could be potentially attributed to an electron P{1H} NMR spectrum displays two peaks, the first corre- transfer process related with the p-TsOH.[56] At the anodic sponding to the initial CoI(L) complex and a second one at scan, a clear decrease in the intensity of the CoIII(L)-S/ 36.5 ppm corresponding to the new CoIII(L)-H species (Fig- CoI(L) feature is observed, which suggests a faster formation ure S8). Taking all the information into account, a symmet- of the CoIII(L)-H in the presence of TFSI during the short ric structure is proposed in Figure 3b, with a k5-N,N,N,P,P- timescale of the CV. Additionally, the dependence of the coordination by the pentadentate ligand and the hydrido intensity of the catalytic process, located at Ecat = 1.98 V, ligand occupying a putative axial position of an octahedral with the concentration and the scan rate was investigated. type of geometry. This is further supported by DFT The plot of Icat vs [1 + ] clearly shows a linear dependence of calculations, where a singlet CoIII(L)-H hydride species is the catalytic process with the concentration of 1 +, which optimized with an octahedral geometry upon coordination confirms that the rate determining step (rds) for the hydro- of a proton to the initial CoI(L) complex. Considering p- gen evolution reaction is taking place before the interaction TsOH as the acid, the calculated free energy of the CoIII(L)- of two CoII(L)-H units to form the H H bond, most likely during the formation of CoIII(L)-H or the reduction to form the CoII(L)-H. The experimental trend matches with the mechanistic model proposed by CV simulations at different catalyst concentrations, which reproduces the linear rela- tionship between Icat and [1 + ] (Figure S35). Additionally, an enhancement of the current centred at the CoI(L)/CoI(L ) * redox feature (E = 2.04 V, Figure S37) is observed when 1 + does not consume all the protons in the double layer, evidencing catalysis also gated by this couple (CVs per- formed at fast scan rates and low catalyst concentration, Figure S37). In the electrocatalytic event the main process is dominated by the reduction of CoIII(L)-H (22 +) to the active CoII(L)-H (2 +) species. However, the CoI(L)/CoI(L ) * couple can become important in the catalytic process at lower potentials, through the binding of a H + forming the CoII(L)-H intermediate. The latter is the species capable of producing hydrogen via the homolytic mechanism described herein. Spectroscopic Characterization of the Cobalt Hydride 22 + Intermediate To further investigate the processes observed during the electrochemical studies with 1 +, UV/Vis and NMR spectros- copies were utilized to monitor and characterize the species Figure 3. a) 1H NMR spectra (500 MHz, CD3CN, 298 K) of 1 + (bottom generated from 1 + in the presence of p-TsOH. The line) and in situ generation of the corresponding CoIII(L)-H, 22 +, derivative upon the addition of 25 equiv p-TsOH in CD3CN, after 1 h, evolution of the electronic spectra is shown in Figures S20 6 h (middle lines) and 14 h (top line), presenting the aromatic region and S21 in acetonitrile and DMF, respectively. In resonances and amplified triplet corresponding to the hydride at acetonitrile, the band at 432 nm and the broad bands at 482– 9.43 ppm, arrows indicate the growing of new signals attributed to 600 nm and 600–800 nm decreases gradually while a new 22 +. b) 1H NMR spectra of 22 + (400 MHz, CD3CN, 298 K) recorded band at 379 nm appears. The process is slow and incomplete immediately after excess TFSI was added to the CoI(L) complex, 1 +. after prolonged reaction times, but it proceeds with * Unidentified species formed after the addition of TFSI. Angew. Chem. Int. Ed. 2022, 61, e202209075 (5 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie H formation is ΔGcalc = 1.5 kcal mol 1, which is only slightly exothermic (Figure S30), in agreement with the experimen- tally observed equilibrium character of this reaction. Full spectroscopic characterization of the cobalt hydride was achieved by using a stronger acid to push the reaction to completion, using trifluoromethanesulfonimide. Thus, full assignment of the 1H NMR spectrum of 22 + was achieved by combined 1D and 2D experiments (Figure 3b and Figur- es S9–S11). It displays C2 symmetry in solution, with the N of the pyridine ring, the central Co atom and the generated hydride H23 situated in the axis of symmetry. A ddt resonance and doublets integrating to 1H and 2H were located at 8.22 ppm and 8.08 ppm, corresponding to p- pyridine and m-pyridine protons, respectively. Other reso- nances integrating to 20H between 7.90 and 7.60 ppm were assigned to the PPh2 moieties. The full conversion process of 1 + into 22 + in the presence of excess TFSI, monitored by UV/Vis spectroscopy is presented in Figure S22. The absorption spectrum of the final pale orange solution corresponding to the CoIII(L)-H species 22 + exhibits two transitions at 297 nm and 379 nm with ɛ � 10 000 and 5000 M 1cm 1, respectively (blue line in Figure S22). Elucidation of Hydrogen Evolution Reaction Mechanism Figure 4. a) 1H NMR spectra (400 MHz, CD3CN, 298 K) of CoIII(L)-H The cobalt hydride species 22 + is stable for several hours species, 22 +, before (bottom line) and after adding overall 0.3 equiv, under inert conditions in the presence of excess acid source 0.7 equiv (middle lines) and 1 equiv of CoCp*2 (top line), presenting (Figure S16). We attribute the high stability of 22 + to the π- the aromatic region resonances and amplified triplet corresponding to acceptor nature of the phosphine groups of the ligand, which the hydride at 9.43 ppm. ! indicates peaks decreasing in intensity lower the electron density on the metal center anodically from CoIII-H, 22 +, ~ indicates growing peaks from CoI(L) species, 1 +. shifting the reduction potentials and, therefore, stabilizing b) 1H NMR spectra (400 MHz, CD3CN, 298 K) of the CoI(L) complex 1 + (bottom line) and the species formed by reacting the CoIII(L)-H the CoIII(L)-H species by decreasing its hydricity.[57] The species 22 + with 1 equiv of CoCp*2 (top line). high stability rules out the direct hydrogen evolution through heterolytic or homolytic pathways from CoIII(L)-H (B and C in Scheme 1, respectively). Thus, hydrogen should be produced by a reduced CoII(L)-H (D in Scheme 1). species after reacting with the reducing agent. A close Direct formation of CoII(L)-H from reduction of CoIII(L)-H examination and comparison of the new spectrum shows with presence of CoI(L) has been observed before for a that it is the same as that of the parent CoI(L), complex 1 + cobalt complex containing the triphos ligand (triphos = 1,1,1- (Figure 4b). The same conclusion can also be extracted from tris(diphenylphosphinomethyl)ethane)[13] but this reaction is the 31P{1H} NMR spectra, where the peak at 36.5 ppm from ruled out in the present case as both species are stable in CoIII(L)-H decreases upon adding CoCp*2 and the charac- solution for prolonged reaction times (> 14 h, see Figure 3a). teristic peak at 43.2 ppm from 1 + raises (Figure S14 and Indeed, the process is not thermodynamically feasible S15). considering the redox chemistry of the two species, where The clean transformation of CoIII(L)-H, 22 +, to initial E2(CoIII/I) > E1(CoII/I-H) (Figure 2). Co (L) complex, 1 +, upon reduction with 1 equiv of CoCp*2 I A strong reducing agent, that is, decamethyl-cobaltocene suggests that hydrogen gas must be generated through the (CoCp*2, E1/2 = 1.91 V vs. Fc/Fc + in MeCN),[58] was used to so-called homolytic pathway F in Scheme 1. Any other generate the one-electron reduced species from 22 +, as catalytic route, such as the heterolytic pathway E in follows. First, pure CoIII(L)-H, 22 +, was prepared in situ in Scheme 1 would lead to a paramagnetic CoII(L)-S species, CD3CN solution by reacting 1 + with 1 equiv of TFSI acid. which would be unstable towards disproportionation as The 1H and 31P{1H} NMR spectra in Figure S12 and S13 discussed above. The result of a disproportion process would show the complete transformation from CoI(L) to CoIII(L)- give rise to a mixture of both CoI(L), 1 +, and CoIII(L)-S, 33 +, H, as expected. Then, 1 equiv of CoCp*2 was added in three species as indicated with the grey dotted arrows of batches to the former mixture. As shown in Figure 4a, the Scheme 1, both fully characterized by NMR spectroscopy signals of the CoIII(L)-H in the 1H NMR spectra gradually (Figure S1 and Figure S7). As clearly shown in the top of disappear with the addition of CoCp*2 what is concomitant Figure 4b, a unique product that coincides with pure CoI(L) with the growing of new resonances, confirming that the complex, 1 +, appears in the spectrum, supporting the CoIII(L)-H derivative is converted to a new diamagnetic homolytic pathway. In order to check the formation of H2 Angew. Chem. Int. Ed. 2022, 61, e202209075 (6 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie through the proposed homolytic pathway, the experiment was repeated by mixing once more 1 + with a strict stoichiometric amount of TFSI followed by CoCp*2 and the headspace was analyzed by gas chromatography coupled to a thermal conductivity detector (CG-TCD) confirming the immediate formation of H2 gas upon addition of the reducing agent (Figure S23). The formation of hydrogen in the absence of an extra equivalent of H + confirms the involvement of a homocoupling H2 formation mechanism F in Scheme 1. These results are relevant since homocoupling H2 formation involves a bimolecular reductive elimination, which in principle should be favored by higher oxidation states in the metal center or by unimolecular intramolecular pathways.[32, 33, 38–40] Thus, metal-hydride species in III oxida- tion states (MIII-H) have shown to undergo bimolecular pathways for H H bond formation, invoking certain radical Figure 5. a) Proposed homocoupling catalytic cycle for 1 +. Free energy character in the H atom responsible for the coupling. On the changes are indicated as black number in kcal mol 1 and calculated other hand, one-electron reduced homologues remain stable redox potential in volts vs NHE. b) Calculated structures of 22 + and 2 + against bimolecular coupling and, instead, normally behave species with a representation of the SOMO localized in the ligand π- orbitals. as good hydride donors.[34] In sharp contrast, for complex 1 + the CoIII(L)-H remains stable while its reduced derivative is the one allowing the reductive elimination towards H2 formation. We attribute the incapability of the CoIII(L)-H erating hydrogen at the CoIII(L)-H/CoII(L)-H reduction species to undergo radical coupling to the high cationic potential (Eonset = 1.6 V, Figure 2). overall charge of the complex responsible for a significant kinetic barrier due to electrostatic repulsions. DFT calcu- lations provided important insights into the electronic Conclusion structure of the reduced intermediate derived from CoIII(L)- H. We found that this species features a doublet electronic The Co complex 1 + containing a k-N3P2 pentadentate ligand structure, where the SOMO spin density is delocalized over that is based on the redox active 2,6-bis(imino)pyridine the Co dz2 orbitals, the bis(imino)pyridine ligand and the scaffold and two phosphine arms, has been successfully hydride, evidencing its active role in the HER catalysis prepared. It is formally a CoI complex, which adopts pseudo (Figure S31). With this result, we conclude that contribution trigonal-bipyramidal geometry in the solid state (XRD) and of the ligand-based orbitals to the stabilization of reduced C2 symmetry in solution (NMR). Complex 1 + is stable under oxidation states allows to access reactive intermediates air in the solid state thanks to the π-accepting character of required for hydrogen evolution (i.e., CoII(L)-H). Moreover, the two phosphine ligands, which stabilize the low valent the partial contribution of hydride-centered orbitals to the metal center. Cyclic voltammetry analysis in acetonitrile SOMO suggest its proclivity to undergo bimolecular H H shows two redox couples at E11/2 = 2.02 V and E21/2 = bond formation. 0.24 V vs. Fc/Fc + that have been assigned to the one- Thus, the proposed complete catalytic pathway towards electron ligand-based reduction CoI(L )/CoI(L), and to the * the formation of molecular hydrogen from organic acids two-electron CoIII(L)-S/CoI(L) couple, respectively. The catalyzed by 1 + is summarized in Figure 5. It starts with the two-electron nature of the CoIII/I oxidation underlines the generation of the key CoIII(L)-H hydride intermediate from capacity of the CoII(L)-S species to disproportionate and the initial CoI(L) compound that reacts with a proton. This accounts for the isolation of the CoI(L) derivative in the species is stable and needs a further 1e reduction to form synthetic process, even though a CoII precursor is used as the corresponding CoII(L)-H species, which is very reactive starting material. and releases H2 through a homolytic pathway regenerating Cyclic voltammetry experiments in the presence of the initial CoI(L) compound, 1 +, closing the catalytic cycle. organic acids together with complementary spectroscopic Hydrogen evolution from the CoII-H or CoII(L)-H species is analysis demonstrate the involvement of a CoIII(L)-H proposed for several cobalt catalysts reported in the species, 22 +, and its reduced homologue in electrochemically literature but most of the examples undergo heterolytic induced HER. Compound 22 + is generated by protonation pathways by reaction with protons in the medium, yet this is of the CoI(L) complex 1 + and its formation is dependent on highly dependent on the catalytic conditions including acid the acid source strength achieving full conversion with TFSI strength and concentration (pH in aqueous conditions), in acetonitrile solution. concentration of catalyst as well as the reaction activation The hydride species 22 + is stable under inert conditions process (electrochemically vs photochemically induced and in the presence of 1 + since they both co-exist in solution reactions).[5, 7, 13, 15, 21] Importantly, the stoichiometric experi- over a timescale of hours. On the other hand, by using ment between the CoIII(L)-H species, 22 +, and the reducing stoichiometric amounts of a strong reducing agent, CoCp*2, agent CoCp*2, unequivocally prove the feasibility of gen- the CoIII(L)-H species is reduced and cleanly transformed Angew. Chem. Int. Ed. 2022, 61, e202209075 (7 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angewandte Research Articles Chemie into the initial 1 +. This transformation, when put in context [7] L. Tong, L. Duan, A. Zhou, R. P. Thummel, Coord. Chem. within the HER mechanism scenario of Scheme 1, implies Rev. 2020, 402, 213079. an homolytic pathway from a CoIII(L)-H derivative com- [8] J.-W. Wang, K. Yamauchi, H.-H. Huang, J.-K. Sun, Z.-M. Luo, D.-C. Zhong, T.-B. Lu, K. Sakai, Angew. Chem. Int. Ed. 2019, pound, leading to fast electrocatalysis at Eonset = 1.6 V vs. 58, 10923–10927; Angew. Chem. 2019, 131, 11039–11043. Fc/Fc +. Computational characterization of the species able [9] J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, H. B. Gray, to promote bimolecular HER points to an active CoIII- Acc. Chem. Res. 2009, 42, 1995–2004. hydride complex where the ligand is reduced by one- [10] P. Du, J. Schneider, G. Luo, W. W. Brennessel, R. Eisenberg, electron. 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Berben, Accepted manuscript online: August 3, 2022 Dalton Trans. 2016, 45, 5989–5998. Version of record online: August 24, 2022 Angew. Chem. Int. Ed. 2022, 61, e202209075 (9 of 9) © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH