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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Role of Anation on the Mechanism of Proton Reduction Involving a Pentapyridine Cobalt Complex: A Theoretical Study Murugesan Panneerselvam†,‡ and Madhavan Jaccob*,†,‡ †

Department of Chemistry, Loyola College, Chennai 600 034, Tamil Nadu, India Computational Chemistry Laboratory, Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600 034, Tamil Nadu, India

‡

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S Supporting Information *

ABSTRACT: Kinetic and thermodynamic aspects of proton reduction involving pentapyridine cobalt(II) complex were investigated with the help of quantum chemical calculations. Free energy profile of all possible mechanistic routes for proton reduction was constructed with the consideration of both anation and solvent bound pathways. The computed free energy profile shows that acetate ion plays a significant role in modulating the kinetic aspects of Co(III)−hydride formation which is found to be the key intermediate for proton reduction. Upon replacing solvent by acetate ion, one electron reduction and protonation of CoI species become more rapid along with slow displacement reaction. Most favorable pathways for hydrogen evolution from Co(III)−hydride species is also investigated. Among the four possible pathways, reduction followed by protonation of Co(III)−hydride (RPP) is found to be the most feasible pathway. On the basis of QTAIM and NBO analyses, the electronic origin of most favorable pathway is explained. The basicity of cobalt center along with thermodynamic stability of putative CoIII/II−H species is essentially a prime factor in deciding the most favorable pathway for hydrogen evolution. Our computed results are in good agreement with experimental observations and also provided adequate information to design cobalt-based molecular electrocatalysts for proton reduction in future.

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onset of electrocatalysis occurs near the CoII/CoI redox couple reveals the involvement of reduced Co(I) species in the catalytic cycle. The pioneering work of Chang and coworkers24 provided the possibility of tuning the catalytic H2 production at much lower overpotentials of 660 mV in aqueous solution. At neutral pH conditions, the [Co(PY5Me2)]2+ complex is found to have a turnover frequency of 300 mmol H2 per mole catalyst per second. Further, peripheral substitutions on the PY5Me2 ligand show a prominent shifts in potential at which the catalytic hydrogen production occurs. There is no clear-cut idea about the putative intermediates and mechanistic aspects of hydrogen production. In this regard, King et al. performed comprehensive electrochemical and spectrophotometric analyses about the mechanism of the proton reduction involving the[Co(PY5Me2)(NCMe)]2+ complex using acetic acid as a proton donor.22,24 They observed the two different mechanistic pathways, both leading to the formation of a Co(III)−H intermediate. The involvement of an anion-bound pathway really has a significant impact on the redox features of cobalt complexes. In particular, coordination of acetate ion with [Co(PY5Me2)(NCMe)]2+ leads to change in the redox

INTRODUCTION In recent years, finding an efficient way of producing synthetic hydrogen is the major concern for most of the researchers since it is an alternative potential molecular fuel. In this context, electrochemical oxidation of hydrogen using earth abundant metals plays an alternative route for proton reduction.1,2 Molecular electrocatalyst is the one which involves mainly in the conversion of chemical energy into electrical energy occurred by the electrochemical oxidation of hydrogen using a metal.3−8 In particular, the use of the mostabundant metal-based water-stable molecular electrocatalysts is a proper chemical design for the efficient hydrogen production at low overpotential.4,9−11 In this connection, diverse ligand skeletons with iron-, cobalt-, and molybdenum-based complexes were found to generate molecular hydrogen in organic and aqueous media.2,12−15 But they are found to produce hydrogen relatively at high overpotentials and often require the organic additives.16,17In order to overcome these issues, recent reports reveal that cobalt(II) complexes containing polypyridine ligand framework is found to produce robust hydrogen in both aqueous and acetonitrile mixtures.13,18−22 Bigi and coworkers23 show that tetradentate polypyridyl (CoIIPY4) complexes are observed to perform catalytic hydrogen production in 1:1 water and NCMe mixtures where the proton reduction occurs at an overpotential 400 mV. The © XXXX American Chemical Society

Received: February 1, 2018

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DOI: 10.1021/acs.inorgchem.8b00286 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Proposed Reaction Mechanism for the Formation of the Cobalt(III) Hydride Intermediate

proton reduction by pentapyridine cobalt complex with the following objectives (Scheme 12): (i) How far does acetate ion influence the formation of Co(III)−hydride species? (ii) Which is the most favorable pathway for CoIII−hydride formation? (iii) How does one identify the predominant pathway for proton reduction from CoIII−Hydride species? (iv) How does one pinpoint the electronic origin of the most favorable pathway for proton reduction?

potential of the Co(II)/Co(I) couple, and thereby, the activity of Co(I) species is enhanced. There were no noteworthy changes observed for other ligand scaffolds.13,25 So it is essential to understand the exact role of acetate ion in the formation of Co(III)−hydride species. Even though, the protonation of the Co(I) species is expected to be the ratelimiting step but there is no clear-cut information about the formation of putative Co(III)−hydride species. Also the evolution of hydrogen from Co(III)−hydride species is not clearly established. Mainly, four different pathways are anticipated for proton reduction from Co(III)−hydride species. Among them, choosing an appropriate pathway for proton reduction with different pH condition along with ambient redox couple is not an easy one in the absence of kinetic aspects of proton reduction. Although Gray et al.26 established the necessity of two redox couple for proton reduction using Co triphos and glyoxime complexes, the exact pathway for the proton reduction is not clearly established. Despite the preliminary attempts to locate the transition states for the above steps by Muckerman et al.,6,27,28 most of the earlier theoretical investigations are based only on computation of relative reaction free energies.29,30 To determine the favorable mechanistic pathway for proton reduction, it is warranted to have the free energy barriers of all the above steps. In this connection, we are aiming to perform the detailed DFT calculations to understand the reaction mechanism of

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COMPUTATIONAL DETAILS All the computations were carried out using the Gaussian 09 program.31 To ascertain the ground-state spin multiplicity, molecular geometries were optimized with the consideration of all possible spin state configurations using exchange-correlation B3LYP functional.32,33 The relative energies of all possible spin states for all intermediates present in the potential energy surface were provided in the Supporting Information (Table S1) and we have considered the lowest energy spin state for discussion. The reliability of B3LYP functional is well established through earlier reports in estimating the barrier heights and redox potentials and computation of spectroscopic properties. In order to describe H, C, N, and O atoms, 631G(d)34 basis set was employed and Los Alamos effective core potential (LANL2DZ)35,36 basis set for Co metal center. Further, harmonic vibrational frequencies were computed on the gas phase optimized geometries to extract thermodynamic B


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Inorganic Chemistry Scheme 2. Possible Mechanistic Pathways for Proton Reduction from Co(III)−Hydride intermediates

describing relative energies, pKas, and redox potentials of transition metal complexes.38 In order to consider the 1 M standard state in acetonitrile solution from the gas phase optimized geometries, a concentration correction of 1.89 kcal/ mol [RT ln(24.5)] is included in the computed reaction profile.47,48 For the computation of inner-sphere reorganization energies,13 the following equation was used as per the earlier report:

properties and to characterize the minima and transition state along the potential energy surfaces(PES). Intrinsic reaction coordinate (IRC) calculations were performed to ensure the particular transition state along the reaction trajectories (All IRC results are provided in the Supporting Information, Figures S1−S4.) The potential energy surfaces involving proton and electron transfer phenomenon in the course of the process is constructed based on the earlier reports.29,30,37−42 To assess the implication of solvent effect, single-point calculations were carried out on the gas-phase optimized geometries in acetonitrile medium using the selfconsistent reaction field-polarizable continuum model (SCRFPCM)43,44 at B3LYP/TZVP level.38,45,46 Furthermore, the B3LYP functional has been found to perform better in

λin = G of non‐optimized [CoI−VS]+ species − G of optimized [CoI−VS]+ species

The free energy of nonoptimized [CoI−VS]+ species is obtained through running the frequency calculation on the CoI species by removing hydrogen bound to cobalt center from C


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Inorganic Chemistry

Figure 1. Computed free energy profile for the formation of Co(III)−hydride. All energies are reported here in kcal/mol.

CoIII−H species. Redox potentials and pKa values were calculated from the oxidized and reduced species by using the Born−Haber cycle:49−51 E=

were generated using Gaussian 09 program. All QTAIM calculations were performed by using AIM 2000 package.55

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RESULTS AND DISCUSSION Mechanism of Co(III)−Hydride Formation. Here we have investigated the mechanistic aspects of Co(III)−hydride formation involving pentapyridyl cobalt complexes with a special emphasis on the role of acetate anion in the reaction mechanism. Scheme 1 shows the reaction mechanism which is proposed based on the available experimental observations.24 Two different pathways (solvent bound/anation pathway) were considered based on the nature of coordinating ligand with Co(II) species (either acetate ion or acetonitrile). The involvement of anion especially acetate ion shows a prominent role in altering the catalytic mechanism of proton reduction. So we have considered the anation bound pathway (AP) in addition to solvent-bound pathway (SBP). In SBP, [CoII(PY5Me2)(NCMe)]2+ complex undergoes one electron reduction along with homolytic dissociation of Co(II)−NCMe bond leading to the formation of five coordinated [CoI(PY5Me2)]+ species (Int1SBP). Subsequently, protonation of Int1SBP yield a Co(III)−hydride species. In AP, displacement of NCMe species by acetate leading to the formation of acetate-bound [CoII(PY5Me2)(OAc)]+ species (Int1AP) which further undergoes one electron reduction to yield a [CoI(PY5Me2)(OAc)] species (Int2AP). Subsequent protonation of Int2AP via TS2AP leads to the formation of Co(III)− hydride species and acetate anion. Here, we are planning to discuss the formation mechanism of Co(III)−hydride species via SBP and AP pathways in separate sections. Solvent-Bound Pathway. The computed free energy profile for Co(III)−hydride formation involving both SBP and AP pathways is shown in Figure 1. All energies reported herein are obtained from PCM (NCMe) B3LYP/TZVP//B3LYP/ LANL2DZ(6-31G(d)) calculations with free energy correction. We have taken high spin [CoII(PY5Me2)(NCMe)]2+ octahedral complex as a starting geometry. First step of the SB pathway is the concomitant dissociation of acetonitrile with

−ΔG − SHE nF

Here, F is the Faraday constant (23.06 kcal mol−1 V−1), n is the number of electrons in the reaction, ΔG is calculated from the change in the Gibbs free energy in the gas-phase and solvation free energies of oxidized and reduced complexes of the redox half-reaction in solution. On the basis of the earlier literature data, the experimental proton solvation free energy in acetonitrile medium is found to be −260.2 kcal/mol.52 In the present work, we have calculated as −260.55 kcal/mol using B3LYP/LANL2DZ(6-31G(d)) level of theory. Similarly, the standard hydrogen electrode (SHE) reference value is computed as 4.28 V using same calculation set up. This is quite comparable with the experimental value of 4.44 V (absolute SHE).50,53 Experimentally,24 a potential of −0.9 V vs. SHE is applied so that we have included −0.9 V for constructing free energy profile. Besides, energies obtained from B3LYP-D329 dispersion correction for Scheme 1 (see Supporting Information, Table S3) and B3LYP-D3 single-point energies from B3LYP optimized geometries for both Scheme 1 and 2 were provided in the Supporting Information (Table S4). On the basis of the computed data and earlier reports29,40 by Liao et al, single-point calculation using B3LYP-D3 dispersion correction on the B3LYP-optimized geometries was found to provide similar energetic trend without having much deviation and therefore the above method was adopted for Scheme 2. The ground state electronic excitation phenomenon of key intermediates were obtained through the computation of absorption properties using time dependent-density functional theory (TD-DFT)54 framework at B3LYP/LANL2DZ(631G(d)) level in acetonitrile continuum. In order to examine the bonding nature of CoIII/II−hydride species, Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses were performed at the same level of theory used for optimization. The wave functions for QTAIM analysis D


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Inorganic Chemistry

Figure 2. Optimized geometries of key intermediates, transition states involved for Co(III)−hydride formation and hydrogen production along with notable geometrical parameters, spin density plot and imaginary frequencies. Bond lengths and bond angles are given in Å and degrees (deg) respectively. The crystal structural parameters are shown in parentheses (italic), and unimportant hydrogen atoms are not shown.

kcal/mol, which shows that decomposition along with reduction is more favorable than the stepwise process. The tendency of stabilizing CoI center by pentadentate PY5Me2 ligand scaffold is much more pronounced when compared with other ligand scaffolds such as tertpyridine-amine cobalt

one electron reduction of stability geometry to yield pentacoordinated Co(I) complex (Int1SBP). This particular step is exoergic by −51.1 kcal/mol. The higher thermodynamic stability of the Co(I) species is clear when considering the stepwise process; the dissociation of acetonitrile ligand from the [CoII(PY5Me2)(NCMe)]2+ complex is endoergic by 18.9 E


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Inorganic Chemistry

Figure 3. TD-DFT computed UV/Visible absorption bands of Int1SBP and [CoIII(PY5Me2)(H)]2+ along with experimental absorption bands.

complex (only by 14 kcal/mol)13 and pentadentatetetrapyridyl N4Py ligand.9 To find the kinetic barrier associated with Co(II)−NCMe cleavage, a relaxed potential energy surface scan is performed by varying the Co−NCMe distances and results reveal that there is no significant barrier height in dissociation of CoNCMe bond (Supporting Information, Figure S5). This indicates that ligand dissociation along with one electron reduction is found to be thermodynamically driven and no significant kinetic barrier. Upon comparing Int1SBP and starting geometry, the switching of distorted octahedral to penta-coordinated square pyramidal geometry is clearly reflected in their structural parameters where Co−Naxial and Co−Nequatorial bond lengths were contracted to 2.04 Å from 2.13 Å and to 2.14 Å from 2.18 Å, respectively (Figure 2). The calculated reduction potential for the CoII to CoI couple is −1.811 V, whereas the corresponding experimental potential value is −0.830 V. The considerable difference between the computed and experimental redox potential value may be due to the ion-pairing effect on the reduction of the CoII to CoI couple.56,57 In accordance with the earlier theoretical calculations,24,58 Int1SBP was found to possess S = 1 ground state with two unpaired electrons in eg orbital and S = 0 spin state lying at 24.5 kcal/mol higher. The slight differences in spin state energetics mainly arises upon employing different basis set with B3LYP functional. Int1SBP were found to display two distinct absorption bands 836.9 and 687.5 nm with oscillator strength of 0.014 and 0.068 respectively (Figure 3). This is in good agreement with experimentally observed absorption bands at 650 and 825 nm, respectively. These bands are mainly originated from metal to ligand charge transfer. Thus, our computed structural and spectroscopic parameters are in

good agreement with the experimental and X-ray crystallographic geometrical data.24 The next step of the mechanism is the direct protonation of Int1SBP, which leads to the formation of the Co(III)−hydride intermediate. This particular step is found to have a significant reaction barrier of 14.8 kcal/mol with respect to Int1SBP. The computed pKa for the direct protonation of Int1SBP leading to the formation of CoIIIH species is 12.03, which is quite comparable with other ligands.13,26,28 Comparing the structural parameters of TS2SBP with Int1SBP indicates that there is a elongation of the Co−Naxial bond (2.121 Å), and the Co− Nequatorial bonds are contracted considerably with a range of 1.973−2.133 Å (Figure 2). The calculated Co−H bond distance in TS2SBP is 2.149 Å. Also there is a substantial reduction of the spin density of the cobalt center from 2.12 to 1.94, which shows that there is a considerable spin delocalization on ligand skeleton upon direct protonation of Int1SBP. Upon examination of the structural aspects of TS2SBP, it is clear that it closely resembles the Int1SBP geometry, which indicates that TS2SBP is an early transition state. The higher stability of cobalt(III)−hydride intermediate is clearly reflected from their high exoergic nature (−88.0 kcal/mol). From our computed free energy profile involving the formation of Co(III)−H species, protonation of five-coordinated CoI species (Int1SBP) is found to have substantial energy barrier relative to Int1SBP. This is consistent with experimental observation9,53 where A. E. King et al. suggested that the protonation of the CoI species is the rate-determining step.24 This is mainly originated by the successful crystallization of the particular intermediate (Int1SBP). A similar kind of trend is observed in the case of triphos ligand, where the protonation of CoI species produces a most stable transient CoIII−H complex with an driving force of 30 meV/Co.26,58 However, with our F


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Inorganic Chemistry

Electronic Structure of the CoIII−Hydride Intermediate. In the absence of apparent details about the formation of Co(III)−hydride species, here we would like to present their structural and spectroscopic features. On the basis of our B3LYP/TZVP calculations, the energy gap between ground S = 0 state and first excited S = 1 spin state is 49.4 kcal/mol. This indicates that there is no involvement of low-lying excited state (S = 1) in the catalytic cycle. The calculated Co(III)−H bond distance is 1.463 Å and Co−Naxial bond length is 2.059 Å and contraction of the Co−Nequatorial bonds in the range of 1.996− 1.998 Å. On the basis of the computed bond distances and bond angles are found to possess ideal octahedral geometrical parameters. For regular octahedral low-spin CoIIIH species, it found to display a set of LMCT bands ranging from 299.2 to 319.2 nm (oscillator strength of 0.096 and 0.100 respectively) and their respective d−d transitions are less intense than LMCT transitions. So we consider the most intense LMCT transitions in the present manuscript (Figure 3). The most intense absorption band at 319.2 nm is mainly originated from the electronic excitation from HOMO−6 to LUMO level. The low energy absorption band at 299.2 nm is found to be less intense with an oscillator strength of 0.096 which involves the electronic excitation from the HOMO−11 to LUMO level. The computed absorption spectra of Co(III)−H complex is in close resemblance with UV−visible spectrum of DPA-Bpybased Co(III)−H complexes13 and earlier DFT calculations.28 On the basis of the controlled potential electrolysis (CPE) experiments, 24 the catalytic activity of electrocatalytic production of hydrogen by Co(II)-PY5Me2 complex was lasting over 60 h and there was no evidence for catalytic decomposition. In the case of DPA-Bpy based cobalt complexes,13,52 the complex remain active only for more than 5 h and CoIIIH species was found to stable for only about 5 s by pulse radiolysis method and they could not observed absorbance feature of CoIIIH species at pH 3 on the 50 ms time scale. Similarly for N4Py ligand skeleton,9 the cobalt complex decompose much faster and hydrogen evolution was lasting within 1 h which is due to the low stability of CoI species. Unlike other ligand skeletons aforementioned, ancillary PY5Me2 scaffold provides a unique platform for stabilizing the high valent Co(III) center along the catalytic cycle and also ability to stabilizes the low-valent CoI species substantially. It is clearly witnessed from successful crystallization of CoI species derived from pentapyridyl ligand scaffold. This is clearly observed from our computed results where large thermodynamic drive accompanied by the formation of putative Co(III)−hydride species. This would be the reason for high turnover number for proton reduction involving pentapyridylcobalt(II) complexes. Mechanistic Pathways for Proton Reduction. On the basis of the available experimental and theoretical results, reduction of proton by pentapyridine cobalt complexes involves the following four main pathways (Scheme 2): (a) reduction of Co(III)−hydride followed by protonation (RPP) and (b) heterolytic cleavage of Co(III)−hydride (HEP), which leads to forming H 2 and Co(III)(OAc); (c) unimolecular homolytic cleavage of Co(III)−H (UMHP); (d) bimolecular homolytic cleavage of two Co(III)−hydride species to give 2Co(II) and H2 (BMHP). In this section, we are aiming to provide a detailed discussions about the reaction profile and geometries of the each transition states for all the above pathways. All energies reported herein are obtained from PCM (CH3CN) B3LYP/TZVP//B3LYP/LANL2DZ level

computed profile, the formation of most stable Co(III)−H species along the SBP pathway is found to be activation-less process. This could be attributed with the remarkable ability of pentapyridyl ligand scaffold in stabilizing the transient CoIIIH species. Anation Pathway. On the basis of the pioneering work of Chang et al.,24 the influence of acetate on the formation of Co(III)−hydride species is pivotal in proton reduction by [CoI(PY5Me2)(CH3COO)] complex. So it is essential to examine that how far acetate ion competing the easily accessible solvent molecule (NCMe) on the formation of Co(III)−H species. On the basis of the on the proposed reaction mechanism (Scheme 1), the first step is the displacement of acetonitrile by acetate anion. From Figure 1, the computed barrier for this step (TS1AP) is 29.5 kcal/mol. This particular step is thermodynamically accessible one. Inspection of geometrical aspects of TS1AP reveals that the elongation and contraction of Co−Naxial and Co−Nequatorial bond lengths at 2.100−2.241 Å occurs with Co(OAc) and Co(NCMe) bond lengths of 2.700 and 2.500 Å, respectively (Figure 2). The formation of Int1AP is exoergic by 5.7 kcal/ mol. So the formation of Int1AP is less favorable than Int1SBP by 45.4 kcal/mol. This shows that homolytic dissociation of Co(II)(NCMe) bond along with one electron reduction is thermodynamically and kinetically favorable by 45.4 kcal/mol over displacement of acetonitrile by acetate ion. Subsequently, Int1AP undergoes one electron reduction leads to the formation of Int2AP which is thermodynamically favorable with reaction energy of −59.4 kcal/mol. This particular intermediate is more exoergic by 8.3 kcal/mol over Int1SBP. So that our computed results clearly corroborate the experimental observations24 where the involvement of sixcoordinated CoI(OAc) species in the proton reduction mechanism is associated with large thermodynamic stability. The computed reduction potential for CoIIOAc/CoIOAc couple is estimated at −1.710 V and it is quite comparable with experimental value of −1.1 V.24 Furthermore, Int2AP undergoes rapid protonation with a barrier of only 2.0 kcal/ mol, which leads to the formation of Co(III)−hydride species. This particular step is much faster than the direct protonation of Int1SBP by 14.8 kcal/mol. It is clearly consistent with the earlier report by Polyansky et al.13 where they show that slow kinetic step associated with the direct protonation of CoI−VS species in the absence of anions. Prior to protonation of CoI− VS, there may be significant structural reorganization. Upon protonating to give CoIII−H species, the computed innersphere reorganization energies (λin) species is only 0.49 eV whereas λin is 1.02 eV for the protonation of [CoIVS(DPABpy)] →[CoIII−H(DPA-bpy)].13 The low value of λin in the NPY5Me2 ligand scaffold shows that a fast kinetic step is associated with the direct protonation of CoI−VS species when compared with other cobalt-based molecular electrocatalysts such as [CoIII−(H2O)(DPA-bpy)]3+. With their catalyst, there is no significant change observed in the redox behavior upon the addition of anions. But in the case of PY5Me2 scaffold, there is a dramatic change in the kinetic aspects upon addition of acetate anion. Hence, initial displacement step is a ratedetermining step with rapid protonation in the anation pathway. This clearly indicates that anation really plays a pivotal role in reducing the activation barrier of protonation step which leads to form most stable putative intermediate (CoIII−H) which is responsible for hydrogen production. G


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Inorganic Chemistry

Figure 4. (a) Computed free energy profile for proton reduction from Co(III)−hydride intermediates. All energies are reported here in kcal/mol. (b) Transition states of TS1FHEP (Four membered ring) and TS1SHEP (Six membered ring).

This shows that the formation of Co(II)−hydride is thermodynamically more favorable than CoIII−H and Int1HEP. The major distinction between RPP and HEP is mainly originated through enhancing the basicity of cobalt center and thereby it could be less reactive toward proton.13 This is the prime factor to ease the proton reduction through CoII−H rather than CoIII−H species. This particular fact is explained in the next section. Hence the nonelectrochemical thermal activation process (TS1FHEP/SHEP) becomes activation less process. This is in good accordance with earlier theoretical reports and specifically consistent with Mukermann’s finding.28 Also, attempts were made to locate the transition state for bimolecular homolytic cleavage of two CoIII/II−H species (BMHP), but we could not able to identify the particular transition state due to larger size.28 On the basis of the calculated reaction free energies, proton reduction through BMHP is exothermic by −8.8 kcal/mol. Less exothermicity of Int1BMHP clearly portray that there should be a kinetic barrier associated with BMHP. This finding is consistent with earlier finding of Marinescu and co-workers.26 Further, we have also calculated the reaction free energy associated with the homolytic cleavage of two CoII−H species. This step is

with free energy correction. The computed free energy profile is provided in Figure 4a. The heterolytic rupture of the Co(III)−hydride species by acetate ion to evolve hydrogen molecule and Int1HEP is either through six (TS1SHEP) or four membered (TS1FHEP) ring transition states. Among them, TS1SHEP is slightly favorable than (TS1FHEP) by 0.7 kcal/mol. This particular step is exoergic by −10.4 kcal/mol. These transition states are late in nature with the incipient H−H and Co−OAc bond distances of 0.77 (0.80) and 3.04 (2.81) Å respectively (Figure 4b. Whereas, there is no kinetic barrier associated with the direct protonation of reduced cobalt(II) hydride from CoIII−H species through RPP. Calculated reduction potential for CoIIIH/CoIIH couple was found to be −1.301 V. Although there is no direct experimental value for the particular ligand system studied in the present work, but this value is in line with the earlier reported values for other ligand skeletons.13,17,26,28 As well as, the associated reaction free energy for the formation of Co(II)−hydride via one electron reduction of Co(III)−hydride species is exoergic by −64.9 kcal/mol. The formation of Co(II)−hydride species is found to possess larger thermodynamic drive than that of Int1HEP by 54.5 kcal/mol. H


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Inorganic Chemistry

Figure 5. Molecular graph along with QTAIM based topological properties at the bond critical point (BCP). All topological parameters are given in atomic units (au).

bond involves the 52.0% contribution from the Co-dz2 orbital and the remainder from the H-s orbital (Supporting Information, Figure S6). Furthermore, we computed NBO, Wiberg, natural atomic orbital (NAO), and molecular orbital (MO) bond orders for CoIIIH and CoIIH species (Supporting Information, Table S2). The MO bond order for CoIII−H species was found to be 0.6849 and 0.4708 for CoII−H. Among them, MO bond order found to provide the same stability trend between CoIIIH and CoIIH species as we found in AIM and NBO analyses. This clearly illustrates that the CoIII−H σ-bond is stronger than the CoII−H σ-bond. The difference in the strength of the CoIII/II−H bond is further evaluated by consideration of their computed stretching frequencies of the CoIII/II−H bond and respective force constants. The computed unscaled stretching frequency and force constant for CoIII−H bond are 1983.65 cm−1 and 2.37 mdyn Å−1 whereas the CoII−H bond stretch is found at 1791.1 cm−1 with 1.94 mdyn Å−1. This clearly demonstrates that CoIII−H σ-bond is much stronger than the CoII−H σ-bond. On the basis of the QTAIM analysis (Figure 5), the computed electron density ρ(r) value at the CoIII/II−H bcp shows that CoII−H bond is weaker than the CoIII−H bond which is associated with highest ρ(r) value of 0.143 au Less negative L(r) and more positive value of H(r) at bcp between cobalt(III) and hydrogen bond indicate that CoIII−H bond is purely covalent in character. This situation could easily understand by high endothermicity associated with the direct protonation of CoIII−H bond through UMH pathway. Rather, protonation of CoII−H by acetic acid is found to be the barrier-less process. It is exactly pinpointed by earlier theoretical24,26,28 and experimental observations.13,24,53

endothermic by 49.7 kcal/mol. So the bimolecular rupture of two Co(II)−H species is unlikely to occur. In addition to this, we have located the transition state for homolytic cleavage of Co(III)−H species in unimolecular fashion (UMHP) produce 1/2H2 and Co(II)(NCMe) as it is depicted in experimental observations.30 The barrier for UMH pathway is found to be 102.0 kcal/mol and the resulting product is highly endoergic by 113.2 kcal/mol. This shows that proton reduction by homolytic cleavage of two CoIII/II−H species is energetically unfavorable. This is consistent with experimental43,59 and earlier theoretical observations.52,59 Particularly Polyansky et al.13 concluded that evolution of hydrogen is not feasible through homolytic rupture of CoIII/II−hydride species on the account of higher thermodynamic stability of putative CoIII/II− hydride species under electrochemical conditions. On the basis of our theoretical calculations, proton reduction of Co(III)−H involving PY5Me2 scaffold occurs via reduction followed by protonation pathway (RPP) and not through other two possible pathways such as homolytic and heterolytic manner. Stability of CoIII/II−H Bond versus Hydrogen Evolution Pathways. According to computed free energy profile, the most plausible route for hydrogen evolution is one electron reduction and subsequent protonation of CoIII−H species. In this situation, the relative basicity of putative CoIII/II−hydride species along with their thermodynamic stability is vital for generating hydrogen via RPP and other possible pathways (HEP, HP). Owing to the higher thermodynamic stability of putative intermediates, homolytic rupture of two CoIII/II− hydride species could not occur. This is in good agreement with earlier theoretical and experimental predictions.13,53 So, what factor really plays a role in favoring the proton reduction from CoII−H via RPP and not from CoIII−H species through HEP? The diverse reactivity mainly originated from the strength of the CoIII/II−hydride bond. Also, the cobalt metal center has to acquire an umpolung nature where it can have a reversible binding with proton and heterolytic cleavage of Co− H bond. This kind of character is clearly visualized through the NBO and electron densities derived from QTAIM analysis. NBO analysis shows that the CoIII−H σ-bond is found to have 41.5% of Co-dz2 and 58.5% of H-s orbitals while the CoII−H σ-

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CONCLUSIONS Density functional theory calculations were performed to investigate the reaction mechanism of proton reduction involving pentapyridylcobalt(II)complex with a special emphasis on understanding the role of acetate ion on the formation of cobalt(III) hydride species. The computed profile of solvent bound pathway clearly shows that protonation of I


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Inorganic Chemistry

2LCTOI14CHM001) for high-performance computing facilities.

penta-coordinated Co(I)species is found to be the ratedetermining step with respect to Int1SBP, and there is no kinetic barrier for initial dissociation of solvent molecules along with one electron reduction. Upon anation of [Co(PY5Me2)(NCMe)]2+ with acetate ion, this was found to play a significant role in altering the kinetic and thermodynamic aspects of the proton reduction. In contrast to the experimental observations, initial displacement of acetonitrile by acetate ion leads to the formation of Int1AP which is the rate-determining step in the anation pathway. Subsequently, the most favorable one electron reduction accompanied by rapid protonation of CoI−OAc leads to formation of the cobalt(III)−hydride. It is clearly confirmed that anation plays a significant role in changing the kinetic aspects of the cobalt(III)−hydride formation. Hydrogen production through heterolytic cleavage of CoIII−H species (HEP) is accompanied by significant energy barrier whereas there is no kinetic barrier associated with the direct protonation of CoII−H species after the one electron reduction of CoIII−H species (RPP). Similarly, homolytic rupture of CoII/III−H species in bimolecular fashion is not a favorable process owing to the larger thermodynamic stability of CoIII−hydride species. The preference of RPP over other pathways is mainly originated from the stability and bond strength of cobalt−hydride species. The conclusions arrived from computed free energy barrier is further quantified through QTAIM based topological properties and NBO analyses. Our computed results are in good agreement with experimental observations. Through our theoretical study, one could easily understand the kinetic aspects of cobalt based molecular electrocatalyst, and this would provide a proper platform in designing a new catalytic system for proton reduction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00286.

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REFERENCES

Relative energies of all possible spin states, computed bond orders, intrinsic reaction coordinate (IRC) analyses, geometry scan for Co-N bond breaking mechanism, bonding orbital contributions, and Cartesian coordinates of key intermediates and transition states (PDF)

AUTHOR INFORMATION

Corresponding Author

*(M.J.) E-mail: [email protected]. ORCID

Madhavan Jaccob: 0000-0002-2632-4076 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to Prof. Peter Comba for his 65th birthday. M.J. and M.P.S. sincerely thank the Department of Science and Technology (DST), New Delhi, India, for financial support through a DST-Inspire Faculty award (Ref. No. IFA13-CH100]. M.J. also thanks the Loyola College-Times of India (TOI) Major Research Project (Ref. No. J


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