nickel(II) Coordination Complex: an Efficient ... - ACS Publications

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

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Polyoxometalate-Supported Bis(2,2′-bipyridine)mono(aqua)nickel(II) Coordination Complex: an Efficient Electrocatalyst for Water Oxidation Chandani Singh, Subhabrata Mukhopadhyay, and Samar K. Das* School of Chemistry, University of Hyderabad, Hyderabad-500046, India S Supporting Information *

ABSTRACT: A polyoxometalate (POM)-supported nickel(II) coordination complex, [NiII(2,2′-bpy)3]3[{NiII(2,2′bpy)2(H2O)}{HCoIIWVI12O40}]2·3H2O (1; 2,2′-bpy = 2,2′bipyridine), has been synthesized and structurally characterized. We could obtain a relatively better resolved structure from dried crystals of 1, Ni II (2,2′-bpy) 3 ] 3 [{Ni II (2,2′bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (D1). Because the title compound has been characterized with a {NiII(2,2′bpy)2(H2O)}2+ fragment coordinated to the surface of the Keggin anion ([H(CoIIW12O40]5−) via a terminal oxo group of tungsten and the [NiII(2,2′-bpy)3]2+ coordination complex cation sitting as the lattice component in the concerned crystals, the electronic spectroscopy of compound 1 has been described by comparing its electronic spectral features with those of [NiII(2,2′-bpy)2(H2O)Cl]Cl, [NiII(2,2′-bpy)3]Cl2, and K6[CoIIW12O40]·6H2O. Most importantly, compound 1 can function as a heterogeneous and robust electrochemical water oxidation catalyst (WOC). To gain insights into the water oxidation (WO) protocol and to interpret the nature of the active catalyst, diverse electrochemical experiments have been conducted. The mode of action of the WOC during the electrochemical process is accounted for by confirmation that there was no formation/ participation of metal oxide during various controlled experiments. It is found that the title compound acts as a true catalyst that has NiII (coordinated to POM surface) acting as the active catalytic center. It is also found to follow a proton-coupled electrontransfer pathway (two electrons and one proton) for WO catalysis with a high turnover frequency of 18.49 (mol of O2)(mol of NiII)−1 s−1.

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INTRODUCTION

advances in a thermodynamically uphill manner with the involvement/requirement of 4e− and 4H+ (refer to eq 1),

Energy, which we derive from various sources to meet our daily needs, has nonrenewable sources as the major ones. In a recent scenario, the rate of depletion of these sources has accelerated with the threat of an exponential decrease of our nonrenewable sources of energy.1 Thus, finding alternative ways to produce energy from renewable sources has been the central theme of research. One such promising alternative method could be mimicking the water splitting (WS) process that occurs in plants during photosynthesis to convert solar energy into chemical energy.2 A successful WS reaction can generate H2 and O2 from H2O. Importantly, H2 is considered to be one of the most promising alternative sources of conventional fossil fuels. Hydrogen chemical energy can be converted into electrical energy using a fuel cell; it is also the major component of feedstock in many chemical industries (including CO2 recycling via a chemical process, such as methanol synthesis).3 Hence, considering the need to develop a sustainable and green energy source for the future, the discovery of an easy method for H2 production seems to be fundamentally crucial. However, water oxidation (WO) is considered to be the bottleneck process of WS because it © XXXX American Chemical Society

2H 2O → 4H+ + O2 + 4e−,

E° = −1.23 V

(1)

where E° is the standard Nernst potential for WO; here a negative potential signifies the thermodynamic uphill nature of the process. The thermodynamic potential (E) of the half-cell reaction of WO depends on the pH. According to the Nernst equation, the thermodynamic potential for WO can be stated as (from eq 1) E = E° + 0.059pH. Thus, at pH 7, for the process 2H2O → 4H+ + O2 + 4e−, E = −0.817 V (from eq 1). Understanding Nature’s route of WO is important for designing a new catalyst. Until now, researchers have been working to find the chemistry behind the process of photosynthesis.4 During this time frame, scientists contributed substantially by discovering the structure of the oxygenevolving complex (OEC) and providing insight into the pathway involved during WO in photosynthesis.5 In 2011, Received: March 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b00541 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

synthesize cost-effective catalysts, scientists have employed 3d transition metals, where Co, Mn, and Cu have gained more attention. Instead of the high abundance of nickel on earth’s crust and the potential biological importance of its derivatives, very few reports of Ni-based WOC are available.18,19 Especially, molecular catalysts containing Ni2+ as an active catalyst are even less explored.9b−d,19 To the best of our knowledge, the utilization of POM-supported Ni2+-centered (or any other early-transition-metal ion as an active catalytic center) hybrid structures as electrochemical WOCs is rather less explored.20 Hybrid compounds, having metal complexes coordinated to the POM unit,14 are not new to the chemical society. There are quite a few reports where metal complexes are coordinated to [CoW12O40]6− as well as other Keggin POMs,14 but until now, there is only one report where this type of hybrid structure is employed as a WOC.20 In 2003, Liu and co-workers reported a hybrid structure containing [SiW12O40]4− as the Keggin POM to which [Co(2,2′-bpy)2(H2O)]2+ is coordinated, but they did not report any electrochemical studies.14c Later, in 2009, Wang and co-workers reported a hybrid structure using [CoW12O40]6− as the Keggin anion to which the [Zn(2,2′bpy)2(H2O)]2+ complex is supported.14d In this case, ZnII being the d10 system is electrochemically inactive and hence it should not have any role as an electrocatalyst for WO. In 2016, Chen and co-workers reported two different hybrid structures, out of which one has [CoW12O40]6− as the Keggin anion, forming the compound having the formula (Hbipy)2[Co(bipy)2(H2O)4]2(CoW12O40)·2bipy·7H2O, but did not explore any electrochemical properties.14a So, in this context the only report where such a hybrid compound has been utilized as the electrocatalyst is by Wang and co-workers,20 which lacks complete electrochemical analysis for WO catalysis and did not address important issues, such as the robustness of the catalyst under the operational potential for electrochemical WO. Thus, in literature precedents, the report of such a system lacks kinetic and mechanistic studies, which requires significant attention. Herein, for the first time, we have provided a comprehensive study of a new Ni-based inorganic−organic hybrid, [Ni I I (2,2′-bpy) 3 ] 3 [{Ni I I (2,2′-bpy) 2 (H 2 O)}{HCoIIWVI12O40}]2·3H2O (1), as the WOC, which can be truly helpful in providing insights into such systems and developing this class of catalysts further. Compound 1 (with three lattice H2O molecules) has been studied in this work as far as its spectral characterization and electrochemical studies are concerned. However, its crystal structure is not so wellresolved in terms of the data completion and disorder found. More dried crystals (with one lattice water) [NiII(2,2′bpy)3]3[{NiII(2,2′-bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (D1) have been included in this work, mainly in the context of crystallography.

Shen, Kamiya, and their co-workers successfully obtained and solved the crystal structure of OEC involved in photosynthesis, verifying the cubane structure of the Mn4CaO5 unit.6 These findings motivated scientists toward functional mimicking of OEC to advance artificial photosynthesis.7 Over the course of time, not only biomimicking but also issues with the structural stability of the catalyst seemed to be crucial.8 A good WO catalyst (WOC) should be robust under high anodic potential and stable toward oxidation by in situ generated oxygen. Over the last few decades, researchers have explored various homogeneous as well as heterogeneous catalysts, where polyoxometalates (POMs) and metal coordination complexes have evolved as two high potential fields for homogeneous WO catalysis.3,9 In the case of heterogeneous catalysts, metal− organic frameworks (MOFs), metal oxides [e.g., Co(OH)2] supported on the solid surface, in situ generated CoOx, in situ generated cobalt phosphate, POM encapsulated in the ZIF-8 cavity, etc., are a few examples to name.10 POMs are one of the well-established WOCs because of their complete inorganic skeleton and the scope of fast as well as reversible electron transfer.11,3 However, there are only a very few reports where POMs have been found to be stable because they usually show a high tendency toward degradation and form metal oxides (which act as active catalysts) under the electrochemical condition required for WO (i.e., under high anodic potential).3,12 In the case of coordination complexes, the oxidative degradation of organic ligands and the formation of metal oxides are the major drawbacks, which necessitate further advancement.13 To overcome the stability issue, molecular-level grafting of the POM with a coordination complex can be considered to be an effective alternative because the coordinate covalent bond, attached to POM can enhance the mechanical rigidity of the concerned coordination complex and increase the rate of electron transfer. These two issues are crucial regarding the lack of electrochemical stability of unsupported metal complexes during WO. In such hybrid structures, we can also exploit the advantages of both POM and metal coordination complexes.14 This allows possible synergistic interaction between inorganic POM units and attached coordination complex(es), leading to an improved functional behavior. This enhances its potential application in WO catalysis along with other fields of sciences (such as materials sciences, biological sciences, and pharmaceutical sciences).15 Inspired by the recent reports from our group, where a mononuclear Co2+-aqua coordination complex acts as a WOC,10e,16 we envisioned to develop a new catalytic system, where we have the scope to achieve WO with the aid of a mononuclear Ni2+-aqua coordination complex. Here, we have successfully synthesized a Ni2+-based POM-supported complex, in which NiII is bound to a Keggin POM ([CoW12O40]6−) through a terminal O atom of the POM unit. Out of the remaining five coordination sites of NiII, one is occupied by a water (H2O) molecule and two 2,2′-bipyridine (2,2′-bpy) units occupy the other four. The POM coordinated NiII has a distorted octahedral geometry around it. In this context, it should be mentioned that Hill and coworkers have studied intensely various Co(II) as well as other first-row transition metals containing sandwiched-POM compounds and found many of them to act as WOCs.17 Among these, a Co2+-containing sandwiched POM with the formula [Co4(H2O)2(α-PW9O34)2]10− appeared to be a major breakthrough. It was proven to be an efficient and stable true molecular catalyst unlike many other POMs.17b In order to

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RESULTS AND DISCUSSION

Synthesis. Compound 1 was synthesized through a solvothermal method at 160 °C as green block crystals with 56% yield (based on tungsten). The relevant synthesis includes the reaction of K6[CoW12O40]·6H2O (CoW12) with Ni(OAc)2· 4H2O and 2,2′-bpy in an acidic aqueous medium. Thus, during hydrothermal synthesis, the Keggin anion retains its molecular structure, coordinating a {NiII(2,2′-bpy)2(H2O)} complex fragment through a terminal oxo group of the Keggin cluster. Interestingly, the tris(bpy)NiIII coordination complex [Ni(bpy)3]2+ is formed during this hydrothermal synthesis of 1. B


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Inorganic Chemistry Looking at the formula of 1, the following reactions can be proposed during the synthesis of compound 1:

Table 1. Crystal Data and Structure Refinement for Compound D1

5Ni(OAc)2 · 4H 2O + 13(2,2′‐bpy) + 10H+

empirical formula fw temperature/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z ρcalc/(g cm−3) μ/mm−1 F(000) radiation 2θ range for data collection/ deg index ranges reflns collected indep reflns data/restraints/param GOF on F2 final R indexes [I ≥ 2σ(I)] final R indexes (all data) largest diff peak/hole/(e Å−3)

→ 3[Ni II(2,2′‐bpy)3 ]2 + + 2[Ni II(2,2′‐bpy)2 (H 2O)]2 + + 10AcOH + 18H 2O

2[CoIIWVI12O40 ]6 − + 2H+ → 2[HCoIIWVI12O40 ]5 −

(2) (3)

3[Ni II(2,2′‐bpy)3 ]2 + + 2[Ni II(2,2′‐bpy)2 (H 2O)]2 + + 2[HCoIIWVI12O40]5 − + 3H 2O → [Ni II(2,2′‐bpy)3 ]3 [{Ni II(2,2′‐bpy)2 (H 2O)}{HCoIIWVI12O40}]2 ·3H 2O

(4)

Thus, the formation of compound 1 is highly dependent upon the pH of the reaction (see eqs 2 and 3). The detailed synthetic procedure is provided in the Experimental Section. The powder X-ray diffraction (PXRD) pattern, generated from CIF file of single-crystal data of compound 1 (simulated PXRD pattern), matches well with that obtained from the bulk PXRD pattern of the sample (Figure S1). This confirms the purity in the bulk sample. Crystallography. Compounds 1 (with three lattice H2O molecules) and D1 (with one lattice H2O molecule) were characterized through X-ray crystallography at room temperature (293 K). 1 and D1 are essentially identical molecular compounds, differing only in the number of solvent H2O molecules present in their respective crystals. In this work, we have described the crystal structure of compound D1 because of its better crystal data (relatively less disorder). An isomorphous system to D1 is known14f that is differed from D1 in terms of the central element of the Keggin anion and oxidation states of the Keggin constituents. The particulars of structural refinement and data collection of compound D1 are provided in Table 1. Selected bond distances and bond angles of compound D1 are provided in Table S1. Compound D1 crystallizes in the monoclinic unit cell with the C2/c space group. The crystal structure shows that the [CoIIW12O40]6− cluster unit is attached to the [NiII(2,2′-bpy)2(H2O)]2+ complex moiety via coordination of the terminal oxo group of the Keggin anion to NiII ion. The remaining five coordination sites of the Ni II center in {Ni II (2,2′bpy)2(H2O)}2+ are occupied by coordinating to two 2,2′-bpy units and one H2O molecule. This coordinated H2O molecule plays an important role as far as the catalytic activity of this compound is concerned (vide infra). The concerned asymmetric unit contains one POM cluster unit coordinating bis(bpy)(aqua)NiII species, [{NiII(2,2′bpy)2(H2O)}{HCoIIWVI12O40}]3−, 1.5 units of [NiII(2,2′bpy)3]2+, and 0.5 solvent H2O molecules, thereby bringing about the formula of compound D1 as [Ni II (2,2′bpy)3]3[{NiII(2,2′-bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (Figure 1). In other words, in this system, three discrete units of [NiII(2,2′-bpy)3]2+ complexes are shared between two adjacent unit cells. As far as the asymmetric unit is concerned, the CoIIcentered Keggin POM cluster has six negative charges (6−) and the summation of the positive charges of 1.5 [NiII(2,2′bpy)3]2+ and one coordinated {NiII(2,2′-bpy)2(H2O)}2+ is 5+. Thus, in order to fulfill the electroneutrality of the overall system (compound 1 or D1), we have proposed the presence

C130H104N26O88.20Co2Ni5W24 8265.04 298 monoclinic C2/c 46.624(4) 14.3807(11) 26.2047(18) 90 90.079(2) 90 17570(2) 4 3.1244 16.442 14966.1 Mo Kα (λ = 0.71073) 4.96−52.82 −58 ≤ h ≤ 58, −17 ≤ k ≤ 17, −32 ≤ l ≤ 32 77175 17883 [Rint = 0.0294, Rσ = 0.0243] 17883/12/1034 1.069 R1 = 0.0364, wR2 = 0.0807 R1 = 0.0451, wR2 = 0.0876 3.34/−3.04

Figure 1. ORTEP diagram of the asymmetric unit of compound D1 (H atoms are omitted for clarity). Thermal ellipsoids are set to the 50% probability level.

of a proton along with the POM cluster anion (see the formulas of compounds 1 and D1). The 2+ oxidation state of the Co center of the Keggin POM anion is consistent with the bondvalence-sum (BVS) calculation. According to this BVS calculation, interatomic distances of “Co−O” suggest the valence of Co to be 2 (segment 2.2 of section S2 and Table S2). Spectroscopy. The Fourier transform infrared (FTIR) spectra of 2,2′-bpy and K6[CoW12O40]·6H2O (POM) have been recorded separately to correlate with the FTIR spectrum of compound 1 (Figures S2 and S3). The following peaks were assigned to the frequencies (in cm−1) that are characteristic for the bpy unit, as reported in the literature:21 1591, 1562, 1462, 1430, 1309, 1247, 1019, and 775. The other main stretchings C


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Inorganic Chemistry

Figure 2. (a) K−M-converted UV−vis DRS spectra of 1 and K6[CoW12O40]. (b) K−M-converted UV−vis DRS spectra of 1 compared with those of [Ni(bpy)3]Cl2 and [Ni(bpy)2(H2O)Cl]Cl coordination complexes.

arising from the POM unit are as follows:14d 940, 865, 750, and 450 (Figures S2 and S3). As shown in the IR spectra of 2,2′-bpy and K6[CoW12O40]·6H2O themselves, there is a sharp and medium peak at around 780 cm−1 for bipyridine only and there a broad IR band at around 770 cm−1 for the Keggin compound K6[CoW12O40]·6H2O only. Compound 1 shows a broad and strong peak at around 777 cm−1. Thus, the broad and strong IR peak appearing at around 777 cm−1 in the IR spectrum of compound 1 can be assigned to the combination of W−O−W fragments and bipyridine, where the broad IR band of the Keggin cluster masks the sharp IR peak of bipyridine (see also section S3). The electronic absorption spectrum of compound 1 is converted into a Kubelka−Munk (K−M)-derived plot. The spectrum thus obtained for 1 is compared with those of K6[CoW12O40]·6H2O (CoW12), [NiII(2,2′-bpy)2(H2O)Cl]Cl, and [NiII(2,2′-bpy)3]Cl2. All of the spectra were recorded in the solid-state diffuse-reflectance spectroscopy (DRS) mode and were K−M-converted for the sake of comparison. As shown in Figure 2a, a direct comparison with CoW12 shows that, in the case of 1, transitions in the near-IR range (1000−1200 nm) are due to charge transfer from e(Co) to t2(W).22 Again, this study shows a blue shift of 12 nm compared to that of CoW12, which can be attributed to the changes in the energy levels of e(Co) and t2(W). This change in the electronic energy level is expected as a result of structural modification of the CoW12 unit in 1 (Figure 1). In the 700−1000 nm range (Figure 2), a broad peak is seen in the DRS spectrum of 1, which is comparable to that found in the UV−vis DRS spectrum of [NiII(2,2′-bpy)2(H2O)Cl]Cl, as shown in Figure 2.23 This supports the presence of a {NiII(2,2′-bpy)2(H2O)}2+ unit in the title compound 1. In the visible range of the spectrum of hybrid 1, a broad peak at 630 nm and two fine shoulders at 592 and 560 nm are observed (Figure 2) that are characteristic transitions of tetrahedral Co2+: (4A2 → 4T1), t2(Cotd) → e(W), and e(Cotd) → e(W), respectively.22a In this range, a d−d transition (which is Laporte-forbidden) is expected because of Ni2+ of {NiII(2,2′bpy) 2(H2 O)}2+ (comparing the electronic spectrum of [NiII(2,2′-bpy)2(H2O)Cl]Cl; Figure 2b), but it is masked by the d−d transition of Co2+ of [CoW12O40]6−, which is Laporteallowed (because of the tetrahedral geometry at CoII).23 At around 408 nm in the DRS spectrum of 1, a broad shoulder is observed (Figure 2) because of the d−d transition arising from

[NiII(2,2′-bpy)3]2+ units (compared to the DRS spectrum of [NiII(2,2′-bpy)3]Cl2; Figure 2b).24 In the UV region of the DRS spectrum of 1 (below 300 nm), peaks are mainly due to O2p → W5d charge transfer.22a Apart from this, peaks are seen in the same range (below 300 nm) and are mainly due to metalto-ligand charge transfer of the [NiII(2,2′-bpy)3]Cl2 unit and the π−π* transition of the bpy ligand.24 Thus, the absorption spectrum of 1 is consistent with the structure, as obtained by single-crystal X-ray crystallography. The Co 2p3/2 and Co 2p1/2 energy levels in compound 1 have binding energies of 781.08 and 796.84 eV, respectively. The energy difference between these energy levels is 14.6 eV, which supports the oxidation state of Co as 2+ (Figure S4).25 In the case of W, a shift of 0.7 and 0.8 eV has been observed for W 4f5/2 and W 4f7/2, respectively, in comparison with those of CoW12. This shift in the binding energy can be attributed to the changes in the electronic level of W in 1 arising as a result of coordination of the concerned Keggin POM to Ni through its WO bond (Figure S5). The thermal stability of the hybrid compound was studied by thermogravimetry analysis (TGA). Compound 1 was taken for thermal studies without prior activation in high-temperature vacuum. Initially, with an increase in the temperature (T ≤ 400 °C), a steady slope for weight loss is observed (Figure S7). This decrease in the weight loss of the compound can be due to the presence of a lattice H2O molecule in 1. Degradation of the organic linker leads to a weight loss of 28.2% (calcd 28.8%) at a temperature of 490 °C. The POM unit in 1 starts to degrade at higher temperature (T > 600 °C). This leads to a weight loss of 35.5% (calcd 35.9%). The elemental composition on the surface was also studied with energy-dispersive X-ray (EDX) elemental mapping analysis (Figure S8). All of the elements (Co, Ni, W, C, N, and O) were detected on the surface by EDX techniques with weight percentages that match well with the formula obtained from the crystal structure (Table S3). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of 1 confirmed the presence of Co (calcd, 1.43%; found, 1.40%) and Ni (calcd, 3.57%; found, 3.50%). The empirical formula obtained from ICP-AES is [(CoW11.8O39.8NiC20N4H16)· 1.5NiC30H24N26·H2O]. The ICP-AES report is provided in Figure S9. Electrocatalytic WO. Cyclic voltammograms of compound 1 were recorded in a 0.1 M phosphate buffer having pH 7 D


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Inorganic Chemistry

Figure 3. (a) Cyclic voltammogram of compound 1 in a 0.1 M phosphate buffer (pH 7). (b) Differential-pulse voltammogram of compound 1 in a 0.1 M phosphate buffer (pH 7). (c) Differential-pulse (black ) and cyclic (red ) voltammograms of compound 1 showing two peaks. Cyclic and differential-pulse voltammograms of 1 were obtained using a 0.1 M phosphate buffer (pH 7) as the supporting electrolyte after coating the sample on a glassy carbon electrode (3 mm diameter). See the text for assignments. (d) Cyclic voltammograms of CoW12 recorded from a 1 mM solution of CoW12 prepared in 0.1 M Na2SO4 (pH 7; black ) and of a blank glassy carbon electrode in 0.1 M Na2SO4 (pH 7; red ).

accompanied by simultaneous reduction of the high-valent Ni species back to NiIII species. Furthermore, on the reduction side, a broad peak C1 is present. This broad peak in the cyclic voltammogram on the reduction side can be a result of the close proximity of the reduction peaks of CoIII → CoII and NiIII → NiII reduction.9c,26 The shift in the peak position of the CoIII/CoII couple in compound 1 with respect to that of CoW12 as such, is observed. This shift in peak position can be assumed to be due to a change in the distribution of the electron density in the CoW12 unit due to sharing of a terminal O atom of CoW12 to the bis(bpy)(aqua)NiII unit. This electronic redistribution causes a change in the electronic environment of the bis(bpy)(aqua)NiII unit as well. The NiIII → NiII peak is shifted to 1.1 V in compound 1 from 1.39 V of [NiIIbis(bpy)(H2O)Cl]Cl (Figure S26) as a result of enhancement of the electronic density around the active catalytic NiII center of the bis(bpy)(aqua)NiII unit. Stability of the catalyst (compound 1) was observed by performing constant-potential electrolysis (CPE) measurements at the onset potential (1.23 V) for 20 h and an accelerated durability test of 1000 cycles of CV scans. For CPE, the change in the current was negligible over a period of 20 h (Figure 4a). CV was recorded before and after CPE measurement for a coated sample of 1. Negligible changes in the WO onset current (ca. 20 μA) and no alteration in the onset potential were observed in CV recorded after CPE measurement of 20 h (Figure 4b). The initial and final cycles of 1000 cycles of CV scans were also in agreement with CPE observation because it showed imperceptible changes in the catalytic current (Figure 4c). This negligible loss of the catalytic

(unless specified otherwise). The cyclic voltammogram of 1 shows a broad oxidation peak A1 at 1.01 V and a reduction peak C1 at 0.91 V (Figure 3a). Differential-pulse voltammetry (DPV) was used to understand the redox phenomenon with higher precision that takes place during the anodic scan. DPV was carried out with a step width and a step height of 100 ms and 5 mV, respectively, and a pulse width and a pulse height of 200 ms and 25 mV, respectively. From DPV measurements, two distinct peaks were found for oxidation at 0.99 V (A1′) and 1.1 V (A2′) (Figure 3b,c). This observation was correlated with the cyclic voltammogram of CoW12, which was recorded in 0.1 M Na2SO4 (because CoW12 is unstable in a phosphate buffer) having pH 7 by dissolving 1 mmol of CoW12 in it. A quasireversible redox couple (A2/C2) was observed (Figure 3d). The oxidation peak A2 represents the oxidation of CoII → CoIII at 0.95 V, and the reduction peak C2 represents the reduction of CoIII → CoII at 0.83 V. Cyclic voltammetric (CV) features of CoW12 provide an approximate guideline to assign the CoIII/ CoII peak in the cyclic voltammogram of compound 1. Similarities between the peak positions of the cyclic voltammograms of CoW12 and compound 1 suggest that the oxidative peak A1′ in the cyclic voltammogram of compound 1 can be logically assumed to be the result of CoII → CoIII oxidation of the CoW12 unit and peak A2′ of NiII → NiIII oxidation. As a result of the surge of current (due to liberation of 2e− from each H2O molecule during catalytic WO) at higher potential, peaks for the further oxidation of NiIII could not be observed in the cyclic voltammogram of 1. With the application of the potential, once the reactive high-valent Ni species is formed, it catalyzes the oxidation of H2O molecules. This catalytic WO is E


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Inorganic Chemistry

Figure 4. (a) CPE of 1 recorded at 1.23 V for 20 h in 0.1 M phosphate buffer (pH 7). (b) Cyclic voltammograms of 1 recorded before (black ) and after (red ) CPE measurements in a 0.1 M phosphate buffer (pH 7). Inset: Enlarged region of the same voltammograms showing 20 μA catalytic current differences in the cyclic voltammograms before and after CPE was performed. (c) Stability check of 1 by 1000 cycles of CV scans in a 0.1 M phosphate buffer (pH 7) representing the (red ) 2nd and (black ) 1000th cycles of the CV scan of 1. Inset: Enlarged region of the same voltammograms showing only a 58 μA difference in the catalytic current.

H2O molecule to perform catalytic WO. Involvement of the central CoII of the POM unit as a catalytic active center would probably lead to degradation of the concerned POM unit, thereby collapsing the whole structure.12 The [Ni(bpy)3]2+ unit, present in the crystal lattice, if participates in catalytic WO, would convert into NiOx. Because of its coordination to the rigid ligand sphere forming an octahedral geometry, it cannot bind to water molecule. In order to negate the possibility of the Keggin POM unit to function as the catalytic center in the present work, we recorded CV of CoW12 (1 mmol in an aqueous homogeneous medium) at a neutral pH using a phosphate buffer (0.1 M). The formation of a precipitate was observed in the cell prior to the application of any external potential. CPE measurements were conducted for 8 h using the same cell without any changes. After the completion of CPE measurements, the working electrode (FTO electrode) was taken out and slowly washed with Milli-Q water. A dark deposit was observed on the surface of the working electrode after CPE measurement for 8 h. The chemical composition of this coating, obtained from the degradation of CoW12, is confirmed to be composed of Co, P, and O atoms from EDX elemental analysis and, henceforth, it is referred to as “CoPxOy generated from CoW12” (Figure S13). Fresh CV and CPE measurements were recorded for 6 h with this deposited moiety using conditions (0.1 M phosphate buffer at pH 7) similar to those used for the CV of 1. The nature of the CV obtained from this differs in terms of the peak potential, peak current, and onset potential from that obtained for compound 1 (Figure 5a). CPE for “CoPxOy generated from CoW12” shows a steady increase in the current with time (Figure 5b). This nature of CPE confirms the lack of stability in

current, observed in both 1000 cycles of CV scans as well as CV recorded before and after 20 h of CPE measurements, is a result of the loss of sample from the electrode surface due to oxygen bubble evolution. In order to check the leaching of Co2+/Ni2+ ions in electrolytes, ICP-AES analysis was carried out for the electrolyte after CPE measurements. ICP-AES data of the electrolyte, collected after 8 h of CPE measurements of 1, confirm no leaching of Co2+ and Ni2+, thus making it evident that compound 1 is a true catalyst and does not degrade during WO catalysis (Figure S12). For a further validation of the results obtained so far, we recorded PXRD and field-emission scanning electron microscopy (FESEM) images and EDX elemental analysis for the coated sample of 1 [sample coated on a fluorine-doped tin oxide (FTO) electrode] before and after 1000 cycles of CV scans (Figures S11 and S14) were recorded. All of these observations of controlled experiments provide convincing information about the robustness and stability of compound 1 as a WOC. Finding the Actual Active Site in Catalyst 1. The crystal structure of compound 1 confirms the presence of three types of metal ions, which may catalyze electrochemical WO. These incude NiII from two different sources, i.e., [Ni(bpy)2H2O]2+ and [Ni(bpy)3]2+, and the CoII center from the POM unit. The mechanism of WO for a mononuclear Co2+ complex is well established. The requirement to form a CoIV intermediate in the case of a molecular Co-based WOC has been validated by Nocera and co-workers27a and Berlinguette and co-workers,27b a few of the pioneers in this field. Because of the structural restriction for the central Co2+ (tetrahedral CoO4 unit caged by 12 WO3 units of the Keggin POM) in 1, it cannot bind to the F


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Inorganic Chemistry

Figure 5. (a) Cyclic voltammograms of CoPxOy generated from CoW12 and compound 1 recorded in a 0.1 M phosphate buffer (pH 7). (b) CPE of the sample obtained after 500 cycles of CV scans by preparing 1 mM CoW12 in a 0.1 M phosphate buffer (pH 7) for 6 h at 1.3 V, denoted by a black donut symbol and labeled as CoPxOy obtained from CoW12. Similarly for NiCl2 salt, we obtained a coating on the FTO electrode, which was subjected to CPE at 1.3 V for 6 h, denoted by NiPxOy (red donut symbol). (c) Cyclic voltammograms of NiPxOy generated from NiCl2 and of compound 1 in a 0.1 M phosphate buffer (pH 7).

“CoPxOy generated from CoW12” and the generation of more active species during this time course. It is noteworthy that the active species generated here loses its catalytic activity after some time. These observations do not have any similarities with the electrochemical properties of compound 1. This confirms that there is no participation of either CoW12 as such or any degraded product formed from CoW12 while using 1 as the electrochemical WOC. In order to find out whether any participation is coming from the degraded product of the NiII-containing units ({NiII(2,2′bpy)2(H2O)]2+ and [NiII(2,2′-bpy)3]2+), we conducted the following controlled experiments: (i) We have synthesized [NiII(2,2′-bpy)2(H2O)Cl]Cl and [NiII(2,2′-bpy)3]Cl2 following the reported protocols21 and conducted CPE and CV for both of them (Figures S26−S29). (ii) A 1 mmol solution of NiCl2 was prepared in 0.1 M KOH and subjected to a high anodic potential (0−1.6 V) for 200 cycles of CV. These CV scans were recorded with FTO as the working electrode. The application of such a high anodic potential during CV is known to form NiOx from NiCl2 salt (a detailed synthetic protocol is provided in section S10).9c The formation of NiOx was confirmed by EDX elemental mapping (Figure S16). In a similar fashion, CoOx was synthesized by using 1 mmol of CoCl2 instead of NiCl2 and characterized (Figure S15). For these species, generated electrochemically on

FTO, CV measurements were carried out in 0.1 M Na2SO4 at pH 7. A iR (cell resistance)-corrected Tafel plot was also constructed for each of these compounds to have an idea about their WO kinetics. (iii) NiPxOy was developed by preparing 1 mmol of NiCl2 in a 0.1 M phosphate buffer having pH 10 and applying a high anodic potential (0−1.6 V). A dark coating formed on the FTO electrode (detailed synthetic protocol is provided in section S10). Prior to indulging in any electrochemical study by using this coating, it was washed slowly with Milli-Q water. The formation of NiPxOy on FTO was confirmed by EDX elemental mapping (Figure S18). Using the same protocol, CoPxOy was synthesized and characterized (Figure S17). Modification of the procedure was just to replace NiCl2 by CoCl2. For these species generated electrochemically on FTO, CV was recorded and a iR-corrected Tafel plot was constructed in galvanostatic mode in a 0.1 M phosphate buffer (pH 7). For NiPxOy, CPE was conducted at 1.3 V for 6 h. FESEM images of all of these electrochemically generated species (oxides and phosphates) were taken and compared with the coated sample of compound 1 obtained after 1000 cycles of CV scans on the FTO electrode (Figures S13 and S15−S18). Observations Obtained from These Experiments and Our Rationale. (i) CPE was recorded for 1 mmol of solution of both [NiII(2,2′-bpy)2(H2O)Cl]Cl and [NiII(2,2′-bpy)3]Cl2 G


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Inorganic Chemistry

chemical measurements (i.e., either a large number of CV scans or CPE analyses) were compared with FESEM images and EDX mapping of the coated sample of 1 after 8 h of CPE (Figures S13 and S15−S18). They were found to be completely different in terms of their morphologies as well as compositions (Tables S4−S7). Thus, it can be concluded that synergistic interaction between the Ni center of the coordinated [NiII(2,2′bpy)2(H2O)]2+ and the CoW12 Keggin POM helps the former to show its WOC activity and provides the later with extra stability. After confirming the true catalytic center/active metal site in compound 1 for its WOC activity, we studied the mechanistic insights. Thus, an iR-corrected Tafel plot was devised with 0.1 M phosphate buffer in galvanostatic mode. An overpotential (η) of 475.6 mV was observed for 1 at a current density of 1 mA cm−2 (Figure 6), and a high turnover frequency (TOF) of 18.49 s−1 was calculated. Calculation details of the TOF are provided in section S15. It can be mentioned, in this context, that Yao, Zhang, and coworkers20 reported in 2016 four similar hybrids with four different Keggin POMs containing the [Ni(bpy)2H2O]2+ unit. Three of the four hybrids have a structural resemblance with 1 (present work); they only differ in the POM unit. They have reported WO in a basic medium using a borate buffer (pH 8), with the onset potential greater than 1.34 V (vs NHE) and the η value being greater than 590 mV for all three cases.20 This work20 lacks detailed electrochemical analysis (for example, Tafel plots, CPE, etc.), thereby TOFs for the electrochemical WO. On the other hand, in the present study, WO by 1 in a phosphate buffer (pH 7) is observed to be highly efficient, with a high TOF [18.49 (mol of O2)(mol of NiII)−1 s−1] as mentioned earlier. The required overpotential for a current of 1 mA cm−2 is also found to be less (475.6 mV) in our case. We should also mention that, in the present work (compound 1), the onset potential for catalytic WO is found to be 0.14 V less anodically shifted in comparison to similar compounds reported by Wang et al.20 These beneficial observations, in the present study, can be explained by considering the synergistic interaction between the NiII center of {NiII(2,2′bpy)2(H2O)}2+ and the CoW12 POM unit in 1, which facilitates the speed and ease of electron transfer in the concerned electrochemical processes. To deduce the mechanism of WO, a CV study was conducted in a wide pH range (acidic and neutral pH; Figure 7a). A derived plot was constructed from these cyclic voltammograms of 1 obtained by varying the pH, where the required potential values for a fixed current density of 0.76 mA cm−2 is plotted against varied pH values. The reason behind the choice of the large current density is to negate the interference of the diffusional peak current. A linear plot is obtained over the pH range 3−7 with a slope of −27.4 mV pH−1 (Figure 7b). This suggests that the present system 1 catalyzes WO via a proton-coupled electron-transfer pathway where two electrons and one proton are involved.9c The ratedetermining step is confirmed by the changes observed in the normalized current density (j/v1/2) with variation of the scan rate. With increasing scan rate, the normalized current density is found to decrease (Figure S25). This infers a chemical process, i.e., the formation of a O−O bond as the rate-limiting step.28 The Faradaic efficiency of 1 as the WOC was calculated by performing constant-current electrolysis in a 0.1 M phosphate buffer (pH 7). A home-built volumetric setup was used to measure the oxygen evolved per hour by separating the evolved gas mixture (Figure S31). From the result observed in

separately in a 0.1 M phosphate buffer (pH 7) for 6 h at 1.35 and 1.4 V, respectively (Figures S28 and S29). From CPE of [Ni(bpy)2(H2O)Cl]Cl and [Ni(bpy)3]Cl2, the current was found to be increasing with time (Figures S28 and S29). This nature of CPE infers the instability of [Ni(bpy)2(H2O)Cl]Cl and [Ni(bpy)3]Cl2 under operational conditions, which is mainly due to structural changes during a high anodic potential in both cases. In the cyclic voltammograms of both these complexes, an irreversible peak at high anodic potential (1.39 and 1.55 V for bis and tris complexes, respectively) was observed with an even higher catalytic current onset following the peak (Figures S26 and S27). The features obtained from CPE and CV measurements of both compounds suggest the irreversible conversion of NiOx from the parent compounds, which was not the case for compound 1 (as mentioned earlier, a stable CPE was obtained for 1, which showed a complete reversible response in terms of the peak current, peak potential, onset potential, and onset current). (ii) The peak and onset potentials for NiOx, CoOx, NiPxOy, and CoPxOy in the cyclic voltammograms recorded vary prominently from that of compound 1 (Figures 5c and S19− S22). Because none of the cyclic voltammograms recorded (as mentioned above) is equivalent to that obtained from compound 1 (in terms of CV features), we can argue that these species are not formed during the electrochemical activity of compound 1 as a WOC. CPE at 1.3 V for 6 h conducted for NiPxOy shows the increasing nature of the current with time, which unveils the unstable nature of NiPxOy under operational electrochemical conditions (Figure 5b). To trail the kinetics followed by all of these species, a Tafel plot was constructed for each of them, and the slope so obtained from the plot was compared with that obtained from compound 1 (Figures 6 and S23). The slopes of the Tafel plots

Figure 6. Galvanostatic iR-corrected Tafel plot of 1 at pH 7 (0.1 M phosphate buffer).

for all of these species, including that for compound 1, are different from each other. To be precise, the Tafel slopes of NiOx, NiPxOy, CoOx, CoPxOy, and compound 1 were found to be 228.3, 331.03, 377.17, 60.02, and 168.41 mV decade−1, respectively (Table S8). This could be elucidating the different kinetics followed by each of these species, which thereby support our claim of compound 1 as the true catalyst. (iii) FESEM images along with EDX elemental mapping recorded from NiPxOy-, CoPxOy-, CoOx-, and NiOx-modified FTO electrodes (each separately) after long-term electroH


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Inorganic Chemistry

Figure 7. (a) Cyclic voltammograms of 1 in a 0.1 M phosphate buffer by varying pH. A fresh coating of the sample (compound 1) was used before running CV in each pH. (b) Derived plot of the potential (at j = 0.76 mA cm−2) versus pH for compound 1. °C for 5 days under solvothermal conditions. It was cooled to room temperature (34 °C) in 24 h to obtain block-shaped green crystals. Yield: 56% (based on W). Anal. Calcd for C130H104Co2N26Ni5O85W24: C, 19.01; H, 1.28; N, 4.43; Co, 1.43; Ni, 3.57; W, 53.72. Found: C, 20.36; H, 1.41; N, 4.65; Co, 1.40 (ICP); Ni, 3.50 (ICP), W, 53.03 (ICP) (see sections S9 and S10). The synthesized compound was characterized by single-crystal X-ray diffraction, PXRD, FTIR, UV−vis, FESEM imaging techniques, X-ray photoelectron spectroscopy, TGA, and electrochemistry. Crystal Data Collection. Single crystals suitable for the structural determination of compounds D1 and 1 were mounted on a threecircle Bruker SMART APEX CCD area detector system under a Mo Kα (λ = 0.71073 Å) graphite-monochromated X-ray beam with a crystal-to-detector distance of 60 mm, and a collimator of 0.5 mm width (at 273 K temperature). The scans were recorded with a ω scan width of 0.3°. Data reduction was performed by SAINT PLUS,30 and empirical absorption corrections using equivalent reflections were performed by the program SADABS.31 Structure solutions were done using SHELXS-97,32 and full-matrix least-squares refinement was carried out using SHELXL-97.33All of the non-H atoms were refined anisotropically. H atoms on the C atoms were introduced on calculated positions and included in the refinement, riding on their respective parent atoms. The crystal data and structural refinement parameters for compounds D1 and 1 are summarized in Tables 1 and S9, respectively. Selected bond lengths and bond angles for compounds D1 and 1 are summarized in Tables S1 and S10, respectively. CCDC 1838003 (compound D1) and CCDC 1826902 (compound 1) contain the supplementary crystallographic data for this paper. The actual molecular formula of D1 and the crystallographic value differ by 5.2 O atoms (see Table below), which is the contribution from two clusters. The excess electron density observed was disordered, as exhibited by the whole {CoW12O40} cluster. We are unable to model this excess density within the CoW12O40 cluster. Hence, these are refined as an isotropic model and included in the CIF file. However, this does not indicate the presence of extra atoms in the molecular formula. The molecular formula is also confirmed by other characterization techniques.

this experiment, the Faradaic efficiency of 1 has been calculated to be 88.7%. The method for calculation of the Faradaic efficiency is explained in section S13.

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CONCLUSION To conclude, we are able to synthesize a new hybrid compound 1 containing POM as the inorganic unit and 2,2′-bpy as the organic unit linked together with a NiII metal ion. This is the first report of such a class of compounds where insights of WO during the electrochemical process are explained by conf irming no formation of metal oxide during the process. NiII from the {NiII(2,2′-bpy)2(H2O)} unit, attached to POM cluster, acts as a catalytically active center. The title compound is found to be highly stable and robust under the electrochemical conditions required for WO at pH 7. It shows a moderate value of the overpotential of 475.6 mV accompanied by a very high TOF of 18.49 s−1. The catalytic cycle proceeds with O−O bond formation as the rate-limiting step. Overall, this work has provided a glimpse that a “wide-range” class of POM-supported transition-metal complexes with (at least) one metal-coordinated H2O molecule (essential for WO), for example, compound [Ni II (2,2′-bpy) 3 ] 3 [{Ni II (2,2′-bpy) 2 (H 2 O)}{HCoIIWVI12O40}]2·3H2O (1) in the present work, have the potential to act as electrocatalysts for efficient WO.

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EXPERIMENTAL SECTION

General Materials and Methods. The starting materials were purchased as analytical grade and used as received. PXRD patterns were recorded on a Bruker D8-Advance diffractrometer using graphitemonochromated Cu Kα1 (1.5406 Å) and Kα2 (1.55439 Å) radiation. IR spectra of solid samples were obtained as KBr pellets on a JASCO 5300 FTIR spectrophotometer. UV−vis DRS electronic absorption spectra were recorded using a Shimadzu-2600 spectrophotometer. TGA was carried out on a STA 409 PC analyzer. FESEM imaging with EDX spectroscopy was carried out on a Carl Zeiss model Ultra 55 microscope, and EDX spectra and maps were recorded using an Oxford Instruments X-MaxN SDD (50 mm2) system and INCA analysis software. Synthesis of 1. The Keggin compound K6[CoW12O40]·6H2O (POM) was synthesized using the reported protocol.29 This synthesized and characterized POM was used as such for the synthesis of title compound [Ni II (2,2′-bpy) 3 ] 3 [{Ni II (2,2′-bpy) 2 (H 2 O)}{HCoIIWVI12O40}]2·3H2O. A mixture of Ni(OAc)2·4H2O (71 mg, 0.28 mmol), 2,2′-bpy (114 mg, 0.73 mmol), and K6[CoW12O40]·6H2O (32 mg, 0.01 mmol) in 3 mL of methanol and 2 mL of water, pH 2 [using acetic acid (AcOH; 50% w/v)], was placed in a 23 mL Teflonlined reactor and stirred for 45 min. The solution was heated to 160

Electrochemical Studies. All electrochemical experiments were conducted using a Zahner Zanium electrochemical workstation operated with Thales software. Complete electrochemical experiments were accomplished using a three-electrode electrochemical cell with compound 1 modified glassy carbon as the working electrode, homemade Ag/AgCl (1 M) as the reference electrode, and a Pt flag as the counter electrode in acidic and neutral pH in an aqueous medium. I


Inorganic Chemistry

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For a basic medium, homemade Hg/HgO (0.1 M KOH) was used as the reference electrode, while the other two electrodes were the same as mentioned above. For the preparation of a 1-modified electrode, 4 mg of 1 and 1 mg of acetylene black carbon were placed in 1 mL of a 3:2 ethanol/H2O mixture, and to it was added 10 μL of 5 wt % Nafion (aqueous). The mixture was kept under sonication for 30 min to make a homogeneous suspension. Then 20 μL of this mixture was coated on a 3-mm-diameter glassy carbon electrode (geometrical area = 0.0706 cm2), resulting in essentially 40 μg of sample (compound 1) in each coating on the glassy carbon electrode. The same amount on the electrode surface was maintained for all electrochemical experiments unless otherwise mentioned. In the case of FTO as the working electrode, an area of 0.07 cm2 was used to coat the sample or to generate electrochemically the oxides/phosphates on it. The coating mixture on the electrode was dried under an IR lamp (temperature ∼70 °C) prior to use. All of the electrochemical experiments were performed at an ambient temperature. Electrode potentials were converted to the normal hydrogen electrode (NHE) scale using the relationship E(NHE) = E(Ag/AgCl) + 0.1263 V when Ag/AgCl was used as the reference electrode and E(NHE) = E(Hg/HgO) + 0.143 V for Hg/HgO as the reference electrode. CV scans were initiated at the open-circuit potential, and the anodic side was scanned first, followed by the cathodic scan. Five cycles were taken consecutively for each set of CV measurements in a quiescent solution. Cyclic voltammograms were also recorded at various scan rates.

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REFERENCES

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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.8b00541. Figures S1−S31 (PXRD plots, FTIR spectra, TGA plot, FESEM images, figures related to electrochemical work), Tables S1−S7 (elemental mapping data from EDX), calculation for the TOF, Faradaic efficiency, and experimental details of the quantitative determination of evolved O2 (PDF) Accession Codes

CCDC 1826902 and 1838003 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or samar439@gmail. ORCID

Chandani Singh: 0000-0001-9913-0304 Samar K. Das: 0000-0002-9536-6579 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SERB, DST, Government of India (Project EMR/ 2017/002971) for financial support. C.S. thanks UGC-BSR for a fellowship, and S.M. thanks DST-INSPIRE, New Delhi, India. The authors also thank to Dr. Joyashish Debgupta for help with electrochemical analysis and Dr. Sateesh Kurapati for help with crystal structure determination. We are also thankful to DST PURSE, UPE-II, and UGC-CAS. We are grateful to Dr. Bharat Kumar Tripuramallu for helping us with crystal structure analysis. J


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