Computational Approach to Understanding the Electrocatalytic Reaction Mechanism for the Process of Electrochemical Oxidation of Nitrite at a Ni-Co-Based Heterometallo-Supramolecular Polymer.

Here, we report a semiempirical quantum chemistry computational approach to understanding the electrocatalytic reaction mechanism (ERM) of a metallic supramolecular polymer (SMP) with nitrite through UV/vis spectral simulations of SMP with different metal oxidation states before and after interactions with nitrite. In one of our recent works, by analyzing the electrochemical experimental data, we showed that computational cyclic voltammetry simulation (CCVS) can be used to predict the possible ERM of heterometallo-SMP (HMSMP) during electrochemical oxidation of nitrite (Islam T.ACS Appl. Polym. Mater.2020, 2( (2), ), 273-284). However, CCVS cannot predict how the ERM happens at the molecular level. Thus, in this work, we simulated the interactions between the repeating unit (RU) of the HMSMP polyNiCo and nitrite to understand how the oxidation process took place at the molecular level. The RU for studying the ERM was confirmed through comparing the simulated UV/vis and IR spectra with the experimental spectra. Then, the simulations between the RU of the polyNiCo and various species of nitrite were done for gaining insights into the ERM. The simulations revealed that the first electron transfer (ET) occurred through coordination of NO2 - with either of the metal centers during the two-electron-transfer oxidation of nitrite, while the second ET followed a ligand-ligand charge transfer (LLCT) and metal-ligand charge transfer (MLCT) pathway between the NO2 species and the RU. This ET pathway has been proposed by analyzing the transition states (TSs), simulated UV/vis spectra, energy of the optimized systems, and highest occupied molecular orbital-lowest occupied molecular orbital (HOMO-LUMO) interactions from the simulations between the RU and nitrite species.

Introduction

Of late, supramolecular polymers (SMPs) have become one of the most active research areas in the field of supramolecular chemistry.[1−7] Extending beyond the normal covalent bonding system usually observed in polymer molecules, SMPs incorporate non-covalent-type interactions for creating supramolecular monomer (SMM) assemblies, which in turn control SMP conformation and behavior.[2,8] These unique traits have given rise to the functional SMPs, where SMPs have ordered shape through self-assembly of designed SMMs.[4,7] These functional SMPs are capable of self-healing or inherent degradation, have good biocompatibility, and have been shown to have excellent applications in the fields of electronics.[4,9] Of these functional SMPs, metallo-SMPs (MSMPs) that form through coordination bonding have been extensively applied as electrocatalysts in the field of electrochemistry for fabricating electrochromic devices, supercapacitors, molecular CO2 reduction photocathodes, O2 evolution, dye-sensitized solar cells, etc.[5,9−11] It is essential to understand the electrocatalytic reaction mechanism (ERM) of these SMPs for designing effective electrocatalysts to reduce production cost, lower material wastage, and increase catalytic efficacy of the SMP. Recently, Leung et al. studied the ERM of a cobalt (Co)-bis(terpyridine)-based photocathode for CO2 reduction using Raman and attenuated total reflection-infrared (ATR-IR) spectroelectrochemical techniques.[10] Through analyzing the results, they showed that the catalytic mechanism of immobilized and soluble Co-bis(terpyridine) SMPs differs significantly.[10]

Another area of the electrochemistry where MSMPs can be applied as potent electrocatalysts is in the field of electrochemical sensing.[1,11] In our recent work, we have shown that Ni–Co-based heterometallo-SMP (HMSMP) (polyNiCo) can be used for nonenzymatic oxidation of nitrite (NO2–).[1] NO2– is an important inorganic environmental pollutant.[12,13] In this particular case, the HMSMP itself is electrochemically active (both metal centers and the surrounding ligand) and therefore the most preferable way for transferring electrons from surrounding analytes is to allow metal–analyte coordination into the SMP moiety under the applied potential through metal–ligand charge transfer (MLCT) and ligand–ligand charge transfer (LLCT).[1]

Like most works involving electrochemistry, we too predicted the catalytic process of this HMSMP electrocatalyst based on the electrochemical experimental results. Others also used in vivo spectroscopic analysis and educated guesses.[10,14] However, both the educated guess method and electrochemical experimental results fail to account for the physicochemical processes at the electrode–electrolyte interface accurately.[15] On top of that, the in vivo spectroscopic analysis requires expert handling and highly controlled environment.[15]

Computational simulation (CPS) of the electron transfer (ET) process at the electrode–electrolyte interface can help elucidate the accurate ERM of electrocatalysts during the ET phenomena while overcoming the difficulties faced by the in vivo spectroscopic analysis.[15−17] To address this challenging task, one would have to verify the structure of the MSMP with its characteristic properties at the molecular level. This can be done by finding the lowest-energy molecular system through optimizing the geometry of the possible structures of the repeating unit (RU) of the synthesized SMP.[15,18] However, using density functional theory (DFT)-based methods for CPS for systems that have many atoms and different types of bonds requires very high processing power, and even then they are time-consuming.[15,17] The alternative to this is using semiempirical quantum chemistry methods (SEQCMs) that can give good approximation within a short period of time.[18] Hakan, K. & Timothy, C. showed that the AM1* parameter of the neglect of diatomic differential overlap (NDDO) can provide more accurate information about the catalytic process of organometallic compounds containing Ni and Co transition metals compared to other semiempirical quantum chemistry method parameters.[18] Thus, the optimization of the RU can be accomplished using the AM1* NDDO (neglect of diatomic differential overlap) CPS method for the polyNiCo HMSMP.[18,19] Then, it is possible to simulate the UV/vis and IR spectra of these optimized structures through electronic and vibrational frequency excitation calculations to verify whether they match with the experimental findings.[20] Zerner et al.[20] and Li et al.[21] showed that the spectroscopic data for the Ni- and Co-based organic systems can be simulated using the INDO/2 ZINDO (Zerner’s intermediate neglect of differential overlap) SEQCM. After establishing the molecular model for the SMP, the ERM of the SMP with the analyte at the interfacial region can be established through analysis of the highest occupied molecular orbital–lowest occupied molecular orbital (HOMO–LUMO), UV/vis spectral changes, thermodynamic properties, charge distribution, and the TS optimization of the interactions between the SMP and the analyte using the previously mentioned methods.

Hence, in this work, we have attempted to establish the ERM of polyNiCo HMSMP for nonenzymatic oxidation of NO2– using the SEQC CPS method. For this purpose, we prepared several possible molecular models of the RU of the polyNiCo and optimized them using the AM1* NDDO Hamiltonian. We simulated the spectroscopic data of the optimized polyNiCo HMSMPs using the INDO/2 ZINDO method and compared it with the experimental results. Finally, we studied the interaction of the polyNiCo with NO2– through studying UV/vis data that illustrated the energy changes associated with the frontier molecular orbitals (HOMO–LUMO) in the polyNiCo system, along with charge distribution. Based on the results of our analysis, we have proposed the ERM pathway for the nonenzymatic NO2– oxidation at the polyNiCo HMSMP. Here, we have used the SEQC-based general CPS method for determining the ERM of SMPs that can be applied to future works involving MSMPs or HMSMPs and redox processes.

Results and Discussion

Confirming the Structure of the RU through Spectroscopic Analysis

To verify the structure of the repeating unit (RU) of the simulated polyNiCo, we have compared the simulated UV/vis spectrum of the optimized polyNiCo RU structures with the experimental spectrum. In our recent work, through UV/vis spectrophotometric titrations, we confirmed that the synthesized polymer had a linear chain structure with random distribution of metals into the polymer chain.[1] Also, the electrochemical characterization of the polyNiCo showed that both metals are electrochemically active and have different oxidation states (+1, +2, and +3 oxidation states) while in the SMP system.[1] Thus, to understand the molecular properties of the polyNiCo, we have performed simulations of the RU containing metal atoms of different oxidation states. We have prepared and optimized six different molecular models of RU by changing metal positions for the computational simulations. These were, namely, (a) Co(II)LNi(II)LCo(II) (Figure S1a), (b) Ni(II)LCo(II)LNi(II) (Figure S2a), (c) Ni(II)LCo(I) (Figure S3a), (d) Ni(I)LCo(I) (Figure S4a), (e) Ni(II)LCo(III) (Figure S5a), and (f) Ni(II)LCo(II) (Figure a). The Roman numeral indicates the oxidation state of the metals in the molecular system. Later, the simulated UV/vis spectra were derived for all six simulated RUs using the theory detailed in Section . It was observed that the RU system (f) Ni(II)LCo(II) showed a better match with the experimental UV/vis spectrum, confirming that the synthesized polymer had Ni and Co metals with similar oxidation states. This is also in accordance with the result that we obtained from the electrochemical characterization experiment of polyNiCo.[1] The experimentally simulated UV/vis spectra for Ni(II)LCo(II) are shown in Figure S6b.

Figure 1

Computational simulation of the Ni(II)LCo(II) repeat unit. (a) Optimized structure of the Ni(II)LCo(II) repeating unit. UV/vis spectra of the real (polyNiCo) (b) and the simulated (Ni(II)LCo(II)) (c) supramolecular polymer. (d) Simulated IR spectrum of Ni(II)LCo(II).

As seen in Figure a, the optimized Ni(II)LCo(II) structure for the Ni and Co system shows octahedral complexation. This is the common complexation way for both the Ni2+ ([Ar] 3d8) and Co2+ ([Ar] 3d7) atomic systems when coordinating with ligands through six sites.[9,10] For comparison, the simulated UV/vis spectra of the Ni(II)LCo(II) structure and experimentally obtained UV/vis spectra of polyNiCo are shown in Figure b,c. The experimentally obtained UV/vis spectrum shows two distinct absorptions bands at 285.94 and 330.29 nm. These absorptions bands are assigned to the metal-to-ligand charge transfer (MLCT) (dπ–pπ* transition) processes for the complexation of Ni(II) and Co(II) ions with the ligand.[1,15,22] A single small d–d transition band is also observed at 525 nm. The simulated UV/vis spectrum of the Ni(II)LCo(II) system shows spectral bands almost similar to those of that observed from the experiment. The first characteristic band is found at around 285 nm, which matches with the experimentally observed band for the complexation of the Ni(II) ion with the ligand terpyridine moiety. However, the second band shows a slight red shift compared to the experimental band and is observed at around 394 nm wavelength. This band is likely due to the MLCT process for the complexation of the Co(II) ion to the terpryridine moiety of the ligand. However, in the simulated spectrum, there is no peak at around 525 nm for the d–d transition. Hence, it is likely that in the simulated spectrum the peaks for MLCT for Co(II) ions and d–d transitions were not resolved. As a result, we got a slightly shifted broad peak at around 394 nm wavelength instead of two individual peaks at around 330 and 525 nm. We have also simulated the ligand structure and generated the corresponding UV/vis spectrum using the same level of the theory (Figure S7). The spectrum showed a large number of peaks mainly responsible for the intraligand charge transfer (ILCT) process, indicating a greater number of electronic transitions (ETs) between different HOMOs and LUMOs. This again confirms that the resulting bands observed at 285 and 394 nm in the Ni(II)LCo(II) system are mainly due to the MLCT-type ETs as shown in Figure c.

For the simulated system Co(II)LNi(II)LCo(II) (Figure S1f), a sharp absorption band was observed at 300 nm, indicating the MLCT process, while the Ni(II)LCo(II)LNi(II) system (Figure S2f) showed a sharp band at around 330 nm. Both of these systems showed only a single peak. The reason for this might be the presence of two metallic ions of same type in these systems. As a result, the peak for MLCT with one type of metal ion (Ni or Co) became more pronounced and the simulation could not resolve the peaks. This caused the simulated spectrum to show a single peak corresponding to the metal that had two ions in the Co(II)LNi(II)LCo(II) and Ni(II)LCo(II)LNi(II) systems. These findings indicate that the ratio of the metals in the polymer chain has a great influence on the spectrum. The Ni(II)LCo(I) system displayed three bands for the ILCT process at 180, 230, and 250 nm. A MLCT band at 272 nm and a d–d transition band at 541 nm were also observed. (d) Ni(I)LCo(I) and (e) Ni(II)LCo(III) showed mainly MLCT and d–d transition bands (Figures S3–S5).[22,23]

Electrostatic potential mappings (ESPMs) for six RU simulations are shown in Figures S1–S6. The Ni(II)LCo(II) system mostly has blue coloring in the ESPM (Figure S6). This even distribution of potential indicates a stable RU with octahedral shape. Other simulated RU showed similar blue coloring except for the RU with Ni metals. In this case, the ESP distribution is uneven, resulting in greenish red color over the molecular system (Figure S2).

Figure d represents the simulated IR spectrum obtained for the Ni(II)LCo(II) system. The simulated IR spectrum of the Ni(II)LCo(II) system matched considerably with the experimental Fourier transform infrared (FT-IR) spectrum of polyNiCo.[1] The assignments of different bands of the real IR spectrum are tabulated in Table . The simulated spectrum shows several bands for stretching and bending motions in the following regions: 1400–1600, 1250–1000, and 600 cm–1. However, the real experimental FT-IR spectrum showed a prominent broad band at 1200–1000 cm–1 and a sharp band at 800 cm–1.[1] On the other hand, the fingerprint region (1400–1600 cm–1) regarding the terpyridine unit was more intense in the simulated spectrum. This difference in the spectra was perhaps due to the fact that the simulated spectrum is obtained for a single RU, whereas the experimentally observed spectrum was for the polymer as a whole. These analyses confirm that the Ni(II)LCo(II) system would be the best approximation for the polyNiCo supramolecular polymer (SMP). Henceforth, we will focus our analysis on this Ni(II)LCo(II) molecular system.

Table 1

Assignment of IR Data of the Simulated and Experimental Spectra[10]

IR
ν (cm–1)	430a	512a	571	635a	630–850a	1020–1300a
assignment	Ni―N	ν(C―C), ν(C―N), δ(C―H)	ring breathing	Co―N	γ(C―H)	ν(C―C), υ(C―N)

Observed spectra for the experimental data.

Frontier Molecular Orbital and Charge Distribution Analyses of the RU

The molecular orbital analysis shows that the HOMO and LUMO of the Ni(II)LCo(II) system are located mostly around the Co atom. As can be seen from Figure a,b, the highest density of the delocalized HOMO is over the Co atom, with the rest being contributed by the surrounding N and C atoms. Thus, it is likely that the d and p orbitals of the Co, N, and C atoms would contribute to the ET process mostly. On the other hand, the LUMO is mostly distributed over the benzene ring adjacent to the Co atom, with very little distribution over the Co atom itself (Figure b). However, this analysis also raises the question that how this distribution of the HOMO–LUMO might alter in an actual chain of the polymer. Figures S1c,d and S2c,d show how the HOMO–LUMO distribution might have taken place at the actual polymer chains with random metal atom distribution. As we see from Figure S1, the HOMO is distributed mostly over the Co atom, but in this case, the LUMO is distributed over the Ni atom. This may be due to the fact that the Co atom contains a 3d7-like configuration while the Ni atom contains a 3d8 configuration, thus allowing the LUMO to be centered on it.[24]Figure S2 shows almost the opposite conditions for HOMO–LUMO distribution compared to Figures and S1. In this case, the HOMO is located at the benzene ring at the side of the Ni atom, while the LUMO is centered on the Co atom, mostly. The reason for such behavior may be that the two Ni atoms surround the Co atom. The metal-to-metal-type interaction between the Ni and Co species might be responsible here. Based on these analyses, we can expect that the HOMO is most likely to be centered on the Co atom except when it is flanked by Ni atoms on both sides of the ligand.

Figure 2

(a, b) HOMO and LUMO on Ni(II)LCo(II), respectively. (c) Coulson charges for each atom in the Ni(II)LCo(II) system. A reversed rainbow color mapping was used for all of the figures. In this scheme, the color changes from blue to yellow with decreasing negative charge.

The Coulson charge distribution shows the noninteger charge of atoms as a part of the molecule. Figure c shows that the Ni and Co atoms have charges around 0.523 and 0.323e–, respectively. Figures S1 and S2 show that the charges on the Ni atom vary from ∼0.5 to 0.6e–, while they are ∼0.3–0.5e– for the Co atoms. At the same time, the charges on the N atoms coordinating with the Co and Ni species are almost always negative, indicating that charge transfer takes place from the metal to the ligand. However, a few positive-charge-carrying N atoms point at back-donation by the ligand to the metal during this charge transfer process, which in turn strengthens the coordination complex.

Computational Analysis for Describing the Reaction Pathway

To investigate how the NO2– ion interacted with the polyNiCo prior to the irreversible electron transfer, we have utilized two different RUs based on the metal’s oxidation states around ∼0.8 V as the substrate, namely, Ni(II)LCo(II) and Ni(II)LCo(III). The optimized structure and UV/vis spectra of the Ni(II)LCo(III) system are shown in Figure S5. These RUs were chosen due to the fact that the Co center was likely to exist in the +3 oxidation state in our experimental potential window during NO2– oxidation.[1] Based on our analysis of the electrochemical experimental data, we previously proposed the following probable pathway for NO2– oxidation at the polyNiCo HMSMP[1]Thus, in this work, we have used semiempirical quantum chemistry theory to understand how the ET process during the oxidation of NO2– occurred at the molecular level and determine the electrocatalytic reaction mechanism (ERM) pathway.

ET through MLCT and LLCT Pathway for Nitrite Oxidation

According to this reaction pathway, the active sites are located on the ligand sphere at which nitrite species attacks through the interfacial region. Since the ligand itself is electrochemically active, it is likely to interact with nitrite species for facilitating the ET process. Through computational simulations, we have analyzed the following properties:

The changes associated with the distributions of the HOMO–LUMO of the optimized systems before and after placing analytes in different positions near the RUs.

The changes in the UV/vis spectra before and after interactions between the RUs and the analytes.

The nature of ET processes based on the observed UV/vis spectra.

The HOMO–LUMO distribution for the interaction between Ni(II)LCo(III) and NO2– at position 1 (P1) is shown in Figure a, and it is denoted as the Ni(II)LCo(III)_NO2– system. It can be seen that the HOMO in this case mostly resides on the NO2– ion and the pyridine ring of the terpyridine moiety connected with the Ni(II) metal, while the LUMO is concentrated around the Co(III) center of the RU. This finding is important as it suggests the possible pathway for ET between the polymer RU and nitrite ion. According to the observed pattern of HOMO–LUMO, when nitrite closely approaches the RU at position 1, it is likely that the ET occurs from the HOMO of NO2– to the LUMO located around the Co center. For all of the molecular systems, the eigenvalues related to the HOMO–LUMO and the next four orbitals located after HOMO and LUMO are given in Table S1.

Figure 3

HOMO and LUMO distributions for (a) Ni(II)LCo(III)_NO2–, (b) Ni(II)LCo(III)_NO2 + H2O, and Ni(II)LCo(II)_NO2 + H2O (c); Ni(II)LCo(III)_NO2 (d); and Ni(II)LCo(II)_NO2 (e). The figures show the analytes (NO2–, NO2 + H2O, and NO2) when they were placed at a distance of approximately 3.5 Å from the RU systems. This was denoted position 1 (P1) among the seven positions where analytes were placed. A reversed rainbow color mapping was used for all of the figures. In this scheme, the color changes from blue to yellow with decreasing negative charge.

The simulated UV/vis spectrum for this Ni(II)LCo(III)_NO2– system exhibits four bands. The two bands at around 360 and 510 nm (Figure S8a) are readily observable from the spectrum, while the other two bands have very low intensity and are almost invisible in the spectrum. This is completely different than the UV/vis spectra of the Ni(II)LCo(II) (285 and 330 nm) and Ni(II)LCo(III) (360 and 570 nm) systems as shown in Figures and S5. The most intense band at 510 nm corresponds to the n–π* transition, indicating the charge transfer ability from the nonbonding HOMO of NO2– to the Co-centered LUMO. The other band at 360 nm indicates the π–π* transition corresponding to the MLCT and LLCT processes. This electronic transition for Ni(II)LCo(III)_NO2– is shown in Figure c, which reveals HOMO–LUMO+1 and HOMO–1–LUMO+1 transitions, indicating n–π* and π–π* transitions. The other two transitions require a high excitation energy, which is likely responsible for their weak absorption intensity. The corresponding ETs in the Ni(II)LCo(III) system also showed n–π* and π–π* transitions (Figure b). However, the n–π* transition in the presence of NO2– resulted in a blue shift and the π–π* transition in a red shift.

Figure 4

Diagram representing electronic transitions in different orbitals for the Ni(II)LCo(II) (a), Ni(II)LCo(III) (b), Ni(II)LCo(III)_NO2– (c), Ni(II)LCo(II)_NO2 + H2O (d), Ni(II)LCo(III)_NO2 + H2O (e), Ni(II)LCo(II)_NO2 (f), and Ni(II)LCo(III)_NO2 (g) systems.

Figure b,c shows the HOMO–LUMO distributions for the systems Ni(II)LCo(III)_NO2 + H2O and Ni(II)LCo(II)_NO2 + H2O, respectively. This pair of NO2 + H2O is chosen because these species are likely involved in the process of transferring the second electron to the electrode. The HOMO–LUMO pattern of these two systems shows the NO2 as the HOMO, while the LUMO is mostly seen around the terpyridine moiety attached to the Ni (II) center in the Ni(II)LCo(III)_NO2 + H2O (Figure b) system and randomly on the pyridine ring over the whole RU in the Ni(II)LCo(II)_NO2 + H2O (Figure c) system.

The simulated UV/vis spectrum of the Ni(II)LCo(III)_NO2 + H2O (Figure b) system showed ETs mostly corresponding to π–π* bands, while the Ni(II)LCo(II)_NO2 + H2O (Figure c) system displayed peaks corresponding to both n–π* and π–π* transitions. The ETs observed for these systems are shown in Figure , represented as Ni(II)LCo(II)_NO2 + H2O (Figure d) and Ni(II)LCo(III)_NO2 + H2O (Figure e). The transitions observed are from HOMO to LUMO+4 (2.04 eV), HOMO–1 to LUMO+1 (2.36 eV), and HOMO–2 to LUMO (3.82 eV). These are responsible for the UV/vis bands in the Ni(II)LCo(II)_NO2 + H2O system at 580 (n–π*), 540 (d–d), and 360 (π–π*) nm, respectively (Figure d). However, the Ni(II)LCo(III)_NO2 + H2O (Figure e) system shows two high-energy ETs from HOMO to LUMO and HOMO–1 to LUMO, among which the HOMO–LUMO transition indicates the π–π* transition observed at 300 nm in the UV/vis spectrum (Figure S8). Overall, NO2 + H2O showed relatively significant interaction with the Ni(II)LCo(II) system compared to Ni(II)LCo(III). The calculated binding energy (Eb) for the Ni(II)LCo(II)_NO2 + H2O system for all positions was more negative compared to that for the Ni(II)LCo(III)_NO2 + H2O system. The calculated energy values from all of the optimized systems are given in Table S2. The Eb value calculations for the different interaction systems were done using the following equations[16,17]In these equations, the Eb values for different systems were calculated using the total energy values from Table S2. The results of Eb for these systems are shown in Table . The highly negative Eb values for systems including RU and NO2– indicate a strong chemical interaction between them.[15,17] This highly negative Eb value could be due to the distortion of the coordination sphere during the interaction between the RU and NO2–. On the other hand, the less negative Eb values of systems containing RU, H2O, and NO2 indicate that the interaction only happened weakly and that there was probably no distortion of the ligand sphere. These Eb values further confirm our assumptions regarding the oxidation ERM pathway of NO2– at the polyNiCo HMSMP interface.

Table 2

Eb Values for the Optimized Systems Calculated Using Equations –4

system	binding energy (eV)
Eb(Ni(II)LCo(II)_NO2–_P1)	–11.72629
Eb(Ni(II)LCo(II)_NO2–_P2)	–10.631244
Eb(Ni(II)LCo(II)_NO2–_P3)	–10.462161
Eb(Ni(II)LCo(II)_NO2–_P4)	–10.630841
Eb(Ni(II)LCo(II)_NO2–_P5)	–10.105292
Eb(Ni(II)LCo(II)_NO2–_P6)	–10.586003
Eb(Ni(II)LCo(II)_NO2–_P7)	–10.212243
Eb(Ni(II)LCo(III)_NO2–_P1)	–11.189306
Eb(Ni(II)LCo(III)_NO2–_P2)	–11.20155
Eb(Ni(II)LCo(III)_NO2–_P3)	–11.171779
Eb(Ni(II)LCo(III)_NO2–_P4)	–11.171779
Eb(Ni(II)LCo(III)_NO2–_P5)	–11.164901
Eb(Ni(II)LCo(III)_NO2–_P6)	–11.195933
Eb(Ni(II)LCo(III)_NO2–_P7)	–11.106205
Eb(Ni(II)LCo(II)_NO2_H2O_P1)	–3.950345
Eb(Ni(II)LCo(II)_NO2_H2O_P2)	–3.473489
Eb(Ni(II)LCo(II)_NO2_H2O_P3)	–3.940463
Eb(Ni(II)LCo(II)_NO2_H2O_P4)	–3.445143
Eb(Ni(II)LCo(II)_NO2_H2O_P5)	–3.211066
Eb(Ni(II)LCo(II)_NO2_H2O_P6)	–3.814611
Eb(Ni(II)LCo(II)_NO2_H2O_P7)	–3.895515
Eb(Ni(II)LCo(III)_NO2_H2O_P1)	–1.005134
Eb(Ni(II)LCo(III)_NO2_H2O_P2)	–0.420827
Eb(Ni(II)LCo(III)_NO2_H2O_P3)	–0.95545
Eb(Ni(II)LCo(III)_NO2_H2O_P4)	–1.000385
Eb(Ni(II)LCo(III)_NO2_H2O_P5)	–1.216798
Eb(Ni(II)LCo(III)_NO2_H2O_P6)	–0.486939
Eb(Ni(II)LCo(III)_NO2_H2O_P7)	–0.98457

We have also analyzed the interactions of NO2 with the Ni(II)LCo(III) and Ni(II)LCo(II) RUs based on HOMO–LUMO distribution (Figure d,e), UV/vis spectral analysis (Figure S8), and the ETs based on their corresponding eigenvalues due to interactions (Figure f,g). It can be seen that NO2 alone can show ETs due to interactions with the RUs.

The Coulson charge analysis of the optimized species also confirms the changes in the net noninteger charges of the individual atoms of the RUs and the nitrite species (Figures and S9–S11). While the nitrite species become neutral, the metal centers bear a negative charge at the optimized system. This further confirms our analysis that the MLCT and the LLCT play an important role in enhancing the electrocatalytic activity of the metallic SMPs.

These computational simulations demonstrate that the NO2 can interact with the ligand sphere or the metal coordination sphere while keeping the metal coordination sphere and ligand hapticity unaltered, while the NO2– is likely to interact through coordination with metal centers. The mechanism of such indirect irreversible ET process probably involves transferring of electrons from NO2 to the coordinated bisterpyridine ligand via the LLCT process followed by the transfer of charge from the ligand to the metal via the LMCT process.

Direct Coordination of Nitrite with the Metal in the Complex via Opening Coordination Site

In this direct metal–NO2– interaction pathway, NO2– coordinates with either of the two metals at the expense of breaking one or two coordination bonds. For the oxidation of NO2–, it is important for NO2– to enter into the coordination sphere. Subsequently, the metal octahedral coordination structure and bisterpyridine ligation are attained after the oxidation. Many of the previous works based on metal SMPs proposed a process of insertion of analytes into the complex for subsequent oxidation/reduction via breaking of the already existing metal–ligand bond or a complete loss of ligand.[10,14,25] In our current work, we have computationally simulated TSs for the reaction of NO2– ion with the preoptimized bimetallic complexes, namely, Ni(II)LCo(II) and Ni(II)LCo(III), using the nonlinear least-squares (NLLSQ) method. TS searches were performed for the close placement of the NO2– ion at Ni(II), Co(II), and Co(III) centers separately in Ni(II)LCo(II) and Ni(II)LCo(III) systems. The imaginary frequency (cm–1) system was determined as the TS from the TS optimization process. Based on this, the following TSs were obtained from the simulations:

TS for placing NO2– close to the Co(II) center of Ni(II)LCo(II): The resultant TS loses a coordination site, while the NO2– ion enters, and coordinates with the Co(II) center keeping the octahedron arrangement unaltered. The vibration around the center (animation) is shown in Video S1.

TS for placing NO2– close to the Ni(II) center of Ni(II)LCo(II): The TS loses a pyridine coordination for incorporating the NO2– ion with the metal empty site. The vibration around the center for such TS tends to result in the NO coordinated with the Ni(II) metal. The animation of how the N=O bond forms in the NO2– ion is shown in Video S2.

TS for placing NO2– close to the Co(III) center of Ni(II)LCo(III): The coordination of the NO2– ion with the Co(III) center tends to break two coordination bonds between the Co(III) and two pyridine rings of the terpyridine moiety. The second pyridine ring tends to break only after the formation of a new coordination bond with Co(III) as shown in Video S3.

TS for placing NO2– close to the Ni(II) center of Ni(II)LCo(III): From the TS analysis, it seems that in this case the NO2– ion tends to break down after coordinating with the metal center and also two coordination bonds with the pyridine rings get broken. The TS is shown in Video S4.

From our analysis of the indirect and direct ET processes to the metal center, it becomes certain that both pathways can occur. Based on the analysis of total energy changes of all of the systems, it is likely that the NO2– species first attacked a metal center for a direct ET, while the NO2 species generated underwent an indirect ET via the MLCT and LLCT pathway (Figure ). Oxidation of NO2– might happen via either the following pathways:

Figure 5

ERM pathway of NO2– oxidation in terms of energy value changes for different systems. All energy values were determined from the optimized systems. The numbers indicate different systems. The energy value of the (1) Ni(II)LCo(II) system, (2) Ni(II)LCo(III) system, (3a) Ni(II)LCo(II)_NO2– system, (3b) Ni(II)LCo(III)_NO2– system, (4a) TS when NO2– is close to the Ni(II) center of the Ni(II)LCo(II) system, (4b) TS when NO2– is close to the Co(II) center of the Ni(II)LCo(II) system, (4c) TS when NO2– is close to the Co(III) center of the Ni(II)LCo(III) system, (4d) TS when NO2– is close to the Ni(II) center of the Ni(II)LCo(III) system, (5a) Ni(II)LCo(II)_NO2 system, (5b) Ni(II)LCo(III)_NO2 system, (6a) Ni(II)LCo(III)_ NO2_ H2O system, (6b) Ni(II)LCo(II)_ NO2_ H2O system, and (7) Ni(II)LCo(III)_ NO3–_ 2H+ system.

ERM pathway 1: (1) → (3b) → (4a,b) → (5a) → (6b) → 7

ERM pathway 2: (1) → (2) → (3a) → (4c,d) → (5a,b) → (6a,b) → 7

The ERM pathway 2 is likely to be followed when the Co2+ of the Ni(II)LCo(II) system becomes oxidized to Co3+, forming the Ni(II)LCo(III) system at high positive potential. On the other hand, the ERM pathway 1 is the likely pathway when NO2– coordinates with the Ni(II)LCo(II) system during the first electron transfer. In both cases, the first e– transfer takes place through a TS formation. In these cases, the NO2– species coordinates with either of the Ni(II), Co(II), or Co(III) metal centers. While coordinating with the metal centers, the metal loses at least one of its coordination sites with the ligand. This is illustrated in Figure . From Figure , it becomes clear that the TSs have the least negative energy values of all of the systems. The least negative energy value of these systems further confirms their role as the TS besides the imaginary frequency assignments. However, the most negative energy value of the product shows that the proposed reaction pathway is energetically favorable. Hence, our analyses of the HOMO–LUMO distribution, UV/vis spectra, charge distribution, and energy value changes of the optimized systems confirm that the ERM for NO2– oxidation follows either ERM pathway 1 or 2. The two cycles in Figure show the most possible ET pathway for the NO2– and NO2 species at the polyNiCo HMSMP interface.

Figure 6

Schematic illustration for the possible catalytic nitrite oxidation mechanism at the polyNiCo interface. Here, cycle 1 and cycle 2 indicate the possible catalytic pathway at Ni(II)LCo(II) and Ni(II)LCo(III), respectively, for the nonenzymatic oxidation of nitrite.

Conclusions

In this work, we showed that it is possible to predict the electrocatalytic reaction mechanism (ERM) of heterometallo-supramolecular polymers (HMSMPs) using semiempirical quantum chemistry (SEQC) computational simulation (CS) methods. We optimized the geometry of the repeating unit (RU) using the AM1* parameterization and simulated its UV/vis spectra using the ZINDO Hamiltonian. Later on, we simulated the interactions between the RU and nitrite species using the same methods. This allowed us to predict the ERM of polyNiCo HMSMP during the oxidation of nitrite. Our analysis from CS matched the one that we previously anticipated from experimental analysis of electrochemical data. However, the CSs allowed us to understand the physicochemical phenomena at the HMSMP electrode interface during the ERM for nitrite oxidation at the molecular level with greater clarity. The transition state (TS) analysis from the CS showed that the NO2– species first coordinated with either of the metal centers to initiate the electron transfer process for the oxidation. The TS analysis also showed that both the type of metals and oxidation states of these metal centers profoundly influenced how the NO2– species coordinated with them. Again, analysis of the UV/vis spectra, HOMO–LUMO, and energy of the RU–nitrite species systems after the simulation showed that the second electron transfer process likely took place through LLCT and MLCT pathway. Through combining the analysis of the CS results, we have proposed the most likely pathway for the ERM of nitrite oxidation at the polyNiCo HMSMP. This work shows that it is possible to predict the ERM of SMPs or HMSMPs with various types of analytes using SEQC methods. Such simulations can be used to predict the electrocatalytic activity of SMPs and help in designing these SMPs for higher electrocatalytic activity without the hassles of trial-and-error methods.

Computational Simulation Methodology

Molecular properties of the polyNiCo material were investigated by taking a repeating unit (RU) of the polymer. The RU was drawn using the three-dimensional (3D) atomistic document feature of Materials Studio 2017 software. Geometry optimization, thermodynamic properties of the optimized structures, and spectroscopic properties of the RU were investigated using semiempirical quantum chemistry methods (SEQCMs). The geometry optimization process, transition state (TS) optimization, and thermodynamic properties’ calculations were carried out with the neglect of diatomic differential overlap (NDDO) Hamiltonian, while the spectroscopic properties were investigated with Zerner’s intermediate neglect of differential overlap (ZINDO) Hamiltonian.[18,20] The two Hamiltonians were used because while the NDDO Hamiltonian is better suited for the geometry optimization process and thermodynamic properties’ evaluation, the ZINDO Hamiltonian is parameterized for more accurate calculations of spectroscopic properties.[18,21] The geometry optimization and thermodynamic properties’ calculations of all of the molecular systems were carried out with the AM1* NDDO Hamiltonian, the spin multiplicity was allowed to be determined during the calculations, and the unrestricted Hartree–Fock (UHF) spin state was assigned. The AM1* Hamiltonian was used because it contains upgraded optimization parameters for the Ni and Co species over its previous version of AM1 Hamiltonian, and the UHF spin state was assigned since it improves the geometry optimization accuracy for coordination complexes.[18] The Hessian calculation was set for Exact, i.e., the Hessian matrix was calculated during the optimization process. The TS optimization process was carried out with the same parameters as those used for the geometry optimization process. The nonlinear least-squares (NLLSQ) method was used for searching the TS with gradient norm being set to 0.1 kcal/(mol Å).

For spectroscopic properties’ calculations, the intermediate neglect of differential overlap (INDO/2) parameter of the ZINDO Hamiltonian was utilized. The restricted open-shell Hartree–Fock (ROHF) spin state was assigned, with spin multiplicity being set to auto-determination.

The electronic properties were calculated by setting the self-consistent field (SCF) tolerance to 5e–7 eV/atom, with max cycles of 1000. The SCF convergence scheme from Badziag and Solm was used (IIS) since it gives the most reliable convergence. For the study of spectroscopic properties, the configuration iteration (CI) type was set to RumerCI. The spectroscopic properties of all of the optimized species were studied using these parameters.

The frequency analysis allowed the calculations of the IR vibrational spectra for the individual molecular systems. The studies of spectroscopic properties permitted the UV/vis spectrum plotting with contribution from the frontier molecular orbitals (HOMO–LUMO).

To study the effects of potential change on the polyNiCo_GCE, the RU was optimized with different charges on the Co and Ni atoms in the complex. The electrocatalytic reactions take place in the electrochemical double layer (EDL) region. The inner Helmholtz plane (IHP) molecules at the interfacial region are most likely to take part in the electron transfer (ET) process. The IHP is usually one molecule thick in length. However, the outer Helmholtz plane or diffused plane molecules can also reach the surface of the electrode and take part in the ET process. That is why the interactions during the nitrite oxidation were studied by placing the nitrite species at seven different positions (3–7 Å) around the RU. In all systems, during energy evaluation, a water-based solvation scheme was used using the self-consistent reaction field (SCRF) method since the experiments were carried out in aqueous medium. The full reaction pathway was determined through optimizing the simulation models that contained the RU and the nitrite species along with individual water and nitrogen dioxide (NO2) molecules.