Nitric Oxide Oxidation Mediated by Substituted Nickel

Article pubs.acs.org/JPCC

Nitric Oxide Oxidation Mediated by Substituted Nickel Phthalocyanines: A Theoretical Viewpoint Gloria I. Cárdenas-Jirón,*,† Cristhy Gonzalez,‡ and Julie Benavides§ †

Laboratorio de Química Teórica, Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile ‡ Departamento de Química, Facultad de Ciencias Básicas, Universidad Metropolitana de Ciencias de la Educación, José Pedro Alessandri 774, Santiago, Chile § Facultad de Química, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile ABSTRACT: The oxidation of nitric oxide (NO) mediated by substituted nickel phthalocyanines (NiPc(X)n) has been investigated for 19 complexes at the B3LYP and M05-2X/LANL2DZ/6-31G(d) levels of theory. Unimolecular reactivity descriptors like chemical potential and electrophilicity were calculated for NiPc(X)n, giving an account of the substituent effect and indicating that electron-withdrawing groups favor the reduction of NiPc(X)n. Bimolecular reactivity descriptor as donor−acceptor hardness was used to get information about the affinity of NO by NiPc(X)n and suggests that Ni(III) has better affinity than Ni(II). A mechanism where Ni(III) catalyzes the oxidation of NO instead of Ni(II) was proposed. Optimized molecular structures calculated for the NO···NiPc(X)n adducts confirm that a larger charge transfer occurs from NO to Ni(III)Pc(X)n in comparison with Ni(II)Pc(X)n and that the reduction in the former complexes happens on the phthalocyanine ligand. oxidation5,26,30 using the carbon fiber as electrode, and they found that the activity follows the order: H2Pc ≪ CuPc < ZnPc < MnPc < CoPc ∼ NiPc < FePc, with H2Pc as metal-free. These results indicated that the catalysis is dependent on the metal atom and suggest that the reduction site, when the NO oxidation occurs, could vary depending of the metal atom. Nyokong et al. have shown that FePc presents the highest activity for the NO oxidation constituting the best electrocatalyst for the detection of NO; however, it does not have good stability in the sense that the complex is not regenerated.5,6 Taking into account the experimental problems with FePc, we chose to study here the compound NiPc and a set of substituted derivatives. The good catalytic activity observed with NiPc5,6 and NiTSPc (nickel tetrasulphophthalocyanine) has been a controversial subject because this complex does not undergo a metal-based redox process.31 It has been proposed in the literature for different nickel complexes that a film of these complexes deposited on an electrode could be demetalated and the interaction could occur between an organic polymer and NO without a specific interaction between the ligand (porphyrin, phthalocyanine) and NO.5,28 In this article, we present a theoretical investigation to rationalize at a quantum chemistry molecular level the NO oxidation mediated by substituted nickel phthalocyanines

1. INTRODUCTION Nitric oxide (NO) has received considerable attention in the literature in the last two decades. NO is a free radical that is produced by the interaction between molecular oxygen and nitrogen at high temperatures in the inside of combustion engines. In the atmosphere, it is a toxic compound and chemically unstable, with a lifetime of 5 s. In low dose in the human body, it is beneficial and it is essential in physiological processes, such as regulation of the blood pressure and neurotransmission.1,2 In high dose, it can provoke septic shock, Parkinson’s, and Alzheimer’s.3 However, its importance in biological systems is widely recognized because it is an essential molecule for life.3,4 NO is a molecule presenting rich redox properties, its first oxidation leads to NO+ and NO2− and a second oxidation leads to NO3−, and its reduction produces NO− and N2O, which can be again reduced to NH2OH and NH3.5 Measurement of NO in biological systems is not easy because of its low concentration and its short lifetime, a fact that has led to an increased interest to design electrochemical sensors capable of measuring small amounts of NO.5−14 Electrodes modified with metalloporphyrins have been used by several authors as electrochemical sensors in the study of the oxidation or reduction of NO.5,12,13,15−29 When the metalloporphyrins are deposited on an electrode (graphite, glassy carbon, metal), they act as catalyst in the oxidation or reduction process occurring. Nyokong et al. have measured the catalytic activity of metallophthalocyanines (MePc), which are metallotetrabenzoporphyrins, in the NO © 2012 American Chemical Society

Received: March 15, 2012 Revised: June 28, 2012 Published: July 10, 2012 16979

dx.doi.org/10.1021/jp3025232 | J. Phys. Chem. C 2012, 116, 16979−16984

The Journal of Physical Chemistry C

Article

(NiPc(X)n). We propose an oxidation mechanism for the NO based in the interaction of this one with a Ni(III)Pc(X)n. The mechanism is supported by a bimolecular reactivity descriptor as the donor−acceptor hardness (ηDA) obtained from the isolated compounds (NO, NiPc(X)n) and by studies of electronic population provided by the calculation of the adducts NO···NiPc(X)n. The use of the donor−acceptor hardness32 is supported for the successful application in previous redox studies.33−37 We think that the present theoretical study is very useful because it provides a detailed quantitative basis justifying the experimental facts, and it allows further better understanding of the behavior that the substituted nickel phthalocyanines present toward the NO oxidation. The methodology can also be applied to other kind of redox processes.

Scheme 1. Structural View of Substituted Nickel Phthalocyanine (NiPc(X)n) and the Corresponding Substituents

where the substitution is done on the β position of the benzenic ring that corresponds to the outer position of the ring. For these optimizations, a B3LYP exchange correlation functional41−43 that contains the Becke’s three-parameter hybrid exchange functional; the exact Hartree−Fock, Slater, and Becke exchanges and the LYP correlation functional; a Vosko, Wilk, and Nusair (VWN) local functional; and a Lee, Yang, and Parr (LYP) nonlocal functional, were used. All calculations were performed with the LANL2DZ44−46 pseudopotential for the nickel atom and a 6-31G(d) basis set47 for the other atoms (C, N, H, O, S, F) using the package Gaussian03.48 Ni(II)Pc(X)n complexes were calculated having a total charge Q = 0 with a singlet multiplicity. The substituents (X) used in the Ni(II)Pc(X)n complexes differ in the ability as electron donor or electron-withdrawing group: F, Cl, SO3H, CN, SH, OH, CH3, NH2, and CH3O. To investigate how the properties are affected by the presence of aza nitrogens, we included some atoms of this type in the benzenic rings of the phthalocyanine macrocycle. Therefore, we studied a total of 19 complexes including the unsubstituted complex. Frequency calculations with numerical second derivatives were carefully performed for each complex Ni(II)Pc(X)n to confirm that every molecule corresponds to a minimum energy state. The behavior of these complexes was also assessed with the electronic population using the approximation of natural atomic orbital (NAO).49,50 The conditions of the calculations for the NO were similar to the nickel(II) complexes, but in this case only a 6-31G(d) basis set was applied.

2. THEORETICAL AND COMPUTATIONAL ASPECTS 2.1. Reactivity Descriptors. To characterize the reactivity of the Ni(II)Pc(X)n complexes, we use some typical reactivity descriptors derived from the density functional theory (DFT) such as electrophilicity (ω), chemical potential (μ), and molecular hardness (η). Electrophilicity, defined by Parr et al.,38 measures how energetically favorable it is to saturate a system with electrons ω=

μ2 2η

(1) 39

Chemical potential (μ) is a global index and measures the escaping tendency of an electron. Its relevance is related to the electronegativity (χ) concept by μ=

⎛ ∂E ⎞ ⎛I + A⎞ ⎟ = −χ ⎜ ⎟ = −⎜ ⎝ 2 ⎠ ⎝ ∂N ⎠ν(r)

(2)

where E is the electronic energy, ν(r) is the external potential, and N corresponds to the total number of electrons. The ionization potential (I) and the electronic affinity (A) are calculated as ΔSCF (self-consistent field) processes; it means that I = E(N − 1) − E(N) and A = E(N) − E(N + 1). ΔSCF calculations correspond to energy differences for two electronic states where each one of the energies are calculated through a procedure of SCF. Molecular hardness40 gives account of the resistance of the electronic cloud to be modified and is defined by ⎛ ∂ 2E ⎞ ⎛I − A⎞ ⎟ η = ⎜ 2⎟ = ⎜ ⎝ ∂N ⎠ν(r) ⎝ 2 ⎠

3. RESULTS AND DISCUSSION 3.1. Theoretical Characterization of Ni(II)Pc(X)n. The substituted nickel(II) phthalocyanine complexes are stable compounds and present different behavior when the substituent is changed. First, chemical potential and electrophilicity were investigated on these complexes, and the results are shown in Figure 1. The numeration for the complexes used in this graphic as in the other discussed later corresponds to that showed in Scheme 1. Therefore, the unsubstituted complex, Ni(II)Pc, corresponds to the compound 17. As it can be seen, the chemical potential increases from 1 (−0.43 au) to 19 (−0.24 au) going from the complexes with electron-withdrawing groups toward the complexes with electron donor groups. It implies a maximum difference between 1 and 19 of ∼119 kcal/mol, which is not negligible. It means that the former substituents help to decrease the escaping of the electrons of the complex and would be compounds with better ability to be reduced. Complexes like 18 and 19 have electron donor substituents, CH3 and NH2, respectively, and clearly can

(3)

For simplicity, we use eqs 2 and 3 without the factor 1/2. In the case of studying the affinity of NO by the Ni(II)Pc(X)n complex, we apply the bimolecular index donor−acceptor hardness (ηDA) that was first defined by Parr and Zhou as the hardness associated with a two-partner electron-transfer chemical reaction,32 where a donor (D) species and an acceptor (A) species are present

ηDA = ID − AA

(4)

ID corresponds to the ionization potential of NO because it is the donor species and AA is the electronic affinity of the Ni(II)Pc(X)n complex that is the acceptor species. 2.2. Theoretical Calculations. Full geometry optimizations without symmetry restrictions were carried out on a set of substituted Ni(II) phthalocyanines (Ni(II)Pc(X)n) (Scheme 1), 16980



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useful tool in our study because it has been possible to identify the type of substituent (electron-withdrawing) that could favor the oxidation of NO. 3.2. Oxidation of NO by NiPc(X)n. Once the correlation between the type of substituent and the ability of the Ni(II)Pc(X)n complexes for a reduction process is investigated, we explore how they behave when faced to the NO. To do that, we determined the donor−acceptor hardness that is shown in Figure 3. Following the experimental works related to the

Figure 1. Chemical potential (μ) and electrophilicity (ω) (a.u.) calculated for the substituted nickel phthalocyanines obtained at the B3LYP/LANL2DZ/6-31G(d) level of theory.

retain fewer electrons as they present the higher value of chemical potential. Therefore, they have more trends to be oxidized. Electrophilicity shows the opposite behavior; a decrease from 1 to 19 is found and means that it goes from the complexes with more ability to accept electrons to those ones able to receive a smaller amount of electrons. Therefore, ω predicts that the complexes with the higher values are potential compounds to be reduced and correspond to those having electron-withdrawing groups. It is worth mentioning that between the complexes with better ability for a reduction process are those having one and two aza nitrogen atoms, giving account that this kind of substitution could increase the reduction in these complexes. As it can be seen, there exists coherence among reactivity descriptors, chemical potential, and electrophilicity. We also investigated the substituent effect on the nickel atom by means of the electronic charges. As an illustration, we used those obtained with the Mulliken approximation, and the results obtained for the set of Ni(II)Pc(X)n are shown in Figure 2. It is observed a trend where the value of qNi(II) decreases

Figure 3. Donor−acceptor hardness of Ni(II)Pc(X)n and Ni(III)Pc(X)n complexes with regression coefficients of R2 = 0.95 and 0.90, respectively, obtained at the B3LYP/LANL2DZ/6-31G(d) level of theory.

oxidation of NO, where it is suggested that the oxidation state participating in this process would be Ni(III) instead of Ni(II),5,51,52 we determined the ηDA values for the [Ni(III)Pc(X)n]+ complexes and added them in the same Figure. The latter complexes are states calculated for vertical transition, where the optimized geometries correspond to the complexes with Ni(II) and the electronic wave functions obtained with the complexes with Ni(III). [Ni(III)Pc(X)n]+ complexes were calculated with a total charge Q = +1 and a doublet multiplicity. A trend is found for both oxidation states, an increase in the ηDA values from 1 to 19. Because ηDA is interpreted as the affinity between the donor species (NO) and the acceptor species (substituted nickel complex), a lower value of ηDA indicates a higher affinity between the species favoring its interaction. The results in Figure 3 can be understood in terms of the complexes bearing electron-withdrawing substituents (aza nitrogen atoms, F, SO3H, Cl, CN, OH) presenting the best affinity toward NO because the difference between the ionization potential of NO and the electronic affinity of the Ni(II) Pc(X)n or [Ni(III)Pc(X)n]+ complex (eq 4) corresponds to the smaller values. For Ni(II) complexes, the ηDA values vary from 0.24 (1) to 0.36 (19) a.u., giving a difference between them of 75 kcal/mol, and for Ni(III) complexes, that difference goes between 1 and 19, and it is of 69 kcal/mol. These results indicate that the substituent effect is very similar in the Ni(II) and Ni(III) complexes. The most interesting point is that coherence among the reactivity descriptors, bimolecular (ηDA) and unimolecular (μ, ω), is found. A sample of that is observed in Figure 4, where a linear relationship is produced between ηDA and ω, with a regression coefficient of R2 = 0.99. A similar result was found for the correlation ηDA versus qNi(II) (Figure 5), a linear relationship with a value of R2 = 0.96. These results give account of which complexes would contribute in a best way in the oxidation of NO. It is interesting to highlight that the complexes with Ni(III) present the smaller value of ηDA, indicating that this oxidation state favors the affinity with NO

Figure 2. Mulliken charges of nickel atom in Ni(II)Pc(X)n complexes calculated at the B3LYP/LANL2DZ/6-31G(d) level of theory.

from 1 (∼0.59) to 19 (∼0.53), indicating that although a substituent effect on the nickel atom occurs this is not dramatic. The electron-withdrawing substituents can produce an electron shifting from the metal atom to itself, leading to a nickel atom more deficient in electrons and more able to be reduced. The theoretical characterization of the Ni(II)Pc(X) n complexes by unimolecular and bimolecular descriptors is a 16981



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Figure 4. Linear relationship between donor−acceptor hardness and electrophilicity of Ni(II)Pc(X)n complexes (R2 = 0.99) at the B3LYP/ LANL2DZ/6-31G(d) level of theory. Figure 6. Frontier molecular orbitals at the B3LYP/LANL2DZ/631G(d) level of theory.

Ni(II)Pc, the HOMO is localized on the phthalocyanine ligand and the oxidized state would be Ni(II)Pc(+1). These results show that the properties of this kind of complexes can be modulated by the substitution in the β position of the ligand. Then, the oxidized state [Ni(III)Pc(X)n]+ would be reduced in the SOMO molecular orbital, which in all cases is localized completely on the phthalocyanine ligand (Figure 6), leading to the state Ni(III)Pc(−1)(X) n. The latter results would demonstrate that when the substituted nickel(III) phthalocyanine is reduced, the nickel atom would not participate in the reduction and in consequence would not have a change in the oxidation state of the metal atom. To have more background to confirm the mechanism proposed for the oxidation of NO, we explored the formation of the NO-Ni(II)Pc(X)n and [NO-Ni(III)Pc(X)n]+ adducts and analyzed the charge transfer from NO toward the nickel phthalocyanine. First, we optimized the corresponding adducts for Ni(II) and Ni(III) with the substituents F16, (CH3O)4, and the unsubstituted complex. The optimized molecular geometry of each adduct is shown in Figure 7. Because the interaction distance between NO and the nickel complex is not a covalent distance, we changed the density functional for the calculation of the adducts and used one proposed in the literature for noncovalent distances, M05-2X.55,56 For comparison purpose, we calculated the electronic structure of NO and NiPc (NiPc(X)n) (Ni(II) and Ni(III)) with M05-2X over the B3LYP optimized geometry. For the adducts with oxidation state of Ni(II), we found that the nitrogen atom of NO approaches the nickel atom as looking for a reduction in NO considering that in Ni(II)Pc(X)n HOMO is localized in Ni. In the case of the adducts with Ni(III), the NO approaches the ligand and not the metal atom as if NO was oxidized, which is coherent with the SOMO above analyzed; the reduction of the Ni(III) complex is over the ligand. For the adducts NO-Ni(II)Pc(X)n and NO-Ni(III)Pc(X), we calculated the atomic charges for determining the charge transfer between the donor and acceptor species; the results are presented in Table 1 and compared with the corresponding isolated species, nickel phthalocyanines complexes, and NO. With the aim to visualize clearly the charge transfer, we do the analysis per fragment Ni, Pc(X)n, N, and O of NO for the

Figure 5. Linear relationship between donor−acceptor hardness and Mulliken charge of nickel atom of Ni(II)Pc(X)n complexes (R2 = 0.96) at the B3LYP/LANL2DZ/6-31G(d) level of theory.

and suggesting that the oxidation of NO would be preferred with these complexes. According to the obtained results, we can propose a mechanism for the oxidation of NO Ni(II)Pc(X)n → [Ni(III)Pc(X)n ]+

(5)

[Ni(III)Pc(X)n ]+ + NO· → Ni(III)Pc(− 1)(X)n + NO+ (6)

where the Ni(II)Pc(X)n complex is first oxidized to [Ni(III)Pc(X)n]+, which just now is ready to accept the electron provided for the oxidation of NO. This is in agreement with related experimental works previously reported5,51,52 in the sense that it has been suggested that the oxidation of NO is mediated by nickel(III) phthalocyanine complexes. It is interesting to mention that a similar mechanism was proposed and confirmed by us in previous works for understanding the oxidation of several chlorophenols on sulphonated nickel(III) phthalocyanine complexes.53,54 A view of the frontier molecular orbitals (FMOs) of the Ni(II) and Ni(III) complexes can help us to understand the mechanism proposed here. As an illustration, we analyze the FMOs for three cases: two substituted complexes, one of them with an electron-withdrawing substituent (NiPcF16) and the other one bearing an electron donor substituent (NiPc(CH3O)4), and the unsubstituted complex NiPc. The oxidation of Ni(II)Pc(X)n (closed shell) would occur in the HOMO molecular orbital (Figure 6), where in the substituted cases it is localized on the nickel atom. Therefore, this oxidation would lead to the formation of [Ni(III)Pc(X)n]+ (open shell). For 16982



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Figure 7. Optimized structure of the NO-Ni(II)(X)n and NO-Ni(III)(X)n adducts calculated at the M05-2X/LANL2DZ/6-31G(d) level of theory.

oxidation of one electron instead a partial amount of electron.53,54 In the case of the nickel atom that happens in all adducts, these are reduced in comparison with the isolated nickel complex. For the adduct having Ni(II), we found that the electron loss presented by the nitrogen of NO favors an electron gain on nickel atom and in the adduct of Ni(III) that electron gain mainly occurs on the phthalocyanine ligand. For the three adducts of Ni(II), we found that the electrons transferred from NO are accepted in 100% by the nickel atom, and the Pc ligand is even oxidized. In the case of adducts of Ni(III), the situation is different, and we observe an effect of the substituent. In the NO-Ni(III)PcF16 adduct, the charge transferred from NO to the nickel complex produces a reduction in both Ni and Pc, and the charge rather localized on the phthalocyanine ligand (91%). For the NO-Ni(III)Pc(CH3O)4 and NO-Ni(III)Pc adducts, the charge transferred from NO is localized over the Pc ligand (80%) and the Ni (20%). It means that for the oxidation of NO, an electronwithdrawing substituent such as fluorine atom favors a reduction more localized on the Pc ligand. All of these results confirm the mechanism proposed in eq 6: the oxidation of NO on substituted nickel(III) phthalocyanine provokes a reduction that is mainly localized in the phthalocyanine ligand and a little reduction occurs on the metal.

Table 1. NAO Electronic Charge (Q) of the Fragments Belonging to the NiPc(X)n Complexs and to the NONiPc(X)n Adducts Calculated at the M05-2X/LANL2DZ/631G(d) Level of Theory and Distance (rN···Ni) between the Nitrogen Atom (NO) and the Nickel Atom (NiPc(X)n)a

Ni(II)PcF16 NO-Ni(II)PcF16 Ni(III)PcF16 NO-Ni(III)PcF16 Ni(II)Pc(CH3O)4 NO-Ni(II)Pc(CH3O)4 Ni(III)Pc(CH3O)4 NO-Ni(III) Pc(CH3O)4 Ni(II)Pc NO-Ni(II)Pc Ni(III)Pc NO-Ni(III)Pc NO a

Q-Ni

QPc(X)n

0.792 0.735 0.805 0.784 0.773 0.626 0.786 0.762

−0.792 −0.787 0.195 −0.023 −0.774 −0.738 0.214 0.119

0.774 0.626 0.787 0.761

−0.774 −0.738 0.213 0.113

QN(NO)

QO(NO)

rN···Ni /Å

0.237

−0.185

2.94

0.350

−0.110

0.244

−0.133

0.284

−0.165

0.244

−0.132

0.282 0.213

−0.156 −0.213

1.95

1.95

Q > 0: electron loss; Q < 0: electron gain.

adducts of Ni(II) and Ni(III). Note that in a previous work,57 where we studied the adsorption of nickel phthalocyanine and tetrasulphonated nickel phthalocyanine on glassy carbon, we calculated a different value for the charge of nickel atom in both ligands (≈ −1.0), but this value was obtained with a semiempirical method (PM3) that only considers the valence electrons in the calculation and does not include the core electrons. However, in that work, this approximation was used along of the whole study ensuring coherence in the analysis. We found that in all cases (six adducts) the nitrogen atom of NO is more oxidized than in isolated NO, showing that an electron loss occurs (Q > 0), but the larger oxidation is observed for the Ni(III) adducts. This behavior is more important for the nickel complex bearing electron-withdrawing substituent such as NO-Ni(III)PcF16 (Q = 0.35). The adducts NO-Ni(III)Pc(CH3O)4 and NO-Ni(III)Pc present very similar values, Q = 0.28. The results suggest that Ni(III) is the oxidation state that best oxidizes the NO. Note that the oxidation that undergoes NO does not correspond to the net charge of one electron but ∼0.2 to 0.3 electrons. We have demonstrated in previous works for the oxidation of a set of chlorophenols mediated by tetrasulphonated nickel phthalocyanine that a through-space charge transfer would allow an

4. CONCLUSIONS A theoretical investigation of the oxidation of NO mediated by substituted nickel phthalocyanine (NiPc(X)n) was performed at the B3LYP and M05-2X/LANL2DZ/6-31G(d) levels of theory. Unimolecular (chemical potential, electrophilicity) and bimolecular (donor−acceptor hardness) reactivity descriptors were determined for studying the substituent effect (Xn) and the oxidation state effect (Ni(II), Ni(III)) of NiPc(X)n on the oxidation of NO. The results indicate that electron-withdrawing groups favor the reduction in the complexes NiPc(X)n and Ni(III) presents the best affinity for NO. Optimized molecular structures for the adducts NO···NiPc(X)n were calculated for Ni(II) and Ni(III) and show a charge transfer from NO to the phthalocyanine complex, where Ni(III) is the oxidation state that leads to a larger oxidation of NO.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.I.C.-J. acknowledges the financial support from CONICYTCHILE from Project FONDECYT No. 1090700 and DICYTUSACH from Project Apoyo Complementario by computational time.

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