Elsevier

Chemical Physics

Volumes 469–470, 1–13 May 2016, Pages 38-48
Chemical Physics

How the change of the ligand from L = porphine, P2−, to L = P4-substituted porphine, P(P)42−, affects the electronic properties and the M–L binding energies for the first-row transition metals M = Sc–Zn: Comparative study

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Abstract

We performed comparative DFT study, including Natural Bond Orbitals (NBO) analysis, of the binding energies between all the first-row transition metals Mn+ (M = Sc–Zn) and two ligands of the similar type: porphine, P2−, and its completely P-substituted counterpart, P(P)42−. The main findings are as follows: (i) complete substitution of all the pyrrole nitrogens with P-atoms does not affect the ground spin state of metalloporphyrins; (ii) generally, for the MP(P)4 compounds the calculated HOMO/LUMO gaps and optical gaps are smaller than for their MP counterparts; (iii) the trends in the change of the binding energies between Mn+ and P(P)42−/P2− are very similar for both ligands. The complete substitution of the pyrrole nitrogens by the P-atoms decreases the Mn+-ligand binding energies; all the MP(P)4 compounds studied are stable according to the calculated Ebind values and therefore can be potentially synthesized.

Introduction

Metalloporphyrins have been of great interest because of being cofactors in numerous enzymes [1], [2], [3], [4] and due to their various technological applications [5], [6], [7], [8], [9], [10], [11], [12]. The porphyrins biological functions include: (i) O2 transport (hemoglobins) and storage (myoglobins); (ii) xenobiotic detoxification (cytochrome P450s); (iii) oxidative metabolism (cytochrome c oxidase); (iv) gas sensing (soluble guanylate cyclases); (v) input/regulation of the circadian clock (nuclear hormone receptor, Reverb α, mPER2); (vi) microRNA processing (DGCR8); (vii) antibactericides/microbicides (myeloperoxidase); (viii) thyroid hormone synthesis (thyroperoxidase); (ix) collection and transport of light energy (antennae complexes); (x) conversion of solar energy to chemical energy (photosynthetic reaction centers); (xi) electron transfer (cytochromes); (xii) oxidative phosphorylation; (xiii) NO scavenging, and a large number of other enzymatic reactions (peroxidases, catalases, cytochromes P450, methylreductases, methyltransferases, etc.) [1], [2], [3], [4].

Technological applications of porphyrins and their derivatives include: catalysis [1], [2], [5], [6], [7], molecular photonic devices [6], [8], medicine [1], [2], [6], artificial photosynthesis [9], [10], [11], sensitizers for dye-sensitized solar cells [12], and sensor devices [6], [13]. In Nature, aerobic oxidation processes are carried out in a highly selective manner by mono- or dioxygenases under mild conditions [5], [7]. Cytochrome P450, a well-known type of monooxygenase, possesses an Fe-porphyrin core and can catalyze a wide variety of oxidation reactions: epoxidation, hydroxylation, dealkylation, dehydrogenation, and oxidation of amines, sulfides, alcohols and aldehydes [5], [7]. This fact stimulated extensive studies of metalloporphyrins, with a core structure closely resembling that of the iron porphyrin core of cytochrome P450, as effective catalysts for oxidation reactions [1], [2], [5], [7]. Also, the rich and extensive absorptions (i.e., π–π transitions) in porphyrins, which are essentially ‘the pigments of life’ [9], hold strong promise for an efficient use of the solar spectrum. Over the recent decades, porphyrins have attracted ever growing attention as light harvesting building blocks in the construction of molecular architectures [9], [10], [11], [12].

The structural and electronic properties along with the binding ability of porphyrins can be easily and broadly tuned by replacing one or several pyrrole nitrogens with other elements [14], [15], [16]. Until recently, the effects of the pyrrole nitrogen replacement in porphyrins with phosphorus were investigated for just a few porphyrins and their derivatives [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Thus, the study by Delaere and Nguyen [17] performed using the density functional theory (DFT) on the P-containing porphyrins with one or two pyrrole nitrogens replaced by phosphorus showed that upon substitution of a NH- by a PH-unit the carbon skeleton of the porphyrin remained essentially planar, whereas replacement of a N- by a P-atom was found to weakly distort the P-containing five-membered ring. Nearly equal red shifts of Q- and B-bands were calculated upon substituting NH- by PH-units, whereas Q-bands were calculated to be red-shifted much larger than B-bands upon substitution of an N-atom by a P-atom. Later, Matano and colleagues reported synthesis and characterizations of the various phosphaporphyrins and their derivatives with only one pyrrole nitrogen replaced by a P-atom [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The incorporation of the P-center in the porphyrin core was shown to cause structural distortions [18]. The UV–vis absorption spectra of the P-substituted porphyrins showed the characteristic porphyrin transitions with significant red shifts [18]. In another study, DFT calculations on model compounds showed that the P,X,N2-porphyrins (X = N, S) possessed considerably small HOMO/LUMO gaps as compared with N4- and S,N3-porphyrins, which was reflected in the red-shifted absorptions, low oxidation potentials, and high reduction potentials [25]. The structures and coordination chemistry of phosphole-containing calixphyrins and the catalytic activities of their Pd and Rh complexes were studied both experimentally and computationally [23]. It is also worthwhile to mention the 2009 DFT investigation of electronic structure and reactivity of the Pd complex of P,S-containing hybrid calixphyrin [27].

As can be seen, the inclusion of one or several P-atom(s) in the porphyrin core would undoubtedly produce unusual structures, reactivities, electronic, optical, and coordinating properties of metalloporphyrins. Of course, it would be extremely interesting to receive answers about possible structures, properties, and reactivities of such compounds, and to figure out what differences would complete substitution of the pyrrole nitrogens with phosphorus cause. Even more significant and intriguing would be to find out: what potential novel applications would these metalloporphyrins with all pyrrole nitrogens replaced with phosphorus (MP(P)4) have? Could we build catalytic systems based on them? Would it be possible to use MP(P)4 compounds as building blocks for nanotechnology? What about their potential complexes with fullerenes? The list of such questions could be growing and growing, and computational studies, of course, can be of great help here.

With this in mind, we should notice that so far computational studies of metalloporphyrins with all pyrrole nitrogens replaced with phosphorus (MP(P)4) have been relatively scarce and are generally represented by our previous reports on the NiP(P)4 compound [28] and on the series of metalloporphyrins MP(P)4, where M = Sc, Ti, Fe, Ni, Cu, and Zn [29]. For the current research, we were motivated by the previous studies of metalloporphyrins with one and four phosphorus atoms and by the 2013 report by Hirao and co-workers [30] on comparative study of binding energies between several transition metals (Cr2+, Mn2+, Fe2+, Co2+, Ni2+, and Cu2+) and porphine/porphine-derived ligands.

In the current study, we decided to extend our previous work in order to cover all the elements of the first transition metal row. We report the comparative computational (DFT) study of electronic properties (ground spin states and low-lying spin states, HOMO/LUMO gaps and optical gaps) and binding energies between the first-row transition metal M2+ (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two ligands L: porphine P2− and its completely P-substituted counterpart, P(P)42−. We wanted to check thoroughly if different DFT approaches would affect electronic properties and Mn+-L binding energies and thus employed five DFT functionals (see below): three hybrid functionals and two generalized gradient approximation (GGA) functionals. Also, for more complete picture, we included in our study the cationic species, with Sc3+, Mn4+, and Ni3+, thus covering the beginning, the middle, and the end of the first row of transition metals. Also, we performed the NBO analysis of the MP(P)4 species studied and applied the results of this analysis in consideration of the bonding between Mn+ and P42− species.

The paper is organized as follows: in the next section, we describe the computational approaches employed; then we address comparison of the ground spin states and the low-lying spin states calculated for MP(P)4 and MP; next, we address the comparison of the electronic properties of the MP(P)4 and MP species (HOMO/LUMO gaps and optical gaps); then, we compare the binding energies between the transition metal ions and two ligands investigated and provide the relevant results of the NBO analysis; finally, we summarize the research findings and discuss further research perspectives.

Section snippets

Computational details

The study described here was performed using the Gaussian09 package [31]. Geometries of all the MP(P)4 species were first optimized within the C2 symmetry constraints, and the resulting structures were assessed using vibrational frequency analysis to probe whether or not the MP(P)4 structures represent true minimum-energy geometries. If any imaginary frequency was observed, we performed further optimizations along those normal coordinates (without symmetry constraints). For all the species

Results and discussion

In this study, we extended our computational research further to cover the P(P)42− compounds of all the transition metals of the first row: Sc2+P(P)4, Ti2+P(P)4, V2+P(P)4, Cr2+P(P)4, Mn2+P(P)4, Fe2+P(P)4, Co2+P(P)4, Ni2+P(P)4, Cu2+P(P)4, Zn2+P(P)4−, and, in order to clarify if there could be any differences for charged MP(P)4 species, we included into consideration the following cations from the beginning, center and end of the transition metals row: Sc3+P(P)4, Mn4+P(P)4, and Ni3+P(P)4. In Sc2+

Conclusions and perspectives

We performed systematic comparative DFT study employing three hybrid and two GGA density functionals on the electronic properties (ground spin states, HOMO/LUMO and optical gaps) and the binding energies between the first-row transition metals M2+ (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two similar ligands: porphine, P2−, and its completely P-substituted counterpart, P(P)42−. In this study, we covered the MP(P)4 and MP compounds following the sequence of electronic configurations: 3d1

Conflict of interest

No conflict of interest exist in this paper.

Acknowledgments

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Grant “Estudo Teórico Computacional de Sistemas Nanoestruturados com Potencial Aplicação Tecnológica”, number 402313/2013-5, approved in the call N° 70/2013 Bolsa de Atração de Jovens Talentos – BJT – MEC/MCTI/CAPES/CNPq/FAPs/Linha 2 – Bolsa de Atração de Jovens Talentos – BJT. The computational resources of the centers Centro Nacional de Processamento de Alto Desempenho em São Paulo, Núcleo de

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