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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.1 Concepción mar. 2004

http://dx.doi.org/10.4067/S0717-97072004000100017 

AZA NITROGENS EFFECT ON THE ELECTRONIC PROPERTIES
OF COBALT PORPHYRINE AND DERIVATIVES

Gloria I. Cárdenas-Jirón

Laboratorio de Química Teórica, Departamento de Ciencias Químicas, Facultad de Química y Biología,
Universidad de Santiago de Chile, Santiago, CHILE E-mail: gcardena@lauca.usach.cl

(Received: August 6, 2003 - Accepted: December 5, 2002)

ABSTRACT

A set of electronic properties calculated at the B3LYP/LACVP(d) level of theory for four cobalt macrocycles containing a ligand like porphyrin (CoP) and a benzoporphyrin (CoTBP) has been analyzed. The effect of aza nitrogen atoms (aza-N) present in the ligand (CoTAP, CoPc) on the electronic properties chosen has been studied. For the pair CoP/CoTAP we found a notorious effect of the presence of aza-N atoms in each electronic property studied. However, for the pair CoTBP/CoPc a different result was obtained. The dipole moment and the atomic charges, the latter evaluated on the cobalt atom and on the aza-N atoms, do not show the isolated effect of the aza nitrogen atoms and rather it appears to be mixed with the effect produced by the ligand size. On the contrary, electronic properties such as spin density and electrostatic potential evaluated on the cobalt atom clearly show the effect of the aza nitrogen atoms predicting that the latter present an electron withdrawing behavior in this kind of molecules.

Keywords: Cobalt Porphyrine, Aza Nitrogen, Electronic Properties.

INTRODUCTION

Porphyrins have a wonderful repertoire of optical and electronic properties that allow them to play a variety of roles in nature and in technical applications [1]. Porphyrins are the most commonly occurring biological tetrapyrrole macrocyclic ligands and have been the subject of extensive investigation and review [2-6]. In addition to the obvious interest in the biological properties of metalloporphyrins [7] there is also great interest in their catalytic behavior [8-16]. Fused porphyrin arrays have emerged as potential molecular wires as well as non-linear optical materials [17-18]. Porphyrin like macrocycles have also shown merit in medicine as phototherapeutic agents and radiation enhancers, benefiting various medical fields, such as oncology, cardiology and dermatology [19-21]. The basic structure of the porphyrine framework can be described as a planar aromatic system but steric and substituent effects around the periphery often lead to nonplanar forms of porphyrine. The basic structure consist of four pyrrole units which are linked in a circular manner by methine or azamethine bridges. The hole in the center of these macrocycles can accomodate either metals (e.g., K, Ti, Fe, Co, Pb, etc.), semi-metals (e.g., Si, Sb, etc.) or hydrogens, where the central atom coordinates with the pyrrole nitrogens. The size of the hole depends on the kind of bridges connecting the pyrrole units. A macrocycle with azamethine bridges has a smaller inner cavity than a macrocycle with methine bridges. In the last years, our attention have been focused to apply the quantum chemistry on transition metal macrocycles to rationalize the reactivity at a global and local level [22-30]. In this work, the effect that the azamethine bridges or aza nitrogen atoms produce on some electronic properties of porphyrine and derivatives calculated at a quantum chemistry level will be analyzed.

COMPUTATIONAL DETAILS

Theoretical calculations reported in this paper were carried out using gradient-corrected density functional theory (DFT) with the Becke-Lee-Yang-Parr composite exchange correlation functional (B3LYP) as implemented in the TITAN package [31]. The LACVP(d) basis set was employed in all of the theoretical calculations which uses a LACVP pseudopotential for the heavy atoms as the cobalt atom and a 6-31G for the lighter atoms such as H, C and N atoms. In order to improve the electronic description a set of polarization functions constituted by five d atomic orbitals denoted by (d) was also included for each atoms. Thus, a number of 382, 374, 622 and 614 basis functions was used in the theoretical calculations of CoP, CoTAP, CoTBP and CoPc, respectively. A LACVP(d) basis set has been successfully used in previous calculations on metalloporphyrins and derivatives and transition metal complexes [23,27,32]. Full geometry optimization without symmetry restrictions was performed on the chemical systems: cobalt porphyrin (CoP), cobalt tetraazaporphyrin (CoTAP), cobalt tetrabenzoporphyrin (CoTBP) and cobalt phthalocyanine (CoPc) (tetraazabenzoporphyrin). Figure 1 shows a structural view of the cobalt porphyrin and derivatives studied in this paper. A doublet multiplicity was used for each molecule because to the +2 oxidation state of the cobalt atom. Thus, the ROB3LYP (Restricted Open B3LYP) method was used instead of a UB3LYP method in order to avoid the spin contamination which is important in this kind of macrocycles.


Fig. 1. Structure of: a) cobalt porphyrine (CoP); b) cobalt benzoporphyrine (CoTBP). The aza nitrogen atoms replacing a C-H group are included to denote their position along the macrocycle. The inclusion in CoP is named as CoTAP and the inclusion in CoTBP is named as CoPc.

 

RESULTS AND DISCUSSION

CoP and CoTAP Systems. Resulting electronic properties obtained from the theoretical calculations are included in Table 1. In particular, atomic properties associated to the atoms that are directly bonded to the cobalt atom have been considered. This because they directly receive the effect that the cobalt atom presents when a change on the periphery of the ligand has occurred. A comparison of the results between CoP and CoTAP shows an important change in some properties. The atomic charges that present the four nitrogen atoms (N3, N5, N7, N8) bonded to the cobalt atom show a decrease from CoP to CoTAP in » 0.3 charge units. The atomic charges have been calculated using the partition method for the electronic density named as natural population analysis (NAP) [33]. The natural population analysis is an alternative to conventional Mulliken population analysis and seems to exhibit improved numerical stability and to better describe electron distribution in compounds of high ionic character, such as those containing metal atoms [33]. The results obtained for the atomic charges indicate that when the aza nitrogen atoms are included in CoTAP, the nucleophilicity degree of the (N3, N5, N7, N8) atoms is modified and decrease suggesting that the aza nitrogen atoms have a behavior of electronwithdrawing atom. A similar result is observed for the cobalt atom, where the positive value increases going from CoP to CoTAP. In this case, as the charge has a positive value, it may be concluded that the cobalt site is more electrophilic when the aza nitrogen atoms are included. This situation occuring in CoTAP may be favorable for the cobalt atom when the macrocycle is submitted to a reduction process due to the decrease in charge with respect to that found in CoP. A dramatic change by the aza nitrogen atoms effect is observed for the dipole moment that reflects in some measure the symmetry along the molecule. The results show that the dipole moment increases from CoP to CoTAP indicating that the latter is a most polar system than the former. It is interesting to note that the same effect is viewed with the electrostatic potential. The electrostatic potential is an electronic property that has been used as reactivity descriptor [34] because it provides the electron rich and electron poor regions and it is defined as the energy of interaction of a positive point charge located at a given point in the space with the nuclei and electrons of a molecule. The electron rich regions are obtained calculating the interaction between the electrons of the molecule and a positive point charge. If one draw a surface of the electrostatic potential, one can see that in the case of CoP one negative region is extended along the ligand but mainly located around of the cobalt atom denoting that the electronic distribution is nearly symmetric. However, when a surface of the electrostatic potential for CoTAP is drawed, the one negative region is now modified to four negative regions located on the aza nitrogen atoms leading to a symmetry less. In particular, the values of the electrostatic potential for the cobalt atom are included in Table 1. A negative value of -8.570 for CoP is modified to a positive value of 24.000 for CoTAP. Againly, the effect of the presence of aza nitrogen atoms is important when a property as the electrostatic potential is analyzed. Due to that these molecules are open shell and present a doublet multiplicity, an interesting property to analyze is the spin density (r s). Results of r s are also included in Table 1 but only for the cobalt atom and the (N3, N5, N7, N8) atoms. It may be observed that the spin density is mainly located on the cobalt atom with a 95% in the case of CoP and 93% in CoTAP. Note that as these molecules present a doublet they present one unpaired electron in its SOMO (Single Occupied Molecular Orbital) frontier molecular orbital. Here, it as may be seen the effect of the aza nitrogen atoms is slight. Finally the bond lengths of N-Co were also analyzed. We observed that in all four bonds the bond length decrease from CoP to CoTAP in a range of 0.07 Å -0.08 Å indicating that the bond force associated to the N3-Co, N5-Co, N7-Co and N8-Co bonds increase doing stronger the interaction between these atoms by effect of the presence of aza nitrogen atoms. These results are an indicator of the electronwithdrawing capacity of the aza nitrogen atoms.

Table 1. Electronic properties calculated with full geometry optimization at B3LYP/LACVP(d) level of theory. qx: atomic (x) charges obtained using the method of Natural Population Analysis (NAP); r sx: atomic spin density; m : dipole moment (Debye); VCo: electrostatic potential (kcal/mol) of the cobalt atom and rNx-Co: nitrogen-cobalt bond length (Å)


Property

CoP

CoTAP

CoTBP

CoPc


qN3

-0.701

-0.326

-0.636

-0.727

qN5

-0.656

-0.320

-0.677

-0.682

qN7

-0.656

-0.325

-0.676

-0.682

qN8

-0.700

-0.320

-0.636

-0.727

qCo

1.345

1.632

1.361

1.307

r sN3

0.007

0.020

0.000

0.008

r sN5

0.000

0.020

0.005

0.000

r sN7

0.000

0.020

0.005

0.000

r sN8

0.007

0.020

0.000

0.008

r sCo

0.952

0.931

0.957

0.937

m

0.228

1.642

0.041

0.010

VCo

-8.570

24.000

-3.800

-0.600

rN3-Co

1.996

1.919

2.010

1.937

rN5-Co

1.989

1.919

2.025

1.932

rN7-Co

1.990

1.919

2.026

1.932

rN8-Co

2.001

1.919

2.011

1.937


CoTBP and CoPc Systems. The results obtained from the theoretical calculations are displayed in Table 1. A comparison of the atomic charges between CoTBP and CoPc shows that for the (N3, N5, N7, N8) atoms an increase of negative charge is produced when the aza nitrogen atoms are included. The (N3, N5, N7, N8) atoms are converted in most nucleophilic sites going from CoTBP to CoPc. Note that an opposite behavior is found for the pair CoP/CoTAP, a decrease in the nucleophilicity degree of the (N3, N5, N7, N8) atoms is obtained for the system containing aza nitrogen atoms (CoTAP). In the case of the cobalt atom, where the charge has a positive value, it decrease going from CoTBP to CoPc indicating a less electrophilicity for the latter. An opposite result is obtained for the pair CoP/CoTAP where the electrophilicity of the cobalt atom is highest when the aza nitrogen atoms are present. In addition, the atomic charges analyzed for the pair CoTBP/CoPc show an effect of the aza nitrogen atoms that is opposite to that obtained for the pair CoP/CoTAP, which may be explained as a sum or coupling of two effects. On the first hand, the effect that produces the presence of aza nitrogen atoms that are tipically electrowithdrawing atoms, and on the other hand, the effect that have the benzoporphyrine rings (CoTBP or CoPc) that present a higher electron delocalization than a porphyrine ring in a CoP or CoTAP. From Table 1, if one analyze only the effect of the ligand size, for example the pair CoP/CoTBP without the presence of the aza nitrogen atoms, one find that the N3 and N8 sites present less negative charges meaning that the sites have lost charge. The same occurs for the cobalt atom that presents a decrease in its charge, from 1.345 to 1.361. However, the change in the ligand size also produces an electron gain in the N5 and N7 sites. If now the charges of the pair CoTAP/CoPc are analyzed, where the presence of the aza nitrogen atoms is constant but the ligand size has been changed, one find that in all atomic sites an electron gain was produced. On the basis of these results, it may be concluded that a larger ligand leads to an electron gain in the atoms studied in this paper. Thus, the results of atomic charges obtained for CoPc may be explained in terms that the effect produced by a ligand such as the benzoporphyrine, donating charge to the (N3, N5, N7, N8) atoms and to the cobalt atom, is more important than the effect produced by the aza nitrogen atoms. In contrast to that obtained for the pair CoP/CoTAP where the system is most polar when the aza nitrogen atoms are present, here the dipole moment with a small value for both CoTBP and CoPc systems decreases to a near zero value for CoPc. The effect of the aza nitrogen atoms is opposite to the pair CoP/CoTAP leading to a system rather apolar (CoPc). Although a local property, such as the atomic charge, and a global property, such as the dipole moment, calculated for the pair CoTBP/CoPc show an opposite behavior with respect to the pair CoP/CoTAP, the electrostatic potential evaluated on the cobalt atom predicts the same trend in both pair of molecules. VCo decrease from CoTBP to CoPc indicating that the presence of the aza nitrogen atoms convert to the cobalt atom in a site less nucleophilic. Note that the effect of the aza nitrogen atoms produces more dramatic changes in the CoP/CoTAP pair with values of -8.570/24.000, that is a nucleophilic site is changed to an electrophilic site. Other property that predicts the same trend in the two pair of molecules is the spin density. For the pair CoTBP/CoPc, the theoretical calculations predict a r s mainly centered on the cobalt atom with a slight difference between both molecules, 0.957 and 0.937 for CoTBP and CoPc, respectively. Although the effect produced by the aza nitrogen atoms is small, it may be noted that a decrease in r s for CoPc is found. Finally, the analysis of the bond geometric parameters (rN-Co) analyzed in this work for the pair CoTBP/CoPc shows that a decrease in each of them due to the effect of the aza nitrogen atoms is presented. The variations from CoTBP to CoPc are in the range of 0.07 Å-0.09 Å indicating that an increase of the bond force is produced.

CONCLUSIONS

An analysis of some electronic properties calculated at the B3LYP/LACVP(d) level of theory has been carried out for cobalt macrocycles containing a porphyrine or a benzoporphyrine as ligand. In particular, the effect of the presence of aza nitrogen atoms in the ligand has been studied. For the pair CoP/CoTAP, all properties studied here indicate that these atoms present an electronwithdrawing behavior. In the case of the pair CoTBP/CoPc, the atomic charges and the dipole moment show that an effect related to the ligand size seems to be more important than the effect of the aza nitrogen atoms. However, properties such as spin density and electrostatic potential evaluated on the cobalt atom, and bond lengths associated to the N-Co bond clearly show the effect of the presence of the aza nitrogen atoms and therefore the electronwithdrawing capacity of these atoms.

ACKNOWLEDGMENTS

This article is dedicated to Dr. Fernando Zuloaga Vargas with whom worked through the Project FONDECYT Postdoctorate N° 3980031 during the years 1998-1999 and save a beautiful memory. GICJ thanks financial support from Project FONDECYT Lineas Complementarias N° 8010006 and to Vicerrectoría de Investigación y Desarrollo (USACH) for a position of Research Associate.

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