A theoretical study of the conformation of meso-tetraphenylporphyrin (TPPH₂), its anions, cations and metal complexes (Mg²⁺, Ca²⁺ and Zn²⁺)

Abstract

The conformational aspects of meso-tetraarylporphyrins, exemplified by TPP, determine most of their properties, such as metal complexation, electronic spectrum, and light emission [1][2][3]. Although there is abundant literature on these aspects, there are almost no theoretical papers on the influence of tautomerism, deprotonation, protonation and metal complexation.The Madrid group has devoted some papers to porphyrins, preparing for the first time meso-tetrapyrazolylporphyrins and studying their atropisomerism [4], determining their X-ray structures [5,6], and preparing some perfluorinated analogs [7]. Later, we used theoretical calculations to study the acid properties of porphyrins [8] and the interaction of indazoles with zinc porphyrins as a nitric oxide synthase (NOS) inhibition model [9]. Other authors have studied the mechanism of inner-hydrogen migration in free-base porphyrin (tautomerism) [10]. In 2009, two theoretical studies of TPPH 2 (see Fig. 1) using the GW approximation were reported: one to calculate its ionization potential [11] and the other to calculate its electronic and optical spectra [12].During the last decade, the Aveiro group has been involved in the development of new methods for the structural modification of porphyrins, namely by cycloaddition reactions [13]. DFT studies allowed us to justify the differences observed on the site selectivity of successive 1,3-dipolar cycloadditions to meso-tetraarylporphyrins [14]. We have also reported the synthesis of [1,2,3]-triazolo[4,5-b]porphyrins, including the single crystal X-ray diffraction of a porphyrin pentamer [15] and the synthesis and photophysical studies of porphyrinphthalocyanine dyads with hindered rotation [16].However, by far the most important publication in relation with the present paper is one by Okuno et al. [17]. These authors calculated at the B3LYP/6-31G(d) level the rotation barrier about the phenyl-porphyrin 1 and the ortho-methoxyphenyl-porphyrin 2 bonds (Fig. 2). For compound 1 the calculated barriers were 69.5 kJ.mol -1 (no corrections), 67.4 kJ.mol -1 (zero-point energy and thermal corrections) and 75.7 kJ.mol -1 (free energy). Since for meso-tetrakis(meta-methoxyphenyl)porphyrin, metaMeOTPPH 2 (3) a value of 44.8 kJ.mol -1 was measured [18], the authors conclude that the calculated and experimental values are consistent considering that the model compound 1 has only one phenyl ring. For ortho-substituted compound 2 the calculated uncorrected barrier, 105.0 kJ.mol -1 , as expected, is considerably higher. ABSTRACT:A theoretical study has been carried out on TPPH 2 (both tautomers), its deprotonated and protonated forms, as well as complexes of TPP containing Mg 2+ , Ca 2+ and Zn 2+ ions. Two properties have been analyzed. The first one considers the conformation of the meso-phenyl rings and the deformation of the porphyrin macrocycle, the second one relates to the barriers of rotation of the meso-phenyl rings (atropisomerism). In the case of the Zn complex (ZnTPP), the coordination effects with the N3 of 1H-imidaz...