Theoretical study of structures of alkaline-earth metal (M=Be, Mg) complexes of porphyrin (MP), porphyrazine (MPz) and phthalocyanine (MPc)

Abstract

In the present work systematic study of the electronic and geometrical structures of MP, MPz and MPc complexes is performed. Density functional theory (three-parameter hybrid functional B3LYP), Moller-Plesset perturbation theory (MP2, only for BeP) and correlation-consisted valence triple-zeta basis sets (cc-pvtz) are used. All calculations were carried out using PC GAMESS program. The geometrical configuration of D 4h symmetry were optimized by B3LYP/cc-pvtz method for all complexes and free anions P -2, Pz -2 and Pc -2. For all compounds except BeP complex the planar D 4h configuration corresponds to the minimum on the potential energy surface. According to both B3LYP and MP2 methods the equilibrium structure of BeP possesses the D 2d symmetry and originates from ruffling-distorted D 4h configuration. The difference of total energies of D 4h and D 2d structures is 0.46 and 0.14 kJ/mol according to B3LYP and MP2 calculations, respectively. Small barrier of the ruffling intramolecular rearrangement allows to consider BeP complex as the non-ridged structure. For D 4h configuration the analysis of different trends is performed. All geometrical parameters of the macrocycle except R e (N-C α) systematically increase in BeL → MgL (L=P, Pz, Pc) rows. Natural net charges on the alkali-earth atoms in all studied compounds are in the small range 1.65 ÷ 1.77 ē. Extra negative charge (~2 ē) of the macrocycle is located on four pyrrol cycles in MP complexes or on four nitrogen meso-atoms in MPc and MPz complexes. Small (0.10 ÷ 0.15) M-N bond order points to the predominantly ionic character of M-N bonding. The macrocycle cavity size decreases in BeL and MgP complexes and increases in MgPz and MgPc in comparison with ones in free anions L -2. Probably the smallest size of Be +2 cation and the electrostatic attraction between the cation and four N - anions result in the strong contraction of the macrocycle cavity of BeP complex and ruffling distortion of the planar D 4h structure. The comparison with available for BePc and MgPc X-ray data shows that the theoretical values of the macrocycle geometrical parameters are close to the experimental data. Differences of the R e (M-N) distances are 0.016 and 0.027 Å for BePc and MgPc, respectively. The electronic spectra in the visible and near UV regions of ML complexes and free anions L -2 were calculated by TDDFT method. According to TDDFT calculations electronic spectra arise from the transition between bonding and anti-bonding π-orbitals of the macrocycle. It is known that positions and intensities bands in electronic spectra of metal porphyrin complexes were explained by Gouterman four-orbital model based on occasional degeneracy of two highest occupied molecular orbitals (a 2u and a 1u). The comparison of MO energies calculated for ML complexes and free anions L -2 shows that occasional degeneracy of these orbitals is the results of indirect effect of the metal atom. Moreover some trends of transition energies in MP → MPz → MPc rows can be explained by this effect. Simulated electronic spectra of MPz and MPc complexes show that absorption bands in near UV region are the result of the superposition of some electronic transitions. The comparison of calculated and experimental spectra was performed and new interpretation was suggested for some observed bands of MgPc. Five bands (670, 361, 338, 282 and 246 nm) were observed in experimental spectra of MgPc complex and assigned to electronic transitions from the ground state to five degenerated excited states. Simulated electronic spectrum of MgPc contains also five bands at 606, 370, 325, 287 and 216 nm. However simulated bands near 325 and 216 nm are the results of the superposition of electronic transitions from the ground state to four and eight degenerated excited states, respectively. The assignment of the experimental phosphorescence spectrum of MgPz was made. The most intensive observed band (915 nm) in the phosphorescence spectrum was assigned to the transition 1 3 E u → 1 A 1g (calculated wavelength - 858 nm). Relative positions of all other bands in the phosphorescence spectrum correspond to vibrational transitions of the macrocycle in the ground state of MgPz complex. © ISUCT Publishing.