Theoretical Study of Alkali Metal Complexes of Porphyrin (М2Р), Porphyrazine (М2Рz) and Phthalocyanine (М2Рc), М=Li, Na, K

Abstract

The information about geometrical and electronic structure of free molecules M 2 Pz, Na 2 Pc, K 2 Pc is not available. The data concerning structure of Li 2 P complex are not definite. The electronic spectra of M 2 P, M 2 Pz, Na 2 Pc and K 2 Pc are not reported. Here the systematic study of the electronic and geometrical structures of M 2 P, M 2 Pz and M 2 Pc complexes is performed. Density functional theory (three-parameter hybrid functional B3LYP), MollerPlesset perturbation theory (MP2) and correlation-consisted valence triple-zeta basis sets (cc-pvtz) are used. All calculations are carried out using PC GAMESS program. For all molecules several geometrical configurations with different positions of alkali atoms relatively the macrocycle plane are optimized. The structures with alkali atoms adjacent to the same side of the macrocycle (C 2v symmetry) and with alkali atoms lying inside of the macrocycle (D 2h ) correspond to saddle points on PES and possess high relative energies (>150 kJ/mol). More energetically favourable configuration of all complexes excepting Li 2 P possesses D 4h symmetry with alkali atoms taking up symmetrical positions at different sides of the macrocycle. According to B3LYP method the equilibrium structure of Li 2 P is the C 2h geometry originating from distorted D 4h configuration. However MP2 calculation leads to the conclusion about D 4h symmetry of equilibrium structure. Perhaps more sophisticated calculations are needed to determine shape equilibrium configuration in the case of Li 2 P. Analysis of all trends is performed for D 4h configuration and is based on the data obtained at B3LYP level. The geometrical parameters of the macrocycle change systematically in the line Li 2 L → Na 2 L → K 2 L (L=P, Pz, Pc): R e (N-C α ) and ϕ e (C α NC α ) increase by ~0.01 A and ~2o, respectively; R e (C α C β ) decreases by ~0.01 A. R e (C α -X m ) value and the macrocycle cavity size (R e (N⋅⋅⋅N)) increase by 0.011÷0.016 A and 0.11÷0.15 A, respectively. R e (Li-Li) does not change practically in the row Li 2 P → Li 2 Pz → Li 2 Pc (increasing 0.006A). Unlike the R e (Li-Li) trend the magnitudes of R e (Na-Na) and R e (K-K) distances in the similar rows increase by 0.134 and 0.249 A. Perhaps, so different trends can be explained by decreasing of the macrocycle cavity size in M 2 P → M 2 Pc → M 2 Pz line and increasing of the alkali atom radius. Natural net charges on the alkali atoms in all studied compounds are situated in the small range 0.90 ÷ 0.96 ē. Extra negative charge (~2ē) of the macrocycle is located on four pyrrol cycles in M 2 P complexes or on four nitrogen meso-atoms in M 2 Pc and M 2 Pz complexes. According to NBO analysis the bond orders of M-M and М-N are close to zero. Thus the М-N bond is ionic bond. The