The pi complexation of alkali and alkaline earth ions by the use of meso-octaalkylporphyrinogen and aromatic hydrocarbons.

Abstract

The full metallation of meso-octaalkylporphyrinogens [R8N4H4] (R=Et, 1; nBu, 2; CH2Ph, 3; (CH2)4, 4) with heavy alkali metals (M = K, Rb, Cs) leads to the porphyrinogen-M4 compounds, in which the solvation of the alkali cations is largely assured by the intra- and intermolecular phi-interactions with the pyrrolyl anions. Such a mode of complexation results in a structural diversity as a function of the meso substituents, the size of the metal ion, and the solvent. The structure of the unsolvated polymers [R8N4M4]n (R= Et, M=K, 5; M=Rb, 6; M=Cs, 7; R= (CH2)4, M = Rb, 8; M = Cs, 9) have been clarified through the X-ray analysis of 7 recrystallized from diglyme. The structure shows that the tetraanion binds two Cs ions inside the cavity, which display in one case eta1:eta1:eta1:eta1 and in the other eta5:eta5:eta5:eta5 interaction modes. Bidimensional polymerization is assured by four Cs ions, which each bind at the eta5 position on the exo face of each pyrrole. With bulkier meso substituents, different polymeric forms are obtained (R = nBu, M = K, 10; M = Rb, 11; M = Cs, 12), and their structures were clarified through the X-ray analysis of 10, which was recrystallized from dimethoxyethane. The polymeric units are made up by the monomeric units [Bu8N4K2]2-, in which one potassium is eta1:eta1:eta1:eta5 and the other eta5:eta1:eta5:eta1 bonded inside the porphyrinogen cavity. In the case of R=CH2Ph, the monomeric anion [(PhCH2)8N4K2]2- (13) has been structurally identified. The metallation of 1 and 2 with active forms of alkaline earth metals (M' = Ca, Sr, Ba) led to dinuclear compounds [R,N4M'2] (R = Et, M' = Ca, 14; M'=Sr, 15; M'=Ba, 16; R=nBu. M'=Ba, 18), in which both metals inside the cavity are eta1:eta3:eta1:eta3 (Ca) and eta1:eta5:eta1:eta5 (Sr and Ba) bonded to the porphyrinogen tetraanion. The coordination sphere of each metal ion is completed by two THF molecules, which, in the case of Ba, are easily replaced by an arene ring [Bu8N4Ba2(eta6-arene)2] (arene=durene, 22; naphthalene, 23; toluene, 24; benzene, 25). The X-ray structures of 14, 15, 18, 22, and 23 are described in detail. We have tried to establish a relationship between the solid-state and solution structures by analyzing the 1H NMR spectra of the porphyrinogen complexes.