Cation-induced crown porphyrin dimers of oxovanadium(IV)

Abstract

Oxovanadium(1V) porphyrins appended with crown ether (benzo-15-crown-5) at the 5 (mono), the 5 and 10/15 (cis/trans bis), the 5, 10, and 15 (tris), and the 5, 10, 15, and 20 (tetrakis) positions have been synthesized. The cation complexation behavior of these cavity-bearing porphyrins has been studied by using optical aborption and ESR spectral methods. The cations K⁺, Cs⁺, NH₄⁺, and Ba²⁺, which require two crown ether cavities for complexation, induce dimerization of the porphyrins. The cation-induced dimerization constants for a representative tetrasubstituted porphyrin vary as K⁺ > Ba²⁺ > Cs⁺ ~ NH₄⁺, and the relative stabilities of the dimers are dependent on the type of the substitution, tetrakis > tris > cis bis. ESR spectra recorded at a sample temperature of 77 K have low-field components attributed to ΔM_h = ± 2 transitions, providing further evidence for the existence of dimers in solutions. The eclipsed sandwich dimers have V-V distances in the range 4.70 Å. The relative distributions of oxovanadium crown porphyrins in terms of monomeric and dimeric forms rest on the geometric dispositions of the crown ether appendages.