A reinforced polyaza[n.n]paracyclophane containing piperazine rings

Abstract

The new [n.n]paraazacyclophane 3,7,10,14,21,25,28,32-octaazapentacyclo[32.2.2^7,10.2^16,19.2^25,28]tetratetraconta-1(37),16(41),17,19(40),34(38),35-hexaene (L) has been prepared. Its protonation has been studied by means of potentiometry and direct microcalorimetry in 0.15 mol dm^–3 NaClO₄ at 298.1 K. A NMR analysis shows that protons bind alternately at both sides of L, the benzylic nitrogens being the first sites to be protonated. From dynamic variable-temperature NMR analysis an activation enthalpy of 61(2) kJ mol^–1 has been derived for the chair–chair interconversion of the piperazine ring. The entropy is almost negligible. Compound L forms with Cu²⁺ in aqueous solution complexes [Cu(H₃L)]³⁺ and [Cu₂L]⁴⁺ with stability constants log β_CuH3L= 33.20(6) and log β_Cu2L= 16.6(1) in 0.15 mol dm^–3 NaClO₄ at 298.1 K, respectively. The low stability of the binuclear complex is attributed to the energy loss due to interconversion between the chair and boat conformers of the piperazine moieties. The interaction of Pd²⁺ with L has been monitored by NMR analysis. The spectral features show formation of strong binuclear complexes with the involvement of all eight nitrogens of the macrocycle.