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

Abstract

The new [n.n]paraazacyclophane 3,7,10,14,21,25,28,32-octaazapentacyclo[32.2.27,10.216,19.225,28]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 βCuH3L= 33.20(6) and log βCu2L= 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.