Synthesis of (NH)_<i>m</i>(NMe)_4−<i>m</i>-Bridged Calix[4]pyridines and the Effect of NH Bridge on Structure and Properties

The (NH)m(NMe)4−m-bridged calix[4]pyridines (m = 1−4) 19−23 were synthesized in excellent yields from deprotection of N-allyl groups of (NAllyl)m(NMe)4−m-bridged calix[4]pyridine derivatives 8 and 15−18, which were prepared in moderate yields by macrocyclic 2+2 and 1+3 coupling reactions between simple diamino- and dibromo-substituted fragments. In the solid state, (NH)m(NMe)4−m-bridged calix[4]pyridines adopted different 1,3-alternate conformations due to mainly the formation of varied conjugation systems of bridging NH units with their neighboring pyridines. In solution, all (NH)m(NMe)4−m-bridged calix[4]pyridines were very fluxional and the rates of interconversion of various conformational structures were very rapid relative to the NMR time scale. While (NH)4-bridged calix[4]pyridine 23 formed the strongest conjugation system, (NH)2(NMe)2-bridged calix[4]pyridine 21 acted as a selective fluorescence probe in the recognition of zinc(II) ion in solution with the dramatic enhancement of fluorescence intensity.