Synthesis and structure of macrocyclic amides containing a 2,2'-dipyridylmethane unit. A new class of chiral macrocyclic ligands

Abstract

A novel class of 13-membered tetraaza rings containing dipyridylmethaneamide units was obtained by condensation of 1,2-diamines with 1,1-bis[6-(chloroformyl)-2pyridyl]-1-methoxypropane. By use of trans-1,2-diaminocyclohexane, a chiral macrocyclic amide was obtained, while the cis isomer afforded two diastereomeric pseudochiral compounds. The chiral ligand yielded a square-planar complex with Ni(II) by loss of the amide protons, with the chirality close to the metal center. The structures of the two macrocyclic dipyridylmethane amides derived from (R,R)- and (R,S)-diaminocyclohexane (5a and 6a, respectively), and that of 1,1-bis(6-carboxy-2-pyridyl)-1-methoxypropane (1) were studied X-ray diffraction. In the crystal of 1-monohydrate, the dipyridylmethane moiety adopts an anti conformation with the two aromatic rings nearly perpendicular to one another. The crystal of 5a and that of 6a contain two and three crystallographically independent molecules, respectively. These five macrocycles are very similar in their overall bowllike shape but exhibit minor conformational differences. 1 H NMR studies demonstrate that similar conformations are maintained in solution