Template synthesis of a tetraazamacrocyclic ligand with two pendant pyridinyl groups: Properties of the isomers of the metal-free ligand and of theirfirst-row transition metal compounds

The one-step reaction of [Cu(en)(2)](2+) (en = 1,2-diaminoethane) with form aldehyde, ethyl 2-pyridyl acetate, and base produces a mixture of [Cu(s-pyp ymac)](2+) and [Cu(a-pypymac)](2+) (s-pypymac = syn-6,13-bis(2-pyridinyl)-1 ,4,8,11-tetraazacyclotetradecane, a-pypymac = anti-6,13-bis(2-pyridinyl)-1, 4,8,11-tetraazacyclotetradecane; syn-to-anti ratio approximately 1:9) in lo w yield (6%). Ion exchange chromatography is used for isomer separation, an d the two isomers of the metal-free ligand are obtained by reduction of the copper(II) complexes and subsequent ion exchange chromatography. Crystal s tructure analyses of the metal-free a-pypymac ligand, of two isomeric coppe r(II) compounds of a-pypymac and one of s-pypymac, and of the cobalt(III) c omplexes of a- and s-pypymac and nickel(II), as well as zinc(II) complexes of a-pypymac, are reported and discussed on the basis of the expectations f rom force field calculations and from published experimental data of the tr ansition metal compounds of the bis-pendant amine derivative diammac.