Methyl 2-[N-(2 '-pyridylmethyl)carbamyl]pyridine-6-carboxylate: A precursor for unsymmetrical diamide ligands

Methyl 2-[N-(2'-pyridylmethyl) carbamyl] pyridine-6-carboxylate (H1), prepared by reaction of a 1: 1 ratio of the methyl diester of pyridine-2,6-dicarboxylic acid and 2- aminomethylpyridine, can form 1: 2 M: L complexes by acting as a tridentate ligand, as exemplified by a crystal structure analysis of [Co-III( 1)(2)](ClO4) where each ligand is coordinated by the amido group and its two flanking pyridine groups, with the ester group, a relatively poor donor, remaining unbound. The acid formed upon ester hydrolysis, hydrogen 2-[N( 2'-pyridylmethyl) carbamyl] pyridine-6-carboxylate (H(2)2), has been isolated as the ammonium salt, and forms a 2: 2 M: L dimer [Cu-2(II) ( 2)(2)(OH2)(2)] where each ligand binds to one copper through the amido group, its flanking pyridine groups, and to the other copper by the carboxylate group, to form a 10-membered ring that incorporates both metals. Onward reaction of the monoester (H1) with a different amine leads to potentially pentadentate ligands centred on a 2,6-disubstituted pyridine but with unsymmetrical arms, as exemplified by synthesis of 2-[N-(2'-pyridylmethyl) carbamyl]-6-[N-(2'-pyridylethyl) carbamyl] pyridine (H(2)3), 2-[N-(2'-pyridylmethyl) carbamyl]-6-[N-(1'-piperidinethyl) carbamyl] pyridine (H(2)4), and 2-[N-(2'-pyridylmethyl) carbamyl]6-[ N-(2'-hydroxypropyl) carbamyl] pyridine (H(2)5). Study of possible helicate formation with the unsymmetrical ligands has been initiated, with the ability to form polymetallic species probed by an ESI-MS study