Methyl 2-[N-(2′-Pyridylmethyl)carbamyl]pyridine-6-carboxylate: A Precursor for Unsymmetrical Diamide Ligands

Abstract

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 (H₂2), has been isolated as the ammonium salt, and forms a 2:2 M:L dimer [Cu^II₂(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 (H₂3), 2-[N-(2′-pyridylmethyl)carbamyl]-6-[N-(1′-piperidinethyl)carbamyl]pyridine (H₂4), and 2-[N-(2′-pyridylmethyl)carbamyl]-6-[N-(2′-hydroxypropyl)carbamyl]pyridine (H₂5). 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.