THE COORDINATION OF AMINOPHOSPHONATES TO CHROMIUM(III)

Abstract

Attempts to induce coordination of the amine group in aminophosphonate complexes of chromium(III) were unsuccessful. However, N-phosphonomethylglycine forms a stable complex in which the amine, the carboxylate, and one phosphonate oxygen are coordinated. A similarly coordinated complex of N,N-diphosphonomethylglycine is unstable and reverts to an isomer in which the amine group is free. Several potential reasons for the failure of the amine group to coordinate in aminophosphonates in contrast to analogous aminocarboxylate complexes were explored. Steric factors appear unimportant. Contributing factors are (1) there is a driving force for the transfer of a proton from a phosphonate oxygen to the amine, establishing a preference for oxygen coordination; (2) coordination of the phosphonate end of an aminophosphonate appears to lower the basicity of the amine group much more than carboxylate coordination in an aminocarboxylate; (3) in an aminophosphonate the nitrogen lone pair orbitals lie lower in energy than they do in aminocarboxylates, decreasing the energy of the donor-acceptor interaction with metal orbitals.

Key Words:

• chromium(III)
• aminophosphonates
• N-phosphonomethylglycine
• N,N-diphosphono-methylglycine