2,6-Di(pyrimidin-4-yl)pyridine Ligands with Nitrogen-Containing Auxiliaries: The Formation of Functionalized Molecular Clefts upon Metal Coordination

A series of ligands (1−4) based on a 2,6-di(pyrimidin-4-yl)pyridine scaffold have been synthesized, and their abilities
to form complexes with Zn(II) and Cu(II) have been determined using UV/vis spectroscopy in buffered aqueous
solution (0.01 M N-[2-hydroxyethyl]piperazine-N‘-[3-ethanesulfonic acid] (HEPES) at pH = 6.8). The Zn(II) complex
of 1 was determined to have a formation constant of 8.4 × 103 M-1 while the formation constant of the Cu(II)
complex was found to be 1 × 106 M-1. The presence of auxiliary amines in 2 increased the stability of the Zn(II)
complex relative to that of 1 by a factor of over 40, suggesting possible coordination of the auxiliaries to the Zn(II)
center. The guanidinium and 2-amino-4,5-dihydro-imidazolinium groups of 3 and 4 considerably diminished the
stability of the Zn(II) and Cu(II) complexes relative to those of 1. X-ray crystal structures of 1-Zn, 3-Zn, 4, and 4-Zn
were obtained and are discussed. A significant increase in the stability of 3-Zn, but not in the stability 1-Zn, was
observed upon the addition of 1 equiv of sodium phosphate, implicating a stabilizing interaction of the guanidinium
groups of 3-Zn and the phosphate anion.