A New Branched Phenanthroline Derivative Ligand: Synthesis, Solution Chemistry, and Crystal Structures of Copper(II) and Zinc(II) Complexes

The synthesis and characterization of the new ligand 2,9-bis[N,N-bis(2-aminoethyl)aminomethyl]-1,10-phenanthroline
(L) are reported. L contains two diethylenetriamine units connected on the central nitrogen atom by a
1,10-phenanthroline group forming a symmetrical branched ligand. The basicity and binding properties of L toward
Cu(II) and Zn(II) in aqueous solution were determined by means of potentiometric, UV−vis, fluorescence, and 1H
and 13C NMR techniques. L behaves as pentaprotic base under the experimental conditions used; from HL+ to
H4L4+ species it is the secondary amine functions that are protonated while in the H5L5+ species also the phenanthroline is involved in protonation. L does not show fluorescence properties in the range of pH (0−14) investigated.
It forms both mono- and dinuclear stable species where the phenanthroline is directly involved with both nitrogens
in the coordination of the first metal which is coordinated in a pentacoordination environment also by one dien unit.
The other dien unit undergoes easy protonation in the mononuclear complex while it binds the second metal in the
dinuclear species. For this reason, L, in providing two different binding areas for metal coordination, behaves as
an unsymmetrical compartmental ligand; one area is formed by one dien unit and by the phenanthroline, and the
other by the remaining dien unit. This produces unsymmetrical metal complexes both for the mono- and dinuclear
species; however, the role of the binding areas is fast exchanging in aqueous solution, at least on the NMR time
scale. Solution studies and the three crystal structures of the [Zn(H2L)]4+, [[Cu(H2L)](ClO4)]3+, and [[Cu2LCl2](ClO4)]+
species highlight the unsymmetrical compartmental behavior of L as well as the host properties of the complexes
in adding exogenous ligands such as hydroxide, pherchlorate, and chloride anions.