Modulating the M−M Distance in Dinuclear Complexes. New Ligand with a 2,2‘-Biphenol Fragment as Key Unit: Synthesis, Coordination Behavior, and Crystal Structures of Cu(II) and Zn(II) Dinuclear Complexes

The synthesis and characterization of the new polyamino-phenolic ligand 3,3‘-bis[N,N-bis(2-aminoethyl)aminomethyl]-2,2‘-dihydroxybiphenyl (L) are reported. L contains two diethylenetriamine units linked by a 1,1‘-bis(2-phenol) group
(BPH) on the central nitrogen atom which allows two separate binding amino subunits in a noncyclic ligand. The
basicity and binding properties of L toward Cu(II) and Zn(II) were determined by means of potentiometric
measurements in aqueous solution (298.1 ± 0.1 K, I = 0.15 mol dm-3). L behaves as a pentaprotic base and as
a monoprotic acid under the experimental conditions used, yielding the H5L5+ or H-1L- species, respectively. L
forms both mono- and dinuclear species with both metal ions investigated; the dinuclear species are largely prevalent
in aqueous solution with a L/M(II) molar ratio of 1:2 at pH higher than 7. L shows different behavior in Cu(II) and
Zn(II) binding, affecting the dinuclear species formed and the distance between the two coordinated metal ions,
which is greater in the Zn(II) than in the Cu(II) dinuclear species. This difference can be attributed to the different
degree of protonation of BPH which influences the angle between the phenyl rings in the two systems. In this way,
it is possible to modulate the M(II)−M(II) distance by the choice M(II) and to space the two M(II) farther away than
was possible with the previously synthesized ligands. L does not saturate the coordination sphere of the coordinated
M(II) ions in the dinuclear species, and thus, these latter species are prone to add guests. 1H and 13C NMR
experiments carried out in aqueous solution, as well as the crystal structures of the dinuclear Cu(II) and Zn(II)
species formed in aqueous solution, aided in elucidating the involvement of L and BPH in Zn(II) and Cu(II) stabilization.