Addition of Small Molecules by Zn(II) and Cu(II) Dinuclear Complexes Obtained by an Amino-Phenolic Ligand. Crystal Structures of the Dinuclear Zinc Complex Assembling Butanolate and Azide Anions

The coordination properties of the ligand 2,6-bis-{[bis-(2-aminoethyl)-amino]methyl}-phenol (L) toward the zinc
ion were determined by means of potentiometric measurements in aqueous solution (298.1 ± 0.1 K, I = 0.15 mol
dm-3). L forms mono- and dinuclear complexes with zinc; the stable dinuclear complexes are practically the
only existing species using L/Zn(II), molar ratio of 1:2. In these species, each dien subunit binds one zinc ion,
while the phenolate moiety bound to both ions allows the two metal centers to be in close proximity with an
incomplete coordination environment. The role of this metal−metal distance in binding secondary ligands was
investigated for the dinuclear systems 2Zn/L and 2Cu/L by potentiometric (298.1 ± 0.1 K, I = 0.15 mol dm-3)
and NMR experiments in aqueous and methanol solution with a series of small guests having nitrogen donor
atoms. The coordination sphere of the two metals was completed by adding 1 equiv of only those guests, which
showed at least two contiguous donor atoms or two lone pairs on the same atom, to exactly fit the metal−metal
distance without modifying the metal cluster. To do this, the imidazole molecule which shows the highest addition
constants to the [M2H-1L]3+ species probably forms a μ-1,1-amino. These results are in agreement with the two
crystal structures reported herein [Zn2(H-1L)(CH3CH2CH2CH2O)](ClO4)2 (space group P21nb, a = 11.483(5), b
= 14.166(5), c = 18.279(5) Å, V = 2973(19) Å3, Z = 4, R = = 0.0701, wR2 = 0.1611) and [Zn2(H-1L)N3](ClO4)2
(space group C 2/c, a = 14.460(3), b = 12.814(3), c = 14.875(3) Å, β = 105.35(2)°, V = 2658(10) Å3, Z = 4,
R = 0.0783, wR2 = 0.1806). In fact, these structures display a butanolate or azide guest linked to both Zn(II)
ions of the dinuclear complex, resulting in a μ-1,1-oxo and μ-1,1-azido bridge, respectively.