The new ligand L has been prepared by appending three 2,2'-bipyridine molec
ules to a tren (tris(2-aminoethyl)amine) unit. It binds a Fe2+ cation with
its three bipyridines, imparting to it an octahedral co-ordination. The obt
ained [FeL](2+) molecular cation has a closed shape, whose nature has been
ascertained by X-ray diffraction studies on the complex [Fe(HL)][PF6](3). T
he tren unit can thus be described as one half of a bis-tren cage. The kine
tic inertness of the complex between Fe2+ and the three bipyridines has bee
n verified: in the pH 2-12 range the [Fe-II(bipyR)(3)] unit is stable and p
lays only an architectural role. According to this it has been possible to
determine the protonation and complexation constants (in the presence of 1
equivalent of Cu2+) of the tren fragment, which is in fact able to interact
with protons and metal cations, behaving as a tight and rigid half of a bi
s-tren ligand. High percentages (70-100%) of the [FeLHn]((n + 2)+) species
can be found in sharp pH ranges, while, in the presence of 1 equivalent of
Cu2+, the metal-containing species [FeCu(HL)](5+), [FeCuL](4+), [FeCuL(OH)]
(3+) and [FeCuL(OH)(2)](2+) exist in the pH 2-12 range, each one prevailing
in distinct pH intervals, reaching high percentages (70-100%). The variati
on of the spectral and electrochemical properties of the [Fe(bipyR)(3)] fra
gment have been studied as a function of pH, and related with the protonati
on degree or complexation of the tren unit.