A monometallic and kinetically inert complex of a ditopic open ligand as a tight polyaza cage

The new ligand L has been prepared by appending three 2,2'-bipyridine molecules 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 obtained [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). The tren unit can thus be described as one half of a bis-tren cage. The kinetic inertness of the complex between Fe2+ and the three bipyridines has been verified: in the pH 2-12 range the [Fe-II(bipyR)(3)] unit is stable and plays 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 bis-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 variation of the spectral and electrochemical properties of the [Fe(bipyR)(3)] fragment have been studied as a function of pH, and related with the protonation degree or complexation of the tren unit.