Redox-adaptable copper hosts. Pyridazine-linked Cryptands accommodate copper in a range of redox states

A series of structurally characterised copper complexes of two pyridazine-spaced cryptands in redox states + (I,I), (II,I), (II), (II,II) are reported. The hexa-imine cryptand LI [formed by the 2+3 condensation of 3,6-diformylpyridazine with tris(2-aminoethyl)amine (tren)] is able to accommodate two non-stereochemically demanding copper(I) ions, resulting in [CuI2LI](BF4)2 1, or one stereochemically demanding copper(II) ion, resulting in [CuIILI](BF4)2 3. Complex 3 crystallises in two forms, 3a and 3b, with differing copper(II) ion coordination geometries. Addition of copper(I) to the monometallic complex 3 results in the mixed valence complex [CuICuIILI](X)3 (X = PF6- 2a, X = BF4- 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (Kcom = 2.1 × 1011). The, structurally characterized, octa-amine cryptand LA, prepared by sodium borohydride reduction of LI, is more flexible than LI and can accommodate two stereochemically demanding copper(II) ions, generating the dicopper(II) cryptate [CuII2LA](BF4)4 4. Electrochemical studies indicate that LA stabilizes the copper(II) oxidation state more effectively than LI; no copper redox state lower than II,II has been isolated in the solid state using this ligand