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

A series of structurally characterized copper complexes of two pyridazine-spaced cryptands in redox states + (I,I), (II,I), (II) (II,II) are reported. The hexaimine cryptand L-I [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(l) ions, resulting in [(Cu2LI)-L-I](BF4)(2) 1, or one stereochemically demanding copper(II) ion, resulting in [(CuLI)-L-II](BF4)(2) 3. Complex 3 crystallizes 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 [(CuCuLI)-Cu-I-L-II](X)(3) (X = PF6- 2a; X = BF4-, 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (K-com x 2.1 x 10(11)). The structurally characterized, octaamine cryptand L-A, prepared by sodium borohydride reduction of L-I, is more flexible than L, and can accommodate two stereochemically demanding copper(II) ions, generating the dicopper(II) cryptate [Cu(2)(II)LA](BF4)(4) 4. Electrochemical studies indicate that L-A stabilizes the copper(II) oxidation state more effectively than L-I; no copper redox state lower than II,II has been isolated in the solid state using this ligand.