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 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 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 [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, octaamine 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.