Redox-Adaptable Copper Hosts. Pyridazine-Unked Cryptands Accommodate Copper in a Range of Redox States

A series of structurally characterized copper complexes of two pyridazine-spaced cryptands in redox states + (1,1), (11,1), (II), (11,11) 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(l) ions, resulting in [Cu12L1](BF4)21, or one stereochemically demanding copper(ll) ion, resulting in [CullL1](BF4)2 3. Complex 3 crystallizes in two forms, 3a and 3b, with differing copper(ll) ion coordination geometries. Addition of copper(l) to the monometallic complex 3 results in the mixed-valence complex [CuʹCu"Li](X)3 (X = PF6-, 2a; X = BF4-, 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (Kcom = 2.1 x 10^11). The structurally characterized, octaamine cryptand LA, prepared by sodium borohydride reduction of L1, is more flexible than L1 and can accommodate two stereochemically demanding copper(ll) ions, generating the dicopper(ll) cryptate [Cull2L](BF4)4 4. Electrochemical studies indicate that LA stabilizes the copper(ll) oxidation state more effectively than Li; no copper redox state lower than 11,11 has been isolated in the solid state using this ligand.