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(I) ions, resulting in [Cu(I)(2)L(I)](BF(4))(2) 1, or one stereochemically demanding copper(II) ion, resulting in [Cu(II)L(I)()](BF(4))(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 [Cu(I)Cu(II)L(I)](X)(3) (X = PF(6)(-), 2a; X = BF(4)(-), 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (K(com) = 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(I) and can accommodate two stereochemically demanding copper(II) ions, generating the dicopper(II) cryptate [Cu(II)(2)L(A)](BF(4))(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.