Electrochemically switched anion translocation in a multicomponent coordination compound

Complex [3a](2+) has been synthesized, which contains one Ni-II cyclam-like and one bipyridine fragment (cyclam = 1,4,8,11-tetraazacyclotetradecane). Spectrophotometric and potentiometric titration experiments in aqueous solution demonstrated that in the examined pH range (2-10), the Ni-II cyclam-like subunit remains square planar, i.e., it does not interact with the bipyridine unit, which is instead allowed to bind external metal centers. In particular, in the presence of 0.5 equiv of Cu-II the trinuclear multicomponent coordination compound [Cu-II(3a)(2)-(H2O)](6+) is the only species existing in solution over a wide pH range. The water molecule bound to the Cu-II center can be substituted by inorganic anions, forming [Cu-II(3a)(2)(anion)](5+) species, and the binding constants of a series of mononegative anions have been determined, demonstrating the effectiveness of [Cu-II(3a)(2)(H2O)](6+) as an anion receptor. Voltammetric experiments have shown that the two Ni-II centers in [Cu-II(3a)(2)(H2O)](6+) can be reversibly oxidized to Ni-III, a d(7) cation which needs further coordination in its axial positions. In the case of the [Cu-II(3a)(2)(anion)](5+) species, differential-pulse voltammetry experiments demonstrated that on oxidation of the peripheral Ni-II centers, the anion bound to the central Cu-II cation jumps onto one of the peripheral Ni-III centers. Thus, the translocation of an anion from the center to the periphery (and vice versa) can be electrochemically switched for the multicomponent coordination compound considered here.