By modifying the mouth of a macrocyclic dicobalt Pacman complex, it is possible to both isolate new bridging-superoxo and hydroxyl complexes and to tune the reactivity of this system towards catalytic fourelectron reduction of dioxygen to water.

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The synthesis and structures of two new octadentate, Schiff-base calixpyrrole macrocycles are presented in which modifications at the meso-substituents (L(1)) or the aryl spacer between the two pyrrole-imine donor compartments (L(2)) are introduced. The outcomes of these changes are highlighted in the structures of binuclear Pacman complexes of these macrocycles, [M(2)(L(1))] and [M(2)(L(2))]. Both palladium and cobalt complexes of the fluorenyl-meso-substituted macrocycle H(4)L(1) adopt rigid, but laterally twisted geometries with enclosed bimetallic microenvironments; a consequence of this spatial constraint is an exo-exo-bonding mode of pyridine in the dicobalt complex [Co(2)(py)(2)(L(1))]. In contrast, the use of an anthracenyl backbone between the two donor compartments (H(4)L(2)) generates a binuclear palladium complex in which the two PdN(4) environments are approximately cofacial and separated by 5.3 A, so generating a bimetallic complex that is structurally very similar to binuclear compounds of cofacial diporphyrins.

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