Structural Variation in Mellitate Complexes of First-Row Transition Metals: What Chance for Design?

Eight compounds of Co, Ni, and Cu with mellitate ligands display a wide variety of structures with metal–mellitate coordination polymer dimensionality 0–3. Usually mellitate is fully deprotonated (mel6–), but there is one example of Hmel5– and one of H2mel4–. [M3(mel)(OH2)12]­·6H2O (M = Co or Ni) are chain polymers with octahedral M, while [Cu7(OH2)19­(OH)2(mel)2]­·9H2O has a 2D polymer sheet structure with square-based pyramidal Cu. Addition of KOH produces different compounds. Two incorporate K+ in the structures: K+ 2(OH2)5­[{Ni(OH2)5}2­(mel)]2–­·2H2O contains discrete nickel–mellitate anionic units, and K+ 2(OH2)6­[{Cu(OH2)3}2­(mel)]2–­·H2O has a copper–mellitate two-dimensional (2D) polymeric anion. For Co the product is [Co(OH2)6]2+­[{Co(OH2)4}5­(mel)2]2–·4H2O, with a 2D polymeric anion and discrete cations. A gel-supported synthesis leads to [Cu3(OH2)10­(Hmel)]­[Cu2(OH2)6(Hmel)]­·7H2O, with two different copper–mellitate polymeric sheets arranged alternately in a stack. [{Cu(OH2)­(EtOH)­(4,4′-bipy)}2­(H2mel)] contains a three-dimensional copper–mellitate network with hexagonal channels, occupied by 4,4′-bipyridyl molecules coordinated to Cu at one end and hydrogen bonded to H2mel4– at the other. While some of these features are familiar from other structures, some are new, raising the question of how far design principles can be applied to the synthesis of mellitate complexes.