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.