No abstract

The protolytic equilibria of copper(II), 5, 5′‐dimethyl‐2, 2′‐bipyridine (bpy) and iminodiacetate (IDA) have been investigated by potentiometric titration between pH 2 and 8. Potentiometric titration suggests the presence of [Cu(IDA)(bpy)]0 at a fraction of 28 % of all copper species in solution at pH 8 with a stability constant logβ = 13.9. The crystal structures of [Cu(IDA)(bpy)] · 4H2O (1) and [Cu(IDA)(H2O)(phen)] · 4H2O (2) show the IDA ligand coordinated in the fac‐NO+O mode. The structure of 1 features “dimeric” entities of symmetry‐independent five‐coordinated Cu‐complexes through a long Cu···O contact, hydrogen bonding and π‐stacking, including a “pyridyl—metal‐chelate ring” π‐contact as evidence for metalloaromaticity in aromatic α, α′‐diimine‐copper(II) chelates. The structure of 2 has the tetragonally six‐coordinated molecular Cu‐complexes arranged by hydrogen bonding and π‐stacking. The crystal water in 2 forms infinite corrugated double chains composed of alternating four‐ and six‐membered, edge‐sharing rings of hydrogen‐bonded water molecules.

The protolytic equilibria of piperazine (C4H10N2) and phosphate have been investigated in the presence of cobalt or nickel chloride or nitrate by potentiometric titrations between pH 2 and 8. Potentiometric titrations suggest the presence of [M2+(H2O)5(C4H11N2+)]3+ and [M2+(H2O)5(C4H10N2)]2+ in solution with stability constants logK of 3.1 and 3.8 for M = Co and 3.1 and 3.6 for M = Ni, respectively. Crystallization experiments were then conducted at selected pH values to isolate desired species from the known solution composition. Crystallization afforded [M(H2O)6]2+(C4H12N22+)(HPO42—)2 at pH 3.5 and 6.2 (M = Co, Ni), and Co3(PO4)2·8H2O at pH 10.5. No crystals with the dihydrogenphosphate anion or a metal‐bound piperazine ligand could be isolated under the reaction conditions. The solid‐state assembly in the isomorphous structures of [M(H2O)6](C4H12N2)(HPO4)2 with M = Co and Ni is based on an extended hydrogen bonded network between the three ionic building blocks.