Outer-sphere coordination, N-coordination and O-coordination of the deprotonated saccharin in copper(II) saccharinato complexes. Implications for the saccharinato carbonyl stretching frequency

Tetraaquabis(o-sulfobenzimidato-N)copper(II) reacts with neutral N-heterocycles to form complexes in which the o-sulfobenzimide (saccharin) entity interacts directly with the copper atom (through either the endocyclic nitrogen atom or the exocyclic oxygen atom) or indirectly, through coordinated water molecules. With 4-aminopyridine, it yields diaquatetrakis(4-aminopyridine)copper(II)di(o-sulfobenzimidate)dihydrate, whose metal atom shows trans-N₄O₂ octahedral coordination. The o-sulfobenzimidate anions interact with the copper atom through the coordinated water molecules, and they link with the lattice water molecules to furnish three-dimensional network architecture. The reagent when treated with pyrazole affords tetrakis(pyrazole)bis[1,2-benzisothiazolyl-3-olato 1,1-dioxide]copper(II); in this neutral compound, the metal atom and the o-sulfobenzimidate moieties are linked by covalent copper-oxygen bonds. The aqua complex with di-2-pyridylamine has the copper atom in a square-pyramidal configuration: one of the o-sulfobenzimidate ligands binds through its nitrogen atom whereas the other binds through the exocyclic oxygen atom in aqua(di-2-pyridylamine)[1,2-benzisothiazolyl-3-olato 1,1-dioxide](o-sulfobenzimidato-N)copper(II), which adopts a linear hydrogen-bonded chain motif. When treated with nicotinamide, tetraaquabis(o-sulfobenzimidato-N)copper(II) affords a monohydrated di(nicotinic acid) adduct, the amide group being oxidized to a carboxylic group. In this square-pyramidal complex, the molecules are linked by hydrogen bonds involving the two carboxylic acid ends into a linear chain that propagates along the a-c diagonal of the unit cell. The coordination mode of the o-sulfobenzimidate entities in the complexes is reflected in the stretching frequencies of the carbonyl groups, the respective band(s) being blue-shifted for N-coordination and red-shifted for O-coordination relatively to the o-sulfobenzimidate ions, leading to the frequency order N-coordinated > uncoordinated > O-coordinated. These shifts should be considered in vibrational frequency versus bond order correlations.