Polymorphism of Molecular Organometallic Crystals. A Third Form of the Supramolecular Hydrogen-Bonded Dimer {[FeII(η5-C5H4COOH)2]}2

A third polymorphic form of the ferrocene dicarboxylic acid [Fe II (η 5 -C 5 H 4 COOH) 2 ], already known as a monoclinic (form I) and a triclinic (form II) crystal form, has been obtained and structurally characterized. This third form III presents dimers {[Fe II (η 5 -C 5 H 4 COOH) 2 ]} 2 of doubly hydrogen-bonded carboxylic rings in the solid state, with O(H)···O distances in the range 2.613(9)−2.646(10) Å. The hydrogen-bonded dimer {[Fe II (η 5 -C 5 H 4 COOH) 2 ]} 2 is the fundamental supramolecular unit present also in forms I and II. The difference between the three crystal forms arises from the relative orientation of the hydrogen-bonded dicarboxylic dimers in the solid state. In form III, the cyclopentadienyl rings of neighboring molecules are at graphitic separation (3.40 Å vs an intrasandwich distance of 3.30 Å).