The unprecedented bridging coordination mode of 1,1-cyclobutane dicarboxylate (mu-cbdc-O,O') stabilized by intramolecular hydrogen bonds in ruthenium(II) complexes.

The 1,1-cyclobutane dicarboxylate ligand (cbdc), that normally binds to metal centres as a chelate (eta(2)-cbdc-O,O'), prefers to bind in an unprecedented bridging fashion (micro-cbdc-O,O') on cationic Ru(ii) centres bearing ancillary ligands (e.g. H(2)O, NH(3)) capable of making intramolecular H-bonds with the non-coordinated oxygen atoms of the carboxylate groups. Thus, the thermodynamic product of the reaction between cis,fac-RuCl(2)(dmso-S)(3)(dmso-O) and cbdc in a number of different reaction conditions is the dinuclear species with two bridging cbdc units fac-Ru(micro-cbdc-O,O')(dmso-S)(3)(H(2)O) (2). Similarly, reaction of cis,fac-[RuCl(2)(dmso-S)(3)(NH(3))] (3) with cbdc yielded the corresponding dinuclear species fac-Ru(micro-cbdc-O,O')(dmso-S)(3)(NH(3)) (4), in which ammonia occupies the position of the water molecule in 2. Both dinuclear species 2 and 4 were characterized by X-ray crystallography and have an anti geometry with respect to the H(2)O or NH(3) ligands. The results from the X-ray studies are consistent with the NMR spectroscopic data, indicating that the dinuclear structures observed in the solid state are maintained in solution. The mononuclear anionic complex with a chelating cbdc unit, K{fac-[RuCl(eta(2)-cbdc-O,O')(dmso-S)(3)]}(5), was isolated under appropriate conditions form the reaction of 1 with K(2)(cbdc) and was demonstrated to be an intermediate in the formation of 2.