The unprecedented bridging coordination mode of 1,1-cyclobutane dicarboxylate (μ-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 (mu-cbdc-O,O') on cationic Ru(II) centres bearing ancillary ligands (e.g. H2O, NH3) 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-[RuCl2(dmso-S)(3)(dmso-O)] (1) and cbdc in a number of different reaction conditions is the dinuclear species with two bridging cbdc units fac-[Ru(mu-cbdc-O,O')(dmso-S)(3)(H2O)](2) (2). Similarly, reaction of cis,fac-[RuCl2(dmso-S)(3)(NH3)] (3) with cbdc yielded the corresponding dinuclear species fac-[Ru(mu-cbdc-O,O')(dmso-S)(3)(NH3)](2) (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 H2O or NH3 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.