Preparation, structure and coordination properties of (η6-p-cymene)ruthenium(II) sandwich complexes of catechol and 5-hydroxyindole derivatives

Bis(η6-arene)ruthenium(II) complexes of the type [(η6-cymene)Ru(η6-L)](CF3SO3)n (2–7), n=2–3, containing catechol and 5-hydroxyindole derivatives L may be prepared by treatment of [(η6-cymene)Ru(acetone)3](CF3SO3)2 with the appropriate compound in CF3COOH. For the l-Dopa and H-dopamine complexes 2 (n=2) and 3 (n=3), η6-coordination leads to a dramatic reduction of more than 6 units in pKa to values of, respectively 3.67(3) and 3.98(7) for the first deprotonation of a phenyl hydroxo substituent. An even greater increase in acidity on η6-coordination is observed for the indole N1 and O5 donor atoms in the 5-hydroxytryptophan complex 4 (n=2). A pH potentiometric and 1H NMR study of the interaction of the 5-hydroxy-3-indolyl acetic acid and N-acetyl-tryptophan (AcTrpOH) complexes 6 and 7 (n=2) with [Pd(dien)(H2O)]2+ confirms facile metalation of the indole nitrogen N1 at pH>4, leading to predominance of dinuclear complexes such as of [(η6-cymene)Ru(η6-AcH−1TrpO-κN1)Pd(dien)]2+ in neutral and weakly alkaline solution. X-ray structural analyses are reported for complexes 3 and 4. A significant distortion towards an η5-oxohexadienyl coordination is indicated for 4 by the short C5O5 distance of 1.29(2) Å and the tilting of the C5 bonding plane at an angle of 12.4° to the coordination plane of the remaining five indole C-atoms.