Structural and solution equilibrium studies on half-sandwich organorhodium complexes of (N,N) donor bidentate ligands

Abstract

Complex formation equilibrium processes of [Rh(η⁵-C₅Me₅)(H₂O)₃]²⁺ with N,N′-dimethylethylenediamine (dmen), N,N,N′,N′-tetramethylethylenediamine (tmeda), 2-picolylamine (pin) and 1,10-phenanthroline (phen) were studied in aqueous solution by ¹H NMR spectroscopy, UV-vis spectrophotometry and pH-potentiometry. Formation and deprotonation of [Rh(η⁵-C₅Me₅)(L)(H₂O)]²⁺ complexes and exchange process of the aqua to chlorido ligand were characterized in addition to single-crystal X-ray diffraction analysis of [Rh(η⁵-C₅Me₅)(L)(Cl)]⁺ complexes (L = dmen, tmeda and pin). Formation of complexes with significantly high stability was found except tmeda due to the sterical hindrance between the methyl groups of the chelating ligand and the arenyl ring resulting in an increased methyl group-ring plane torsion angle. [Rh(η⁵-C₅Me₅)(L)(H₂O)]²⁺ complexes of dmen, pin, phen predominate at pH 7.4 without decomposition even in the micromolar concentration range. The complexes were characterized by relatively high chloride affinity and a strong correlation was obtained between the log K′ (H₂O/Cl⁻) and pK_a of [Rh(η⁵-C₅Me₅)(L)(H₂O)]²⁺ constants for a series of (O,O), (O,N) and (N,N)-chelated complexes. For this set of 12 complexes a relationship between log K′ (H₂O/Cl⁻) values and certain crystallographic parameters was found using multiple linear regression approach. DNA binding of these complexes was also monitored and compared by ultrafiltration and fluorimetry.