Acid–base behaviour of organopalladium complexes [Pd(CNN)R]BF4

Abstract

Protonation of the orthopalladated carbon of the cyclometallated complex [Pd(CNN)L]⁺, where L = P(OMe)₃ and HCNN is the C-deprotonated form of the tricoordinated chelant donor ligand 2-acetylpyridinephenylhydrazone, has been studied at 25, 35, 40 and 45 °C in highly acidic 10% v∶v ethanol–water, and at 25 °C in 30% and 50% v∶v ethanol–water. The acidity constants pK_SH²⁺ remained essentially constant with temperature, whereas these values noticeably increased with decreasing water content. Likewise, the complexes with L = P(OPh)₃, PPh₃, and SC₄H₈ were also studied at 25 °C in 10% v∶v ethanol–water. Under such conditions the pK_SH²⁺ values were more negative in the order SC₄H₈ < PPh₃ < P(OMe)₃ < P(OPh)₃; in the case of phosphines, this feature can be attributed to the substituent π-acceptor ability. These complexes behave as very weak bases and the reactions occur in sulfuric acid stronger than 1.3 M; the medium effects observed in the spectral curves were corrected to reliably determine the pK_SH²⁺ values. The observed difference in the solvation parameter m* can be related to the differing hardness of the ligands bound to Pd.