The electronic effect of quinoline moieties on the lability of platinum(II) complexes of tridentate N^N^N and N^C^N ligands: a kinetic, mechanistic and theoretical analysis

Abstract

The rate of the chloride ligand displacement by three thiourea neutral nucleophiles (Nu) of different steric demands, namely thiourea (Tu), N,N’-dimethylthiourea (Dmtu) and N,N,N,’N-tetramethylthiourea (Tmtu) in the complex 2,6-bis(8-quinolyl)-pyridine chloroplatinum(II) (Pt3), was investigated under pseudo-first-order conditions as a function of concentration and temperature using UV–visible spectrophotometry and compared with the literature data of complexes: 2,6-bis(2-pyridyl)pyridine chloroplatinum(II) (Pt1), 1,3-bis(pyridyl)phenyl chloroplatinum(II) (Pt2) and 1,3-bis(8-quinolyl)phenyl chloroplatinum(II) (Pt4). The observed pseudo-first-order rate constants for substitution reactions obeyed the simple rate law $$k_{{{\text{obs}}}} = k_{2} \left[ {Nu} \right]$$ . The results demonstrated that the lability of the chloride ligand is dependent on the degree of synergy between electronic character and the planarity of architectural frame work of the ligands around the platinum centre. The second-order kinetics and large negative activation entropies (ΔS#) assert an associative mode of activation. DFT calculations were performed to support the interpretation and discussion of the experimental data. The retardation in lability of quinoline systems; 2,6-bis(8-quinolyl)pyridine chloroplatinum(II) (Pt3) and 1,3-bis(8-quinolyl)phenyl chloroplatinum(II) (Pt4) is attributable to cis σ-donor effect and twisting of the quinoline rings that offsets the π-acceptor ability on the extended π-system. Conversely, high reactivity of pyridine systems; 2,6-bis(2-pyridyl)pyridine chloroplatinum(II) (Pt1) and 1,3-bis(pyridyl)phenyl chloroplatinum(II) (Pt2), is due to their rigid planar structure that facilitates effective π-acceptor ability.