Slowing the reactivity of dicyclometalated Pd(II) complexes through the 2,6-bis(N-heterocyclic carbene)pyridine (C^N^C) non-leaving ligands: kinetic and computational study

Abstract

In this study, the kinetic and mechanistic studies of the substitution of chloride ligand of [(chloro)(2,6-bis(N-heterocyclic carbene)pyridine)Pd(II)]BF₄ complexes, namely Pd1, Pd2, Pd3 and Pd4, by thiourea nucleophiles viz Tu, Dmtu and Tmtu were investigated. The rate of chloride substitution of dicyclometalated complexes was monitored in aqueous media containing 20 mM LiCl using stopped-flow spectrophotometry as a function of concentration and temperature under pseudo-first-order conditions. The kinetic data fitted to the pseudo-first-order rate law, k_obs = k₂[Nu]. The rate of chloride substitution decreased in the order Pd1 ˃ Pd2 ˃ Pd4 ˃ > Pd3. The reactivity of Pd1 was lower by two orders of magnitude compared to [Pd(terpy)Cl]⁺ (terpy = terpyridine). Both complexes have strong π-acceptor non-leaving ligands that promote efficient back bonding of charge into the aromatic bis(NHC) chelates of its non-leaving ligand. Contrastingly, the lutidine-bridged complexes, (Pd2-4) form 6-membered and non-aromatic bis(NHC) chelates which cause steric influence on either side of the square plane. Their substituents also impart additional steric effects and σ-inductive effects in the rings. The combined effect significantly lowers rates of substitution. Consequently, Pd3 was the least reactive. The substitution mechanism is associative since no evidence of a mechanistic change over to the dissociative substitution was observed, despite the complexes coordinated with tridentates with two cis-σ-bound carbon donors.