The reactivity of Pd(II) complexes with C^N^N cyclometalated polypyridylphenyl ligands: crystal structures, kinetics and computational studies
摘要
The rate and mechanism of substitution of the labile chloride ligand from cyclometalated Pd(II) complexes coordinated to C^N^N polypyridylphenyl ligands (PdL1–PdL4) are reported. The reactions were performed under pseudo-first order conditions with thiourea nucleophiles as a function of concentration and temperature using the stopped-flow spectrophotometric technique. The observed rates of substitution can be expressed as kobs = k2[Nu] and the associative rate constants (k2) for the nucleophilic substitution decreased in the order: PdL1 ˃ PdL2 ˃ PdL3 ˃ PdL4 as controlled by the electronic properties of the C^N^N tridentate ligand. The rate constants for the PdL4 reactions were found to be 3–5 times lower than that of PdL1. The cis-positioned and deprotonated C of the phenyl or the naphthyl moiety of the C^N^N ligand reduces the rate by accumulating electron density on the cis-Pd–C bond. The cis σ-donation towards the Pd(II) centre is slightly stronger for the coordinated naphthyl moiety than for the phenyl ring. The observed trends indicated that increasing the conjugated π-surface for the cis-coordinated naphthyl or isoquinolyl groups of the C^N^N ligand does not necessarily increase the electrophilicity of the Pd(II) centre because both are relatively weaker π-acceptors groups. By using a C^N^N core chelate for Pd coordination as well as benzannulation of its cis rings, the rates of substitutions were lower than that of [Pd(N^N^Nterpy)Cl]+, a reactivity balance required to circumvent their in vivo deactivation by ubiquitous S-bio-nucleophiles. A limiting associative mechanism applies to the substitution process as supported by the low but positive activation enthalpy change (ΔH≠) and negative activation entropy change (ΔS≠) values. The solid-state crystal structures of PdL2 and PdL3 belong to P-1 and Pbca space groups, respectively. The trends in the DFT-calculated chemical potential (μ) and other descriptors support the observed reactivity trends of the complexes.