The π-acceptor effect in the substitution reactions of tridentate N-donor ligand complexes of platinum(ii): a detailed kinetic and mechanistic study

Abstract

The nucleophilic substitution reactions of complexes [Pt{4'-(2'''-CH(3)-phenyl)-2,2':6',2''-terpyridine}Cl]CF(3)SO(3), [CH(3)PhPtCl], [Pt{4'-(2'''-CH(3)-phenyl)-6-(3''-isoquinoyl)-2,2'bipyridine}Cl]SbF(6), [CH(3)PhisoqPtCl], [Pt{2-(2'-pyridyl)-1,10-phenanthroline}Cl]Cl, [pyPhenPtCl], and [Pt(terpyridine)Cl](+), [PtCl] with a series of nucleophiles: thiourea (TU), N,N-dimethylthiourea (DMTU), N,N,N,N-tetramethylthiourea (TMTU), I(-), Br(-), and SCN(-) were studied in 0.1 M LiCF(3)SO(3) in methanol (in the presence of 10 mM LiCl). The reactivity of the investigated complexes follows the order pyPhenPtCl > PtCl > CH(3)PhPtCl > CH(3)PhisoqPtCl. The lability of the chloride ligand is dependent on the strength of π-backbonding properties of the spectator ligands around the platinum centre. The experimental data is strongly supported by DFT calculations. The dependence of the second-order rate constants on concentration of the nucleophiles as well as the large negative values reported for the activation entropy (ΔS(‡)) confirmed an associative mechanism of substitution.

Kinetic and mechanistic studies of 1,3-bis(2-pyridylimino)isoindolate Pt(ii) derivatives. Experimental and new computational approach

“…Similarly, the more perfect squareplanar structures of the six-membered chelate complexes under study confer stronger field strength from the ligand owing to minimal ring strain due to a more optimal bite angle at the metal ion close to 180°5 3,65,83 as shown in Table 1, encumbers reactivity compared to five-membered chelates. 1,4,7,8,41,43,45 The mappings of frontier orbitals (Fig. 1) compare satisfactorily with those determined previously by the Hanson 53 and Wen 65 research groups.…”

“…4,42,44 Moreover, it emerged that the cis-π acceptor has a stronger influence on the reactivity of the metal centre than the trans π-acceptor whilst the π-acceptor effects of a conjugated system are multiplicative. 4,[40][41][42] Against this backdrop, DFT calculations have been employed to give an insight into the electronic and steric properties of complexes; in addition, NBO charge on the metal center has been used effectively by Jaganyi et al 1,8,9,45 to explain the degree of electron deficiency in a given complex. Nonetheless, the challenge of NBO charge has always been the small magnitude of difference and sometimes inconsistency with reactivity.…”

Reactivity of cis‐platinum(II) complexes with 2‐(4‐substituted)phenylthiomethyl)quinoline nonleaving ligands toward thiourea nucleophiles

“…The decreased reactivity for Pt(L2) when compared to Pt4 is due to a decreased π‐back‐donation into the quinolyl group orbitals, which increases electron density on the Pt atom. Similar trends have been reported previously …”

“…In principle, extending the π‐surface of a strong π‐acceptor ring such as pyridine to a quinoline moiety is expected to accelerate the rate of substitution due to an increased propensity of the latter moiety to receive electron density into its lower energy, extended molecular orbitals. However, recent reports have demonstrated an antithesis of this observation. Relative to the reactivity of Pt(II)(terpy)Cl, terpy = 2,2′:6‐terpyridine, Pt(II) complexes bearing 2,6‐bis(iso/quinolyl)pyridine have been found to have surprisingly lower rates of substitution .…”

Structural and ethylene oligomerization studies of chelating (imino)phenol Fe(ii), Co(ii) and Ni(ii) complexes: an experimental and theoretical approach

“…On the other hand, the higher metal charges, on complexes supported on ligands L1 À and L2 À , in comparison to those of ligand L3 À , may be attributed to the pyridine ring being a better p-acceptor than the quinoline ring. 59,66 An opposite trend was interestingly observed on examinations of the different metal atoms, containing the Fig. 4 Influence of the metal NBO charge on the catalytic activities of the complexes in ethylene oligomerization reactions.…”