Cyclometalated Analogues of Platinum Terpyridine Complexes: Kinetic Study of the Strong σ-Donor Cis and Trans Effects of Carbon in the Presence of a π-Acceptor Ligand Backbone

The substitution kinetics of the complexes [Pt(N−N−C)Cl] (N−N−CH = 6-phenyl-2,2‘-bipyridine), [Pt(N−C−N)Cl]
(N−CH−N = 1,3-di(2-pyridyl)benzene), and [Pt(N−N−N)Cl]Cl (N−N−N = 2,2‘:6‘,2‘ ‘-terpyridine) with the nucleophiles
Br-, I-, and, for the first two complexes, also thiourea, N,N-dimethylthiourea, and N,N,N‘,N‘-tetramethylthiourea,
have been studied in methanol as solvent. In case of the thioureas, the activation parameters ΔH⧧, ΔS⧧, and ΔV⧧
were also determined from the temperature and pressure dependence of the reactions. Two crystal structures of
[Pt(N−N−C)Cl] were determined (yellow and red polymorphs); the intense red color of the latter polymorph results
from Pt−Pt interactions (Pt−Pt distance = 3.366 Å). The data enable an analysis of the cis and trans effects and
the influence of the strong σ-donor carbon in the presence of an electron withdrawing π-acceptor ligand backbone.
The results indicate that the intrinsic reactivity is enhanced greatly by the labilizing effect of the trans carbon donor,
but the nucleophilic discrimination is dramatically reduced due to the decrease in electrophilicity on the metal
center. However, although the electron withdrawing π-acceptor effect is partly counteracted by the σ-donor effect,
the complex still benefits from a higher nucleophilic discrimination than in the comparable Pt(II) trans carbon donor
complexes, where no or fewer π-acceptors are present. In the case of the cis carbon donor complex, the intrinsic
reactivity remains unchanged, but the nucleophilic discrimination is reduced and leads to a reduced reactivity of
the [Pt(N−N−C)Cl] complex in comparison to [Pt(N−N−N)Cl]Cl. On the basis of these results, a more detailed
treatment of the nature of the cis effect is offered.