Cyclometalated analogues of platinum terpyridine complexes: kinetic study of the strong sigma-donor cis and trans effects of carbon in the presence of a pi-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 DeltaH, DeltaS, and DeltaV 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 A). The data enable an analysis of the cis and trans effects and the influence of the strong sigma-donor carbon in the presence of an electron withdrawing pi-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 pi-acceptor effect is partly counteracted by the sigma-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 pi-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.