Synthesis and Structural Aspects of Metalated Complexes of the Platinum Metals Containing Pi-Deficient Heterocyclic Amines.

David Wesley Evans, Louisiana State University and Agricultural & Mechanical College


Various ligands were synthesized containing $\pi$-deficient heterocyclic amines. These amines included pyridine, bipyridine and biquinoline. The goal of these studies was to determine the factors important to cyclometalation with platinum metals. These factors included chelate ring size, ligand rigidity, carbon acidity and steric congestion at the binding locus. The first study of this series involved biquinoline-based ligands, in which the binding locus was highly hindered for attack by a transition metal. Coupling of 2-bromo-8-methyquinoline with Ni(PPh$\sb3$)$\sb4$ led to the formation of 8,8$\sp\prime$-dimethylbiquinoline in good yield. $\sp{13}$C and $\sp1$H NMR analyses indicated that subsequent metalation could not be achieved due to severe distortions about the coordination sphere caused by the extreme steric congestion. The next two studies involved the inclusion of ligands capable of forming 5-, 6- and 5-membered cumulative chelate rings (5.6.5), in which the central 6-membered ring contained a saturation carbon, rather than our previosly synthesized sp$\sp2$-carbon bridge. In addition, a previously synthesized saturated 5.7.5-ligand was modified to obtain an unsaturated ethylene bridge. These modifications provided clear trends in the steric requirements of the various platinum metals, since these studies also extended the metals incorporated from Pd(II) to Pt(II) and Ni(II). In a slightly different direction, the extrusion of phosphorus from triarylphosphine complexes of Pd(II) was studied. In an earlier report by us on the behavior of dipyridylphosphine oxides, the extrusion of phosphorus leading to the coupling of the pyridines was observed. At this time, it was noted that three contiguous electron-poor centers appeared to be a prerequisite for this extrusion. In our study, this prerequisite was met in the form of the $\alpha$-carbon of a pyridine, a coordinated phosphine (analogous to a phosphonium salt) and a Pd(II). In the presence of good nucleophiles (e.g. Cl$\sp-$ for attack of P to give a pentavalent intermediate), extrusion of phosphorus occurred leading to the formation of a C -pyridinium Pd(II) complex. Higher concentrations of nucleophiles and increased temperatures led to a subsequent extrusion, apparently leading to an unstable bis- C pyridinium Pd(II) complex, which then underwent a reductive elimination leading to the formation of bipyridine. (Abstract shortened with permission of author.).