Trans coordinating bidentate ligands

Abstract

A number of novel bidentate ligands have been designed, which would, because of steric constraints, coordinate exclusively across the trans positions in a square planar complex.
Such complexes could show optical activity centred on the metal if the four ligand groups (A,B,C,IJ in the diagram) were all different. They would then have exciting potential applications as catalysts for asymmetric organic synthesis. Their specificity could be very high, because coordination to the metal by substrate organic molecules would be blocked on one side of the square plane by the framework of the trans coordinating bidentate ligand. Coordinating on the other side,
the substrate molecule would encounter a highly asymmetric environment provided by four different groups A,B,C,IJ ordered anticlockwise around the metal.
Synthetic routes to the ligands of suitable design for trans coordination were investigated. Many designs had to be abandoned. Finally two new ligands were successfully isolated, 3,3'-bis(diphenylphosphinomethyl) biphenyl (denoted by PP), and the arsenic analogue 3,3' -bis(diphenylarsinomethyl)biphenyl, (AsAs). These compounds were readily prepared in good yields as colourless needles.
Although PP and AsAs were symmetrical ligands and would not therefore, produce optically active complexes, they could be used to test the ligand design with regard to formation of complexes and catalytic properties of these complexes. The ligands were found to form many complexes with metal ions, representative examples being RhCl(PPh3)(PP), RhCl(cycloocta-1,5-diene)(AsAs), [Rhcol(PPh3)(PP)](cl0), RuC12(FPh3)(PP, and NiBr2(PP).
The complexes were screened for catalytic activity. Most of the rhodium(I) complexes prepared were found to be quite efficient catalysts for the hydrogenation of simple alicenes.
Following the successful demonstration of catalysis by rhodium(l) complexes of the new ligand system, attempts to prepare related unsynunetrical ligands were made. These were not successful, but the investigations indicated that future work should be directed towards conversion of PP into a terdentate ligand.