Novel pathways for ligand substitution reactions of dinuclear platinum(I) complexes

Abstract

The terminal halide and/or triphenylphosphine ligands L in dinuclear platinum(I) complexes (Pt/sub 2/L/sub 2/(..mu..dppm)/sub 2/)/sup n /sup +// (where dppm = Ph/sub 2/PCH/sub 2/PPh/sub 2/) undergo ready substitutions. The reaction of (Pt/sub 2/Cl/sub 2/(..mu..-dppm)/sub 2/), abbreviated (Cl--Cl), with PPh/sub 3/ follows a second-order rate law, with kappa = 15.2 +/- 0.7 M/sup -1/ s/sup -1/ at 25/sup 0/C in CH/sub 2/Cl/sub 2/. The product is exclusively (Cl--PPh/sub 3/)/sup +/; (Ph/sub 3/P--PPh/sub 3/)/sup 2 +/ forms only in a more polar solvent like methanol. The complex (Ph/sub 3/P--PPh/sub 3/)/sup 2 +/ reacts in CH/sub 2/Cl/sub 2/ with halide ions (X/sup -/ = Cl/sup -/, Br/sup -/, and I/sup -/) also to yield (X--PPh/sub 3/)/sup +/. In the presence of inert salts such as perchlorates, the reaction proceeds by rate-limiting dissociation of PPh/sub 3/, unusual for square-planar, 16-electron platinum, but further evidence for the large trans influence of the metal-metal bond. In the absence of such salts, however, the reaction occurs in three distinct kinetic stages, although eventually leading to (Cl--PPh/sub 3/)/sup +/. One of the intermediates can be stabilized at low temperature; its /sup 31/P NMR spectrum identifies it (for X/sup -/ = I/sup -/ and Cl/sup -/) as a speciesmore » containing both bridging and chelating dppm (Pt/sub 2/(X)(PPh/sub 3/)(..mu..-dppm)(eta-dppm))/sup +/. Substitution of PPh/sub 3/ in (Ph/sub 3/--PPh/sub 3/)/sup 2 +/, like the A-frame forming reactions of the same complex, starts with the dissociation of one arm of a bridging dppm ligand.« less