Selective functionalisation. Part 11. Selective hydrogenation by a novel palladium salicylidene-ethylenediamine complex and the properties of derivatives of some square planar homogeneous hydrogenation catalysts

Abstract

Analogues of two reported homogeneous hydrogenation catalysts based upon square planar palladium complexes have been prepared with a view to modifying the structures to permit the control of selectivity by micellar interactions. Derivatives of bisacetylacetonatopalladium(II) in which the diketone was alkylated at C-3 or C-5 were prepared but no useful catalytic activity was observed for the reduction of nitrobenzene to aniline in the presence of pyridine. Complexes were also prepared from bisacetylacetonatopalladium(II) with 4-substituted pyridines as ligands; 4-tridecylpyridine afforded an unstable complex but 4-decylaminopyridine afforded a stable complex with low catalytic activity. A series of salicylidene imine palladium(II) complexes was prepared. Contrary to previous reports, the well-known salicyclidene-ethylenediaminepalladium(II) complex was not a catalyst for homogeneous hydrogenation but a new oligomeric green heterogenous complex with selective hydrogenation properties was discovered. This green complex was selective for the hydrogenation of alkynes, especially terminal alkynes, and reduced few other functional groups (ArNO₂, ArCHO). Many variations on this structure were investigated in an attempt to discover a soluble or crystalline analogue of the green complex but no complexes with improved properties were isolated.