Coordination behavior of phosphino-phosphaferrocenes: monodentate versus bidentate coordination to divalent palladium

Two novel phosphino-phosphaferrocenes [η5-C5H4(CH2)nPPh2]Fe(η5-PC4H2-2,5-Cy2) (PP1: n=1; PP2: n=2) have been designed and prepared in order to clarify weak chelate effect in the previously reported (η5-C5H4CH2PPh2)Fe[η5-PC4H2-2,5-((–)-menthyl)2] (1). 31P NMR studies of reactions of PP1 with PdCl2(cod) (6) revealed that PP1 showed stronger tendency to coordinate to the PdII center in bidentate fashion compared to 1. On the other hand, chelate effect in PP2 was negligibly weak and a reaction of PP2 with 6 in a PP2/6 = 2/1 molar ratio gave a complex PdCl2(PP2)2 (10) cleanly in which PP2 coordinated to the palladium center at the PPh2 moiety as a monodentate ligand. X-ray crystal structure studies of chelate complexes PdCl2(PP1) (7) and PdCl2(PP2) (9) showed that 9 had deviations from an idealized geometry in the square planar complex which could be attributed to a larger chelate ring of PP2, while PP1 in 7 constructed nearly ideal geometry for the square planar complex.