LINKING METAL CENTERS WITH 1,4-BIS(DIPHENYLPHOSPHINO)-2,5-DIFLUOROBENZENE (DPFB) - SYNTHESES AND MOLECULAR-STRUCTURES OF [(MO(CO)(4)(MU-DPFB))(2)] AND [(MO(CO)(4))(2)(MU-DPFB)(MU-DPPA)] (DPPA=PH(2)PC-CPPH(2))

The rigid diphosphine 1,4-bis(diphenylphosphino)-2,5-difluorobenzene ( dpfb) has been prepared via addition of 2 equivalents of lithium diphe nylphosphide to 1,2,4,5-tetrafluorobenzene and characterised by multin uclear NMR spectroscopy. Reaction with cis-[Mo(CO)(4)(pip)(2)] (pip = piperidine) afforded doubly bridged [{Mo(CO)(4)(mu-dpfb)}(2)] 1, while with equimolar amounts of dpfb and bis(diphenylphosphino)acetylene (d ppa) the mixed-ligand complex [{Mo(CO)(4)}(2)(mu-dpfb)(mu-dppa)] 2 was isolated. Both 1 and 2 have been characterised crystallographically. The dfpb ligand always adopts a syn-type configuration. Complex 1 show s a weak pi-stacking interaction between the bridging aryl rings, whil e in 2 the dppa ligand adopts an anti conformation. Variable-temperatu re multinuclear NMR experiments revealed that 1 undergoes two fluxiona l processes in solution; twisting of the central core of the molecule and a low-energy process involving the flipping up and down of molybde num tetracarbonyl units which interconverts the anti and syn conformat ions of the diphosphines.