Linking metal centres with 1,4-bis(diphenylphosphino)-2,5-difluorobenzene (dpfb): syntheses and molecular structures of [{Mo(CO)4(µ-dpfb)}2] and [{Mo(CO)4}2(µ-dpfb)(µ-dppa)](dppa = Ph2PCCPPh2)

Abstract

The rigid diphosphine 1,4-bis(diphenylphosphino)-2,5-difluorobenzene (dpfb) has been prepared via addition of 2 equivalents of lithium diphenylphosphide to 1,2,4,5-tetrafluorobenzene and characterised by multinuclear NMR spectroscopy. Reaction with cis-[Mo(CO)₄(pip)₂](pip = piperidine) afforded doubly bridged [{Mo(CO)₄(µ-dpfb)}₂]1, while with equimolar amounts of dpfb and bis(diphenylphosphino)acetylene (dppa) the mixed-ligand complex [{Mo(CO)₄}₂(µ-dpfb)(µ-dppa)]2 was isolated. Both 1 and 2 have been characterised crystallographically. The dfpb ligand always adopts a syn-type configuration. Complex 1 shows a weak π-stacking interaction between the bridging aryl rings, while in 2 the dppa ligand adopts an anti conformation. Variable-temperature multinuclear NMR experiments revealed that 1 undergoes two fluxional processes in solution; twisting of the central core of the molecule and a low-energy process involving the flipping up and down of molybdenum tetracarbonyl units which interconverts the anti and syn conformations of the diphosphines.