Mechanism of the Catalytic Hydrodefluorination of Pentafluoropyridine by Group Six Triangular Cluster Hydrides Containing Phosphines: A Combined Experimental and Theoretical Study

The catalytic hydrodefluorination (HDF) of pentafluoropyridine in the presence of arylsilanes is catalyzed by the tungsten and molybdenum(IV) cluster hydrides of formula [M3S4H3(dmpe)(3)](+), W-1(+) for M = W and Mo-1(+) for M = Mo (dmpe = 1,2-(bis)dimethylphosphinoethane). The reaction proceeds regioselectively at the 4-position under microwave radiation to yield the 2,3,5,6-tetrafluorpyridine. Catalytic activity is higher for the tungsten complexes with turnover numbers close to 100, while reactions catalyzed by molybdenum compounds are faster. A mechanism for the HDF reaction has been proposed that explains these differences based on DFT calculations. The mechanism involves partial decoordination of the diphosphine ligand that generates an empty position in the metal coordination sphere. This position together with its neighbor M-H site are used to activate the C-F bond of the pentafluoropyridine through a M-H/C-F sigma-bond metathesis mechanism involving a four-center transition state to give 2,3,5,6-tetrafluoropyridine. Subsequent coordination of the dangling diphosphine affords the para-substituted product and the [M3S4F3(dmpe)(3)](+), W-2(+) for M = W and Mo-2(+) for M = Mo, cluster fluoride. The structure of W-2(+) has been determined by single-crystal X-ray diffraction experiments. In the presence of silanes the calculated mechanism for the cluster hydride regeneration also implies three steps: (i) partial decoordination of the diphosphine, (ii) M-F/Si-H sigma-bond metathesis, and (iii) coordination of the dangling diphosphine, to afford the cluster hydride.