Bis(diphenylphosphino)acetylene as Bifunctional Ligand in Dicobalt Carbonyl Complexes

Treatment of bis(diphenylphosphino)acetylene (DPPA) with a bis(diphenylphosphino)methylene (DPPM)-bridged dicobalt complex, [{μ-P,P-PPh2CH2PPh2}Co2(CO)6], 3, yielded a
DPPA-bridged dicobalt compound, [{μ-P,P-PPh2CH2PPh2}Co2(CO)4{μ-PPh2C⋮CPPh2}], 5.
Two oxidized complexes, [{μ-P,P-PPh2CH2PPh2}Co2(CO)4{μ-PPh2C⋮CP(O)Ph2}], 6, and [{μ-P,P-PPh2CH2PPh2}Co2(CO)4{μ-P(O)Ph2C⋮CP(O)Ph2}], 7, were obtained along with 5
during the chromatographic separation process. Further reaction of 5 with selenium powder
in THF at 25 °C produced [{μ-P,P-PPh2CH2PPh2}Co2(CO)4{μ-P(O)Ph2C⋮CP(Se)Ph2}], 8,
a compound with two phosphorus atoms of 5 oxidized by oxygen and selenium, respectively.
The 31P−77Se coupling constant of the Se−P bond is 723 Hz. The reaction of 5 with Mo(CO)6
in toluene at 110 °C yielded [{μ-P,P-PPh2CH2PPh2}Co2(CO)4{μ-P,P-PPh2C⋮CPPh2}Mo(CO)4],
9. Compounds 5−9 were characterized by spectroscopic means as well as by X-ray diffraction
studies. Crystal structures of 6−8 reveal that the distance between the oxide and the adjacent
methylene proton is within the range of an intramolecular hydrogen bond. The structure of
9 reveals that 5 behaves as a diphosphine chelating ligand toward the Mo(CO)4 fragment.
The fluxional behavior of the DPPM ligands in 5 and 9 were examined by variable-temperature 1H NMR experiments. The calculated activation energies for the fluxional
processes are 19.2 and 7.1 kcal/mol for 5 and 9, respectively.