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

Abstract

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.