Divergent Reactivity of a Phosphinidene-Bridged Dimolybdenum Complex Toward 1‑Alkynes: P–C, P–H, C–C, and C–H Couplings

The ability of complex
[Mo2Cp2{μ-P­(2,4,6-C6H2tBu3)}­(CO)4] to
promote P–C and subsequent coupling processes
has been analyzed by examining reactions with alkynes. The title compound
reacted at room temperature with different terminal alkynes HCCR
under visible-UV light irradiation to give, in a selective way, the
corresponding phosphapropenediyl derivatives [Mo2Cp2(μ–κ1C:η3C,C,P-CRCHPR*)­(CO)4] for R groups of
varied electron-withdrawing nature, but low to medium size, such as
Pr, CO2Me, and p-tol. These products follow
from formal insertion of the alkyne into one of the Mo-phosphinidene
bonds in the parent compound, with selective P–C coupling to
the terminal carbon of the alkyne. In contrast, when R was the bulky tBu group, the corresponding photochemical
reactions yielded a mixture of the cis and trans isomers of the phosphanyl- and formylalkenyl-bridged
complex [Mo2Cp2{μ–κ2C,O:η2C,C-CHC­(tBu)­C­(O)­H}­{μ-P­(CH2CMe2)­C6H2tBu2}­(CO)2], which follow from a complex reaction sequence
involving an H–C­(sp3) bond cleavage
along with different P–C, P–H, C–C, and C–H
bond formation steps. Density functional theory calculations were
carried out to rationalize the preceding observations and suggested
that an isomer of the parent complex displaying a bent terminal phosphinidene
ligand might be involved in all the above photochemical reactions.