Isolation of Elusive Electrophilic Phosphinidene Complexes with π‑Donor N‑Heterocyclic Vinyl Substituents

Phosphinidene
complexes of the general formula RPM­(CO)n (R = an alkyl or aryl group; M = a transition metal)
are electrophilic and thermally unstable. Thus, the isolation of these
elusive species for structural elucidations remains a challenge. Herein,
we report the first terminal phosphinidene complexes [{(NHC)­C­(Ph)}­P]­Fe­(CO)4 [NHC = IPr = C­{(NDipp)­CH}2 for 3;
Me-IPr = C­{(NDipp)­CMe}2 for 4; Dipp = 2,6-iPr2C6H3; NHC = N-heterocyclic
carbene] as red crystalline solids containing a π-donor N-heterocyclic
vinyl (NHV) substituent at the phosphorus atom. Calculations reveal
donor–acceptor type bonding between phosphorus and iron atoms
in 3 and 4. The P → Fe donation represents
∼70% of the orbital interaction, whereas the Fe → P
π-back-donation corresponds to ∼15% of the orbital interaction.
The phosphorus atoms in 3 and 4 carry charges
of +0.65e and +0.64e, respectively, indicating the electrophilic character
of the phosphinidene {(NHC)­C­(Ph)}P moiety. Accordingly, 3 reacts with an NHC nucleophile (IMe4) to yield the Lewis
adduct [{(NHC)­C­(Ph)}­P­(IMe4)]­Fe­(CO)4 (5) [IMe4 = C­(NMeCMe)2]. The coordination of
an electron-rich NHC (IMe4) to the phosphorus atom in 5 precludes the π-electron density transfer from the
NHV to the phosphorus atom. Thus, the CIPr–Cvinyl and Cvinyl–P bonds of 5 become shorter and longer, respectively, compared to those of 3.