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

Phosphinidene complexes of a 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 so far a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO)4 (NHC = IPr = C{(NDipp)CH}2, 3; Me-IPr = C{(NDipp)CMe}2, 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 the donor-acceptor type bonding between phosphorus and iron atoms in 3 and 4. The P→Fe donation represents ~70 % whereas the Fe→P π-back donation corresponds to ~15 % of the orbital interaction. The phosphorus atom in 3 and 4 carries +0.65e and +0.64e charge, respectively, indicating an 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 respectively shorter and longer compared to those of 3.