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

Abstract

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.