4-Phosphino-Substituted N-Heterocyclic Carbenes (NHCs) from the Abnormal Reaction of NHCs with Phosphaalkenes

The activation of the PC bond of phosphaalkenes
with N-heterocyclic
carbenes (NHCs) offers a convenient means to introduce new functionality
at the 4-position of an NHC. Treatment of MesPCRR′
(2a: R = R′ = Ph; 2b: R = Ph, R′
= 2-C5H4N; 2c: R = R′ =
4-C6H4F) with 1,3-dimesitylimidazol-2-ylidene
[:C­(NMesCH)2, IMes, Mes = 2,4,6-Me3C6H2] affords 4-phosphino-2-carbenes :C­(NMes)2CHC­(PMesCHRR′)NMes
[1a: R = R′ = Ph; 1b: R = Ph, R′
= 2-C5H4N; 1c: R = R′ =
4-C6H4F]. Significantly, these functional NHCs
retain the parent carbene functionality and, in addition, contain
a phosphine moiety. The preparation and spectroscopic characterization
of the complexes [(1a)­M­(cod)­Cl] (4) and cis-[(1a)­M­(CO)2Cl] (5) (M = Ir, Rh) are reported. The average CO stretching frequencies
(ν̅COav) for 5M=Ir, 5M=Rh, and authentic samples of cis-[(IMes)­M­(CO)2Cl] (M = Ir, Rh) are presented as a means to evaluate the
donor properties of 4-phosphino-2-carbene 1a. The average
CO stretching frequency is lower energy for 5M=Ir than for cis-[(IMes)­Ir­(CO)2Cl] (Δν̅COav = −2 cm–1), whereas
the opposite is observed between 5M=Rh and cis-[(IMes)­Ir­(CO)2Cl] (Δν̅COav = 2 cm–1). The molecular structures are reported for 1a, 1b, 1c, 4M=Ir, and 5M=Ir.