Imidazoliophosphines are True N-Heterocyclic Carbene (NHC)-Phosphenium Adducts

Whereas the external nucleo philic reactivity of alpha amidiniophos phines has been previously illustrated by their complexation to transition metal centers, their internal electro philic reactivity is herein investigated by using BIMIONAP (BIMIONAP=N methylated BIMINAP cation, BIMI NAP=formal contraction of the acronyms BIMIP=2,2 bis(diphenylphos phino) 1,1 -bibenzimidazole and BINAP=2,2` bis(diphenylphosphino) 1,1 binaphthyl) Reaction of tetraethyl-ammonium chloride with free BIMIO NAP is found to Induce heterolytic cleavage of the N2C-P bond to give chlorodiphenylphosphine and a transient phosphine-N heterocyclic carbene (NHC) species that is trapped in situ by protonation to the corresponding phosphine-benzimidazolium cation When the chloride anion reacts with the cationic [Pd(eta(2) BIMIONAP)Cl-2] complex, the same cleavage occurs and the phosphine-NHC moiety is trapped in the corresponding [PdCl2(eta(2) phosphine-NHC)] complex When the chloride anion reacts with the dicationic [Pd(pi allyl)(eta(2)-BIMIONAP)](+) complex, allyldiphenylphosphine is produced, and the [PdCl(eta(2) phosphine-NHC)(PPh2CH2CH=CH2)](+) complex is obtained Reaction of free BIMIO NAP with the harder n butyllithium nucleophile also induces heterolytic cleavage of the N2C-P bond, from which the phosphine-NHC moiety is trapped by hydrolysis of the benzimidazole ring or by P,C sulfurization Cleavage of a C-P bond with the weak Cl- nucleophile to release the reactive NHC moiety (according to the unusual scheme C-P+C1(-)-> C + Cl-P) is a definite experimental indication of the dative nature of the N2C-P bond of amidiniophosphines, which are, there fore, better described as NHC -> phosphenium adducts This interpretation is supported by the calculation, at the DFT level, of a heterolytic dissociation mode of the N2C-P bond lower in energy than the homolytic one A mesomeric description of the NHC -> phosphenium entity is also proposed on the basis of electron localization function (ELF) and atoms in molecules (AIM) analyses Finally ELF and AIM-based Fukui Indices molecular orbitals, and MESP analyses show that the initial attack of Cl- takes place at the car benic atom of BIMIONAP