Imidazoliophosphines are true N-heterocyclic carbene (NHC)-phosphenium adducts

Article abstract of DOI:10.1002/chem.201001721

Whereas the external nucleophilic reactivity of α-amidiniophosphines has been previously illustrated by their complexation to transition-metal centers, their internal electrophilic reactivity is herein investigated by using BIMIONAP (BIMIONAP=N-methylated BIMINAP cation, BIMINAP=formal contraction of the acronyms BIMIP=2,2′-bis(diphenylphosphino)-1,1′-bibenzimidazole and BINAP=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl). Reaction of tetraethylammonium chloride with free BIMIONAP 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(η²-BIMIONAP)Cl ₂] complex, the same cleavage occurs and the phosphine-NHC moiety is trapped in the corresponding [PdCl₂(η²-phosphine-NHC)] complex. When the chloride anion reacts with the dicationic [Pd(π-allyl)(η²-BIMIONAP)]⁺ complex, allyldiphenylphosphine is produced, and the [PdCl(η²-phosphine- NHC)(PPh₂CH₂CH=CH₂)]⁺ complex is obtained. Reaction of free BIMIONAP 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+Cl^-→C:+Cl-P) is a definite experimental indication of the dative nature of the N₂C-P bond of amidiniophosphines, which are, therefore, 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 carbenic atom of BIMIONAP. Addition of anionic nucleophiles (Cl^-, nBu^-) to free BIMIONAP and BIMIONAP-containing palladium complexes results in selective cleavage of the N₂C-P bond, from which the released N-heterocyclic carbene (NHC) fragment can be trapped by protonation, hydrolysis, sulfurization, or coordination to Pd^II centers (see scheme).