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Article Dans Une Revue Chemistry - A European Journal Année : 2010

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

Ibrahim Abdellah
Christine Lepetit
Yves Canac
Rémi Chauvin

Résumé

Whereas the external nucleophilic reactivity of a-amidiniophosphines has been previously illustrated by their complexation to transitionmetal 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(h2-BIMIONAP)Cl2] complex, the same cleavage occurs and the phosphine–NHC moiety is trapped in the corresponding [PdCl2(h2-phosphine– NHC)] complex. When the chloride anion reacts with the dicationic [PdACHTUNGTRENUNG(p-allyl)(h2-BIMIONAP)]+ complex, allyldiphenylphosphine is produced, and the [PdCl(h2-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 AIMbased Fukui indices, molecular orbitals, and MESP analyses show that the initial attack of Cl_ takes place at the carbenic atom of BIMIONAP.

Dates et versions

hal-02907440 , version 1 (27-07-2020)

Identifiants

Citer

Ibrahim Abdellah, Christine Lepetit, Yves Canac, Carine Guyard-Duhayon, Rémi Chauvin. Imidazoliophosphines are True N-Heterocyclic Carbene (NHC)-Phosphenium Adducts. Chemistry - A European Journal, 2010, 16 (44), pp.13095-13108. ⟨10.1002/chem.201001721⟩. ⟨hal-02907440⟩
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