A modular approach to carbene-stabilized diphosphorus species

Abstract

Heteroleptic N-heterocyclic dicarbene-diphosphorus species were prepared by reaction of the carbene-phosphinidene adduct (IPr)PSiMe₃ (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with the carbene-phosphorus trichloride adduct (IMes)PCl₃ (2, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), which furnished the dichloride [(IPr)PPCl(IMes)]Cl (3). Reduction of 3 with potassium graphite (KC₈) afforded [(IPr)PP(IMes)] (4). The corresponding radical cation [(IPr)PP(IMes)]˙⁺ (5˙⁺) is isolated as [5]PF₆ by reaction of 4 with ferrocenium hexafluorophosphate, whereas complexes containing the corresponding dication [(IPr)PP(IMes)]²⁺ (6²⁺) can be isolated as the gallate and borate salts [6](GaCl₄)₂ and [6](BAr^F)₂ by chloride abstraction from 3 with GaCl₃ or sodium tetrakis[bis(3,5-trifluoromethyl)phenyl]borate (NaBAr^F), respectively. The asymmetric set of N-heterocyclic carbene ligands allows to establish ¹J_PP coupling constants of 249 Hz for 4 and 543 Hz for [6](GaCl₄)₂. Based on X-ray diffraction analyses, the molecular structures of 4, 5˙⁺ and 6²⁺ reveal a consecutive shortening of the P-P bond lengths, in agreement with the presence of a phosphorus-phosphorus single bond in 4 and a double bond in 6²⁺, which is best described as a dicationic diphosphene according to density functional theory (DFT) calculations.