A modular approach to carbene-stabilized diphosphorus species.

Heteroleptic N-heterocyclic dicarbene-diphosphorus species were prepared by reaction of the carbene-phosphinidene adduct (IPr)PSiMe ₃ (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with the carbene-phosphorus trichloride adduct (IMes)PCl ₃ (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 (KC ₈ ) afforded [(IPr)PP(IMes)] (4). The corresponding radical cation [(IPr)PP(IMes)]˙ ⁺ (5˙ ⁺ ) is isolated as [5]PF ₆ by reaction of 4 with ferrocenium hexafluorophosphate, whereas complexes containing the corresponding dication [(IPr)PP(IMes)] ²⁺ (6 ²⁺ ) can be isolated as the gallate and borate salts [6](GaCl ₄ ) ₂ and [6](BAr ^F ) ₂ by chloride abstraction from 3 with GaCl ₃ or sodium tetrakis[bis(3,5-trifluoromethyl)phenyl]borate (NaBAr ^F ), respectively. The asymmetric set of N-heterocyclic carbene ligands allows to establish ¹ J _PP coupling constants of 249 Hz for 4 and 543 Hz for [6](GaCl ₄ ) ₂ . Based on X-ray diffraction analyses, the molecular structures of 4, 5˙ ⁺ and 6 ²⁺ 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 6 ²⁺ , which is best described as a dicationic diphosphene according to density functional theory (DFT) calculations.