Proton Affinities of Cationic Carbone Adducts [AC(PPh3)2]+ (A = Halogen, Hydrogen, Methyl) and Unusual Electronic Structures of the Cations and Dications [AC(H)(PPh3)2]2+

This work reports the syntheses and the first crystal structures of the cationic carbone adducts [FC(PPh3)(2)](+) and [BrC(PPh3)(2)](+) and the protonated dication [FC(H)(PPh3)(2)](2+), which are derived from the carbone C(PPh3)(2). Quantum chemical calculations and bonding analyses were carried out for the series of cations [AC(PPh3)(2)](+) and dications [AC(H)(PPh3)(2)](2+), where A = H, Me, F, Cl, Br, I. The bonding analysis suggests that the cations are best described as phosphane complexes L -> (CA)(+) -> L (L= PPh3), which are related to the neutral borylene adducts L -> (BA) <- L (L= cyclic carbene; A = H, aryl) that were recently isolated. The carbone adducts [AC(PPh3)(2)](+) possess a pi electron lone pair at carbon and they can easily be protonated to the dications [AC(H)(PPh3)(2)](2+). The calculations of the di-cations indicate that the molecules are best represented as complexes L -> (CHA)(2+) <- L (L= PPh3) where a carbene dication is stabilized by the ligands. The central carbon atom in the cations and even in the dications carries a negative partial charge, which is larger than the negative charge at fluorine. There is also the peculiar situation in which the carbon-fluorine bonds in [FC(PPh3)(2)](+) and [FC(H)(PPh3)(2)](2+) exhibit the expected polarity with the negative end at fluorine, but the carbon atom has a larger negative charge than fluorine. Given the similarity of carbodiphosphorane C(PPh3)(2) and carbodicarbene C(NHC)(2), we expect that analogous compounds [AC(NHC)(2)](+) and [AC(H)(NHC)(2)](2+) with similar features as [AC(PPh3)(2)](+) and [AC(H)(PPh3)(2)](2+) can be isolated.