SET Oxidation of Li/X Phosphinidenoid Complexes by TEMPO

Oxidation of in situ generated Li/X phosphinidenoid complexes
[(OC)5W­{(Me3Si)2CHP­(X)­Li/12-crown-4}]
(X =
F (2), Cl (6)) with TEMPO at −78
°C led to the lithium halogenophosphinite complexes [(OC)5W­{(Me3Si)2CHP­(X)­OLi/12-crown-4}] (4) via an unknown pathway possibly involving transient P-nitroxyl complex derivatives (3). Whereas 4a (X = F) could be isolated directly, 4b (X
= Cl) underwent facile hydrolysis to give the lithium phosphonite
complex [(OC)5W­{(Me3Si)2CHP­(OH)­OLi/12-crown-4}]
(7) as the final product; the structures of complexes 4a and 7 were firmly established by single-crystal
X-ray analysis. DFT calculations on the assumed intermediate P-nitroxyl complexes 3a,b reveal
a preference for homolytic N–O over P–O bond cleavage,
as underlined by the small bond dissociation energies (in kcal/mol: 3a, P–O 53.2 and N–O 26.0; 3b,
P–O 35.4 and N–O 20.8).