Fourier-transform heteronuclear magnetic triple resonance in complex spin systems. Part 2. Acyclic ditertiary phosphines and their selenides

Abstract

Phosphorus–phosphorus nuclear spin–spin coupling has been studied in a series of symmetrical bis(diphenylphosphino)-compounds with aliphatic, olefinic, and acetylenic carbon groups linking the two phosphorus atoms. ¹³C-{¹H,³¹P} and ³¹P-{¹H,⁷⁷Se} magnetic triple resonance experiments allow the signs of these coupling constants to be obtained, and in some cases prove essential for the measurement of their magnitudes. Different patterns of behaviour clearly emerge for phosphorus in the +3 and +5 oxidation states. In the diselenide series the various geminal and vicinal couplings show marked similarities in their trends to the much more extensively studied couplings J(HH) and J(P^VH). When, however, at least one of the phosphorus atoms possesses a lone pair of electrons the same rules no longer apply, and certain conformations (frequently the most favoured ones) apparently make an exceptionally large positive contribution to the coupling constant. Factors affecting selenium-77 shielding in the selenides and diselenides are discussed.