Solid-state phosphorus-31 NMR study of phosphine- and phosphite-substituted cobaloximes

Abstract

Solid-state 31P NMR spectra of two phosphite- and 17 phosphine-substituted cobaloximes have been acquired under conditions of magic-angle spinning (MAS) and cross polarization (CP) at two applied magnetic fields. In the majority of cases, eight-peak multiplets are observed in the 31P CPMAS NMR spectra, arising from one-bond indirect spin-spin coupling to cobalt (spin S = 7/2). The spacings between adjacent peaks gradually increase or decrease from low to high frequency, due to the presence of residual dipolar coupling between the cobalt and phosphorus nuclei. Values of 1J(59Co,31P) were estimated from the spacing between the central peaks of the eight-peak multiplets. For the phosphine-substituted cobaloximes, values of 1J(59Co,31P) range from 225 to 372 Hz, while in the phosphite derivatives this coupling is considerably larger, from 420 to 615 Hz. From comparison with cobaloximes for which the cobalt nuclear quadrupole coupling interaction is fully characterized, the residual dipolar shift, d, and sense of the 31P NMR spectrum can be utilized to infer the electric field gradient (EFG) orientations at the cobalt nucleus in cobaloximes for which such data are unavailable. The magnitudes of 1J(59Co,31P) and d, as well as the sense of the spectra, are shown to be dependent upon the nature of the axially-substituted ligands. Phosphorus-31 CPMAS NMR spectra of several cobaloximes exhibit broad peak shapes at room temperature. Variable-temperature 31P NMR experiments reveal that the cobalt nucleus is effectively self-decoupled at room temperature; however, splittings due to 1J(59Co,31P) were observed in spectra acquired at low temperatures.Key words: solid-state 31P NMR, 59Co-31P spin-spin coupling constants, 59Co electric field gradient tensors, cobaloximes.