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

Solid-state P-31 NMR spectra of two phosphite- and 17 phosphine-substituted cobaloximes have been acquired under conditions of magic-angle spinning (M AS) and cross polarization (CP) at two applied magnetic fields. In the majo rity of cases, eight-peak multiplets are observed in the P-31 CPMAS NMR spe ctra, arising from one-bond indirect spin-spin coupling to cobalt (spin S = 7/2). The spacings between adjacent peaks gradually increase or decrease f rom low to high frequency, due to the presence of residual dipolar coupling between the cobalt and phosphorus nuclei. Values of (1)J(Co-59, P-31) were estimated from the spacing between the central peaks of the eight-peak mul tiplets. For the phosphine-substituted cobaloximes, values of (1)J(Co-59, P -31) range from 225 to 372 Hz, while in the phosphite derivatives this coup ling is considerably larger, from 420 to 615 Hz. From comparison with cobal oximes for which the cobalt nuclear quadrupole coupling interaction is full y characterized, the residual dipolar shift, d, and sense of the P-31 NMR s pectrum can be utilized to infer the electric field gradient (EFG) orientat ions at the cobalt nucleus in cobaloximes for which such data are unavailab le. The magnitudes of (1)J(Co-59, P-31) and d, as well as the sense of the spectra, are shown to be dependent upon the nature of the axially-substitut ed ligands. Phosphorus-31 CPMAS NMR spectra of several cobaloximes exhibit broad peak shapes at room temperature. Variable-temperature P-31 NMR experi ments reveal that the cobalt nucleus is effectively self-decoupled at room temperature; however, splittings due to (1)J(Co-59, P-31) were observed in spectra acquired at low temperatures.