Synthesis and 15N nuclear magnetic resonance shift tensors of bent nitrosyl complexes with N-substituted salicylideneiminate coligands; the shift tensor as a criterion of MNO bond angle

Abstract

A series of nitrosylcobalt–Schiff base complexes [Co(NO)(asal)₂][asal =N-alkyl- or N-aryl-salicylideneiminate, o-OC₆H₄CHNR, R = Me(msal), Et(esal), Buⁿ(bsal), Ph(phsal) or CH₂Ph(bzsal)] has been prepared. Single-crystal X-ray structure analysis of the esal derivative shows the molecule to be pyramidal with a bent apical nitrosyl group [Co–N 1.738(12), N–O 1.16(2)Å, Co–N–O 129(1)°] and a trans arrangement of the basal ligands [Co–O 1.833(6), Co–N 1.950(6)Å]. The crystals are orthorhombic, space group Pbca, a= 9.544(3), b= 17.000(3), c= 11.359(3)Å, Z= 4; the structure was refined to R= 0.059 for 822 observed reflections. The compounds are not very stable in solution, but high-resolution cross-polarisation magic-angle spinning ¹⁵N NMR spectroscopy of the [Co(¹⁵NO)(asal)₂] solids, and also of [Co(¹⁵NO)(ketox)₂]( ketox =o-OC₆H₄CMeNOH), shows large isotropic shifts (ca. 730 ppm relative to nitromethane) characteristic of bent nitrosyls. Also characteristic are the nitrogen shift tensors, with one very large component (1100–1960 ppm) reflecting the low energy of the n(N)→π*(NO) circulation. The large ranges of the shielding anisotropy Δ(540–1150 ppm) and the asymmetry parameter η(1.25–2.40) reflect the sensitivity (because of the low excitation energies) of the tensor components to relatively small changes in the geometry of the MNO group, and its relation to the ligands in the basal plane.