Enhanced Cross Polarization in Magic Angle Spinning NMR of Metal Complexes

Solid-state NMR measurements of S = 1/2 metal nuclei, such as ¹¹³Cd and ¹⁹⁹Hg, report directly on the nature and coordination geometry of the ligands in metal complexes. The combination of cross polarization (CP) and magic angle spinning (MAS) has been the standard approach for obtaining high-resolution NMR spectra of these compounds in the solid state. High-speed MAS is usually desirable in order to average the extremely large chemical shift anisotropy of metal nuclei and increase signal intensity. However, the interference of MAS with the CP efficiency at high speeds normally results in weak signals as well as an increased dependence on the exact Hartmann-Hahn matching condition. As a result, it is difficult in practice to locate and maintain an optimal CP match. In this paper, we describe the successful application of variable-amplitude cross polarization (VACP: Peersen et al. J. Magn. Reson., Ser. A. 1993, 104, 334–339) to CP-MAS NMR studies of two metal complexes, [Me₄N]₂[Cd₂(SⁱPr)₆] and [Me₄N][Hg(SⁱPr)₃]. In VACP, the strength of the spin-lock field is varied to generate multiple matching conditions in a single CP contact period, giving rise to high signal intensity that is independent of the exact Hartmann-Hahn match. VACP can also increase the signal intensity by compensating for the effect of B₁ field inhomogeneity across the sample coil. The results presented in this paper illustrate how VACP can greatly facilitate data acquisition of metal complexes at high MAS speeds and can generally be applied with improved performance to those systems where conventional CP is effective.