SOLID-STATE CO-59 NMR IN TETRAHEDRAL CLUSTERS

A series of tetrahedral mixed-metal dusters A[MCo3(CO)12] with M = Fe, Ru and A = NEt4, H has been studied by solid-state Co-59 NMR at two different magnetic field strengths (4.7 and 7.1 T). Due to the large value of the quadrupole coupling constant, only the central transition (-1/2 <-> 1/2) is detected experimentally. We have developed a moment analysis of the central transition powder pattern that permits a direct evaluation of the quadrupole coupling constant as well as the isotropic part and anisotropy of the chemical shift tensor without having to assume coincident chemical shift (CS) and electric field gradient (EFG) tensors. Using the method of the moments to obtain starting guess parameter sets, both the magnitudes and relative orientation of the CS and EFG tensors have been determined by nonlinear least-squares fitting of the line shapes at the two magnetic field strengths. The CS tensor is almost completely axially symmetric for the whole series of clusters. While the substitution of the apical metallic atom M does not affect the CS anisotropy, all the clusters can clearly be distinguished by their quadrupole coupling constant. The EFG tensor is found to be significantly less symmetric than the CS tensor. Furthermore, the line-shape fits demonstrate that the CS and EFG tensors are noncoincident and that their relative orientation mainly depends on the nature of the counterion A. The solid-state isotropic chemical shifts and quadrupole coupling constants are consistent with Co-59 NMR data previously obtained in solution.