Solid-State 45Sc NMR Studies of Cp*2Sc-OR (R = CMe2CF3, CMe(CF3)2, C(CF3)3, SiPh3) and Relationship to the Structure of Cp*2Sc-Sites Supported on Partially Dehydroxylated Silica

Cp*Sc-2-OR (R = CMe2CF3, CMe(CF3)(2), C(CF3)(3), SiPh3) was synthesized to determine how the alkoxide affects the quadrupole coupling (C-Q) obtained from solid-state Sc-45{H-1} NMR spectroscopy. These alkoxides are characterized by large (>29 MHz) C-Q values, which are similar to C-Q values obtained for Cp*Sc-2-R (R = Me, Et, Ph) and Cp*Sc-2-X (X = F, Cl, Br, I) but significantly larger than C-Q values obtained for Cp*Sc-2-X(THF). The NMR properties from these alkoxides were used to understand the NMR properties of Cp*Sc-2-supported on silica partially dehydroxylated at 700 degrees C. This material contains two species from the solid-state Sc-45{H-1} NMR spectra assigned to Cp*2ScOSi (5a) and Cp*Sc-2(OSi )0(SiOx)(2) (5b). The solid-state Sc-45{H-1} NMR spectrum of 5a is considerably broader than 5b, which relates to the magnitude of the quadrupolar coupling (C-Q) in these two different sites. Density functional theory (DFT) optimized structures of Cp*Sc-2-OR and small cluster approximations of 5a and Sb follow similar trends as the experimental C-Q values for this family of organoscandium complexes. Analysis of the origin of C-Q using DFT methods shows that sigma- and pi-bonding orbitals from the Sc-O bond in Cp*Sc-2-OR and 5a are major contributors to C-Q, whereas different orbitals contribute to C-Q in 5b. These studies show that quadrupolar solid-state NMR spectroscopy can distinguish between surface sites on partially dehydroxylated silica.