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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

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posted on 25.02.2020, 20:46 by Damien B. Culver, Winn Huynh, Hosein Tafazolian, Matthew P. Conley
Cp*2Sc–OR (R = CMe2CF3, CMe­(CF3)2, C­(CF3)3, SiPh3) was synthesized to determine how the alkoxide affects the quadrupole coupling (CQ) obtained from solid-state 45Sc­{1H} NMR spectroscopy. These alkoxides are characterized by large (>29 MHz) CQ values, which are similar to CQ values obtained for Cp*2Sc–R (R = Me, Et, Ph) and Cp*2Sc–X (X = F, Cl, Br, I) but significantly larger than CQ values obtained for Cp*2ScX­(THF). The NMR properties from these alkoxides were used to understand the NMR properties of Cp*2Sc-supported on silica partially dehydroxylated at 700 °C. This material contains two species from the solid-state 45Sc­{1H} NMR spectra assigned to Cp*2ScOSi (5a) and Cp*2Sc­(OSi)­O­(SiOx)2 (5b). The solid-state 45Sc­{1H} NMR spectrum of 5a is considerably broader than 5b, which relates to the magnitude of the quadrupolar coupling (CQ) in these two different sites. Density functional theory (DFT) optimized structures of Cp*2Sc–OR and small cluster approximations of 5a and 5b follow similar trends as the experimental CQ values for this family of organoscandium complexes. Analysis of the origin of CQ using DFT methods shows that σ- and π-bonding orbitals from the Sc–O bond in Cp*2Sc–OR and 5a are major contributors to CQ, whereas different orbitals contribute to CQ in 5b. These studies show that quadrupolar solid-state NMR spectroscopy can distinguish between surface sites on partially dehydroxylated silica.