Solid-state ⁴⁵Sc NMR studies of Cp*₂Sc–X and Cp*₂ScX(THF)

Abstract

Cp*Sc-X, where X is a halide, were synthesized and studied by solid-state Sc NMR to determine how the Sc-X bond affects quadrupolar NMR parameters. The experimental quadrupolar coupling constants (C) show that the fluoride has the largest coupling constant and that the iodide has the smallest coupling constant. DFT analysis of this data indicates that the C of these compounds is related to core scandium and halide orbitals, which is related to polarizability of the halide and the Sc-X distance. Cp*ScX(THF) were also investigated by solid-state Sc NMR spectroscopy, and have much smaller C values than the base-free halides. This is related to the change in structure of the THF adduct and occupation of orbitals of π-symmetry that reduce C.

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

“…The smaller C Q for 5b and 6b than in 5a and 6a is reminiscent of similar trends observed in Cp* 2 Sc−X and Cp* 2 ScX(THF) (X = F, Cl, Br, I). 56 In this study the THF adducts consistently show smaller C Q values than the base free compounds. This is a consequence of the more tetrahedral geometry at scandium in Cp* 2 ScX(THF), which results in πoverlap between group fragment orbitals in these compounds.…”

“…The calculated LUMO in 6b is shown in Figure 5d, and corresponds to a π*bonding orbital between an oxygen lone pair and the empty orbital on scandium. The LUMO and EFG tensor orientation in 1, as well as 2−4 and 6a, is similar to Cp* 2 Sc−X, while the LUMO and EFG tensor orientation in 6b is similar to Cp* 2 ScX(THF) (X = F, Cl, Br, I), 56 The C Q originates from the interaction of the quadrupolar moment of the scandium nucleus and the EFG tensor, which is sensitive to local structure. This coupling is electric in origin, indicating that the magnitude of V 33 will be related to core and valence scandium orbitals.…”

“…66 These spectroscopic data do not distinguish between 5a and 5b, which is typical of these particular techniques. However, the examples mentioned above and results showing that Cp* 2 ScX(THF) complexes (X = H, alkyl, halide) are isolable, 41,56 suggest that 5b would be present on partially dehydroxylated silica surfaces.…”

“…These spectra show characteristic broad second order quadrupolar powder patterns associated with solid-state 45 Sc NMR spectra of Cp* 2 Sc−X complexes (X = alkyl, halide). 56 Simulations of these spectra are shown in red in Figure 6, and the NMR parameters associated with these simulations are summarized in Table 1. The C Q values decrease following the trend 4 > 1 > 2 > 3.…”

“…The basefree halides have much larger C Q values than the THF adducts. 56 Density functional theory (DFT) studies of these compounds showed that the smaller C Q values for the THF adducts are due to the higher symmetry at Sc, which results in some degree of π-bonding from the halide to scandium.…”