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Solid-State NMR Spectroscopy and Computational Modeling for Structure Elucidation


Multinuclear solid-state NMR spectroscopy and computational modeling are powerful techniques for the elucidation of chemical structure. Multinuclear 45Sc{1H} and 13C{1H} CPMAS NMR spectra unambiguously assigns a β-CH agostic interaction in Cp*2Sc-CH2CH3, a compound synthesized by Bercaw and co-workers in 1987. The quadrupolar coupling constant (CQ) obtained in 45Sc{1H} solid-state NMR for three-coordinate scandocenes Cp*2Sc-R (R = Ph, Me, OSiPh3, OCMe2CF3, OCMe(CF3)2, C(CF3)3), Cp*2Sc-X (X = F, Cl, Br, I), and Cp*2Sc-OR are large (CQ > 27MHz), but for four-coordinate THF adducts of Cp*2Sc-X are small (CQ < 26 MHz). The CQ (45Sc) of Cp*2Sc-X and Cp*2Sc-X(THF) are affected by the local coordination environment at Sc. When supported on silica partially dehydroxylated at 700 oC, Cp*2ScMe reacts to form methane and, Cp*2ScOSi≡ (CQ = 35.4 MHz), and Cp*2Sc(OSi≡)O(SiOx)2 (CQ = 21.9 MHz). These differences are related to the change in symmetry at scandium. The CQ is also affected by coordination of a metal to boron in complexes containing the phosphine substituted 9,10-diboraantracene ligand B2P2. Coordination of B2P2 to a coinage metal in a cationic environment forms [M(B2P2)][PF6] (M = Cu, Ag, Au), in which the borane is three-coordinate, exhibits trigonal planar geometry, and contains a large CQ > 4 MHz. Formation of the boro-auride [K(18-c-6)][Au(B2P2)] forms a tetrahedral boron, and the 11B NMR signals for this compound a small CQ of 1.5 MHz. The natural localized molecular orbitals for each can be calculated, and the contribution of each bonding orbital can be analyzed. The 29Si NMR chemical shift of R3Si-X (R = iPr) is also studied via solid-state NMR, and calculations reveal that the magnitude of the coupling of the occupied σ(Si-R) to the unoccupied σ(Si-X) / pSi is responsible for the deshielding of 29Si. These results allow for the elucidation of structure in [iPr3Si][MOx] (MOx = sulfated zirconia and Al(OC(CF3)3)3 activated SiO2-700).

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