3-center-2-electron (3c-2e) interactions between a metal and its carbyl ligand are common in electrophilic organometallic complexes; if the pair of electrons are in a σ-C–H bond they are then labeled agostic.[1] Agostic interactions are proposed in the transition state for olefin insertion and as intermediates in polymerization catalysis.[2] Several structural features are characteristic of ground state β-C–H agostic interactions in d-block transition metals. These features for (dmpe)Ti(Et)Cl3 are summarized below: the agostic C–H bond is elongated, the M-HC distance is short, and the M–H–C angle is acute compared to organometallic complexes lacking such an interaction.[1] Also, the 1H NMR chemical shift of the H-atom involved in an agostic interaction shifts upfield from the free alkane and the 1JCH becomes substantially lower than ca. 125 Hz in sp3 C-H bonds. The low coordinate lanthanide bis(silyl)alkyls Ln[CH(SiMe3)3]3 contain a short Ln-Cγ distance and an acute Ln–Cγ–Siβ angle between the metal and one –SiMe3 group of the pendant alkyl in their X-ray crystal structures. We describe the nature and the strength of the secondary interactions in Lu[CH(SiMe3)2][O–2,6–tBu2C6H3] (x = 0,1,2) and silica-supported (OSi)OLu[CH(SiMe3)2]2. The monomeric organolutetium complexes have short Ln-Cγ distances in their X-ray crystal structures. Solution and solid-state NMR spectroscopies show that the secondary interaction is a 3c-2e Ln…Cγ–Siβ interaction, better described to be a pseudo-bridging methyl between Lu and Si, rather than an agostic Lu…H–C. This is supported by NBO charge analysis. The model developed from the molecular compounds is used to rationalize the structure of the (OSi)OLu[CH(SiMe3)2]2, obtained from X-ray absorption spectroscopy. The combination of solution and solid-state spectroscopic studies, supported by the calculation of the NBO charges, provides a detailed understanding of the weak secondary interactions in these molecular and silica-supported organolutetium compounds. [1] M. Brookhart, M. L. H. Green, G. Parkin, Proc. Nat. Acad. Sci. USA 2007, 104, 6908-6914. [2] a. G. W. Coates, Chem. Rev. 2000, 100, 1223-1252; b. S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169-1204.

Nature of secondary interactions in molecular and silica-supported organolutetium complexes from solid-state NMR spectroscopy

251st National Meeting (2016)

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