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Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
Agostic interactions play a central role in organometallic chemistry. The word agostic is derived from the Greek word to hold on to oneself first coined by Malcolm Green and later used by Maurice Brookhart and others. Agostic interaction in organometallic chemistry is used to represent the σ-like interaction of the electron density associated with the C-H bond with the coordinative unsaturated transition metal, resulting in the three center, two electron bond (Figure 1.28). The agostic interactions are believed to facilitate many important catalytic transformations such as oxidative addition, reductive elimination, chain transfer, or chain termination reactions in polymerization. Agostic interactions help in temporarily stabilizing the transition state in a catalytic reaction, which possesses less than 16 valence electrons before they take up ligands to saturate their valency.
Reactivity of a β-diketiminate ytterbium(II) hydride with cyclopentadiene derivatives
Published in Journal of Coordination Chemistry, 2022
Georgia M. Richardson, Jesse Howarth, Matthew J. Evans, Alison J. Edwards, Scott A. Cameron, Mathew David Anker
In the solution state, the pentamethylcyclopentadieneyl derivative, 2, displays similar features to 1. The 1H NMR spectrum, obtained at 22 °C, of 2 demonstrated a methine resonance of the β-diketiminate ligand at δH 4.66 ppm is a 1:15 ratio with the sharp Me5C5 methyl proton singlet at δH 4.69 ppm. Despite the increased bulk of the pentamethylcyclopentadieneyl ligand, no broadening of any of the proton signals was observed. Crystallization of 2 from a saturated n-hexane solution at −30 °C provided dark brown blocks suitable for single crystal X-ray diffraction (Figure S10). The solid-state structure of 2 is monomeric and contains one molecule of [(BDIDipp)Yb(η5-Cp*)] in the asymmetric unit. Both the bidentate β-diketiminate ligand and η5-Cp* ligand coordinate directly to the Yb center, with a distinct Yb···η3-H2C agostic interaction to a methyl group of the Dipp substituent (Yb1···H13a: 2.6900(4) Å, Yb1···H13b: 2.5154(4) Å, Yb1···C13: 3.021(3) Å). Agostic interactions to the Dipp substituent on the BDIDipp framework have been observed in a related ytterbium(III) complex, [(BDIDipp)Yb (Cl)(N{SiMe3}2)] [26], which is postulated to help saturate the coordination sphere of the Yb center. The Yb–N bond distances range between 2.398(2) Å and 2.418(2) Å, which is slightly longer than those observed in [(BDIDipp)Yb(N{SiMe3}2) (2.344(2) Å–2.347(2) Å) [13], consistent with an increase in the coordination number of 2 due to the agostic interaction. The Yb···Cp* interaction (2.4485(14) Å) is within the range reported for ytterbium complexes containing Cp* ligand (2.27 Å–2.50 Å) and is elongated, which suggests the presence of an ytterbium(II) center [27]. “Non-classical” M···η3-H2C agostic interactions have only been previously characterized by neutron diffraction studies on transition metal complexes [28, 29]. This prompted our investigation into the solid state structure of 2 using single crystal neutron diffraction experiments. Crystals of 2 (>0.2 mm3) suitable for neutron diffraction were grown from the slow evaporation of an n-hexane solution stored at room temperature (Figure 1b)). In the solid state, the two agostic Yb···H interactions (Yb1···H131: 2.69(2) Å, Yb1···H132: 2.57(2) Å, Yb1···C13: 3.033(16) Å) were definitively identified, along with an additional Yb···H interaction to a second methyl group (Yb1···H281: 2.71(3) Å, Yb1···C28: 3.71(2) Å). To the best of our knowledge, this study represents the first example of a neutral ytterbium(II) complex characterized by single crystal neutron diffraction [30, 31].