Nonclassical vs Classical Metal···H3C−C Interactions: Accurate Characterization of a 14-Electron Ruthenium(II) System by Neutron Diffraction, Database Analysis, Solution Dynamics, and DFT Studies

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Abstract
A neutron diffraction study of the complex RuCl2[PPh2(2,6-Me2C6H3)]2 (1) defines the precise nature of the δ agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH3 carbon atoms lie in the RuP2 equatorial plane with Ru···C distances of 2.637(7) and 2.668(6) Å, whereas four short Ru···H distances (from 2.113(11) to 2.507(11) Å) indicate that each methyl group interacts with two C−H bonds. A survey of the X-ray structures with β, γ, δ, and ε M···H3C−C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M···C−C and the torsion of the methyl group about the C−C bond. Thus, the agostic interactions span the range between the classical (M···η2-HC) and the nonclassical (M···η3-H2C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH3 groups. Activation parameters, evaluated from the analysis of 1H NMR line shape as a function of temperature, are ΔH⧧ = 9.6 ± 0.2 kcal mol-1 with ΔS⧧ = −15.4 ± 0.7 eu (CDCl3). The related 14-electron complexes RuX2[PPh2(2,6-Me2C6H3)]2 (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl2[PH2(2,6-Me2C6H3)]2 (1a) reproduces well the nonclassical Ru···η3-H2C agostic mode, whereas the classical Ru···η2-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol-1. A similar barrier (ca. 3.8 kcal mol-1) is calculated for the xylyl rotation in the further simplified model RuCl2[PH2(2,6-Me2C6H3)][PH2CHCHCH3] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl2(PH3)2 and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.