A Static Agostic α-CH⋯M Interaction Observable by NMR Spectroscopy: Synthesis of the Chromium(II) Alkyl [Cr₂(CH₂SiMe₃)₆]²⁻ and Its Conversion to the Unusual “Windowpane” Bis(metallacycle) Complex [Cr(k²C,C′-CH₂SiMe₂CH₂)₂]²⁻

Alkylation of CrCl₂(thf) with 3 equiv of LiCH₂SiMe₃ yields the dinuclear chromium(II) alkyl anion [Li(thf)₂]₂[Cr₂(CH₂SiMe₃)₆]. This essentially diamagnetic compound possesses a “bent” Cr‒Cr quadruple bond in which two of the six CH₂SiMe₃ ligands bridge the metal‒metal bond; each chromium center also bears two terminal CH₂SiMe₃ ligands. NMR spectroscopy clearly indicates that the bridging CH₂SiMe₃ groups engage in agostic Cr⋯H—C interactions that, somewhat surprisingly, are static on the NMR time scale at −80 °C. Analysis of the NMR line shapes as a function of temperature indicates that the molecule undergoes two different dynamic processes. One process, which has activation parameters of ΔH^‡ = 10.6 ± 0.5 kcal mol^-1 and ΔS^‡ = −4 ± 2 eu, has been tentatively ascribed to exchange between the terminal and bridging CH₂SiMe₃ ligands; the other process, which has activation parameters of ΔH^‡= 14.1 ± 0.6 kcal mol⁻¹ and ΔS^‡ = 17 ± 3 eu, is ascribed to rotation of the bridging CH₂SiMe₃ ligands about their Cr—C bonds. The latter values give an estimate of the strength of an agostic Cr⋯H bond. Addition of N,N,N′,N′-tetramethylethylenediamine (tmed) to [Li(thf)₂]₂[Cr₂(CH₂SiMe₃)e] results in cleavage of the metal‒metal bond and activation of γ-hydrogen atoms of the CH₂SiMe₃ ligands; the unusual bis(metallacycle) compound [Li(tmed)]₂[Cr(k²CC′-CH₂SiMe₂CH₂)₂] results. The X-ray crystal structure of this high-spin (S = 2) chromium(II) complex shows that it possesses an unusual “windowpane” structure, in which the square-planar chromium center is the common vertex of two metallacyclobutane rings and the lithium cations form close contacts with the carbon atoms of the α-CH₂ groups. Important bond distances and angles: Cr‒C = 2.235(3) Å, Li⋯C = 2.163(6) Å; Si‒C = 1.853(3) A; C‒Cr‒C(cis) = 79.0(1)°; Li⋯C‒Si = 158.3(2)°; Cr‒C‒H = 105(2), 137(2)°; Li⋯C‒H = 76(2), 83(2)°; Si‒C‒H = 109(2), 115(2)°. Crystal data for C₂₀H₅₂-CrLi₂N₄Si₂ at −75 °C: triclinic, space group P1, with a = 7.811(4) Å, b = 10.336(4) Å, c = 10.720(5) Å, α = 95.12(3)°, β = 110.87(4)°, γ = 108.58(5)°, V = 746(2) ų, Z = 1, R_F = 0.043, and R_wF = 0.058 for 212 variables and 2261 data with I > 2.58σ(I).