A STATIC AGOSTIC ALPHA-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(KAPPA-2C,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-degrees-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 DELTAH(double dagger) = 10.6 +/- 0.5 kcal mol-1 and DELTAS(double dagger); = -4 +/- 2 eu, has been tentatively ascribed to exchange between the terminal and bridging CH2SiMe3 ligands; the other process, which has activation parameters of DELTAH(double dagger) = 14.1 +/- 0.6 kcal mol-1 and DELTAS(double dagger) = 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)6] results in cleavage of the metal-metal bond and activation of gamma-hydrogen atoms of the CH2SiMe3 ligands; the unusual bis(metallacycle) compound [Li(tmed)]2[Cr(kappa2CC'-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 alpha-CH2 groups. Important bond distances and angles: Cr-C = 2.235(3) angstrom, Li...C = 2.163(6) angstrom; si-C = 1.853(3) angstrom; C-Cr-C(cis) = 79.0(1)-degrees; Li...C-Si = 158.3(2)-degrees; Cr-C-H = 105(2),137(2)-degrees; Li...C-H = 76(2), 83(2)-degrees; Si-C-H = 109(2), 115(2)-degrees. Crystal data for C20H52-CrLi2N4Si2 at -75-degrees-C: triclinic, space group P1BAR, with a = 7.811(4) angstrom, b = 10.336(4) angstrom, c = 10.720(5) angstrom, alpha = 95.12(3)-degrees, beta = 110.87(4)-degrees, gamma = 108.58(5)-degrees, V = 746(2) angstrom3, Z = 1, R(F) = 0.043, and R(wF) = 0.058 for 212 variables and 2261 data with I > 2.58sigma(I).