CHROMIUM(II) ORGANOCHROMATES - PREPARATION, CHARACTERIZATION, AND STABILITY

The synthesis, characterization, and stability properties of a novel series of Cr(II) alkylchromates together with their transformation into unprecedented alkylidene (Schrock-type) Cr(III) species and Cr(II) and Cr(III) metallacycles is described. Reaction of CrCl2(THF)2 with 4 equiv of RLi [R = Me, PhCH2, (CH3)3CCH2, (CH3)2C(Ph)CH2] led to the initial formation of the corresponding R8Cr2Li4(THF)4 (1) which were isolated as thermally labile and almost diamagnetic crystalline solids. Treatment of these complexes with TMEDA [N,N,N'-tetramethylethylenediamine] gave different species containing either Cr(II) or Cr(III), depending on the nature of the alkyl. While in the case of the methyl derivative the monomeric Me4Cr[Li(TMEDA)]2 (2) was reversibly formed, another diamagnetic Cr(II) complex (PhCH2)6Cr2[Li(TMEDA)]2 (3), likely dimeric, was obtained in the case of the benzyl derivative. Addition of TMEDA to the in situ prepared diamagnetic [(CH3)3CCH2]8Cr2[Li(THF)]4 gave the chromacyclobutane species [(CH3)3CCH2]2[CH2C(CH3)2CH2]Cr[Li(TMEDA)]2 (4). In the case of the corresponding neophyl derivative a Cr(III) complex [o-C6H4C(CH3)2CH2]2Cr(TMEDA)] [Li(TMEDA)2] (5) was obtained via an unexplained oxidation reaction. The utilization of a RLi:Cr stoichiometric ratio of 2:1, in the presence of TMEDA, led to the formation of the corresponding paramagnetic dialkyl species (TMEDA)CrR2 [R = CH2Ph (7), CH2C(CH3)3 (8), CH2C(Ph)(CH3)2 (9)]. In these three cases there is evidence that the reaction proceeds via formation of the intermediate monosubstitution product(TMEDA)CrR(mu-Cl)[LiCl(TMEDA)]. In the case of the crystalline benzyl derivative (6) the product was isolated in analytically pure form. Finally, the employment of a PhCH2Li:Cr stoichiometric ratio of 3:1 formed a Cr(III) complex, probably an alkylidene species (TMEDA)Cr(CHPh)CH2Ph (10). The structures of 4-7, 9, and 10 were clarified by X-ray analysis. Crystal data are as follows. 4: C27H64N4Li2Cr, M = 510.71, monoclinic, C2/c, a = 14.72(4) angstrom, b = 14.100(6) angstrom, c = 16.94(2) angstrom, beta = 106.6(1)degrees, V = 3367 angstrom3, Z = 4, T = -160 -degrees-C, Mo Kalpha, R = 0.061, R(w) = 0.047 for 1197 reflections out of 1726 unique. 5: C38H72N6CrLi, M = 671.96, monoclinic, P2(1)/n, a = 11.629(2) angstrom, b = 23.259(5) angstrom, c = 15.726(3) angstrom, beta = 107.89(1)degrees, V = 4048(1) angstrom3, Z = 4, T = -160-degrees-C, Mo Kalpha, R = 0.059, R(w) = 0.069 for 4003 reflections out of 7344 unique. 6: C25H55N6CrCl3Li2, M = 611.99, triclinic, P1BAR, a = 11.976(5) angstrom, b = 17.251(5) angstrom, c = 8.828(3) angstrom, alpha = 92.43(3)degrees, beta = 101.79(3)degrees, gamma = 84.05(3)degrees, V = 1775(1) angstrom3, Z = 2, T = -160 -degrees-C, Mo Kalpha, R = 0.066, R(w) = 0.071 for 4189 reflections out of 6258 unique. 7: C20H30N2Cr, M = 350.47, monoclinic, P2(1)/n, a = 9.596(1) angstrom, b = 18.128(2) angstrom, c = 11.803(1) angstrom, beta = 110.692(9)degrees, V = 1980.8(4) angstrom3, Z = 4, T = -157-degrees-C, Mo Kalpha, R = 0.029, R(w) = 0.041 for 2901 reflections out of 3503 unique. 9: C26H42N2Cr, M = 434.63, orthorhombic, Pbcn, a = 15.808(8) angstrom, b = 9.562(4) angstrom, c = 16.036(5) angstrom, V = 2424(3) angstrom3, Z = 4, T = -160-degrees-C, Mo Kalpha, R = 0.041, R(w) = 0.057 for 1604 reflections out of 2439 unique; 10: C20H29N2Cr, M = 349.46, orthorhombic, Pbca, a = 23.085(5) angstrom, b = 14.455(6) angstrom, c = 11.243(3) angstrom, V = 3751(3) angstrom3, Z = 8, T = -158-degrees-C, Mo Kalpha, R = 0.064, R(w) = 0.072 for 2477 reflections out of 3725 unique.