ansa-metallocene derivatives XXXVI. Dimethylsilyl-bridged permethyl chromocene carbonyl complexes — syntheses, crystal structures and interconversion reactions

Abstract

The ring-bridged chromocene carbonyl complex Me z Si(C sMe 4)zCr(CO) 1 is obtained by reaction of Me~ Si(C, MC 4)2 Li ~ with CrCl z. THF in the presence of CO. Reaction with CO transforms the monocarbonyl complex 1 to a dicarbonyl complex MezSi(CsMe4)(Cs(endo-3-H)(2-CHz)Me3)Cr(CO)2 2; in this reaction, a hydrogen atom is shifted from one of the cx-CH 3 groups to the adjacent l3-position of a C s ring ligand. The rate of this reaction is first order in complex and CO concentration, with an activation enthalpy of H oF = 51 ± 3 kJ mol-I and an activation entropy of S oF = 136 ± 10 J mol-I K-I. In toluene solution at 50 cC, complex 2 isomerizes to Me2Si(CsMe4)(Cs(exo-3-H)(2-CH2)Me3)Cr(CO)2 3, then to Me2Si(CsMe4)(Cs(exo-I-H)(2-CHz)Me3)Cr(CO)2 4 and further to Me2Si(CsMe4)(Cs(endo-5-HX2-CHz)Me3)Cr(CO)2 5. The kinetics of these isomerizations, which involve hydrogen migrations and ring rotation reactions, indicate monomolecular reactions. Possible reaction paths and transition states are discussed. The structures of complexes 1-5 were determined by lH NMR spectroscopy, those of 1, 2, 4 and 5 also by single-crystal X-ray diffraction.

The Ansa Effect in Permethylmolybdenocene Chemistry: A [Me2Si] Ansa Bridge Promotes Intermolecular C−H and C−C Bond Activation

“…Such perturbation presumably reduces the degree of electron donation to the Mo center in the ansa system, analogous to that observed for the related zirconium system . In accord with this notion, the ν(CO) stretching frequency for [Me 2 Si(C 5 Me 4 ) 2 ]Mo(CO) (1900 cm -1 ) is higher than that in Cp* 2 Mo(CO) (1890 cm -1 ). , …”

ansa-Chromocene Complexes. 2. Isocyanide Derivatives of Cr(II) and Cr(III), Their Syntheses, X-ray Crystal Structures, and Physical Properties

“…Ligand substitution on ansa -chromocene carbonyl precursors is a general route to a variety of ansa- chromocene isocyanide derivatives. We have yet to determine the mechanism for this ligand substitution reaction; however, we suspect that the substitution of CO by isocyanide ligands is associative in nature for the following reasons: (a) Brintzinger and co-workers have identified ansa -chromocene dicarbonyl complexes, , showing that ring slippage can occur to accommodate a second 2 e -donating ligand on the metal. Furthermore, this ring slippage is catalyzed by small amounts of oxidants, indicating that one-electron oxidation of the ansa -chromocene carbonyl complex facilitates ring slippage; (b) it is difficult to remove CO from the neutral and cationic complexes by either thermal or photochemical means…”

Thermal stability of Group 6 bis(cyclopentadienyl) and ethylene bridged bis(cyclopentadienyl) monocarbonyl complexes; a theoretical study

“…These arguments about the effect of an ansa-bridge are supported by the observation that the silicon bridged species [Cr{Me 2 Si(C 5 Me 4 ) 2 }(CO)] is also stable to thermolysis, showing no change in the 1 H NMR spectrum after 10 d at 60 ЊC. 11 The relative stabilities of the unbridged and ansa-chromocene carbonyls bear comparison with the thermal stabilities of unbridged and ansa-tungstenocene methyl hydrides. The unbridged species loses methane at 48 ЊC 12 whereas the methylene bridged analogue is stable up to 120 ЊC.…”