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

The ring-bridged chromocene carbonyl complex Me2Si(C5Me4)2Cr(CO) 1 is obtained by reaction of Me2Si(C5Me4)2Li2 with CrCl2 · THF in the presence of CO. Reaction with CO transforms the monocarbonyl complex 1 to a dicarbonyl complex Me2Si(C5Me4)(C5(endo-3-H)(2-CH2)Me3)Cr(CO)22; in this reaction, a hydrogen atom is shifted from one of the α-CH3 groups to the adjacent β-position of a C5 ring ligand. The rate of this reaction is first order in complex and CO concentration, with an activation enthalpy of H≠ = 51 ± 3 kJ mol−1 and an activation entropy of S≠ = 136 ± 10 J mol−1 K−1. In toluene solution at 50°C, complex 2 isomerizes to Me2Si(C5Me4)(C5(exo-3-H)(2-CH2)Me3)Cr(CO)23, then to Me2Si(C5Me4)(C5(exo-1-H)(2-CH2)Me3)Cr(CO)24 and further to Me2Si(C5Me4)(C5 (endo-5-H)(2-CH2)Me3)Cr(CO)25. 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 1H NMR spectroscopy, those of 1, 2, 4 and 5 also by single-crystal X-ray diffraction. Complex 1: space group P21/n, a = 13.257(7)Å, b = 10.186(5)Å, c = 14.739(7) Å, β = 90.89(4)°; V = 1990.1(7)Å3; Z = 4. Complex 2: space group Pbca, a = 8.699(3) Å, b = 14.713(6) Å, c = 32.801(14)Å; V = 4198(3)Å3; Z = 8. Complex 4: space group P21, a = 9.497(2)Å, b = 9.895(3) Å, c = 11.371(2)Å, β = 103.919(8)°; V = 1037.1(4) Å3; Z = 2. Complex 5: space group P21/c, a = 16.555(4)Å, b = 13.036(3)Å, c = 10.044(2)Å, β = 107.45(1)°; V = 2067.8(8)Å3; Z = 4