RING HYDROGEN C-H ACTIVATION IN (CP-STAR-2)LNCH(SIME3)2 (LN=Y, LA, CE) - X-RAY CRYSTAL-STRUCTURES OF [CP-STAR-3(MU-3-ETA-5,ETA-1,ETA-1-C5ME3(CH2)2)CE2]2 AND (CP-STAR-2)CECH2C6H5

Thermolysis of complexes Cp*2LnCH(SiMe3)2 (1, Ln = Y; 2, Ln = La; 3, Ln = Ce) in cyclohexane leads to extrusion of CH2(SiMe3)2 by hydrogen abstraction from a Cp* ligand and postulated formation of a pentamethylcyclopentadienyl fulvene species Cp*FvLn. For yttrium thermolysis occurs under conditions where Cp*FvY reacts further, e.g. with solvent and CH2(SiMe3)2, to produce a complicated mixture. In cyclohexane-d12 extensive H/D scrambling between organometallic species and solvent is observed. For lanthanum and cerium secondary reactions take place between molecules of Cp*FvLn, but solvent and CH2(SiMe3)2 are not involved. The stable end product for these metals is a tetranuclear complex [Cp*3(mu-3-eta-5,eta-1,eta-1-C5Me3(CH2)2)Ln2]2 (4, Ln = La; 5, Ln = Ce), formed by intermolecular hydrogen transfer. For 5 the molecular structure was determined by X-ray diffraction. The compound crystallizes in the monoclinic space group C2/c (No. 15) with a = 23.849 (9) angstrom, b = 10.846 (4) angstrom, c = 30.11 (1) angstrom, and beta = 104.29 (3)-degrees. The unit cell contains four molecules of 5 and two cyclohexane-d12 solvate molecules (Z = 4). Compound 5 contains a novel type of bridging ligand, a doubly metalated pentamethylcyclopentadienyl group, eta-5,eta-1,eta-1-C5(CH2)2Me3, bridging three cerium atoms in a unique manner. The tetranuclear complex is essentially a tightly fitting assembly of four units Cp*2LnR. Thermolysis of 2 and 3 or heating of tetranuclear compounds 4 and 5 in toluene leads to the formation of the benzyl compounds Cp*2LnCH2Ph (6, Ln = La; 7, Ln = Ce). Thermolysis of 3 in toluene-d8 shows that the alkane CH2(SiMe3)2 has no deuterium built-in, indicating that 6 and 7 are formed through hydrogen abstraction from a pentamethylcyclopentadienyl ligand and that the solvent is not involved here. Compound 7 crystallizes in the orthorhombic space group Pna2(1) with a = 16.730 (4) angstrom, b = 11.387 (3) angstrom, c = 12.506 (3) angstrom, and Z = 4. The bonding of the benzyl group is best described as a benzyl anion with regular Sp2 hybridization of the methylene group eta-3-bonded to a Cp*2Ce+ cation. The metal has a strong interaction with the methylene, the ipso, and an ortho carbon atom of the benzyl ligand.