Carbon-Hydrogen Activation in Zerovalent Bis(1,5-cyclooctadiene) Complexes of the First Row Transition Metals: A Theoretical Study.

Stepwise interaction of first row transition metal atoms with 1,5-cyclooctadiene to give (C8H12)2M complexes is studied using the M06-L/DZP density functional method. The experimentally known (C8H12)2Ni is the thermodynamically most favorable complex, with a predicted geometry consistent with its experimental structure as determined by X-ray crystallography. The other transition metal atoms from scandium to zinc also interact exothermically with 1,5-cyclooctadiene to give (C8H12)2M derivatives, but these exhibit lower symmetry than the S4 symmetry exhibited by (C8H12)2Ni. Carbon-hydrogen activation of CH2 groups in a C8H12 ligand is predicted for most systems. Thus, conversion of (η2,2-C8H12)2M to (η3,2-C8H11)(η2,1-C8H13)M, through a hydride intermediate (η3,2-C8H11)(η2,2-C8H12)MH, is predicted for scandium, vanadium, chromium, manganese, and cobalt. For titanium with a low-lying empty orbital, further C-H activation through a hydride intermediate (η6-C8H10)(η2,1-C8H13)TiH is predicted, leading ultimately to (η6-C8H10)(η1,1-C8H14)Ti, in which the hexahapto η6-C8H10 ligand is shown by NICS to be aromatic. These two C-H activation processes on a titanium center represent the dehydrogenation of 1,5-cyclooctadiene to 1,3,5-cyclooctatriene with the second 1,5-cyclooctadiene ligand as the hydrogen acceptor. For zinc C-H activation terminates at (η1-C8H11)(C8H12)ZnH, which has a C-Zn-H three-center bond. No energetically favorable C-H activation processes are predicted for the iron, nickel, and copper (η2,2-C8H12)2M derivatives.