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

Abstract

Stepwise interaction of first row transition metal atoms with 1,5-cyclooctadiene to give (CH)M complexes is studied using the M06-L/DZP density functional method. The experimentally known (CH)Ni 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 (CH)M derivatives, but these exhibit lower symmetry than the S symmetry exhibited by (CH)Ni. Carbon-hydrogen activation of CH groups in a CH ligand is predicted for most systems. Thus, conversion of (η-CH)M to (η-CH)(η-CH)M, through a hydride intermediate (η-CH)(η-CH)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 (η-CH)(η-CH)TiH is predicted, leading ultimately to (η-CH)(η-CH)Ti, in which the hexahapto η-CH 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 (η-CH)(CH)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 (η-CH)M derivatives.