Triple-Decker Sandwiches and Related Compounds of the First-Row Transition Metals Containing Cyclopentadienyl and Benzene Rings
Haibo Liu, Qian-shu Li, Yaoming Xie, R. Bruce King and Henry F. Schaefer
J. Phys. Chem. A, Article ASAP
Publication Date (Web): July 21, 2010
The triple-decker sandwich compound trans-Cp2V2(η6:η6-μ-C6H6) has been synthesized, as well as “slipped” sandwich compounds of the type trans-Cp2Co2(η4:η4-μ-arene) and the cis-Cp2Fe2(η4:η4-μ-C6R6) derivatives with an Fe−Fe bond (Cp = η5-cyclopentadienyl). Theoretical studies show that the symmetrical triple-decker sandwich structures trans-Cp2M2(η6:η6-μ-C6H6) are the global minima for M = Ti, V, and Mn but lie 10 kcal/mol above the global minimum for M = Cr. The nonbonding M···M distances and spin states in these triple decker sandwich compounds can be related to the occupancies of the frontier bonding molecular orbitals. The global minimum for the chromium derivative is a singlet spin state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a very short CrCr distance of 2.06 Å, suggesting a formal quadruple bond. A triplet state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a predicted Cr≡Cr distance of 2.26 Å lies only 3 kcal/mol above this global minimum. For the later transition metals the global minima are predicted to be cis-Cp2M2(η6:η6-μ-C6H6) structures with a metal−metal bond, rather than triple decker sandwiches. These include singlet cis-Cp2Fe2(η4:η4-μ-C6H6) with a predicted Fe═Fe double bond distance of 2.43 Å, singlet cis-Cp2Co2(η3:η3-μ-C6H6) with a predicted Co—Co single bond distance of 2.59 Å, and triplet cis-Cp2Ni2(η3:η3-μ-C6H6) with a predicted Ni—Ni distance of 2.71 Å.