Tetranuclear rhodium clusters with facial cyclooctatetraene ligands

Abstract

Molecular tetrarhodium cluster complexes with facial (μ₃-) cyclooctatetraene ligands are synthesized from [Rh₄(CO)₁₂] 2 and 1,3,5,7-cyclooctatetraene (cot) or 1,4-(SiMe₃)₂C₈H₆, respectively. The reactions proceed in a cooperative fashion, with all four metal atoms of the cluster being involved. Reaction of 2 with cot at low temperature (boiling n-pentane) only gives the mono-cot complex [Rh₄(CO)₈(μ₃-C₈H₈)] 4, whereas at higher temperature (boiling n-heptane) the bis-cot cluster complex [Rh₄(CO)₆(μ₃-C₈H₈)(η⁴-C₈H₈)] 5 is formed exclusively. Complex 4 is also prepared from 5 and [Fe(CO)₅]. From 2 and 1,4-(SiMe₃)₂C₈H₆ in boiling n-pentane only the complex [Rh₄(CO)₆{μ₃-C₈H₆(SiMe₃)₂}{η⁴-C₈H₆(SiMe₃)₂}] 6 is obtained in quantitative yield. Substitution of the apical ligands in 4 with 1,4-(SiMe₃)₂C₈H₆, and in 5 with 1,3-cyclohexadiene or 1,5-cyclooctadiene gives the complexes [Rh₄(CO)₆(μ₃-C₈H₈){η⁴-C₈H₆(SiMe₃)₂}] 7, [Rh₄(CO)₆(μ₃-C₈H₈)(η⁴-C₆H₈)] 8 and [Rh₄(CO)₆(μ₃-C₈H₈)(η⁴-C₈H₁₂)] 9, respectively. The crystal and molecular structures of 4, 5, 6, 8 and 9 were determined. The apical C₈H₆R₂ ligands in 5, 6 and 7 attain the 1,2,5,6-η⁴-coordination mode through non-adjacent double bonds.