Tetranuclear rhodium clusters with facial cyclooctatetraene ligandsDedicated to Professor Hansgeorg Schnöckel on the occasion of his 60th birthday. Electronic supplementary information (ESI) available: packing diagrams for 4 and 6. See http://www.rsc.org/suppdata/dt/b1/b105485f/

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.