Indenyl carbametallaboranes. Part 2. Compounds with phenyl or ether groups substituted at cage carbon atoms within a 3-(η-C₉H₇)-3,1,2-closo-CoC₂B₉ framework

Reaction of [Co(acac)₃] (acac = acetylacetonate), Li[C₉H₇] and Tl₂[C₂B₉H₉R¹R ²] in tetrahydrofuran (thf) affords the substituted indenyl carbacobaltaboranes [1-R¹-2-R²-3-(η-C₉H ₇)-3,1,2-closo-CoC₂B₉H₉] (R¹ = Ph, R² = H 2; R¹ = CH₂OMe, R² = H 3; R¹ = R² = CH₂OMe 4). The indenyl ligand of 4 (and also, by inference, that of 3) is, in solution, in rapid rotation about the metal-carbaborane cage axis, even at 185 K. In contrast, the room-temperature solution fluxionality of 2, which involves synchronous rotation of the indenyl ligand as in 3 and 4 with rotation of the phenyl group about the phenyl-C(1) axis, may be arrested by cooling to 185 K. The limiting low-temperature structure of 2 is in full agreement with the molecular conformation determined in the solid state. For both compounds 2 and 3, cisoid conformations are observed, in which the ring-junction atoms of the indenyl ligand are as near as they can be to the carbaborane carbon atoms, given an overall staggered relationship between metal-bonded η⁵ rings. In contrast the steric influence of two substituent ether functions in 4 is sufficient to push round the six-membered ring of the indenyl ligand to the next best staggered conformation. All the molecular conformations observed, and the relative heights of the barriers to indenyl rotation, are supported by the results of molecular orbital calculations at the extended-Hückel level. Crystal structure data at 185 ± 1 K: 2, a = 17.732(11), b = 13.186(3), c = 17.739(7) Å, β = 117.00(4)°, space group C2/c, R = 0.0439 for 2741 observed reflections; 3, a = 12.382(3), b = 8.914(5), c = 15.523(9) Å, β = 103.60(4)°, space group P2₁/c, R = 0.0269 for 2651 reflections; 4, a = 10.927(4), b = 14.743(4), c = 11.789(6) Å, β = 90.61(4)°, space group P2₁/n, R = 0.0290 for 3087 reflections.