Tetracarbon Metallocarboranes 2

Alternative Title

Thermal rearrangement of 14-vertex cage systems. Crystal structures of three tetracarbon diferracarborane isomers, (η⁵-C₅H₅)₂Fe₂(CH₃)₄C₄B₈H₈


The stereochemistry of 14-vertex polyhedral cages has been explored via a study of the crystal structures of several isomers of (η5-C5H5)2Fe2(CH 3)4C4B8H8 and their thermal rearrangement. In contrast to other metallocarborane systems, isomerism in these species involves changes in gross polyhedral geometry. Isomers I and II, formed by treatment of the (CH3)4C4B8H8 2- ion with FeCl2 and NaC5H5 below 25°C, have sharply different open-cage (nido) structures with a five-and a four-sided open face, respectively; these results violate the skeletal electron-count theory for polyhedral compounds, which predict closo geometry for this system. Both I and II rearrange below 170°C to a common isomer. V, which has a structure similar to that of I with a BH and a CCH3 unit interchanged. In I, II, and V the distribution of framework carbon atoms is unusual, with carbon residing in both low- and high-coordinate polyhedral vertices. Isomer V rearranges at 300°C to still other isomers, VII and VIII, whose 11B and 1H NMR spectra (supported by an x-ray study in the case of VIII) indicate closo structures consisting of a bicapped hexagonal antiprism with the metal atoms in the capping locations. In VII, the arrangement of methyl groups produces C2 symmetry, but VIII has pseudo-D2d symmetry with all eight boron atoms equivalent. Continued heating of VII at 300°C generates only VIII and decomposition products. In the conversion of V to VII two nonadjacent cage carbon atoms evidently migrate to adjacent vertices, indicating that the drive to achieve cage closure takes precedence over the normal tendency for framework carbons to separate from each other on thermal rearrangement. Crystal data: isomer I, space group P21/n, Z = 4, a = 10.676 (2), b = 14.009 (5), c = 13.667 (5) Å, β = 93.97 (3)°, V = 2039 Å, and R = 4.1% for 2796 reflections; isomer II, space group C2/c, Z = 8, a = 16.338 (3), b - 8.210 (2), c = 31.55 (1) Å, β= 103.74 (2)°, V = 4111 Å3, and R = 4.2% for 2849 reflections; isomer V, space group P21/c, Z = 4, a = 14.805 (4), b = 10.547 (2), c = 14.682 (5) Å, β = 109.20 (2)°, V = 2060 Å3, and R = 4.7% for 1660 reflections.



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