TRICARBON CARBORANE CHEMISTRY .2. SYNTHESES AND STRUCTURAL CHARACTERIZATIONS OF COMMO IRON AND COBALT BIS(TRICARBABORANE) COMPLEXES

A series of commo metalla bis(tricarbaborane) complexes has been generated via the reaction of the metal halide with the 6-CH3-5,6,9-C3B7H9- anion. Reaction of 6-CH3-5,6,9-C3B7H9- with FeCl2 yielded the isomeric complexes commo-Fe-(1-Fe-2-CH3-2,3,5-C3B7H9)2 (6), commo-Fe-(1-Fe-5-CH3-2,3,5-C3B7H9)2 (7), and commo-Fe-(1-Fe-5-CH3-2,3,5-C3B7H9)(1-Fe-4-CH3-2,3,4-C3B7H9) (8). Thermolysis of 6, 7, or 8 was found to produce a new isomer, commo-Fe-(1-Fe-10-CH3-2,3,10-C3B7H9)2 (9). Single-crystal X-ray determinations of 6-9 have confirmed that in each structure the iron atom is sandwiched between two tricarbon carborane cages. Compounds 6 and 7 differ in the site of substitution of the exopolyhedral methyl group. In 6 the methyl group is bound to a four-coordinate carbon as in the 6-CH3-5,6,9-C3B7H9- anion. In 7 the methyl group has rearranged to an adjacent five-coordinate cage carbon. Variable-temperature NMR studies of 7 also indicate that it is fluxional in solution. In 8, like 7, a methyl rearrangement to the five-coordinate carbon has occurred, but unlike 7, the two tricarbon carborane cages are of two different enantiomeric forms. In 9, a cage-carbon rearrangement has maximized the separation of the skeletal carbons. Although the ferratricarbaborane fragments Fe(CH3)C3B7H9 in 6-9 each exhibit gross octadecahedral geometries consistent with their closo 24-skeletal-electron counts, 6-8 also show cage distortions that result in the formation of open four-membered Fe-C-C-B faces. Reaction of 6-CH3-5,6,9-C3B7H9- with CoCl2 yielded two compounds, commo-Co-(1-Co-2-CH3-2,3,5-C3B7H9)(1-Co-5-CH3-2,3,5-C3B7H9) (10) and commo-Co-(1-Co-2-CH3-2,3,5-C3B7H9)2 (11). Both 10 and 11 are 19-electron, air-stable analogues of cobaltocene, and their structures were established by single-crystal X-ray studies. Both compounds have sandwich structures, but the cage units show cage distortions resulting in open structures that are intermediate between the normal 11-vertex 24-skeletal-electron closo and 26-skeletal-electron nido structures.