Nickelation of [3-Et-7,8-Ph2-7,8-nido-C2B9H8]2-:: synthesis and characterization of 1,2→1,2 and 1,2→1,7 isomerized products

The reaction between [3-Et-7,8-Ph(2)-7,8-nido-C(2)B(9)H(8)](2-) and Ni(dppe)Cl(2) in tetrahydrofuran affords major and minor products, identified spectroscopically and crystallographically. The major species is 1,2-Ph(2)-4-dppe-6-Et-4,1,2-closo-NiC(2)B(9)H(8), which arises from the presumed initial product 1,2-Ph(2)-3-dppe-6-Et-3,1,2-closo-NiC(2)B(9)H(8) by a sterically induced 1,2 --> 1,2 C atom isomerization, typical of overcrowded nickelacarboranes. The minor species is 1,8-Ph(2)-2-dppe-4-Et-2,1,8-closo-NiC(2)B(9)H(8), arising from an unexpected 1,2 --> 1,7 carbon-atom isomerization of the initial product. The fact that one boron atom is labelled with an Et group, coupled with the fact that these isomerizations occur at low temperatures, allows comment on the isomerization. mechanisms. The minor isomer cannot be explained by reference to Wales' sequential diamond-square-diamond mechanism for 1,2-closo-C(2)B(10)H(12) --> 1,7-closo-C(2)B(10)H(12). A simple rationalization of the formation of both major and minor isomers is available through triangle face rotation. Copyright (C) 2003 John Wiley Sons, Ltd.