Steric versus electronic factors in metallacarborane isomerisation: nickelacarboranes with 3,1,2-, 4,1,2- and 2,1,8-NiC2B9 architectures and pendant carborane groups, derived from 1,1′-bis(o-carborane)

Abstract

Metalation of the [7-(1′-1′,2′-closo-C₂B₁₀H₁₁)-7,8-nido-C₂B₉H₁₀]²⁻ dianion with various {NiPP²⁺} or {NiP₂²⁺} fragments (PP = chelating diphosphine; P = monodentate phosphine or phosphite) leads either to unisomerised 3,1,2-NiC₂B₉ species or to isomerised 4,1,2-NiC₂B₉ or 2,1,8-NiC₂B₉ species, all with a pendant C₂B₁₀ substituent. The products [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-3-dppe-3,1,2-closo-NiC₂B₉H₁₀] (1), [2-(1′-1′,2′-closo-C₂B₁₀H₁₁)-4-dppe-4,1,2-closo-NiC₂B₉H₁₀] (2), [8-(1′-1′,2′-closo-C₂B₁₀H₁₁)-2-dmpe-2,1,8-closo-NiC₂B₉H₁₀] (3), [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-3,3-(PMe₃)₂-3,1,2-closo-NiC₂B₉H₁₀] (4), [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-3,3-(PMe₂Ph)₂-3,1,2-closo-NiC₂B₉H₁₀] (6), [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-3,3-{P(OMe)₃}₂-3,1,2-closo-NiC₂B₉H₁₀] (9) and [1-(1′-1′,2′-closo-C₂B₁₀H₁₁)-2,2-{P(OMe)₃}₂-2,1,8-closo-NiC₂B₉H₁₀] (10) were fully characterised spectroscopically and crystallographically, whilst [2-(1′-1′,2′-closo-C₂B₁₀H₁₁)-4,4-(PMePh₂)₂-4,1,2-closo-NiC₂B₉H₁₀] (8) was characterised spectroscopically. Overall the results suggest that an important factor in a 3,1,2 to 4,1,2 isomerisation is the relief gained from steric crowding, whereas a 3,1,2 to 2,1,8 isomerisation appears to be favoured by strongly electron-donating ligands on the metal.