Synthesis, Structure, and Dynamics of Nickelacarboranes Incorporating the [<i>n</i><i>ido</i>-7,9-C₂B₉H₁₁]^2- Ligand

The nickelacarboranes [NEt4][2-(η3-C3H4R)-closo-2,1,7-NiC2B9H11] (R = H (1a), Ph (1b)) have been synthesized via reaction between [Na]2[nido-7,9-C2B9H11] and [Ni2(μ-Br)2(η3-C3H4R)2] in THF (THF = tetrahydrofuran), followed by addition of [NEt4]Cl. Protonation of 1a in the presence of a donor ligand L affords the complexes [2,2-L2-closo-2,1,7-NiC2B9H11] (L = CO (2), CNBut (3)). Addition of PEt3 (1 equiv) to 2 produces quantitative conversion to [2-CO-2-PEt3-closo-2,1,7-NiC2B9H11], 4. Species 2−4 exhibit in solution hindered rotation of the NiL2 fragment with respect to the η5-C2B9 cage unit. Protonation of 1a in the presence of a diene affords the neutral complexes [2-(η2:η2-diene)-closo-2,1,7-NiC2B9H11] (diene = C5Me5H (5), dcp (6), cod (7), nbd (8), chd (9), and cot (10a); dcp = dicyclopentadiene, cod = 1,5-cyclooctadiene, nbd = norbornadiene, chd = 1,3-cyclohexadiene, and cot = cyclooctatetraene). Variable temperature 1H NMR experiments show that the {Ni(diene)} fragments are freely rotating even at 193 K. A small quantity of the di-cage species [2,2‘-μ-(1,2:5,6-η-3,4:7,8-η-cot)-(closo-2,1,7-NiC2B9H11)2] (10b) is formed as a coproduct in the synthesis of 10a. This species can be rationally synthesized by protonation of 1a and subsequent addition of 10a.