Arene-Ruthenium Complexes of 1,1'-Bis(ortho-carborane): Synthesis, Characterization, and Catalysis

Deprotonation of 1,1'-bis(ortho-carborane) with BuLi in THF followed by reaction with [RuCl₂(p-cymene)]₂ affords, in addition to the known compound [Ru(κ³-2,2',3'-{1-(1'-closo-1',2'-C₂B₁₀H₁₀)-closo-1,2-C₂B₁₀H₁₀)}(p-cymene)] (I), a small amount of a new species, [Ru(κ³-2,2',11'-{1-(7'-nido-7',8'-C₂B₉H₁₁)-closo-1,2-C₂B₁₀H₁₀)}(p-cymene)] (1a), with two B-agostic B-H⇀Ru bonds, making the bis(carborane) unit a closo-nido-X(C)L₂ ligand, a previously unreported bonding mode. Similar species were also formed with arene = benzene (1b), mesitylene (1c), and hexamethylbenzene (1d), although in the last two cases the metallacarborane-carborane species [1-(1'-closo-1',2'-C₂B₁₀H₁₁)-3-(arene)-closo-3,1,2-RuC₂B₉H₁₀)], 2c and 2d, were also isolated. With the bis(ortho-carborane) transfer reagent [Mg(κ²-2,2'-{1-(1'-closo-1',2'-C₂B₁₀H₁₀)-closo-1,2-C₂B₁₀H₁₀)}(DME)₂], the target compounds [Ru(κ³-2,2',3'-{1-(1'-closo-1',2'-C₂B₁₀H₁₀)-closo-1,2-C₂B₁₀H₁₀)}(arene)], 4b and 4d, were prepared in reasonable-to-good yields, although for arene = benzene and mesitylene small amounts of the unique paramagnetic species [{Ru(arene)}₂(μ-Cl)(μ-κ⁴-2,2',3,3'-{1-(1'-closo-1',2'-C₂B₁₀H₉)-closo-1,2-C₂B₁₀H₉})], 3b and 3c, were also formed. In compounds 3, the bis(carborane) acts as a closo-closo-X₄(C,C',B,B') ligand to the Ru₂ unit. In I, 4b, and 4d, the B-agostic B-H⇀Ru bond is readily cleaved by MeCN, affording compounds [Ru(κ²-2,2'-{1-(1'-closo-1',2'-C₂B₁₀H₁₀)-closo-1,2-C₂B₁₀H₁₀})(arene)(NCMe)] (5a, 5b, and 5d) and suggesting that I, 4b, and 4d could act as Lewis acid catalysts, which is subsequently shown to be the case for the Diels-Alder cycloaddition reactions between cyclopentadiene and methacrolein, ethylacrolein and E-crotonaldehyde. All new species were characterized by multinuclear NMR spectroscopy and 1a, 1c, 1d, 2c, 2d, 3b, 3c, 4b, 4d, 5a, 5b, and 5d were also characterized crystallographically.