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

Deprotonation of 1,1′-bis­(ortho-carborane) with n 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.