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

Deprotonation of 1,1'-bis(-carborane) with BuLi in THF followed by reaction with [RuCl(-cymene)] affords, in addition to the known compound [Ru(κ-2,2',3'-{1-(1'--1',2'-CBH)--1,2-CBH)}(-cymene)] (), a small amount of a new species, [Ru(κ-2,2',11'-{1-(7'--7',8'-CBH)--1,2-CBH)}(-cymene)] (), with two -agostic B-H⇀Ru bonds, making the bis(carborane) unit a --X()L ligand, a previously unreported bonding mode. Similar species were also formed with arene = benzene (), mesitylene (), and hexamethylbenzene (), although in the last two cases the metallacarborane-carborane species [1-(1'--1',2'-CBH)-3-(arene)--3,1,2-RuCBH)], and , were also isolated. With the bis(-carborane) transfer reagent [Mg(κ-2,2'-{1-(1'--1',2'-CBH)--1,2-CBH)}(DME)], the target compounds [Ru(κ-2,2',3'-{1-(1'--1',2'-CBH)--1,2-CBH)}(arene)], and , were prepared in reasonable-to-good yields, although for arene = benzene and mesitylene small amounts of the unique paramagnetic species [{Ru(arene)}(μ-Cl)(μ-κ-2,2',3,3'-{1-(1'--1',2'-CBH)--1,2-CBH})], and , were also formed. In compounds , the bis(carborane) acts as a --X(,',,') ligand to the Ru unit. In , , and , the -agostic B-H⇀Ru bond is readily cleaved by MeCN, affording compounds [Ru(κ-2,2'-{1-(1'--1',2'-CBH)--1,2-CBH})(arene)(NCMe)] (, , and ) and suggesting that , , and 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 -crotonaldehyde. All new species were characterized by multinuclear NMR spectroscopy and , , , , , , , , , , , and were also characterized crystallographically.