Unprecedented flexibility of the 1,1′-bis(o-carborane) ligand: catalytically-active species stabilised by B-agostic B–H⇀Ru interactions

Abstract

Doubly-deprotonated 1,1′-bis(o-carborane) reacts with [RuCl₂(p-cymene)]₂ to afford [Ru(κ³-2,2′,3′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(p-cymene)] (1) in which 1,1′-bis(o-carborane) acts as an X₂(C,C′)L ligand where “L” is a B3′–H3′⇀Ru B-agostic interaction, fluctional over four BH units (3′, 6′, 3 and 6) at 298 K but partially arrested at 203 K (B3′ and B6′). This interaction is readily cleaved by CO affording [Ru(κ²-2,2′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(p-cymene)(CO)] (2) with the 1,1′-bis(o-carborane) simply an X₂(C,C′) ligand. With PPh₃ or dppe 1 yields [Ru(κ³-2,3′,3-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(PPh₃)₂] (3) or [Ru(κ³-2,3′,3-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(dppe)] (4) via unusually facile loss of the η-(p-cymene) ligand. In 3 and 4 the 1,1′-bis(o-carborane) has unexpectedly transformed into an X₂(C,B′)L ligand with “L” now a B3–H3⇀Ru B-agostic bond. Unlike in 1 the B-agostic bonding in 3 and 4 appears non-fluctional at 298 K. With CO the B-agostic interaction of 3 is cleaved and a PPh₃ ligand is lost to afford [Ru(κ²-2,3′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(CO)₃(PPh₃)] (5), which exists as a 1 : 1 mixture of isomers, one having PPh₃trans to C2, the other trans to B3′. With MeCN the analogous product [Ru(κ²-2,3′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(MeCN)₃(PPh₃)] (6) is formed as only the former isomer. With CO 4 affords [Ru(κ²-2,3′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(CO)₂(dppe)] (7), whilst with MeCN 4 yields [Ru(κ²-2,3′-{1-(1′-1′,2′-closo-C₂B₁₀H₁₀)-1,2-closo-C₂B₁₀H₁₀})(MeCN)₂(dppe)] (8). In 5 and 6 the three common ligands (CO or MeCN) are meridional, whilst in 7 and 8 the two monodentate ligands are mutually trans. Compound 1 is an 18-e, 6-co-ordinate, species but with a labile B-agostic interaction and 3 and 4 are 16-e, formally 5-co-ordinate, species also including a B-agostic interaction, and thus all three have the potential to act as Lewis acid catalysts. A 1% loading of 1 catalyses the Diels–Alder cycloaddition of cyclopentadiene and methacrolein in CH₂Cl₂ with full conversion after 6 h at 298 K, affording the product with exo diastereoselectivity (de >77%). Compounds 1–8 are fully characterised spectroscopically and crystallographically.