title: 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 PPh3 trans 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.
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: Laura E
authors.initials: LE
authors.lastname: Riley
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: Antony P Y
authors.initials: AP
authors.lastname: Chan
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: James
authors.initials: J
authors.lastname: Taylor
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: Wing Y
authors.initials: WY
authors.lastname: Man
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: David
authors.initials: D
authors.lastname: Ellis
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: Georgina M
authors.initials: GM
authors.lastname: Rosair
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS UK. a.j.welch@hw.ac.uk.
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Welch
authors.affiliation: A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119991, Moscow, Russia.
authors.forename: Igor B
authors.initials: IB
authors.lastname: Sivaev