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Issue 3, 2016
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Unprecedented flexibility of the 1,1′-bis(o-carborane) ligand: catalytically-active species stabilised by B-agostic B–H⇀Ru interactions

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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.

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

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Publication details

The article was received on 02 Sep 2015, accepted on 13 Nov 2015 and first published on 17 Nov 2015


Article type: Paper
DOI: 10.1039/C5DT03417E
Author version available: Download Author version (PDF)
Citation: Dalton Trans., 2016,45, 1127-1137
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    Unprecedented flexibility of the 1,1′-bis(o-carborane) ligand: catalytically-active species stabilised by B-agostic B–H⇀Ru interactions

    L. E. Riley, A. P. Y. Chan, J. Taylor, W. Y. Man, D. Ellis, G. M. Rosair, A. J. Welch and I. B. Sivaev, Dalton Trans., 2016, 45, 1127
    DOI: 10.1039/C5DT03417E

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