The bis(carboranyl)phosphines [μ-2,2'-PPh-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (I) and [μ-2,2'-PEt-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (1) have been prepared and spectroscopically and structurally characterised. Crystallographic and DFT computational studies of 1 suggest that the orientation of the ethyl group, relative to the bis(carborane), is the result of intramolecular dihydrogen bonding. This orientation is such that the magnitudes of the J coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed J close to zero. The steric properties of I, 1 and their derivatives [μ-2,2'-P(Ph)AuCl-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (2) and [μ-2,2'-P(Et)AuCl-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (3) have been assessed by Tolman cone angle and percent buried volume calculations, from which it is concluded that the bis(carboranyl)phosphines I and 1 are comparable to PCy in their steric demands. The selenides [μ-2,2'-P(Ph)Se-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (4) and [μ-2,2'-P(Et)Se-{1-(1'-1',2'-closo-CBH)-1,2-closo-CBH}] (5) have also been prepared and characterised. The J coupling constants for 4 and 5 are the largest reported so far for carboranylphosphine selenides and indicate that I and 1 are very weakly basic.

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.

Deprotonation of 1,1′-bis(ortho-carborane) with n BuLi in THF followed by reaction with [RuCl 2 (p-cymene)] 2 affords, in addition to the known compound [Ru(κ 3 -2,2′,3′-{1-(1′-closo-1′,2′-C 2 B 10 H 10 )-closo-1,2-C 2 B 10 H 10 )}(pcymene)] (I), a small amount of a new species, [Ru(κ 3 -2,2′,11′-{1-(7′-nido-7′,8′-C 2 B 9 H 11 )-closo-1,2-C 2 B 10 H 10 )}(p-cymene)] (1a), with two B-agostic B− H⇀Ru bonds, making the bis(carborane) unit a closo-nido-X(C)L 2 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′-C 2 B 10 H 11 )-3-(arene)-closo-3,1,2-RuC 2 B 9 H 10 )], 2c and 2d, were also isolated. With the bis(ortho-carborane) transfer reagent [Mg(κ 2 -2,2′-{1-(1′-closo-1′,2′-C 2 B 10 H 10 )-closo-1,2-C 2 B 10 H 10 )}(DME) 2 ], the target compounds [Ru(κ 3 -2,2′,3′-{1-(1′-closo-1′,2′-C 2 B 10 H 10 )-closo-1,2-C 2 B 10 H 10 )}(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-κ 4 -2,2′,3,3′-{1-(1′closo-1′,2′-C 2 B 10 H 9 )-closo-1,2-C 2 B 10 H 9 })], 3b and 3c, were also formed. In compounds 3, the bis(carborane) acts as a closocloso-X 4 (C,C′,B,B′) ligand to the Ru 2 unit. In I, 4b, and 4d, the B-agostic B−H⇀Ru bond is readily cleaved by MeCN, affording compounds [Ru(κ 2 -2,2′-{1-(1′-closo-1′,2′-C 2 B 10 H 10 )-closo-1,2-C 2 B 10 H 10 })(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.