Examples of singly-metallated derivatives of 1,1'-bis(o-carborane) have been prepared and spectroscopically and structurally characterised. Metallation of [7-(1'-1',2'-closo-C2B10H11)-7,8-nido-C2B9H10](2-) with a {Ru(p-cymene)}(2+) fragment affords both the unisomerised species [1-(1'-1',2'-closo-C2B10H11)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (2) and the isomerised [8-(1'-1',2'-closo-C2B10H11)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (3), and 2 is easily transformed into 3 with mild heating. Metallation with a preformed {CoCp}(2+) fragment also affords a 3,1,2-MC2B9-1',2'-C2B10 product [1-(1'-1',2'-closo-C2B10H11)-3-Cp-3,1,2-closo-CoC2B9H10] (4), but if CoCl2/NaCp is used followed by oxidation the result is the 2,1,8-CoC2B9-1',2'-C2B10 species [8-(1'-1',2'-closo-C2B10H11)-2-Cp-2,1,8-closo-CoC2B9H10] (5). Compound 4 does not convert into 5 in refluxing toluene, but does do so if it is reduced and then reoxidised, perhaps highlighting the importance of the basicity of the metal fragment in the isomerisation of metallacarboranes. A computational study of 1,1'-bis(o-carborane) is in excellent agreement with a recently-determined precise crystallographic study and establishes that the {1',2'-closo-C2B10H11} fragment is electron-withdrawing compared to H.

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.

Deboronation of [8-(1'- closo-1',2'-CBH)- closo-2,1,8-MCBH] affords diastereoisomeric mixtures of [8-(7'- nido-7',8'-CBH)- closo-2,1,8-MCBH] anions (1, M = Ru( p-cymene); 2, M = CoCp) isolated as [HNMe] salts. Deprotonation of 1 and reaction with CoCl/NaCp followed by oxidation yields [8-(1'-3'-Cp -closo-3',1',2'-CoCBH)-2-( p-cymene)- closo-2,1,8-RuCBH] isolated as two separable diastereoisomers, namely, 3α and 3β, the first examples of heterometalated derivatives of 1,1'-bis( ortho-carborane). Deprotonation of [7-(1'- closo-1',2'-CBH)- nido-7,8-CBH], metalation with CoCl/NaCp* and oxidation affords the isomers [1-(1'- closo-1',2'-CBH)-3-Cp*- closo-3,1,2-CoCBH] (4) and [8-(1'- closo-1',2'-CBH)-2-Cp*- closo-2,1,8-CoCBH] (5) as well as a trace amount of the 13-vertex/12-vertex species [12-(1'- closo-1',2'-CBH)-4,5-Cp*- closo-4,5,1,12-CoCBH] (6). Reduction then reoxidation of 4 converts it to 5. Deboronation of either 4 or 5 yields a diastereoisomeric mixture of [8-(7'- nido-7',8'-CBH)-2-Cp*- closo-2,1,8-CoCBH] (7), again isolated as the [HNMe] salt. Deprotonation of this followed by treatment with [RuCl( p-cymene)] produces [8-(1'-3'-( p-cymene)- closo-3',1',2'-RuCBH)-2-Cp*- closo-2,1,8-CoCBH] (8) as a mixture of two diastereoisomers in a 2:1 ratio, which could not be separated. Diastereoisomers 8 are complementary to 3α and 3β in which {CoCp} and {Ru( p-cymene)} in 3 were replaced by {Ru( p-cymene)} and {CoCp*}, respectively, in 8. Finally, thermolysis of mixture 8 in refluxing dimethoxyethane yields [8-(8'-2'-( p-cymene)- closo-2',1',8'-RuCBH)-2-Cp*- closo-2,1,8-CoCBH] (9), again as a 2:1 diastereoisomeric mixture that could not be separated. All new species were characterized by multinuclear NMR spectroscopy, and 3α, 3β, 4, 5, 6, and 9 were also characterized crystallographically.

Double deboronation of 1,1'-bis(ortho-carborane) results in a mixture of racemic and meso diastereoisomers which are sources of the [7-(7'-7',8'-nido-CBH)-7,8-nido-CBH] tetraanion. Consistent with this, metalation of the mixture with {Ru(p-cymene)} affords the diastereoisomers α-[1-(8'-2'-(p-cymene)-2',1',8'-closo-RuCBH)-3-(p-cymene)-3,1,2-closo-RuCBH] (3α) and β-[1-(8'-2'-(p-cymene)-2',1',8'-closo-RuCBH)-3-(p-cymene)-3,1,2-closo-RuCBH] (3β) in which the primed cage has undergone a spontaneous 3',1',2' to 2',1',8'-RuCB isomerisation. Analogous cobaltacarboranes α-[1-(8'-2'-Cp-2',1',8'-closo-CoCBH)-3-Cp-3,1,2-closo-CoCBH] (4α) and β-[1-(8'-2'-Cp-2',1',8'-closo-CoCBH)-3-Cp-3,1,2-closo-CoCBH] (4β) are formed by metalation with CoCl/NaCp followed by oxidation, along with a small amount of the unique species [8-(8'-2'-Cp-2',1',8'-closo-CoCBH)-2-Cp-2,1,8-closo-CoCBH] (5) if the source of the tetraanion is [HNMe][7-(7'-7',8'-nido-CBH)-7,8-nido-CBH]. Two-electron reduction and subsequent reoxidation of 4α and 4β afford species indistinguishable from 5. The reaction between [Tl][1-(1'-3',1',2'-closo-TlCBH)-3,1,2-closo-TlCBH] and [CoCpI(CO)] leads to the isolation of a further isomer of (CpCoCBH), rac-[1-(1'-3'-Cp-3',1',2'-closo-CoCBH)-3-Cp-3,1,2-closo-CoCBH] (6), which displays intramolecular dihydrogen bonding. Thermolysis of 6 yields 4α, allowing a link to be established between the α and β forms of 3 and 4 and racemic and meso forms of the [7-(7'-7',8'-nido-CBH)-7,8-nido-CBH] tetraanion, whilst reduction-oxidation of 6 again results in a product indistinguishable from 5.

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