Bridges and Vertices in Heteroboranes.

A number of (hetero)boranes are known in which a main group atom X 'bridges' a B-B connectivity in the open face, and in such species X has previously been described as simply a bridge or, alternatively, as a vertex in a larger cluster. In this study we describe an approach to distinguish between these options based on identifying the best fit of the experimental {Bx} cluster fragment with alternate exemplar {Bx} fragments derived from DFT-optimized [BnHn]2- models. In most of the examples studied atom X is found to be better regarded as a vertex, having 'a 'verticity' of ca. 60-65%. Consideration of our results leads to the suggestion that the radial electron contribution from X to the overall skeletal electron count is more significant than the tangential contribution.

C,C'-Ru to C,B'-Ru isomerisation in bis(phosphine)Ru complexes of [1,1'-bis(ortho-carborane)].

We report herein the first example of the controlled isomerisation of a C,C'-bound (to metal) bis(ortho-carborane) ligand to C,B'-bound with no other change in the molecule. Since the C and B vertices of carboranes have different electron-donating properties this transformation allows the reactivity of the metal centre to be fine-tuned.

Exopolyhedral Ligand Orientation Controls Diastereoisomer in Mixed-Metal Bis(carboranes).

Heterobimetallic derivatives of a bis(carborane), [µ7,8-(1',3'-3'-Cl-3'-PPh3-closo-3',1',2'-RhC2B9H10)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (1) and [µ7,8-(1',3'-3'-Cl-3'-PPh3-closo-3',1',2'-RhC2B9H10)-2-Cp-closo-2,1,8-CoC2B9H10] (2) have been synthesised and characterised, including crystallographic studies. A minor co-product during the synthesis of compound 2 is the new species [8-{8'-2'-H-2',2'-(PPh3)2-closo-2',1',8'-RhC2B9H10}-2-Cp-closo-2,1,8-CoC2B9H10] (3), isolated as a mixture of diastereoisomers. Although, in principle, compounds 1 and 2 could also exist as two diastereoisomers, only one (the same in both cases) is formed. It is suggested that the preferred exopolyhedral ligand orientation in the rhodacarboranes in the non-observed diastereoisomers would lead to unacceptable steric crowding between the PPh3 ligand and either the p-cymene (compound 1) or Cp (compound 2) ligand of the ruthenacarborane or cobaltacarborane, respectively.

Bis(phosphine)hydridorhodacarborane Derivatives of 1,1'-Bis(ortho-carborane) and Their Catalysis of Alkene Isomerization and the Hydrosilylation of Acetophenone.

Deprotonation of [7-(1'-closo-1',2'-C2B10H11)-nido-7,8-C2B9H11]- and reaction with [Rh(PPh3)3Cl] results in isomerization of the metalated cage and the formation of [8-(1'-closo-1',2'-C2B10H11)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (1). Similarly, deprotonation/metalation of [8'-(7-nido-7,8-C2B9H11)-2'-(p-cymene)-closo-2',1',8'-RuC2B9H10]- and [8'-(7-nido-7,8-C2B9H11)-2'-Cp*-closo-2',1',8'-CoC2B9H10]- affords [8-{8'-2'-(p-cymene)-closo-2',1',8'-RuC2B9H10}-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (2) and [8-(8'-2'-Cp*-closo-2',1',8'-CoC2B9H10)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (3), respectively, as diastereoisomeric mixtures. The performances of compounds 1-3 as catalysts in the isomerization of 1-hexene and in the hydrosilylation of acetophenone are compared with those of the known single-cage species [3-H-3,3-(PPh3)2-closo-3,1,2-RhC2B9H11] (I) and [2-H-2,2-(PPh3)2-closo-2,1,12-RhC2B9H11] (V), the last two compounds also being the subjects of 103Rh NMR spectroscopic studies, the first such investigations of rhodacarboranes. In alkene isomerization all the 2,1,8- or 2,1,12-RhC2B9 species (1-3, V) outperform the 3,1,2-RhC2B9 compound I, while for hydrosilylation the single-cage compounds I and V are better catalysts than the double-cage species 1-3.

Do Gold(III) Complexes Form Hydrogen Bonds? An Exploration of AuIII Dicarboranyl Chemistry.

The reaction of 1,1'-Li2 [(2,2'-C2 B10 H10 )2 ] with the cyclometallated gold(III) complex (C^N)AuCl2 afforded the first examples of gold(III) dicarboranyl complexes. The reactivity of these complexes is subject to the trans-influence exerted by the dicarboranyl ligand, which is substantially weaker than that of non-carboranyl anionic C-ligands. In line with this, displacement of coordinated pyridine by chloride is only possible under forcing conditions. While treatment of (C^N)Au{(2,2'-C2 B10 H10 )2 } (2) with triflic acid leads to Au-C rather than Au-N bond protonolysis, aqueous HBr cleaves the Au-N bond to give the pyridinium bromo complex 7. The trans-influence of a series of ligands including dicarboranyl and bis(dicarboranyl) was assessed by means of DFT calculations. The analysis demonstrated that it was not sufficient to rely exclusively on geometric descriptors (calculated or experimental) when attempting to rank ligands for their trans influence. Complex (C^N)Au(C2 B10 H11 )2 containing two non-chelating dicarboranyl ligands was prepared similar to 2. Its reaction with trifluoroacetic acid also leads to Au-N cleavage to give trans-(Hpy^C)Au(OAcF )(C2 B10 H11 )2 (8). In crystals of 8 the pyridinium N-H bond points towards the metal centre, while in 7 it is bent away. The possible contribution of gold(III)⋅⋅⋅H-N hydrogen bonding in these complexes was investigated by DFT calculations. The results show that, unlike the situation for platinum(II), there is no evidence for an energetically significant contribution by hydrogen bonding in the case of gold(III).

On the Basicity of Carboranylphosphines.

Three new carboranylphosphines, [1-(1'-closo-1',7'-C2B10H11)-7-PPh2-closo-1,7-C2B10H10], [1-(1'-7'-PPh2-closo-1',7'-C2B10H10)-7-PPh2-closo-1,7-C2B10H10], and [1-{PPh-(1'-closo-1',2'-C2B10H11)}-closo-1,2-C2B10H11], have been prepared, and from a combination of these and literature compounds, eight new carboranylphosphine selenides were subsequently synthesized. The relative basicities of the carboranylphosphines were established by (i) measurement of the 1JPSe NMR coupling constant of the selenide and (ii) calculation of the proton affinity of the phosphine, in an attempt to establish which of several factors are the most important in controlling the basicity. It is found that the basicity of the carboranylphosphines is significantly influenced by the nature of other substituents on the P atom, the nature of the carborane cage vertex (C or B) to which the P atom is attached, and the charge on the carboranylphosphine. In contrast, the basicity of the carboranylphosphines appears to be relatively insensitive to the nature of other substituents on the carborane cage, the isomeric form of the carborane, and whether the cage is closo or nido (insofar as that does not alter the charge on the cluster). Such information is likely to be of significant importance in optimizing future applications of carboranylphosphines, e.g., as components of frustrated Lewis pairs.

Arene-Ruthenium Complexes of 1,1'-Bis(ortho-carborane): Synthesis, Characterization, and Catalysis.

Deprotonation of 1,1'-bis(ortho-carborane) with nBuLi in THF followed by reaction with [RuCl2(p-cymene)]2 affords, in addition to the known compound [Ru(κ3-2,2',3'-{1-(1'-closo-1',2'-C2B10H10)-closo-1,2-C2B10H10)}(p-cymene)] (I), a small amount of a new species, [Ru(κ3-2,2',11'-{1-(7'-nido-7',8'-C2B9H11)-closo-1,2-C2B10H10)}(p-cymene)] (1a), with two B-agostic B-H⇀Ru bonds, making the bis(carborane) unit a closo-nido-X(C)L2 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'-C2B10H11)-3-(arene)-closo-3,1,2-RuC2B9H10)], 2c and 2d, were also isolated. With the bis(ortho-carborane) transfer reagent [Mg(κ2-2,2'-{1-(1'-closo-1',2'-C2B10H10)-closo-1,2-C2B10H10)}(DME)2], the target compounds [Ru(κ3-2,2',3'-{1-(1'-closo-1',2'-C2B10H10)-closo-1,2-C2B10H10)}(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-κ4-2,2',3,3'-{1-(1'-closo-1',2'-C2B10H9)-closo-1,2-C2B10H9})], 3b and 3c, were also formed. In compounds 3, the bis(carborane) acts as a closo-closo-X4(C,C',B,B') ligand to the Ru2 unit. In I, 4b, and 4d, the B-agostic B-H⇀Ru bond is readily cleaved by MeCN, affording compounds [Ru(κ2-2,2'-{1-(1'-closo-1',2'-C2B10H10)-closo-1,2-C2B10H10})(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.

Crystal structure of 1-hepta-fluoro-tolyl-closo-1,2-dicarbadodeca-borane.

The mol-ecular structure of the title compound 1-(2',3',5',6'-tetra-fluoro-4'-trifluoro-methyl-phen-yl)-closo-1,2-dicarbadodeca-borane, C9H11B10F7, features an intra-molecular ortho-F⋯H2 hydrogen bond [2.11 (2) Å], which is responsible for an orientation of the hepta-fluoro-tolyl substituent in which the plane of the aryl ring nearly eclipses the C1-C2 cage connectivity.

Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs.

The first example of a carborane with a catecholborolyl substituent, [1-Bcat-2-Ph-closo-1,2-C2B10H10] (1), has been prepared and characterized and shown to act as the Lewis acid component of an intermolecular frustrated Lewis pair in catalyzing a Michael addition. In combination with B(C6F5)3 the C-carboranylphosphine [1-PPh2-closo-1,2-C2B10H11] (IVa) is found to be comparable with PPh2(C6F5) in its ability to catalyze hydrosilylation, whilst the more strongly basic B-carboranylphosphine [9-PPh2-closo-1,7-C2B10H11] (V) is less effective and the very weakly basic species [µ-2,2'-PPh-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (IX) is completely ineffective. Base strengths are rank-ordered via measurement of the ¹J 31P-77Se coupling constants of the phosphineselenides [1-SePPh2-closo-1,2-C2B10H11] (2), [9-SePPh2-closo-1,7-C2B10H11] (3), and [SePPh2(C6F5)] (4).

Heterometalation of 1,1'-Bis( ortho-carborane).

Deboronation of [8-(1'- closo-1',2'-C2B10H11)- closo-2,1,8-MC2B9H10] affords diastereoisomeric mixtures of [8-(7'- nido-7',8'-C2B9H11)- closo-2,1,8-MC2B9H10]- anions (1, M = Ru( p-cymene); 2, M = CoCp) isolated as [HNMe3]+ salts. Deprotonation of 1 and reaction with CoCl2/NaCp followed by oxidation yields [8-(1'-3'-Cp -closo-3',1',2'-CoC2B9H10)-2-( p-cymene)- closo-2,1,8-RuC2B9H10] isolated as two separable diastereoisomers, namely, 3α and 3ß, the first examples of heterometalated derivatives of 1,1'-bis( ortho-carborane). Deprotonation of [7-(1'- closo-1',2'-C2B10H11)- nido-7,8-C2B9H11]-, metalation with CoCl2/NaCp* and oxidation affords the isomers [1-(1'- closo-1',2'-C2B10H11)-3-Cp*- closo-3,1,2-CoC2B9H10] (4) and [8-(1'- closo-1',2'-C2B10H11)-2-Cp*- closo-2,1,8-CoC2B9H10] (5) as well as a trace amount of the 13-vertex/12-vertex species [12-(1'- closo-1',2'-C2B10H11)-4,5-Cp*2- closo-4,5,1,12-Co2C2B9H10] (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'-C2B9H11)-2-Cp*- closo-2,1,8-CoC2B9H10]- (7), again isolated as the [HNMe3]+ salt. Deprotonation of this followed by treatment with [RuCl2( p-cymene)]2 produces [8-(1'-3'-( p-cymene)- closo-3',1',2'-RuC2B9H10)-2-Cp*- closo-2,1,8-CoC2B9H10] (8) as a mixture of two diastereoisomers in a 2:1 ratio, which could not be separated. Diastereoisomers 8 are complementary to 3α and 3ß 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'-RuC2B9H10)-2-Cp*- closo-2,1,8-CoC2B9H10] (9), again as a 2:1 diastereoisomeric mixture that could not be separated. All new species were characterized by multinuclear NMR spectroscopy, and 3α, 3ß, 4, 5, 6, and 9 were also characterized crystallographically.

Large, weakly basic bis(carboranyl)phosphines: an experimental and computational study.

The bis(carboranyl)phosphines [µ-2,2'-PPh-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (I) and [µ-2,2'-PEt-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (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 2JPH coupling constants are approximately equal but of opposite sign, and fast exchange between the methylene protons in solution leads to an observed 2JPH close to zero. The steric properties of I, 1 and their derivatives [µ-2,2'-P(Ph)AuCl-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (2) and [µ-2,2'-P(Et)AuCl-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (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 PCy3 in their steric demands. The selenides [µ-2,2'-P(Ph)Se-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (4) and [µ-2,2'-P(Et)Se-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (5) have also been prepared and characterised. The 1JPSe 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.

Double deboronation and homometalation of 1,1'-bis(ortho-carborane).

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-C2B9H10)-7,8-nido-C2B9H10]4- 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-RuC2B9H10)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (3α) and ß-[1-(8'-2'-(p-cymene)-2',1',8'-closo-RuC2B9H10)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (3ß) in which the primed cage has undergone a spontaneous 3',1',2' to 2',1',8'-RuC2B9 isomerisation. Analogous cobaltacarboranes α-[1-(8'-2'-Cp-2',1',8'-closo-CoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (4α) and ß-[1-(8'-2'-Cp-2',1',8'-closo-CoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (4ß) are formed by metalation with CoCl2/NaCp followed by oxidation, along with a small amount of the unique species [8-(8'-2'-Cp-2',1',8'-closo-CoC2B9H10)-2-Cp-2,1,8-closo-CoC2B9H10] (5) if the source of the tetraanion is [HNMe3]2[7-(7'-7',8'-nido-C2B9H11)-7,8-nido-C2B9H11]. Two-electron reduction and subsequent reoxidation of 4α and 4ß afford species indistinguishable from 5. The reaction between [Tl]2[1-(1'-3',1',2'-closo-TlC2B9H10)-3,1,2-closo-TlC2B9H10] and [CoCpI2(CO)] leads to the isolation of a further isomer of (CpCoC2B9H11)2, rac-[1-(1'-3'-Cp-3',1',2'-closo-CoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (6), which displays intramolecular dihydrogen bonding. Thermolysis of 6 yields 4α, allowing a link to be established between the α and ß forms of 3 and 4 and racemic and meso forms of the [7-(7'-7',8'-nido-C2B9H10)-7,8-nido-C2B9H10]4- tetraanion, whilst reduction-oxidation of 6 again results in a product indistinguishable from 5.

Steric versus electronic factors in metallacarborane isomerisation: nickelacarboranes with 3,1,2-, 4,1,2- and 2,1,8-NiC2B9 architectures and pendant carborane groups, derived from 1,1'-bis(o-carborane).

Metalation of the [7-(1'-1',2'-closo-C2B10H11)-7,8-nido-C2B9H10](2-) dianion with various {NiPP(2+)} or {NiP2(2+)} fragments (PP = chelating diphosphine; P = monodentate phosphine or phosphite) leads either to unisomerised 3,1,2-NiC2B9 species or to isomerised 4,1,2-NiC2B9 or 2,1,8-NiC2B9 species, all with a pendant C2B10 substituent. The products [1-(1'-1',2'-closo-C2B10H11)-3-dppe-3,1,2-closo-NiC2B9H10] (1), [2-(1'-1',2'-closo-C2B10H11)-4-dppe-4,1,2-closo-NiC2B9H10] (2), [8-(1'-1',2'-closo-C2B10H11)-2-dmpe-2,1,8-closo-NiC2B9H10] (3), [1-(1'-1',2'-closo-C2B10H11)-3,3-(PMe3)2-3,1,2-closo-NiC2B9H10] (4), [1-(1'-1',2'-closo-C2B10H11)-3,3-(PMe2Ph)2-3,1,2-closo-NiC2B9H10] (6), [1-(1'-1',2'-closo-C2B10H11)-3,3-{P(OMe)3}2-3,1,2-closo-NiC2B9H10] (9) and [1-(1'-1',2'-closo-C2B10H11)-2,2-{P(OMe)3}2-2,1,8-closo-NiC2B9H10] (10) were fully characterised spectroscopically and crystallographically, whilst [2-(1'-1',2'-closo-C2B10H11)-4,4-(PMePh2)2-4,1,2-closo-NiC2B9H10] (8) was characterised spectroscopically. Overall the results suggest that an important factor in a 3,1,2 to 4,1,2 isomerisation is the relief gained from steric crowding, whereas a 3,1,2 to 2,1,8 isomerisation appears to be favoured by strongly electron-donating ligands on the metal.

14-Vertex Heteroboranes with 14 Skeletal Electron Pairs: An Experimental and Computational Study.

Three isomers of [(Cp*Ru)2 C2 B10 H12 ], the first examples of 14-vertex heteroboranes containing 14-skeletal electron pairs, have been synthesized by the direct electrophilic insertion of a {Cp*Ru(+) } fragment into the anion [4-Cp*-4,1,6-RuC2 B10 H12 ](-) . All three compounds have the same unique polyhedral structure having an approximate Cs symmetry and featuring a four-atom trapezoidal face. X-ray diffraction studies could confidently identify only one of the two cage C atoms in each structure. The other C atom position has been established by a combination of i) best fitting of computed and experimental (11) B and (1) H NMR chemical shifts, and ii) consideration of the lowest computed energy for series of isomers studied by DFT calculations. In all three isomers, one cage C atom occupies a degree-4 vertex on the short parallel edge of the trapezium.

Further studies of the Enhanced Structural Carborane Effect: tricarbonylruthenium and related derivatives of benzocarborane, dihydrobenzocarborane and biphenylcarborane.

Detailed comparison of the molecular structures of [1,2-µ-(C4H4)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (1) and [1,2-µ-(C4H6)-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (2) reveals evidence for an Enhanced Structural Carborane Effect in 1 arising from the involvement of the cage pπ orbitals in the exopolyhedral ring to some degree. A minor co-product in the synthesis of 2 is [η-{1,2-µ-(C4H6)}-3,3-(CO)2-3,1,2-closo-RuC2B9H9] (3). Compounds 2 and 3 are readily interconverted, since heating 2 to reflux in THF or reaction with Me3NO affords 3 which readily reacts with CO to regenerate 2. The η-ene bonding in 3 is also displaced by PMe3, P(OMe)3 and t-BuNC to yield [1,2-µ-(C4H6)-3,3-(CO)2-3-PMe3-3,1,2-closo-RuC2B9H9] (4), [1,2-µ-(C4H6)-3,3-(CO)2-3-P(OMe)3-3,1,2-closo-RuC2B9H9] (5) and [1,2-µ-(C4H6)-3,3-(CO)2-3-t-BuNC-3,1,2-closo-RuC2B9H9] (6), respectively. Structural studies of 4-6, focussing on the Exopolyhedral Ligand Orientation of the {Ru(CO)2L} fragment relative to the C2B3 carborane face, are discussed in terms of the structural trans effects of PMe3, P(OMe)3 and t-BuNC relative to that of CO. An improved synthesis of [1,2-µ-(C6H4)2-1,2-closo-C2B10H10], "biphenylcarborane", is reported following which the first transition-metal derivatives of this species, [1,2-µ-(C6H4)2-3-Cp-3,1,2-closo-CoC2B9H9] (7) and [1,2-µ-(C6H4)2-3,3,3-(CO)3-3,1,2-closo-RuC2B9H9] (8), are prepared. Comparisons of the structures of 7 and 8 with the corresponding benzocarborane derivatives [1,2-µ-(C4H4)-3-Cp-3,1,2-closo-CoC2B9H9] and 1, respectively, suggest that Clar's rule for aromaticity can be applied to polycyclic aromatic hydrocarbons fused onto carborane cages.

Carborane Substituents Promote Direct Electrophilic Insertion over Reduction-Metalation Reactions.

Two-electron reduction of 1,1'-bis(o-carborane) followed by reaction with [Ru(η-mes)Cl2 ]2 affords [8-(1'-1',2'-closo-C2 B10 H11 )-4-(η-mes)-4,1,8-closo-RuC2 B10 H11 ]. Subsequent two-electron reduction of this species and treatment with [Ru(η-arene)Cl2 ]2 results in the 14-vertex/12-vertex species [1-(η-mes)-9-(1'-1',2'-closo-C2 B10 H11 )-13-(η-arene)-1,13,2,9-closo-Ru2 C2 B10 H11 ] by direct electrophilic insertion, promoted by the carborane substituent in the 13-vertex/12-vertex precursor. When arene=mesitylene (mes), the diruthenium species is fluxional in solution at room temperature in a process that makes the metal-ligand fragments equivalent. A unique mechanism for this fluxionality is proposed and is shown to be fully consistent with the observed fluxionality or nonfluxionality of a series of previously reported 14-vertex dicobaltacarboranes.

Developing nitrosocarborane chemistry.

The new nitrosocarboranes [1-NO-2-R-1,2-closo-C2B10H10] [R = CH2Cl (1), CH3OCH2 (2) p-MeC6H4 (3), SiMe3 (4) and SiMe2tBu (5)] and [1-NO-7-Ph-1,7-closo-C2B10H10] (6) were synthesised by reaction of the appropriate lithiocarborane in diethyl ether with NOCl in petroleum ether followed by quenching the reaction with aqueous NaHCO3. These bright-blue compounds were characterised spectroscopically and, in several cases, crystallographically including structural determinations of 2 and 6 using crystals grown in situ on the diffractometer from liquid samples. In all cases the nitroso group bonds to the carborane as a 1e substituent (bent C­N­O sequence) and has little or no influence on <δ11B>, the weighted average 11B chemical shift, relative to that in the parent (monosubstituted) carborane. Mono- and dinitroso derivatives of 1,1'-bis(m-carborane), compounds 7 and 8 respectively, were similarly synthesised but attempts to prepare the mononitroso 1,1'-bis(o-carborane) by the same protocol led only to the hydroxylamine species [1-(1'-1',2'-closo-C2B10H11)-2-N(H)OH-1,2-closo-C2B10H10] (9); the desired compound [1-(1'-1',2'-closo-C2B10H11)-2-NO-1,2-closo-C2B10H10] (10) was only realised by switching to a non-aqueous work-up. The involvement of water in effecting the net reduction of the NO function in 10 to N(H)OH in 9 was confirmed by a series of experiments involving [1-N(H)OH-2-Ph-1,2-closo-C2B10H10] (11), [1-(1'-2'-D-1',2'-closo-C2B10H10)-2-D-1,2-closo-C2B10H10] (12) and [1-(1'-2'-D-1',2'-closo-C2B10H10)-2-N(H)OH-1,2-closo-C2B10H10] (13). It is suggested that during aqueous work-up a water molecule, H-bonded to the acidic C2'H of 10, is "delivered" to the adjacent C2NO unit. The ability of the NO group in nitrosocarboranes to undergo Diels-Alder cycloaddition reactions with cyclic 1,3-dienes was established via the syntheses of [1-(NOC10H14)-1,2-closo-C2B10H11] (14) and [1-(NOC6H8)-2-Ph-1,2-closo-C2B10H10] (15). This strategy was then utilised to prepare derivatives of the elusive dinitroso compounds of [1,2-closo-C2B10H12] and 1,1'-bis(o-carborane) leading to the sterically-crowded products [1,2-(NOC6H8)2-1,2-closo-C2B10H10] (16, prepared as meso and racemic diastereoisomers), [1-{1'-2'-(NOC6H8)-1',2'-closo-C2B10H10}-2-(NOC6H8)-1,2-closo-C2B10H10] (17) and [1-(1'-1',2'-closo-C2B10H11)-2-(NOC6H8)-1,2-closo-C2B10H10] (18).

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

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 B3­H3⇀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.

Synthesis and crystal structures of the racemic and meso forms of [1-{1'-4'-cyclopentadienyl-4'-cobalta-1',12'-dicarba-closo-dodecaboranyl(10)}-4-cyclopentadienyl-4-cobalta-1,12-dicarba-closo-dodecaborane(10)], the former as its tetrahydrofuran disolvate.

Both rac-[1-(1'-4'-Cp-4',1',12'-closo-CoC2B10H11)-4-Cp-4,1,12-closo-CoC2B10H11]·2THF (Cp is cyclopentadienyl and THF is tetrahydrofuran) or [Co2(C5H5)2(C4H22B20)]·2C4H8O, (1), and meso-[1-(1'-4'-Cp-4',1',12'-closo-CoC2B10H11)-4-Cp-4,1,12-closo-CoC2B10H11] or [Co2(C5H5)2(C4H22B20)], (2), were prepared by thermolysis of a rac/meso mixture of the precursor species [1-(1'-4'-Cp-4',1',6'-closo-CoC2B10H11)-4-Cp-4,1,6-closo-CoC2B10H11] and were separated, spectroscopically characterized and studied crystallographically. Cage C-atom identification was accomplished by both the vertex-to-centroid distance and boron-hydrogen distance methods, and, in both cases, the structure established crystallographically is fully consistent with the spectroscopic data. Both the rac-(1) and meso-(2) forms share the same overall conformation (Co-C-C'-Co' ca 136°) and show clear evidence of intramolecular steric crowding resulting in tilted cyclopentadienyl ligands.

Reduction-induced facile isomerisation of metallacarboranes: synthesis and crystallographic characterisation of 4-Cp-4,1,2-closo-CoC2B9H11.

One-electron reduction of 3-Cp-3,1,2-closo-CoC2B9H11 followed by heating to reflux in DME (b.p. 85 °C) induces isomerisation to 4-Cp-4,1,2-closo-CoC2B9H11, a compound previously only synthesised at much higher temperatures (>380 °C). The 4,1,2-isomer has been thoroughly characterised both spectroscopically and crystallographically.