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.

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).

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.

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.

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.

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.

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).

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.

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.

Crystal structure of 1,1'-bis[1,7-dicarba-closo-dodeca-borane(11)].

In the title compound, C4H22B20, the two {1,7-closo-C2B10H11} cages are linked across a centre of inversion, with C-C = 1.5401 (16) Å. The position of the second non-linking cage C atom was established unambiguously by geometric and crystallographic methods and there is no evidence of C/B disorder.

Definitive crystal structure of 1,1'-bis-[1,2-dicarba-closo-dodeca-borane(11)].

In the title compound, C4H22B20, the two {1,2-closo-C2B10H11} cages are linked across a centre of inversion with a C-C distance of 1.5339 (11) Å. By careful analysis of the structure, it is established that the non-linking cage C atom is equally disordered over cage vertices 2 and 3.

How to make 8,1,2-closo-MC2B9 metallacarboranes.

Three examples of the rare 8,1,2-closo-MC2B9 isomeric form of an icosahedral metallacarborane have been isolated as unexpected trace products in reactions. Seeking to understand how these were formed we considered both the nature of the reactions that were being undertaken and the nature of the coproducts. This led us to propose a mechanism for the formation of the 8,1,2-closo-MC2B9 species. The mechanism was then tested, leading to the first deliberate synthesis of an example of this isomer. Thus, deboronation of 4-(η-C5H5)-4,1,8-closo-CoC2B10H12 selectively removes the B5 vertex to yield the dianion [nido-(η-C5H5)CoC2B9H11](2-), oxidative closure of which affords 8-(η-C5H5)-8,1,2-closo-CoC2B9H11 in moderate yield.

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.

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.

Unprecedented steric deformation of ortho-carborane.

The reduction and subsequent oxidation of meta-carboranes containing bulky groups attached to the cage C atoms affords sterically-crowded ortho-carboranes with unprecedentedly long C-C connectivities.

Unexpectedly facile isomerisation of [7,10-Ph2-7,10-nido-C2B10H10]2- to [7,9-Ph2-7,9-nido-C2B10H10]2-.

The 2e-reduction of 1,12-Ph2-1,12-closo-C(2)B(10)H(10) followed by oxidation or metallation gives products that arise from [7,9-Ph2-7,9-nido-C(2)B(10)H(10)](2-), formed by unexpectedly facile isomerisation of the kinetic 7,10-isomer: the 4,1,6-MC(2)B(10) compounds which result are progressively isomerised to 4,1,8- and 4,1,12-isomers for M = {CpCo} but to an equilibrium mixture of 4,1,8- and 4,1,12-isomers for M = {(arene)Ru}.

Symmetric and asymmetric 13-vertex bimetallacarboranes by polyhedral expansion.

Symmetric 4,5,2,3-M(2)C(2)B(9) 13-vertex bimetallacarboranes of cobalt and ruthenium with 14 skeletal electron pairs are afforded by reduction and metallation of 3,1,2-MC(2)B(9) icosahedra; the symmetric species can be converted to their asymmetric 4,5,1,6-M(2)C(2)B(9) isomers by heat, but an easier route is by thermolysis of the reduced species before metallation.

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.