The Photochemical Mediated Ring Contraction of 4H-1,2,6-Thiadiazines To Afford 1,2,5-Thiadiazol-3(2H)-one 1-Oxides.

1,2,6-Thiadiazines treated with visible light and 3O2 under ambient conditions are converted into difficult-to-access 1,2,5-thiadiazole 1-oxides (35 examples, yields of 39-100%). Experimental and theoretical studies reveal that 1,2,6-thiadiazines act as triplet photosensitizers that produce 1O2 and then undergo a chemoselective [3 + 2] cycloaddition to give an endoperoxide that ring contracts with selective carbon atom excision and complete atom economy. The reaction was optimized under both batch and continuous-flow conditions and is also efficient in green solvents.

Regioselective electrophilic aromatic borylation as a method for synthesising sterically hindered benzothiadiazole fluorophores.

Regioselective stepwise phenylation of 4,7-diarylbenzo[c][1,2,5]thiadiazole fluorophores has been achieved through a facile one-pot, three-step synthetic strategy involving sequential borylation, hydroxydechlorination and Suzuki-Miyaura cross-coupling reactions. Crucial to the selectivity was the use of BCl3 to regioselectively install a boronic acid group in the ortho-position of only one of the diaryl groups. The subsequent introduction of ortho-phenyl groups through Suzuki-Miyaura cross-coupling gave rise to twisted structures with hindered intramolecular rotation, providing a structural lever with which the fluorophore absorption and emission properties could be adjusted.

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.

Amino acid-derived bisphenolate palladium complexes as C-C coupling catalysts.

A series of amine bisphenol (ABP) pro-ligands featuring amino acid ester pendant arms were prepared. Optimisation of the synthetic method allowed the facile incorporation of naturally occurring, chiral amino acids into the ABP scaffold with minimal racemisation. Reaction of the pro-ligands (LH2) with Pd(OAc)2, in the presence of amines, led to the formation of complexes with an unprecedented pincer-like O,N,O coordination mode around the PdII centre. The complexations in the presence of trialkylamines (NR3) afforded a mixture of LPdNR3 and LPdNHR2 species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc)2(NHR2)2 intermediates. Using pyridine as base eliminated this dealkylation and resulted in the exclusive formation of LPd(py) complexes in high yields. In total, seven novel PdII ABP complexes were prepared, exhibiting distorted square-planar geometries with the asymmetric ligand moieties orientated towards the metal centre. The air- and moisture-stable LPd(py) complexes were successfully employed as catalysts in two types of C-C coupling reactions. The Suzuki-Miyaura coupling of 4'-bromoacetophenone and phenylboronic acid reached high yields (up to 81%), while a scope of further alkyl bromides was also efficiently converted using low catalyst loadings (1 mol%) and mild temperatures (40 °C). Furthermore, a Pd-pyridine complex achieved high activity in the Mizoroki-Heck coupling of styrene and 4'-bromoacetophenone.

Crystal structure and Hirshfeld surface analysis of one-dimensional copper(II) coordination polymer incorporating succinate and tetra-methyl-ethylene-diamine ligands.

The reaction of copper nitrate with succinic acid (succH) and N,N,N',N'-tetra-methyl-ethylenedi-amine (TMEDA) in basic solution produces the complex catena-poly[[[(N,N,N',N'-tetra-methyl-ethylenedi-amine-κ2 N,N')copper(II)]-µ-succinato-κ2 O 1:O 4] tetra-hydrate], {[Cu(C4H4O4)(C6H16N2)]·4H2O} n or {[Cu(succ)(tmeda)]·4H2O} n . Each carboxyl-ate group of the succinate ligand coordinates to a CuII atom in a monodentate fashion, giving rise to a distorted square-planar geometry. The succinate ligands bridge the CuII centres, forming one-dimensional polymeric chains. Hydrogen bonds between the ligands and water mol-ecules link these chains into sheets that lie parallel to the ac plane. Hirshfeld surface analysis, d norm and two-dimensional fingerprint plots were examined to verify the contributions of the different inter-molecular contacts within the supra-molecular structure.

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.

Crystal structure and Hirshfeld surface analysis of a copper(II) complex with ethyl-enedi-amine and non-coordinated benzoate.

In the title compound, di-aqua-bis-(ethyl-enedi-amine-κ2 N,N')copper(II) bis-(2-nitro-benzoate), [Cu(C2H8N2)2(H2O)2](C7H4NO4)2, two di-aqua-bis-(ethyl-enedi-amine)-copper(II) cations and four nitro-benzoate anions are present in the asymmetric unit. All four anions are 'whole-mol-ecule' disordered over two sets of sites. The major components have refined occupancies of 0.572 (13), 0.591 (9), 0.601 (9) and 0.794 (10). The CuII ions exhibit slightly distorted octa-hedral geometries. In the crystal, cations and anions are connected to each other via N-H⋯O and O-H⋯O hydrogen bonds, forming a two-dimensional network parallel to (200). The inter-molecular contacts in the crystal were further analysed using Hirshfeld surface analysis, which indicates that the most significant contacts are O⋯H/H⋯O (42.9%), followed by H⋯H (35.7%), C⋯H/H⋯C (14.2%), C⋯C (2.9%), C⋯O/O⋯C (2.2%), N⋯H/H⋯N (0.9%) and N⋯O/O⋯N (0.3%).

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.

Crystal structure and Hirshfeld surface analysis of a zinc xanthate complex containing the 2,2'-bi-pyridine ligand.

In the title compound, (2,2'-bi-pyridine-κ2 N,N')bis-(2-meth-oxy-ethyl xanthato-κS)zinc(II), [Zn(C4H7O2S2)2(C10H8N2)], the ZnII ion is coordinated to two N atoms of the 2,2'-bi-pyridine ligand and two S atoms from two 2-meth-oxy-ethyl xanthate ligands. The ZnII ion lies on a crystallographic twofold rotation axis and has distorted tetra-hedral coordination geometry. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds, forming supramolecular chains propagating along the a-axis direction. Weak intra-molecular C-H⋯S hydrogen bonds are also observed. The inter-molecular contacts in the crystal were further analysed using Hirshfield surface analysis, which indicates that the most significant contacts are H⋯H (36.3%), followed by S⋯H/H⋯S (24.7%), C⋯H/H⋯C (15.1%), O⋯H/H⋯O (14.4%), N⋯H/H⋯N (4.1%) and C⋯C (2.9%).

Synthesis and optoelectronic properties of benzoquinone-based donor-acceptor compounds.

Herein, we report a mild and efficient palladium-catalyzed C-H functionalization method to synthesize a series of benzoquinone (BQ)-based charge-transfer (CT) derivatives in good yields. The optoelectronic properties of these compounds were explored both theoretically and experimentally and correlations to their structures were identified as a function of the nature and position of the donor group (meta and para) attached to the benzoquinone acceptor. Compound 3, where benzoquinone is para-conjugated to the diphenylamine donor group, exhibited thermally activated delayed fluorescence (TADF) with a biexponential lifetime characterized by a prompt ns component and a delayed component of 353 µs.

Crystal structure and DFT study of a zinc xanthate complex.

In the title compound, bis-(2-meth-oxy-ethyl xanthato-κS)(N,N,N',N'-tetra-methyl-ethylenedi-amine-κ2 N,N')zinc(II) acetone hemisolvate, [Zn(C4H7O2S2)2(C6H16N2)]·0.5C3H6O, the ZnII ion is coordinated by two N atoms of the N,N,N',N'-tetra-methyl-ethylenedi-amine ligand and two S atoms from two 2-meth-oxy-ethyl xanthate ligands. The amine ligand is disordered over two orientations and was modelled with refined occupancies of 0.538 (6) and 0.462 (6). The mol-ecular structure features two C-H⋯O and two C-H⋯S intra-molecular inter-actions. In the crystal, mol-ecules are linked by weak C-H⋯O and C-H⋯S hydrogen bonds, forming a three-dimensional supra-molecular architecture. The mol-ecular structure was optimized using density functional theory (DFT) at the B3LYP/6-311 G(d,p) level. The smallest HOMO-LUMO energy gap (3.19 eV) indicates the suitability of this crystal for optoelectronic applications. The mol-ecular electrostatic potential (MEP) further identifies the positive, negative and neutral electrostatic potential regions of the mol-ecules. Half a mol-ecule of disordered acetone was removed with the solvent-mask procedure in OLEX2 [Dolomanov et al. (2009 ▸). J. Appl. Cryst. 42, 339-341] and this contribition is included in the formula.

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).

Effect of temperature and ligand protonation on the electronic ground state in Cu(ii) polymers having unusual secondary interactions: a magnetic and catechol oxidase study.

Two new copper(ii) polymeric complexes, {[Cu(HPymat)(H2O)](NO3)}n (1) and [Cu2(Pymat)2(H2O)3]n (2), have been synthesized using the Schiff base ligand H2Pymat [H2Pymat = (E)-2-(1-(pyridin-2-yl)-methyleneamino)terephthalic acid]. Complex 1 is a cationic 1D polymer, whereas complex 2 is a two dimensional polymer. Both complexes were crystallographically, spectroscopically and magnetically characterized. Theoretical studies were performed and the catecholase activity of the complexes was also examined. Complex 1 is a ferromagnetically coupled complex with J = 2.8 cm-1 and 2 shows antiferromagnetic coupling with J = -1.6 cm-1. Both complexes show notable features in the EPR study. They show rhombic spectra at 77 K in the solid state, but by varying the temperature or solvents the nature of the spectra can be changed or inverted. This behaviour indicates a change of the ground state from dx2-y2 to dz2 orbitals. Theoretical calculations of 1 focus on the evaluation and characterization of interesting anion-π-anion assemblies that are formed in the solid state. In 2 we have analysed the unconventional chelate ringchelate ring π-stacking interactions that govern its solid state architecture. Both complexes act as functional models and show catechol oxidase activity with a kcat value of the order of 103 h-1.

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.

Gold(I)-Catalysed Hydroarylation of 1,3-Disubstituted Allenes with Efficient Axial-to-Point Chirality Transfer.

Hydroarylation of enantioenriched 1,3-disubstituted allenes has the potential to proceed with axial-to-point chirality transfer to yield enantioenriched allylated (hetero)aryl compounds. However, the gold-catalysed intermolecular reaction was previously reported to occur with no chirality transfer owing to competing allene racemisation. Herein, we describe the development of the first intermolecular hydroarylations of allenes to proceed with efficient chirality transfer and summarise some of the key criteria for achieving high regio- and stereoselectivity.

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.