Supraicosahedral indenyl cobaltacarboranes

Abstract

13-vertex indenyl cobaltacarboranes with 4,1,6-, 4,1,10- and 4,1,2-CoC(2)B(10) architectures have been synthesised by reduction of the corresponding closo carborane and metallation with an {(eta-C(9)H(7))Co} fragment. Variants of the 4,1,6-isomer were prepared with no, one and two methyl groups on cage C atoms, whilst 4,1,2-species were obtained both with two methyl groups and a trimethylene tether on the cage C atoms. Thermolysis of the 4,1,6-isomers yielded the corresponding 4,1,8-isomers, which in turn were converted to 4,1,12-isomers by thermolysis at higher temperatures. Alternatively relatively mild heating of the 4,1,10-isomer led to the 4,1,12-isomer directly. Products were characterised by mass spectrometry, (1)H and (11)B NMR spectroscopies and, in most cases, elemental analysis, and nine compounds were studied crystallographically. The 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- species have docosahedral cages whilst the 4,1,2-species are henicosahedral. In the structural studies attention focused on the orientation of the indenyl ligand with respect to the carborane ligand since this affords experimental information on the metal-cage bonding through the structural indenyl effect. There is a general tendency for the indenyl ligand to adopt orientations in which the ring junction C atoms lie trans to cage B atoms. In cases where the orientation is not compromised by the presence of a non-H substituent on the face of the carborane there is generally good agreement between the experimental orientation and that computed by DFT calculations for the related naphthalene ferracarboranes (eta-C(10)H(8))FeC(2)B(10)H(12). The presence of C-methyl substituents in the indenyl cobaltacarboranes tends to override this preference except in the case of 1,6-Me(2)-4-(eta-C(9)H(7))-4,1,6-closo-CoC(2)B(10)H(10) where the indenyl ligand instead is forced to incline away from the cage methyl groups. In DCM solution the 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- isomers of (eta-C(9)H(7))CoC(2)B(10)H(12) exhibit two, stepwise, 1-electron reductions assigned to Co(III)/Co(II)/Co(I) couples at less negative potentials than those of the corresponding Cp compounds. Moreover these reductions are easier for those isomers (4,1,6- and 4,1,10-) in which there are two cage C atoms in the carborane face to which the metal atom is bound. By spectroelectrochemical and EPR measurements it is concluded that the reductions of these indenyl cobaltacarboranes are largely metal-based.

The synthesis and characterisation of homo- and heterobimetallic 1,14,2,9- and 1,14,2,10-M2C2B1014-vertex metallacarboranes

“…At first this implication that methyl groups are electron-withdrawing seems anomalous, but in fact such a situation has already been clearly established for heteroborane clusters by Viñas, Teixidor and coworkers. 20 Note that the effect of dimethylation (at cage C) on <δ 11 B> here is somewhat larger than that seen in 4,1,6-, 4,1,8and 4,1,12-13-vertex cobaltacarboranes with both Cp 16 and indenyl 17 exopolyhedral ligands, 1.8-3.4 ppm. Finally we note…”

“…16 The indenyl species 4-(η-C 9 H 7 )-4,1,8-closo-CoC 2 B 10 H 12 is available by a 73% conversion (refluxing THF) of 4-(η-C 9 H 7 )-4,1,6-closo-CoC 2 B 10 H 12 which is afforded in 47% yield from 1,2-closo-C 2 B 10 H 12 . 17 The three precursors 4-Cp-4,1,8-closo-CoC 2 B 10 H 12 , 1,8-Me 2 -4-Cp-4,1,8-closo-CoC 2 B 10 H 10 and 4-(η-C 9 H 7 )-4,1,8-closo-CoC 2 B 10 H 12 were each reduced, treated with CoCl 2 /NaCp, CoCl 2 /NaCp and CoCl 2 /Li[C 9 H 7 ], respectively, and finally aerially oxidised to afford the 1,14,2,9-Co 2 C 2 B 10 species 1,14-Cp 2 -1,14,2,9-closo-Co 2 C 2 B 10 H 12 (3), 1,14-Cp 2 -2,9-Me 2 -1,14,2,9-closo-Co 2 C 2 B 10 H 10 (4) and 1,14-(η-C 9 H 7 ) 2 -1,14,2,9-closo-Co 2 C 2 B 10 H 12 (5), respectively, in all cases isolated in low yields from complex mixtures of products.…”

Untethered 4,1,2-MC2B10 supraicosahedral metallacarboranes, their C,C′-dimethyl 4,1,6-, 4,1,8- and 4,1,12-MC2B10 analogues, and DFT study of the (4,)1,2- to (4,)1,6-isomerisations of C2B11 carboranes and MC2B10 metallacarboranes

“…This complements our earlier report of the related species 1,2-Me 2 -4-(η-C 9 H 7 )-4,1,2-closo-CoC 2 B 10 H 10 obtained by reduction of 1,2-μ-(CH 2 SiMe 2 CH 2 )-1,2-closo-C 2 B 10 H 10 and metallation with a source of {(η-C 9 H 7 )Co} 2+ . 12 We also report C,C′-…”

Synthesis and Characterisation of Sigma‐ and Pi‐Bonded Metallaphosphacarboranes

“…In targeting compound 1 for synthesis and structural study we deliberately chose to incorporate the indenyl ligand to probe the relative structural trans effect (STE) 18 (trans influence) of the two types of cage heteroatom, C and P. The factors that influence the orientational preference of the indenyl ligand are concisely explained by the structural indenyl effect 19 and by the fact that the frontier molecular orbitals of nido heteroborane ligands are localised predominantly on the boron atoms of the open face 20. Using these principles we have previously established that boron has a stronger STE than either carbon 15,19 or phosphorus 4. In 1 we have both carbon and phosphorus heteroatoms, so the indenyl orientation may provide information on the relative STEs of these two atoms.…”