Issue 22, 2010

Supraicosahedral indenyl cobaltacarboranes


13-vertex indenyl cobaltacarboranes with 4,1,6-, 4,1,10- and 4,1,2-CoC2B10 architectures have been synthesised by reduction of the corresponding closo carborane and metallation with an {(η-C9H7)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, 1H and 11B 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 (η-C10H8)FeC2B10H12. The presence of C-methyl substituents in the indenyl cobaltacarboranes tends to override this preference except in the case of 1,6-Me2-4-(η-C9H7)-4,1,6-closo-CoC2B10H10 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 (η-C9H7)CoC2B10H12 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.

Graphical abstract: Supraicosahedral indenyl cobaltacarboranes

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Article information

Article type
15 Feb 2010
09 Apr 2010
First published
04 May 2010

Dalton Trans., 2010,39, 5286-5300

Supraicosahedral indenyl cobaltacarboranes

G. Scott, A. McAnaw, D. McKay, A. S. F. Boyd, D. Ellis, G. M. Rosair, S. A. Macgregor, A. J. Welch, F. Laschi, F. Rossi and P. Zanello, Dalton Trans., 2010, 39, 5286 DOI: 10.1039/C003067H

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