Homoleptic carbene complexes: Part VIII. Hexacarbene complexes

A modified in situ preparation of the chelating monoanionic tricarbene ligands hydrotris(3-alkyl-imidazoline-2-yliden-1-yl)borate (TRISR, R=Me, Et) is reported along with the syntheses of the hexacarbene iron(III) [Fe(TRISR)2](BF4) (R=Me (3a), Et (3b)) and cobalt(III) complexes [Co(TRISR)2](BF4) (R=Me (4a), Et (4b)) and the usual spectroscopic data (IR, 1H-, 13C- and 11B-NMR, MS (pos-FAB)). The X-ray structure determinations of 3b and 4b reveal their spherical quasi-C6-cage structures of approximate S6 geometry and the absence of any metal to carbon multiple bonding as expected for the combination of high valent metals with electron-rich, doubly heteroatom-stabilized carbenes. In addition, electrochemical studies have been undertaken in order to understand the spontaneous formation of the hexacarbeneiron(III) species 3 from iron(II) starting materials and TRISR. CV and CPC measurements show that both, the reduced Fe(II)- and the oxidized Fe(IV)-complex are accessible and stable, i.e. no chemical decomposition follows the electrochemical reduction and oxidation processes. However, Fe(TRISR)2 appears very sensitive towards any oxidation means. A second approach to hexacarbene metal complexes, viz. that via functional isocyanides — the 2-hydroxyalkylisocyanides CNCH2CH2OH and CNCH2CHMeOH — is described in summary: these isocyanides which can be regarded as ‘masked cyclic N,O-carbenes’ isomerize (cyclisize) in the presence of higher valent metals such as Co(III), Rh(III) and probably also Pt(IV) and Cr(III) to give the complexes [M()6]Cl3·5H2O (M=Co, R=H (5a), Me (5b); M=Rh, R=H (9a), Me (9b)), [M()6](PF6)3 (M=Co, R=H (6a), Me (6b); M=Rh, R=H (10a), Me (10b)), [Co()()5](BPh4)2 (R=H (7a), Me (7b)), [Pt()6](PF6)4 (R=H (11a), Me (11b)) and [Cr()6](PF6)3 (R=H (12a), Me (12b)). The X-ray structure determination of 5a shows a highly symmetric carbene species (3 bar) co-existing in the crystal with lattice water and chloride anions all of which are involved in a complex three-dimensional network of hydrogen bonds.