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, H-1-, C-13- and B-11-NMR, MS (pos-FAB)). The X-ray structure determinations of 3b and 4b reveal their spherical quasi-C-6-cage structures of approximate Sg 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(CN(H)CH,CHRO)(6)]Cl-3. 5H(2)O (M = Co, R = H (5a), Me (5b); M = Rh, R = H (9a), Me (9b)), [M(CN(H)CH2CHRO)(6)](PF6)(3) (M = Co, R = H (6a), Me (6b); M = Rh, R = H (10a), Me (10b)), [Co(C=NCH2CHRO)(CN(H)CH2CHRO)(5)](BPh4)(2) (R = H (7a), Me (7b)), [Pt(CN(H)CH2CHRO)(6)](PF6)(4) (R= H (11a), Me (11b)) and [Cr(CN(H)CH2CHRO)(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. (C) 2001 Elsevier Science B.V. All rights reserved.