Multimetal Fischer carbene complexes of Group VI transition metals: synthesis, structure and substituent effect investigation

Abstract

Fischer carbene complexes of tungsten with substituents containing up to two additional different transition metals, with all the metals in electronic contact with the carbene carbon atom, were synthesised and studied both in solution and in the solid state. For the complexes of the type [W(CO) 5 {C(OR¢)R}], the substituents chosen were heteroaromatic 2-benzo[b]thienyl (2-BT), or 2-BT p-bonded to a chromium tricarbonyl fragment ([Cr(CO) 3 (2-h 6 -BT)]) or ferrocenyl (Fc) as the R-substituent, while the OR¢-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1b (R¢ = Et, R = 2-BT), 2b (R¢ = Et, R = [Cr(CO) 3 (2-h 6 -BT)]), 3b (R¢ = TiCp 2 Cl, R = 21-BT), 4b (R¢ = TiCp 2 Cl, R = [Cr(CO) 3 (2-h 6 -BT)]), 5b (R¢ = Et, R = Fc) and 6b (R¢ = TiCp 2 Cl, R = Fc). The structural features and their relevance to bonding in the multimetal carbene compounds of both these tungsten and the analogous chromium complexes were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry.

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…In all cases, only one reduction and only one oxidation were observed (so that both metal atoms participate in one molecular orbital) although there are two redox active metal atoms in the molecule [139]. This was in direct contrast to the recent electrochemical results obtained where heterodimetallic carbene complexes with a π-aryl metal carbene substituent showed that the two metal moieties in each complex function as separate, localized redox centres [140]. This implies that a direct comparison of substituent effects on the pentacarbonyl metal can be made.…”

“…Structure data suggest a higher contribution of the acyl resonance structure in Fig. 43 to the bonding than in the alkoxycarbene complexes, with shorter M´-O bond lengths [140,151,159,170,186,187].…”

“…the "carbonyl wall" [18,210] and thus the alkyl group is orientated towards the carbonyls and normally nestles in a staggered position between two carbonyls (see, for example: [65][66][67]140]). This orientation may lead to an interaction between the alkoxy oxygen atom and the hetero atom (Y) of a hetero-aryl ring substituent.…”

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes

“…3.34 μA), and since the ferrocenyl couple represents a one-electron transfer process, wave I must also represent a one-electron reduction process to initially generate − Cr-C˙, not only in 1, but also in the other carbene complexes of this study. This assignment is possible because it is known that the LUMO is mainly carbene-ligand based, 17 and single electron transfer reactions followed by ESI-MS have also indicated the formation of such − Cr-C˙species. 36 Since the orbitals associated with carbenes are delocalized over metal, carbon, and heteroatom, one can envisage the − Cr-C˙species as being stabilised by distributing charge and radical all over the ligand system.…”

Redox Behaviour of Cymantrene Fischer Carbene Complexes in Designing Organometallic Multi‐tags

“…[13] Fine-tuning of the steric and electronic properties of the carbene moiety can be effected by modulation of the carbene substituents (X = heteroatom substituent, R = aryl substituent). [14] The introduction of a metal-containing substituent, such as ferrocenyl, opens the door for the design of organometallic multi-tag complexes. To this end, mono-and biscarbene ferrocenyl complexes were also synthesized.…”