The electrochemical activity of heteroatom-stabilized Fischer-type carbene complexes

Electronic effect of ligands vs. reduction potentials of Fischer carbene complexes of chromium: a molecular electrostatic potential analysis

Molecular electrostatic potential at the chromium centre (V_Cr) emerges as a powerful predictor of reduction potential (E⁰).

Triphenylstibine-substituted Fischer carbene complexes of tungsten(0): synthesis, structure, DFT and electrochemistry

Structure and electrochemical behaviour of triphenylstibine-substituted W(0) Fischer ethoxycarbene complexes containing a heteroaryl-ring as a carbene substituent.

An investigation into the photochemistry of, and the electrochemically induced CO-loss from, [(CO)5MC(OMe)Me](M = Cr or W) using low-temperature matrix isolation, picosecond infrared spectroscopy, cyclic voltammetry, and time-dependent density functional theory

The photophysics and photochemistry of [(CO)5MC(OMe)Me] (M = Cr or W) were investigated using picosecond time-resolved infrared spectroscopy (M = Cr or W), low-temperature matrix isolation techniques (M = Cr), and time-dependent density functional calculations (M = Cr or W). These studies provide unambiguous evidence for the photochemical formation of a long-lived, 18-electron metallaketene species capable of acting as a synthetically useful intermediate. For the Cr complex, an intermediate metallacyclopropanone singlet excited state was detected on the reaction path to the metallaketene species. This metallacyclopropanone excited state species has a lifetime of less than 100 ps and a characteristic bridging carbonyl band at 1770 cm(-1). The tungsten ketene species was also detected but in contrast to the chromium system, this forms directly from a low-lying triplet excited state. The electrochemical release of CO showed a greater efficiency for the chromium complex when compared to the tungsten.

Nucleophilic substitution in ionizable Fischer thiocarbene complexes: steric effect of the alkyl substituent on the heteroatom

A detailed kinetic study has been carried out for the aminolysis of ionizable Fischer thiocarbene complexes (CO)5M[double bond, length as m-dash]C(SR)CH3 (M = Cr, W; R = iPr, nBu, cHex, tBu) with five primary amines and one secondary amine in aqueous acetonitrile solutions (50% MeCN-50% water (v/v)). The observed rate constants for the reaction with primary amines showed a first-order dependence on the amine concentration, while with morpholine, the rate constant has second-order dependence. The general base catalysis process was confirmed by the variation of the rate constants with the concentration of an external catalyst and the pH. The results agree with a stepwise mechanism where the nucleophilic addition to the carbene carbon to produce a tetrahedral intermediate (T±) is the first step, followed by a rapid deprotonation of to form the anion T- which leads to the products by general-acid catalysed leaving group (-SR) expulsion. In general, it was found that the chromium complexes are less reactive than the tungsten analogues. The obtained Brønsted parameters for the nucleophilic addition (βnuc) indicate that C-N bond formation has made little progress at the transition state. By using Charton's correlation, the role that the steric factor plays throughout the mechanism has been unraveled. The nucleophilic addition to the thiocarbenes is less sensitive to steric effects than the alkoxycarbenes regardless of the nature of the metal centre. Conversely, the steric effects on the general-base catalysis can be strong depending on the volume of the catalyst and the metal centre. On the basis of the structure-reactivity coefficients β and ψ and comparison with alkoxycarbene complexes, esters and thiolesters, insights into the main factors ruling the reactivity in terms of transition state imbalances are discussed.

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries

Fifty years after their introduction, Fischer-type carbene complexes still enthral synthetic and theoretical chemists interested in their preparation and characterization.

Electronic excitations in Fischer-type Cr and W aminocarbene complexes: a combined ab initio and experimental study

The influence of the substitution on the carbene ligand in the series of Fischer-type Cr and W aminocarbene complexes was studied experimentally by UV-vis spectroscopy and theoretically by comparative ab initio SA-CASSCF/MS-CASPT2 and TD-DFT methods. Both calculations interpreted the experimental UV-vis spectra and their variations caused by substitution effects well. TD-DFT analysis of individual transitions using electron density redistributions indicated that the variation of the absorption spectra due to substitution is accompanied by a change in the character of the low-lying excited states participating in the visible bands. Correlated MS-CASPT2 calculations confirmed the TD-DFT assignments of the lowest-lying transitions in the visible region almost quantitatively.