Substituent Effects on the Electrochemical, Spectroscopic, and Structural Properties of Fischer Mono- and Biscarbene Complexes of Chromium(0)

Abstract

Supporting Information PlaceholderA series of ten ferrocenyl, furyl and thienyl mono-and biscarbene chromium(0) complexes were synthesized and characterized spectroscopically and electrochemically. The single crystal structure of the biscarbene complex [(CO) 5 Cr=C(OEt)-Fu'-(OEt)C=Cr(CO) 5 ] (4a) was determined: C 20 H 12 Cr 2 O 13 ; triclinic; P1; a = 6.2838(5), b = 12.6526(9), c = 29.1888(19) Å, α = 89.575(2), β = 88.030(2), γ = 87.423(2)°; Z = 4. Results from an electrochemical study in CH 2 Cl 2 were mutually consistent with a computational study in showing that the carbene double bond of 1 -6 is reduced to an anion radical, -Cr-C• at formal reduction potentials < -1.7 V vs. FcH/FcH + . The Cr centers are oxidized in two successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. Only Cr(I) oxidation is electrochemically irreversible. Dicationic Cr(II) species formed upon two consecutive oneelectron oxidation processes are characterized by a peculiar bonding situation as they are stabilized by genuine CH···Cr agostic interactions. With respect to aryl substituents, carbene redox processes occurred at the lowest potentials for ferrocene derivatives followed by furan complexes. Redox process in the thiophene derivatives occurred at the highest potentials. This result is mutually consistent with a 13 C NMR study that showed the Cr=C functionality of furyl complexes were more shielded than thienyl complexes. The NHBu carbene substituent resulted in carbene complexes showing redox processes at substantially lower redox potentials than carbenes having OEt substituents.

Electrochemical and Computational Study of Tungsten(0) Ferrocene Complexes: Observation of the Mono-Oxidized Tungsten(0) Ferrocenium Species and Intramolecular Electronic Interactions

“…In contrast to our findings that the Cr I/0 and Cr II/I redox couples in Fischer carbene complexes are at potentials that may differ as much as 0.75 V, 10,11 tungsten(0) carbene complexes were reported to be oxidized to tungsten(II) carbene complexes either by a single two-electron transfer process Scheme 1. Synthesis of ferrocenyl mono-and biscarbene tungsten complexes; Reagents and conditions: (a) …”

“…This observation also suggests some form of electronic interaction between two carbene moieties observed for 2 but completely contrasts the observations described for the chromium complex [(CO) 5 Cr=C(OEt)-Fc¢-(OEt)C=Cr(CO) 5 ]. 10,11 For this analogous Cr carbene complex, under the same experimental conditions, no splitting of Cr=C carbene reduction into two components "a" and "b" could be detected. Very few studies describe the electrochemical reduction of the W=C double bond, 14 although the in situ generation of the radical anion by chemical single electron transfer agents have been reported.…”

Electrochemistry of Fischer alkoxycarbene complexes of chromium: The use of density functional theory to predict and understand oxidation and reduction potentials

“…It is clear that the reduction centre is mainly localized on the carbene ligand. This interpretation is in agreement with literature [11] and ESR experiments on [(CO) 5 Cr=C(OEt)Ph], where addition of an electron forms a carboncentred radical with the odd electron localized on the carbene ligand [47].…”

“…Comparing peak currents and LSV data, a one electron reduction process is thus found for the chromium ethoxycarbene complexes of this study, reducing the neutral carbene complex to a radical anion species. A one electron reduction process was also found for the chromium ethoxycarbene and aminocarbene complexes in literature in DCM as solvent [11], while a series of chromium aminocarbene complexes showed a two electron reduction process utilizing the same solvent/electrolyte system as this study but a hanging mercury drop electrode (HMDE) [7,8,9,10] for CV measurements. (η is viscosity and r is the radius of the spherical particle).…”

Fac and mer dppe-substituted Fischer carbene complexes of chromium: X-ray, DFT and electrochemical study

“…the dppe-containing complexes [(CO) 3 [45,46,47,47,52]. Since the oxidation centre for all these Fischer Cr-carbene complexes is mainly located on the Cr-metal centre, it is expected that the electronic influence of the dppe ligand directly attached to the Cr atom will be more pronounced than the influence of the different substituents on the carbene ligand.…”