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

A 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)₅CrC­(OEt)-Fu′-(OEt)­CCr­(CO)₅] (4a) was determined: C₂₀H₁₂Cr₂O₁₃; triclinic; P1̅; 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 CH₂Cl₂ 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 one-electron 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 ¹³C 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.