A series of ferrocenyl and thienyl mono-and biscarbene chromium(0) complexes 1-6 were synthesised.The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH 2 Cl 2 revealed 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 + . A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at E pa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent CrvC species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the CrvC centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

A range of ferrocenyl aryl ethers of type Fc−O−Ar (Fc = Fe(η 5 -C 5 H 5 )(η 5 -C 5 H n X 4−n ); n = 2−4; X = H, P(O)(OEt) 2 , SO 2 CF 3 ; Ar = 2,4-(NO 2 ) 2 -C 6 H 3 , 4-NO 2 -C 6 H 4 ) have been successfully prepared by using the nucleophilic aromatic substitution reaction (S N Ar) of Fc−OLi (1a−Li) and electron-deficient aryl fluorides, enabling a new pathway to this rarely described family of organometallic compounds. Initial studies of 1a−Li and ophosphonato-substituted hydroxyferrocenes (1b−Li) have also been performed, indicating a low nucleophilicity of the oxygen atom. The S N Ar reaction protocol tolerates ortho substituents, e.g. phosphonato and sulfonyl groups, resulting in 1,2-X,O (X = S, P) ferrocenyl ethers that can be obtained in a onepot synthesis procedure including 1,3-O → C anionic phospho-and thia-Fries rearrangements. Within these studies, the first 1,3-diortho-functionalized ferrocenyl aryl ether could be synthesized. An electrochemical study of the redox potentials of the obtained compounds allows conclusions on the reactivity of the varying electronic properties and the electrophilicity of different aryl fluorides and the nucleophilicity of the functionalized hydroxyl ferrocenes, which is directly reflected by the potential of the first ferrocene related redox processes. Chiral-pool-based phosphonates rearrange to the aryl ethers with diastereomeric excesses of 74 and 81% after the anionic Fries rearrangement. The usage of aryl 1,2-/1,3-difluorides resulted in the synthesis of the first examples of bis(FcO)-substituted benzenes. Their electrochemical investigations reveal a splitting of the two redox processes of the ferrocenyls that decreases from 155 mV (1,2-substitution) to 130 mV (1,3-substitution) on the basis of electrostatic interactions.

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.

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