Structural Rearrangements in Triple-Decker-Like Complexes with Mixed Group 15/16 Ligands: Synthesis and Characterization of the Redox Couple [Cp2Fe2As2Se2]/[Cp2Fe2As2Se2]+ (Cp=C5Me5)

• Published in: Chemistry : a European journal Vol. 7; no. 6; pp. 1342 - 1349
• Main Authors: Blacque, Olivier, Brunner, Henri, M. Kubicki, Marek, Leis, Franz, Lucas, Dominique, Mugnier, Yves, Nuber, Bernhard, Wachter, Joachim
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag GmbH 16.03.2001; WILEY‐VCH Verlag GmbH
• Subjects: arsenic; iron; selenium; structure elucidation
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20010316)7:6<1342::AID-CHEM1342>3.0.CO;2-D

The reaction of As4Se4 with stoichiometric amounts of [Cp$\rm{^{{\ast}}_{2}}$Fe2(CO)4] (Cp*=C5Me5) in boiling toluene forms [Cp$\rm{^{{\ast}}_{2}}$Fe2As2Se2] (1) in good yield. X‐ray crystallography shows 1 to have a triple‐decker structure which comprises a tetraatomic μ,η4:4‐As2Se2 ligand. Density functional theory (DFT) and extended Hückel molecular orbital (EHMO) calculations confirm that the As2Se2 ligand behaves as a four‐electron π donor. Oxidation of 1 with equimolar amounts of [(C5H5)2Fe]PF6, Br2 and I2, respectively, gave compounds 2–4. According to X‐ray crystallographic investigations that were carried out on 2 and 4, the oxidation state has a considerable influence on the structure of the Fe2As2Se2 core: significant shortening of the Fe−Fe distance (Δd(Fe−Fe)>0.3 Å) and weakening of the As−As bond length ((Δd(As−As)>0.3 Å) suggests the formal presence of two diatomic AsSe ligands and a Fe−Fe bond. DFT and EHMO calculations confirm that an electron is removed from an occupied Fe−Fe orbital of antibonding character during oxidation. All molecular orbitals lower their energies upon oxidation, but the energy drop is relatively small for those involving the As−As bond. An additional structural feature in 4 consists of an electronic interaction of the iodide with both As atoms which suggests a formally neutral ion pair. Electrochemical studies confirm that the oxidation of 1 is a reversible one‐electron process with E1/2=+0.07 V (in THF). These studies also reveal that 4 dissociates in polar solvents, such as THF, into [1]+ and I−, which is followed by transformation into 1 and I3. The reversible dimerization of diatomic AsSe ligands is observed in the redox couple [Cp$\rm{^{{\ast}}_{2}}$Fe2As2Se2]/[Cp$\rm{^{{\ast}}_{2}}$Fe2As2Se2]+ (1/[1]+; see scheme). Complex 1, which is obtained in good yield from [Cp$\rm{^{{\ast}}_{2}}$Fe2(CO)4] and As4Se4, has a triple‐decker structure comprising the novel tetraatomic μ,η4:4‐As2Se2 ligand. Chemical or electrochemical oxidation of 1 effects considerable structural changes in the inorganic core of the molecule: significant shortening of the Fe−Fe distance and weakening of the As−As bond in the oxidation product [1]+ suggest formation of two diatomic AsSe ligands and of a Fe−Fe bond.