Reactive Sigma-Aryliron Complexes or Iron-Promoted Coupling of Two Phenyl Anions to One Bis(cyclohexadienylidene) Ligand: Synthesis, Structure, Mass Spectrometry, and DFT Calculations

A reaction of the bulky alkylcyclopentadienyliron(II) high-spin complex [Cp′′′Fe(μ-Br)]₂ (1a) (Cp′′′ = C₅H₂(CMe₃)₃-1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex [{Cp′′′Fe}₂(μ,η⁵:η⁵-H₅C₆C₆H₅)] (2) with a bridging bis(cyclohexadienylidene) ligand. Its structural analysis shows a centrosymmetric dimer. Each tri(tert-butyl)cyclopentadienyliron fragment is η⁵-coordinated to a cyclohexadienylidene moiety in which one carbon atom is bent out of the plane by 0.39 Å, exhibiting a bond length of 1.370 Å to its symmetry equivalent. Electrospray ionization mass spectra (ESI-MS) from acetonitrile solution confirm nicely the elemental composition of 2 by way of their isotope patterns. Reaction of 1a or its tetraisopropylcyclopentadienyl analogue [⁴CpFe(μ-Br)]₂ (1b) (⁴CpC₅H(CHMe₂)₄) with 2,6-diisopropylphenylmagnesium bromide affords the extremely air-sensitive, paramagnetic σ-aryl complexes [Cp′′′Fe(C₆H₃ⁱPr₂)] (3a) or [⁴CpFe(C₆H₃ⁱPr₂)] (3b), whose ⁴Cp−Fe distance of 1.92 Å is typical for cyclopentadienyliron high-spin complexes. In reactions with copper(I) halides 3a is rearranged to a diamagnetic π complex and coordinated via the ipso carbon atom of the six-membered ring to copper(I) halide fragments to form heterodinuclear complexes [Cp′′′Fe(μ,η⁵:η¹-C₆H₃ⁱPr₂)CuCl] (4-Cl) and [Cp′′′Fe(μ,η⁵:η¹-C₆H₃ⁱPr₂)CuBr] (4-Br). ESI mass spectra of complexes 4 do not show the molecular cations, but fragmentation to cyclopentadienyliron arene cations and formation of the hexa(tert-butyl)ferrocenium cation on one hand and fusion of complex fragments to oligonuclear complexes with or without inclusion of oxygen or fragments of solvent molecules on the other hand. Three of these oligonuclear complexes formed under the conditions of the ESI-MS experiment, whose elemental composition could be derived from isotope patterns, have been interpreted as [Cp′′′Fe(μ,η⁵:η¹-C₆H₃ⁱPr₂)Cu(μ,η¹:η⁵-OC₆H₃ⁱPr₂)FeCp′′′]⁺ and [{Cp′′′Fe(μ,η⁵:η¹-C₆H₃ⁱPr₂)Cu}₂X]⁺ (X = Cl, Br). DFT calculations support the structural analysis of 2 and predict the structure of the dication 2²⁺. The crystal structures obtained by X-ray diffraction for 2 and 3b are reported.