Chemistry of low-valent metal isocyanide complexes. Part 3. The synthesis, structure, and dynamic behaviour of nonakis(ethyl and isopropyl isocyanide)di-iron and -ruthenium complexes. Crystal structure of [Fe2(µ-CNEt)3(CNEt)6]

Abstract

Ultra-violet irradiation of diethyl ether solutions of [Fe(CNR)₅](R = Et or Prⁱ) affords the dinuclear complexes [Fe₂(CNR)₉]. In contrast, sodium amalgam reduction of trans-[RuCl₂(CNPrⁱ)₄] in the presence of an excess of isopropyl isocyanide affords directly [Ru₂(CNPrⁱ)₉]. In order to establish the molecular structure of these dimetal complexes, an X-ray diffraction study was carried out on [Fe₂(CNEt)₉], crystals of which are monoclinic, space group P2₁/n, Z= 4, in a unit cell with lattice parameters a= 22.193(14), b= 12.762(6), c= 12.332(8)Å, and β= 104.60(5)°. The structure has been refined to R 0.08 (R′ 0.11) for 3 062 observed intensities to 2.9 ⩽ 2θ⩽ 50°(Mo-K_αX-radiation) collected at 223 K. The structure is similar to that previously established for [Fe₂(CO)₉], i.e. two iron atoms separated by 2.461 (2)Å are spanned by three isocyanide ligands in bridging modes, while the remaining isocyanide molecules are terminally bound, three to each metal atom. Whereas for the terminal ligands the angles C–N–C range over 165–179°, for the bridging groups the angles C–N–C are 123(1)°. In the bridge system one ligand bends at its nitrogen atom in a direction anti with respect to the other two CNEt groups, with the Fe–C distances syn to the ethyl groups being ca. 0.03 Å longer than those which are anti. Examination of the ¹H and ¹³C n.m.r. spectra of the di-iron and -ruthenium complexes at various temperatures reveals dynamic behaviour in solution involving bridge–terminal isocyanide exchange with ΔG_Tc‡= 63 ± 1, 65 ± 1, and 61 ± 1 kJ mol^–1 for [Fe₂(CNEt)₉], [Fe₂(CNPrⁱ)₉], and [Ru₂(CNPrⁱ)₉] respectively, this process occurring via synchronous pair-wise exchange with inversion at nitrogen. Reaction of [Fe₂(CNEt)₉] with methyl or ethyl iodide affords salts [Fe₂-(CNEt)₇{CNEt(R)}₂]l₂(R = Me or Et), which are formulated as having bridging carbyne ligands. Ultraviolet irradiation of solutions of [Fe(CNBu^t)₅] yields trans-[Fe(CN)₂(CNBu^t)₄], whereas in the presence of cyclo-octatetraene the complex [Fe(CNBu^t)₃(η⁴-C₈H₈)] is produced.