Characterisation of a new solid-state isomer of the [Fe5N(CO)14]– ion

Abstract

An example of cation-induced solid-state isomerisation in a carbonyl cluster has been found. Whereas the structure of the [Fe₅N(CO)₁₄]^– cluster anion contains five different types of Fe(CO)_n sites when prepared as the [N(PPh₃)₂]⁺ salt, the [NMe₄]⁺ salt crystallises with two symmetry-related bridging carbonyls on adjacent iron–iron bonds in the basal plane and contains four different iron sites: monoclinic, space group P2₁/m, a= 9.973(2), b= 11.498(2), c= 12.401(2)Å, β= 113.53(1)° and Z= 2. The structure was refined on the basis of 2441 observed reflections, converging to a final R value of 0.029. The molecule has a plane of symmetry through the apical iron site and both the unbridged and the doubly bridged iron sites in the basal plane. Analysis of the Mössbauer spectra of the [N(PPh₃)₂]⁺, [NEt₄]⁺ and [NMe₄]⁺ salts indicates that the overall arrangement of the carbonyl ligands is different in each case. The electrochemistry of the [Fe₅N(CO)₁₄]^– cluster anion in acetonitrile, tetrahydrofuran and dichloromethane solutions was examined. It is shown that, independent of the cation, the anion undergoes two one-electron reductions. The first cathodic step leads to the relatively stable dianion [Fe₅N(CO)₁₄]^2–, whereas the second step initiates cluster degradation.