Synthesis and X-ray structure of the [{Fe3(CO)9(µ3-O)}2H]3− trianion: dimerization of a metal carbonyl cluster via formation of an exceptionally short hydrogen bond

Abstract

The synthesis, structure and characterization of the [{Fe₃(CO)₉(µ₃-O)}₂H]³⁻ trianion in its [Cs(THF)_0.33]⁺ and [NEt₄]⁺ salt are reported. The title dimeric cluster has been obtained by protonation in water or in organic solvent of the [Fe₃(CO)₉(µ₃-O)]²⁻ dianion to the hydroxo [Fe₃(CO)₉(µ₃-OH)]⁻ derivative and crystallization. The solid state structure of [Cs(THF)_0.33]₃[{Fe₃(CO)₉(µ₃-O)}₂H] is based on ionic packing of [Cs(THF)_0.33]⁺ cations and [{Fe₃(CO)₉(µ₃-O)}₂H]³⁻ trianions. The fractional formula is due to the particular packing of Cs⁺ cations, which are at the vertices of fused cuboctahedral and trigonal antiprismatic polyhedrons. Each cuboctahedron encapsulates a [{Fe₃(CO)₉(µ₃-O)}₂H]³⁻ trianion, whereas each trigonal antiprism encapsulates a THF molecule. The possibility that the structure of the [{Fe₃(CO)₉(µ₃-O)}₂H]³⁻ trianion could be affected by its confinement in the cuboctahedral cage of Cs⁺ ions and the heavy disorder of the THF molecule urged a further structural determination of the trianion with a completely different cation. The corresponding [NEt₄]₃[{Fe₃(CO)₉(µ₃-O)}₂H] salt has been, therefore, prepared and structurally characterized. The [{Fe₃(CO)₉(µ₃-O)}₂H]³⁻ trianion displays an identical structure and almost coincident molecular parameters in both salts. Its most notable feature is represented by the unique hydrogen atom symmetrically bridging the µ₃-O atoms of two different [Fe₃(CO)₉(µ₃-O)]²⁻ molecules and displaying one of shortest O⋯H⋯O interaction so far reported in organic, inorganic and organometallic literature. The structure of [Cs(THF)]₂[Fe₄(CO)₁₃], which has been obtained as a by-product of the synthesis of [Cs(THF)_0.33]₃[{Fe₃(CO)₉(µ₃-O)}₂H], is also briefly reported.