Stabilization of Pentaphospholes as η⁵ -Coordinating Ligands

Electrophilic functionalisation of [Cp*Fe(η⁵ -P₅ )] (1) yields the first transition-metal complexes of pentaphospholes (cyclo-P₅ R). Silylation of 1 with [(Et₃ Si)₂ (μ-H)][B(C₆ F₅ )₄ ] leads to the ionic species [Cp*Fe(η⁵ -P₅ SiEt₃ )][B(C₆ F₅ )₄ ] (2), whose subsequent reaction with H₂ O yields the parent compound [Cp*Fe(η⁵ -P₅ H)][B(C₆ F₅ )₄ ] (3). The synthesis of a carbon-substituted derivative [Cp*Fe(η⁵ -P₅ Me)][X] ([X]^- =[FB(C₆ F₅ )₃ ]^- (4 a), [B(C₆ F₅ )₄ ]^- (4 b)) is achieved by methylation of 1 employing [Me₃ O][BF₄ ] and B(C₆ F₅ )₃ or a combination of MeOTf and [Li(OEt₂ )₂ ][B(C₆ F₅ )₄ ]. The structural characterisation of these compounds reveals a slight envelope structure for the cyclo-P₅ R ligand. Detailed NMR-spectroscopic studies suggest a highly dynamic behaviour and thus a distinct lability for 2 and 3 in solution. DFT calculations shed light on the electronic structure and bonding situation of this unprecedented class of compounds.