Borabenzene derivatives. 20. (Boratabenzene) (hexamethylbenzene)iron complexes: Synthesis, structure, and reactivity

Article abstract of DOI:10.1021/om00031a040

The 19e complex CpFe(HMB) (HMB = hexamethylbenzene) (1a) reacts with dihaloboranes RBX₂ (R = Me, Ph, NEt₂; X = Cl, Br) to give paramagnetic 19e (boratabenzene)-(hexamethylbenzene) iron complexes (C₅H₅BR)Fe(HMB) (R = Me, Ph, NEt₂) (2a-c). Oxidation with [FeCp₂]PF₆ affords the corresponding diamagnetic salts [(C₅H₅BR)Fe(HMB)]PF₆ (2a-c·PF₆), while oxidation of the diethylamino derivative 2c with FeCl₃ in Et₂O/H₂O produces the hydroxo compound [(C₅H₅BOH)Fe(HMB)]PF₆ (2d·PF₆). The structures of both the neutral methyl compound 2a and the corresponding salt 2a·PF₆ have been determined by X-ray crystallography. A comparison of the two structures demonstrates the labilizing effect of the excess electron. Both structures show distortions of the ring ligands which can be explained by qualitative MO considerations. The arene ligand possesses a boat conformation in 2a and an inverted boat conformation in 2a⁺. Cyclovoltammetric investigation of the cations [(C₅H₅-BR)Fe(HMB)]⁺ (2a-c⁺) shows a reversible (+/0) and a quasireversible (0/-) reduction wave; compared to CpFe(HMB) these waves are shifted anodically. The phenyl derivative 2b reacts with MeI to form the iodide 2b·I as well as diamagnetic methyl addition products (C₅H₅BPh)-Fe(C₆Me₇) (6a) and three isomers (C₅H₅MeBPh)Fe(HMB) (3a-5a) with the methyl group added to the borabenzene ring. The cation 2b⁺ undergoes hydride addition with NaBH₄ to produce (C₅H₅BPh)Fe(1,2,3,4,5,6-endo-C₆HMe₆) (6b) and two isomers (C₅H₆BPh)Fe(HMB) (3b, 4b) with bora-2,4- and bora-2,5-cyclohexadiene ligands.