Borole derivatives. 20. Three-center Fe-H-B bonding in (borole) (cyclopentadienyl)hydridoiron derivatives

Article abstract of DOI:10.1021/om00014a037

(Borole)tricarbonyliron complexes Fe(CO)₃(3,4-R¹₂C₄H₂BPh) (3a, R¹ = H; 3b, R¹ = Me) react with cyclopentadiene under irradiation to give the hydrides CpFeH(3,4-R¹₂C₄H₂BPh) (1a,b). Deprotonation by NaH in THF produces the borataferrocene anions [CpFe(3,4-R¹₂C₄H₂BPh)]-(5a ^-, 5b^-). These can be alkylated in the 2-/5-position by alkyl iodides in the presence of NaH to give 2,5-dialkyl hydride derivatives: e.g. CpFeH(2,5-R²₂C₄H₂BPh) (1c, R² = Me; 1g, R² = CH₂=CH(CH₂)₄; 1h, R² = (CH₂)₂CHCH₂). The structures of CpFeH(2,3,4,5-Me₄C₄BPh) (1d; Fe-B = 215.1(4) pm, Fe-H = 147(3) pm, B-H = 151(3) pm) and (C₅H₄Me)FeH(2,5-Me₂C₄H ₂-BPh) (1f; Fe-B = 215.5(2) pm, Fe-H = 147.5(23) pm, B-H = 146.1(26) pm) as determined by low-temperature X-ray diffraction show the presence of an Fe-H-B three-center bond with a weak B-H interaction. In solution 1a establishes a mobile equilibrium between the ground state 1a-G with Fe-H-B three-center bonding and the agostic isomer 1a-A with Fe-H-C-2(C₄H₄B) three-center bonding with an estimated ΔG = 1.4 kcal/mol. The hydridic proton and the 2-/5-protons of the borole ring undergo fast intramolecular exchange (T_c = 333 K at 80 MHz and ΔG?₃₃₃ = 17.0 ± 0.7 kcal/mol for 1a). A merry-go-round of the three H atoms involved with one hydrogen crossing the exo face of the borole ligand is proposed as an exchange mechanism. The bonding in CpFeH(C₄H₄BH) is analyzed by means of extended Hu?ckel calculations, which confirm the Fe-H-B three-center bonding. Exploratory extended Hu?ckel calculations also support the existence of agostic isomers as well as the proposed exchange mechanism. Cyclic voltammetry revealed the existence of the 17e complex CpFe(C₄H₄BPh) (5a), which is characterized by its EPR and paramagnetic ¹H NMR spectra. 1a reacts with CNBu^t to give the boracyclopentenyl complex CpFe(CNBu^t)(C₄H₅BPh) (9a) and, probably via the intermediate Fe(CNBU^t)(C₄H₄BPh)(C₅H₆) (10), the borole complex Fe(CNBU^t)₃(C₄H₄BPh) (11a) and 1 equiv of cyclopentadiene. Labeling experiments show that the shift of the hydridic hydrogen to the borole and Cp ligands, respectively, is an intramolecular process. 1c reacts analogously.