Borole derivatives. 20. Three-center Fe-H-B bonding in (borole)(cyclopentadienyl)hydridoiron derivatives.
(Borole)tricarbonyliron complexes Fe(CO)3(3,4-R12C4H2BPh) (3a, R1 = H; 3b, R1 = Me) react with cyclopentadiene under irradn. to give the hydrides CpFeH(3,4-R12C4H2BPh) (1a, b). Deprotonation by NaH in THF produces the borataferrocene anions [CpFe(3,4-R12C4H2BPh)]- (5a, b). These can be alkylated in the 2-/5-position by alkyl iodides in the presence of NaH to give 2,5-dialkyl hydride derivs.: 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) and (C5H4Me)FeH(2,5-Me2C4H2BPh) (1f) as detd. by low-temp. x-ray diffraction show the presence of an Fe-H-B three-center bond with a weak B-H interaction. In soln. 1a establishes a mobile equil. between the ground state with Fe-H-B three-center bonding and the agostic isomer with Fe-H-C-2(C4H4B) three-center bonding with an estd. DG = 1.4 kcal/mol. The hydridic proton and the 2-/5-protons of the borole ring undergo fast intramol. exchange. 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 of CpFeH(C4H4BH) is analyzed by means of extended Hueckel calcns., which confirm the Fe-H-B three-center bonding. Exploratory extended Hueckel calcns. 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 CNBut to give the boracyclopentenyl complex CpFe(CNBut)(C4H5BPh) and, probably via the intermediate Fe(CNBut)(C4H4BPh)(C5H6), the borole complex Fe(CNBut)3(C4H4BPh) and 1 equiv of cyclopentadiene. Labeling expts. show that the shift of the hydride hydrogen to the borole and Cp ligands, resp., is an intramol. process. 1C reacts analogously.
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