BOROLE DERIVATIVES .20. 3-CENTER FE-H-B BONDING IN (BOROLE)(CYCLOPENTADIENYL)HYDRIDOIRON DERIVATIVES

(Borole)tricarbonyliron complexes Fe(CO)3(3,4-R1(2)C4H2BPh) (3a, R1 = H; 3b, R1 = Me) react with cyclopentadiene under irradiation to give t he hydrides CpFeH(3,4-R1(2)C4H2BPh) (1a,b). Deprotonation by NaH in TH F produces the borataferrocene anions [CpFe(3,4-R1(2)C4H2BPh)]-(5a-, 5 b-). These can be alkylated in the 2-/5-position by alkyl iodides in t he presence of NaH to give 2,5-dialkyl hydride derivatives: e.g. CpFeH (2,5-R2(2)C4H2BPh) (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-Me2C4H2-B Ph) (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 o f 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 wit h Fe-H-B three-center bonding and the agostic isomer 1a-A with Fe-H-C- 2(C4H4B) three-center bonding with an estimated DELTAG = 1.4 kcal/mol. The hydridic proton and the 2-/5-protons of the borole ring undergo f ast intramolecular exchange (T(c) = 333 K at 80 MHz and DELTAG(double dagger)333 = 17.0 +/- 0.7 kcal/mol for 1a). A merry-go-round of the th ree H atoms involved with one hydrogen crossing the exo face of the bo role ligand is proposed as an exchange mechanism. The bonding in CpFeH (C4H4BH) is analyzed by means of extended Huckel calculations, which c onfirm the Fe-H-B three-center bonding. Exploratory extended Huckel ca lculations also support the existence of agostic isomers as well as th e proposed exchange mechanism. Cyclic voltammetry revealed the existen ce of the 17e complex CpFe(C4H4BPh) (5a), which is characterized by it s EPR and paramagnetic H-1 NMR spectra. 1a reacts with CNBu(t) to give the boracyclopentenyl complex CpFe(CNBu(t)) (C4H5BPh) (9a) and, proba bly via the intermediate Fe(CNBu(t))(C4H4BPh) (C5H6) (10), the borole complex Fe(CNBu(t))3(C4H4BPh) (11a) and 1 equiv of cyclopentadiene. La beling experiments show that the shift of the hydridic hydrogen to the borole and Cp ligands, respectively, is an intramolecular process. 1c reacts analogously.