MIGRATION OF TRIPHENYLBORON FROM IRON TO AN ETA-5-CYCLOPENTADIENYL LIGAND

Triphenylborane and (C2H5)4N+(η5-C5H5)Fe(CO)2- form a 1:1 adduct Ph3BFe(CO)2Cp-NEt4+, 1. Analyses of spectroscopic data (IR, 1H NMR, and 11B NMR) from 1 and the analogous (n-C4H,)4N+ salt, including comparisons with Ph3BOCH3-NEt4+ and Ph3AlFe(CO)2Cp-NEt4+, strongly indicate the presence of an iron-to-boron bond and suggest that the negative charge is localized on the boron atom. In dilute tetrahydrofuran solutions, 1 spontaneously converted to Ph3BC5H4Fe2-(CO)4Cp-NEt4+-1/2THF, 2, and Ph3BH-NEt4+, 3. From X-ray diffraction analysis, the crystal structure of 2 was refined to R = 0.097 in C2/c with a = 40.357 (6) Å, b = 9.943 (2) Å, c = 20.453 (2) Å, and β = 111.72 (1)° with Z = 8. Partial disorder in the cation required two-state parameterization. The anion consists of a central di-μ-carbonyl-diiron unit (Fe-Fe = 2.540 (2) Å) with a terminal carbonyl and an η5-cyclopentadienyl ligand on each metal atom; the configuration of the rings is cis. The nearly tetrahedral Ph3B group is η1 bonded to one of the rings at the position farthermost from the other ring. The bonding geometry around the iron atoms is essentially that found in cis-[CpFe(CO)2]2- In acetone, the conductivity of 2 is similar to that of Ph4B-NEt4+. The IR spectra of 2 in dimethyl sulfoxide and in methylene chloride show different intensities of corresponding v(CO), indicative of a cis-trans isomer distribution. Compound 3 is identical with Ph3BH NEt4+ prepared from Ph3B and KH followed by metathesis with Et4N+Br-. The single B-H bond in 3 was identified from the stretching frequency of 2198 ± 1 cm-1, observed by Raman and infrared spectroscopy and from 1H and 11B NMR spectroscopy. A prominent absorption at 2136 cm-1 seen only in the infrared was assigned as a combination band. A mechanism is proposed for the conversion of 1 to 2 and 3 via an intramolecular nucleophilic attack on the Cp ring by Ph3B-, followed by hydride elimination. © 1979, American Chemical Society. All rights reserved.