Synthesis, electronic structure, and novel reactivity of strained, boron-bridged [1]ferrocenophanes

Andrea Berenbaum, Holger Braunschweig, Regina Dirk, Ulli Englert, Jennifer C. Green, Frieder Jäkle, Alan J. Lough, Ian Manners

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The first boron-bridged [1]ferrocenophanes, Fe(η-C5H4)2BN(SiMe3)2 (4a), Fe(η-C5H4)2BN(tBu)SiMe3 (4b), and Fe(η-C5H4)2BNiPr2 (4c), have been synthesized via reaction of Fe(η-C5H4Li)2·nTMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine) with the aminodichloroboranes Cl2BN(SiMe3)2, Cl2BN(tBu)SiMe3, and Cl2BNiPr2, respectively. Species 4a, 4b, and 4c represent the first [1]ferrocenophanes containing a bridging first row element and were isolated in 35-44% yield as dark red crystalline solids which were characterized by multinuclear NMR and UV - vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction (4a,c). The tilt-angle between the planes of the cyclopentadienyl rings in 4a and 4c was found to be 32.4(2)°and 31.2(2)°(average), respectively. The angle of 32.4(2)° in 4a represents the largest tilt reported to date for any [n]ferrocenophane. The ring-tilting results in a considerable redshift of the lowest energy absorbances in the UV - vis spectra recorded in hexanes (λ(max) 479 (4a), 489 (4b), 498 nm (4c)) in comparison to ferrocene (λ(max) = 440 nm). However, interestingly, the transitions are not as low energy as that for Fe(ηC5H4)2S (3; λ(max) = 504 nm), which shows a slightly smaller tilt angle of 31.1(1)°. As the nature of the bridging element appeared to influence the HOMO-LUMO gap, a comparative investigation of the electronic structures of 4a and 3 was performed using Density Functional Theory (DFT). These calculations revealed that the HOMO-LUMO gap is indeed slightly larger in 4a than in 3 as a consequence of mixing of the Fe-Cp antibonding orbitals with the B-C orbitals in 4 and the Fed orbitals with the sulfur lone pair in 3. The boron-bridged [1]ferrocenophanes 4b and 4c were found to readily undergo unprecedented ring-opening and insertion reactions with transition metal carbonyls at room temperature. Thus, reaction of 4c with Fe(CO)4- (THF) or Fe2(CO)9 led to insertion of an Fe(CO)4 fragment into the Fe-Cp bond rather than the B-C bond yielding (CO)2(μ-CO)2Fe2(η- C5H4)2BNiPr2 (7), a novel boron-bridged analogue of cis-[CpFe(CO)2]2. Reaction of the analogous compound 4b with Co2(CO)8 led to the formation of compound 8, which consists of a CpCo(CO)2 fragment, and a Cp(CO)2Fe- Co(CO)4 fragment linked by a boron bridge. Compounds 4a, 4b, and 4c undergo thermal ring-opening polymerization (ROP) at 180-200 °C to give the insoluble poly(ferrocenylborane)s [Fe(η-C5H4)2BN(SiMe3)2](n) (9a) and [Fe(η-C5H4)2BN(tBu)SiMe3](n) (9b) and the soluble, low molecular weight polymer [Fe(η-C5H4)2BNiPr2](n) (9c) together with small quantities of cyclic dimers and trimers. The ROP enthalpy for 4a was estimated to be ca. 95 kJ mol-1 by DSC measurements. This value is lower than expected, which we attribute to the presence of bulky substituents at boron.

Original languageEnglish (US)
Pages (from-to)5765-5774
Number of pages10
JournalJournal of the American Chemical Society
Issue number24
StatePublished - Jun 21 2000
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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