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

The first boron-bridged [1] ferrocenophanes, Fe(eta-C5H4)(2)BN(SiMe3)(2) (4a), Fe(eta-C5H4)(2)BN(tBu)SiMe3 (4b), and Fe(eta-C5H4)(2)BNiPr2 (4c), have been synthesized via reaction of Fe(eta-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 MMR 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)degrees and 31.2(2)degrees (average), respectively. The angle of 32.4(2)degrees 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 (lambda(max) 479 (4a), 489 (4b), 498 nm (4c)) in comparison to ferrocene (lambda(max) = 440 nm). However, interestingly, the transitions are not as low energy as that for Fe(eta-C5H4)(2)S (3; lambda(max) = 504 nm), which shows a slightly smaller tilt angle of 31.1(1)degrees. 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 Fe d 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 Fez(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)(mu-CO)(2)Fe-2(eta-C5H4)(2)BNiPr2 (7), a novel boron-bridged analogue of cis-[CpFe(CO)(2)](2). Reaction of the analogous compound 4b with Co-2(CO)(4) led to the formation of compound 8, which consists of a CpCo(CO)(2) fragment, and a Cp(CO)(2)Fe-Co(CO)(4) fragment linked by a boron bridge. Compounds 4a, 4b, and 4c undergo thermal ring-opening polymerization (ROP) at 180-200 degrees C to give the insoluble poly(ferrocenylborane)s [Fe(eta-C5H4)(2)BN(SiMe3)(2)](n) (9a) and [Fe(eta-C5H4)(2)BN(tBu)SiMe3](n) (9b) and the soluble, low molecular weight polymer [Fe(eta-C5H4)(2)BNiPr2](n) (9c) together with small quantities of cyclic dimers and trimers. The ROP enthalpy for 4a was estimated to be ca. 95 kl mol(-1) by DSC measurements. This value is lower than expected, which we attribute to the presence of bulky substituents at boron.