Synthesis, Electronic Structure, and Novel Reactivity of Strained, Boron-Bridged [1]Ferrocenophanes

Citations Over TimeTop 10% of 2000 papers

Abstract

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 (λmax 479 (4a), 489 (4b), 498 nm (4c)) in comparison to ferrocene (λmax = 440 nm). However, interestingly, the transitions are not as low energy as that for Fe(η-C5H4)2S (3; λmax = 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 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 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.

Related Papers

• → Visible light induced tetramethylethylenediamine assisted formylation of imidazopyridines(2018)66 cited
• → Metalated Tetra‐ and Penta(cyclopentadienyl)‐cyclopentadienyl Compounds: Syntheses by Multiple Pd‐Catalyzed Cyclopentadienylations(1996)27 cited
• → Infrared spectra (700–200 cm.–1) of some π-cyclopentadienyl–metal carbonyls and their substituted derivatives(1970)23 cited
• → Cyclopentadienyl ring metalation in cyclopentadienyliron carbonyl complexes(1983)23 cited
• → The synthesis and structure of silica-supported bis(h5-cyclopentadienyl)dichlorotitanium(IV) complexes(1986)9 cited