Article pubs.acs.org/Organometallics
Synthesis and Spectral Electrochemical Properties of a Symmetrical Tin-Bridged [3.3]Ferrocenophane Jon Ward,† Saif Al-Alul,† Matthew W. Forbes,‡ Timothy E. Burrow,§ and Daniel A. Foucher*,† †
Department of Chemistry and Biology, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B-2K3 AIMS Mass Spectrometry Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto Ontario, Canada, M5S 3H6 § CSICOMP, The Center for Spectroscopic Investigation of Complex Organic Molecules and Polymers, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 ‡
S Supporting Information *
ABSTRACT: The symmetrical bisferrocene 1,1,14,14-tetra-n-butyl-2,2,13,13,15,15,26,26-octamethyl-1,2,13,14,15,26-hexastanna[3.3]ferrocenophane ([(η5-C5H4)Fe(η5-C5H4SnMe2Sn(nBu)2SnMe2)]2, 8) was synthesized in good yield (82%) from the reductive coupling reaction of 1,1′-bis(dimethylstannyl)ferrocene, 6, with one equivalent of di-n-butylbis(diethylamino) stannane, 7a, at 0 °C in diethyl ether solution. Compound 8 is stable to ambient light but decomposes in the presence of moisture or chlorinated solvents. The title compound 8 was characterized by NMR (1H, 13C, 117Sn, 119Sn) spectroscopy, HRMS, and elemental analysis. A statistical model to account for the 117Sn/119Sn coupling patterns, observed in both the 117Sn and 119Sn NMR spectra, was undertaken and is a good fit to experimental data. Investigations by UV−vis spectroscopy display a visible d−d transition at λmax at 455 nm (ε = 74 L mol−1 cm−1), while cyclic voltammetry reveals two reversible oxidation events at 0.01 and 0.23 V (relative to the ferrocene/ferrocenium couple), indicative of strong electronic communication between the ferrocene units. A DFT analysis of compound 8 supports the symmetrical nature of the molecule with a calculated intramolecular distance of ≃8.5 Å between iron centers. This is the first example of a [3.3]ferrocenophane bearing three σ-conjugated group 14 spacers between the ferrocene units. Furthermore, an improved route to 1,1,2,2-tetramethyldistannanediyl[2]ferrocenophane 4a has been found, through an intermolecular dehydrogenative coupling of 6 with catalytic amounts of Pd2(dba)3, that leads to superior yields of 4a over previously published protocols.
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INTRODUCTION [n.n]Ferrocenophanes are macrocyclic organometallic ring systems that contain two ferrocene moieties linked to each other through their cyclopentadienyl rings. Several examples of symmetrical [1.1]ferrocenophanes containing group 12 (Hg),1 13 (B, Ga, In),2−4 14 (C, Si, Sn),5−8 or 15 (P, As)9,10 linking moieties have been described since the late 1960s. Very recently, several new [1.1]ferrocenophanes containing Al or Ga bridging elements containing intramolecularly coordinating ligands have been synthesized by the Müller group.11 Larger symmetrical macrocyclic [1n]ferrocenophanes were also reported by Manners et al. for species with either five or six alternating linked ferrocenes and dimethylsilyl groups.12 Such [n.n]- or [1n]ferrocenes with one or two heteroatom spacer atom(s) usually display strong electronic communication between the ferrocenyl centers, as evidenced by cyclic voltammetry, with sequential, well-separated, multistep oxidation potentials as a result of the intimate structural connectivity of the metal centers. The first tincontaining [1.1]ferrocenophane (1a, R = n-Bu) was reported by Seyferth,8 who isolated, in very low yield (3%), a crystalline product in the attempted preparation of the strained di-nbutylstanna[1]ferrocenophane (2a, R = n-Bu). The groups of Manners13 and Pannell14 successfully prepared and polymerized examples of ring-strained stanna[1]ferrocenophanes (2b−d) into polyferrocenylstannanes (3b−d). Also isolated from the © XXXX American Chemical Society
solution polymerizations of monomers (2b, 2c) were tincontaining [1.1]ferrocenophanes (1b, 1c) in modest yields (20− 30%). Concurrently, Herberhold and co-workers15 described the synthesis of the closely related [2]- and [3]ferrocenophanes with tetramethyldistannanediyl (4a) and hexamethyltristannanediyl (5a) bridges via the reductive coupling reactions of the 1,1′bis(dimethylstannyl)ferrocene 6 with dialkylstannyldiamines (7a = (n-Bu)2Sn(NEt2)2, 7b = Me2Sn(NEt2)2). The more sterically hindered distanna[2]ferrocenophane 4b was recently prepared by Braunschweig via salt elimination of dilithioferrocene·tmeda (tmeda = N,N,N′,N′-tetramethylethylenediamine) with the bulky dichlorodistannane t-Bu4Sn2Cl2 in good yield.16 There have been a few examples of larger [n.n]ferrocenophanes in the literature17−20 including a tin-containing [5.5]ferrocenophane with two (Me2SiCH2)2SnMe2 bridges isolated by Jurkschat and co-workers17 in a 60% yield. We report herein, the isolation, in modest yield, of the first example of a symmetrical tin-containing [3.3]ferrocenophane, its characterization by NMR spectroscopy and identification by mass spectrometry, and detail the electronic properties investigated by UV−vis spectroscopy and cyclic voltammetry. Received: November 7, 2012
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dx.doi.org/10.1021/om301065q | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
Scheme 1a
Conditions: (i) Et2O (0.1 M 6, 7a), 0 °C 1 h, reflux 3 h, hexanes/ silica col. (82% yield of 8), (ii) Et2O (0.02 M 6, 7a) 0 °C 1 h, reflux 2 h, 3:2 DCM/hexanes/silica col. (52% yield of 4a). a
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RESULTS AND DISCUSSION Our interest in group 14 metallocenes has been focused on the preparation of oligo- and polymetallocenes with at least two bridging atoms between ferrocene units. In particular, dehydrogenative coupling reactions of suitable group 14 hydrides, such as 6, and metal-catalyzed ring-opening polymerization of novel monomers, such as the [2]ferrocenophane 4a, have been targeted. Following procedures outlined by Herberhold and Wrackmeyer,15 we readily prepared 6 in comparable yields and purity. Room-temperature dehydrogenative coupling of 6 to yield 4a was then attempted with a few group 9 and 10 metal catalysts. These results are listed in Table 1. Of the three catalysts
purified by silica gel chromatography eluting with neat hexanes, with two distinct fractions being collected. This accounted for approximately 87% of the product mass (8 and 4a); the remainder of the reaction material remained on the column and could not be recovered. Surprisingly, analysis by NMR (119Sn; C6D6) confirmed the smaller second fraction (5% of the recovered materials) was 4a, displaying a resonance at −43 ppm. By contrast, the first fraction, recovered as a bright, orange oil (stable for short periods in air), was isolated in an 82% yield. Initial attempts to carry out the NMR analysis of both the first and second fractions in CDCl3 resulted in broad, unresolved resonances in the Cp and n-butyl regions, indicative of immediate decomposition in this solvent. When the NMR (1H, Figure 1) of the first fraction was analyzed in C6D6, a highly symmetrically substituted [n.n]ferrocenphane, with a detectable trace of another 1,1′-disubstituted ferrocene- or ferrocenophane-containing compound (
