Synthesis and Spectral Electrochemical Properties of a Symmetrical Tin-Bridged [3.3]Ferrocenophane

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 ([(η 5 -C 5 H 4 )Fe(η 5 -C 5 H 4 SnMe 2 Sn( n -Bu) 2 SnMe 2 )] 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 ( 1 H, 13 C, 117 Sn, 119 Sn) spectroscopy, HRMS, and elemental analysis. A statistical model to account for the 117 Sn/ 119 Sn coupling patterns, observed in both the 117 Sn and 119 Sn NMR spectra, was undertaken and is a good fit to experimental data. Investigations by UV–vis spectroscopy display a visible d–d transition at λ max 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 Pd 2 (dba) 3, that leads to superior yields of 4a over previously published protocols.