(18-Crown-6)potassium [(1,2,5,6-η)-cycloocta-1,5-diene][(1,2,3,4-η)-naphthalene]ferrate(−I)

The title salt, [K(C12H24O6)][Fe(C8H12)(C10H8)], is the only known naphthalene complex containing iron in a formally negative oxidation state. Each (naphthalene)(1,5-cod)ferrate(−I) anion is in contact with one (18-crown-6)potassium cation via K⋯C contacts to the outer four carbon atoms of the naphthalene ligand (cod = 1,5-cyclooctadiene, 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane). When using the midpoints of the coordinating olefin bonds, the overall geometry of the coordination sphere around iron can be best described as distorted tetrahedral. The naphthalene fold angle between the plane of the iron-coordinating butadiene unit and the plane containing the exo-benzene moiety is 19.2 (1)°.


Related literature
For the known complexes that contain iron in a formally negative oxidation state with solely olefinic ligands, see: Jonas (1979Jonas ( , 1981

Experimental
Crystal data [K(C 12  To date there are very few reported complexes of iron in a formally negative oxidation state and supported solely by olefinic ligands. In the 1970s Klaus Jonas and coworkers devised a way to synthesize (L) 2 Li 2 Fe(C═C) 2 , where L = tetramethylethylenediamine and C═C = 2(ethylene) or 1,5-cyclooctadiene (cod) or L = 1,2-dimethoxyethane (dme) and C═C = 1,5-cod (Jonas, 1979(Jonas, , 1981Jonas et al., 1979;Jonas & Krüger, 1980), for which the cod complex is a direct derivative of the ethylene complex. Because ferrocene (FeCp 2 ) was the starting material, the synthesis of the homoleptic ethylene complex required pressurized ethylene gas to fully displace both cyclopentadienyl ligands (5 atm prior to heating to 323 K in a closed vessel; Jonas, 1979). To avoid the need for the superambient pressures necessary with ferrocene, we devised syntheses from a ferrous halide, FeBr 2 . Recently we reported the syntheses of new ferrate anions bis(anthracene)ferrate(-I), bis(butadiene)ferrate(-I), and mixed-ligand (anthracene)(1,5-cod)ferrate(-I) (Brennessel et al., 2007). The title complex is unique because it is the sole example of a naphthalene complex containing iron in a formally negative oxidation state.
Unlike what was observed in the cobalt system, in which the reduction of CoBr 2 by three equivalents of potassium naphthalene in the presence of excess 1,5-cod led to the homoleptic 1,5-cod anion [Co(η 4 -1,5-cod) 2 ] - (Brennessel et al., 2006;Brennessel & Ellis, 2012), only one molecule of 1,5-cod is found coordinating to the iron atom regardless of excess 1,5-cod. This was not the only product, since carbonylation of the bulk material showed ν CO stretching frequencies corresponding to [Fe 2 (CO) 8 ] 2-(major) and [Fe(CO) 4 ] 2-(minor). If the naphthalene radical anion is reducing enough to afford an Fe(-II) species directly, then that species could be the precursor to the minor carbonylation product, the Fe (-II) carbonyl. However, since the yield of the title complex is modest (40-50%), there is likely excess reducing agent left over from the initial reduction which easily could have reduced the Fe(-I) carbonyl to Fe(-II). Unfortunately, it has proved very difficult to separate the title complex from the naphthalene radical anion, and no further optimizations or characterizations have been performed to date.
The bond lengths of the metal-coordinating olefins (C1═C2 and C3═C4, Figure 1) of the naphthalene ligand (1.424 (3) Å, avg.) are statistically identical to those found in the related anthracene-cod ferrate anion, [Fe(C 14 H 10 )(C 8 H 12 )] -(1.422 (4) Å, avg.; Brennessel et al., 2007), which suggests that naphthalene is performing an equivalent role in supporting the low-valent iron atom. Even so, anthracene quantitatively displaces naphthalene at room temperature in THF solution (i.e., the title complex can be converted to the anthracene-cod ferrate with ease), a result that can be justified with Dewar's resonance energies (Milun et al., 1972). Both the title complex and the anthracene-cod ferrate have supplementary materials an essentially tetrahedral geometry about their iron atoms and have similar polyaromatic hydrocarbon fold angles (for the title structure the fold angle between the planes defined by atoms C1, C2, C3, C4 and C1, C4, C5, C6, C7, C8, C9, C10, respectively, amounts to 19.2 (1) °.) The packing of the molecular entities is shown in Figure 2.

Experimental
Details on the preparation and purification of reagents and solvents, and descriptions of the equipment and techniques can be found elsewhere (Brennessel, 2009). Under argon, an orange slurry of anhydrous FeBr 2 (0.500 g, 2.32 mmol) in THF (50 ml, 195 K) was added to a deep green solution of K[C 10 H 8 ] (6.86 mmol) and excess 1,5-cyclooctadiene (0.882 g, 8.15 mmol) in THF (50 ml, 195 K). The resulting reddish-yellow solution was warmed slowly to room temperature, when it was filtered to remove KBr. 18-crown-6 (0.613 g, 2.32 mmol) in THF (30 ml) was added to the deep red filtrate and the solvent was removed in vacuo. Pentane (40 ml) was added and the solid was carefully scraped off the flask wall with the stir bar. The slurry was then filtered, and the product was washed with pentane (30 ml) and dried in vacuo, yielding a dark red solid (0.607 g, 44% assuming the uni-negative title complex: see Comment above). An analytically pure bulk sample has not been obtained to date. Dark red blocks were grown from a pentane-layered THF solution at 273 K.

Refinement
Hydrogen atoms on the naphthalene ligand and on the metal-coordinating carbon atoms of the 1,5-cod ligand were found from a difference Fourier map, and their positional and isotropic displacement parameters were refined independently from those of their respective bonded carbon atoms. All other hydrogen atoms were placed geometrically, and refined relative to their respective bonded carbon atoms with a bond lengths of 0.99 Å and U iso [H] = 1.2 . U eq [C].   Unit cell packing plot that features the cation-anion contacts.   (7) C11-C18-H18A 108.9 (10)