A cyclooctatrienone complex of diiron hexacarbonyl

In the title compound, [μ-(2,6,7-η:3,4,5-η)-cycloocta-2,4,6-trienone]bis(tricarbonyliron)(Fe—Fe), [Fe2(C8H8O)(CO)6], the diiron hexacarbonyl moiety has a sawhorse arrangement, with the OC—Fe—Fe—CO fragment forming the horizontal bar of the horse, and the other four carbonyl groups the legs. The Fe—Fe distance is 2.795 (2) Å. Each Fe atom is also bonded to three C atoms of the cyclooctatrienone ring. One Fe atom forms a σ-bond with one ring C atom, with Fe—C = 2.109 (2) Å, and also a metal–olefin π-bond with two C atoms on the other side of the ring, with Fe—C distances of 2.238 (2) and 2.236 (3) Å. The second Fe atom forms a η3-allyl bond with three other ring atoms, with Fe—C bond lengths of 2.158 (2), 2.062 (2), and 2.123 (3) Å. Counting the π- and π-allyl interactions as one bond, the coordinations of the Fe atoms can, respectively, be approximated as octahedral and trigonal bipyramidal.


Related literature
The title compound was synthesized as part of a study on reactions of various cyclooctatetraene iron carbonyls (Paquette et al., 1975). The first reported synthesis of the compound was by King (1963). The structure of the corresponding cyclooctatriene complex was reported by Cotton & Edwards (1969), and that of a closely related derivative by Kerber et al. (1984), who also review other related structures.

Synthesis and crystallization
Details of the synthesis of the title compound are given in Paquette et al. (1975), which describes how the structure relates to mechanistic studies on cycloadditions of substituted cyclooctatetraenes to iron carbonyl complexes. A previous synthesis by a different route is given by King (1963).

Refinement
Each of the 18 standard reflections was measured 17-18 times during the 97 hours of data collection. Decay of individual standards during data collection was relatively isotropic, ranging from 6.9 (2)%-8.3 (5)%. Data were collected in two shells, θ=0-20° and θ=20-30°. The average decay during collection of data in the first shell was 0.8 (1)%, so that most of the decay occurred during collection of weaker intensities in the higher angle shell. No correction was made for the falloff in standard intensities.
The original data reduction deleted 536 reflections with I<2σ(I)out of a total of 4226 measurements, and their details are no longer available. Near the end of the final refinements, the missing reflections were reinserted into the data file, with F 2 values set equal to the σ(F 2 ) found for reflections with F 2 <3σ(F 2 ), averaged over ten ranges of theta values.
This report is based upon refinements that include the reinserted weaker reflections. The necessarily arbitrary assignment of F 2 values for these reflections with I<2σ(I) is the reason for the high K value for the weakest reflections in the final refinement. The structure was also refined without these weak reflections. The average Δ/σ for all parameters between refinements with and without the missing reflections was 0.21, with a maximum of 0.67 for U 11 for Fe1.
One reflection, (-2, 1, 3), was omitted from the final refinements, as the records clearly indicate an error during the scan.
As noted by the checkCIF/ PLATON report, alert level B, there are two reflections which show large Δ(F 2 )/σ values in the final refinements, (-2, 0, 2) and (0, 1, 5). The calculated F 2 values for these weaker reflections are near zero. However, as the chart record clearly shows peaks during these scans, there seemed no reason to delete them from the reflections file.
Positions of the two Fe atoms and the atoms of the six carbonyl groups were found by superposition methods.
H atoms were constrained to idealized positions with C-H distances of 0.98Å for the tertiary H atoms on C2-7 and 0.97Å for the secondary H atoms on C8. The U eq values for all H atoms were fixed at 1.2 times the U iso of their bonded C atoms.
Initial refinements with anisotropic temperature factors for Fe, O and C atoms and constrained hydrogen atom parameters converged smoothly, but a difference Fourier synthesis at this stage showed a pattern of peaks and holes 0.8-0.9 Å from the iron atoms, with maximum and minimum density values of 0.66 and -0.42 e/A 3 . The intensity data were smoothed by a 12 parameter model with XABS2 (Parkin et al., 1995), to allow for systematic anisotropies that might have existed in the data collection. After ensuing refinements, the maximum and minimum residual electron densities supplementary materials were reduced to 0.34 and -0.49 e/A 3 .

Comment
The title compound was synthesized as part of a study on the mechanism of cycloaddition of tetracyanoethylene to various cyclooctatetraene iron carbonyls. (Paquette et al., 1975). Determination of this structure clearly showed that Fe1 was σ bonded to C2 and π-bonded to C6 and C7, distinguishing the compound from a possible isomer with Fe1 σ bonded to C7 and π-bonded to the other side of the ring.
The structure of the corresponding cyclooctatriene complex was reported by Cotton and Edwards (1969), and that of a related carboxylate derivative by reported by Kerber et al. (1984), who also review two other related structures involving a bicyclo arrangement at C8 and C1.
In the present compound, the di-iron hexacarbonyl moiety has a sawhorse arrangement, with the fragment O11-C11-Fe1-Fe2-C14-O14 forming the horizontal bar of the horse, and the other four carbonyl groups the legs. The Fe-Fe distance is 2.795 (2) Å, somewhat longer than in similar structures reviewed by Kerber et al. (1984), in which the Fe-Fe distances range from 2.764 (3)Å to 2.786 (2)Å.
Counting the π-coordination as one bond, Fe1 can be considered as octahedrally coordinated. The shorter σ-bond length and the longer distances to the π-bonded C atoms are similar to those reported by Kerber et al. (1984).
The cyclooctatrienone ring is buckled in a complex way due to the trihapto bonding to each of the two Fe atoms. One might expect the six bonded C atoms to be held closer to the Fe atoms and the remaining two ring C atoms to be bent away from the Fe atoms, and this indeed appears to be the case. The bonded ring atoms C2-C7 form a rough plane, with rms deviation of 0.26 Å, and atoms C8 and C1 are displaced 1.543 (3)Å and 1.002 (3)Å respectively from this plane, away from the Fe atoms. Distances and angles for the ring are given in Table 1.