Abstract:
When dibenzo[a,e]cyclooctatetraene and hexacarbonylchromium are refluxed in di-n-butyl ether /THF for an extended period, both the syn,anti and anti,anti isomers of the bis tricarbonylchromium complex are obtained. After separation by column chromatography, the syn,anti:anti,anti isomer ratio is approximately 12:1. The anti,anti arrangement of the Cr(CO)3 groups was verified by X-ray crystallographic structure determination. The orientations of the two tricarbonylchromium tripods relative to the 1,2-disubstituted arene rings differ. One is exo
staggered while the other is eclipsed. Intermolecular C-H···O hydrogen bonding is prevalent in
the crystal and is reminiscent of the syn,anti isomer. A dimeric motif in the solid resulting from
C-H···π interactions is observed. DFT calculations on both isomers confirms that the syn,anti
isomer is favored. The preferred orientation for the anti Cr(CO)3 groups in both cases is exo
staggered. However, the barrier to rotation is low, allowing hydrogen bonding to readily
overcome this barrier and control the orientation of the tripod. While thermodynamic
consideration support the preference for the syn,anti isomer, kinetic factors may also be
important as this isomer would be formed preferentially due to steric inhibition of the diene
coordination site by the mononuclear anti isomer. The stabilization of the syn,anti isomer occurs by an electrostatic attraction of the syn carbonyl groups with the anti Cr(CO)3-coordinated arene
carbon atoms. A reexamination of the structural parameters verifies nonbonded carbon-oxygen
distances less than the sum of the van der Waals radii in the syn,anti isomer. A search of the Cambridge Structural Database results in a significant number of interactions of this nature for
arenetricarbonylchromium complexes.
