Synthesis, Structure and Conformation of cis- and trans-(μ-1,6-Dimethylheptalene)bis(tricarbonyliron)

The reaction of 1,6-dimethylheptalene (2) with Fe₂(CO)₉ affords two binuclear iron complexes 4a and 4b, each bearing two tricarbonyliron moieties, in trans and cis orientation, respectively. The reaction leads additionally to the formation of the monocomplexed compound 3. NMR studies indicate that the heptalene moiety of the new compounds is a π-bond isomer of 2, and this is clearly attributable to steric factors. Single-crystal X-ray structure analyses of the metal complexes 4a and 4b confirm that the Fe(CO)₃ fragments adopt trans and cis geometries, respectively, with respect to the plane of the carbocycle. The ring systems of the two isomers adopt different conformations in the solid state. In the case of 4a, a hitherto unknown chair-like conformation is determined, whereas in complex 4b the ring system shows a twisted double boat conformation.