Cofacial metallocenes. Synthesis and crystal structure of 1,8-diferrocenylnaphthalene

Article abstract of DOI:10.1021/om00122a019

The synthesis of 1,8-diferrocenylnaphthalene (3) is described. This substance is of interest as a model and potential precursor for columnar polymeric metallocenes in which cyclopentadienyl rings of adjacent metallocene nuclei are held proximate and cofacial. Although the molecule is very crowded, 1,8-diferrocenylnaphthalene may be made in high yield by coupling 1,8-diiodonaphthalene and ferrocenylzinc chloride in the presence of dichloro[1,1′-bis(diphenylphosphino)ferrocene]palladium(II) (Pd(dppf)Cl₂). An X-ray crystal structure determination of the compound shows the naphthalene ring and bonds from the peri-carbon atoms to the cyclopentadienyl rings to be highly distorted. The two substituted cyclopentadienyl rings are rotated by ～46° from a conformation which would place them perpendicular to the naphthalene ring. Cyclic voltammetry measurements, carried out on 3 in methylene chloride, show two reversible, one-electron waves at +0.013 and 0.208 V vs. a Ag/0.1 M AgNO₃ reference electrode. The monocation, derived by oxidation of 3 with silver tetrafluoroborate, exhibits an intervalence transfer band at 1600 nm (ε_max 200), not shown by the corresponding dication of 3. A comparison of these properties with those of biferrocenyl, diferrocenylacetylene, and 1,4-diferrocenylbutadiyne suggests that 3⁺ is best represented as a weakly interacting mixed-valence system.