Synthesis, Structure, and Reactivity of Alkyl-Substituted Half-Sandwich η⁵-Pentadienyl Complexes of Cobalt

Alkyl-substituted η 5 -pentadienyl complexes of cobalt have been reported to undergo [5 + 2] cycloaddition reactions with alkynes to form substituted η 2,η 3 - and η 5 -cycloheptadienyl products, providing a new route to the synthesis of substituted cycloheptadienes. A series of cyclopentadienyl and pentamethylcyclopentadienyl cobalt(III) η 5 -pentadienyl complexes have been prepared, incorporating alkyl and aryl substituents at various positions on the pentadienyl ligand. The crystalline complexes have been completely characterized spectroscopically and, in the solid state, by X-ray crystallography. The alkyl-substituted pentadienyl complexes can be prepared by a range of methodologies, most generally by acid-promoted dehydration of in situ -derived η 2 - or η 4 -dienol complexes. Two variations on this classic strategy have been developed, starting from either conjugated (1,3-) or nonconjugated (1,4-) dienyl alcohols. For both cyclopentadienyl and pentamethylcyclopentadienyl ancillary ligands, the substituted η 5 -pentadienyl complexes are obtained in reasonable to good isolated yields, limited by the extent of substitution on the starting allylic alcohol. The cationic cobalt(III) η 5 -pentadienyl complexes are indefinitely stable to air and moisture; isolation and purification is accomplished by chromatography on the bench. The substitutional lability of the pentadienyl ligand has been investigated using both neutral and anionic donor ligands (CO, isonitrile, acetonitrile, and halide salts). The results reveal that η 5 -pentadienyl complexes react by equilibrium dissociation of the most substituted end of the pentadienyl moiety, providing the corresponding η 3 -coordinated pentadienyl adducts. A comparative study of 11 X-ray crystal structures covering a range of pentadienyl substitution patterns is also reported, revealing long Co−C1 and Co−C5 bonds and, consistently, short Co−C3 bond lengths. The internal bond angles of substituent-bearing carbon atoms are smaller than those observed at unsubstituted carbons, with the compression as much as 10° at the substituted center. As expected, alkyl substituents invariably deviate out of the pentadienyl plane, toward the metal. In unbiased cases, greater deviations are noted for C1 substituents than for C2 substituents, although the deviations at C2 increase significantly for 1,2-disubstituted ligands. In some cases, the carbon−carbon bond distances display distortions toward η 2,η 3 - or η 1,η 4 -hapticity, although no particular correlation between structural distortion and solution reactivity is evident.

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