Cobalt Complexes Containing Dimethyldihydropyrene-Substituted Cyclobutadiene Ligands

CpCo(CO)2-induced cyclization of the dihydropyrenyl methyl acetylene 12 selectively gave cis (head to head) cyclobutadiene (Cbd) cobalt Cp complexes, 15, in 64% yield as a mixture of three isomers, which differ in the orientation of the internal DHP methyl substituents with respect to the CoCp unit. Cyclization of the ester-substituted alkyne 13 likewise gave three cis isomers of 16 in 30% yield. The bis-DHP substituted alkyne 9, however, gave 50% of a mixture of cis and trans (head to tail) isomers of 17. An X-ray crystal structure for complex 15 showed exclusively the cis regioisomer; however, the CpCo fragment was disordered over two main sites in a 56:41 ratio and the internal methyl groups were also disordered with respect to the CoCp unit. These two major sites are related by an approximately 180° rotation of the CpCo(Cbd) unit around the edge of the Cbd ring bearing the two DHP substituents. The aromaticity of the cyclobutadiene cobalt fragment was studied by examination of the DHP internal methyl chemical shifts, coupling constants, and bond distance alternation. Comparison of the 3J9′,10′ values for complex 16 and the phenyl derivative 5 and the sum of the bond length deviations from average for complexes 15 and 5 are consistent with the idea that, quantitatively, the aromaticity of the cyclobutadiene cobalt fragment is on the same order as that of benzene. Study of the photoopening of the naphthoyl dihydropyrene cobalt complex 26 indicated that the organometallic fragment slowed the reaction by about 12 times relative to the precursor, alkyne 25.