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Multiple Aromatic C–H Bond Activations by an Unsaturated Dirhenium Carbonyl Complex

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journal contribution
posted on 11.01.2019, 00:00 by Richard D. Adams, Poonam Dhull, Mark D. Smith, Jonathan D. Tedder
Reactions of Re2(CO)8(μ-C6H5)­(μ-H), 1, with naphthalene and anthracene have yielded the first multiply-CH activated arene products through the reductive elimination of benzene from 1 and multiple oxidative-additions of the dirhenium octacarbonyl grouping to these polycyclic aromatic compounds under very mild conditions. In addition, compound 1 was found to react with itself to yield the bis-Re2-metalated C6H4 bridged compound Re2(CO)8(μ-H)­(μ-1,μ-3-C6H4)­Re2(CO)8(μ-H), 3. Reaction of 1 with naphthalene yielded two doubly CH activated isomers, Re2(CO)8(μ-H)­(μ–η2-1,2-μ–η2-3,4-C10H6)­Re2(CO)8(μ-H), 4, 41% yield, and Re2(CO)8(μ-H)­(μ–η2-1,2-μ–η2-5,6-C10H6)­Re2(CO)8(μ-H), 5, via the mono CH activated intermediate Re2(CO)8(μ–η2-C10H7)­(μ-H), 2. Compound 4 contains two Re2(CO)8(μ-H) groups on one C6 ring formed by CH activations at the 2- and 4-positions. Compound 5 contains two Re2(CO)8(μ-H) groups; one formed by CH activation at the 2-position on one C6 ring and the other formed by CH activation at the 6-position (or centrosymmetrically related 2′-position) on the second C6 ring. The Re2(CO)8(μ-H) groups are coordinated to the C6 rings by binuclear σ + π coordination to two adjacent carbon atoms in the rings. Compound 1 reacts with anthracene to yield the mono-CH activated compound Re2(CO)8(μ–η2-1,2-C14H9)­(μ-H), 6, and two doubly CH activated compounds, Re2(CO)8(μ-H)­(μ–η2-1,2-μ–η2-3,4-C14H8)­Re2(CO)8(μ-H), 7, and Re2(CO)8(μ-H)­(μ–η2-1,2-μ–η2-5,6-C14H8)­Re2(CO)8(μ-H), 8. Compounds 7 and 8 are isomers that are structurally similar to 4 and 5. Compounds 7 and 8 can also be obtained in good yields from the reaction of 6 with 1. In the presence of a 5/1 ratio of 1/anthracene, a small amount (5% yield) of the tetra-substituted anthracene product [Re2(CO)8(μ-H)]4(μ–η2-1,2-μ–η2-3,4-μ–η2-5,6-μ–η2-7,8-C14H6), 9, was formed. Compound 9 contains four σ + π coordinated Re2(CO)8(μ-H) groups formed by oxidative additions of the CH bonds of anthracene to the Re2(CO)8 groups at the 2, 4, 6, and 8 positions of the three ring system. Molecular orbital calculations have been performed for all new compounds in order to develop an understanding of the bonding of the ring systems to the Re2(CO)8(μ-H) groups. All new compounds were characterized by single-crystal X-ray diffraction analyses.