A New Approach to the Photochemically Controlled Crown Ethers : (Tetramethylcyclobutadiene)cobalt Complexes with Benzo-15-Crown-5 and Dibenzo-18-Crown-6

The photochemical substitution of benzene in [Cb*Co(C₆H₆)]⁺ (Cb* = C₄Me₄) with benzo-15-crown-5 (1) gives the corresponding π-complex [Cb*Co(η⁶-C₆H₄(OCH₂CH₂C)CH₂CH₂)₂O)]⁺ (2). A similar reaction with dibenzo-18-crown-6 (3) produces cation [Cb*Co(η⁶-C₆H₄(OCH₂CH₂OCH₂CH₂O)₂C₆H₄)]⁺ (4). Arene exchange is reversible, and both 1 and 3 can be regenerated by irradiation of 2 and 4 with excess benzene. The reaction of [Cb*Co(C₆H₆)]⁺ with 0.5 equiv of 3 gives dinuclear complex [trans-Cb*Co(μ-η⁶, η⁶-C₆H₄(OCH₂CH₂OCH₂CH₂O)₂C₆H₄)CoCb*]²⁺ (5). The structures of cations 4 and 5 were confirmed by X-ray diffraction. The electrochemical behavior of complexes prepared was studied. The sodium-binding ability of cobalt complexes 2, 4, and 5 (determined by NMR titration; K_a = 2500, 800, and 50, respectively) was found to be much smaller than that of the free crown ethers 1 and 3 (K_a, -200 000 and 5 000 000). DFT calculations suggest that this decrease is mainly due to electrostatic repulsion of Co⁺ and Na⁺ centers. © 2008 American Chemical Society.