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(OCH2CH2OCH2CH2)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; Ka = 2500, 800, and 50, respectively) was found to be much smaller than that of the free crown ethers 1 and 3 (Ka = 200 000 and 5 000 000). DFT calculations suggest that this decrease is mainly due to electrostatic repulsion of Co+ and Na+ centers.