Redox-active crown ethers. Electrochemical and electron paramagnetic resonance studies on alkali metal complexes of quinone crown ethers

Article abstract of DOI:10.1021/ja00049a032

Structural studies on [M(NCS)·(5QC-HQDME)] (M = Li, Na) as well as free 6QC-HQDME and [M(NCS)· (6QC-HQDME)] (M = Na, K) (where 5QC-HQDME is 15,17-dimethyl-16,18-dimethoxy-3,6,9,12-tetraoxabicyclo-[12.3.1]octadeca(1,14, 16)triene, and 6QC-HQDME is 15,17-dimethyl-16,18-dimethoxy-3,6,9,12,15-pentaoxabicyclo-[15.3.1]heneico(1,14, 16)triene) show that in all cases the metal ion binds to the anisole oxygen atom in the 1-position. Only in the case of [K(NCS)·(6QC-HQDME)] do both benzylic O atoms bind to the metal ion; in the other complexes only one of these O atoms interacts with M⁺. In each complex all of the non-benzylic crown O atoms coordinate. These results indicate that the benzylic O atoms contribute suboptimally to complexation. Crystallographic data are as follows: [Li(NCS)· (5QC-HQDME)], monoclinic, C₁₉H₂₈NO₆SLi, space group P2}/n, a = 14.103 (4) A?, b = 8.493 (4) A?, c = 19.128 (8) A?, β = 108 70 (9)°, Z = 4; [Na(NCS)·(5QC-HQDME)], monoclinic, C₁₉H₂₈NO₆SNa, space group P2₁/c, a = 10.182 (4) A?, b = 8.601 (1) A?, c = 25.631 (3) A?, β= 97.29 (3)°, Z = 4; 6QC-HQDME, orthohombic, C₂₀H₃₂O₇, space group P2₁2₁2₁, a = 8 195 (1) A?, b = 11.541 (1) A?, c = 22.449 (3) A?, Z = 4; [Na(NCS)·(6QC-HQDME)]·MeCN, monoclinic, C₂₃H₃₅N₂O₇SNa, space group P2₁/c, a = 11.308 (1) A?, b = 14.521 (2) A?, c = 16.440 (4) A?, β= 91.56 (1)°, Z = 4; [K(NCS)·(6QC-HQDME)], monoclinic, C₂₁H₃₂NO₇SK, space group P2₁/c, a = 17.377 (3) A?, b = 10.600 (2) A?, c = 27.538 (7) A?, β= 102.41 (3)°, Z = 8. Electrochemical and EPR studies show that redox-active crown ethers incorporating quinone groups successfully couple ion binding by the crown ether to the redox state of the quinone group. Alkali metal ions cause potential shifts that establish-differential redox-induced complexation that qualitatively and quantitatively differs from ion-pairing effects. They also perturb the EPR hyperfine splittings in the semiquinone moieties in a characteristic fashion, as well as in one case giving rise to ²³Na superhyperfine splitting.