Zinc and cadmium tropocoronand complexes: Effect of metal ion radius on macrocyclic ligand twist and fold

The four-coordinate [Zn(TC-n,m)], n + m = 7, 8, 9, 10, and 12, and [Cd(TC-n,m)], n + m 8, 9, 10, 12, complexes and the five-coordinate complex [Zn(py)(TC-3,3)] were synthesized and characterized by H-1 NMR and W-vis spectroscopy as well as by X-ray crystallography. The dihedral angle Theta between five-membered N-M-N chelate ring planes increases monotonically as a function of the number of methylene linker units, n + m. The Theta angles range from 36.4 degrees in [Zn(TC-3,4)] to 84.7 degrees in [Zn(TC-6,6)] and from 36.5 degrees in [Cd(TC-4,4)] to 78.9 degrees in [Cd(TC-6,6)]. For a given value of n + m, a larger dihedral angle is observed in the [M(TC-n,m)] complex having the smaller metal ion radius. Although changes in Theta largely reflect a twisting motion of the two halves of the macrocycle with respect to one another, in [Cd(TC-4,4)] and [Zn(py)(TC-3,3)] the tropocoronand ligand folds in order to accommodate the metal ion, which is too large to fit into the hole of the macrocycle. Cyclic voltammetric studies in THF and CH2Cl2 revealed a window from -2.25 < E-1/2 < +0.5 V vs ferrocenium/ferrocene where the TC-n,m ligands are not redox active. The structural and electrochemical information afforded by this detailed investigation is essential for interpreting ligand field effects on the physical and chemical properties of tropocoronand complexes of transition metal ions having partially filled d-shells. Crystallographic information: [Zn(py)(TC-3,3)], C2/c, a 19.637(2) Angstrom, b = 8.6418(7) Angstrom, c = 13.5516(11) Angstrom, beta = 108.598(1)degrees, V = 2179.6(3) Angstrom(3), Z = 4, R = 0.0580, wR(2) = 0.1036; [Zn(TC-3,4)], P2(1)/c, a = 10.5582(4) Angstrom, b = 8.5280(2) Angstrom, c 20.1471(8) Angstrom, beta = 93.587(1)degrees, V = 1810.5(1) Angstrom(3), Z = 4, R = 0.0284, wR(2) = 0.0663; [Zn(TC-4,4)], Fdd2, a = 19.012(2) Angstrom, b = 46.316(2) Angstrom, c = 8.517(2) Angstrom, V = 7500(3) Angstrom(3), Z = 16, R = 0.0561, wR(2) = 0.1444; [Zn(TC-4,5)], P2/c, a = 22.0993(3) Angstrom, b = 9.3733(2) Angstrom, c = 19.9053(2) Angstrom, beta = 106.552(1)degrees, V = 3952.39(11) Angstrom(3), Z = 8, R = 0.0905, wR(2) = 0.1811; [Zn(TC-5,5)], P6(1)22, a = 10.955(1) Angstrom, c = 30.622(4) Angstrom, V = 3183(1) Angstrom(3), Z = 6, R = 0.0593, wR(2) = 0.1160; [in(TC-6,6)], P2(1)2(1)2(1), a = 10.647(2) Angstrom, b = 11.4171(2) Angstrom, c = 19.027(4) Angstrom, V = 2312.9(8) Angstrom(3), Z = 4, R = 0.0473, wR(2) = 0.0951; [Cd(TC-4,4)1, P2(1)/c, a = 10.2448(9) Angstrom, b = 19.4345(2) Angstrom, c = 9.9038(9) Angstrom, beta = 101.107(1)degrees, V = 1935.0(3) Angstrom(3), Z = 4, R = 0.0675, wR(2) = 0.1516; [Cd(TC-4,5)1, P2(1)/n, a = 9.6688(5) Angstrom, b = 22.9915(12) Angstrom, c = 10.3002(5) Angstrom, beta = 117.268(1)degrees, V = 2035.3(2) Angstrom 3, 2 = 4, R = 0.0778, wR(2) = 0.1381; [Cd(TC-5,5)], C2/c, a = 12.9224(4) Angstrom, b = 17.0359(5) Angstrom, c = 9.5653(3) Angstrom, beta = 97.562(1)degrees, V 2087.4(4) Angstrom(3), Z = 4, R = 0.0203, wR(2) = 0.0504; [Cd(TC-6,6)], P2(1)2(1)2(1), a = 11.1429(1) Angstrom, b = 11.5325(2) Angstrom, c = 18.3954(3) Angstrom, V = 2363.91(6) Angstrom(3), Z = 4, R = 0.0206, wR(2) = 0.0475.