Structural evaluation and solution integrity of alkali metal salt complexes of the manganese 12-metallacrown-4 (12-MC-4) structural type

The preparation of a variety of salt complexes of [12-MC(Mn(III)N(shi))-4] (1) provides the structural basis for the first quantitative investigation of the cation and anion selectivity of metallacrowns. The preparation, X-ray crystal structures, and solution integrities of crystalline salts (LiCl2)[12-MC(Mn(III)N(shi))-4](-) ([(LiCl2). 1](-)), (Li(trifluoroacetate))]12-MC(Mn(III)N(shi))-4] ([(LiTFA). 1]), (Li)[12-MC(Mn(III)N(shi))-4](+) ([(Li). 1](+)), (NaBr)(2)[12-MC(Mn(III)N(shi))-4] ([(NaBr)(2) . 1]), and (KBr)(2)[12-MC(Mn(III)N(shi))-4] ([(KBr)(2) . 1]) of the metallacrown [12-MC(Mn(III)N(shi))-4] (1) are described. Each salt complex of the metallacrown forms from a generic one-step, high-yield synthesis giving 1:1 metal:metallacrown adducts with lithium and 2:1 metal:metallacrown complexes with sodium and potassium ions. On the basis of synthetic preference, the trend for the cation affinity is Li+ > Na+ > K+ and that for anion affinity is Cl- > Br- > TFA(-) > F- approximate to I-3(-). The 12-metallacrown-4, structural parameters compare favorably with those of 12-crown-4, an organic crown ether, as well as with those of the topologically similar alkali metal complexes of porphyrin and phthalocyanine dianions, solidifying the structural analogy between metallacrowns and crown ethers. The solution integrities of the alkali metal halide salts of [12-MC(Mn(III)N(shi))-4] were confirmed by using paramagnetically shifted H-1 NMR, FAB-MS, ESI-MS, and UV-vis spectroscopies. Analysis of the H-1 NMR spectra and ESI-MS of the complexes proves that both the halide ions and the cations remain bound to the metallacrown upon dissolution. Investigations of metallacrown ligand exchange rates demonstrate that the metallacrowns are inert to ligand exchange in DMF and acetonitrile. This broad series of salt complexes of a single metallacrown allows for comparison of the structural features influencing the stability and specificity of joint cation/anion binding in this relatively new molecular class. X-ray parameters: (LiCl2)[12-MCh(Mn(III)N(shi))-4](-) ([(LiCl2). 1](-)), triclinic space group, P (1) over bar, a 12.516(2) Angstrom, b = 13.780(2) Angstrom, c = 19.943(3) Angstrom, alpha = 85.20(1)degrees, beta = 84.57(1)degrees, gamma = 72.18(1)degrees, V = 5059(2) Angstrom(3), Z = 2, R = 0.0773, R(w) = 0.0887; (Li(trifluoroacetate))[12-MC(Mn(III)N(shi))-4] ([(LiTFA). 1), (monoclinic space group, Cc, a = 20.261(6) Angstrom, b = 19.577(5) Angstrom, c = 16.134(5) Angstrom, beta = 99.81(2)degrees, Z = 4, refined on \F\(2), wR(2) = 0.271; (Li)[12-MC(Mn(III)N(shi))-4](+) ([(Li). 1](+)), (monoclinic space group, P2(1)/n, a = 14.814(4) Angstrom, b = 14.909(3) Angstrom, c = 32.26(1) Angstrom, beta = 102.42(2)degrees, Z = 4, refined on \F\(2), wR(2) = 0.0684; (NaBr)(2)[12-MC(Mn(III)N(shi))-4] ([(NaBr)(2) . 1]), monoclinic space group, P2(1)/n, a = 14.131(4) Angstrom, b = 13.845(3) Angstrom, c = 16.539(4) Angstrom, beta = 96.17(2)degrees, Z = 2, R = 0.0416, R(w) = 0.0419; (KBr)(2)[12-MC(Mn(III)N(shi))-4] ([(KBr)2 . 1), monoclinic space group, P2(1)/n, a = 11.654(3) Angstrom, b = 17.392(5) Iq, c = 16.786(5) Angstrom, beta = 98.40(2)degrees, Z = 2, R = 0.0649, R(w) = 0.0850.