A Cobalt Metallacrown Anion Host with Guest-Dependent Redox Activity



Two bowl-shaped cavities, each having three OH hydrogen-bond donors at its base, are present in double-cone-shaped metallacrown anion host [Co6(μ-OH)6(μ-L)6]m+ (1m+; HL=3{5}-(pyrid-2-yl)-5{3}-(tert-butyl)pyrazole). Depending on its affinity for the anions present, it can be isolated in its CoIII3CoII3 (m=3; e.g., 1(ClO4)3) and CoIII2CoII4 (m=2; e.g., 1(BF4)2n H2O) oxidation states. See picture for photographs of isolated salts.

original image

Reaction of cobalt(II) salts with one equivalent of 3{5}-(pyrid-2-yl)-5{3}-(tert-butyl)pyrazole (HL) and NaOH in EtOH affords salts of [Co6(μ-OH)6(μ-L)6]m+ (2m+, m=2 or 3). This is formed from alternating tetrahedral and octahedral cobalt centers, giving a double-cone-shaped molecule with two bowl-shaped cavities. Each cavity has three OH hydrogen-bond donors at its base, giving them an affinity for inorganic anions. Reactions with CoX2 (X=ClO4 or CF3SO3) afford products with the stoichiometry 2X3. In contrast, when salts of fluorinated anions are used, the products have the formulae 2Y2n H2O (Y=BF4, PF6, or SbF6; n≈2). The connectivity and topology of the metallacrown were confirmed by crystal structures of three of these salts. Magnetic measurements imply that the tricationic products have the oxidation state CoIII3CoII3, while the dications are formulated as CoIII2CoII4. Solutions of 2X3 contain purely 23+ according to NMR spectroscopy, ES mass spectrometry, and UV/Vis/NIR spectroscopy, but solutions of 2Y2n H2O contain more than one significant component. The 23+/22+ couple in MeCN is irreversible at room temperature by cyclic voltammetry, occurring near −1.1 V versus ferrocene/ferrocenium. Importantly, the 2Y2n H2O compounds do not exhibit this process in the presence of 0.1 M [NBu4][BPh4] as base electrolyte, but they do show it when the appropriate [NBu4]Y salt is used (where the Y ions are present in 103-fold excess). Conversely, the 2X3 complexes exhibit a strong 23+/22+ reduction under both experimental conditions. Hence, the isolation of 23+ or 22+ in the presence of different anions appears to be controlled by the affinity of those anions for the metallacrown.