A Cobalt Metallacrown Anion Host with Guest‐Dependent Redox Activity

Abstract

Two bowl-shaped cavities, each having three OH(-) hydrogen-bond donors at its base, are present in double-cone-shaped metallacrown anion host [Co(6)(mu-OH)(6)(mu-L)(6)](m+) (1(m+); 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 Co(III) (3)Co(II) (3) (m=3; e.g., 1(ClO(4))(3)) and Co(III) (2)Co(II) (4) (m=2; e.g., 1(BF(4))(2)n H(2)O) oxidation states. See picture for photographs of isolated salts.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 [Co(6)(mu-OH)(6)(mu-L)(6)](m+) (2(m+), 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 CoX(2) (X(-)=ClO(4) (-) or CF(3)SO(3) (-)) afford products with the stoichiometry 2X(3). In contrast, when salts of fluorinated anions are used, the products have the formulae 2Y(2)n H(2)O (Y(-)=BF(4) (-), PF(6) (-), or SbF(6) (-); n approximately 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 Co(III) (3)Co(II) (3), while the dications are formulated as Co(III) (2)Co(II) (4). Solutions of 2X(3) contain purely 2(3+) according to NMR spectroscopy, ES mass spectrometry, and UV/Vis/NIR spectroscopy, but solutions of 2Y(2)n H(2)O contain more than one significant component. The 2(3+)/2(2+) couple in MeCN is irreversible at room temperature by cyclic voltammetry, occurring near -1.1 V versus ferrocene/ferrocenium. Importantly, the 2Y(2)n H(2)O compounds do not exhibit this process in the presence of 0.1 M [NBu(4)][BPh(4)] as base electrolyte, but they do show it when the appropriate [NBu(4)]Y salt is used (where the Y(-) ions are present in 10(3)-fold excess). Conversely, the 2X(3) complexes exhibit a strong 2(3+)/2(2+) reduction under both experimental conditions. Hence, the isolation of 2(3+) or 2(2+) in the presence of different anions appears to be controlled by the affinity of those anions for the metallacrown.