Premium
A Cobalt Metallacrown Anion Host with Guest‐Dependent Redox Activity
Author(s) -
Jones Leigh F.,
Kilner Colin A.,
Halcrow Malcolm A.
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802368
Subject(s) - redox , cobalt , chemistry , host (biology) , ion , combinatorial chemistry , polymer chemistry , inorganic chemistry , organic chemistry , biology , ecology
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 (μ‐OH) 6 (μ‐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 (μ‐OH) 6 (μ‐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 ≈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.