Metal Ion Cages: Capping Reactions With Bifunctional Methylene Compounds and Formaldehyde

Abstract

[(1,1,1-Tris(4-amino-2-azabutyl)ethane)cobalt(III)](3+) ([co(sen)]³⁺) and [(tris(4-amino-2- azabutyl)amine)cobalt(III)](3+) ([Co(azasen)]³⁺) react with formaldehyde and diethyl malonate or cyanoacetic acid ethyl ester in the presence of base to form macrobicyclic cages [ox- osarcophagine (oxosar) and oxoazasarcophagine (azaoxosar)] about the metal ion. The cage structure of the complexes has been established by an X-ray crystallographic analysis of [(1-carboxy-8-methyl-2-oxo-3,6,10,13, 16,19-hexaazabicyclo[6.6.6]icosanato)cobalt(III)] diperchlorate [Co(Me,CO₂H-oxosar- H)]²⁺ which crystallizes in the orthorhombic space group Pbcn, with cell parameters a 33.61, b 10.42, c 13.743 Å , and V 4813 Å ³ , and Z 8. The chiral cobalt(III) complex is characteristically inert to substitution and racemization, but the cobalt(II) complex, obtained by reduction of the appropriate cobalt(III) compound, is surprisingly stable both to loss of Co²⁺ ion and to racemization at 25°C (pH 7.1, t_½4.5 days). The syntheses, spectroscopic and chemical properties are reported. The kinetics of the electron self exchange for the cage system [Co(Me,CO2Et-oxosar-H)]^+/2+, and of H₂O₂ formation from [co^II(Me,co₂Et-oxosar - H)]⁺ and O² are reported. Similar syntheses have been carried out to half-cap the [co(en)₃]³⁺ ion (en = ethane-1,2-diamine). These and related reactions have allowed substituents such as COOR, COOH, CN, COCl, CONR₂, NH₂ and NO₂ to be placed on the bridgehead carbon atoms, and have altered the redox potentials of the systems by at least 0.3 V. The oxosar CO^II ions are useful as powerful reducing agents [from c. -0.3 to -0.6 V (v. n.h.e.)], and the cages are capable of further derivatization to build larger macromolecules.