Formation and Base Hydrolysis of Oxidimethaneamine Bridges in CoIII–Amine Complexes

cis-[CoL2]3+ (1a3+), trans-[CoL2]3+ (2a3+), cis-[Co­(MeL)2]3+ (1b3+), and trans-[Co­(MeL)2]3+ (2b3+), L = 1,4-diazepan-6-amine (daza) and MeL = 6-methyl-1,4-diazepan-6-amine (Medaza), were allowed to react as templates in acetonitrile with paraformaldehyde and triethylamine. Several CoIII complexes, where two adjacent amino groups of two ligand moieties are interlinked by an oxidimethaneamine bridge, were obtained. Connection of a primary with a secondary amino group (prim–sec bridging) was found to be predominant. The singly and doubly bridged daza- and Medaza-derivatives 7a3+, 9a3+ and 7b3+, 9b3+ were characterized by crystal-structure analysis. The bridging process resulted in a slight lengthening of the mean Co–N distance, a red shift of the A1g–T1g transition, and an increase of the CoIII/CoII reduction potential. Several minor components, which could be only partially separated by chromatographic methods, were also formed. The daza-derivatives 6a3+ (prim–prim bridged) and 10a3+ (bidentate coordination of one daza frame) formed in small quantities. The Medaza derivatives 3b3+ and 4b3+ (trans configuration of the Medaza frames, with additional pending carbinolamino groups), and 8b3+ (with a methylideneimino group) represent intermediates of the condensation process. Their structure was again corroborated by X-ray diffraction. All bridged species (6a3+, 7a3+, 7b3+, 8b3+, 9a3+, 9b3+, and 10a3+) exhibited exclusively a cis orientation of the two diazepane frames, even if the trans configured 2a3+ or 2b3+ were used as starting materials. Molecular mechanics calculations indicate that in the bridged species with a trans configuration steric strain is substantially more pronounced. In alkaline aqueous media, 9a3+ and 9b3+ revealed a complete degradation of the bridges whereby the original 1a3+ and 1b3+ reformed. The pseudo-first-order rate constant kobs of the degradation reaction was found to depend linearly on OH– concentration. The degradation of the first bridge is about 100 times faster than the degradation of the second. The mechanism of formation and degradation of such oxidimethaneamine bridges is discussed.