Palladium(II) as a versatile template for the formation of tetraaza macrocycles via Mannich-type reactions

The versatility of palladium(II) as a template for Mannich-type macrocyclization is illustrated. Reaction of (bis(3-aminopropyl)piperazine)palladium(II) with formaldehyde and nitroethane in basic aqueous solution yields the ‘reinforced’ macrocycle 7-methyl-7-nitro-1,5,9,13-tetraazabicyclo[11.2.2]heptadecane as its palladium(II) complex. The crystal structure shows the palladium ion lies in a slightly tetrahedrally distorted square plane of four nitrogen donors, with distances to the two tertiary donors [av. 2.059(3) Å] slightly shorter than those to the secondary amines [av. 2.066(3) Å]. The 3-methyl-3-nitro-1,5,9,13-tetraazacyclohexadecane as its palladium(II) complex was prepared by an analogous route. In a separate reaction based on the [Pd(en)(chxn)]²⁺ (en = ethane-1,2-diamine; chxn = cyclohexane-1,2-diamine) intermediate, an unsymmetrical macrocycle with a fused cyclohexane ring, 4,11-dimethyl-4,11-dinitro-2,6,9,13-tetraazabicyclo[12.4.0]octadecane was isolated as its palladium(II) complex. Accessibility to an isolable mixed-ligand precursor is a key to this reaction, provided by using palladium(II) as the templating metal. Reaction of (4,8-diazaundecane-1,11-diamine)palladium(II) with formaldehyde and diethyl malonate in basic aqueous solution yields, with ester hydrolysis and decarboxylation, the carboxylate-pendant macrocycle 1,5,9,13-tetraazacyclohexadecane-3-carboxylic acid as its palladium(II) complex. The crystal structure is comprised of hydrogen-bonded dimers {[Pd(L)][Pd(L-H)]}³⁺ where the pair of inversion related square-planar complexes share a single proton between their pendant carboxylates. Bis(3-aminopropyl)(piperazine)palladium(II) yields the macrocyclic complex ion (1,5,9,13-tetraazabicyclo[11.2.2]heptadecane-7-carboxylic acid)palladium(II), in a similar reaction.