The influence of alkyl substituents on carbon acid-formaldehyde condensation reactions of bis(alkane-1,2-diamine)copper(II) ions

The reaction of the bis(1,2-diamine)copper(II) complexes of ethane-1,2-diamine (en), propane-1,2-diamine (pn) and 2-methylpropane-1,2-diamine (dmen) with formaldehyde and nitroethane in methanol under basic conditions yields macrocyclic and acyclic condensation products with two or one new -NH-CH2-C(Me)(NO2)-CH2-NH- links introduced respectively between pairs of cis-disposed primary amines, and where the ratio of products is, surprisingly, significantly altered by C-methyl substitution on the parent ethane-1,2-diamine chelate. Under identical reaction conditions, the ratio of acyclic to macrocyclic product varied from 2:3 (en) to 10:1 (pn) to 50:1 (dmen) as first one methyl and then gem-dimethyl groups are attached to each parent chelate ring. That the reaction is influenced by substituent steric effects is supported by the structural characterization of the zinc-acid reduced and recomplexed acyclic species, which indicates that first ring formation occurs at amine pairs distant from the C-methyl substituents. The mu-chloro-bis(6-methyl-4,8-diazaundecane-2,6,10-triamine)dicopper(II) perchlorate crystallized in the space group C 2/c, a 23.003(6), b 12.641(3), c 19.120(6) Angstrom, beta 127.88(2)degrees, whereas the (2,6,10-trimethyl-4,8-diazaundecane-2,6,10-triamine)copper(II) perchlorate crystallized in the space group Cc2a, a 43.769(3), b 14.509(3), c 17.750(8) Angstrom. The copper ion is found in a distorted octahedral environment in the former with two complex units weakly bridged by a chloride anion, and in a square-based pyramidal environment in the latter.