Facially coordinating cyclic triamines, Part 1 - The coordination chemistry of cis-3,5-diaminopiperidine and substituted derivatives

An efficient and convenient method for the preparation of cis-3,5-diaminopi peridine (dapi) has been established and the coordination chemistry of this ligand with Co-II, Co-III, Ni-II, Cu-II, Zn-II, and Cd-II has been investi gated in the solid state and in aqueous solution. Potentiometric measuremen ts revealed a generally high stability for the bis complexes of the divalen t cations with maximum stability for Ni-II (log beta(2) = 21.2, beta(2) = [ M(dapi)(2)][M](-1) [dapi](-2), 25 degrees C, mu = 0.1 mol dm(-3)). Cyclic v oltammetry established quasi-reversible formation of [Ni(dapi)(2)](3+) with a redox potential of 0.91 V (versus NHE) for the Ni-II/III couple. [Co(dap i)(2)](3+) was prepared by aerial oxidation of the corresponding Co-II prec ursor. The two isomers trans[Co(dapi)(2)](3+) (1(3+), 26%) and cis[Co(dapi) (2)](3+) (2(3+), 74%), have been separated and isolated as solid Cl- and CF 3SO3- salts. In a non-aqueous medium 1(3+) and 2(3+) reacted with paraforma ldehyde and NEt3 to give the methylidene-imino derivatives 3(3+) and 4(3+), in which the two piperidine rings are bridged by two or one N-CH3-O-CH2-N bridges, respectively. Crystal structure analyses were performed for H(3)da pi[ZnCl4]Cl, 1Cl(3). 2H(2)O, 2Cl(3). H2O 3[ZnCl4]Cl, 4[ZnCl4]Cl, [Ni(dapi)( 2)]Cl-2. H2O, [Cu(dapi)(2)](NO3)(2), [Cu(dapi)Cl-2], [(dapi)ClCd-(mu-Cl)(2) -CdCl(dapi)], and [Co(dapi)(NO2)(CO3)]. The stability of [M-II(dapi)](2+) a nd [M-II(dapi)(2)](2+) complexes in aqueous solution, particularly the rema rkably high tendency of [M(dapi)](2+) to undergo coordinative disproportion ation is discussed in terms of the specific steric requirements of this lig and, Molecular mechanics calculations have been performed to analyze the di fferent types of strain in these complexes. A variety of alkylated derivati ves of dapi have been prepared by reductive alkylation with formaldehyde, b enzaldehyde, salicylaldehyde, and pyridine-2-carbaldehyde. The Ni-II comple xes of the pentadentate N-3,N-5-bis-(2-pyridinylmethyl)-cis-3,5-diaminopipe ridine (py(2)dapi) and the hexadentate N-3,N-5,1-tris(2-pyridinylmethyl)-ci s-3,5-diaminopiperidine (py(3)dapi) have been isolated as crystalline ClO4- salts [Ni(py(2)dapi)Cl]ClO4 and [Ni(py(3)dapi)](ClO4)(2). H2O and characte rized by crystal structure analyses.