Complexation of Constrained Ligands Piperazine, N-substituted Piperazines, and Thiomorpholine*

Complexation of the symmetric cyclic diamine piperazine (1,4-diazacyclohexane) has been examined in dry dimethyl formamide by spectrophotometric titrations (with Cu2+, Ni2+) to define formation constants, and by stopped-flow kinetics to define the complexation rates and reaction pathway. Initial formation of a rarely observed η1-piperazine intermediate occurs in a rapid second-order reactions. This intermediate then undergoes two competing reactions: formation of (chelated) η2-piperazine (ML) or the formation of (bridging) μ-piperazine (in M2L and M2L3, speciation depending on relative concentrations). Protonation constants and formation constants for complexation in water of N-ethylpiperazine and thiomorpholine (1-aza-4-thiocyclohexane, tm) have been determined by potentiometric titration; 1:1 complexes with first-row M2+ display a log K from ~4 to 6, with speciation that suggests chelation of the heterocycles may be involved. Complexation of thiomorpholine has been further probed by the synthesis of PdII complexes. The N-monodentate coordination mode has been confirmed in trans-[Pd(tm)2Br2] by an X-ray crystal structure. Complexation of N-(2-aminoethyl)piperazine to CuII as a bidentate ligand involving the primary and tertiary amines is also defined by an X-ray crystal structure.