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

Sarah E. Clifford; Vanny Tiwow; Aleasia Gendron; Marcel Maeder; Monica Rossignoli; Geoffrey A. Lawrance; Peter Turner; Alexander J. Blake; Martin Schröder

doi:10.1071/CH09313

RESEARCH FRONT

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

Sarah E. Clifford ^A , Vanny Tiwow ^A , Aleasia Gendron ^A , Marcel Maeder ^A , Monica Rossignoli ^A , Geoffrey A. Lawrance ^A ^D , Peter Turner ^B , Alexander J. Blake ^C and Martin Schröder ^C

+ Author Affiliations

- Author Affiliations

^A Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia.

^B Crystal Structure Analysis Facility, School of Chemistry, The University of Sydney, NSW 2006, Australia.

^C School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK.

^D Corresponding author. Email: Geoffrey.Lawrance@newcastle.edu.au

Australian Journal of Chemistry 62(10) 1196-1206 https://doi.org/10.1071/CH09313
Submitted: 2 June 2009 Accepted: 19 June 2009 Published: 13 October 2009

Abstract

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 Pd^II 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 Cu^II as a bidentate ligand involving the primary and tertiary amines is also defined by an X-ray crystal structure.

Acknowledgements

Support of the Australian Research Council is gratefully acknowledged. G.A.L. thanks the University of Nottingham for provision of facilities while on sabbatical leave. S.E.C. and V.T. thank the University of Newcastle and AusAID respectively for doctoral scholarship support.

References

^* Dedicated to the memory of Professor Alan M. Sargeson FAA FRS – exceptional Australian chemist and mentor.

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

Abstract

Acknowledgements

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