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Copper(II) Coordination Chemistry of Westiellamide and Its Imidazole, Oxazole, and Thiazole Analogues
Author(s) -
Comba Peter,
Gahan Lawrence R.,
Haberhauer Gebhard,
Hanson Graeme R.,
Noble Christopher J.,
Seibold Björn,
van den Brenk Anna L.
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701778
Subject(s) - chemistry , copper , oxazole , imidazole , electron paramagnetic resonance , coordination complex , crystallography , thiazole , coordination sphere , stereochemistry , molecule , square pyramidal molecular geometry , metal , crystal structure , organic chemistry , physics , nuclear magnetic resonance
The copper( II ) coordination chemistry of westiellamide (H 3 L wa ), as well as of three synthetic analogues with an [18]azacrown‐6 macrocyclic structure but with three imidazole (H 3 L 1 ), oxazole (H 3 L 2 ), and thiazole (H 3 L 3 ) rings instead of oxazoline, is reported. As in the larger patellamide rings, the N heterocycle ‐N peptide ‐N heterocycle binding site is highly preorganized for copper(II) coordination. In contrast to earlier reports, the macrocyclic peptides have been found to form stable mono‐ and dinuclear copper(II) complexes. The coordination of copper(II) has been monitored by high‐resolution electrospray mass spectrometry (ESI‐MS), spectrophotometric and polarimetric titrations, and EPR and IR spectroscopies, and the structural assignments have been supported by time‐dependent studies (UV/Vis/NIR, ESI‐MS, and EPR) of the complexation reaction of copper(II) with H 3 L 1 . Density functional theory (DFT) calculations have been used to model the structures of the copper(II) complexes on the basis of their spectroscopic data. The copper(II) ion has a distorted square‐pyramidal geometry with one or two coordinated solvent molecules (CH 3 OH) in the mononuclear copper(II) cyclic peptide complexes, but the coordination sphere in [Cu(H 2 L wa )(OHCH 3 )] + differs from those in the synthetic analogues, [Cu(H 2 L)(OHCH 3 ) 2 ] + (L=L 1 , L 2 , L 3 ). Dinuclear copper(II) complexes ([Cu II 2 (HL)(μ‐X)] + ; X=OCH 3 , OH; L=L 1 , L 2 , L 3 , L wa ) are observed in the mass spectra. While a dipole–dipole coupled EPR spectrum is observed for the dinuclear copper(II) complex of H 3 L 3 , the corresponding complexes with H 3 L (L=L 1 , L 2 , L wa ) are EPR‐silent. This may be explained in terms of strong antiferromagnetic coupling (H 3 L 1 ) and/or a low concentration of the dicopper(II) complexes (H 3 L wa , H 3 L 2 ), in agreement with the mass spectrometric observations.