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Ligand Reprogramming in Dinuclear Helicate Complexes: A Consequence of Allosteric or Electrostatic Effects?
Author(s) -
Jeffery John C.,
Rice Craig R.,
Harding Lindsay P.,
Baylies Christian J.,
RiisJohannessen Thomas
Publication year - 2007
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.200700261
Subject(s) - allosteric regulation , reprogramming , ligand (biochemistry) , chemistry , biophysics , stereochemistry , biology , biochemistry , receptor , cell
The ditopic ligand 6,6′‐bis(4‐methylthiazol‐2‐yl)‐3,3′‐([18]crown‐6)‐2,2′‐bipyridine ( L 1 ) contains both a potentially tetradentate pyridyl‐thiazole (py‐tz) N ‐ donor chain and an additional “external” crown ether binding site which spans the central 2,2′‐bipyridine unit. In polar solvents (MeCN, MeNO 2 ) this ligand forms complexes with Zn II , Cd II , Hg II and Cu I ions via coordination of the N donors to the metal ion. Reaction with both Hg II and Cu I ions results in the self‐assembly of dinuclear double‐stranded helicate complexes. The ligands are partitioned by rotation about the central pypy bond, such that each can coordinate to both metals as a bis‐bidentate donor ligand. With Zn II ions a single‐stranded mononuclear species is formed in which one ligand coordinates the metal ion in a planar tetradentate fashion. Reaction with Cd II ions gives rise to an equilibrium between both the dinuclear double‐stranded helicate and the mononuclear species. These complexes can further coordinate s‐block metal cations via the remote crown ether O ‐ donor domains; a consequence of which are some remarkable changes in the binding modes of the N‐donor domains. Reaction of the Hg II ‐ or Cd II ‐containing helicate with either Ba 2+ or Sr 2+ ions effectively reprogrammes the ligand to form only the single‐stranded heterobinuclear complexes [MM′( L 1 )] 4+ (M=Hg II , Cd II ; M′=Ba 2+ , Sr 2+ ), where the transition and s‐block cations reside in the N‐ and O‐donor sites, respectively. In contrast, the same ions have only a minor structural impact on the Zn II species, which already exists as a single‐stranded mononuclear complex. Similar reactions with the Cd II system result in a shift in equilibrium towards the single‐stranded species, the extent of which depends on the size and charge of the s‐block cation in question. Reaction of the dicopper(I) double‐stranded helicate with Ba 2+ shows that the dinuclear structure still remains intact but the pitch length is significantly increased.