Rigidity extremes in coordination chemistry: a fine energetic balance in the isolation of a trapped ligand inversion intermediateElectronic supplementary information (ESI) available: Experimental, crystallographic and computational details. See http://www.rsc.org/suppdata/cc/b4/b405358c/

Abstract

An unusual copper(ii) complex of a highly rigid and bulky ligand (a macrocycle-glyoxal condensate) has been synthesized and investigated via DFT calculations and structural characterisation.

Probing the Limits of Tetraazamacrocycle‐Glyoxal Condensates as Bidentate Ligands for Cu2+

“…Turning to the dichlorido complexes of ligands 5 – 7 , we find similar cyclic voltammograms to that depicted in Figure 6, even though the coordination of ligands 5 and 6 , as discussed above (see Figure 3), involves two adjacent nitrogen atoms of the same seven‐membered ring. An X‐ray crystal structure of Cu( 7 )Cl 2 has been published26 and shows that the Cu 2+ ion is coordinated to three nitrogen atoms of the highly distorted glyoxal condensate as well as to two chlorido ligands. This distortion is caused by the highly substituted macrocycle ring, which is forced to go through an inversion intermediate in order to produce a conformation capable of sterically binding the Cu 2+ ion 26…”

“…An X‐ray crystal structure of Cu( 7 )Cl 2 has been published26 and shows that the Cu 2+ ion is coordinated to three nitrogen atoms of the highly distorted glyoxal condensate as well as to two chlorido ligands. This distortion is caused by the highly substituted macrocycle ring, which is forced to go through an inversion intermediate in order to produce a conformation capable of sterically binding the Cu 2+ ion 26…”

“…From this example, we learned that changes to the macrocycle skeleton can greatly modify the coordination chemistry of the ligand, while still allowing the glyoxal condensate to coordinate Cu 2+ . As part of an investigation of the potential for distortion of the coordination geometry away from planarity, we reported a similar complexation using a highly substituted macrocycle–glyoxal condensate (Scheme , structure 7 ) in order to modify the cavity in which the Cu 2+ binds 26. This complex showed a conformational departure from the free ligand structure that was not predicted.…”

“…We,10,19 and others,15,20–22 have also exploited the predictable regioselective alkylation of these interesting molecules to produce linked analogues containing two or three tetraazamacrocycles joined together. Yet, our previous work23–26 appears to remain the only examples of exploiting these macrocycle condensates themselves as ligands for transition metal ions.…”

“…This complex showed a conformational departure from the free ligand structure that was not predicted. We proposed that the structurally characterized product was a trapped intermediate in the known inversion equilibrium of macrocycle‐glyoxal condensates of this type 26…”