865255-93-2Relevant articles and documents
A modular approach toward regulating the secondary coordination sphere of metal ions: Differential dioxygen activation assisted by intramolecular hydrogen bonds
Lucas, Robie L.,Zart, Matthew K.,Murkerjee, Jhumpa,Sorrell, Thomas N.,Powell, Douglas R.,Borovik
, p. 15476 - 15489 (2007/10/03)
Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure-function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea-carboxamide ligands that provide varying intramolecular hydrogen bond (C-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N′-tert-butylureayl)-N- ethyl]-bis(N″-R-carbamoylmethyl)amine (H41R) and bis[(N′-tert-butylureayl)-N-ethyl]-(N″-R-carbamoylmethyl)amine (H52R), where R = isopropyl, cyclopentyl, and (S)-(-)-α-methylbenzyl. The ligands with isopropyl groups H 41iPr and H52iPr were combined with tris[(N′-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)-amine (H3OiPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH 22iP7]- (two H-bond donors), [Co IIH1iPr]- (one H-bond donor), and [Co IIOiPr]- (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S = 3/2 ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (Epa) with [CoIIH22iPr]- being the most negative at -0.385 V vs [Cp2Fe]+/[Cp2Fe]. The properties of [CoIIH3buea]- (H3buea, tris[(N′-tert-butylureaylato)-N-ethyl]aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea]- and [Co IIH22iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH22iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: [CoIIIH3buea(16OH)] - has a ν(O-H) band at 3589 cm-1 that shifts to 3579 cm-1 in [CoIIIH3buea(18OH)] -; [CoIIIH22iPr(OH)]- has ν(16O-H) = 3661 and ν(18O-H) = 3650 cm -1. [CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating [CoIIH1 iPr]- with excess dioxygen initially produces a species with an X-band EPR signal at g = 2.0 that is assigned to a Co-O2 adduct, which is not stable and converts to a species having properties similar to those of the CoIII-OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (kint = 2.5 × 10-7 M min-1). Moreover, the stability of the CoIII-OH complexes is correlated with the number of H-bond donors within the secondary coordination sphere; [CoIIIH3buea(OH) ]- is stable in solution for days, whereas [CoIIIH 22iPr(OH)]- decays with a kint = 5.9 × 10-8 M min-1. The system without any intramolecular H-bond donors [CoIIOiPr]- does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H-bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the CoIII-OH complexes with [CoIIIH 3buea(OH)]- being the most stable. We suggest that the greater number of intramolecular H-bonds involving the hydroxo ligand reduces the nucleophilicity of the CoIII-OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.
