A modular approach toward regulating the secondary coordination sphere of metal ions: differential dioxygen activation assisted by intramolecular hydrogen bonds

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 (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands (N'-tert-butylureayl)-N-ethyl]-bis(N' '-R-carbamoylmethyl)amine (H(4)1R) and bis(N'-tert-butylureayl)-N-ethyl]-(N' '-R-carbamoylmethyl)amine (H(5)2R), where R=isopropyl, cyclopentyl, and (S)-(-)-alpha-methylbenzyl. The ligands with isopropyl groups H(4)1iPr and H(5)2iPr were combined with tris(N'-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H(3)0iPr) to prepare a series of Co(II) complexes with varying H-bond donors. CoIIH(2)2iPr]- (two H-bond donors), CoIIH1iPr]- (one H-bond donor), and CoII0iPr]- (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 CoIIH(2)2iPr]- 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 CoIIH(2)2iPr]- react with 0.5 equiv of dioxygen to afford CoIIIH3buea(OH)]- and CoIIIH(2)2iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: CoIIIH3buea(16OH)]- has a -(O-H) band at 3589 cm-1 that shifts to 3579 cm-1 in CoIIIH3buea(18OH)]-; CoIIIH(2)2iPr(OH)]- has -(16O-H)=3661 and -(18O-H)=3650 cm-1. CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating CoIIH1iPr]- 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=2.5x10-7 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(2)2iPr(OH)]- decays with a kint=5.9x10-8 M min-1. The system without any intramolecular H-bond donors CoII0iPr]- 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 CoIIIH3buea(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.