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 (H4 1 R) and bis[(N‘-tert-butylureayl)-N-ethyl]-(N‘ ‘-R-carbamoylmethyl)amine (H5 2 R), where R = isopropyl, cyclopentyl, and (S)-(−)-α-methylbenzyl. The ligands with isopropyl groups H4 1 iPr and H5 2 iPr were combined with tris[(N‘-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H3 0 iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH2 2 iPr]- (two H-bond donors), [CoIIH1 iPr]- (one H-bond donor), and [CoII 0 iPr]- (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 (E pa) with [CoIIH2 2 iPr]- 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 [CoIIH2 2 iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH2 2 iPr(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)]-; [CoIIIH2 2 iPr(OH)]- has ν(16O−H) = 3661 and ν(18O−H) = 3650 cm-1. [CoIIH1 iPr]- 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 instabil