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 (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 H41iPr and H52iPr
were combined with tris[(N‘-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H30iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH22iPr]- (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 [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 [CoIIH22iPr]- 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 [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 = 2.5 ×
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
[CoIIIH22iPr(OH)]- decays with a kint = 5.9 × 10-8 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.