Tuning affinity and reversibility for O2 binding in dinuclear Co(ii) complexes

Abstract

The O₂ binding affinity of a series of dicobalt(II) complexes can be tuned between p(O₂)_50% = 2.3 × 10⁻³ and 700 × 10⁻³ atm at 40 °C by varying the number of H and Cl atoms in the bridging acetato ligands of [Co₂(bpbp)(CH_(3−n)Cl_nCO₂)(CH₃CN)₂]²⁺, where bpbp⁻ = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolate and n = {0, 1, 2, 3}. O₂ binds most strongly to the deoxy complex containing the acetato bridge and the O₂ affinity decreases linearly as the number of Cl atoms is increased from 0 to 3 in [Co₂(bpbp)(O₂)(CH₃CO₂)]²⁺, [Co₂(bpbp)(O₂)(CH₂ClCO₂)]²⁺, [Co₂(bpbp)(O₂)(CHCl₂CO₂)]²⁺ and [Co₂(bpbp)(O₂)(CCl₃CO₂)]²⁺. The O₂ affinities can be qualitatively correlated with both the pK_a value of the parent acetic or chloroacetic acid and the redox potential of the O₂²⁻/O₂˙⁻ couple measured for the peroxide-bridged complexes. The redox potential varies between 510 mV (vs. Fc^0/+) for the acetato-bridged complex to 696 mV for the trichloroacetato-bridged system. Despite the clear difference in reactivity in solution, there are no clear trends which can be correlated to O₂ affinity in the O–O bond lengths in the X-ray crystal structures at 180 K (1.415(4)–1.424(2) Å) or in the frequencies of the peroxido O–O stretch in the solid-state resonance Raman spectra at 298 K (830–836 cm⁻¹). Using density functional theory calculations, we conclude that the Co(II) atoms of the deoxy complexes coordinate solvent molecules as auxiliary ligands and that a conformation change of the ligand is involved in the reversible O₂ binding process. The alternative of five coordination in the deoxy Co(II) complexes is therefore seen as less likely. The crystal structure and p(O₂)_50% are also reported for the 1-naphthoato-bridged oxy complex [Co₂(bpbp)(O₂)(C₁₀H₇O₂)]²⁺, and the O₂ binding affinity in that case is also qualitatively consistent with the expectation from the pK_a of the parent 1-naphthoic acid.