Tuning affinity and reversibility for O sub(2) binding in dinuclear Co(ii) complexes

• Published in: Dalton transactions : an international journal of inorganic chemistry 2013-06, Vol.42 (27), p.9921-9929
• Main Authors: Vad, Mads S, Johansson, Frank B, Seidler-Egdal, Rune Kirk, McGrady, John E, Novikov, Sergey M, Bozhevolnyi, Sergey I, Bond, Andrew D, McKenzie, Christine J
• Format: Article
• Language: English
• Subjects: Affinity; Atomic properties; Binding; Crystal structure; Electrode potentials; Ligands; Parents; Tuning
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c3dt50617g

The O sub(2) binding affinity of a series of dicobalt(ii) complexes can be tuned between p(O sub(2)) sub(50%) = 2.3 10 super(-3) and 700 10 super(-3) atm at 40 degree C by varying the number of H and Cl atoms in the bridging acetato ligands of [Co sub(2)(bpbp)(CH sub((3-)nCl sub(n)CO sub(2))(CH sub(3)CN) sub(2)] super(2)+ where bpbp super(-) = 2,6-bis(N,N-bis(2-pyridylmethyl)a minomethyl)-4-tert-butylphenolate and n= {0, 1, 2, 3}. O sub(2) binds most strongly to the deoxy complex containing the acetato bridge and the O sub(2) affinity decreases linearly as the number of Cl atoms is increased from 0 to 3 in [Co sub(2)(bpbp)(O sub(2))(CH sub(3)CO sub(2))] super(2+), [Co sub(2)(bpbp)(O sub(2))(CH sub(2)ClCO sub(2))] super(2+), [Co sub(2)(bpbp)(O sub(2))(CHCl sub(2)CO sub(2))] super(2+) and [Co sub(2)(bpbp)(O sub(2))(CCl sub(3)CO sub(2))] super(2+). The O sub(2) affinities can be qualitatively correlated with both the pK sub(a) value of the parent acetic or chloroacetic acid and the redox potential of the O sub(2) super(2-)/O sub(2)&z.rad; super(-) couple measured for the peroxide-bridged complexes. The redox potential varies between 510 mV (vs.Fc super(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 O sub(2) affinity in the O-O bond lengths in the X-ray crystal structures at 180 K (1.415(4)-1.424(2) Aa) or in the frequencies of the peroxido O-O stretch in the solid-state resonance Raman spectra at 298 K (830-836 cm super(-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 O sub(2) binding process. The alternative of five coordination in the deoxy Co(ii) complexes is therefore seen as less likely. The crystal structure and p(O sub(2)) sub(50%) are also reported for the 1-naphthoato-bridged oxy complex [Co sub(2)(bpbp)(O sub(2))(C sub(10)H sub(7)O sub(2))] super(2+), and the O sub(2) binding affinity in that case is also qualitatively consistent with the expectation from the pK sub(a) of the parent 1-naphthoic acid.