Correlated four-component EPR g-tensors for doublet molecules.

The first correlated ab initio four-component calculations of electron paramagnetic resonance (EPR) g-tensors for doublet radicals are reported. We have implemented a first-order degenerate perturbation theory approach based on the four-component Dirac-Coulomb Hamiltonian and fully relativistic configuration interaction wave functions in the DIRAC program package. We find that the correlation effects on the g-tensors can be sufficiently well described with manageable basis sets of triple-zeta quality and manageable configuration spaces. The new fully relativistic EPR module in DIRAC should be useful for benchmarking density functional theory approaches, however, with future optimization of the code we believe it will also be useful for applications.

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

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)ClnCO2)(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 pKa value of the parent acetic or chloroacetic acid and the redox potential of the O2(2-)/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 pKa of the parent 1-naphthoic acid.

Switching on oxygen activation by cobalt complexes of pentadentate ligands.

The monoanionic N(4)O ligand N-methyl-N,N'-bis(2-pyridylmethyl)ethylenediamine-N'-acetate (mebpena(-)) undergoes oxidative C-N bond cleavage in the presence of Co(II) and O(2). The two resultant fragments are coordinated to the metal ion in the product [Co(III)(2-pyridylformate)(mepena)]ClO(4) (mepena(-) = N-methyl-N'-(2-pyridylmethyl)ethylenediamine-N'-acetato). Bond cleavage does not occur in the presence of chloride ions and [Co(III)(mebpena)Cl](+), containing intact mebpena(-), can be isolated. The oxidative instability of the mebpena(-) in the presence of Co(II) and air stands in contrast to the oxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobalt complexes. Cyclic voltammetry on the matched pair [Co(III)Cl(mebpena)](+) and [Co(II)Cl(bztpen)](+), bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridine donor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in the E°(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metal oxidation state in the complexes isolated under ambient conditions. DFT calculations support a proposal that [Co(II)(mebpena)](+) reacts with O(2) to form a Co(III)-superoxide complex which can abstract an H atom from a ligand methylene C atom as the initial step towards the observed oxidative C-N bond cleavage.