Preparation of organocobalt(iii) complexes via O2 activation

Pressure-induced single-crystal-to-single-crystal nitrite ligand isomerisation accompanied by a piezochromic effect

High-pressure structural study of a piezochromic crystal of a rare di-exo-nitrito nickel(II) complex supported by computational analysis is presented. The examined system is not photoswitchable, however, in the 0-6.2 GPa pressure range the crystal undergoes two phase-transitions accompanied by a colour change and the nitrite ligand isomerisation, which is unique.

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

Increasing the electron donation in a dinucleating ligand family: molecular and electronic structures in a series of CoIICoII complexes

We have developed a family of dinucleating ligands with varying terminal donors to generate dinuclear peroxo and high-valent complexes and to correlate their stabilities and reactivities with their molecular and electronic structures as a function of the terminal donors. It appears that the electron-donating ability of the terminal donors is an important handle for controlling these stabilities and reactivities. Here, we present the synthesis of a new dinucleating ligand with potentially strong donating terminal imidazole donors. As CoII ions are sensitive to variations in donor strength in terms of coordination number, magnetism, UV-Vis-NIR spectra, redox potentials, we probe the electron donation ability of this new ligand in CoIICoII complexes in comparison to the parent CoIICoII complexes with terminal pyridine donors and we synthesize the analogous CoIICoII complexes with terminal 6-methylpyridines and methoxy-substituted pyridines. The molecular structures show indeed strong variations in coordination numbers and bond lengths. These differences in the molecular structures are reflected in the magnetic properties and in the d-d transitions demonstrating that the molecular structures remain intact upon dissolution. The redox potentials are analyzed with respect to the electron donation ability and are the only handle to observe an effect of the methoxy-substituted pyridines. All data taken together show the following order of electron donating ability for the terminal donors: 6-methylpyridines ≪ pyridines < methoxy-substituted pyridines ≪ imidazoles.

Structure Activity Relationships for Reversible O2 Chemisorption by the Solid Phases of Co(salen) and Co(3F-salen)

The potential of solid-state materials comprising Co(salen) units for concentrating dioxygen from air was recognized over 80 years ago. While the chemisorptive mechanism at the molecular level is largely understood, the bulk crystalline phase plays important, yet unidentified roles. We have reverse crystal-engineered these materials and can for the first time describe the nanostructuring requisite for achieving reversible O₂ chemisorption by Co(3R-salen) R = H or F, the simplest and most effective of the many known derivatives of Co(salen). Of the six phases of Co(salen) identified, α-ζ: α = ESACIO, β = VEXLIU, γ, δ, ε, and ζ (this work), only γ, δ, ε, and ζ are capable of reversible O₂ binding. Class I materials (phases γ, δ, and ε) are obtained by desorption (40-80 °C, atmospheric pressure) of the co-crystallized solvent from Co(salen)·(solv), solv = CHCl₃, CH₂Cl₂, or 1.5 C₆H₆. The oxy forms comprise between 1:5 and 1:3 O₂:[Co] stoichiometries. Class II materials achieve an apparent maximum of 1:2 O₂:Co(salen) stoichiometries. The precursors for the Class II materials comprise [Co(3R-salen)(L)·(H₂O)_x], R = H, L = pyridine, and x = 0; R = F, L = H₂O, and x = 0; R = F, L = pyridine, and x = 0; R = F, L = piperidine, and x = 1. Activation of these depends on the desorption of the apical ligand (L) that templates channels through the crystalline compounds with the Co(3R-salen) molecules interlocked in a Flemish bond brick pattern. The 3F-salen system produces F-lined channels proposed to facilitate O₂ transport through the materials through repulsive interactions with the guest O₂. We postulate that a moisture dependence of the activity of the Co(3F-salen) series is due to a highly specific binding pocket for locking in water via bifurcated hydrogen bonding to the two coordinated phenolato O atoms and the two ortho F atoms.

Hydrogen Bonding Effect on the Oxygen Binding and Activation in Cobalt(III)-Peroxo Complexes

Cobalt(III)peroxo complexes serve as model metal complexes mediating oxygen activation. We report a systematic study of the effect of hydrogen bonding on the O₂ binding energy and the O-O bond activation within the cobalt(III)peroxo complexes. To this end, we prepared a series of tris(pyridin-2-ylmethyl)amine-based cobalt(III)peroxo complexes having either none, one, two, or three amino groups in the secondary coordination sphere. The hydrogen bonding between the amino group(s) and the peroxo ligand was investigated within the isolated complexes in the gas phase using helium tagging infrared photodissociation spectroscopy, energy-resolved collision-induced dissociation experiments, and density functional theory. The results show that the hydrogen bonding stabilizes the cobalt(III)peroxo core, but the effect is only 10-20 kJ mol^-1. Introducing the first amino group to the secondary coordination sphere has the largest stabilization effect; more amino groups do not change the results significantly. The amino group can transfer a hydrogen atom to the peroxo ligands, which results in the O-O bond cleavage. This process is thermodynamically favored over the O₂ elimination but entropically disfavored.

Solid Phase Nitrosylation of Enantiomeric Cobalt(II) Complexes

Accompanied by a change in color from red to black, the enantiomorphic phases of the cobalt complexes of a chiral salen ligand (L2−, Co(L)·CS2, and Co(L) (L = LS,S or LR,R)) chemisorb NO (g) at atmospheric pressure and rt over hours for the CS2 solvated phase, and within seconds for the desolvated phase. NO is installed as an axial nitrosyl ligand. Aligned but unconnected voids in the CS2 desorbed Co(LR,R)·CS2 structure indicate conduits for the directional desorption of CS2 and reversible sorption of NO, which occur without loss of crystallinity. Vibrational circular dichroism (VCD) spectra have been recorded for both hands of LH2, Zn(L), Co(L)·CS2, Co(L), Co(NO)(L), and Co(NO)(L)·CS2, revealing significant differences between the solution-state and solid-state spectra. Chiral induction enables the detection of the νNO band in both condensed states, and surprisingly also the achiral lattice solvent (CS2 (νCS at 1514 cm−1)) in the solid-state VCD. Solution-state spectra of the paramagnetic Co(II) complex shows a nearly 10-fold enhancement and more extensive inversion of polarity of the vibrations of dominant VCD bands compared to the spectra of the diamagnetic compounds. This enhancement is less pronounced when there are fewer polarity inversions in the solid state VCD spectra.