Activation of Dioxygen and Nitric Oxide by Transition Metal Materials

The activation of small abundant inert molecules, like CO2, CH4, O2, H2O and N2 forreactions that selectively give specific products (e.g., MeOH, H2O2, NH3) in an energyand atom economic way, is highly desirable. Predominantly activation is promotedby direct coordination to redox active and spin-flexible d-block elements. Ligands,proteins and cofactors in enzymes, will tune this reactivity. The work in this thesisfocuses on activation of the homoatomic dioxygen (O2) and heteroatomic nitric oxide(NO) molecules. Both have biological, medical, industrial, and environmental importance. Appealing reactions addressed include: (i) Coupling of selective C–H activation with O2 activation - an important goal for organic synthesis; (ii) Reversible O2binding for gas separations; and (iii) Capture and conversion of toxic NO.

Crystalline solids of cationic tetracobalt complexes containing dicobalt sites not only reversibly bind O2, but also chemisorb and transform NO. For the nitrate salts {[(bpbp)CoIII2(O2)]2(bdcNH2)}(NO3)4 and {[(bpbp)CoII2(NO3)]2(bdcNH2)}(NO3)2 (bpbp = 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato,
bdcNH2 = 2-amino-1,4-benzenedicarboxylato), a cascade of reactions occurs on exposure to NO (1 atm, room temperature): A bridging nitrite and a nitrate counter anion per Co2 site are generated in-crystal and an arylamine group on the ligand scaffold is (at least partially) oxidised to a nitro group, resulting in formation of {[(bpbp)CoIII2(µ-(η1 -O:η1 -N)-ONO)]2(bdcNO2)}(NO3)6. Remarkably, all this occurs via single-crystal to single-crystal transformations. For the hexafluorophosphate salts {[(bpbp)CoIII2(O2)]2(bdc)}(PF6)4 and {[(bpbp)CoII2]2(bdcNH2)}(PF6)4 (bdc = 1,4-benzenedicarboxylato) the same reaction instead results in loss of single crystallinity and formation of the mixed valent {[(bpbp)CoIICoIII(µ-(η1 -O:η1 -N)-ONO)]2(bdc)}4+.
This implies that the nitrate counter ions are non-innocent with respect to the cascade of in-crystal reactions.

Enantiomorphic phases of cobalt(II) complexes of a chiral salen ligand L2– , Co(L)∙CS2 and Co(L), (L = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino) can reversibly chemisorb NO at atmospheric pressure and room temperature
and store it for prolonged periods, with release triggered by heating (100 °C). In addition, they induce chirality into sorbed CS2 and chemisorbed NO.

The potential of solid-state Co(salen) for concentrating O2 from air was recognized over 80 years ago. For this process bulk crystalline phase plays important, yet unidentified, roles. Through reverse crystal engineering the nanostructuring, requisite for achieving reversible O2 chemisorption, is described for Co(3R-salen) R = H, F, the simplest and most effective of the many known derivatives of Co(salen).

Co(salen) also binds O2 in solution, with the reactivity and stability of the various species depending greatly on the solvent and axial ligand. A common holistic, mechanistic, framework is outlined that can rationalise the formation of peroxo, superoxo, organo and alkoxide complexes of CoIII(salen) through coupling of O2 binding with the C–H activation of selected solvents.