In recent years the interest of shape-persistent organic cage compounds synthesized by dynamic covalent chemistry (DCC) has risen, because these cages are potentially interesting for gas sorption or -separation. One such reaction in DCC is the condensation of boronic acids with diols to form boronic esters. Most interestingly, the variety of geometries and sizes for boronic ester cages is much lower than that of, for example, imine-based cages. Here, a small series of shape-persistent [4+6] tetrahedral boronic ester cages is introduced. One cage has a high specific surface area of 511 m g and selectively adsorbs ethane over ethylene and acetylene.

Iron complexes supported by novel π-acidic bis­(imino)­pyrazine (PPzDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resulting complexes. Dearomatization is observed in the presence of cobaltocene, which reversibly reduces the pyrazine core and not the imine functionality, as observed in the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the formation of a long carbon–carbon bond, which can be subsequently cleaved under mild oxidative conditions.

A detailed investigation of the electronic structure of diazinediimine iron complexes and their comparison with the pyridine analogues reveals subtle but important differences, imparted by the supporting heterocycle. In the case of LFe­(CO)2 complexes (L = pyrazine- and pyrimidine­diimine), the characterization of three available redox states confirmed that whereas the nature of the electron-transfer processes is similar, the differences in π-acidity of the supporting heterocycle significantly affect the redox potentials. The reduction of LFe­(CO)2 can yield either a ligand-centered radical (for L = pyrimidine) or a C–C-bonded dimer (for L = pyrazine), supported by a dearomatized core. In the latter case, the C–C bond can be reversibly cleaved oxidatively. Compared to the carbonyl analogues, employing weak-field N2 ligands triggers changes in electronic structure for the neutral and reduced LFe­(N2) complexes (L = pyrimidine­diimine). En route to the synthesis of the nitrogen complexes, the square-planar LFeCl (L = pyrimidine­diimine) was isolated. The monoradical character of the supporting chelate triggers the asymmetric distribution of electron density around the heterocycle.

An ew redox-active N-heterocyclic carbene (NHC) architecture is obtained using N-methylated pyrazinediimine iron complexes as precursors.The new species exhibit strong paccepting/s-donating properties and are able to ligate two metal centres simultaneously.T he redox activity was demonstrated by the reversible chemicalo xidation of ah eterobimetallic Fe 0 /Rh I example,w hich affords an isolable ligand-based radical cation. The reversible redox process was then applied in the catalytic hydrosilylation of 4,4'-difluorobenzophenone, where the reaction rate could be reversibly controlled as af unction of the catalyst oxidation state.T he new NHC exhibits high electrophilicity and nucleophilicity,w hich was demonstrated in the reversible activation of alcohols and amines.The electronic structure of the resulting complexes was investigated through various spectroscopic and computational methods.

Metal–ligand cooperativity and redox-active ligands enable the use of open-shell first-row transition metals in catalysis. However, the fleeting nature of the reactive intermediates prevents direct inspection of the relevant catalytic species. By employing phosphine α-iminopyridine (PNN)-based complexes, we show that chemical and redox metal–ligand cooperativity can be combined in the coordination sphere of iron dinitrogen complexes. These systems show dual activation modes either through deprotonation, which triggers reversible core dearomatization, or through reversibly accepting one electron by reducing the imine functionality. (PNN)­Fe­(N2) fragments can be obtained under mildly reducing conditions. Deprotonation of such complexes induces dearomatization of the pyridine core while retaining a terminally coordinated N2 ligand. This species is nevertheless stable in solution only below −30 °C and undergoes unusual ligand-assisted redox disproportionation through proton-coupled electron transfer at room temperature. The origin of this phenomenon is the significant lability of the α-imine C–H bonds in the dearomatized species, where the calculated bond dissociation free energy is 48.7 kcal mol–1. The dispropotionation reaction yields an overreduced iron compound, demonstrating that the formation of such species can be triggered by mild bases, and does not require harsh reducing agents. Reaction of the dearomatized species with dihydrogen yields a rare anionic Fe hydride that binds dinitrogen and features a rearomatized core.