The development of pyrimidine-based analogues of the well-known pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant methodology for the catalytic trimerization of terminal aliphatic alkynes. Remarkably, in contrast to established alkyne trimerization protocols, the 1,3,5-substituted arenes are the main reaction products. Preliminary mechanistic investigations suggest that the enhanced π-acidity of the pyrimidine ring, combined with the hemilability of the imine groups coordinated to the iron center, facilitates this transformation. The entry point in the catalytic cycle is an isolable iron dinitrogen complex. The catalytic reaction proceeds via a 1,3substituted metallacycle, which explains the observed 1,3,5-regioselectivity. Such a metallacycle could be isolated and represents a rare 1,3-substituted ferracycle obtained through alkyne cycloaddition.

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.

We present a highly efficient cobalt-mediated hydroboration reaction of alkynes and alkenes enabled by a πacidic and redox-active pyrimidinediimine (P Pym DI) core. The entry point in the catalytic cycle is on a cobalt dinitrogen complex rather than cobalt hydride species, which are commonly postulated in hydroelementation reactions. Stoichiometric studies have demonstrated that both cobalt hydrides and dinitrogen complexes can be generated from the same precursors and under the same reaction conditions, with the sole difference being the reaction atmosphere (argon vs N 2 ). Nevertheless, while the P Pym DI-based cobalt dinitrogen complex is highly active (TOF = 1100 h −1 at t 1/2 , RT), the hydride analogue displays only modest conversions at slow reaction rates. Under the optimized conditions, a wide range of vinyl and alkyl organoboron derivatives can be obtained with high catalytic efficiency. Mechanistic studies suggest that, due to the increased π-acidity of the P Pym DI core, initial formation of Co-π-complexes is preferred, followed by the oxidative addition of borane, which is the turnover-limiting step. This sequence of catalytic steps is supported by a Hammett analysis, which shows that the hydroboration reaction proceeds more rapidly for electron-rich substrates. The electronic structure of all relevant species was investigated in detail by computational, crystallographic, and spectroscopic means, revealing ligand involvement in the redox processes.

The ability to control catalytic properties, such as selectivity and rate during the reaction, represents an attractive tool which can potentially deliver the synthesis of specialized products from a large pool of building blocks. This can be achieved by the incorporation of an on / off switch into the backbone of (pre‐)catalysts. The electronic or solubility properties of the switch can be then reversibly modified through the action of external triggers, a process which in turn modulates the catalytic activity. Incorporation of switches that can be reversibly reduced or oxidized represents one of the most versatile methods for controlling the catalytic activity in situ. This article describes the current state of research in the field of redox‐switchable catalysis, with a focus on homogenous metal‐mediated processes. The first part presents an overview of the types of redox switches and of the methods employed for delivering the redox stimuli. The second part then focuses on the catalytic transformations which can be controlled in this fashion, which include polymerization catalysis, metathesis, hydroelementation, and other catalytic reactions for the synthesis of fine chemicals.