The CoopClickCat project features Cooperatively operating Click-derived metal Catalysts for the efficient oxidation of organic and inorganic substrates with high rates and turnover numbers. For this purpose, triazolylidene ligands, which are readily available through...
The CoopClickCat project features Cooperatively operating Click-derived metal Catalysts for the efficient oxidation of organic and inorganic substrates with high rates and turnover numbers. For this purpose, triazolylidene ligands, which are readily available through click-chemistry, will be functionalized with a cooperative donor site for improving the catalytic activity of the bound ruthenium center. The ligand design involves a sterically constraint position of the donor site, thus preventing this group from coordination to the metal center. The availability of a Lewis acid (metal center) and base in a confined arrangement will enable the two sites to act cooperatively, thus providing access to concerted proton and electron transfer processes. This synergistic behavior lowers activation barriers and hence leads to a smoother potential energy surface for reduction and oxidation reactions. This unique arrangement will lead to new oxidative coupling processes and reduction pathways for the activation of abundant starting materials such as alcohols and amines, and eventually also water to reach unprecedented activity in metal-catalyzed water oxidation catalysis.
Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines,inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such NpyrÂ·Â·Â·H-N interactions are demonstrated by X-ray diffraction analysis, 1H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligandâ€“metal cooperative substrate activation.
A bimetallic [Ir3+]2 complex was synthesized based on a bridging 1,2,3-triazole ligand that coordinates to one Cp*Ir unit as N,N-bidentate chelate, and to the other as a C,C-bidentate ligand. When compared to monometallic homologues, the bimetallic complex shows greatly enhanced product selectivity for the acceptorless dehydrogenation of alcohols; spectroscopic and electrochemical analysis suggest significant alteration of the metal properties in the bimetallic system compared to the monometallic species, which offers a rationale for the observed high selectivity.
The main progress beyond the state of the art in complexes active for homogeneous catalysis involves the following projectâ€™s achievement:
- We have synthesized suitable ligands and their iridium complexes, and have identified and assessed their bifunctional reactivity.
- The catalytic activity of this bifunctional catalyst has been investigated and evaluated in amine dehydrogenation and in the acceptor-free alcohol dehydrogenation catalysis.
- Stoichiometric experiments with the catalyst precursor were carried out allowing us to understand the mechanistic details. That will help us to design further modifications for catalyst improvement.