By Valentinos Mouarrawis & Sander Kluwer
Transition metal catalysis occupies a pivotal role in the modernization of our chemical industry because it ensures more efficient use of scarce natural resources and also aids in the minimization of waste production. Besides these economic reasons, the development of efficient catalytic processes is of crucial importance for sustainable-oriented applications, which in turn positions catalysis at the heart of our quality of life. Typically, in the fine chemistry industry complex transformations are done by transition metals, and as a result, there is high demand for more efficient technologies aiming for sustainable and economically valuable processes.
Palladium is probably the most versatile and exploited transition metal in catalysis due to its capability to promote a myriad of organic transformations, both on laboratory and industrial scale. Natural alkaloids, bioactive compounds, pharmaceutical agents, agrochemicals, specialty polymers, etc., can be efficiently accessed by means of palladium catalysts. The large demand for the metal has increased the price to a staggering $62.000 per kilogram. Alternatively, nickel catalysts ($22 per kilogram) has been coined as alternative, albeit that the catalysts are more often complementary than a substitution, thus replacing the palladium for nickel using the same ligands, solvents and additives result in inferior results.
There are some key differences that need to be addressed for the development of high-performance nickel catalysts. First of all, nickel has significantly lower electronegativity and thus nickel-catalysts are typically more prone to oxidation with various oxidizers and less stable than corresponding Pd(0) counterparts. As a consequence, nickel catalysts display higher reactivity in the activation of reluctant electrophiles such as aryl chlorides, phenol derivatives, and alkyl halides. Reversibly, starting from Ni(II) precursors, it is more difficult to enter the catalytic cycle requiring accessing a Ni(0)-intermediate and this typically requires special reductants. These properties demand a more stringent control over air and other oxidizers. Secondly, nickel has more accessible oxidation states than palladium (Ni(0), Ni(I), Ni(II), Ni(III), Ni(IV)) which provide remarkable possibilities in terms of reactivity as different catalytic cycles are possible). Under catalytical conditions, both Pd and Ni catalysts can lose ancillary ligands via side reductive elimination or dissociation to give ligandless Pd and Ni species of various nuclearities (metal particles, nanoclusters). In many reactions, the ligandless Pd species are catalytically active and often display superior activities. Nickel, on the other hand, is more prone to aggregation but these Ni(0) species are too unstable toward various oxidizers, such as alkyl or aryl halides reagents and solvents, and lose their catalytic activity. For these reasons, ligand selection is even more important for nickel-based catalysis.
To address these specific properties of nickel, it is clear that the typical ligands used for palladium catalysis are not sufficient. New ligands need to be developed to balance the oxidative addition/ reduction elimination processes and to control the number of accessible oxidation states of the nickel catalyst. To speed up the discovery process, high throughput screening is essential to quickly identify new ligands and unravel structure-activity relationships. This will lead to important advances in nickel catalysis. Factors such as reducing cost, scalability, and environmental concerns are key to the chemical industry, and therefore approaches to provide higher robustness of nickel systems are highly desirable. For that, lower nickel loadings and orthogonal selectivity still need to be further developed for many important chemical transformations.
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