Abstract
The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.
Highlights
First-row transition metal compounds have emerged in recent years as homogeneous catalysts for numerous organic transformations,[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] such as de-/hydrogenation,[18,20,21,22,23,24,25,26,27,28,29,30,31,32,33] and other hydroelementation[34,35,36,37,38] reactions that were traditionally the realm of precious metal catalysis
This review aims at covering the application of functional pincer ligands in 3d metal catalyzed de-/hydrogenation
Open-shell first-row catalysts proved instrumental in C–C coupling[125] and multi-state reactivity that crosses through low-lying spin surfaces along the reaction coordinate can accelerate catalytic transformations, which further complicates predictions.[134,135]
Summary
First-row transition metal compounds have emerged in recent years as homogeneous catalysts for numerous organic transformations,[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] such as de-/hydrogenation,[18,20,21,22,23,24,25,26,27,28,29,30,31,32,33] and other hydroelementation[34,35,36,37,38] reactions that were traditionally the realm of precious metal catalysis. In case of weak spin-orbit coupling (z(Fe3+) = 460 cm–1, z(Ru3+) ≈ 1250 cm–1, z(Os3+) ≈ 3000 cm–1)[130] and strong distortion in the crossing region, an additional kinetic barrier with low transmission probability may arise.[131,132] The implications of these aspects on substrate binding are further exemplified by the diffusion dynamics of the 16-electron species 3Fe(CO)[4] vs 1Cr(CO)[5] in hydrocarbon solvents, which is considerably slower for the singlet compound due to alkane coordination.[133] From these considerations, rigid strong-field ligands, such as diphosphino pincers, that avoid trapping of active first-row catalysts in unproductive high-spin states should be beneficial for hydrogenation catalysis.[15,54,70,122,123,124] redox-active weakfield pincer ligands (see below) provide an alternative approach.[126] Notably, open-shell first-row catalysts proved instrumental in C–C coupling[125] and multi-state reactivity that crosses through low-lying spin surfaces along the reaction coordinate can accelerate catalytic transformations, which further complicates predictions.[134,135]
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