Diazines and Triazines as Building Blocks in Ligands for Metal-Mediated Catalytic Transformations

Abstract

Pyridine is a ubiquitous building block for the design of very diverse ligand platforms, many of which have become indispensable for catalytic transformations. Nevertheless, the isosteric pyrazine, pyrimidine, and triazine congeners have enjoyed thus far a less privileged role in ligand design. In this review, several applications of such fragments in the design of new catalysts are presented. In a significant number of cases described, diazine- and triazine-based ligands either outperform their pyridine congeners or offer alternative catalytic pathways which enable new reactivities. The potential opportunities unlocked by using these building blocks in ligand design are discussed, and the origin of the enhanced catalytic activity is highlighted where mechanistic studies are available.

Figure 1

Brief overview of role and binding modes of pyridine ligands in transition metal chemistry.

Figure 2

(a) NICS data; optimization: B3LYP/def2-TZVP/D3BJ, NMR: pcSseg-3. (b) Data from ref (16). (c) Data from ref (14). (d) Values are given relative to vertical excitation energy of pyridine: 4.94 eV.

Figure 3

Remote-site functionalization effects in catalysis exemplified on diazine ligands.

Figure 4

Generation of “abnormal” NHCs from pyrazines and pyrimidines and application in redox-switchable catalysis.

Figure 5

Ligand-design elements for pyridine-diimines (PDI), their diazine-analogues, and other examples of diazine-based redox-active pincers.

Figure 6

Calculated frontier orbital energies in (PDI)Fe(CO)₂ and their diazine analogues. Comparison of experimental redox-potentials and ν_CO stretching frequencies from IR spectroscopy. ^aData taken from ref (53).

Figure 7

Pyrmidinediimine (P^PymDI) iron and cobalt complexes and examples of enhanced stability compared to the pyridine analogues under [2 + 2]-cycloaddition and hydroboration conditions.

Figure 8

1,3,5-Selective trimerization of alkynes catalyzed by (P^PymDI)Fe-systems.

Figure 9

Observed deactivation pathways and limitations of (P^PymDI)Fe-complexes.

Figure 10

Chemical MLC mechanisms of pyridine-based PNP-ligands and examples of known diazine and triazine analogues.

Figure 11

Diazine- and triazine-based PNP ligands as actor ligands.

Figure 12

Pyrazine-based PNP active ligands in CO₂ hydrogenation catalysis.

Figure 13

Triazine-based PNP complexes in MLC-type bond activation reactions and their applications in catalysis.

Figure 14

Key steps in imine reduction, highlighting the role of secondary triazine···K···substrate interactions.

Figure 15

Active triazine-based PNP ligands.