Given that the central void space surrounded by four pyridyl nitrogen atoms in TEQs is reminiscent of porphyrins and phthalocyanines, which are tetradentate ligands ubiquitous in nature, we turned our attention to the scope of metal cations that can be accommodated by TEQs. In contrast to the entirely planar structure of porphyrins, the lone-pair orbitals of the four nitrogen atoms of the saddle-shaped TEQs face each other in a twisted configuration due to the tight concatenation of the quinoline units, suggesting that TEQ might be reluctant to coordinate metal cations in a cooperative tetradentate fashion (Fig. ). However, despite their twisted structure, 8a and 8b accommodate a series of transition-metal cations (M) to form discrete metal complexes in mostly acceptable yields; the structures of these complexes were examined by single-crystal X-ray diffraction analysis (Fig. , Supplementary Figs. 32-39). The diagonal N-N distance decreases upon metal complexation, and the truncated (anisyl groups not shown) superimposed structures of 8a and its metal complexes show that metal coordination decreases the dihedral angles between the quinoline units to produce a more flattened overall architecture (Fig. ). The four nitrogen atoms and the central metal cation adopt a largely coplanar arrangement with the cis ∠N-M-N angle close to 90°, except in the 8a/Zn(II) complex obtained from the addition of ZnCl2, which furnished a monochloro-ligated complex as established by X-ray crystallography (Fig. ). In the pyramidalized MN4 fragment, the trans ∠N-M-N angle (150.3°) strongly deviates from 180°, and each atom is located relatively far away from the least-square plane (LSP) of MN4 (Fig. ). The preferential apical position of the chloride ligand at the same side as the anisyl groups were reproduced computationally (1.78 kcal•mol -1 ; Supplementary Fig. ). Coordination of the nitrogen atom to the Zn cation appears to be frustrated, and the quinoline plane and the N-Zn dative bond are not parallel and include a large acute angle (25.5°). An NBO analysis allowed for a quantitative estimation the second-order perturbation energy (27.7 kcal•mol -1 ) with a Wiberg bond index of 0.21, indicating an effective overlap of a nitrogen orbital exhibiting significant p-character (sp ) with a Zn orbital (Fig. ). On the other hand, the crystal structure of the 8a/Pd(II) complex obtained from the reaction with PdBr2 is characterized by a high degree of MN4 planarity, in which the maximum deviation from the LSP is only 0.05 Å (Fig. ). The characteristic downfield shift of the proton peaks upon addition of ZnCl2 or Pd(CH3CN)4[BF4]2 indicates that the metal complexes of TEQs are readily formed in solution. Neither exposure to TFA or pyridine nor the attempted addition of Pd(CH3CN)4 [BF4]2 to 8a/Zn(II) or ZnCl2 to 8a/Pd(II) resulted in changes in the spectral pattern, suggesting that the metal complexes of TEQs are kinetically stable and unsusceptible to facile exchange or loss of the accommodated metal cations (Supplementary Figs. ). The robustness of the TEQ metal complexes was harnessed for catalytic applications in chemical transformations. The fully characterized bench-stable darkred 8a/FeCl2 complex exhibited catalytic proficiency in the oxidation of tetrahydroisoquinolines (12) using cumene hydroperoxide (CHP) (Fig. ). Intriguingly, a comparative study using 1 mol% of various other Fe catalysts indicated that the 8a/FeCl2 complex outperformed a tetraphenylporphyrin (TPP 14)/Fe(III)Cl complex and the Fe(II) complexes of bipyridine (15) and phenanthroline (16), affording the corresponding dihydroquinoline 13a in 59% yield (More detailed data are summarized in Supplementary Section 5). The catalyst loading could be reduced to as little as 0.1 mol% to effectively promote the oxidation of a variety of tetrahydroisoquinolines 12. An electron-rich substrate bearing a methoxy group at the 6-position was amenable to oxidation to give 13f even at -25 °C. This new class of tetradentate ligand/Fe(II) catalysts also engaged indoline 14 and dihydroanthracene 15 in dehydrogenation and dioxygenation reactions to give indole 18 and anthraquinone 20, respectively .