Cyclometallated complexes incorporating a heterocyclic donor atom; the interface of coordination chemistry and organometallic chemistry

A cyclometalated ruthenium complex (1) with 2-phenylbenzimidazole as the ligand was synthesized and used as a colorimetric chemosensor to detect Hg²⁺ and Cu²⁺. It is interesting that the presence of Hg²⁺ caused a blue-shifted absorption of 1 from 560 to 450 nm resulting in a solution color change from red to yellow, while the addition of Cu²⁺ resulted in an attenuated absorption along with the solution color changing from red to colorless. Their corresponding detection limits were calculated to be 0.25 µM for Hg²⁺ and 0.45 µM for Cu²⁺, respectively. Moreover, the slips of paper were dyed with complex 1 and then successfully used to recognize Hg²⁺ and Cu²⁺ by naked eyes, respectively. Furthermore, 1 was also used to determine Hg²⁺ and Cu²⁺ in some spiked water samples, respectively.

Ten newly synthesized ortho-palladated complexes of the type [PdCl(bhq)L] (1–10) (where bhp = benzo[H]quinolate, L = 2-aminobenzothiazole (2-abt), 2-amino-benzimidazole(2-abi), 2-aminothiazole(at), imidazole(im), 2-aminopyridine(2-apy), 2-amino-3-methylpyridine(2-ampy), 2-amino-5-chloropyridine(2-acpy), 3-methyl-pyridine (3-mpy), 2-methylpyridine(2-mpy)or2-pyridinemethaneamine (2-pyma) have been prepared by bridge opening reaction of dimeric [PdCl(bhq)]₂ with two equivalent of various aromatic N-donor ligands(L). The prepared complexes were characterized using IR, ¹H n.m.r, mass spectroscopy, molar conductance, and X-ray crystallography. The results indicate that all complexes have a neutral square planar shape except complex (10) is ionic have electric conductance (40 µs), then approved by the addition of AgNO₃ solution to afford white precipitate of AgCl. The biological activity of the prepared complexes was examined against Pseudomonas aeruginosa, Escherichia coli and staphylococcus aurous. The complexes showed good to moderate activity against the three pathogenic bacteria. Further more complex 4, displays the highest anti-bacterial activity than the other complexes. Some of the synthesized complexes were evaluated as anticancer, screening against A2780 and HCT-116 cell lines using the MTT colorimetric assay. The [Pd(bhq)Cl(im)] (4) complex exhibited significant cytotoxicity against the HCT-116 and A2780 cell lines with IC₅₀ values are 8.67 ± 1.04 µM and 6.32 ± 0.86 µM, respectively.

In this work, different bipyridine-analogue ligands bearing a dimethylamino group in the meta-position of one of the heterocyclic rings were synthesized and reacted with palladium(II) acetate under identical conditions. Cyclometallated palladium(II) complexes with C,N- or C,N,N’-coordinating chelate ligands are formed which were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, and single crystal X-ray diffraction analysis. In the case of the mononuclear, C,N,N’-coordinated complex, which is formed by an attack of the palladium(II) site at of the N-methyl groups, the primarily coordinating acetato ligand is exchanged against a chlorido ligand, which is liberated from the solvent dichloromethane by a nucleophilic substitution reaction. In contrast, cyclometallation occurring at one of the six-membered heterocycles leads to dinuclear acetato-bridged palladium(II) complexes.

In 1966, we first found a fact that a dicarboxylic oxygen atom coordinates to tin atom by IR spectra in cyclometalation reactions. The five-membered ring products obtained from the cyclometalation reactions are found to be more chemically stable compared with four- and six-membered ring products. Therefore, in the cyclometalation reaction, the five-membered ring products of the cyclometalation reactions are easily produced, and the great many number of papers involving the cyclometalation reaction five-membered ring products have been published since 1970s. The five-membered ring products of cyclometalation reactions have various functions, because they contain five kinds of hetero atoms (N, P, As, O and S) and most of the metal atoms belonging to both main-group and transition metal group (69 kinds of metals). This review reports on their functions utilized as catalysts, anticancer agents, OLEDs, carbon dioxide utilizations, dye-sensitized solar cells, sensors, hydrogen productions and other pharmaceuticals.

Cyclometalated six-membered ring products have been reported in applications as catalysts and OLEDs. The 2-benzylpyridine gold dichloride for the formation of tri-substituted oxazoles, the similar 2-benzylpyridine PEG-linked compound for the synthesis of propargylic amines, a biphenyl palladium compound for polycondensation using the Suzuki-Miyaura cross-coupling reaction, a naphthalene palladacycle for the ring-opening reaction of 7-oxabenzonorbornadiene with phenylacetylene, a sulfone oxygen donor ruthenium compound for the ring-closing metathesis of unhindered diene, and the non-symmetric NCN-Pd pincer compound for the Suzuki-Miyaura couplings of para-substituted bromobenzenes with phenylboronic acid have all been shown to be efficient catalysts.

Quinoline iridium compounds show deep red phosphorescent emissions with short lifetimes, which is promising for the uses in OLED displays. Bis-pyridine, bis-carbazole tetradentate platinum compounds were proved to be successful for achieving deep blue emission onsets as low as 420 nm, with estimated lifetimes of greater than 500 h at 1000 cd/m².

Organic light-emitting diodes (OLEDs) are manufactured primarily by synthesizing five-membered ring products using cyclometalation reactions. Highly photoluminescent quantum efficient emitters are primarily prepared by utilizing the following five types of iridium compounds: 1. N-Heterocyclic carbene compounds such as phenylimidazolinate-type carbene compounds and 2-phenylpyridine N-heterocyclic carbene compounds; 2. NCN-tridentate type compounds; 3. Polycyclic compounds, such as phenanthro-imidazole and fluorenylpyrazole compounds; 4. Heterocyclic compounds, such as 2-phenylpyridines, 2-phenylpyridine fluorinated compounds, phenyl-imidazole, -pyrazole, and -triazole compounds, 2,3⿲-bispyridines and phosphites; and 5. High-molecular-weight cyclometalated compounds such as the dendrimers of phenyltriazoles.

In particular, the presence of a high field ligand such as an N-heterocyclic carbene in these compounds facilitates high energy emission, which results in the desired blue-colored light required for OLED applications.