Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Metallaaromatic Chemistry: History and Development.

New heteroleptic compounds of platinum(II)-containing cyclometalated N-heterocyclic carbenes, [PtCl(R-C^C*)(PPh3)] [R-CH^C*-κC* = 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene (R-C = Naph; 1A), 1-[4-(ethoxycarbonyl)phenyl]-3-methyl-1H-imidazol-2-ylidene (R = CO2Et; 1B), and [Pt(R-C^C*)(py)(PPh3)]PF6 (py = pyridine; R-C = Naph, 2A; R = CO2Et, 2B], have been prepared and fully characterized. All of them were obtained as the trans-(C*,PPh3) isomer in high yields. The selectivity of their synthesis has been explained in terms of the degree of transphobia (T) of pairs of ligands in trans positions. X-ray diffraction studies on both 2A and 2B revealed that only in 2A, containing a C^C* with a more extended π system, do the molecules assemble themselves into head-to-tail pairs through intermolecular π···π contacts. The photophysical properties of 2A and 2B and those of the related compounds [Pt(NC-C^C*)(PPh3)L]PF6 [NC-CH^C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene; L = pyridine (py; 2C), 2,6-d...

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Heteroleptic Cycloplatinated N-Heterocyclic Carbene Complexes: A New Approach to Highly Efficient Blue-Light Emitters

The synthesis of 1-(4-cyanophenyl)-1H-imidazol (1) has been carried out by an improved method. Then its corresponding imidazolium iodide salt, 2, has been used to prepare the N-heterocyclic carbene (NHC) cycloplatinated compound [{Pt(μ-Cl)(C∧C*)}2] (4) (HC∧C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene) following a step-by-step protocol. The intermediate complex [PtCl(η3-2-Me-C3H4) (HC∧C*-κC*)] (3) has also been isolated and characterized. Using 4 as precursor, several heteroleptic complexes of stoicheometry [PtCl(C∧C*)L] (L = PPh3 (5), pyridine (py, 6), 2,6-dimethylphenyl isocyanide (CNXyl, 7), and 2-mercapto-1-methylimidazole (MMI, 8)) and [Pt(C∧C*)LL′]PF6 (L = PPh3, L′ = py (9), CNXyl (10), and MMI (11)) have been synthesized. Complexes 6–8 were obtained as a mixture of cis- and trans-(C*,L) isomers, while trans-(C*,L) isomer was the only one observed for complexes 5 and 9–11. Their geometries have been discussed in terms of the degree of transphobia (T) of pairs of trans ligands and support...

/pdf/exploring-the-transphobia-effect-on-heteroleptic-nhc-45hxi0fos3.pdf

Exploring the transphobia effect on heteroleptic NHC cycloplatinated complexes

Cyclometalated rollover complexes of the type [PtMe(κ2 N,C-bipyO-H)(L)] [bipyO-H=cyclometalated 2,2'-bipyridine N-oxide; L=tricyclohexylphosphine (PCy3 , 2 a), 2-(diphenylphosphino)pyridine (PPh2 py, 2 b), P(OPh)3 (2 c)] were synthesized by treating [PtMe(κ2 N,C-bipyO-H)(SMe2 )] (1) with various monodentate phosphine and phosphite ligands. These complexes were characterized by NMR spectroscopy, and the structure of 2 a was confirmed by single-crystal X-ray diffraction. Complex 1 was treated with bis(diphenylphosphino)methane (dppm) at a 1:1 ratio to give the corresponding [PtMe(κ2 N,C-bipyO-H)(κ1 P-dppm)] (3 b) complex, in which the dppm ligand acts as a monodentate pendant ligand. The biological activities of these complexes were evaluated against a panel of four standard cancer cell lines: lung carcinoma (A549), ovarian carcinoma (OV-90 and SKOV3), and breast carcinoma (MCF-7). Complexes 2 c and especially 3 b indicated effective potent cytotoxic activity regarding the cell lines. Electrophoresis mobility shift assays and molecular-modeling investigations were performed to determine the specific binding mode and the binding orientation of these alkylating agents to DNA. Detection of cellular reactive oxygen species was also determined.

Synthesis, Biological Evaluation, and Molecular Docking Studies on the DNA Binding Interactions of Platinum(II) Rollover Complexes Containing Phosphorus Donor Ligands.

Cycloplatinated complexes based on 2-(4-substituted)benzothiazole ligands of type [Pt(R-PBT-κC,N)Cl(L)] (PBT=2-phenylbenzothiazole; R=Br (1), Me2N (2); L=dimethyl sulfoxide (DMSO; a), 1,3,5- triaza-7-phosphaadamantane (PTA; b), triphenylphosphine 3,3′,3′′-trisulfonate (TPPTS; c)) and [Pt(Br-PBT-κC)Cl(PTA)2] (3) are presented. On the basis of the photophysical data and time-dependent (TD)-DFT calculations (1 a and 2 a), the low-lying transitions (absorption and emission) were associated with ligand-center (LC) charge transfer, with minor metal-to-ligand charge transfer (MLCT), and intraligand charge transfer (ILCT) [Me2N-PBT→PBT] excited states, respectively. Simultaneous fluorescence/phosphorescence bands were found in fluid solutions (and also in the solid state for 2 a), which become dominated by triplet emission bands in rigid media at 77 K. The effect of the concentration on emissive behavior of 2 a, b indicated the occurrence of aggregation-induced luminescence properties related to the occurrence of metal–metal and π⋅⋅⋅π interactions, which are more enhanced in 2 a because of the less bulky DMSO ligand. The behavior of 2 a toward para-toluenesulfonic acid (PTSA) in aerated acetonitrile and to hydrogen chloride gas in the solid state has been evaluated, thus showing a clear reversible change between the 1ILCT and 3LC/3MLCT states due to protonation of the NMe2 group (theoretical calculations on 2 a-H+). Solid 2 a undergoes a surprising oxidation of the PtII center to PtIV with concomitant deoxygenation of DMSO, under prolonged reaction with hydrogen chloride gas to afford the PtIV/dimethyl sulfide complex (mer-[Pt(Me2N-PBT-κC,N)Cl3(SMe2)]; mer-4), which evolves in solution to fac-4, as confirmed by X-ray studies. Cytotoxic activity studies on A549 and HeLa cell lines indicated cytotoxic activity of 1 b and 2 a, b. In addition, fluorescent cell microscopy revealed cytoplasmic staining, more visible in perinuclear areas. Inhibition of tubulin polymerization by 1 b in both cells is presented as a preliminary mechanism of its cytotoxic action. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Benzothiazole-Based Cycloplatinated Chromophores: Synthetic, Optical, and Biological Studies.

The electron-rich platinum(II) derivatives [Pt(R)2(DMSO)2] (R = Me, Ph) react with 2-vinylpyridine (L) to give the cyclometalated complexes [Pt(L–H)(R)(DMSO)] through C–H bond activation. Cyclometalation also occurs with [Pt(Me)(Cl)(DMSO)2]. From the starting compounds, a series of cyclometalated complexes was obtained and characterised by analytical and spectroscopic methods. The molecular structures of [Pt(L–H)(Me)(PPh3)] and [Pt(L–H)(Ph)(PCy3)] were solved by X-ray diffraction analysis, and the results unambiguously demonstrate cyclometalation of 2-vinylpyridine. The five-membered cyclometalated ring is particularly stable, likely due to partial electronic delocalisation.

Cyclometalated Complexes of Platinum(II) with 2‐Vinylpyridine

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Cyclometalated Complexes of Platinum(II) with 2‐Vinylpyridine