Assembly of symmetrical and unsymmetrical platinum(II) rollover complexes with bidentate phosphine ligands

A Heteropolynuclear Pt-Ag System Having Cycloplatinated Rollover Bipyridyl Units

The synthesis and photophysical properties of the heteropolynuclear Pt-Ag complex having cyclometalated rollover bipyridine ligands (bpy*) and bridging pyrazolato ligands are reported. The Pt₂Ag₂ complex was synthesized by two step reactions from a Pt(II) complex precursor having the rollover bpy* ligand, [Pt(bpy*)(dmso)Cl], with 3,5-dimethylpyrazole (Me₂pzH) and a subsequent replacement of NH protons in the Me₂pzH moieties with the Ag(I) ion. The Pt₂Ag₂ complex exists as a mixture of U- and Z-shaped isomers in solution, whose structures were clearly determined by single-crystal X-ray structural analyses. NMR studies using the single crystals revealed rapid isomerization of the Pt₂Ag₂ complexes in solution, although the Pt₂Ag₂ structures were supported effectively by the multiple metal-metal interactions. Furthermore, the Pt₂Ag₂ framework can capture a Ag(I) ion during the U-Z isomerization to afford a Pt₂Ag₃ core with the formation of Pt → Ag dative bonds. The Pt₂Ag₃ complex showed further aggregation to form a dimer structure in the presence of coordinating solvent via the crystallization process. The formation of Pt → Ag dative bonds significantly changes the emission energy from the Pt₂Ag₂ complex, while the emission spectra of U- and Z-isomers of Pt₂Ag₂ complex almost coincide with each other and their emissive properties are very similar. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed the effects of additional Ag(I) ion on the photophysical properties of the heteropolynuclear Pt-Ag complexes bearing the rollover bpy* ligands.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.

A novel approach for rhodium(iii)-catalyzed C-H functionalization of 2,2'-bipyridine derivatives with alkynes: a significant substituent effect

We described a novel approach for the C-H functionalization of 2,2'-bipyridine derivatives with alkynes. DFT calculations and experimental data showed a significant substituent effect at the 6-position of 2,2'-bipyridine, which weakened the adjacent N-Rh bond and provided the possibility of subsequent rollover cyclometalation, C-H activation, and functionalization.

Rh(iii)-Catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes

We reported a Rh(iii)-catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes, which provided diversiform functionalized N,N-bidentate chelating compounds.

Rhodium(III)-Catalyzed Regioselective C3-H Acylmethylation of [2,2'-Bipyridine]-6-carboxamides with Sulfoxonium Ylides

A rhodium(III)-catalyzed C-H acylmethylation of tridentate [2,2'-bipyridine]-6-carboxamides was developed. A variety of [2,2'-bipyridine]-6-carboxamides could be monoalkylated exclusively at the C3 position with sulfoxonium ylides as carbene precursors, giving 3-alkylated products in high yields. This protocol proceeds through a rollover cyclometalation pathway, has a broad range scope of substrates, and exhibits excellent functional group tolerance.

A double rollover cycloplatinated(ii) skeleton: a versatile platform for tuning emission by chelating and non-chelating ancillary ligand systems

Described here is the synthesis and characterization of heteroleptic binuclear platinum(ii) complexes of the type [Pt2(μ-bpy-2H)(S^S)2] and [Pt2(μ-bpy-2H)(L)2(X)2], containing a 2,2'-bipyridine-based double rollover cycloplatinated core (Pt(μ-bpy-2H)Pt), in combination with the anionic S^S- chelate ligands di(ethyl)dithiocarbamate (dedtc) and O,O'-di(cyclohexyl)dithiophosphate (dcdtp) or non-chelating L/X ancillary ligands (PPh3/Me, t-BuNC/Me, PPh3/SCN and PPh3/N3). The new complexes were characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and some of them additionally using single crystal X-ray diffraction. The absorption and photoluminescence of the complexes show a strong dependence on the ancillary ligands. Upon excitation at 365 nm, in a CH2Cl2 rigid matrix (77 K), the complexes exhibit structured emission bands with λmax between 488 nm and 525 nm and vibrational spacing around 1350 cm-1, indicating the excited states centered on the cyclometalated ligand (3ILCT) with some mixing 3MLCT characteristics. In the case of the PPh3/N3 complex, a dual emission band (orange color) is observed in the solid state at 298 K for which the low energy band arises from an aggregation-induced emission (AIE). Upon lowering the temperature (77 K), thermochromism is observed (orange to yellow) which is accompanied by the intensification of the high energy band (ligand-centered structured band). Finally, in order to rationalize the obtained photophysical data, complete DFT (density functional theory) and TD-DFT (time-dependent DFT) calculations were performed on the selected complexes.

Palladium-Catalyzed Site-Selective C-H Arylation of 2,2'-Bipyridine-6-carboxamides via a Rollover Cyclometalation Pathway

The first example of Pd(II)-catalyzed arylation of 2,2'-bipyridine-6-carboxamides via a rollover cyclometalation pathway is reported. A diverse range of (hetero)aryl iodides could be directly coupled with various 2,2'-bipyridine-6-carboxamides in exclusive site-selectivity, which offers a distinct entry to arylated 2,2'-bipyridine-6-carboxamides. A preliminary mechanistic study supports the Pd(II)-rollover cyclometalation pathway.

Cyclometalated platinum(ii) complexes of 2,2'-bipyridine N-oxide containing a 1,1'-bis(diphenylphosphino)ferrocene ligand: structural, computational and electrochemical studies

The preparation and characterization of new heteronuclear-platinum(ii) complexes containing a 1,1'-bis(diphenylphosphino)ferrocene (dppf) ligand are described. The reaction of the known starting complex [PtMe(κ²N,C-bipyO-H)(SMe₂)], A, in which bipyO-H is a cyclometalated rollover 2,2'-bipyridine N-oxide, with the dppf ligand in a 2 : 1 ratio or an equimolar ratio led to the formation of the corresponding binuclear complex [Pt₂Me₂(κ²N,C-bipyO-H)₂(μ-dppf)], 1, or the mononuclear complex [PtMe(κ¹C-bipyO-H)(dppf)], 2, respectively. According to the reaction conditions, the dppf ligand in 1 and 2 behaves as either a bridging or chelating ligand. All complexes were characterized by NMR spectroscopy. The solid-state structure of 2 was determined by the single-crystal X-ray diffraction method and it was shown that the chelating dppf ligand in this complex was arranged in a "synclinal-staggered" conformation. Also, the occurrence of intermolecular C-H_CpO_bipyO-H interactions in the solid-state gave rise to an extended 1-D network. The electronic absorption spectra and the electrochemical behavior of these complexes are discussed. Density functional theory (DFT) was used for geometry optimization of the singlet states in solution and for electronic structure calculations. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in 2 was performed.

Reactivity of a new aryl cycloplatinated(ii) complex containing rollover 2,2′-bipyridine N-oxide toward a series of diphosphine ligands

A new rollover cycloplatinated(ii) complex was prepared. The reactivity of this complex was investigated towards a wide range of diphosphine ligands.

Palladium-Catalyzed Directed Halogenation of Bipyridine N-Oxides

The palladium-catalyzed directed C-H halogenation of bipyridine N-oxides was investigated. Using NCS or NBS (N-chloro- or N-bromosuccinimide) and 5 mol % Pd(OAc)₂ in chlorobenzene (0.10 molar) at 110 °C, pyridine-directed functionalization took place and 3-chloro- or 3-bromobipyridine N-oxides were obtained in high yields. The reaction is sensitive to steric hindrance by 4- and 6'-substituents. Only in the latter case, where coordination of palladium by the pyridine is hindered, 3'-halogenation directed by the N-oxide function was observed. The halogenated products were deoxygenated by PCl₃ or PBr₃.

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

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-dimethylphenylisocyanide (CNXyl; 3C), and 2-mercapto-1-methylimidazole (MMI; 4C)] have been examined to analyze the influence of the R substituent on R-C^C* (R-C = Naph; R = CO₂Et, CN) and that of the ancillary ligands (L) on them. Experimental data and time-dependent density functional theory calculations showed the similarity of the electronic features associated with R-C^C* (R = CN, CO₂Et) and their difference with respect to R-C^C* (R-C = Naph). All of the compounds are very efficient blue emitters in poly(methyl methacrylate) films under an argon atmosphere, with QY values ranging from 68% (2B) to 93% (2C). In the solid state, the color of the emission changes to yellowish-orange for compounds 2A (λ_max = 600 nm) and 3C (λ_max = 590 nm) because of the formation of aggregates through intermolecular π···π interactions. 2C and 3C were chosen to fabricate fully solution-processed electroluminescent devices with blue-light (2C), yellow-orange-light (3C), and white-light (mixtures of 2C and 3C) emission from neat films of the compounds as emitting layers.

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

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.

Photophysical study on unsymmetrical binuclear cycloplatinated(ii) complexes

A general synthetic method has been designed to prepare a new series of unsymmetrical binuclear cyclometalated organoplatinum(ii) complexes and the effect of each part on their luminescence properties were investigated.

Reactivity of a half-lantern Pt2(ii,ii) complex with triphenylphosphine: selectivity in a protonation reaction

Reaction of PPh₃ with half-lantern complex [{Pt(ppy)(μ₂-Spy)}₂], 1, gave complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 2, as a result of platinum–nitrogen bonds dissociation. Complex 2 revealed anionic selectivity upon protonation reaction with acidic species.

"Aggregation induced phosphorescence" active "rollover" iridium(III) complex as a multi-stimuli-responsive luminescence material

On reaction of 2,2'-bipyridine with iridium(iii), an "aggregation induced phosphorescence (AIP)" active "rollover" complex, [Ir(PPh3)2(bipy-H)(Cl)(H)] (bipy-H = κ(2)-N,C-2,2'-bipyridine) or [Ir(bipy-H)], is obtained. The emission colour changes from bluish-green to yellowish-orange and vice versa after repeated protonation and deprotonation of [Ir(bipy-H)], respectively, which unequivocally supports its reversible nature. [Ir(bipy-H)] is sensitive to acids with different pKa values. Tuning of the emission properties can be achieved in the presence of acids with different pKas. This behaviour allows the complex, [Ir(bipy-H)], to function as a phosphorescent acid sensor in both solution and the solid state, as well as a chemosensor for detecting acidic and basic organic vapours. The protonated form, [Ir(bipy-H)H(+)], which is generated after protonation of [Ir(bipy-H)] can be used as a solvatochromic probe for oxygen containing solvents, and also shows vapochromic properties. The emission, absorption and (1)H NMR spectra of [Ir(bipy-H)] under acidic and basic conditions demonstrate its reversible nature. DFT based calculations suggest that changes in the electron affinity of the pyridinyl rings are responsible for all these processes.