Cyclometalated Pt Complexes of CNC Pincer Ligands: Luminescence and Cytotoxic Evaluation

One Dianionic Luminophore with Three Coordination Modes Binding Four Different Metals: Toward Unexpectedly Phosphorescent Transition Metal Complexes

This work reports on a battery of coordination compounds featuring a versatile dianionic luminophore adopting three different coordination modes (mono, bi, and tridentate) while chelating Pd(II), Pt(II), Au(III), and Hg(II) centers. An in-depth structural characterization of the ligand precursor (H2 L) and six transition metal complexes ([HLPdCNtBu], [LPtCl], [LPtCNtBu], [LPtCNPhen], [HLHgCl], and [LAuCl]) is presented. The influence of the cations and coordination modes of the luminophore and co-ligands on the photophysical properties (including photoluminescence quantum yields (ΦL ), excited state lifetimes (τ), and average (non-)radiative rate constants) are evaluated at various temperatures in different phases. Five complexes show interesting photophysical properties at room temperature (RT) in solution. Embedment in frozen glassy matrices at 77 K significantly boosts their luminescence by suppressing radiationless deactivation paths. Thus, the Pt(II)-based compounds provide the highest efficiencies, with slight variations upon exchange of the ancillary ligand. In the case of [HLPdCNtBu], both ΦL and τ increase over 30-fold as compared to RT. Furthermore, the Hg(II) complex achieves, for the first time in its class, a ΦL exceeding 60% and millisecond-range lifetimes. This demonstrates that a judicious ligand design can pave the way toward versatile coordination compounds with tunable excited state properties.

Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh3 and SbPh3

In the frame of our research aiming to develop efficient triplet-emitting materials, we are exploring the concept of introducing additional heavy atoms into cyclometalated transition metal complexes to enhance intersystem-crossing (ISC) and thus triplet emission through increased spin-orbit coupling (SOC). In an in-depth proof-of-principle study we investigated the double cyclometalated Pt(II) complexes [Pt(C^N^C)(PnPh3)] (HC^N^CH = 2,6-diphenyl-pyridine (H2dpp) or dibenzoacridine (H2dba); Pn = pnictogen atoms P, As, Sb, or Bi) through a combined experimental and theoretical approach. The derivatives containing Pn = P, As, and Sb were synthesised and characterised comprehensively using single crystal X-ray diffraction (scXRD), UV-vis absorption and emission spectroscopy, transient absorption (TA) spectroscopy and cyclic voltammetry (CV). Across the series P < As < Sb, a red-shift is observed concerning absorption and emission maxima as well as optical and electrochemical HOMO-LUMO gaps. Increased photoluminescence quantum yields ΦL and radiative rates kr from mixed metal-to-ligand charge transfer (MLCT)/ligand centred (LC) triplet states are observed for the heavier homologues. Transient absorption spectroscopy showed processes in the ps range that were assigned to the population of the T1 state by ISC. The heavy PnPh3 ancillary ligands are found to enhance the emission efficiency due to both higher Pt-Pn bond strength and stronger SOC related to increased MLCT character of the excited states. The experimental findings are mirrored in hybrid (TD-)DFT calculations. This allowed for extrapolation to the rather elusive Bi derivatives, which were synthetically not accessible. This shortcoming is attributed to the transmetalation of phenyl groups from BiPh3 to Pt, as supported by experimental NMR/MS as well as DFT studies.

Pt(II) Complexes with Tetradentate C^N*N^C Luminophores: From Supramolecular Interactions to Temperature-Sensing Materials with Memory and Optical Readouts

A series of four regioisomeric Pt(II) complexes (PtLa-n and PtLb-n) bearing tetradentate luminophores as dianionic ligands were synthesized. Hence, both classes of cyclometallating chelators were decorated with three n-hexyl (n = 6) or n-dodecyl (n = 12) chains. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry. Steady-state and time-resolved photoluminescence spectroscopy as well quantum chemical calculations show that the effect of the regioisomerism on the emission colour and on the deactivation rate constants can be correlated with the participation of the Pt atom on the excited state. The thermal properties of the complexes were studied by DSC, POM and temperature-dependent steady-state photoluminescence spectroscopy. Three of the four complexes (PtLa-12, PtLb-6 and PtLb-12) present an intriguing thermochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Each material has different transition temperatures and memory capabilities, which can be tuned at the intermolecular level. Hence, dipole-dipole interactions between the luminophores and disruption of the crystalline packing by the alkyl groups are responsible for the final properties of the resulting materials.

Non-Palindromic C∧C∧P Platinum and Palladium Pincer Complexes Showing Intense Phosphorescence via Direct Spin-Forbidden S0 → T1 Excitation

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of PtII and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of PtII as well as PdII emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S0 → T1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S0 → T1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S0 → Sn → T1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.

A new family of luminescent [Pt(pbt)2(C6F5)L]n+ (n = 1, 0) complexes: synthesis, optical and cytotoxic studies

Given the widely recognized bioactivity of 2-arylbenzothiazoles against tumor cells, we have designed a new family of luminescent heteroleptic pentafluorophenyl-bis(2-phenylbenzothiazolyl) PtIV derivatives, fac-[Pt(pbt)2(C6F5)L]n+ (n = 1, 0) [L = 4-Mepy 1, 4-pyridylbenzothiazole (pybt) 2, 4,4'-bipyridine (4,4'-bpy) 3, 1,2-bis-(4-pyridyl)ethylene (bpe) 4 (E/Z ratio: 90/10), 1,4-bis-(pyridyl)butadiyne (bpyb) 5, trifluoroacetate (-OCOCF3) 6] and a dinuclear complex [{Pt(pbt)2(C6F5)}2(μ-bpyb)](PF6)27, in which the trans ligand to the metalated C-(pbt) was varied to modify the optical properties and lipophilicity. Their photophysical properties were systematically studied through experimental and theoretical investigations, which were strongly dependent on the identity of the N-bonded ligand. Thus, complexes 1, 3 and 6 display, in different media, emission from the triplet excited states of primarily intraligand 3ILCT nature localized on the pbt ligand, while the emissions of 2, 5 and 7 were ascribed to a mixture of close 3IL'(N donor)/3ILCT(pbt) excited states, as supported by lifetime measurements and theoretical calculations. Irradiation of the initial E/Z mixture of 4 (15 min) led to a steady state composed of roughly 1 : 1.15 (E : Z) and this complex was not emissive at room temperature due to an enhanced intramolecular E to Z isomerization process of the 1,2-bis-(4-pyridyl)ethylene ligand. Complexes 1-3 and 6 showed excellent quantum yields for the generation of singlet oxygen in aerated MeCN solution with the values of ϕ(1O2) ranging from 0.66 to 0.86 using phenalenone as a reference. Cationic complexes 1-3 exhibited remarkable efficacy in the nanomolar range against A549 (lung carcinoma) and HeLa (cervix carcinoma) cell lines with notable selectivity relative to the non-tumorigenic BEAS-2B (bronchial epithelium) cells. In the A549 cell line, the neutral complex 6 showed low cytotoxicity (IC50: 29.40 μM) and high photocytotoxicity (IC50: 5.75) when cells were irradiated with blue light for 15 min. These complexes do not show evidence of DNA interaction.

Bichromophoric Photosensitizers: How and Where to Attach Pyrene Moieties to Phenanthroline to Generate Copper(I) Complexes

Pyrene is a polycyclic aromatic hydrocarbon and organic dye that can form superior bichromophoric systems when combined with a transition metal-based chromophore. However, little is known about the effect of the type of attachment (i.e., 1- vs 2-pyrenyl) and the individual position of the pyrenyl substituents at the ligand. Therefore, a systematic series of three novel diimine ligands and their respective heteroleptic diimine-diphosphine copper(I) complexes has been designed and extensively studied. Special attention was given to two different substitution strategies: (i) attaching pyrene via its 1-position, which occurs most frequently in the literature, or via its 2-position and (ii) targeting two contrasting substitution patterns at the 1,10-phenanthroline ligand, i.e., the 5,6- and the 4,7-position. In the applied spectroscopic, electrochemical, and theoretical methods (UV/vis, emission, time-resolved luminescence and transient absorption, cyclic voltammetry, density functional theory), it has been shown that the precise choice of the derivatization sites is crucial. Substituting the pyridine rings of phenanthroline in the 4,7-position with the 1-pyrenyl moiety has the strongest impact on the bichromophore. This approach results in the most anodically shifted reduction potential and a drastic increase in the excited state lifetime by more than two orders of magnitude. In addition, it enables the highest singlet oxygen quantum yield of 96% and the most beneficial activity in the photocatalytic oxidation of 1,5-dihydroxy-naphthalene.

Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C2− heterocyclic ligand dba2− (H2dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh3, CNtBu and Me2Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh3, CNtBu and Me2Imd derivatives and characterized them by elemental analysis, 1H (and 31P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between −1.89 and −2.10 V and increasing along the series Me2Imd < dmso ≈ PPh3 < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me2Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me2Imd < CNtBu < PPh3 < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields ΦL ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high ΦL and long τ, the dmso and PPh3 complexes outperform the CNtBu and Me2Imd derivatives. This is remarkable in view of the higher ligand strength of Me2Imd, compared with all other coligands, as concluded from the electrochemical data.

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h’]diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around −1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around −1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around −2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed ³LC/³MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields Φ_L massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed ³LC/³MLCT with an emission lifetime of around 3 µs.

Monoanionic C^N^N Luminophores and Monodentate C-Donor Co-Ligands for Phosphorescent Pt(II) Complexes: A Case Study Involving Their Photophysics and Cytotoxicity

A family of Pt(II) complexes bearing monoanionic C^N^N ligands as luminophoric units as well as a set of monodentate ligands derived from allenylidene and carbene species were synthesized and characterized in terms of structure and photophysical properties. In addition, we present the extraordinary molecular structure of a phosphorescent complex carrying an allenylidene ligand. Depending on the co-ligand, an effect can be observed in the photoluminescence lifetimes and quantum yields as well as in the radiative and radiation less deactivation rate constants. Their correlation with the substitution pattern was analyzed by comparing the photoluminescence in fluid solution at room temperature and in frozen glassy matrices at 77 K. Moreover, in order to gain a deeper understanding of the electronic states responsible for the optical properties, density functional theory calculations were performed. Finally, the cytotoxicity of the complexes was evaluated in vitro, showing that the cationic complexes exhibit strong effects at low micromolar concentrations. The calculated half-maximum effective concentrations (EC₅₀ values) were 4 times lower in comparison to the established antitumor agent oxaliplatin. In contrast, the neutral species are less toxic, rendering them as potential bioimaging agents.

Computational, Mechanistic, and Experimental Insights into Regioselective Catalytic C-C Bond Activation in Linear 1-Aza-[3]triphenylene

C-C bond activation by transition metal complexes in ring-strained compounds followed by annulation with unsaturated compounds is an efficient approach to generate structurally more complex compounds. However, the site of catalytic C-C bond activation is difficult to predict in unsymmetrically substituted polycyclic systems. Here, we report a study on the (regio)selective catalytic cleavage of selected C-C bonds in 1-aza-[3]triphenylene, followed by annulation with alkynes, forming products with extended π-conjugated frameworks. Based on density functional theory (DFT) calculations, we established the stability of possible transition metal intermediates formed by oxidative addition to the C-C bond and thus identified the likely site of C-C bond activation. The computationally predicted selectivity was confirmed by the following experimental tests for the corresponding Ir-catalyzed C-C cleavage reaction followed by an alkyne insertion that yielded mixtures of two mono-insertion products isolated with yields of 34-36%, due to the close reactivity of two bonds during the first C-C bond activation. Similar results were obtained for twofold Ir- or Rh-catalyzed insertion reactions, with higher yields of 72-77%. In a broader context, by combining DFT calculations, which provided insights into the relative reactivity of individual C-C bonds, with experimental results, our approach allows us to synthesize previously unknown pentacyclic azaaromatic compounds.

Enantioselective Deaminative Alkylation of Amino Acid Derivatives with Unactivated Olefins

Herein, we report the first Ni-catalyzed enantioselective deaminative alkylation of amino acid and peptide derivatives with unactivated olefins. Key for success was the discovery of a new sterically encumbered bis(oxazoline) ligand backbone, thus offering a de novo technology for accessing enantioenriched sp³-sp³ linkages via sp³ C-N functionalization. Our protocol is distinguished by its broad scope and generality across a wide number of counterparts, even in the context of late-stage functionalization. In addition, an enantioselective deaminative remote hydroalkylation reaction of unactivated internal olefins is within reach, thus providing a useful entry point for forging enantioenriched sp³-sp³ centers at remote sp³ C-H sites.

Computational Study of Photochemical Relaxation Pathways of Platinum(II) Complexes

A series of functional platinum(II) complexes (Pt1-Pt3), which present high activity in four-photon absorption, in vivo imaging, and precise cancer therapy, as previously reported by the experimental work of Zhang et al. (Inorg. Chem. 2021, 60, 2362-2371), are computationally investigated in the article. We find that after the complex goes through four-photon absorption to the S₁ state, it undergoes intersystem crossing to the T₂ state and eventually reaches the T₁ state through internal conversion. On the T₁ state, both radiative and nonradiative decay to S₀ exit. The radiative decay forms the basis for the phosphorescence imaging in tissues as reported in the original paper. In addition, the nonradiative decay can simultaneously generate cytotoxic singlet oxygen by the excited energy transfer process, also known as triplet oxygen's quenching of triplet states. We conclude that the phosphorescence property as well as the photosensitizer character jointly bring high activity of in vivo imaging and photodynamic therapy to these complexes.

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH)

The three complexes [M(Me₂dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me₂dpb^-) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal d_yz, phenyl C4, C2, C1, and C6, and Cl p_z orbitals were found. As expected, the d_z² (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d_x²_-y² orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced d_z²-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d_z² orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d_yz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me₂dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called "dark", i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the "dark" states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

Investigation on Optical and Biological Properties of 2-(4-Dimethylaminophenyl)benzothiazole Based Cycloplatinated Complexes

The optical and biological properties of 2-(4-dimethylaminophenyl)benzothiazole cycloplatinated complexes featuring bioactive ligands ([{Pt(Me 2 N-pbt)(C 6 F 5 )}L] [L = Me 2 N-pbtH 1 , p -dpbH (4-(diphenylphosphino)benzoic acid) 2 , o -dpbH (2-(diphenylphosphino)benzoic acid) 3) , [Pt(Me 2 N-pbt)( o -dpb)] 4 ,  [{Pt(Me 2 N-pbt)(C 6 F 5 )} 2 (µ-PR n P)] [PR 4 P = O(CH 2 CH 2 OC(O)C 6 H 4 PPh 2 ) 2 5 , PR 12 P = O{(CH 2 CH 2 O) 3 C(O)C 6 H 4 PPh 2 } 2 6 ] are presented. Complexes 1-6 display 1 ILCT and metal perturbed 3 ILCT dual emissions. The ratio between both bands is excitation dependent, accomplishing warm-white emissions for 2 , 5 and 6 .  The phosphorescent emission is lost in aerated solutions owing to photoinduced electron transfer to 3 O 2 and formation of 1 O 2 , as confirmed in complexes 2 and 4 . They also exhibit photoinduced phosphorescence enhancement in non-degassed DMSO, due to local oxidation of DMSO by sensitized 1 O 2 , which causes a local degassing. Me 2 N-pbtH and the complexes exhibit specific accumulation in the Golgi apparatus although only 2 , 3 and 6 were active against A549 and HeLa cancer cell lines, being 6 highly selective respect to nontumoral cells. The potential photodynamic property of these complexes was demonstrated with complex 4 .

Synthesis, structure and reactivity of iridium complexes containing a bis-cyclometalated tridentate C^N^C ligand

In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of [Hg(HC^N^C)Cl] (1) (H2C^N^C = 2,6-bis(4-tert-butylphenyl)pyridine) with various organoiridium starting materials has been studied. The treatment of 1 with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) in acetonitrile at room temperature afforded a hexanuclear Ir4Hg2 complex, [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)Cl2]2 (2), which features Ir-Hg-Ir and Ir-Cl-Ir bridges. Refluxing 2 with sodium acetate in tetrahydrofuran (thf) resulted in cyclometalation of the bidentate HC^N^C ligand and formation of trinuclear [(C^N^C)(cod)IrHgIr(cod)Cl2] (3). On the other hand, refluxing [Ir(cod)Cl]2 with 1 and sodium acetate in thf yielded [Ir(C^N^C)(cod)(HgCl)] (4). Chlorination of 4 with PhICl2 gave [Ir(C^N^C)(cod)Cl]·HgCl2 (5·HgCl2) that reacted with tricyclohexylphosphine to yield Hg-free [Ir(C^N^C)(cod)Cl] (5). Chloride abstraction of 5 with silver(i) triflate (AgOTf) gave [Ir(C^N^C)(cod)(H2O)](OTf) (6) that can catalyze the cyclopropanation of styrene with ethyl diazoacetate. Reaction of 1 and [Ir(CO)2Cl(py)] (py = pyridine) with sodium acetate in refluxing thf afforded [Ir(C^N^C)(HgCl)(py)(CO)] (7), in which the carbonyl ligand is coplanar with the C^N^C ligand. On the other hand, refluxing 1 with (PPh4)[Ir(CO)2Cl2] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ2C,N-HC^N^C)(CO)] (8), the carbonyl ligand of which is trans to the pyridyl ring of the bidentate HC^N^C ligand. Upon irradiation with UV light 8 in thf was isomerized to 8', in which the carbonyl is trans to a phenyl group of the bidentate HC^N^C ligand. The isomer pair 8 and 8' exhibited emission at 548 and 514 nm in EtOH/MeOH at 77 K with lifetime of 84.0 and 64.6 μs, respectively. Protonation of 8 with p-toluenesulfonic acid (TsOH) afforded the bis(bidentate) tosylate complex [Ir(κ2C,N-HC^N^C)2(CO)(OTs)] (9) that could be reconverted to 8 upon treatment with sodium acetate. The electrochemistry of the Ir(C^N^C) complexes has been studied using cyclic voltammetry. Reaction of [Ir(PPh3)3Cl] with 1 and sodium acetate in refluxing thf led to isolation of the previously reported compound [Ir(κ2P,C-C6H4PPh2)2(PPh3)Cl] (10). The crystal structures of 2-5, 8, 8', 9 and 10 have been determined.

Photophysics and Excited State Dynamics of Cyclometalated [M(Phbpy)(CN)] (M = Ni, Pd, Pt) Complexes: A Theoretical and Experimental Study

Cyclometalated complexes [M(Phbpy)(CN)] (HPhbpy = 6-phenyl-2,2'-bipyridine) of the group 10 metals (Ni, Pd, and Pt) bearing a carbanionic ^-C^∧N^∧N pincer ligand were synthesized and studied in a combined experimental and computational DFT approach. All three complexes were crystallographically characterized showing closely packed dimers with head-to-tail stacking and short metal-metal contacts in the solid state. The computational models for geometries, excited states, and electronic transitions addressed both monomeric (Ni-mono, Pd-mono, and Pt-mono) and dimeric (Ni-dim, Pd-dim, and Pt-dim) entities. Photophysical properties and excited state dynamics of all title complexes were investigated in solution and in the solid at 298 and 77 K. [Ni(Phbpy)(CN)] and [Pd(Phbpy)(CN)] are virtually nonemissive in solution at 298 K, whereas [Pt(Phbpy)(CN)] shows phosphorescence in CH₂Cl₂ (DCM) solution (λ_em = 562 nm) stemming from a mixed ³MLCT/ILCT (metal-to-ligand charge transfer/intraligand charge transfer) state. At 77 K in a glassy frozen DCM:MeOH matrix, [Pd(Phbpy)(CN)] shows a remarkable emission (λ_em = 571 nm) with a photoluminescence quantum yield reaching almost unity, whereas [Ni(Phbpy)(CN)] is again nonemissive. Calculations on the monomeric models M-mono show that low-lying metal-centered states (MC, i.e., d-d* configuration) with dissociative character quench the photoluminescence. In the solid state, the complexes [M(Phbpy)(CN)] show defined photoluminescence bands (λ_em = 561 nm for Pd and 701 nm for Pt). Calculations on the dimeric models M-dim shows that the axial M···M interactions alter the photophysical properties of Pd-dim and Pt-dim toward MMLCT (metal-metal-to-ligand charge transfer) excited states with Pd-dim showing temperature-dependent emission lifetimes, suggesting thermally activated delayed fluorescence, whereas Pt-dim displayed phosphorescence with excimeric character. The metal-metal interactions were analyzed in detail with the quantum theory of atoms in molecules approach.

DFT Investigation of the Molecular Properties of the Dimethylglyoximato Complexes [M(Hdmg)2] (M = Ni, Pd, Pt)

Important applications of the NiII, PdII and PtII complexes [M(Hdmg)2] (H2dmg = dimethylglyoxime) stem from their metal...metal stacked virtually insoluble aggregates. Given the virtual insolubility of the materials, we postulated that the rare reports on dissolved species in solution do not represent monomolecular species but oligomers. We thus studied the structural and spectral properties of the monomolecular entities of these compounds using density functional theory (DFT) and time-dependent DFT computations in dimethyl sulfoxide (DMSO) as a solvent. The molecular geometries, IR and UV-vis spectra, and frontier orbitals properties were computed using LANL2DZ ecp and def2TZVP as basis sets and M06-2X as the functional. The results are compared with the available experimental and other calculated data. The optimised molecular geometries proved the asymmetric character of the two formed O–H…O bonds which connect the two Hdmg‒ ligands in the completely planar molecules. Calculated UV-vis spectra revealed the presence of three absorptions in the range 180 to 350 nm that are red-shifted along the series Ni–Pd–Pt. They were assigned to essentially ligand-centred π−π* transitions in part with metal(d) to ligand(π*) charge transfer (MLCT) contributions. The notorious d‒p transitions dominating the colour and electronics of the compounds in the solid-state and oligomeric stacks are negligible in our monomolecular models strongly supporting the idea that the previously reported spectroscopic observations or biological effects in solutions are not due to monomolecular complexes but rather to oligomeric dissolved species.

Antitumor activity of tridentate pincer and related metal complexes

Pincer complexes featuring tunable tridentate ligand frameworks are one of the most actively studied classes of metal-based complexes. Currently, growing attention is devoted to the cytotoxicity of pincer and related metal complexes. The antiproliferative activity of numerous pincer complexes has been reported. Pincer tridentate ligand scaffolds show different coordination modes and offer multiple options for directed structural modifications. This review summarizes the significant progress in the research studies of the antitumor activity of pincer and related platinum(ii), gold(iii), palladium(ii), copper(ii), iron(iii), ruthenium(ii), nickel(ii) and some other metal complexes, in order to provide a reference for designing novel metal coordination drug candidates with promising antitumor activity.

Luminescent cyclometalated platinum(ii) complexes with acyclic diaminocarbene ligands: structural, photophysical and biological properties

Four new cyclometalated Pt(ii) complexes bearing acyclic diaminocarbene (ADC) ligands, [Pt(C^N)Cl{C(NHXyl)(NHR)}] [C^N = 2,6-difluorophenylpyridine (dfppy), phenylquinoline (pq); R = Pr 3a, 4a, CH₂Ph 3b, 4b], were prepared by the nucleophilic attack on the isocyanide [Pt(C^N)Cl(CNXyl)] (C^N = dfppy 1, pq 2) by the corresponding amine RNH₂ (R = Pr, CH₂Ph). Complexes 3 show in their ¹H NMR spectra in CDCl₃ a notable concentration dependence, with a clear variation of the δ_H (NHXyl) signal, suggesting an assembling process implying donor-acceptor NH_XylCl bonding, also supported by 1D-PGSE (Pulse Field Gradient Spin Echo) and 2D-DOSY (Diffusion Ordered Spectroscopy) NMR experiments in solution and X-ray diffraction studies. The intermolecular interactions in compounds 3a and 3b were studied by using Hirshfeld surface analysis and Non-Covalent Interaction (NCI) methods on their X-ray structures. Their photophysical properties were investigated by absorption and emission spectroscopies and also by TD-DFT calculations performed on 3a and 4b. These complexes show green (3) or orange (4) phosphorescence, attributed to a mixed ³IL/³MLCT excited state. The carbene ligand does not affect the emission maxima but it produces an increase of the quantum yields in relation to the isocyanide in the precursors. In fluid solutions, the emission is not concentration-dependent, but the complexes may show aggregation induced emission as detailed for complexes 3a and 4a. In addition, cytotoxicity studies in the human cell lines A549 (lung carcinoma) and HeLa (cervix carcinoma) showed good activity for these complexes and 3a, 3b and 4a exhibit a strong effect on DNA electrophoretic mobility. To the best of our knowledge, compounds 3 and 4 represent the first examples of cycloplatinated complexes bearing acyclic diamino carbenes with antiproliferative properties.

Functional Platinum(II) Complexes with Four-Photon Absorption Activity, Lysosome Specificity, and Precise Cancer Therapy

Multiphoton materials are in special demand in the field of photodynamic therapy and multiphoton fluorescence imaging. However, rational design methodology for these brands of materials is still nascent. This is despite transition-metal complexes favoring optimized nonlinear-optical (NLO) activity and heavy-atom-effected phosphorescent emission. Here, three four-photon absorption (4PA) platinum(II) complexes (Pt1-Pt3) are achieved by the incorporation of varied functionalized C^N^C ligands with high yields. Pt1-Pt3 exhibit triplet metal-to-ligand charge-transfer transitions at ∼460 nm, which are verified multiple times by transient absorption spectra, time-dependent density functional theory calculations, and low-temperature emission spectra. Further, Pt1-Pt3 undergo 4PA. Notably, one of the complexes, Pt2, has maximum 4PA cross-sectional values of up to 15.2 × 10^-82 cm⁸ s³ photon^-3 under excitation of a 1600 nm femtosecond laser (near-IR II window). The 4PA cross sections vary when Pt2 is binding to lecithin and when it displays its lysosome-specific targeting behavior. On the basis of the excellent 4PA property of Pt2, we believe that those 4PA platinum(II) complexes have great potential applications in cancer theranostics.

Influence of Ligand and Nuclearity on the Cytotoxicity of Cyclometallated C^N^C Platinum(II) Complexes

A series of cyclometallated mono- and di-nuclear platinum(II) complexes and the parent organic ligand, 2,6-diphenylpyridine 1 (HC^N^CH), have been synthesized and characterized. This library of compounds includes [(C^N^C)PtII (L)] (L=dimethylsulfoxide (DMSO) 2 and triphenylphosphine (PPh3 ) 3) and [((C^N^C)PtII )2 (L')] (where L'=N-heterocycles (pyrazine (pyr) 4, 4,4'-bipyridine (4,4'-bipy) 5 or diphosphine (1,4-bis(diphenylphosphino)butane (dppb) 6). Their cytotoxicity was assessed against four cancerous cell lines and one normal cell line, with results highlighting significantly increased antiproliferative activity for the dinuclear complexes (4-6), when compared to the mononucleated species (2 and 3). Complex 6 is the most promising candidate, displaying very high selectivity towards cancerous cells, with selectivity index (SI) values >29.5 (A2780) and >11.2 (A2780cisR), and outperforming cisplatin by >4-fold and >18-fold, respectively.

A Bis-Chelating O N O ^ / N N ^ Ligand for the Synthesis of Heterobimetallic Platinum(II)/Rhenium(I) Complexes: Tools for the Optimization of a New Class of Platinum(II) Anticancer Agents

The two independent and N N ^ coordination sites of a newly synthesized bis[2-(hydroxyphenyl)-1,2,4-triazole] platform have been exploited to prepare four monometallic neutral ()Pt^II complexes carrying DMSO, pyridine, triphenylphosphine, or N-heterocyclic carbene as the fourth ligand. Then, the second N N ^ coordination site was used to introduce an IR-active rhenium tricarbonyl entity, affording the four corresponding heterobimetallic neutral Pt^II /Re^I complexes, as well as a cationic Pt^II /Re^I derivative. X-ray crystallographic studies showed that distortion of the organic platform occurred to accommodate the coordination geometry of both metal centers. No ligand exchange or transchelation occurred upon incubation of the Pt^II complexes in aqueous environment or in the presence of Fe^III , respectively. The antiproliferative activity of the ligand and complexes was first screened on the triple-negative breast cancer cell line MDA-MB-231. Then, the IC₅₀ values of the most active candidates were determined on a wider panel of human cancer cells (MDA-MB-231, MCF-7, and A2780), as well as on a nontumorigenic cell line (MCF-10A). Low micromolar activities were reached for the complexes carrying a DMSO ligand, making them the first examples of highly active, but hydrolytically stable, Pt^II complexes. Finally, the characteristic mid-IR signature of the {Re(CO)₃ } fragment in the Pt/Re heterobimetallic complexes was used to quantify their uptake in breast cancer cells.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Synthetic access to a phosphorescent non-palindromic pincer complex of palladium by a double oxidative addition – comproportionation sequence

A highly phosphorescent non-palindromic (C^C^N) palladium complex may be prepared by means of a double oxidative addition – comproportionation sequence, which is a new approach for the synthesis of non-palindromic pincer complexes.