Synthesis, characterization and a reactivity study of some allyl palladium complexes bearing bidentate hemi-labile carbene or mixed carbene/PPh3 ligands

Palladium(II)-Indenyl Complexes Bearing N-Heterocyclic Carbene (NHC) Ligands as Potent and Selective Metallodrugs toward High-Grade Serous Ovarian Cancer Models

In this study, we synthesized novel Pd(II)-indenyl complexes using various N-heterocyclic carbene (NHC) ligands, including chelating NHC-picolyl, NHC-thioether, and diNHC ligands, and two monodentate NHCs. Transmetalation reactions between a Pd(II)-indenyl precursor and silver-NHC complexes were generally employed, except for chelating diNHC derivatives, which required direct reaction with bisimidazolium salts and potassium carbonate. Characterization included NMR, HRMS analysis, and single-crystal X-ray diffraction. In vitro on five ovarian cancer cell lines showed notable cytotoxicity, with IC50 values in the micro- and submicromolar range. Some compounds exhibited intriguing selectivity for cancer cells due to higher tumor cell uptake. Mechanistic studies revealed that monodentate NHCs induced mitochondrial damage while chelating ligands caused DNA damage. One chelating NHC-picolyl ligand showed promising cytotoxicity and selectivity in high-grade serous ovarian cancer models, supporting its consideration for preclinical study.

Rational Design of Palladium(II) Indenyl and Allyl Complexes Bearing Phosphine and Isocyanide Ancillary Ligands with Promising Antitumor Activity

A new class of palladium-indenyl complexes characterized by the presence of one bulky alkyl isocyanide and one aryl phosphine serving as ancillary ligands has been prepared, presenting high yields and selectivity. All the new products were completely characterized using spectroscopic and spectrometric techniques (NMR, FT-IR, and HRMS), and, for most of them, it was also possible to define their solid-state structures via X-ray diffractometry, revealing that the indenyl fragment always binds to the metal centre with a hapticity intermediate between ƞ3 and ƞ5. A reactivity study carried out using piperidine as a nucleophilic agent proved that the indenyl moiety is the eligible site of attack rather than the isocyanide ligand or the metal centre. All complexes were tested as potential anticancer agents against three ovarian cancer cell lines (A2780, A2780cis, and OVCAR-5) and one breast cancer cell line (MDA-MB-231), displaying comparable activity with respect to cisplatin, which was used as a positive control. Moreover, the similar cytotoxicity observed towards A2780 and A2780cis cells (cisplatin-sensitive and cisplatin-resistant, respectively) suggests that our palladium derivatives presumably act with a mechanism of action different than that of the clinically approved platinum drugs. For comparison, we also synthesized Pd-ƞ3-allyl derivatives, which generally showed a slightly higher activity towards ovarian cancer cells and lower activity towards breast cancer cells with respect to their Pd-indenyl congeners.

Cationic palladium(II)-indenyl complexes bearing phosphines as ancillary ligands: synthesis, and study of indenyl amination and anticancer activity

The reactivity of palladium(II) indenyl derivatives and their applications are topics relatively less studied, though in recent times these compounds have been used as pre-catalysts able to promote challenging cross-coupling processes. Herein, we propose the first systematic study concerning the nucleophilic attack on the palladium(II) coordinated indenyl fragment and, for this purpose, we have prepared a library of new Pd-indenyl complexes bearing mono- or bidentate phosphines as spectator ligands, developing specific synthetic strategies. All novel compounds are thoroughly characterized, highlighting that the indenyl ligand presents always a hapticity intermediate between η³ and η⁵. Secondary amines have been chosen as nucleophiles for the present study and indenyl amination has been monitored by UV-Vis and NMR spectroscopies, deriving a second order rate law, with dependence on both complex and amine concentrations. The rate-determining step of the process is the initial attack of the amine to the coordinated indenyl fragment, and this conclusion has been supported also by DFT calculations. The determination of second order rate constants has allowed us to assess the impact of the phosphine ligands on the kinetics of the process and identify the steric and electronic descriptors most suitable for predicting the reactivity of these systems. Finally, in vitro tests have proven that these organometallic compounds promote antiproliferative activity towards ovarian cancer cells better than cisplatin and possibly by adopting a different mechanism of action.

Reactions of proteins with a few organopalladium compounds of medicinal interest

Pd compounds form a promising class of experimental anticancer drug candidates whose mechanism of action is still largely unknown; in particular, a few organopalladium compounds seem very attractive. To gain mechanistic insight into medicinal palladium compounds, we have explored here – through ESI MS analysis – the interactions of four organopalladium agents (1–4) – showing remarkable in vitro antiproliferative properties – with a few representative model proteins, i.e., lysozyme (HEWL), ribonuclease A (RNase), and carbonic anhydrase (hCAI). The tested panel included three Pd allyl compounds with one or two carbene ligands and a palladacyclopentadienyl complex. Notably, the Pd allyl compounds turned out to manifest, on the whole, a modest tendency to react with the above proteins. Only complex 3 produced small amounts of characteristic adducts with hCAI bearing either one or two Pd allyl groups. In contrast, the palladacyclopentadienyl complex 4 manifested a greater and peculiar reactivity with all the above proteins generating invariably protein adducts with a mass increase of +256 Da where a butadienyl group – with no associated Pd – is attached to the proteins. Afterwards, we extended our investigations to the C-terminal dodecapeptide of thioredoxin reductase bearing the –Cys–Sec– reactive motif. In this latter case adducts were formed with all tested Pd compounds; however, complex 4 manifested towards this dodecapeptide a type of reactivity deeply different from that observed with HEWL, RNase A and hCAI. The mechanistic implications of these findings are discussed.

Pd compounds form a promising class of experimental anticancer drug candidates whose mechanism of action is still largely unknown; in particular, a few organopalladium compounds seem very attractive.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

Conversion of Pd(I) off-cycle species into highly efficient cross-coupling catalysts

We report on the facile conversion of [Pd2(μ-Cl)(μ-η3-R-allyl)(NHC)2] complexes, which are commonly considered undesirable off-cycle species in cross-coupling reactions, into active [PdCl(μ-Cl)(NHC)]2 pre-catalysts. All reactions proceed under mild conditions (40 °C, 1-2 hours in acetone) using inexpensive HCl as both an oxidant and chloride source. DFT calculations were performed to explore the possible mechanism of this transformation, which appears to involve a combination of two different pathways. Moreover this study provides insights into factors favoring and hindering Pd(i) dimer formation undesirable in catalysis.

Palladium(II)-η3 -Allyl Complexes Bearing N-Trifluoromethyl N-Heterocyclic Carbenes: A New Generation of Anticancer Agents that Restrain the Growth of High-Grade Serous Ovarian Cancer Tumoroids

The first palladium organometallic compounds bearing N-trifluoromethyl N-heterocyclic carbenes have been synthesized. These η³ -allyl complexes are potent antiproliferative agents against different cancer lines (for the most part, IC₅₀ values fall in the range 0.02-0.5 μm). By choosing 1,3,5-triaza-7-phosphaadamantane (PTA) as co-ligand, we can improve the selectivity toward tumor cells, whereas the introduction of 2-methyl substituents generally reduces the antitumor activity slightly. A series of biochemical assays, aimed at defining the cellular targets of these palladium complexes, has shown that mitochondria are damaged before DNA, thus revealing a behavior substantially different from that of cisplatin and its derivatives. We assume that the specific mechanism of action of these organometallic compounds involves nucleophilic attack on the η³ -allyl fragment. The effectiveness of a representative complex, 4 c, was verified on ovarian cancer tumoroids derived from patients. The results are promising: unlike carboplatin, our compound turned out to be very active and showed a low toxicity toward normal liver organoids.

Synthesis, characterization, catalytic and biological application of half-sandwich ruthenium complexes bearing hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

A series of cationic Ru(ii)(η⁶-p-cymene) complexes with thioether-functionalised N-heterocyclic carbene ligands have been prepared and fully characterized. Steric and electronic influence of the R thioether substituent on the coordination of the sulfur atom was investigated. The molecular structure of three of them has been determined by means of X-ray diffractrometry and confirmed the bidentate (κ²-C,S) coordination mode of the ligand. Interestingly, only a single diastereomer, as an enantiomeric couple, was observed in the solid state for complexes 1c, 1i and 1j. DFT calculations established a low energy inversion barrier between the two diastereomers through a sulfur pyramidal inversion pathway with R donating group while a dissociative/associative mechanism is more likely with R substituents that contain electron withdrawing group, thus suggesting that the only species observed by the ¹H-NMR correspond to an average resonance position of a fluxional mixtures of isomers. All these complexes were found to catalyse the oxydant-free double dehydrogenation of primary amine into nitrile. Ru complex bearing NHC-functionalised S-tBu group was further investigated in a wide range of amines and was found more selective for alkyl amine substrates than for benzylamine derivatives. Finally, preliminary results of the biological effects on various human cancer cells of four selected Ru complexes are reported.

Synthesis and in-depth studies on the anticancer activity of novel palladacyclopentadienyl complexes stabilized by N-Heterocyclic carbene ligands

New palladacyclopentadienyl complexes with bis-N-heterocyclic carbenes as spectator ligands have been synthesized and exhaustively characterized. The crystal structure of complex 1a has been also determined by X-ray diffraction analysis. Their in vitro cytotoxicity and that of other palladacyclopentadienyl derivatives coordinating different ancillary ligands has been determined against different cancer cell lines. Many complexes have shown an antiproliferative activity toward tumor cells often definitely better than cisplatin, whereas they have resulted practically inactive against the non-cancer MRC-5 cell line. The mechanism of action of bis-NHC derivative 1a, particularly active against ovarian cancer cell lines was studied in depth. Through a longitudinally analysis, it is shown that compound 1a induces apoptosis via DNA damage and release of cytochrome C. We propose compound 1a as a powerful and specific drug for the therapy of a deadly disease such as high grade serous ovarian cancer.

Synthesis of new allyl palladium complexes bearing purine-based NHC ligands with antiproliferative and proapoptotic activities on human ovarian cancer cell lines

A series of new palladium allyl complexes bearing purine-based carbenes derived from caffeine, theophylline and theobromine have been prepared and characterized by NMR spectroscopy, and elemental analysis and in two cases by single crystal X-ray diffraction. The cytotoxic and proapoptotic activities of compounds have been determined in vitro on human ovarian cancer A2780 and SKOV-3 cell lines. These experiments have shown that the palladium-allyl fragment induces a general cytotoxicity, but the choice of the supporting ligands is of paramount importance for achieving the best results. In particular complexes 4c, 4d and 5d exhibit a higher antiproliferative effect (IC50: 0.09, 0.81 and 0.85 μM respectively) than cisplatin (IC50: 1.5 μM) on A2780 cells, and 4d (IC50: 1.7 μM vs. 5.94 μM) on SKOV-3 cell line. Moreover in many cases it has been proved that the cytotoxicity of our complexes is associated with the induction of apoptosis.