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Treatment of bis(iminophosphorane)phosphane ligands 2a–2e with Li2PdCl4 gave a set of novel diphosphane-derived complexes bearing two metallacycle rings, each one enclosing a P=N double bond: the unprecedented bis(iminophosphorane)phosphane-[C,N,S] palladacycles. In the case of the ligand derived from bis(diphenylphosphino)methane, 2a, both the single and the double palladacycle complexes were obtained. Reaction of 3a with bis(diphenylphosphino)ethane did not yield the expected product with the diphosphane bonded to both palladium atoms, but rather the novel coordination compound 5. The crystal structures of 3c and 5 are described.

Palladacycles are versatile organometallic compounds that show potential for therapeutic use. Here are described the synthesis and characterization of mono- and dinuclear palladacycles bearing diphosphines. Their biological effect was investigated in A2780, an ovarian-derived cancer line, and in normal dermal fibroblasts. The compounds displayed selective cytotoxicity toward the A2780 cell line. Compound 3 decreased the cell viability through cell cycle retention in G0/G1, triggered apoptosis through the intrinsic pathway, and induced autophagy in A2780 cells. Compound 9 also induced cell cycle retention, apoptosis, and cellular detachment. Notably, compound 9 induced the production of intracellular reactive oxygen species (ROS). Our work demonstrated that compound 3 enters A2780 cells via active transport, which requires energy, while compound 9 enters A2780 cells mostly passively. The potential effect of palladacycles in angiogenesis was investigated for the first time in an in vivo chorioallantoic membrane model, showing that while compound 3 displayed an antiangiogenic effect crucial to fighting cancer progression, compound 9 promoted angiogenesis. These results show that palladacycles may be used in different clinical applications where pro- or antiangiogenic effects may be desirable.

A theoretical and experimental study of tetradentate [C,N : C,N] iminophosphorane palladacycles was carried out for the purpose of elucidating their behavior as compared to the parent Schiff base analogues to determine the prospect of encountering new A-frame structures for the iminophosphorane derivatives. The DFT calculations were in agreement with the experimental results regarding the performance of these ligands. New insights into the chemistry of the related dinuclear species have been obtained.

The synthesis of palladium cyclometallated compounds with thiosemicarbazone ligands is described,as well as their reactivity with bidentate phosphine ligands. The synthesis of the ligands is carried out by a condensation reaction between a ketone and a thiosemicarbazide. Subsequently, metallation proceeds, and the resulting product is reacted with the bis(diphenylphosphino)methane (dppm) using the appropriate conditions to obtain the compound with phosphine in a monodentate coordination mode.

A theoretical and experimental study of tetradentate [C,N : C,N] iminophosphorane palladacycles was carried out for the purpose of elucidating their behavior as compared to the parent Schiff base analogues to determine the prospect of encountering new A‐frame structures for the iminophosphorane derivatives. The DFT calculations were in agreement with the experimental results regarding the performance of these ligands. New insights into the chemistry of the related dinuclear species have been obtained., The experimental evidence, together with the DFT theoretical calculations prove that the tetradentate [C,N : C,N] iminophosphorane ligands behaves differently compared with the analogous diimine substrates. The results obtained from NMR spectroscopy suggest the existence in solution of E and Z conformers for the dinuclear complexes bearing chelated diphosphane donors.

A solution of compound 3 in dichloromethane produced a crystalline solid whose crystal structural analysis yielded a molecular structure with an asymmetric unit comprising two different palladacycles: the acetylacetonate complex 3 and the chloro–bridged dinuclear compound 2. The parent solution was examined by IR and NMR spectroscopy showing the conversion of 3 into 2, which may be due to traces of chloride ion in the form of acid present in the solvent, whereas an acid‑free solution of compound 3 remained unchanged. DFT calculations of the transformation have been carried out.

Herein we report on the synthesis, characterization and the ensuing chemistry of iminophosphorane palladacycles. Treatment of Ph3P[double bond, length as m-dash]N-(2-OHC6H4), 1, with sodium tetrachloropalladate gives 2 with the ligand as terdentate [C,N,O] allowing for only one μ-Cl ligand bonding the metal centers, resulting in a dinuclear complex. Treatment of 2 with PPh3 gives the mononuclear complex 3, whereas the reaction of 2 with diphosphanes Ph2P(CH2)nPPh2 in 1 : 2 ratio gives mixtures of 4 and 5 (n = 2) and 6 and 7 (n = 3). From them, the mononuclear complexes 4 and 6, and the dinuclear compounds, 5 and 7, were obtained with the parent ligand as bidentate [C,N]. The former two are of zwitterionic nature void of any counterion, with the phosphane ligand in the chelating mode. In a remarkable case of chemical serendipity, a solution of 2 left to stand produced crystals of complex 8: this is the missing link in the series of triphenylphosphane chalcogenide metallacycles. The experiment is repeatable; however, direct metallation of triphenylphosphane oxide was not possible.

Palladacycles of the type [Pd2(Ph2PCH2PPh2-P,P)2(C,N:C,N)] (C,N:C,N = bis(N-2,3,4-trimethoxybenzylidene)-4,4'-sulfonyldianiline or -4,4'-oxydianiline) can undergo a spontaneous slow chelate-to-bridging diphosphane coordination shift in solution. Following this strategy a tailor-made synthetic procedure was devised that culminates in isolation of double A-frame tetranuclear palladium complexes.

The Cover Feature shows four structures depicting (clockwise from the top‐left corner) a beautiful tetranuclear layered compound, a bis‐pseudopentacoordinated palladium moiety, the only known trinuclear thiosemicarbazone palladacycle, and the sandwiched coordination of the potassium cation by small crown ether rings attached to the metallated phenyl enclosures of a dinuclear diphosphine‐bridged palladium complex. All of these unexpected findings put forward the presence of serendipity in the reactivity of palladacycles and highlight the relevance of such an experience in their chemistry. More information can be found in the Review by A. Fernández‐Figueiras et al. on page 754 in Issue 10, 2018 (DOI: https://doi.org/10.1002/open.201800036).