Metal Phosphorus Complexes as Antitumor Agents

Pd(II)- and Pt(II)-Assisted P-C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine

There is unceasing interest toward transformations of phosphine derivatives, which are facilitated by transition metals. We report a facile Pd(II)- and Pt(II)-assisted P-C bond cleavage in a luminescent 2-phenylbenzothiazole-based α-methylaminophosphine (PCN, 1). Specifically, reactions between 1 and [M(COD)Cl2] (M = Pd, Pt; COD = cycloocta-1,5-diene) in different solvents (methylene chloride, acetonitrile, pyridine, toluene) resulted in the formation of PPh2-, captured either as a bridging ligand in binuclear complexes with a {M2(PPh2)2} moiety or as an adduct to COD in [Pt2(PPh2COD)2Cl2]. The heterocyclic part transforms to annulated c-CN+ species with a 1,2-dihydroquinazoline cycle formed. In the presence of pyridine as a base, annulated form c-CN+ destabilizes and undergoes reverse cyclization transforming to deprotonated CN form. Quantum-chemical density functional theory (DFT) calculations predict that a crucial step in the reactions involves proton transfer from the N atom of the amino group of PCN to a neighboring molecule. A combination of high photophysical sensitivity of c-CN+ toward its immediate environment and rich structural capabilities in assembling (c-CN)22+ pairs in different crystal packings in a family of phases with the general formula (c-CN)2[M2(PPh2)2Cl4] allows one to fine-tune the luminescence properties of the latter. The results were rationalized as a variation of π-π intercationic spacings, which tunes the degree of excited-state charge transfer between c-CN+ cations. As a result, compounds with relatively short interplanar π-π-separation between the cations show a stronger charge-transfer-mediated bathochromic shift.

Fluorination and hydrolytic stability of water-soluble platinum complexes with a borane-bridged diphosphoramidite ligand

The high fluorophilicity of borane-containing ligands offers promise for accessing new metallodrug candidates capable of bifunctional [¹⁸F]-positron emission tomography (PET) imaging, but this requires water soluble and hydrolytically stable ligands that can be fluorinated under mild conditions. Toward this goal, here we report the synthesis and characterization of water-soluble Pt(II) complexes containing a triaminoborane-bridged diphosphoramidite ligand called ^MeOTBDPhos that can be fluorinated using simple fluoride salts. NMR and XRD studies show that (^MeOTBDPhos)PtCl₂ (1) dissolves in water with cooperative H-OH addition across the bridgehead N-B bond to form 1-H2O. The B-OH bond in 1-H2O undergoes rapid displacement with fluoride (<10 min) when treated with CsF in MeCN to form 1-HF. 1-HF can also be prepared in <10 min by addition of KF to 1 in the presence Kryptofix® 222 and (HNEt₃)Cl in MeCN. In addition to using fluoride salts, we show how mononuclear 1 can be fluorinated with HBF₄·Et₂O to form dinuclear [(^MeOTBDPhos-HF)Pt(μ-Cl)]₂(BF₄)₂ (4-HF). Comparative studies show that the B-F bond in 1-HF undergoes hydrolysis as soon as it is dissolved in water or saline, but the B-F bond persists for hours when the pH of the solution is lowered to pH ≤ 2. In contrast to 1-HF, the B-F bond in dinuclear 4-HF persists for days when dissolved in water, which may be attributed to slow, sacrificial release of fluoride from the BF₄^- anion. The results show how cooperative N-B reactivity on the ligand can be leveraged to rapidly fluorinate water-soluble ^MeOTBDPhos complexes under mild conditions and afford suggestions for how to enhance hydrolytic B-F stability, as required for use in biomedical applications.

Microwave-assisted synthesis of N,N-bis(phosphinoylmethyl)amines and N,N,N-tris(phosphinoylmethyl)amines bearing different substituents on the phosphorus atoms

A family of N,N-bis(phosphinoylmethyl)amines bearing different substituents on the phosphorus atoms was synthesized by the microwave-assisted and catalyst-free Kabachnik-Fields reaction of (aminomethyl)phosphine oxides with paraformaldehyde and diphenylphosphine oxide. The three-component condensation of N,N-bis(phosphinoylmethyl)amine, paraformaldehyde and a secondary phosphine oxide affording N,N,N-tris(phosphinoylmethyl)amine derivatives was also elaborated. This method is a novel approach for the synthesis of the target products.

Synthesis and characterization of a new class of anti-angiogenic agents based on ruthenium clusters

New triruthenium-carbonyl clusters derivatized with glucose-modified bicyclophosphite ligands have been synthesized. These compounds were found to have cytostatic and cytotoxic activity and depending on the number of bicyclophosphite ligands, and could be tuned for either anti-cancer or specific anti-angiogenic activity. While some compounds had a broad cellular toxicity profile in several cell types others showed endothelial cell specific dose-dependent anti-proliferative and anti-migratory efficacy. A profound inhibition of angiogenesis was also observed in the in vivo chicken chorioallantoic membrane (CAM) model, and consequently, these new compounds have considerable potential in drug design, e.g. for the treatment of cancer.

New ruthenium(II)-letrozole complexes as anticancer therapeutics

Novel ruthenium-letrozole complexes have been prepared, and cell viability of two human cancer cell types (breast and glioblastoma) was determined. Some ruthenium compounds are known for their cytotoxicity to cancer cells, whereas letrozole is an aromatase inhibitor administered after surgery to post-menopausal women with hormonally responsive breast cancer. A significant in vitro activity was established for complex 5·Let against breast cancer MCF-7 cells and significantly lower activity against glioblastoma U251N cells. The activity of 5·Let was even higher than that of 4, a compound analogous to the well-known drug RAPTA-C. Results from the combination of 5·Let (or 4) with 3-methyladenine (3-MA) or with curcumin, respectively, revealed that the resultant cancer cell death likely involves 5·Let-induced autophagy.