Synthesis and Structure of Amino-Functionalized Cyclopentadienyl Vanadium Complexes and Evaluation of Their Butadiene Polymerization Behavior

Mechanistic and cytotoxicity studies of group IV β-diketonate complexes

Group IV metal complexes have previously shown promise as novel anticancer agents. Here, we discuss the mechanistic and cytotoxic nature of a series of group IV β-diketonate coordination complexes. Clear evidence that the ligands are exchangeable on the metal centre and that the β-diketonate ligands can act as potential drug delivery vehicles of the group IV metal ions was obtained. When evaluated for the cytotoxicity against human colon adenocarcinoma (HT-29) and human breast adenocarcinoma (MCF-7) cell lines, a general trend of decreasing potency down the group IV metals was observed. The most promising results obtained were for the hafnium complexes, with the tris diphenyl β-diketonate hafnium complex exhibiting IC50 values of 4.9 ± 0.9 μM and 3.2 ± 0.3 μM against HT-29 and MCF-7, respectively, which are comparable with the activity of cisplatin against the same cell lines. This tri β-diketonate hafnium complex is the first to show potent in vitro cytotoxic activity. The results reported show that ligand design has a significant effect on the cytotoxic potential of the complexes, and that these group IV complexes warrant further evaluation as novel metal-containing anticancer agents.

Novel Achiral Aryl-Substituted Vanadocenes: Synthesisand Preliminary Cytotoxicity Studies

From the reaction of 6‐(4‐methoxymethyl‐phenyl) fulvene (1a), 6‐(N‐methyl‐pyrrole) fulvene (1b), and 6‐(4‐dimethylamino‐phenyl) fulvene (1c) with LiBEt3H, the corresponding lithium cyclopentadienide intermediates (2a–c) were synthesized. These intermediates were then transmetallated to vanadium with VCl4 to yield the substituted vanadocene dichlorides bis‐[(4‐methoxymethyl‐benzyl)cyclopentadienyl] vanadium(iv) dichloride (3a), bis‐[(N‐methylpyrrole)2‐methylcyclopentadienyl] vanadium(iv) dichloride (3b), and bis‐[(p‐dimethylaminobenzyl)cyclopentadienyl] vanadium(iv) dichloride (3c). The vanadocene dichloride 3a was characterized by single crystal X‐ray diffraction. All three vanadocenes had their cytotoxicity investigated through MTT based preliminary in vitro testing on the human renal cell line Caki‐1 in order to determine their IC50 values. Vanadocenes 3a–b were found to have IC50 values of 6.5 and 5.2 µM, respectively, while 3c displayed a superior value of 1.7 µM.

Functionalised cyclopentadienyl zirconium compounds as potential anticancer drugs

New neutral and ionic functionalised zirconocene dichloride compounds have been isolated and characterised. The ionic zirconocene exhibits excellent cytotoxicity against a range of human tumour cell lines, which represents the first active anticancer zirconocene dichloride compound.

Antitumour bis(cyclopentadienyl) metal complexes: titanocene and molybdocene dichloride and derivatives

This Perspective will focus on recent developments in the field of antitumour metallocenes structurally related to titanocene dichloride. Despite extensive testing of titanocene dichloride which culminated in phase I and II clinical trials, further trials have been abandoned. While DNA has been implicated as the major target related to anticancer activity, identification of the active species and mechanism of action has been poorly understood and hence the design of second generation titanocene derivatives has not been possible. Recent mechanistic studies have provided a plausible mechanism for delivery of Ti to cancer cells via transferrin mediated endocytosis. This mechanism requires the presence of labile Cp-Ti bonds that hydrolyse on a time scale to deliver Ti to transferrin. A large range of titanocene derivatives in which the cyclopentadienyl rings have been substituted by both electron withdrawing and donating groups, including aromatic, alkyl and cyclic amines, have been prepared and tested for activity in the last 5 years. These results have shown that subtle structural effects can have a significant effect on biological activity and that biological activity is highly cell line dependent. However, the biological chemistry and cellular studies required to determine the mechanism of action of these new titanocenes have not been reported. In contrast, the bioorganometallic chemistry and cellular studies of molybdocene dichloride have implicated interaction with cellular thiols as the key reaction related to biological activity. Tailoring of the pseudohalide ligands by tuning the strength of the Mo-S bonds provides the opportunity to enhance cell uptake. Further research is required to establish the origin of antitumour activity.

Synthesis of mono-amino-functionalised ferrocene, ferrocene salts and ferrocenium salts

A number of mono-amino-functionalised ferrocenes, ferrocene salts and a mono-amino-functionalised ferrocenium salt have been synthesised and characterised. This represents a novel method of accessing these important classes of molecules. In the cases of some of the ferrocene salts, there are some interesting structural features in the solid state. These include N-H...O and N-H...F hydrogen bonds.