Tetrametallic rectangular box complexes assembled from heteroligated macrocycles

Novel {Cu4} and {Cu4Cd6} clusters derived from flexible aminoalcohols: synthesis, characterization, crystal structures, and evaluation of anticancer properties

Two new copper clusters, {Cu₄} and {Cu₄Cd₆}, with polydentate aminoalcohol ligands, diethanol propanolamine (H₃L¹) and bis-tris{2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol} (H₆L²), have been synthesized under mild conditions and characterized thoroughly by single-crystal X-ray diffraction (XRD), infrared spectroscopy, elemental analysis, powder XRD, magnetic and DFT studies, and absorption and fluorescence spectroscopy. The cluster {Cu₄} exhibits a rare tetranuclear copper cubane core whereas {Cu₄Cd₆} forms an unusual heterometallic cage owing to the introduction of the second metal Cd into the ligand. A hexapodal ligand (H₆L²) with N and O donor atoms was chosen deliberately for the construction of a high-nuclearity cluster, i.e., {Cu₄Cd₆}. Interestingly, both the clusters displayed significant cytotoxicity towards human cervical (HeLa) and lung (A549) cancer cells as evident from the shallow IC₅₀ values [15.6 ± 0.8 μM (HeLa), 18.5 ± 1.9 μM (A549) for {Cu₄}, and 11.1 ± 1.5 μM (HeLa), 10.2 ± 1.3 μM (A549) for {Cu₄Cd₆}] obtained after a 24 h incubation. However, moderate toxicity was observed toward immortalized lung epithelial normal cells (HPL1D) with IC₅₀ values of 32.4 ± 1.2 μM for {Cu₄} and 27.6 ± 1.7 μM for {Cu₄Cd₆}. A cellular apoptotic study using HeLa cells revealed that the {Cu₄} cluster triggered apoptosis at both the early and late phases while the {Cu₄Cd₆} cluster facilitate apoptosis mainly at the late apoptotic stage. A standard 2',7'-dichlorodihydrofluorescein-diacetate (DCFH-DA) test affirms that both the clusters enhanced ROS production inside the cancer cells, responsible for promoting cell apoptosis. The decanuclear {Cu₄Cd₆} clusters demonstrated better anticancer activity compared to the tetranuclear {Cu₄} clusters, indicating the role of high nuclearity and additional Cd metal in the enhanced intracellular production of ROS.

Host–guest capability of a three-dimensional heterometallic macrocycle

Three-dimensional heterometallic macrocycles with half-sandwich Rh corners were studied for their ability to trap planar and non-planar guests. Furthermore, these heterometallic macrocycles can be destroyed in the presence of a soft base to form hexanuclear triangular prism complexes.

Reversible Ligand Pairing and Sorting Processes Leading to Heteroligated Palladium(II) Complexes with Hemilabile Ligands

Halide-induced ligand pairing and sorting processes have been observed in the context of Pd(II) complexes with hemilabile P,S and P,O ligands. Mixing of the ligands Ph₂PCH₂CH₂SMe (7) and Ph₂PCH₂CH₂SPh (8) with a Pd(II) precursor in CH₂Cl₂ results in a mixture of [(7)₂ClPd]Cl, [(8)₂Cl₂Pd], and [(7)(8)ClPd]Cl complexes at 20 °C. This equilibrium can be driven toward the heteroligated structure [(7)(8)ClPd]Cl by (1) cooling the mixture or (2) precipitation with hexanes, leading to the exclusive formation of semiopen heteroligated complex cis-[κ ²-(7)-κ ¹-(8)ClPd]Cl (9a), as confirmed by a single-crystal X-ray diffraction study and solid state CPMAS ³¹P{¹H} NMR spectroscopy. Dissolution of 9a in CH₂Cl₂ leads to the original mixture of complexes, which illustrates the reversible nature of this ligand pairing and sorting process. Similar processes occur when a combination of P,S and P,O ligands is used. The semiopen heteroligated complexes can be chemically manipulated in a reversible fashion to form closed complexes, allowing for control of the relative position and flexibility between neighboring substituents in these "tweezer"-like structures. Control experiments suggest these ligand sorting and pairing processes occur via a halide-induced ligand rearrangement (HILR) reaction.

Heteroligated supramolecular coordination complexes formed via the halide-induced ligand rearrangement reaction

Supramolecular coordination chemistry allows researchers to synthesize higher-order structures that approach the nanoscale dimensions of small enzymes. Frequently, such structures have highly symmetric macrocyclic square or cage shapes. To build functional structures that mimic the complex recognition, catalytic, and allosteric properties of enzymes, researchers must do more than synthesize highly symmetric nanoscale structures. They must also simultaneously incorporate different functionalities into these structures and learn how to regulate their relative arrangement with respect to each other. Designing such heteroligated coordination complexes remains a significant challenge for supramolecular chemists. This Account focuses on the discovery and development of a novel supramolecular reaction known as the halide-induced ligand rearrangement (HILR) reaction. Two hemilabile ligands with different binding strengths combine with d(8) transition metal precursors that contain halide ions. The reaction spontaneously results in heteroligated complexes and is highly modular and general. Indeed, it not only can be used to prepare tweezer complexes but also allows for the rapid and quantitative formation of heteroligated macrocyclic triple-decker/step and rectangular box complexes from a variety of different ligands and transition metal ions. The relative arrangement between functional groups A and B in these structures can be regulated in situ using small ancillary ligands such as halides, CO, and nitriles. Based on this reaction, zinc- and magnesium-porphyrin moieties can be incorporated into heteroligated macrocyclic or tweezer scaffolds. These examples demonstrate the convergent and cofacial assembly of functional sites that are known to be involved in numerous processes in enzymes. They also show how the relative spatial and lateral distances of these sites can be varied, in many cases reversibly. Researchers can use such complexes to study a wide range of enzymatic processes, including catalysis, molecular recognition, electron transfer, and allosteric signal transfer.

Spontaneous formation of heteroligated PtII complexes with chelating hemilabile ligands

The spontaneous formation of the heteroligated complex [PtCl(kappa(2)-Ph(2)PCH(2)CH(2)SMe)(Ph(2)PCH(2)CH(2)SPh)]Cl (8 a) by a novel ligand rearrangement process has been observed. By using the weak-link approach, the relative arrangement of the alkyl and aryl groups can be controlled by abstraction of chloride from 8 a to form the closed complex [Pt(kappa(2)-Ph(2)PCH(2)CH(2)SMe)(kappa(2)-Ph(2)PCH(2)CH(2)SPh)][BF(4)](2) (5) and reopening using halide ions to form semi-open complexes [PtX(kappa(2)-Ph(2)PCH(2)CH(2)SMe)(Ph(2)PCH(2)CH(2)SPh)]BF(4) (8 b; X=Cl(-)) and (8 c; X=I(-)). Analogous procedures using Ph(2)PCH(2)CH(2)SMe and 1,4-(Ph(2)PCH(2)CH(2)S)(2)C(6)H(4) lead to heteroligated bimetallic complexes 7 and 9, illustrating that this ligand rearrangement process can be used as a tool for the assembly of complementary metallosupramolecular structures.