James D. Crowley

Stimuli-responsive Pd2L4 metallosupramolecular cages: towards targeted cisplatin drug delivery

Metallosupramolecular cages are an emerging, but as of yet relativity unexplored, drug delivery vector. Herein we show that discrete dipalladium(II) molecular cages of the formula [Pd2L4](X)4 can be quantatively self-assembled from a simple tripyridyl ligand (2,6-bis(pyridin-3-ylethynyl)pyridine) and [Pd(CH3CN)4](X)2 (X = BF4− or SbF6−). The cages have been fully characterised using 1H, 13C and DOSY NMR spectroscopy, elemental analysis, IR spectroscopy, and high resolution electrospray mass spectrometry (HR-ESMS). Additionally, the molecular structure of the [Pd2L4](SbF6)4 cage was confirmed unequivocally using X-ray diffraction. These [Pd2L4](X)4 cages are stimuli-responsive and can be reversibly disassembled/reassembled upon the addition/removal of suitable competing ligands. The central cavities of the [Pd2L4](X)4 cages are lined with four hydrogen bond accepting pyridine units which enable the encapsulation of two cisplatin molecules within the metallosupramolecular architecture through hydrogen bonding interactions between the cage and the amine ligands of the cisplatin guest. The structure of the [Pd2L4⊃(cisplatin)2](BF4)4 host–guest adduct has been confirmed by 1H NMR spectroscopy, HR-ESMS and X-ray crystallography. Additionally we have demonstrated that the cage–cisplatin host–guest adduct can be quantatively disassembled upon the addition of a competing ligand, releasing the cisplatin guest. This is the first crystallographically characterised example of a discrete metallosupramolecular cage encapsulating an FDA-approved inorganic drug molecule. This host–guest chemistry could open the way to relatively unexplored methods of drug delivery, which circumvent the malicious side effects and drug resistance associated with cisplatin and other anticancer therapeutics.

1,3,4-Trisubstituted-1,2,3-Triazol-5-ylidene 'Click' Carbene Ligands: Synthesis, Catalysis and Self-Assembly

This review examines the use of the Cu(I)-catalyzed 1,3-cycloaddition of organic azides with terminal alkynes (the CuAAC ‘click’ reaction) for development of a novel family of abnormal/mesoionic N-heterocyclic carbenes and their corresponding metal complexes. These 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes have donor properties that are intermediate between the traditional Arduengo-type imidazol-2-ylidenes and more highly σ-donating abnormal carbenes, such as imidazol-4-ylidenes or pyrazolin-4-ylidenes. Metal complexes of the 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes have been used as catalysts for a variety of reactions including the CuAAC cycloaddition, Pd cross-couplings, and ring closing/ring opening metathesis. Additionally, ‘click’ carbene ligands have been used to generate self-assembled metallo-macrocycles and novel photosensitizers. The mild, modular CuAAC approach to these ligands should allow the rapid generation of libraries of 1,3,4-trisubstituted-1,2,3-triazol-5-ylidenes that can be further exploited to generate novel catalysts, metallo-pharmaceuticals and materials.

An Unusual Nickel-Copper-Mediated Alkyne Homocoupling Reaction for the Active-Template Synthesis of [2]Rotaxanes

We report on an unusual Ni-/Cu-mediated alkyne homocoupling reaction, directed through the cavity of a bidentate macrocyclic ligand by chelated metal ions to furnish [2]rotaxanes in excellent (up to 95%) yields. This is the first active metal template reaction to employ an octahedral coordination geometry metal ion, Ni(II), and the study provides some interesting mechanistic insights into the mixed bimetallic reaction mechanism. The mixed-metal catalyst system was discovered serendipitously when Cu(I) was added to a Ni(II)-catalyzed alkyne homocoupling reaction in an attempt to facilitate chloride-acetylide ligand exchange. The role of Cu(I) in the reaction is, in fact, quite different from that originally intended. The effectiveness of having both nickel and copper present can be rationalized by the nature of a pi-activated, sigma-bonded, bimetallic intermediate in which the substitution of Ni(II) for one Cu(I) ion in the classic bimetallic Glaser reaction mechanism apparently aids reductive elimination of the acetylide ligands. The system may prove useful for the development of general mixed-metal protocols for catalytic alkyne coupling reactions as well as being a highly effective route to rotaxanes with bis-acetylene threads, which are potentially useful for materials applications (insulated molecular wires) and in molecular machines (rigid, nonfolding axles).

‘Click’ to functionalise: synthesis, characterisation and enhancement of the physical properties of a series of exo- and endo-functionalised Pd2L4 nanocages

The synthesis of self-assembled metallosupramolecular architectures has been of steadily growing interest in recent years due to their diverse applications. Appending additional functionality to the ligands of these architectures has been limited as this often involves incorporation of coordinating groups that can potentially disrupt formation of the desired structure. Herein we report the use of the facile, functional group tolerant and high yielding CuAAC ‘click’ reaction to attach a variety of functional moieties to a tripyridyl ligand system. Despite the presence of the potentially coordinating 1,2,3-triazole rings, self-assembly of quadruply-stranded dipalladium(II) cage architectures in the presence of Pd(II) ions was almost universally observed for the functionalised “click” ligands. The only system which did not assemble into the expected cage featured a 2-(1,2,3-triazol-4-yl)pyridine binding pocket which sequestered the Pd(II) ions. Blocking this chelating pocket with an inert [Re(CO)3Cl] moiety restored the ability of the ligand to self-assemble into the desired quadruply-stranded dipalladium(II) cage, generating a heterometallic cage architecture. All ligands and cage architectures have been characterised using 1H, 13C and DOSY NMR, IR, UV-Vis and emission spectroscopies, mass spectrometry and in some cases by X-ray crystallography. Whilst the parent cage system is devoid of useful physical properties and displays a limited range of solubility, the CuAAC methodology provides a facile method to enhance the cages properties. A variety of fluorescent, redox active and biologically relevant species have been appended to the external surface of these cages. These groups enabled the generation of a series of aqueous soluble, fluorescent and electrochemically active Pd2L4 cages in a modular fashion.

Gold(I) and Palladium(II) Complexes of 1,3,4-Trisubstituted 1,2,3-Triazol-5-ylidene “Click” Carbenes: Systematic Study of the Electronic and Steric Influence on Catalytic Activity

The synthesis of a small family of six electronically and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene gold(I) chloride complexes is described. Additionally, the corresponding trans-[PdBr2(iPr2-bimy)(1,3,4-trisubstituted 1,2,3-triazol-5-ylidene)] complexes are also generated and used to examine the donor strength of the 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene ligands. All compounds have been characterized by 1H and 13C NMR and IR spectroscopy, high-resolution electrospray mass spectrometry (HR-ESI-MS), and elemental analysis. The molecular structures of four of the gold(I) and four of the palladium(II) complexes were determined using X-ray crystallography. Finally, it is demonstrated that these 1,2,3-triazol-5-ylidene gold(I) chloride complexes (Au(trz)Cl) are able to catalyze the cycloisomerization of 1,6-enynes, in high yield and regioselectivity, as well as the intermolecular direct etherification of allylic alcohols. Exploiting the Au(trz)Cl precatalysts allowed the etherification of allylic alcohols to be carried out under milder conditions, with better yield and regioselectivity than selected commercially available gold(I) catalysts.

Toward the self-assembly of metal-organic nanotubes using metal-metal and π-stacking interactions: bis(pyridylethynyl) silver(I) metallo-macrocycles and coordination polymers.

Shape-persistent macrocycles and planar organometallic complexes are beginning to show considerable promise as building blocks for the self-assembly of a variety of supramolecular materials including nanofibers, nanowires, and liquid crystals. Here we report the synthesis and characterization of a family of planar di- and tri-silver(I) containing metallo-macrocycles designed to self-assemble into novel metal-organic nanotubes through a combination of π-stacking and metal-metal interactions. The silver(I) complexes have been fully characterized by elemental analysis, high resolution electrospray ionization mass spectrometry (HR-ESI-MS), IR, (1)H and (13)C NMR spectroscopy, and the solution data are consistent with the formation of the metallo-macrocycles. Four of the complexes have been structurally characterized using X-ray crystallography. However, only the di-silver(I) complex formed with 1,3-bis(pyridin-3-ylethynyl)benzene is found to maintain its macrocyclic structure in the solid state. The di-silver(I) shape-persistent macrocycle assembles into a nanoporous chicken-wire like structure, and ClO(4)(-) anions and disordered H(2)O molecules fill the pores. The silver(I) complexes of 2,6-bis(pyridin-3-ylethynyl)pyridine and 1,4-di(3-pyridyl)buta-1,3-diyne ring-open and crystallize as non-porous coordination polymers.

“Click-Triazole” Coordination Chemistry: Exploiting 1,4-Disubstituted-1,2,3-Triazoles as Ligands

Access to readily functionalized ligand architectures is of crucial importance in a range of different areas including catalysis, metallopharmaceuticals, bioimaging, metallosupramolecular chemistry, mechanically interlocked architectures, and molecular machines. The mild and modular Cu(I)-catalyzed 1,3-cycloaddition of terminal alkynes with organic azides (the CuAAC “click” reaction) allows the ready formation of functionalized 1,4-disubstituted-1,2,3-triazole scaffolds, and this has led to an explosion of interest in the coordination chemistry of these heterocycles. The parent 1,4-disubstituted-1,2,3-triazole units can potentially act as monodentate or bridging ligands. Examples of both the monodentate (through either the N3 nitrogen or C5 carbon positions of the 1,2,3-triazole) and bridging (through the N2 and N3 nitrogen atoms) coordination modes have been structurally characterized. A diverse array of bi-, tri-, and polydentate ligands incorporating 1,4-disubstituted-1,2,3-triazole units have also been synthesized and characterized. When the chelate pocket involves coordination through the N3 nitrogen atom of the 1,2,3-triazole, these are called “regular” click ligands. While these are the most common type of “click” chelate, “inverse” ligands in which the 1,2,3-triazole unit coordinates through the less electron-rich N2 nitrogen atom have also been synthesized and characterized. The resulting “click” complexes are beginning to find applications in catalysis, metallosupramolecular chemistry, photophysics, and as metallopharmaceuticals and bioimaging agents.

Molecular recognition. Self-assembly of molecular trigonal prisms and their host–guest adducts

Two supramolecular trigonal prisms, each bearing three molecular clefts are shown to form 1:6 and 1:7 host-guest complexes with 9-methylanthracene and one of the prisms forms a 1:2 host-guest complex with a tritopic tri-anthracene guest that registers with the recognition sites of the host.

[Fe2L3]4+ cylinders derived from bis(bidentate) 2-pyridyl-1,2,3- triazole "click" ligands: Synthesis, structures and exploration of biological activity

A series of metallosupramolecular [Fe2L3](BF4)4 “click” cylinders have been synthesized in excellent yields (90%–95%) from [Fe(H2O)6](BF4)2 and bis(bidentate) pyridyl-1,2,3-triazole ligands. All complexes were characterized by elemental analysis, IR, UV-vis, 1H-, 13C- and DOSY-NMR spectroscopies and, in four cases, the structures confirmed by X-ray crystallography. Molecular modeling indicated that some of these “click” complexes were of similar size and shape to related biologically active pyridylimine-based iron(II) helicates and suggested that the “click” complexes may bind both duplex and triplex DNA. Cell-based agarose diffusion assays showed that the metallosupramolecular [Fe2L3](BF4)4 “click” cylinders display no antifungal activity against S. cerevisiae. This observed lack of antifungal activity appears to be due to the poor stability of the “click” complexes in DMSO and biological media.

Supramolecular Recognition: Use of Cofacially Disposed Bis-terpyridyl Square-Planar Complexes in Self-Assembly and Molecular Recognition

A molecular receptor consisting of a molecular spacer that constrains two terpyridyl-palladium(II) complexes to be disposed in a parallel cofacial geometry has been prepared. The separation between the two terpyridyl-palladium units is enforced to be ca. 7 A, a distance sufficient to incarcerate aromatic molecules and square-planar complexes. A number of molecules are shown to associate with this spacer-chelator complex. In particular, 9-methylanthracene (9-MA) is found to form a 1 : 2 host-guest complex. A crystal structure of this complex shows one 9-MA in the molecular cleft formed by the two terpyridyl-palladium units and the other 9-MA molecule to lie above one of the terpyridyl-palladium units. Nuclear Overhauser effects on analogous molecules that contain two anthracene guests tethered intramolecularly indicate that the structure found in the solid is similar to that in solution. Low-temperature 1H-NMR studies indicate rapid exchange between the two binding sites. The spacer-chelator complexes, when combined with appropriate molecular linkers, readily form molecular rectangles, trigonal prisms, and tetragonal prisms. One molecular rectangle is shown to associate with up to five 9-MA molecules.