Toward Self-Assembled Surface-Mounted Prismatic Altitudinal Rotors. A Test Case: Molecular Rectangle

Transporting and shielding photosensitisers by using water-soluble organometallic cages: a new strategy in drug delivery and photodynamic therapy

Skin photosensitivity remains one of the main limitations in photodynamic therapy. In this Concept article a strategy to overcome this limitation is described, in which the photosensitizer is hidden inside the hydrophobic cavity of a water-soluble organometallic cage. The metallacage not only protects the photosensitizer from light, it also facilitates its delivery to cancer cells.

Coordination-driven self-assembly of ruthenium-based molecular-rectangles: synthesis, characterization, photo-physical and anticancer potency studies

A suite of eight cationic, tetra-metallic molecular rectangles (1-8) was generated via coordination-driven self-assembly using four dicarboxylate-bridged arene-Ru precursors (A1-A4) with one of two dipyridyl ligands (D1, D2). The high-yielding (84-92%) rectangles were characterized by (1)H NMR and HR-ESI-MS to support their structural assignments. The molecular structure of 5 was determined by single crystal X-ray analysis, which indicated that two D2 ligands bridge two A1 acceptors to form a rectangular construct. The photophysical properties of these metalla-rectangles and their molecular precursors were also investigated, as well as an MTT assay to evaluate the in vitro cytotoxicities relative to two chemotherapeutic agents, cisplatin and doxorubicin. MTT assays were conducted using SK-hep-1 (liver cancer) and HCT-15 (colon cancer) human cancer cell lines. Compounds 3, 4, 7 and 8 showed significant activity, with IC(50) values comparable to those of cisplatin and doxorubicin.

DYNAMICS OF SINGLE-MOLECULE ROTATIONS ON SURFACES DEPEND ON SYMMETRY, INTERACTIONS AND MOLECULAR SIZES

Rotating surface-mounted molecules have attracted attention of many research groups as a way to develop new nanoscale devices and materials. However, mechanisms of motion of these rotors at the single-molecule level are still not well understood. Theoretical and experimental studies on thioether molecular rotors on gold surfaces suggest that the size of the molecules, their flexibility and steric repulsions with the surface are important for dynamics of the system. A complex combination of these factors leads to the observation that the rotation speeds have not been hindered by increasing the length of the alkyl chains. However, experiments on diferrocene derivatives indicated that a significant increase in the rotational barriers for longer molecules. We present here a comprehensive theoretical study that combines molecular dynamics simulations and simple models to investigate what factors influence single-molecule rotations on the surfaces. Our results suggest that rotational dynamics is determined by the size and by the symmetry of the molecules and surfaces, and by interactions with surfaces. Our theoretical predictions are in excellent agreement with current experimental observations.

Construction of coordination-driven self-assembled [5 + 5] pentagons using metal-carbonyl dipyridine ligands

The coordination-driven self-assembly of two metal-carbonyl-cluster-coordinated dipyridyl donors, (4-C(5)H(4)N)(2)C[triple bond]CCo(2)(CO)(6) (1) and (4-C(5)H(4)N)(2)C[triple bond]CMo(2)Cp(2)(CO)(4) (2), with a linear diplatinum(II) acceptor ligand was investigated. The structures of the resulting self-assembled polygons were found to be controlled by the steric bulk of the metal-carbonyl cluster adduct. The use of a sterically less imposing ligand 1 resulted in a pentagon-hexagon mixture, which was characterized by electrospray ionization time-of-flight mass spectroscopy. The exclusive formation of a [5 + 5] pentagon was achieved by the self-assembly of the bulkier molybdenum donor ligand 2 with a linear organoplatinum(II) acceptor ligand. Molecular force field modeling was used to study the structural details of the pentagonal and hexagonal architectures. The first Fe(3)-Co(6)-Pt(6) trimetal [3 + 3] hexagon was also synthesized via the combination of 1 with a 120 degrees ferrocenyldiplatinum(II) acceptor.

Coordination-driven face-directed self-assembly of trigonal prisms. Face-based conformational chirality

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy DeltaH(double dagger) of approximately 12 kcal/mol and activation entropy DeltaS(double dagger) of approximately -15 cal/mol x K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to approximately 318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a DeltaH(double dagger) of approximately 35 kcal/mol and DeltaS(double dagger) of approximately 60 cal/mol x K. This highly unusual result is further investigated and discussed in the following companion paper.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

195Pt NMR--theory and application

This critical review highlights the progress in (195)Pt NMR over the last 25 years. In particular, some of the recent applications of (195)Pt NMR in catalytic and mechanistic studies, intermetallics and drug binding studies are discussed. (195)Pt NMR chemical shifts obtained from both theoretical studies and experiments are presented for Pt(0), Pt(II), Pt(III) and Pt(IV) complexes. (195)Pt coupling with various nuclei (viz. coupling constants) have also been collected in addition to data on (195)Pt relaxation. The latest developments in the theoretical knowledge and experimental advances have made (195)Pt NMR into a rich source of information in many fields. (164 references.).

Tetrametallic rectangular box complexes assembled from heteroligated macrocycles

The reaction of a heteroligated Rh(I) bimetallic macrocycle with rigid ditopic ligands (1,4-dicyanobenzene, 4-4'-dicyanobiphenyl, or dipyridyl terminated salen ligand 5) results in the formation of tetrametallic rectangular box complexes.

Incorporation of 2,6-Di(4,4‘-dipyridyl)-9-thiabicyclo[3.3.1]nonane into Discrete 2D Supramolecules via Coordination-Driven Self-Assembly

The synthesis and characterization of three new supramolecular complexes 6-8 (a rhomboid and two hexagons) via coordination-driven self-assembly are reported in excellent yields (>90%). These assemblies have 2,6-di(4,4'-dipyridyl)-9-thiabicyclo[3.3.1]nonane 2 as the bridging tecton. All assemblies were characterized by multinuclear NMR (1H and 31P), mass spectrometry (ESI-MS and ESI-FT-ICR), and elemental analysis. The X-ray structure of the 120 degrees tecton 2 is also discussed.

Facile Synthesis of Enantiopure Chiral Molecular Rectangles Exhibiting Induced Circular Dichroism

[structure: see text] The facile syntheses of enantiopure molecular rectangles using 1,8-bis(trans-Pt(PEt(3))(2)(NO(3)))anthracene and optically pure d- or l-tartrate are reported in high yields. These self-assembled macrocycles are unique examples where the phenomenon of induced chiral dichroism (ICD) has been observed in chiral metallosupramolecular assemblies.

Synthesis and Structural Characterization of Carborane-Containing Neutral, Self-Assembled Pt-Metallacycles

[structure: see text] We have combined carborane chemistry with the newly developed directional bonding strategy to synthesize neutral macrocycles. The m- and p-carborane dicarboxylates were utilized as the donor linkers in conjunction with 1,8-bis[trans-Pt(PEt3)2NO3]anthracene 3, 2,9-bis[trans-Pt(PEt3)2NO3]phenanthrene 5, and cis-Pt(PEt3)2(NO3)2 unit 6. Three new platinum-based macrocycles, 4, 7, and 8, were thus synthesized. 31P{1H} NMR as well as the X-ray characterization of Pt-metallacycles reveal the formation of single highly symmetrical neutral species.

Self-Organization of a Self-Assembled Supramolecular Rectangle, Square, and Three-Dimensional Cage on Au(111) Surfaces

The structure and conformation of three self-assembled supramolecular species, a rectangle, a square, and a three-dimensional cage, on Au111 surfaces were investigated by scanning tunneling microscopy. These supramolecular assemblies adsorb on Au111 surfaces and self-organize to form highly ordered adlayers with distinct conformations that are consistent with their chemical structures. The faces of the supramolecular rectangle and square lie flat on the surface, preserving their rectangle and square conformations, respectively. The three-dimensional cage also forms well-ordered adlayers on the gold surface, forming regular molecular rows of assemblies. When the rectangle and cage were mixed together, the assemblies separated into individual domains, and no mixed adlayers were observed. These results provide direct evidence of the noncrystalline solid-state structures of these assemblies and information about how they self-organize on Au111 surfaces, which is of importance in the potential manufacturing of functional nanostructures and devices.

Disilver(I) Rectangular-Shaped Metallacycles: X-ray Crystal Structure and Dynamic Behavior in Solution

Reaction of the ditopic semirigid ligand 1,2-bis(imidazolylmethyl)benzene (1,2-bImb) or the flexible ligand 1,4-bis(2-benzimidazolyl)butane (C4BIm) with AgX (X = ClO4-, BF4-, CF3CO2-) afforded five new complexes, namely, [Ag2(1,2-bImb)2](ClO4)2 (1), [Ag2(1,2-bImb)2](BF4)2 (2), [Ag2(1,2-bImb)2](CF3CO2)2.2CH3OH (3.2CH3OH), [Ag2(C4BIm)2](ClO4)2.2DMF (4.2DMF), and [Ag2(C4BIm)2](CF3CO2)2.2H2O (5.2H2O), all of which contain a centrosymmetric, rectangular-shaped cationic disilver(I) metallacycle [Ag2(L)2]2+. In 1-3, a pair of 1,2-bImb ligands takes on the syn conformation to connect two Ag(I) ions to give a compressed rectangle with a transannular Ag...Ag separation of 3.27-3.36 angstroms, whereas in 4 and 5, the pair of planar C4BIm ligands acts in the cis conformation to connect two Ag(I) ions to yield a normal rectangle with a transannular Ag...Ag separation of 7.67-7.91 angstroms. The anions form Ag...O or Ag...F weak interactions in 1-3 and O-H...O or N-H...O hydrogen bonds in 4 and 5 in crystal packing but exhibit no significant influence on the formation of the disilver(I) macrocycles. The solution structure and dynamic behavior of the complexes studied by electrospray ionization mass spectrometry, 1H NMR, and variable-temperature NMR indicated that the dynamic equilibrium between the [Ag2(L)2]2+ cation and the open-ring oligomers or other potential species occurs via solvent-assisted dissociative exchange. The metal-ligand exchange barrier was estimated to be 54.5 kJ mol(-1).

Coordination-Driven Self-Assemblies with a Carborane Backbone

The design and self-assembly of five new supramolecular complexes (a rectangle, a triangle, a hexagon, and two squares) are described. These assemblies incorporate carborane building blocks and were prepared in excellent yields (>85%). The assemblies and building blocks were characterized with multinuclear NMR spectroscopy, electrospray ionization mass spectrometry, and elemental analysis. Isotopically resolved mass spectrometry data confirm the existence of the rectangle, triangle, and hexagon, and NMR data are consistent with the formation of all five assemblies. The X-ray structures of two linear carborane building blocks, 1,12-(4-CC(C(5)H(4)N)(2)-p-C(2)B(10)H(10) (1) and 1,12-(trans-(Pt(PEt(3))(2)I)CC)(2)-p-C(2)B(10)H(10) (2), are reported: 1 is monoclinic, P2(1)/c, a = 10.6791(4) A, b = 8.0091(14) A, c = 11.6796(4) A, beta = 107.8461(15) degrees , V = 950.89(5) A(3), Z = 2; 2 is monoclinic, C2/c, a = 62.1128(10) A, b = 22.0071(3) A, c = 14.0494(2) A, beta = 89.9411(8) degrees , V = 19204.4(5) A(3), Z = 16. Crystals of the linear linker 1 exhibit close pi-pi pyridine and pyridine-B(carborane) interactions, which are discussed.

Toward Self-Assembled Surface-Mounted Prismatic Altitudinal Rotors. A Test Case: Trigonal and Tetragonal Prisms

[structure: see text] A self-assembly path toward prismatic molecular rotors based on transversely reactive terminally metalated molecular rods and pyridine-terminated star connectors has been extended. The concept has been tested on the assembly of trigonal and tetragonal prisms from the biphenyl rod, [Ph2P(CH2)3PPh2]Pt+ -C6H4-C6H4-Pt+ [Ph2P(CH2)3PPh2], and the star-shaped connectors, 1,3,5-tris(4-ethynylpyridyl)benzene and [tetrakis(4-pyridyl)cyclobutadiene]cyclopentadienylcobalt, respectively. The prisms have been fully characterized by NMR and MS, including diffusion-ordered NMR and collision-induced dissociation, and their chiral structures optimized by molecular mechanics are discussed.

Self-Assembled Trigonal Prismatic Altitudinal Rotors with Triptycene Paddle Wheels

We describe the synthesis of a trigonal prismatic molecular rotor by self-assembly from paddlewheel carrying molecular rods and trigonal star connectors in a 3:2 ratio. The rod is 9,10-diethynyltriptycene terminated in transversely reactive [Pt(dppp)]+ groups (dppp = 1,3-bis(diphenylphosphino)propane) and the connector is 1,3,5-tris[(4-pyridyl)ethynyl]benzene. The structure of the molecular rotor has been established by 2-D NMR and MS, including diffusion-ordered NMR and collision-induced dissociation.