Coordination-Driven Self-Assemblies with a Carborane Backbone

Self-assembly of a family of suprametallomacrocycles: revisiting an o-carborane bisterpyridyl building block

Reaction of an o-carborane-based, bisterpyridyl building block with a labile metal gives mixtures of the enthalpically and entropically favored ring architectures.

New ligand platforms featuring boron-rich clusters as organomimetic substituents

200 years of research with carbon-rich molecules have shaped the development of modern chemistry. Research pertaining to the chemistry of boron-rich species has historically trailed behind its more distinguished neighbor (carbon) in the periodic table. Notably, a potentially rich and, in many cases, unmatched field of coordination chemistry using boronrich clusters remains fundamentally underdeveloped. Our work has been devoted to examining several basic concepts related to the functionalization of icosahedral boron-rich clusters and their use as ligands, aimed at designing fundamentally new hybrid molecular motifs and materials. Particularly interesting are icosahedral carboranes, which can be regarded as 3D analogs of benzene. These species comprise a class of boron-rich clusters that were discovered in the 1950s during the "space race" while researchers were developing energetic materials for rocket fuels. Ultimately, the unique chemical and physical properties of carborane species, such as rigidity, indefinite stability to air and moisture, and 3D aromaticity, may allow one to access a set of properties not normally available in carbon-based chemistry. While technically these species are considered as inorganic clusters, the chemical properties they possess make these boron-rich species suitable for replacing and/or altering structural and functional features of the organic and organometallic molecules-a phenomenon best described as "organomimetic". Aside from purely fundamental features associated with the organomimetic chemistry of icosahedral carboranes, their use can also provide new avenues in the development of systems relevant to solving current problems associated with energy production, storage, and conversion.

Self-assembly of Cationic, Hetero- or Homo-nuclear Ru(II) Macrocyclic Rectangles and their Photophysical, Electrochemical and Biological Studies

A series of supramolecular rectangles, including two mixed-metal Ru/Pt complexes, have been formed by the coordination-driven self-assembly of a range of arene-Ru "molecular clip" acceptors (1a-1d) with rigid dipyridyl-based ligands (2a-2d) over the course of 10 hours in solution. The isolated products were characterized by multinuclear NMR ((1)H and (13)C or (31)P), HR-ESI-MS and an X-ray diffraction study to support the ascribed two-component rectangular structures. The rectangles were further characterized by UV-Vis and fluorescence studies. The redox behaviors of rectangles 3ca and 3da were also determined using cyclic voltammetry. Additionally, the antitumor activities of the suite of rectangles were determined against various human cancer cell lines and significant activity was shown by complexes 3ca, 3da, 3cb, 3cc and 3cd, with IC(50) values as low as 2.65 μM.

Transition metal-carboryne complexes: synthesis, bonding, and reactivity

The construction and transformation of metal-carbon (M-C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M-C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal-carbon (M-C(cage)) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M-C(cage) bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal-carboryne complexes. Our work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M-C(cage) bonds, we prepared a series of group 4 and group 10 transition metal-carboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M-C(cage) bonds with electrophiles. Structural and theoretical studies on metal-carboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M-C σ and M-C π bonds, similar to that observed in metal-benzyne complexes. The nickel-carboryne complex (η(2)-C(2)B(10)H(10))Ni(PPh(3))(2) can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes. The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium-carboryne species, generated in situ from Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2), reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations. Studies of metal-carboryne complexes remain a relatively young research area, particularly in comparison to the rich literature of metal-benzyne complexes. Other transition metal-carborynes are expected to be prepared and structurally characterized as the field progresses, and the results detailed here will further that effort by providing easy access to a wide range of functionalized carborane derivatives.

New insights into the factors that govern the square/triangle equilibria of Pd(II) and Pt(II) supramolecules. Unexpected participation of a mononuclear species in the equilibrium

The new rigid fluorinated ligand 4,4'-bis(4-pyridyl)octafluorobiphenyl (L1) has been synthesized by a nucleophilic substitution reaction between the organolithium derivative of the 4-bromopyridine and the compound decafluorobiphenyl. The use of L1 as building block of supramolecular species containing diphosphane or ethylenediamine Pd(II) and Pt(II) fragments has been explored, and the results have been compared with those previously reported for the smaller ligand 1,4-bis(4-pyridyl)tetrafluorobenzene (L2). The observed differences (particularly, square/triangle ratio) are discussed in terms of different parameters such as the nature of the ancillary ligands, solvent, or reaction temperature. The synthesis of water-soluble palladium metallamacrocycles derived from L1 has been attempted using ethylenediamine (en) or tetramethylethylenediamine (tmen) as ancillary ligand. Interestingly, in the latter case, the resulting solutions of the compounds showed, together with the square/triangle equilibrium, the unexpected presence of an additional lower nuclearity species, whose nature has been determined by means of bidimensional (1)H NMR experiments.

Synthesis and properties of carborane-appended C(3)-symmetrical extended pi systems

A series of C(3)-symmetric pi-conjugated compounds containing three to six o-carborane clusters have been synthesized by employing palladium-catalyzed Suzuki coupling reactions and palladium-catalyzed acetylation reactions, followed by silicon tetrachloride mediated trimerization reactions. Carborane-containing extended trimers were found to emit blue light. Incorporation of o-carborane clusters into extended pi-conjugated systems led to 22-70% enhancement of their relative fluorescence quantum yields. Decapitation of o-carborane clusters made these extended trimers water soluble, and their aqueous solutions were also found to be fluorescent, but with a reduced fluorescence intensity. The carborane-appended pi-conjugated compounds are found to be extremely thermally stable, and for some of these compounds only 10% mass loss occurred at temperatures close to 500 degrees C. The DSC thermograms of smaller C(cage)-appended trimers indicate the occurrence of solid-solid phase transitions.

Self-assembled Pd(II) metallocycles using an ambidentate donor and the study of square-triangle equilibria

The self-assembly reaction of a cis-blocked 90 degrees square planar metal acceptor with a symmetrical linear flexible linker is expected to yield a [4 + 4] self-assembled square, a [3 + 3] assembled triangle, or a mixture of these. However, if the ligand is a nonsymmetrical ambidentate, it is expected to form a complex mixture comprising several linkage isomeric squares and triangles as a result of different connectivities of the ambidentate linker. We report instead that the reaction of a 90 degrees acceptor cis-(dppf)Pd(OTf)(2) [where dppf = 1,1'-bis(diphenylphosphino)ferrocene] with an equimolar amount of the ambidentate unsymmetrical ligand Na-isonicotinate unexpectedly yields a mixture of symmetrical triangles and squares in the solution. An analogous reaction using cis-(tmen)Pd(NO(3))(2) instead of cis-(dppf)Pd(OTf)(2) also produced a mixture of symmetrical triangles and squares in the solution. In both cases the square was isolated as the sole product in the solid state, which was characterized by a single crystal structure analysis. The equilibrium between the triangle and the square in the solution is governed by the enthalpic and entropic contributions. The former parameter favors the formation of the square due to less strain in the structure whereas the latter one favors the formation of triangles due to the formation of more triangles from the same number of starting linkers. The effects of temperature and concentration on the equilibria have been studied by NMR techniques. This represents the first report on the study of square-triangle equilibria obtained using a nonsymmetric ambidentate linker. Detail NMR spectroscopy along with the ESI-mass spectrometry unambiguously identified the components in the mixture while the X-ray structure analysis determined the solid-state structure.

Construction of endo-functionalized two dimensional metallacycles via coordination-driven self-assembly

The synthesis of three endofunctionalized two-dimensional supramolecular metallacycles including two [2 + 2] rhomboids (5 and 6) and a [3 + 3] hexagon (7) is reported. The resulting self-assembled supramolecular structures, containing several nitrobenzyl moieties at their interior surface, have been fully characterized by multinuclear NMR ((31)P and (1)H) and electrospray ionization mass spectrometry. A significant C-H...O hydrogen bonding between the nitrobenzyl acceptor and the edge molecules of the supramolecular architecture is observed in the small rhomboid 5 and this interaction gradually decreases upon the enlargement of the resulting polygonal structures from a small rhomboid 5 through a large rhomboid 6 to a hexagon 7. Molecular modeling with the MMFF force field gives a possible conformation of each self-assembly in different solvents and shows that the hydrophilic nitrobenzyl moiety prefers to be buried in the cavity of the resulting polygonal structures in nonpolar solvents, thus forming hydrogen bonds with the peripheral component building units.

Synthesis and characterization of molecular rectangles of half-sandwich p-cymene ruthenium complexes bearing oxamidato ligands

A series of binuclear half-sandwich p-cymene ruthenium complexes bearing oxamidato ligands [(p-cymene)(2)Ru(2)(mu-N,N'-bis(aryl)oxamidato)Cl(2)] (1-3) was synthesized by the reactions of the lithium salts of oxamide with [(p-cymene)RuCl(2)](2), respectively. Treatment of the binuclear complexes (1-3) with bidentate ligands such as 4,4'-bipyridine (4,4'-bpy) and trans-1,2-bis(4-pyridyl)ethylene (bpe) in the presence of AgOTf (OTf = CF(3)SO(3)) gave the corresponding tetranuclear complexes generally formulated as [(p-cymene)(4)Ru(4)(mu-N,N'- bis(aryl)oxamidato)(2)(mu-4,4'-bpy)(2)](OTf)(4) (4a-c) and [(p-cymene)(4)Ru(4)(mu-N,N'-bis(aryl)oxamidato)(2)(mu-bpe)(2)](OTf)(4) (5a-c) in high yields. All compounds (1-3, 4a-5c) have been characterized by NMR and IR spectra and elemental analyses. The molecular structures of , and have been determined by single-crystal X-ray analyses. The molecular structures of tetranuclear complexes and showed that two binuclear fragments as building blocks were connected by 4,4'-bpy or bpe to construct a rectangular cavity with the dimensions 5.57 x 11.28 A (4a) and 5.56 x 13.65 A (5a).

Self-assembly of a triangle-shaped, hexaplatinum-incorporated, supramolecular amphiphile in solution and at interfaces

The self-assembly and characterization of a novel supramolecular amphiphile built from a new 60 degree amphiphilic precursor that incorporates hydrophilic platinum(II) metals and hydrophobic dioctadecyloxy chains is reported. The amphiphilic macrocycle and its precursor compound have been characterized by multinuclear NMR spectroscopy, ESI-MS, and other standard techniques. The coacervate morphology of the amphiphile at the liquid-liquid interface has been studied by using confocal optical microscopy and in situ Raman spectroscopy. The self-assembly of the amphiphilic macrocycle at the air-water interface has been investigated through Langmuir-trough techniques. The study indicates the possible formation of surface micelle-like aggregates. The disparity between the experimental molecular areas and those derived from molecular models support the idea of aggregation. AFM images of the surface aggregates show the formation of a flat topology with arbitrary ridgelike patterns. Reasonable molecular-packing arrangements are proposed to explain the molecular organization within the observed structures.