Jeffrey W. Kampf

para-Octaiodophenylsilsesquioxane, [p-IC6H4SiO1.5]8, a Nearly Perfect Nano-Building Block

The cubic symmetry of octafunctional octaphenylsilsesquioxanes [ROPS, (RC6H4SiO(1.5))8] coupled with a 1 nm diameter offers exceptional potential to assemble materials in three dimensions with perfect control of periodicity and the potential to tailor global properties at nanometer length scales. OPS itself is very inert and insoluble and can only be functionalized via electrophilic reactions with difficulty and with poor substitutional selectivity. However, functionalized OPS products are robust and highly soluble, offering easy purification and processing. In contrast to previous studies, we report here that OPS reacts with ICl at sub-ambient temperatures to provide (following recrystallization) [p-IC6H4SiO(1.5)]8, or I8OPS, in good yields and with excellent selectivity: >99% mono-iodo substitution with >93% para substitution as determined by H2O2/F- cleavage of the Si-C bonds to produce iodophenols. I8OPS in turn can be functionalized using conventional catalytic coupling reactions to provide sets of >93% para-substituted, functionalized compounds (alkynes, alkenes, aryl amines, phosphonates, aryl amines, polyaromatics, etc.), suggesting the potential to develop diverse nano-building blocks for the assembly of a wide variety of materials, some with novel photonic, electronic, and structural properties.

Nano building blocks via iodination of [PhSiO1.5]n, forming [p-I-C6H4SiO1.5]n (n = 8, 10, 12), and a new route to high-surface-area, thermally stable, microporous materials via thermal elimination of I2.

We describe the synthesis and characterization of the homologous p-iodophenylsilsesquioxanes (SQs) [p-I-C(6)H(4)SiO(1.5)](n) (n = 8, 10, 12) via ICl-promoted iodination (-40 to -60 degrees C) with overall yields of 80-90% and > 95% para selectivity following recrystallization. Characterization by NMR, FTIR, TGA, and single-crystal X-ray diffraction are reported and compared to data previously published for I(8)OPS. Coincidentally, we report a new synthesis of the elusive pentagonal decaphenyl SQ (dPS) [C(6)H(4)SiO(1.5)](10) and its characterization by NMR and single-crystal X-ray studies. These unique macromolecules possess equivalent chemical functionality but varying symmetries (cubic, pentagonal, and D(2d) dodecahedral), offering the potential to develop homologous series of functionalized star and dendrimer compounds with quite different core geometries and thereby providing the potential to greatly vary structure-property relationships in derivative compounds and nanocomposites made therefrom. We find that all three compounds decompose on heating to approximately 400 degrees C/N(2) with loss of I(2) to form robust, microporous materials with BET surface areas of 500-700 m(2)/g, pore volumes of 0.25-0.31 cm(3)/g, average pore widths of 8 A, and oxidative stabilities > or = 500 degrees C and with solid-phase morphologies varying from crystalline to mostly amorphous, as indicated by powder XRD and SEM studies. These latter findings point to important symmetry effects relating directly to packing in the crystalline phase prior to thermolysis.

Development of bifunctional stilbene derivatives for targeting and modulating metal-amyloid-β species.

Amyloid-β (Aβ) peptides and their metal-associated aggregated states have been implicated in the pathogenesis of Alzheimers disease (AD). Although the etiology of AD remains uncertain, understanding the role of metal-Aβ species could provide insights into the onset and development of the disease. To unravel this, bifunctional small molecules that can specifically target and modulate metal-Aβ species have been developed, which could serve as suitable chemical tools for investigating metal-Aβ-associated events in AD. Through a rational structure-based design principle involving the incorporation of a metal binding site into the structure of an Aβ interacting molecule, we devised stilbene derivatives (L1-a and L1-b) and demonstrated their reactivity toward metal-Aβ species. In particular, the dual functions of compounds with different structural features (e.g., with or without a dimethylamino group) were explored by UV-vis, X-ray crystallography, high-resolution 2D NMR, and docking studies. Enhanced bifunctionality of compounds provided greater effects on metal-induced Aβ aggregation and neurotoxicity in vitro and in living cells. Mechanistic investigations of the reaction of L1-a and L1-b with Zn(2+)-Aβ species by UV-vis and 2D NMR suggest that metal chelation with ligand and/or metal-ligand interaction with the Aβ peptide may be driving factors for the observed modulation of metal-Aβ aggregation pathways. Overall, the studies presented herein demonstrate the importance of a structure-interaction-reactivity relationship for designing small molecules to target metal-Aβ species allowing for the modulation of metal-induced Aβ reactivity and neurotoxicity.

Compositional and geometrical isomers of 15-metallacrowns-5 complexes

Abstract Two mixed valent, hexanuclear manganese clusters of general composition Mn(acetate)2[Mn(III)(salicylhydroximate)]5(pyridine)6, Mn(II)(OAc)2(15-MCMn3+N-5) have been prepared in ≈ 70% yield. Structural characterization of these complexes using X-ray crystallography provides the first examples of 15 membered ring metallacrowns. A 15 membered chelate ring is constructed of five [Mn(III)NO] linkages. The five manganese(III) ions are designated Mn(1)Mn(5). The Mn(1), Mn(2) and Mn(3) atoms adopt propeller configurations with Λ,Δ,Λ absolute stereochemistry, respectively. The Mn(4) and Mn(5) atoms are restricted to planar configurations. Both complexes contain a seven coordinate manganese(II) [Mn(6)] encapsulated in the core of this 15 membered ring. The Mn(6) polyhedron is best described using an 8 coordinate reference frame in which one of the vertices of a trigonal faced dodecahedron is vacant. The Mn(6) coordination sphere contains five hydroximate oxygen donors, provided by the metallacrown ring, and two oxygen from acetates that bridge between ring manganese(III) and the encapsulated manganese(II). Two geometrical isomers are formed that differ in the bridging acetate modes. In 1, the bridging acetate forms a bifurcated bridged between Mn(2) and Mn(6) while complex 2 has a bifurcated acetate bridge between Mn(5) and Mn(6). A compositional isomer, Mn(II)(acetate)(HSal)(15-MCMn3+N-5), 4, which substitutes the carboxylate function of salicylate for one of the acetates has also been prepared. This isomer adopts the same bifurcation mode, using the salicylate carboxylate, as complex 2. Studies to define the solution integrity of the metallacrowns are presented in addition to the solid state structure.

The selective dissolution of rice hull ash to form [OSiO1.5]8[R4N]8(R = Me, CH2CH2OH) octasilicates. Basic nanobuilding blocks and possible models of intermediates formed during biosilicification processes

Rice hull ash (RHA) silica (80–98 wt.% amorphous, >25 m2 g−1 silica, 2–20% porous amorphous C) can be depolymerized in aqueous alcohol with [NR4]8OH (R = Me, CH2CH2OH) under ambient conditions with the selective formation of octasilicate anions, [NR4]8[OSiO1.5]8. Dissolution kinetics were studied as a function of base and water concentration and temperature. Dissolution rates were determined by conversion of the octaanion to [HMe2OSiO1.5]8, OHS. Activation energies for dissolution were 5 ± 1 kcal mol−1, much lower than for typical base-promoted silica dissolution. Furthermore, dissolution was not catalytic in base as found for other base-promoted silica dissolution reactions. Reaction rates were dependent on ammonium base and water concentrations and temperature. Dissolution was optimal at approximately one equivalent of [NR4]8OH and three equivalents of water for choline hydroxide and five equivalents of water for [NMe4]8OH. A single crystal study of the octacholine [Me3NCH2CH2OH]8 derivative indicates that the compound crystallizes with three equivalents of water per SiO1.5 suggesting that the rate limiting step in the dissolution process may be formation of the octaanion which is in accord with theoretical Ea’s calculated for the condensation of polyhydroxyl siloxanes. Octasilicate anions offer access to novel polyfunctional silsesquioxane platforms with each functional group occupying a single octant in Cartesian space. These platforms offer potential as precursors to dendrimers and hyperbranched polymers, and as nanobuilding blocks for the formation of nanocomposites. Furthermore, choline is structurally similar to: (1) e-N,N,N-trimethyl-γ-hydroxy-lysine, (2) the oligomeric N-methylpropylamine components found in silafins, and (3) possibly the hydroxyamino acid units found in sponge filament proteins; all of which are thought to play a role in silica accretion, transport and deposition in diatoms and sponges. Thus, the octasilicate structure may reflect the structure(s) of species involved in silica transport and/or deposition in biosilicification processes.

Manganese carbonyl complexes of 2,5-dimethylbismolyl. The crystal and molecular structure of (η1-2,5-dimethylbismolyl) manganese pentacarbonyl

Abstract (η-1-2,5-Dimethylbismolyl)manganese pentacarbonyl (8) has been obtained from the reaction of 1-phenyl-2,5-dimethylbismole with lithium followed by BrMn(CO)5. Heating 8 to its melting point causes the loss of CO to produce (η5-2,5-dimethylbismolyl)manganese tricarbonyl. Compound 8 crystallizes in the monoclinic crystal system, space group P 21 (No. 4) with a = 6.838(2) A, b = 6.424(1) A, c = 15.787(4) A, β = 95.68(2)° V= 6901.(2) A3 and Z = 2. A full structure has been determined and is compared with those of analogous compounds.

Synthesis of Penta-alkoxy- and Penta-aryloxy Silicates Directly from SiO2

It is likely that the utility of silicon chemistry would be greatly expanded, if new, general routes to organosilicon compounds could be developed. In particular, synthetic routes stemming from the use of silica (sand) would be particularly attractive because of the modest cost of pure starting material. To this end, we have developed novel chemistry that offers considerable opportunity as a general synthetic technique for the synthesis of unique hypervalent, penta-alkoxy and penta-aryloxy silicates. These species can be further elaborated to form novel compounds that exhibit a wide variety of properties including charge transfer from anionic silicon to cationic pyridinium counterions.

CCDC 875641: Experimental Crystal Structure Determination

Related Article: J.J.Braymer, Jung-Suk Choi, A.S.DeToma, Chen Wang, Kisoo Nam, J.W.Kampf, A.Ramamoorthy, Mi Hee Lim|2011|Inorg.Chem.|50|10724|doi:10.1021/ic2012205

Modeling vanadium bromoperoxidase: Synthesis, structure and reactivity of vanadium-imidazole complexes.

Studies of vanadium coordination compounds containing imidazole ligands have been undertaken to understand the relationship between the structure and spectroscopy of vanadium coordination compounds as related to vanadium dependent enzymes [l]. The ligand H2SALIM [4-(2-, salicylideneiminoethyl)imidazole], prepared by the condensation of histamine and salicylaldehyde, reacts with various vanadium starting materials to form several new complexes. These have been structurally characterized and contain imidazole in the vanadium coordination sphere as shown in Figure 1 for VNO(HSALIM);l. Two other complexes, 3

Metallacrowns: A new class of metal binding agents

Metal sequestration by agents such as crown ethers [l], siderophores [2] and polyaminocarboxylic acids have been studied extensively. We have commented [3] on a new class of molecules, metallacrowns, that may present a unique way to generate in a controlled manner unique homoand heterometallic clusters, The metallacrowns structurally resemble crown ethers clic structures that orient multiple, formally neutral, oxygen atoms into a cavity that can capture a free metal ion. However, unlike crown ethers, metallacrowns form 9-, 12or 15 membered rings using only heteroatom linkages such as -[-Mn(lll)NO-]4, The metallacrowns also are related to siderophores in that the primary functional groups that bind the ring metals are hydroxamic acids, specifically salicylhydroxamic acid, Two examples of metallacrowns are illustrated. The hexanuclear, mixed valence manganese cluster Mn(II)(acetate MCM,~(JJI>N-~] illustrated as Figure 1 is the first example of a 15-metallacrown-5 complex. The central Mn(I1) is encapsulated by five hydroximate oxygen atoms of the ring and two acetate oxygens. Metallacrowns can also be fused to make mixed metal metallacryptates such as Naz[Na( [ 12-MC in Figure 2. A Na(I) ion is complexed by Ga ([~~N_4l~(~-~H)4) 1 as iilustmted