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Biomedical subjects

Mark D Smith

Publications and source records attributed to Mark D Smith.

At least 19 recordsLinked to original sources
PubMed2006

Self-assembling poly(dioxaborole)s as blue-emissive materials.

Conjugated, borole-linked polymers have been self-assembled based on boronate ester formation between a fluorene-2,7-diboronic acid and 1,2,4,5-tetrahydroxybenzene. Readily soluble polymers with molecular weights approaching 60 000 have been generated with materials exhibiting extended conjugation over approximately 3-5 borole linkages. Emission from these polymers reaches into the visible (blue) range, representing a novel class of blue-emitting materials.

Borates↗
PubMed2006

Janus scorpionates: supramolecular tectons for the directed assembly of hard-soft alkali metallopolymer chains.

A new scorpionate ligand [HB(mtda)3-] containing mercaptothiadiazolyl (mtda) heterocyclic rings with both hard nitrogen donors and soft sulfur donors has been prepared. This new ligand, the Janus scorpionate, is a hybrid of a tris(pyrazolyl)borate and a tris(mercaptoimidazolyl)borate. The differential hard/soft character of the dissimilar donor groups in this bridging ligand was exploited for the controlled solid-state organization of homometallic and heterometallic alkali metal coordination polymers. Remarkably, in the case of sodium, coordination polymers with both acentric (with NaS3N3H kernels) and centric (with alternating NaN6 and NaS6H2 kernels) chains are found in the same crystal (where the centricity is defined by the relative orientations of the B-H bonds of the ligands along the lattice). For the homometallic potassium congener, the larger cation size, compared to sodium, induced significant distortions and favored a polar arrangement of ligands in the resulting coordination polymer chain. An examination of the solid-state structure of the mixed alkali metal salt system revealed that synergistic binding of smaller sodium cations to the nitrogen portion and of the larger potassium cations to the sulfur portion of the ligand minimizes the ligand distortions relative to the homometallic coordination polymer counterparts, a design feature of the ligand that likely assists in thermodynamically driving the self-assembly of the heterometallic chains. The effect of alkali metal complexation on the solution properties of the ligand was studied by comparing NMR chemical shifts, B-H stretching frequencies, and electrochemical properties with those of the noncoordinating tetrabutylammonium salt of the scorpionate. The similarity of these data regardless of cation indicates that the salts are likely dissociated in solution rather than maintaining their solid-state polymeric structures. This data is augmented by the ESI(+/-) mass spectral data for a series of mixed alkali metal tris(mercaptothiadiazolyl)borates that also indicate that dissociation occurs in solution.

Journal Article↗
PubMed2006

Layered heterometallic iodoplumbate containing a novel Pb3Cu6I16 net: structure and optical properties.

A new heterometallic iodoplumbate was synthesized solvothermally. The complex, [Co(phen)3][Pb3Cu6I16].C2H5OH, contains a novel Pb3Cu6I16 net made up of linked Pb3I11 and Cu6I11 clusters. The clusters form a BN-type layer, where the Pb3I11 and Cu6I11 clusters take the place of B and N. The layers, which are separated by [Co(phen)3]2+ cations, contain cavities in which ethanol molecules are located.

Journal Article↗
PubMed2006

Silver(I) complexes of fixed, polytopic bis(pyrazolyl)methane ligands: influence of ligand geometry on the formation of discrete metallacycles and coordination polymers.

Reactions of the arene-linked bis(pyrazolyl)methane ligands m-bis[bis(1-pyrazolyl)methyl]benzene, (m-[CH(pz)2]2C6H4, Lm), p-bis[bis(1-pyrazolyl)methyl]benzene, (p-[CH(pz)2]2C6H4, Lp), and 1,3,5-tris[bis(1-pyrazolyl)methyl]benzene (1,3,5-[CH(pz)2]3C6H3, L3) with AgX salts (pz = 1-pyrazolyl; X = BF4- or PF6-) yield two types of molecular motifs depending on the arrangement of the ligating sites about the central arene ring. Reactions of the m-phenylene-linked Lm with AgBF4 and AgPF6 afford complexes consisting of discrete, metallacyclic dications: [Ag2(mu-Lm)2](BF4)2 (1) and [Ag2(mu-Lm)2](PF6)2 (2). When the p-phenylene-linked Lp is treated with AgBF4 and AgPF6, acyclic, cationic coordination polymers are obtained: {[Ag(mu-Lp)]BF4}infinity (3) and {[Ag(mu-Lp)]PF6}infinity (4). Reaction of the ligand L3, containing three bis(pyrazolyl)methane units in a meta arrangement, with an equimolar amount of AgBF4 again yields discrete metallacyclic dications in which one bis(pyrazolyl)methane unit on each ligand remains unbound: [Ag2(mu-L3)2](BF4)2 (5). Treatment of L3 with an excess of AgBF4 affords a polymer of metallacycles, {[Ag3(mu-L3)2](BF4)3}infinity (6), with one of the bis(pyrazolyl)methane units on each ligand bound to a silver cation bridging two metallacycles. The supramolecular structures of the silver(I) complexes 1-6 are organized by noncovalent interactions, including weak hydrogen bonding, pi-pi, and anion-pi interactions.

Crystallography, X-Ray↗
PubMed2006

Metallacycles of iron, zinc, and cadmium assembled by polytopic bis(pyrazolyl)methane ligands and fluoride abstraction from BF4-.

Reactions of the arene-linked bis(pyrazolyl)methane ligands m-bis[bis(1-pyrazolyl)methyl]benzene (m-[CH(pz)2]2C6H4, Lm) and 1,3,5-tris[bis(1-pyrazolyl)methyl]benzene (1,3,5-[CH(pz)2]3C6H3, L3) with BF4- salts of divalent iron, zinc, and cadmium result in fluoride abstraction from BF4- and formation of fluoride-bridged metallacyclic complexes. Treatment of Fe(BF4)2.6H2O and Zn(BF4)2.5H2O with Lm leads to the complexes [Fe2(mu-F)(mu-Lm)2](BF4)3 (1) and [Zn2(mu-F)(mu-Lm)2](BF4)3 (2), in which a single fluoride ligand and two Lm molecules bridge the two metal centers. The reaction of [Cd2(thf)5](BF4)4 with Lm results in the complex [Cd2(mu-F)2(mu-Lm)2](BF4)2 (3), which contains dimeric cations in which two fluoride and two Lm ligands bridge the cadmium centers. Equimolar amounts of the tritopic ligand L3 and Zn(BF4)2.5H2O react to give the related monofluoride-bridged complex [Zn2(mu-F)(mu-L3)2](BF4)3 (4), in which one bis(pyrazolyl)methane unit on each ligand remains unbound. NMR spectroscopic studies show that in acetonitrile the zinc metallacycles observed in the solid-state remain intact in solution.

Journal Article↗
PubMed2006

Structure-function correlations in Iron(II) tris(pyrazolyl)borate spin-state crossover complexes.

Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO) behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion greatly influences the properties of these complexes. The structure of Fe[B(3-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = cyclopropyl ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior. Magnetic and Mössbauer spectral measurements on Fe[B(3-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)3]2 indicate that both complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed, this change dominates the SCO behavior. A comparison of the Mössbauer spectral properties of these two complexes and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the Mössbauer-effect isomer shift and the average Fe-N bond distance and between the quadrupole splitting and the average FeN-NB intraligand dihedral torsion angles and the distortion of the average N-Fe-N intraligand bond angles.

Borates↗
PubMed2006

Crystal retro-engineering: structural impact on silver(I) complexes with changing complexity of tris(pyrazolyl)methane ligands.

The preparation and structures of seven new silver(I) complexes involving the parent tris(pyrazolyl)methane unit, [C(pz)(3)], as the donor set, {[C6H5CH2OCH2C(pz)3]Ag}(BF4), {[C6H5CH2OCH2C(pz)3]2Ag3}(CF3SO3)3, {[HOCH2C(pz)3]Ag}(BF4), {[HOCH2C(pz)3]Ag}(CF3SO3), {[HC(pz)3]2Ag2(CH3CN)}(BF4)2, {[HC(pz)3]Ag}(PF6), and {[HC(pz)3]Ag}(CF3SO3), are reported. This project is based on a retro-design of our multitopic C6H(6-n)[CH2OCH2C(pz)3]n (pz = pyrazolyl ring, n = 2, 3, 4, and 6) family of ligands in such a way that each new ligand has one fewer organizational feature. The kappa2-kappa1 bonding mode of the [C(pz)3] units to two silvers, also observed with the multitopic ligands, is the dominant structural feature in all cases. Changing the counterion has important effects on the local structures and on crystal packing. When these structures are compared to similar ones based on the multitopic C6H(6-n)[CH2OCH2C(pz)3]n ligands, it has been shown that the presence of the rigid parts (central arene core and the [C(pz)3] units) are important in order to observe highly organized supramolecular structures. The presence of the flexible ether linkage is also crucial, allowing all noncovalent forces to manifest themselves in a cumulative and complementary manner.

Journal Article↗
PubMed2006

Ligand-promoted solvent-dependent ionization and conformational equilibria of Re(CO)3Br[CH2(S-tim)2] (tim = 1-methylthioimidazolyl). Crystal structures of Re(CO)3Br[CH2(S-tim)2] and {Re(CO)3(CH3CN)[CH2(S-tim)2]}(PF6).

The compounds Re(CO)3Br[CH2(S-tim)2] (1) and {Re(CO)3(CH3CN)[CH2(S-tim)2]}(PF6) (2), where tim is 1-methylthioimidazolyl, were prepared in high yields and characterized both in the solid state and in solution. The solid-state structures show that the ligand acts in a chelating binding mode where the eight-member chelate ring adopts twist-boat conformations in both compounds. A comparison of both solid-state IR data for CO stretching frequencies and the solution-phase voltammetric measurements for the Re(1+/2+) couples between 1, 2, and related N,N-chelates of the rhenium tricarbonyl moiety indicate that the CH2(S-tim)2 ligand is a stronger donor than even the ubiquitous dipyridyl ligands. A combination of NMR spectroscopic studies and voltammetric studies revealed that compound 1 undergoes spontaneous ionization to form {Re(CO)3(CH3CN)[CH2(S-tim)2]+}(Br-) in acetonitrile. Ionization does not occur in solvents such as CH2Cl2 or acetone that are less polar and Lewis basic (less coordinating). The equilibrium constant at 293 K for the ionization of 1 in CH3CN is 4.3 x 10(-3). The eight-member chelate rings in each 1 and 2 were found to be conformationally flexible in all solvents, and boat-chair conformers could be identified. Variable-temperature NMR spectroscopic studies were used to elucidate the various kinetic and thermodynamic parameters associated with the energetically accessible twist-boat to twist-boat and twist-boat to boat-chair interconversions.

Crystallography, X-Ray↗
PubMed2006

A new 2-carboxylate-substituted 4,4'-bipyridine ligand: coordination chemistry of 4,4'-bipyridine-2-carboxylic acid and its synthetic intermediate 2-methyl-4,4'-bipyridine.

The first monocarboxylate-substituted 4,4'-bipyridine ligand, 4,4'-bipyridine-2-carboxylic acid (4-(pyridin-4-yl)pyridine-2-carboxylic acid (PPCAH)), has been successfully synthesized from 4,4'-bipyridine. Reactions with transition metals zinc and manganese were used to establish the coordination characteristics of the product, 4,4'-bipyridine-2-carboxylic acid, and of the synthetic intermediate, 2-methyl-4,4'-bipyridine, by single-crystal X-ray diffraction. The ligand PPCAH is useful for the formation of metal-containing building blocks that can be used in the assembly of mixed-metal framework materials. The synthesis and structure of one such mixed-metal coordination polymer, Cu(PPCA)(2)HgI(2), is also presented.

Journal Article↗
PubMed2006

Tetrakis[2-(2-pyridyl)pyridinium] tetra-mu3-iodo-hexa-mu2-iodo-dodecaiodohexabismuthate and bis[tris(2,2'-bipyridine)ruthenium(II)] di-mu4-iodo-octa-mu2-iodo-dodecaiodohexabismuthate.

Crystals of the title compounds were grown solvothermally in an ethanol-water solvent mixture using ruthenium triiodide, 2,2'-bipyridine and bismuth triiodide as starting materials. Tetrakis[2-(2-pyridyl)pyridinium] tetra-mu3-iodo-hexa-mu2-iodo-dodecaiodohexabismuthate, (C10H9N2)4[Bi6I22], crystallizes in the triclinic space group P-1 and is the major reaction product. The asymmetric unit of this compound consists of half a centrosymmetric [Bi6I22]4- anion and two independent 2,2'-bipyridinium cations. The minor product of the reaction is bis[tris(2,2'-bipyridine)ruthenium(II)] di-mu4-iodo-octa-mu2-iodo-dodecaiodohexabismuthate, [Ru(C10H8N2)3]2[Bi6I22], which also crystallizes in the triclinic space group P-1. For this compound, the asymmetric unit consists of one full [Ru(2,2'-bipyridine)3]2+ cation and half a centrosymmetric [Bi6I22]4- anion. Although both compounds contain a centrosymmetric [Bi6I22]4- anion, the polyhedral arrangement of the distorted BiI6 octahedra in the two compounds is quite different, and the anion of the latter compound has not previously been observed in iodobismuthate chemistry.

Journal Article↗
PubMed2006

New N,N,N-heteroscorpionates based on 2,2'-bis(pyrazolyl)ethanamine and its derivatives. Ligands designed for probing supramolecular interactions.

The successful design and synthesis of the new bis(pyrazolyl)ethanamine ligand and its copper(I) triphenylphosphine complex is reported. The ligand coordinates to the copper(I) center in a fac tridentate fashion, through both the pyrazolyl rings and the nitrogen atom from the NH2 group. In the solid state, the compound is organized in a 2D noncovalent network by N-H...pi and C-H...pi interactions and hydrogen bonds. The analogous ligand with a benzyl group substituted on the amine forms a complex with the same copper(I) center that has a similar 2D supramolecular structure and, in addition, is organized by the benzyl synthon into a 3D architecture.

Journal Article↗
PubMed2006

Anion- and solvent-directed assembly in silver bis(thioimidazolyl)methane chemistry and the silver-sulfur interaction.

The effect of metal complexation on the structure and properties of the electroactive bis(1-methylthioimidazolyl)methane linkage isomers CH2(N-tim)2 (L1) and CH2(S-tim)2 (L2) has been explored. Coordination polymers {[Ag(L1)2]X}n (X = BF4, PF6) are formed by bridging L1 between tetrahedral silver centers giving two-dimensional cationic sheets composed of AgS(4) linkages; the anions are sandwiched between sheets. Cyclic dimers {[Ag2(L2)2]X2} (X = BF4, PF6, OSO2CF3) are formed when L2:AgX ratios are lower than 1.5. When L2:AgPF6 was 1.5 or higher, the complex [Ag4(L2)5](PF5)4 could be isolated as a solvate. The NMR, IR, electrochemical, and ESI+ mass spectral data of this latter compound indicate that extensive dissociation to the cyclic dimer and free ligand occurs in solution. Finally, a Cambridge Structural Database search was performed to provide insight into reasonable silver-sulfur bond distances, since literature values appeared to vary widely between 2.3 and 3.2 A. It was found that these distances increase with increasing coordination number of silver. The average distances for 2-, 3-, 4-, 5-, and 6-coordinate silver were found to be 2.40, 2.52, 2.62, 2.70, and 2.75 A, respectively.

Journal Article↗
PubMed2006

Synthesis of a peralkynylated pyrazino[2,3-g]quinoxaline.

[structure: see text] The synthesis of a hexaethynyl[2,3-g]pyrazinoquinoxaline and its crystal structure are reported. Starting from tetraaminobenzoquinone, condensation to bis(triisopropylsilyl)hexadiyne-2,3-dione affords 2,3,7,8-tetrakis(triisopropylsilylethynyl)pyrazino[2,3-g]quinoxaline-5,10-dione. Reaction with TIPS-CC-Li followed by reduction with hypophosphite in the presence of KI furnished the title molecule in a yield of 62%. Pd catalysis is not involved in any of these steps.

Journal Article↗
PubMed2006

A rationale for determining, testing, and controlling specific impurities in pharmaceuticals that possess potential for genotoxicity.

The synthesis of pharmaceutical products frequently involves the use of reactive reagents and the formation of intermediates and by-products. Low levels of some of these may be present in the final drug substance and drug product as impurities. Such chemically reactive impurities may have at the same time the potential for unwanted toxicities including genotoxicity and carcinogenicity and hence can have an impact on product risk assessment. This paper outlines a procedure for testing, classification, qualification, toxicological risk assessment, and control of impurities possessing genotoxic potential in pharmaceutical products. Referencing accepted principles of cancer risk assessment, this document proposes a staged threshold of toxicological concern (TTC) approach for the intake of genotoxic impurities over various periods of exposure. This staged TTC is based on knowledge about tumorigenic potency of a wide range of genotoxic carcinogens and can be used for genotoxic compounds, for which cancer data are limited or not available. The delineated acceptable daily intake values of between approximately 1.5 microg/day for approximately lifetime intake and approximately 120 microg/day for < or = 1 month are virtually safe doses. Based on sound scientific reasoning, these virtually safe intake values do not pose an unacceptable risk to either human volunteers or patients at any stage of clinical development and marketing of a pharmaceutical product. The intake levels are estimated to give an excess cancer risk of 1 in 100,000 to 1 in a million over a lifetime, and are extremely conservative given the current lifetime cancer risk in the population of over 1 in 4 (http://seer.cancer.gov/statfacts/html.all.html). The proposals in this document apply to all clinical routes of administration and to compounds at all stages of clinical development. It is important to note that certain types of products, such as those for life-threatening indications for which there are no safer alternatives, allow for special considerations using adaptations of the principles outlined in this paper.

Animals↗