Biomedical subjects
Publications and source records attributed to David J Williams.
The reaction of [Rh(C6H5)Cl2(PPh3)2] with Na[HB(mt)3](mt = methimazolyl) provides [RhCl(PPh3){B(mt)3}](Rh-->B) the first authentic example of a compound with a rhodium-boron dative bond.
Three constitutionally isomeric bis(naphthylmethyl)ammonium ions, in which the two naphthyl groups are substituted 1) both at their 1-positions, 2) one at its 1-position and the other at its 2-position, and 3) both at their 2-positions, have been investigated separately in solution for their propensities to undergo spontaneous self-assembly with three different [24]crown-8 derivatives, namely, pyrido[24]crown-8 (P24C8), dipyrido[24]crown-8 (DP24C8) and dibenzo[24]crown-8 (DB24C8), in turn to form [2]pseudorotaxanes. The strengths of the 1:1 complexes depend on the composition of the secondary dialkylammonium ions and on the nature of the crown ether hosts; generally, as far as the guest cation is concerned, the 1/1- and 2/2-isomers form stronger complexes, as indicated by stability constant measurements, than the 1/2-isomer and, as far as the crown ethers are concerned, the more flexible P24C8 is a much more efficient host than either DP24C8 or DB24C8. The rates of formation of the [2]pseudorotaxanes are fast (i.e., taking no more than a few minutes) in solution with the exception of one case, that is, in which the crown ether host is DB24C8 and the guest cation is the 1/1-isomer, when it can take upwards of one month for the complexation-decomplexation equilibrium to be established at room temperature. In all cases, the equilibrium between complexed and uncomplexed species is slow on the NMR timescale, allowing the determination of stability constants to be made readily using the single-point method. X-ray crystallography and molecular modeling have been used to gain insight into ground and transition state interactions, respectively, in some of the [2]pseudorotaxanes. The relative stabilities of the three [2]pseudorotaxanes formed by each guest cation in the presence of the three crown ether hosts were also evaluated in solution by competition experiments that were monitored by (1)H NMR spectroscopy. By and large the results of the competition experiments could be predicted on the basis of the derived stability constants for the individual [2]pseudorotaxanes.
Bis(imino)pyridine iron complexes bearing ether and thioether backbone substituents have been synthesized and evaluated for the polymerization of ethylene. The methoxy derivative is inactive whereas bulky phenoxides or thioether derivatives afford activities as high as the most active systems reported to date.
Group 4 metal complexes containing phenoxy-amide ligands bearing soft pendant donors are shown to give more highly active ethylene polymerisation catalysts than counterparts containing hard donors or systems without a pendant donor.
Reaction of potassium diphenylphosphide with elemental selenium is shown to give [K(Se(2)PPh(2))(THF)(2)](2) 1, which further reacts with InCl(3) to yield [In(Se(2)PPh(2))(3)] 2. Crystallization of 2 from either THF or PhMe gave 2.THF or 2.PhMe, respectively, both of which form loosely linked dimers in the solid state via Se...Se intermolecular van der Waals interactions. Decomposition of 2 has been studied by TGA.
Two new unsymmetrical 1'-substituted hydroxyferrocene ligands featuring either phosphine or phosphine oxide substituents have been synthesised and the phosphine oxide derivative has been structurally characterised. A nickel complex of the hydroxyl/phosphine ligand has been formed, along with preliminary evaluation of the complex for catalysis of ethylene polymerisation.
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Reaction of Me(3)Al (one equivalent) with the bis(imino)phenol, [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] (I)(Ar = 2,6-Pr(i)(2)C(6)H(3)) in toluene at ambient temperature yields the yellow complex [Me(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](1). Interaction of two equivalents of Me(3)Al in refluxing toluene affords the red complex [(Me(2)Al)(2)[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](2). Similar interaction (two equivalents, refluxing toluene) of MeAlCl(2) or (i)Bu(3)Al with [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] affords [ClAl[2,6-(ArNCH)(2)-4-MeC(6)H(2)O](2)](3) or [(i)Bu(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](4), respectively. Hydrolysis of 2 readily affords the iminoaminophenol ligand [2-(ArN=CH)-6-ArNHCH(Me)-4-MeC(6)H(2)OH](II), which reacts further with Me(3)Al to afford [Me(2)Al[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](5). An X-ray study on reveals bidentate imino-alkoxide ligation about the distorted aluminium centre, whereas is a binuclear structure with tetrahedral aluminiums ligated by imino-alkoxide and amido-alkoxide ligand fragments, respectively. For and bidentate imino-alkoxide ligation is observed.
A new family of aluminum complexes bearing tetradentate bis(aminophenoxide) ligands is reported and shown to initiate the living ring-opening polymerization of rac-lactide. The microstructures of the polylactide products are found to be highly dependent upon the ancillary ligand substituents, ranging from highly isotactic (Pm = 0.79) to very highly heterotactic (Pr = 0.96).
The synthesis of a range of highly functionalized peptidomimetic macrocycles has been accomplished using ring-closing metathesis and enyne tandem cross-metathesis-ring-closing metathesis reactions. This approach gives access to rigidified macrocycles modeled on the structures of cyclic peptides and designed to be biologically stable. The potential for peripheral functionalization of these templates has been demonstrated using Diels-Alder reactions, palladium(0) coupling reactions, and amide formation both in the solution phase and using polymer-supported syntheses.
The boron-bridged constrained geometry titanium complexes [Ti[eta5:eta1-(C5H4)B(NR2)NPh](NMe2)2][R = iPr (3), SiMe3(4)] and [Ti[eta5:eta1-(C9H6)B(NiPr2)NPh](NMe2)2](12) have been prepared in good yields by amine elimination reaction from [Ti(NMe2)4]. Subsequent deamination-chlorination with excess Me3SiCl yielded the corresponding dichloro-complexes (5, 6, 13). Reaction of the analogous ligand precursors (C5H5)B(NiPr2)N(H)R (R = Cy, tBu) with [Ti(NMe2)4] did not result in the expected bridged compounds, but rather in the half-sandwich complexes [Ti[(eta5-C5H4)B(NiPr2)N(H)R](NMe2)3][R = Cy (9), tBu (10)]. All compounds were fully characterised by means of multinuclear NMR spectroscopy. Thorough investigation of substituent effects was achieved by comparative X-ray diffraction studies on complexes 3, 5, 6 and 12.
The isomorphous polymeric complexes [M(mu-C(6)H(5)NHC(4)O(3))(2)(CH(3)OH)(2)](n) [M = Mn (1), Co (2), Cu (4), Zn (5)] are produced by reacting the anilinosquarate anion with the appropriate metal nitrates in a methanolic solution. Each of these complexes contains the central metal atom in a slightly distorted octahedral environment, with the coordination polyhedron consisting of four mu-1,2-bridging anilinosquarate ligands and two trans-oriented methanols. The polymer chains propagate to form a two-dimensional net of metal centers, with the conformation of the component sheets in the net being controlled by intramolecular N-H...O and O-H...O hydrogen bonds. Under reaction conditions similar to those used in the synthesis of the polymers 1, 2, 4, and 5, the nickel(II) monomer [Ni(C(6)H(5)NHC(4)O(3))(2)(H(2)O)(4)].2H(2)O (3) is produced in which each nickel center is attached to two cis-coordinated anilinosquarate and four aqua ligands in a distorted octahedral arrangement. The ligand conformation in 3 is stabilized by both intra- and intermolecular hydrogen bonding, which results in the formation of a sheet polymer having distinct hydrophobic and hydrophilic surfaces. Magnetochemical analysis of 1 and 4 reveals normal paramagnetic behavior for 1 and a very weak ferromagnetic interaction in 4; the absence of significant magnetic interactions is attributed to the distortion of the C(4) cycle of the anilinosquarate ligand (lower than C(2)(v) symmetry) in these complexes. Reaction of anisolesquarate with M(NO(3))(2).xH(2)O in acetonitrile produced the set of isomorphous salts [M(H(2)O)(6)][CH(3)OC(6)H(5)C(4)O(3)](2) [M = Mn (6), Co (7), Ni (8), Zn (9)]. The anisolesquarate anions in 6-9 are hydrogen bonded to the [M(H(2)O)(6)](2+) ions to form polymer chains, which are further linked by hydrogen bonds to form complex sheets. Complexation of the anisolesquarate ligand was not observed even when other solvents and reaction conditions were employed.
The synthesis of the unsymmetrically substituted beta-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N'-bis(2,6-diisopropylphenyl) beta-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex, (BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds, and reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).
A number of metal trifluoroacetates were reacted with the olefin 3beta-acetoxyandrost-5-en-17-one (6). Palladium(II) trifluoroacetate afforded bis[micro-trifluoroacetato(alpha-5,7-eta-3beta-acetoxyandrostenyl-17-one)palladium(II)] (20), a new ring B pi-allyl steroid-palladium complex, in quantitative yield. Thallium(III) trifluoroacetate gave 3beta-acetoxy-5alpha-hydroxy-6beta-trifluoroacetoxyandrostan-17-one (16), 3beta-acetoxy-6beta-trifluoroacetoxyandrost-4-en-17-one (9), 3beta-acetoxy-4beta-trifluoroacetoxyandrost-5-en-17-one (10), and 3beta-acetoxy-5alpha,6beta-dihydroxyandrostan-17-one (17). Lead(IV) trifluoroacetate yielded 9, 10 and 16. 3beta-Acetoxy-5alpha,6beta-bis(trifluoroacetoxy)androstan-17-one (15), a new compound, was also formed in this reaction. During the course of the lead(IV) studies the dichlorosteroid 21 and the rearranged allylic oxidation product 24 were formed. Their formation was attributed to the generation of lead(IV) chloride in the reaction. Silver(I) and copper(II) trifluoroacetates proved to be unreactive towards 6.
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The polymeric lanthanide complexes (Ln(mu-CH3OC6H5C4O3)(CH3OC6H5C4O3)2 (H2O)4.xH2O)n [Ln=La (1), Eu (2), Gd (3)], formed from the reaction of aqueous solutions of anisolesquarate and Ln(NO3)3.xH2O, are all structurally similar with only subtle differences between the lanthanum complex and the isomorphous pair of europium and gadolinium analogues. The lanthanum atom in 1 has a square antiprismatic coordination geometry comprising two pendant and two mu-1,3-bridging anisolesquarate groups and four aqua ligands. Complexes 2 and 3 have two independent metal atoms in their asymmetric units compared to one for the lanthanum complex. However, the gross structures of 1-3 are essentially the same. The asymmetric unit of the terbium complex ((CH3OC6H5C4O3)3Tb(H2O)4(mu-CH3OC6H5C4O3)(CH3OC6H5C4O3)2Tb(H2O)5).H2O (4) contains two independent binuclear units which hydrogen bond to form an extended structure very similar to those of 1-3. The ionic polymers ([Ln(mu2-C4O4)(H2O)6][C6H5NHC4O3].4H2O)n [Ln=Eu (5), Gd (6), Tb (7)] result from the incomplete hydrolysis of the anilinosquarate ion during the attempted synthesis of Eu(III), Gd(III), and Tb(III) anilinosquarate complexes. However, complete hydrolysis of the substituent is accomplished by La(III) ions, and the neutral polymer (La2(mu2-C4O4)2(mu3-C4O4)(H2O)11.2H2O)n (8) is formed. In complexes 5-7, the central lanthanide atom has a square antiprismatic geometry, being bonded to two mu-1,2-bridging squarate and six aqua ligands. Two anilinosquarate counteranions participate in second-sphere coordination via direct hydrogen bonding to aqua ligands on each metal center. These counteranions, and the included waters of crystallization, serve to link neighboring cationic polymer chains via an extensive array of O-H...O hydrogen bonds to form a 3-dimensional network. The polymeric lanthanum complex 8 contains two different metal environments, each having distorted monocapped square antiprismatic geometry. For one lanthanum atom the coordination polyhedron comprises five aqua and four squarate ligands, while for the other the polyhedron consists of six aqua and three squarate ligands; in each case one of the aqua ligands occupies the capping position. The squarate ligand exhibits two coordination modes in 8 (mu-1,2- and mu-1,3-bridging), and neighboring polymer chains are cross-linked by hydrogen bonds to form a 3-dimensional network.
[reaction: see text] Interaction of a novel pyrene-based tweezer molecule with a macrocyclic ether-imide-sulfone results in formation of a strongly bound complex (K(a) = 24 000 M(-)(1)) in which binding results not only from pi-pi stacking interactions involving pyrene units as donors and macrocyclic naphthalene-tetracarboximide and biphenylenedisulfone groups as acceptors but also from N-H.O and C-H.O hydrogen bonds and from "reverse" pi-stacking of the electron-poor isophthaloyl residue of the tweezer with an electron-rich 3-aminophenoxy residue of the macrocyclic imide.
We have investigated the (1)H NMR spectra, the absorption spectra, the fluorescence spectra and decays, and the electrochemical properties of i). a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core, ii). a trifurcated trication wherein three dibenzylammonium ions are linked 1,3,5 to a central benzenoid core, and iii). their 1:1 adduct which constitutes a triply-threaded, two-component supramolecular bundle. X-Ray crystallography has established the precise geometry of this paucivalent recognition motif in the solid state. In addition to [N(+)-H...O] hydrogen bonding and [C-H...O] interactions between the NH(2) (+) centers on the three dibenzylammonium ion containing arms of the trication and the three crown ether rings in the tritopic receptor, there is a stabilizing [pi...pi] stacking interaction between the two aromatic cores. Mass spectrometry and (1)H NMR spectroscopy have confirmed the integrity of the 1:1 adduct beyond the solid state, provided the solvents are relatively apolar (e.g., chloroform and acetonitrile). The intense fluorescence emissions of the two recognition components are quenched upon association with the concomitant appearance of a lower energy, broad fluorescence band originating from the pi-pi stacking in the 1:1 adduct of the aromatic cores in the two matching components. Titration experiments, including Job plots, establish the 1:1 stoichiometry of the adduct, an observation which is also confirmed by electrochemical experiments. The electrochemical results show that, both in the tritopic receptor and in the superbundle itself, the first oxidation process is associated with the hexaalkoxytriphenylene core. The successive oxidation processes of the peripheral dioxybenzene units are affected by charge-transfer interactions in the tritopic receptor, whereas, in the superbundle, such units are not interacting. In acetonitrile solution, dethreading/rethreading of the 1:1 adduct can be controlled quantitatively by addition of base and acid. Dethreading and rethreading is also observed by (1)H NMR spectroscopy when dimethylsulfoxide is added to a solution of the 1:1 adduct in equal volumes of acetonitrile and chloroform. A trifurcated trication where methyl groups are located on the para positions of the three dibenzylammonium ions, which are linked 1,3,5 to the neutral benzenoid core, has been employed to demonstrate that dethreading of the 1:1 adduct involves doubly-threaded and singly-threaded species, that is, the paucivalent site is dismembered in a sequence of logical steps involving stable intermediates. This molecular recognition system is a rare example of a supramolecular entity based on a cooperative binding motif that can be switched on and off by chemical means.