Biomedical subjects
Publications and source records attributed to Stuart L James.
In the absence of coordinating solvents and anions, silver salts and triphosphines form viscous, thixotropic coordination polymers in solution, which aggregate into unusual wormlike macroscopic structures and undergo cage-polymer interconversions depending on the stoichiometry and presence of templating anions.
We present a highly unusual example of self-assembly, specifically a polar, mixed-ligand cage which forms in preference to symmetrical homo-ligand products, and which suggests that steric effects can be exploited to obtain novel non-uniform polyhedral cages. In particular, reaction between the bulky tripodal triphosphine 2,4,6-tris(diphenylphosphino)triazine, L1, the non-bulky tripodal trinitrile 2,4,6-tris(cyanomethyl)trimethylbenzene, L2 and silver hexafluoroantimonate, AgSbF6, in a 3:1:4 ratio gives the mixed-ligand aggregate [Ag4(L1)3(L2)(SbF6)]3+, 1-SbF6, instantly as the only product in quantitative yield. The X-ray crystal structure of complex 1-SbF6 is consistent with the suspected solution-state structure. The cage derives from trigonal-pyramidal geometry, the basal vertices of which are defined by three bulky triphosphines, L1, and the apical vertex by the non-bulky trinitrile, L2. There is apical elongation amounting to 19% in comparison to the ideal uniform tetrahedron. The cage also encapsulates an SbF6 anion. 19F NMR spectra in solution for the analogous PF6 complex [Ag4(L1)3(L2)(PF6)]3+, 1-PF6, confirm that one anion is also encapsulated in solution. The synthesis of the analogous CF3SO3(-) complex, [Ag4(L1)3(L2)(OTf)]3+, 1-OTf, in solution is also described, although 1-PF6 and 1-OTf could not be isolated due to slow decomposition in solution. The selective formation of these mixed-ligand cages is discussed in terms of ligand-ligand and ligand-included anion steric repulsions, which we propose prevent the formation of the competing hypothetical homo-ligand tetrahedral structure [Ag4(L1)4(SbF6)]3+, and thus favour the mixed ligand cage. "Cage cone angles" for L1 and L2 are estimated at 115 degrees and 101 degrees, respectively. Variable-temperature 31P NMR spectroscopy shows that complex 1-SbF6 and the related previously reported partial tetrahedral complex [Ag4(L1)3(anion)]3+ undergo dynamic twisting processes in solution between enantiomeric C3-symmetry conformations.
The nanoporous structure of collagen-glycosaminoglycan (CG) hydrogels was studied using 1H NMR spectroscopy and thermally stimulated depolarisation (TSD) current with layer-by-layer freezing-out of bulk and interfacial water. The depression of the freezing point of water is related to the size of the nanopore, to which it is confined. Changes in the Gibbs free energy of the unfrozen interfacial water are related to the amount of bound water in the hydrogel matrix and to the re-arrangement of the 3D network structure of the biopolymer. Analysis of the thermodynamic properties of bulk and interfacial water using the layer-by-layer freezing-out technique combined with NMR and TSDC provides valuable information about the structural features of CG hydrogels that can be used for characterisation of different types of hydrogels and soft tissue scaffolds, artificial skin substitutes and other biomaterials.
A polymeric metal-organic gel is described, which acts as a template in the preparation of macroporous polymethylmethacrylate, and can be easily removed post-polymerisation.
The combination of directionality and unsaturated metal sites in bowl-shaped coordination clusters [Ag(4)L(3)(SbF(6))](3+){L = 2,4,6-tris(diphenylphosphino)triazine} results in the formation of discrete dimers on reaction with additional bridging ligands.
The metal-organic framework [Co(INA)(2)]x0.5EtOH (INA = isonicotinate, NC(5)H(4)-4-CO(2)(-)), was synthesised under solvothermal conditions. Its X-ray crystal structure shows channels containing ethanol guests which are hydrogen-bonded to carboxylate oxygens of the framework. The pyridyl rings of the framework alternate between 'open' and 'closed' positions along the channels resulting in large variation in the channel cross-sectional area from ca. 1.4 by 2.3 A at the narrowest point to 4.9 by 5.3 A at the widest. Despite the very small windows, the ethanol guests (of van der Waals diameter ca. 4.2-6.1 A) may be reversibly desorbed and sorbed into the structure quantitatively, as shown by in situ variable-temperture IR spectroscopy and XRPD. The single-crystal structure of the desolvated form [Co(INA)(2)] shows that there is no change in the overall connectivity on desolvation, but the rotational positions of the pyridine rings are altered. This suggests that pyridyl rotation may occur to allow guests to pass in and out. When the synthesis was conducted in 1-propanol solvent [Co(INA)(2)]x0.5Pr(n)OHxH(2)O, was obtained, and a single-crystal X-ray structure revealed the same overall connectivity as in but with pyridine rings disordered over closed and open positions. There was no evidence of included guests from X-ray crystallography, suggesting that they are also highly disordered. Variable-temperature XRPD performed on bulk samples showed peaks which were unsymmetrical and exhibited shoulders, suggesting that for each pattern obtained the material actually consisted of several closely-related phases. The movements of the peaks during desolvation showed the presence of intermediate phases before the final desolvated product was formed. The peak positions of the intermediate phases matched more closely with the calculated pattern for than with or, suggesting that they may have disordered structures similar to. The results also suggest that the intermediate phase represents an initial increase in volume before a larger decrease in volume occurs to give the final desolvated material.
Metal-organic frameworks are a recently-identified class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands, and are a new development on the interface between molecular coordination chemistry and materials science. A range of novel structures has been prepared which feature amongst the largest pores known for crystalline compounds, very high sorption capacities and complex sorption behaviour not seen in aluminosilicate zeolites. The development of synthetic approaches to these materials and investigations of their properties are reviewed.
Silver salts and triphosphine ligands with biphenyl substituents assemble to give coordination cages with four external aromatic channel receptors in a pseudo-tetrahedral arrangement.
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Addition of anionic benzylsulfate dendrons to dynamic mixtures of Ag+ and triphosphine ligands results in the assembly of loosely-bonded cage-core dendrimers.