(20e) Amphiphilic Poly(organosiloxane) Nanocages: Preparation, Characterization, and Formation of Unusual Co Complexes in a Confined Space
AIChE Annual Meeting
Sunday, November 3, 2013 - 4:30pm to 4:45pm
Inspired by the remarkable properties of protein cavities in enzyme, a soluble discrete core-shell nanocage structure was designed and synthesized, and its properties characterized. To synthesize this structure, a pentacyclooctasiloxane was first converted to a spherosilicate with vinyl termination at every corner, [Si8O12]-(OSiMe2CH2CH2CH2CHCOOSi(CH=CH2)3)8. The vinyl groups were used for hydrosilylation with 1,4-bis-dimethylsilylbenzene that served as a linker to form the shell, and subsequent hydroxylation of the internal silyl ester bonds created a core-shell nanocage structure. The resulting 2 – 5nm diameter core-shell nanocages could be dispersed homogeneously in non-polar organic solvents, had a hydrophilic interior with carboxylic acid on the core and silanol groups on the shell, and a hydrophobic carbosilane exterior. Metal salts (e.g. Pd ions) could be accommodated in the cage interior while the cages were dispersed in a hydrophobic hydrocarbon solvent (e.g. toluene). Mixing a toluene suspension of nanocages with a stoichiomeric excess of Co2(CO)8 resulted in the formation of a bright orange product. The IR spectrum of this product showed a CO stretch at 1959cm-1 that was different from any CO stretch in Co2(CO)8, indicating the presence of a terminal CO on a reduced cobalt center (Co(I) or Co(0)). The IR spectrum also indicated the formation of carboxylate (COO-) from carboxylic acid (COOH) groups on the core structure, and shifts in peaks in 1H NMR implied interaction between Cox(CO)y and the shell of the nanocage. Cyclic voltammetry results indicated that the Co complex resided inside the nanocage. This Co complex reacted readily with oxygen, displacing the CO, and UV-vis spectroscopy suggested that oxidation of Co. The data suggested that this nanocage environment can stabilize unusual reactive cobalt carbonyl species with a possible oxidation state of Co(I), which usually are supported by pi-acids or soft donors. Such Co(I) is well known to be active species or intermediate in many catalytic cycles. This nanocage- Cox(CO)y was found to be able to active dioxygen and transfor O atom or O atom contained group to small organic molecules.