(20e) Amphiphilic Poly(organosiloxane) Nanocages: Preparation, Characterization, and Formation of Unusual Co Complexes in a Confined Space

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 Co_x(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- Co_x(CO)_y  was found to be able to active dioxygen and transfor O atom or O atom contained group to small organic molecules.