Andrew Solovyov

Synthesis and Characterization of Accessible Metal Surfaces in Calixarene-Bound Gold Nanoparticles

Use of organic ligands to partially passivate nanoparticles against sintering yet retain a degree of small molecule accessibility to the metal surface has been a lofty goal in functional materials synthesis, which in principle also enables the design of preferred electronic and steric environments on a nanoparticle surface. Catalysis using gold in particular requires donor ligands that facilitate an electron-rich metal surface and generalizable strategies for dealing with deactivation due to sintering. Here, synthesis and characterization of gold nanoparticles postsynthetically modified with the chelating ligand cone-5,11,17,23,29,35-hexa(tert-butyl)-37,39,41-tris(diphenylphosphinomethoxy)-38,40,42-trimethoxycalix[6]arene (1) is reported. In solution as well as when supported on the surface of TiO2, nanoparticles modified with tripodal calix[6]arene phosphine ligand 1 demonstrate enhanced protection against sintering relative to unmodified, tetraoctylammonium bromide-surfactant-stabilized gold nanoparticles. In between adsorbed calixarene ligands, there is accessible gold surface area in these nanoparticles, and this is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. Ligand steric bulk critically influences amount of accessible surface on the metal nanoparticle since the use of a smaller calix[4]arene ligand (MBC) results in a 7-fold lower accessible surface area relative to using 1 under otherwise similar conditions. In addition, surface coverage of 1 controls accessible surface area in an unintuitive fashion: a 4-fold increase in accessible metal surface area is observed upon increasing the surface coverage of 1 to be 1.5-fold higher than the minimum required for surface saturation. This is presumably the result of a more open ligand packing of 1 at higher surface coverages, which allows greater accessibility to 2-napthalenethiol.

Postsynthetic modification of gold nanoparticles with calix[4]arene enantiomers: origin of chiral surface plasmon resonance.

Gold nanoparticles postsynthetically modified with chiral 1,3-disubstituted diamino calix[4]arene ligands 2a and 2b are shown to exhibit a circular dichroism (CD)-active surface plasmon resonance absorption (SPR) band. Electronic communication between adsorbed ligand and the gold nanoparticle surface is evidenced in an almost 10-fold increase in the ligand molar ellipticity in the pi-pi* transition spectral range when bound to the gold surface relative to free solution. The footprint of ligand on the gold nanoparticle surface at saturation is measured to be 91 A(2) via monitoring the red shift in the SPR band accompanying ligand adsorption. At ligand concentrations above that required for surface saturation, the ellipticity of the band in the SPR spectral range plateaus to a constant value, whereas the ellipticity of the band in the pi-pi* transition spectral range continues to increase in a manner that corresponds to free ligand in solution. This critical observation correlates ligand adsorption and the onset of the CD-active SPR band. On the basis of the packing characteristics of the bulky calixarene ligand, which are controlled by achiral tert-butyl groups, and the postsynthetic nature of nanoaprticle surface modification of 4.7 nm gold cores used in this study, which precludes synthesis of chiral arrangements of gold atoms, a mechanism responsible for the CD-active SPR bands is proposed, which is based upon the influence of the asymmetric center of the chiral adsorbate on the electronic states of the metal nanoparticle core-an explanation supported by the observed interactions between the gold surface and adsorbed ligand.

Patterned metal polyhedra using calixarenes as organizational scaffolds: Ir4-based cluster assemblies

Isolated and patterned assemblies of Ir(4)-based metal polyhedra are described, in which a coordinated calixarene phosphine ligand enforces the desired cluster organization. The compounds are characterized by single-crystal X-ray diffraction and infrared spectroscopy.

Upper-rim calixarene phosphines consisting of multiple lower-rim OH functional groups: synthesis and characterisation

The synthetic approaches to mono-isopropoxytrihydroxycalix[4]arenes bearing two or three diphenylphosphino groups at the upper macrocycle rim were elaborated. Due to reactive hydroxyl groups at the lower rim capable of binding with silica gel surface, the calixarenes, especially the enantiomerically pure inherently chiral diphosphinocalixarene, are promissing ligands for creation of hybride organic–inorganic metallocomplexing catalysts of organic (asymmetric) reactions.

Role of N-Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHCs behave as stronger σ donors compared with COs and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the COs and the decrease in the LDEs of the NHCs as more NHCs are substituted for COs are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging COs and the μ3-COs can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.