Synthesis of gold clusters with flexible and rigid diphosphine ligands and the effect of spacer and solvent on the size selectivity

Abstract

The relationship between ligand rigidity and solvent polarity and the size and dispersity of Au cluster suspensions was investigated. Solutions were prepared which contained 10−3mol/L of Chloro(triphenylphosphine) gold(I) [Au(PPh3)Cl], the same concentration of diphosphine ligand, and 5×10−3mol/L of borane-tert-butylamine (BTBC) as the reducing agent. Diphosphine ligands were used which consisted of phosphorus atoms connected by chains with increasing length and rigidity. Specifically we employed a short flexible chain PPh2(CH2)3PPh2, i.e., 1,3-bis(diphenylphosphino)propane, denoted by L3, a long, flexible chain, PPh2(CH2)6PPh2 i.e., 1,6-bis(diphenylphosphino)hexane denoted by L6, two rigid ligands, 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) denoted by LAr, and 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP) denoted by LBn, and a moderately flexible spacer 2,2’-Bis(diphenylphosphino)biphenyl (Biphenyl) denoted by LBp. Materials were dissolved in solvents of increasing polarity; from highest to lowest, methanol, ethanol, propanol, butanol and chloroform were used. Samples were characterized using UV–Visible spectroscopy and electrospray ionization mass spectrometry. We observed the formation of stable monodisperse clusters with the shortest ligand, (L3), independent of solvent. With a longer flexible ligand, (L6), we observed primarily Au8−10 cores depending on the ratio of L6/PPh3. All other ligands yielded polydisperse distributions. These dispersions contained clusters with a nuclearity between 8 and 11, for example [Au10(PPh3)9]3+ in LBn and [Au8(PPh3)7]2+ in LBp, were observed in the initial stages, but they were not stable and precipitated out or plated the glass vial. We also observed that the polarity of the solvent did not play a significant role in the formation of MPC’s, however a correlation between the size of the solvent and MPC formation was observed. Selectivity was observed in the smallest solvents, methanol and chloroform, which are the most and least polar solvents, respectively.