Cationic gold clusters ligated with differently substituted phosphines: effect of substitution on ligand reactivity and binding

Abstract

We present a systematic study of the effect of the number of methyl (Me) and cyclohexyl (Cy) functional groups in monodentate phosphine ligands on the solution-phase synthesis of ligated sub-nanometer gold clusters and their gas-phase fragmentation pathways. Small mixed ligand cationic gold clusters were synthesized using reactions between pre-formed triphenylphosphine ligated (PPh3) gold clusters and monodentate Me- and Cy-substituted phosphine ligands in solution and characterized using electrospray ionization mass spectrometry (ESI-MS) and collision-induced dissociation (CID) experiments. Under the same experimental conditions, larger gold-PPh3 clusters undergo efficient exchange of unsubstituted PPh3 ligands for singly Me- and Cy-substituted PPh2Me and PPh2Cy ligands. The efficiency of reaction decreases with an increasing number of Me or Cy groups in the substituted phosphine ligands. CID experiments performed for a series of mixed-ligand gold clusters indicate that loss of a neutral Me-substituted ligand is preferred over loss of a neutral PPh3 ligand while the opposite trend is observed for Cy-substituted ligands. The branching ratio of the competing ligand loss channels is strongly correlated with the electron donating ability of the phosphorous lone pair as determined by the relative proton affinity of the ligand. The results indicate that the relative ligand binding energies increase in the order PMe3 < PPhMe2 < PPh2Me < PPh3 < PPh2Cy < PPhCy2 < PCy3. Furthermore, the difference in relative ligand binding energies increases with the number of substituted PPh(3-m)Me(m) or PPh(3-m)Cy(m) ligands (L) on each cluster. This study provides the first experimental determination of the relative binding energies of ligated gold clusters containing differently substituted monophosphine ligands, which are important to controlling their synthesis and reactivity in solution. The results also indicate that ligand substitution is an important parameter that must be considered in theoretical modeling of these complex systems.

Collision-Induced Dissociation of Undecagold Clusters Protected by Mixed Ligands [Au11(PPh3)8X2]+ (X = Cl, C≡CPh)

“…33,34 Ligand loss, ligand activation, and core fission were observed in the CID of phosphine-protected Au clusters. 35−38…”

Understanding ligand effects in gold clusters using mass spectrometry

“…79 PPh 3 and L is observed. It is important to note here that ESI-MS is only sensitive to charged clusters in solution and does not provide insight into potential neutral species.…”

“…30,51 The number of ligands exchanged varies for different size clusters and with the extent of ligand substitution. 79 For example, (7,6) 2+ exchanges only one or two ligands with PPh 3-m (CH 3 ) m .…”

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

“…This trend may be correlated with the larger electron density at the σ‐donating phosphorus of the L Cy ligands compared with L Ph . In the same study, the analogous deuterido complex [Ag 3 (μ 3 ‐D)(μ 2 ‐L Ph ) 3 ](BF 4 ) 2 was successfully crystallized and the position of the deuterido ligand was found to be slightly shifted out of the Ag 3 plane by around 0.3 Å, as elucidated by neutron diffraction experiments . The analogous copper(I) congener of [ P ](PF 6 ) 2 , [Cu 3 (μ 3 ‐H)(μ 2 ‐L Cy ) 3 ](PF 6 ) 2 , was successfully isolated in 2005 by Mao et al .…”

“…This was motivated by preliminary studies on related triangular Ag/Cu CMHs and also binuclear [M 2 (L Cy ) 2 ] 2+ (M=Ag, Au) complexes . In addition, the relative binding energy of L Cy is advantageously higher than that of L Ph , as revealed by an ion‐trap CID investigation of [Au n (PR 3 ) m ] +/2+ ( n =1–11, m =1–8, R=Me, Ph, Cy) . Thus, the expected stability of [ P ] 2+ facilitates parallel studies in the gas phase and in solution.…”