pubs.acs.org/JPCL Synthetic Approach for Tunable, Size-Selective Formation of Monodisperse, Diphosphine-Protected Gold Nanoclusters † John M. Pettibone and Jeffrey W. Hudgens* Chemical and Biochemical Reference Data Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ABSTRACT We report a new strategy that provides stringent control for the size and dispersity of ultrasmall nanoclusters through preparation of gold complex ( ) distributions formed from the precursor, AuClPPh3 PPh3 = triphenylphosphine , and the L6 (L6 = 1,6-bis(diphenylphosphino) hexane) ligand prior to reduction with NaBH4 in 1:1 methanol/chloroform solutions. Monodisperse nanoclusters of [ 6] [ ] distinct nuclearity are obtained for specific ligand ratios; L / PPh3 = 4 yields [ 6 ]2þ [ 6] [ ] [ 6 ]2þ [ 6] [ ] Au8L 4 , L / PPh3 = 0.4 yields Au9L 4Cl , and L / PPh3 = 8 yields ligated 6 2þ 6 2þ Au10 cores in the form of [Au10L 4] and [Au10L 5] . Polyhedral skeletal electron [ 6 ]2þ pair theory accounts for the stability of Au9L 4Cl , which is the smallest closed- shell chlorinated cluster reported. Electrospray mass spectrometry and UV-vis 6 2þ 6 2þ spectra indicate that [Au9L 4Cl] and [Au10L x] (x =4,5) result from reactions [ 6 ]2þ involving Au8L 4 . Syntheses of small gold clusters containing chloride ligands open the possibility of constructing larger clusters via ligand exchange. SECTION Nanoparticles and Nanostructures [ 6 ]2þ ynthetic methods for the production of phosphine- the novel Au9L 4Cl , which is the smallest known, closed- protected Au clusters have been the topic of extensive shell, chlorinated cluster. S research for the last 30 years, with a significant portion Using an electrospray ionization mass spectrometer (ESI- devoted to nanoclusters capped by triphenylphosphine MS), we have measured the cationic reaction products vola- ( ) 1-4 PPh3 ligands. Solution-phase properties of gold nanopar- tilized from 1:1 methanol/chloroform solutions. Prior to addi- ticles are conveniently tuned by exchanging the phosphine tion of a reducing agent, sets of reaction vials containing AuCl- 5-7 6 cap with ligands possessing desired properties. In contrast, PPh3 were also loaded with increasing amounts of L .The [ 6] [ ] aimed syntheses of monodisperse closed-shell clusters of ligand content in each vial is described by L / PPh3 ,where 6 6 chosen core nuclearity remain difficult and subject to the luck L is the free and complexed L ligands in solution, AuClPPh3 7 [ ] and skill of the experimentalist. Some improvements of is the only source of PPh3,and PPh3 is the total concentration nanoparticle dispersity are obtained by controlling growth of free and complexed PPh3 ligands in solution. Reduction of 6 2þ rates through adjustments of physical parameters (e.g., sol- the solutions generally yields the cationic products [Au8L 4] ) [ 6 ]2þ [ 6 ]2þ vent, temperature, reductant strength, and stir rate .More and Au9L 4Cl and the Au10 species, Au10L 4 and [ 6 ]2þ - recently, diphosphine ligands have shown promise for facilit- Au10L 5 . These clusters display distinct UV vis spectra - ating the formation of monodisperse gold clusters.8 10 Bertino and simple ESI mass spectra (Figure S1, Supporting In- et al.10 found that the mean gold core nuclearity obtained formation) comprising their molecular ion peaks, which from synthesis solutions containing Ln diphosphine ligands exhibit no evidence for ion fragmentation or neutral loss changes with the length of the carbon chain, n,inthe1,n- as the in-source ESI cone voltage is increased from þ50 to bis(diphenylphosphino)n-alkane ligand. Thus, metal core size þ250 V. Such stability is consistent with the closed skeletal - selectivity appears to be an inherent property of these biden- valence shells of these species.11 15 Figure 1 shows the frac- tate ligands. Since solution metal complex equilibria are tional product distribution among the cationic reaction pro- 6 2þ 6 2þ 6 2þ affected by the molar ratios among ligands, we hypothesized ducts [Au8L 4] and [Au9L 4Cl] ,andthesumof[Au10L x] ( ) [ 6] [ ] that cluster nuclearity and dispersity might be tunable thro- x = 4 and 5 as a function of the ligand ratio L / PPh3 . The ugh manipulation of the solution complex distribution. The graph readily shows that nearly monodisperse nanoclusters present study shows that this strategy works for the case of L6 and AuI phosphine complexes. By controlling the complex distribution in solution, the synthetic products are able to be Received Date: July 9, 2010 tuned, forming nearly monodisperse ligand-capped Au8,Au9, Accepted Date: July 30, 2010 or Au10 clusters. This study also describes the preparation of Published on Web Date: August 10, 2010 r 2010 American Chemical Society 2536 DOI: 10.1021/jz1009339 |J. Phys. Chem. Lett. 2010, 1, 2536–2540 pubs.acs.org/JPCL [ 6 ]2þ dominant solution complex, and only traces of Au2L 2 , 6 2þ 6 2þ [Au2L 2Cl] ,and[Au2L 3Cl] are observed; hence, during re- [ ( ) ]þ ( duction, the synthesis chemistry of Au PPh3 2 and Au- ) ClPPh3 dominates, and the final products are similar to those obtained without the presence of L6.AsL6 is added, the [ ( ) ]þ [ 6] [ ] g Au PPh3 2 diminishes until L / PPh3 3, where its signal falls below the ESI-MS detection limit. (Note, L3 and L5 com- [ ( ) n ]þ plexes combine with gold, forming Au PPh3 xL 2-x com- plexes,16 whereas, the analogous Au:L6 complexes are not observed.) Further, addition of L6 causes the fractional abun- [ 6 ]þ [ 6 ]þ dances of Au2L 2Cl and Au2L 3Cl to increase initially and 6 attain maxima of 0.18 and 0.3, respectively, for [L ]/[PPh3] = 0.8, concurrent with the formation of the stable ligand- protected Au8,Au9,andAu10 cluster species. Then, the frac- tional abundances of chlorinated complexes steadily diminish [ 6] [ ] to nearly 0 at L / PPh3 = 16. The fractional abundance of [ 6 ]2þ 6 Au2L 2 rapidly increases as L is added until it accounts for Figure 1. Fractional product distribution as a function of [ 6] [ ] [ 6] [ ] nearly all ion signal at L / PPh3 = 16 and correlates with L / PPh3 . The symbols are assigned as follows: red circles, þ [ 6 ]2þ [ 6 ]2þ [ 6 ]2 Au8L 4 ; blue squares, Au9L 4Cl ; and green diamonds, Au8L 4 observation in the ESI-MS spectra after reduction. [ 6 ]2þ (x ) Au10L x = 4 and 5 . The product distribution was deter- Thus, by selecting the initial metal complex distribution, we mined from fractional ion intensities measured with ESI-MS. See can control product formation. text for more details. Dynamic light scattering measurements (DLS) of synthesis e [ 6] [ ] e solutions prepared with the ratios 0.4 L / PPh3 16 show no evidence for any species in solution larger than the detection limit of the instrument (e1nm). This result indi- cates either their absence or that the concentration of clusters above ∼1 nm is below the concentration detection 6 2þ 6 2þ 6 2þ limit for the [Au8L 4] , [Au9L 4Cl] , and [Au10L x] (x =4, ) [ 6] [ ] 5 products. However, reduction of solutions L / PPh3 = 0.25 shows formation of colloids with hydrodynamic dia- meters larger than 100 nm. In accord with this observation, the UV-vis spectrum (Figure S1, trace A, Supporting In- formation) shows a faint 520 nm band that may arise from a plasmon resonance of a nanoparticle formed through the - reduction of the gold precursor.17 19 The contrasting beha- viors among the syntheses are accounted for by examining the metal complex distributions. 6 2þ Although [Au9L 4Cl] is the primary final product formed [ 6] [ ] at L / PPh3 = 1, the succession of species appearing during syntheses suggests similarities of the initial synthetic path- n way with other L syntheses that do not form Au9 clusters (Figure 3).10,16 Initially (t e 10 min), the major products [ 6 ]2þ [ 6 ]2þ observed in the ESI-MS data are Au8L 4 and Au10L 4 ( ) Figure 2. Fractional total ion current measured via ESI-MS as a Figure 3 , corresponding to the final ligated Au8 and Au10 6 10 function of the ratio [L ]/[PPh3], where all PPh3 originates from the 6 clusters reported previously by Bertino et al.; the present chlorinated gold precursor, AuClPPh3.AsL is added incremen- UV-vis spectra (Figure S2, Supporting Information) support tally to a solution comprising 10.0 mg of AuClPPh3 dissolved in 1:1 6 CH OH/CHCl , PPh ligands are increasingly replaced on the metal the previous assignments. At t = 6 days, the distribution of L 3 3 3 [ 6 2þ complexes. The symbols are assigned as follows: red circles, Au- protected clusters in solution becomes monodisperse [Au8L 4] . ( ) ]þ [ 6 ]2þ [ 6 ]þ þ PPh3 2 ; blue diamonds, Au2L 2 ; green wedges, Au2L 2Cl ; ( [ 6 ]2 ) [ 6 ]þ Presumably, the nascent larger clusters e.g., Au10L x have and black triangles, Au2L 3Cl . The inset displays the fractional e [ 6] [ ] e become depleted by ligand etching, leaving almost entirely total ion current for 0 L / PPh3 3. 6 2þ 6 2þ [Au8L 4] . In turn, by t = 14 days, [Au8L 4] is largely de- - [ 6 of distinct nuclearity are obtained for reaction mixtures contain- pleted, and the ESI-MS and UV vis data indicate that Au9L 4- [ 6] [ ] [ 6 ]2þ ]2þ ( ) [ 6 ]2þ ing specific ligand ratios; L / PPh3 =0.4yields Au9L 4Cl , Cl is the principal product Figure 3 and Au10L 4 and [ 6] [ ] [ 6 ]2þ [ 6] [ ] [ 6 ]2þ [ n ]2þ L / PPh3 =4yields Au8L 4 ,and L / PPh3 = 8 yields Au10L 5 are minor products. Previously, Au8L 4 species in [ 6 ]2þ ( ) n ( ) Au10L x x = 4 and 5 . solutions for L n =3,5 did not persist in solution and The ligand-metal complex distribution as a function of behaved as transient species only;16 the current study shows [ 6] [ ] [ n ]2þ L / PPh3 is measured at equilibrium by observing the con- the Au8L 4 as a stable product for specific complex dis- stant fractional ion intensities of the complexes prior to re- tributions, which may also be observable with other Ln ligands 6 þ duction (Figure 2).For[L ]/[PPh3] ≈ 0.25, [Au(PPh3)2] is the by changing the complex distribution prior to reduction.