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. Author manuscript; available in PMC: 2017 Feb 1.
Published in final edited form as: Inorg Chem. 2016 Jan 13;55(3):999–1001. doi: 10.1021/acs.inorgchem.5b02106

Crystal Structure of the PdAu24(SR)180 Superatom

Marcus A Tofanelli 1, Thomas W Ni 1, Billy D Phillips 1, Christopher J Ackerson 1,✉
PMCID: PMC4809668  NIHMSID: NIHMS767687  PMID: 26760220

Abstract

The single-crystal x-ray structure of Pd doped Au25(SR)18 was solved. The crystal structure reveals that in PdAu24(SR)18, the Pd atom is localized only to the centroid of the Au25(SR)18 cluster. This single crystal x-ray structure shows that PdAu24(SR)180 is well conceptualized with superatom theory. The PdAu24(SR)180 charge state is structurally isoelectronic with Au25(SR)18+1 as determined by a first order Jahn-Teller effect of similar magnitude and by electrochemical comparison. The previously reported increased stability of PdAu24(SR)18 can be rationalized in terms of Pd-Au bonds that are shorter than the Au-Au bonds in Au25(SR)18.

Many ligand protected gold clusters, including Au11(PPh3)7Cl3, Au25(SR)18, Au39(PPh3)14Cl6 and Au102(SR)44 can be described electronically as superatoms.1 Superatom theory can be extended to describe properties such as stability,2 magnetism3 and certain optical properties.4 Superatom theory approximates metal clusters as spheres containing the free or valence electrons of the metal atoms comprising the cluster. A valence electron count of 2, 8, 18, 20, 34, 40, 58… corresponds to total filling of superatom orbitals, resulting in noble-gas-like electron configurations associated with high stabilty.5 Ligands can either withdraw one electron each from the superatom, or serve as dative ligands that neither add to nor subtract from the superatom electron count.1 Overall, this simple approximation remarkably predicts the symmetry and degeneracy of the frontier orbitals of many ligated metal clusters. 6 Au25(SR)18 and isostructural compounds are intensively studied currently, with detailed understanding of electronic properties available.7

An emergent question in the experimental literature is of how dopant atoms such as Pt or Pd impact the superatom electron count in ligated bimetallic clusters, such as in the recently reported PdAu24(SR)18, PtAu24(SR)18, Pd2Au36(SR)24, and AgxAu25-x(SR)18, and PdxAu144–x(SR)60 clusters. 8-10 All of the single crystal x-ray determined doped or alloy clusters of thiolate protected metal replace Au with Ag or vice versa, resulting in clusters such as Ag32Au12(SR)24, and AgxAu144–x(SR)60.11-15 In the case of coinage metals (Cu, Ag, Au) each metal is now understood to donate one electron (i.e., the 6s1 electron of Au). The electron donation behavior of other metals to the superatom is a matter of conjecture. In the case of Pd, for instance, it is expected that the d10 metal atom will neither add to nor subtract from the superatom electron count.16 There is no empirical structural evidence so-far to support this conjecture.

Another open question in the recent literature concerns the position of dopant or alloy atoms in the cluster. In Au25(SR)18 based clusters there are three possible positions for these dopants: (1) in the centroid of the 13 atom icoshedron that forms the core of the cluster; (2) as one of the vertices of the central icosahedron; (3) in the semiring (i.e., replacing an Au atom in the SR-Au(I)-SR-Au(I)-SR structure. In the case of PdAu24(SR)18, DFT and EXAFS studies place the dopant atom at the centroid position for Au24X(SR)18 clusters where X is Pt or Pd. 14,16 However, the position of Pd in the Pd2Au36(SR)24 structure is not speculated as found at the centroid positions of each fused bi-icosihedra.17 In each case, the positions of heteroatoms are inferred from indirect methods; single-crystal x-ray structures are not yet available for any alloy structure of a gold cluster that does not contain coinage metals.

Here we present the single crystal x-ray structure of PdAu24(SR)18, localizing the Pd dopant atom to a single location in the crystal structure. Through analysis of electrochemical/ spectroscopic data, we assign the solved structure as the 1S21P4 superatom configuration, suggesting that the Pd heteroatom donates no electrons to the superatom electronic structure.

We synthesized PdAu24(PET)18 (PET = phenylethanethiol) by methods adopted from Negishi.18 The PdAu24(PET)18 compound was crystallized as discussed in the SI. Crystal diffraction data was collected on Advanced Light Source Beamline 4.2.2; Synchrotron flux was required for timely collection of data especially at higher angle diffraction. PdAu24(PET)180 forms a triclinic lattice in the space group P1(bar), as observed for every other crystallographically resolved Au25(SR)18 cluster structure.19-23 The crystal structure was solved in SHELXTL. The single crystal x-ray structure of PdAu24(PET)18 is shown in Figure 1, left panel. The structure reveals identical connectivity to the other Au25(SR)18 crystal structures so-far reported, with a filled 13-atom icosahedral core protected by 6 SR-Au- SR-Au-SR semirings.19-23 Static substitution disorder refinement in SHELX was used to refine the occupancy of Pd in all metal atom positions; In this refinement strategy Pd refines to less than 10% occupancy or fails to refine to any occupancy except in the centroid of the cluster, where it refines to 92.6% occupancy. Au could completely account for electron density in every other electron density peak, without resulting in ‘negative density.’ (Figure S1) We assign the material that crystallized as neutral because no counterions were observed in the crystal lattice.

Figure 1.

Figure 1

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Shown in A is the crystal structure of PdAu24(PET)18 with gold in yellow, thiol in red and palladium in blue. The carbon chains have been removed for clarity. Panel 1B is illustrates bond lengths of the gold icosahedron.

Linear absorption spectroscopy and electrochemical measurements suggest that the PdAu24(PET)18 cluster is isoelectronic with the Au25(SR)18 cluster. Electrochemically, PdAu24(PET)18 and Au25(PET)18 show the same multiplicity of charging events with almost identical spacing between each reduction/oxidation wave. The difference in the voltammograms is a shift of -534 mV for the potentials of PdAu24(PET)18 compared to Au25(PET)18 (Figure 2)24 Likewise, the linear absorption spectra of PdAu24(PET)180 are similar to Au25(PET)180; each compound has a broad band peak centered at 650 nm and 688 nm with a sharp feature at 380 nm and 400 nm for PdAu24(PET)180 and Au25(PET)180, respectively. The voltammograms are similar to those previously reported by Murray for the same compound.24 Previous theoretical reports describe modification of the electronic spectra of Au25(SR)18 upon doping with Pd. DFT description of the electronic structure suggests that removal of electrons is “softer” for PdAu24(SR)18 as a result of the electronic structure modification upon doping, resulting in the substantially shifted electrochemical response.25

Figure 2.

Figure 2

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Shown above are the square wave voltammograms of PdAu24(PET)18 and Au25(PET)18. The current axis is offset and normalized for comparison.

Despite the similarities, it is obscure which oxidation state of Au25(PET)18 is formally isoelectronic with PdAu24(PET)180. In general only the s and p electrons of a metal are donated to the superatom. With an electron configuration of 5s0 4d10, it is expected that Pd makes no contribution to the superatom electron count. Thus, PdAu24(PET)180 is expected to be “superatom-isoelectronic” with Au25(PET)18+.

Assuming this, each event in the square wave voltammogram of PdAu24(SR)18 can be assigned with the superatomic configuration shown in Figure 2. The resting potential of the PdAu24(PET)180 used in formation of single-crystals is at 50 mV vs SCE suggesting a 1S21P4 superatomic configuration for the single-crystal structure. This ‘open-shell’ superatom electron configuration is one in which a Jahn-Teller effect should be observable, analogous to our recent observations of a Jahn-Teller effect that increases with increasing oxidation state for Au25(SR)18-1/0/+1.26

Indeed, analysis of the central icosahedron of PdAu24(SR)18 reveals that the structure is distorted away from idealized icosahedral symmetry with remarkable similarity to the distortion observed previously in the Au25(SR)181+ (1S21P4) superatom. In the 1S2 1P4 superatom structure of Au25(SR)181+, bond lengths in the icosahedral core varied from 2.7 to 3.3Å, whereas for the 1S21P6 configuration, bond lengths vary only from 2.8Å to 3.0Å. The variability in bond lengths observed in PdAu24(SR)180 span an identical range to those of Au25(SR)18+1. Continuous Symmetry Measurement (CSM) can be used to quantify distortion from idealized geometry in terms of root mean squares.27 The CSM values for the central icosahedron of Au25(SR)18-1/0/+1 are 0.67, 0.201 and 0.524. The CSM value for the corresponding structure in PdAu24(SR)180 is 0.350, falling between the values previously observed for the neutral and cationic Au25(SR)18 species.

We observe that the Pd-Au bond lengths in this structure are shorter than Au-Au bond lengths. Overall, this appears to result in a contraction around the Pd centroid with resulting shortening of the gold-gold bonds of the PdAu24(SR)18 icosahedron by an average of 0.04 Å. Thus, the higher stability of PdAu24(PET)180 compared to Au25(SR)18 suggested by previous results may be attributed to an overall stronger bonding in the inorganic core. 2,28

We isolated the PdAu24(PET)18 as the neutral (1S21P4) compound without taking any measures to preserve the oxidation state of the cluster. We suggest that the as-synthesized PdAu24(PET)18 is in the −2 charge state (1S21P6, presumably more stable). We suggest that, similar to Au25(SR)18, the compound we studied may be oxidized by ambient atmosphere into the crystallized oxidation state. Because the reduction potentials are shifted to more negative values for PdAu24(PET)18 compared to Au25(PET)18, oxidaiton in the presenece of atmospheric oxygen is expected to be more facile.

Herein we report the crystal structure of PdAu24(PET)180 revealing that Pd is localized to the cluster core, which retains the same atomic connectivity and nearly identical geometry to Au25(PET)18−,0,+ clusters. The Pd dopant results in shorter bonds in the 13-atom core and a blue-shift in the UV/Vis spectrum. Overall we suggest that PdAu24(PET)18, like Au25(SR)18 is well predicted by a spherical superatom model.

During revision of this manuscript we became aware of a similar report of structures of doped XAg24 (SR)18 clusters, where X is Pt or Pd.29 Notably these authors arrive at the same conclusion regarding the contribution of Pt or Pd to the superatom electron count.

Supplementary Material

SI

Acknowledgments

We acknowledge NIH R01 GM 112225. Portions of this research were carried out at Advanced Light Source Beam-line 4.2.2 with the help of Dr. Jay Nix The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Footnotes

Associated Content: The Supporting Information is available free of charge on the ACS Publications website. A detailed experimental procedure, electron density map, and UV/Vis are reported in the SI. (PDF)

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Associated Data

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Supplementary Materials

SI
NIHMS767687-supplement-SI.pdf (206.7KB, pdf)

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