Structural characterisation of the giant organometallic platinum cluster Pt309(phen*)36O30 using EXAFS1

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Abstract

The structure of the high-nuclearity organometallic platinum cluster material Pt309(phen*)36O30 has been investigated by X-ray absorption spectroscopy. Analysis of Pt L3-edge EXAFS data, using platinum foil as a reference, shows that the platinum clusters have cubic close packed geometry. A slight contraction of the Pt–Pt bond length from the bulk was observed. XANES analysis shows that the platinum atoms in the clusters have a low mean oxidation state, very close to that in metallic platinum.

Introduction

Organometallic cluster compounds have become one of the most intensely studied fields of chemistry. Clusters containing several tens of atoms have been synthesised and characterised 1, 2. Transition metal carbonyl clusters form the most extensive series of these compounds; there are also many phosphine halide clusters of gold. The structural parallels between these molecular cluster compounds and small particles of metal have long been recognised 1, 3. The arrangements of ligands on the cluster surfaces have also been studied [4].

The spectroscopic and physical properties of these metal cluster compounds have become of intense interest. Appropriate measurements can characterise the way in which the well-separated energy levels of a small metal atom cluster develop into an effective band structure in larger clusters [5]. An important question is whether there is a smooth transition with increasing cluster size from ‘molecular’ towards ‘bulk metallic’ behaviour, or whether the transition is more complex, with an intermediate ‘metametallic’ region showing unusual properties different from either limit and displaying quantum-size effects 6, 7, 8.

It has also been recognised that there are many different criteria for ‘metallic’ behaviour, and that these may be attained at greatly different nuclearities. For example, in gold clusters of 55 atoms, the ‘interband’ 5d→6s, 6p electronic transition in the UV-visible is quite similar to that of colloidal gold; but in the same clusters, the plasma resonance absorption of delocalised 6s conduction electrons, characteristic of gold colloids, is almost entirely absent 9, 10.

Recently, several types of non-crystalline high-nuclearity metal clusters have been prepared. These materials, stabilised by ligands which may be monodentate, bidentate, or act as bridges between clusters, represent an interface between molecular clusters and colloids 11, 12, 13, 14.

In interpreting the physical and spectroscopic properties of these high-nuclearity organometallic materials, structural characterisation is important. However, they cannot be obtained in a form suitable for X-ray crystallographic study. Their structures and interatomic distances must be determined by methods appropriate for amorphous materials, such as EXAFS [15].

There are two key structural issues in these giant organometallic clusters. Firstly, is their geometric structure cubic close-packed, or do they adopt some other geometry such as icosahedral? Secondly, are the metal–metal distances in the clusters the same as in the corresponding bulk metals, or is there a contraction reflecting a changed bonding environment and the effect of a high surface:bulk ratio?

EXAFS can answer both these questions by probing the local environment of the metal atoms in a cluster. At the same time, the X-ray absorption spectrum used for the EXAFS analysis can give information from its near-edge region (XANES) on the effective oxidation state of the metal atoms in the cluster.

We have previously reported the results of EXAFS studies on organometallic gold and palladium clusters. EXAFS confirmed the crystallographically known icosahedral-based structure of a molecular Au11 cluster by resolving the 0.2 Å splitting between the radial and tangential nearest-neighbour interatomic distances, which differ in an icosahedron by about 5%. No such splitting could be resolved in Au55(PPh3)12Cl6, which was therefore concluded to have a cubic close-packed cluster structure [16]. The Au–Au nearest neighbour distance in Au55(PPh3)12Cl6 was found to be about 4% contracted from that in bulk gold 16, 17. The palladium cluster material Pd561(phen)36O200, was also found to be cubic close-packed, but in this case no significant contraction of the metal–metal distance was observed 16, 18. Consistent with this, differential scanning calorimetry measurements of the thermal decomposition of the clusters have shown that the Au–Au bonds in Au55(PPh3)12Cl6 are substantially stronger than in bulk gold [19], but the Pd–Pd bonds in Pd561(phen)36O200 are slightly weaker than in bulk palladium [20]. This parallels the behaviour of the corresponding metals in the absence of ligands. Lattice contractions are consistently found in small gold particles [21]. For small palladium particles, most experimental measurements have shown lattice expansions from the bulk [22], although lattice contractions have also been reported [23].

We now report the results of Pt L3-edge EXAFS analysis of the cluster material of ideal formulation Pt309(phen*)36O30 (where phen* represents a sulphonated phenanthroline ligand, which gives the cluster high aqueous solubility). High-resolution electron microscopy and X-ray powder diffraction have previously shown that Pt309(phen*)36O30 contains nearly monodisperse platinum atom clusters of diameter 17.5 Å, which have a cubic close-packed structure of cuboctahedral geometry with the Pt–Pt distance very close to that in the bulk metal [24]. Mössbauer studies, using a novel method in which platinum atoms are transmuted to 197Au by neutron irradiation, have shown that the metal atoms in the inner core of the Pt309 cluster have substantially metallic character [25]. NMR studies on the 195Pt nucleus have shown a Knight-shifted peak from the core atoms, for which metallic behaviour was inferred [26]from the temperature-dependence of the relaxation time T1.

Section snippets

Experimental

Pt309(phen*)36O30 was prepared by controlled reduction of Pt(II) acetate with hydrogen in the presence of protecting ligands, followed by oxidation with oxygen [24]. Finely ground samples were diluted with boron nitride powder and pressed into pellets for EXAFS measurements in transmission mode. A 25-micron thick platinum foil (Goodfellow Metals) was used as a reference sample. X-ray absorption spectra were collected at the SRS facility (Daresbury Laboratory, UK), on beamline 7.1. Data were

Data analysis

EXAFS data analysis was performed using both calculated and experimental phase-shifts and backscattering amplitudes (δ(k) and A(k), respectively). Two analysis program packages were employed, namely GNXAS [27]and WINXAS [28]. GNXAS allows a calculation of phase shifts employing a complex Hedin-Lundqvist potential. WINXAS allows analysis using experimental δ(k) and A(k), which are extracted performing a fit on the EXAFS signal of the reference compound, extracting reliable information from the

EXAFS refinements

Results of the EXAFS structural refinements for Pt foil and Pt309(phen*)36O30 are shown in Table 1. The EXAFS signal of bulk platinum was successfully fitted up to the fourth shell. The refined parameters agree well with crystallographic data.

For the Pt309 cluster, the data collected at 190 and 300 K showed a lower signal to noise ratio than those from 80 K, because of greater dynamic disorder. They could be fitted only over a relatively limited k-region, with a consequent loss of resolution in

XANES analysis

X-ray absorption L-edges correspond to the excitation of an electron from a core 2p level to a vacant d-orbital. The intensity of these so-called ‘white lines’ gives a measure of the extent to which the d-orbitals are vacant, and can be correlated with the real oxidation state of the absorbing atom [37]. Specifically, the difference between Pt L3 and L2 spectra yields information on the density of unoccupied states in the 5d5/2 band [30]. The ‘L3kL2’ method employed here minimises problems

Discussion

A significant outcome of our EXAFS studies is that we have found substantially different degrees of lattice contraction in giant ligand-stabilised clusters of different metals: 4% in Au55(PPh3)12Cl6, 1% in Pt309(phen*)36O30, and well under 1% in Pd561(phen)36O200. Re-refinement of the Au55(PPh3)12Cl6 data using GNXAS has confirmed the earlier results, which were obtained using EXCURV. All these studies have now accounted in the same way for the possibility of asymmetric bond length

Acknowledgements

For financial support, we thank the Royal Society, the EPSRC, the EU SCIENCE Plan and the EU HCM and TMR Programmes.

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    Dedicated to Professor Brian Johnson on the occasion of his 60th birthday, in recognition of his outstanding contributions to organometallic, inorganic and cluster chemistry.

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