Michael
Gatchell
*ab,
Marcelo
Goulart
a,
Lorenz
Kranabetter
a,
Martin
Kuhn
a,
Paul
Martini
a,
Bilal
Rasul
ac and
Paul
Scheier
a
aInstitut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria. E-mail: michael.gatchell@uibk.ac.at
bDepartment of Physics, Stockholm University, 106 91 Stockholm, Sweden. E-mail: gatchell@fysik.su.se
cDepartment of Physics, University of Sargodha, 40100 Sargodha, Pakistan
First published on 27th February 2018
We have studied complexes of gold atoms and imidazole (C3N2H4, abbreviated Im) produced in helium nanodroplets. Following the ionization of the doped droplets we detect a broad range of different AumImn+ complexes, however we find that for specific values of m certain n are “magic” and thus particularly abundant. Our density functional theory calculations indicate that these abundant clusters sizes are partially the result of particularly stable complexes, e.g. AuIm2+, and partially due to a transition in fragmentation patterns from the loss of neutral imidazole molecules for large systems to the loss of neutral gold atoms for smaller systems.
Clusters and nanoparticles of gold in particular have been shown to be remarkably good catalysts, debunking the previous assumption that gold had little chemical activity.10–12 Much of this focus has been on the roll of gold complexes in driving hydrogenation and oxidization reactions in organic chemistry.10–12 A category of such catalysts that have found interest recently are gold nanoclusters protected by organic ligands, such as Aum(SPh)n (Ph = C6H5) systems, where a core consisting of a Au84+ cluster is stabilized by the surrounding ligands.13,14 These types of complexes are used as catalysts for effective site-selective hydrogenation.13,14
Organometallic complexes first sparked significant interest with the discovery of the ferrocene in the 1950s.15–17 Work in this field has since continued with the interactions between metals and organic systems being deduced from studies of their interactions with individual molecular building blocks. However, relatively little work has been done investigating the interactions between gold—in particular gold clusters—and many groups of organic molecules. New results on this matter can help improve our understanding as to how gold interacts with biological systems and may also play a role in the development of new gold-based catalysts. This work is the first part of a series of studies we are carrying out to investigate the properties of gold nanoparticles in various chemical environments.
In this work we have studied complexes consisting of one or more gold atoms/ions and imidazole (C3N2H4, abbreviated Im) molecules. Imidazole is a simple nitrogen-containing heterocycle consisting of the pentagonal structure shown in Fig. 1. An aromatic molecule, imidazole forms the basis for a wide range of organic molecules such as the DNA bases guanine and adenine, histidines, histamines, and N-heterocyclic carbene (NHC) complexes to name a few.18 We produce neutral AumImn complexes in superfluid helium nanodroplets which are then ionized. Using mass spectrometry to study the AumImn+ products, we identify a number of seemingly “magic” combinations of m and n, i.e. structures that are particularly abundant compared to those with n ± 1 imidazole molecules for a given m Au atoms. We have also performed quantum chemical structure calculations to identify the origin of these abundant structures and find that certain structures, e.g. AuIm2+ and Au3Im3+, are indeed much more stable than systems with more imidazole molecules. We also find that clusters larger (i.e. containing more imidazole rings) than the most abundant sizes predominantly dissociate through the loss of neutral imidazole molecules, while at and below the these sizes (i.e. fewer imidazoles) they mainly lose one or more neutral gold atoms.
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Fig. 1 The molecular structure of imidazole. The labels next to each C and N atom are used to denote the locations of bond sites when discussing our results. |
The SCC-DFTB calculations were performed using the DFTB+ package (release 17.1)28 and the DFT calculations were run using the ORCA suite (version 4.0.1).29
Fig. 3 shows an overview of the integrated intensities of AumImn+ complexes from the mass spectrum in Fig. 2. In each of the panels of Fig. 3 (each number of Au atoms) we see that a specific number of imidazole molecules (n = 2 in the case of AuImn+) is significantly more abundant than the rest for a given gold cluster size. Complexes with a single Au atom seem to prefer to contain two imidazole molecules while those with 2 or 3 Au atoms preferably carry 3 imidazole molecules. Systems with 4 Au atoms stand out with two strong features coming from complexes with 3 or 4 imidazole molecules. Finally, the largest systems studied here, with 5 and 6 Au atoms, mainly contain 4 imidazole rings. Other combinations of gold and imidazole have significantly lower abundances, with broad distributions that could be remnants of the size distributions prior to ionization. There is also a trend that odd-numbered gold clusters (n = 1, 3, 5) show somewhat more prominent intensity maximas that the even-numbered clusters (n = 2, 4, 6) which could indicate that the standout odd-numbered structures are particularly stable. Due to the processes in which the cations are formed and the behavior of the mass spectra, we expect the “magic” sizes in each series to mainly be produced through the decay of larger counterparts (e.g. those with more imidazole molecules) instead of forming in the neutral droplets. It is thus difficult to directly compare the intensities of complex sizes above and below the maxima in each cluster series as this appears to be a bottle-neck in the decay process.
We have investigated the structures of AuImn+, where n = 1, 2, 3; Au2Imn+, where n = 1, 2, 3, 4; Au3Imn+, where n = 1, 2, 3, 4; and Au4Imn+, where n = 1, 2, 3, 4, 5, using DFT structure calculations in order to better understand the experimental results. We have performed calculations on several geometries for each cluster size, and the geometries that are shown here are those with the lowest energies and those that lie within 0.15 eV of the lowest energy structure (other than a few exceptions for educational purposes, e.g. highlighting specific AuImm geometries). The dissociation energy given with each structure in Fig. 4–7 is the lowest dissociation energy found for that cluster geometry. The corresponding dissociation pathways are discussed in the text and summarized in Fig. 9. The results of the calculations are covered in the next sections and a summary is given in the discussion.
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Fig. 4 Proposed structures of AuImn+, n = 1, 2, 3, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations. |
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Fig. 5 Proposed structures of Au2Imn+, n = 1, 2, 3, 4, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations. |
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Fig. 6 Proposed structures of Au3Imn+, n = 1, 2, 3, 4, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations. |
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Fig. 7 Proposed structures of Au4Imn+, n = 1, 2, 3, 4, 5, and lowest dissociation energies from M06/def2-TZVP//M06/def2-SVP calculations. |
This trend is repeated with the addition of a second imidazole molecule (structures (e) and (f) in Fig. 4). The second ring preferentially forms a bond via the N3 atom to the Au center, resulting in a dumbbell shaped ImAuIm+ structure (panel (e)). Alternative structures, like the one shown in panel (f), follow the same order in binding energies as in the case of AuIm+. Noteworthy is that the binding energy of the second imidazole ring is higher than that of the first one by a few tenths of an eV for the most stable structures.
The lowest energy AuIm3+ system found in our calculations is shown in panel (g) in Fig. 4. Compared to the first two rings, the third imidazole is only weakly bound to the Au core, in agreement with the experimental results where AuImn>2+ are much less abundant than AuIm2+ complexes.
The addition of a third imidazole ring (structure (k)) stabilizes the Au2Im3+ system relative to clusters with only two rings. The lowest energy structure that we have found forms a T-shape, with two rings attached to one Au atom and a single ring to the other. There are two competing dissociation pathways for this system separated by less than 0.1 eV, the Au2Im3+ → Au2Im2+ + Im and Au2Im3+ → AuIm2+ + AuIm, with the prior having the lower dissociation energy. We have identified a few different Au2Im4+ structures, all with the same properties and similar binding energies. The lowest energy isomer found is shown as structure (l). Here the fourth imidazole ring forms a weak bond at a N–H–N site, essentially a hydrogen bond, with one of the other rings. Other positions are possible too, however none results in the fourth imidazole ring forming a covalent bond with the Au2 core.
Like with the Au2Im3+ systems, Au3Im3+ (structure (o)) is stabilized by the presence of a third imidazole molecule resulting in as structure with near C3 symmetry (depending on the orientation of the imidazole rings). The lowest energy dissociation channel for Au3Im3+ is through the loss of a neutral imidazole ring resulting in Au3Im2+ as the charged fragment.
A proposed structure of Au3Im4+ is shown as panel (p) in Fig. 6. The fourth imidazole interacts here directly with the Au3 core, however it lowers the overall stability of the cluster such that the dissociation energy for losing a neutral imidazole ring is only about 0.4 eV.
Our calculations indicate that the Au4Im3+ structure strongly prefers the tetrahedron geometry ((u) in Fig. 7). Attempts to optimize Au4Im3+ with a rhombic Au4 substructure all result in the folding of the gold atoms into a tetrahedron. The most stable Au4Im3+ structure has a threefold symmetry similar to Au3Im3+ and the lowest energy dissociation pathway identified is the loss of a single neutral Au atom. For Au4Im4+ we identify both tetrahedron (v) and rhombic (w) geometries, with the prior being energetically preferred. The tetrahedron Au4Im4+ consists of two linear Au2Im2 elements arranged perpendicularly to each other with their axes offset by about 2.25 Å. The most stable Au4Im5+ complex we have identified is labeled (x) in Fig. 7. This consists of the basic tetrahedron Au4 core decorated with four imdazoles, structure (v), with the fifth ring forming a hydrogen bond with one of the NH sites of a neighboring ring. It is the loss of the fifth ring which has the lowest dissociation energy for this system. Similar structures are observed with a rhombic Au4 core, although these are less stable by at least a few tenths of an eV.
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Fig. 8 Lowest dissociation energies determined by our structure calculations at M06/def2-TZVP//M06/def2-SVP level of theory. The lines connecting the data points are to guide the reader. The corresponding charged fragments are given in Fig. 9. |
A contributing factor to the observed “magic” numbers in the experiments is likely the specific fragmentation patterns exhibited when the different systems dissociate. In Fig. 9 we present the charged products produced when each AumImn+ cluster size dissociates along its lowest energy pathway (disregarding potential reaction barriers). For large systems, with more imidazole rings than the observed maxima in each gold cluster series, the lowest energy dissociation pathway is consistently the loss of neutral, loosely bound imidazole molecules. After losing a sufficient number of imidazole molecules, systems that remain hot enough can then dissociate through the loss of neutral gold atoms, possibly together with one or more imidazole rings. This transition between losing neutral imidazole and neutral gold correlates well with the experimentally observed “magic” number of imidazole molecules for each of the four sizes of Aum clusters that we have studied theoretically. Thus in an assumed top-down decay process, the systems with lower numbers n of imidazole molecules in each of the panels of Fig. 3 will likely have been depleted by the loss of Au atoms, contributing among other pathways to the strong signals from AuIm2+ and Au3Im3+ that we detect.
An interesting feature that stands out in the experiments is the pair of Au4Im3+ and Au4Im4+, which are about equally abundant. It is not clear from the dissociation energies (Fig. 8) why this is the case, although some clue might be given by the structures in Fig. 7. We find no stable rhombic structure for Au4Im3+, although there is one for Au4Im4+. There could be some reaction barrier that stabilizes the rhombic Au4Im4+ → Au4Im3+ + Im dissociation pathway that gives the double peak, although this remains speculative.
The structures of AumImn+ that we have identified with out calculations are distinctively different than the sandwich and riceball structures found in studies of complexes consisting of transition metals and aromatic molecules like benzene31–36 and cyclopentadienyl.17 This distinction is perhaps most clear when comparing the dumbbell AuIm2+ (structure (e), Fig. 4) with, for example, ferrocene Fe(C5H5)2. Ferrocene is made up of an iron atom sandwiched between two cyclopentadienyl rings with Fe forming equally distributed η5 bonds with the carbon atoms in each molecule.17 Imidazole is essentially a cyclopentadiene molecule with two nonadjacent CH groups replaced with N and NH (N3 and N1 in Fig. 1), respectively, and it is the lone electron pair from the N3 site that gives the strongest bonds with gold. This preference is observed for all of the larger AumImn+ systems that we have studied here and is the reason why sandwiched structures are disfavored.
It is thus clear that gold and imidazole form strong chemical bonds, so strong that imidazole is able to break down smaller gold clusters. However, a complete theoretical picture of properties of these systems will require more advanced calculations than presented here, such as dynamical simulations, to properly include reaction barriers and the role of different cluster geometries, especially for larger systems. Nonetheless, our experimental results clearly show that different sizes of gold clusters preferentially bind specific numbers of imidazole molecules. The structures of these systems are given by our calculations, which also gives insight into the stabilities of these systems and the reasons for their abundance. It will be interesting to compare these results with others where imidazole is replaced by other molecular systems, insights that will improve our understanding of the chemical nature of gold nanoparticles.
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