Thesis
Reference
On the stereochemistry of atomically defined gold clusters: synthesis, size-selection and stereochemical characterization of
thiolate-protected gold clusters
KNOPPE, Stefan
Abstract
The thesis describes the synthesis of chiral thiolate-protected gold clusters. Au25(SR)18 clusters were protected with chiral thiolate ligands and the induced Cotton effects are ligand-dependent. A chromatography method was developed to isolate Au38(SR)24 and Au40(SR)24 clusters. Both clusters were separated into their enantiomers using HPLC. This predicts chirality for Au40(SR)24. The first CD spectra of intrinsically chiral gold clusters were measured and the activation barrier for racemization of Au38(SR)24 was determined. Ligand exchange experiments show that - using 1,1'-binaphthyl-2,2'-dithiol (BINAS) - the exchange is regio-selective. The data can be used to predict structural features in Au40(SR)24. The ligand exchange reaction between racemic Au38(SCH2CH2Ph)24 and R-BINAS shows a clear preference for the left-handed enantiomer of Au38. First experiments suggest a significant, but minor influence of the chiral ligand on the CD spectra and a drastically enhanced stability against thermally induced inversion compared to Au38(SCH2CH2Ph)24, making the BINAS-substituted Au38 cluster a suitable canditate for catalytic [...]
KNOPPE, Stefan. On the stereochemistry of atomically defined gold clusters:
synthesis, size-selection and stereochemical characterization of thiolate-protected gold clusters. Thèse de doctorat : Univ. Genève, 2012, no. Sc. 4491
URN : urn:nbn:ch:unige-263506
DOI : 10.13097/archive-ouverte/unige:26350
Available at:
http://archive-ouverte.unige.ch/unige:26350
Disclaimer: layout of this document may differ from the published version.
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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Section de chimie et biochimie
Département de chimie physique Professeur T. Bürgi
On the Stereochemistry of Atomically Defined Gold Clusters -
Synthesis, Size-Selection and Stereochemical Characterization of Thiolate-Protected Gold Clusters
THÈSE
présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention chimie
par
Stefan KNOPPE
de
Werl (Allemagne)
Thèse No 4491
GENÈVE
Atelier d'Impression ReproMail ; Uni Mail, Genève 2012
i
Acknowledgement
The present work would not have been possible without the valuable direct or indirect contribution of numerous people. First of all, I would like to thank my advisor Thomas Bürgi for his never-ending support and enthusiasm and motivating me to work on this fascinating field. His trust and patience created a free and stimulating environment. Thomas also wrote the Matlab routine that was used.
I would like to thank my jury members Hannu Häkkinen, Ulrich Heiz and Jerome Lacour for accepting this thesis and being board members.
A fundamental contribution to the success of this project has been made by Amal Dass and his group members at OleMiss. Amal and his students Asantha Dharmaratne, Praneeth Reddy Nimmala, Vijay Reddy Jupally and Nuwan Kothalawala have measured an incredible amount of MALDI and ESI spectra. The discussions and meetings with Amal have sharpened my view and opened new paths that we went in our projects. The detours have been worth it!
Julien Boudon at Université de Bourgogne measured some TEM images. Other collaborations and enlightening discussions with the groups of Hannu Häkkinen, Georg Mehl and Richard Palmer shall be mentioned as well.
Special gratitude goes to the students that I supervised over the past years: Florian Hinderer, Ella Schreiner, Alina Misiewicz and Cyril Etignard. Their help greatly contributed to the success of this project. I would like to thank Isabelle Garin for administrative help, Raymond Azoulay for the synthesis of the BINAS ligand, Dominique Lovy for his help with IT issues and Patric Barman for his assistance concerning mechanical problems. The NMR section is
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greatly acknowledged as well. Crucial contribution came from Stefan Matile and Naomi Sakai who provided and took care of the CD spectrometer.
I would like to thank all the group members for the good atmosphere, all the support in major and minor issues, for their help with problems in chemistry, physics, maths and discussions on science and apart from science. These are: Alastair Cunningham, Ahmed Bouhekka, Thomas Kriesche, Leo Pöttinger, Nils Salingue, Rita Garsuch, Florian Wölzl, Barbara Völker, Georg Kobiela, Igor Dolamic, Mahshid Chekini, Harekrishna Ghosh, Patric Oulevey, Noelia Barrabés, Andrea Seehuber, Samuele Lo Piano, Ugo Cataldi, Gerard Klein and Birte Varnholt. Special gratitude is owed to Igor for his help in the development of the HPLC method. His intuition and persistence have been crucial. In particular, I would like to thank Alastair, Thomas and Leo for all the activities and discussions that allowed us to free our minds from work. Thank you, Patric and Julien, for your help to translate the abstract into French!
In Heidelberg, there are numerous people to mention: Gabriele Fabry (administration), Dominic Riedel (discussions on ligand synthesis), Andres Jäschke and Matthias Mayer (CD spectrometer), Jürgen Graf (NMR). I am fairly sure that I have forgotten to mention hundreds of other people.
Last but not least, I would like to thank my parents, my sister and my brother for all the support over all the years, in all possible respect. This work is dedicated to them.
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Curriculum Vitae
Stefan Knoppe, Born October 8 1983, Werl (Germany)
Scientific Education
2003 – 2005: Chemistry (Diploma), Christian-Albrechts-Universität Kiel, Germany 2005 – 2009: Chemistry (Diploma), Ruprecht-Karls-Universität Heidelberg, Germany 2009 – 2010: PhD thesis, Institute for Physical Chemistry, Ruprecht-Karls-
Universität Heidelberg, Germany; Supervisor: Thomas Bürgi
2010 – 2012: PhD thesis (continued), Département de Chimie Physique, Université de Genève, Switzerland; Supervisor: Thomas Bürgi
Internships
2006: Institute for Organic Chemistry, Ruprecht-Karls-Universität Heidelberg, Supervisor: Janet Blümel
2006 - 2007: Institute for Inorganic Chemistry, Ruprecht-Karls-Universität Heidelberg, Supervisor: Peter Comba
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2008: Institute for Physical Chemistry, Ruprecht-Karls-Universität Heidelberg, Supervisor: Joachim Spatz
Conferences
08/2010: 240th Fall Meeting of the American Chemical Society, Boston, United States (Oral presentation)
09/2010: Research Seminar of the Department of Chemistry and Biochemistry, University of Mississippi, United States (Oral Presentation)
06/2011: International Symposium on Monolayer-protected Clusters, University of Jyväskylä, Finland (Poster Presentation)
07/2011: 23rd International Symposium on Chiral Discrimination – Chirality 2011, Liverpool, United Kingdom (Oral Presentation)
07/2011: 13th International Conference on Chiroptical Spectroscopy, Oxford, United Kingdom (Oral Presentation)
01/2012: Geneva Chemistry Days 2012, Geneva, Switzerland (Oral Presentation) 07/2012: 16th International Symposium on Small Particles and Inorganic
Clusters, Leuven, Belgium (Poster Presentation)
09/2012: SCS Fall Meeting 2012, Zürich, Switzerland (Oral Presentation) Metrohm Award for the Best Oral Presentation in Inorganic Chemistry
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Publications
Publications related to the thesis
1) S. Knoppe, A. C. Dharmaratne, E. Schreiner, A. Dass, T. Bürgi, J. Am. Chem. Soc. 2010, 132, 16783 – 16789.
2) S. Knoppe, J. Boudon, I. Dolamic, A. Dass, T. Bürgi, Anal. Chem. 2011, 81, 5056 – 5061.
3) S. Knoppe, N. Kothalawala, V. R. Jupally, A. Dass, T. Bürgi, Chem. Commun. 2012, 48, 4630 – 4632.
4) I. Dolamic, S. Knoppe, A. Dass, T. Bürgi, Nat. Commun. 2012, 3, 798.
5) S. Knoppe, A. Dass, T. Bürgi, Nanoscale 2012, 4, 4211 – 4216.
6) S. Knoppe, I. Dolamic, A. Dass, T. Bürgi, Angew. Chem. Int. Ed. 2012, 51, 7589 – 7591.
7) S. Knoppe, I. Dolamic, T. Bürgi, J. Am. Chem. Soc. 2012, 134, 13114 – 13120.
8) S. Knoppe, R. Azoulay, A. Dass, T. Bürgi, submitted.
Other Publications:
9) P. Comba, S. Knoppe, B. Martin, G. Rajaraman, C. Rolli, B. Shapiro, T. Storck, Chem.
Eur. J. 2007, 14, 344 – 357.
10) D. Aydin, M. Schwieder, I. Louban, S. Knoppe, J. Ulmer, T. L. Haas, H. Walczak, J. P.
Spatz, Small 2009, 5, 1014 – 1018.
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Résumé
Depuis la première étude de chiralité de surfaces métalliques étendues, la chiralité des métaux est un centre d'intérêt croissant. Des surfaces métalliques peuvent montrer une chiralité soit intrinsèque soit induite ; ceci est dû aux sites intrinsèquement chiraux ou induits par liaison d'adsorbants (chiraux). Leur utilité pour des applications comme sondes et en catalyse est à l’étude.
Les nanoparticules sont développées comme un système modèle pour les molécules chirales adsorbées sur les surfaces métalliques ; le rapport surface/volume augmente quand la taille de particule diminue – ce qui permet d'étudier un nombre plus élevé de molécules d'adsorbant par atome de métal. La solubilité des nanoparticules a comme avantage de permettre l'utilisation de méthodes comme la spectroscopie de dichroïsme circulaire (CD).
Ces méthodes sont moins contraignantes comparées aux techniques sous ultravide et de microscopie à balayage.
D’autre part, une réduction de la taille des structures métalliques influence fortement leurs propriétés. D'après la théorie de Mie, les résonances du plasmon de surface se propageant dans un métal subissent un décalage vers le visible dès que la taille de particule est réduite. Les résonances de plasmon deviennent restreintes et localisées. Ce fait peut être observé avec la fameuse couleur rouge des nanoparticules d'or pour une plage de tailles comprises entre 3 et 100 nm. Si la taille est réduite davantage, à une valeur d'environ 2 nm (qui correspond à peu près à 200 atomes d'or) ou inférieure, la résonance de plasmon disparaît au profit d’un comportement moléculaire. Ces nanoparticules ultra compactes sont surnommées « clusters » et leurs propriétés ont été examinées en phase gazeuse et sur des
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supports. Les propriétés ne peuvent pas être quantifiées (par exemple : les propriétés optiques, les structures électroniques, la forme, la stabilité, l’activité catalytique et ainsi de suite).
Les clusters métalliques stabilisés par des ligands connaissent un intérêt particulier puisqu'ils peuvent être examinés en solution et leur préparation est basée sur l'approche classique de « chimie par voie humide » avec, par exemple, la réduction d'un sel métallique en présence de ligands protecteurs. L'adsorption de ligands sur la surface de clusters augmente leur stabilité grâce à l'agrégation évitée et elle permet également d'introduire de nouvelles fonctionnalités en modifiant la couche de ligands. Dans le cas de l'or, des thiolates peuvent être utilisés comme une procédure simple pour former des clusters stables. En ce qui concerne ce travail, des thiolates chiraux sont utilisés pour conférer une chiralité aux clusters d'or et des réponses intenses peuvent être obtenues dans les spectres de dichroïsme circulaire. Des caractéristiques de ces spectres peuvent être attribuées aux transitions électroniques liées à l'or. La diffraction de rayons X d'un monocristal a dévoilé que les clusters d'or protégés par des thiolates peuvent présenter des propriétés intrinsèquement chirales. Les ligands sont organisés d'une manière chirale mais la cellule unitaire de clusters est racémique (puisque des ligands achiraux ont été utilisés).
Ce travail couvre trois questionnements principaux : 1) La quantité de ligands thiolates chiraux est faible et l'influence de ces ligands sur l'effet Cotton des clusters n'a pas été étudiée en détail à cause d’un manque de composés. La préparation de clusters d'or à l'échelle atomique constitués de ligands chiraux différents est un point-clé. 2) Est-ce possible de séparer les énantiomères de clusters intrinsèquement chiraux protégés par des ligands achiraux ? Si c'est le cas, la mesure de leurs spectres CD et la détermination de la barrière d'activation de la conversion entre énantiomères est d'un très grand intérêt. 3) Quelle est l'influence d'un ligand chiral sur le spectre CD d'un cluster d'or ? La réaction d'échange de
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ligands thiolates peut mener à une réponse en commençant avec les clusters protégés par des ligands achiraux et par l'introduction de ligands chiraux.
Au départ, une méthode pour la préparation et l'isolement de clusters d'or de différentes tailles protégés par des thiolates a été développée. Alors que la synthèse des clusters a été adaptée à partir de protocoles publiés, la chromatographie d’exclusion stérique (SEC) a été mise en œuvre pour l'isolement efficace de clusters de haute monodispersité et en grande quantité. La synthèse et l'isolement des clusters Au25(SR)18, Au38(SR)24 et Au40(SR)24 (SR : thiolate) est décrite dans le chapitre B-II. Dans le cas de Au25, une certaine dépendance des ligands est observée dans la distribution de taille des produits si le protocole qui est sensé amener à des clusters monodisperses avec une diversité de thiols est appliqué.
Les propriétés chiroptiques des clusters Au25(SR)18 protégés par une nouvelle classe de ligands sont décrites dans chapitre B-III. En utilisant le camphorthiol, on obtient des spectres CD qui ont une forte ressemblance à ceux des clusters Au25 protégés par le glutathionate, mais qui sont, par contre, clairement différents de ceux obtenus à partir de Au25 protégé par le 2- methyl-2-phenylethylthiolate. La chromatographie liquide de haute performance (HPLC) chirale a été utilisée pour séparer les énantiomères des clusters Au38(SCH2CH2Ph)24 intrinsèquement chiraux (chapitre B-III). Ces clusters sont suffisamment stables pour mesurer leurs spectres CD qui sont en très bon accord avec les spectres calculés et publiés auparavant : cela permet d’attribuer de la chiralité aux clusters. L’HPLC chirale a été utilisée pour déterminer la chiralité intrinsèque du cluster Au40(SCH2CH2Ph)24. Il est noté que la réponse chiroptique des clusters intrinsèquement chiraux est d'environ un ordre de magnitude plus élevé que la réponse des clusters protégés avec des thiolates chiraux (mais qui ne sont pas intrinsèquement chiraux). La racémisation induite thermiquement du cluster Au38(SCH2CH2Ph)24 énantiopure a été suivie par CD et sa barrière d'activation a été déterminée comme remarquablement basse, environ 28 kcal/mol. Les réactions d'échange de
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ligands pour les clusters racémiques Au38(SCH2CH2Ph)24 et Au40(SCH2CH2Ph)24 ont été réalisées et analysées par spectrométrie de masse (Matrix-assisted Laser Desorption/Ionization, MALDI) et CD (chapitre B-IV). La méthode MALDI permet d'identifier les produits de réaction avec une résolution au niveau de la population de ligands.
Un échange limité est observé lorsque le 1,1’-binaphthyl-2,2’-dithiol (BINAS) énantiopure est utilisé. Cela peut être assimilé à un échange spécifique aux sites présentant une courte chaîne de protection (SR-Au-SR). La comparaison de la population de ligands et de l'activité optique augmentée mène à des courbes non linéaires qui sont interprétées comme le résultat d'échanges diastéréosélectifs (un énantiomère du cluster réagit plus vite avec un énantiomère de BINAS donné). Cette dernière supposition a été confirmée par un suivi in situ de la réaction d'échange entre rac-Au38(SCH2CH2Ph)24 et R-BINAS qui provoque une préférence du BINAS par rapport à l'énantiomère gauche de Au38. De plus, la deuxième étape d'échange présente un taux de réaction considérablement plus bas (environ six fois plus faible) : le cluster est désactivé par l’introduction d'un ligand BINAS. Les produits de la réaction diastéréomérique sont collectés et leurs spectres CD ont suggéré que l'influence du ligand BINAS sur la forme spectrale et sur le signe est conséquente, mais la chiralité de l'énantiomère initial peut tout de même être attribuée. Les tests initiaux sur la barrière d'activation pour la conversion de produits diastéréomériques montrent que le ligand BINAS plutôt rigide a tendance à supprimer la flexibilité de l'interface or-thiolate. Par contre, le camphorthiol monodenté ne mène pas à un échange limité et l'activité optique résultante est plutôt faible.
En conclusion, le travail a démontré que la chiralité intrinsèque de clusters d'or protégés par des thiolates mène à des réponses chiroptiques fortes dues à l'arrangement des ligands de protection. Des ligands chiraux semblent avoir très peu d'influence sur les spectres CD de ce type de clusters. Si des clusters sans chiralité intrinsèque sont étudiés, des ligands chiraux
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induisent un spectre CD aux clusters, mais ils sont d'environ un ordre de magnitude plus faible. Les clusters intrinsèquement chiraux ont une faible barrière d'activation pour leur racémisation, ce qui démontre la flexibilité de l'interface or-thiolate. La configuration des clusters d'or intrinsèquement chiraux peut être « fixée » en utilisant des thiols chiraux rigides et bidentés.
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Summary
Chirality of metals has been an ever-increasing field of interest, since the first studies on the chirality of extended metal surfaces have been carried out. Metal surfaces can show either intrinsic or induced chirality, due to chiral kink sites (intrinsic) or adsorption of (chiral) adsorbates (induced). Their use in sensing applications and catalysis is under investigation.
Nanoparticles began to evolve as model systems for adsorbed chiral molecules on metal surfaces; the surface-to-volume ratio increases with decreasing particle size and more adsorbate molecules per metal atom can be studied. A further advantage is the solubility of the nanoparticles, allowing the use of techniques such as Circular Dichroism (CD) spectroscopy. These techniques afford less effort compared to ultra high vacuum and scanning microscopy techniques.
On the other hand, the reduction of size in a metal structure has strong implications on its properties. According to standard Mie theory, the propagating surface plasmon resonances of a bulk metal shift into the visible when the particle size is decreased. The plasmon resonances become confined and localized. This is manifested in the famous red color of gold nanoparticles in the size regime of ca 3 – 100 nm. At even smaller sizes, the plasmon resonances collapse and molecular behavior is found. A standard value is about 2 nm in diameter (corresponding to ca 200 gold atoms). These ultrasmall nanoparticles are commonly termed ‘clusters’ and their properties have been investigated in the gas phase and on supports.
The properties are non-scalable (e.g. optical properties, electronic structures, shape, stability, catalytic activity, etc).
Ligand-stabilized metal clusters have gained interest since they can be studied in solution and their preparation is based on classic ‘wet chemistry’, e.g. reduction of a metal salt in the
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presence of the protecting ligand. The adsorption of ligand on the surface of the clusters increases their stability due to prevention of aggregation and allows for the introduction of functionalities by manipulation of the ligand layer. In the case of gold, thiolates have been found to form stable clusters and the preparation is easily achieved. In the scope of this work, chiral thiolates can be used to bestow chirality to gold clusters, which leads to strong responses in the CD spectra. The spectra show features that are ascribed to gold-related electronic transitions. X-ray diffraction of single crystals also showed that thiolate-protected gold clusters can exhibit intrinsically chiral features. The ligands are arranged in a chiral fashion and – since achiral ligands have been used – the unit cell of the clusters is racemic.
This work covers three main issues: 1) the pool of chiral thiolate ligands is small and the influence of the ligands on the Cotton effects of the clusters has not been studied in detail due to lack of compounds. A key issue is the preparation of atomically defined gold clusters that bear different chiral ligands. 2) Is it possible to separate the enantiomers of intrinsically chiral clusters that are protected with achiral ligands? If so, the measurement of their CD spectra and determination of the activation barrier of the interconversion between the enantiomers is of great interest. 3) What is the influence of a chiral ligand on the CD spectrum of a gold cluster?
Thiolate-for-thiolate ligand exchange reactions starting from clusters protected with achiral ligands and introduction of chiral ligands may help in answering this question.
At first, a method for the preparation and isolation of thiolate-protected gold clusters of various sizes had to be developed. Whereas the synthesis of the clusters was adapted from published protocols, Size-Exclusion Chromatography (SEC) was found to be an effective way to isolate clusters at high monodispersity and in greater amounts. The synthesis and isolation of Au25(SR)18, Au38(SR)24 and Au40(SR)24 (SR: thiolate) clusters is described in Chapter B-II.
In the case of Au25, a certain ligand-dependence in the size-distribution of the products is
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found when applying a protocol that claims to lead to monodisperse clusters with a variety of thiols.
The chiroptical properties of Au25(SR)18 clusters protected with a new class of chiral ligands are described in Chapter B-III. The use of camphorthiol leads to CD spectra that bear great resemblance to those of glutathionate-protected Au25 clusters, but are distinctively different from those of 2-methyl-2-phenylethylthiolate-protected Au25. Chiral High Performance Liquid Chromatography (HPLC) was used to separate the enantiomers of the intrinsically chiral Au38(SCH2CH2Ph)24 cluster (Chapter B-II). The clusters are stable enough to measure their CD spectra. The spectra are in very good agreement with computed ones published earlier, which enabled the assignment of the handedness of the cluster. The HPLC method was used to predict intrinsic chirality for Au40(SCH2CH2Ph)24 clusters. Of note, the chiroptical response of intrinsically chiral clusters is about one order of magnitude stronger than of clusters that are protected with chiral thiolates (but are not intrinsically chiral).
Thermally induced racemization of enantiopure Au38(SCH2CH2Ph)24 was monitored with CD and the activation barrier was found to be remarkably low (ca 28 kcal/mol). Ligand exchange reactions were performed on racemic Au38(SCH2CH2Ph)24 and Au40(SCH2CH2Ph)24 clusters and analyzed with mass spectrometry (Matrix-assisted Laser Desorption/Ionization, MALDI) and CD (Chapter B-IV). MALDI allows for the identification of the reaction products with resolution of the ligand population. If enantiopure 1,1’-binaphthyl-2,2’-dithiol (BINAS) is used, limited exchange is observed. This is ascribed to a site-specific exchange with short protecting units (SR-Au-SR). Comparison of the ligand population with the arising optical activity leads to nonlinear curves, which is interpreted as a result of diastereoselective exchanges (one enantiomer of the cluster reacts faster with a given enantiomer of BINAS).
The latter assumption was confirmed by in situ reaction monitoring of the exchange reaction between rac-Au38(SCH2CH2Ph)24 and R-BINAS, which gives rise to a preference of R-
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BINAS towards the left-handed enantiomer of Au38. Furthermore, the second exchange step has a considerably lower (about six times) reaction rate; the cluster is deactivated by introduction of one BINAS ligand. The diastereomeric reaction products were collected and their CD spectra suggest that influence of the BINAS ligand on the spectral shape and sign is significant, but the handedness of the parent enantiomer can still be ascribed. Initial tests on the activation barrier for the interconversion of the diastereomeric products show that the rather rigid BINAS ligand seems to suppress the flexibility of the gold-thiolate interface. In contrast, monodentate camphorthiol does not lead to limited exchange and the resulting optical activity is rather weak.
In summary, it was shown that intrinsic chirality of thiolate-protected gold clusters (due to arrangement of the protecting ligands) leads to strong chiroptical responses. Chiral ligands seem to have minor influence on the CD spectra of such clusters. If clusters without intrinsic chirality are studied, chiral ligands induce a CD spectrum to the cluster, but the resulting spectra are weaker (by about one order of magnitude). The intrinsically chiral clusters have low activation barriers for their racemization, which demonstrates the flexibility of the gold- thiolate interface. The configuration of intrinsically chiral gold clusters can be ‘locked’ by using rigid, bidentate chiral thiols.
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Table of Contents
I. Introduction: An Asymmetric Third Dimension in the Periodic Table ... 1
A. Theoretical Part ... 4
I. Gold ... 5
II. Gold Clusters ... 7
III. Chirality ... 12
1) Circular Dichroism ... 12
2) Chirality in Gold Clusters ... 16
IV. Motivation ... 24
B. Results and Discussion ... 29
I. General Remarks ... 30
II. Synthesis and Size-Selection of Thiolate-Protected Gold Clusters ... 31
1) One-Phase Synthesis of Au25(SR)18 Clusters ... 35
2) Thermal Etching Towards Au38(2-PET)24 and Au40(2-PET)24 ... 43
3) Conclusions and Outlook ... 51
III. Chiroptical Properties of Gold Clusters ... 53
1) Chiral ligands: Camphor-10-thiol and 1-Phenylethylthiol... 56
2) Intrinsic Chirality: Au38(2-PET)24 and Au40(2-PET)24 ... 61
3) Racemization Barrier of Au38(2-PET)24 ... 69
4) Conclusion and Outlook ... 80
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IV. Ligand Exchange Reactions ... 83
1) Au38(2-PET)24/Au40(2-PET)24 and BINAS ... 86
2) Size-selected Au38(2-PET)24 and Au40(2-PET)24 with BINAS and CamSH ... 95
3) HPLC Separation and Kinetics of the Ligand Exchange between Au38(2-PET)24 and R-BINAS ... 104
4) Conclusions ... 119
V. General Conclusion and Outlook ... 122
C. Experimental ... 129
I. Materials and Methods ... 130
1) Materials ... 130
2) Methods ... 133
II. Synthesis of Ligands ... 136
1) Camphor-10-thiol150 ... 136
2) 1-Phenylethanethiol ... 138
III. Synthesis of Clusters ... 141
1) Synthesis of Au38(2-PET)24 and Au40(2-PET)24 ... 141
2) Racemization of Au38(2-PET)24 ... 143
3) Synthesis of Au25(CamS)18148 ... 144
4) Synthesis of Aum(1-PET)n... 145
IV. Ligand Exchange Reactions ... 146
1) Au25(2-PET)18 and BINAS ... 146
2) Au38(2-PET)24/Au40(2-PET)24 and BINAS ... 146
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3) Au38(2-PET)24/Au40(2-PET)24 and Thiophenol ... 147
4) Au38(2-PET)24/Au40(2-PET)24 and BINAS ... 147
5) Au38(2-PET)24/Au40(2-PET)24 and Camphorthiol ... 147
V. HPLC separation of Au38(2-PET)24 and Au40(2-PET)24 ... 148
1) Au38(2-PET)24 ... 148
2) Au40(2-PET)24 ... 148
3) in situ-monitoring of the ligand exchange between rac-Au38(2-PET)24 and R- BINAS ... 148 D. Appendix ... A I. Spectra ... B 1) 1-Phenylethylthiol ... B 2) Camphorthiol ... G II. Program Code of the Matlab Routine used for the Kinetic Fits of the Ligand Exchange Reaction between Au38(2-PET)24 and R-BINAS ... K III. References ... P
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Abbrevations
BINAS 1,1’-binaphthyl-2,2’-dithiol
BINOL 1,1’-binaphthyl-2,2’-diol
CamSH Camphor-10-thiol
CD Circular Dichroism
DCTB
trans-2-[3-(4-tert-butylphenyl)-2-methyl-2- propenylidene]malononitrile
ee Enantiomeric Excess
ESI Electrospray Ionization
FT Fourier Transformation
HPLC High Performance Liquid Chromatography
LSPR Localized Surface Plasmon Resonance
MALDI Matrix-assisted Laser Desorption/Ionization
p-MBA para-mercaptobenzoic acid
(N)IR (Near) Infrared
1-PET, 2-PET 1-phenylethylthiol, 2-phenylethylthiol
pet* 2-methyl-2-phenylethylthiol(ate)
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PTFE Polytetrafluoroethylene
SACM Superatom Complex Model
SAM Self-Assembled Monolayer
SEC Size-Exclusion Chromatography
SG glutathionate
SR thiolate
TEM Transmission Electron Microscopy
THF tetrahydrofuran
UV Ultraviolet
VCD Vibrational Circular Dichroism
Vis visible
Introduction
1
I. Introduction: An Asymmetric Third Dimension in the Periodic Table
‘[…] A very small - but I think important! - control of nanostructures: making them asymmetric. Symmetry [...] is what chemistry gives you. Destroying the symmetry of the structures in a controlled manner is an interesting challenge.’
Gil Markovich (introducing his talk at the 6th Workshop of the Center for Nanoscience and Nanotechnology, Tel Aviv University, 2010)a
The research on metal clusters Mx has always been accompanied with the idea of adding a
‘third dimension’ to the periodic table.1,2 It has been observed very early that several sizes in a mixture of metal clusters are more abundant and stable than others.3 As both theory and experiments advanced in accuracy, these stable clusters were termed ‘superatoms’,4 since these assemblies of atoms rather behave as one single atom (with new properties) than a molecule or a small piece of the bulk metal. The identification of superatoms for different elements and exploration of their properties was connected with the idea to integrate a series of superatoms of a given element into Mendeleev’s periodic system of the elements (PSE). As in the PSE, these superatoms would allow for predicting trends, and the discovery of new superatoms may lead to the design of materials with properties that cannot be created by using single atoms, molecules or even the bulk.
But not only bare metal clusters Mx can be understood as superatoms. Certain sizes of ligand- protected clusters [Mx(L)y] were found to be more stable than others as well.5 In particular,
a The talk can be accessed online at www.youtube.com/watch?v=l9LN0oDK4OA; October 17 2012.
Introduction
2
thiolate-protected gold clusters (with the general formula [Aux(SR)y]z; z: charge) aroused significant interest, because the affinity of sulfur towards gold allows for the preparation of stable clusters that can be studied with a variety of analytical techniques. Just as in bare clusters, the stability of certain cluster sizes can be explained by the formation of superatoms.6 These ‘superatom complexes’ behave similar to complex compounds of metal atoms and coordinating ligands, which are well-known to each chemist. Moreover, it was shown that some of the clustersb (as to date the majority of clusters with known structures, actually) contain inherently chiral features.7,8 Chirality is the absence of symmetryc in an object and is of paramount importance in biology and thus in drug design, sensing and organic synthesis and all related fields. An object is chiral if it cannot be superimposed with its mirror image.
The object and its mirror image are called enantiomers. The inherent chirality of gold clusters does not require the presence of chiral ligands that induce the property. The use of achiral ligands leads to the formation of racemates; that is a 1:1 mixture of the enantiomers.
However, the use of chiral ligands to stabilize the clusters is another approach to bestow chirality onto gold clusters.9 Both strategies allow for the preparation of ‘asymmetric superatoms’, a completely new class of superatoms. It should be stressed that the asymmetry is a consequence of the interplay between the cluster and the protecting ligands. Removal of the ligands leads to loss of chirality.
Single atoms cannot be asymmetric (since they are spherical) and the idea of ‘asymmetric superatoms’ as building blocks in novel materials may lead to manifold properties (e.g. in nonlinear optics) that cannot be achieved in a classical way. The field is young and a lot has to be done. In the following, it will be demonstrated how (racemic) asymmetric superatoms can be split into their enantiomers, how they can be transformed into each other (which equals the
b In this work, the word ‘cluster’ shall usually refer to either bare gold clusters or thiolate-protected gold clusters.
This, of course, depends on the context.
c More precisely: the absence of symmetry elements of higher order.
Introduction
3
loss of chiral information) and how they can be stabilized against this interconversion. These questions are of fundamental interest and hopefully contribute to future developments of the field.
The work is divided into several parts: In Part A (Theory), the underlying concepts will be briefly discussed. This includes the stability and structures as well as (potential) applications of thiolate-protected gold clusters, chirality (and its experimental observation) in general and the chirality of gold clusters in detail. In Part B, the results of the project are presented and discussed. This is followed by an experimental part (Part C).
Stefan Knoppe
Geneva, November 2012
A. Theoretical Part
Theoretical Part
5
I. Gold
Gold is the 79th element in the periodic table and as such a transition metal.10 It belongs to the 6th period in the table and to the 11th group; the ground state configuration is [Xe] 4f14 5d10 6s1. It is usually associated with chemical inertness (E0(Au/Au3+) = +1.48 V vs. standard hydrogen electrode). The chemical inertness and its yellow color (in bulk) are ascribed to strong relativistic effects. The effect leads to a contraction (stabilization) of the 6s orbital (direct relativistic effect) and expansion of the 5d orbitals (indirect relativistic effect). The result is an interband gap that absorbs blue light (absorption onset at 2.4 eV,11 calculated 2.38 eV12) and leads to the distinct yellow color of metallic gold.11-14 Without consideration of relativistic effects, calculations on the color of gold would predict a silverish appearance. Due to the high mass of the nucleus of the gold atom, the effect is much stronger than for silver (the corresponding interband gap is 3.7 eV). Another consequence is the high ionization potential of gold and the chemical inertness associated to it (IP = 9.22 eV for a single Au atom).
Nevertheless, in the recent years, gold has gained attention as a catalyst in both homogeneous15-17 and heterogeneous18-24 catalysis, bio applications,25-28 sensing29-33 and in metamaterials.34-38 Gold surfaces and Self-Assembled Monolayers (SAMs) of thiolates on gold surfaces are another field that attracts significant interest.39-41 Gold nanoparticles combine the properties of SAMs with interesting optical properties (such as plasmon resonances) and allow studying these in solution.42-44 The Localized Surface Plasmon Resonance (LSPR), which is responsible for the famous red color of gold nanoparticles,45 can be described by application of Mie theory.46,47 Plasmon resonances are propagating waves of surface electrons that are stimulated by incident light. If the dimension of the object is below
Theoretical Part
6
the wavelength of the incident light, the plasmon resonance becomes localized and can be described as a collective oscillation of electrons in the particle. In order to be excited by visible light, the object has to have dimensions of 100 nm or below. For gold, a typical extinction maximum is found around 520 nm (Figure 1, left). The exact properties depend on size and shape of the particle and the local refractive index.47,48 When reducing the system size of the nanoparticles, the LSPR collapses and the nanoparticle shows molecular behavior due to quantum confinement (Figure 2, right; see below).5,49 It is generally believed that a particle diameter of about 2 nm in diameter is the threshold.50 Very recently, Dass and Jin reported (independently) on nanoparticles consisting of about 300 gold atoms that show LSPRs.51,52 On the other hand, it is known that particles consisting of ca 144 gold atoms do not exhibit plasmon resonances.a A good rule of thumb seems transition from molecular to plasmonic behavior in the range of 200 gold atoms.49 Nanoparticles that show molecular behavior are often termed ‘clusters’.
Figure 1. Absorption spectra of a plasmonic gold nanoparticle (20 nm in diameter, left) and Au38(SCH2CH2Ph)24 (right).
a In fact, a very recent article suggests the presence of a plasmon resonance in Au144(SR)60 clusters (Philip et al., Nano Lett. 2012, 12, 4661).
Theoretical Part
7
II. Gold Clusters
Clusters are defined as metal compounds that show direct bonds between at least two atoms of the same metal.53 By definition, the smallest cluster of a metal M is its dimer M2. For several reasons, the study of cluster properties is of fundamental interest. For instance, cluster properties are strongly size-dependent and non-scalable (e.g., stability, shape, ionization potential, electron affinity, magnetism, even-odd-alternations…) for very small systems (up to several hundred atoms).54 The identification of threshold sizes for the emergence of the metallic state is enlightening, whereas the size-dependency allows for tailoring of materials with novel properties that could be of use in sensing55,56 and catalytic applications.57,58
Clusters are usually studied in the gas phase (or deposited on supports) and size-selection is achieved in mass spectrometers.54 It was found that certain sizes of a metal cluster are more stable than others.3 Two main reasons were identified for this: i) geometric and ii) electronic.
Concerning geometric effects, the majority of metals adopt either a fcc or bcc bulk structure.
A commonly found motif in clusters is the icosahedron, which fits into the fcc lattice (but – due to its five-fold symmetry – is not cut out of the bulk).59 It is intuitive to assume metals to arrange in spheroidal geometries in order to minimize surface effects. Closing geometric shells of icosahedrons or cuboctahedrons (the series for both structures obeys the formula:
N(k) = 1/3(10k3 – 15k2 + 11k -3), in which N is the number of atoms in the cluster and k the shell closing number) leads to stable structures.60 However, not only geometric but also electronic effects have to be taken into account. For instance, a gold atom donates only its 6s1 electron to the valence shell; the other electrons are localized to the nucleus of the atom. A cluster Aun is then composed of a delocalized fermion gas composed of n electrons within the potential of the atomic nuclei.61,62 The Aufbau principle for such systems is
Theoretical Part
8
1S2 | 1P6 | 1D10 | 2S2 1F14 | 2P6 1G18 | 2D10 3S2 1H22 | …, (| denoting shell closing) with special stability associated to n = 2, 8, 18, 34, 58, 92, …(‘magic numbers’). These clusters are often termed as ‘superatoms’.b Depending on the potential in which the electrons are confined, also 20 and 40 are considered as magic numbers.62 With increasing size of the clusters, the discrete states move closer in energy and a transition from confined energy states towards bands is observed (Figure 1).
Figure 1. Degeneracy of states and their energies for electrons in a spherical potential. Each bar represents one energy level. Reprinted with permission from Martin et al., J. Phys. Chem. 1991, 95, 6421. Copyright 1991 American Chemical Society.
At very large cluster sizes, the semiconducting particles become metallic. This is also observed in the optical properties, which have strongly size-dependent features that converge into the emergence of surface plasmon resonances.63 At cluster sizes that do not obey magic
b Gold forms, in contrast to other metals, a monoanion Au-. In terms of the superatom model, this can be considered as a ‘minicluster’ with an electron count of 2 (which is a magic number).
Theoretical Part
9
numbers, other shapes than spherical are expected.62 However, with increasing cluster size (hundreds to thousands of atoms), the geometric shell closing overrules the electronic shell closing.60 The properties of metal clusters are usually studied in the gas phase or the clusters are deposited onto solid supports (especially for catalytic applications). Heterogeneous catalysts composed of clusters and (mainly) oxidic supports have been studied extensively. In general, it was found that the catalytic activity strongly depends on the size of the clusters.64-66 The stability of gold clusters can be drastically enhanced by protection of the clusters with suitable ligands, such as thiolates,50,67-69 phosphines,70-74 selenolates75-77 or amines.78 The method allows preparing the clusters in greater amounts (10 – 100 mg scale) and studying their properties in solution. Ligand-protection prevents the clusters from aggregation and manipulation of the ligands can introduce functionalities, such as binding of liquid crystalline molecules,79-82 chirality,9,71,83,84 or coordination of other metals.85,86 The introduction of functionalities can be achieved in either a direct synthesis or in post-synthetic processes (ligand exchange).82 In the latter case, it should be verified that the cluster does not decompose under the applied conditions.87,88 Depending on the ligand type, the clusters have different solubilities as well (of which water-soluble ligands are of great interest for bio- applications).
Concerning thiolate-protected gold clusters, a breakthrough was made in 1995 when Brust et al. presented the first large-scale method for their preparation.67 The molecular behavior of the clusters was recognized in 1996 by Whetten et al.5 It was observed very soon,5 that not all possible combinations (x, y, z) in a mixture of clusters with the composition [Aux(SR)y]z (z:
charge) are formed.69 A comprehensive explanation of this can be made with the ‘Superatom Complex Model’ (SACM).6 Briefly, each gold atom donates its 6s electron to the cluster (as in unprotected clusters). In addition, the protecting ligands are usually assumed to withdraw
Theoretical Part
10
one electron per ligand. Charges have to be considered as well, e.g. the Au25(SR)18 cluster is stable as an anion.89-91 The modified electron count NSAC for a cluster [Aux(L)y]z is:
Eqn. 1
Exceptional stability is ascribed to NSAC = 2, 8, 18, 34, 58, 92, … The validity of the concept has been demonstrated for a number of clusters such as [Au25(SR)18]-,92 Au102(SR)44,6 Au144(SR)60 (the structure of this cluster is not confirmed experimentally)93 and for phosphine-stabilized clusters such as Au11(PR3)7(SR)3, [Au13(PR3)10X2]3+ and [Au39(PR3)14X6]- and rod-shaped [Au25(PR3)10(SR)5X2]2+ (X: halide anion, note that neutral ligands such as PR3 do not contribute to the electron count).6,94,95 It can also be applied to explain the stability of group 13 metal clusters.96-99
Figure 2. Left: Crystal structure of TOA[Au25(2-PET)18]. The tetraoctylammonium (TOA) cation and the CH2CH2Ph groups are not shown for clarity. Reprinted with permission from Heaven et al., J. Am. Chem. Soc.
2008, 130, 3754 - 3755. Copyright 2008 American Chemical Society. Right: Density-of-States analysis of the structure. The HOMO-LUMO gap is about 1.25 eV and the transition from P- (HOMO) to D-symmetry (LUMO) is indicated by a color change. Reprinted with permission from Akola et al., J. Am. Chem. Soc. 2008, 130, 3756 - 3757. Copyright 2008 American Chemical Society.
Theoretical Part
11
As an example, the structure of [Au25(2-PET)18]- (2-PET: 2-phenylethylthiolate) has recently been solved.90 The cluster consists of a Au13 core of icosahedral symmetry (geometric shell closing) and six dimeric protecting units Au2(SR)3 are arranged on its surface (Figure 2, left).
The structure is slightly distorted, which is due to the packing of tetraoctylammonium cations in the unit cell of the crystal. The crystal structure of the neutral Au25(SR)18 cluster does not show this distortion.91 Density functional theory reveals the superatomic nature of the cluster.92 The electron count is NSAC = 25 – 18 – (-1) = 8, which is a magic number. The core localizes the negative charge and the formula can be rewritten as (Au13)-(Au2SR3)6 (the bridged binding will be discussed later). The SACM predicts a shell structure of 1S2| 1P6 is predicted and thus, the HOMO should be of P-symmetry, whereas the LUMO should have D- character. Density-of-States analysis of the cluster confirms these predictions (Figure 2, right).
Applications of ligand protected gold clusters are still rare, but the tunability of properties (optical, electronic and in the ligands layer) gives rise to the assumption that they will gain in importance. Notable examples are catalytic applications.57,58 One can follow two approaches:
in the first approach, the particle as is acts as catalyst (either in solution or deposited on oxidic supports);71,100-110 in the second approach, the clusters are deposited on supports and the ligands are removed by thermal treatment.111,112 The latter method is a facile alternate way compared to deposition of size-selected clusters from the gas phase. However, key features such as chirality are lost, since – up to know – chirality of gold cluster is strictly associated with the ligands (may it be by the use of chiral ligands or due to their arrangement, see below). Applications in halide anion (Br-, I-) sensing seem possible for Au25 clusters,55,56 electrochemical sensing has also been demonstrated.113 The anchoring of mesogenes to gold clusters may provide materials with interesting opportunities.79-82,114
Theoretical Part
12
III. Chirality
Chirality is one of the most-studied topics in natural sciences.115 It is well known that virtually all building blocks of biological systems are chiral. Moreover, only one of the two possible enantiomers of these building blocks is found in nature. It is unknown how this homochirality evolved. However, the fact that chiral compounds react specifically with other chiral compounds has strong implications on all fields that touch life sciences, for instance drug discovery and organic synthesis.
1) Circular Dichroism
One commonly used method for the detection of chirality is Circular Dichroism (CD) spectroscopy.116 The CD effect is based on the difference in absorption (ΔA) of left- and right- handed circular-polarized light (AL and AR, respectively) by a chiral compound (Cotton effect):
Eqn. 1
The difference in absorbance is equal in extent but of opposite sign for the two enantiomers of a chiral compound (Figure 1). When passing the chiral probe, the circular-polarized light becomes polarized in an elliptical fashion (which is a superposition of left- and right-handed circular-polarized light). The ellipticity θ is expressed in millidegree (mdeg).c
c Note, that the ellipticity θ is usually measured in mdeg. It is connected with the differential absorbance by θ = 32980*ΔA = 32980*Δε*cl. The molar ellipticity [θ], however, is defined as [θ] = 3298*Δε. This often leads to confusion.
Theoretical Part
13
Figure 1. CD spectra of 1R, 4S- (black) and 1S, 4R-camphorthiol (red). The enantiomeric relationship between the two isomers is reflected in the mirror-imaged CD responses.
Similarly to linear absorption, the difference in the absorbance depends on concentration of the sample c and pathlength l, as in the Beer-Lambert law:
Eqn. 2
The CD signal can be converted into a molecular property, the anisotropy factor g (or g- value), by dividing by the ordinary absorbance:
Eqn. 3
The CD effect can be described by the rotatory strength R, in analogy to the (electric) dipole strength D in linear absorption:
Eqn. 4
For the formulation of R, one has to know the magnetic dipole strength G as well:
Eqn. 5
Theoretical Part
14 And R is
Eqn. 6
As one can see, the rotatory strength R depends on the electric and magnetic transition dipole moments (μel, μmagn) and their angle (cos(μel, μmagn)). There are three conditions under which R can be zero:
1) µel = 0 2) µmagn = 0
3) both µel and µmagn ≠ 0, but µel µmagn
Since enantiomers are mirror images, the vectors point into opposite directions and this leads to opposite signs (as a result of the cosine function).
The strength of a CD spectrum depends on both concentration and enantiomeric excess (ee) in the sample:
Eqn. 7
For a known CD spectrum, the determination of the enantiomeric excess is possible by comparison of the intensity with the one of the pure enantiomer. This requires knowledge of the concentration. The anisotropy factor on the other hand is independent of the concentration and does not require this knowledge (but still, the value for the pure enantiomer is needed).
Furthermore, the CD spectrum is sensitive to different conformers of a molecule, since in these, the electric and magnetic dipoles differ. This is often used for the determination of secondary structures in proteins.117
In a typical CD experiment, the light is first linearly polarized (which is a 1:1 superposition of left-and right-handed circular-polarized light). It is then passed through a Photoelastic
Theoretical Part
15
Modulator (PEM), whose refractive index is modulated in a sinusoidal way at a certain frequency. The speed of left- and right-handed circular-polarized light is sensitive to differences in the refractive index (this is used in Optical Rotarory Dispersion (ORD) measurements). The result is an alternatively left- and right-handed circularly-polarized light beam (due to the phase differences). Passing the sample, the light becomes elliptically polarized. The detector is coupled to the PEM (lock-in-amplification) and the signal is demodulated. The basic setups for both Electronic Circular Dichroism (ECD) in the UV/Vis and Vibrational Circular Dichroism (VCD) in the Infrared are in principal the sample.
Whereas in ECD the spectrum is usually obtained in continuous scanning of the spectral region, VCD uses FT-IR spectrometer.118
Figure 2. CD spectrum (top) and absorption spectrum (bottom) of L-SG-protected gold clusters. Reprinted with permission from Schaaff et al., J. Phys. Chem. B 2008, 102, 10643. Copyright (1998) American Chemical Society.
Theoretical Part
16 2) Chirality in Gold Clusters
Chirality in thiolate-protected gold clusters was first observed by Whetten and co-workers in 1998.83 Gold clusters were prepared with L-glutathione (L-SG) as protecting ligand, a tripeptide (N-γ-Glu-Cys-Gly, Scheme 1). The central amino acid is cysteine and provides a thiol group. After purification of the clusters, CD spectra were recorded and Cotton effects were observed for transitions at higher wavelengths than compared to the free glutathione ligand (Figure 2). These bands were ascribed to the cluster core, thus involving gold based electronic transitions, but the authors also stated that the “[…] exact origin of the chiroptical effects has yet to be determined for this class of compounds.”83 In a later study, L-SG- protected clusters were size-selected by polyacrylamide gel electrophoresis (PAGE) and it was found that the optical activity is size-dependent in both shape of the spectrum and strength of the chiroptical effect.119
Scheme 1. L-glutathione, composed of glutamic acid (red), cysteine (black) and glycine (blue).
Theoretical Part
17
Figure 3. Anisotropy factors of 1,1’-binaphthyl-2,2’-dithiolate-protected gold clusters of different sizes. The maximum anisotropy factors vary between 4 * 10-3 (sample 3) and 5 * 10-4 (sample 5). The shape of the spectra is size-dependent as well. From Gautier et al., Chirality 2008, 20, 486 – 493. Copyright 2008 John Wiley and Sons.
In the following years, a number of studies was carried out, exploring the origin of chiroptical effects. Several models have been discussed. It was proposed that the clusters either have intrinsically chiral cores,120,121 the electrons might feel the dissymmetric field that is created by the chiral adsorbates,122 or that – in analogy to surfaces123 – a chiral local distortion is created (‘chiral footprint’).124 A major breakthrough was made in 2007, when the first crystal structure of a thiolate-protected gold cluster was published.7 The crystal structure bears intrinsic chirality due to the arrangement of the ligands on the surface of the cluster core. In the following, the basic features and levels of chirality in gold clusters will be briefly discussed.
Theoretical Part
18 a) Chiral Ligands
The easiest way to bestow chirality to gold clusters is the use of chiral protecting ligands.71,74,83,84,114,119,124-134
This induces optical activity to the gold clusters and their strength and shape is ligand-dependent (Figure 3).9 It should be noted, that also phosphines71,74 (instead of thiols) can be used as ligands and that other metals71,130-135 (such as silver or palladium) have been studied. In all cases, CD spectroscopy served as method of choice for the observation of induced optical activity. Also, it was shown that the ligand can be studied using VCD, which allows conformational analysis.124,126,128,134
b) Inherent Chirality
Chirality cannot only be imparted to a gold cluster by protection with a chiral ligand, but the clusters themselves bear some intrinsically chiral features that shall be briefly addressed.
First, the binding motif between gold and sulfur was found to be bridged.7,136 Instead of
‘standard’ terminal binding, two sulfur atoms stabilize a gold atom, forming monomers -SR- (Au-SR)- or oligomers –SR-(Au-SR)n- (n = 2 or 3; the trimer is predicted only137-139). These oligomers can be interpreted as bidentate ligands with an overall charge of -1 (the Au atoms were found to be positively charged and the thiolates are negatively charged). The sulfur atoms become stereogenic centers; since they are binding to four different substituents (see below). Furthermore, they can be arranged in chiral patterns along the surface of the cluster cores, adding a second chiral feature.7,8 It has been suggested that the ground state structures of clusters have chiral core geometries.120,121,140-142
Concerning the intrinsically chiral thiolate- protected clusters, it remains open whether the chiral arrangement of the protecting units is a consequence of inherently chiral cluster cores or if the chiral distortion of the cores is a result of the chiral arrangement of the ligand overlayer.
Theoretical Part
19 i. The Staple Motif
The staple motifs –SR-(Au-SR)n- (n = 1, 2, 3) are commonly found in the structures of thiolate protected gold clusters.7,8,69,90 In the case of the monomer (n = 1), it can occur in cis and trans configurations (Scheme 2). The sulfur atoms in these staples are stereogenic centers since they are ligated by four different ligands (Ausurface > AuAdatom > organic rest > electron lone pair; in decreasing priority following CIP143). In a monomeric staple, overall four isomers ((S, S), (S, R), (R, S) and (R, R)) can be considered, since 2m (m: number of stereogenic centers) permutations are possible. In a dimeric staple –SR-(Au-SR)2-, eight isomers can occur (since 23 = 8). It should be noted that the central sulfur atom in a dimeric staple is a pseudochiral center if the absolute configuration at the outer sulfur atoms are different. If the outer sulfur atoms have the same descriptor, the descriptor of the central atom is not readily assignable. The assignment of the stereodescriptors is based on the five- or seven-membered rings that are formed by the staple motif only.
The problem of assigning all absolute configurations in gold clusters becomes very complex in larger clusters. For instance, the Au38(SR)24 cluster has 224 = 16,777,216 different configurations for the sulfur atoms (which partly can be ruled out due to steric reasons). In Au102(SR)44, already 244 = 17,592,186,044,416 possible combinations are found. This makes the prediction of CD spectra extremely complicated. It has been shown that certain configurations lead to chiral overlayers which can induce a CD spectrum to a cluster that does not bear other forms of chirality.144 However, in a cluster that is protected by achiral ligands, the CD spectra are expected to cancel and it seems unlikely that it is possible to separate enantiomers of such chiral overlayers. Nevertheless, when protecting a cluster with a chiral ligand, it might be possible to stabilize certain configurations in these staples.
