Bronsted and Lewis acid adducts of triazenes

The synthetic utility of triazenes rests on the fact that the triazene function can be cleaved by BrOnsted or Lewis acids, liberating diazonium compounds. However, the preferred coordination site of the acid is still a matter of debate. We have analyzed triflic acid, B(C6F5)(3), and PdCl2 adducts of triazenes by NMR spectroscopy and single crystal X-ray crystallography. In all cases, we observe coordination of the acid to the N1 atom of the triazene. This finding is not only of relevance for acid-induced cleavage reactions, but also for metal-catalyzed reactions with triazenes, which are increasingly being used in synthetic organic chemistry.

Synthetic chemistry with nitrous oxide and triazenes

Iris Roswitha Landman

Nitrous oxide (N2O) has gained much interest because of its physiological effects ("laughing gas") and its negative environmental impact ("greenhouse gas", "ozone-depleting substance"): It has a lifetime of more than 100 years in the atmosphere. Its persistent character shows how challenging it is to convert N2O into more valuable products. From a synthetic point of view, N2O has potential as diazo (N2) transfer reagent, but applications in synthetic organic chemistry are scarce. The main challenges are to capture N2O, avoid the loss of N2, and to overcome poor-selectivity and -yields. Herein, we describe two examples of the incorporation of two nitrogen atoms into the final product. Firstly, we use N2O as diazo transfer reagent in the preparation of triazolopyridines, which are valuable starting materials for heterocyclic compounds. The reactions can be performed under mild conditions and give synthetically interesting triazoles in moderate to good yields. Secondly, general methods to N1 acylated triazenes were missing previously. We developed routes to 1-acyl triazenes via acid-catalyzed hydration, or by gold- or iodine-catalyzed oxidation of 1-alkynyl triazenes. The latter compounds are prepared from N2O. The properties of the N1 acylated triazenes are distinct compared to other triazenes, displaying different physical and chemical properties. Under strong acidic conditions, 1-acyl triazenes act as acylation reagents. Finally, despite that triazene chemistry is known for more than 160 years, the preferred coordination site of acids still remained a matter of debate (N1, N2, or N3). In search of experimental evidence, we have analyzed triflic acid, B(C6F5)3, and PdCl2 adducts of triazenes by IR, NMR spectroscopy and single crystal X-ray crystallography. For all cases, we found the acid coordinating to the N1 atom of the triazene. This preference should be taken into account for future acid- and transition metal-catalyzed reactions with triazenes. Altogether, this thesis contributes to triazene chemistry and synthetic chemistry with N2O. In this way, we provide further evidence that N2O can be used as a diazo donor to get to synthetically interesting N-containing products.

EPFL2022

Self-assembly of organometallic macrocycles and cages

Thomas Brasey

Neutral metallamacrocycles with four or ten metal centers were obtained by reaction of [(R3P)PdCl2]2 with 2,3-dihydroxypyridine and derivatives or 2-hydroxynicotinic acid in the presence of base. In all these assemblies, the two O-donor atoms of the ligand bind to one metal center to form a chelate ring, while the N-donor atom binds to another metal center, linking the subunits together. The tetrameric complexes showed a low affinity for small aromatic diols. The procedure was adapted for the synthesis of water-soluble macrocycles: ligands which contain both the 2,3-dihydroxypyridine motive and a tertiary amine function that can be protonated were reacted in water with the acetato-bridged complex [(Et3P)Pd(CH3COO)2]2 to give tetrameric macrocycles in over 90% yields. Scrambling experiments showed that the Pd(II) macrocycles are highly inert. The reaction of ReBr3(CO)32 with 3-hydroxy-1,2,3-benzotriazine-4(3H)-one in the presence of base gave the ionic rhenium complex [ReBr(C7H4N3O2)(CO)3][NEt4], from which a neutral metallamacrocycle was obtained upon abstraction of (NEt4)Br. Its connectivity is comparable to that of the Pd(II) macrocycles. In coordinating solvents, this macrocycle disrupts and the monomeric solvent adducts are formed. In the presence of BF4-, it undergoes an anion-templated rearrangement to give a new, trimeric macrocycle which cocrystallizes with an unusual C3-symmetric [Ag(η2-benzene)3(OH2)][BF4] complex, the tetrafluoroborate anion being encapsulated in the cavity of the macrocycle. By reacting [(cymene)RuCl2]2 with 3,5-pyridinedicarboxylic acid, a trifunctional ligand in which the three donor atoms are unable to coordinate to the same metal center, an hexameric coordination cage was obtained. This complex has a trigonal antiprism geometry. Half of the carbonyl O-atoms of the bridging ligands are positioned in close proximity to each other, forming two binding sites on the surface. The cage can thus act as an exo-receptor for alkali metal cations (Na+, K+, Cs+). 1H and 13C NMR investigations as well as a single crystal X-ray analysis revealed that upon addition of alkali metal salts, the hexanuclear assembly partly rearranges to form a dodecameric coordination cage with icosahedral geometry. Although entropically disfavored, this process takes place because in this new complex all carbonyl O-atoms are part of a binding site (eight metal cations can be accommodated on the surface of the cage). Furthermore, due to the reduced surface curvature of the dodecanuclear assembly, these O-atoms are closer together, forming more suited binding sites for the alkali guest cations. To best of our knowledge, a coordination cage with an icosahedral geometry has not been reported so far.

EPFL2006

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007