Counting unlabelled toroidal graphs with no K33-subdivisions

We provide a description of unlabelled enumeration techniques, with complete proofs, for graphs that can be canonically obtained by substituting 2-pole networks for the edges of core graphs. Using structure theorems for toroidal and projective-planar graphs containing no K33-subdivisions, we apply these techniques to obtain their unlabelled enumeration.Comment: 25 pages (some corrections), 4 figures (one figure added), 3 table

Characterization and enumeration of toroidal K_{3,3}-subdivision-free graphs

We describe the structure of 2-connected non-planar toroidal graphs with no K_{3,3}-subdivisions, using an appropriate substitution of planar networks into the edges of certain graphs called toroidal cores. The structural result is based on a refinement of the algorithmic results for graphs containing a fixed K_5-subdivision in [A. Gagarin and W. Kocay, "Embedding graphs containing K_5-subdivisions'', Ars Combin. 64 (2002), 33-49]. It allows to recognize these graphs in linear-time and makes possible to enumerate labelled 2-connected toroidal graphs containing no K_{3,3}-subdivisions and having minimum vertex degree two or three by using an approach similar to [A. Gagarin, G. Labelle, and P. Leroux, "Counting labelled projective-planar graphs without a K_{3,3}-subdivision", submitted, arXiv:math.CO/0406140, (2004)].Comment: 18 pages, 7 figures and 4 table

A new insight into the oxidative mechanism of caffeine and related methylxanthines in aprotic medium: May caffeine be really considered as an antioxidant?

Background: Antioxidant properties have been recently suggested for caffeine that seems showing protective effects against damages caused by oxidative stress. In particular, a HO% scavenging activity has been ascribed to caffeine. Even if the oxidation of caffeine has been widely studied, the antioxidant mechanism is still far to be understood. Methods: The electrochemical behavior of caffeine, theobromine and theophylline was studied in aprotic medium by cyclic voltammetry and electrolysis in UV–vis cell; a computational analysis of the molecular structures based on the Density Functional Theory was performed; the reactivity of all substrates towards lead dioxide, superoxide and galvinoxyl radical was followed by UV–vis spectrophotometry. Results: Results supported the mono-electronic oxidation of the C4]C5 bond for all substrates at high oxidation potentials, the electron-transfer process leading to a radical cation or a neutral radical according to the starting methylxanthine N7-substituted (caffeine and theobromine) or N7-unsubstituted (theophylline), respectively. A different following chemical fate might be predicted for the radical cation or the neutral radical. No interaction was evidenced towards the tested reactive oxygen species. Conclusions: No reactivity via H-atom transfer was evidenced for all studied compounds, suggesting that an antiradical activity should be excluded. Some reactivity only with strong oxidants could be predicted via electron- transfer. The acclaimed HO% scavenging activity should be interpreted in these terms. The study suggested that CAF might be hardly considered an antioxidant. General significance: Beyond the experimental methods used, the discussion of the present results might provide food for thought to the wide audience working on antioxidants

Systematic manipulation of siliconoid clusters : from peripheral functionalization to heteroatom doping

Unsaturated silicon clusters (siliconoids) arise as transient intermediates during the industrial preparation of bulk silicon by chemical vapor deposition of silane precursors. Stable representatives reiterate surface features of silicon materials. The present thesis deals with the systematic manipulation of the ligand periphery and the cluster core of Si6 siliconoids. The attachment of representative electrophiles at that particular silicon scaffold is now possible at two distinct vertices and their influence on the electronic structure is rationalized. The isolation of boron- and phosphorus-substituted derivatives prompted investigations on the incorporation of the dopant atoms in the cluster core itself. An NHC-stabilized, cyclic Si2P phosphasilene is shown to dimerize upon Lewis-acid induced abstraction of the donating NHC-ligand. The resulting Si4P2 species exhibit cluster structures reminiscent of the corresponding Si6 siliconoids despite their apparent saturated nature. Access to unsaturated ESi5 heterosiliconoids (E = B, P) was finally gained via a novel dianionic Si5 cluster, which is obtained by the formal reductive cleavage of a SiTip2 moiety from the Si6 siliconoid. The successful incorporation of dopant atoms in siliconoids extends the siliconoid/silicon surfaces analogy to the technological process of silicon doping.Ungesättigte Siliciumcluster (Silicoide) treten als kurzlebige Zwischenprodukte bei der industriellen Herstellung von Bulk-Silicium durch chemische Gasphasenabscheidung von Silan-Vorstufen auf. Stabile Vertreter weisen Oberflächenmerkmale von Siliciummaterialien auf. Die vorliegende Arbeit befasst sich mit der systematischen Manipulation der Ligandenperipherie und des Clusterkerns von Si6-Silicoiden. Die Bindung repräsentativer Elektrophile an diesem bestimmten Siliciumgerüst ist nun an zwei verschiedenen Positionen möglich und ihr Einfluss auf die elektronische Struktur wird rationalisiert. Die Isolierung von Bor- und Phosphor-substituierten Derivaten veranlasste Untersuchungen zum Einbau der Dotierelemente in den Clusterkern selbst. Es wurde gezeigt, dass ein NHC-stabilisiertes, cyclisches Si2P-Phosphasilen bei Lewis-Säure-induzierter Abstraktion des Donor-NHC-Liganden dimerisiert. Die resultierenden Si4P2-Spezies weisen trotz ihrer augenscheinlich gesättigten Natur Clusterstrukturen auf, die an die entsprechenden Si6-Silicoide erinnern. Ungesättigte ESi5-Heterosilicoide (E = B, P) wurden schließlich über einen neuen dianionischen Si5-Cluster zugänglich, der durch die formale reduktive Abspaltung einer SiTip2-Einheit vom Si6-Silicoid erhalten wird. Der erfolgreiche Einbau von Dotierelementen in Silicoide erweitert die Silicoid/Silicium-Oberflächen Analogie um den technologischen Prozess der Siliciumdotierung

Stable functionalized unsaturated siliconoids : from a Tetrylene / Siliconoid hybrid to application in homogeneous catalysis

Siliconoids have received tremendous attention due to the prominent role of silicon in technologies of the modern society. The presence of the partially unsubstituted cluster scaffold resembles silicon surface materials at molecular regimes. While siliconoids play important roles as presumed intermediates during chemical vapor deposition processes or in heterogeneous catalysis, the possibility to graft functional groups to the Si6 benzpolarene scaffold is a prerequisite for their incorporation as building blocks into extended systems. Main attention of this thesis is the introduction and transformation of functionalities in the periphery of Si6 siliconoids. Dichlorinated metallocenes of Group 4 are shown to be suitable reagents for the transfer of metals to the Si6 siliconoid. The electrophilic transfer of a chlorinated amidinato tetrylene to the Si6 siliconoid facilitates the coordination to transition metal fragments. As will be shown, depending on the nature of the substituents of the transition metal fragments, different and unprecedented structural motifs can be obtained. In context of the application of siliconoids in homogeneous catalysis, the reactivity of a silylene/siliconoid hybrid towards chalcogens and carbon monoxide was investigated resulting in unprecedented and unsaturated chalcogen-expanded heterosiliconoids as well as in the full cleavage of the C≡O triple bond under formation of an Si=C Enol ether bridge in the peripherie of the cluster scaffold.Silicoide erregten aufgrund der herausragenden Rolle von Silicium in den Technologien der modernen Gesellschaft eine enorme Aufmerksamkeit. Das Vorhandensein des teilweise unsubstituierten Clustergerüsts ähnelt Siliciumoberflächenmaterialien im molekularen Bereich. Während Silicoide als vermutete Zwischenprodukte bei chemischen Gasphasenabscheidungsprozessen oder bei der heterogenen Katalyse eine wichtige Rolle spielen, ist die Möglichkeit, funktionelle Gruppen auf das Si6 Benzpolarengerüst zu übertragen eine Voraussetzung für deren Einbau als Bausteine in erweiterte Systeme. Das Hauptaugenmerk dieser Arbeit liegt auf der Einführung und Transformation von Funktionalitäten in der Peripherie von Si6 Silicoiden. Es wurde gezeigt, dass dichlorierte Metallocene der Gruppe 4 geeignete Reagenzien für die Übertragung von Metallen auf das Si6 Siliciumgerüst sind. Der elektrophile Transfer eines chlorierten Amidinatotetrylens auf ein Si6 Silicoid erleichtert die Koordination an Übergangsmetallfragmente. Wie gezeigt werden wird, können unabhängig von der Art der Substituenten der Übergangsmetallfragmente unterschiedliche und beispiellose Strukturmotive erhalten werden. Im Zusammenhang mit der Anwendung von Silicoiden in der homogenen Katalyse wurde die Reaktivität eines Silylen/Silicoid Hybrids gegenüber Chalkogenen und Kohlenmonoxid untersucht, was zu beispiellosen, ungesättigten Chalkogenexpandierten Heterosilicoiden, sowie zur vollständigen Spaltung der C≡O Dreifachbindung unter Bildung einer Si=C Enoletherbrücke in der Peripherie des Clustergerüsts führte

Chalcogen-Expanded Unsaturated Silicon Clusters: Thia-, Selena-, and Tellurasiliconoids

Reactions of silylenes with heavier chalcogens (E) typically result in Si=E double bonds or their π‐addition products. In contrast, the oxidation of a silylene‐functionalized unsaturated silicon cluster (siliconoid) with Group 16 elements selectively yields cluster expanded siliconoids Si7E (E=S, Se, Te) fully preserving the unsaturated nature of the cluster scaffold as evident from the NMR signatures of the products. Mechanistic considerations by DFT calculations suggest the intermediacy of a Si6 siliconoid with exohedral Si=E functionality. The reaction thus may serve as model system for the oxidation of surface‐bonded silylenes at Si(100) by chalcogens and their diffusion into the silicon bulk

Siliconoid Expansion by a Single Germanium Atom through Isolated Intermediates

The growth of (semi-)metal clusters is pivotal for nucleation processes in gaseous and condensed phases. We now report the isolation of intermediates during the expansion of a stable unsaturated silicon cluster (siliconoid) by a single germanium atom through a sequence of substitution, rearrangement and reduction. The reaction of ligato-lithiated hexasilabenzpolarene LiSi6Tip5 (1Li⋅(thf)2, Tip=2,4,6-triisopropylphenyl) with GeCl2⋅NHC (NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) initially yields the product with exohedral germanium(II) functionality, which then inserts into an Si−Si bond of the Si6 scaffold. The concomitant transfer of the chloro functionality from germanium to an adjacent silicon preserves the electron-precise nature of the formed endohedral germylene. Full incorporation of the germanium heteroatom to the Si6Ge cluster core is finally achieved either by reduction under loss of the coordinating NHC or directly by reaction of 1Li⋅(thf)2 with GeCl2⋅1,4-dioxane

Atomically Precise Expansion of Unsaturated Silicon Clusters

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to the stabilizing ligand periphery, virtually excluding the systematic variation of the property-determining cluster scaffold. We now report the deliberate expansion of a stable unsaturated silicon cluster from six to seven and finally eight vertices. The consecutive application of lithium/naphthalene as the reducing agent and decamethylsilicocene as the electrophilic source of silicon results in the expansion of the core by precisely one atom with the potential of infinite repetition

FHBC, a Hexa‐\u3cem\u3eperi\u3c/em\u3e‐hexabenzocoronene–Fluorene Hybrid: A Platform for Highly Soluble, Easily Functionalizable HBCs with an Expanded Graphitic Core

Materials based upon hexa‐peri‐hexabenzocoronenes (HBCs) show significant promise in a variety of photovoltaic applications. There remains the need, however, for a soluble, versatile, HBC‐based platform, which can be tailored by incorporation of electroactive groups or groups that can prompt self‐assembly. The synthesis of a HBC–fluorene hybrid is presented that contains an expanded graphitic core that is highly soluble, resists aggregation, and can be readily functionalized at its vertices. This new HBC platform can be tailored to incorporate six electroactive groups at its vertices, as exemplified by a facile synthesis of a representative hexaaryl derivative of FHBC. Synthesis of new FHBC derivatives, containing electroactive functional groups that can allow controlled self‐assembly, may serve as potential long‐range charge‐transfer materials for photovoltaic applications

New chemistry of sterically-crowded carboranes

Chapter one gives an introduction into heteroborane chemistry focussing on the areas of icosahedral and supraicosahedral (metalla)carboranes and their isomerisation mechanisms. Chapter two describes attempts at placing bulky substituents onto a 1,7-closo-C2B10 species so that upon reduction and subsequent oxidation the carbon atoms are unable to connect thus potentially preventing oxidation and allowing for easier capitation to form a supraicosahedral carborane. Initial efforts to synthesise such 1,7- bulky species, using tertiary alcohol substituents, did not lead to polyhedral expansion due to undesired reactivity. Compounds with bulky substituents incorporating cobaltacarborane clusters were successfully prepared but in insufficient yield for further expansion chemistry. Chapter three discusses the synthesis and characterisation of 1,7-closo-C2B10 species which incorporate a ferrocenyl unit on each substituent. Polyhedral expansion to form a 13-vertex carborane was unsuccessful, instead upon oxidation forming significantly sterically deformed 1,2 species with unprecedentedly elongated C-C connectivities. Chapter four investigates the redox chemistry of the ferrocenyl 1,2 and 1,7 species. The electronic spectrum of the 1,2 species were compared to that given by TD DFT calculations which suggest a degree of charge transfer. Associated electrochemistry suggests this may be a sterically-induced charge transfer within the 1,2 species. The steric deformation of the bis-ferrocenyl 1,2 species leads to a relatively lowtemperature isomerisation to the 1,7 species. This has renewed interest in the elucidation of the isomerisation mechanism of icosahedral carboranes. Chapter five details the synthesis and characterisation of labelled species with labels which can be reliably tracked upon isomerisation. A comparison of the experimental findings to the results of theoretical studies on the subject is examined. Chapter six contains experimental procedures and characterisation details for all the new compounds reported herein. Crystallographic data is listed in Appendices A and B (CD) along with structure solution and refinement details