TY  - THES
A1  - Budiman, Yudha Prawira
T1  - Applications of Fluorinated Aryl Boronates in Organic Synthesis
T1  - Die Anwendungen von fluorinierten Arylboronaten in der organischen Synthese
N2  - Fluorinated compounds are an important motif, particularly in pharmaceuticals, as one-third of the top performing drugs have fluorine in their structures. Fluorinated biaryls also have numerous applications in areas such as material science, agriculture, crystal engineering, supramolecular chemistry, etc. Thus, the development of new synthetic routes to fluorinated chemical compounds is an important area of current research. One promising method is the borylation of suitable precursors to generate fluorinated aryl boronates as versatile building blocks for organic synthesis.
Chapter 1
In this chapter, the latest developments in the synthesis, stability issues, and applications of fluorinated aryl boronates in organic synthesis are reviewed. The catalytic synthesis of fluorinated aryl boronates using different methods, such as C–H, C–F, and C–X (X = Cl, Br, I, OTf) borylations are discussed. Further studies covering instability issues of the fluorinated boronate derivatives, which are accelerated by ortho-fluorine, have been reported, and the applications of these substrates, therefore, need special treatment.
Numerous groups have reported methods to employ highly fluorinated aryl boronates that anticipate the protodeboronation issue; thus, polyfluorinated aryl boronates, especially those containing ortho-fluorine substituents, can be converted into chloride, bromide, iodide, phenol, carboxylic acid, nitro, cyano, methyl esters, and aldehyde analogues. These substrates can be applied in many cross-coupling reactions, such as the Suzuki-Miyaura reaction with aryl halides, the Chan-Evans-Lam C–N reaction with aryl amines or nitrosoarenes, C–C(O) reactions with N-(aryl-carbonyloxy)phthalamides or thiol esters (Liebskind-Srogl cross-coupling), and oxidative coupling reactions with terminal alkynes. Furthermore, the difficult reductive elimination from the highly stable complex [PdL2(2,6-C6F2+nH3-n)2] was the next challenge to be targeted in the homocoupling of 2,6-di-fluoro aryl pinacol boronates, and it has been solved by conducting the reaction in arene solvents that reduce the energy barrier in this step as long as no coordinating solvent or ancillary ligand is employed.
 
Chapter 2
In this chapter, phenanthroline-ligated copper complexes proved to be efficient catalysts for the Suzuki-Miyaura cross-coupling of highly fluorinated aryl boronate esters (ArF–Bpin) with aryl iodides or bromides. This newly developed method is an attractive alternative to the traditional methods as copper is an Earth-abundant metal, less toxic, and cheaper compared to the traditional methods which commonly required palladium catalysts, and silver oxide that is also often required in stoichiometric amounts. A combination of 10 mol% copper iodide and 10 mol% phenanthroline, with CsF as a base, in DMF, at 130 ˚C, for 18 hours is efficient to cross-couple fluorinated aryl pinacol boronates with aryl iodides to generate cross-coupled products in good to excellent yields. This method is also viable for polyfluorophenyl borate salts such as pentafluorophenyl-BF3K. Notably, employing aryl bromides instead of aryl iodides for the coupling with fluorinated aryl–Bpin compounds is also possible; however, increased amounts of CuI/phenanthroline catalyst is necessary, in a mixture of DMF and toluene (1:1). 
A diverse range of π···π stacking interactions is observed in the cross-coupling products partly perfluorinated biaryl crystals. They range from arene–perfluoroarene interactions (2-(perfluorophenyl)naphthalene and 2,3,4-trifluorobiphenyl) to arene–arene (9-perfluorophenyl)anthracene) and perfluoroarene–perfluoroarene (2,3,4,5,6-pentafluoro-2’methylbiphenyl) interactions.

Chapter 3
In this chapter, the efficient Pd-catalyzed homocoupling reaction of aryl pinacol pinacol boronates (ArF–Bpin) that contain two ortho-fluorines is presented. The reaction must be conducted in a “noncoordinating” solvent such as toluene, benzene, or m-xylene and, notably, stronger coordinating solvents or ancillary ligands have to be avoided. Thus, the Pd center becomes more electron deficient and the reductive elimination becomes more favorable. The Pd-catalyzed homocoupling reaction of di-ortho-fluorinated aryl boronate derivatives is difficult in strongly coordinating solvents or in the presence of strong ancillary ligands, as the reaction stops at the [PdL2(2,6-C6F2+nH3-n)2] stage after the transmetalations without the reductive elimination taking place. It is known that the rate of reductive elimination of Ar–Ar from [ML2(Ar)(Ar)] complexes containing group-10 metals decreases in the order Arrich–Arpoor > Arrich–Arrich > Arpoor–Arpoor. Furthermore, reductive elimination of the most electron-poor diaryls, such as C6F5–C6F5, from [PdL2(C6F5)2] complexes is difficult and has been a challenge for 50 years, due to their high stability as the Pd–Caryl bond is strong. Thus, the Pd-catalyzed homocoupling of perfluoro phenyl boronates is found to be rather difficult. 
 
	Further investigation showed that stoichiometric reactions of C6F5Bpin, 2,4,6-trifluorophenyl–Bpin, or 2,6-difluorophenyl–Bpin with palladium acetate in MeCN stops at the double transmetalation step, as demonstrated by the isolation of cis-[Pd(MeCN)2(C6F5)2], cis-[Pd(MeCN)2(2,4,6-C6F3H2)2], and cis-[Pd(MeCN)2(2,6-C6F2H3)2] in quantitative yields. Thus, it can be concluded that the reductive elimination from diaryl-palladium complexes containing two ortho-fluorines in both aryl rings, is difficult even in a weakly coordinating solvent such as MeCN. Therefore, even less coordinating solvents are needed to make the Pd center more electron deficient. Reactions using “noncoordinating” arene solvents such as toluene, benzene, or m-xylene were conducted and found to be effective for the catalytic homocoupling of 2,6-C6F2+nH3-nBpin. The scope of the reactions was expanded. Using toluene as the solvent, the palladium-catalyzed homocoupling of ArF–Bpin derivatives containing one, two or no ortho-fluorines gave the coupled products in excellent yields without any difficulties.

	DFT calculations at the B3LYP-D3/def2-TZVP/6-311+g(2d,p)/IEFPCM // B3LYP-D3/SDD/6-31g**/IEFPCM level of theory predicted an exergonic process and lower barrier (< 21 kcal/mol) for the reductive elimination of Pd(C6F5)2 complexes bearing arene ligands, compared to stronger coordinating solvents (acetonitrile, THF, SMe2, and PMe3), which have high barriers ( > 33.7 kcal/mol). Reductive elimination from [Pd(ηn-Ar)(C6F5)2] complexes have low barriers due to: (i) ring slippage of the arene ligand as a hapticity change from η6 in the reactant to ηn (n  ≤  3) in the transition state and the product, which led to less σ-repulsion; and (ii) more favorable π-back-bonding from Pd(ArF)2 to the arene fragment in the transition state.

Chapter 4
In this chapter, the efficient Pd-catalyzed C–Cl borylation of aryl chlorides containing two ortho-fluorines is presented. The reactions are conducted under base-free conditions to prevent the decomposition of the di-ortho-fluorinated aryl boronates, which are unstable in the presence of base. A combination of Pd(dba)2 (dba = dibenzylideneacetone) with SPhos (2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl) as a ligand is efficient to catalyze the C–Cl borylation of aryl chlorides containing two ortho-fluorine substituents without base, and the products were isolated in excellent yields. The substrate scope can be expanded to aryl chloride containing one or no ortho-fluorines and the borylated products were isolated in good to very good yield. This method provides a nice alternative to traditional methodologies using lithium or Grignard reagents.
N2  - Fluorierte Verbindungen sind insbesondere in der Pharmazie wichtige Bausteine, da ein Drittel der wirksamen Medikamente Fluorsubstituenten beinhalten. Fluorierte Biaryle haben auch zahlreiche Anwendungen in Bereichen wie der Materialwissenschaft, der Landwirtschaft, dem Design molekularer Festkörperstrukturen, der supramolekularen Chemie etc. Daher ist die Entwicklung neuer synthetischer Wege zu fluorierten chemischen Verbindungen sehr gefragt. Eine der vielversprechenden Methoden ist die Borylierung geeigneter Vorstufen zur Erzeugung fluorierter Arylboronate, die als vielseitige Bausteine für die organische Synthese dienen können. ...
KW  - Chemistry
KW  - Homogeneous Catalysis
KW  - borylation
KW  - boronates
KW  - fluorine
KW  - C-C coupling
KW  - Homogene Katalyse
KW  - Borylierung
KW  - Fluorierung
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-217579
ER  - 
TY  - THES
A1  - Eck, Martin
T1  - Iron- and Copper-catalyzed Borylation of Alkyl and Aryl Halides and B–B Bond Activation and NHC Ring-expansion Reactions of the Diboron(4) Compound Bis(ethylene glycolato)diboron (B\(_2\)eg\(_2\))
T1  - Eisen- und kupferkatalysierte Borylierung von Alkyl- und Arylhalogeniden und B-B Bindungsaktivierung und NHC Ringerweiterungsreaktionen der Diboran(4) Verbindung Bis(ethylenglykol)diboran (B\(_2\)eg\(_2\))
N2  - The purpose of the present work was, in the first part, to investigate the potential of iron-based metal complexes in catalytic borylation reactions with alkyl halides as substrates and B2pin2 as the borylation reagent. Moreover, extended studies of the recently reported, copper mediated borylation reactions of aryl halides were performed, including the screening of substrates and alkoxy bases as well as ligand-screening. Investigations were undertaken on the role of Cu-nanoparticles, which might be involved in this catalytic reaction. Furthermore, Cu-phosphine complexes were synthesized as precursors, but attempts to isolate Cu-boryl species which are intermediates in the proposed catalytic cycle were unsuccessful, although 11B NMR evidence for a Cu-boryl complex was obtained.
In the second part of this work, the alternative, Lewis-acidic diboron(4) compound bis(ethylene glycolato)diboron (B2eg2) was synthesized to compare its reactivity with the reactivity of other diboron(4) compounds (e.g. B2neop2, B2cat2, B2pin2 and B2(NMe2)4). Therefore, reactions of B2eg2 with different Lewis-bases, such as NHCs and phosphines, were performed to investigate the possible formation of sp2-sp3 or sp3-sp3 adducts and ring-expansion reactions (RERs).
The aim was to obtain a better general insight into the reactivity of diboron(4) compounds with Lewis-bases because they are both used as reactants in transition metal-catalyzed and metal-free borylation reactions. Understanding the B–B bond activation process promoted by Lewis-bases provides a new perspective on the reaction pathways available for various borylation reactions.
N2  - Im ersten Teil der vorliegenden Arbeit wurde das Potential eisenkatalysierter Borylierungsreaktionen von Alkylhalogeniden (Substrate) mit B2pin2 als Borylierungsreagenz untersucht. Weiterhin wurden detaillierte und intensive Untersuchungen zur literaturbekannten kupferkatalysierten Borylierung von Arylhalogeniden durchgeführt, einschließlich eines Screenings von unterschiedlich funktionalisierten Substraten und diversen Alkoxybasen. Es wurde ebenfalls ein sehr umfangreiches Ligandenscreening durchgeführt. Des Weiteren wurden die mögliche Entstehung und der mögliche Einfluss von Kupfernanopartikeln auf die Borylierungsreaktion untersucht.
Um Intermediate der kupferkatalysierten Borylierung zu untersuchen wurden Kupferphosphankomplexe als Vorläufermoleküle für die Synthese von Kupferborylkomplexen hergestellt. Aufgrund der sehr hohen Reaktivität gelang es jedoch nicht, die entsprechenden Kupferborylkomplexe zu isolieren und zu charakterisieren. Es gelang allerdings in einem in situ 11B{1H}-NMR-Experiment, ein 11B{1H}-Signal zu detektieren, welches in dem zu erwartendem Bereich für einen Kupferborylkomplex lag und einen ersten Hinweis für die Bildung eines solchen Kupferborylkomplexes lieferte.
Im zweiten Teil der vorliegenden Arbeit wurde das alternative, lewissaure Diboran(4)-Derivat Bis(ethylenglykol)diboran (B2eg2) synthetisiert, um dessen Reaktivität mit der Reaktivität von anderen Diboran(4)-Verbindungen (z.B. B2neop2, B2cat2, B2pin2 und B2(NMe2)4) zu vergleichen. Hierfür wurden Reaktionen von B2eg2 mit unterschiedlichen Lewisbasen wie NHCs und Phosphanliganden durchgeführt und die mögliche Bildung von sp2-sp3 oder sp3-sp3 hybridisierten mono- bzw. bis-Addukten sowie mögliche NHC-Ringerweiterungsreaktionen untersucht.
Im Allgemeinen wurde im zweiten Teil der Arbeit versucht ein besseres Verständnis über die Reaktivität von Diboran(4)-Verbindungen mit Lewisbasen zu erlangen, da beide als Reaktanten in übergangsmetallkatalysierten und metallfreien Borylierungsreaktionen verwendet werden. Dies macht es zwingend erforderlich die B–B-Bindungsaktivierung durch Lewisbasen zu verstehen, da hierdurch eine komplett neue Perspektive auf mögliche Reaktionspfade vieler Borylierungsreaktionen eröffnet wird.
KW  - Boron
KW  - Catalysis
KW  - Chemistry
Y1  - 2018
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-149791
ER  - 
TY  - JOUR
A1  - Betts, Jonathan
A1  - Nagel, Christopher
A1  - Schatzschneider, Ulrich
A1  - Poole, Robert
A1  - La Ragione, Robert M.
T1  - Antimicrobial activity of carbon monoxide-releasing molecule [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br versus multidrug-resistant isolates of Avian Pathogenic \(Escherichia\) \(coli\) and its synergy with colistin
JF  - PLoS ONE
N2  - Antimicrobial resistance is a growing global concern in human and veterinary medicine, with an ever-increasing void in the arsenal of clinicians. Novel classes of compounds including carbon monoxoide-releasing molecules (CORMs), for example the light-activated metal complex [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br, could be used as alternatives/to supplement traditional antibacterials. Avian pathogenic \(Escherichia\) \(coli\) (APEC) represent a large reservoir of antibiotic resistance and can cause serious clinical disease in poultry, with potential as zoonotic pathogens, due to shared serotypes and virulence factors with human pathogenic \(E.\) \(coli\). The \(in\) \(vitro\) activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br against multidrug-resistant APECs was assessed via broth microtitre dilution assays and synergy testing with colistin performed using checkerboard and time-kill assays. \(In\) \(vivo\) antibacterial activity of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br alone and in combination with colistin was determined using the \(Galleria\) \(mellonella\) wax moth larvae model. Animals were monitored for life/death, melanisation and bacterial numbers enumerated from larval haemolymph. \(In\) \(vitro\) testing produced relatively high [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br minimum inhibitory concentrations (MICs) of 1024 mg/L. However, its activity was significantly increased with the addition of colistin, bringing MICs down to \(\geq\)32 mg/L. This synergy was confirmed in time-kill assays. \(In\) \(vivo\) assays showed that the combination of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br with colistin produced superior bacterial killing and significantly increased larval survival. In both \(in\) \(vitro\) and \(in\) \(vivo\) assays light activation was not required for antibacterial activity. This data supports further evaluation of [Mn(CO)\(_3\)(tpa-\(\kappa^{3}N\))]Br as a potential agent for treatment of systemic infections in humans and animals, when used with permeabilising agents such as colistin.
KW  - Chemistry
KW  - Larvae
KW  - Antibacterials
KW  - Antibiotics
KW  - Birds
KW  - Bacterial pathogens
KW  - Manganese
KW  - Antibiotic resistance
KW  - Antibacterial therapy
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-173687
VL  - 12
IS  - 10
ER  - 
TY  - THES
A1  - Macha, Bret B.
T1  - Boron-Containing Aromatics as Communicating and Communicative Units in π-Conjugated Systems
T1  - Borhaltige Aromaten als Kommunikations-vermittelnde Einheiten in π-konjugierten Systemen
N2  - Project Borylene

A new borylene ligand ({BN(SiMe\(_3\))(t-Bu)}) has been successfully synthesized bound in a terminal manner to base metal scaffolds of the type [M(CO)\(_5\)] (M = Cr, Mo, and W), yielding complexes [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19), [(OC)\(_5\)Mo{BN(SiMe\(_3\))(t- Bu)}] (20), and [(OC)\(_5\)W{BN(SiMe\(_3\))(t-Bu)}] (21) (Figure 5-1). Synthesis of complexes 19, 20, and 21 was accomplished by double salt elimination reactions of Na\(_2\)[M(CO)\(_5\)] (M = Cr (11), Mo (1), and W (12)) with the dihaloborane Br\(_2\)BN(SiMe\(_3\))(t-Bu) (18). This new “first generation” unsymmetrical borylene ligand is closely akin to the bis(trimethylsilyl)aminoborylene ligand and has been shown to display similar structural characteristics and reactivity. The unsymmetrical borylene ligand {BN((SiMe\(_3\))(t-Bu)} does display some individual characteristics of note and has experimentally been shown to undergo photolytic transfer to transition metal scaffolds in a more rapid manner, and appears to be a more reactive borylene ligand, than the previously published symmetrical {BN(SiMe\(_3\))\(_2\)} ligand, based on NMR and IR spectroscopic evidence.

Photolytic transfer reactions with this new borylene ligand ({BN((SiMe\(_3\))(t-Bu)}) were conducted with other metal scaffolds, resulting in either complete borylene transfer or partial transfer to form bridging borylene ligand interactions between the two transition metals. The unsymmetrical ligand’s coordination to early transition metals (up to Group 6) indicates a preference for a terminal coordination motif while bound to these highly Lewis acidic species. The ligand appears to form more energetically stable bridging coordination modes when bound to transition metals with high Lewis basicity (beyond Group 9) and has been witnessed to transfer to transition metal scaffolds in a terminal manner and subsequently rearrange in order to achieve a more energetically stable bridging final state.

Figure 5-2 lists the four different transfer reactions conducted between the chromium borylene species [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19) and the transition metal complexes [(η\(^5\)-C\(_5\)H\(_5\))V(CO)\(_4\)] (51), [(η\(^5\)-C\(_5\)Me\(_5\))Ir(CO)\(_2\)] (56), [(η\(^5\)-C\(_5\)H\(_4\)Me)Co(CO)\(_2\)] (59), and [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\){μ-(CO)\(_2\)}] (53). These reactions successfully yielded the new “second generation” borylene complexes [(η\(^5\)-C\(_5\)H\(_5\))(OC)\(_3\)V{BN(SiMe\(_3\))(t-Bu)}] (55), [(η\(^5\)-C\(_5\)Me\(_5\))Ir{BN(SiMe\(_3\))(t-Bu)}\(_2\)] (58), [{(η\(^5\)-C\(_5\)H\(_4\)Me)Co}\(_2\)(μ-CO)\(_2\){μ- BN(SiMe\(_3\))(t-Bu)}] (61), and [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\)(μ-CO){μ-BN(SiMe\(_3\))(t-Bu)}] (62), respectively.

Analysis of the accumulated data for all of the terminal borylene species discussed in this section, particularly bond distances, infrared spectroscopy, and \(^{11}\)B{\(^1\)H} NMR spectroscopic data, has been performed, and a trend in the data has led to the following conclusions:
[1] NMR spectroscopic data for the \(^{11}\)B{\(^1\)H} boron and \(^{13}\)C{\(^1\)H} carbonyl environments of the first generation borylene species ([(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Cr (19), Mo (20), and W (21))) all show progressive up-field shifting as the Group 6 metal becomes heavier (Cr (19) to Mo (20) to W (21)), indicating maximum deshielding for these nuclei in the [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19) complex.
[2] The boron-metal-trans-carbon (B-M-C\(_{trans}\)) axes of the first generation borylene complexes [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Mo (20), and W (21)) are not completely linear, preventing direct IR spectroscopic comparison. The chromium analog [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19), however, is essentially linear and displays the expected three carbonyl IR stretching frequencies, all at higher energy than those of the chromium bis(trimethylsilyl)aminoborylene complex [(OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13), indicating that the ({BN(SiMe\(_3\))(t-Bu)}) ligand is either a stronger σ-donor or a poorer π-acceptor compared to the chromium metal center.
[3] In transfer reactions, the {BN(SiMe\(_3\))(t-Bu)} fragment appears to be more stable as a terminal ligand when bound to more Lewis acidic first row transition metals and appears to prefer coordination in a bridging motif when coordinated to more Lewis basic first row transition metals.

Project Borirene

The synthesis of the first platinum bis(borirene) complexes are presented along with findings from structural and electronic examination of the role of platinum in allowing increased coplanarity and conjugation of twin borirene systems. This series of trans-platinum-linked bis(borirene) complexes (119/120, 122/123, and 125/126) all show coplanarity in the twin ring systems and stand as the first verified structural representations of two coplanar borirene systems across a linking unit. The role of a platinum atom in mediating communication between chromophoric ligands can be generalized by an expected bathochromic (red) shift in the absorption spectrum due to an increase in the electronic delocalization between the formerly independent aromatic systems when compared to the platinum mono-σ-borirenyl systems. The trans-platinum bis(borirene) scaffold serves as a simplified monomeric system that allows not only study of the effects of transition metals in mitigating electronic conjugation, but also the tunability of the overall photophysical profile of the system by exocyclic augmentation of the three-membered aromatic ring.

A series of trans-platinum bis(alkynyl) complexes were prepared (Figure 5-3) to serve as stable platforms to transfer terminal borylene ligands {BN(SiMe\(_3\))\(_2\)} onto 95, 102, 106, and 63. Mixing of cis-[PtCl\(_2\)(PEt\(_3\))\(_2\)] (93) with two equivalents of corresponding alkynes in diethylamine solutions successfully yielded trans-[Pt(C≡C-Ph)\(_2\)(PEt\(_3\))\(_2\)] (95), trans-[Pt(C≡C-p-C\(_6\)H\(_4\)OMe)\(_2\)(PEt\(_3\))\(_2\)] (102), trans-[Pt(C≡C-p-C\(_6\)H\(_4\)CF\(_3\))\(_2\)(PEt\(_3\))\(_2\)](106), and trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)] (63) through salt elimination reactions.

Three of the trans-platinum bis(alkynyl) complexes (95, 102, and 106) successfully yielded trans-platinum bis(borirenyl) complexes 119/120, 122/123, and 125/126 through photolytic transfer of two equivalents of the terminal borylene ligand {BN(SiMe\(_3\))\(_2\)} from [(OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13) (Figure 5-4). Attempted borylene transfer reactions to the trans-platinum bis(alkynyl) complex trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)] (63) failed due to the complex’s photoinstability. Although a host of other variants of platinum alkynyl species were prepared and attempted, these three were the only ones that successfully yielded trans-platinum bis(borirenyl) units. Attempts were also made to create a cis variant for direct UV-vis comparison to the trans-platinum bis(borirenyl) variants, however, these attempts were also not successful. Gladysz-type platinum end-capped alkynyl species were also synthesized to serve as transfer platforms for borirene synthesis in sequential order, however, these species were also shown to not be photolytically stable.

A host of new monoborirenes: Ph-(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph (148), trans- [PtCl{(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph}(PEt\(_3\))\(_2\)] (149), and [(η\(^5\)-C\(_5\)Me\(_5\))(OC)\(_2\)Fe(μ- {BN(SiMe\(_3\))(t-Bu)}C=C)Ph] (150) were synthesized by photo- and thermolytic transfer of the unsymmetrical {BN(SiMe\(_3\))(t-Bu)} ligand from the complexes [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Cr (19), Mo (20), and W (21)) to organic and organometallic alkynyl species to verify that the borylene complexes all display similar reactivity to the symmetrical terminal borylenes of the type [(OC)\(_5\)M{BN(SiMe\(_3\))\(_2\)}] (M = Cr (13), Mo (14), and W (15)). These monoborirenes are all found to be oils when in their pure states and X-ray structural determination was impossible for these species.

Project Boratabenzene

The bis(boratabenzene) complex [{(η\(^5\)-C\(_5\)H\(_5\))Co}\(_2\){μ:η\(^6\),η\(^6\)-(BC\(_5\)H\(_5\))\(_2\)}] (189) was successfully prepared by treatment of tetrabromodiborane (65) with six equivalents of cobaltocene (176) in a unique reaction that utilized cobaltocene as both a reagent and reductant (Figure 5-5). The bimetallic transition metal complex features a new bridging bis(boratabenzene) ligand linked through a boron-boron single bond that can manifest delocalization of electron density by providing an accessible LUMO orbital for π-communication between the cobalt centers and heteroaromatic rings.

This dianionic diboron ligand was shown to facilitate electronic coupling between the cobalt metal sites, as evidenced by the potential separations between successive single-electron redox events in the cyclic voltammogram. Four formal redox potentials for complex 189 were found: E\(_{1/2}\)(1) = −0.84 V, E\(_{1/2}\)(2) = −0.94 V, E\(_{1/2}\)(3) = −2.09 V, and E\(_{1/2}\)(4) = −2.36 V (relative to the Fc/Fc+ couple) (Figure 5-6). These potentials correlate to two closely-spaced oxidation waves and two well-resolved reduction waves ([(189)]\(^{0/+1}\), [(189)]\(^{+1/+2}\), [(189)]\(^{0/–1}\), and [(189)]\(^{–1/–2}\) redox couples, respectively). The extent of metal-metal communication was found to be relative to the charge of the metal atoms, with the negative charge being more efficiently delocalized across the bis(boratabenzene) unit (class II Robin-Day system). Magnetic studies indicate that the Co(II) ions are weakly antiferromagnetically coupled across the B-B bridge.

While reduction of the bis(boratabenzene) system resulted in decomposition of the complex, oxidation of the system by one- and two-electron steps resulted in isolable stable monocationic (194) and dicationic (195) forms of the bis(boratabenzene) complex (Figure 5-7). Study of these systems verified the results of the cyclic voltammetry studies performed on the neutral species. These species are unfortunately not stable in acetonitrile or nitromethane solutions, which until this point are the only solvents that have been observed to dissolve the cationic species. Unfortunately, this instability in solution complicates reactivity studies of these cationic complexes.

Finally, reactivity studies were performed on the neutral bis(boratabenzene) complex 189 in which the compound was tested for: (A) cleavage of the boratabenzene (cyclo-BC\(_5\)H\(_5\)) ring from the cobalt center, and (B) oxidative addition of the B-B bond to a transition metal scaffold to attempt synthesis of the first ever L\(_x\)M-η\(^1\)-(BC\(_5\)H\(_5\)) complex. Both of these reactivity studies, however, proved unsuccessful and typically witnessed decomposition of the bis(boratabenzene) complex or no reactivity. After repeated attempts of these reactions, no oxidative addition of the bis(boratabenzene) system could be confirmed.
N2  - Projekt: Borylene
Ein neuer Borylen-Ligand ({BN(SiMe\(_3\))(t-Bu)}) konnte dargestellt werden, der terminal an Metallfragmente der Form [M(CO)\(_5\)] (M = Cr, Mo, und W) bindet. Die Komplexe [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19), [(OC)\(_5\)Mo{BN(SiMe\(_3\))(t- Bu)}] (20), und [(OC)\(_5\)W{BN(SiMe\(_3\))(t-Bu)}] (21) konnten durch doppelte Salzeliminierung von Na\(_2\)[M(CO)\(_5\)] (M = Cr (11), Mo (1), und W (12)) mit dem Dihalogenboran Br\(_2\)BN(SiMe\(_3\))(t-Bu) (18) erfolgreich dargestellt werden (Abbildung 6-1). Der neue, unsymmetrische Borylen-Ligand der “ersten Generation” ist eng verwandt mit dem Bis(trimethylsilyl)aminoborylen-Liganden, zu dem er, wie gezeigt werden konnte, ähnliche Strukturmerkmale und Reaktivitäten aufweist. Der unsymmetrische Borylen-Ligand {BN((SiMe\(_3\))(t-Bu)} zeigt jedoch auch einige Besonderheiten. So verläuft der photolytische Transfer auf Übergangsmetallfragmente im Vergleich zum symmetrischen {BN(SiMe\(_3\))\(_2\)}-Liganden schneller, was ihn auf der Grundlage von NMR- und IR-spektroskopischen Daten zu einem reaktiveren Borylen macht.
Photolytische Transferreaktionen des neuen Borylens ({BN((SiMe\(_3\))(t-Bu)}) mit verschiedenen Metallkomplexen verlaufen entweder unter vollständigem oder teilweisem Transfer zu verbrückenden Borylen-Komplexen. Im Falle von Lewis-aciden, frühen Übergangsmetallkomplexen (bis zur Gruppe 6) wird eine terminale Koordination bevorzugt. Bei Lewis-basischen Übergangsmetallen (jenseits der Gruppe 9) hingegen scheint der verbrückende Koordinationsmodus energetisch bevorzugt zu sein. Es wurde beobachtet, dass bei Transferreaktionen anfänglich terminale Borylen-Komplexe gebildet werden, die sich in Komplexe mit verbrückenden Borylen-Liganden umlagern. 

Abbildung 6-2 gibt eine Übersicht über die durchgeführten Transferreaktionen zwischen der Chromborylen-Spezies [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19) und den Übergangsmetallkomplexen [(η\(^5\)-C\(_5\)H\(_5\))V(CO)\(_4\)] (51), [(η\(^5\)-C\(_5\)Me\(_5\))Ir(CO)\(_2\)] (56), [(η\(^5\)-C\(_5\)H\(_4\)Me)Co(CO)\(_2\)] (59), und [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\){μ-(CO)\(_2\)}] (53). Bei diesen Umsetzungen konnten die Borylen-Komplexe der “zweiten Generation” [(η\(^5\)-C\(_5\)H\(_5\))(OC)\(_3\)V{BN(SiMe\(_3\))(t-Bu)}] (55), [(η\(^5\)-C\(_5\)Me\(_5\))Ir{BN(SiMe\(_3\))(t-Bu)}\(_2\)] (58), [{(η\(^5\)-C\(_5\)H\(_4\)Me)Co}\(_2\)(μ-CO)\(_2\){μ- BN(SiMe\(_3\))(t-Bu)}] (61), und [{(η\(^5\)-C\(_5\)H\(_5\))Ni}\(_2\)(μ-CO){μ-BN(SiMe\(_3\))(t-Bu)}] (62) erfolgreich erhalten werden.
Eine Analyse der gesammelten spektroskopischen (NMR, IR) und strukturellen Daten erlaubt folgende Schlussfolgerungen für die terminalen Borylen-Komplexe:
[1] Die NMR-spektroskopischen Daten für die Gruppe-6-Metall-Komplexe der “ersten Generation” ([(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}]  (M = Cr (19), Mo (20), und W (21)) zeigen, dass die \(^{11}\)B{\(^1\)H}- und \(^{13}\)C{\(^1\)H}-NMR Signale für die Carbonyle bei Gang zu den schwereren Homologen zu höherem Feld verschoben sind. Daraus resultiert die größte Entschirmung der Kerne für den Komplex [(OC)\(_5\)Cr{BN(SiMe\(_3\))(t-Bu)}] (19).
[2] Die Bor-Metall-trans-Kohlenstoff-Achse (B-M-C\(_{trans}\)) in den Borylen-Komplexen der “ersten Generation” [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Mo (20) und W (21)) ist nicht linear, was einen direkten Vergleich der IR-Schwingungsbanden miteinander verhindert. Die Chrom-Verbindung ist nahezu linear und zeigt die erwarteten drei IR-Schwingungsbanden für die Carbonyle, die im Vergleich zum Chrom-Bis(trimethylsilyl)aminoborylen-Komplex (OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13) zu höheren Energien verschoben sind. Die Befunde deuten darauf hin, dass der ({BN(SiMe\(_3\))(t-Bu)})-Ligand ein etwas stärkerer σ-Donor ist.
[3] Wie in Transferreaktionen gezeigt werden konnte, bevorzugt der Borylen-Ligand {BN(SiMe\(_3\))(t-Bu)} einen terminalen Koordinationsmodus bei Lewis-aciden, frühen Übergangsmetallen und einen verbrückenden Koordinationsmodus bei Lewis-basischen, späten Übergangsmetallen.

Projekt: Borirene
Die Synthese der ersten Bis(borirenyl)platin-Komplexe wurde vorgestellt, zusammen mit strukturellen und elektronischen Auswirkungen der koplanar angeordneten Borirenringe. Die Reihe der trans-Bis(borirenyl)platin-Komplexe 119/120, 122/123, und 125/126 stellt die ersten Beispiele für Bis(boriren)-Komplexe dar, in denen die Ringsysteme koplanar zueinander angeordnet sind (Abbildung 6-4). Die Rolle des Platinatoms als Vermittler elektronischer Kommunikation zwischen den chromophoren, aromatischen Borliganden manifestiert sich in einem bathochromen Shift der Boriren-Absorptionsbanden im Vergleich zu Mono-σ-Borirenyl-Systemen, als Folge einer ausgedehnten Elektronendelokalisierung. Das trans-Bis(boriren)platin-Gerüst dient dabei als vereinfachtes monomeres System, um sowohl die Effekte des Übergangmetallatoms in der Vermittlung elektronischer Konjugation als auch die Abstimmbarkeit der photophysikalischen Eigenschaften in Abhängigkeit der dreigliedrigen, aromatischen Ringe zu studieren.

Eine Reihe von trans-Bis(alkinyl)platin-Komplexen (95, 102, 106, und 63) wurde als Plattform für den Borylentransfer von {BN(SiMe\(_3\))\(_2\)} dargestellt (Abbildung 6-3). Durch Salzeliminierungsreaktionen von cis-[PtCl\(_2\)(PEt\(_3\))\(_2\)] (93) mit zwei Äquivalenten des entsprechenden Alkins in Diethylamin ließen sich die Komplexe "trans-[Pt(C≡C-Ph)\(_2\)(PEt\(_3\))\(_2\)]"  (95), "trans-[Pt(C≡C-p-C\(_6\)H\(_4\)OMe)\(_2\)(PEt\(_3\))\(_2\)] "  (102), "trans-[Pt(C≡C-p-C\(_6\)H\(_4\)CF\(_3\))\(_2\)(PEt\(_3\))\(_2\)]"  (106), und "trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)]"  (63) synthetisieren.
Drei trans-Bis(borirenyl)platin-Komplexe (119/120, 122/123, und 125/126) konnten erfolgreich durch photolytischen Transfer des terminalen Borylen-Liganden {BN(SiMe\(_3\))\(_2\)} vom Chrom-Komplex [(OC)\(_5\)Cr{BN(SiMe\(_3\))\(_2\)}] (13) auf die trans-Bis(alkinyl)platin-Komplexe (95, 102, und 106) dargestellt werden (Abbildung 6-4). Versuche des Borylentransfers auf den trans-Bis(alkinyl)platin-Komplex "trans-[Pt(C≡C-9-C\(_{14}\)H\(_9\))\(_2\)(PEt\(_3\))\(_2\)]"  (63) scheiterten an der Photoinstabilität des Komplexes. Obwohl eine weitere Vielzahl an Platin-Alkinyl-Komplexen dargestellt wurde, verlief der Borylentransfer nur bei den drei eben erwähnten Komplexen erfolgreich. Weitere Bestrebungen, ein cis-Isomer zum direkten Vergleich mit den trans-konfigurierten Komplexen darzustellen, verliefen ebenfalls erfolglos. Außerdem wurden terminal funktionalisierte Platin-Komplexe vom Gladysz-Typ als Plattform zum Borylentransfer synthetisiert, zeigten sich aber ebenfalls instabil unter photolytischen Bedingungen.

Eine Reihe neuer Monoborirene Ph-(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph (148), trans- [PtCl{(μ-{BN(SiMe\(_3\))(t-Bu)}C=C)-Ph}(PEt\(_3\))\(_2\)] (149), und [(η\(^5\)-C\(_5\)Me\(_5\))(OC)\(_2\)Fe(μ- {BN(SiMe\(_3\))(t-Bu)}C=C)Ph] (150) wurde durch photolytischen und thermischen Transfer des unsymmetrischen {BN(SiMe\(_3\))(t-Bu)}-Liganden ausgehend von den Komplexen [(OC)\(_5\)M{BN(SiMe\(_3\))(t-Bu)}] (M = Cr (19), Mo (20), und W (21)) auf organische und organometallische Alkin-Substrate dargestellt. Ziel dieser Umsetzungen war es zu untersuchen, ob der unsymmetrische Borylen-Ligand eine andere Reaktivität im Vergleich zu den symmetrischen, terminalen Borylenen der Form [(OC)\(_5\)M{BN(SiMe\(_3\))\(_2\)}] (M = Cr (13), Mo (14), und W (15)) aufweist. Die entsprechenden Monoborirene wurden als analytisch reine Öle isoliert. Zur Röntgenstrukturanalyse geeignete Einkristalle konnten demnach nicht erhalten werden.

Projekt: Boratabenzole
Der Bis(boratabenzol)-Komplex [{(η\(^5\)-C\(_5\)H\(_5\))Co}\(_2\){μ:η\(^6\),η\(^6\)-(BC\(_5\)H\(_5\))\(_2\)}](189) wurde durch Umsetzung von Tetrabromdiboran (65) mit sechs Äquivalenten Cobaltocen erfolgreich dargestellt. Die Reaktion ist in der Hinsicht besonders, da Cobaltocen (176) sowohl als Reagens als auch als Reduktionsmittel fungiert (Abbildung 6-5). Der bimetallische Übergangsmetallkomplex besteht aus einem verbrückenden Bis(boratabenzol)-Liganden, der die beiden Metallzentren durch eine Bor-Bor-Einfachbindung verbindet. Es konnte gezeigt werden, dass der neuartige Ligand eine Delokalisierung der π-Elektronen zwischen den Cobaltatomen ermöglicht, begünstigt durch das zur Bor-Bor-Bindung zugehörige LUMO.
Wie durch die Separation aufeinanderfolgender Redoxprozesse im Cyclovoltammogramm gezeigt werden konnte, vermittelt der dianionische Diborligand die elektronische Kopplung zwischen den Cobaltatomen. Vier verschiedene Redoxpotentiale wurden für Komplex 189 gefunden:E\(_{1/2}\)(1) = −0.84 V, E\(_{1/2}\)(2) = −0.94 V, E\(_{1/2}\)(3) = −2.09 V, und E\(_{1/2}\)(4) = −2.36 V (referenziert gegen das Fc/Fc+-Paar) (Abbildung 6-6). Die Potentiale können zwei dicht beieinander liegenden Oxidationsprozessen ([(189)]\(^{0/+1}\) und [(189)]\(^{+1/+2}\) und zwei gut getrennten Reduktionsprozessen [(189)]\(^{+1/+2}\) und [(189)]\(^{–1/–2}\) zugeordnet werden. Das Ausmaß der Metall-Metall-Kommunikation ist dabei abhängig von der Ladung der Metallatome. Die negative Ladung wird effektiver durch die Bis(boratabenzol)-Einheit delokalisiert (Robin-Day-Klasse II). Magnetische Messungen deuten darauf hin, dass die Cobalt(II)-Ionen schwach antiferromagnetisch über die B-B-Brücke gekoppelt sind. 
Während die Reduktion des Bis(boratabenzol)-Systems zur Zersetzung des Komplexes führte, resultierte die Ein- und Zwei-Elektronen-Oxidation in isolierbaren, stabilen monokationischen (194) bzw. dikationischen (195) Formen des Bis(boratabenzol)-Komplexes (Abbildung 6-7). Die präparativen Arbeiten bestätigen damit die elektrochemischen Ergebnisse an der neutralen Verbindung. Die kationischen Spezies zeigten sich instabil in Acetonitril oder Nitromethan, den einzigen Lösungsmitteln worin sie sich lösten, was weitere Reaktivitätsstudien erschwerte.
Schließlich wurde die Reaktivität des neutralen Bis(boratabenzol)-Komplexes 189 eingehender untersucht. Versuche zur (A) Abspaltung des freien Bis(boratabenzol)-Liganden durch Dekomplexierung der Cyclopentadienylcobalt-Fragmente und zur (B) oxidativen Addition von Übergangsmetall-Komplexen an die B-B-Bindung, um die ersten L\(_x\)M-η\(^1\)-(BC\(_5\)H\(_5\))Komplexe darzustellen, wurden unternommen. Alle Versuche zeigten jedoch, dass die eingesetzten Reagenzien entweder zu keinen Umsetzungen oder zur Zersetzung des Bis(boratabenzol)-Komplexes führten. Bisher konnten somit noch keine Hinweise auf eine oxidative Addition der B-B-Bindung im Bis(boratabenzol)-Komplex erhalten werden.
KW  - Borverbindungen
KW  - Boron Chemistry
KW  - Aromatic Systems
KW  - Transition Metals
KW  - Aromaten
KW  - Übergangsmetall
KW  - Chemistry
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-137498
ER  -