546 Anorganische Chemie
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- Fraunhofer-Institut für Chemische Technologie (ICT) (1)
- Institut Ruđer Bošković, Zagreb, Croatia (1)
- Institut für Organische Chemie, RWTH Aachen (1)
- Institute of Transformative Bio-Molecules, Nagoya University, Nagoya, Japan (1)
- Université de Bordeaux, Bordeaux, France (1)
- Université de Montréal (1)
N-heterocyclic carbenes (NHC) are utilized for the stabilization of reactive compounds, for the activation of strong bonds, and as ligands in transition metal chemistry. In contrast to neutral NHCs, few examples of anionic or even dianionic NHCs are known. One approach for the synthesis of anionic carbenes is the deprotonation of neutral or anionic precursors, bearing Lewis acids instead of alkyl or aryl substituents. Following this strategy, novel anionic and dianionic NHCs, featuring weakly coordinating fluorinated borane and phosphorane substituents or coordinating tricyanoborane substituents were synthesized within the scope of this thesis. These carbenes possess unprecedented stabilities compared to related species. Furthermore, their electronic and steric properties can be directly adjusted by the type of Lewis acid attached. Their potential as ligands with highly shielding weakly coordinating substituents next to the carbene coordination center was demonstrated by the syntheses of the respective NHC selenium adducts and NHC gold(I) complexes. In contrast anionic NHCs with coordinating tricyanoborane moieties have an outstanding potential as ditopic ligands with coordination being possible at the carbene center and via the cyano groups. Their beneficial ligand properties were demonstrated by the syntheses of the respective NHC selenium adducts and NHC nickeltricarbonyl complexes. The combination of electronic properties, the large buried volume, the negative charge, the possibility to act as ditopic or ligands with weakly coordinating groups, and the ease of accessibility render borane- and phosphorane functionalized NHCs unique novel ligands. A further project of this PhD thesis deals with the steric properties of Lewis acids. Therefore, an easy-to-apply model was designed to quantify the steric demand of Lewis acids. Using the results of this evaluation, a second model was developed which judges the steric repulsion in Lewis acid/base adduct formation for arbitrary sets of acids and bases.
Anionic Adducts
Sp2-sp3 tetraalkoxy diboron compounds have gained attention due to the development of new, synthetically useful catalytic reactions either with or without transition-metals. Lewis-base adducts of the diboron(4) compounds were suggested as possible intermediates in Cu catalyzed borylation reactions some time ago. However, intermolecular adducts of tetraalkoxy diboron compounds have not been studied yet in great detail. In preliminary studies, we have synthesized a series of anionic sp2-sp3 adducts of B2pin2 with alkoxy-groups (L = [OMe]–, [OtBu]–), a phenoxy-group (L = [4-tBuC6H4O]–) and fluoride (L = [F]–, with [nBu4N]+ as the counter ion) as Lewis-bases.
Neutral Adducts
Since their isolation and characterization, applications of N-heterocyclic carbenes (NHCs) and related molecules, e.g., cyclic alkylaminocarbenes (CAACs) and acyclic diaminocarbenes (aDCs), have grown rapidly. Their use as ligands in homogeneous catalysis and directly in organocatalysis, including recently developed borylation reactions, is now well established. Recently, several examples of ring expansion reactions (RER) involving NHCs were reported to take place at elevated temperatures, involving Be, B, and Si.
Furthermore, preliminary studies in the group of Marder et al. showed the presence of neutral sp2-sp3 diboron compounds with B2pin2 and the NHC Cy2Im. In this work, we focused on the synthesis and characterization of further neutral sp2-sp3 as well as sp3-sp3 diboron adducts with B2cat2 and B2neop2 and different NHCs. Whereas the mono-NHC adduct is stable for several hours at temperatures up to 60 °C, the bis-NHC adducts undergo thermally induced rearrangement to form the ring expanded products compound 26 and 27. B2neop2 is much more reactive than B2cat2 giving ring expanded product 29 at room temperature in quantitative yields, demonstrating that NHC ring expansion and B–B bond cleavage can be very facile processes.
Whereas the mono-NHC adduct is stable for several hours at temperatures up to 60 °C, the bis-NHC adducts undergo thermally induced rearrangement to form the ring expanded products compound 26 and 27. B2neop2 is much more reactive than B2cat2 giving ring expanded product 29 at room temperature in quantitative yields, demonstrating that NHC ring expansion and B–B bond cleavage can be very facile processes.
Persistent room temperature phosphorescent (RTP) luminophores have gained remarkable interest recently for a number of applications in security printing, OLEDs, optical storage, time-gated biological imaging and oxygen sensors. We report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. We also have prepared 3 isomeric (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers (o-, m- and p-BrTAB) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. At last, we checked the RTP properties from the boric acid. We found that the pure boric acid does not show RTP in the solid state.
Organoboron compounds are important building blocks in organic synthesis, materials science, and drug discovery. The development of practical and convenient ways to synthesize boronate esters attracted significant interest. Photoinduced borylations originated with stoichiometric reactions of arenes and alkanes with well-defined metal-boryl complexes. Now photoredox-initiated borylations, catalyzed either by transition-metal or organic photocatalysts, and photochemical borylations with high efficiency have become a burgeoning area of research. In this chapter, we summarize research in the field of photocatalytic C-X borylation, especially emphasizing recent developments and trends, based on transition-metal catalysis, metal-free organocatalysis and direct photochemical activation. We focus on reaction mechanisms involving single electron transfer (SET), triplet energy transfer (TET), and other radical processes.
We developed a highly selective photocatalytic C-F borylation method that employs a rhodium biphenyl complex as a triplet sensitizer and the nickel catalyst [Ni(IMes)2] (IMes = 1,3-dimesitylimidazolin-2-ylidene) for the C-F bond activation and defluoroborylation process. This tandem catalyst system operates with visible (400 nm) light and achieves borylation of a wide range of fluoroarenes with B2pin2 at room temperature in excellent yields and with high selectivity. Direct irradiation of the intermediary C-F bond oxidative addition product trans-[NiF(ArF)(IMes)2] leads to fast decomposition when B2pin2 is present. This destructive pathway can be bypassed by indirect excitation of the triplet states of the nickel(II) complex via the photoexcited rhodium biphenyl complex. Mechanistic studies suggest that the exceptionally long-lived triplet excited state of the Rh biphenyl complex used as the photosensitizer allows for efficient triplet energy transfer to trans-[NiF(ArF)(IMes)2], which leads to dissociation of one of the NHC ligands. This contrasts with the majority of current photocatalytic transformations, which employ transition metals as excited state single electron transfer agents. We have previously reported that C(arene)-F bond activation with [Ni(IMes)2] is facile at room temperature, but that the transmetalation step with B2pin2 is associated with a high energy barrier. Thus, this triplet energy transfer ultimately leads to a greatly enhanced rate constant for the transmetalation step and thus for the whole borylation process. While addition of a fluoride source such as CsF enhances the yield, it is not absolutely required. We attribute this yield-enhancing effect to (i) formation of an anionic adduct of B2pin2, i.e. FB2pin2-, as an efficient, much more nucleophilic {Bpin-} transfer reagent for the borylation/transmetalation process, and/or (ii) trapping of the Lewis acidic side product FBpin by formation of [F2Bpin]- to avoid the formation of a significant amount of NHC-FBpin and consequently of decomposition of {Ni(NHC)2} species in the reaction mixture.
We reported a highly selective and general photo-induced C-Cl borylation protocol that employs [Ni(IMes)2] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes. This photo-induced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B2pin2 at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes)2] undergoes very fast chlorine atom abstraction from aryl chlorides to give [NiI(IMes)2Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [NiI(IMes)2Cl] with aryl chlorides generating additional aryl radicals and [NiII(IMes)2Cl2]. The aryl radicals react with an anionic sp2-sp3 diborane [B2pin2(OMe)]- formed from B2pin2 and KOMe to yield the corresponding borylation product and the [Bpin(OMe)]•- radical anion, which reduces [NiII(IMes)2Cl2] under irradiation to regenerate [NiI(IMes)2Cl] and [Ni(IMes)2] for the next catalytic cycle.
A highly efficient and general protocol for traceless, directed C3-selective C-H borylation of indoles with [Ni(IMes)2] as the catalyst was achieved. Activation and borylation of N-H bonds by [Ni(IMes)2] is essential to install a Bpin moiety at the N-position as a traceless directing group, which enables the C3-selective borylation of C-H bonds. The N-Bpin group which is formed is easily converted in situ back to an N-H group by the oxidiative addition product of [Ni(IMes)2] and in situ-generated HBpin. The catalytic reactions are operationally simple, allowing borylation of of a variety of substituted indoles with B2pin2 in excellent yields and with high selectivity. The C-H borylation can be followed by Suzuki-Miyaura cross-coupling of the C-borylated indoles in an overall two-step, one-pot process providing an efficient method for synthesizing C3-functionalized heteroarenes.
This thesis describes the synthesis and reactivity of bis-NHC ligated nickel(0)-complexes and their application in catalytic cyclization and borylation reactions of alkynes. The focus of the presented work lies on the investigation of the electronic and steric impact of different NHC ligands on the reactivity and catalytic activity of [Ni(NHC)2] complexes. Since d10 ML2 complexes play a decisive role for numerous catalytic reactions, such as the Suzuki-Miyaura cross-coupling, the first chapter provides an overview about the general properties of NHCs and the chemistry of NHC-ligated nickel complexes, their synthesis, characterization, reactivity, and application in catalysis.
The aim of this work was to synthesize and functionalize different bio-relevant nanomaterials like silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) as contrast agents for T2 magnetic resonance imaging (MRI) and detonation nanodiamond (DND) with the neurohormone peptide allatostatin 1 (ALST1) and a fluorescent dye. Analytical techniques for the determination and quantification of surface functional groups like amines, azides, and peptides were also developed and established.
Thus, in the first part of the work, a TGF-1 binding peptide and allatostatin 1 (ALST1), both supposed to act as active tumour targeting vectors, were synthesized by solid-phase peptide synthesis (SPPS) and characterized by high pressure liquid chromatography (HPLC) and mass spectrometry. Then, azide-functionalized silica nanoparticles were synthesized by the Stöber process and characterized by transmission electron microscopy (TEM) and infrared spectroscopy (IR). The surface loading of amine and azide groups was determined by a new protocol. The azide groups were reduced with sodium boronhydride to amine and then functionalized with Fmoc-Rink Amide linker according to a standard SPPS protocol. Upon cleavage of Fmoc by piperidine, the resulting dibenzofulvene and its piperidine adduct were quantified by UV/Vis spectroscopy and used to determine the amount of amine groups on the nanoparticle surface. Then, ALST1 and related tyrosine- and phenylalanine substituted model peptides were conjugated to the azide-functionalized silica nanoparticles by copper(I)-catalyzed azide-alkyne dipolar cycloaddition (CuAAC). The successful peptide conjugation was demonstrated by the Pauly reaction, which however is only sensitive to histidine- and tyrosine-containing peptides. As a more general alternative, the acid hydrolysis of the peptides to their individual amino acid building blocks followed by derivatization with phenyl isothiocyanate (PITC) allowed the separation, determination, and quantification of the constituent amino acids by HPLC.
In the second part of the work, amine- and azide-functionalized silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized by co-precipitation and subsequent silica-coated based on the Stöber process and characterized by TEM and IR. The amine surface loading was determined by the method already established for the pure silica systems. The azide surface loading could also be quantified by reduction with sodium boronhydride to amine groups and then conjugation to Fmoc-Rink amide linker. Upon cleavage of Fmoc with piperidine, the total amine surface loading was obtained. The amount of azide surface groups was then determined from the difference of the total amine surface loading and the amine surface loading. Thus, it was possible to quantify both amine and azide surface groups on a single nanoparticle system. Superparamagnetic iron oxide nanoparticles (SPIONs) are potent T2 contrast agents for magnetic resonance imaging (MRI). Due to their natural metabolism after injection into the blood stream, SPIONs mostly end up inside macrophages, liver, spleen or kidneys. To generate a potential target-specific SPION-based T2 contrast agent for MRI, the neurohormone peptide ALST1 was conjugated by CuAAC to the azide- and amine functionalized superparamagnetic iron oxide nanoparticles, since ALST1 is supposed to target difficult-to-treat neuroendocrinic tumours due to its analogy to galanin and somastatin receptor ligands. The organic fluorescent dye cyanine 5 (Cy5) was also conjugated to the silica-coated superparamagnetic iron oxide nanoparticles (SPIONs) via a NHS-ester to the amines to enable cell uptake studies by fluorescence microscopy. These constructs were characterized by TEM, dynamic light scattering (DLS), and IR. The amino acids of the conjugated ALST1 were determined by the HPLC method as described before for peptide-modified silica nanoparticle surfaces. Then, the relaxivity r2 was measured at 7 T. However, a r2 value of 27 L/mmolFe·s for the dual ALST1-/Cy5-functionalized silica-coated SPIONs was not comparable to T2 contrast agents in clinical use, since their relaxivity is commonly determined at 1.5 T, and no such instrument was available. However, it can be assumed that the synthesized dual
ALST1-/Cy5-functionalized silica-coated SPION would show a lower r2 at 1.5 T than at 7T. Commercial T2 MRI contrast agents like VSOP-C184 from Ferropharm show at r2 values of about 30 L/mmolFe·s at 1.5 T. Still, the relaxivity of the new material has some potential for application as a T2 contrast agent. Then, the material was used in cell uptake studies by fluorescence microscopy with the conjugated Cy5 dye as a probe. The dual
ALST1-/Cy5-functionalized silica-coated SPION showed a high degree of agglomeration with no cellular uptake unlike described for ALST1-functionalized nanoparticles in literature. It is assumed that upon agglomeration of the particles, constructs form which are unable to be internalized by the cellular endocytotic pathways anymore. As a future perspective, the tendency of the particle to agglomerate should be reduced by changing the coating material to polyethylene glycol (PEG) or chitosan, which are known to be bio-compatible, bio-degradable and prevent agglomeration.
In the third part of the work, the rhenium compound [ReBr(CO)3(L)] with L = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline and its manganese analogue were synthesized by heating the ligand and rhenium pentacarbonyl bromide or and manganese pentacarbonyl bromide respectively, in toluene. However, [MnBr(CO)3(L)] was unstable upon illumination by UV light at 365 nm. Thus, it was dismissed for further application. The photophysical properties of [ReBr(CO)3(L)] were explored, by determination of the excited-state life time by the time-correlated single-photon counting (TCSPC) method and the quantum yield by a fluorescence spectrometer equipped with an integration sphere. A value of = 455 ns, a Stokes shift of 197 nm and a rather low quantum yield =were found. Metal complexes are supposed to have superior properties compared to organic dyes due to their large Stokes shifts, long excited-state life times, and high quantum yields. Thus, amine- and azide-functionalized detonation nanodiamond (DND) as an alternative biological inert carrier system was functionalized with ALST1 to enhance its cell uptake properties. A luminescent probe for cell uptake studies using fluorescence microscopy was also attached, either based on the new rhenium complex or the commercially available organic dye Cy5, respectively. The aldehyde-functionalized rhenium complex was conjugated to the DND via oxime ligation, which is known to be a mild and catalyst-free conjugation method. The amount of peptide ALST1 on the DND was analyzed and quantified after acid hydrolysis and PITC derivatization by HPLC as described before. Then, the ALST1-/luminescent probe-functionalized DND was investigated for its photophysical properties by fluorescence spectroscopy. The Cy5-functionalized material showed a slightly lower fluorescence performance in aqueous solution than reported in literature and commercial suppliers with a life time < 0.4 ns and quantum yields not determinable by integration sphere due to the week signal intensity. The rhenium complex-functionalized material had a very low signal intensity in only aqueous medium, and thus determination of life times and quantum yield by fluorescence spectroscopy was not possible. After incubation with MDA-MB 231 cells, the Cy5-functionalized DND could easily be detected due to its red fluorescence. However, it was not possible to visualize the rhenium complex-functionalized DND with fluorescence microscopy due to the low fluorescence intensity of the complex in aqueous medium and the lack of proper filters for the fluorescence microscope. Cy5-functionalized DND did not show any cellular uptake in fluorescence microscopy after conjugation with ALST1. Since the nanodiamond surface is known to strongly adsorb peptides and proteins, it is assumed that the peptide chain is oriented perpendicular to the nanoparticle surface and thus not able to interact with cell membrane receptors to promote cell uptake of the particles. As a future perspective, the ALST1-promoted cellular uptake of the DND should be improved by using different linker systems for peptide conjugation to prevent adsorption of the peptide chain on the particle surface.
The new analytical methods for amino-, azide-, and peptide-functionalized nanoparticles have great potential to assist in the quantification of nanoparticle surface modifications by UV/Vis spectroscopy and HPLC. The determination of surface amine and azide groups based on the cleavage of conjugated Fmoc-Rink amide linker and detected by UV/Vis spectroscopy is applicable to all amine-/azide-functionalized nanomaterials. However, particles which form very stable suspension with the cleavage mixture can cause quantification problems due to scattering, making an accurate quantification of dibenzofulvene and its piperidine adduct impossible. The detection of tyrosine- and histidine-containing peptides based on the Pauly reaction is well-suited as a fast and easy-to-perform qualitative demonstration of successful peptide surface conjugation. However, its major drawback as a colourimetric approach is that coloured particles cannot be evaluated by this method. The amino acid analysis based on HPLC after acid hydrolysis of peptides conjugated to nanoparticle surfaces to its individual building blocks and subsequent derivatization with PITC, can be used on all nanomaterials with peptide or protein surface modification. It allows detection of amino acids down to picomolar concentrations and even enables analysis of very small peptide surface loadings. However, the resulting HPLC traces are difficult to analyze.
Three new analytical methods based on UV/Vis and HPLC techniques have been developed and established. They assisted in the characterization of the synthesized DND and SPIONs with dual functionalization by ALST1 and Cy5 or [ReBr(CO)3(L)], respectively. However, the nanomaterials showed no cellular uptake due to a high tendency to agglomerate. The cellular uptake should be improved and the tendency to agglomerate of the SPIONs should be reduced by changing the surface coating from silica to either PEG or chitosan. Furthermore, different linker systems for connecting peptides to DND surfaces should be synthesized and evaluated to reduce potential peptide chain adsorption.
The photochemistry and photophysics of transition metal complexes are of great interest, since such materials can be exploited for a wide range of applications such as in photocatalysis, sensing and imaging, multiphoton-absorption materials and the fabrication of OLEDs. A full understanding of the excited state behavior of transition metal compounds is therefore important for the design of new materials for the applications mentioned above. In principle, the luminescence properties of this class of compounds can be tuned by changing the metal or subtle changes in the ligand environment.
Furthermore, transition-metal complexes continue to play a major role in modern synthetic chemistry. In particular, they can realize selective transformations that would either be difficult or impossible by conventional organic chemistry. For example, they enable the efficient and selective formation of carbon–carbon bonds. One famous example of these types of transformations are metal-catalyzed cyclization reactions. Herein, metallacyclopentadiene complexes are considered as key intermediates in a number of metal-mediated or -catalyzed cyclization reactions, i.e. the [2+2+2] cyclotrimerization of alkynes. Recent research has focused on the synthesis and characterization of these metallacyclic intermediates such as MC4 ring systems. Metallacyclopentadienes are structurally related to main group EC4 systems such as boroles, siloles, thiophenes and phospholes. Overall, this group of compounds (EC4 analogues) is well known and has attracted significant attention due to their electron-transport and optical properties. Unlike transition metal analogues, however, these EC4 systems show no phosphorescence, which is due to inefficient SOC compared to 2nd and 3rd row transition metals, which promoted us to explore the phosphorescence potential of metallacyclopentadienes.
In 2001, Marder et al. developed a one-pot high-yield synthesis of luminescent 2,5 bis(arylethynyl)rhodacyclopentadienes by reductive coupling of 1,4-diarylbuta-1,3-diynes at a suitable rhodium(I) precursor. Over the past years, a variety of ligands (e.g. TMSA, S,S’ diethyldithiocarbamate, etc.) and 1,4-bis(p-R-phenyl)-1,3-butadiynes or linked , bis(p-R-arylethynyl)alkanes (R = electron withdrawing or donating groups) were investigated and always provided a selective formation of 2,5 bis(arylethynyl)rhodacyclopentadienes, which were reported to be fluorescent despite presence of the heavy atom. To examine the influence of the ligand sphere around the rhodium center on the intersystem-crossing (ISC) processes in the above-mentioned fluorescent rhodacyclopentadienes and to increase the metal character in the frontier orbitals by destabilizing the Rh filled d-orbitals, a -electron donating group was introduced, namely acetylacetonato (acac). Interestingly, in 2010 Tay reacted [Rh(κ2-O,O-acac)(PMe3)2] with ,-bis(p-R-arylbutadiynyl)alkanes and observed not only the fluorescent 2,5 bis(arylethynyl)rhodacyclopentadienes, but also rhodium 2,2’-bph complexes as products, which were reported to be phosphorescent in preliminary photophysical studies.
In this work, the reaction behavior of [Rh(κ2-O,O-acac)(L)2] (L = PMe3, P(p-tolyl)3) with different ,-bis(p-R-arylbutadiynyl)alkanes was established. Furthermore, the separation of the two isomers 2,5-bis(arylethynyl)rhodacyclopentadienes (A) and rhodium 2,2’-bph complexes (B), and the photophysical properties of those were explored in order to clarify their fundamentally different excited state behaviors.
Reactions of [Rh(κ2-O,O-acac)(P(p-tolyl3)2)] with ,-bis(arylbutadiynyl)alkanes gives exclusively weakly fluorescent 2,5-bis(arylethynyl)rhodacyclopentadienes. Changing the phosphine ligands to PMe3, reactions of [Rh(κ2-O,O-acac)(PMe3)2] and , bis(arylbutadiynyl)alkanes afford two isomeric types of MC4 metallacycles with very different photophysical properties, as mentioned before.
As a result of a normal [2+2] reductive coupling at rhodium, 2,5 bis(arylethynyl)rhodacyclopentadienes (A) are formed, which display intense fluorescence. Rhodium 2,2’-bph complexes (B), which show phosphorescence, have been isolated as a second isomer originating from an unusual [4+2] cycloaddition reaction and a subsequent -H-shift. Control of the isomer distribution, of 2,5-bis(arylethynyl)rhodacyclopentadienes (A) and rhodium biphenyl complexes (B), is achieved by modification of the linked , bis(arylbutadiynyl)alkane.
Changing the linker length from four CH2 to three CH2 groups, dramatically favors the formation of the rhodium biphenyl isomer B, providing a fundamentally new route to access photoactive metal biphenyl compounds in good yields. This is very exciting as the photophysical properties of only a limited number of bph complexes of Ir, Pd and Pt had been explored. The lack of photophysical reports in the literature is presumably due to the limited synthetic access to various substituted 2,2’-bph transition metal complexes.
On the other hand, as the reaction of [Rh(κ2-O,O-acac)(P(p-tolyl)3)2] with , bis(arylbutadiynyl)alkanes provides a selective reaction to give weakly fluorescent 2,5 bis(arylethynyl)rhodacyclopentadiene complexes with P(p-tolyl)3 as phosphine ligands, a different synthetic access to 2,5-bis(arylethynyl)rhodacyclopentadiene complexes with PMe3 as phosphine ligands was developed, preventing the time-consuming separation of the isomers. The weak rhodium-phosphorus bonds of 2,5-bis(arylethynyl)rhodacyclopentadiene complexes bearing P(p tolyl)3 as phosphine ligands, relative to those of related PMe3 complexes, allowed for facile ligand exchange reactions. In the presence of an excess of PMe3, a stepwise reaction was observed, giving first the mono-substituted, mixed-phosphine rhodacyclopentadiene intermediates and, subsequently, full conversion to the highly fluorescent 2,5 bis(arylethynyl)-rhodacyclopentadienes bearing only PMe3 ligands (by increasing the reaction temperature).
With spectroscopically pure 2,5-bis(arylethynyl)rhodacyclopentadiene complexes A (bearing PMe3 as phosphine ligands) and rhodium 2,2-bph complexes B in hand, photophysical studies were conducted. The 2,5-bis(arylethynyl)rhodacyclopentadienes (A) are highly fluorescent with high quantum yields up to 54% and very short lifetimes (τ = 0.2 – 2.5 ns) in solution at room temperature. Even at 77 K in glass matrices, no additional phosphorescence is observed which is in line with previous observations made by Steffen et al., who showed that SOC mediated by the heavy metal atom in 2,5-bis(arylethynyl)rhodacyclopentadienes and 2,5 bis(arylethynyl)iridacyclopentadienes is negligible. The origin of this fluorescence lies in the pure intra-ligand (IL) nature of the excited states S1 and T1. The HOMO and the LUMO are nearly pure and * ligand orbitals, respectively, and the HOMO is energetically well separated from the filled rhodium d orbitals. The absence of phosphorescence in transition metal complexes due to mainly IL character of the excited states is not unusual, even for heavier homologues than rhodium with greater SOC, resulting in residual S1 emission (fluorescence) despite ISC S1→Tn being sufficiently fast for population of T1 states. However, there are very few complexes that exhibit fluorescence with the efficiency displayed by our rhodacyclopentadienes, which involves exceptionally slow S1→Tn ISC on the timescale of nanoseconds rather than a few picoseconds or faster.
In stark contrast, the 2,2’-bph rhodium complexes B are exclusively phosphorescent, as expected for 2nd-row transition metal complexes, and show long-lived (hundreds of s) phosphorescence (Ф = 0.01 – 0.33) at room temperature in solution. As no fluorescence is detected even at low temperature, it can be assumed that S1→Tn ISC must be faster than both fluorescence and non-radiative decay from the S1 state. This contrasts with the behavior of the isomeric 2,5-bis(arylethynyl)rhodacyclopentadienes for which unusually slow ISC occurs on a timescale that is competitive with fluorescence (vide supra). The very small values for the radiative rate constants, however, indicate that the nature of the T1 state is purely 3IL with weak SOC mediated by the Rh atom. The phosphorescence efficiency of these complexes in solution at room temperature is even more impressive, as non-radiative coupling of the excited state with the ground state typically inhibits phosphorescence. Instead, the rigidity of the organic -system allows the ligand-based excited triplet state to exist in solution for up to 646 s and to emit with high quantum yields for biphenyl complexes. The exceptionally long lifetimes and small radiative rate constants of the rhodium biphenyl complexes are presumably a result of the large conjugated -system of the organic ligand. According to TD DFT studies, the T1 state involves charge-transfer from the biphenyl ligand into the arylethynyl moiety away from the rhodium atom. This reduces the SOC of the metal center that would be necessary for fast phosphorescence. These results show that the π-chromophoric ligand can gain control over the photophysical excited state behavior to such an extent that even heavy transition metal atoms like rhodium participate in increasing the fluorescence such as main-group analogues do. Furthermore, in the 2,2’-bph rhodium complexes, the rigidity of the organic -system allows the ligand-based excited triplet state to exist in solution for up to hundreds of s and to emit with exceptional quantum yields.
Therefore, investigations of the influence of the ligand sphere around the rhodium center have been made to modify the photophysical properties and furthermore to explore the reaction behavior of these rhodium complexes. Bearing in mind that the P(p-tolyl)3 ligands can easily be replaced by the stronger -donating PMe3 ligands, ligand exchange reactions with N heterocyclic carbenes (NHCs) as even stronger -donors was investigated. Addition of two equivalents of NHCs at room temperature led to the release of one equivalent of P(p-tolyl3) and formation of the mono-substituted NHC rhodium complex. The reaction of isolated mono-NHC complex with another equivalent of NHC at room temperature did not result in the exchange of the second phosphine ligand. Moderate heating of the reaction to 60 °C, however, resulted in the formation of tetra-substituted NHC rhodium complex [Rh(nPr2Im)4]+[acac]-. To circumvent the loss of the other ligands in the experiments described above, a different approach was investigated to access rhodacyclopentadienes with NHC instead of phosphine ligands.
Reaction of the bis-NHC complex [Rh(κ2-O,O-acac)(nPr2Im)2] with , bis(arylbutadiynyl)alkanes at room temperature resulted 2,5-bis(arylethynyl)-rhodacyclopentadienes with the NHC ligands being cis or trans to each other as indicated by NMR spectroscopic measurements and single-crystal X-ray diffraction analysis. Isolation of clean material and a fundamental photophysical study could not be finished for reasons of time within the scope of this work.
Furthermore, shortening of the well conjugated -system of the chromophoric ligand (changing from tetraynes to diynes) was another strategy to examine the reaction behavior of theses ligands with rhodium(I) complexes and to modify the excited state behavior of the formed rhodacyclopentadienes. The reaction of [Rh(κ2-O,O-acac)(PMe3)2] with 1,7 diaryl 1,6-heptadiynes (diynes) leads to the selective formation of 2,5 bis(aryl)rhodacyclopentadienes. These compounds, however, are very weakly fluorescent with quantum yields ФPL < 1, and very short emission lifetimes in toluene at room temperature. Presumably, vibrational modes of the bis(phenyl)butadiene backbone leads to a higher rate constant for non-radiative decay and is thus responsible for the low quantum yields compared to their corresponding PMe3 complexes with the bis(phenylethynyl)butadiene backbone at room temperature. No additional phosphorescence, even at 77 K in the glass matrix is observed.
Chancing the phosphine ligands to P(p-tolyl)3, reactions of [Rh(κ2-O,O-acac)(P(p-tolyl3)2)] with 1,7-diaryl-1,6-heptadiynes, however, resulted in a metal-mediated or -catalyzed cycloaddition reaction of alkynes and leads to full conversion to dimerization and trimerization products and recovery of the rhodium(I) starting material. This is intuitive, considering that P(Ar)3 (Ar = aryl) ligands are considered weaker -donor ligands and therefore have a higher tendency to dissociate. Therefore, rhodium(I) complexes with aryl phosphines as ligands have an increasing tendency to promote catalytic reactions, while the stronger -donating ligands (PMe3 or NHCs) promote the formation of stable rhodium complexes.
Finally, in Chapter 4, the findings of the work conducted on N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) is presented. These compounds have unique electronic and steric properties and are therefore of great interest as ligands and organo-catalysts. In this work, studies of substitution reactions involving novel carbonyl complexes of rhodium and nickel are reported. For characterization and comparison of CAACmethyl with the large amount of data available for NHC and sterically more demanding CAAC ligands, an overview on physicochemical data (electronics, sterics and bond strength) is provided.
The reaction of [Rh(-Cl)(CO)2]2 with 2 equivalents of CAACmethyl at low temperature afforded the mononuclear complex cis-[(RhCl(CO)2(CAACmethyl)]. However, reacting [Rh( Cl)(CO)2]2 with CAACmethyl at room temperature afforded a mixture of complexes. The mononuclear complex [(RhCl(CO)(CAACmethyl)2], the chloro-bridged complexes [(Rh2( Cl)2(CO)3(CAACmethyl)], [Rh(-Cl)(CO)(CAACmethyl)]2 and a carbon monoxide activation product were formed. The carbon monoxide activation product is presumably formed via the reaction of two equivalents of the CAAC with CO to give the bis-carbene adduct of CO, and subsequent rearrangement via migration of the Dipp moiety. While classical N-heterocyclic carbenes are not electrophilic enough to react with CO, related diamidocarbenes and alkyl(amino)carbenes undergo addition reactions with CO to give the corresponding ketenes. Consequently, to obtain the CAAC-disubstituted mononuclear complex selectively, 8 equivalents of CAACmethyl were reacted with 1 equivalent of [Rh(-Cl)(CO)2]2. For the evaluation of TEP values, [Ni(CO)3(CAAC)] was synthesized in collaboration with the group of Radius. With the complexes [(RhCl(CO)(CAACmethyl)2] and [Ni(CO)3(CAAC)] in hand, it was furthermore possible to examine the electronic and steric parameters of CAACmethyl. Like its bulkier congeners CAACmenthyl and CAACcy, the methyl-substituted CAAC is proposed to be a notably stronger -donor than common NHCs. While it has a very similar TEP value of 2046 cm-1, it additionally possess superior -acceptor properties (P = 67.2 ppm of phosphinidene adduct).
CAACs appear to be very effective in the isolation of a variety of otherwise unstable main group and transition metal diamagnetic and paramagnetic species. This is due to their low-lying LUMO and the small singlet-triplet gap. These electronic properties also allow free CAACs to activate small molecules with strong bonds. They also bind strongly to transition metal centers, which enables their use under harsh conditions. One recent development is the use of CAACs as ligands in transition metal complexes, which previously were only postulated as short-lived catalytic intermediates.[292,345] The availability of these reactive species allows for a better understanding of known catalytic reactions and the design of new catalysts and, moreover, new applications. For example Radius et al.[320] prepared a CAAC complex of cobalt as a precursor for thin-film deposition and Steffen et al.[346] reported a CAAC complex of copper with very high photoluminescent properties, which could be used in LED devices. With the development of cheap and facile synthetic methods for the preparation of CAACs and their corresponding transition metals complexes, as well as the knowledge of their electronic properties, it is safe to predict that applications in and around this field of chemistry will continue to increase.
Luminescent organotransition metal complexes are of much current interest. As the large spin-orbit coupling of 2nd and 3rd row transition metals usually leads to rapid intersystem crossing from S1 to T1, which enables phosphorescence, there is a special interest in using triplet-emitting materials in organic or organometallic light emitting diodes (OLEDs). Marder et al. have found that, reductive coupling of both para-R-substituted diarylbutadiynes and diaryldodecatetraynes on Rh(PMe3)4X leads to quantitative yields of bis(arylethynyl)-rhodacyclopentadienes with complete regiospecificity (R = BMes2, H, Me, OMe, SMe, CF3, CN, CO2Me, NMe2, NO2, C≡C-TMS and X = -C≡C-TMS, -C≡C-C6H4-4-NMe2, -C≡C-C≡C-C6H4-4-NPh2, Me, Cl).47,49 Unexpectedly, these compounds show intense fluorescence rather than phosphorescence (ɸf = 0.33-0.69, t = 1.2 3.0 ns). The substituent R has a significant influence on the photophysical properties, as absorption and emission are both bathochromically shifted compared to R = H, especially for R = π-acceptor.
To clarify the mechanism of the formation of the rhodacyclopentadienes, and to investigate further their unique photophysical properties, a series of novel, luminescent rhodacyclopentadienes with dithiocarbamate as a bidentate ligand at the rhodium centre has been synthesised and characterised (R = NO2, CO2Me, Me, NMe2, SMe, Ar = C6F4-4-OMe). The rhodacyclopentadienes have been formed via reductive coupling of diaryl undecatetraynes with [Rh(k2-S,S`-S2CNEt2)(PMe3)2]. The structures of a series of such compounds were solved by single crystal X-ray diffraction and are discussed in this work. The compounds were fully characterised via NMR, UV/Vis and photoluminescence spectroscopy as well as by elemental analysis, high-resolution mass spectrometry (HRMS) and X-ray diffraction.
When heating the reactions, another isomer is formed to a certain extent. The so-called dibenzorhodacyclopentadienes already appeared during earlier studies of Marder et al., when acetylacetonate (acac) was employed as the bidentate ligand at the Rh-centre. They are probably formed via a [4+2] cycloaddition reaction and C-H activation, followed by a β-H shift.
Use of the perfluorinated phenyl moiety Ar = C6F4-4-OMe provided a total new insight into the mechanism of formation of the rhodacyclopentadiene isomers and other reactions. Besides the formation of the expected rhodacyclopentadiene, a bimetallic compound was generated, isolated and characterised via X-ray crystallography and NMR spectroscopy, elemental analysis and high resolution mass spectrometry.
For further comparison, analogous reactions with [Rh(k2 S,S` S2CNEt2)(PPh3)2] and a variety of diaryl undecatetraynes (R = NO2 CO2Me, Me, NMe2, SMe, Ar = C6F4-4-OMe) were carried out. They also yield the expected rhodacyclopentadienes, but quickly react with a second or even third equivalent of the tetraynes to form, catalytically, alkyne cyclotrimerisation products, namely substituted benzene derivatives (dimers and trimers), which are highly luminescent. The rhodacyclopentadienes (R = NO2, CO2Me, Me, SMe, Ar = C6F4-4-OMe) are stable and were isolated. The structures of a series of these compounds were obtained via single crystal X-ray crystallography and the compounds were fully characterised via NMR, UV/Vis and photoluminescence spectroscopy as well as by elemental analysis and HRMS.
Another attempt to clarify the mechanism of formation of the rhodacyclopentadienes involved reacting a variety of diaryl 1,3-butadiynes (R = CO2Me, Me, NMe2, naphthyl) with [Rh(k2 S,S` S2CNEt2)(PMe3)2]. The reactions stop at an intermediate step, yielding a 1:1 trans π-complex, confirmed by single crystal X-ray diffraction and NMR spectroscopy. Only after several weeks, or under forcing conditions (µw / 80 °C, 75 h), the formation of another major product occurs, having bound a second diaryl 1,3-butadiyne. Based on earlier results of Murata, the product is identified as an unusual [3+2] cycloaddition product, ϭ-bound to the rhodium centre.
Industrially used semiconducting materials, building blocks of modern electronics and computer industry, are mostly based on inorganic, crystalline solids, which have the drawback of relatively high production costs. As an alternative, organic pi-conjugated systems show enhanced flexibility and processability as well as the opportunity to obtain light-weight materials. They have emerged as attractive candidates, especially since elements beyond hydrogen and carbon can be used to create pi-conjugated frameworks. In recent years, pi-conjugated oligomers and polymers with tricoordinate boron centers incorporated into the main chain of such organic polymers have attracted considerable attention as the interaction of the vacant p orbital on boron with an adjacent pi system of the chain leads to extended conjugated materials. These materials show intriguing optical and electronic properties and potential applications in organic electronics and optoelectronics (OLEDs, OFETs, photovoltaics) or as sensory materials.
In this thesis, a catalytic Si/B exchange reaction protocol is used as a facile and highly effective B-C bond formation method to synthesize organoboron molecules, oligomers, polymers and macrocycles. This reaction is applied to synthesize a series of thienyl- and furylborane based materials. Special focus is on furylborane based materials, which, in general, have been only scarcely explored so far. This is mainly due to synthetic challenges since furan decomposes readily in the presence of light and oxygen. Our mild and highly selective reaction protocol in combination with sufficient kinetic protection of the boron centers gives access to a series of extended organoboranes featuring furylborane units in the main chain. Furthermore, kinetically stabilized furylboranes are established as highly robust and versatile building blocks for pi conjugated materials. The obtained materials reveal remarkable luminescence properties. The scope of potential starting materials was investigated by a catalyst screening, demonstrating that the Si/B exchange reaction can also be performed for less reactive aryldichloroboranes. Furthermore, borazine-based hybrid cyclomatrix microspheres have been synthesized via a Si/B exchange condensation reaction under precipitation polymerization conditions. Finally, synthetic routes to tetrabora- and diboraporphyrinogens were attempted in a multi-step reaction procedure. In the case for tetraboraporphyrinogens, the final macrocyclization reaction under pseudo high-dilution conditions afforded a mixture of macrocycles with different ring sizes. UV-vis and fluorescence spectroscopic analysis indicated significant differences in comparison to their linear congeners.
Sensitivity and selectivity remain the central technical requirement for analytical devices, detectors and sensors. Especially in the gas phase, concentrations of threat substances can be very low (e.g. explosives) or have severe effects on health even at low concentrations (e.g. benzene) while it contains many potential interferents. Preconcentration, facilitated by active or passive sampling of air by an adsorbent, followed by thermal desorption, results in these substances being released in a smaller volume, effectively increasing their concentration.
Traditionally, a wide range of adsorbents, such as active carbons or porous polymers, are used for preconcentration. However, many adsorbents either show chemical reactions due to active surfaces, serious water retention or high background emission due to thermal instability. Metal-organic frameworks (MOFs) are a hybrid substance class, composed inorganic and organic building blocks, being a special case of coordination polymers containing pores. They can be tailored for specific applications such as gas storage, separation, catalysis, sensors or drug delivery.
This thesis is focused on investigating MOFs for their use in thermal preconcentration for airborne detection systems. A pre-screening method for MOF-adsorbate interactions was developed and applied, namely inverse gas chromatography (iGC). Using this pulse chromatographic method, the interaction of MOFs and molecules from the class of explosives and volatile organic compounds was studied at different temperatures and compared to thermal desorption results.
In the first part, it is shown that archetype MOFs (HKUST-1, MIL-53 and Fe-BTC) outperformed the state-of-the-art polymeric adsorbent Tenax® TA in nitromethane preconcentration for a 1000 (later 1) ppm nitromethane source. For HKUST-1, a factor of more than 2000 per g of adsorbent was achieved, about 100 times higher than for Tenax. Thereby, a nitromethane concentration of 1 ppb could be increased to 2 ppm. High enrichment is addressed to the specific interaction of the nitro group as by iGC, which was determined by comparing nitromethane’s free enthalpy of adsorption with the respective saturated alkane. Also, HKUST-1 shows a similar mode of sorption (enthalpy-entropy compensation) for nitro and saturated alkanes.
In the second part, benzene of 1 ppm of concentration was enriched with a similar setup, using 2nd generation MOFs, primarily UiO-66 and UiO-67, under dry and humid (50 %rH) conditions using constant sampling times. Not any MOF within the study did surpass the polymeric Tenax in benzene preconcentration. This is most certainly due to low sampling times – while Tenax may be highly saturated after 600 s, MOFs are not. For regular UiO-66, four differently synthesized samples showed a strongly varying behavior for dry and humid enrichment which cannot be completely explained. iGC investigations with regular alkanes and BTEX compounds revealed that confinement factors and dispersive surface energy were different for all UiO-66 samples. Using physicochemical parameters from iGC, no unified hypothesis explaining all variances could be developed.
Altogether, it was shown that MOFs can replace or add to state-of-the-art adsorbents for the enrichment of specific analytes with preconcentration being a universal sensitivity-boosting concept for detectors and sensors. Especially with iGC as a powerful screening tool, most suitable MOFs for the respective target analyte can be evaluated. iGC can be used for determining “single point” retention volumes, which translate into partition coefficients for a specific MOF × analyte × temperature combination.