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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.
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.
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.
Synthese und Charakterisierung neuartiger Silicium-, Germanium- und zinnorganischer Riechstoffe
(2014)
Aufbauend auf dem Konzept der C/Si-Bioisosterie beschreibt die vorliegende Arbeit die Synthese und Charakterisierung siliciumhaltiger Derivate der Riechstoffe Galaxolide, Lilial, Bourgeonal, 5,7,7-Trimethyl-4-methylenoctanal und α-Galbanone sowie Beiträge zur Synthese eines silicium¬haltigen Derivats von δ-Damascone. Basierend auf der C/Ge- und C/Sn-Bioisosterie wurden zudem die Germa- und Stanna-Analoga von Lilial und Bourgeonal synthetisiert. Die entsprechenden Zielverbindungen sowie alle isolierten Zwischenstufen wurden durch NMR-Spektroskopie (1H, 11B, 13C, 15N, 29Si, 119Sn) und Elementaranalyse (C, H, N) charakterisiert. In einigen Fällen erfolgte zusätzlich eine Charakterisierung durch Einkristall-Röntgenstrukturanalyse oder Infrarot-Spektroskopie.
Einfach Lewis-Basen stabilisierte Borylene wurden durch Reduktion in situ hergestellt und in Gegenwart von Kohlenstoffmonoxid oder Distickstoff umgesetzt. Die entstandenen Verbindungen wurden mittels NMR-, ESR-, UV/Vis- und IR-Spektroskopie sowie Einkristallröntgenstrukturanalyse charakterisiert. Im Zuge dessen konnten für die erhaltenen Spezies Eigenschaften ermittelt werden, die denen analoger Übergangsmetallkomplexe ähneln. Ferner konnten die zugrundeliegenden mechanistischen Vorgänge der Reaktionen durch gezielte Variation der Reaktionsparameter aufgeklärt werden. Zudem wurden Redoxverhalten und Reaktivitäten der isolierten Produkte in weiterführenden Studien näher untersucht.