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- URN to cite this document:
- urn:nbn:de:bvb:355-epub-592829
- DOI to cite this document:
- 10.5283/epub.59282
| Item type: | Thesis of the University of Regensburg (PhD) |
|---|---|
| Open Access Type: | Primary Publication |
| Place of Publication: | Regensburg |
| Date: | 19 September 2025 |
| Referee: | Prof. Dr. Manfred Scheer |
| Date of exam: | 20 September 2024 |
| Institutions: | Chemistry and Pharmacy > Institut für Anorganische Chemie > Chair Prof. Dr. Manfred Scheer |
| Keywords: | Group 13/15 compounds, N-heterocyclic carbenes, bismuth, antimony, gallium, aluminium |
| Dewey Decimal Classification: | 500 Science > 540 Chemistry & allied sciences |
| Status: | Published |
| Refereed: | Yes, this version has been refereed |
| Created at the University of Regensburg: | Yes |
| Item ID: | 59282 |
Abstract (English)
This work investigated the synthesis and reactivity of heavy group 13/15 element combinations with the focus on monomeric 13/15 parent compounds stabilized by N-heterocyclic carbenes. In the following, the individual results of the chapters presented in this work are presented and the most important findings discussed. Coordination of Pnictogenylboranes Towards Tl(I) Salts and a Tl-mediated ...
Abstract (English)
This work investigated the synthesis and reactivity of heavy group 13/15 element combinations with the focus on monomeric 13/15 parent compounds stabilized by N-heterocyclic carbenes. In the following, the individual results of the chapters presented in this work are presented and the most important findings discussed.
Coordination of Pnictogenylboranes Towards Tl(I) Salts and a Tl-mediated P-P coupling
The chemistry of Tl(I) phosphine complexes remained underdeveloped throughout the literature, as only a handful of examples are known. One objective within this work was to extend this library by complexation of Tl(I) with pnictogenylboranes. By reaction of donor stabilized pnictogenylboranes with Tl(I) salts of weakly coordinating anions (WCAs), unprecedented Tl(I) coordination compounds could be synthesized. Depending on the counter ion and the stoichiometry, monovalent [Tl(EH2BH2·NMe3)][BArCl] (1a: E = P, 1b: E = As) or three-valent Tl(I) phosphine and arsine complexes [Tl(EH2BH2·NMe3)3][WCA] (2a: E = P, WCA = TEFCl; 2b: E = As, WCA = TEF) were prepared and comprehensively characterized. Unexpectedly, at a stoichiometry of two equivalents of phosphanylborane to one Tl[WCA] a P-P coupling with involvement of the solvent dichloromethane takes place, leading to [Me3N·BH2PH2PHBH2·NMe3][WCA] (3a: WCA = TEF, 3b: WCA = BArCl, 3c: WCA = TEFCl) independently of the used WCA. As verified by powder X-ray diffraction measurements, elimination of TlCl leads to the formation of 3a-c. The coupling only takes place in dichloromethane and cannot be induced by, e.g. hydride abstracting agents such as [Ph3C][TEF], which simply leads to coordination of the phosphanylborane to [Ph3C]+, as in [Me3N·BH2PH2CPh3][TEF] (4). According to quantum chemical calculations, a concerted mechanism at the intermediate [Tl(PH2BH2·NMe3)2(CH2Cl2)]+ takes place in the coupling process rather than a stepwise pathway. The analogue reactions with arsanylboranes did not result in As-As bond formation, so far. The obtained complexes 1a-2b represent rare examples of phosphine thallium complexes, which are surprisingly scarce throughout the literature.
NHC-Stabilized Parent Tripentelyltrielanes
Based on previous results, the synthesis and isolation of heavy NHC stabilized parent tripentelyltrielanes appeared to be possible, which would fill an important gap in the literature as last missing type of compounds featuring a E(PnH2) substitution pattern. Thus, it was targeted to prepare these compounds by triple salt metathesis of NHC stabilized trichloride precursors IDipp·ECl3 (E = Ga, Al, In) with LiPH2·DME or KAsH2, affording the tripentelyltrielanes IDipp·E(PnH2)3 (1a: E/Pn = Ga/P, 1b: Ga/As, 2a: Al/P, 2b: Al/As, 3: In/P). The compounds display extreme sensitivity to air and decompose slowly at room temperature. Regarding the triphosphanylindiumane, the isolation proved highly difficult, but the existence could be unambiguously proven by multinuclear NMR spectroscopy. Investigation on the reactivity of the tripentelyltrielanes has shown, that in many cases E-Pn bond fission is observed rather than coordination. With a very soft acceptor such as (HgC6F4)3, coordination can still be achieved, however. Reaction of IDipp·Ga(PH2)3 with (HgC6F4)3 in o-DFB led to the very air sensitive compound [IDipp·Ga(PH2)2(µ3-PH2{HgC6F4}3)], which could be characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction structure determination. Besides slow decomposition at room temperature, the compound displayed high lability and even CH2Cl2 is strong enough to substitute IDipp·Ga(PH2)3 from (HgC6F4)3. The bonding situation of [IDipp·Ga(PH2)2(µ3-PH2{HgC6F4}3)] could be characterized by quantum chemical calculations which revealed, that electrostatic binding of the ligand rather than covalent bonding takes place at the {Hg}3 unit. In subsequent studies, the homologue NHC stabilized triphosphanylboranes could be synthesized, as well, though the triple salt metathesis reactions do not proceed as cleanly as in the other cases. Nevertheless, IPr2Me2·B(PH2)3 (5) could be accessed, and the structure elucidated by multinuclear NMR spectroscopy and single-crystal X-ray diffraction structure determination. The triarsanylborane however could not be obtained by this pathway as only intractable product mixtures occur and thus this compound remains elusive, so far.
A general Pathway towards NHC·GaH₂(OTf) adducts – The key for the Synthesis of NHC-stabilized cationic 13/15 Chain Compounds of Gallium
By addition of D·BH2PR2 to IDipp·GaH2(OTf), the unprecedented mixed chain compounds [IDipp·GaH2PR2BH2·D][OTf] (3a: D = IDipp, R = H; 3b: D = NMe3, R = H, 3c: D = NMe3, R = Ph) could be obtained. All chain compounds display high sensitivity towards air and decompose slowly at room temperature in solution or in the solid state. The compounds could be characterized by multinuclear NMR spectroscopy, ESI-MS and by single crystal X-ray structure determination. However, reactions with any arsanylboranes have failed in producing the desired chain compounds, so far. Surprisingly though, when the phosphanylgallanes IDipp·GaH2PH2 was reacted with IDipp·GaH2(OTf), the parent chain compound [IDipp·GaH2PH2GaH2·IDipp][OTf] (3d) was obtained and characterized. 3d displays extreme sensitivity towards air and decomposes quickly at room temperature as solids or in solution. Furthermore, the compound could only be dissolved in ortho-difluorobenzene, as any other suitable solvent will result in reconversion back into the starting materials, due to the low nucleophilicity of IDipp·GaH2PH2. The chain compounds presented here represent the first examples of catena extension of the parent phosphanylboranes/gallanes by a heavy [D·GaH2]+ synthon. It yet remains to be tested, if a similar strategy can be applied to obtain the corresponding aluminium chain compounds.
Characterization of the Ligand Properties of Donor stabilized Pnictogenyltrielanes
Despite being known for almost two decades, the donor strength of Lewis base stabilized pnictogenyltrielanes has not been classified, so far. A classification of this type of compound regarding their steric and electronic properties would fill an important gap in this field and was a central task within this thesis. In this work, the library of NHC stabilized alkyl substituted phosphanylboranes has been extended by salt metathesis reactions of NHC·BH2I with alkalimetalpnictogenides and the donor strength of the phosphanyl- and arsanylboranes, as well as of the parent phosphanylgallane IDipp·GaH2PH2 was evaluated by experimental comparison of the Tolman Electronic Parameter (TEP). Therefore, the compounds were reacted with Ni(CO)4 in order to obtain [Ni(CO)3(D·EH2PnR2)] complexes. The scope of the investigations focussed on the influence of the applied donor D at the triel moiety (NMe3 vs. IDipp), the triel atom itself (B vs. Ga) and the pnictogen atom (P vs. As). The reactions of the NHC-stabilized pnictogenylboranes with Ni(CO)4 selectively led to the substitution of one CO ligand, while the reactions with NMe3 stabilized pnictogenylboranes (Scheme 12.8, left) partially led to the substitution of multiple CO ligands, which can be avoided by short reaction times. Reacting IDipp·GaH2PH2 with Ni(CO)4 (Scheme 12.8, bottom) also selectively led to the formation of the desired [Ni(CO)3(IDipp·GaH2PH2)] complex, which to this date remains the only example of successful coordination of IDipp·GaH2PH2 to a metal centre without Ga-P bond fission. All complexes could be isolated in medium yields and the TEPs were recorded by IR spectroscopic measurements. Comparison of the TEPs showed that the pnictogenyltrielanes can be considered as of similar donor strength than commonly used NHCs, which underlines the unique donor properties of 13/15 compounds compared to regular phosphines. The donor atom D contributes strongly to the donor strength of the pnictogen with the trend being IDipp > NMe3 in general for all compounds. Moreover, it can be concluded that the donor strength decreases by Ga > B for the triel, P > As for the pnictogen and overall R > H as substituents at the pnictogen atom. According to the percent buried volumes %Vbur and Tolman Cone Angles (TCA). The steric demand can be considered comparably low, with the parent compounds displaying a similar demand as PH3. Quantum chemical calculations could show that the Tolman Electronic Parameter depends on the HOMO energies of the pnictogenyltrielanes. A similar correlation could be observed for the proton affinities of the 13/15 compounds. The study overall fills an important gap in the studies of monomeric 13/15 compounds by providing an overview of the electronic and steric nature of these compounds.
The parent GaH2SbH2 monomer stabilized by a N-heterocyclic carbene
As the pnictogenylgallane monomers IDipp·GaH2PH2 and IDipp·GaH2AsH2 were described previously, the question remained, if the even heavier homolog IDipp·GaH2SbH2 can be prepared, as well, which would represent the heaviest parent ethylene analogue of a group 13/15 element combination. By using the novel reagent K(18c6)SbH2 by von Hänisch and co-workers, direct salt metathesis leading to parent stibanyltrielanes has been discovered. Beforehand, this class of compounds could only be accessed by the methanolytic treatment of the corresponding silylated precursor compounds. Although the reaction of KSb(SiMe3)2 with IDipp·GaH2(OTf) afforded the stibanylgallane IDipp·GaH2Sb(SiMe3)2 (1), conversion to the parent compound by methanolysis was unsuccessful, as no reaction occurred. In contrast, the analogue reaction of IDipp·GaH2(OTf) with K(18c6)SbH2 led to the, so far, heaviest parent donor stabilized 13/15 element combination, IDipp·GaH2SbH2 (3). 3 shows high sensitivity and decomposes and room temperature, under light and at air. 3 could be crystallized alongside the inseparable side product IDipp·GaH3. Furthermore, K(18c6)SbH2 could be reacted with D·BH2I (D = NMe3, IDipp) to directly yield the corresponding stibanylboranes D·BH2SbH2 (2a: D = NMe3, 2b: D = IDipp), which display extreme sensitivity towards air and light and decompose slowly in solution even at -30°C. 2b could previously only be observed detected as an intermediate during the reaction of Me3N·BH2Sb(SiMe3)2 with MeOH and the salt metathesis reaction presented in this work represents the first direct synthetic pathway towards 2b. The obtained compounds are rare examples for primary stibanes and compound 3 represents the heaviest parent ethylene analogue up to this date. Investigations on the reactivity of 2a and 3 were hampered by the low nucleophilicity and low stability of these primary stibanes. Reactions with Ni(CO)4, W(CO)5(thf), BEt3 and IDipp·GaH2(OTf) either yielded intractable mixtures as a result of side reactions or no reaction was observed. Only the reaction of 3 with BH3·SMe2 towards catenation seems promising according to NMR spectroscopic investigation, but clear determination of the resulting product is still pending. To address the question, whether di- and tristibanylgallanes can be prepared by salt metathesis, as well, IDipp·GaCl3 was reacted with an excess amount of K(18c6)SbH2 (Scheme 12.11). According to 1H NMR spectroscopy, a mixture of IDipp·GaHx(SbH2)3-x (x = 1-3) can likely be seen as a result of hydride transfer from the stibanide salt, though only 1 and IDipp·GaH(SbH2)2 can clearly be identified.
Synthesis of Bismuthanyl-substituted monomeric Triel Hydrides
Throughout the literature, Lewis base stabilized parent phosphanyl-, arsanyl- and even stibanylboranes have been reported. However, the missing link in this perspective still remained the bismuthylboranes, which represent the combination of the lightest group 13 and heaviest group 15 element. Together with the experimental works by Marquardt and Hegen, a new class of 13/15 compounds stabilized only by a Lewis base containing the heavy group 15 element bismuth (bismuthanyltrielanes) could be prepared and characterized. Salt metathesis reactions of KBi(SiMe3)2(THF)0.3 with IDipp·BH2I, IMe4·BH2I, DMAP·BH2I and IDipp·GaH2(OTf) (IDipp = 1,3-Bis-(2,6-diisopropylphenyl)-imidazolin-2-ylidene, IMe4 = 1,3-Bis-(dimethyl)-4,6-dimethyl-imidazolin-2-ylidene) led to the formation of the unprecedented bismuthyltrielanes IDipp·BH2Bi(SiMe3)2 (1a), IMe4·BH2Bi(SiMe3)2 (1b), DMAP·BH2Bi(SiMe3)2 (1c) and IDipp·GaH2Bi(SiMe3)2 (2). All compounds could be characterized by multinuclear NMR spectroscopy, single-crystal X-ray structure determination and partially by LIFDI-MS. 1a-2 display drastic sensitivity towards air and light and decompose as solids and in solution at temperatures above -30 °C. Even below -30 °C, slow decomposition of 1a-2 in solution is observed under deposition of elemental bismuth. Compounds 1a-2 represent the first examples of B-Bi bonds where the boron atom is not stabilized within a cyclic system. The stability of the Lewis acid/base adducts decreases according to IMe4 > IDipp > DMAP, which is in line with calculated dissociation enthalpies and isolated yields. The obtained compounds represent the first examples of molecules containing triel-bismuth σ-bonds in chain-like compounds. Unfortunately, any desilylation attempts have not been successful and the parent bis-muthylborane remains elusive, so far. However, it cannot be excluded that such a transient species was formed during the attempts, but any attempt at isolating or spectroscopic observation did not prove fruitful. The most promising reaction of all attempts was the conversion of IDipp·BH2Bi(SiMe3)2 with MeOH-d4 in the presence of 1,4-benzoquinone, which resulted in the par-tial consumption of the starting material and presumably cleavage of at least one {SiMe3} group. However, clear identification of any Bi-D/H containing species was not possible.
Electrophilic Functionalization of Donor stabilized Phosphanylboranes
The functionalization of phosphanylboranes by electrophiles has been investigated in the past. However, these investigations were mostly limited towards group 13 electrophiles. A study towards catenation by main group electrophiles other than group 13 has not yet been reported in the literature and this was a further aim of this work. The possible scope of functionalization of phosphanylboranes by main group electrophiles could be successfully expanded to a broader range of the periodic table. First, protonation of IDipp·BH2PH2 with [H(OEt2)2][BArF] ([BArF] = [B(C6F5)4]-) selectively led to [IDipp·BH2PH3][BArF] (1a). While this method works for NHC stabilized phosphanylboranes, the amine stabilized analogues required a different approach. Reacting Me3N·BH2Cl with H3P·AlCl3 results in the very sensitive compound [Me3N·BH2PH3][AlCl4] (1b), which slowly decomposes at room temperature under release of PH3. When A was reacted with main group electrophiles from group 14, a plethora of novel cationic 13/15/14 chain compounds [IDipp·BH2PH2E][WCA] (2: E = CH3, WCA = OTf; 3: E = SiEt3, WCA = TEF; 4: E = Ge(SiMe3)3, WCA = TEF; 5: E = SnMe3, WCA = TEF) could be obtained, ranging from the group 14 elements C to Sn. The heavier group 14 homologs 3-5 display very high sensitivity towards air and decompose upon contact with untreated glass surfaces due to the lability of the P-E bond. Interestingly, reactions of A with in situ generated [Cy2Pn][PF6] (Pn = P, As) did not yield the corresponding phosphinophosphonium cation, but rather a P-P coupling reaction took place. Both reactions exclusively resulted in the formation of [IDipp·BH2PH2PHBH2·IDipp][PF6] (6) as evident by single-crystal X-ray structure determination, NMR spectroscopy and ESI-MS. To probe the reactivity of the obtained products, 1a and 1b were reacted with the main group hydrides LiGaH4 and LiAlH4. The reaction with LiAlH4 yielded simple deprotonation towards the parent phosphanylboranes, which represents an alternative synthetic approach towards this class of compounds. However, when 1a/b are reacted with LiGaH4, catenation occurs leading to the neutral elongated chain compounds D·BH2PH2GaH3 (7a: D = IDipp, 7b: D = Me3N). These compounds represent the first known examples of compounds containing terminal {GaH3} units bound to a primary phosphine.
Translation of the abstract (German)
In dieser Arbeit wurde die Synthese und Reaktivität von schweren 13/15-Elementkombinationen untersucht, wobei der Schwerpunkt auf monomeren 13/15-Stammverbindungen lag, die durch N-heterozyklische Carbene stabilisiert wurden.
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