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1.
Angew Chem Int Ed Engl ; : e202406438, 2024 Jul 01.
Article in English | MEDLINE | ID: mdl-38946322

ABSTRACT

Mixed N-heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth-abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal-to-ligand charge transfer state (3MLCT), which typically decays via a triplet metal-centered (3MC) state back to the ground state. We reveal by variable-temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans- and cis-[Fe(pdmi)2]2+with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans-[Fe(pdmi)2]2+possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm-1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi-classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

2.
Materials (Basel) ; 15(23)2022 Nov 22.
Article in English | MEDLINE | ID: mdl-36499789

ABSTRACT

As a lightweight construction material, aluminum plays a key role in weight reduction and, thus, sustainability in the transport industry. The brazing of aluminum and its alloys is impeded by the natural passivating oxide layer, which interferes with the brazing process. The presented study investigates the possibility of using a thermal silane-doped argon plasma to reduce this oxide layer in situ and thus eliminating the need to use hazardous chemical fluxes to enable high-quality brazing. Using plasma spectroscopy and an oxygen partial pressure probe, it was shown that a silane-doped argon plasma could significantly reduce the oxygen concentration around the plasma in a thermal plasma brazing process. Oxygen concentrations below 10-16 vol.-% were achieved. Additionally, metallographic analyses showed that the thickness of an artificially produced Al2O3-Layer on top of AlMg1 samples could be substantially reduced by more than 50%. With the oxide layer removed and inhibition of re-oxidation, silane-doped plasma brazing has the potential to become an economically efficient new joining method.

3.
Dalton Trans ; 51(41): 15792-15796, 2022 Oct 25.
Article in English | MEDLINE | ID: mdl-36185003

ABSTRACT

The reaction of sodium tert-butoxide with trimethylsilyl compounds enables the encapsulation of small anions in an icosahedral arrangement of 12 alkoxide units and 13 dynamically moving sodium cations. Advancement of this method in connection with larger anions results in the formation of mixed aggregates involving the tetraflouoroborate anion or the butadiyne-dianion.

4.
Materials (Basel) ; 15(4)2022 Feb 14.
Article in English | MEDLINE | ID: mdl-35207958

ABSTRACT

Hydrogen-assisted cracking is a major challenge in underwater wet welding of high-strength steels with a carbon equivalent larger than 0.4 wt%. In dry welding processes, post-weld heat treatment can reduce the hardness in the heat-affected zone while simultaneously lowering the diffusible hydrogen concentration in the weldment. However, common heat treatments known from atmospheric welding under dry conditions are non-applicable in the wet environment. Induction heating could make a difference since the heat is generated directly in the workpiece. In the present study, the thermal input by using a commercial induction heating system under water was characterized first. Then, the effect of an additional induction heating was examined with respect to the resulting microstructure of weldments on structural steels with different strength and composition. Moreover, the diffusible hydrogen content in weld metal was analyzed by the carrier gas hot extraction method. Post-weld induction heating could reduce the diffusible hydrogen content by -34% in 30 m simulated water depth.

5.
Chemistry ; 27(49): 12693-12701, 2021 Sep 01.
Article in English | MEDLINE | ID: mdl-33938049

ABSTRACT

Alkali metal alkoxides are widely used in chemistry due to their Brønsted basic and nucleophilic properties. Potassium alkoxides assist alkyllithium in the metalation of hydrocarbons in Lochmann-Schlosser-bases. Both compounds form mixed aggregates, which enhance the thermal stability, solubility, and the basic reactivity of these mixtures. A very unusual spherical mixed alkoxy aggregate was discovered by Grützmacher et al., where a central dihydrogen phosphide anion is surrounded by a highly dynamic shell of thirteen sodium atoms and a hull of twelve tert-butoxide groups. This structural motif can be reproduced by a reaction of trimethylsilyl compounds of methane, halogens, or pseudo-halogens with excess sodium tert-butoxide. A nucleophilic substitution releases the corresponding anion, which is then encapsulated by the sodium alkoxide units. The compounds are soluble in hydrocarbon solvents, enabling studies of solutions by high-resolution NMR spectroscopy and IR/Raman studies of the crystalline materials.

6.
Chemistry ; 27(3): 888-904, 2021 Jan 13.
Article in English | MEDLINE | ID: mdl-33165981

ABSTRACT

Lochmann-Schlosser superbases (LSB) are a standard reagent in synthetic chemistry to achieve an exchange of a proton on an organic framework with an alkali metal cation, which in turn can be replaced by a wide range of electrophilic groups. In standard examples, the deprotonating reagent consists of an equimolar mixture of n-butyllithium and potassium t-butoxide. However, the nature of the reactive species could not be pinned down either for this composition or for similar mixtures with comparable high reactivity. Despite the poor solubility and the fierce reactivity, some insights into this mixture were achieved by some indirect results, comparison with chemically related systems, or skillful deductions. Recent results, mainly based on new soluble compounds, delivered structural evidence. These new insights lead to advanced and more detailed conclusions about the interplay of the involved components.

7.
Materials (Basel) ; 13(17)2020 Aug 25.
Article in English | MEDLINE | ID: mdl-32854263

ABSTRACT

The European standard ISO 3690 regulates the measurement of diffusible hydrogen in arc-welded metal. It was designed for different welding methods performed in dry atmosphere (20% humidity). Some details of the standard are not applicable for wet underwater welding. The objective of this study was to extend the applicability of DIN EN ISO 3690:2018-12 to underwater wet-shielded metal arc welding (SMAW). Four different aspects regulated within the standard were accounted for: (1) sample dimensions and number of samples taken simultaneously; (2) time limitations defined by the standard regarding the welding and the cleaning process; (3) time, temperature, and method defined for analysis of the diffusible hydrogen content; (4) normalization of the hydrogen concentration measured. Underwater wet welding was performed using an automated, arc voltage-controlled welding machine. The results are discussed in light of standard DIN EN ISO 3690, and recommendations are provided for the analysis of diffusible hydrogen content upon underwater wet welding.

8.
Beilstein J Org Chem ; 14: 1004-1015, 2018.
Article in English | MEDLINE | ID: mdl-29977372

ABSTRACT

A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (1), 1,1'-bis(methoxycarbonyl)- (2), 1,1',3-tris(methoxycarbonyl)- (3) and 1,1',3,3'-tetrakis(methoxycarbonyl)ferrocene (4), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in 1-4 on the electrochemical potential E1/2 is correlated with the sum of Hammett constants. The 1/1+ -4/4+ redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV-vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV-vis absorptions of 1-4 and 1+ -4+ correlate with the number of ester groups. Paramagnetic 1H NMR redox titration experiments give access to the chemical shifts of 1+ -4+ and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.

9.
Dalton Trans ; 47(36): 12582-12586, 2018 Sep 18.
Article in English | MEDLINE | ID: mdl-29900470

ABSTRACT

A series of alkali metal (Li-Cs) alkoxides of tert-pentanol (1,1-dimethylpropan-1-ol) have been prepared by reaction of the corresponding metal with the alcohol in n-hexane or n-heptane. The compounds were purified by vacuum sublimation and crystallised in n-hexane to produce crystals suitable for single-crystal X-ray diffraction studies. The structures of the potassium, rubidium, and caesium compounds revealed tetrameric units with additional intra- and intermolecular interactions between the metal atom and alkoxide methyl groups, increasing with the size of the metal involved.

10.
Z Anorg Allg Chem ; 644(4): 194-204, 2018 03 01.
Article in English | MEDLINE | ID: mdl-29540939

ABSTRACT

The element lithium has been discovered 200 years ago. Due to its unique properties it has emerged to play a vital role in industry, esp. for energy storage, and lithium-based products and processes support sustainable technological developments. In addition to the many uses of lithium in its inorganic forms, lithium has a rich organometallic chemistry. The development of organometallic chemistry has been hindered by synthetic problems from the start. When Wilhelm Schlenk developed the basic principles to handle and synthesize air- and moisture-sensitive compounds, the road was open to further developments. After more information was available about the stability and solubility of such compounds, they started to play an essential role in other fields of chemistry as alkyl or aryl transfer reagents.

11.
Chemistry ; 24(30): 7605-7609, 2018 May 28.
Article in English | MEDLINE | ID: mdl-29529351

ABSTRACT

Lochmann-Schlosser superbases are formed by mixing alkyllithium with potassium alkoxides. These reagents could prove their synthetic usefulness and reliability in many reactions over five decades. However, despite many efforts, the real source of the exceptional reactivity remained a secret. The seemingly manageable system of four components (lithium, potassium atoms, alkyl groups, and alkoxy groups) and their interaction is obscured by poor solubility and fierce reactivity. Recent progress was achieved by using neopentyllithium, leading to alkane-soluble aggregates with varying lithium/potassium content and a flexible alkyl/alkoxy ratio. Herein, we isolated two new alkane-soluble alkyl/alkoxy mixed aggregates, [Li4 KNp2 (OtBu)3 ] and [K4 Np(OtAm)3 ]. The latter compound is a thermally stable three-component potassium alkyl/alkoxy base with well-defined stoichiometry, in contrast to lithium-containing Lochmann-Schlosser bases with variable metal and alkyl/alkoxy content. In a simple protocol, this potassium-base gave tetrametalated ferrocene, which was converted into 1,1',3,3'-ferrocenetetracarboxylic acid by reaction with CO2 . A subsequent conversion into the methyl ester allowed its separation from accompanying di- and tri-substituted ferrocenes.

12.
Angew Chem Int Ed Engl ; 55(36): 10886-9, 2016 08 26.
Article in English | MEDLINE | ID: mdl-27392232

ABSTRACT

Mixtures of alkyllithium and heavier alkali-metal alkoxides are often used to form alkyl compounds of heavier alkali metals, but these mixtures are also known for their high reactivity in deprotonative metalation reactions. These organometallic mixtures are often called LiC-KOR superbases, but despite many efforts their constitution remains unknown. Herein we present mixed alkali-metal alkyl/alkoxy compounds produced by reaction of neopentyllithium with potassium tert-butoxide. The key to success was the good solubility and temperature-stability of neopentyl alkali-metal compounds, leading to hexane-soluble mixtures, which allowed handling at ambient temperatures and isolation by crystallization. The compounds in solid state and in solution were identified by X-ray crystallography and NMR spectroscopy as mixtures of lithium/potassium neopentyl/tert-butoxy aggregates of varying compositions Lix Ky Npz (OtBu)x+y-z .

13.
Chem Commun (Camb) ; 50(85): 12859-62, 2014 Nov 04.
Article in English | MEDLINE | ID: mdl-25209651

ABSTRACT

Using structurally well-defined dipotassium-tetra(alkyl)magnesiates, a new straightforward methodology to promote regioselective Mg-H exchange reactions of a wide range of aromatic and heteroaromatic substrates is disclosed. Contacted ion pair intermediates are likely to be involved, with K being the key to facilitate the magnesiation processes.

14.
Chem Commun (Camb) ; 50(59): 7929-32, 2014 Jul 28.
Article in English | MEDLINE | ID: mdl-24737047

ABSTRACT

Treatment of tribenzotriquinacene with trimethylsilylmethyl sodium [NaCH2SiMe3] or potassium hydride [KH] leads to the formation of monometalated products. The sodium atom interacts with the carbanion in an exo fashion, while the potassium atom is found in an endo position inside the rigid bowl-shaped molecule.

15.
Dalton Trans ; 42(7): 2512-9, 2013 Feb 21.
Article in English | MEDLINE | ID: mdl-23212388

ABSTRACT

Five novel neutral homometallic complexes which contain the anthracen-9-olate anion have been prepared and characterised by treating anthrone with an organometallic base to induce aromatisation. Three of these complexes [(donor)·M(OC(14)H(9))](2) (2-4) are dimeric and contain sodium, lithium and potassium respectively. For 2 and 3 the donor is N,N,N',N'-tetramethylethylenediamine (TMEDA) whilst for 4 it is N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA). When bimetallic reagents such as [(TMEDA)·Na(µ-(t)Bu)(µ-TMP)Zn((t)Bu)] are employed, only the alkali-metal-containing species is isolated (e.g., 2 in this case). Providing a contrast, complexes 5 [(TMEDA)·Mg(OC(14)H(9))(n)Bu] and 6 [(TMEDA)·Zn(OC(14)H(9))Et] are monomeric. The alkali metal complexes were prepared by reacting anthrone with one molar equivalent of either n-butylsodium, n-butyllithium or (trimethylsilyl)methylpotassium in the presence of the required donor solvent. For 5 and 6, equimolar quantities of di-n-butylmagnesium and diethylzinc, respectively, were reacted with anthrone in the presence of TMEDA. The solid-state structures and the arene solution structures of 2-6 have been determined by X-ray crystallography and NMR spectroscopy respectively.


Subject(s)
Anthracenes/chemistry , Organometallic Compounds/chemical synthesis , Zinc/chemistry , Anthracenes/chemical synthesis , Crystallography, X-Ray , Magnetic Resonance Spectroscopy , Models, Molecular , Molecular Structure , Organometallic Compounds/chemistry , Protons
17.
Chem Commun (Camb) ; 47(13): 3772-4, 2011 Apr 07.
Article in English | MEDLINE | ID: mdl-21350770

ABSTRACT

Treating fluorenone or 2-benzoylpyridine with the sodium zincate [(TMEDA)·Na(µ-(t)Bu)(µ-TMP)Zn((t)Bu)] in hexane solution, gives efficient (t)Bu addition across the respective organic substrate in a highly unusual 1,6-fashion, producing isolable organometallic intermediates which can be quenched and aerobically oxidised to give 3-tert-butyl-9H-fluoren-9-one and 2-benzoyl-5-tert-butylpyridine respectively.


Subject(s)
Fluorenes/chemistry , Ketones/chemistry , Organometallic Compounds/chemistry , Pyridines/chemistry , Fluorenes/chemical synthesis , Ketones/chemical synthesis , Models, Molecular , Organometallic Compounds/chemical synthesis , Pyridines/chemical synthesis
18.
Chem Commun (Camb) ; 47(1): 388-90, 2011 Jan 07.
Article in English | MEDLINE | ID: mdl-20820529

ABSTRACT

The study of co-complexation reactions between NaCH(2)SiMe(3) and Mg(CH(2)SiMe(3))(2) has allowed the isolation and structural elucidation of the first solvent-free alkali-metal alkylmagnesiate [{NaMg(CH(2)SiMe(3))(3)}(∞)] (1) as well as the related solvent-free sodium alkyl [{(NaCH(2)SiMe(3))(4)}(∞)] (3).


Subject(s)
Alkanes/chemistry , Magnesium/chemistry , Organometallic Compounds/chemistry , Organometallic Compounds/chemical synthesis , Sodium/chemistry , Crystallography, X-Ray , Models, Molecular , Molecular Structure
20.
Nat Chem ; 2(7): 588-91, 2010 Jul.
Article in English | MEDLINE | ID: mdl-20571579

ABSTRACT

The cleavage of ethers is commonly encountered in organometallic chemistry, although rarely studied in the context of new, emerging bimetallic reagents. Recently, it was reported that a bimetallic sodium-zinc base can deprotonate cyclic tetrahydrofuran under mild conditions without opening its heterocyclic (OC(4)) ring. In marked contrast to this synergic sedation, herein we show that switching to the more reactive sodium-magnesium or sodium-manganese bases promotes cleavage of at least six bonds in tetrahydrofuran, but uniquely the ring fragments are captured in separate crystalline complexes. Oxide fragments occupy guest positions in bimetallic, inverse crown ethers and C(4) fragments ultimately appear in bimetallated butadiene molecules. These results demonstrate the special synergic reactivity that can be executed by bimetallic reagents, which include the ability to capture and control, and thereby study, reactive fragments from sensitive substrates.


Subject(s)
Furans/chemistry , Crown Ethers/chemical synthesis , Crown Ethers/chemistry , Crystallography, X-Ray , Molecular Conformation , Organometallic Compounds/chemistry
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