A Tetracarbene Iron(II) Complex with a Long-lived Triplet Metal-to-ligand Charge Transfer State due to a Triplet-Triplet Barrier.

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.

Structural Motifs of Alkali Metal Superbases in Non-coordinating Solvents.

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.

Endohedral Mixed Aggregates: Sodium Alkoxide Cages with Organic or Inorganic Central Anions and Variable Hull.

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.

Towards the Next Generation of Lochmann-Schlosser Superbases: A Potassium Neopentyl/Alkoxy Aggregate used in the Tetra-Functionalization of Ferrocene.

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.

Polysubstituted ferrocenes as tunable redox mediators.

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.

Combining Neopentyllithium with Potassium tert-Butoxide: Formation of an Alkane-Soluble Lochmann-Schlosser Superbase.

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 .

Size- and shape-adaptable mixed-alkoxide-aggregates: encapsulation of tetrafluoroborate and butadiynediide.

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.

Induction Heating in Underwater Wet Welding-Thermal Input, Microstructure and Diffusible Hydrogen Content.

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.

Investigations into Flux-Free Plasma Brazing of Aluminum in a Local XHV-Atmosphere.

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.

The Applicability of the Standard DIN EN ISO 3690 for the Analysis of Diffusible Hydrogen Content in Underwater Wet Welding.

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.

Structural and magnetic insights into the trinuclear ferrocenophane and unexpected hydrido inverse crown products of alkali-metal-mediated manganation(II) of ferrocene.

With the aim of introducing the diisopropylamide [NiPr(2)](-) ligand to alkali-metal-mediated manganation (AMMMn) chemistry, the temperature-dependent reactions of a 1:1:3 mixture of butylsodium, bis(trimethylsilylmethyl)manganese(II), and diisopropylamine with ferrocene in hexane/toluene have been investigated. Performed at reflux temperature, the reaction affords the surprising, ferrocene-free, hydrido product [Na(2)Mn(2) (mu-H)(2){N(iPr)(2)}(4)]2 toluene (1), the first Mn hydrido inverse crown complex. Repeating the reaction rationally, excluding ferrocene, produces 1 in an isolated crystalline yield of 62 %. At lower temperatures, the same bimetallic amide mixture leads to the manganation of ferrocene to generate the first trimanganese, trinuclear ferrocenophane, [{Fe(C(5)H(4))(2)}(3){Mn(3)Na(2)(NiPr(2))(2) (HNiPr(2))(2)}] (2) in an isolated crystalline yield of 81 %. Both 1 and 2 have been characterised by X-ray crystallographic studies. The magnetic properties of paramagnetic 1 and 2 have also been examined by variable-temperature magnetisation measurements on powdered samples. For 1, the room-temperature value for chiT is 3.45 cm(3) K mol(-1), and on lowering the temperature a strong antiferromagnetic coupling between the two Mn ions is observed. For 2, the room-temperature value for chiT is 4.06 cm(3) K mol(-1), which is significantly lower than the expected value for three isolated paramagnetic Mn(II) ions.

Direct lateral metallation using alkali-metal mediated zincation (AMMZn): SiC-H vs.Si-O bond cleavage.

A new application of zincate [(THF)Li(TMP)(tBu)Zn(tBu)] (1) in alkali-metal mediated zincation (AMMZn) is reported by isolating and structurally defining the first intermediates of direct lateral zincation (DlZn) of trimethyl(phenoxy)silane (2) both of which adopt novel six-element {ZnCSiOLiN} ring structures.

Structural variations within group 1 (Li-Cs)+ (2,2,6,6-tetramethyl-1-piperidinyloxy)- complexes made via metallic reduction of the nitroxyl radical.

Treatment of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) with a group 1 metal (Li, Na, K, Rb, or Cs), resulted in the reduction of this important radical to the TEMPO(-) anion--the first examples of elemental-metal single electron reduction of the radical to its anionic form. The synthesis and characterization of seven alkali metal TEMPO(-) complexes are reported. A variety of structural motifs are encountered depending on the choice of metal and/or solvent. (THF)(2) x [Li(+)(TEMPO(-))](4) 1 crystallized from THF as a cyclic (Li(4)O(4)) molecule. Two Li centers are stabilized by coordination to a THF molecule; the others by intramolecular coordination to N(TEMPO) atoms. [(THF) x Na(+)(TEMPO(-))](4) 2 exists as a distorted cubane where each Na center is coordinated to a THF molecule. No appreciable Na-N(TEMPO) coordination is observed. [(THF)(2) x Na(+)(3)(TEMPO(-))(2)(OH)](2) 3 was serendipitously prepared and exists as a distorted bis(cubane). It is envisaged that 3 is formed from 2 by insertion of a (Na-OH)(2) double bridge into its framework. [Na(+)(4)(mu(3)-TEMPO(-))(2)(mu(2)-TEMPO(-))(2)(TMEDA)(2)] 4, adopts a four-runged ladder structure, whereby the two outer Na centers are coordinated to TMEDA, in addition to two mu(2)-O and a N atom. The inner metal atoms are bound to three mu(3)-O atoms and a N atom. [(THF) x K(+)(TEMPO(-))](4) 5 resembles the motif found for 2; however, presumably because of the larger size of the metal, K-N(TEMPO) interactions are present in 5. The asymmetric unit of [(TMEDA) x Rb(+)(2)(TEMPO(-))(2)](2) 6 comprises a Rb(4)O(4) cubane with half a molecule of TMEDA coordinated to each metal. From a supramolecular perspective, 6 exists as a polymeric array of cubane units connected by TMEDA bridges. Completing the series, [Cs(+)(TEMPO)](infinity) 7 crystallizes from hexane to form a donor-free polymeric complex. Complexes 1, 2, and 4-7 are soluble in D(8)-THF solution, and their NMR spectra are reported. The solution structures in donor solvent appear virtually identical.

Direct C-H metalation with chromium(II) and iron(II): transition-metal host/benzenediide guest magnetic inverse-crown complexes.

Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low-polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.

Structural complexity of the magnesiation of furan: an octadecanuclear product with a subporphyrin-like Mg3(2,5-fur-di-yl)3 substructure.

Subjecting furan to sodium-mediated magnesiation via the new heteroleptic alkyl-amido reagent [(TMEDA).Na(CH(2)SiMe(3))(TMP)Mg(TMP)] produces the remarkable dodecasodium-hexamagnesium molecule [{(TMEDA)(3)Na(6)Mg(3)(CH(2)SiMe(3))(2,5-C(4)H(2)O)(3)(2-C(4)H(3)O)(5)}(2)], built upon a bridge network of 10 monodeprotonated and 6 twofold-deprotonated furan ligands.

Manganese(II)-lithium and -sodium inverse crown ether (ICE) complexes.

Extending to transition metals, the class of compounds known as inverse crown ethers, two mixed alkali metal-manganese(II) amide ring compounds with oxo cores have been synthesised and crystallographically characterised, together with an oxo-free alkyl-amido precursor.

Structurally-defined potassium-mediated regioselective zincation of amino- and alkoxy-substituted pyridines.

The new synergic base [PMDETA.K(TMP)(Et)Zn(Et)] selectively zincates 4-(dimethylamino)pyridine at the 2-position and 4-methoxypyridine at the 3-position, to afford bimetallic potassium pyridylzinc complexes each displaying a novel, but remarkably different, structure.

200 Years of Lithium and 100 Years of Organolithium Chemistry.

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.

The hetero-cubane structures of the heavy alkali metal tert-amyloxides [MOCMe2Et]4 (M = K, Rb, Cs).

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.

Lewis base stabilized lithium TMP-aluminates: an unexpected fragmentation and capture reaction involving cyclic ether 1,4-dioxane.

Three Lewis base variations of the synthetically useful aluminate [L x Li(TMP)((i)Bu)Al((i)Bu)2], where L is TMPH, Et3N or PhC(=O)N(i)Pr2, are reported, together with the reaction of the benzamide complex with 1,4-dioxane, which surprisingly leads to fragmentation of the cyclic ether and capture of its alkoxy vinyl ether residue within a novel dilithium dialuminium hexaalkyl aggregate.