Metal Vapour Synthesis of an Organometallic Barium(0) Synthon.

A hydrocarbon-soluble barium anthracene complex was prepared by means of metal vapour synthesis. Reaction of 9,10-bis(trimethylsilyl)anthracene (Anth'') with barium vapour gave deep purple Ba(Anth'') which after extraction with diethyl ether crystallised as the cyclic octamer [Ba(Anth'')⋅Et2 O]8 . Dissolution in benzene or toluene led to replacement of the Et2 O ligand with a softer arene ligand and isolation of Ba(Anth'')⋅arene. Diffusion ordered spectroscopy (DOSY NMR ) measurements in benzene-d6 indicate solution species with a molecular weight that equals a trimeric constitution. Natural population analysis (NPA) assigned charges of +1.70 and -1.70 to Ba and Anth'', respectively, relating to highly ionic Ba2+ /Anth''2- bonding. Preliminary reactivity studies with air, Ph2 C=NPh, or H2 show that the complex reacts as a Ba0 synthon by release of neutral Anth''. This soluble molecular Ba0 /BaII redox synthon provides new routes for the syntheses of barium complexes under mild conditions.

Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis.

Alkenes that normally do not react with LiAlH4 (3-hexene, cyclohexene, 1-Me-cyclohexene), can be reduced to the corresponding alkanes by a mixture of LiAlH4 and Fe0 (the iron was activated by Metal-Vapour-Synthesis). This alkene-to-alkane conversion with a stoichiometric quantity of LiAlH4 /Fe0 does not need quenching with water or acids, implying that both H's originate from LiAlH4 . The LiAlH4 /Fe0 combination is also a remarkably potent cooperative catalyst for hydrogenation of multi-substituted alkenes and benzene or toluene. An induction period of circa two hours and the minimally required temperature of 120 °C, suggests that the actual catalyst is a combination of Fe0 and the decomposition product of LiAlH4 (LiH and Al0 ). A thermally pre-activated LiAlH4 /Fe0 catalyst did not need an induction time and is also active at room temperature and 1 bar H2 . A combination of AliBu3 and Fe0 is an even more active hydrogenation catalyst. Without pre-activation, tetra-substituted alkenes like Me2 C=CMe2 and toluene could be fully hydrogenated.

Teaming up main group metals with metallic iron to boost hydrogenation catalysis.

Hydrogenation of unsaturated bonds is a key step in both the fine and petrochemical industries. Homogeneous and heterogeneous catalysts are historically based on noble group 9 and 10 metals. Increasing awareness of sustainability drives the replacement of costly, and often harmful, precious metals by abundant 3d-metals or even main group metals. Although not as efficient as noble transition metals, metallic barium was recently found to be a versatile hydrogenation catalyst. Here we show that addition of finely divided Fe0, which itself is a poor hydrogenation catalyst, boosts activities of Ba0 by several orders of magnitude, enabling rapid hydrogenation of alkynes, imines, challenging multi-substituted alkenes and non-activated arenes. Metallic Fe0 also boosts the activity of soluble early main group metal hydride catalysts, or precursors thereto. This synergy originates from cooperativity between a homogeneous, highly reactive, polar main group metal hydride complex and a heterogeneous Fe0 surface that is responsible for substrate activation.

Interobserver reliability of scapula fracture classifications in intra- and extra-articular injury patterns.

BACKGROUND: Morphology and glenoid involvement determine the necessity of surgical management in scapula fractures. While being present in only a small share of patients with shoulder trauma, numerous classification systems have been in use over the years for categorization of scapula fractures. The purpose of this study was to evaluate the established AO/OTA classification in comparison to the classification system of Euler and Rüedi (ER) with regard to interobserver reliability and confidence in clinical practice. METHODS: Based on CT imaging, 149 patients with scapula fractures were retrospectively categorized by two trauma surgeons and two radiologists using the classification systems of ER and AO/OTA. To measure the interrater reliability, Fleiss kappa (κ) was calculated independently for both fracture classifications. Rater confidence was stated subjectively on a five-point scale and compared with Wilcoxon signed rank tests. Additionally, we computed the intraclass correlation coefficient (ICC) based on absolute agreement in a two-way random effects model to assess the diagnostic confidence agreement between observers. RESULTS: In scapula fractures involving the glenoid fossa, interrater reliability was substantial (κ = 0.722; 95% confidence interval [CI] 0.676-0.769) for the AO/OTA classification in contrast to moderate agreement (κ = 0.579; 95% CI 0.525-0.634) for the ER classification system. Diagnostic confidence for intra-articular fracture patterns was superior using the AO/OTA classification compared to ER (p < 0.001) with higher confidence agreement (ICC: 0.882 versus 0.831). For extra-articular fractures, ER (κ = 0.817; 95% CI 0.771-0.863) provided better interrater reliability compared to AO/OTA (κ = 0.734; 95% CI 0.692-0.776) with higher diagnostic confidence (p < 0.001) and superior agreement between confidence ratings (ICC: 0.881 versus 0.912). CONCLUSIONS: The AO/OTA classification is most suitable to categorize intra-articular scapula fractures with glenoid involvement, whereas the classification system of Euler and Rüedi appears to be superior in extra-articular injury patterns with fractures involving only the scapula body, spine, acromion and coracoid process.

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage.

Low-valent (Me BDI)Al and (Me BDI)Ga and highly Lewis acidic cations in [(tBu BDI)M+ ⋅C6 H6 ][(B(C6 F5 )4 - ] (M=Mg or Zn, Me BDI=HC[C(Me)N-DIPP]2 , tBu BDI=HC[C(tBu)N-DIPP]2 , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [(tBu BDI)Mg-Al(Me BDI)+ ], [(tBu BDI)Mg-Ga(Me BDI)+ ] and [(tBu BDI)Zn-Ga(Me BDI)+ ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [(tBu BDI)Zn-Al(Me BDI)+ ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak AlI (or GaI )→Mg2+ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give (tBu BDI)ZnPh and (Me BDI)AlF+ which is extremely Lewis acidic and was not observed, but (Me BDI)Al(F)-(µ-F)-(F)Al(Me BDI)+ was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route.

Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst.

Ba metal was activated by evaporation and cocondensation with heptane. This black powder is a highly active hydrogenation catalyst for the reduction of a variety of unactivated (non-conjugated) mono-, di- and tri-substituted alkenes, tetraphenylethylene, benzene, a number of polycyclic aromatic hydrocarbons, aldimines, ketimines and various pyridines. The performance of metallic Ba in hydrogenation catalysis tops that of the hitherto most active molecular group 2 metal catalysts. Depending on the substrate, two different catalytic cycles are proposed. A: a classical metal hydride cycle and B: the Ba metal cycle. The latter is proposed for substrates that are easily reduced by Ba0 , that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba0 and BaH2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH2 ) confirm that the presence of metallic Ba has an accelerating effect.

Boosting Low-Valent Aluminum(I) Reactivity with a Potassium Reagent.

The reagent RK [R=CH(SiMe3 )2 or N(SiMe3 )2 ] was expected to react with the low-valent (DIPP BDI)Al (DIPP BDI=HC[C(Me)N(DIPP)]2 , DIPP=2,6-iPr-phenyl) to give [(DIPP BDI)AlR]- K+ . However, deprotonation of the Me group in the ligand backbone was observed and [H2 C=C(N-DIPP)-C(H)=C(Me)-N-DIPP]Al- K+ (1) crystallized as a bright-yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+ ⋅⋅⋅DIPP interactions. The rather short Al-K bonds [3.499(1)-3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6 H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6 H6 , two C-H bond activations are observed and a product with a para-phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPP BDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

d-d Dative Bonding Between Iron and the Alkaline-Earth Metals Calcium, Strontium, and Barium.

Double deprotonation of the diamine 1,1'-(tBuCH2 NH)-ferrocene (1-H2 ) by alkaline-earth (Ae) or EuII metal reagents gave the complexes 1-Ae (Ae=Mg, Ca, Sr, Ba) and 1-Eu. 1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1-Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1-Ca, 1-Sr, and 1-Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1 H NMR chemical-shift difference between the Cp α and ß protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1-Ae complexes shows that the heavier species 1-Ca, 1-Sr, and 1-Ba possess genuine Fe→Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In 1-Mg, a weak Fe→Mg donation into vacant p-orbitals of the Mg atom is observed.

Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings.

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)2 ]2 (1-Ae) and Ae[N(TRIP)(DIPP)]2 (2-Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr3 , DIPP=2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized. Monomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene.

Lewis acidic alkaline earth metal complexes with a perfluorinated diphenylamide ligand.

Alkaline earth metal (Ae) chemistry with the anion [N(C6F5)2]- has been explored. Deprotonation of the amine (C6F5)2NH, abbreviated in here as NFH, with 0.5 equivalent of AeN''2 (N'' = N(SiMe3)2) is fast and gave, dependent on the solvent, the complexes AeNF2, AeNF2·(THF)2 and AeNF2·(Et2O)2 (Ae = Mg, Ca, Sr). Using a 1/1 ratio, mixed amide complexes were obtained: NFAeN'' (Ae = Mg, Ca, Sr). Crystal structures of the monomers AeNF2·(THF)2 (Ae = Mg, Ca, Sr) and AeNF2·(Et2O)2 (Ae = Mg, Ca) are presented and compared with those of AeN''2·(THF)2. In addition, crystal structures of the homoleptic dimer (MgNF2)2 and the heteroleptic dimers (NFAeN'')2 (Ae = Mg, Ca, Sr) are discussed. All structures are strongly influenced by very short AeF contacts down to circa 2.11 Å (Mg), 2.50 Å (Ca) and 2.73 Å (Sr). AIM analysis illustrates that, although AeF contacts are short, there is no bond-critical-point along this axis, indicating an essentially electrostatic interaction. The monomeric complexes feature strong C6F5C6F5π-stacking, resulting in unusually acute NF-Ae-NF angles as small as 95°. Heteroleptic (NFAeN'')2 complexes retain their dimeric structure in C6D6 solution and there is no indication of ligand scrambling by the Schlenk equilibrium, suggesting that an electron withdrawing ligand may stabilize heteroleptic complexes. According to DFT calculations, the heteroleptic arrangement is 70 kJ mol-1 more stable than the homoleptic dimers. The Lewis acidity of MgNF2 has been quantified with the Gutmann-Beckett method and by calculation of the Fluoride-Ion-Affinity. The latter calculations show that the Lewis acidity of MgNF2 and CaNF2 is comparable to that of B(C6F5)3. Dimeric (MgNF2)2 fully abstracts Et3PO from Et3PO·B(C6F5)3 and may have potential in Lewis acid catalysis.

LiAlH4 : From Stoichiometric Reduction to Imine Hydrogenation Catalysis.

Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH4 is demonstrated (85 °C, catalyst loading≥2.5 mol %, pressure≥1 bar). The effects of temperature, pressure, solvent, and catalyst modifications, as well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ: LiAlH4 +Ph(H)C=NtBu→LiAlH2 [N(tBu)CH2 Ph]2 . A cooperative mechanism in which Li and Al both play a prominent role is proposed.

Simple Access to the Heaviest Alkaline Earth Metal Hydride: A Strongly Reducing Hydrocarbon-Soluble Barium Hydride Cluster.

Reaction of Ba[N(SiMe3 )2 ]2 with PhSiH3 in toluene gave simple access to the unique Ba hydride cluster Ba7 H7 [N(SiMe3 )2 ]7 that can be described as a square pyramid spanned by five Ba2+ ions with two flanking BaH[N(SiMe3 )2 ] units. This heptanuclear cluster is well soluble in aromatic solvents, and the hydride 1 H NMR signals and coupling pattern suggests that the structure is stable in solution. At 95 °C, no coalescence of hydride signals is observed but the cluster slowly decomposes to undefined barium hydride species. The complex Ba7 H7 [N(SiMe3 )2 ]7 is a very strong reducing agent that already at room temperature reacts with Me3 SiCH=CH2 , norbornadiene, and ethylene. The highly reactive alkyl barium intermediates cannot be observed and deprotonate the (Me3 Si)2 N- ion, as confirmed by the crystal structure of Ba14 H12 [N(SiMe3 )2 ]12 [(Me3 Si)(Me2 SiCH2 )N]4 .

A Simple Route to Calcium and Strontium Hydride Clusters.

The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe3 )2 ]2 with PhSiH3 in the presence of PMDTA. The Sr complex Sr6 H9 [N(SiMe3 )2 ]3 ⋅(PMDTA)3 crystallizes as an "inverse cryptand": an interstitial H- is surrounded by a Sr6 H84+ cage decorated with amide and PMDTA ligands. The analogous Ca complex could also be obtained and both retain their solid-state structures in solution: 1 H NMR spectra in C6 D6 show two doublets and one nonet (4:4:1). Up to 90 °C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+1.14) and H (-0.59) which signifies an unexpectedly low ionicity. AIM analysis shows hydride⋅⋅⋅hydride bond paths with considerable electron densities in the bond critical point. The clusters thermally decompose into larger, undefined, metal hydride aggregates.

Bora-amidinate as a cooperative ligand in group 2 metal catalysis.

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes DIPPNBN-Mg·(THF)3 and DIPPNBN-Ca·(THF)4 are presented; DIPPNBN = HB[N(2,6-iPr2-C6H3)]2. The simplicity of their 1H NMR spectra in THF-d8 suggest that their monomeric solid state structures are retained in solution. DIPPNBN-Mg·(THF)3 in C6D6, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in DIPPNBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

Phosphido complexes derived from 1,1'-ferrocenediyl-bridged secondary diphosphines.

This paper focuses on ferrocene-based secondary diphosphines of the type [Fe{η5-C5H4(PHR)}2] with P-substituents of distinctly different steric and electronic properties, namely methyl, neopentyl (Np), tert-butyl, phenyl and 3,5-bis(trifluoromethyl)phenyl (XyF). The reaction of [Fe{η5-C5H4(PHPh)}2] (H21a) and [Fe{η5-C5H4(PHt-Bu)}2] (H21b) with n-BuLi in the presence of TMEDA afforded lithium diphosphides of the type [Li2(µ-1)(TMEDA)2], which contain a cyclic non-planar Li2P2 core. The analogous reactions of [Fe{η5-C5H4(PHMe)}2] (H21c) and [Fe{η5-C5H4(PHNp)}2] (H21d) furnished dimeric aggregates exhibiting a ladder-type Li4P4 motif, viz. [Li4(µ-1c)2(TMEDA)3] and [Li2(µ-1d)(TMEDA)]2. H21e (R = XyF) did not afford a stable lithium diphosphide. A Brønsted metathesis with Zr(NMe2)4 was possible with the aryl-substituted compounds H21a and H21e, leading to products of the type [{Zr(NMe2)3}2(µ-1)]. In contrast, the alkyl-substituted congeners H21b-H21d were inert towards Zr(NMe2)4. The reaction of [Fe{η5-C5H4(PHR)}2] with nickelocene afforded intractable mixtures of numerous products in the case of H21c and H21e. In the other three cases, compounds of the type [(NiCp)2(µ-1)] were isolated. For H21b and H21d a two-stepped reaction via a phosphino-phosphido intermediate of the type [NiCp(H1)] was observed, which could be isolated and fully characterised in the case of [NiCp(H1b)].

ß-Diketiminate calcium hydride complexes: the importance of solvent effects.

A series of (DIPPnacnac)CaN(SiMe3)2·S complexes (DIPPnacnac = HC[C(Me)N(2,6-iPr-C6H3)]2; S = solvent) could be obtained by the addition of S = THF, DME or N-Me-morpholine (Morph) to (DIPPnacnac)CaN(SiMe3)2·OEt2 or (DIPPnacnac)CaN(SiMe3)2. Crystal structures for complexes with S = DME and Morph are compared to literature-known structures with S = none, THF or Et2O. Bulkier and weaker Lewis bases like the tertiary amines Et3N, TMEDA and DABCO did not interact with (DIPPnacnac)CaN(SiMe3)2. The reaction of (DIPPnacnac)CaN(SiMe3)2 with PhSiH3 gave conversion to a calcium hydride complex that dismutated in (DIPPnacnac)2Ca and CaH2. The reaction of (DIPPnacnac)CaN(SiMe3)2·S with PhSiH3 gave [(DIPPnacnac)CaH·S]2 for S = THF, Et2O or N-Me-morpholine (Morph). For S = DME, high reaction temperatures were needed and dismutation into (DIPPnacnac)2Ca and CaH2 was observed. Extensive NMR investigations (VT-NMR and PGSE) confirm the dimeric nature of [(DIPPnacnac)CaH·THF]2 in aromatic solvents or in THF. Thermal decomposition of [(DIPPnacnac)CaH·THF]2 (release of H2 at 200 °C) is compared to that of Mg and Zn analogues. Weakly coordinating Et2O in [(DIPPnacnac)CaH·OEt2]2 could be replaced by THF, Morph or DABCO but not with Et3N. The addition of TMEDA led to the formation of CaH2 and unidentified products. The addition of DME led to the decomposition of Et2O and complex [(DIPPnacnac)CaOEt]2 was obtained. Crystal structures of the following compounds are presented: (DIPPnacnac)CaN(SiMe3)2·S (S = Morph, DME), [(DIPPnacnac)CaH·S]2 (S = Et2O, Morph and DABCO) and [(DIPPnacnac)CaOEt]2. Although bulky ligands have long been thought to be the key to the stabilization of calcium hydride complexes, the presence of a polar, strongly coordinating, co-solvent is also crucial.

Unsymmetrical N-heterocyclic carbenes with a 1,1'-ferrocenediyl backbone.

This paper focuses on ferrocene-based expanded-ring N-heterocyclic carbenes (NHCs) of the type [Fe(C5H4-NR-C-NR'-C5H4)] (), which contain two different N-substituents. Three combinations were addressed, with R = neopentyl (Np) in each case and R' being either 2-adamantyl (Ad), phenyl (Ph) or 9-anthracenylmethyl (Acm). The NHCs were generated by reaction of the corresponding formamidinium tetrafluoroborates [H-][BF4] with lithium diisopropylamide (LDA). While only was sufficiently stable for isolation, and could be efficiently trapped in situ by complexation reactions. Two series of Rh(I) complexes were prepared, viz. [RhCl(cod)()] (cod = 1,5-cyclooctadiene) by reacting [{Rh(µ-Cl)(cod)}2] with and cis-[RhCl(CO)2()] by reacting [RhCl(cod)(')] with CO. All complexes exhibit pronounced anagostic α-CHRh interactions, both in solution and in the solid state, in accord with a strong influence of the N-substituents on the steric ligand properties, as is chemically illustrated by the huge reactivity difference of [RhCl(cod)()] (R = R' = Ad) and [RhCl(cod)()] towards CO, the former complex being inert. Tolman electronic parameter (TEP) values are 2050 ± 1 cm(-1) for the unsymmetrical NHCs studied, indicating only a weak influence of the N-substituents on the electronic ligand properties.

Diastereoselective synthesis of a bicyclic ß-lactam with penicillin G-like spectrum of activity by carbonylation of an acyclic diaminocarbene.

Diisopropylamino-cis-2,6-dimethylpiperidinocarbene reacts regio- and diastereoselectively with CO to afford a bicyclic ß-lactam with 100% atom efficiency, whose spectrum of activity resembles that of penicillin G or amoxicillin.

Carbonylation of the simplest persistent diaminocarbene.

The reactions of the acyclic diaminocarbenes (Me2N)2C and (Ph2N)(iPr2N)C with CO proceed in a 2 : 1 stoichiometric ratio, affording unprecedented betainic oxyallyl species of type [(R2N)2C]2CO.

Unexpected thermal decomposition of the "Alder carbene" (iPr2N)2C.

The "Alder carbene" (iPr(2)N)(2)C undergoes a ß-fragmentation in solution already at room temperature, affording propene and N,N,N'-triisopropylformamidine. This stands in sharp contrast to the indefinite stability previously claimed for this iconic compound.