Synthesis and Modular Reactivity of Low Valent Al/Zn Heterobimetallics Supported by Common Monodentate Amides.

Recent advances on low valent main group metal chemistry have shown the excellent potential of heterobimetallic complexes derived from Al(I) to promote cooperative small molecule activation processes. A signature feature of these complexes is the use of bulky chelating ligands which act as spectators providing kinetic stabilization to their highly reactive Al-M bonds. Here we report the synthesis of novel Al/Zn bimetallics prepared by the selective formal insertion of AlCp* into the Zn-N bond of the utility zinc amides ZnR2 (R=HMDS, hexamethyldisilazide; or TMP, 2,2,6,6-tetramethylpiperidide). By systematically assessing the reactivity of the new [(R)(Cp*)AlZn(R)] bimetallics towards carbodiimides, structural and mechanistic insights have been gained on their ability to undergo insertion in their Zn-Al bond. Disclosing a ligand effect, when R=TMP, an isomerization process can be induced giving [(TMP)2AlZn(Cp*)] which displays a special reactivity towards carbodiimides and carbon dioxide involving both its Al-N bonds, leaving its Al-Zn bond untouched.

Beryllium-Mediated Halide and Aryl Transfer onto Silicon.

The reactivity of hexamethylcyclotrisiloxane (D3 ) towards BeCl2 , BeBr2 , BeI2 and [Be3 Ph6 ]3 was investigated. While BeCl2 only showed unselective reactivity, BeBr2 , BeI2 and [Be3 Ph6 ] cleanly react to the trinuclear complexes [Be3 Br2 (OSiMe2 Br)4 ], [Be3 I2 (OSiMe2 I)4 ] and [Be3 Ph2 (OSiMe2 Ph)4 ]. These unprecedented bromide, iodide and phenyl transfer reactions from a group II metal onto silicon offer a versatile access to previously unknown diorgano bromo and iodo silanolates.

Evidence for Carbene Intermediates in Isocyanide Homologation by Aluminium(I).

The C-C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer-Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral AlI complex (Me NacNac)Al (1, Me NacNac=HC[(CMe)(NDipp)]2 , Dipp=2,6-diisopropylphenyl) and various isocyanides. The step-by-step coupling mechanism was studied in detail by low-temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6-bis(benzhydryl)-4-Me-phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri-, tetra-, and even pentamerization products were isolated with the sterically less congested phenyl and p-methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT-type chemistry of aluminium(I) and isocyanides.

On the Reactivity of Phosphaalumenes towards C-C Multiple Bonds.

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe3 gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles.

Metal-Free N-H Bond Activation by Phospha-Wittig Reagents.

N-containing molecules are mostly derived from ammonia (NH3 ). Ammonia activation has been demonstrated for single transition metal centers as well as for low-valent main group species. Phosphinidenes, mono-valent phosphorus species, can be stabilized by phosphines, giving so-called phosphanylidenephosphoranes of the type RP(PR'3 ). We demonstrate the facile, metal-free NH3 activation using ArP(PMe3 ), affording for the first time isolable secondary aminophosphines ArP(H)NH2 . DFT studies reveal that two molecules of NH3 act in concert to facilitate an NH3 for PMe3 exchange. Furthermore, H2 NR and HNR2 activation is demonstrated.

Construction of Inorganic Crown Ethers by s-Block-Metal-Templated Si-O Bond Activation.

We herein report the synthesis, structures, coordination ability, and mechanism of formation of silicon analogs of crown ethers. An oligomerization of 2 D2 (I) (2 Dn ,=(Me4 Si2 O)n ) was achieved by the reaction with GaI3 and MIx (M=Li, Na, Mg, Ca, Sr). In these reactions the metal cations serve as template and the anions (I- /[GaI4 ]- ) are required as nucleophiles. In case of MIx =LiI, [Li(2 D3 )GaI4 ] (1) is formed. In case of MIx =NaI, MgI2 , CaI2 , and SrI2 the compounds [M(2 D4 )(GaI4 )x ] (M=Mg2+ (3), Ca2+ (4), Sr2+ (5) are obtained. Furthermore the proton complex [H(2 D3 )][Ga2 I7 ] (6) was isolated and structurally characterized. All complexes were characterized by means of multinuclear NMR spectroscopy, DOSY experiments and, except for compound 3, also by single crystal X-ray diffraction. Quantum chemical calculations were carried out to compare the affinity of M+ to 2 Dn and other ligands and to shed light on the formation of larger rings from smaller ones.

A Second Modification of Beryllium Bromide: ß-BeBr2.

The synthesis of a second beryllium bromide modification, ß-BeBr2, was accomplished through recrystallization of α-BeBr2 from benzene in the presence of cyclo-decamethylpentasiloxane. This phase was analyzed via single-crystal X-ray diffraction and IR and Raman spectroscopy as well as density functional theory calculations. This enabled a comparison to α-BeBr2 and the α and ß phases of beryllium chloride and iodide.

Alkaline Earth Metal Template (Cross-)Coupling Reactions with Hybrid Disila-Crown Ether Analogues.

Alkaline earth metal iodides were used as templates for the synthesis of novel silicon-based ligands. Siloxane moieties were (cross-)coupled and ion-specific, silicon-rich crown ether analogues were obtained. The reaction of 1,2,7,8-tetrasila[12]crown-4 (I) and 1,2-disila[9]crown-3 (II) with MgI2 yielded exclusively [Mg(1,2,7,8-tetrasila[12]crown-4)I2 ] (1). The larger Ca2+ ion was then employed for cross-coupling of I and II and yielded the complex [Ca(1,2,7,8-tetrasila[15]crown-5)I2 ] (2). Cross-coupling of I and 1,2,4,5-tetrasila[9]crown-3 (III) with SrI2 enables the synthesis of the silicon-dominant 1,2,4,5,10,11-hexasila[15]crown-5 ether complex of SrI2 (3). Further, the compounds [Sr(1,2,10,11-tetrasila[18]crown-6)I2 ] (4), [Sr(1,2,13,14-tetrasila[24]crown-8)I2 ] (5), and [Sr(1,2,13,14-tetrasila-dibenzo[24]crown-8)I2 ] (6) were obtained by coupling I, 1,2-disila[12]crown-4 (IV) or 1,2-disila-benzo[12]crown-4 (V), respectively. Using various anions, the (cross-)coupled ligands were also observed in an X-ray structure within the mentioned complexes. These template-assisted (cross-)couplings of various ligands are the first of their kind and a novel method to obtain macrocycles and/or their metal complexes to be established. Further, the Si-O bond activations presented herein might be of importance for silane or even organic functionalization.

Insights into the Coordination Ability of Siloxanes Employing Partially Silicon Based Crown Ethers: A Comparative Analysis of s-Block Metal Complexes.

Herein we present the synthesis and coordination chemistry of the partially silicon based crown ether analogues 1,2,4,5-tetrasila-benzo[15]crown-5 (1) and 1,2,4,5-tetrasila[18]crown-6 (7). Stable complexes of alkali and alkaline earth metal iodides could be obtained showing the good coordination ability of these ligands. The complexes [M A(1,2,4,5-tetrasila-benzo[15]crown-5)I] (M A = Li+ (2), Na+ (4)) and [M EA(1,2,4,5-tetrasila-benzo[15]crown-5)I2] (M EA = Mg2+ (3), Ca2+ (5), Sr2+ (6)) were obtained by equimolar reaction of 1 with the respective alkali or alkaline earth metal iodide. Depending on the ionic radii of the respective cations, the coordination modes of the siloxane backbone are significantly different and could be well-represented by means of 29Si NMR spectroscopy. In addition, we were able to generate the unusual dinuclear complex [Ba2(1,2,4,5-tetrasila[18]crown-6)2I4] (8) by reaction of 7 with BaI2. All compounds were characterized via single crystal X-ray diffraction (XRD) analysis.

Not Non-Coordinating at All: Coordination Compounds of the Cyclodimethylsiloxanes Dn (D = Me2SiO; n = 6, 7) and Group 2 Metal Cations.

We present the coordination chemistry of the cyclodimethylsiloxanes D6 and D7 toward alkaline earth metal salts. The coordination chemistry of these macrocycles toward alkaline earth metals has been unprecedented to date, and we could show that these ligands coordinate better than previously thought. Direct reaction of alkaline earth metal salts with these ligands yields stable complexes even with a relatively strongly coordinating iodide anion. A handful of counterintuitive coordination compounds could be characterized by single-crystal X-ray diffraction analysis. Quantum chemical calculations of suited Born-Haber cycles showed that these complexes are indeed stable, for Mg2+ and Ca2+ even with iodide employed as the anion and for Sr2+ and Ba2+ in the presence of GaI3.

Alkali and Alkaline Earth Metal Derivatives of Disila-Bridged Podands: Coordination Chemistry and Structural Diversity.

Within this study, the synthesis and coordination chemistry of open-chain ligands bearing disila-units is presented. Instead of basic 1:1 complexes, structural diversity was discovered in the variety of ligand and salt. Stable complexes of alkali and alkaline earth metal complexes were obtained by equimolar reactions of different salts with the disila-bridged podands 8,9-disila-EO5 (1) and 11,12-disila-EO7 (2) (EO5 = pentaethylene glycol; EO7 = heptaethylene glycol). The respective alkaline earth metal complexes of the type [Ca(8,9-disila-EO5)(OTf)2] (3), [Sr(8,9-disila-EO5)I2] (5), [Sr(11,12-disila-EO7)I]I (6), and [Ba(11,12-disila-EO7)OTf2] (7) (OTf = CF3SO3-) were characterized via single-crystal X-ray diffraction analyses. Within the reaction of the alkali metal salt NaPF6 with 1, the sodium ion acts as a template during the complexation process. Under elimination of one molecule of diethylene glycol, the dinuclear species [Na2(8,9,17,18-tetrasila-EO8)(PF6)2]·EO2 (4) (EO8 = octaethylen glycol, EO2 = diethylene glycol) is obtained, in which the sodium cations are 7-fold coordinated within a disilane-bearing framework. The reaction of 2 with CsOTf failed, leading to recrystallization of anhydrous CsOTf. By means of DFT calculations it was shown that the disila-bearing ligands are burdened with negative hyperconjugation interactions between the silicon and the oxygen atoms, but the coordination by sufficiently hard cations can easily overcompensate the competing polarization. In contrast, soft Lewis acids barely share interactions with silicon-bonded oxygen atoms. All findings are consistent with observations made in solution according to 29Si NMR spectroscopical studies.

Pd8(PDip)6: Cubic, Unsaturated, Zerovalent.

Atomically precise nanoclusters hold promise for supramolecular assembly and (opto)electronic- as well as magnetic materials. Herein, this work reports that treating palladium(0) precursors with a triphosphirane affords strongly colored Pd8(PDip)6 that is fully characterized by mass spectrometry, heteronuclear and Cross-Polarization Magic-Angle Spinning (CP-MAS) NMR-, infrared (IR), UV-vis, and X-ray photoelectron (XP) spectroscopies, single-crystal X-Ray diffraction (sc-XRD), mass spectrometry, and cyclovoltammetry (CV). This coordinatively unsaturated 104-electron Pd(0) cluster features a cubic Pd8-core, µ4-capping phosphinidene ligands, and is air-stable. Quantum chemical calculations provide insight to the cluster's electronic structure and suggest 5s/4d orbital mixing as well as minor Pd─P covalency. Trapping experiments reveal that cluster growth proceeds via insertion of Pd(0) into the triphosphirane. The unsaturated cluster senses ethylene and binds isocyanides, which triggers the rearrangement to a tetrahedral structure with a reduced frontier orbital energy gap. These experiments demonstrate facile cluster manipulation and highlight non-destructive cluster rearrangement as is required for supramolecular assembly.

Triphosphiranes as phosphinidene-transfer agents - synthesis of regular and chelating NHC phosphinidene adducts.

In this contribution we describe the general use of aryl-substituted triphosphiranes (Ar3P3; Ar = Mes, Dip, Tip) as phosphinidene transfer reagents towards N-heterocyclic carbenes (NHCs) to give a library of twelve N-heterocyclic carbene phosphinidene adducts of the type ArPNHC (NHCPs), in which the NHCs have varying steric profiles, allowing a systematic evaluation of their structural and NMR-spectroscopic properties. In the next series of experiments we utilized 1,3- and 1,4-phenylene bridged bis-NHCs to access a new class of chelating bis(NHCP)s, of which three derivatives could be structurally characterized. The 1,4-phenylene derivatives were shown to be susceptible to P-CNHC bond cleavage when irradiated with an LED (396 nm), providing a rare example of phosphinidene release from NHCPs. The coordination chemistry of 1,3-phenylene bridged bis(NHCP)s towards GeCl2(dioxane) and GaI3 was investigated and revealed the formation of ion-separated cationic complexes, with significant charge transfer from the ligand to the metal center according to NBO analyses.

Swift C-C bond insertion by a 12-electron palladium(0) surrogate.

The selective activation of C-C bonds holds vast promise for catalysis. So far, research has been primarily directed at rhodium and nickel under harsh reaction conditions. Herein, we report C-C insertion reactions of a 12-electron palladium(0) surrogate stabilized by a cyclic(alkyl)(amino) carbene (CAAC) ligand. Benzonitrile (1), biphenylene (2), benzocyclobutenone (3), and naphtho[b]cyclopropene (4) were studied. These substrates allow elucidation of the effect of ring strain as well as hybridization encompassing sp3, sp2 and sp hybridized carbon atoms. All reactions proceed quantitatively at or below room temperature. This work therefore outlines perspectives for mild C-C bond functionalization catalysis.

Modulating the reactivity of phosphanylidenephosphoranes towards water with Lewis acids.

Phosphanylidenephosphoranes of the type R-P(PR'3), also known as phospha-Wittig reagents, can be utilized in a variety of bond activation reactions exploiting their phosphinidenoid reactivity. Herein, we thoroughly show that a facile PMe3 for H2O exchange gives access to various primary phosphine oxides of the general formula RP(H)2O (R = Mes*, MesTer, DipTer) and the molecular structure of DipTerP(O)H2 was determined. However, phosphanylidenephosphoranes are described to be highly nucleophilic as well. We show that the attachment of main group Lewis acids such as GaCl3 and GaI3 to R-P(PMe3) yielded the highly sensitive, yet stable coordination compounds [(RPGaX3)PMe3] (R = Mes*, DipTer) or [(RPPMe3)2GaCl2]GaCl4 (R = MesTer). In contrast to the free phosphanylidenephosphoranes, these species reacted differently with H2O, which was demonstrated for [(Mes*PPMe3)GaI3]. Here the formation of the phosphino-phosphonium cation [Mes*P(H)PMe3]+ and different anions was observed with combined NMR spectroscopic and SC-XRD (SC-XRD = single crystal X-ray diffraction analysis) studies. This work demonstrates that the ambiphilic character of phosphanylidenephosphoranes can be utilized to manipulate the reactivity of R-P(PMe3) towards water, giving primary phosphine oxides, whereas the Lewis acid adducts [(RPGaX3)PMe3] gave phosphino-phosphonium species.

Deoxygenation of chalcogen oxides EO2 (E = S, Se) with phospha-Wittig reagents.

In here we present the deoxygenation of the chalcogen oxides EO2 (E = S, Se) with R-P(PMe3), so-called phospha-Wittig reagents. The reaction of DABSO (DABCO·2SO2) with R-P(PMe3) (R = Mes*, 2,4,6-tBu3-C6H2; MesTer, 2,6-(2,4,6-Me3-C6H2)2-C6H3) resulted in the formation of thiadiphosphiranes (RP)2S (1:R), while selenadiphosphiranes (RP)2Se (2:R) were afforded with SeO2, both accompanied by the formation of OPMe3. Utilizing the sterically more encumbered DipTer-P(PMe3) (DipTer = 2,6-(2,6-iPr2-C6H3)2-C6H3) a different selectivity was observed and (DipTerP)2Se (2:DipTer) along with [Se(µ-PDipTer)]2 (3:DipTer) were isolated as the Se-containing species in the reaction with SeO2. Interestingly, the reaction with DABSO (or with equimolar ratios of SeO2 at elevated temperatures) gave rise to the formation of the OPMe3-stabilized dioxophosphorane (phosphinidene dioxide) DipTerP(O)2-OPMe3 (4:DipTer) as the main product. This contrasting reactivity can be rationalized by two potential pathways in the reaction with EO2: (i) a Wittig-type pathway and (ii) a pathway involving oxygenation of the phospha-Wittig reagents and release of SO. Thus, phospha-Wittig reagents are shown to be useful synthetic tools for the metal-free deoxygenation of EO2 (E = S, Se).

A diarsene radical anion.

The isolation of the first diarsene radical anion by reduction of a neutral diarsene is presented. Comprehensive characterisation in conjunction with DFT calculations reveals unpaired spin density residing in the antibonding π*-orbital with involvement of the terphenyl ligands. First reactivity studies reveal no pronounced radical, but rather reducing properties.

Coordination polymers of alkali metal cyclosiloxazanides with one- and two-dimensional structures.

The reaction of O(Si2Me4Cl)2 with ammonia yielded the cyclic siloxazane O(Si2Me4)2NH (1), which was used as a precursor for the synthesis of siloxazanide-type alkali metal salts. The metalation of 1 with the strong bases BzA (A = Na, K, Rb, Cs and Bz = benzyl) results in different dimensional structures depending on the alkali metal ion used. These results give new insights into framework design with inorganic building blocks and the coordination ability of siloxanes.

Synthesis and crystal structures of novel tertiary butyl substituted (pseudo-)halogen bismuthanes.

Herein we present the synthesis and characterization of di-tertiary butyl substituted (pseudo-)halogen bismuthanes tBu2BiX (X = Cl (1), Br (2), I (3), CN (4), N3 (5), SCN (6)). These compounds were obtained via different reaction paths. Compound 1 was obtained by a Grignard reaction of BiCl3 with two equivalents of tBuMgCl, whereas compounds 2, 3, 4 and 6 were synthesised by a oxidative addition/reductive elimination pathway starting from tBu3Bi and X2 (X = Br, I, CN, SCN). Finally, azide 5 was obtained by the reaction of 1 and NaN3. Secondary bonding interactions in the solid state within all the investigated compounds (1-6) cause additional stabilisation. Starting from tBu2BiCl, the completely tbutyl substituted ternary interpnictogen compound tBu2Bi(tBuP)SbtBu2 (7) was synthesized through the reaction with [tBu2SbP(tBu)Li(Et2O)]2. All new compounds were characterized by means of X-ray diffraction and mass spectrometry as well as NMR and IR spectroscopy.

The coordination behaviour and reactivity of partially silicon based crown ethers towards beryllium chloride.

Through reactions of 1,2-disila[12]crown-4 or 1,2-disila-benzo[12]crown-4 ethers with BeCl2 eight-membered Be-O-heterocycles, which are annulated by two six-membered Be-O-cycles were obtained and characterised. The reactions leading to these unusual ring systems have been investigated by NMR and IR spectroscopy as well as reactions with further ligand derivatives.