Matthias Driess

Methanol synthesis over ZnO: A structure-sensitive reaction?

In order to identify active sites on ZnO powdered catalysts in methanol synthesis a total of five ZnO samples with different degrees of crystallinity were characterized by means of N2 physisorption, XRD, TEM, and EXAFS. With respect to catalysis, high-pressure methanol synthesis was performed as a test reaction. A linear area-activity relationship for the highly crystalline materials was obtained, but at high BET surface areas a strong deviation from linearity was found. The observed phenomena provide evidence for a structure-sensitivity, suggesting that a specific active site is favored for methanol formation. Based on earlier work with polycrystalline ZnO powder and with respect to the current theoretical and experimental work with ZnO single crystals, the polar ZnO faces are assumed to be highly relevant for the catalytic activity under methanol synthesis conditions.

Low-temperature approach to high surface ZnO nanopowders and a non-aqueous synthesis of ZnO colloids using the single-source precursor [MeZnOSiMe3]4 and related zinc siloxides

We present the temperature-dependent thermolysis of siloxy-substituted ZnO single-source precursors into zinc and zinc oxide, respectively. The solid-state pyrolysis at low temperatures leads to the formation of ZnO powder with a very high surface area whereas the thermolysis in solution yields ZnO colloids. The materials are characterized by UV/VIS, photoluminescence, X-Ray Diffraction (XRD), and Transmission Electron Microscopy (TEM).

Synthesis and Structure of Siloxy‐Substituted ZnO Aggregates Having (ZnO)n (n = 2, 4) and Zn3O4 Cores

Convenient syntheses and X-ray crystallographic characterizations of the first bis(trimethylsilyl)amido-, methyl-, and iodozinc triorganosiloxide aggregates 1−5 are described. They are accessible by the simple reaction of ZnR′2 [R = Me, N(SiMe3)2] with the respective silanols R3SiOH (R = Me, Et, iPr), which affords the dimeric [(Me3Si)2NZnOSiR3]2 (1a: R = iPr; 1b: R = Et), trinuclear [(MeZn)2Zn(OSiiPr3)4] (2a), {[(Me3Si)2NZn]2Zn(OSiR3)4} (2b: R = Et; 2c: R = Me), and tetranuclear heterocubanes [MeZnOSiR3]4 (3a: R = Me; 3b: R = Et), respectively. The latter were oxidized with four equivalents of elemental iodine to form the tetraiodo derivatives [IZnOSiR3]4 (4a: R = Me; 4b: R = Et) in 82 and 88% yield, respectively. Due to the higher polarity of the Zn−I vs. Zn−C σ-bond, the Zn−O distances of the almost regular Zn4O4 core in 4a are 2−6 pm shorter than those observed in the less Lewis-acidic cluster 3b. However, the Zn−O distances in 3b and 4a are ca. 10−15 pm longer than those in 1a, 2a, and 2c, due to different coordination numbers at Zn and the effects of ring strain. Remarkably, the iodo derivatives 4a,b undergo dissociation in THF to give the respective dimeric THF solvates [IZn(THF)OSiR3]2 (5a: R = Me; 5b: R = Et), whereas the Zn4O4 cores in 3a and 3b are retained even in aprotic polar solvents.

P(SiiPr3)3: the First Trisilylphosphane Derivative with an almost Planar Three‐Coordinated Phosphorus Atom

The title compound (1) was obtained by salt-metathesis reaction of iPr3SiPLi2 with two molar equiv. of iPr3SiOTf (OTf = OSO2CF3) in 34% yield. Surprisingly, (1) consists of an 4 : 1 mixture of the two diastereomers (1 a) and (1 b), which do not interconvert to each other even at their decomposition temperature (> 70 °C). They represent different iPr-rotational isomers which are separated by an unusual high rotational barrier (> 25 kcal mol–1), resulting from hindered rotations around the Si–C and C–C bonds. The unexpectedly small magnitude of the 1J(Si, P) coupling constant of (1 a) (9.4 Hz) and (1 b) (9.0 Hz) reflects unusual electronic properties of the Si3P skeleton. Hitherto only (1 a) could be isolated in the form of single-crystals and its structure was determined by X-ray diffraction analysis. The P-atom in (1 a) is almost planar coordinated (sum of bond angles = 359.789(3)°), but the Si–P-distance (2.264(7) A) resembles those values of related silylphosphanes with pyramidally coordinated P atoms. Although MNDO calculations revealed two other iPr-rotational isomers with similar energy, they prove that the Si3P skeleton prefers the trigonal-planar arrangement due to steric congestion. P(SiiPr3)3: Das erste Trisilylphosphanderivat mit einem fast planar dreifachkoordinierten Phosphoratom Die Titelverbindung (1) wurde durch Salzmetathesereaktion von iPr3SiPLi2 mit zwei Molaquivalent iPr3SiOTf (OTf = OSO2CF3) in 34% Ausbeute erhalten. Uberraschenderweise besteht (1) aus einer 4 : 1 Mischung der zwei Diastereomere (1 a) und (1 b), die sich auch bis zu ihrer Zersetzungstemperatur (70 °C) nicht ineinander umwandeln. Sie reprasentieren verschiedene iPr-Rotamere, die durch eine ungewohnlich hohe Rotationsbarriere (> 25 kcal · mol–1) voneinander getrennt sind und die von gehinderten Rotationen um die Si–C- und C–C-Bindungen herruhren. Der unerwartet kleine Betrag der 1J(Si, P)-Kopplungskonstante von (1 a) (9.4 Hz) und (1 b) (9.0 Hz) spiegelt die ungewohnlichen elektronischen Eigenschaften des Si3P-Gerusts wider. Bisher konnte nur (1 a) in einkristalliner Form isoliert werden, und seine Struktur wurde durch Rontgenbeugungsanalyse bestimmt. Das P-Atom in (1 a) ist annahernd planar koordiniert (Summe der Bindungswinkel = 359.789(3)°), aber der Si–P-Abstand (2.264(7) A) gleicht den Werten von verwandten Silylphosphanen mit pyramidal-koordinierten P-Atomen. Obwohl MNDO-Berechnungen zwei andere Rotamere ergaben, beweisen sie, dass das Si3P-Gerust die trigonal-planare Anordnung wegen der sterischen Uberladung bevorzugt.

Novel Molecular Clusters Having Aluminum–Phosphorus, Aluminum–Arsenic, and Gallium–Arsenic Skeletons, and Synthesis of an Al4As6Li4 Rhombododecahedron

Starting from small building blocks, namely the primary silylarsanes/phosphanes and H3M–NMe3 (M=Al, Ga) and [LiAlH4], the M2E2 heterocycles 1 (M =Al, Ga, E=P,S) and the unusual aggregate 2 are accessible. Compound 1 readily loses NMe3 during thermolysis to give the title compounds 3, which possess a hexagonal prismatic M6E6 cluster skeleton. They are potentially interesting as cocatalysts for the co-oligomerization of ethene and CO.

Modular Chemistry with Aluminum Phosphanides: Cluster Formation of (AlP)n (n=3,6,7), Al4P3, and Al4Li4P6 Frameworks

The Al3P3 heterocycle 1 is formed in 94% yield by the reaction of the primary silylphosphane 6a with Me3Al in toluene at 70 degrees C. While 1 crystallizes in an isomerically pure form, in which the six-membered Al3P3 ring prefers the chair conformation and the P-H hydrogen atoms adopt exo positions, it isomerizes in solution to give different diastereomers as shown by 1H and 31P NMR spectroscopy. Intermolecular cyclocondensation of 1 at 110 degrees C in toluene leads, under liberation of methane, to the distorted hexameric-prismatic (AlP)6 cluster 2 in 98% yield. The capability of 1 to function as a building block was further used for the synthesis of the solvent-separated ion pair [Li(thf)2]+ [(Me2Al)4(PR)3]- (3) which was prepared by a one-pot reaction of 1 with nBuLi and Me2AlCl in 15% yield. The structure of 3 was established by an X-ray diffraction analysis. Double deprotonation at phosphorus in 1 with RPLi2 (R = iPr3Si) (molar ratio 1:2), and subsequent transformation of the reaction mixture with Me3Al afforded the novel donor-solvent-free cluster 4 in 62% yield. The latter consists of a rhombododecahedral Al4Li4P6 framework, in which the Li centers are three-coordinate. The reaction of the silylphosphane 6b with the trimethylamine adduct of alane furnishes not only the hexamer (RPAIH)6 (R = (iPrMe2C)-Me2Si) but also the corresponding heptamer 5, which has a nonregular polyhedral (AIP)7 framework and represents the first cluster of this type.

E(SiMe3)4+ Ions (E=P, As): Persilylated Phosphonium and Arsonium Ions

Only strong Lewis acidic, arene-solvated Me3 Si+ ions react with E(SiMe3 )3 compounds (E=P, As) to give the crystallographically characterized E(SiMe3 )4+ onium ions 1 (left hand picture), which contain highly negative polarized P and As atoms, respectively. The masked Me3 Si+ ions in 1 can be easily transferred to Et2 O, to give the first structurally characterized planar silyloxonium ion [Et2 (Me3 Si)O]+ 2 (right hand picture).

Orthotelluric Acid as Substitute for Crystal-Water: Syntheses and Crystal Structure of the Co-crystallate [Te(OH)6 · 2 Adenine · 4 H2O] and the Disodium Ditellurate(VI) Aggregate {[Te2O2(OH)6(ONa)2]2[NaOH · 12.5 H2O]}

Supramolecular aspects on Te(OH)6 as substitute for crystal-water in adenine hydrate complexes and the first disodium ditellurate(VI) are reported. The co-crystallate [Te(OH)6 · 2 adenine · 4 H2O] (1) has been prepared in 41% yield from the 1 : 1 mixing of Te(OH)6 with the nitrogenous base adenine. The adduct of infinite stacks of adenine molecules, Te(OH)6 and water not only proves that Te(OH)6 mimicks the role of water in the related hydrate adenine · 3 H2O but also shows that the inclusion of Te(OH)6 raises the number of HO–H and N–HO contacts and therefore increases the distance between the adenine rings to 3.31 A in 1 in comparison to that in adenine trihydrate (3.22 A). Additionally, the disodium ditellurate(VI) aggregate {[Te2(O)2(OH)6(ONa)2]2 [NaOH · 12.5 H2O]} (2) resulted from the reaction of 1 with 2 molar equivalents of aqueous NaOH. Dinuclear 2 represents the first X-ray diffraction characterized example of a sodium tellurate(VI) constructed from [Te2O4(OH)6]2– dianions. Orthotellursaure als Ersatz fur Kristallwasser: Synthese und Kristallstruktur des Cokristallisats [Te(OH)6 · 2 Adenine · 4 H2O] und des Dinatriumditellurate(VI) Aggregats {[Te2O2(OH)6(ONa)2]2[NaOH · 12.5 H2O]} Supramolekulare Aspekte uber Te(OH)6 als Ersatz fur Kristallwasser in Adeninhydratkomplexen und das erste Dinatriumditellurat(VI) werden berichtet. Das Cokristallisat (1) mit der Zusammensetzung [Te(OH)6 · 2 Adenin · 4 H2O] wurde aus einer 1 : 1-Mischung von Te(OH)6 mit der Stickstoffbase Adenin in 41% Ausbeute hergestellt. Das Addukt aus unendlichen Stapeln von Adenin-, Te(OH)6- und Wassermolekulen beweist nicht nur, dass Te(OH)6 die Rolle von Wassermolekulen im verwandten Hydrat Adenin · 3 H2O nachahmen kann, sondern es zeigt auch, das der Einbau von Te(OH)6 die Anzahl der HO–H und N–HO Kontakte erhoht, so das sich der Abstand der Adeninringe in 1 im Vergleich zum Adenintrihydrate (3.22 A) auf 3.31 A vergrosert. Auserdem resultiert das Dinatriumditellurat(VI)-Aggregat {[Te2(O)2(OH)6(ONa)2]2[NaOH · 12.5 H2O]} (2) aus der Reaktion von 1 mit 2 Molaquivalent wassriger NaOH. Dinucleares 2 reprasentiert das erste, durch Rontgendiffraktion charakterisierte Beispiel eines Natriumditellurats(VI), das aus [Te2O4(OH)6]2–-Anionen aufgebaut ist.

Synthesis of the first fluoro(phosphanyl)- and diphosphanyl-stannanes and surprising formation of [P(SnMe3)4]+SiF5−

Monophosphanylation of the difluorostannane Is2SnF2 (Is = 2,4,6-triisopropylphenyl) with Si(PH2)4 furnishes the first isolable fluoro(phosphanyl)stannane Is2Sn(F)PH2 1, which readily undergoes unique SiO2-assisted decompostion to [Is2Sn(PH)]2 2 and HF, while further phosphanylation of 1 leads to the remarkably stable diphosphanylstannane Is2Sn(PH2)2 3; the latter reacts with Me3SnF at the glass-wall to give the first perstannylphosphonium pentafluorosilicate [P(SnMe3)4]+ SiF5- 4.

Ungewöhnliche Reaktivität der Silicium-Phosphor-Doppelbindung in einem Silyliden(fluorsilyl)phosphan: intramolekulare C,H-Inserierung und seine Umwandlung in ein neues Silyliden(silyl)phosphan

Die Thermolyse des (Fluor-tert.butyl-2,4,6-triisopropylphenylsilyl)-tert.butyl-2,4,6-triisopropylphenylsilanylidenphosphans (“Phosphasilenes”) (1) in Toluol bei 130 °C fuhrt in 92% Ausbeute unter C,H-Aktivierung einer Methylgruppe einer ortho-Isopropylgruppe zum konstitutionsisomeren Silyl(fluorsilyl)phosphan (2), dessen Struktur kristallographisch bestimmt wurde. Es kristallisiert racemisch und besitzt eine Benzosilacyclopent-2-en Einheit als wesentliches Strukturmotiv. Lithiierung von 2 fuhrt zum korrespondierenden Lithiumphosphanid (3), das bei 80 °C in Toluol LiF eliminiert und gleichzeitig das farblose neue Silyliden(silyl)phosphan (4) in Form seiner beiden Diastereomere (E : Z ca. 2 : 1) bildet. Die 31P-NMR chemischen Verschiebungen von δ = –29.95 und –31.75 sind praktisch identisch mit dem Wert von 1, und das 29Si-NMR Spektrum zeigt Resonanzsignale bei charakteristisch tiefem Feld (δ = 222.5 (1J(Si, P) = 160 Hz) und 221.8 (1J(Si, P) = 161 Hz)). Eine Einkristall-Rontgenbeugungsanalyse einer enantiomeren Form des E-Isomers von 4 ergab einen Si=P-Abstand von 2.063(2) A, wahrend der Si–P-Einfachbindungsabstand mit 2.246(2) A ca. 8% langer ist. Das niedrigkoordinierte Si-Zentrum ist trigonal planar umgeben und der Si–P–Si-Bindungswinkel betragt 108.09(8)°. Unusual Reactivity of the Silicon-Phosphorus Double Bond in a Silylidene(fluorosilyl)phosphane: Intramolecular C, H Insertion and its Conversion in a New Silylidene(silyl)phosphane Thermolysis of the (fluoro-tert.butyl-2,4,6-triisopropylphenylsilyl)-tert.butyl-2,4,6-triisopropylphenylsilylidenephosphane (“Phosphasilene”) (1) in toluene at 130 °C leads, under C,H-activation of a methyl group of a ortho-isopropyl group, to the constitutional isomeric silyl(fluorosilyl)phosphane (2), whose structure has been crystallographically established. It crystallizes racemically and possesses a benzosilacyclopent-2-ene moiety as major structural motif. Lithiation of 2 leads to the corresponding lithium phosphanide (3), which eliminates LiF at 80 °C in toluene and, concomitantly, furnishes the colorless new silylidene(silyl)phosphane (4) in the form of its two diasteromers (Z : E ca. 1 : 2). The 31P NMR chemical shifts of δ = –29.95 and –31.75 are practically identical with the value of 1, and the 29Si NMR spectrum shows resonance signals at characteristically low field (δ = 222.5 (1J(Si, P) = 160 Hz), 221.8 (1J(Si, P) = 161 Hz)). An single-crystal X-ray diffraction analysis of an enantiomeric form of the E-isomer reveals a Si=P distance of 2.063(2) A, whereas the Si–P single bond distance of 2.246(2) A is ca. 8% longer. The low coordinated silicon center is trigonal planar surrounded and the Si–P–Si angle is 108.09(8)°.