Moshe Kapon

Some 60 new space-group corrections

Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort.

First utilization of a homochiral ruthenium porphyrin as enantioselective epxidation catalyst

Abstract The enantioselectivity in the first catalytic conversion of styrene to its epoxide by a homochiral ruthenium porphyrin- 1 -Ru(O) 2 - displayed a remarkable sensitivity to the solvent and the identity of the oxidant. The latter phenomenon clearly indicates that several high valent intermediates with different selectivities participate in oxygen atom transfer from catalyst to substrate. These observations are anticipated to significantly affect research of metal complexes of other homochiral porphyrins as well.

Catenated and non-catenated inclusion complexes of trimesic acid

Trimesic acid (benzene-1, 3,5-tri-carboxylic acid; TMA) can in principle form two-dimensional hydrogen-bonded hexagonal networks in which central holes of the network have net diameters of 14 Å. Although such holes would be expected to be natural locations for guest molecules, non-catenated single networks have not been found in any of the crystals containing TMA studied in the last sixteen years. Instead, anhydrous α-TMA, TMA pentaiodide (TMA.I5) and (so-called) γ-TMA have mutually triply-catenated structures in which triplets of networks are interlaced [3,4,5], while the hydrated complexes are based on non-catenated nets of composition TMA.H2O [6]. We have now found conditions under which single networks are preserved without catenation, the cavities being occupied by guests such as n-tetradecane, n-heptanol, n-octanol, n-decanol, octene, cyclooctane and isooctane. The structures of 2TMA. n-tetradecane and 2TMA. isooctane have been solved and refined to R=13.0% and R=11.3%, respectively, disorder of the guest molecules having prevented further refinement of the room-temperature data. Determination of the crystal structures of the other complexes, which are isostructural with 2TMA. n-tetradecane, is now in progress. We are also investigating other potential guests.

Intramolecular cycloaddition of carbonyl ylides generated from α-diazo ketones

Abstract Two cases of intramolecular cycloaddition of carbonyl ylides, formed from the α-diazo-ketones1 and8, to a C=C bond within the molecule are described. The structures of the products4a and10 have been established from chemical and spectroscopic evidence and by single crystal X-ray crystallographic analysis of6a and10.

Oligomerization and hydroamination of terminal alkynes promoted by the cationic organoactinide compound [(Et2N)3U][BPh4].

The three ancillary amido moieties in the cationic complex [(Et2N)3U][BPh4] are highly reactive and are easily replaced when the complex is treated with primary amines. The reaction of [(Et2N)3U][BPh4] with excess tBuNH2 allows the formation of the cationic complex [(tBuNH2)3(tBuNH)3U][BPh4]. X-ray diffraction studies on the complex indicate that three amido and three amine ligands are arranged around the cationic metal center in a slightly distorted octahedral mer geometry. The cationic complex reacts with primary alkynes in the presence of external primary amines to primarily afford the unexpected cis dimer and, in some cases, the hydroamination products are obtained concomitantly. The formation of the cis dimer is the result of an envelope isomerization through a metal-cyclopropyl cationic complex. In the reaction of the bulkier alkyne tBuC identical to CH with the cationic uranium complex in the presence of various primary amines, the cis dimer, one trimer, and one tetramer are obtained regioselectively, as confirmed by deuterium labeling experiments. The trimer and the tetramer correspond to consecutive insertions of an alkyne molecule into the vinylic CH bond trans to the bulky tert-butyl group. The reaction of (TMS) C identical to CH with the uranium catalyst in the presence of EtNH2 followed a different course and produced the gem dimer along with the hydroamination imine as the major product. However, when other bulkier amines were used (iPrNH2 or tBuNH2) both hydroamination isomeric imines Z and E were obtained. During the catalytic reaction, the E (kinetic) isomer is transformed into the most stable Z (thermodynamic) isomer. The unique reactivity of the alkyne (TMS) C identical to CH with the secondary amine Et2NH is remarkable because it afforded the trans dimer and the corresponding hydroamination enamine. The latter probably results from the insertion of the alkyne into a secondary metal-amide bond, followed by protonolysis.

Protein-induced, previously unidentified twin form of calcite

Using single-crystal x-ray diffraction, we found a formerly unknown twin form in calcite crystals grown from solution to which a mollusc shell-derived 17-kDa protein, Caspartin, was added. This intracrystalline protein was extracted from the calcitic prisms of the Pinna nobilis shells. The observed twin form is characterized by the twinning plane of the (108)-type, which is in addition to the known four twin laws of calcite identified during 150 years of investigations. The established twin forms in calcite have twinning planes of the (001)-, (012)-, (104)-, and (018)-types. Our discovery provides additional evidence on the crucial role of biological macromolecules in biomineralization.

Old and new studies of the thermal decomposition of potassium permanganate

The overall chemical equation representing the thermal decomposition of potassium permanganate up to ≈300°C is given approximately by: 10 KMnO4→2.65 K2MnO4+[2.35 K2O·7.35·MnO2.05]+6O2, the bracketed material being δ-MnO2. The experimental mass loss in air is ≈12% and the enthalpy of decomposition is ≈10 kJ/mol of KMnO4. Analysis of published kinetic studies of the decomposition show that most of the results can be represented by the Prout-Tompkins equation ln (x/(1−x))=kTt+constant, and insertion of the rate constants into the Arrhenius equation gives an activation energy for decomposition of ≈150 kJ/mol of KMnO4. Although the kinetic studies have always been interpreted in terms of a single type of chemical decomposition, with the different rates encountered during the course of the decomposition ascribed to physical effects, X-ray diffraction studies by Boldyrev and co-workers have shown that the reaction actually occurs in two stages, with essentially all the KMnO4 transformed into K3(MnO4)2, δ-MnO2 and O2 in the first stage, and the K3(MnO4)2 then decomposing into K2MnO4 and more δ-MnO2 and O2 in the second stage. We have confirmed the Boldyrev diffraction results and extended them by measuring the kinetics of the appearance and disappearance of K3(MnO4)2 by an X-ray diffraction method. Our earlier isotopes studies have shown that the oxygen molecules come from oxygen atoms produced by breaking Mn−O bonds in different permanganate ions i.e. the decomposition mechanism is interionic. We conclude by summarising what is, and is not, currently known about the thermal decomposition of potassium permanganate up to ≈300°C.ZusammenfassungDie Bruttoreaktionsgleichung für die thermische Zersetzung von Kaliumpermanganat bis etwa 300°C lautet ungefähr: 10 KMnNO4→2.65 K2MnO4+[2.35 K2O·7.35 MnO2.05]+6O2, wobei der in Klammern angeführte Ausdruck für δ-MnO2 steht. Im Experiment erfolgt in Luft ein Masseverlust von etwa 12% und die Enthalpie der Zersetzung beträgt 10 kJ/mol KMnO4. Eine Answertung der publizierten kinetischen Untersuchungen der Zersetzung ergaben, daß die meisten Resultate mit Hilfe der Prout-Tompkins-Gleichung ln (x/(1−x))=kTt+Konstante beschrieben werden können und Einsetzen der Geschwindigkeitskonstanten in der Arr-heniusschen Gleichung ergibt für die Zersetzung eine Aktivierungsenergie von etwa 150 kJ/mol KMnO4. Obwohl die kinetischen Untersuchungen bereits als Einzeltyp einer chemischen Zersetzung interpretiert wurden, bei deren Verlauf aufgrund physikalischer Effekte verschiedene Geschwindigkeiten auftreten, zeigten Röntendiffraktions-untersuchungen von Boldyrev und Mitarb., daß die Reaktion eigentlich in zwei Schritten verläuft, wobei im ersten Schritt alles KMnO4 zu K3(MnO4)2, δ-MnO2 und O2 umgesetzt wird, und im zweiten Schritt zersetzt sich dann K3(MnO4)2 zu K2MnO4 und noch mehr δ-MnO2 und O2. Wir bestätigten Boldyrev’s Diffraktionsergebnisse und erweiterten diese durch die Messung der Kinetik des Erscheinens und Verschwindens von K3(MnO4)2 mittels einer Röntgendiffraktionsmethode. Unsere früheren Isotopenuntersuchungen zeigten, daß die Sauerstoffmoleküle aus Sauerstoffatomen entstehen, die aus der Spaltung von Mn−O-Bindungen unterschiedlicher Permanganationen stammen, d.h. es handelt sich um einen interionischen Zersetzungsmechanismus.

Structures of three crystals containing approximately — linear chains of triiodide ions

The structures of the ordered crystals of tetra(«-butyl)ammonium triiodide [I: a = 15.791(8), b = 15.993(8), c = 9.578(5) Α, α = 74.48(8)°, 0 = 101.52(9), y = 96.63(9)°, P I , Ζ = 4)] and (benzamide)2 · ΗΙ3; [II: a = 20.824(10), b == 9.874(5), c = 9.620(5) A, α = 95.58(9), β = 102.10(10), y = 94.72(9)°, PU Ζ = 4] have been fully refined, while that of caffeine · H 2 0 · HI3 [III: a = 14.043(5), b = 12.202(5), c = 9.701(5) Α, β = 106.5(1), P2Ja, Ζ = 4] has been determined in outline because of unresolved problems of disorder. Intensity measurements were made on a four-circle diffractometer using graphite-monochromated MoATa. The numbers of reflections used in the final refinement cycle were: 15370, II5295, III 2971.1 has cations of D2¿ symmetry (i.e. in the form of flattened crosses) which interleave to form channels of rectangular cross-section with axes along [001]; these channels contain single, almost-linear chains of triiodide ions, with weak 13 — 13 interactions within the chains. II is a pseudo-Type A basic salt, with pairs of benzamide molecules, (joined by a short, presumably symmetrical, proton bond between carbonyl groups) forming the cations; these cations are arranged so as to leave channels (axes along [001]) of rectangular cross-section which contain double, almost-linear chains of triiodide ions. In III the caffeine molecules are presumably protonated at N(9) and are hydrogen bonded via water molecules to form corrugated sheets, the molecular plane being in the sheets. These sheets are so disposed as to leave channels of rectangular cross-section which contain double chains of polyiodide ions. One of the three iodines is disordered along the chain direction and it was not possible to identify the nature of the chains ( — IJ * Pa r t i l i of Crystal Structures of Polyiodide Salts and Molecular Complexes; for Pa r t i i see Herbstein and Kapon, 1979 12 F. H. Herbstein et al.: Structures of three triiodides Table 1. Crystal data (n -C 4 H 9 ) ,NI 3 (Benzamide)2 • HI 3 Caffeine · Η 2 0 · Η Ι 3 f W t 615.12 624.00 593.92 F(000) 116

Cross dimerization of terminal alkynes catalyzed by [(Et2N)3U][BPh4]

Abstract The cationic complex [(Et2N)3U][BPh4] reacts with a mixture of terminal alkynes inducing the synthesis of the cross dimerization products. For equimolar amounts of aliphatic alkynes (iPrCCH, tBuCCH) the head-to-tail geminal dimer of iPrCCH and the geminal cross dimer resulting from the insertion of iPrCCH into the UCCR (R=iPr, tBu) moiety are obtained. When a mixture of PhCCH is reacted with either iPrCCH or tBuCCH, different products are obtained depending on the molar ratio of the alkynes. The dimerization of iPrCCH with an excess of PhCCH produces the geminal head-to-tail cross dimer issued from the insertion of the aliphatic alkyne into the UCCPh moiety, and the geminal dimer of PhCCH. Inverting the molar ratio of the alkynes and using the deuterium labeled aliphatic alkyne iPrCCD, the deuterated geminal head-to-tail cross dimer is obtained preferentially with small amounts of the deuterated head-to-tail dimer of iPrCCD. The mixture of tBuCCH and PhCCH is converted into the geminal head-to-tail cross dimer in good yield if the former alkyne is in large excess. The addition of external EtNH2 in the cross dimerization of iPrCCH with PhCCH induces a different chemoselectivity producing mainly the cis-dimer of PhCCH. The use of a bulky amine, tBuNH2, with tBuCCH causes the decomposition of the catalytic complex, forming the salt [tBuNH3][BPh4]·tBuNH2.

X-ray and neutron diffraction study of benzoylacetone in the temperature range 8–300 K: comparison with other cis-enol molecules

The crystal structure of benzoylacetone (1-phenyl-1,3-butanedione, C(10)H(10)O(2); P2(1)/c, Z = 4) has been determined at 300, 160 (both Mo Kalpha X-ray diffraction, XRD), 20 (lambda = 1.012 Å neutron diffraction, ND) and 8 K (Ag Kalpha XRD), to which should be added earlier structure determinations at 300 (Mo Kalpha XRD and ND, lambda = 0.983 Å) and 143 K (Mo Kalpha XRD). Cell dimensions have been measured over the temperature range 8-300 K; a first- or second-order phase change does not occur within this range. The atomic displacement parameters have been analyzed using the thermal motion analysis program THMA11. The most marked change in the molecular structure is in the disposition of the methyl group, which has a librational amplitude of approximately 20 degrees at 20 K and is rotationally disordered at 300 K. The lengths of the two C-O bonds in the cis-enol ring do not differ significantly, nor do those of the two C-C bonds, nor do these lengths change between 8 and 300 K. An ND difference synthesis (20 K) shows a single enol hydrogen trough (rather than two half H atoms), approximately centered between the O atoms; analogous results were obtained by XRD (8 K). It is inferred that the enol hydrogen is in a broad, flat-bottomed single-minimum potential well between the O atoms, with a libration amplitude of approximately 0.30 Å at 8 K. These results suggest that at 8 K the cis-enol ring in benzoylacetone has quasi-aromatic character, in agreement with the results of high-level ab initio calculations made for benzoylacetone [Schiøtt et al. (1998). J. Am. Chem. Soc. 120, 12117-12124]. Application [in a related paper by Madsen et al. (1998). J. Am. Chem. Soc. 120, 10040-10045] of multipolar analysis and topological methods to the charge density obtained from the combined lowest temperature X-ray and neutron data provides evidence for an intramolecular hydrogen bond with partly electrostatic and partly covalent character, and large p-delocalization in the cis-enol ring. This is in good agreement with what is expected from the observed bond lengths. Analysis of the total available (through the Cambridge Structural Database, CSD) population of cis-enol ring geometries confirms earlier reports of correlation between the degree of bond localization in the pairs of C-C and C-O bonds, but does not show the dependence of bond localization on d(O.O) that was reported earlier for a more restricted sample. It is suggested that the only reliable method of determining whether the enol hydrogen is found in a single or double potential well is by low-temperature X-ray or (preferably) neutron diffraction.