Structural Isomers of Bis(diphenylarsino)methane‐dichloroplatinum(II)

From a range of preparative methods, three geometric isomers of bis(diphenylarsino)methane‐dichloroplatinum(II) have been isolated, viz. cis‐PtCl₂(dpam), trans, trans‐Pt₂Cl₄(dpam)₂ and cis, trans‐Pt₂Cl₄(dpam)₂. Their structures were determined by single‐crystal X‐ray diffraction.     

Chirality due to deuterium substitution: synthesis and circular dichroism of (+)(R) p -2,7-dideuterio-1,6-methano[10]annulene

Summary The title compound was prepared from (–)(S) _p -2,7-dibromo-1,6-methano[10]annulene by treatment withn-Bu-Li and subsequent quenching with D₂O. The optical rotations at four wavelengths and the circular dichroism spectrum are reported.

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Chiralität durch Deuterium-Substitution: Synthese und Circulardichroismus von (+)(R) _p -2,7-Dideuterio-1,6-methano[10]annulen (Kurze Mitt.)

Zusammenfassung Die Titelverbindung wurde aus (–)(S) _p -2,7-Dibrom-1,6-methano[10]annulen durch Umsetzung mitn-Bu-Li und nachfolgende Reaktion mit D₂O dargestellt. Optische Rotationen bei vier Wellenlängen und das Circulardichroismus-Spektrum wurden bestimmt.

     

X‐Ray Single Crystal Refinement and Superconductivity of La3Ni2B2N3

Crystalline samples of La₃Ni₂B₂N₃ were synthesized using solid state metathesis reactions from combinations of La, LaCl₃, NiCl₂ together with Li₃BN₂. The structure was determined by single crystal X‐ray diffraction (I4/mmm (No. 139), a = 372.95(2) pm, c = 2056.3(2) pm, R₁ = 0.027, wR₂ = 0.062) and confirmed earlier results from neutron powder diffraction. La₃Ni₂B₂N₃ contains BN units capping square planar Ni layers. Isolated nitrogen atoms reside in La₆ octahedra. Magnetic measurements on several bulk samples exhibit superconductivity at temperatures below 14.6 K.     

Persistent bissilylated arenium ions

A series of bissilylated arenium ions 1 with different substitution patterns on the aryl ring have been synthesized by hydride abstraction from 2-aryl-substituted 2,6-dimethyl-2,6-disilaheptanes (2) via transient silylium ions. The arenium ions have been identified by their characteristic NMR chemical shifts, (delta(29)Si=19.1-25.6, delta(13)C(ipso) =89.0-102.4, delta(13)C(ortho)=160.9-182.0, delta(13)C(meta)=132.5-146.9, delta(13)C(para)=150.2-169.9) supported by quantum mechanical calculations of structures, energies, and magnetic properties at the B3LYP/6-311G(d,p)//B3LYP/6-31G(d) + DeltaZPVE level of theory. The calculations clearly reveal the charge dispersing and stabilizing effect of the silyl substituents in arenium ions 1. The bissilylated benzenium ion 1a is more stable than the parent benzenium ion (C(6)H(7)(+)) by 37.6 kcalmol(-1). The synthesized arenium ions 1 are stable in solution at room temperature for periods ranging from a few hours to days. This unusual stability is attributed to: 1) the thermodynamic stabilization of the arenium ion by two beta-silyl substituents and 2) the essentially non-nucleophilic reaction conditions (the use of the weakly coordinating [B(C(6)F(5))(4)](-) anion and aromatic hydrocarbons as solvents). Addition of stronger nucleophiles than aromatic hydrocarbons (for example, acetonitrile) results in desilylation of the arenium ion 1 and recovery of the 2-aryl-2,6-disilaheptane moiety.     

Cyclische Kieselsäurederivate

Cyclic Silicic Acid Derivatives By reaction of SiCl₄ with oxygen at elevated temperatures a number of chlorsiloxanes consisting of more or less fused rings can be prepared of which those components of low molecular weight can be separated by combining distillation and crystallization. Methyl and ethyl esters of silicic acids are obtained by reaction with methyl or ethyl nitrite, resp. The compounds are characterized by silicon n.m.r. and mass spectroscopy.     

Metalations—XVI : Polylithiation at benzylic positions

Metalation of mesitylene with butyllithium in hexane in the presence of tetramethylethylenediamine yields mono-, di- and trilithiated benzylic compounds (1–5), characterized as their silylated derivatives. Dilithiation occurred partly at the same but preferentially at different methyls. The symmetrical tris-1,3,5-(lithiomethylene)benzene was the main product. Similar metalation occurred with m-xylene. o-Xylene gave also dilithiated products, but at a slower rate, and p-xylene gave slowly and exclusively the gem-dilithio-derivative.The results were explained by greater stabilization of polylithio derivatives with charges all on the starred carbons of the conjugated system, rather than those with charges uniformly dispersed on all carbons. CNDO/2 calculations of the energy of the anions account only partially for the order of stability of most of the studied compounds, but a good correlation between this experimental order and the energies of the lithium compounds was found.     

Thermal analysis and microstructure of solids and solid state reactions

Thermoanalytical and microstructural measurement techniques allow the characterization of solids, their transformation, decomposition, specific surface and pore structure. These techniques are of importance for solids with high specific surface and/or porosity. Thermoanalytical techniques are applied to detect solid state reactions, to determine reaction temperatures and to monitor the progress of the reaction. They are useful to simulate thermal processes in the industry and to receive meaningful results with small samples in a short time. Specific surface, cumulative pore volume and pore volume distribution are obtained by intrusion and absorption techniques. Here selected applications of thermoanalytical and microstructure investigations in modern fields of materials research will be presented, as:

-pore analysis of sintering steps of ceramics,

-secondary structure of catalyst granules,

-template decomposition in zeolites,

-firing process in a composide ceramic tape,

-ignition of coke.

     

Mechanistic control of product selectivity. Reactions between cis-/trans cis-/trans-[OsVI(tpy)(Cl)2(N)]+ and triphenylphosphine sulfide

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)2(N)]+ (tpy is 2,2':6',2"-terpyridine) and triphenylphosphine sulfide, SPPh3, give the corresponding Os(IV)-phosphoraniminato, [Os(IV)(tpy)(Cl)2(NPPh3)]+, and Os(II)-thionitrosyl, [Os(II)(tpy)(Cl)2(NS)]+, complexes as products. The Os-N bond length and Os-N-P angle in cis-[Os(IV)(tpy)(Cl)2(NPPh3)](PF6) are 2.077(6) A and 138.4(4) degrees. The rate law for formation of cis- and trans-[Os(IV)(tpy)(Cl)2(NPPh3)]+ is first order in both [Os(VI)(tpy)(Cl)2(N)]+ and SPPh3 with ktrans(25 degrees C, CH3CN) = 24.6 +/- 0.6 M(-1) s(-1) and kcis(25 degrees C, CH3CN) = 0.84 +/- 0.09 M(-1) s(-1). As found earlier for [Os(II)(tpm)(Cl)2(NS)]+, both cis- and trans-[Os(II)(tpy)(Cl)2(NS)]+ react with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3. For both complexes, the reaction is first order in each reagent with ktrans(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M(-1) s(-1) and kcis(25 degrees C, CH3CN) = (2.30 +/- 0.07) x 10(2) M(-1) s(-1). The fact that both reactions occur rules out mechanisms involving S atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)2(NSPPh3)]+, which undergoes further reaction with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3 or with [Os(VI)(tpy)(Cl)2(N)]+ to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and [Os(II)(tpy)(Cl)2(NS)]+.     

31P- und 195Pt-NMR-Untersuchungen an Diplatin(I)-Komplexen des Typs [Pt2(μ-SPR2)2L2] (L = PR3, PhP(OPh)2, P(OPh)3, CNR)

³¹P and ¹⁹⁵Pt N.M.R. Investigations on Diplatinum (I) Complexes of the Type [Pt₂(μ-SPR₂)₂L₂] (L = PR₃, PhP(OPh)₂, P(OPh)₃, CNR) ³¹P-, ¹⁹⁵Pt-chemical shifts and ¹⁹⁵Pt–³¹P- resp. ³¹P–³¹P-coupling constants of a series of doubly bridged diplatinum(I) complexes are reported. ³¹P-coordination chemical shifts of the terminal ligands of complexes of type [Pt₂(μ-SPR₂)₂(P′R₃′)₂] and some of the various coupling constants are strongly influenced by the π-acceptor strength of these ligands. J(¹⁹⁵Pt–¹⁹⁵Pt) is found to change the sign among the series of complexes investigated. Thermal singlett triplet exitation giving rise to the paramagnetism of these complexes observed by preliminary EPR-measurements and confirmed by EHT-calculations is deduced from the large values of ²J(P–P′) and ³J(P′P′) as well as the unusually high temperature dependence of some coupling constants and other NMR features. The chemical stability of the doubly bridged core, the coordination shifts of the bridging phosphorus atoms and EHT-calculations suggest a view of aromaticity of the [Pt₂(μ-SPR₂)₂](M–M) unit of these complexes.