In situ NMR observation of mono- and binuclear rhodium dihydride complexes using parahydrogen-induced polarization

Starting from the binuclear complex [RhCl(NBD)]2 (NBD = 2,5-norbornadiene) in the presence of the phosphines L = PMe3, PMe2Ph, PMePh2, PEt3, PEt2Ph, PEtPh2, or P(n-butyl)3, various mononuclear dihydrides of the type Rh(H)2CIL3, i.e., those of the homogeneous hydrogenation catalysts RhCIL3, have been obtained upon addition of parahydrogen, and their 1H NMR spectra have been investigated using parahydrogen-induced polarization (PHIP). Furthermore, the two binuclear complexes (H)(Cl)Rh(PMe3)2(mu-Cl)(mu-H)Rh(PMe3) and (H)(Cl)Rh(PMe2Ph)2(mu-Cl)(mu-H)Rh(PMe2Ph) have been detected and characterized by means of this in situ NMR method. Analogous complexes with trifluoroacetate instead of chloride, i.e., Rh(H)2(CF3COO)L3, have been generated in situ starting from Rh(NBD)(acac) in the presence of trifluoroacetic acid in combination with the phosphines L = PPh3, PEt2Ph, PEt3, and P(n-butyl)3, and their 1H NMR parameters have been determined.     

Photometrische Bestimmung von Antimon in Eisen und Stahl

Zusammenfassung Zur Bestimmung von 2,5–100 ppm Sb in Eisen, unlegierten und niedrig legierten Stählen sowie in Automatenstählen wird das Antimon zuerst in 2 Stufen vorisoliert mit Hilfe der Spurenfällung mit Mangan(IV)-oxidhydrat als Spurenfänger und anschließender Jodidextraktion. Nach der Rückextraktion erfolgt die photometrische Bestimmung mit Methylfluoron in der wäßrigen Phase. Der relative Fehler beträgt ±5%, der Zeitbedarf für eine Einzelbestimmung 1,5 h.Herrn H. Sifirin danke ich für die sorgfältige Durchführung der Versuche.     

Using ultrafast infrared multidimensional correlation spectroscopy to aid in vibrational spectral peak assignments

Ultrafast infrared heterodyne detected vibrational stimulated echoes with full phase information are used to obtain the vibrational correlation spectrum from a mixture of metal-carbonyl compounds. The linear absorption spectrum displays four peaks in the carbonyl stretching region. In the absence of knowledge of the molecules that make up the mixture, the absorption spectrum could arise from four molecules that each produces a single peak to one molecule with four peaks. In contrast, the correlation spectrum displays four peaks on the diagonal and off-diagonal peaks that make it straightforward to determine which peaks belong to a particular molecule.     

X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.     

Photometrische Bestimmung von Antimon mit Methylfluoron

Zusammenfassung Es wird eine Arbeitsvorschrift zur photometrischen Bestimmung kleiner Mengen Antimon mit Methylfluoron beschrieben. Die günstigsten Arbeitsbedingungen und die Störung durch Fremdionen werden untersucht. Das Extinktionsmaximum liegt bei 530 nm. Die photometrische Bestimmung wird bei 495 nm durchgeführt. Das Beersche Gesetz ist im Bereich von 0,2– 4 g Sb/ml erfüllt, die Bestimmungsgrenze liegt bei 0,21 g Sb/ml · cm. Der Zeitbedarf einer Einzelbestimmung beträgt 15–20 min.Für die sorgfältige Durchführung der zahlreichen Versuche sprechen wir Herrn H. Siffrin unseren Dank aus.     

A general organocatalyst for direct alpha-functionalization of aldehydes: stereoselective C-C, C-N, C-F, C-Br, and C-S bond-forming reactions. Scope and mechanistic insights

The development of a general organocatalyst for the alpha-functionalization of aldehydes, via an enamine intermediate, is presented. Based on optically active alpha,alpha-diarylprolinol silyl ethers, the scope and applications of this catalyst for the stereogenic formation of C-C, C-N, C-F, C-Br, and C-S bonds are outlined. The reactions all proceed in good to high yields and with excellent enantioselectivities. Furthermore, we will present mechanistic insight into the reaction course applying nonlinear effect studies, kinetic resolution, and computational investigations leading to an understanding of the properties of the alpha,alpha-diarylprolinol silyl ether catalysts.     

The Dynamic Behaviour and NMR Solution structures of complexes of the type (Bisphosphine)(cycloocta-1,5-diene)iridium(I) and the X-ray crystal structure of (cycloocta-1,5-diene)((−)-norphos)iridium(I) hexafluorophosphate

A square-planar coordination geometry was found for the complex [Ir(cod){(?)-norphos}][PF₆] ( 1b [PF₆]; cod = cylcoocta-1,5-diene and (?)-norphos = [(2R,3R)-8-9-10-trinorborn-5-ene-2,3-diyl]bis(diphenylphosphine)) in the solid state by X-ray diffraction. Crystal data: monoclinic, space group P2₁, a = 10.751 (6), b = 18.669(14), c = 12.037(8) Å, β = 114.82(5)°, Z = 2. A total structural assignment including the configurational and conformational aspects of this and the related compounds [Ir(bishosphine)(cod)]X (bisphosphine = (?)-chiraphos = (2S,3S)-2,3-bis(diphenylphosphino)butane and (?)-norphos, X = Cl, CF₃SO₃, or PF₆) was carried out in solution by one- and two-dimensional NMR spectroscopy. The complexes containing the CF₃SO₃^? and PF₆^? anions are four-coordinate cations with square-planar geometry, whereas the chlorides are five-coordinate neutral compounds showing solvent-dependent dynamic behaviour. In toluene, two diastereoisomers of [IrCl(cod){(?)-norphos}] ( 2b ) exist and interconvert slowly at room temperature. This interchange is fast in CDCl₃ solution, and it is likely to involve Cl dissociation and the formation of the cation [Ir(cod){(?)-norphos}]⁺ as an intermediate.     

Synthese und Charakterisierung funktionalisierter β‐Hydroxydithiozimtsäuren und deren Ester. Komplexchemisches Verhalten gegenüber Nickel(II), Palladium(II) und Platin(II)

Synthesis and Analytical Characterization of Functionalized β‐Hydroxydithiocinnamic Acids and their Esters. Complex Chemistry towards Nickel(II), Palladium(II), and Platin(II) Starting from silyl‐protected 4‐hydroxy acetophenone ( 1 ) the 1,1‐ethenedihiolato complexes 3 – 5 were synthesised using carbon disulfide and potassium‐tert‐butylate as a base. After being deprotected, the resulting 4‐hydroxy‐substituted complexes 6 – 8 were esterified with DL‐α‐lipoic acid to obtain the compounds 9 – 11 . The resulting complexes were characterized using NMR spectroscopy, mass spectrometry and IR spectroscopy. 3‐substituted β‐hydroxydithiocinnamic acid methyl ester ( 12 ) was obtained via an analogous path of reaction using silyl‐protected 3‐hydroxy acetophenone ( 2 ), carbon disulfide and methyl iodide. After removing of the silyl group the resulting hydroxy group was esterified with DL‐α‐lipoic acid. Using the dithioacid ester 14 as a ligand the Ni^II ( 15 ), Pd^II ( 16 ) and Pt^II ( 17 ) [O,S] complexes were obtained.     

Restricted rotation about partial C,N double bonds

The restricted rotation about the partial C,N double bond in 2-chloro-6-NR₂-pyran-4-ones is discussed in the light of NMR spectroscopic data and theoretical calculations.Ab initio calculations at the HF/6-31G^* level were carried out using a continuum model to take solvent effects into account. The delocalization of-electron density [described by natural bond orbital analysis (NBO)] was applied to determine the degree of conjugation in the ground state (GS) and in the transition state (TS) for the restricted rotation of the compounds studied. The reason for the different barriers to rotation of the NR₂ substituents (pyrrolidino > dimethylamino > morpholidino > piperidino) at the 2-chloro-pyran-4-one ring appears to be the different steric hindrance of the NR₂ substituents in the GS for the restricted rotation.     

Gel-phase 19F NMR spectral quality for resins commonly used in solid-phase organic synthesis; a study of peptide solid-phase glycosylations

The spectroscopic properties for seven different commercial resins used in solid-phase synthesis were investigated with (19)F NMR spectroscopy. A fluorine-labeled dipeptide was synthesized on each resin, and the resolution of the (19)F resonances in CDCl(3), DMSO-d(6), benzene-d(6) and CD(3)OD were measured with a conventional NMR spectrometer, i.e. without using magic angle spinning. In general, resins containing poly(ethylene glycol) chains (ArgoGel, TentaGel and PEGA) were found to be favorable for the (19)F NMR spectral quality. Three serine containing tri-, penta-, and heptapeptides were then prepared on an ArgoGel resin functionalized with a fluorine-labeled linker. The resin bound peptides were glycosylated utilizing a thiogalactoside glycosyl donor carrying fluorine-labeled protective groups. Monitoring of the glycosylations with gel-phase (19)F NMR spectroscopy allowed each glycopeptide to be formed in similar 80% yield, using a minimal amount of glycosyl donor (3 x 2 equivalents). In addition, it was found that the glycosylation yields were independent of peptide length.