Janet Blümel

Immobilized rhodium hydrogenation catalysts

Abstract The showcase catalyst for olefin hydrogenations is Wilkinsons complex ClRh(PPh 3 ) 3 ( 1 ). Here, the Wilkinson-type catalyst ClRh[Ph 2 P(CH 2 ) 3 Si(OEt) 3 ] 3 ( 2 ), trans -ClRh(CO)[Ph 2 P(CH 2 ) 3 Si(OEt) 3 ] 2 ( 3 ), and the chelate complex ClRh(PPh 3 )[Ph 2 PCH 2 CHOHCH 2 PPh 2 ] ( 4 ) have been synthesized using the bifunctional ligands Ph 2 P(CH 2 ) 3 Si(OEt) 3 ( 5 ) and Ph 2 PCH 2 CHOHCH 2 PPh 2 ( 6 ), and fully characterized. The dihydride complex 4 ( H ) 2 has been generated in situ and characterized with NMR. Complexes 2 , 3 , and 4 have been immobilized on silica, giving 2i , 3i , and 4i , respectively, which have been studied by solid-state NMR. The catalytic activities of 2i – 4i for the hydrogenation of 1-dodecene ( 7 ), 2-cyclohexen-1-one ( 8 ), and 4-bromostyrene ( 9 ) are compared with those of the homogeneous analogs 2 and 3 , as well as with 1 . While there are no substantial differences between 1 and 2 concerning TON and TOF, 2i shows reversed activity for the three test samples: dodecene gives the lowest TOF, followed by bromostyrene and cyclohexenone. However, TON and maximal yield are the same for 1 , 2 , and 2i . In contrast to 1 and 2 , the immobilized catalyst 2i can be recycled seven times, 4i three times. Catalysts 3 and 3i give maximal TOF for the hydrogenation of 7 , followed by 8 and 9 . The yields are below 100% for the homogeneous catalyst 3 , but immobilization ( 3i ) gives maximum yields for all substrates.

Solid state NMR study supporting the lithium vacancy defect model in congruent lithium niobate

Abstract 93Nb and 7Li wideline- as well as MAS-NMR measurements were performed on powdered and single crystal lithium niobate of defective congruent composition (48.4% Li2O; 51.6% Nb2O5) using a magnetic field strength of 7.05 Tesla with the aim to distinguish between a lithium vacancy defect model (1% of the Nb atoms in a different site) and a niobium vacancy defect model (5% of the Nb atoms in a different site). Although the line width of the 7Li signal of powdered lithium niobate could be reduced to 0.6 kHz by using MAS-NMR with a rotational frequency of 4000 Hz, there was no second 7Li signal apparent, indicating different chemical surroundings for 7Li nuclei caused by the defect structure. The wideline spectra of 93Nb NMR measurements on lithium niobate single crystals in various orientations do not exhibit a second resonance and therefore contradict several earlier 93Nb NMR studies from which a niobium defect model was substantiated. Additionally, earlier NMR measurements could be explained on the basis of the complex 93Nb MAS spectra of powdered material.

Polymer-bound osmium oxide catalysts

Abstract Polymer-supported oxidic osmium catalysts based on cross-linked poly(4-vinyl pyridine) were synthesized by various routes and characterized by a number of physical techniques (Raman, IR, XPS, 13 C and 15 N solid-state NMR spectroscopy). Model compounds of type Os 2 O 6 L 4 (L = pyridine, 4- iso -propyl pyridine, and 4- tert -butyl pyridine) were obtained under the conditions of the catalyst synthesis. The catalytic systems were successful in the dihydroxylation of alkenes.

Easily immobilized di- and tetraphosphine linkers: rigid scaffolds that prevent interactions of metal complexes with oxide supports.

Di- and tetraphosphines with rigid phenyl-, biphenyl-, and tetraphenylstannane, -silane, and -methane scaffolds, and various substituents R, have been synthesized and immobilized via triethoxysilane-propagated formation of one or two surface-bound phosphonium moieties. The remaining phosphine groups can be coordinated to metal complexes. All the detective work and proof is done by solid-state NMR spectroscopy.

Immobilization and chelation of metal complexes with bifunctional phosphine ligands: a solid-state NMR study

29 Si CP MAS NMR shows that a transition-metal complex with two bifunctional phosphine ligands such as (CO)2Ni[PPh2(C6H4)SiMe2OEt]2, when attached to a silica surface according to a commonly used procedure, is mainly anchored by just one ligand.

Easy one-pot synthesis of new dppm-type linkers for immobilizations

New synthetic routes to symmetric and unsymmetric dppm-type chelate linkers containing the ethoxysilane group are described and the compounds were characterized also by solid-state NMR spectroscopy and X-ray structures.

The Union Carbide catalyst (Cp2Cr + SiO2), studied by solid-state NMR☆

Abstract The Union Carbide catalyst, that can be synthesized by reaction of chromocene (Cp 2 Cr) with silica (SiC 2 ), allows the polymerization of olefins at low pressure and low temperature without any cocatalyst. Despite the interest in this heterogeneous catalyst, the nature of the active species is still unknown. In this contribution it is demonstrated that 1 H solid-state NMR serves as a powerful method for the investigation of the surface species. The spectra are evidence for the presence of one mononuclear and two dinuclear surface-attached complexes. The nature of these species is discussed on the basis of molecular model compounds and their NMR characteristics.

Immobilized nickel catalysts for cyclotrimerizations of acetylenes: enhancement of activities, stabilities, and lifetimes

Solid-state 31P NMR is used to optimize the stability and minimize leaching of nickel catalysts for alkyne cyclotrimerization immobilized by mono- and bidentate phosphine linkers on silica.

New linker systems for superior immobilized catalysts

The rigid linkers E(p-C6H4PPh2)4 (E = Si, Sn) have been synthesized from the corresponding precursors E(p-C6H4Br)4 and immobilized by forming one (E = Si) or three (E = Sn) phosphonium groups that are attached to the silica support by electrostatic interactions. The remaining unquaternized phosphines are coordinated to Wilkinson-type Rh complexes. The catalyst immobilized by the rigid scaffold with three surface-bound phosphonium groups (E = Sn) displays superior catalytic activity and lifetime with respect to the hydrogenation of dodecene. This catalyst can be recycled 30 times in a batchwise manner under standardized conditions.

Synthesis, structure, electronic properties and thermal behavior of butadiynyl substituted phenylCr(CO)3-complexes

Butadiynyl-substituted h 6 -benzeneCr(CO)3 complexes 3 are very efficiently prepared by a copper catalyzed coupling of the phenylacetylene complex 1 and bromo alkynes 2. The transmission of electronic effects through the butadiynyl bridge operates by