Reinhard Benn

Transition metal allyls : IV. The (η3-allyl)2M complexes of nickel, palladium and platinum: reaction with tertiary phosphines

Abstract A series of 1 : 1 adducts have been prepared by treating the bis-η3-allyl complexes of nickel, palladium and platinum with tertiary phosphines. Investigations of their structure in solution as well as in the crystal have shown that both 18-electron (η3-allyl)2ML complexes as well as 16-electron (η1-allyl)-(η3-allyl)ML complexes may be formed.

27Al-NMR-Spektroskopie zur Charakterisierung von Organoaluminium-Verbindungen

Abstract 27Al NMR parameters of organoaluminium compounds of the type (RnAlX3-n)m (n = 0, 1, 2, 3; m = 1, 2, 3) and (R2 AlO(CH 2 ) 2 Y )2 (Y = OR′, NR′2) were determined. In related organoaluminium compounds the chemical shift δ(27Al) is an indicator for the coordination number of aluminium atoms: In the investigated monomeric compounds R3Al, with three-coordinated aluminium δ(27Al) lies between 280 and 210 ppm, whereas for four-coordinated aluminium in (RnAlX3-n)m complexes (n = 1, 2, 3; m = 2,3) δ(27Al) is between 180 and 125 ppm. In organoaluminium compounds with five-coordinated aluminium atoms δ(27Al) was found between 125 and 100 ppm. Inorganic complexes of the type XAl(OAlX2)2-(OBR)4 have distinct 27Al shift ranges for their four- (δ ∼ 90) and five- (δ ∼ 45) coordinated aluminium atoms. Owing to the relationship between δ(27Al) and the coordination number, organoaluminium compounds of unknown structure and the adducts of RnAlX3-n units with donors can be easily characterised by 27Al NMR spectroscopy.

Transition metal allyls ☆: III. The (η3-allyl)2M complexes of nickel, palladium and platinum: structural considerations

Abstract The structures of a series of (η 3 -allyl) 2 M complexes of nickel, palladium and platinum have been investigated with the help of 13 C NMR, 1 H NMR and Raman spectroscopy. The crystal structure of (η 3 -cyclooctatrienyl) 2 Ni has been determined by X-ray methods; the two nickel-bonded η 3 -allyl groups are mutually trans .

Drei- oder Vierfach-Koordination des Aluminiums in Alkylaluminiumphenoxiden und deren Unterscheidung durch 27Al-NMR-Spektroskopie

The 27Al NMR spectra for a series of aryloxyaluminium compounds [RnAl(OAr)3−n]m (R  methyl, isobutyl; n  0–2; m  1, 2, 3) with various alkyl substituents in the 2 and 6 positions of the phenoxy rings have been measured. The δ(27Al) resonances give an indication of the number of aryloxy ligands and the coordination number of each Al atom. Monomeric R2AlOAr compounds have δ(27Al) resonances around 190 ppm whereas dimeric analogues give signals at 167 ppm. The 27Al NMR resonances for monomeric RAl(OAr)2 are found at ca. 100 ppm while those for [Al(OAr)3]2, where the aluminium atoms are solely bonded to aryloxy groups, lie at ca. 50 ppm. Characteristic shifts of the resonances were also observed for complexes of these compounds with THF, whereby it was found that the resulting change Δδ(27Al) decreases with increasing number of aryloxy ligands. The crystal structures of three selected compounds, 3, 8 and 11, were determined by X-ray crystallography.

The structure and fluxional behaviour of butadiene in butadiene-transition metal complexes of T i, Zr, Hf, Mo, W and Co

Abstract The 1 H NMR spectra of the [(butadiene) (cyclooctatetraene)]metal complexes [(η 4 -C 4 H 6 )COT]M (I, M = Ti; II, M = Zr; III, M  Hf), trisbutadiene-metal complexes (η 4 -C 4 H 6 ) 3 M (IV, M = Mo; V, M = W), [(η 4 -butadiene) (η 5 -cyclopentadienyl)]cobalt (VI), and [(butadiene) (cyclopentadienyl) 2 ]metal complexes (η 4 -C 4 H 6 ) (η 5 -C 5 H 5 ) 2 M (VII, M = Zr; VIII, M = Hf) in toluene- d 8 solvent were measured at various temperatures. Sub-spectral analysis and iterative computer simulation were carried out to yield chemical shifts and coupling constants. These parameters provide direct evidence for a s- cis -η 4 -butadiene conformation in I-VI and a s- trans -η 4 -butadiene conformation in VII, VIII. Furthermore, the 1 H NMR data confirm an increasing sp 3 rehybridisation at the terminal carbon atoms in I-V and VII, VIII when descending the relevant groups of the periodic table. II, III and V have temperature dependent 1 H NMR spectra. In III the terminal butadiene protons begin to equilibrate above 30°C (80 MHz) and the transition of an AA′MM′XX′ spin system to an AA′X 2 X 2 ′ spin system is observed and calculated by line shape analysis including all the couplings. Whilst II and V are also fluxional the rate constant for equilibration of the terminal protons is low (compared to III) and the exchange process can be demonstrated only by magnetisation transfer experiments at temperatures above +40 and +90°C, respectively (80 MHz). The exchange process in II, III, and V is interpreted as proceeding through a metallacyclopentene transition state. Parallels to the fluxional behaviour of η 3 -allyl-transition metal compounds are briefly pointed out.

Fluxional behaviour in η3-allyl complexes of cr, mo and w as shown by magnetisation transfer difference spectroscopy (MTDS)

Abstract (η3-allyl)4M2 (M  Cr, I;M  Mo, II) exist in solution as mixtures of two isomers (Ia/Ib  60/40; IIa/IIb  90/10). Each isomer has two inequivalent allyl groups which are trans to one another and bridge the two metal atoms. The other allyl groups are bonded to each one of the metals. In the major isomer these two allyl groups are cis, whereas in the minor isomer they are trans. At temperatures above 30°C for I (8O°C for II), interconversion of the isomers Ia and Ib (IIa and IIb) can be detected by magnetisation transfer difference spectroscopy (MTDS) (1H NMR 400 MHz). This interconversion proceeds by exchange of syn (anti) protons of the non-bridging allyl groups of Ia (IIa) with syn (anti) protons of the non-bridging allyl groups of Ib (IIb). In addition, in Ia and Ib syn—syn (anti—anti) exchange of the protons of the bridging allyl groups can be detected. (η3-allyl)4M (M  Mo, III; M  W, IV) shows fluxional behaviour at temperatures above 70°C for III (90°C for IV). Syn—anti and also syn—syn (anti—anti) exchange takes place. Each of these processes can be monitored separately by MTDS.

Developing potentially useful organometallic Lewis acid catalysts: (η-cyclopentadienyl)zirconium trichloride derivatives

Abstract The organometallic Lewis acid (η-cyclopentadienyl)zirconium trichloride ( 2 ) was obtained from bis(cyclopentadienyl)zirconium dichloride ( 1 ) and chlorine by means of a radical induced metal-Cp bond cleavage. Reaction of 2 with tetrahydrofuran gave the CpZrCl 3 -bis(tetrahydrofuran) adduct 3 ; treatment of either 2 or 3 with excess dimethylformamide furnished CpZrCl 3 (dmf) 2 ( 4 ). (η-Methylcyclopentadienyl)bis(tetrahydrofuran)zirconium trichloride ( 7 ) was prepared by treatment of ZrCl 4 (thf) 2 ( 6 ) with one molar equivalent of the (methylcyclopentadienyl)thallium reagent 5 . Complexes 3, 4 and 7 were characterized by X-ray diffraction studies and their dynamic NMR spectra were recorded. Complex 3 crystallizes in space group P 2 1 / n with cell parameters a 8.155(1), b 13.342(2), c 15.775(1) A, β 104.81(1)°, Z = 4, R = 0.041, R w = 0.056. CpZrCl 3 (dmf) 2 ( 4 ) crystallizes in space group P 2 1 / c with cell parameters a 7.790(1), b 15.445(2), c 13.893(1) A, β 93.94(1)°, Z = 4, R = 0.032, R w = 0.052. Complex 7 crystallizes in space group P 2 1 / n with cell parameters a 10.242(2), b 12.836(2). c 13.683(1) A, β106.37 (1)°, Z = 4, R = 0.037, R w = 0.039. The (cyclopentadienyl)bis(tetrahydrofuran)zirconium trichloride adduct 3 selectively catalyzed the Diels-Alder addition of 2,3-dimethylbutadiene and methacrolein at ambient temperature to give 1,3,4-trimethyl-3-cyclohexenyl-1-carbaldehyde ( 13 ). However, the CpZrCl 3 -Lewis acid catalyzed rearrangement of 13 to 1,3,4-trimethylbicyclo[2.2.1]heptan-2-one ( 14 ) does not take place at a significant rate until ca. 150°C.

Transition metal allyls. VI: The stoichiometric reaction of 1,3-dienes with ligand modified zerovalent-nickel systems

Abstract Butadiene and methyl substituted 1,3-dienes react with zerovalent-nickel-ligand complexes in a stoichiometric manner to give octadienediylnickel-ligand complexes. The structure, rearrangement and reactions with CO and P-donor ligands of these species have been studied with the help of 1 H and 13 C NMR spectroscopy. The results provide an insight into the mechanism of the nickel-catalysed cyclodimerization of 1,3-dienes.

Structure, thermal stability and decomposition of bis-allyl-zinc compounds

Abstract The structure, thermal stability and decomposition of solutions of diallylzinc (I), bis(2-methylallyl)zinc (II), bis(3-methylallyl)zinc (III) and bis(3,3-dimethylallyl)zinc (IV) in deuterated solvents, have been investigated by1H NMR and by kinetic measurements at temperatures between −125 and +180°C. At room temperature I, II, III and IV are dynamic systems and are best described as being rapidly equilibrating mixtures of all isomeric σ-allyl forms; the NMR spectra are averages weighted according to the relative concentrations of the respective forms. I displays a1H NMR spectrum of a static σ-allyl system only below −125°C and II only below −115°C. At temperatures above 100°C the thermal decomposition of I–IV results in coupling of the allyl groups, decomposition via radicals being the major process. The coupled products exhibit CIDNP, in which the multiplet polarisations confirm a decomposition via randomly diffusing allyl radicals. In the allyl radicals CH2CR1CR2R3 an alternating spin density was proved experimentally. The thermal stability decreases in the order I > II > III > IV.

Optisch aktive übergangsmetall-komplexe: LXXXV. Darstellung, eigenschaften und konformationsanalyse von optisch aktiven CpCo-Komplexen☆

Abstract The Schiff bases from 2-pyridine-carbaldehyde, 2-acetylpyridine, 2-benzoylpyridine, 2-pyrrole-carbaldehyde, and 2-acetylpyrrole with (−)-1-phenylethylamine and (−)-3-aminomethylpinane were synthesized. The pyridine Schiff bases αγ were used as neutral compounds and the pyrrole Schiff bases δHζH were used as anions δζ in the reaction with CpCo(CO)I 2 and CpCo(CO)(C 3 F 7 )I. In place of the covalently bonded iodine ligand different monodentate ligands bp were incorporated. Compounds of the type CpCo(αζ)(ap) and [CpCo(αζ)(ap)]X with X = I, PF 6 are formed. All the complexes consist of pairs of diastereomers differing only in the Co configuration. The diastereomers exhibit different 1 H NMR spectra. The Co configuration in all the compounds is labile except the C 3 F 7 derivatives. A conformational analysis establishes which of the diastereomers is the thermodynamically more stable one, on the basis of the following arguments and methods: NOE-difference spectroscopy, diastereomer ratios at equilibrium, chemical shifts of the Cp signals, and CD spectra.