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CHIM03 Abstract: The coordination chemistry of the chiral bioxazoline ligand (4S,4'S)-2,2'-bis(4-isopropyl-4,5-dihydrooxazole) to Pd-II provides evidence that the ligand bonding can occur either through chelation of one Pd-II ion leading to a mononuclear species with the expected cis geometry, or by double bridging of two Pd-II ions giving a dinuclear complex with trans geometry. The species in solution are identified by H-1 NMR spectroscopy Both the mononuclear and the dinuclear complexes promote the CO/styrene copolymerization, yielding the corresponding polyketone with a fully or a predominantly isotactic microstructure, depending on the reaction medium. The nature of the anion present in the palladium precatalysts affects the polyketone stereochemistry. MALDI-TOF analysis of the copolymers synthesized reveals the presence of p-hydroxyphenolic end-groups, thus confirming and explaining the role of 1,4-hydroquinone as a molecular weight regulator.

Some pentamethylcyclopentadienyl ruthenium(II) diphosphine chloride complexes have been prepared by ligand exchange starting with the parent triphenylphosphine derivatives and their reactivities compared with those of the corresponding cyclopentadienyl compounds. The pentamethyl ligand causes a greater extent of asymmetric induction when the (R)-prophos and (R)-phenphos ligands are used as well as a higher lability of the stereochemistry at the stereogenic ruthenium centre. A shift of about 200 mV in the oxidation potential is caused by the substitution at the penta-hapto ligand. The order of basicity of the diphosphine ligands was also evaluated and was found to be consistent with previous determinations. The crystal structure of(eta(5)-C5Me5)Ru {(S,S)-chiraphos}Cl shows a coordination around the ruthenium atom similar to that found for the (eta(5)-C5H5)Ru{(S,S) -chiraphos}Cl complex. (C) 1998 Elsevier Science S.A. All rights reserved.

Cyclopentadienyl and indenyl complexes of the type (eta5-C5H5)Rh(L2) and (eta5-C9H7)Rh(L2) (L2 = chiral diphosphine) have been synthesized and characterized through multinuclear NMR spectroscopy. These formally planar complexes react with methyl iodide to give the pseudotetrahedral methyl derivatives [(eta5-C5H5)Rh(CH3)(L2)]I and [(eta5-C9H7)Rh(CH3)(L2)]I. The stereochemistry of these complexes, as far as the stereogenic metal center is concerned, is inferred by the difference of chemical shifts of the two phosphorus atoms in the P-31 NMR spectra of the diastereomeric pairs. The crystal structure of (R)C,(S)Rh- [ (eta5-C9H7)Rh(Ph2PCH-(CH3)CH2PPh2)(CH3)]BPh4 has been determined and confirms the above assignment. The electrochemical reduction of the same complex is interpreted in terms of a charge-transfer process followed by the homolytic fission of the Rh-CH3 sigma-bond of the electrogenerated rhodium-(II) complex.

The effects of six ortho -substituents (Me, Et, F, Cl, Br and NO 2 ), showing different electronic and proximity effects, on the rate of the title reaction have been studied at 313.15 K in dioxane/water at various p S + values (3.8–12.7) and in benzene at various piperidine concentrations. The kinetic data obtained have been treated by using a Fujita-Nishioka approach with a dissection of the ortho -substituent effect in ‘ordinary polar effect’, ‘proximity polar effect’ and ‘primary steric effect’. The different contributions calculated have been discussed in the framework of the various mechanisms proposed for the studied rearrangement.

A series of new bis(indenyl)zirconium(IV) complexes of the general formula (η5-4,7-X2C9H5)2Zr(CH2C6H5)2 (where X = H, CH3, OCH3, and F) were synthesized in order to investigate the electronic effects of substituents on the nature of the catalytic species in homogeneous Ziegler‒Natta catalysts. By use of poly(methylaluminoxane) as cocatalyst, the activity of these complexes as catalyst precursors in the polymerization of ethylene was examined. The substitution with electron-withdrawing groups resulted in a decrease of both molecular weights and productivity, whereas in the case of electron donors, like methyl groups, no significant change in productivity and average molecular weight was observed. These effects support the hypothesis of a polar or ionic character of the catalytic species. The molecular structure of (η5-4,7-F2C9H5)2Zr(CH2C6H5)2was determined by X-ray diffraction. The complex crystallizes in space group P21/c (No. 14) with cell parameters a = 10.786 (1) Å, b = 11.084 (2) Å, c = 21.602 (2) Å, β = 98.30 (1)°, Z = 4, R = 0.0220, and Rw = 0.0295. © 1990, American Chemical Society. All rights reserved.

The optically active indenyl complexes ((η5-C9H7)Ru(LL)Cl (where LL is either (S,S)-1,2-dimethyl-1,2-ethanediylbis(diphenylphosphine) (chiraphos) or (R,R)-1,2-cyclopentanediylbis(diphenylphosphine) (cypenphos)) have been synthesized and spectroscopically characterized and compared with the corresponding cyclopentadienyl complexes. Reaction of the new complexes with 2-e-donors give cationic adducts in which the pentahaptocoordination of the indenyl ligand is maintained. The crystal structures of (S,S)-(η5-C9H7)Ru{Ph2PCH(CH3)CH(CH3)PPh2}Cl (1) and (S,S)-η5-C5H5Ru{Ph2PCH(CH3)CH(CH3)PPh2}Cl (3) have been determined. © 1989.