Abstract

Abstract

Abstract

Taktgeber: Die hoch stereoselektive Ringöffnungspolymerisation von rac-Lactid unter Bildung eines heterotaktischen Polymers gelang erstmals durch Einsatz eines Mono-Germaniumalkoxid-Initiators. Die C₃-Symmetrie des Liganden spielt möglicherweise eine wichtige Rolle für die Steuerung der stereoselektiven Kettenverlängerung ausgehend vom racemischen Monomer.

Ge whiz: Highly stereoselective bulk ring-opening polymerization of rac-lactide to afford a heterotactic polymer was achieved for the first time through the use of a single-site germanium alkoxide initiator. The C₃ symmetry of the ligand may play an important role in controlling stereoselective chain propagation from the racemic monomer.

This paper reports the reaction of Ti(OiPr)₄ with a series of Schiff-base ligands. The Schiff-base proligands 1a–l are synthesised by the condensation of salicylaldehyde with a range of primary alkylamine and aniline derivatives. The treatment of 1a–f with Ti(OiPr)₄ yields the octahedral bis(aryloxy–imine)Ti(OiPr)₂ complexes 2a–f. X-ray crystal structure analysis of 2c, 2d and 2e reveals complexes with a trans-aldiminato oxygen atom and cis-N,cis-alkoxide ligand arrangement about the central metal atom. The reactions of Ti(OiPr)₄ with the ligands 1g and 1h result in a sterically induced ligand rearrangement to form the octahedral complexes 2g and 2h, also characterised by X-ray diffraction experiments, in which the nitrogen atoms of the O,N-chelate are now trans-orientated at the titanium centre. ¹H NMR analysis reveals significant deshielding of the isopropoxide methine proton, induced by this coordination mode. In contrast, reactions of 1i and 1j with Ti(OiPr)₄ result in the formation and isolation of the complexes 2i and 2j. X-ray crystal structure analysis shows complex 2j to have a previously unobserved ligand orientation, in which both ligands are trans-orientated, with respect to the aldiminato oxygen atoms, about the titanium centre, but steric bulk of the ligand inhibits the bidentate coordination of both O,N-ligands. Further increase in thesteric bulk of the imine substituent results in a reduced reactivity for ligands 1k and 1l, such that Ti(OiPr)₄ reacts with 1k to form the dimeric mono(Schiff-base) complex 2k (characterised by X-ray analysis). No reaction is observed for 1l. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The coordination modes of various azines (C=N–N=C) and hydrazones (C=N–NR₂) with titanium(IV) are investigated. Five ligands were studied, two hydrazones 1H–2H and three azines 3H₂–5H₂, and it was found that hydrazones form monomers and azines form dimers in the solid state. The hydrogen-bonded arrays observed for the hydrazones were significantly different for the two ligands, with one forming a ribbon like structure and the other a chain. For the azines the supramolecular structures are also discussed. A pronounced twist is observed in the azine ligand upon complexation to the metal centre, which is rationalised in terms of the favourable interactions observed in the solid-state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Complexation of the salt lithium triflamide [LiNTf₂; NTf₂ = N(SO₂CF₃)₂], with a range of nitrogen donor ligands results in the formation of a series of coordination complexes [(Tf₂N)Li·N(H)PPh₂Me] (1), [(Tf₂N)Li·N(H)PPh₃] (2), [(Tf₂N)Li·TMEDA] (3), [(Tf₂N)Li·PMDTEA] (4) and [(Bz-TAC)₂Li][NTf₂] (5). The molecular structures of 1, 3, 4 and 5 have been determined by single-crystal X-ray diffraction. The resulting solvated structures are discussed in terms of solvent-separated ion pairs, contact ion pairs and higher aggregates, with the degree of aggregation and ion contact being dependent on the relative coordinating abilities and steric demands of the donor ligand and the anion, and provide an insight into the possible structures of lithium aggregates present in complex systems such as polymer electrolytes and synthetic reaction media. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Ungewöhnliche Wasserstoffbrücken wurden bei den Addukten beobachtet, die aus einem sterisch anspruchsvollen N-heterocyclischen Carben mit einem Phenol und einem Diarylamin erhalten wurden. Im ersten Fall – hier wird das Carben protoniert – enthält das Addukt 1 die bislang kürzeste C−H⋅⋅⋅O-Brücke; im zweiten Fall entsteht kein Ionenpaar, sondern das neutrale Carben-Amin-Addukt 2 mit einer einzigartigen N−H⋅⋅⋅C-Brücke.

Two unprecedented hydrogen bonds were observed in the adducts obtained by the reactions of an N-heterocyclic carbene with a bulky phenol and with diphenylamine. In the former case, proton transfer yields an imidazolium aryloxide (1) containing the shortest C−H⋅⋅⋅O interaction seen to date and in the latter, the hydrogen atom remains bound to the organic acid but forms a unique N−H⋅⋅⋅C hydrogen bond (2).