Bis(oxazolinylmethyl) Derivatives C4H4E Heterocycles (E = NH, O, S) as C2-Chiral Meridionally Coordinating Ligands for Nickel and Chromium

The synthesis of the three ligands employed in this study is based on the condensation of two molar equivalents of (S)valinol with the diester precursors pyrrole-2,5-bis(ethyl)acetate (2a), furan-2,5-bis(ethyl)acetate (2b) and thiophene-2,5-bis (ethyl) acetate (2c). This gave the corresponding bis(oxazolinyhnethyl)pyrrole (iPrL(O), 4a), bis(oxazolinylmethyl)furan (iPrL(O), 4b) and bis(oxazolinylmethyl)thiophene (iPrL(S), 4c) ligands. Stirring 4a in MeOD at ambient temperature in the presence of a catalytic amount of acetic acid (1 mol-%) led to complete hydrogen/deuterium exchange in the two bridging methylene groups of the ligand. This behaviour is explained by an acetate-mediated reversible proton transfer between the oxazoline N atom and the methylene bridge, a conjecture which was supported by a DFT study of the process. Deprotonation of iPrL(N)H (4a) with tBuLi at -78 degrees C and subsequent stirring with NiCl2 yielded the square planar nickel(II) complex [Ni(iPrL(N))Cl] (5). However, on stirring iPrL(N)H (4a) with nickel acetate in methanol, a deep red nickel acetato complex [Ni(iso-iPrL(N))(OAc)] (6) bearing the isomerized tridentate pincer ligand was obtained. Reaction of acetato complex 6 with Me3SiCl in dichloromethane cleanly gave the corresponding chlorido complex [Ni(iso-PrLN)Cl] (7), which is the isomer of compound 5. The intra-ligand rearrangement was explained by acetate-mediated proton transfer between the methylene bridges and the 3/4-positions of the pyrrole ring, the computed thermodynamic driving force being Delta G = -9.8 kcalmol(-1). Neither thiophene derivative 4c nor furan-derived ligand 4b gave robust, isolable complexes with nickel(II). However, reaction of iPrL(O) (4b) with [CrCl3(thf)(3)] in thf yielded the yellow-green complex [CrCl3(iPrL(O))] (8), whereas no complexation occurred with the analogous thiophene-derived bisoxazolme 4c. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)