Bis(oxazolinylmethyl) Derivatives of C 4 H 4 E Heterocycles (E = NH, O, S) as C 2 ‐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(oxazolinylmethyl)pyrrole ( i PrL N H, 4a ), bis(oxazolinylmethyl)furan ( i PrL O , 4b ) and bis(oxazolinylmethyl)thiophene ( i PrL 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 i PrL N H ( 4a ) with t BuLi at –78 °C and subsequent stirring with NiCl 2 yielded the square planar nickel(II) complex [Ni( i PrL N )Cl] ( 5 ). However, on stirring i PrL N H ( 4a ) with nickel acetate in methanol, a deep red nickel acetato complex [Ni( iso ‐ i PrL N )(OAc)] ( 6 ) bearing the isomerized tridentate pincer ligand was obtained. Reaction of acetato complex 6 with Me 3 SiCl in dichloromethane cleanly gave the corresponding chlorido complex [Ni( iso ‐ i PrL N )Cl] ( 7 ), which is the isomer of compound 5 . The intraligand 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 Δ G = –9.8 kcal mol –1 . Neither thiophene derivative 4c nor furan‐derived ligand 4b gave robust, isolable complexes with nickel(II). However, reaction of i PrL O ( 4b ) with [CrCl 3 (thf) 3 ] in thf yielded the yellow‐green complex [CrCl 3 ( i PrL O )] ( 8 ), whereas no complexation occurred with the analogous thiophene‐derived bisoxazoline 4c . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)