SYNTHESIS, STRUCTURE, AND SPECTROSCOPIC PROPERTIES OF CHIRAL RHENIUM AROMATIC ALDEHYDE COMPLEXES [(ETA(5)-C5H5)RE(NO)(PPH3)(O=CHAR)]+X- - EQUILIBRIA BETWEEN PI-ALDEHYDE AND SIGMA-ALDEHYDE BINDING MODES
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Reactions of [(5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+BF4 and ArCHO (Ar = a, C6F5; b, 4-C6H4CF3; c, 3-C6H4CF3; d, 3-C6H4OCH3; e, 2-C6H4OCH3; f, 4-C6H4Cl; g, 4-C6H4F; h, 1-naphthyl; i, C6H5; j, 3,4,5-C6H2(OCH3)3; k, 4-C6H4C6H5; l, 4-C6H4CH3; m, 2,4-C6H3(OCH3)2; n, 3,4-C6H3(OCH3)2; o, 4-C6H4OCH3) give aromatic aldehyde complexes [(5- C5H5)Re(NO)(PPh3)(O=CHAr)]+BF4 (4a-o+BF4, 8097%). IR analyses (CH2Cl2) 26 C) show 4a-o +BF4 to be >96:<4 to 15:85 mixtures of / isomers (NO 17451733/17011692 cm1). Electron withdrawing substituents (which enhance aldehyde acidity and lower basicity) favor the binding mode. Electron donating substituents have an opposite effect. The / ratios increase in more polar solvents and at lower temperatures. Van't Hoff plots give H (4i,l,o+BF4; ) of 3.6, 2.8, and 1.8 kcal/mol and S of 9.2, 9.5, and 9.1 eu. IR features unique to both binding modes, and visible absorptions characteristic of isomers, are identified. Complex 4o+PF6 crystallizes as a isomer, with a conjugated Ar/C=O linkage, a lengthened C=O bond (1.271 (8) ), and a 0 N-Re-O-C torsion angle. This Re-O conformation maximizes overlap of the d orbital HOMO of the rhenium fragment with C=O * orbital lobes on oxygen. The CPMAS 13C NMR spectrum of 4m+BF4 exhibits a HC=O resonance at 196.2 ppm, consistent with a binding mode. 1993, American Chemical Society. All rights reserved.
