Synthesis, structure, and spectroscopic properties of chiral rhenium aromatic aldehyde complexes [(.eta.5-C5H5)Re(NO)(PPh3)(O:CHAr)]+X-: equilibria between .pi. and .sigma. aldehyde binding modes Academic Article uri icon

abstract

  • Reactions of [(η5-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 [(η5- C5H5)Re(NO)(PPh3)(O=CHAr)]+BF4− (4a-o+BF4−, 80–97%). IR analyses (CH2Cl2) 26 °C) show 4a-o +BF4− to be >96:<4 to 15:85 mixtures of π/σ isomers (νNO 1745−1733/1701−1692 cm−1). Electron withdrawing substituents (which enhance aldehyde π acidity and lower σ basicity) favor the π binding mode. Electron donating substituents have an opposite effect. The π/σ ratios increase in more polar solvents and at lower temperatures. Van't Hoff plots give ΔH (4i,l,o+BF4−; π → σ) of 3.6, 2.8, and 1.8 kcal/mol and ΔS of 9.2, 9.5, and 9.1 eu. IR features unique to both binding modes, and visible absorptions characteristic of σ isomers, are identified. Complex 4o+PF6− crystallizes as a σ isomer, with a conjugated Ar/C=O linkage, a lengthened C=O bond (1.271 (8) Å), 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 π* orbital lobes on oxygen. The CPMAS 13C NMR spectrum of 4m+BF4− exhibits a HC=O resonance at 196.2 ppm, consistent with a σ binding mode. © 1993, American Chemical Society. All rights reserved.

author list (cited authors)

  • Quiros Mendez, N., Seyler, J. W., Arif, A. M., & Gladysz, J. A.

citation count

  • 40

publication date

  • March 1993