SYNTHESIS, CHARACTERIZATION, AND PHOTOBEHAVIOR OF DELTA-FAC-TRIS((S)-TRYPTOPHANATO)CHROMIUM(III) AND LAMBDA-FAC-TRIS((S)-TRYPTOPHANATO)CHROMIUM(III)

The complexes Delta-fac-Cr(S-trp)(3) and Delta-fac-Cr(S-trp)(3) (where S-trp is the unsymmetric bidentate ligand (S)-tryptophanato) have bee n prepared by heating [Cr(en)(3)]Cl-3 with excess S-trpH in water or d imethyl sulfoxide (DMSO) solution. The two diastereoisomeric species a re separated by extracting the initial solid product with acetone, in which only Delta-fac-Cr(S-trp)(3) is soluble. Geometric isomer identif ications are made from an analysis of uv-visible spectral data and ind icate isolation of just the facial isomer. Optical isomer assignments are based on circular dichroism (CD) measurements. photochemical studi es in DMSO solution reveal that on ligand-field (d-d) excitation at 48 8 nm the Delta-fac-Cr(S-trp)(3) isomer is essentially photoinert, whil e the Delta-fac-Cr(S-trp)(3) isomer undergoes complete conversion to D elta-fac-Cr(S-trp)(3) with a quantum yield of 0.03. Complete photoinve rsion demonstrates a key stereochemical role for the asymmetric C of ( S)-tryptophan. The observation of the same inversion process thermally (at elevated temperatures) indicates that the Delta-fac-Cr(S-trp)(3) diastereoisomer has a considerably higher stability than that of the i i species. This conclusion is in accord with molecular mechanics calcu lations, which estimate the Delta diastereoisomer to be more stable by 7 kcal mol(-1). The observation of optical inversion without fac --> mer geometric isomerism is suggestive of a Bailar trigonal twist mecha nism and may constitute the first example of a photoinduced Bailar twi st. Using X-ray crystallographic data available for the glycine analog , a normalized chelate bite value of 1.31 is estimated for the fac-Cr( S-trp)(3) system. Consistent with our experimental data, recent theore tical studies by Rodger and Johnson conclude that a Bailar twist will be energetically preferred over a Ray and Dutt twist for bite values l ess than 1.4.