436 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 4 Jong-Ha Choi High Res ution Optical Spectroscopy of mer- [Cr(dpt)(Gly-Gly)]ClO4 Jong-Ha Choi Department of Chemistry, Andong National University, Andong 760-749, Korea Received January 14, 1999 The highly resolved absorption spectra of mer-[Cr(dpt)(Gly-Gly)]ClO4，H2 [dpt = di(3-aminopropyl)amine, H2Gly-Gly = glycylglycine] have been measured between 13000 cm-1 and 50000 cm-1 temperatures down to 2 K. The vibrational intervals were extracted by recording emission and far-infrared spectra. The characteristic in- frared bands in meridional isomer were discussed. The fourteen electronic origins due to spin-allowed and spin- forbidden transitions were assigned. With the use of this electronic transitions, a ligand field optimization based on the exact known ligand geometry have been performed to determine more detailed bonding properties of Gly- Gly and dpt ligands. It is confirmed that the peptide nitrogen of the Gly-Gly has a weak n-donor property toward chromium(III) ion. Introduction A few investigations have been made on the spectroscopic and structural properties of chromium(III) complexes with the dipeptides series.1〜4 In these complexes with dipeptides as tridentate ligand, the three donor atoms (carboxylate-O, amido-N and amino-N) of a dipeptides ligand are located in meridional disposition. Dipeptides such as glycylglycine generally form meridionally coordinated complexes as tri- dentate ligand because the coordinating peptide nitrogen atom is constrained to approximate coplanarity. Thus they can act as strong template ligands to synthesize meridionally coordinated complexes with other tridentate ligands. Di(3- aminopropyl)amine (dpt) has a moderate preference for meridional coordination, though facial coordination has been found in at least one mononuclear compound.5,6 Recently a considerable amount of data concerning sharp- line electronic spectroscopy of chromium(III) complexes has been accumulated.*16 It has been recognized that the intraconfiguration d-d transitions may be useful to determine metal-ligand bonding property as well as molecular geome- try. Especially the sharp-line splittings are very sensitive to the exact bond angles around the metal. Thus it is possible to extract structural information from sharp-line electronic spectroscopy without a full X-ray structure determina- tion.17,18 In this study the low temperature absorption and emission spectra, and the room temperature infrared spectra of mer- [Cr(dpt)(Gly-Gly)]ClO4，H2。have been measured. The pure electronic origins were assigned by analyzing the vibrational and absorption spectra. Using the observed electronic transi- tions, a ligand field analysis have been performed to deter- mine the metal-ligand bonding properties for the coordinat- ed atoms toward chromium(III) ion. Experiment지 Section Synthesis. The chemicals were of reagent grade or better quality and used without further purification. The complex mer-[Cr(dpt)(Gly-Gly)]ClO4，H2。was prepared as described previously.3 The compound was recrystallized three times for spectroscopic measurements. Physical Measurements. The low-temperature absorp- tion spectra were measured on polyethylene matrix by using a Cary 5e spectrometer. The pellets were cooled to 2 K by a Cryvac helium cryostat. The emission spectra were obtained using an argon-ion laser (Spectra Physics 2045) pumped Ti:sapphire laser (Schwartz Electrooptics) in standing wave configuration. Wavelength control was achieved by an inch- worm driven (Burleigh PZ 501) birefringent filter. The sam- ple emission was dispersed by a 0.85m double monochro- mator (Spex 1402) and detected by a cooled photomultiplier (RCA 31034) using a photoncounting system (Standard Research 400). Sample cooling for emission measurements at 10 K was achieved using the helium-gas flow technique.19 The far-infrared spectra were recorded with a Bruker 113v instrument on a microcrystalline sample pressed in to a poly- ethylene pellet. The mid-infrared spectra were recorded with a Mattson 2020 Galaxy FT-IR spectrometer on a KBr disk. The 2 mg compound was pressed into 100 mg of polyethyl- ene or KBr disks by using a Spex 3624B X-Press. Results and Discussion Infrared and Emission Spectra. The infrared spectros- copy is frequently useful in elucidating structures and deter- mining the number of functional groups involved in coordination by a multidentate ligand. In some cases stretch- ing and bending modes of amino groups in metal-amino acid complexes are sensitive to the stereochemistry of the com- plexes. Meridional coordinated ligands have different stretching, bending and rocking frequencies in the infrared region than facially coordinated ligands. Figure 1 presents the mid-infrared spectrum recorded at room-temperature of mer-[Cr(dpt)(Gly-Gly)] ClO4 H2O. A sharp absorption peak at 3590 cm-1 can easily assigned to the O-H stretching of H2O molecule in the hydrate com- plex. The strong bands in the regions of 3100-3300 cm-1 and 2800-3000 cm-1 due to the symmetric and antisymmetric N- H and C-H stretching modes, respectively. Infrared spectra