New trans-dichloropalladium(II) complexes of 7-azaindole: Crystal and molecular structures, FT-IR, FT-Raman and DFT studies

• New palladium complexes with 7-azaindole (7AI) have been synthesized.
• The trans structure was confirmed by single crystal XRD, FT-IR, FT-Raman studies.
• A detailed interpretation of the IR and Raman spectra is reported.
• Controversial assignment of the Pd–N stretching vibrations is elucidated.
• NBO analysis reveals strong electron delocalization in the complex.

New complexes of Pd(II) with 7-azaindole (7AI), trans-[PdCl2(7AI)2] (1) and 1⋅DMF were synthesized. A single crystal X-ray diffraction of 1⋅DMF has shown that the complex crystallizes in the monoclinic system (space group P21/n with a = 7.65743 (13), b = 11.9426 (2), c = 13.3870 (2) Å, β = 91.2583 (16)°, V = 1223.94 (3) (Å3) and Z = 2). Palladium is bound to two pyridine nitrogen atoms of the 7AI ligands and to two chloride ions, in a square-planar trans arrangement. The FT-Raman and infrared spectra of 1 were recorded in the 3600–50 cm−1 range. The molecular structure of 1 was confirmed by elemental analysis and vibrational spectra. Comprehensive theoretical calculations (including the optimized structural parameters, the HOMO–LUMO energy gap, harmonic frequencies and vibrational intensities) were performed for 1 by using the B3LYP and PBE0 density functional methods with the 6-311G++(d,p)/LanL2DZ basis sets. A natural bond orbital (NBO) analysis was carried out. A detailed assignment of the experimental vibrational spectra was performed on the basis of the potential energy distribution (PED). The Pd–Cl stretching vibrations were assigned to bands at 339 (IR) and 292 cm−1 (Raman). The bands at 273 and 260 cm−1 (IR) and 195 cm−1 (Raman) correspond to the Pd–N(pyridine) stretching vibrations. The unequivocal assignment of these modes is necessary for the structural characterization of other Pd(II) and Pt(II) complexes with the pyridine-based ligands, by using vibrational spectroscopy.

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