Synthesis, spectral and structural characterization and computational studies of rhenium(I)-tricarbonyl nitrito complexes of 2,2′-bipyridine and 2,9-dimethylphenanthroline ligands: π-Accepting character of the diimine ligands

Abstract

The synthesis, characterization, structural and computational studies of mononuclear Re(I) tricarbonyl complexes of 2,2′-bipyridine (2,2′-bpy) and 2,9-dimethylphenantroline (2,9-Me2Phen), [Re(CO)3(NN)(X)], where NN=2,2′-bpy, X=Br (1) and X=ONO (3); NN=2,9-Me2Phen, X=Br (2) and X=ONO (4), are reported. The complexes characterized by crystallographic and spectroscopic methods and elemental analyses. In each complex the Re(I) centre shows the distorted octahedral geometry. Single crystal X-ray diffraction data revealed the endo-nitrito (κ1–ONO) coordination in complexes 3 and 4. It has been shown that the replacement of the bromo ligand in complexes 1 and 2, either by AgOTf/NaNO2 in a mixture of CH3CN/H2O or AgNO2 in CH2Cl2 solution under dark and inert atmosphere conditions, were resulted the corresponding endo-nitrito (κ1–ONO) complexes. Density functional theory (DFT) was used for geometry optimization of the singlet and triplet states in gas phase and the electronic structure calculations. DFT calculations showed that the HOMO-LUMO energy gap is increased by replacing of the axial bromo with the electron withdrawing nitrito ligand. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in each complex was performed by AOMix program. Charge decomposition analysis (CDA) based on the electron-donation and back-donation revealed that the π-accepting character of 2,2′-bpy is greater than 2,9-Me2phen in these similar complexes. The singlet excited states are examined by TD-DFT showed that the occupied orbitals involved in the transitions have a significant mixture of Re and X, and the lowest unoccupied orbital is a π∗ orbitals of the diimine ligands. Details of the excited state character are revealed by TD-DFT, calculated changes of electron density distribution using electron density difference map (EDDM) and confirmed the MLCT/LLCT and IL character of the transitions. NCIPLOT analysis based on promolecular densities showed that the intermolecular CH⋯O, O⋯O and π⋯π interactions in complexes 3 and 4 are attractive based on their electron density and reduced electron density gradient.