DFT Study on Spectra of Mn-Carbonyl Molecular Complexes

The reduction of CO2 is still a challenge in energy sources and the environmental field. Diamine coordinated manganese tricarbonylcatalysts containing diamine ligands, as inexpensive molecule based inorganic materials, become a potentially interesting catalyst in the photoreduction of CO2. The ultraviolet-visible (UV-Vis) and infrared (IR) spectra are beneficial to the modulation of catalysts for the photoreduction of CO2. TheUV-Vis and IR spectra of a series of diamine coordinated manganese tricarbonyl catalysts, [Mn(bpy)(CO)(3)Br], (1), [Mn(phen)(CO)(3)Br](2), [Mn(phen-dione)(CO)(3)Br](3), [Mn(phen-dione)(CO)(3)CH3CN](+)(4) (bpy=2,2'-bipyridine, phen=1,10-phenan-throline, phen-dione=phenanthroline-5,6-dione) has been investigated using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations. The UV-Vis spectrawere obtained using several TD-DFT calculations. The simulated UV-Vis spectra of 1 and 2 all show two peaks centered at 371 nm (1), 408 nm (1) and 361 nm (2), 414 nm (2), respectively. Absorption peaks of 1 and 2 arise from a metal-to-ligand charge transfer (MLCT) transition. The simulated UV-Vis spectra of 3 and 4 show three absorption peaks centered around 290 nm (3), 337 nm (3), 431 nm (3) and 294 nm (4), 319 nm (4), 371 nm (4), respectively. Herein, except for the peak of 294 nm (4) generated by the ligand-to-ligand pi-pi* transition, the remain absorption peaks of 3 and 4 all arise from MLCT absorption. The increased electronegativity of diamine ligands is responsible for shifting absorption peak from the UV region to the visible region (red). The increased electronegativity of the Mn-centeredligand is responsible for shifting absorption peak from the visible region to the UV region (blue). Once an electron is transferred from the Mn-centered unit to the diamine ligands, the Mn center will become anelectron-deficient unit. Herein, the Mn-centeredunit contributes from the sigma antibonding orbital between the Mn atom and ligand in the MLCT states. Thus, when the populated excited MLCT states are hit, the release of Mn-centered ligand (Br-/CH3CN) becomes favorable, and the system can form the active species. The simulated IR spectra show that two kinds of characteristic peaks are obtained, that is, (1) stretching vibration of Mn-centered C-O unit at 1 920 similar to 2020 cm(-1) (1, 2, 3, and 4) and (2) stretching vibration of C=O unit of diamine ligands at 1 690 cm(-1) (3) and 1 694 cm(-1) (4), respectively. The increased electronegativity of diamine ligands and Mn-centered ligand strengthens the C=O bond, which increases the stretching vibration frequency of the C=O units from 1 to 4. The calculation results, including molecular structures, the UV and IR spectra show good agreement with those obtained by experiment and provide the reasonable theoretical guide for the synthesis and regulation of diamine-coordinated manganese tricarbonyl catalysts.

Automated summary

In the reduction of CO2, the use of diamine coordinated manganese tricarbonylcatalysts shows potential in being an effective and inexpensive catalyst. The UV-Vis and IR spectra play a crucial role in the modulation and understanding of the catalysts for CO2 photoreduction.