The structure of the NiO2N2 coordination center in the [Ni(Salen)] complex and its polymer: a comparative study by X-ray absorption spectroscopy and quantum-chemical calculations

Abstract

The atomic-electronic structure of the [NiO₂N₂] coordination center in the [Ni(Salen)] complex and its polymer was studied using X-ray absorption spectroscopy (EXAFS and NEXAFS) techniques, supplemented by quantum chemical calculations. Density functional theory (DFT) calculations were performed to construct the initial models of the [Ni(Salen)] complex and identify structural fragments of the poly-[Ni(Salen)] polymer that determine its properties in both reduced (Red) and oxidized (Ox) states. Based on the analysis of the Ni 1s EXAFS spectra, a d–d dimer model was found to best describe the structure and properties of the complex in the condensed state and can also be used as a simplified model for the polymer. It has been established that when moving from the complex to the poly-[Ni(Salen)]-Red, the square-planar structure of the [NiO₂N₂] coordination center remains practically unchanged. However, during the oxidation of the monomers, the coordination center becomes distorted, which is most likely caused by the phenolate–quinone transformation of the ligand. The absorption bands in the Ni 1s NEXAFS spectra of the complex and its polymer are attributed to Ni 1s electron transitions to vacant molecular orbitals (MOs) of the NiO₂N₂ quasi-molecule. In order to determine the final structural fragments responsible for the properties of the complex and polymer in both charge states, we compared the experimental Ni 1s spectra with the model spectra calculated using the self-consistent Green's function method. It was found that the d–d stacked dimer is the basic structural unit that best describes the complex in its condensed state. In the case of the polymers, these structures are tetramers formed by the cross-linking of d–d dimers with total charges of 0 (reduced state) and +3 (with the BF₄⁻ counterion, oxidized state). A joint analysis of the Ni 1s and 2p_3/2 NEXAFS spectra revealed that the local electronic structure of the coordination center in the complex and poly-[Ni(Salen)]-Red is characterized by a similar vacant antibonding MO. In contrast, for poly-[Ni(Salen)]-Ox, a new low-energy band appears in the Ni 2p_3/2 spectrum. This is due to Ni 2p_3/2 electron transitions to σb_1g MOs localized on Ni atoms with a reduced effective charge due to interactions with BF₄⁻ counterions.