X-ray emission spectroscopy to study ligand valence orbitals in Mn coordination complexes - PubMed (original) (raw)

X-ray emission spectroscopy to study ligand valence orbitals in Mn coordination complexes

Grigory Smolentsev et al. J Am Chem Soc. 2009.

Abstract

We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H(2)O)(6)](2+), [Mn(H(2)O)(5)OH](+), and [Mn(H(2)O)(5)NH(3)](2+). An application of the method, with comparison between theory and experiment, is presented for the solvated Mn(2+) ion in water and three Mn coordination complexes, namely [LMn(acac)N(3)]BPh(4), [LMn(B(2)O(3)Ph(2))(ClO(4))], and [LMn(acac)N]BPh(4), where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B(2)O(3)Ph(2) represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.

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Figures

Figure 1

Figure 1

Theoretical valence-to-core XES (left panel) and Mn K-edge XANES (right panel) of [Mn(H2O)6]2+ (black solid line), [Mn(H2O)5OH]+ (red dash-dotted line) and [Mn(H2O)5NH3]2+ (blue dashed line). Spectra are shown with optimized local structures (top) and with fixed metal-ligand distances (bottom).

Figure 2

Figure 2

Theoretical valence-to-core XES, with some important molecular orbitals contributing to the spectra of [Mn(H2O)6]2+ (top), [Mn(H2O)5OH]+ (middle) and [Mn(H2O)5NH3]2+ (bottom). Sticks show contributions of individual molecular orbitals. Isosurfaces are colored red (blue) for positive (negative) values of the wavefunction. The geometry of the molecule is shown with O (red), H (grey), N (blue) and Mn (magenta).

Fig. 3

Fig. 3

Experimental (top) valence-to-core XES of solvated Mn2+ ion in water and theoretical spectrum of [Mn(H2O)6]2+ (bottom).

Figure 4

Figure 4

Schematic representation of [LMn(acac)N3]BPh4 (top) and [LMn(B2O3Ph2)(ClO4)] (middle) and [LMn(acac)N] BPh4 (bottom) with C (black), O (red), N (blue), B (yellow), Cl (green), H (grey) and Mn (magenta) atoms. Distant counter ion BPh4 is not shown for the top and bottom structures.

Figure 5

Figure 5

Experimental (top) and theoretical (bottom) valence-to-core XES of [LMn(acac)N3]BPh4 (black solid lines), [LMn(B2O3Ph2)(ClO4)] (red dashed lines) and [LMn(acac)N]BPh4 (blue short-dashed lines).

Figure 6

Figure 6

Theoretical valence-to-core XES and some important molecular orbitals contributing to the XES spectra of [LMn(acac)N3]BPh4 (top) and [LMn(B2O3Ph2)(ClO4)] (bottom). Sticks show contributions of individual molecular orbitals. Isosurfaces corresponding to the positive (red) and negative (blue) values of the wave function are plotted. Parts of the MOs are shown as a mesh for clarity. The geometry of the molecules is shown with O (red), N (blue), C (black), B (yellow), Cl (green), H (grey) and Mn (magenta) atoms.

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