X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

Abstract

Manganese K-edge X-ray absorption (XAS) and Kβ emission (XES) spectroscopies were used to investigate the factors contributing to O–O bond activation in a small-molecule system. The recent structural characterization of a metastable peroxo-bridged dimeric Mn(III)2 complex derived from dioxygen has provided the first opportunity to obtain X-ray spectroscopic data on this type of species. Ground state and time-dependent density functional theory calculations have provided further insight into the nature of the transitions in XAS pre-edge and valence-to-core (VtC) XES spectral regions. An experimentally validated electronic structure description has also enabled the determination of structural and electronic factors that govern peroxo bond activation, and have allowed us to propose both a rationale for the metastability of this unique compound, as well as potential future ligand designs which may further promote or inhibit O–O bond scission. Finally, we have explored the potential of VtC XES as an element-selective probe of both the coordination mode and degree of activation of peroxomanganese adducts. The comparison of these results to a recent VtC XES study of iron-mediated dintrogen activation helps to illustrate the factors that may determine the success of this spectroscopic method for future studies of small-molecule activation at transition metal sites.

Cobalt Kβ valence-to-core X-ray emission spectroscopy: a study of low-spin octahedral cobalt(iii) complexes

“…In the current work, we develop a thorough understanding of cobalt Kβ V2C X-ray emission spectral features of a structurally diverse set of low-spin Co(III) complexes for the first time through combined experimental and computational studies. The experimental Co Kβ V2C features were found to be sensitive to valence orbital composition and cobalt-ligand environment, consistent with previous studies for iron, 11,13–25 manganese, 26–31 titanium, 32 chromium 33 and vanadium. 34 The ORCA single particle model was used to compute the V2C features of the cobalt complexes in this study.…”

Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification

“…43 We are interested in obtaining benchmark parameters and looking for correlations between key emission features and O–O bond lengths in Mn–peroxo compounds. As shown in Figure 15, fits to the XES spectrum of peroxo-bridged 4 (black) show two features in the region 6514–6521 eV 20 that are not present in the spectrum of the Mn(II) precursor 2 (blue) and lie above and below that involving the O 2s orbital of oxo-bridged 5 (red). These features correspond to emission from the peroxo σ 2s (O–O) and ${\sigma }_{2s}^{\ast }false(\mathrm{normalO}-\mathrm{normalO}false)$ orbitals, the separation of which reflects the peroxo O–O bond strength.…”

“…Prior to our work, 12,20,30,31 there were no characterized examples of non-heme Mn intermediates A , B , C , and D (Figure 1), although there were a handful of examples of deprotonated versions of B containing an η 2 -side-on peroxo (Figure 3). 18,32–35 Only one of these is derived from O 2 , however.…”

“…20 X-ray absorption spectroscopy (XAS) involves the promotion of a metal ion 1s electron into either discrete orbitals (the pre-edge) or the continuum. XES involves the relaxation of a metal 3p electron (the K β 1,3 and K β ′ mainline region) or a ligand-based 2s or 2p electron (the K β ″ and K β 2,5 valence-to-core (VtC) region) into the 1s hole.…”

“…These features correspond to emission from the peroxo σ 2s (O–O) and ${\sigma }_{2s}^{\ast }false(\mathrm{normalO}-\mathrm{normalO}false)$ orbitals, the separation of which reflects the peroxo O–O bond strength. DFT-calculated fits to the experimental XES data were used to generate a bonding picture of 4 (Figure 16), 20 which shows that the thiolate and peroxo ligands dominate the ligand field, resulting in an empty ${\mathrm{d}}_{x^2-y^2}{\sigma }^{\ast }$ orbital and leaving the pyridine nitrogens pointing toward the half-occupied ${\mathrm{d}}_{z^2}$ orbital, thereby providing a mechanism for modulating the metal ion Lewis acidity. Pseudo- σ overlap between the thiolate sulfur, half-occupied Mn d xy , and peroxo ${\pi }_{\text{ip}}^{\ast }$ orbitals (Figure 17) provides an additional mechanism for elongating the peroxo O–O bond.…”