Abstract
Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl]pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (MII) and the one-electron oxidized (MIII) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.
Highlights
The optimization of multi-step redox catalysis performed by molecular complexes requires a fundamental understanding of the factors that determine performance
We reported the first comprehensive analysis of the effects of substituting the central base metal, the auxiliary ligand, and distal modification to the ligand framework have been on the redox potential of Py5 coordinated complexes
The trends in structure, spin-state energies, redox potentials, and ligand-exchange reactivity were rationalized in a molecular orbital picture, supported by density-functional theory (DFT) calculations
Summary
The family of pentapyridyl ligands stemming from Py5, (2,6-bis-[(2-pyridyl)methyl]pyridine) has been extensively adopted in the past two decades to complex different first-row. Using the Py5OH ligand framework (Fig. 1; R = OH) we synthesized here a series of metal complexes with the general formula [MII(Py5OH)Cl](PF6) (abbreviated as [MII–Cl]), where M = Mn, Fe, Co and Ni. Using the Py5OH ligand framework (Fig. 1; R = OH) we synthesized here a series of metal complexes with the general formula [MII(Py5OH)Cl](PF6) (abbreviated as [MII–Cl]), where M = Mn, Fe, Co and Ni Their one-electron redox potentials were determined by cyclic voltammetry, and the geometric and electronic structures of both the reduced [MII–Cl] and oxidized [MIII–Cl] species were investigated in powder and/or dissolved form by employing a combination of singlecrystal X-ray diffraction (XRD), synchrotron X-ray absorption spectroscopy (XAS) and density-functional theory (DFT) calculations. The effect of the ligand sphere on the redox potential was studied by exchange of the apical chloride ligand This is especially interesting with regard to catalytic reactions as its exchangeability is likely important for substrate activation. Combined with the careful experimental and theoretical analysis of a well-defined one-electron redox event, this provides fundamental insight into the electrochemical processes of transition metal complexes
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