Unsymmetrical Dinuclear Ru^II Complexes with Bridging Polydentate Nitrogen Ligands as Potential Water Oxidation Catalysts

Abstract

Mononuclear RuII complex [RuCl(κ3N‐tpm)(κ2N‐bptz)]Cl, [1]Cl [tpm=tris(1‐ pyrazolyl)methane; bptz=3,6‐di(2‐pyridyl)‐1,2,4,5‐tetrazine], and dinuclear complexes [RuCl(κ3N‐tpm)(μ‐κ2N:κ2N‐bptz)Ru(κ2N‐bipy)2][PF6]3, [3][PF6]3, [RuCl(η6‐p‐cymene)(μ‐κ2N:κ2N‐dpp)Ru(κ2N‐bipy)2][PF6]2, [4][PF6]2, and [RuCl(η6‐p‐cymene)(μ‐κ2N:κ2N‐dpp)Ru(κ2N‐biqn)2][PF6]3, [5][PF6]3 [dpp=2,3‐bis(2′‐pyridyl)‐pyrazine; bipy=2,2′‐bipyridine; biqn=2,2′‐quinoline], incorporating both potentially catalytic and photosensitive subunits, were synthesized and characterized by means of elemental analysis, mass spectrometry, and spectroscopic methods. The molecular structures of the new compounds were also investigated and compared by means of DFT calculations. The absorption spectra of all the compounds are dominated by metal‐to‐ligand charge‐transfer bands in the visible (which in most cases largely extend over the red portion of the spectrum) and ligand‐centered bands in the UV region. The oxidation behavior is based on metal‐centered RuII to RuIII oxidation processes, which in phosphate buffer solution are followed by a catalytic water oxidation wave for [1]Cl and [3][PF6]3. For these compounds, the mechanism of water oxidation is proposed to consist in water nucleophilic attack, according to chemical experiments with Ce(IV) salts, so demonstrating for the first time that the bptz ligand can be profitably used to build ruthenium(II) complexes with catalytic properties. On the contrary, no catalytic process is observed for 4 and 5, most likely due to the high positive potential for RuII oxidation induced by the presence of the p‐cymene moiety.

Triazine Chalcogenones from Thiocyanate or Selenocyanate Addition to Tetrazine Ligands in Ruthenium Arene Complexes

“…, formation of the bimetallic compound and incomplete binding of the free ligand) during the preparation of other monometallic complexes of 3,6-di(2-pyridyl)-1,2,4,5-tetrazine was reported ( Chart 3 ). 51 …”

Observation of parallel intersystem crossing and charge transfer-state dynamics in [Fe(bpy)3]2+ from ultrafast 2D electronic spectroscopy

“…Transition metal-based polypyridyl complexes represent an important class of inorganic chromophores with photophysical properties amenable for applications ranging from solar energy conversion [1][2][3][4][5] to organic transformations. [6][7][8] Amongst this class of complexes, the prototypical chromophore is tris(2,2 ′ -bipyridine)ruthenium(II), i.e., [Ru(bpy) 3 ] 2+ .…”

“…12,[22][23][24] The dramatic attenuation in MLCT-state lifetime observed for the Fe(II) complexes results from an inversion in the relative energies of the charge-transfer and ligand-eld excited states compared to what exists in the second-and third transition series due to the so-called primogenic effect. [25][26][27] These ligand-eld states are characterized by large geometric distortions relative to both the ground-and MLCT excited states, thereby facilitating rapid non-radiative decay out of the charge transfer-state manifold and the eventual formation of the high-spin 5 T 2 excited state on a timescale of ∼200 fs. Following conversion from the MLCT excited state manifold to the lowest-energy ligand-eld excited state, specically the 5 T 2 state, ground state recovery (i.e., 5 T 2 / 1 A 1 relaxation) occurs on a timescale of ∼1 ns.…”

“…[25][26][27] These ligand-eld states are characterized by large geometric distortions relative to both the ground-and MLCT excited states, thereby facilitating rapid non-radiative decay out of the charge transfer-state manifold and the eventual formation of the high-spin 5 T 2 excited state on a timescale of ∼200 fs. Following conversion from the MLCT excited state manifold to the lowest-energy ligand-eld excited state, specically the 5 T 2 state, ground state recovery (i.e., 5 T 2 / 1 A 1 relaxation) occurs on a timescale of ∼1 ns. Recently, Miller and McCusker identied solvent dependent kinetics for this ground-state recovery.…”