Ruthenium–Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru–Ag Coordination Polymer and Its Enhanced Water-Splitting Feature

Abstract

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[Ru^III(acac)₂(μ-bis-η¹-<i>N</i>,η¹-N<i>-</i>BTD)₂Ag^I(ClO₄)]ClO₄}_<i>n</i> (<b>3</b>) via the oxidation of the monomeric building block <i>cis</i>-[Ru^II(acac)₂(η¹-N-BTD)₂] (<b>1</b>) by AgClO₄ (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric <i>cis</i>-[Ru^II(acac)₂(η¹-N-BTD)₂] (<b>1</b>) and [Ru^II(acac)₂(η¹-<i>N</i>-BTD)(CH₃CN)] (<b>2</b>) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor Ru^II(acac)₂(CH₃CN)₂ in refluxing CH₃CN. Molecular identities of <b>1</b>-<b>3</b> were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric <b>3</b>. Ru(III) (<i>S</i> = 1/2)-derived <b>3</b> displayed metal-based anisotropic EPR with ⟨<i>g</i>⟩/Δ<i>g</i> = 2.12/0.56 and paramagnetically shifted ¹H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of <b>1</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = 1+, 0, 1-) determined a metal-based (Ru^II/Ru^III) oxidation and BTD-based reduction (BTD/BTD^•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of <b>3</b> with respect to <b>1</b> could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of <b>3</b> were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric <b>3</b> was highlighted by its degradation to the η¹-<i>N</i>-BTD coordinated monomeric fragment <b>1</b>, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric <b>1</b> and polymeric <b>3</b> exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of <b>3</b> in comparison to <b>1</b>.