Carboxylate tolerance of the redox-active platform [Ru(μ-tppz)Ru]ⁿ, where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine, in the electron-transfer series [(L)ClRu(μ-tppz)RuCl(L)]ⁿ, n = 2+, +, 0, -, 2-, with 2-picolinato, quinaldato, and 8-quinolinecarboxylato ligands (L^-)

Abstract

The neutral title complexes [(L_1-3)ClRu^II(μ-tppz)Ru^IICl(L_1-3)] [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine with L₁ = 2-picolinate, L₂ = 2-quinolinecarboxylate (quinaldate) and with the hitherto little used L₃ = 8-quinolinecarboxylate] have been structurally characterized as approximately anti- (1 and 3) and syn-configured isomers (2) with L ligand N (1 and 3) or O atoms (2) trans to the pyrazine N atoms of tppz. In contrast to 1 and 2 with five-membered chelate rings, complex 3 (which is isomeric with 2) contains six-membered chelate rings. Each system 1-3 thus features a significantly different coordination situation, and all complexes exhibit a considerably distorted tppz bridge, including a twisted central pyrazine ring. In spite of this, double one-electron reduction of the bridge is always possible, as is evident from electron paramagnetic resonance (EPR) and UV/vis spectroelectrochemistry. Two separate (ΔE ∼0.4 V and K_c∼ 10⁷) one-electron oxidations occur on the metals, producing invariably EPR-silent (4 κ) Ru^IIIRu^II intermediates with moderately intense near-IR absorptions, ranging from 1500 to 1900 nm. IR spectroelectrochemistry of the carboxylato carbonyl stretching bands did not result in any noticeable shift, in contrast to what was observed with dipyridyl ketones and related coligands. Density functional theory (DFT)/time-dependent DFT calculations confirm the experimental structures and explain the noted spectroscopic trends: destabilized and closer-spaced frontier orbitals for 3 over 2, with the comparison to 1 suggesting that the configuration is a major factor. Nevertheless, the rather unperturbed electronic structure of the [Ru(μ -tppz)Ru]ⁿ entity, despite different coordination situations at the metal sites, is remarkable and suggests further use of this entity as a robust, carboxylate-tolerant redox-active platform in extended frameworks.