Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Abstract

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Mixed Donor–Acceptor‐Derived N,N′‐Diarylpyrazine‐2,5‐dicarboxamido‐Bridged Diruthenium Systems: Structures, Magnetic Properties, and Electronic Forms in Multiredox States

“…The {[Ru III (acac) 2 ] + } fragment in the mononuclear complex 3 is linked to one set of pyrazine and anionic amido nitrogen donors of the monodeprotonated HL 1 – , and the hydrogen‐bonded free NH group and the pyrazine nitrogen atom at the back face of HL 1 form a N4–H4 ··· N3 angle and N4–N3 distance of 117(3)° and 2.750(4) Å, respectively. The Ru III –N(amido), Ru III –N*(pyrazine),[3d], [3f], average Ru III –O(acac), and C=O (L 2– )[5c], [10c] bond lengths in the complexes matched well with those for previously reported analogous systems. The Ru–N(pyrazine) bond lengths observed for the three complexes described here are slightly longer (Table ) than those observed in the Creutz–Taube ion [1.998(4)–2.002(2) Å].…”

“…As expected, the magnetic properties of ruthenium compounds containing two {Ru III (acac) 2 } fragments connected by O‐ or N‐donor‐based bridging ligands are strongly dependent on the nature of the bridge. Thus, similar antiferromagnetic coupling constants ( J ) are usually observed,[3c], [6a], but some dependence of the magnetic properties on the magnetic field has also been noted . However, for 1 , the bridging ligand (L 1 2– ) favors only intramolecular antiferromagnetic interactions, and the magnetic properties are independent of the applied magnetic field.…”

“…The pertinent question of valence localization ( A ) or delocalization ( B ) in the pyrazine‐bridged mixed‐valent Creutz–Taube ion {[(NH 3 ) 5 Ru II (NC 4 H 4 N)Ru III (NH 3 ) 5 ] 5+ } has initiated experimental and theoretical explorations of a wide variety of diruthenium(II/III) (d 6 d 5 ) complexes containing pyrazine or modified pyrazine‐derived bridges in combination with different ancillary ligands In this context, a recent investigation with diverse molecular frameworks introduced the concept of the borderline (hybrid class II–class III) mixed‐valent situation and revealed the effective role of the ancillary ligands and the bridge in fine tuning the electronic coupling in the mixed‐valent state . The scope for further expansion to the less explored Ru III Ru IV (d 5 d 4 ) mixed‐valent state as well as the application potential of the mixed‐valent species in catalysis, molecular electronics and energy‐based research have also been explored.…”

“…Some of us have recently reported diruthenium complexes bridged by the doubly deprotonated form of the ligand pyrazine‐2,5‐dicarboxylic acid (H 2 L). [3c] As the bridge substituents have a profound influence on the redox and spectroscopic properties of ligand‐bridged, redox‐active diruthenium complexes, we have now turned our attention to the modification of such substituents. In this regard, in the present article, we aim to deal with the hitherto unexplored pyrazine‐derived deprotonated N , N′ ‐diarylpyrazine‐2,5‐dicarboxamide (H 2 L 1 /H 2 L 2 ) bridge embedded with amide functions to form a rather unusual bridge blended with π‐accepting (pyrazine) and σ‐donating (–NHAr) components.…”

“…In this regard, in the present article, we aim to deal with the hitherto unexplored pyrazine‐derived deprotonated N , N′ ‐diarylpyrazine‐2,5‐dicarboxamide (H 2 L 1 /H 2 L 2 ) bridge embedded with amide functions to form a rather unusual bridge blended with π‐accepting (pyrazine) and σ‐donating (–NHAr) components. [5b], [3c], [3d]…”