The new, potentially ambidentate heterocyclic ligand 2,3-bis(1-methylimidazol-2-yl)quinoxaline (bmiq) was obtained from 2,3-bis(1-methylimidazol-2-yl)glyoxal and 1,2-diaminobenzene. Its coordination to PtCl(2) and to the isoelectronic [AuCl(2)](+) in [AuCl(2)(bmiq)](AuCl(4)) occurs via the imine N donors of the imidazolyl groups, leading to the formation of seven-membered chelate rings with boat conformation. According to the spectroelectrochemistry (UV-vis-NIR, EPR), the reversible electron addition to the [PtCl(2)(bmiq)] and the free ligand takes place in the (non-coordinated) quinoxaline part of the molecule, similarly as for related complexes of dipyrido[3,2-a:2',3'-c]phenazines (dppz), 2,3-bis(2-pyridyl)quinoxalines (bpq) and 2,3-bis(dialkylphosphino)quinoxalines (QuinoxP). DFT calculations confirm the experimental results (structures, spectroscopy) and also point to the coordination potential of the quinoxaline N atoms. The electron addition to [AuCl(2)(bmiq)](+) takes place not at the ligand but at the metal site, according to experimental and DFT results.

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Ruthenium nitrosyl complexes have been isolated in the {RuNO}6 and {RuNO}7 configurations, employing the following reaction pathway for [Ru(trpy)(bik)(X)]n+: X = Cl–, [1](ClO4) → X = CH3CN, [2](ClO4)2 → X = NO2–, [3](ClO4) → X = NO+, [4](ClO4)3 → X = NO·, [4](ClO4)2. The single‐crystal X‐ray structures of [1](ClO4)·(C6H6)·H2O, [2](ClO4)2·H2O, and [3](ClO4)·H2O have been determined. The successive NO+/NO· (reversible) and NO·/NO– (irreversible) reduction processes of [4]3+ appear at +0.36 and –0.40 V vs. SCE, respectively. While the ν(C=O) frequency of the bik ligand at about 1630 cm–1 is largely invariant on complexation and reduction, the ν(NO) frequency for the {RuNO}6 state in [4]3+ at 1950 cm–1 shifts to about 1640 cm–1 on one‐electron reduction to the {RuNO}7 form in [4]2+, reflecting the predominant NO+ → NO· character of this electron transfer. However, a sizeable contribution from ruthenium with its high spin‐orbit coupling constant to the singly occupied molecular orbital (SOMO) is apparent from the enhanced g anisotropy in the EPR spectrum [4]2+ (g1 = 2.015, g2 = 1.995, g3 = 1.881; gav = 1.965; Δg = 0.134). The {RuNO}6 unit in [4]3+ reacts with OH– via an associatively activated process (ΔS# = –126.5 ± 2 J K–1 mol–1) with a second‐order rate constant of k = 3.3 × 10–2 M–1 s–1, leading to the corresponding nitro complex [3]+. On exposure to light both {RuNO}6 and {RuNO}7 in [4]3+ and [4]2+ undergo Ru–NO photocleavage in CH3CN via the formation of [Ru(trpy)(bik)(CH3CN)]2+, [2]2+. The rate of photocleavage of the RuII–NO+ bond in [4]3+ (kNO, 8.57 ×10–1 s–1, t1/2 = 0.80 s) is found to be much faster than that of the RuII–NO· bond in [4]2+, [kNO·, 5.45 × 10–4 s–1, t1/2 = 21.2 min (= 1272 s)]. The photoreleased nitrosyl can be trapped as an Mb‐NO adduct.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Complexes cis-MCl2(big), big=bis(1-methylimidazol-2-yl)glyoxal, M=Pt, Pd, were prepared and characterized through electrochemistry, spectroscopy, and for M=Pt, by X-ray structure analysis. The seven-membered chelate ring formed through N,N' coordination of the ligand big shows a boat conformation in agreement with density functional theory (DFT) calculation results. No significant intermolecular interactions were observed for the platinum compound. Both the PdII and PtII complexes undergo reversible one-electron reduction in CH2Cl2/ 0.1 M Bu4NPF6; the reduced palladium compound disintegrates above -30 degrees C. Electron paramagnetic resonance (EPR), UV-vis, and IR spectroelectrochemistry studies were employed to study the monoanions. The anion radical complex [cis-PtCl2(big)]*- exhibits a well-resolved EPR spectrum with small but well-detectable g anisotropy and an isotropic 195Pt hyperfine coupling of 12.2 G. DFT calculations confirm the spin concentration in the alpha-semidione part of the radical complex with small delocalization to the bis(imidazolyl)metal section. The results show that EPR and electroactive moieties can be linked to the cis-dichloroplatinum(II) group via imidazole coordination.