the very different redox behaviour of isoelectronic complexes with [PtCl2] and [AuCl2]+

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 PtCl2 and to the isoelectronic [AuCl2]+ in [AuCl2(bmiq)](AuCl4) 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 [PtCl2(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 [AuCl2(bmiq)]+ takes place not at the ligand but at the metal site, according to experimental and DFT results

Stabilization of {RuNO}<SUP>6</SUP> and {RuNO}<SUP>7</SUP> states in [Ru<SUP>II</SUP>(trpy)(bik)(NO)]<SUP>n+</SUP> {trpy = 2,2':6',2" -terpyridine, bik = 2,2'-bis(1-methylimidazolyl) ketone} - formation, reactivity, and photorelease of metal-bound nitrosyl

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) &#8594; X= CH3CN, [2](ClO4)2 &#8594; X= NO2-, [3](ClO4) &#8594; X= NO+, [4](ClO4)3 &#8594; 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 &#957;(C=O) frequency of the bik ligand at about 1630 cm-1 is largely invariant on complexation and reduction, the &#957;(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+ &#8594; 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; &#916;g = 0.134). The {RuNO}6 unit in [4]3+reacts with OH- via an associatively activated process (&#916;S# = -126.5 &#177; 2 J K-1 mol-1) with a second-order rate constant of k = 3.3 &#215; 10-2M-1 s-1, leading to the corresponding nitro complex [3]+. On exposure to light both {RuNO}6 and {RuNO}7in [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 &#215; 10-1 s-1, t&#189;= 0.80 s) is found to be much faster than that of the RuII-NO. bond in [4]2+, [kNO., 5.45 &#215; 10-4 s-1, t&#189; = 21.2 min (= 1272 s)]. The photoreleased nitrosyl can be trapped as an Mb-NO adduct