Stabilization of {RuNO}6 and {RuNO}7 States in [RuII(trpy)(bik)(NO)]n+ {trpy = 2,2':6',2''-terpyridine, bik = 2,2'-bis(1-methylimidazolyl) ketone} - Formation, Reactivity, and Photorelease of Metal-Bound Nitrosyl

Abstract

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.