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

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 center dot, [4](ClO4)(2). The single- crystal X-ray structures of [1](ClO4)center dot(C6H6)center dot H2O, [2](ClO4)(2)center dot H2O, and [3](ClO4)center dot H2O have been determined. The successive NO+/NO- (reversible) and NO center dot/NO- (irreversible) reduction processes of [4](3+) appear at +0.36 and -0.40 V vs. SCE, respectively. While the nu(C=O) frequency of the bik ligand at about 1630 cm(-1) is largely invariant on complexation and reduction, the nu(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 center dot character of this electron transfer. However, a sizeable contribution from ruthenium with its high spin-orbit coupling constant to the singly occupied molecular or bital (SOMO) is apparent from the enhanced g anisotropy in the EPR spectrum [4](2+) (g(1) = 2.015, g(2) = 1.995, g(3) = 1.881; g(av) = 1.965; Delta g = 0.134). The {RuNO}(6) unit in [4](3+) reacts with OH- via an associatively activated process (Delta S-# = -126.5 +/- 2JK(-l) mol(-1)) with a second-order rate constant of k = 3.3 X 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 Ru-II-NO+ bond in [4](3+) (k(NO), 8.57 x 10(-1) s(-1), t(1/2) = 0-80 s) is found to be much faster than that of the Ru-II-NO center dot bond in [4](2+), [k(NO center dot), 5.45 x 10 s(-1), t(1/2) = 21.2 min (= 1272 s)]. The photoreleased nitrosyl can be trapped as an Mb-NO adduct. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)