Stabilization of {RuNO} 6 and {RuNO} 7 States in [Ru II (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 ](ClO 4 ) → X = CH 3 CN, [ 2 ](ClO 4 ) 2 → X = NO 2 – , [ 3 ](ClO 4 ) → X = NO + , [ 4 ](ClO 4 ) 3 → X = NO · , [ 4 ](ClO 4 ) 2 . The single‐crystal X‐ray structures of [ 1 ](ClO 4 ) · (C 6 H 6 ) · H 2 O, [ 2 ](ClO 4 ) 2 · H 2 O, and [ 3 ](ClO 4 ) · H 2 O 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+ ( g 1 = 2.015, g 2 = 1.995, g 3 = 1.881; g av = 1.965; Δ g = 0.134). The {RuNO} 6 unit in [ 4 ] 3+ reacts with OH – via an associatively activated process (Δ S # = –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 CH 3 CN via the formation of [Ru(trpy)(bik)(CH 3 CN)] 2+ , [ 2 ] 2+ . The rate of photocleavage of the Ru II –NO + bond in [ 4 ] 3+ ( k NO , 8.57 ×10 –1 s –1 , t 1/2 = 0.80 s) is found to be much faster than that of the Ru II –NO · bond in [ 4 ] 2+ , [ k NO ·, 5.45 × 10 –4 s –1 , t 1/2 = 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)