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

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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Electronic structures and reactivity aspects of ruthenium–nitrosyls

“…The distances between the Ru(II) ion and the pyrazolyl nitrogen atoms of tpm as well as pyridine nitrogen donor of pap match fairly well with those reported for the analogous complexes [55,56,43,77]. Similarly, Ru II -Cl, Ru II -CH 3 CN and Ru II -NO 2 bond distances in the complexes are in good agreement with those reported in the literature for the analogous systems [43][44][45][46][47][48][49][50][51][52]. The following are the striking features associated with the structures of the complexes: (i) The bite angle associated with the five-membered chelate ring involving the pap ligand, N1-Ru-N2, $76°is much shorter than that of the six-membered chelate ring involving the tpm ligand, N4-Ru-N6 or N6-Ru-N8, $84°.…”

“…The presence of strongly p-acidic pap ligand along with the weakly p-acidic tpm as ancillary ligands in the complex framework of 4 3+ has been reflected in the high m(NO) frequency which in turn signifies the nitrosonium character of the coordinated NO [42]. On the other hand the reduced nitrosyl bent geometry (138.0°, sp 2 -hybridization) in 4 2+ as observed earlier in analogous complexes [42][43][44][45][46][47][48][49][50][51][52][87][88][89][90][91]. The one-electron reduced radical species [Ru II (tpm)(pap)(NO Å )] 2+ (4 2+ ) exhibits the EPR spectrum with g = 1.989 (Fig.…”

“…It may be noted that ruthenium-nitrosyl species involving meridionally coordinated trpy (trpy: 2,2 0 :6 0 ,2 00 -terpyridine) in combination with a wide variety of bidentate ancillary ligands have been reported in recent years [42][43][44][45][46][47][48][49][50][51][52], but the ruthenium-nitrosyl derivatives encompassing facially coordinated tpm ligand have been confined to only two molecular frameworks of [Ru II (tpm)(bpy)(NO)] n [55] and [Ru II (tpm)(Cl) 2 (NO)] n [56].…”

“…The present work originates from our interest [42][43][44][45][46][47][48][49][50][51][52] in exploring the aforesaid aspects (i-iii) of the coordinated nitrosyl function, using the specific ruthenium-nitrosyl framework of [Ru(tpm)(pap)(NO)] n+ (n = 3, {RuNO} 6 (4 3+ ) and n = 2, {RuNO} 7 (4 2+ )). Facially coordinated r-donor and weak p-donor/acceptor tridentate scorpionate ligand, tris(1-pyrazolyl)methane (tpm) [53] and strongly p-accepting bidentate 2-phenylazopyridine (pap) [54] have been selectively chosen as ancillary ligands in developing the desired Ru-NO derivatives, [Ru(tpm)(pap)(NO)] n+ .…”

Comparison of Carbazole and Fluorene Donating Effects on the Two‐Photon Absorption and Nitric Oxide Photorelease Capabilities of a Ruthenium–Nitrosyl Complex

“…Hydrogen atoms have been omitted for clarity. [22] 2.029(2), [23,24] and 2.029(3) Å. [25] [13a] (non-alkylated version of [1] 3+ ), in particular, at the terpyridine, for which the greatest deviation from the mean plane (18 atoms) is 0.143 Å, a value reduced to 0.090 Å in the present case.…”