Preparation of a Nitroxyl (HNO) Complex of Rhenium by Selective Oxidation of Coordinated Hydroxylamine

Abstract

The discovery that nitric oxide (NO) plays key roles in mammalian biochemistry has spurred significant recent work aimed at elucidating the bioinorganic chemistry of NO. The diatomic radical has been shown to be responsible for numerous physiological functions, and it is particularly important as a cellsignaling agent, which includes its role as the endotheliumderived relaxing factor (EDRF) responsible for triggering smooth muscle relaxation. In this sense, NO functions as a vasodilator, helping to lower blood pressure; in the stomach, it promotes the relaxation component of peristalsis; and it is the neurotransmitter in males responsible for translating sexual excitement into tumescence. 1 It has been suggested that the 1-eredox relatives of NO, the nitrosonium cation (NO + ), and the nitroside anion (NO -) as well as the conjugate acid of NO -, nitroxyl (HNdO), may be responsible for certain aspects of the rich biological chemistry of nitric oxide. 1a,b In addition to its biological relevance, HNdO has been postulated as an intermediate in photochemical and free-radical reactions, 2 and its formation and decomposition may play a role in mechanisms for the combustion of nitrogen-containing fuels and the oxidation of atmospheric nitrogen. 3 Nitroxyl is a reactive molecule that was first proposed in 1903 as an intermediate in the decomposition of sodium trioxodinitrate(II) (Na 2 N 2 O 3 ) in aqueous media. 4 The first reports of direct spectroscopic observation of HNdO appeared in 1958 from gasphase work by Dalby 2 and, independently, from argon-matrixisolation experiments (by the photolysis of methyl nitrite) by Pimentel. 5 However, despite its very simple constitution and structure, a facile synthesis for this triatomic molecule remains elusive. While HNdO is thought to readily decompose in a bimolecular fashion to give N 2 O (eq 1), 6 its stability, like that of the isoelectronic 1,2-diazene (NHdNH), 7-9 can be greatly enhanced by coordination to a transition metal. There are a few reported examples of nitroxyl complexes, 10,11 most notably Roper's OsCl 2 (CO)(NHdO)(PPh 3 ) 2 , produced by reaction of OsCl(CO)(NO)(PPh 3 ) 2 with anhydrous HCl. 11 Our group has been actively involved in synthetic and reactivity studies of cationic complexes of 1,2-diazene in d 6transition-metal systems (W, Re, Ru, Os). 7-9 We have shown NHdNH can indeed be stabilized by complexation and, furthermore, that these complexes can serve as clean sources of the free NHdNH molecule (as illustrated for W in eq 2). Since HNdO is isoelectronic with diazene, we have explored the use of a similar synthetic approach for the preparation of stable coordination complexes of nitroxyl, with an eye on ultimately using such complexes as HNdO synthons.Of the W, Re, Ru, and Os series of cationic, d 6 diazene complexes, the Re derivative [mer,trans-Re(CO) 3 (NHdNH)-(PPh 3 ) 2 + ][SO 3 CF 3 -] is by far the most thermally robust, 8 so our initial synthetic efforts have concentrated on preparing an HNdO analogue of this compound. Herein we report the...

Coordination chemistry of the HNO ligand with hemes and synthetic coordination complexes

“…[142]. NO)(PPh 3 ) 2 as well as by theoretical modeling [139][140][141]. The effect of spectator ligands is seen by comparison of Re(Cl)(CO) 2 (HNO)(PR) 2 where R equals phenyl (Ph) or cyclohexyl (Cy).…”

“…The initial reports of Roper demonstrated that CO can displace the HNO ligand in OsCl 2 (CO)(H-NO)(PPh 3 ) 2 , giving the dicarbonyl OsCl 2 (CO) 2 (PPh 3 ) 2 [135]. Likewise, the cationic complex [Re(CO) 3 (H-NO)(PPh 3 ) 2 ] + reacted with (t-But) 4 NBr to give ReBr(CO) 3 (PPh 3 ) 2 [140]; and when NOPF 6 or NOBF 4 were used in the synthesis of [Re(HNO)(CO) 3 (PPh 3 ) 2 ] + , the adduct was unstable and decomposed to the neutral fluoride complex ReF(CO) 3 (PPh 3 ) 2 [141]. Likewise, the HNO ligand in [HNORu(ttp)(1-MeIm)] may be displaced by nitrosobenzene to give [Ru(PhNO)(ttp)(1-MeIm)] in high yield [127].…”

“…A cationic nitroxyl complex, [Re(CO) 3 (HNO)-(PPh 3 ) 2 ][SO 3 CF 3 ], has been synthesized by two different routes: the first by oxidizing a hydroxylamine adduct, [Re(CO) 3 (NH 2 OH)(PPh 3 ) 2 ] + , with Pb(OAc) 4 [140]; the second involve 1,1-insertion of NO + into metal-hydride bond, by reaction of [NO][SO 3 CF 3 ] with Re-H(CO) 3 (PPh 3 ) 2 . When PF À 6 or BF À 4 counteranions were used instead of SO 3 CF À 6 the nitroxyl complex was unstable and decomposed to a neutral fluoride complex Re-F(CO) 3 (PPh 3 ) 2 [144].…”

Reactivity of iron(II)-bound nitrosyl hydride (HNO, nitroxyl) in aqueous solution

“…Well characterized complexes with HNO and NO -as ligands have been prepared in the last decade, noticeably with d 6 low-spin metals such as Ru II [25][26][27][28], Os II [27,[29][30][31], Re I [31][32][33], Ir III [29] (for HNO), and with Co III [34][35][36][37][38], Cr III [39][40][41], Pt IV [42], Ru II [27], and Fe II [43][44][45][46][47] (for NO -), generally by using chelating macrocyclic coligands (viz., with porphyrins, aza-macrocycles, and thioether binding species) [24]. In general, a main drawback relates to the low solubility in water and the very limited presence of iron complexes.…”

A nitrosyl hydride complex of a heme model [Ru(ttp)(HNO)(1-MeIm)] (ttp=tetratolylporphyrinato dianion)

“…2 This is surprising, since nucleophilic attack at bound nitrosyls to give related metal-{N(Nu)@O} derivatives are well-known [37]. Other methods to generate metal-HNO complexes involve (i) formal protonation of bound NO [22,23,[29][30][31][38][39][40], (ii) proton abstraction from coordinated H 2 NO [33], (iii) oxidation of bound hydroxylamine [27], (iv) formal coupling of a H-atom with the NO ligand in a metal nitrosyl radical complex [24], and (v) NO þ insertion into metal-hydride bonds [26]. In addition, the [Ru(ttp)(H-NO)(1-MeIm)] complex is air-and thermally sensitive.…”