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...

The reactions of hydrochloric and triflic acids with the five-coordinate nitrosyl complexes trans-Re(CO)(2)(NO)(PR(3))(2) (2a, R = Ph; 2b, R = Cy) have been investigated. Reaction of anhydrous HCl with 2 results in a formal protonation of the nitrosyl ligand and addition of chloride to the metal, giving the neutral nitroxyl complex cis,trans-ReCl(CO)(2)(NH=O)(PR(3))(2) (3a, R = Ph; 3b, R = Cy). Reaction of Brønsted bases with 3a or 3b results in clean conversion of 3 to 2 when the base is appropriately strong (pK(b) approximately 7). Addition of HOSO(2)CF(3) to solutions of 2a results in protonation at the metal and formation of the cationic rhenium hydride [trans,trans-ReH(CO)(2)(NO)(PPh(3))(2)(+)][SO(3)CF(3)(-)] (4) in 74% yield; the deuteride [trans,trans-Re((2)H)(CO)(2)(NO)(PPh(3))(2)(+)][SO(3)CF(3)(-)] (4-d) was analogously prepared from (2)HOSO(2)CF(3). 4 crystallized from CH(2)Cl(2)/Et(2)O solution in the orthorhombic space group Pnma, with a = 17.2201(2) A, b = 23.6119(3) A, c = 9.2380(2) A, and Z = 4. The least-squares refinement converged to R(F) = 0.039 and R(wF(2)()) = 0.063 for the 4330 unique data with I > 2 sigma(I). The structure of 4 shows that the hydride (Re-H = 1.74 A) occupies the position trans to the linear nitrosyl ligand (Re-N-O = 178.1(4) degrees ) in the pseudooctahedral complex cation. Complex 4 does not react with chloride to give 3a. DFT calculations carried out on free nitroxyl and its model complexes [Re(CO)(5)(NH=O)(+)] (5), [mer,trans-Re(CO)(3)(NH=O)(PH(3))(2)(+)] (6), and cis,trans-ReCl(CO)(2)(NH=O)(PH(3))(2) (7) indicate that coordinated nitroxyl acts as both a sigma-donor and pi-acceptor ligand, consistent with the observed trend for nu(NO) in free HN=O (1563 cm(-1)), [mer,trans-Re(CO)(3)(NH=O)(PPh(3))(2)(+)] (1, 1391 cm(-1)), 3a (1376 cm(-1)), and 3b (1335 cm(-1)).

Nitrosonium triflate reacts with cold methylene chloride solutions of mer,trans-ReH(CO)3(PPh3)2 (1) with 1,1-insertion of NO+ into the Re-H bond to give the orange nitroxyl complex [mer,trans-Re(NH=O)(CO)3(PPh3)2][SO3CF3] (3) in 86% isolated yield. Use of [NO][PF6] or [NO][BF4] gives analogous insertion products at low temperature, which decompose on warning to ambient temperature to the fluoride complex mer,trans-ReF(CO)3(PPh3)2 (4). A related 1,1-insertion is observed in the reaction of 1 with [PhN2][PF6] in acetone that affords the yellow-orange phenyldiazene salt [mer,trans-Re(NH=NPh)(CO)3(PPh3)2][PF6] (2), which has been characterized by X-ray crystallographic methods. The methyl derivative mer,trans-Re(CH3)(CO)3(PPh3)2 (5) also undergoes a 1,1-insertion reaction with [NO][SO3CF3] to give the nitrosomethane adduct [mer,trans-Re{N(CH3)=O}(CO)3(PPh3)2][SO3CF3] (6) as red crystals in 75% yield. The nitroxyl complex [cis,trans-OsBr(NH=O)(CO)2(PPh3)2][SO3CF3] (8) can be similarly prepared as orange crystals in 52% yield by reaction of cis,trans-OsHBr(CO)2(PPh3)2 (7) with [NO][SO3CF3] in cold methylene chloride solution.

The tris(diphenylmethyl)phosphine molybdenum complex mer-Mo(Cl)(CO)(NO)(PMePh2)3 (2), prepared by the reaction of 4 equiv of PMePh2 with trans-Mo(ClAlCl3)(NO)(CO)4 (1), undergoes a metathesis reaction with LiBH4 to afford the hydrido nitrosyl complex mer-Mo(H)(CO)(NO)(PMePh2)3 (3). The reactions of several olefins (ethylene, propylene, styrene) with 3 have been examined, and these studies have uncovered interesting stoichiometric coupling reactions involving hydride, CO, olefin, acyl, and alkyl ligands which bear on alternating CO/olefin oligomerization chemistry. Reaction of 3 with C2H4 (50 °C, 50 psi, 48 h) results in the loss of a PMePh2 ligand and incorporation of 3 equiv of C2H4 to give trans-Mo{CH2CH2C(O)CH2CH3}(C2H4)(NO)(PMePh2)2 (4) in good isolated yield. A slower reaction is observed between 3 and propylene with incorporation of only one olefin in the metal coordination sphere, giving isomeric mer-Mo{η2-C(O)CH2CH2CH3}(NO)(PMePh2)3 (5) and mer-Mo{η2-C(O)CH(CH3)2}(NO)(PMePh2)3 (6). Treatment of solutions of 5 and 6 with HCl results in their conversion to two new aldehyde complexes, mer-MoCl(η1-OCHCH2CH2CH3)(NO)(PMePh2)3 (7) and mer-MoCl{η1-OCHCH(CH3)2}(NO)(PMePh2)3 (8), respectively. Treatment of 7 and 8 with CO results in their ultimate conversion to Mo(Cl)(CO)2(NO)(PMePh2)2 (11), butyraldehyde, and isobutyraldehyde via the bis(phosphine) aldehyde intermediates trans-MoCl(η1-OCHCH2CH2CH3)(CO)(NO)(PMePh2)2 (9) and trans-MoCl{η1-OCHCH(CH3)2}(CO)(NO)(PMePh2)2 (10). Styrene reacts slowly (5 d, 50 °C) with benzene solutions of 3 to give 1 equiv of ethylbenzene and emerald-green trans-Mo[PMePh{C6H4C(O)CH2CH2C6H4}](NO)(PMePh2)2 (12) in high yield.