Reactivity of the Hydrido/Nitrosyl Radical MHCl(NO)(CO)(PⁱPr₃)₂, M = Ru, Os

Abstract

The reaction of equimolar NO with the 16 electron molecule RuHCl(CO)L(2) (L = P(i)Pr(3)) proceeds, via a radical adduct RuHCl(CO)(NO) L(2), onward to form RuCl(NO)(CO)L(2) (X-ray structure determination) and RuHCl(HNO)(CO)L(2), in a 1:1 mole ratio. The HNO ligand, bound by N and trans to hydride, is rapidly degraded by excess NO. The osmium complex behaves analogously, but the adduct has a higher formation constant, permitting determination of its IR spectrum; both MHCl(CO)(NO)L(2) radicals are characterized by EPR spectroscopy, and DFT calculations on the Ru system show it to have a "half-bent" Ru-N-O unit with the spin density mainly on nitrogen. DFT (PBE) energies rule out certain possible mechanistic steps for forming the two products. A survey of the literature leads to the hypothesis that NO should generally be considered as a (neutral) Lewis base (2-electron donor) when it binds to a 16 electron complex which is resistant to oxidation or reduction, and that the resulting N-centered radical has a M-N-O angle of approximately 140 degrees, which distinguishes it from NO(-) (bent at <140 degrees ) and from NO(+) (>170 degrees ).

Electronic structures and reactivity aspects of ruthenium–nitrosyls

“…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°. (ii) The geometrical constraint arises due to two different chelating ligands (tridentate, tpm and bidentate, pap) and one monodentate ligand X (X = Cl, 1(ClO 4 ); CH 3 CN, 2(ClO 4 ) 2 ; NO 2 , 3(ClO 4 )) around the metal ion has been reflected in the smaller trans angles (172-177° [42,[82][83][84][85]. Interestingly, the DFT calculated bond parameters for 4 3+ and 4 2+ predict the lengthening and shortening of Ru-N2(azo, pap) and N@N bond distances, respectively (Table S2 and Fig.…”

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

“…When substoichiometric NO is used, equal molar amounts of RuHCl(HNO)-(CO)(P i Pr 3 ) 2 and RuCl(NO)(CO)(P i Pr 3 ) 2 are produced [144]. The HNO adduct is stable for $12 h, but this time is dramatically shortened by the presence of NO, yielding the same products as from generated from N-N coupling reactions of free HNO with NO in solution.…”

“…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].…”

“…Initially, an intermediate nitrosyl hydride is formed, MHCl(NO)-(CO)(P i Pr 3 ) 2 , which undergoes bimolecular H-atom transfer to give equal molar mixtures of the six coordinate HNO adduct, MHCl(HNO)(CO)(P i Pr 3 ) 2 , and the five coordinate nitrosyl, MCl(NO)(CO)(P i Pr 3 ) 2 . The 1 H NMR of RuHCl(HNO)(CO)(P i Pr 3 ) 2 shows the nitroxyl proton at 20.9 ppm, but at À20°C the peak appears as a doublet (J NH 9.8 Hz), due to coupling between the trans nitroxyl and hydride protons [143,144].…”

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.…”