An Electron‐Rich {RuNO}⁶ Complex: trans‐[Ru(DMAP)₄(NO)(OH)]²⁺ – Structure and Reactivity

Abstract

The pseudo-octahedral nitrosyl species trans-[Ru(DMAP) 4 -(NO)(OH)] 2+ [1 2+ , DMAP = 4-(dimethylamino)pyridine] was prepared by the reaction between [Ru(DMAP) 5 (H 2 O)] 2+ and NaNO 2 under mild conditions (room temperature, pH = 6-8), and precipitated with NaBF 4 or NaPF 6 . Single-crystal X-ray diffraction data of 1(BF 4 ) 2 ·2H 2 O point to a {RuNO} 6 electronic configuration of the cation, although there is a significant deviation from the expected linear arrangement of the RuNO moiety (Ru-N-O angle: 169.3°). The remarkably low wavenumber of the NO stretching band (ν NO ) of 1832 cm -1 in the solid state corresponds to an electron-rich ligand environment with strong backbonding interactions between d π metal orbitals and π* NO orbitals of the (formal) NO + (nitrosonium) ligand. Consistently, the complex acts as a poor electrophile:[a] 4301 it is extremely unreactive toward OHand toward the stronger nucleophile cysteine. Cyclic voltammetry (CV) experiments show three reduction processes in acetonitrile. The first one at -0.50 V [vs. Ag/AgCl, KCl(s)] is reversible on the CV time scale. Spectroelectrochemical experiments (IR and UV/Vis) suggest that the reduced complex has a {RuNO} 7 configuration. The ν NO wavenumber of 1603 cm -1 agrees with a reduction mostly centered at the nitrosyl ligand. This species appears to be inert toward substitution in the coordination sphere, and the original {RuNO} 6 ion can be quantitatively recovered upon oxidation. In contrast, further reduction of this species is completely irreversible, even on the CV timescale, which is probably due to the lability of the presumably formed nitroxyl (NO -) ligand.Scheme 1. Preparation of trans-[Ru(DMAP) 4 (NO)(OH)] 2+ . www.eurjic.org

Three Redox States of Metallonitrosyls in Aqueous Solution

“…For the [Ru(Me 3 [9]aneN 3 )(bpy)(NO)] 3+ ion (Table 1), a combined experimental UV-vis/(TD)DFT approach allowed assigning the detailed ordering of frontier orbitals (15), with the HOMO defined as a π(bpy) orbital. On the other hand, the LUMO and LUMO + 1 are mostly located on the nitrosyl moiety with $27% contribution from metal center orbitals, reflecting a substantial π-backbonding, comparable to the one calculated in [Ru(tpm)(bpy)(NO)] 3+ (25-30%) (30) and [Ru(DMAP) 4 (OH) (NO)] 2+ (30%) (38). As a comparison, recent calculations with [Fe(por) (MI)(NO)] + afforded a pair of LUMO and LUMO + 1 orbitals with composition close to 68% π* x ,π y * and 27% d xz ,d yz (69).…”

“…The results for the different species can be grouped into two sets of parallel lines. The displacement to lower values of k OH for the same potential in one of them is probably due to steric restrictions of the tetrapyridine-nitrosyl complexes, an explanation that would also account the lack of reactivity of [Ru(DMAP) 4 (OH)(NO)] 2+ toward OH À and even HS À (38). The correlation spans an impressive range larger than 1.0 V in the redox potentials and $10 orders of magnitude in the values of k OH .…”

Nitrosyl-Centered Redox and Acid–Base Interconversions in [Ru(Me3[9]aneN3)(bpy)(NO)]3,2,1+. The pKaof HNO for its Nitroxyl Derivative in Aqueous Solution

“…A thorough comparison reveals that the theoretical Ru−N bond lengths are slightly (and consistently) overestimated, a behavior already reported when comparing experimental and theoretically produced bond lengths at this level of theory in transition metal complexes 62 and in particular in other {RuNO} 6 systems. 57,61,63,64 The DFT computations reproduce the principal features observed in the vibrational spectra of all the species and allow for a complete assignment of the vibrational modes. 65 In general terms, there is a very good agreement between experimental and theoretically derived magnitudes, a fact that contributes to validation of the theoretical methodology.…”

“…The downward shift in ν NO to 1861 cm −1 with labeled 15 N agrees with values found for other nitrosyl-complexes. 59,64,66 {RuNO} 6 Species in Aqueous Solution. Solutions of the nitro-complex [1] + are intensely red colored.…”

“…The LUMO and LUMO+1 MOs are mostly located on the nitrosyl moiety with ∼27% contribution from metal center orbitals, reflecting a substantial π-backbonding comparable to the one calculated in [Ru(tpm)(bpy)NO] 3+ (25−30%) 61 and [Ru(DMAP) 4 (OH)-(NO)] 2+ (30%). 64 This back-donation mechanism partially transfers electron density form the metal center to the π* orbitals of the coordinated NO + , 15 and is responsible for the observed lowering of ν NO from 2377 cm −1 in free NO + 3 to 1899 cm −1 in [2] 3+ . The light-yellow color of the aqueous solutions of [2] 3+ is mostly due to the low energy tail of intraligand-and ligand-to-ligand transitions.…”