A family of organoruthenium nitrites: alkyne insertion, linkage isomerization and ring nitration

Abstract

The reaction of [Ru(C₆H₂OH-2-CHNR-3-Me-5)(PPh₃)₂(CO)(NO₂)] 1 (R = Et, p-MeC₆H₄ or p-ClC₆H₄) with an excess of alkyne HC≡CX (X = H, Ph or CH₂OH) in CH₂Cl₂–MeOH medium was accompanied by linkage isomerization of nitrite (O,O′-bonded → N-bonded) and formation of a six-membered vinyl–phenolato chelate ring to give [Ru(η²-C₆H₂CXCH-1-O-2-CHNHR-3-Me-5)(PPh₃)₂(CO)(NO₂)] 4. The active substrate is the solvate 1·MeOH, and the 2 + 2 addition of the bulky ≡CX (X = Ph or CH₂OH) group proceeds regiospecifically to the carbon end of the Ru–C bond. Compound 4 has also been obtained metathetically by treating 3 (the chloro analogue of 4) with NaNO₂ in neutral media. However in acid media ring nitration of 3 (R = Et, X = Ph) occurs furnishing [Ru(^η2-C₆HCPhCH-1-O-2-CHNHEt-3-NO₂-4-Me-5)(PPh₃)₂(CO)Cl] 7 which can metathetically be converted into the corresponding N-bonded NO₂ analogue, 8. The iminium proton is hydrogen bonded to the phenolato oxygen in 4, 7 and 8 and also weakly to a nitro oxygen in 4 and 8 (IR and ¹H NMR data). All the species display a quasireversible cyclic voltammetric Ru^III–Ru^II couple, the E_1/2 of which shifts to higher potential by ≈200 mV upon replacing chloride by nitrite (3 → 4; 7 → 8) as well as upon aromatic nitration (3 → 7). The crystal structures of the solvate 4b·C₆H₆ in which R = p-MeC₆H₄ and X = H, 4h in which R = p-ClC₆H₄ and X = CH2OH and 7 have been determined. The σ-vinyl–phenolato chelate ring is approximately planar. The Ru–N bond in the planar RuNO₂ fragment of 4b·C₆H₆ and 4h is lengthened by ≈0.1 Å due to the trans influence of the vinyl group. The Ru–C(vinyl) bond in 7 is significantly shortened due to electron withdrawal by the nitro group, thus promoting Ru–ligand back bonding. The distances of the iminium nitrogen from phenolic oxygen and a nitrito oxygen (in 4b·C₆H₆ and 4h) lie in the ranges 2.55–2.67 and 2.90–2.98 Å respectively.