A novel triruthenium nitrosyl bearing a quinolinic ligand: a comparison of its spectroscopic behavior with its pyridine analogues

Abstract

We present the novel compound [Ru₃O(CH₃COO)₆(iq)₂NO]PF₆, iq = isoquinoline. Its electronic spectrum displays three charge-transfer bands between 350 and 550 nm and a band due to intra-cluster transitions centered at metallic levels at 701 nm (842 L mol⁻¹ cm⁻¹). This compound releases NO_(g) under UV irradiation (λ_irrad = 365 nm), with ϕ = 0.23 in acetonitrile solution. Its ¹H NMR spectrum shows attenuated paramagnetic anisotropy of the [Ru₃O] unit. As for IR spectroscopy, the ν(NO) stretching band (1904 cm⁻¹) arises at a relatively high frequency, suggesting that the NO ligand has important NO⁺ character. To understand the nature of the [Ru₃O]–NO metallic core, we have compared the spectroscopic data of [Ru₃O(CH₃COO)₆(iq)₂NO]PF₆ with the data of pyridinic analogues bearing 4-acetylpyridine, 3-methylpyridine, pyridine, 4-methylpyridine, or 4-tert-butylpyridine as ancillary ligands. Except for the value of ν(NO), the introduction of a ligand (iq) bearing a second aromatic ring does not impact the other spectroscopic properties of the metallic core. The nitrosyl compounds with the general formula [Ru₃O(CH₃COO)₆(L)₂NO]PF₆ share similarities with both symmetric compounds ([Ru^IIIRu^IIIRu^IIIO(CH₃COO)₆(L)₃]PF₆) and with the isoelectronic carbonyl compounds ([Ru^IIIRu^IIIRu^IIO(CH₃COO)₆(L)₂CO]) and there is no clear evidence for the role of π-backbonding in their spectroscopic characteristics. These findings point to the presence of two metallic subunits: a {RuNO}⁶ portion with high covalence and electron pairing, and thus less sensitivity to variations in L; and a second subunit, Ru₂^III,III, which is susceptible to perturbations to its electronic density by σ-donation from L. This analysis finally suggests that the triruthenium nitrosyl compounds constitute multiconfigurational systems with relevant contributions from both Ru^IIIRu^IIIRu^III–NO⁰ and Ru^IIIRu^IIIRu^II–NO⁺ configurations.