Cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds.

Alessio Vidal,Enzo Alessio,Gilles Gasser,Roger Alberto,Gabriele Balducci,Rudy Calligaro

doi:10.1021/acs.inorgchem.1c00240

Abstract

We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)2(L2)]2+ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2–7) of the general formula [Y] fac-[RuX(DMSO–S)3(O–O)]n (where O–O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO–O or Cl–; n = −1/0/+1 depending on the nature and charge of X and O–O; when present, Y = K+ or PF6–) were efficiently prepared from the well-known cis-[RuCl2(DMSO)4] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2–7 afforded the target [Ru(L1)2(O–O)]0/+ complex together with the undesired (and unexpected) [Ru(L1)3]2+ species. Nevertheless, we found that the formation of [Ru(L1)3]2+can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)2(O–O)]0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100–200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)2(O–O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)3]2+ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O–O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O–O in [Ru(L1)2(O–O)]0/+ by L2, selectively affording [Ru(L1)2(L2)]2+, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF6.

Highlights

Ruthenium(II) polypyridyl complexes are well-known to the inorganic chemistry community, mainly because of their interesting photophysical and photochemical properties.[1]
In the Introduction, we reviewed the main synthetic routes to heteroleptic polypyridyl Ru(II) complexes, evidencing advantages and limits
We tested experimentally the route that uses cis-[RuCl2(DMSO)4] (1) as a precursor, finding that in general this complex is not reactive and well-behaved for this type of reaction and that what is reported in the literature for a particular diimine ligand is not always reproducible or automatically extensible even to similar ligands

Summary

Introduction

Ruthenium(II) polypyridyl complexes are well-known to the inorganic chemistry community, mainly because of their interesting photophysical and photochemical properties (e.g., strong absorption and emission bands in the visible-to-NIR region, long-lived triplet excited states).[1]. The synthetic procedures leading to Ru(II) polypyridyl complexes bearing three equal or different diimine chelating ligands (L) were thoroughly reviewed by Spiccia and co-workers in 2004.34 Quite obviously, upon going from homoleptic [Ru(L1)3]2+ to tris-heterlopetic [Ru(L1)(L2)(L3)]2+ compounds, the synthetic procedures become more challenging. Optical resolution of the chiral-at-metal Λ and Δ enantiomers adds an additional level of complexity.[35] The reader interested in this specific topic is referred to the relatively recent review by Meggers and co-workers.[36]

Methods

Results

Conclusion