Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine

Abstract

Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of the thioindigo-N,N'-diphenyldiimine chelating ligand 3 have been prepared. The thioindigo diimine ligand binds to ruthenium in a bidentate fashion in the mononuclear compound 2 and serves as a bidentate chelating bridging ligand in the diruthenium complexes 1a and 1b. Compound 2 was isolated as a racemic mixture while the diruthenium complexes were isolated as the meso (ΔΛ) 1a and rac (ΔΔ and ΛΛ) 1b diastereomers. In-depth structural characterization of the compounds was performed, including X-ray crystallography, H,C, and F nuclear magnetic resonance (NMR) spectroscopy, and 2D NMR correlation experiments. Electrochemical properties were evaluated utilizing cyclic voltammetry. Ground state optical properties of the complexes were examined using UV-visible spectroscopy and spectroelectrochemistry. The excited state dynamics of the series were investigated by ultrafast transient absorption spectroscopy. Variable temperature NMR experiments demonstrated that the rac diruthenium compound 1b undergoes conformational exchange with a rate constant of 8700 s at 298 K, a behavior that is not observed in the meso diastereomer 1a. The series of complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λ 689-783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes for 1a, 1b, and 2, and spectroelectrochemical experiments confirmed assignment of the transient species as arising from MLCT transitions. Unexpectedly, the transient absorption measurements revealed disparate time constants for the excited state decay of diastereomers 1a and 1b.

Mononuclear and Dinuclear Ruthenium Complexes of cis- and trans-Thioindigo: Geometrical and Electronic Structure Analyses

“…The Ru−S bond lengths are in the expected range. 8 According to DFT calculations (Table S3) they change on oxidation but not on reduction.…”

“…7 The availability of several charge forms of L n (n = 0, 1−, 2−) was reported 7 in 2017, and the bis(bidentate) bridging capacity was shown for a bis-phenylimino derivative of thioindigo in 2018 (Figure 1). 8 Our present report uses unaltered L as a chelate and bischelate ligand for the {Ru(acac) 2 } complex fragment (acac − = acetylacetonato) to allow for a structural and electronic comparison with complexes of related indigoid ligands. 9 −11 The compounds obtained were 1 and 2 (Figure 2), and the methods for study included experimental techniques (X-ray diffraction, electrochemistry, UV−vis−NIR−IR (NIR = nearinfrared) spectroelectrochemistry, electron paramagnetic resonance (EPR)) and computations (time-dependent (TD) density functional theory (DFT)).…”

Metal Complexes of Indigo and of Some Related Ligands

“…Hicks and co‐workers reported the synthesis of mono and diruthenium complexes of the thioindigo‐ N,N '‐diphenyldiimine, [(hfac) 2 Ru(thioindigo‐ N,N '‐diphenylimine)Ru(hfac)] and [(hfac) 2 Ru(thioindigo‐diimine)], which were characterized by X‐ray crystallography and they emphasized the absence of thioindigo complexes. Coordination of the neutral thioindigo‐imine occurs through the S and the imine N atoms.…”

Efficient Convergent Energy Transfer in a Stereoisomerically Pure Heptanuclear Luminescent Terpyridine-Based Ru(II)–Os(II) Dendrimer

“…Increased light absorption can be achieved by introducing intermediate metal subunits between the peripheral Ru­(II) and central Os­(II), thereby increasing the nuclearity of the dendrimer. However, crucially, the thermodynamics of the intermediate metal units must be in between those of the peripheral Ru­(II) subunits and the Os­(II) center, or the convergent energy transfer becomes inefficient or inoperative. , Furthermore, most Ru­(II)–Os­(II) polynuclear complexes reported to date were obtained by using bidentate ligands that, unless stereospecific synthetic approaches are used, do not allow for stereo (Λ and Δ) and geometric ( mer and fac ) control. − Hence, mixtures of several diastereoisomers resulted, complicating structural characterization and, in some rare cases, impacting the excited-state properties − and vectorial energy transfer or interaction with chiral molecules. − …”