Electronic Structures of Divinylchalcogenophene‐Bridged Biruthenium Complexes: Exploring Trends from O to Te

Abstract

An homologous series of divinylchalcogenophene‐bridged binuclear ruthenium complexes [{(PMe3)3Cl(CO)Ru}2(µ‐CH=CH‐C4H2E‐CH=CH)] (4a–4d, E = O, S, Se, Te) have been synthesised and fully characterised by X‐ray crystallography and various spectroscopic techniques. The single‐crystal X‐ray diffraction results reveal a distinct short/long bond‐length alternation along the polyene‐like hydrocarbon backbone, with geometric constraints imposed by the chalcogenophene leading to an increasing distance between the two metal centres (dRu–Ru) in complexes 4a–4d as the heteroatom in the five‐membered ring is changed from oxygen (9.980 Å in 4a) to tellurium (11.063 Å in 4d). The complexes undergo two sequential one‐electron oxidation processes, the half‐wave potential and separation of which appear to be sensitive to a range of factors, including aromatic stabilisation and re‐organisation energies. Analysis of [4a–4d]n+ (n = 0, 1, 2) by UV/Vis/NIR and IR spectroelectrochemical methods, supported by DFT calculations (n = 0, 1), revealed that the redox character of the complexes is dominated by the polyene‐like backbone with the chalcogenide playing a subtle but influential, structural rather than electronic, role. In the radical cations [4a–4d]+, the charge is rather effectively delocalised over the 10‐atom Ru–[4‐s‐cis‐all‐trans‐(CH=CH)4]–Ru chain, giving rise to a species with spectroscopic properties not dissimilar to oxidised polyaceylene.

Influence of Quinoidal Distortion on the Electronic Properties of Oxidized Divinylarylene-Bridged Diruthenium Complexes

“…1000–1300 nm in the near-infrared (NIR), while their associated dications absorb strongly in the borderline régime of the visible (vis) and the NIR. Significantly different absorption profiles of three individually addressable redox states thus render such complexes polyelectrochromic. − ,− ,,, These absorptions are further amplified in macrocyclic architectures due to restricted rotations around the Ru–C α – and C β –arylene bonds. ,− The electronic spectra of the new complexes of this study were therefore probed by UV/vis/NIR spectroscopy.…”

“…They usually undergo two consecutive, well-behaved one-electron oxidations, which gives rise to three individually addressable oxidation states. Redox splittings, Δ E 1/2 = E 1/2 +/2+ – E 1/2 0/+ , between the individual redox waves depend on the arylene linker, − the points of vinylruthenium attachment to the arylene “bridge”, , and, quite strongly, on the electronic properties of the arene substituents. − The latter is a direct consequence of dominant bridge contributions to their HOMO, which renders the bridging ligands redox noninnocent. ,,,,,− This has several interesting consequences. One is the observation of charge and spin delocalization in their mixed-valent (MV) state(s) despite rather modest values of Δ E 1/2 , in particular in complexes with π-extended bridging ligands.…”

“…These hfs patterns provide complementary information on the spin density distribution. One further consequence of large bridge contributions to the HOMO is the appearance of intense, structured absorption bands in the visible (vis) and the near-infrared (NIR) for their associated radical cations and dications, which correspond to electronic transitions between delocalized orbitals of the oxidized, π-conjugated metal–organic chromophore. − ,− ,,, …”

Redox Isomeric Ferrocenyl Styrylruthenium Radical Cations with Diphenyl-Substituted β-Ketoenolato Ligands

“…The carbonyl ligand at the Ru atom provides a sensitive and convenient spectroscopic label to monitor any change of the electron density at the styrylruthenium entity accompanying a redox process. Oxidation-induced shifts of the energy of the CO stretching vibration v ̃(CO) have proven highly useful for probing the metal versus ligand contributions to the HOMO of alkenylruthenium complexes − and the degree of charge (de)­localization in the one-electron oxidized, mixed-valent forms of divinylarylene-bridged ruthenium complexes with identical ,− or chemically different ,,,− redox sites.…”

“…Hence, oxidized alkenylruthenium complexes display a prominent NIR absorption, which is due to a transition from the β-HOSO (the HOMO–1 of the neutral complexes) to the β-LUSO (the former HOMO). Both of these orbitals are delocalized over the entire π-conjugated metal-arylalkenyl backbone. ,− ,,,,,− However, ferrocenium ions with an attached π-conjugated aryl substituent also exhibit a fairly intense NIR band in a similar energy régime as has been pointed out by Simpson et al − Overlap of only one orbital of the set of two mutually mixed metal d π /d δ //Cp/ π/δ orbitals with the π-MOs of the corresponding substituent lifts the degeneracy of the two π and δ MOs and renders the otherwise symmetry-forbidden transition between the doubly occupied π orbital and the δ SOMO of the radical cations observable. In the present situation, the underlying band will most likely be associated with some charge transfer from the bridging ligand ,− and the remaining nonoxidized redox site to the oxidized one, such that its assignment as an intervalence charge-transfer transition (IVCT) seems appropriate.…”