Simultaneous Bridge-Localized and Mixed-Valence Character in Diruthenium Radical Cations Featuring Diethynylaromatic Bridging Ligands

Multistep Oxidation of Diethynyl Oligophenylamine-Bridged Diruthenium and Diiron Complexes

Homo-dinuclear nonlinear complexes [{M(dppe)Cp*}₂{μ-(-C≡C)₂X}] (dppe = 1,2-bis(diphenylphosphino)ethane; Cp* = η⁵-C₅Me₅; X = triphenylamine (TPA), M = Ru (1a) and Fe (1b); X = N,N,N',N'-tetraphenylphenylene-1,4-diamine (TPPD), M = Ru (2a)) were prepared and characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy and single-crystal X-ray diffraction (1a, 2a). Attempts to prepare the diiron analogue of 2a were not successful. Experimental data obtained from cyclic voltammetry, square wave voltammetry, UV-vis-NIR (NIR = near-infrared) spectro-electrochemistry, and very informative IR spectro-electrochemistry in the C≡C stretching region, combined with density functional theory calculations, afford to make an emphasizing assessment of the close association between the metal-ethynyl termini and the oligophenylamine bridge core as well as their respective involvement in sequential one-electron oxidations of these complexes. The anodic behavior of the homo-bimetallic complexes depends strongly both on the metal center and the length of the oligophenylamine bridge core. The poorly separated first two oxidations of diiron complex 1b are localized on the electronically nearly independent Fe termini. In contrast, diruthenium complex 1a exhibits a significantly delocalized character and a marked electronic communication between the ruthenium centers through the diethynyl-TPA bridge. The ruthenium-ethynyl halves in 2a, separated by the doubly extended and more flexible TPPD bridge core, show a lower degree of electronic coupling, resulting in close-lying first two anodic waves and the NIR electronic absorption of [2a]⁺ with an indistinctive intervalence charge transfer character. Finally, the third anodic waves in the voltammetric responses of the homo-bimetallic complexes are associated with the concurrent exclusive oxidation of the TPA or TPPD bridge cores.

[Al2O4](-), a Benchmark Gas-Phase Class II Mixed-Valence Radical Anion for the Evaluation of Quantum-Chemical Methods

The radical anion [Al2O4](-) has been identified as a rare example of a small gas-phase mixed-valence system with partially localized, weakly coupled class II character in the Robin/Day classification. It exhibits a low-lying C2v minimum with one terminal oxyl radical ligand and a high-lying D2h minimum at about 70 kJ/mol relative energy with predominantly bridge-localized-hole character. Two identical C2v minima and the D2h minimum are connected by two C2v-symmetrical transition states, which are only ca. 6-10 kJ/mol above the D2h local minimum. The small size of the system and the absence of environmental effects has for the first time enabled the computation of accurate ab initio benchmark energies, at the CCSDT(Q)/CBS level using W3-F12 theory, for a class-II mixed-valence system. These energies have been used to evaluate wave function-based methods [CCSD(T), CCSD, SCS-MP2, MP2, UHF] and density functionals ranging from semilocal (e.g., BLYP, PBE, M06L, M11L, N12) via global hybrids (B3LYP, PBE0, BLYP35, BMK, M06, M062X, M06HF, PW6B95) and range-separated hybrids (CAM-B3LYP, ωB97, ωB97X-D, LC-BLYP, LC-ωPBE, M11, N12SX), the B2PLYP double hybrid, and some local hybrid functionals. Global hybrids with about 35-43% exact-exchange (EXX) admixture (e.g., BLYP35, BMK), several range hybrids (CAM-B3LYP, ωB97X-D, ω-B97), and a local hybrid provide good to excellent agreement with benchmark energetics. In contrast, too low EXX admixture leads to an incorrect delocalized class III picture, while too large EXX overlocalizes and gives too large energy differences. These results provide support for previous method choices for mixed-valence systems in solution and for the treatment of oxyl defect sites in alumosilicates and SiO2. Vibrational gas-phase spectra at various computational levels have been compared directly to experiment and to CCSD(T)/aug-cc-pV(T+d)Z data.

Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor-Acceptor Systems by Variation of the Bridge Conformation

Assembling two quadruply bonded dimolybdenum units [Mo2 (DAniF)3 ](+) (DAniF=N,N'-di(p-anisyl)formamidinate) with 1,4-naphthalenedicarboxylate and its thiolated derivatives produced three complexes [{Mo2 (DAniF)3 }2 (μ-1,4-O2 CC10 H6 CO2 )], [{Mo2 (DAniF)3 }2 (μ-1,4-OSCC10 H6 COS)], and [{Mo2 (DAniF)3 }2 (μ-1,4-S2 CC10 H6 CS2 )]. In the X-ray structures, the naphthalene bridge deviates from the plane defined by the two Mo-Mo bond vectors with the torsion angle increasing as the chelating atoms of the bridging ligand vary from O to S. The mixed-valent species exhibit intervalence transition absorption bands with high energy and very low intensity. In comparison with the data for the phenylene analogues, the optically determined electronic coupling matrix elements (Hab =258-345 cm(-1) ) are lowered by a factor of two or more, and the electron-transfer rate constants (ket ≈10(11)  s(-1) ) are reduced by about one order of magnitude. These results show that, when the electron-transporting ability of the bridge and electron-donating (electron-accepting) ability of the donor (acceptor) are both variable, the former plays a dominant role in controlling the intramolecular electron transfer. DFT calculations revealed that increasing the torsion angle enlarges the HOMO-LUMO energy gap by elevating the (bridging) ligand-based LUMO energy. Therefore, our experimental results and theoretical analyses verify the superexchange mechanism for electronic coupling and electron transfer.

Notable differences between oxidized diruthenium complexes bridged by four isomeric diethynyl benzodithiophene ligands

Isomeric benzodithiophenes in the core of a diethynyl bridge have a strong impact on the stability and electronic properties of oxidized diruthenium complexes.

Synthesis and electronic structure of the first cyaphide-alkynyl complexes

The novel complexes trans-[Ru(dppe)2(C≡CR)(C≡P)] (R = CO2Me, C6H4OMe), the first to incorporate cyaphide as part of a conjugated system, are obtained in facile manner. The electronic structure of these compounds is probed by X-ray, DFT and UV/Vis studies.

Synthesis and wire-like performance of diruthenium molecular wire with a C≡C-porphyrin-C≡C linker

A diruthenium complex with a free-base porphyrin linker 1 is synthesized and characterized by 1 H and 31 P NMR, IR, and ESI-TOF-MS spectroscopy. A cyclic voltammogram of 1 shows two reversible waves attributed to the redox processes at the ruthenium centers, and a compropotionation constant (K C ) has been determined to be 1.8 × 105, indicating that a mixed valence state of 1+ is thermodynamically stable. The monocationic complex 1+ obtained by chemical oxidation of 1 by [ Cp 2 Fe ] PF 6 shows an intervalence charge transfer (IVCT) band in the NIR region. On the basis of the electronic coupling ( V ab) of 2644 cm-1 obtained by analysis of the IVCT band, complex 1+• is assigned as a Class III compound according to the Robin–Day classification. DFT calculation and IR study suggest that the strength of π-back donation is one of key determinants for a strong electronic coupling between the two metal centers.

Electronic structure of modelized vs. real carbon-chain containing organometallic dinuclear complexes: similarities and differences

DFT calculations were carried out on the homo- and hetero-bimetallic model wires [(η(5)-C5H5)(dpe)Fe-C≡C-C6H4-C≡C-Fe(dpe)(η(5)-C5H5)] (1'), [(η(7)-C7H7)(dpe)Mo-C≡C-C6H4-C≡C-Mo(dpe)(η(7)-C7H7)] (2'), and [(η(5)-C5H5)(dpe)Fe-C≡C-C6H4-C≡C-Mo(dpe)(η(7)-C7H7)] (3') used to tentatively mimic [(η(5)-C5Me5)(dppe)Fe-C≡C-C6H4-C≡C-Fe(dppe)(η(5)-C5Me5)] (1), [(η(7)-C7H7)(dppe)Mo-C≡C-C6H4-C≡C-Mo(dppe)(η(7)-C7H7)] (2), and [(η(5)-C5Me5)(dppe)Fe-C≡C-C6H4-C≡C-Mo(dppe)(η(7)-C7H7)] (3), respectively in order to analyze the similarities and the differences between models and real compounds previously theoretically and experimentally studied, with respect to their molecular structures and properties. A comparison of the metrical data computed for the models and the real systems shows some slight discrepancy between the metal-ancillary ligand distances - shorter distances are observed in the formers - but comparable metal-Cα and Cα-Cβ distances. Incidentally, distances computed for the model molecules match more closely those measured experimentally. Replacement of a dppe ligand tethered to the metal centers by a dpe group does not much alter the electronic properties. Therefore, overall, data obtained for the Mo2 models 2' compare rather well with those computed for the real systems 2. Larger alteration is noticed when Cp* is substituted by Cp, even if the general trends observed for the real iron species 1 and 3 are kept overall for the iron models 1' and 3'. Indeed, the smaller electron-donor properties of Cp affect somewhat the nodal properties of the HOMOs (less metallic character) and increase the HOMO-LUMO gaps and the ionization potentials. Despite this, similarities between models and real compounds largely overtake differences. It is shown that calculations on models provide quite acceptable results.

New donor–acceptor conjugates based on a trifluorenylporphyrin linked to a redox–switchable ruthenium unit

Spectroelectrochemical studies show four accessible redox states on dyad 3 with the first oxidation process taking place at the Ru(ii) centre and the second oxidation and first reduction processes at the porphyrin centre.

The influence of bridge-reduced state levels on the electron transfer within the 2,7-dinitroanthracene radical anion

The 2,7-dinitroanthracene radical anion has an (apparent) electronic coupling much higher than either itsN,Ndistance or its non-Kekule substitution would suggest. The results can only be explained if a low-lying bridge redox state is influencing the electron transfer.

Conformation-determined through-bond versus through-space electronic communication in mixed-valence systems with a cross-conjugated urea bridge

Bis-triarylamine 2 and cyclometalated diruthenium 6(PF6)2 with a linear trans,trans-urea bridge have been prepared, together with the bis-triarylamine 3 and cyclometalated diruthenium 8(PF6)2 with a folded cis,cis-N,N-dimethylurea bridge. The linear or folded conformations of these molecules are supported by single-crystal X-ray structures of 2, 3, and other related compounds. These compounds display two consecutive anodic redox waves (N(·+/0) or Ru(III/II) processes) with a potential separation of 110-170 mV. This suggests that an efficient electronic coupling is present between two redox termini through the cross-conjugated urea bridge. The degree of electronic coupling has been investigated by using spectroelectrochemical measurements. Distinct intervalence charge-transfer (IVCT) transitions have been observed for mixed-valent (MV) compounds with a linear conformation. The IVCT transitions can also be identified for the folded MV compounds, albeit with a much weaker intensity. DFT results support that the electronic communication occurs by a through-bond and through-space pathway for the linear and folded compounds, respectively. The IVCT transitions of the MV compounds have been reproduced by TDDFT calculations. For the purpose of comparison, a bistriarylamine and a diruthenium complex with an imidazolidin-2-one bridge and a urea-containing mono-triarylamine and monoruthenium complex have been synthesized and studied.

Resonance Raman spectroscopic study of solvent-dependent coexistence of localized and delocalized dinitroaromatic radical anions

Optical absorption spectra of three dinitroaromatic radical anions, 2,7-dinitro-9,9-dimethylfluorene (1•−), 4,4′-dinitrobiphenyl (2•−), and 4,4′-dinitrotolane (3•−) in solvents THF, HMPA, and MeCN show both an unresolved broad band characteristic of charge-localized mixed valence species (Robin-Day classification Class II), and a vibronically structured band of the delocalized species (Class III). With decreasing solvent reorganization energy, a greater portion of the compounds become charge-delocalized. In particular, 1•− with the greatest coupling, is almost entirely composed of the delocalized species in all solvents. An intense Raman mode is used to identify the charge-bearing unit as the nitrobenzene moiety. Resonance Raman profiles are utilized to gain detailed information of vibrational modes. The out-of-phase ring lengthening stretch mode is observed to be enhanced at higher excitation energies, corresponding to the absorption band of the Class II species, while the in-phase ring lengthening stretch mode, a totally symmetric vibration, is most strongly enhanced in the absorption region of the Class III species. Resonance Raman profiles support the solvent-dependent coexistence of Class II and Class III molecules of the same chemical composition.

Three distinct redox states of an oxo-bridged dinuclear ruthenium complex

A series of [{(terpy)(bpy)Ru}(μ-O){Ru(bpy)(terpy)}](n+) ([RuORu](n+), terpy=2,2';6',2''-terpyridine, bpy=2,2'-bipyridine) was systematically synthesized and characterized in three distinct redox states (n=3, 4, and 5 for Ru(II,III)2, Ru(III,III)2, and Ru(III,IV)2, respectively). The crystal structures of [RuORu](n+) (n=3, 4, 5) in all three redox states were successfully determined. X-ray crystallography showed that the Ru-O distances and the Ru-O-Ru angles are mainly regulated by the oxidation states of the ruthenium centers. X-ray crystallography and ESR spectra clearly revealed the detailed electronic structures of two mixed-valence complexes, [Ru(III)ORu(IV)](5+) and [Ru(II)ORu(III)](3+), in which each unpaired electron is completely delocalized across the oxo-bridged dinuclear core. These findings allow us to understand the systematic changes in structure and electronic state that accompany the changes in the redox state.

Tuning the electronic coupling in cyclometalated diruthenium complexes through substituent effects: a correlation between the experimental and calculated results

A common bridging ligand, 3,3',5,5'-tetrakis(N-methylbenzimidazol-2-yl)biphenyl, and four terpyridine terminal ligands with various substituents (amine, tolyl, nitro, and ester groups) have been used to synthesize ten cyclometalated diruthenium complexes 1(2+) -10(2+) . Among them, compounds 1(2+) -6(2+) are redox nonsymmetric, and others are symmetric. These complexes show two Ru(III/II) processes and an intervalence charge transfer (IVCT) transition in the one-electron oxidized state. The potential separation (ΔE) of 1(2+) -10(2+) has been correlated to the energy difference ΔG(0) , the energy of the IVCT band Eop , and the ground-state delocalization coefficient α(2) . Time-dependent (TD)DFT calculations suggest that the absorptions in the visible region of 1(2+) -6(2+) are mainly associated with the metal-to-ligand charge-transfer transitions from both ruthenium ions and to both terminal ligands and the bridging ligand. However, the energies of these transitions vary significantly. DFT calculations have been performed on 1(2+) -6(2+) and 1(3+) -6(3+) to give information on the electronic structures and spin populations of the mixed-valent compounds. The TDDFT-predicted IVCT excitations reproduce well the experimental trends in transition energies. In addition, three monoruthenium complexes have been synthesized for a comparison study.

Quantum-chemical insights into mixed-valence systems: within and beyond the Robin–Day scheme

The application of quantum-chemical methods to both organic and transition-metal mixed-valence systems is reviewed, with particular emphasis on how to describe correctly delocalisation vs. localisation near the borderline between Robin–Day classes II and III.