Electron-Rich Piano-Stool Iron σ-Acetylides. Electronic Structures of Arylalkynyl Iron(III) Radical Cations

Controlling Redox and Wirelike Charge-Delocalization Properties of Dinuclear Mixed-Valence Complexes with MCp*(dppe) (M = Fe, Ru) Termini Bridged by Metalloporphyrin Linkers

Four dinuclear organometallic molecular wire complexes with diethynylmetalloporphyrin linkers 1 MM' , [5,15-bis{MCp*(dppe)ethynyl}-10,20-diarylporphinato]M' (Cp* = η5-C5Me5; dppe = 1,2-bis(diphenylphosphino)ethane; M/M' = Fe/Zn (1 FeZn ), Ru/Zn (1 RuZn ), Fe/Ni (1 FeNi ), Ru/Ni (1 RuNi ); aryl = 3,5-di-tert-butylphenyl), are synthesized and characterized by NMR, CV, UV-vis-NIR, and ESI-TOF mass spectrometry techniques. Electrochemical investigations combined with electronic absorption spectroscopic studies reveal strong interactions among the electron-donating, redox-active MCp*(dppe) termini and the metalloporphyrin moieties. The monocationic species of the four complexes obtained by chemical oxidation have been characterized as mixed-valence Class II/III or Class III compounds according to the Robin-Day classification despite the long molecular dimension (>1.5 nm), as demonstrated by their intense intervalence charge transfer bands (IVCT) in the near IR region. DFT calculations indicate large spin densities on the metalloporphyrin moieties. Furthermore, the wirelike performance can be finely tuned by coordination of appropriate nitrogen donors to the axial sites of the metalloporphyrin.

Functionalised organometallic photoswitches containing dihydropyrene units

Functionalising organic molecular photoswitches with metal complexes has been shown to alter and enhance their switching states. These organometallic photoswitches provide a promising basis for novel smart molecular materials and molecular electronic devices. We have detailed the synthesis and characterisation of mono- and bimetallic half-sandwich ruthenium and iron complexes functionalised with alkynyl dihydropyrenes (DHP). Their electronic and photophysical properties were determined by the use of chemical, electrochemical and spectroelectrochemical techniques. The introduction of the metal alkynyl moiety allows access to additional redox and protonation states not accessible by the DHP alone. An additional metal alkynyl moiety inhibits observable photochromic switching. Analysis of the NIR and IR bands in the mixed valence complexes suggests there is a high degree of charge delocalisation across the DHP.

Iron vs. ruthenium: syntheses, structures and IR spectroelectrochemical characterisation of half-sandwich Group 8 acetylide complexes

A synthetic ‘trick’ affording complexes [M(CCR)(dppe)Cp′] (M = Fe, Ru) in high purity directly from the reaction vessel is described.

Mix and (Mis)match: further studies of the electronic structure and mixed-valence characteristics of 1,4-diethynylbenzene-bridged bimetallic complexes

The 1,4-diethynylbenzene motif is commonly employed as a bridging ligand in bimetallic molecular systems intended to show pronounced intramolecular electronic interactions, delocalized electronic structures and 'wire-like' properties between the metal fragments at the ligand termini. In contrast to these expectations, the donor-acceptor compounds [{Cp'(CO)xM'}(μ-C[triple bond, length as m-dash]CC6H4C[triple bond, length as m-dash]C){M(PP)Cp'}]n+ [n = 0, 1; M'(CO)xCp' = Fe(CO)2Cp, W(CO)3Cp*; M(PP)Cp' = Fe(dppe)Cp, Fe(dppe)Cp*, Ru(PPh3)2Cp, Ru(dppe)Cp, Ru(dppe)Cp*] display remarkably little bridge-mediated electronic interaction between the electron-rich {M(PP)Cp'} and electron-poor {M'(CO)xCp'} fragments in the ground state. However, a relatively high-energy (26 000-30 000 cm-1) M-to-M' charge transfer can be identified. One-electron oxidation is largely localized on the {M(C[triple bond, length as m-dash]CR)(PP)Cp'} fragment and gives rise to a new charge transfer band with bridging-ligand-to-{M(PP)Cp'}+ (M'(CO)xCp' = Fe(CO)2Cp) or M'-to-M(+) (M(CO)xCp' = W(CO)3Cp*) character. The localized electronic ground state of these complexes is better revealed through analysis of the IR spectra, taking advantage of the well-resolved ν(C[triple bond, length as m-dash]C) and ν(CO) bands and IR spectroelectrochemical methods, than through the more classical analysis based on the concepts of Marcus-Hush theory and analysis of the putative IVCT electronic transition. The conclusions are supported by DFT calculations using the BLYP35 functional.

A Spectroscopic and Computationally Minimal Approach to the Analysis of Charge-Transfer Processes in Conformationally Fluxional Mixed-Valence and Heterobimetallic Complexes

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']}²⁺ (M, M'=Ru, Fe; PP=dppe, (PPh₃ )₂ ; Cp'=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d-d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus-Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry.

Redox-Active Chiroptical Switching in Mono- and Bis-Iron Ethynylcarbo[6]helicenes Studied by Electronic and Vibrational Circular Dichroism and Resonance Raman Optical Activity

Introducing one or two alkynyl-iron moieties onto a carbo[6]helicene results in organometallic helicenes (2 a,b) that display strong chiroptical activity combined with efficient redox-triggered switching. The neutral and oxidized forms have been studied in detail by electronic and vibrational circular dichroism, as well as by Raman optical activity (ROA) spectroscopy. The experimental results were analyzed and spectra were assigned with the help of first-principles calculations. In particular, a recently developed method for ROA calculations under resonance conditions has been used to study the intricate resonance effects on the ROA spectrum of mono-iron ethynylhelicene 2 a.

Theoretical investigation on second-order nonlinear optical properties of ruthenium alkynyl-dihydroazulene/vinylheptafulvene complexes

Ru metal acetylide electron donor-acceptor complexes have important applications in the field of nonlinear optics. Herein, in this work, a series of half-sandwich ruthenium-based Cp*(dpe)Ru ([Ru*]) metal complexes with the dihydroazulene/vinylheptafulvene (DHA/VHF) have been investigated by density functional theory (DFT) calculations. The results showed that the position of the [Ru*] acetylide functionality, either para or meta on the phenylene ring to the DHA/VHF core (1c/1o and 2c/2o), and additional a p-phenylene spacer (3c/3o) had a great influence on the second-order nonlinear optical (NLO) responses. The systems 1 and 3 can significantly increased second-order NLO responses compared with system 2. It was attributed to the more obvious charge transfer along y-axis, which is from [Ru*] acetylide functionality to DHA, accompanied by a significant decrease of the transition energy according electron density difference maps and time-dependent DFT calculations. The β_vec values of the open-ring complexes were larger than the corresponding closed-ring complexes owing to the smaller HOMO-LUMO gap in the open-ring complexes. It was also because of the smaller BLA values in open-ring complexes, which had stronger π-conjugation. Especially, the change ratio of β_vec value of system 2 was the largest due to the fact that their charge transfers degree varied greatly. In addition, the frequency-dependent NLO properties of the studied complexes were evaluated at 0.0239 a.u. and 0.0340 a.u. The calculation results demonstrated that the magnitude of the frequency-dependent first hyperpolarizability increased with the increasing frequency. We believe that our present work will be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

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.

Easy and quantitative access to Fe(ii) and Fe(iii) di(aryl)alkynylphosphine oxides featuring [Fe(dppe)Cp*] endgroups: terminal PO functionality blocks the dimerisation of the Fe(iii) derivatives

Preventing the oxidatively-induced dimerization of Fe(iii) metalloalkynyl phosphines can be achieved by sequestering the phosphorus lone pair.

Long range charge transfer in trimetallic mixed-valence iron complexes mediated by redox non-innocent cyanoacetylide ligands

A redox active 9-atom, 10-bond –CC–CN{Fe(dppx)₂}NC–CC– bridge is shown to be effective at promoting electron transfer between metal centres separated by some 15 Å.

Hybrid molecular systems containing tetrathiafulvalene and iron-alkynyl electrophores: five-component functional molecules obtained from C-H bond activation

Treatment of [Cp*(dppe)Fe-C≡C-TTFMe3] (1) with Ag[PF6] (3 equiv) in DMF provides the binuclear complex [Cp*(dppe)Fe=C=C=TTFMe2 =CH-CH=TTFMe2 =C=C=Fe(dppe)Cp*][PF6]2 (2[PF6 ]2) isolated as a deep-blue powder in 69 % yield. EPR monitoring of the reaction and comparison of the experimental and calculated EPR spectra allowed the identification of the radical salt [Cp*(dppe)Fe=C=C=TTFMe2 =CH][PF6]2 ([1-CH][PF6]) an intermediate of the reaction, which results from the activation of the methyl group attached in vicinal position with respect to the alkynyl-iron on the TTF ligand by the triple oxidation of 1 leading to its deprotonation by the solvent. The dimerization of [1-CH][PF6] through carbon-carbon bond formation provides 2[PF6]2. The cyclic voltammetry (CV) experiments show that 2[PF6]2 is subject to two sequential well-reversible one-electron reductions yielding the complexes 2[PF6] and 2. The CV also shows that further oxidation of 2[PF6]2 generates 2[PF6]n (n=3-6) at the electrode. Treatment of 2[PF6]2 with KOtBu provides 2[PF6] and 2 as stable powders. The salts 2[PF6] and 2[PF6]2 were characterized by XRD. The electronic structures of 2(n+) (n=0-2) were computed. The new complexes were also characterized by NMR, IR, Mössbauer, EPR, UV/Vis and NIR spectroscopies. The data show that the three complexes 2[PF6]n are iron(II) derivatives in the ground state. In the solid state, the dication 2(2+) is diamagnetic and has a bis(allenylidene-iron) structure with one positive charge on each iron building block. In solution, as a result of the thermal motion of the metal-carbon backbone, the triplet excited state becomes thermally accessible and equilibrium takes place between singlet and triplet states. In 2[PF6], the charge and the spin are both symmetrically distributed on the carbon bridge and only moderately on the iron and TTFMe2 electroactive centers.