Metal-to-ligand Charge Transfer Band Research Articles

Electronic spectra of solid pentakis(dimethylamido)tantalum(V)

The electronic spectra of the complex Ta V(NMe 2) 5 are characterized by amide → Ta( V) ligand-to-metal charge transfer (LMCT) transitions. In the reflectance and excitation spectrum the longest-wavelength LMCT band appears at λ max∼415 nm. The emission of Ta(NMe 2) 5 which originates from the lowest-energy LMCT triplet occurs at λ max=578 nm and decays with τ=0.9 μs at r.t.

Photoreduction of methemoglobin by irradiation in the near-ultraviolet region.

Ferric metHb can be photoreduced to the ferrous state by direct photoexcitation in the near-ultraviolet region. In this research, we studied the mechanism and facilitating conditions for the photoreduction and the resulting restoration of O(2) binding. MetHb in phosphate-buffered saline or pure water in a CO atmosphere was photoreduced to form HbCO by illuminating the N band (365 nm), one of the porphyrin pi --> pi transitions, whereas the photoreduction did not occur in Ar, N(2), or O(2). The transient absorption spectrum exhibited the generation of deoxyHb within 30 ns in both the CO and Ar atmospheres; however, only in CO did the subsequent CO binding inhibit the back reaction. The photoreduction rate was dependent on the pH and ligand anions, showing that aquametHb in the high-spin state was predominant for the photoreduction. Axial ligand-to-metal charge-transfer (LMCT) bands overlap with the Soret and Q bands in metHb; however, the excitation of these bands showed little photoreduction, indicating that the contribution of these LMCT bands is minimal. Excitation of the N band significantly contributes to the photoreduction, and this is facilitated by the external addition of mannitol, hyaluronic acid, Trp, Tyr, etc. Especially, Trp allowed the photoreduction even in an Ar atmosphere, and the reduced Hb can be converted to HbO(2) by O(2) bubbling. One mechanism of the metHb photoreduction that is proposed on the basis of these results consists of a charge transfer from the porphyrin ring to the central ferric iron to form the porphyrin pi cation radical and ferrous iron by the N band excitation, and the contribution of the amino acid residues in the globin chain as an electron donor or an electron pathway.

Ruthenium(II) complexes of α-diimines: synthesis, spectral characterisation, electrochemical properties and single-crystal X-ray structure of bis(2,2′-bipyridine){1-benzyl-2-( p-tollylazo)imidazole}ruthenium(II) perchlorate

Mixed-ligand tris-chelates, [Ru(bpy) 2(RaaiR′)](ClO 4) 2·H 2O (bpy=2,2′-bipyridine; RaaiR′=1-alkyl-2-(arylazo)imidazole) have been synthesized by a silver-assisted route [Ru(bpy) 2Cl 2+RaaiR′+AgNO 3+NaClO 4/Ru(bpy) 2Cl 2+[Ag(RaaiR′) 2](ClO 4)]. The structures of the complexes have been supported by the single-crystal X-ray diffraction study and the stereochemistry are assessed by 1H NMR spectral data. All the complexes show two metal-to-ligand charge transfer (MLCT) transitions in the visible region at ca. 400 and 500 nm and their emission spectra are also described here. Cyclic voltammetric studies exhibit high-potential Ru III/II (1.4 V vs. SCE) and three successive ligand reductions at negative to SCE in acetonitrile solution. The difference in the first metal and successive ligand redox potentials are linearly correlated with the ν CT of MLCT band.

Resonance Raman study of the solvatochromic electronic transitions of [Ru(NH3)4bipyridine]2+ in methanol and dimethylsulfoxide

The electronic transitions of [Ru(NH3)4bipyridine]2+(R4AB) in the visible region are solvatochromic due to hydrogen bonding interactions with the solvent. In this work, we employ resonance Raman and absorption spectroscopy to separate the static and dynamic contributions to the solvatochromic shift. Raman excitation profiles for R4AB in methanol (MeOH) and dimethylsulfoxide (DMSO) were obtained at wavelengths within the lowest energy absorption band, which comprises two overlapping metal-to-ligand charge transfer (MLCT) transitions (the red band), and preresonant with a higher energy blue band. The absorption and Raman profiles of R4AB were analyzed using time-dependent theory to determine the 0–0, internal, and solvent reorganization energies, the sum of which is the energy of maximum absorption. It is concluded that the solvatochromic shift in the transition energy of each of the two visible MLCT bands is due mainly to changes in the 0–0 energy, while the internal and solvent reorganization energies are found to be similar in MeOH and DMSO. Preresonance enhancement via the blue band is larger in methanol than in DMSO. The question of the composite nature of the visible absorption band is addressed by modeling the low-temperature absorption spectrum, where two peaks in the red are resolved. The dimensionless displacements derived from the Raman modeling are shown to be consistent with attributing the structure to two overlapping electronic transitions rather than a vibrational progression. Depolarization ratio dispersion was found to be difficult to model, probably due to strong preresonance enhancement which is only accounted for phenomenologically in this work.

A spectroscopic, electrochemical and structural study of polarizable, dipolar ruthenium(II) arsine complexes as models for chromophores with large quadratic non-linear optical responses

The new series of complex salts trans-[RuIICl(pdma)2L][PF6]2 [pdma = 1,2-phenylenebis(dimethylarsine); L = N-methyl-4,4′-bipyridinium (MeQ+) 2, N-phenyl-4,4′-bipyridinium (PhQ+) 3, N-(4-acetylphenyl)-4,4′-bipyridinium (4-AcPhQ+) 4, N-(2,4-dinitrophenyl)-4,4′-bipyridinium (2,4-DNPhQ+) 5 or N-(2-pyrimidyl)-4,4′-bipyridinium (2-PymQ+) 6] have been prepared. The known complex salt trans-[RuIICl(pdma)2(4,4′-bpy)]PF6 (4,4′-bpy = 4,4′-bipyridine) 1 exhibits an intense dπ(RuII)→π*(4,4′-bpy) metal-to-ligand charge-transfer (MLCT) absorption with λmax at 418 nm in acetonitrile, whilst 2–6 display dπ(RuII)→π*(L) MLCT bands with λmax values in the region 486–544 nm. The MLCT energy decreases as the electron-accepting ability of L increases, in the order L = MeQ+ < PhQ+ < 4-AcPhQ+ < 2,4-DNPhQ+ < 2-PymQ+. Cyclic voltammetric studies show that within the series 2–6, the energy of the Ru-based HOMO is almost constant, whilst that of the L-based LUMO decreases by ca. 0.4 eV moving from 2 to 6. Single-crystal structures of the complete series 1·DMF, 2, 3·MeCN, 4·Me2CO, 5·MeCN and 6 have been determined. Analysis of bond lengths and dihedral angles provides no evidence for ground state charge-transfer, despite the strongly dipolar, polarizable nature of these complexes.

Synthesis, properties and Mössbauer spectra of bisaxially co-ordinated iron(II) phthalocyanine low-spin complexes: the first semi-quantitative explanation of the influence of the character of axial ligands on the spectral parameters †

A series of bisaxially co-ordinated low-spin iron(II) phthalocyanine complexes (pc)FeL2 with different types of nitrogen bases as axial ligands has been prepared and characterised by electronic, 1H NMR and Mossbauer spectroscopies. The influence of electronic and steric effects of the axial ligands on Mossbauer quadrupole splitting (ΔEQ) and isomer shift (δ), NMR parameters, and metal-to-ligand charge transfer (MLCT) band position are discussed. Mossbauer partial quadrupole splitting (p.q.s.) and partial isomer shift (p.i.s.) parameters for different ligands have been estimated and rationalised with the overall data for (pc)FeL2 and (nx)2FeL2 (nx = nioxime) complexes. A semi-empirical AM1 method and the cone angle concept were used to factorise σ- and π-electronic and steric effects of the ligands. A good correlation between the predicted and experimental MLCT band position of (pc)FeL1L2 complexes, as well as predicted and experimental p.i.s. and p.q.s. values for different ligands, has been observed.

Skirting the oxo-wall: characterization and catalytic reactivity of binuclear Co 2+/3+ 1,2-bis(2-hydroxybenzamido)benzene complexes with comparison to their isostructural Fe 2+/3+ analogs. Implications of d-electron count on oxygen atom transfer catalysis

The reaction of Co 2+( N-MeIm) 2(Cl) 2 ( 1), with Li 2H 2Hbab (H 4Hbab=1,2-bis(2-hydroxybenzamido)benzene) affords the binuclear complex [Co 2 2+ (H 2Hbab) 2( N-MeIm) 2] ( 2). Single crystal X-ray crystallography shows that the two Co 2+ centers in 2 are related by a crystallographically imposed center of symmetry with the coordination sphere around each metal center composed of amide oxygens, terminal and bridging phenolate oxygen atoms and a single nitrogen from N-methylimidazole. 2 is isostructural to the Fe 2+ complex, [Fe 2(H 2Hbab) 2( N-MeIm) 2] ( 5), previously reported from our laboratory (A. Stassinopoulos, G. Schulte, G.C. Papaefthymiou, J.P. Caradonna, J. Am. Chem. Soc. 113 (1991) 8686–8697). Stoichiometric iodometric oxidation of 2 yields the mixed valence complex [Co 2+Co 3+(H 2Hbab) 2( N-MeIm) 2] + ( 3), and the oxidized complex [Co 2 3+ (H 2Hbab) 2( N-MeIm) 2] 2+ ( 4). The UV–Vis spectrum of 2 shows ligand field transitions at 600 nm ( ε M=160) and 540 nm ( ε M=120) and a phenolate-to-Co 2+ ligand-to-metal charge-transfer (LMCT) band at 327 nm ( ε M=23 600). Oxidation of the dicobalt core results in a bathchromic shift of the LMCT band ( 3: 305 nm ( ε M=17 400), 334 nm (sh, ε M=14 500); 4: 295 nm ( ε M=21 000), 357 nm ( ε M=15 000)). EPR spectra of 2 and 3 at 4 K show broad resonances from 500 to 4500 Gauss indicative of the presence of strong zero-field splitting effects, while 4 is EPR silent. Solution magnetic susceptibility measurements for 2 ( S 1= S 2=3/2; μ B=6.90) are consistent with a weakly interacting high-spin Co 2+ dimer. Analogous measurements for 3 ( S 1=3/2, S 2=0; μ B=4.78) and 4 ( S 1= S 2=0; μ B=0.25) indicate the presence of a single high-spin Co 2+ for 3 and a diamagnetic core for 4, consistent with the latter’s 1H NMR spectrum. Complexes 2– 4 exhibit the ability to catalytically oxidize a variety of organic substrates (olefins, sulfides) using iodosylbenzene (OIPh) as oxygen atom donor. Reactions with olefin substrates (norbornene, cyclooctene, styrene) yield solely epoxide products while cyclohexene afforded 85% epoxide with small amounts (15%) of allylic oxidation products. Oxygen atom transfer to olefins occurs with high but not exclusive retention of stereochemistry. The mechanistic implications of the significantly different product distributions obtained with the Co 2+ (d 7) dimer, 2, versus the isostructural Fe 2+ (d 6) dimer, 5 (A. Stassinopoulos, J.P. Caradonna, J. Am. Chem. Soc. 112 (1990) 7071–7073), are presented. These data strongly infer the operation of two independent mechanisms for the Co 2+ and Fe 2+ catalyzed reactions.

Synthesis and Electronic Spectra of Tricarbonylrhenium(I) Chloride Complexes with 9,10-Phenanthrenequinone and 9,10-Phenanthrenequinone Diimine as Acceptor Ligands

The complexes Re(L-L)(CO)3Cl with L-L = 9,10-phenanthrenequinone (PHQ) and 9,10- phenanthrenequinone diimine (PHI) were prepared and characterized spectroscopically. Both compounds show intense Re(I) to L-L metal-to-ligand charge transfer (MLCT) absorptions. In chloroform, the MLCT bands appear at 711 nm (L-L = PHQ) and 553 nm (L-L = PHI).

Electronic spectra of 1,2-diiminetricarbonylrhenium(I)chloride complexes with imidazole derivatives as ligands

A series of Re(CO) 3(1,2-diimine)Cl complexes with bisimidazoles (BIIM) as 1,2-diimines have been synthesized and their electronic spectra studied. Compared with the absorption spectra of the free BIIM ligands, the spectra of the metal complexes display a shoulder at the low energy side of the intraligand (IL) bands. This additional absorption is assigned to the Re(I) to π* (BIIM) metal-to-ligand charge transfer (MLCT) transition. Since imidazoles are weak π-acceptor ligands, the MLCT band occurs at rather short wavelengths and is partially hidden by the intense IL bands of the BIIM moiety. The complexes are photoluminescent both at room temperature and at 77 K.

Detailed Spectroscopic and Theoretical Studies on [Fe(EDTA)(O2)]3-: Electronic Structure of the Side-on Ferric−Peroxide Bond and Its Relevance to Reactivity

A spectroscopic study of the Fe(III)−EDTA−peroxide complex using electron paramagnetic resonance (EPR), low-temperature absorption (LT-ABS), variable-temperature−variable-field (VTVH) magnetic circular dichroism (MCD), and resonance Raman (rR) spectroscopies is reported. Density functional (DFT) and INDO/S-CI molecular orbital (MO) calculations are used to derive an experimentally calibrated bonding scheme. The molecule is described as a 6-coordinate, mononuclear, high-spin ferric−peroxide complex with a side-on η2-FeO2 arrangement. EPR spectroscopy shows that the zero-field splitting (ZFS) is negative with D = −1.0 ± 0.25 cm-1 and E/D = 0.21. MCD and LT-ABS spectroscopies lead to the identification of at least six excited states below 35 000 cm-1. The lowest two are assigned as ligand field sextet→quartet transitions, and the remaining transitions have peroxide to iron ligand-to-metal charge transfer (LMCT) character. The polarization of the LMCT bands in the principal axis system of the D-tensor are der...

Asymmetrically substituted ferrocene derivatives showing redox switching of optical properties

The electrochemical and spectroelectrochemical behaviors of three ferrocene derivatives, ( 1) (C 18H 37) 2NC 6H 4CHCHFcCH 2OH, ( 2) (C 18H 37) 2NC 6H 4CHCHFcCHO, ( 3) (C 18H 37) 2NC 6H 4CHCHFcCHC(CN) 2, were studied and analyzed on basis of frontier orbital interactions. It was shown that all the three derivatives show two oxidation steps. The first oxidized states of 1 and 2 are stable and show strong LMCT (ligand-to-metal charge transfer) bands. This suggests that they may serve as redox switching of optical properties. In contrast, the first oxidized state of 3 becomes unstable due to strong electron-withdrawing effect of the acceptor substitute.

Metal-to-Ligand Charge Transfer in the Gas-Phase Cluster Limit

The solvent dependence of metal-to-ligand charge transfer (MLCT) in the bis(2,2‘,2‘‘-terpyridyl)iron(II) complex, [Fe(terpy)2]2+, isolated in small gas-phase clusters with one or four molecules of the polar, organic solvents acetone, acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, methanol, and pyridazine is reported. The shift in the maximum of the MLCT band, Eop, for [Fe(terpy)2·(solvent)1]2+ clusters, measured using laser photofragmentation mass spectrometry, relative to the corresponding values in bulk solution ranges from +601 cm-1 for acetone to +764 cm-1 for pyridazine. The solvent dependence of the outer-sphere reorganization energy predicted by a dielectric continuum model provides a context for comparing Eop values determined for MLCT in [Fe(terpy)2·(solvent)n]2+ clusters (n = 1, 4) with those measured in solution. A model derived from Kirkwood's equation for the mutual electrostatic interaction energy of an ion and a polar medium predicts that the solvent reorganization energy associated with MLCT in [Fe(terpy)2]2+ is a linear function of (1 − Dop)/(2Dop+ 1), where Dop is the optical dielectric constant of the bulk solvent. A linear relationship between Eop and (1 − Dop)/(2Dop+ 1) is observed not only in the bulk solvents, as anticipated, but also in clusters containing as few as four solvent molecules.

Low-spin octahedral cobalt(II) complexes of CoN6 and CoN4P2 chromophores. Synthesis, spectroscopic characterisation and electron-transfer properties †

The reaction of 2-(arylazo)pyridines (NC5H4)NNC6H4R L1–L7 (R = H, o-Me/Cl, m-Me/Cl, p-Me/Cl) with cobalt(II) perchlorate hexahydrate in absolute ethanol under anaerobic conditions afforded low-spin [CoIIL3]2+ complexes, isolated as ClO4– salts. At room temperature the complexes are one-electron paramagnetic in nature, low-spin CoII, t2g6eg1, S = ½ and behave as 1∶2 electrolytes in acetonitrile solvent. In acetonitrile solvent they show a ligand-to-metal charge-transfer (LMCT) band near 400 nm, an intraligand transition near 300 nm and ligand-field d–d transitions in the range 860–600 nm. The complexes exhibit quasi-reversible CoII–CoIII couples near 1 V and six sequential ligand reductions (NN groups) in the range 0.2 to –1.8 V versus saturated calomel electrode (SCE). At room temperature in the solid state they exhibit isotropic EPR spectra but at 77 K, both in the polycrystalline state and in the dichloromethane solution, display rhombic spectra. Reaction of [CoIIL3]2+ with 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) resulted in complete ligand-exchanged products with concomitant metal oxidation, low-spin [CoIII(bpy)3]3+ and low-spin [CoIII(phen)3]3+ respectively. The reaction of PPh3 with the [CoII(L7)3]2+ [L7 = 2-(p-chlorophenylazo)pyridine] yielded a partial ligand-exchanged product, low-spin [CoII(L7)2(PPh3)2]2+, isolated as its ClO4– salt. The complex is one-electron paramagnet and a 1∶2 electrolyte in acetonitrile solvent. It displays an LMCT band at 401 nm, an intraligand transition at 305 nm and four d–d transitions in the range 870–640 nm. It exhibits irreversible CoII to CoIII oxidation at 1.33 V (Epa) and four successive ligand reductions in the range –0.30 to –1.1 V versus SCE. At 77 K the complex displays an axial EPR spectrum.

Ruthenium(II) bipyridine complexes with modified phenolic Schiff base ligands. Synthesis, spectroscopic characterization and Redox properties

A group of stable new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bpy) 2L]ClO 4 ( 1–6), (bpy = 2,2′-bipyridine; L = deprotonated form of the HL ligands, o-(HO)C 6H 3(R)C(R′)NCH 2C 6H 5 or o-(HO)C 6H 3(R)C(R′)NNHC 6H 5; where R = H, p-NO 2 and R′ = H, CH 3) have been synthesized and characterized. The complexes are essentially diamagnetic and behave as 1 : 1 electrolytes in acetonitrile solution. They display two metal-to-ligand-charge-transfer (MLCT) transitions near 500 and 400 nm respectively and intra ligand π-π ∗ transitions in the UV-region. In acetonitrile solution the complexes exhibit weak emission from the lowest energy MLCT band at room-temperature. The quantum yields of the complexes are found to be in the range 0.0004–0.01. In acetonitrile solution the complexes show quasi-reversible ruthenium(II)-ruthenium(III) oxidation couples in the range 0.33 a ̊ 0.70 V and irreversible ruthenium(III)-ruthenium(IV) oxidations in the range 1.53 a ̊ 1.95 V vs SCE . Two successive reversible bipyridine reductions are observed for each complex in the ranges −1.4 a ̊ −1.62 V and −1.59 a ̊ −1.85 V vs SCE respectively. The presence of trivalent ruthenium in the oxidized solution for one complex 5 is evidenced by the rhombic EPR spectrum with g values, g 1 = 2.389, g 2 = 2.081 and g 3 = 1.810. The EPR spectrum of the coulometrically oxidized species, 5 + has been analyzed to furnish values of axial ( Δ = 4745 cm −1) and rhombic (V = 3692 cm −1) distortion parameters as well as the energies of the two expected ligand field transitions ( ν 1 = 3071 cm −1 and ν 2 = 6819 cm −1) within the t 2 shell. One of the ligand field transitions has been observed experimentally at 6578 cm −1 by near-IR spectrum which is close to the computed ν 2 value.

Complexes Containing 2,9-Bis(p-biphenylyl)-1,10-phenanthroline Units Incorporated into a 56-Membered Ring. Synthesis, Electrochemistry, and Photophysical Properties

We synthesized a new macrocycle (1) which is made of a 56-membered ring and contains two 2,9-bis(p-biphenylyl)-1,10-phenanthroline ligands. Then we prepared the complexes [Cu(1)(2)]+ (a catenate species), [Cu(3)2]+, [Cu(2)(3)]+ (a threaded species), Re(1)(CO)3Cl, and Re(3)(CO)3Cl, where 2 is a 33-membered macrocycle containing a 2,9-di-p-phenyl-1,10-phenanthroline ligand and 3 is an acyclic ligand containing the same 2,9-bis(p-biphenylyl)-1,10-phenanthroline unit which is present in 1. The absorption spectra of the free ligands in CH2Cl2 solution show very intense bands in the near-UV region, characteristic of their aromatic moieties. In the Cu(I) complexes, besides the perturbed ligand-centered bands, weak metal-to-ligand charge-transfer (MLCT) bands can be observed in the 400−700 nm region. In the Re(I) complexes the MLCT bands lie at higher energy because the oxidation potential of Re(I) is much more positive than that of Cu(I). At room temperature, ligands 1 and 3, which contain an extensively conjugated aromatic unit, show a very strong (Φ = 0.89), short-lived (1.7 ns) fluorescence band with λmax = 419 nm. The fluorescence of 2 is blue shifted (λmax = 387 nm) and less intense (Φ = 0.29) because of the less extended conjugation and rigidity of its chromophoric unit. Besides fluorescence, also a long-lived (second time scale) phosphorescence is displayed by the three ligands in a rigid matrix at 77 K. At room temperature the [Cu(1)(2)]+, [Cu(3)2]+, and [Cu(2)(3)]+ species exhibit a weak (Φ ∼ 10-3), relatively long-lived (102 ns time scale) emission with λmax around 750 nm, which can be assigned to a triplet metal-to-ligand charge-transfer (3MLCT) excited state. This emission is slightly red shifted and considerably longer lived (τ ∼ 2 μs) at 77 K. The Re(1)(CO)3Cl and Re(3)(CO)3Cl complexes at room temperature show a moderately weak (Φ ∼ 10-2), long-lived (102 ns time scale) 3MLCT emission with λmax around 610 nm. At 77 K this emission is blue shifted (λmax = 560 nm) and considerably longer lived (τ ∼ 10 μs). In the [Cu(1)(2)]+ catenate and in the Re(1)(CO)3Cl complex one of the two chelating sites of the macrocycle 1 is coordinated to the metal and the other one is free. Besides a 3MLCT emission, these species exhibit the fluorescence band of the 2,9-bis(p-biphenylyl)-1,10-phenanthroline unit, strongly quenched by the nearby metal-based chromophoric unit (kq = 1.6 × 1010 and 1.4 × 1010 s-1 for the Cu(I) and the Re(I) complex, respectively). In aerated solution all of the complexes sensitize the 1Δg(O2) luminescence at 1269 nm.

Wavelength-Dependent Photodissociation of [Fe(bpy)3·(CH3OH)n]2+ Clusters, n = 2−6, Triggered by Excitation of the Metal-to-Ligand Charge-Transfer Transition

This paper reports the first investigation of the energetics of charge transfer in a coordination complex containing a divalent transition-metal ion as a function of the extent of solvation and provides a detailed description of the first instrument designed for spectroscopic characterization of gas-phase ions and clusters produced by electrospray ionization. Metal-to-ligand charge transfer (MLCT) is probed by laser photofragmentation mass spectrometry in clusters of the complex tris(2,2‘-bipyridyl)iron(II), [Fe(bpy)3]2+, with methanol prepared by electrospray ionization. Excitation of the MLCT transition in [Fe(bpy)3]2+ triggers evaporation of methanol solvent molecules from these clusters, permitting indirect detection of absorption. Measurement of ion beam depletion and/or production of charged photofragments as a function of photon energy yields the red edge of the MLCT band for [Fe(bpy)3·(CH3OH)n]2+ clusters, n = 2−6. Increasing the number of methanol molecules in the clusters shifts the onset of the...

Comparing the spectroscopic and electrochemical properties of ruthenium and osmium complexes of the tridentate polyazine ligands 2,2′:6′,2″-terpyridine and 2,3,5,6-tetrakis(2-pyridyl)pyrazine

A number of osmium and ruthenium complexes of the tridentate ligands 2,2′:6′,2″-terpyridine (tpy) and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tpp) have been prepared and characterized by our laboratory. All these complexes possess metal based oxidations and ligand based reductions localized on each polyazine ligand. Polymetallic complexes bridged by the tpp ligand exhibit two sequential tpp based reductions prior to the reduction of other polyazine ligands in these complexes. The spectroscopy of these complexes is dominated by ligand based π-π ∗ transitions in the ultraviolet and MLCT (metal-to-ligand charge transfer) bands terminating on each polyzine ligand in the visible. For the complexes reported herein the lowest lying optical transitionis a M → BL CT band. For most of the complexes reported, occupation of this excited state gives rise to an observable emission at room temperature. For ruthenium complexes of these tridentate ligands, the presence of a low-lying LF state shortens the excited state lifetimes of these chromophores. This gives rise to ruthenium complexes that display shorter excited state lifetimes than the analogous osmium based systems. Incorporation of tpp based chromophores into polymetallic frameworks leads to the production of bimetallic species with long-lived excited states, ∼ 100 ns at room temperature. This makes these chromophores good candidates for the development of stereochemically defined supramolecular complexes. It is possible to measure an electrochemical HOMO-LUMO energy gap and a correlation between this electrochemically measured energy gap and the spectroscopic energy associated with this HOMO→LUMO transition are reported herein (HOMO== highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital).

Nitro-substituted benzoates of dimolybdenum: the Mo 24+ δ to ligand charge transfer band

The preparation and characterization of a series of Mo 2(O 2CAr′) 4 compounds, where Ar′ is a nitrosubstituted phenyl group, are described. The introduction of the NO 2 group in the 4- ( para) position causes a significant red-shift of a metal-to-ligand charge transfer (MLCT) band such that these compounds are intensely purple in solution: λ max ca. 540 nm with ε ∼ 15 000 M −1 cm −1. The cinnamate derivative (O 2CCHCHPh-4-NO 2) shows a further red-shift of this MLCT band while the 3-NO 2 ( meta) derivatives are only slightly red-shifted compared to the parent benzoate Mo 2(O 2CPh) 4. The cyclic voltammograms for Mo 2(O 2CR) 4, where R = Me, tBu, n-octyl, Ph, C 6H 4-4-NO 2, and C 6H 4-3-NO 2 reveal how substituents on R influence the ease of oxidation from the δ orbital. The structure of Mo 2(O 2CC 6H 4-3-NO 2) 4 was determined by a single crystal X-ray diffraction study as its pyridine adduct with additional pyridine in the lattice. Crystal data for Mo 2(O 2CC 6H 4-3-NO 2) 4(py) 2·2py at −174° C: a = 10.982(2) A ̊ , b = 20.457(4) A ̊ , c = 10.615(2) A ̊ , α = 93.82(1)°, β = 90.49(1)°, γ = 98.31(1)°, Z = 2, d calc = 1.65 g cm −3 and space group P 1 . The nature of the MLCT band is discussed in the light of Fenske-Hall MO calculations on Mo 2(O 2C 6H 5) 4, Mo 2(O 2C 6H 4-4-NO 2) 4, Mo 2(O 2CC 6H 4-3-NO 2) 4, and Mo 2(O 2C 6H 3-2-Cl-4-NO 2) 4.

Influence of Solvent on the Spectroscopic Properties of Cyano Complexes of Ruthenium(II)

UV−visible spectra, emission spectra, and RuIII/II reduction potentials have been measured for cis-[Ru(bpy)2(py)(CN)]+ (bpy is 2,2‘-bipyridine; py is pyridine), cis-Ru(bpy)2(CN)2, [Ru(tpy)(CN)3]- (tpy is 2,2‘:6‘,2‘‘-terpyridine), [Ru(bpy)(CN)4]2-, and [Ru(MQ+)(CN)5]2- (MQ+ is N-methyl-4,4‘-bipyridinium cation) in twelve solvents. The shifts in the metal-to-ligand charge transfer (MLCT) absorption (Eabs) or emission (Eem) band energies with solvent increase linearly with the number of cyano ligands and correlate well with the Gutmann “acceptor number” of the solvent. Intraligand π → π* band energies also correlate with acceptor number, but with only ∼30% of the shifts for the MLCT bands. The solvent dependence arises through mixing of the π → π* transitions with lower energy MLCT transitions. MLCT absorption and emission spectra are convolutions of overlapping vibronic components, and a Franck−Condon analysis of emission spectral profiles for cis-Ru(bpy)2(CN)2* has been used to evaluate the energy gap, E0,...

Homoleptic Fe(II) complexes of new 2,2′:6′,2″-terpyridine ligands with 4′- p-hydroquinonyl or 4′- p-quinolyl pendants

Preparations of three new ligands, 4′- p-dimethoxyphenyl-2,2′:6′,2″-terpyridine (L 1), 4′- p-hydroquinonyl-2,2′:6′,2″-terpyridine (L 2) and 4′- p-quinonyl-2,2′:6′,2″-terpyridine (L 3), and their homoleptic iron (II) complexes, [Fe(L) 2] 2+ (L = L 1- L 3), are described. The iron complexes are highly coloured due to metal-to-ligand charge-transfer (MLCT) transitions, the energies of which do not vary significantly. Thus switching the 4′-groups has little effect on the energies of the MLCT bands. The electrochemistries of the complexes have been characterised by cyclic voltammetric and by bulk electrolysis experiments. Each complex displays a combination of the redox processes xpected for the Fe(tpy −) 2 2+ (tpy − = 2,2′:6′, 2″-terpyridyl) core and for the two 4′ groups. Under protic conditions, the following redox reaction is chemically reversible: [Fe(L 2) 2] 2+ ⇄[Fe(L 3) 2] 2+ + 4H + + 4 ϵ −].