A Ru(II) bis-terpyridine-like complex that catalyzes water oxidation: the influence of steric strain.

The complexation of 2,9-dicarboxy-1,10-phenanthroline (DPA) with [Ru(tpy)Cl3] (tpy = 2,2';6,2″-terpyridine) provides a six-coordinate species in which one carboxyl group of DPA is not bound to the Ru(II) center. A more soluble tri-t-butyl tpy analogue is also prepared. Upon oxidation, neither species shows evidence for intramolecular trapping of a seven-coordinate intermediate. The role of the tpy ligand is revealed by the preparation of [Ru(tpy)(phenq)](2+) (phenq = 2-(quinol-8'-yl)-1,10-phenanthroline) that behaves as an active water oxidation catalyst (TON = 334). This activity is explained by the expanded coordination geometry of the phenq ligand that can form a six-membered chelate ring that better accommodates the linear arrangement of axial ligands required for optimal pentagonal bipyramid geometry. When a 1,8-naphthyidine ring is substituted for each of the two peripheral pyridine rings on tpy, increased crowding in the vicinity of the metal center impedes acquisition of the prerequisite reaction geometry.

A molecular light-driven water oxidation catalyst.

Two mononuclear Ru(II) complexes, [Ru(ttbt)(pynap)(I)]I and [Ru(tpy)(Mepy)(2)(I)]I (tpy = 2,2';6,2"-terpyridine; ttbt = 4,4',4"-tri-tert-butyltpy; pynap = 2-(pyrid-2'-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ(max) = 472 nm) LED light source using [Ru(bpy)(3)]Cl(2) (bpy = 2,2'-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components, light, photosensitizer, sodium persulfate, and catalyst, are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions.

Metal-ion-complexing properties of 2-(pyrid-2'-yl)-1,10-phenanthroline, a more preorganized analogue of terpyridyl. A crystallographic, fluorescence, and thermodynamic study.

Some metal-ion-complexing properties of the ligand 2-(pyrid-2'-yl)-1,10-phenanthroline (MPP) are reported. MPP is of interest in that it is a more preorganized version of 2,2';6,2''-terpyridine (tpy). Protonation constants (pK(1) = 4.60; pK(2) = 3.35) for MPP were determined by monitoring the intense π-π* transitions of 2 × 10(-5) M solutions of the ligand as a function of the pH at an ionic strength of 0 and 25 °C. Formation constants (log K(1)) at an ionic strength of 0 and 25 °C were obtained by monitoring the π-π* transitions of MPP titrated with solutions of the metal ion, or 1:1 solutions of MPP and the metal ion were titrated with acid. Large metal ions such as Ca(II) or La(III) showed increases of log K(1) of about 1.5 log units compared to that of tpy. Small metal ions such as Zn(II) and Ni(II) showed little increase in log K(1) for MPP compared to the tpy complexes, which is attributed to the presence of five-membered chelate rings in the MPP complexes, which favor large metal ions. The structure of [Cd(MPP)(H(2)O)(NO(3))(2)] (1) is reported: monoclinic, P2(1)/c, a = 7.4940(13) Å, b = 12.165(2) Å, c = 20.557(4) Å, ß = 96.271(7)°, V = 1864.67(9) Å(3), Z = 4, and final R = 0.0786. The Cd in 1 is seven-coordinate, comprising the three donor atoms of MPP, a coordinated water, a monodentate, and a bidentate NO(3)(-). Cd(II) is a fairly large metal ion, with r(+) = 0.96 Å, slightly too small for coordination with MPP. The effect of this size matching in terms of the structure is discussed. Fluorescence spectra of 2 × 10(-7) M MPP in aqueous solution are reported. The nonprotonated MPP ligand fluoresces only weakly, which is attributed to a photoinduced-electron-transfer effect. The chelation-enhanced-fluorescence (CHEF) effect induced by some metal ions is presented, and the trend of the CHEF effect, which is Ca(II) > Zn(II) > Cd(II) ~ La(III) > Hg(II), is discussed in terms of factors that control the CHEF effect, such as the heavy-atom effect.

Dynamics of closure of zinc bis-porphyrin molecular tweezers with copper(II) ions and electron transfer.

Zinc bis-porphyrin molecular tweezers composed of a N(4) spacer bound through pyridyl units to the meso position of porphyrins were synthesized, and the tweezers are closed by the coordination of a copper(II) ion inside the spacer ligand. The effect of the π-π interaction between the porphyrin rings in the closed conformation on the absorption spectra of multi-electron oxidized species and the reduction potentials were clarified by chemical and electrochemical oxidation of the closed form of the zinc bis-porphyrin molecular tweezers in comparison with the open form without copper(II) ion and the corresponding porphyrin monomer. The shifts in redox potentials and absorption spectrum of the porphyrin dication indicate a strong electronic interaction between the two oxidized porphyrins in the closed form, whereas there is little interaction between them in the neutral form. The dynamics of copper(II) ion coordination and subsequent electron transfer was examined by using a stopped-flow UV/Vis spectroscopic technique. It was confirmed that coordination of copper(II) occurs prior to electron-transfer oxidation of the closed form of the zinc bis-porphyrin molecular tweezers.

Polydentate analogues of 8-hydroxyquinoline and their complexes with ruthenium.

Selective reduction of 2-nitro-3-methoxybenzaldehyde provides 2-amino-3-methoxybenzaldehyde that undergoes the Friedländer condensation with a variety of acetyl-substituted derivatives of pyridine and 1,10-phenanthroline. After cleavage of the methyl ether, the resulting polydentate analogues of 8-hydroxyquinoline are excellent ligands for ruthenium. The resulting oxidation state of the metal center depends on the anionic character of the ligands. The presence of two electron donating anionic ligands results in a Ru(III) complex as evidenced by paramagnetic NMR behavior. The electronic absorption and redox properties of the complexes were measured and found to be consistent with the anionic character of the 8-HQ moieties. A planar pentadentate ligand provides two Ru-O and two Ru-N bonds in the equatorial plane. An X-ray structure shows that the central pyridine of the ligand is oriented toward the metal but held at a distance of 2.44 Å.

Metal ion complexing properties of dipyridoacridine, a highly preorganized tridentate homologue of 1,10-phenanthroline.

DPA (dipyrido[4,3-b;5,6-b]acridine) may be considered as a tridentate homologue of phen (1,10-phenanthroline). In this paper some of the metal ion complexing properties of DPA in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of DPA as a function of pH gave protonation constants at ionic strength (µ) = 0 and 25 °C of pK(1) = 4.57(3) and pK(2) = 2.90(3). Titration of 10(-5) M solutions of DPA with a variety of metal ions gave log K(1) values as follows: Zn(II), 7.9(1); Cd(II), 8.1(1); Pb(II), 8.3(1); La(III), 5.23(7); Gd(III), 5.7(1); Ca(II), 3.68; all at 25 °C and µ = 0. Log K(1) values at µ = 0.1 were obtained for Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II), 1.5(1). The log K(1) values show that the high level of preorganization of DPA leads to complexes 3 log units more stable than the corresponding terpyridyl complexes for large metal ions such as La(III) or Ca(II), but that for small metal ions such as Mg(II) and Zn(II) such stabilization is minimal. Molecular mechanics calculations (MM) are used to show that the best-fit M-N length for coordination with DPA is 2.60 Å, accounting for the high stability of Ca(II) or La(III) complexes of DPA, which are found to have close to this M-N bond length in their phen complexes.

Electrochemistry and spectroelectrochemistry of bismanganese porphyrin-corrole dyads.

A series of homobimetallic manganese cofacial porphyrin-corrole dyads were synthesized and investigated as to their electrochemistry, spectroelectrochemistry, and ligand binding properties in nonaqueous media. Four dyads were investigated, each of which contained a Mn(III) corrole linked in a face-to-face arrangement with a Mn(III) porphyrin. The main difference between compounds in the series is the type of spacer, 9,9-dimethylxanthene, anthracene, dibenzofuran, or diphenylether, which determines the distance and interaction between the metallomacrocycles. Each redox process of the porphyrin-corrole dyads was assigned on the basis of spectroscopic and electrochemical data and by comparison with reactions and properties of the monocorrole and the monoporphyrin which were examined under the same solution conditions. The Mn(III) porphyrin part of the dyad undergoes two major one-electron reductions in pyridine and benzonitrile, the first of which involves a Mn(III)/Mn(II) process and the second the addition of an electron to the conjugated π-ring system of the macrocycle. The Mn(III) corrole part of the dyads also exhibits two major redox processes, one involving Mn(III)/Mn(II) and the other Mn(III) to Mn(IV) under the same solution conditions. The potentials and reversibility of each electron transfer reaction were shown to depend upon the solvent, type of spacer separating the two macrocycles, and the presence or absence of axial ligation, the latter of which was investigated in detail for the case of acetate ion which was found to bind within the cavity of the dyad to both manganese centers, both before and after the stepwise electroreduction to the Mn(II) forms of the two macrocycles. An intramolecular chloride ion exchange between the porphyrin part of the dyads which contain Mn(III)Cl and the singly oxidized corrole in the dyad is observed after the Mn(III)/Mn(IV) reaction of the corrole, suggesting that chloride is coordinated inside the cavity in the neutral compound.

Energy transfers in monomers, dimers, and trimers of zinc(II) and palladium(II) porphyrins bridged by rigid Pt-containing conjugated organometallic spacers.

A series of linear monomers (spacer-M(P)), dimers (M(P)-spacer-M'(P)), and trimers (M(P)-spacer-M'(P)-spacer-M(P)) of spacer/metalloporphyrin systems (M' = Zn, M = Zn, Pd, P = porphyrin, and spacer = trans-C(6)H(4)C[triple bond]CPtL(2)C[triple bond]CC(6)H(4)- (L = PEt(3))) including mixed metalloporphyrin compounds, were synthesized and characterized. The S(1) and T(1) energy transfers Pd(P)*-->Zn(P) occur with rates of approximately 2 x 10(9) s(-1), S(1), and 0.15 x 10(3) (slow component) and 4.3 x 10(3) s(-1) (fast component), T(1). On the basis of a literature comparison with a related dyad, the Pt atom in the conjugated chain slows down the transfers. The excitation in the absorption band of the trans-C(6)H(4)C[triple bond]CPtL(2)C[triple bond]CC(6)H(4)- spacer in the 300-360 nm range also leads to T(1) energy transfer (spacer* --> M(P); M = Zn, Pd) with rates of 10(4) s(-1).

Catalytic activity of biscobalt porphyrin-corrole dyads toward the reduction of dioxygen.

A series of biscobalt cofacial porphyrin-corrole dyads bearing mesityl substituents at the meso positions of the corrole ring were investigated as to their electrochemistry, spectroelectrochemistry, and CO binding properties in nonaqueous media and then applied to the surface of a graphite electrode and tested as electrocatalysts for the reduction of dioxygen to water or hydrogen peroxide in air-saturated aqueous solutions containing 1 M HClO(4). The catalytic reduction of O(2) with the same dyads was also investigated in the homogeneous phase using 1,1'-dimethylferrocene as a reductant in PhCN containing HClO(4). The examined compounds are represented as (PMes(2)CY)Co(2), where P = a porphyrin dianion, Mes(2)C = a corrole trianion with two mesityl groups in trans meso-positions of the macrocycle, and Y is one of three bridging groups separating the two metallomacrocycles in a face-to-face arrangement, either with 9,9-dimethylxanthene, dibenzofuran, or diphenylether as linkers. Cyclic voltammetry and rotating disk electrode voltammetry revealed that the examined compounds are all catalytically active toward the electroreduction of dioxygen in acid media giving H(2)O(2) or H(2)O depending upon the type of linkage (Y) and the initial site of electron transfer which, in nonaqueous media, could be switched between the corrole and the porphyrin metal center by variations of substituents on the corrole macrocycle or the gas above the solution. The homogeneous reduction of dioxygen via a two- or four-electron transfer process was also investigated using 1,1'-dimethylferrocene as reductant in PhCN containing HClO(4).

Solvent, anion, and structural effects on the redox potentials and UV-visible spectral properties of mononuclear manganese corroles.

A series of manganese(III) corroles were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous solvents. Up to three oxidations and one reduction were obtained for each complex depending on the solvents. The main compound discussed in this paper is the meso-substituted manganese corrole, (Mes 2PhCor)Mn, and the main points are how changes in axially coordinated anion and solvent will affect the redox potentials and UV-vis spectra of each electrogenerated species in oxidation states of Mn(III), Mn(IV), or Mn(II). The anions OAc (-), Cl (-), CN (-), and SCN (-) were found to form five-coordinate complexes with the neutral Mn(III) corrole while two OH (-) or F (-) anions were shown to bind axially in a stepwise addition to give the five- and six-coordinate complexes in nonaqueous media. In each case, complexation with one or two anionic axial ligands led to an easier oxidation and a harder reduction as compared to the uncomplexed four-coordinate species.

Clarification of the oxidation state of cobalt corroles in heterogeneous and homogeneous catalytic reduction of dioxygen.

Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15-tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15-dimesitylcorrole)cobalt (F 5PhMes 2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes 3Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O 2 in benzonitrile containing 10 (-2) M HClO 4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E 1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E 1/2 = 0.86 to 1.18 V in CH 2Cl 2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E 1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso -substituted C 6F 5 substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F 5PhMes 2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

Cytotoxicity of cyclometallated ruthenium complexes: the role of ligand exchange on the activity.

The cyclometallated Ru(II) complexes cis-[Ru(phpy)(phen)(CH3CN)2](PF6) (1; phpy(-)=deprotonated 2-phenylpyridine, phen=1,10-phenanthroline) and cis-[Ru(phpy)(bpy)(CH3CN)2](PF6) (2; bpy=2,2'-bipyridine) were investigated as potential agents for photodynamic therapy. The presence of phpy(-) in the coordination sphere results in a red-shift of the Ru→phen and Ru→bpy metal-to-ligand charge transfer of 1 and 2, respectively, thus improving the tissue penetration of light while maintaining the efficient photo-induced ligand exchange required for DNA binding. The 14-fold enhancement of OVCAR-5 cell death that occurs upon irradiation with 690 nm light can be attributed to photo-aquation. The role of glutathione (GSH) on the toxicity of the complex was also explored. Complexes 1 and 2 undergo ligand substitution in the presence of GSH in the dark, such that the metal may covalently bind to biomolecules. The combination of photo-induced ligand exchange and GSH-facilitated ligand exchange may explain the observed cytotoxicity.

Effect of ligands with extended π-system on the photophysical properties of Ru(II) complexes.

Density functional theory calculations were performed on a series of six ruthenium complexes possessing tridentate ligands: [Ru(tpy)(2)](2+) (1; tpy = [2,2';6',2'']-terpyridine), [Ru(tpy)(pydppx)](2+) (2; pydppx = 3-(pyrid-2'-yl)-11,12-dimethyldipyrido[3,2-a: 2',3'-c]phenazine), [Ru(pydppx)(2)](2+) (3), [Ru(tpy)(pydppn)](2+) (4; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraazadibenzo[a,c]naphthacene), [Ru(pydppn)(2)](2+) (5), and [Ru(tpy)(pydbn)](+) (6; pyHdbn = 3-pyrid-2'-yl-4,9,16-triazadibenzo[a,c]naphthacene). The calculations were compared to experimental data, including electrochemistry and electronic absorption spectra. The theoretical results reveal that the lowest-lying singlet and triplet states in 4 and 5 are pydppn-based ππ* in character, which are remarkably different from the lowest-lying metal-to-ligand charge transfer (MLCT) states in 1-3. The calculated lowest triplet states in 4 and 5 are consistent with the (3)ππ* states observed experimentally. However, although the extended π-system of pydbn(-) is similar to that of pydppn, the HOMO of 6 lies above those of 4 and 5, resulting in strikingly different spectroscopic properties. Calculations show that the lowest triplet excited state of 6 is a combination of (3)MLCT and (3)ππ*. This work demonstrates that the electronic structure of the tridentate ligand has a pronounced effect on the photophysical properties of ruthenium(II) complexes and that DFT and TD-DFT methods are a useful tool that can be used to predict photophysical and redox properties of transition metal complexes.

Face-to-face pacman-type porphyrin-fullerene dyads: design, synthesis, charge-transfer interactions, and photophysical studies.

Pacman-type face-to-face zinc-porphyrin-fullerene dyads have been newly synthesized and studied. Owing to the close proximity of the donor and acceptor entities, strong pi-pi intramolecular interactions between the porphyrin and fullerene entities resulted in modulating the spectral and electrochemical properties of the dyads. New absorption and emission bands that correspond to the charge-transfer interactions were observed in the near-IR region. Time-resolved transient absorption studies revealed efficient photoinduced electron transfer from the singlet excited porphyrin to the fullerene entity. The rate constants for photoinduced electron transfer are analyzed in terms of the Marcus theory of electron transfer, which afforded a large electron coupling matrix element (V=140 cm(-1)) for the face-to-face dyads. As a consequence of the large charge-recombination driving force in the Marcus inverted region, a relatively long lifetime of the charge-separated state has been achieved.