Multireversible redox processes in pentanuclear bis(triple-helical) manganese complexes featuring an oxo-centered triangular {Mn(II)2Mn(III)(µ3-O)}5+ or {Mn(II)Mn(III)2(µ3-O)}6+ core wrapped by two {Mn(II)2(bpp)3}-.

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{Mn(II)(µ-bpp)(3)}(2)Mn(II)(2)Mn(III)(µ-O)](3+) (1(3+)) and [{Mn(II)(µ-bpp)(3)}(2)Mn(II)Mn(III)(2)(µ-O)](4+) (1(4+)). The structure consists of a central {Mn(3)(µ(3)-O)} core of Mn(II)(2)Mn(III) (1(3+)) or Mn(II)Mn(III)(2) ions (1(4+)) which is connected to two apical Mn(II) ions through six bpp(-) ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 1(3+) have been investigated in CH(3)CN. A series of five distinct and reversible one-electron waves is observed in the -1.0 and +1.50 V potential range, assigned to the Mn(II)(4)Mn(III)/Mn(II)(5), Mn(II)(3)Mn(III)(2)/Mn(II)(4)Mn(III), Mn(II)(2)Mn(III)(3)/Mn(II)(3)Mn(III)(2), Mn(II)Mn(III)(4)/Mn(II)(2)Mn(III)(3), and Mn(III)(5)/Mn(II)Mn(III)(4) redox couples. The two first oxidation processes leading to Mn(II)(3)Mn(III)(2) (1(4+)) and Mn(II)(2)Mn(III)(3) (1(5+)) are related to the oxidation of the Mn(II) ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical Mn(II) ions. The 1(4+) and 1(5+) oxidized species and the reduced Mn(4)(II) (1(2+)) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (1(2+) to 1(5+)). The spectroscopic characteristics (X-band electron paramagnetic resonance, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 1(3+) and 1(4+) in solid state. The powder X- and Q-band EPR signature of 1(3+) corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm(-1)) attributed to the two apical Mn(II) ions. At 40 K, the magnetic behavior is consistent for 1(3+) with two apical S = 5/2 {Mn(II)(bpp)(3)}(-) and one S = 2 noninteracting spins (11.75 cm(3) K mol(-1)), and for 1(4+) with three S = 5/2 noninteracting spins (13.125 cm(3) K mol(-1)) suggesting that the {Mn(II)(2)Mn(III)(µ(3)-O)}(5+) and {Mn(II)Mn(III)(2)(µ(3)-O)}(6+) cores behave at low temperature like S = 2 and S = 5/2 spin centers, respectively. The thermal behavior below 40 K highlights the presence of intracomplex magnetic interactions between the two apical spins and the central core, which is antiferromagnetic for 1(3+) leading to an S(T) = 3 and ferromagnetic for 1(4+) giving thus an S(T) = 15/2 ground state.

The Ru-Hbpp water oxidation catalyst.

A thorough characterization of the Ru-Hbpp (in,in-{[Ru(II)(trpy)(H(2)O)](2)(mu-bpp)}(3+) (trpy is 2,2':6',2''-terpyridine, bpp is bis(2-pyridyl)-3,5-pyrazolate)) water oxidation catalyst has been carried out employing structural (single crystal X-ray), spectroscopic (UV-vis and NMR), kinetic, and electrochemical (cyclic voltammetry) analyses. The latter reveals the existence of five different oxidation states generated by sequential oxidation of an initial II,II state to an ultimate, formal IV,IV oxidation state. Each of these oxidation states has been characterized by UV-vis spectroscopy, and their relative stabilities are reported. The electron transfer kinetics for individual one-electron oxidation steps have been measured by means of stopped flow techniques at temperatures ranging from 10 to 40 degrees C and associated second-order rate constants and activation parameters (DeltaH() and DeltaS()) have been determined. Room-temperature rate constants for substitution of aqua ligands by MeCN as a function of oxidation state have been determined using UV-vis spectroscopy. Complete kinetic analysis has been carried out for the addition of 4 equiv of oxidant (Ce(IV)) to the initial Ru-Hbpp catalyst in its II,II oxidation state. Subsequent to reaching the formal oxidation state IV,IV, an intermediate species is formed prior to oxygen evolution. Intermediate formation and oxygen evolution are both much slower than the preceding ET processes, and both are first order with regard to the catalyst; rate constants and activation parameters are reported for these steps. Theoretical modeling at density functional and multireference second-order perturbation theory levels provides a microscopic mechanism for key steps in intermediate formation and oxygen evolution that are consistent with experimental kinetic data and also oxygen labeling experiments, monitored via mass spectrometry (MS), that unambiguously establish that oxygen-oxygen bond formation proceeds intramolecularly. Finally, the Ru-Hbpp complex has also been studied under catalytic conditions as a function of time by means of manometric measurements and MS, and potential deactivation pathways are discussed.

Ru complexes that can catalytically oxidize water to molecular dioxygen.

The main objective of this review is to give a general overview of the structure, electrochemistry (when available), and catalytic performance of the Ru complexes, which are capable of oxidizing water to molecular dioxygen, and to highlight their more relevant features. The description of the Ru catalysts is mainly divided into complexes that contain a Ru-O-Ru bridging group and those that do not. Finally a few conclusions are drawn from the global description of all of the catalysts presented here, and some guidelines for future catalyst design are given.

A new Ru complex capable of catalytically oxidizing water to molecular dioxygen.

We have prepared three new dinuclear ruthenium complexes having the formulas [Ru2II(bpp)(trpy)2(mu-L)]2+ (L = Cl, 1; L = AcO, 2) and [Ru2II(bpp)(trpy)2(H2O)2]3+ (3). The three complexes have been characterized through the usual spectroscopic and electrochemical techniques and, in the cases of 1 and 2, the X-ray crystal structures have been solved. In aqueous acidic solution, the acetato bridge of 2 is replaced by aqua ligands, generating the bis(aqua) complex 3 which, upon oxidation to its RuIVRuIV state, has been shown to catalytically oxidize water to molecular oxygen. The measured pseudo-first-order rate constant for the O2-evolving process is 1.4 x 10-2 s-1, more than 3 times larger than the higher one previously reported for Ru-O-Ru type catalysts. This new water-splitting catalyst also has improved stability with regard to any previously described, achieving a total of 18.6 metal cycles.

Synthesis, structure, and acid-base and redox properties of a family of new Ru(II) isomeric complexes containing the trpy and the dinucleating Hbpp ligands.

Three pairs of mononuclear geometrical isomers containing the ligand 3,5-bis(2-pyridyl)pyrazole (Hbpp) of general formula in- and out-[RuII(Hbpp)(trpy)X](n+) (trpy=2,2':6',2' '-terpyridine; X=Cl, n=1, 2a,b; X=H2O, n=2, 3a,b; X=py (pyridine), n=2, 4a,b) have been prepared through two different synthetic routes, isolated, and structurally characterized. The solid state structural characterization was performed by X-ray diffraction analysis of four complexes: 2a-4a and 4b. The structural characterization in solution was performed by means of 1D and 2D NMR spectroscopy for complexes 2a,b and 4a,b and coincides with the structures found in the solid state. All complexes were also spectroscopically characterized by UV-vis which also allowed us to carry out spectrophotometric acid-base titrations. Thus, a number of species were spectroscopically characterized with the same oxidation state but with a different degree of protonation. As an example, for 3a three pKa values were obtained: pKa1(RuII)=2.13, pKa2(RuII)=6.88, and pKa3(RuII)=11.09. The redox properties were also studied, giving in all cases a number of electron transfers coupled to proton transfers. The pH dependency of the redox potentials allowed us to calculate the pKa of the complexes in the Ru(III) oxidation state. For complex 3a, these were found to be pKa1(RuIII)=0.01, pKa2(RuIII)=2.78, and pKa3(RuIII)=5.43. The oxidation state Ru(IV) was only reached from the Ru-OH2 type of complexes 3a or 3b. It has also been shown that the RuIV=O species derived from 3a is capable of electrocatalytically oxidizing benzyl alcohol with a second-order rate constant of kcat=17.1 M(-1) s(-1).

Synthesis, structure, and spectroscopic, photochemical, redox, and catalytic properties of ruthenium(II) isomeric complexes containing dimethyl sulfoxide, chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands.

Two isomeric Ru(II) complexes containing the dinucleating Hbpp (3,5-bis(2-pyridyl)pyrazole) ligand together with Cl and dmso ligands have been prepared and their structural, spectroscopic, electrochemical, photochemical, and catalytic properties studied. The crystal structures of trans,cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2a, and cis(out),cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2b, have been solved by means of single-crystal X-ray diffraction analysis showing a distorted octahedral geometry for the metal center where the dmso ligands coordinate through their S atom. 1D and 2D NMR spectroscopy corroborates a similar structure in solution for both isomers. Exposure of either 2a or 2b in acetonitrile solution under UV light produces a substitution of one dmso ligand by a solvent molecule generating the same product namely, cis(out)-[Ru(II)Cl(2)(Hbpp)(MeCN)(dmso)], 4. While the 1 e(-) oxidation of 2b or cis(out),cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3b, generates a stable product, the same process for 2a or trans,cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3a, produces the interesting linkage isomerization phenomenon where the dmso ligand switches its bond from Ru-S to Ru-O (K(III)(O)(-->)(S) = 0.25 +/- 0.025, k(III)(O)(-->)(S) = 0.017 s(-1), and k(III)(S)(-->)(O) = 0.065 s(-1); K(II)(O)(-->)(S) = 6.45 x 10(9), k(II)(O)(-->)(S) = 0.132 s(-1), k(II)(S)(-->)(O) = 2.1 x 10(-11) s(-1)). Finally complex 3a presents a relatively high activity as hydrogen transfer catalyst, with regard to its ability to transform acetophenone into 2-phenylethyl alcohol using 2-propanol as the source of hydrogen atoms.