Diet composition, food intake, apparent digestibility, and body condition score of the captive Asian elephant (Elephas maximus): a pilot study in two collections in Thailand.

Limited data are available regarding the nutrition and feeding of captive Asian elephants in range countries. In this study, feeding regimens of two collections in northern Thailand and their actual diets shaped by availability of forage and mahout preferences were assessed for nutritional quality. The composition of dietary intake, fecal output, and the dietary regimen were individually recorded for 5 days in 10 elephants. The proportion of forage in the diet represented 41 to 62% of the dry matter intake (DMI) in one collection whereas in the other collections it varied between 68 and 72%. Between 8.5 and 24% of the diet consisted of commercial pellets, and hulled rice represented up to 25% of the DMI in one collection. Sugar cane, corn cobs, and fruits such as bananas were eaten in smaller amounts. Body condition scores and weights were measured, which revealed that nine animals were in good condition. Representative samples of each food as well as fecal samples were analyzed for dry matter, crude protein, fat, crude fiber, gross energy, ash, calcium, and phosphorus. Diet adequacy was assessed by calculating the digestible nutrients in the rations and by comparing them to the recommendations from literature. The digestible energy (DE) intake varied between 0.6 and 1.4 megajoules (MJ) per kg(0.75) per day; therefore, higher than the estimated recommendations of 0.65 MJ per kg(0.75) per day for nine of the elephants. In all elephants the crude protein intake was less than the maintenance recommendations and ranged between 6.01 and 7.56% of the DMI. Calcium intake was low in one collection and there was an inverse calcium: phosphorus ratio, which was inadequate. The present study adds to the knowledge of captive elephant diets in Asia and is a starting point for further research, which is necessary to design optimum diet plans for captive Asian elephants in Thailand.

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.

Oxygen-oxygen bond formation pathways promoted by ruthenium complexes.

The photoproduction of hydrogen from water and sunlight represents an attractive means of artificial energy conversion for a world still largely dependent on fossil fuels. A practical technology for producing sun-derived hydrogen remains an unachieved goal, however, and is dependent on developing a better understanding of the key reaction, the oxidation of water to dioxygen. The molecular complexity of this process is such that sophisticated transition metal complexes, which can access low-energy reaction pathways, are considered essential as catalysts. Complexes based on Mn, Co, Ir, and Ru have been described recently; a variety of ligands and nuclearities that comprise many complex topologies have been developed, but very few of them have been studied from a mechanistic perspective. One step in particular needs to be understood and better characterized for the transition-metal-catalyzed oxidation of water to dioxygen, namely, the circumstances under which the formation of O-O bonds can occur. Although there is a large body of work related to the formation of C-C bonds promoted by metal complexes, the analogous literature for O-O bond formation is practically nonexistent and just beginning to emerge. In this Account, we describe the sparse literature existing on this topic, focusing on the Ru-aqua complexes. These complexes are capable of reaching high oxidation states as a result of the sequential and simultaneous loss of protons and electrons. A solvent water molecule may or may not participate in the formation of the O-O bond; accordingly, the two main pathways are named (i) solvent water nucleophilic attack (WNA) and (ii) interaction of two M-O units (I2M). Most of the complexes described belong to the WNA class, including a variety of mononuclear and polynuclear complexes containing one or several Ru-O units. A common feature of these complexes is the generation of formal oxidation states as high as Ru(V) and Ru(VI), which render the oxygen atom of the Ru-O group highly electrophilic. On the other hand, only one symmetric dinuclear complex that undergoes an intramolecular O-O bond formation step has been described for the I2M class; it has a formal oxidation state of Ru(IV). A special section is devoted to Ru-OH(2) complexes that contain redox active ligands, such as the chelating quinone. These ligands are capable of undergoing reversible redox processes and thus generate a complex but fascinating electron-transfer process between the metal and the ligand. Despite the intrinsic experimental difficulties in determining reaction mechanisms, progress with these Ru complexes is now beginning to be reported. An understanding of recent successes, as well as pitfalls, is essential in the search for a practical water oxidation catalyst.

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.

Water oxidation at a tetraruthenate core stabilized by polyoxometalate ligands: experimental and computational evidence to trace the competent intermediates.

Converging UV-vis, EPR, rRaman, and DFT calculations highlight the evolution of [Ru(4)(H(2)O)(4)(mu-O)(4)(mu-OH)(2)(gamma-SiW(10)O(36))(2)](10-), 1, to high-valent intermediates. In analogy with the natural enzyme, five different oxidation states, generated from 1, have been found to power the catalytic cycle for water oxidation. A high electrophilic tetraruthenium(V)-hydroxo species is envisaged as the competent intermediate, undergoing nucleophilic attack by an external water molecule as a key step in the formation of a new O-O bond under catalytic conditions.

Mononuclear Mn(III) and Mn(IV) bis-terpyridine complexes: electrochemical formation and spectroscopic characterizations.

The electrochemical behavior of two mononuclear Mn(II) bis-terpyridine complexes, [Mn(II)(L)(2)](2+) (L = terpy (2,2':6',2''-terpyridine) and (t)Bu(3)-terpy (4,4',4''-tritert-butyl-2,2':6',2''-terpyridine)), has been investigated in dry CH(3)CN. Under these conditions, the cyclic voltammograms of these complexes exhibit not only the well-known Mn(II)/Mn(III) oxidation system but also a second metal-based oxidation one, corresponding to the Mn(III)/Mn(IV) redox couple. These oxidative processes are located at E(1/2) = +0.96 and +1.77 V vs Ag/Ag(+) (+1.26 and +2.07 V vs SCE) for the terpy complex and E(1/2) = +0.85 and +1.56 V vs Ag/Ag(+) (+1.15 and +1.86 V vs SCE) for the (t)Bu(3)-terpy derivative. The one-electron oxidized form of these complexes, [Mn(III)(L)(2)](3+), has been quantitatively generated by exhaustive electrolyses at E = 1.30 V, as previously observed in the case of the oxidation of [Mn(II)(tolyl-terpy)(2)](2+) (tolyl-terpy = 4'-(4-Methylphenyl)-2,2':6',2''-terpyridine) (Romain, S.; Duboc, C.; Neese, F.; Riviere, E.; Hanton, L. R.; Blackman, A. G.; Lepretre, J.-C.; Deronzier, A.; Collomb, M.-N. Chem.Eur. J. 2009, 15, 980-988). Further electrolyses at E = 1.65-1.80 V of [Mn(III)(L)(2)](3+) solutions have shown that the [Mn(IV)(L)(2)](4+) species is only stable for L = (t)Bu(3)-terpy because of the strong electron-donating properties of the tert-butyl substituents. These electrogenerated high-valent complexes are rare examples of mononuclear Mn(III) and Mn(IV) complexes stabilized solely by neutral N ligands. They have been fully characterized in solution by UV-visible and electron paramagnetic resonance (EPR) spectroscopies. A detailed investigation of the EPR spectra of the [Mn(II)((t)Bu(3)-terpy)(2)](2+) and [Mn(IV)((t)Bu(3)-terpy)(2)](4+) has allowed the determination of the spin Hamiltonian parameters for both systems (for Mn(II): |D| = 0.059 cm(-1); |E| = 0.014 cm(-1); E/D = 0.259; g(x) = g(y) = g(z) = 2.00 and for Mn(IV): |D| = 1.33(6) cm(-1); |E| = 0.36(4) cm(-1); E/D = 0.27; g(x) = 1.96(4); g(y) = 1.97(4); g(z) = 1.98(4)).

Oxygen-oxygen bond formation by the Ru-Hbpp water oxidation catalyst occurs solely via an intramolecular reaction pathway.

A thorough kinetics investigation of the Ru-Hbpp water oxidation catalyst has been carried out at temperatures in the range 10-40 degrees C. Four oxidative electron-transfer processes that take the catalyst from its initial II,II oxidation state up to the formal IV,IV oxidation state were kinetically characterized and the corresponding activation parameters determined. Once the IV,IV oxidation state is reached, two additional slower kinetic processes take place, corresponding to the formation of an intermediate that finally evolves oxygen and regenerates the initial Ru-Hbpp catalyst. These two kinetic processes were also fully characterized with respect to the evaluation of their rate constants and activation parameters. Furthermore, (18)O labeling experiments were performed with different degrees of labeled catalyst and solvent, and the (16)O(2)/(16)O(18)O/(18)O(2) isotopic distribution of the generated molecular oxygen was calculated. These results clearly point to the existence of a single intramolecular reaction pathway for the formation of the oxygen-oxygen bond in the case of the Ru-Hbpp catalyst.

An unusual stable mononuclear Mn(III) bis-terpyridine complex exhibiting Jahn-Teller compression: electrochemical synthesis, physical characterisation and theoretical study.

The mononuclear manganese bis-terpyridine complex [Mn(tolyl-terpy)(2)](X)(3) (1(X)(3); X=BF(4), ClO(4), PF(6); tolyl-terpy=4'-(4-methylphenyl)-2,2':6',2"-terpyridine), containing Mn in the unusual +III oxidation state, has been isolated and characterised. The 1(3+) ion is a rare example of a mononuclear Mn(III) complex stabilised solely by neutral N ligands. Complex 1(3+) is obtained by electrochemical oxidation of the corresponding Mn(II) compound 1(2+) in anhydrous acetonitrile. Under these conditions the cyclic voltammogram of 1(2+) exhibits not only the well-known Mn(II)/Mn(III) oxidation at E(1/2)=+0.91 V versus Ag/Ag(+) (+1.21 V vs. SCE) but also a second metal-based oxidation process corresponding to Mn(III)/Mn(IV) at E(1/2)=+1.63 V (+1.93 V vs. SCE). Single crystals of 1(PF(6))(3)2 CH(3)CN were obtained by an electrocrystallisation procedure. X-ray analysis unambiguously revealed its tetragonally compressed octahedral geometry and high-spin character. The electronic properties of 1(3+) were investigated in detail by magnetic measurements and theoretical calculations, from which a D value of +4.82 cm(-1) was precisely determined. Density functional and complete active space self consistent field ab initio calculations both correctly predict a positive sign of D, in agreement with the compressed tetragonal distortion observed in the X-ray structure of 1(PF(6))(3)2 CH(3)CN. The different contributions to D were calculated, and the results show that 1) the spin-orbit coupling part (+2.593 cm(-1)) is predominant compared to the spin-spin interaction (+1.075 cm(-1)) and 2) the excited triplet states make the dominant contribution to the total D value.

Di(micro-oxo) binuclear manganese(III,IV) poly(bipyridyl) complexes bearing four ruthenium(II) photoactive units: synthesis, characterization, and photoinduced electron-transfer properties.

In order to model the photoinduced electron-transfer reactions from the manganese cluster to the photoactive P680 chlorophylls in photosystem II, three heterohexanuclear complexes, [Mn2III,IVO2[RuII(bpy)2(Ln)]4]11+ [bpy = 2,2'-bipyridine, n = 2 (1a), 4 (1b), 6 (1c)], in which one MnIII,IV(micro-O)2 center is covalently linked to four RuII(bpy)3-like moieties by bridged bis(bipyridine) Ln ligands, have been synthesized and characterized. The electrochemical, photophysical, and photochemical properties of these complexes have been investigated in CH3CN. The cyclic voltammograms and rotating-disk electrode curves of the three complexes show the presence of two very close successive reversible oxidation processes corresponding to the Mn2III,IV/Mn2IV,IV and RuII/RuIII redox couples (estimated E1/2 approximately 0.82 and 0.90 V, respectively). The lower potential of the Mn2III,IV subunit compared to those of the RuII moieties indicates that the RuIII species can act as an efficient oxidant toward the Mn2III,IV core. The two oxidized forms of the complexes [Mn2IV,IVO2[RuII(bpy)2(Ln)]4]12+ (2a-c) and [Mn2IV,IVO2[RuIII(bpy)2(Ln)]4]16+ (3a-c) obtained in good yields (>90% for 2a-c and >85% for 3a-c) by sequential electrolyses are very stable. Photophysical studies show that the 3MLCT excited state of the Ru(bpy)3 centers is moderately quenched by the Mn2III,IV(micro-O)2 core (15-25% depending on the length of the bridging alkyl chain). Nevertheless, this energy transfer can be easily short-circuited in the presence of an external irreversible electron acceptor like the (4-bromophenyl)diazonium cation, by an electron transfer leading, in a stepwise fashion, to the stable one- and five-electron-oxidized species 2a-c and 3a-c, respectively, also in good yields, under continuous irradiation of the solutions. Electro- and photoinduced oxidation experiments have been followed by UV-visible and electron paramagnetic resonance spectroscopy.

Tetranuclear polybipyridyl complexes of Ru(II) and Mn(II), their electro- and photo-induced transformation into di-mu-oxo Mn(III)Mn(IV) hexanuclear complexes.

Three heterotetranuclear complexes, [{Ru(II)(bpy)(2)(L(n))}(3)Mn(II)](8+) (bpy = 2,2'-bipyridine, n = 2, 4, 6), in which a Mn(II)-tris-bipyridine-like centre is covalently linked to three Ru(II)-tris-bipyridine-like moieties using bridging bis-bipyridine L(n) ligands, have been synthesised and characterised. The electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH(3)CN. The cyclic voltammograms of the three complexes exhibit two successive very close one-electron metal-centred oxidation processes in the positive potential region. The first, which is irreversible, corresponds to the Mn(II)/Mn(III) redox system (E(pa) approximately 0.82 V vs Ag/Ag(+) 0.01 M in CH(3)CN-0.1 M Bu(4)NClO(4)), whereas the second which is, reversible, is associated with the Ru(II)/Ru(III) redox couple (E(1/2) approximately 0.91 V). In the negative potential region, three successive reversible four electron systems are observed, corresponding to ligand-based reduction processes. The three stable dimeric oxidized forms of the complexes, [Mn(2)(III,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](11+), [Mn(2)(IV,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](12+) and [Mn(2)(IV,IV)O(2){Ru(III)(bpy)(2)(L(n))}(4)](16+) are obtained in fairly good yields by sequential electrolyses after consumption of respectively 1.5, 0.5 and 3 electrons per molecule of initial tetranuclear complexes. The formation of the di-micro-oxo binuclear complexes are the result of the instability of the {[Ru(II)(bpy)(2)(L(n))](3)Mn(III)}(9+) species, which react with residual water, via a disproportionation reaction and the release of one ligand, [Ru(II)(bpy)(2)(L(n))](2+). A quantitative yield can be obtained for these reactions if the electrochemical oxidations are performed in the presence of an added external base like 2,6-dimethylpyridine. Photophysical properties of these compounds have been investigated showing that the luminescence of the Ru(II)-tris-bipyridine-like moieties is little affected by the presence of manganese within the tetranuclear complexes. A slight quenching of the excited states of the ruthenium moieties, which occurs by an intramolecular process, has been observed. Measurements made at low concentration (<1 x 10(-5) M) indicate that some decoordination of Mn(2+) arises in 1a-c. These measurements allow the calculation of the association constants for these complexes. Finally, photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, acting as a sacrificial oxidant. The three successive oxidation processes, Mn(II)--> Mn(III)Mn(IV), Mn(III)Mn(IV)--> Mn(IV)Mn(IV) and Ru(II)--> Ru(III) are thus obtained, the addition of 2,6-dimethylpyridine in the medium giving an essentially quantitative yield for the two first photo-induced oxidation steps as found for electrochemical oxidation.

Electrochemical and chemical formation of [Mn4(IV)O5(terpy)4(H2O)2]6+, in relation with the photosystem II oxygen-evolving center model [Mn2(III,IV)O2(terpy)2(H2O)2]3+.

To examine the real ability of the binuclear di-mu-oxo complex [Mn2(III,IV)O2(terpy)2(H2O)2]3+ (2) to act as a catalyst for water oxidation, we have investigated in detail its redox properties and that of its mononuclear precursor complex [Mn(II)(terpy)2]2+ (1) in aqueous solution. It appears that electrochemical oxidation of 1 allows the quantitative formation of 2 and, most importantly, that electrochemical oxidation of 2 quantitatively yields the stable tetranuclear Mn(IV) complex, [Mn4(IV)O5(terpy)4(H2O)2]6+ (4), having a linear mono-mu-oxo{Mn2(mu-oxo)2}2 core. Therefore, these results show that the electrochemical oxidation of 2 in aqueous solution is only a one-electron process leading to 4 via the formation of a mono-mu-oxo bridge between two oxidized [Mn2(IV,IV)O2(terpy)2(H2O)2]4+ species. 4 is also quantitatively formed by dissolution of the binuclear complex [Mn2(IV,IV)O2(terpy)2(SO4)2] (3) in aqueous solutions. Evidence of this work is that 4 is stable in aqueous solutions, and even if it is a good synthetic analogue of the "dimers-of-dimers" model compound of the OEC in PSII, this complex is not able to oxidize water. As a consequence, since 4 results from an one-electron oxidation of 2, 2 cannot act as an efficient homogeneous electrocatalyst for water oxidation. This work demonstrates that a simple oxidation of 2 cannot produce molecular oxygen without the help of an oxygen donor.