Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H(2)O)MOO(2+) (L = (H(2)O)(4), (NH(3))(4), or N(4)-macrocycle; M = Cr(III), Rh(III)) react with iodide ions according to the stoichiometry L(H(2)O)MOO(2+) + 3I(-) + 3H(+) --> L(H(2)O)MOH(2+) + 1.5I(2) + H(2)O. The rate law is -d[L(H(2)O)MOO(2+)]/dt = k [L(H(2)O)MOO(2+)][I(-)][H(+)], where k = 93.7 M(-2) s(-1) for Cr(aq)OO(2+), 402 for ([14]aneN(4))(H(2)O)CrOO(2+), and 888 for (NH(3))(4)(H(2)O)RhOO(2+) in acidic aqueous solutions at 25 degrees C and 0.50 M ionic strength. The Cr(aq)OO(2+)/I(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H)()2(O)/k(D)2(O) = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related Cr(aq)OOH(2+)/I(-) reaction has k(H)2(O)/k(D)2(O) = 0.6. The oxidation of (NH(3))(5)Rupy(2+) by Cr(aq)OO(2+) exhibits an [H(+)]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H(+)])[Ru(NH(3))(5)py(2+)][Cr(aq)OO(2+)]. Diiodine radical anions, I(2)(*)(-), reduce Cr(aq)OO(2+) with a rate constant k = 1.7 x 10(9) M(-1) s(-1).     

Kinetics and mechanism of the reduction of a macrocyclic Rh(III) complex by chromium(II) ions: pH-controlled selectivity to rhodium(II) vs. rhodium(III) hydride

Aqueous chromium(II) ions reduce a macrocyclic Rh(III) complex L(1)(H(2)O)(2)Rh(3+) (L(1) = 1,4,8,11-tetraazacyclotetradecane) to the hydride L(1)(H(2)O)RhH(2+) in two discrete, one-electron steps. The first step generates L(1)(H(2)O)Rh(2+) with kinetics that are first order in each rhodium(III) complex and Cr(H(2)O)(6)(2+), and inverse in [H(+)], k/M(-1) s(-1) = 0.065/(0.0031 + [H(+)]). Further reduction of L(1)(H(2)O)Rh(2+) to L(1)(H(2)O)RhH(2+) is kinetically independent of [H(+)], k/M(-1) s(-1) = 0.30. The difference in [H(+)] dependence allows relative rates of the two steps to be manipulated to generate either L(1)(H(2)O)Rh(2+) or L(1)(H(2)O)RhH(2+) as the final product.     

Reduction and oxidation of hydroperoxo rhodium(III) complexes by halides and hypobromous acid

Oxygen atom transfer from trans-L(H(2)O)RhOOH(2+) [L = [14]aneN(4) (L(1)), meso-Me(6)[14]aneN(4) (L(2)), and (NH(3))(4)] to iodide takes place according to the rate law -d[L(H(2)O)RhOOH(2+)]/dt = k(I)[L(H(2)O)RhOOH(2+)][I(-)][H(+)]. At 0.10 M ionic strength and 25 degrees C, the rate constant k(I)/M(-)(2) s(-)(1) has values of 8.8 x 10(3) [L = (NH(3))(4)], 536 (L(1)), and 530 (L(2)). The final products are LRh(H(2)O)(2)(3+) and I(2)/I(3)(-). The (NH(3))(4)(H(2)O)RhOOH(2+)/Br(-) reaction also exhibits mixed third-order kinetics with k(Br) approximately 1.8 M(-)(2) s(-)(1) at high concentrations of acid (close to 1 M) and bromide (close to 0.1 M) and an ionic strength of 1.0 M. Under these conditions, Br(2)/Br(3)(-) is produced in stoichiometric amounts. As the concentrations of acid and bromide decrease, the reaction begins to generate O(2) at the expense of Br(2), until the limit at which [H(+)] 2(NH(3))(4)(H(2)O)RhOH(2+) + O(2); i.e., the reaction has turned into the bromide-catalyzed disproportionation of coordinated hydroperoxide. In the proposed mechanism, the hydrolysis of the initially formed Br(2) produces HOBr, the active oxidant for the second equivalent of (NH(3))(4)(H(2)O)RhOOH(2+). The rate constant k(HOBr) for the HOBr/(NH(3))(4)(H(2)O)RhOOH(2+) reaction is 2.9 x 10(8) M(-)(1) s(-)(1).     

Reaction of nitrogen monoxide with a macrocyclic superoxorhodium(III) complex produces an observable nitratorhodium intermediate

The rapid (k > or = 10(6) M(-1) s(-1)) reaction between NO and L(2)(H(2)O)RhOO(2+) (L(2) = meso-Me(6)-[14]ane-N(4)) generates two strongly oxidizing, scavengeable intermediates, believed to be NO(2) and L(2)(H(2)O)RhO(2+). A mechanism is proposed whereby a peroxynitrito complex L(2)(H(2)O)RhOONO(2+) is formed first. The homolysis of O-O bond produces NO(2) and L(2)(H(2)O)RhO(2+) which were trapped with ABTS(2)(-) and Ni([14]aneN(4))(2+). In the absence of scavengers, the decomposition of L(2)(H(2)O)RhOONO(2+) produces both free NO(3)(-) and a rhodium nitrato complex L(2)(H(2)O)RhONO(2)(2+), which releases NO(3)(-) in an inverse acid-dependent process. The total yield of L(2)(H(2)O)RhONO(2)(2+) is 70%. In a minor, parallel path, NO and L(2)(H(2)O)RhOO(2+) react to give nitrite and the hydroperoxo complex L(2)(H(2)O)RhOOH(2+).     

Visible light-induced release of nitrogen monoxide from a nitrosylrhodium complex

The important roles that nitric oxide (NO) plays in biological environments, and the need for precise and targeted delivery of NO for medicinal and other purposes have led to intense research in the area of metal nitrosyl complexes as thermal and photochemical sources of NO. Complexes with a good combination of chemical stability and high quantum yield for photochemical release of NO upon irradiation with visible light in aqueous solutions are rare. Here we report that a simple macrocyclic nitrosylrhodium complex [L(2)(H(2)O)Rh(NO)](2+) (L(2)=Me(6)[14]aneN(4)) exhibits unique chemical and photochemical properties that make it an excellent photochemical precursor of NO. The complex is highly soluble in water, thermally stable, and resistant toward O(2). Irradiation in the 648 nm band generates NO and [L(2)(H(2)O)Rh](2+) in aqueous solutions with a quantum yield of 1.00±0.07, the highest ever reported for a nitrosyl complex under any conditions. In the absence of O(2), the two fragments combine to regenerate [L(2)(H(2)O)Rh- (NO)](2+), but in O(2)-containing solutions, [L(2)(H(2)O)RhOO](2+) is formed as determined in spectral and kinetic measurements. The kinetics of the reaction of this superoxo complex with NO were measured by laser flash photolysis, k=(3.9±0.4)×10(7) M(-1) s(-1). Steady-state photolysis of [L(2)(H(2)O)Rh(NO)](2+) under O(2) yielded [L(2)(H(2)O)Rh(ONO(2))](2+), a long-lived nitrato intermediate that can also be generated in a direct reaction between NO and genuine [L(2)(H(2)O)RhOO](2+). Thus, visible-light photolysis of the [L(2)(H(2)O)Rh(NO)](2+)/O(2) system converts it to the [L(2)(H(2)O)RhOO](2+)/NO combination.     

Reaction of a superoxochromium(III) ion with nitrogen monoxide: kinetics and mechanism.

The kinetics of the rapid reaction between Cr(aq)OO(2+) and NO were determined by laser flash photolysis of Cr(aq)NO(2+) in O(2)-saturated acidic aqueous solutions, k = 7 x 10(8) M(-1) s(-1) at 25 degrees C. The reaction produces an intermediate, believed to be NO(2), which was scavenged with ([14]aneN(4))Ni(2+). With limiting NO, the Cr(aq)OO(2+)/NO reaction has a 1:1 stoichiometry and produces both free NO(3)(-) and a chromium nitrato complex, Cr(aq)ONO(2)(2+). In the presence of excess NO, the stoichiometry changes to [NO]/[Cr(aq)OO(2+)] = 3:1, and the reaction produces close to 3 mol of nitrite/mol of Cr(aq)OO(2+). An intermediate, identified as a nitritochromium(III) ion, Cr(aq)ONO(2+), is a precursor to a portion of free NO(2)(-). In the proposed mechanism, the initially produced peroxynitrito complex, Cr(aq)OONO(2+), undergoes O-O bond homolysis followed by some known and some novel chemistry of Cr(aq)O(2+) and NO(2). The reaction between Cr(aq)O(2+) and NO generates Cr(aq)ONO(2+), k > 10(4) M(-1) s(-1). Cr(aq)OO(2+) reacts with NO(2) with k = 2.3 x 10(8) M(-1) s(-1).     

Probing the effects of ligand structure on activity and selectivity of Cr(III) complexes for ethylene oligomerisation and polymerisation

A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH(2)CH(2)SC(10)H(21))(2) (L(1)) and O(CH(2)CH(2)SC(10)H(21))(2) (L(2))}, tripodal {MeC(CH(2)S(n)C(4)H(9))(3) (L(3)), MeC(CH(2)SC(10)H(21))(3) (L(4))} and macrocyclic {(C(10)H(21))[9]aneN(3) (L(5)), (C(10)H(21))(3)[9]aneN(3) (L(6)), with [9]aneN(3)=1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl(3)(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl(3)(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me(3)Al investigated similarly. Treatment with excess Me(3)Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the neutral S and S/O-donor ligands.     

Hydrogen atom and hydride transfer in the reactions of chromium(IV) and chromium(V) complexes with rhodium hydrides. Crystal structure of a superoxorhodium(III) product

The aquachromyl ion, Cr(IV)aqO2+, reacts with the hydrides L(H2O)RhH2+ (L = L1 = [14]aneN4 and L2 = meso-Me6-[14]aneN4) in aqueous solutions in the presence of molecular oxygen to yield Cr(aq)3+ and the superoxo complexes L(H2O)RhOO2+. At 25 degrees C, the rate constants are approximately 10(4) M(-1) s(-1) (L = L1) and 1.12 x 10(3) M(-1) s(-1) (L = L2). Both reactions exhibit a moderate deuterium isotope effect, kRhH/kRhD = approximately 3 (L1) and 3.3 (L2), but no solvent isotope effect, kH2O/kD2O = 1. The proposed mechanism involves hydrogen atom abstraction followed by the capture of LRh(H2O)2+ with molecular oxygen. There is no evidence for the formation of L(H2O)Rh2+ in the reaction between L(H2O)RhH2+ and (salen)CrVO+. The proposed hydride transfer is supported by the magnitude of the rate constants (L = L1, k = 8,800 M(-1) s(-1); (NH3)4, 2,500; L2, 1,000) and isotope effects (L = L1, kie = 5.4; L2, 6.2). The superoxo complex [L1(CH3CN)RhOO](CF3SO3)2.H2O crystallizes with discrete anions, cations, and solvate water molecules in the lattice. All moieties are linked by a network of hydrogen bonds of nine different types. The complex crystallized in the triclinic space group P1 with a = 9.4257(5) A, b = 13.4119(7) A, c = 13.6140(7) A, alpha = 72.842(1)degrees, beta = 82.082(1) degrees, gamma = 75.414(1) degrees, V = 1587.69(14) A3, and Z = 2.     

A benzene-core trinuclear GdIII complex: towards the optimization of relaxivity for MRI contrast agent applications at high magnetic field

A novel ligand, H(12)L, based on a trimethylbenzene core bearing three methylenediethylenetriamine-N,N,N',N'-tetraacetate moieties (-CH(2)DTTA(4-)) for Gd(3+) chelation has been synthesized, and its trinuclear Gd(3+) complex [Gd(3)L(H(2)O)(6)](3-) investigated with respect to MRI contrast agent applications. A multiple-field, variable-temperature (17)O NMR and proton relaxivity study on [Gd(3)L(H(2)O)(6)](3-) yielded the parameters characterizing water exchange and rotational dynamics. On the basis of the (17)O chemical shifts, bishydration of Gd(3+) could be evidenced. The water exchange rate, k(ex)(298)=9.0+/-3.0 s(-1) is around twice as high as k(ex)(298) of the commercial [Gd(DTPA)(H(2)O)](2-) and comparable to those on analogous Gd(3+)-DTTA chelates. Despite the relatively small size of the complex, the rotational dynamics had to be described with the Lipari-Szabo approach, by separating global and local motions. The difference between the local and global rotational correlation times, tau(lO)(298)=170+/-10 ps and tau(gO)(298)=540+/-100 ps respectively, shows that [Gd(3)L(H(2)O)(6)](3-) is not fully rigid; its flexibility originates from the CH(2) linker between the benzene core and the poly(amino carboxylate) moiety. As a consequence of the two inner-sphere water molecules per Gd(3+), their close to optimal exchange rate and the appropriate size and limited flexibility of the molecule, [Gd(3)L(H(2)O)(6)](3-) has remarkable proton relaxivities when compared with commercial contrast agents, particularly at high magnetic fields (r(1)=21.6, 17.0 and 10.7 mM(-1)s(-1) at 60, 200 and 400 MHz respectively, at 25 degrees C; r(1) is the paramagnetic enhancement of the longitudinal water proton relaxation rate, referred to 1 mM concentration of Gd(3+)).     

Generation and reactivity of rhodium(IV) complexes in aqueous solutions

At pH = 1 and 25 degrees C, the Fenton-like reactions of Fe(aq)(2+) with hydroperoxorhodium complexes LRh(III)OOH(2+) (L = (H(2)O)(NH(3))(4), k = 30 M(-1) s(-1), and L = L(2) = (H(2)O)(meso-Me(6)-[14]aneN(4)), k = 31 M(-1) s(-1)) generate short-lived, reactive intermediates, believed to be the rhodium(IV) species LRh(IV)O(2+). In the rapid follow-up steps, these transients oxidize Fe(aq)(2+), and the overall reaction has the standard 2:1 [Fe(aq)(2+)]/[LRhOOH(2+)] stoichiometry. Added substrates, such as alcohols, aldehydes, and (NH(3))(4)(H(2)O)RhH(2+), compete with Fe(aq)(2+) for LRh(IV)O(2+), causing the stoichiometry to change to <2:1. Such competition data were used to determine relative reactivities of (NH(3))(4)RhO(2+) toward CH(3)OH (1), CD(3)OH (0.2), C(2)H(5)OH (2.7), 2-C(3)H(7)OH (3.4), 2-C(3)D(7)OH (1.0), CH(2)O (12.5), C(2)H(5)CHO (45), and (NH(3))(4)RhH(2+) (125). The kinetics and products suggest hydrogen atom abstraction for (NH(3))(4)RhO(2+)/alcohol reactions. A short chain reaction observed with C(2)H(5)CHO is consistent with both hydrogen atom and hydride transfer. The rate constant for the reaction between Tl(aq)(III) and L(2)Rh(2+) is 2.25 x 10(5) M(-1) s(-1).     

Stepwise oxidation of anilines by cis-[RuIV(bpy)2(py)(O)]2+

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (bpy is 2,2'-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H(2)O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 degrees C, CH(3)CN) = (2.05 +/- 0.18) x 10(2) M(-1) s(-1) (DeltaH(++) = 5.0 +/- 0.7 kcal/mol; DeltaS(++) = -31 +/- 2 eu). On the basis of competition experiments, k(H)2(O)/k(D)2(O) kinetic isotope effects, and the results of an (18)O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[Ru(IV)(bpy)(2)(py)(O)](2+) (Ru(IV)=O(2+)). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH(+)) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by Ru(IV)=O(2+) to PhNO and PhNHNHPh to PhN=NPh. The rate laws for these reactions are first order in Ru(IV)=O(2+) and first order in reductant with k(14.4 degrees C, H(2)O/(CH(3))(2)CO) = (4.35 +/- 0.24) x 10(6) M(-1) s(-1) for PhNHOH and k(25.1 degrees C, CH(3)CN) = (1.79 +/- 0.14) x 10(4) M(-1) s(-1) for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO(2) and PhN=NPh to PhN(O)=NPh. The oxidation of PhNO is first order in PhNO and in Ru(IV)=O(2+) with k(25.1 degrees C, CH(3)CN) = 6.32 +/- 0.33 M(-1) s(-1) (DeltaH(++) = 4.6 +/- 0.8 kcal/mol; DeltaS(++) = -39 +/- 3 eu). The reaction occurs by O-atom transfer, as shown by an (18)O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.     

Kinetics and mechanism of the reaction of Cr(II) aqua ions with benzoylpyridine N-oxide

Aqueous chromium(II) ions, Cr(aq)(2+), react with benzoylpyridine oxide (BPO) much more rapidly than with other pyridine N-oxides previously explored. The kinetics were studied under pseudo-first order conditions with either reagent in excess. Under both sets of conditions, the major kinetic term exhibits first order dependence on limiting reagent, and second order dependence on excess reagent, i.e.k(Cr) = k2(Cr)[BPO][Cr(aq)(2+)]2 (excess Cr(aq)(2+)), and k(BPO) = k2(BPO)[Cr(aq)(2+)][BPO](2) (excess BPO), where k2(Cr) = (6.90 +/- 0.27) x 10(4) M(-2) s(-1) and k2(BPO) = (3.32 +/- 0.28) x 10(5) M(-2) s(-1) in 0.10 M HClO4. The rate constant k2(Cr) contains terms corresponding to [H+]-independent and [H+]-catalyzed paths. In the proposed mechanism, the initially formed Cr(aq)(BPO)(2+) engages in parallel oxidation of Cr(aq)(2+) and reduction of BPO. The latter reaction provides the basis for a convenient new preparative route for the BPO complex of Cr(III).     

A double arene hydroxylation mediated by dicopper(II)-hydroperoxide species

The dicopper(II) complex [Cu(2)(L)](4+) (L = alpha,alpha'-bis[bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino]-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)-hydroquinone complex in which the xylyl ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated ligand 2,6-bis([bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)benzene-1,4-diol was isolated by decomposition of the product complex. The incorporation of two oxygen atoms from H(2)O(2) into the ligand was confirmed by isotope labeling studies using H(2)(18)O(2). The pathway of the unusual double hydroxylation was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (6) and 2,6-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging micro-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6 and containing a protonated phenol is rapidly hydroxylated by H(2)O(2) and represents the first product formed in the hydroxylation of [Cu(2)(L)](4+). Kinetic studies performed on the reactions of [Cu(2)(L)](4+) and 3 with H(2)O(2) show that the second hydroxylation is faster than the first one at room temperature (0.13 +/- 0.05 s(-1) vs 5.0(+/-0.1) x 10(-3) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (Delta H++ = 39.1 +/- 0.9 kJ mol(-1) and Delta S++ = -115.7 +/- 2.4 J K(-1) mol(-1) for the first hydroxylation; Delta H++ = 77.8 +/- 1.6 kJ mol(-1) and Delta S++ = -14.0 +/- 0.4 J K(-1) mol(-1) for the second hydroxylation). By studying the reaction between [Cu(2)(L)](4+) and H(2)O(2) at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu(2)(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12,000), 444 (epsilon 1200), and 610 nm (epsilon 800 M(-1)cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(III) to oxo-chromium(V) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).     

Carboxy ester hydrolysis promoted by a zinc(II) 2-[bis(2-aminomethyl)amino]ethanol complex: a new model for indirect activation on the serine nucleophile by zinc(II) in zinc enzymes

A complexation study on the new Zn(II) complexes of asymmetric tripodal ligand 2-[bis(2-aminoethyl)amino]ethanol (L) has revealed that the alcoholic OH group of complex ZnL exhibits remarkable acidity with a very low pK(a) value of 7.7 at 25 degrees C. Both the monomeric complex [ZnH(-0.25)L(H(2)O)](ClO(4))(1.75) (I) and the dimeric alkoxide-coordinating complex [Zn(2)(H(-1)L)(2)](ClO(4))(2) (II) were synthesized, and their structures were determined by X-ray diffraction. The Zn(II)-bound alkoxide, as the reactive nucleophile toward the hydrolysis of esters, has shown a second-order rate constant of 0.13 +/- 0.01 M(-1) s(-1) in 10% (v/v) CH(3)CN at 25 degrees C in 4-nitrophenyl acetate (NA) hydrolysis, which is almost the same as the corresponding value for the very reactive alcohol-dependent [12]aneN(3)-Zn complex. Present work shows for the first time that Zn(II) complexes of the asymmetric tripodal polyamine bearing an ethoxyl pod can also serve as good models of Zn(II)-containing enzymes.     

New bis-cresol-bridged bis(1,4,7-triazacyclononane) ligand as receptor for metal cations and phosphate anions

The synthesis and characterization of a new bis([9]aneN3) ligand (H2L) containing two [9]aneN3 macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H2L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H5L)(3+) and (H6L)(4+) species at acidic pH values. The coordination properties of H2L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M2H2L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN3 and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M2H2L](4+) species at alkaline pH values affords [M2L](2+) complexes, where all amine groups of the [9]aneN3 moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN3 moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN3 subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.     

Redox chemistry of morpholine-based Os(VI)-hydrazido complexes: trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+)

The oxidations of benzyl alcohol, PPh3, and the sulfides (SEt2 and SPh2) (Ph = phenyl and Et = ethyl) by the Os(VI)-hydrazido complex trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) (tpy = 2,2':6',2' '-terpyridine and O(CH2)4N(-) = morpholide) have been investigated in CH3CN solution by UV-visible monitoring and product analysis by gas chromatography-mass spectrometry. For benzyl alcohol and the sulfides, the rate law for the formation of the Os(V)-hydrazido complex, trans-[Os(V)(tpy)(Cl)2(NN(CH2)4O)](+), is first order in both trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) and reductant, with k(benzyl) (25.0 +/- 0.1 degrees C, CH3CN) = (1.80 +/- 0.07) x 10(-4) M(-1) s(-1), k(SEt2) = (1.33 +/- 0.02) x 10(-1) M(-1) s(-1), and k(SPh2) = (1.12 +/- 0.05) x 10(-1) M(-1) s(-1). Reduction of trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) by PPh3 is rapid and accompanied by isomerization and solvolysis to give the Os(IV)-hydrazido product, cis-[Os(IV)(tpy)(NCCH3)2(NN(CH2)4O)](2+), and OPPh3. This reaction presumably occurs by net double Cl-atom transfer to PPh3 to give Cl2PPh3 that subsequently undergoes hydrolysis by trace H2O to give the final product, OPPh3. In the X-ray crystal structure of the Os(IV)-hydrazido complex, the Os-N-N angle of 130.9(5) degrees and the Os-N bond length of 1.971(7) A are consistent with an Os-N double bond.