Re-investigation of the reaction of phenylboronic acid (PhB (OH)(2)) with Alizarin Red S (ARS, H2L) was carried out using absorption and emission spectrophotometry to correct the currently proposed mechanism. Kinetic results obtained by both methods, which essentially show no significant differences, showed that the kinetic reactivity of ARS toward PhB OH)(2) was in the order H2L < HL-and that of phenylboronic acid toward HL-was PhB(OH)(2) > PhB(OH)(3) -. Acetic acid/acetate buffer accelerated the present reaction, though HEPES buffer did not. A detailed reaction mechanism involving the acceleration effect of the acetate buffer is proposed for the present reaction on the basis of a recent universal reaction mechanism.

A detailed kinetic study of the reactions of phenylboronic acid (PhB(OH)(2)), 2-methylphenylboronic acid (2-MePhB(OH)(2)), 2-isopropylphenylboronic acid (2-'PrPhB(OH)(2)), and 1-hydroxy-3H-2,1- benzoxaborole (BxB(OH)) with D-fructose was carried out to clarify the nature of the reactive boron species in D-fructose sensing and investigate the corresponding reaction mechanism. Both the boronic acids (RB(OH)(2)) and boronate ions (RB(OH)(3)(-)) were reactive toward D-fructose, while out of the five D-fructose anomers only alpha-D-fructofuranose was reactive toward boron species. The reactions of all substrates proceeded consecutively in two steps (steps 1 and 2). We concluded that the first intermolecular step (step 1) corresponds to the parallel reactions (two parallel reactions for 2-'PrPhB(OH)(2) and three for the other systems) of the boronic acid and the boronate ion with alpha-D-fructofuranose to form bicoordinate complexes (mixture of exo-and endo-isomers), and the second intramolecular step (step 2) corresponds to the formation of a tricoordinate alpha-D-fructofuranose complex from the bicoordinate complexes. It was found that both the boronic acid and the boronate ion were kinetically reactive toward D-fructose, with the latter being more reactive.

The reaction of alpha-fructose with diphenylborinic acid (Ph2B(OH)), to which a-fructose cannot coordinate tridentately, was kinetically investigated in order to solve the reaction mechanism, and to find a clue to resolve the two-step reaction mechanism of boronic acid (RB(OH)(2)) with a-fructose. The title reaction was a single-step reaction under the pseudo first-order conditions with large excess a-fructose over Ph2B(OH), which suggests that the reaction of RB(OH)(2) with a-fructose consists of the fast formation of a two-coordinate complex followed by the slow formation of a three-coordinate complex. The kinetic reactivity of Ph2B(OH) was considerably higher than that of its conjugate base, diphenylborinate ion (Ph2B(OH)(2)(-). It was shown that Ph2B(OH)(2)(-) interacted with CHES buffer to form an outer-sphere complex, whereas no interaction was observed between Ph2B(OH) and CHES buffer. Both the free borinate ion and the restrained borinate ion into the outer-sphere complex react with a-fructose to form the chelate complex via stable binary and ternary outer-sphere complexes, respectively. (C) 2015 Elsevier B.V. All rights reserved.

The axial ligand substitution reactions of head-to-head (HH) pivalamidato-bridged platinum(III) binuclear complexes containing equatorial Br- ligands, ([(H2O)(NH3)(2)Pt(mu-pivalamidato)(2)Pt(Br)(2)(OH2)](2+) (complex 1) and [(Br)(NH3)(2)Pt(mu-pivalamidato)(2)Pt(Br)(2)(Br)] (complex 2)) with acetone were kinetically investigated in acidic aqueous solutions. Both the complexes reacted with acetone slowly and acetonyl Pt-III binuclear complexes were formed in two steps (1 and 2). The observed pseudo first-order rate constants (k(obs1)) for step 1 of the reactions of complexes 1 and 2 were linearly and quadratically dependent on the excess ligand concentration (C-L) at a given [H+], respectively. The rate constants for step 2 in both the reactions were independent of both C-L and [H+]. A reaction mechanism involving an intermediate with one p-coordinated acetone molecule was proposed for complex 1, which is similar to those proposed previously for the reactions of HH tetraammine amidato-bridged diplatinum(III) complexes with acetone. In contrast, a reaction mechanism involving an intermediate with two p-coordinated acetone molecules on the axial sites was proposed for complex 2. (C) 2015 Elsevier B.V. All rights reserved.

The title cyclometalated platinum(II) complex with 2-(4-bromophenyl)pyridinato and acetylacetonato ligands, [Pt(C11H7BrN)(C5H7O2)], consists of two crystallographically non-equivalent dimers, each stacked by π-π interactions with distances of ≃ 3.4Å. In both dimers, the platinum(II) complexes are arranged antiparallel to each other. Each complex exhibits a slightly distorted square-planar coordination environment around the central Pt(II) atom. The dihedral angles between two chelate rings including the PtII atom in these complexes are 0.08(12) and 1.54(9)°.

Six-coordinate [Cu(pdt)(2)(H2O)(2)](2+) and four-coordinate [Cu(pdt)(2)](+) complexes were synthesized and the cross redox reactions were studied in acetonitrile (pdt = 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine). Single crystal analyses revealed that [Cu(pdt)(2)(H2O)(2)](BF4)(2) was of pseudo-D-2h symmetry with two axial water molecules and two symmetrically coordinated equatorial pdt ligands, while the coordination structure of [Cu(pdt)(2)]BF4 was a squashed tetrahedron (dihedral angle = 54.87 degrees) with an asymmetric coordination by two pdt ligands: one pdt ligand was coordinated to Cu(I) through pyridine-N and triazine-N2 while another pdt ligand was coordinated through pyridine-N and triazine-N4, and a stacking interaction between the phenyl ring on one pdt ligand and the triazine ring on another pdt ligand caused the squashed structure and non-equivalent Cu-N bond lengths. The cyclic voltammograms for [Cu(pdt)(2)(H2O)(2)](2+) and [Cu(pdt)(2)](+) in acetonitrile were identical to each other and quasi-reversible. The reduction of [Cu(pdt)(2)(H2O)(2)](2+) by decamethylferrocene and the oxidation of [Cu(pdt)(2)](+) by [Co(2,2'-bipyridine)(3)](3+) in acetonitrile revealed that both cross reactions were sluggish through a gated process (the structural change took place prior to the electron transfer) accompanied by slow direct electron transfer processes. It was found that the triazine ring of the coordinated pdt ligand rotates around the C-C bond between the triazine and pyridine rings with the kinetic parameters k = 51 +/- 5 s(-1) (297.8 K), Delta H-double dagger = 6.2 +/- 1.1 kJ mol(-1) and Delta S-double dagger = -192 +/- 4 J mol(-1) K-1. The electron self-exchange process was directly measured using the line-broadening method: k(ex) = (9.9 +/- 0.5) x 10(4) kg mol(-1) s(-1) (297.8 K) with Delta H-double dagger = 44 +/- 7 kJ mol(-1) and Delta S-double dagger = 0.2 +/- 2.6 J mol(-1) K-1. By comparing this rate constant with the self-exchange rate constants estimated from the cross reactions using the Marcus cross relation, the nonadiabaticity (electronic) factors, kappa(el), for the direct electron transfer processes between [Cu(pdt)(2)](+/2+) and non-copper metal (Fe2+ and Co3+) complexes were estimated as ca. 10(-7), indicating that the electronic coupling between the d orbitals of copper and of non-copper metals is very small.

New iron(II) complexes were synthesized with two tridentate hybrid ligands having phosphorous and nitrogen donor sites, in order to quantitatively estimate the difference of the ligand-field strengths of phosphorous and nitrogen donor sites in cationic metal complexes. Iron(II) complexes with bis(dimethylphosphinoethyl)amine (PNP) and 2,6-bis(diphenylphosphinomethyl)pyridine (PpyP) ligands crystallized as un-symmetric facial-[Fe(PNP)(2)](PF6)(2)center dot CH3NO2 and mer-[Fe(PpyP)(2)](CF3SO3)(2), respectively, as expected from the steric congestion and from the tendency to avoid the mutual trans influence between two phosphorous donor sites. Both complexes are in the low-spin electronic state up to 400 K. The pseudo-D (4h) coordination geometry of the PpyP complex made it possible to separate axial (2 x N) and equatorial (4 x P) contributions to the overall ligand-field by means of a spectrometric method: the difference in the ligand-field strengths by the equatorial Ph2P-donor sites and by the axial 2,6-disubstituted pyridine donor sites is ca. 13,200 cm(-1). A significantly reduced inter-electronic repulsion parameter (425 cm(-1) for both PNP and PpyP complexes) from the value of the free ion (1,060 cm(-1)) indicates covalent interaction between the Fe(II) and P atoms even in these cationic metal complexes. It is shown that the degree of covalency as well as the coordination bond strengths between various metal ions and phosphorous/nitrogen donor atoms is successfully explained by the relative energy levels of interacting atomic orbitals calculated on the basis of the Thomas-Fermi-Dirac potential.

The reduction reaction of the Cu(II)-pitn complex (pitn = 1,3-di(pyridine-2-carboxaldimino)propane) by decamethylferrocene [Fe(Cp*)(2)] was examined in acetonitrile. The observed pseudo-first-order rate constants exhibited saturation kinetics with increasing excess amount of [Fe(Cp*)(2)]. Detailed analyses revealed that the reaction is controlled by a structural change prior to the electron transfer step, rather than a conventional bimolecular electron transfer process preceded by ion pair (encounter complex) formation. The rate constant for the structural change was estimated to be 275 +/- A 13 s(-1) at 298 K (a dagger H* = 33.3 +/- A 1.0 kJ center dot mol(-1), a dagger S* = 86 +/- A 5 J center dot mol(-1)center dot K-1), which is the fastest among gated reactions involving CuN4 complexes. It was confirmed by EPR measurement and Conflex calculations that the dihedral angle between the two N-N planes is significantly large (40A degrees) in solution whereas it is merely 17.14A degrees in the crystal.

To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)(2), and a diol, H2L, in aqueous solution, the acid dissociation constants (C) of boronic acid diol esters (HBLs) were determined based on the well-established concept of conditional formation constants of metal complexes. The pK(a) values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4-difluorophenylboronic acid and chromotropic acid, 3-nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K' based on the main reaction: RB(OH)(2) H2L reversible arrow(k1) RB(L)(OH)(-) + H3O instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B') with uncomplexed diol species (L') to form boronic acid diol complex species (esters,, BL') in aqueous solution at pH 5-11: B' + L' reversible arrow(K') BL'. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.

The following reaction systems, with and without proton ambiguity, were set up for direct measurements of the rate constants of borate and boronate ions: boric acid [B(OH)(3)] and 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (Tiron); 3-nitrophenylboronic acid [3-NO2PhB(OH)(2)] and Tiron; phenylboronic acid [PhB(OH)(2)] and 4-nitrocatechol; boric acid and 4-nitrocatechol. Each rate constant for boric or boronic acid is larger than that for the conjugate borate or boronate ion for the reaction systems without proton ambiguity. These results explicitly indicate that the widespread belief that the rate constants of boronate ions are several orders of magnitude larger than those of the conjugate boronic acids is erroneous. The relative kinetic reactivities of RB(OH)(2) and RB(OH)(3)(-) were investigated in relation to the linear free energy relationships between the rate constants and acidities of the boronic acids and diols.

Reaction systems of boronic acid (RB(OH2), R = phenyl or 3-fluorophenyl) with diols and no proton ambiguity were elaborately set up, and kinetic measurements were conducted to elucidate the relative reactivities of RB(OH)2 and RB(OH)3-. In the reactions of phenylboronic and 3-fluorophenylboronic acids with propylene glycol, the reactivity order was: RB(OH)2 &gt
&gt
RB(OH)3 -, whereas in the reactions of 3-pyridylboronic acid with Tiron and 2,2′-biphenol, the reactivity of RB(OH)2 was comparable to that of RB(OH)3-. These results are in contrast to those that have been previously reported, and widely accepted for over thirty years, that concluded that the reactivity of RB(OH)3- is several orders of magnitude higher than that of RB(OH)2. The reactivity of Tiron with 3-pyridylboronic acid is affected by the protonation of one of its sulfonate groups. © 2013 The Royal Society of Chemistry.

The pKas of 3-pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pKas assignable to the boron center and pyridine moiety. The pKa assignment performed by 11B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3-pyridylboronic acid [3-PyB(OH)2] and the N-methylated derivative [3-(N-Me)Py+B(OH)2] have strong acidities (pKa?=?4.4 for both). It was found that introduction of a substituent to pyridine-C atom in 3-pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine-N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3-pyridinium boronic acid [3-HPy+B(OH)2] with 4-isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3-HPy+B(OH)2 reacts with Hipt and protonated H2ipt+, and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3-HPy+B(OH)2 reacts with Hipt faster than its conjugate boronate [3-HPy+B(OH)3], which is consistent with our recent results. The reactivity of 3-(N-Me)Py+B(OH)2 towards Hipt was also examined kinetically; the reactivities of 3-(N-Me)Py+B(OH)2 and 3-(N-Me)Py+B(OH)3 are almost the same as those of their original 3-HPy+B(OH)2 and 3-HPy+B(OH)3, respectively. Copyright (c) 2012 John Wiley & Sons, Ltd.

The structure of a methanesulfonate salt of 4-(N-methyl)pyridinium boronic acid, [4-(N-Me)PyB(OH)₂]CH₃SO₃·H₂O, was determined by X-ray crystallography, and was characterized as: P1, a = 5.6937(16), b = 7.805(2), c = 13.522(3)Å, α = 99.46(3), β = 91.45(3), γ = 108.014(17)°, Z = 2, V = 561.8(2)ų. In the crystals, the cation and the anion are linked by BOH…O(anion), BOH…O(water), and (water)OH…O(anion) hydrogen bonds to stabilize the 1:1:1-cation:anion:water salt unit.

Reactions of head-to-head (HH) alpha-pyridonato- and pivalamidato-bridged platinum(III) binuclear complexes with acetone were investigated kinetically. The reactions with acetone proceeded slowly to form acetonyl Pt(III) binuclear complexes in two steps. A mechanism, which involves pi-coordination of the enol form of acetone in the first step, followed by the pi-sigma bond conversion in the second step, is proposed. pi-Coordination of the enol was strongly supported by kinetic analyses of a ligand substitution reaction involving the pivalamidato-bridged Pt(III) binuclear complex with (4-hydroxyphenyl) pyruvic acid. Only the enol form was reactive towards the PtIII dimer complex.

Reactions of a pivalamidato-bridged head-to-head (HH) platinum(III) binuclear complex with 2-methyl-1,3-butadiene (isoprene) and p-styrenesulfonate and of an a-pyrrolidonato-bridged HH platinum(III) binuclear complex with p-styrenesulfonate were studied kinetically using UV-vis spectrophotometry and (1)H NMR spectroscopy, and detailed reaction mechanisms are proposed. Pt(III) binuclear complexes react with p-styrenesulfonate in four successive steps with mechanisms similar to that for an HH alpha-pyridonato-bridged Pt(III) binuclear complex with p-styrenesulfonate. In the case of isoprene, four steps were observed on the basis of UV-vis spectrophotometry. However, the reaction kinetics for steps 1 and 2 correspond to those for the previous reaction system, and those for steps 3 and 4 do not correspond to those for the previous system or to those observed by using (1)H NMR spectroscopy for the present isoprene system. By using UV-vis spectrophotometry, it was shown that isoprene preferentially pi-coordinates to the Pt(N(2)O(2)) atom via the double bond adjacent to the methyl group in step 1. In step 2, a second isoprene molecule pi-coordinates to the Pt(N(4)) atom, which is the rate-determining step, followed by nucleophilic attack of a water molecule on the pi-coordinated isoprene on the Pt(N(2)O(2)) atom to form two isomeric sigma-complexes. In the same step, pi-coordinated isoprene on the Pt(N4) atom of the s-complexes is released. This is different from the reaction of the Pt(III) binuclear complexes with other olefins. In step 3, reductive elimination of the s-complexes occurs to form two diols and the HH pivalamidato-bridged Pt(II) binuclear complex. Finally, acid decomposition of the Pt(II) binuclear complex occurs to form monomers in step 4. From (1)H NMR spectroscopic observations, fast isomerization between s-complexes and reductive elimination of the s-complexes occurs in step 3, and isomerization from a 1,4-diol to a 1,2-diol occurs in step 4.

The structure of methanesulfonate salt of 3-(N-methyl)pyridinium boronic acid, [3-(N-Me)PyB(OH)2]CH3SO3, was determined by X-ray crystallography. The boronic acid salt existed in crystals as a monomeric form with a pair of strong O-H...O-S hydrogen bonds, and was characterized as: P21/c, a = 8.9460(18), b = 8.8380(18), c = 13.850(3)Å, β = 105.28(3)°, Z = 4, V = 1056.3(4)Å3. © 2011 The Japan Society for Analytical Chemistry.

Kinetic studies were performed on the reactions of phenylboronic acid with L-lactic acid and mandelic acid in acidic aqueous and alkaline solutions in order to specify reactive species in these reactions. It was confirmed that the diprotonated ligand (H2L: L-lactic acid or mandelic acid) is less reactive than the monoprotonated ligand (HL-: L-lactate ion or mandelate ion), which made possible direct determination of the rate constants of phenylboronic acid (PhB(OH)(2)) and its conjugate base, phenylboronate ion (PhB(OH)(3)(-)). It was found that PhB(OH)(2) is more reactive than PhB(OH)(3)(-). On the basis of kinetic results, it was concluded that the most reactive species are PhB(OH)(2) and HL- at physiological pH 7.4, so the reaction in the boronic acid-based sensor for L-lactate mainly would occur between these species. (C) 2010 Elsevier B.V. All rights reserved.

The rate constants for a boronate ion were determined for the first time using the reaction systems of 3-nitrophenylboronic acid (3-NO2PhB(OH)(2)) with ethylene glycol (EG) and propylene glycol (PG) in an alkaline solution: the rate constants (25 degrees C, I = 0.10 M) for the reactions of 3-NO2PhB(OH)(3)(-) are 1.2 M-1 s(-1) (EG) and 1.5 M-1 s(-1) (PG), which are at least 103 times smaller than those for the reactions of 3-NO2PhB(OH)(2) [1.0 X 10(4) M-1 S-1 (EG) and 5.8 x 10(3) M-1 S-1 (PG)].

Two ruthenium(II) complexes, [Ru(bpy)(2)(dhbpy-H-1)](+) and [Ru(bpy)(2)(dhphen)](2+) (bpy=2,2'-bipyridine, dhbpy=3,3'-dihydroxy-2,2'-bipyridine, dhphen=5,6-dihydroxy-1,10-phenanthroline) were examined for use as a colorimetric reagent for the determination of boron. The reactions of boric acid with these two complexes were thermodynamically and kinetically studied in detail in order to specify the reactive species and to set up optimum condition for the determination of boron. The detailed analysis of the kinetic data for the reaction of the latter water-soluble complex showed that both boric acid and borate ion were reactive in an alkaline solution. The thermodynamically and kinetically optimum pH for the determination of boron was ca. 9 at 25 degrees C. A spectrofluorimetric determination of boron with the latter complex was attempted at 600 nm with excitation at 360 nm, and at pH 8.9 using CHES (N-cyclohexyl-2-aminoethanesulfonic acid) buffer. It was found that a trace amount of boron as low as ca. 2 x 10(-5) M (similar to 1 ppm) could be detectable. (C) 2007 Elsevier B.V. All rights reserved.

That boronic acid is a reactive species toward a diol moiety even in an alkaline solution and that the boronate ion is not very reactive were demonstrated by the estimated upper limit of the rate constants for the reactions of some boronic acids with 2,2'-biphenol and 2,3-dihydroxynaphthalene in a neutral-alkaline solution, which will correct a common misunderstanding in boron chemistry and would renew the idea of effective boronic acid sensor design for carbohydrates.

The axial water ligand substitution reaction on the head-to-head (HH) and head-to-tail (HT) amidato-bridged cis-diammineplatinum(III) binuclear complexes, HH- and HT-[Pt-2(NH3)(4)(mu-amidato)(2)(OH2\)(2)](4+) (amidato = alpha-pyridonato, alpha-pyrrolidonato, or pivalamidato), with halide ions (X- = Cl-, Br-) in acidic aqueous solution reveals a consecutive 2-step kinetics under the pseudo first-order conditions of a large excess concentration of halide ion relative to that of the complex (C-X- >> C-Pt,), corresponding to formation of the monohalo (step 1) and the dihalo complexes (step 2). The first substitution step of all the HH and HT diaqua dimers consists of two parallel reaction pathways: one is the simple substituion path of one of the coordinated aqua ligands with X-, and the other is the replacement in the aquahydroxo complex which is the conjugate base of the diaqua dimer. In step 2, there are three reaction pathway patterns: the direct substitution path, the path via a coordinatively unsaturated intermediate, and the unusual path of OH- replacement. Step 2 proceeds via either one or two paths of the three depending on the nature. of the halide ion as well as the bridging ligand. On the other hand, the substitution reaction of the hydrogenphosphato-bridged lantern-type complex, [Pt-2(mu-HPO4)(4)(OH2)(2)](2-) proceeds in 1-step at C-X- > C-Pt, in which the first aqua ligand substitution to form the monohalo species is rate-determining and the monohalo species is in rapid equilibrium with the dihalo complex. The rate-determining step consists of parallel pathways similar to step I in the amidato-bridged complex systems. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

Head-to-head bis(alpha-pyridonato)-bridged bis(ethylenediamine) dipalladium( II), HH-[Pd-2(en)(2)(alpha-pyridonato) (2)](ClO4)(2), was synthesized and structurally characterized by X-ray crystallography. The H-1 NMR spectra show that the head-to-head (HH) dimer produces the head-to-tail (HT) dimer and monomers ([Pd(en)(alpha-pyridone)(2)](2+), [Pd(en)(H2O)(alpha-pyridone)](2+), [Pd(en)(H2O)(2)](2+), etc.) in aqueous solution, and the relative amount of dimers to monomers is dependent on the total concentration of the HH dimer dissolved as well as the acidity of the solution. It was found that the formation of the HH and HT dimers from the monomers is fast, and the HT dimer is produced from the HH dimer only via coexisting monomers, i.e., there is no direct isomerization path between the HH and HT dimers. The kinetic analyses for the HH = HT isomerization reaction with time-resolved H-1 NMR measurements revealed that the reaction proceeds via first-order kinetics, which was explained based on a relaxation process. The rate determining step for HH = HT isomerization is the reaction step between the mono-alpha-pyridone complex and the bis-alpha-pyridone complex, [Pd(en)(H2O)(alpha-pyridone)](2+) + alpha-pyridone = [Pd(en)(alpha-pyridone)(2)](2+).

reaction of the pivalamidate-bridged platinum(III) dinuclear complex [Pt-2(NH3)(4)-((BuCONH)-Bu-t)(2)(OH2)(2)](4+) with phenol in water gave 2-hydroxyphenyl platinum dinuclear complex [Pt-2(NH3)(4)((BuCONH)-Bu-t)(2)(C6H4(OH))](3+) via ortho C-H bond activation in the axial position. The identical compound and the analogous phenyl complex [Pt-2(NH3)(4)((BuCONH)-Bu-t)(2)(C6H5 could alternatively be synthesized from the reactions Of [Pt-2(NH3)(4)((BuCONH)-Bu-t)(2)(OH2)(2)](4+) with the corresponding arylboronic acids via transmetalation. The reactivities of the novel aryl-Pt-2 compounds toward nucleophiles and the effect of UV light irradiation were investigated. The ESR spectra showed that the axial aryl-Pt bond is homolytically cleaved by LTV light irradiation to give the aryl radical and the Pt(III)-Pt(II) paramagnetic complex.

Which of the step is rate determining in the complexation of boronic acid with bidentate ligands, the first trigonal/tetragonal interconversion step or the following chelate-ring closure step ? No one could have ever answered this simple question clearly. In this study, we for the first-time gave an explicit answer to this question. Reaction systems were devised for kinetic measurements to answer this simple question. At first, pK(a), values of 15 boronic acids were determined spectrophotometrically at I = 1.0 M and 25 degrees C: pK(a) = 6.40 +/- 0.06 for 3,5-Cl2PhB(OH)2, 6.62 +/- 0.03 for 3,5-(CF3)(2)PhB(OH)(2), 6.95 +/- 0.09 for 3,5-Br2PhB(OH)(2), 7.04 +/- 0.03 for 3-NO2PhB(OH)(2), 7.08 +/- 0.03 for 3,5-F2PhB(OH)(2), 7.12 +/- 0.02 for 2,4-F2PhB(OH)(2), 7.39 +/- 0.02 for 4-CF3PhB3(OH)2, 7.50 +/- 0.02 for 3-FPhB(OH)(2), 7.83 +/- 0.02 for 2-FPhB(OH)(2), 7.87 +/- 0.01 for 3-CF2PhB(OH)(2), 8.11 +/- 0.01 for 2-ClPhB(OH)(2), 8.14 +/- 0.03 for 3-(COOH)PhB(OH)(2), 8.15 +/- 0.03 for 3-(CH3CO)Ph(OH)(2), 8.20 +/- 0.01 for 3-ClPhB(OH)(2), and 8.66 +/- 0.05 for 4-FPhB(OH)(2). The kinetic results for the reactions of some of these boronic acids with 4-isopropyltropolone clearly show that the rate-determining step is not the interconversion step but the chelate-ring closure step. (c) 2005 Elsevier B.V. All rights reserved.

The rate constant, k(1), for the following addition reaction between 4-isopropyltropolonate (ipt(-)) and meta-nitrophenylboronic acid (m-NO2PhB(OH)(2)) in acetonitrile was measured at various temperatures by using a low-temperature stopped-flow spectrophotometer:
m-NO2PhB(OH)(2) + ipt(-) reversible arrow m-NO2PhB(OH)(2)(ipt)(-);
k(1) = 7.9 x 10(4)M(-1) s(-1) at 25 degrees C, Delta H-1* = 34.7 +/- 1.0 kJ mol(-1), Delta S-1* = -34.8 +/- 3.8 J mol(-1) K-1, which indicates that the intercoriversion between the trigonal boronic acid and the tetrahedral boronate ion is much faster than the chelate formations for the complexation of boronic acid with bidentate ligands, but much slower than the diffusion-controlled reactions. (C) 2005 Elsevier B.V. All rights reserved.

The reaction volume corresponding to the self-exchange process of the [Ni(tacn)(2)](3+/ 2+) couple was determined in aqueous acidic solution. Theoretical equations on the basis of the Mean Spherical Approximation were proposed for the estimation of reaction volumes for Mn+/(n-1)+ couples in solution, and the calculated reaction volumes were compared with the experimentally estimated values. The activation volume for the [Ni(tacn)(2)](3+/2+) couple was determined in the acidic condition from the cross reaction of [Ni(tacn)(2)](2+) and [Fe(o-phen)(3)](3+) at elevated pressures. The agreement of the experimentally estimated activation volume for the [Ni(tacn)(2)](3+/ 2+) couple, -8.2 +/- 2.4 cm(3) mol(-1), with the theoretically calculated value, -7.5 cm(3) mol(-1), within the allowed uncertainty (+/- 1 cm(3) mol(-1)) indicates that the electron self-exchange reaction of this redox couple obeys the Marcusian behavior in aqueous acidic solution.

Homogeneous electron transfer reactions of the Cu(II)complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the pi-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu2+ was determined as a function of the water concentration under the pseudo first-order conditions where Cu2+ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)(6)](2+)(AN=acetonitrile) by water, and it was concluded that only the Cu2+ species fully solvated by acetonitrile, [Cu(AN)(6)](2+), possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)(2)](3+) (tacn=1,4,7-triazacyclononane) and [Ru(bpy)(3)](3+) (bpy=2,2'-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its pi-cation radical: log(k/M-1 s(-1))=9.5+/-0.5 for TPP and log(k/M-1 s(-1))=11.1+/-0.5 for OEP at 25.0degreesC. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.

The following successive axial ligand substitution reactions of the head-to-head (HH) 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex ([(H2O)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(H2O)](4+)) With halide ions X- (X- = Cl- and Br-) to give monohalogeno and dihalogeno complexes were studied kinetically:
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The acid dissociation constant (K-h1) Of the axial aqua ligand in the HH dimer and the formation constants (K-1(X) and K-2(X)) of the monohalogeno and dihalogeno complexes were determined spectrophotometrically to be -log(K-h1/M) = 1.71 +/- 0.04; log(K-1(Cl)/M-1) = 5.93 +/- 0.02, log(K-2(Cl)/M-1) = 3.71 +/- 0.00 for the reaction with Cl-, and log(K-1(Br)/M-1) = 6.20 +/- 0.05, log(K-2(Br)/M-1) = 4.55 +/- 0.01 for the reaction with Br-. The two platinum atoms are nonequivalent in the HH dimer, in which the first deprotonation occurs selectively to the water molecule on the Pt(N-4) atom and the first nucleophilic substitution with X- occurs preferentially to the Pt(N2O2) atom, where the atoms in parentheses are the coordinating atoms. The first water substitution with X- proceeds via two similar substitution paths, whereas the second substitution proceeds through two dissimilar paths: one is a simple substitution path and the other is via dissociation of the water molecule, providing a coordinatively unsaturated complex. These reaction paths are reasonably explained by the relative strength of the trans effect of the water, hydroxide, and halide ions in the monohalogeno complexes.

Kinetics for the reaction of m-nitrophenyl- (m-NO2Ph-), phenyl- (Ph-), methyl(Me-), and n-butyl- (n-Bu-) boronic acids with H-resorcinol has been studied at various temperatures and pressures. The pseudo first-order rate constant (k(obs)) exhibited saturation in the plot of k(obs) against the total boronic acid concentration (C-B) for the reactions of m-NO2PhB(OH)(2) and PhB(OH)(2), while the plot was linear for the reactions of MeB(OH)(2) and n-BuB(OH)(2), which leads to the following equations, respectively: k(obs) = k*KCB/(l + KCB) and k(obs) = k*KCB = k(f)C(B). It was experimentally shown that the reaction of boronic acids with H-resorcinol proceeds with rate determining second chelate-ring closure. (C) 1999 Elsevier Science S.A. All rights reserved.

A new high-pressure stopped-flow apparatus equipped with a push-pull coupler has been developed which enables one to monitor fast reactions in various organic solvents as well as in strongly acidic media under pressure up to 200 MPa. Sample solutions come into contact only with chemically inert materials: Daiflon, quartz, and a Teflon-coated O-ring. The reactant-syringe and receiver-syringe pistons are driven at the same time with the aid of a push-pull coupler and an actuator. The dead time of this apparatus is similar to 2 ms. (C) 1999 American Institute of Physics. [S0034-6748(99)01701-3].

The reaction volumes for the protonation of 4-isopropyltropolone (Hipt)(Delta V-H degrees: Hipt + H+ reversible arrow H(2)ipt(+)) and for the reaction of the Fe(III) ion with Hipt(Delta V(FeL)degrees: Fe3+ + Hipt reversible arrow Fe(ipt)(2+) + H+) have been determined spectrophotometrically in acidic aqueous media under pressure up to 250 MPa. The obtained values are: Delta V-H degrees (cm(3) mol(-1)) = +2.8 +/- 0.1, Delta V(FeL)degrees (cm(3) mol(-1)) = +7.8 +/- 0.1. The activation volume for the dissociation of Fe(ipt)(2+) was calculated to be -16.5 +/- 0.9 cm(3) mol(-1) by using Delta V(FeL)degrees and the reported activation volume for the formation of Fe(ipt)(2+). The negative volume of activation for the dissociation reaction confirms associative activation for the reaction of the Fe3+ ion.