Satoshi Iwatsuki

Piezochromism and Related Phenomena Exhibited by Palladium Complexes

Piezochromic phenomena are explained by pressure perturbation to the HOMO and/or LUMO energy levels of the related electronic transition. The piezochromism of solid inorganic and organic materials has been investigated by examination of the phase transition phenomena. Specific electronic properties of the solids, acquired by tuning the external pressure, may be used as electronic devices and as pressure sensors. The effects of pressure perturbations on the absorption and emission spectra exhibited by solid palladium complexes are reviewed here. Related phenomena exhibited by platinum complexes and other metal complexes are included for comparison.

Kinetic Evidence for High Reactivity of 3-Nitrophenylboronic Acid Compared to Its Conjugate Boronate Ion in Reactions with Ethylene and Propylene Glycols

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 10(3) times smaller than those for the reactions of 3-NO 2PhB(OH)2 [1.0 x 10(4) M(-1) s(-1) (EG) and 5.8 x 10(3) M(-1) s(-1) (PG)].

Quinoline‐Based, Glucose‐Pendant Fluorescent Zinc Probes

Quinoline‐based tetradentate ligands with glucose pendants, N,N′‐bis[2‐(β‐d‐glucopyranosyloxy)ethyl]‐N,N′‐bis[(6‐methoxyquinolin‐2‐yl)methyl]ethylenediamine (N,N′‐6‐MeOBQBGEN) and its N,N‐counterpart, N,N‐6‐MeOBQBGEN, have been prepared, and their fluorescence‐spectral changes upon Zn binding were investigated. Upon excitation at 336 nm, N,N′‐6‐MeOBQBGEN showed weak fluorescence (ϕ≈ 0.016) in HEPES buffer (HEPES 50 mM, KCl 100 mM, pH 7.5). In the presence of Zn, N,N′‐6‐MeOBQBGEN exhibited a significant increase in fluorescence (ϕ=0.096) at 414 nm. The fluorescence enhancement is specific for Zn and Cd (ICd/IZn of 50% at 414 nm). On the other hand, N,N‐6‐MeOBQBGEN exhibited a smaller fluorescence enhancement upon Zn complexation (ϕ=0.043, λex=334 nm, λem=407 nm) compared with N,N′‐6‐MeOBQBGEN. Fluorescence microscopic analysis using PC‐12 rat adrenal cells revealed that N,N′‐6‐MeOBQBGEN exhibits a 1.8‐fold higher fluorescence‐signal response to Zn ion concentration compared with sugar‐depleted compound 2 (N,N′‐bis[(6‐methoxyquinolin‐2‐yl)methyl]ethylenediamine), due to its enhanced uptake into cells due to the targeting ability of the attached carbohydrates.

Thermosensitive gels incorporating polythioether units for the selective extraction of class b metal ions

Novel temperature-responsive copolymers of N-isopropylacrylamide and monoaza-tetrathioether derivative, were synthesized for the selective extraction of soft metal ions such as silver(I), copper(I), gold(III) and palladium(II) ion. The ratio between N-isopropylacrylamide group and monoaza-tetrathioether group in the copolymer was determined. The ratio between N-isopropylacrylamide group and monoaza-tetrathioether group varied in the range of 66:1-187:1. Each lower critical solution temperature (LCST) of the polymer solution was determined spectrophotometrically by the relative absorbance change at 750 nm via temperature of the polymer solution. Metal ion extraction using the copolymer with appropriate counter anions such as picrate ion, nitrate or perchlorate ion was examined. Soft metal ions such as silver(I), copper(I), gold(III) and palladium(II) ion were extracted selectively into the solid polymer phase. The extraction efficiency of a metal ion such as silver ion increased as the increase of the ratio of the monoaza-tetrathioether group to N-isopropylacrylamide group in the polymer. The quantitative extraction of class b metal ions as well as the liquid-liquid extraction of metal ions with monoaza-tetrathioether molecule was performed.

Kinetics and mechanisms of the axial ligand substitution reactions of platinum(III) binuclear complexes with halide ions

Abstract 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-[Pt2(NH3)4(m-amidato)2(OH2)2]4+ (amidato = α-pyridonato, a-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 (Cx bCpt), 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, [Pt2(m-HPO4)4(OH2)2]2 proceeds in 1-step at Cx bCpt, 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 1 in the amidato-bridged complex systems.

Cobalt(II) phosphine complexes stable in aqueous solution: spectroscopic and kinetic evidence for low-spin Co(II)P6 and Co(II)P3S3 with tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane

Abstract Co(II) species produced by the controlled potential electrochemical reduction of Co(III)P 3 S 3 and Co(III)P 6 complexes with 1,4,7-trithiacyclononane and tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane in aqueous solution were characterized at ambient temperature by the EPR and spectrophotometric methods: the Co(II)P 6 and Co(II)P 3 S 3 species are in the low-spin t 2g 6 e g 1 state with large Jahn–Teller distortion. Kinetic studies of the redox reactions involving these Co(III)/(II) species revealed that the electron self-exchange reactions for the Co(III)/(II) couples are very fast ( k ex ∼10 4 dm 3 mol −1 s −1 ), which is consistent with the results for other low-spin/low-spin Co(III)/(II) couples. It was concluded that the nephelauxetic effect of the P donor atom stabilizes the low-spin state in Co(II).

Pyridinium-based Task-specific Ionic Liquid with a Monothioether Group for Selective Extraction of Class b Metal Ions

A pyridinium-based task-specific ionic liquid (TSIL) with a monothioether group, [3-TPPy][NTf2], extracted typical class b metal ions, such as Ag(I), Cu(I), Pd(II), and Pt(II), in high selectivity. It was found that the composition ratio of the extracted Ag(I) and Cu(I) species depended on the TSIL concentration, and that TSIL extracted these metal ions through mono-S-coordinated complex formation at low TSIL concentrations. [3-TPPy][NTf2] can be recycled in the extraction-recovery process, which is of a great advantage for practical use in environmentally benign separation methods.

Formation mechanism of 2-methyl-2-buten-1,4-diol and 2-methyl-3-buten-1,2-diol from 2-methyl-1,3-butadiene on a head-to-head pivalamidato-bridged cis-diammineplatinum(III) binuclear 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 α-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 α-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 π-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 π-coordinates to the Pt(N(4)) atom, which is the rate-determining step, followed by nucleophilic attack of a water molecule on the π-coordinated isoprene on the Pt(N(2)O(2)) atom to form two isomeric σ-complexes. In the same step, π-coordinated isoprene on the Pt(N(4)) atom of the σ-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 σ-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 σ-complexes and reductive elimination of the σ-complexes occurs in step 3, and isomerization from a 1,4-diol to a 1,2-diol occurs in step 4.

Preparation, crystal structures and isomerization kinetics of cis- and trans-[Co(dtc)2(PHPh2)2]+: thermodynamically and kinetically stable cobalt(III)–P bonds through interplay of σ-donicity, π-acidity, and steric bulkiness

Novel cobalt(III)–diphenylphosphine complexes, cis- and trans-[Co(dtc)2(PHPh2)2]+, were synthesized and structurally characterized by X-ray crystallographic analyses and spectroscopic methods. The Co–P bond lengths in both isomers were shorter than those in the analogous cobalt(III)–bis(tertiary phosphine) complexes with sterically less bulky but more basic phosphine ligands: Co–P(1) = 2.2340(6) and Co–P(2) = 2.2258(7) A for the cis-isomer, and Co–P = 2.276(1) A for the trans isomer. The title complexes also exhibited a unique dynamic behavior: cis to trans isomerization was induced by irradiation with visible light, while thermal trans to cis isomerization took place at elevated temperatures. The absorbance change for the trans to cis isomerization reaction exhibited multi-exponential kinetic traces when no free PHPh2 was present in the solution. Such a complicated kinetic behavior was explained either by the slow dissociation of coordinated PHPh2 or by the abstraction of a P–H proton from coordinated PHPh2 through an acid–base interaction with trace water in the bulk solvent. By addition of an excess amount of PHPh2, the dissociation of coordinated PHPh2 as well as the basicity of impure water was suppressed, and a first-order kinetic trace was observed. Kinetic studies with excess free PHPh2 in acetonitrile revealed that the isomerization reaction takes place via an intramolecular twist mechanism: ΔH* = 120 ± 1 kJ mol−1 and ΔS* = 50 ± 18 J mol−1 K−1. AOM calculations indicate that the twist mechanism involves a spin state change (1A1g to 5A1′) during the activation process. The importance of the π-acidity of PHPh2 together with the cooperative effect of the spectator ligand (dtc−) was suggested to explain the thermodynamic and kinetic behaviors of these complexes.

Kinetic studies on cyclopalladation in palladium(II) complexes containing an indole moiety

Abstract Various Pd–C complexes have been developed to date, affording deep insights into the reaction intermediates in useful catalytic reactions in organic syntheses. Cyclopalladation is one of the most famous Pd–C bond formation reactions to generate the palladacycles. Indole is an electron-rich aromatic ring involved in the side chain of an essential amino acid, tryptophan (Trp), and Trp and its derivatives are important in biological systems, such as electron transfer in protein, cofactors for conversion of biological molecules and so on. Pd catalysts are also useful for syntheses of such indole derivatives, and the mechanisms are considered to be through the Pd–C intermediates. However, the detailed properties and formation mechanisms of Pd–indole species are still unclear. With these points in mind, we focus on Pd(II)–indole-C2 carbon bond formations using various Pd(II) complexes having an indole moiety, especially on the recent studies on the kinetic analyses for these cyclopalladation reactions and their detailed mechanisms.