Arup K. Basak

Hydrolysis of α-amino-acid esters in mixed-ligand complexes with 2,2′-bipyridylpalladium(II)

α-Amino-acid esters, NH2CH(R)CO2R′(L), interact with [Pd(bipy)(OH2)2]2+(bipy = 2,2′-bipyridyl) in aqueous solution according to the equilibrium (i)(R and R′= alkyl). The kinetics of hydrolysis of the ester group in the complexes [Pd(bipy)L]2+ have been studied by pH-stat methods and rate constants [Pd(bipy)(OH2)2]2++ NH3CH(R)CO2R′ [graphic omitted] [Pd(bipy){NH2CH(R)CO2R′}]2++ H3O++ H2O (i) have been obtained for the kinetic processes (ii) and (iii) where A = NH2CH(R)CO2– and L = methyl glycinate, ethyl glycinate, ethyl L-α-alaninate, ethyl L-β-phenylalaninate, ethyl picolinate, ethyl L-cysteinate, and methyl L-histidinate. For the first five esters substantial rate accelerations are [Pd(bipy)L]2++ H2O [graphic omitted] [Pd(bipy)A]++ R′OH + H+(ii), [Pd(bipy)L]2++ OH– [graphic omitted] [Pd(bipy)A]++ R′OH (iii) observed for base hydrolysis (factors of 1.6 × 105 for methyl glycinate to 3.3 × 107 for ethyl picolinate). The effects with methyl L-cysteinate and methyl L-histidinate are much less marked, as the mixed-ligand complexes with these ligands do not involve alkoxycarbonyl donors. Possible mechanisms for these reactions are discussed, and alternative reaction schemes involving a monodentate (N-bonded) ester species are considered. Activation Parameters have been determined for reactions (ii) and (iii) where L = methyl glycinate.

The copper(II) promoted hydrolysis of benzylpenicillin. Evidence for the participation of a Cu–OH species in the hydrolysis of the β-lactam ring

The pKa of benzylpenicillin (HL ⇄ L– or H+) relating to the carboxyl group ionisation has been determined to be 2.67 ± 0.005 at 10°C and 2.60 ± 0.008 at 5 °C and I= 0.1 mol dm–3. At a 1 : 1 ligand-to-metal ratio the interaction of copper(II) with benzylpenicillin can be described by the equilibria (i) and (ii) at 10 °C. The pK for the ionization [CuLH–1]⇄[CuLH–2]–+ H+ is 4.92 at, L–+ Cu2+⇄[CuLH–1]+ H+; log β11 – 1=–1.41 ± 0.1 (i), L–+ Cu2+⇄[CuLH–2]–+ 2H+; logβ11 –2=–6.33 ± 0.08 (ii) 10°C and I= 0.1 mol dm–3. No potentiometric evidence was obtained for a species [CuL]+. Possible structures for [CuLH–1] and [CuLH –2]– are considered involving deprotonation of the side chain amide group and a water molecule co-ordinated to copper(II). The copper(II)-promoted hydrolysis of benzylpenicillin to penicilloic acid has been studied over the pH range 4.3–5.3 at 30 °C and I= 0.5 mol dm–3 using non-complexing 2,6-dimethylpyridine-3-sulphonic acid buffers. The dependence of the rate on the copper concentration and the pH strongly suggests that the complex [CuLH–2]– is the active species in the reaction. Hydrolysis of the lactam ring of benzylpenicillin occurs by intramolecular attack of a Cu–OH species on the carbonyl group and not by intermolecular attack of ‘free’ hydroxide ion on the complex. The hydrolysis of [CuLH–2]– at pH 6 is some 6 × 106 fold faster than the hydrolysis of the anion L– at 30 °C.

Solution Equilibria and Kinetic Studies of the Copper(II) Promoted Hydrolysis of 8-Quinolyl Phosphate. A Large Rate Enhancement for the Hydrolysis of a Phosphate Monoester Dianion

Abstract The dissociation constants of 8-quinolyl phosphate (H3L+) at 10°C and I = 0.1 mol dm−3 (NaC104) have been determined (pKa values < 1.5, 4.12 and 6.46). At a 1:1 ligand to metal ratio, the interaction of copper(II) with 8-quinolyl phosphate can be described by the equilibria given below. The pKa values for the ionisation processes [CuHL]+ ⇄ [CuL] + H+ and [CuL(OH2)] ⇄ [CuL(OH)]− + H+ are 3.75 and 5.71, respectively. Kinetic studies establish that in the copper(II)-promoted hydrolysis the complex [CuL] is the active species with kCuL = 2.7 × 10−3 s_1 at 30°C and I = 0.1 moldm−3. Copper(II) ions promote the hydrolysis of the normally unreactive dianion L2- of the phosphate monoester by a factor of ca 106.

Aquation of α-cis-chloroaquoethylenediamine-N, N'-diacetatocobalt(III) in aqueous and in mixed solvents: A mechanistic study

SummaryThe displacement of chloride ligands from α-cis-chloro-aquoethylenediamine-N,N′-diacetatocobalt(III) in nonacidic aqueous solutions was followed conductimetrically at 30–45° and the products of aquation were characterised by conductance, spectral and ion-exchange techniques. The rate constants for aquation in aqueous media and in 1 : 4 v : v mixed solvents at 25° are: 4.0 × 10−5 s−1 in H2O, 2.71 × 10−5 s−1 in MeOH : H2O, 2.74 × 10−5 s−1 in EtOH: H2,O and 2.58 × 10−5 s−1 n in Me2CO : H2O. The corresponding ΔH* and ΔS* values have also been evaluated. Solvent polarity has a marked influence on the rate of chloride ion release. The aquation rate constants and the activation parameters have been correlated with solvent parameters,e.g. D, Y-values, Dimroths ET and Kosowers Z-values and, based on these correlations, a dissociative interchange (Id) mechanism is proposed rather than dissociative as observed for some other cobalt(III) complexes.

Kinetics and mechanism of the base hydrolysis of [Co(tren)CO3]+ (tren = 2,2′,2″-triaminotriethylamine)

Abstract The reactions of [Co(tren)CO 3 ] + in basic solution have been studied in detail. For opening of the carbonato ring, values of k obs (the observed first order rate constant at constant hydroxide ion concentration) correspond to the expression, K obs = k o + k OH [OH − ], where K o relates to the aquation pathway and k OH to base hydrolysis. At 30°C, k o = 7.0 x 10 −4 s −1 and k OH = 4.7 x 10 −3 M −1 s −1 at I = 1.0 M (KNO 3 ). The corresponding activation parameters are ΔH≠ = 22.7 kcal mol −1 and ΔtS≠ 298 = +2.2 cal K −1 mol −1 (for k o ) and ΔH≠ = 34.1 kcal mol −1 and ΔS≠ 298 = +43.5 cal K −1 mol −1 (for k OH ). The monodentate carbonato complex [Co(tren)- (OH)OCO 2 ] has been prepared in solution and its base hydrolysis studied in isolation. Only a base hydrolysis pathway was observed with k OH = 2.8 x 10 −4 M −1 s −1 at 50.2°C and I = 1.0 M (NaClO 4 ). The requisite activation parameters are ΔH≠ = 27.8 kcal mol −1 and ΔS≠ 298 = 10.9 cal K −1 mol −1 . Literature data on this latter reaction are considered in detail.

KINETICS AND MECHANISM OF THE METAL ION CATALYSED AQUATION OF cis-DIAQUOBIS(MALONATO) CHROMATE(III) ION IN AQUEOUS-ACID MEDIA

Abstract The Cu(II), Ni(II), Co(II) and Zn(II) catalysed dissociation of the cis-diaquobis(malonato)chromate(III) ion has been studied kinetically in perchloric acid solution over a temperature range. Under these conditions k obs = kH[H+] + k cat[M2+] where k obs is the observed first order rate constants and k H and k cat are the appropriate rate constants for the proton and metal ion catalysed pathways respectively. The metal ion exert marked catalytic effects with k cat following the sequence Cu(II) > Ni(II) > Co(II) > Zn(II). A possible mechanism for the metal promoted reaction is proposed and the relevance of these results to the mechanism of the trans → cis isomerisation of [Cr(mal)2(H2O)2]− considered.

Kinetics and mechanism of the copper(II) promoted hydrolysis of the methyl ester of ethylenediaminemonoacetate

SummaryBase hydrolysis of methyl ethylenediaminemonoacetate has been studied at I=0.1 mol dm−3 (NaClO4) over the pH range 7.4–8.8 at 25 °C. The proton equilibria of the ligand can be represented by the equations, where E is the free unprotonated ester species. Values of pK1 and pK2 are 4.69 andca. 7.5 at 25° (I=0.1 mol dm−3). For base hydrolysis of EH+, kOH=1.1×103 dm3 mol−1 s−1 at 25 °C. The species E is shown to undergo lactamisation to give 2-oxopiperazine (klactca. 1×10−3 s−1) at 25 °C. Formation of the lactam is indicated both by u.v. measurements and by isolation and characterisation of the compound.Base hydrolysis of the ester ligand in the complex [CuE]2+ has been studied over a range of pH and temperature, kOH25=9.3×104 dm3 mol−1 s−1 with ΔH≠=107 kJ mol−1 and ΔS298≠=209 JK−1 mol−1. Base hydrolysis of [CuE]2+ is estimated to be some 1055 fold faster than that of the free ester ligand. The results suggest that base hydrolysis occursvia a chelate ester species in which the methoxycarbonyl group of the ligand is bonded to copper(II).

The uncatalysed and the copper(II) promoted hydrolysis of 4-nitrophenyl L-leucinate

Abstract The uncatalysed hydrolysis of 4-nitrophenyl L-leucinate has been studied in detail over a range of pH and temperature at I =0.1 M (KNO 3 ). Base hydrolysis of the ester is strongly promoted by copper(II) ions. Rate constants have been obtained for the following reactions (where EH + is the N- protonated ester and E is the free base form) EH + + OH − → products E + OH − → products E + H 2 O → products CuE 2+ + OH − → products Base hydrolysis of the copper(II) complex CuE 2+ is 3.8 × 10 5 times faster than that of E and 75 times faster than that of EH + at 25 °C and I =0.1 M. Activation parameters for these reactions have been determined and possible mechanisms are considered.

Kinetics and mechanism of the acid-catalysed ring opening of cis[Co(en)2-(β-alaO)]2+

The preparation of cis-[Co(en)2(β-alaO)]I2 is described. Ring opening of the complex occurs in acidic solution. Kinetic studies establish that the reaction shows a first-order dependence on [H+] with rate = kH[Complex][H+]. Values of kH have been determined over a temperature range giving ΔH‡ = 59.9 kJ mol−1 and ΔS‡298 = -50 J K−1 mol−1. The solvent deuterium isotope effect kD,2O/kH,2O, for the ring opening process is 2.4, consistent with a mechanism involving a rapid pre-equilibrium protonation step followed by slow rate-determining ring opening. Mechanism involving concerted attack by H+ and H2O are excluded.

The Mechanism of the base catalysed carbonato ring opening of cis-[Co(cyclen)CO3]+

Abstract The kinetics of carbonato ring opening of cis -[Co(cyclen)CO 3 ] + (cyclen = 1,4,7,10-tetraazacyclododecane) in basic solution has been studied in detail over a temperature range. The reaction shows a complex dependence on the hydroxide ion concentration with both first and second order terms in [OH − ] occurring. The observed first order rate constant k obs is given by the equation Values of k 1 K 1 and k 2 K 1 K 2 have been obtained and the apparent activation parameters determined. A mechanism is developed which can rationalise all the kinetic data.