Mahesh P. Pujari

Complexes of large ring macrocycles. The preparation of Me6[22]dieneN4, C-meso Me6[22]aneN4, C-racemic Me6[22]aneN4 and some binuclear copper(II) and nickel(II) complexes

Abstract The reaction of 1,6-diaminohexane monohydroperchlorate with acetone gives Me6[22]dieneN4· 2HClO4† in high yield (ca.85%). Reaction of the diene with NaBH4 in methanol gives a mixture of C-meso Me6[22]aneN4†† (m.p. 128 °C) and C-racemic Me6[22]aneN4 (m.p. 75 °C) which can be separated by fractional crystallisation from hot xylene. The free base form of Me6[22]dieneN4 has been characterised and molecular weight measurements by vapour pressure osmometry confirm the 22-membered ring structure. The ligands give binuclear methoxy-bridged complexes of the type [M2L(OCH3)2](ClO4)2 with metal ions such as copper(II) and nickel(II) L = C-meso Me6[22]aneN4 or Me6[22]dieneN4).

The palladium(II) promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine

Abstract The palladium(II)-promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine have been studied at 25 °C and I =0.1 M in the pH range 4–5. At a 1:1 metal to ligand ratio the peptide esters act as tridentate ligands, donation occurring via the terminal amino group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Due to the high Lewis acidity of Pd(II) rapid hydrolysis of the ester function by water and hydroxide ion occurs. Rate constants k OH and k H 2 O have been obtained for base hydrolysis and water hydrolysis of the coordinated peptide esters at 25 °C. The rate constants for base hydrolysis are 3.4 X 10 6 M −1 s −1 (L-alaglyOMe), 6.4 X 10 6 M −1 s −1 (gly-L-alaOMe) and 2.3 X 10 7 M −1 s −1 (gly-L-leuOMe). Base hydrolysis of the coordinated peptide esters is at least 10 6 times that of the free unprotonated ligand. Activation parameters have been obtained for both water and base hydrolysis of the Pd(II) complex of methyl L-alanylglycinate and possible mechanisms for the hydrolyses are considered.

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.

Acid dissociation kinetics of the copper(II) Ccomplex of 1,5,8,12-tetra-azacyclo-octadecane ([18]aneN4) and the nickel(II) complex of 1,5,8,12-tetra-azacycloheptadecane ([17]aneN4)

SummaryThe kinetics of the dissociation of [Cu([18]aneN49)]2+ and [Ni([17]aneN48)]2+ in acidic solution have been studied in detail. The dissociation of [Cu([18]aneN48)]2+ displays saturation kinetics beyond 0.4 mol dm−3 HC1O4 with values of kobs becoming independent of [HClO4]. The kinetic behaviour can be rationalised in terms of the scheme,

Metal complexes of cyclic diamines, dissociation kinetics of bis(1,5-diazacyclo-octane)nickel(II) in acidic and basic solution, and electrochemical studies

\begin{gathered} CuL^{2 + } + H^ + \mathop \rightleftharpoons \limits^K CuLH^{3 + } \hfill \\ CuLH^{3 + } \mathop \to \limits^k Cu^{2 + } + protonated ligand \hfill \\ \end{gathered}

The palladium(II) promoted hydrolysis of methyl, ethyl and isopropyl glycylglycylglycinate

with K = 64 dm3mol−1 and k=0.625 s−1 at 25 °C. Saturation kinetics are not observed in the dissociation of [Ni([17]aneN48)]2+ and significant amounts of the protonated complex do not occur even in 0.5 mol dm−3 HClO4. In this case protonation of the complex may be the rate-determining step. Dissociation of [Ni([17]aneN48)]2+ is 1010-fold faster than that of [Ni(cyclam)]2+ at 25 °C.

The preparation and characterisation of chromium(III) complexes of C-meso-5, 12-dimethyl-1, 4, 8, 11-tetraazacyclotetradecane (LM). Aquation and base hydrolysis kinetics ofcis- andtrans-[CrLMCl2]+

SummaryThe macrocyclic mono-oxotetraamine, 5-oxo-1,4,8,11-tetraazacyclotetradecane (mono-oxocyclam=LH) has been prepared by reaction of methyl acrylate with 2,3,2-tetra(1,9-diamino-3,7-diazanonane). The protonation constants of the ligand are log K1=9.40, log K2=6.65 and log K3=2.87 at 25 °C (I=0.1 mol dm−3 NaClO4). Detailed potentiometric studies of the interaction of the base with copper(II) and nickel(II) have been carried out. In the pH range 2.5–7.0 two complexes, [CuLH]2+ and [CuL]+, form; the deprotonated complex being 100% abundant at pH 7. For nickel(II), only [NiL]+ forms (log β11−1 = 3.90), the yellow low spin nickel complex reaching its maximum concentration above pH 6. The [CuL][ClO4] · H2O and [NiL][ClO4] 0.5 H2O complexes have been characterised in the solid state. The nickel(II) complex is square planar with a d-d band at 22625 cm−1.

Complexes of large ring macrocycles. The preparation of the C-meso- and C-racemic-diastereoisomers of Me6[18]aneN4 and characterisation of their ‘blue’ and ‘red’ copper(II) complexes

Abstract The preparation of the planar yellow [Ni([8]aneN 2 ) 2 ](ClO 4 ) 2 is described. The complex dissociates in basic solution, with rate = k OH [NiL][OH − ] (L = 1,5-diazacyclo-octane). At 25 °C, k OH = 4.5 x 10 −2 M −1 s −1 and the corresponding activation parameters are ΔH ≠ = 69.2 kJ mol −1 and ΔS 298 ≠ = −38.6 J K −1 mol −1 . Acid catalysed dissociation in quite slow even in strongly acidic solutions. The kinetic data in this case can be fitted to the expression K obs = k o + K H [H + ], where k o relates to a solvolytic pathway and k H to the acid catalysed pathway. At 60 °C, K o = 2 x 10 −5 s −1 and k H is 2 x 10 −5 M −1 s −1 . Possible mechanisms for these reactions are considered. The Ni(II)/Ni(III) redox couple for NiL n+ is irreversible on Pt using MeCN as solvent.