Robert W. Hay

Kinetic and thermodynamic studies of the copper(II) and nickel(II) complexes of glycylglycyl-L-histidine

The protonation constants of the tripeptide glycylglycyl-L-histidine (L-) have been determined at 25 degrees C and I = 0.1 mol dm-3 as log K 8.06, 6.82, and 2.80. Complexation with copper(II) can be represented by the series of equilibria [formula: see text] in the case of nickel(II) only the species [NiLH]2+, [NiL]+, and [NiLH-2]- are of importance with log beta 111 = 11.33(2); log beta 110 = 4.74(6), and log beta 11-2 = -6.93(1). The tripeptide acts as a quadridentate ligand to give complexes with copper and nickel with an amino group, two deprotonated amide groups and an imidazole pyridine nitrogen (Im-N3) as donors. At 1:1 ligand-to-metal ratios the purple copper(II) complex [CuLH-2]- is essentially 100% abundant above pH 7 and the planar yellow [NiLH-2]- above pH 8. The displacement of the tripeptide ligand from the nickel(II) complex by L-histidine has been studied kinetically over the pH range 7-8. There is a small solvolytic reaction and a reaction which is first-order in the hydrogen ion concentration. Under the experimental conditions employed, the reaction is essentially independent of the L-His concentration and displacement occurs by a proton-assisted nucleophilic pathway with rate-determining cleavage of the first nickel(II)-N(peptide) bond.

Reactions of platinum(II) anticancer drugs. Kinetics of acid hydrolysis of cis-diammine(cyclobutane-1,1-dicarboxylato)platinum(II) “Carboplatin”

Abstract The kinetics of acid hydrolysis of “carboplatin” [ cis -diammine(cyclobutane-1,1-dicarboxylato)platinum(II)] has been studied over a range of acidities at constant ionic strength ( I = 1.0 mol dm −3 ) and various temperatures. Loss of the substituted malonato ligand occurs in a biphasic reaction involving a ring opening step and a slower step in which loss of the monodentate malonato ligand occurs. At low acidities the first step k obs ( 1 ) is approximately ten times faster than the second step ( k obs ( 2 )). At higher acidities this ratio falls to ca four fold. Plots of k obs ( 1 ) vs [H + ] are linear with a positive intercept and the plots can be adequately described by the relationship k obs ( 1 ) = k 0 + k 1 [H + ] with k 0 = 6 x 10 −5 s −1 and k 1 = 3.3 x 10 −4 s −1 at 25°C and I = 1.0 mol dm −3 . The rate constant k obs ( 2 ) = k 2 [H + ] with k 2 = 1.1 x 10 −4 s −1 at 25°C and I = 1.0 mol dm −3 . The antitumour activity of carboplatin is discussed in terms of the kinetic results obtained.

Cyclic voltammetry study of the electrocatalysis of carbon dioxide reduction by bis(polyazamacrocyclic) nickel complexes

Abstract Two series of binuclear macrocyclic nickel(II) complexes with varying lengths of the chain linking the two macrocyclic rings were characterised by cyclic voltammetry under argon and CO 2 . The first series consisted of binuclear complexes [Ni 2 L 2–6 ] 4+ containing pentaaza macrocycles with (CH 2 ) n bridges ( n =2, 3, 4, 6) or a p -xylyl linkage (L 6 ). In general, the two nickel sites in the binuclear complexes behave independently with the currents corresponding to the simultaneous transfer of two electrons. The redox potentials are remarkably constant along this series, but the peak separations increase, reflecting slower electron transfer due to more effective adsorption on the electrode. Electrochemical data for the electrocatalytic reduction of CO 2 in MeCN/10% H 2 O revealed catalytic waves for CO 2 reduction with E p c close to −1.7 V and catalytic currents ( i p c ) which are about half those of the mononuclear complex, proposed to be due to steric constraints allowing strong interaction of only one nickel centre of the binuclear one on the surface. The catalytic currents increased slightly as the linking chain length increased as the stereochemical constrains were relaxed somewhat. There was also a splitting in the catalytic peaks of the bismacrocyclic complexes which could reflect two types of adsorbed catalyst sites. In the more sterically crowded series of complex, [Ni 2 L 7 ] 4+ along with the series of linked heptaaza macrocyclic complexes [Ni 2 L 9–11 ] 4+ much more positive redox potentials were observed due to both alkylation of the coordinated nitrogen atoms, which decreases the ligand field, and the introduction of steric barriers to axial coordination. These steric barriers prevented strong electrode interaction and led to a lower catalytic activity. Indeed, the complex [Ni 2 L 7 ] 4+ did not even show any interaction with CO 2 in dry acetonitrile. The complexes showed well separated peaks due to solution and surface catalytic activity, and the surface catalytic currents were now comparable to mononuclear complexes at the same effective concentration. We proposed that the less effective absorption on the electrode arising from ligand steric interactions places far fewer stereochemical constraints on the adsorption of both nickel centres to the same extent as the binuclear complex, and hence the catalytic currents for binuclear complex and mononuclear complex are comparable.

The preparation, chemistry and crystal structure of the nickel(II) complex of N-hydroxyethylazacyclam [3-(2′-hydroxyethyl)-1,3,5,8,12-penta-azacyclotetradecane nickel(II) perchlorate]. A new electrocatalyst for CO2 reduction

Abstract The reaction of formaldehyde and ethanolamine with the nickel(II) complex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) gives the nickel(II) complex of the macrocycle 3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane (L) which can be readily isolated as the perchlorate salt. The crystal structure of [NiL](ClO4)2 has been determined. The nickel atom is in an essentially planar environment with N1, N5, N8 and N12 acting as donors with NiN bond distances in the range 1.930–1.938 A The ligand has a trans III configuration of the sec-NH centres with chair six-membered and gauche five-membered chelate rings. The hydroxyethyl group on N3 is axial. The two perchlorate anions lie in the axial sites but the NiO(4) and the NiO(7) distances of 2.836(3) and 3.028(3) A indicate that there is no bonding interaction between these centres. In aqueous solution the complex is predominantly square planar. Addition of HCl leads to axial addition of chloride while addition of thiocyanate gives the trans- [NiL(NCS)2] complex. The ligand readily folds to give cis-complexes. Thus addition of ethylenediamine to [NiL]2+ gives cis-[NiL(en)]2+ in solution. The electrochemistry of [NiL]2+ has been studied in detail. The complex is a good electrocatalyst for the reduction of CO2. The complex was less active for hydrogen evolution in an acetate electrolyte than [Ni(cyclam)]2+ and thus appeared to be more active than the cyclam complex for CO2 reduction under these conditions.

A metallomicelle catalysed hydrolysis of a phosphate triester, a phosphonate diester and O-isopropyl methylfluorophosphonate (Sarin)

Abstract A metallomicelle based on the copper(II) complex of N,N,N′ -trimethyl-W-tetradecylethylenediamine is shown to be an excellent catalyst for the hydrolysis of 2,4-dinitrophenyl diethyl phosphate (DNPDEP), 2,4-dinitrophenyl ethyl methylphosphonate (DNPEMP) and O -isopropyl methylfluorophosphonate (Sarin). At pH 8 and35 °C the hydrolysis of DNPDEP is accelerated by 1.3 × 10 5 fold, while that of of DNPEMP is increased by 6.6 × 10 4 fold. The micelle is also an active catalyst for the hydrolysis of the nerve agent Sarin. A mechanism involving binding of the phosphoryl oxygen to copper(II) and intramolecular attack by coordinated hydroxide at the phosphorus centre is consistent with the kinetic data.

Padlock macrocyclic complexes. The synthesis of a range of nickel(II) complexes of N-alkyl azacyclams and the crystal structure of (3-ethyl-1,3,5,8,12-penta-azacyclotetradecane) nickel(II) perchlorate

Abstract A range of (3-alkyl-1,3,5,8,12-penta-azacyclotetradecane)nickel(II) complexes [NiL] (ClO 4 ) 2 (R = Me, Et, n -Pr, n -Bu, n -octyl, n -tridecyl, n -octadecyl) has been prepared and characterised. The planar octahedral equilibrium in solution has been studied in detail for R = Et. For this equilibrium Δ H 0 = −19.2±0.5 kJ mol −1 and Δ S 0 = −76.4±1.5 JK −1 mol −1 , with K = 0.24 at 25°. The crystal structure of 3-ethyl-1,3,5,8,12-penta-azacyclotetradecane)nickel(II) perchlorate has been determined. The complex is square planar and the NiN bond lengths are 1.959(5) and 1.906(5) A. Both six-membered chelate rings adopt a chair conformation and the five-membered rings are gauche with the sec -NH centres having the RSRS configuration. The NEt group is axial.

Copper(II) and zinc(II) complexes of N,N-bis(benzimidazole-2-ylmethyl)-amine. Synthesis, formation constants and the crystal structure of [ZnLCl]2]. MeOH. Catalytic activity of the complexes in the hydrolysis of the phosphotriester 2,4-dinitrophenyl diethyl phosphate

Abstract The copper(II) and zinc(II) complexes of N,N-bis(benzimidazole-2-ylmethyl)amine (L) have been prepared and the crystal structure of [ZnLCl2]. MeOH determined. The complex is five-coordinate with a distorted square pyramidal geometry on zinc. Two pyridine nitrogen donors from the imidazole groups, the secondary amino group and one chloride act as in-plane donors with the axial site occupied by chloride. The zinc(II) lies some 0.65 A above the basal plane. The stepwise protonation constants of the ligand have been obtained, log K11=5.638(3) and log K12=10.12(1) and the stability constants (log β1mh) for the copper(II) and zinc(II) complexes determined using 50% MeOH–H2O as solvent. The hydroxoaqua complexes [ML(OH) (OH2)]+ show only slight catalytic activity in the hydrolysis of the phosphotriester 2,4-dinitrophenyl diethyl phosphate.

Facile synthesis of the nickel(II) complex of a new pendant arm macrocycle

The reaction of nickel(II), ethylenediamine, formaldehyde and primary amines has been shown to give the nickel(II) complex of the macrocycle (I)(1). We have now extended this synthesis to the preparation of the new macrocycle (2), by using ethanolamine as the primary amine. This reaction gives high yields of (2), with pendant hydroxyl groups which allow attachment to solid supports or incorporation into polymer systems.

The copper(II)-2,2′-dipyridylamine promoted hydrolysis of glycine ethyl ester. Kinetic evidence for intramolecular attack by coordinated hydroxide

Abstract The hydrolysis of glycine ethyl ester (EtGly) is catalysed by the copper(II) complex [Cu(DPA)]2+ (DPA = 2,2′-dipyridylamine). The reaction has been studied by pH-stat over the pH range 5.8–7.0 at 25°C and I = 0.1 mol dm−3 (KNO3). The kinetic and equilibrium results can be interpreted in terms of the kinetic scheme Hydrolysis takes place by reaction of coordinated hydroxide with the nitrogen-bonded amino acid ester. The pK for ionization of the coordinated water molecule is 6.78 and K = 1.70×107 dm3 ol−1 determined from the kinetic results. The pK determined by direct potentiometric titration of the complex [Cu(DPA)(OH2)2]2+ is 6.74. The rate constant k for intramolecular attack of coordinated hydroxide on the ester is 4.9×10−4 s−1. At pH 7.7, where the hydroxo complex is completely formed, hydrolysis of the complexed ester is ca 60 times faster than that of the uncomplexed ester. The rate enhancement compared with the free unprotonated ester is ca 103 fold. In this system it has been possible, for the first time, to define the involvement of coordinated hydroxide ion in the copper(II)-catalysed hydrolysis of amino acid esters.

THE CHEMISTRY OF NICKEL(II) COMPLEXES OF 1,4,7,11-TETRA-AZACYCLOTETRADECANE (ISOCYCLAM)

Abstract The cyclic tetradentate ligand 1,4,7,11-tetra-azacyclotetradecane (isocyclam, = L) has been prepared by the Richman-Atkins procedure. The nickel(II) complexes of the ligand have been studied in detail. The complex [NiL](ClO4)2 undergoes a square planar ⇋ octahedral equilibrium in aqueous solution: [NiL]2+ + 2H2O ⇋K [NiL(OH2)2]2+; the equilibrium constant K has been determined as a function of temperature and ΔH° = −22.6 ± 1 kJ mol−1 and ΔS° = −74.9 ± 2 J K−1 mol−1. Reaction of ethylenediamine with [NiL](ClO4)2 leads to folding of the macrocyle and formation of the blue octahedral cis-[NiL(en)](ClO4)2. The complex [NiL]2+ displays a reversible one-electron reduction at −1.45 V versus SCE in aqueous solution and a reversible oxidation to the nickel(III) complex at +0.85 V versus SCE. [Ni(isocyclam)]2+ was found to be a good electrocatalyst for the two-electron reduction of CO2 to CO but is not as efficient as [Ni(cyclam)]2+.