Mark A.W. Lawrence
University of the West Indies
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Featured researches published by Mark A.W. Lawrence.
Journal of Coordination Chemistry | 2007
Mohammed Bakir; Mark A.W. Lawrence; Marvadeen A. Singh-Wilmot
The reaction between dpktch and CdCl2 in refluxing acetonitrile gave [CdCl2(η3-dpktch)] in good yield. Spectroscopic measurements divulge the coordination of dpktch and the elemental analysis confirmed its formulation. Optical measurements in N,N-dimethylformamide (dmf) and dimethylsulfoxide (DMSO) in the absence and presence of a proton donor/acceptor disclosed two highly sensitivity interlocked intra-ligand-charge-transfer transitions (ILCT) that are sensitive to their surroundings. Under basic conditions, a low-energy electronic transition with an extinction coefficient of 17,400 ± 2000 M−1cm−1 appeared at ∼403 nm and a peak minimum appeared at 326 nm. Under acidic conditions, a high energy electronic transition with extinction coefficient of 13,500 ± 2000 M−1cm−1 appeared at ∼330 nm and a shoulder appeared at ∼400 nm. The addition of an acid to a dmf solution of [CdCl2(η3-dpktch)] caused the disappearance of the low energy absorption band at 403 nm and a peak maximum appeared at 330 nm. The reverse was observed when a base was added to a DMSO solution of [CdCl2(η3-dpktch)]. Electrochemical measurements reveal reduction of coordinated CdCl2 and oxidation of electrodeposited cadmium metal along with ligand-based redox processes. X-ray crystallographic analysis on a monoclinic, P21/n single crystal of [CdCl2(η3-dpktch)], confirmed the N,N,O-coordination of dpktch and revealed digitated units of [CdCl2(η3-dpktch)] interlocked via a web of hydrogen bonds.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy | 2015
Mohammed Bakir; Mark A.W. Lawrence; Shameal McBean
The reaction between [dpktch] and [M(OAc)2] (M=group 12 metal atom) in refluxing CH3CN gave [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O (n=0 or 1). The infrared and (1)H NMR spectra are consistent with the coordination of [η(2)-O,O-OAc] and [η(3)-N,N,O-dpktch-H](-) and the proposed formulations. The electronic absorption spectra of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O measured in non-aqueous solvents revealed a highly intense intra-ligand-charge transfer (ILCT) transition due to π-π∗ of dpk followed by dpk→thiophene charge transfer. The electronic transitions of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O are solvent and concentration dependent. Spectrophotometric titrations of dmso solutions of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O with benzoic acid revealed irreversible inter-conversion between [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O and it conjugate acid [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch)]·nH2O pointing to ligand exchange between the acetate and benzoate anions. When CH2Cl2 solutions of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O were titrated with dmso, changes appeared pointing to solvolysis or ligand exchange reactions. Electrochemical measurements on dmso solutions of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O divulged irreversible redox transformations consistent with electrochemical decomposition of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O. The solid state structure of a single crystal of [Cd(η(3)-N,N,O-dpktch-H)2] obtained from a dmso solution of [Cd(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O confirmed the ligand scrambling of [M(η(2)-O,O-OAc)(η(3)-N,N,O-dpktch-H)]·nH2O. The extended structure of [Cd(η(3)-N,N,O-dpktch-H)2] revealed stacks of [Cd(η(3)-N,N,O-dpktch-H)2] locked via a network of hydrogen bonds. A significant amount of empty space (35.5%) was observed in the solid state structure of [Cd(η(3)-N,N,O-dpktch-H)2].
Journal of Coordination Chemistry | 2010
Mark A.W. Lawrence; Paul T. Maragh; Tara P. Dasgupta
The aquation of a series of [Cr2Fe(µ3-O)µ2-(RCO2)6(H2O)3]+ cations, where R = H, CH3, CH3CH2, and (CH3)2CH was investigated in aqueous perchloric acid, 0.01 ≤ [H+] ≤ 0.17 mol dm−3 and 25 ≤ θ ≤ 35°C. The mechanism is postulated as an equilibrium between the protons and the complex cation, where the oxygens of the carboxylate coordinated to the metal center are protonated, followed by Fe–O bond cleavage. This is followed by rapid decomposition to produce aqueous iron(III), a dinuclear chromium(III) species (which is further hydrolyzed), and carboxylate ions. First-order rate constants for the reactions at 25°C and 0.5 mol dm−3 ionic strength (NaClO4) and corresponding activation parameters are: R = (CH3)2CH; k 1 = (7.18 ± 0.07) × 10−5 s−1 (ΔH ‡= 52 ± 2 kJ mol−1, ΔS ‡= −151 ± 8 J K−1 mol−1), R = CH3CH2; k 1 = (13.67 ± 0.02) × 10−5 s−1 (ΔH ‡= 57.8 ± 0.6 kJ mol−1, ΔS ‡= −125 ± 2 J K−1 mol−1), R = CH3; k 1 = (17.6 ± 0.3) × 10−5 s−1 (ΔH ‡= 30 ± 5 kJ mol−1, ΔS ‡= −216 ± 16 J K−1 mol−1), R = H; k 1 = (3.86 ± 3.01) × 10−2 s−1 (ΔH ‡= 75 ± 1 kJ mol−1, ΔS ‡= −22 ± 5 J K−1 mol−1). Spontaneous hydrolysis rate constants and activation parameters were also determined at 25°C and 0.5 mol dm−3 ionic strength (NaClO4): R = (CH3)2CH; k 0 = (3.18 ± 0.05) × 10−5 s−1 (ΔH ‡= 12.0 ± 0.1 kJ mol−1, ΔS‡= −291 ± 1 J K−1 mol−1), R = CH3CH2; k 0 = (4.04 ± 0.01) × 10−5 s−1 (ΔH ‡= 22.4 ± 0.9 kJ mol−1, ΔS ‡= −254 ± 3 J K−1 mol−1), R = CH3; k 0 = (4.05 ± 0.17) × 10−5 s−1 (ΔH ‡= 34.1 ± 0.1 kJ mol−1, ΔS ‡= −214 ± 1 J K−1 mol−1), R = H; k 0 = (3.4 ± 0.2) × 10−3 s−1 (ΔH ‡= 25.3 ± 0.4 kJ mol−1, ΔS ‡= −207 ± 1 J K−1 mol−1).
Australian Journal of Chemistry | 2015
Mark A.W. Lawrence; Yvette A. Jackson; Willem H. Mulder; Per Martin Björemark; Mikael Håkansson
The synthesis and crystal structures of bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (dicarbothioamide I) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L1) as well as the syntheses of the palladium(ii) chloride and acetate pincer complexes are reported. The stability constant for the palladium complex formation at 25°C was found to be (2.04 ± 0.26) × 104 dm3 mol–1 and (2.30 ± 0.19) × 104 dm3 mol–1 with ΔfH = 8 ± 1 kJ mol–1, ΔfSθ = 108 ± 10 J K–1 mol–1, and ΔfH = 17 ± 4 kJ mol–1 and ΔfSθ = 140 ± 20 J K–1 mol–1 for the PdClL1 and Pd(OAc)L1, respectively. The kinetics of formation of the palladium(ii) complexes were investigated and the mechanism is proposed to be associative in nature (ΔH1‡ = 34 ± 2 kJ mol–1 and ΔS1‡ = –113 ± 8 J K–1 mol–1, and ΔH1‡ = 37 ± 3 kJ mol–1 and ΔS1‡ = –100 ± 8 J K–1 mol–1 for the PdClL1 and Pd(OAc)L1 species, respectively). The electrochemical measurements of the acetonitrile solutions revealed irreversible electron transfers consistent with the electrochemical decomposition of the ligand and its coordination complexes.
Journal of Coordination Chemistry | 2017
Mohammed Bakir; Mark A.W. Lawrence; Marhoun Ferhat
Abstract The reaction between di-2-pyridyl ketone thiosemicarbazone (dpktsc) and PdCl2(CH3CN)2, generated in situ from the reaction between PdCl2 and CH3CN, gave the unprecedented [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·2CH3CN (1) complex (py = pyridine, im = imine and am = amide). The identity of 1 was confirmed via its elemental analysis and spectroscopic properties. Infrared and 1H-NMR spectra confirmed the coordination of (dpktsc-H)− to the palladium ions. The electronic absorption spectra measured in dmso and dmf and density functional theory (DFT) calculations revealed metal-to-ligand charge-transfer (MLCT), d–d and intra-ligand charge-transfer (ILCT) electronic transitions. X-ray structural analysis on a crystal of [Pd2Cl3(κ5-Npy,Nim,S,Npy,Nam-dpktsc-H)]·H2O (2) grown from dmf solution of 1 confirmed its formulation and showed the solid-state structure contains a web of molecules locked via a network of non-covalent interactions. Electrochemical measurements on 1 in dmf revealed metal- and ligand-based redox processes. In contrast to the electrochemical decomposition of uncoordinated dpktsc, coordinated (dpktsc-H)− in 1 does not undergo electrochemical decomposition. Electrochemical titrations of 1 with p-toluenesulfonic acid monohydrate (p-TSOH) revealed electro-catalytic proton reduction. Over-potential (η) of 180 mV for the H2 evolution was observed and is comparable to several molecular electro-catalysts for proton reduction. Controlled-potential electrolysis confirmed the electro-catalytic proton reduction by the Pd-complex. Electrochemical reactions of CO2 in the presence of 1 exhibited a proton dependence, and metal- and ligand-based electrochemical reaction.
Journal of Coordination Chemistry | 2018
Mohammed Bakir; Mark A.W. Lawrence
Abstract Di-2-pyridyl ketone acetic acid hydrazone hydrate, dpkaah.0.5H2O (1), prepared from the acid catalyzed condensation of di-2-pyridyl ketone (dpk) with acetic acid hydrazide in refluxing ethanol, undergoes facile coordination to Group 12 metal-chlorides in CH3CN to form [MCl2(κ3-N,N,O-dpkaah)] {M=Zn (3), Cd (4) or Hg(5)}. X-ray structural analysis on single crystals of dpkaah (2) and 3–5 confirmed their identities and revealed pseudo-coordination of the carbonyl group (C=O). Infrared measurements confirmed the pseudo-coordination of the carbonyl group to MCl2. The geometries of 3–5 vary, while 5 adopts a square pyramidal geometry, 4 has a structure halfway between square pyramidal and trigonal bipyramidal and 3 is less distorted from square pyramidal than 3. The extended structures of 3–5 exposed extensive networks of non-covalent interactions, and in the case of 4 chloride bridges of the type Cd(μ-Cl)2Cd were observed. Spectroscopic measurements in different solvents and variable temperature studies confirmed the stability of the keto form of 1 and 3–5. Spectrophotometric titrations of protophilic solutions (dmf or dmso) of 1 with MCl2 revealed facile coordination of MCl2 to 1 and disclosed low concentrations of MCl2 can be detected and determined using protophilic solutions of 1. Electrochemical measurements on dmf solutions divulged electrochemical decomposition of uncoordinated 1, the facile coordination of 1 to MCl2, and the stability of 3–5 decreases as the size of the metal ion increases.
Transition Metal Chemistry | 2011
Mark A.W. Lawrence; Sonia E. Thomas; Paul T. Maragh; Tara P. Dasgupta
Polyhedron | 2017
Mark A.W. Lawrence; Kerry-Ann Green; Peter N. Nelson; Shannen C. Lorraine
Transition Metal Chemistry | 2012
Mark A.W. Lawrence; Paul T. Maragh; Tara P. Dasgupta
Inorganica Chimica Acta | 2012
Mark A.W. Lawrence; Paul T. Maragh; Tara P. Dasgupta