Suman K. Barman

Isostructural Dinuclear Phenoxo-/Acetato-Bridged Manganese(II), Cobalt(II), and Zinc(II) Complexes with Labile Sites: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate

Using the dinucleating phenol-based ligand 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol] (HL(2)), in its deprotonated form, the six new dinuclear complexes [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][PF(6)] (M = Mn (2a), Co (3a), Zn (4a)) and [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][BPh(4)] (M = Mn (2b), Co (3b), Zn (4b)) have been synthesized. Crystallographic analyses on 2b·2MeCN, 3b·2MeCN, and 4b·2MeCN reveal that these complexes have closely similar μ-phenoxo bis(μ-carboxylato) structures. The physicochemical properties (absorption and ESI-MS spectral data, 2a,b, 3a,b, and 4a,b; (1)H NMR, 4a,b) of the cations of 2a-4a are identical with those of 2b-4b. Each metal ion is terminally coordinated by a pyrazole nitrogen and a pyridyl nitrogen from a 3-(pyridin-2-yl)pyrazole unit and a solvent molecule (MeCN). Thus, each metal center assumes distorted-octahedral M(II)N(3)O(3) coordination. Temperature-dependent magnetic studies on Mn(II) and Co(II) dimers reveal the presence of intramolecular antiferromagnetic (J = -8.5 cm(-1)) for 2b and ferromagnetic exchange coupling (J = +2.51 cm(-1)) for 3b, on the basis of the Hamiltonian H = -JS(1)·S(2). The exchange mechanism is discussed on the basis of magneto-structural parameters (M···M distance). Spectroscopic properties of the complexes have also been investigated. The pH titration and kinetics of phosphatase (transesterification) activity on 2-hydroxypropyl-p-nirophenylphosphate (HPNP) were studied in MeOH/H(2)O (33%, v/v) with 2a-4a, due to solubility reasons. This comparative kinetic study revealed the effect of the metal ion on the rate of hydrolysis of HPNP, which has been compared with what we recently reported for [Ni(II)(2)(L(2))(μ-O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (1a). The efficacy in the order of conversion of substrate to product (p-nitrophenolate ion) follows the order 4a > 3a > 2a > 1a, under identical experimental conditions. Notably, this trend follows the decrease of pK(a) values of M(II)-coordinated water (7.95 ± 0.04 and 8.78 ± 0.03 for 1a, 7.67 ± 0.08 and 8.69 ± 0.06 for 2a, 7.09 ± 0.05 and 8.05 ± 0.06 for 3a, and 6.20 ± 0.04 and 6.80 ± 0.03 for 4a). In this work we demonstrate that the stronger the Lewis acidity (Z(eff)/r) of the metal ion, the more acidic is the M(II)-coordinated water and the greater is the propensity of the metal ion to catalyze hydrolysis of the activated phosphate ester HPNP. Notably, the observed k(2) values (M(-1) s(-1)) for Mn(II) (2a, 0.152), Co(II) (3a, 0.208), and Zn(II) (4a, 0.230) complexes (1a, 0.058; already reported) linearly correlate with Z(eff)/r values of the metal ion. In each case a pseudo-first-order kinetic treatment has been done. Kinetic data analysis of complexes 2a-4a were also done following Michaelis-Menten treatment (catalytic efficiency k(cat)/K(M) values 0.170 M(-1) s(-1) for 2a, 0.194 M(-1) s(-1) for 3a and 0.161 M(-1) s(-1) for 4a; for 1a the value is 0.089 M(-1) s(-1)). Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification of HPNP and yielded comparable activation parameters (E(a) (kJ mol(-1)): 71.00 ± 4.60 (1a; reported), 67.95 ± 5.71 (2a), 62.60 ± 4.46 (3a), 67.80 ± 3.25 (4a)) and enthalpy/entropy of activation values (ΔH(‡) (kJ mol(-1)) = 68.00 ± 4.65 (1a; reported), 65.40 ± 5.72 (2a), 60.00 ± 4.47 (3a), 65.29 ± 3.26 (4a); ΔS(‡) (J mol(-1) K(-1)) = -109.00 ± 13 (1a; reported), -107.30 ± 16 (2a), -122.54 ± 14 (3a), -104.67 ± 10 (4a)). The E(a) values for all the complexes are comparable, suggesting a closely similar reaction barrier, meaning thereby similar course of reaction. The ΔS(‡) values are consistent with an associative process. Positive ΔH(‡) values correspond to bond breaking of the activated complex as a result of nucleophilic attack at the phosphorus atom, releasing cyclic phosphate and p-nitrophenolate ion. These data have helped us to propose a common mechanistic pathway: deprotonation of a metal-bound species to form the effective nucleophile, binding of the substrate to the metal center(s), intramolecular nucleophilic attack on the electrophilic phosphorus atom with the release of the leaving group, and possibly regeneration of the catalyst.

Neutral, cationic, and anionic low-spin iron(III) complexes stabilized by amidophenolate and iminobenzosemiquinonate radical in N,N,O ligands.

A brownish-black complex [Fe(III)(L)2] (1) (S = 0), supported by two tridentate redox-active azo-appended o-amidophenolates [H2L = 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol], has been synthesized and structurally characterized. In CH2Cl2 1 displays two oxidative and two reductive 1e(-) redox processes at E1/2 values of 0.48 and 1.06 V and -0.42 and -1.48 V vs SCE, respectively. The one-electron oxidized form [1](+) isolated as a green solid [Fe(III)(L)2][BF4] (2) (S = 1/2) has been structurally characterized. Isolation of a dark ink-blue one-electron reduced form [1](-) has also been achieved [Co(III)(η(5)-C10H15)2][Fe(III)(L)2] (3) (S = 1/2). Mössbauer spectral parameters unequivocally establish that 1 is a low-spin (LS) Fe(III) complex. Careful analysis of Mössbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major LS Fe(III) and a minor LS Fe(II) component (redox isomers): [Fe(III){(L(ISQ))(-•)}2](+) and [Fe(II){(L(IBQ))(0)}{(L(ISQ))(-•)}](+) (2) and [Fe(III){(L(AP))(2-)}2](-) and [Fe(II){(L(ISQ))(-•)}{(L(AP))(2-)}](-) (3). Notably, for both at 8 K mainly the major component exists. Broken-Symmetry (BS) Density Functional Theory (DFT) calculations at the B3LYP level reveals that in 1 the unpaired electron of LS Fe(III) is strongly antiferromagnetically coupled with a π-radical of o-iminobenzosemiquinonate(1-) (L(ISQ))(-•) form of the ligand, delocalized over two ligands providing 3- charge (X-ray structure). DFT calculations reveal that the unpaired electron in 2 is due to (L(ISQ))(-•) [LS Fe(III) (SFe = 1/2) is strongly antiferromagnetically coupled to one of the (L(ISQ))(-•) radicals (Srad = 1/2)] and 3 is primarily a LS Fe(III) complex, supported by two o-amidophenolate(2-) ligands. Time-Dependent-DFT calculations shed light on the origin of UV-vis-NIR spectral absorptions for 1-3. The collective consideration of Mössbauer, variable-temperature (77-298 K) electron paramagnetic resonance (EPR), and absorption spectral behavior at 298 K, and DFT results reveals that in 2 and 3 the valence-tautomerism is operative in the temperature range 80-300 K.

A dinuclear zinc(II) complex of a new unsymmetric ligand with an N5O2 donor set; A structural and functional model for the active site of zinc phosphoesterases.

The dinuclear complex [Zn(2)(DPCPMP)(pivalate)](ClO4), where DPCPMP is the new unsymmetrical ligand [2-(N-(3-((bis((pyridin-2-yl)methyl)amino)methyl)-2-hydroxy-5-methylbenzyl)-N-((pyridin-2-yl)methyl)amino)acetic acid], has been synthesized and characterized. The complex is a functional model for zinc phosphoesterases with dinuclear active sites. The hydrolytic efficacy of the complex has been investigated using bis-(2,4-dinitrophenyl)phosphate (BDNPP), a DNA analog, as substrate. Speciation studies using potentiometric titrations have been performed for both the ligand and the corresponding dizinc complex to elucidate the formation of the active hydrolysis catalyst; they reveals that the dinuclear zinc(II) complexes, [Zn(2)(DPCPMP)](2+) and [Zn(2)(DPCPMP)(OH)](+) predominate the solution above pH4. The relatively high pK(a) of 8.38 for water deprotonation suggests that a terminal hydroxide complex is formed. Kinetic investigations of BDNPP hydrolysis over the pH range 5.5-11.0 and with varying metal to ligand ratio (metal salt:ligand=0.5:1 to 3:1) have been performed. Variable temperature studies gave the activation parameters ΔH(‡)=95.6kJmol(-1), ΔS(‡)=-44.8Jmol(-1)K(-1), and ΔG(‡)=108.0 kJmol(-1). The cumulative results indicate the hydroxido-bridged dinuclear Zn(II) complex [Zn(2)(DPCPMP)(μ-OH)](+) as the effective catalyst. The mechanism of hydrolysis has been probed by computational modeling using density functional theory (DFT). Calculations show that the reaction goes through one concerted step (S(N)2 type) in which the bridging hydroxide in the transition state becomes terminal and performs a nucleophilic attack on the BDNPP phosphorus; the leaving group dissociates simultaneously in an overall inner sphere type activation. The calculated free energy barrier is in good agreement with the experimentally determined activation parameters.

Palladium(II) Complex of a Redox-Active Amidophenolate-Based O,N,S,N Ligand: Its Monocation and Dication and Reactivity with PPh3.

A new potentially tetradentate redox-active o-aminophenol-based ligand, H2L = 2-(2-ethylthio)pyridine-anilino-4,6-di-tert-butylphenol, reacts with Pd(II)(O2CCH3)2 in CH3OH in the presence of air and Et3N affording isolation of a green crystalline solid of composition [Pd(L)] 1. When examined by cyclic voltammetry (CV), 1 exhibits two quasireversible oxidative responses at E1/2 = 0.16 (peak-to-peak separation, ΔEp = 100 mV) and 0.89 V (ΔEp = 90 mV) vs SCE (saturated calomel electrode). Chemical oxidation of 1 by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of two crystalline solids, red [Pd(L)][PF6]·CH2Cl2 2 and dark green [Pd(L)][BF4]2·2CH2Cl2 3, respectively. Single-crystal X-ray crystallography at 100(2) K unambiguously established that the O,N,S,N-coordinated ligand is present in the square-planar complexes [Pd(II){(L(AP))(2-)}] 1, [Pd(II){(L(ISQ))(•-)}][PF6]·CH2Cl2 2, and [Pd(II){(L(IBQ))(0)}][BF4]2·2CH2Cl2 3, as dianionic (L(AP))(2-), monoanionic o-iminobenzosemiquinonate(1-) π-radical (Srad = (1)/2) (L(ISQ))(•-), and neutral o-iminobenzoquinone (L(IBQ))(0) redox level. Reaction of 1 and 2 with PPh3 afforded isolation of two crystalline complexes: dark green [Pd(II)(L)(PPh3)] 4 and red [Pd(II){(L(ISQ))(•-)}(PPh3)][PF6]·CH2Cl2 5. X-ray structure determination of 5 at 100(2) K revealed Pd(II)ON2P coordination environment. The square-planar complexes 1-5 possess an S = 0, (1)/2, 0, 0, and (1)/2 ground-state, respectively, as was established by (1)H NMR and EPR spectroscopy, and room-temperature magnetic moment data. All redox processes are thus shown to be ligand-based. Absorption spectral measurements were done for all complexes. DFT calculations at B3LYP-level of theory adequately describe the electronic structures of 1-3, and 5, containing a spin-paired d(8) Pd(II) ion. Time-dependent-DFT calculations on 1-3 and 5 shed light on the origin of UV-vis-NIR spectral absorptions.

Model Complexes for the Nip Site of Acetyl Coenzyme A Synthase/Carbon Monoxide (CO) Dehydrogenase: Structure, Electrochemistry, and CO Reactivity

Aliphatic thiolato-S-bridged tri- and binuclear nickel(II) complexes have been synthesized and characterized as models for the Nip site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide (CO) dehydrogenase. Reaction of the in situ formed N2Sthiol donor ligands with [Ni(H2O)6](ClO4)2 afforded the trinuclear complexes [Ni{(LMe(S))2Ni}2](ClO4)2·CH3CN (1·CH3CN) and [Ni{(LBr(S))2Ni}2](ClO4)2·5H2O (2·5H2O) following self-assembly. Complexes 1 and 2 react with [Ni(dppe)Cl2] and dppe [dppe = 1,2-bis(diphenylphosphino)ethane] to afford the binuclear [Ni(dppe)Ni(LMe(S))2](ClO4)2·2H2O (3·2H2O) and [Ni(dppe)Ni(LBr(S))2](ClO4)2·0.75O(C2H5)2 [4·0.75O(C2H5)2], respectively. The X-ray crystal structures of 1-4 revealed a central NiIIS4 moiety in 1 and 2 and a NiIIP2S2 moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Nip site of ACS. The electrochemical reduction of both terminal NiII ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni{(LMe(S))2Ni(NO)}2](ClO4)2 (5) and [Ni{(LBr(S))2Ni(NO)}2](ClO4)2 (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit νNO at 1773 and 1789 cm-1, respectively. In the presence of O2, both 5 and 6 transform to nitrite-bound monomers [(LMe(S-S))Ni(NO2)](ClO4) (7) and [(LBr(S-S))Ni(NO2)](ClO4)2 (8). The nature of the ligand modification is evident from the X-ray crystal structure of 7. To understand the origin of multiple reductive responses of 1-4, complex [(LMe(SMe))2Ni](ClO4)2 (9) is considered. The central NiS4 part of 1 is labile like the Nip site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3red-(CO)2 species, as confirmed by Fourier transform infrared (νCO = 1997 and 2068 cm-1) and electron paramagnetic resonance ( g1 = 2.18, g2 = 2.13, g3 = 1.95, and AP = 30-80 G) spectroscopy. The CO binding to NiI of 3red is relevant to the ACS activity.