Iron(III) Complexes of a Hexadentate Thioether-Appended 2-Aminophenol Ligand: Redox-Driven Spin State Switchover.

A green complex [Fe(L3)] (1), supported by the deprotonated form of a hexadentate noninnocent redox-active thioether-appended 2-aminophenolate ligand (H4L3 = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)ethane), has been synthesized and structurally characterized at 100(2) K and 298(2) K. In CH2Cl2, 1 displays two oxidative and a reductive one-electron redox processes at E1/2 values of -0.52 and 0.20 V, and -0.85 V versus the Fc+/Fc redox couple, respectively. The one-electron oxidized 1+ and one-electron reduced 1- forms, isolated as a blackish-blue solid 1(PF6)·CH2Cl2 (2) and a gray solid [Co(η5-C5H5)2]1·DMF (3), have been structurally characterized at 100(2) K. Structural parameters at 100 K of the ligand backbone and metrical oxidation state values unambiguously establish the electronic states as [FeIII{(LAPO,N)2-}{(LISQO,N)•-}{(LS,S)0}] (1) (two tridentate halves are electronically asymmetric-ligand mixed-valency), [FeIII{(LISQO,N)•-}{(LISQO,N)•-}{(LS,S)0}]+ (1+), and [FeIII{(LAPO,N)2-}{(LAPO,N)2-}{(LS,S)0}]- (1-) [dianionic 2-amidophenolate(2-) (LAPO,N)2- and monoanionic 2-iminobenzosemiquinonate(1-) π-radical (Srad = 1/2) (LISQ)•- redox level]. Mössbauer spectral data of 1 at 295, 200, and 80 K reveal that it has a major low-spin (ls)-Fe(III) and a minor ls-Fe(II) component (redox isomers), and at 7 K, the major component exists exclusively. Thus, in 1, the occurrence of a thermally driven valence-tautomeric (VT) equilibrium (asymmetric) [FeIII{(LAPO,N)2-}{(LISQO,N)•-}{(LS,S)0}] ⇌ (symmetric) [FeII{(LISQO,N)•-}{(LISQO,N)•-}{(LS,S)0}] (80-295 K) is implicated. Mössbauer spectral parameters unequivocally establish that 1+ is a ls-Fe(III) complex. In contrast, the monoanion 1- contains a high-spin (hs)-Fe(III) center (SFe = 5/2), as is deduced from its Mössbauer and EPR spectra. Complexes 1-3 possess total spin ground states St = 0, 1/2, and 5/2, respectively, based on 1H NMR and EPR spectra, the variable-temperature (2-300 K) magnetic behavior of 2, and the µeff value of 3 at 300 K. Broken-symmetry density functional theory (DFT) calculations at the B3LYP-level of theory reveal that the unpaired electron in 1+/2 is due to the (LISQ)•- redox level [ls-Fe(III) (SFe = 1/2) is strongly antiferromagnetically coupled to one of the (LISQ)•- radicals (Srad = 1/2)], and 1-/3 is a hs-Fe(III) complex, supported by (L3)4- with two-halves in the (LAP)2- redox level. Complex 1 can have either a symmetric or asymmetric electronic state. As per DFT calculation, the former state is stabilized by -3.9 kcal/mol over the latter (DFT usually stabilizes electronically symmetric structure). Time-dependent (TD)-DFT calculations shed light on the origin of observed UV-vis-NIR spectral absorptions for 1-3 and corroborate the results of spectroelectrochemical experiments (300-1100 nm) on 1 (CH2Cl2; 298 K). Variable-temperature (218-298 K; CH2Cl2) absorption spectral (400-1000 nm) studies on 1 justify the presence of VT equilibrium in the solution-state.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures.

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Single-Molecule-Magnet FeII4FeIII2 and Antiferromagnetic FeIII4 Coordination Clusters.

Supported by endogenous (part of the ligand, in-built) phenoxo bridges provided by the ligand 2,6-bis[{{(5-bromo-2-hydroxybenzyl)}{(2-(pyridylethyl)}amino}methyl]-4-methylphenol) (H3L), in its deprotonated form, exogenous (not part of the ligand, externally added or generated) oxo-/hydroxo- and acetato-bridged [FeII4FeIII2(O)2(O2CMe)4(L)2]·4Et2O (1) and [FeIII4(OH)2(O2CMe)3(L)2](ClO4)·3MeCN·2H2O (2) coordination clusters have been synthesized and structurally characterized. Complexes 1 and 2 have µ4-O and µ3-OH bridges, respectively. Magnetic studies on 1 reveal slow magnetic relaxation below 2 K. Both in-phase ( χ'M) and out-of-phase (χ″M) magnetic susceptibility were found to be frequency dependent. This is typical of a single-molecule magnet (SMM) with τ0 = 1.9(2) × 10-7 s-1 and Ea = 5.1(3) cm-1. Assuming that Ea corresponds to the energy splitting of the ground spin state ( S = 2) by the zero-filed-splitting (zfs), Ea = 4| D| ( D is the axial zfs parameter), D ≈ - 1.3 cm-1 could be estimated. For 2, three types of magnetic interactions are observed: JA = -56.5(3), JB = -71.6(4), and JC = +4.5(2) cm-1. Considering the observed structural parameters, the magnetic behavior for both of the coordination clusters 1 and 2 has been rationalized.

A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions.

From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1) (H3L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N',N″-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH3CN/CH3OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [NiII5(H2L)2(µ3-OH)2(µ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [NiII2(HL)(µ-O2CCH3)(H2O)2][ClO4]·4H2O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H3L with 2 equiv of NiII(O2CCH3)2·4H2O, followed by the usual workup, and recrystallization from CH2Cl2 led to the isolation of [NiII2(H2L)(µ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4). Complex 4 is structurally different from 3, confirming that the reaction of NiII(O2CCH3)2·4H2O with H3L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by NiII-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [NiII2(HL)(µ-O2CCH2CH3)(H2O)2][ClO4] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = -25.0(1) cm-1 for 1, J = -20.0(1) cm-1 for 4, and J = -18.80(8) cm-1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (Ja = +16.02 cm-1, and Jb = +9.02 cm-1) and an antiferromagnetic (Jc = -49.7 cm-1), have been identified.

Nickel(II) Complex of a Hexadentate Ligand with Two o-Iminosemiquinonato(1-) π-Radical Units and Its Monocation and Dication.

Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H4L(3) = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)-ethane, in CH3OH with Ni(II)(O2CCH3)2·4H2O and Et3N afforded isolation of a reddish-brown crystalline solid [Ni(L(3))] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of one-electron oxidized species [1](1+) as purple [1][PF6]·CH2Cl2 and two-electron oxidized species [1](2+) as dark purple [1][BF4]2·CH2Cl2, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [Ni(II){(L(ISQ)O,N)(•-)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}] 1, [Ni(II){(L(IBQ)O,N)(0)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}][PF6]·CH2Cl2, and [Ni(II){(L(IBQ)O,N)(0)}{(L(IBQ)O,N)(0)}{(LS,S)(0)}][BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate(1-) π-radical (Srad = 1/2) (L(ISQ))(•-) and neutral o-iminoquinone (L(IBQ))(0) redox-levels. Complexes 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2-300 K) magnetic behavior of 1 and [1][PF6]·CH2Cl2, and a µeff value of [1][BF4]2·CH2Cl2 at 300 K. Both 1 and [1][PF6]·CH2Cl2 exhibit ferromagnetic exchange-coupling between the two electrons of Ni(II) and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1](1+), and [1](2+). The observed UV-vis-NIR absorptions for 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 have been assigned, based on time-dependent (TD)-DFT calculations.

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.

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.

Ni(II) and Pd(II) complexes of a new redox-active pentadentate azo-appended 2-aminophenol ligand: Pd(II)-assisted intraligand cyclization forms a phenoxazinyl ring.

Square planar complexes of Ni(II) and Pd(II) of a new redox-active pentadentate azo-appended 2-aminophenol ligand (H4L = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino-ortho-azobenzene) in three accessible redox levels [amidophenolate(2-), semiquinonate(1-) π radical, and quinone(0)] were synthesized. The coordinated HL(3-) ligand provides four donor sites [two N(iminophenolates), an N'(azo), and an O(phenolate)], while the phenolic OH group remains free in the three complexes. Cyclic voltammetry on complex [Ni(L)] 1 and its corresponding Pd(II) analogue [Pd(L)] 2 in CH2Cl2 displayed three redox responses (two oxidative at E1/2 = 0.06 V and Epa (anodic peak potential) = 0.80 V and one reductive at -0.77 V for 1 and at E1/2 = 0.08 V and Epa = 0.85 V and at -0.74 V for 2vs. Fc+/Fc). The chemical oxidation of 1 with AgSbF6 afforded [Ni(L)]SbF6·2CH2Cl2 (3·2CH2Cl2). Complex [Pd(L*)] 4, which is coordinated by a phenoxazinyl derivative of L(4-), was obtained via intraligand cyclization in the parent complex 2 under basic oxidizing conditions. The molecular structures of 1, 2, 3·2CH2Cl2 and 4 were elucidated through X-ray crystallography at 100 K. Characterization using 1H NMR, X-band EPR, and UV-VIS-NIR spectroscopy established that the complexes have [NiII{(LISQ)˙2-}] 1, [PdII{(LISQ)˙2-}] 2, [NiII{(LIBQ)-}]SbF6/1+SbF6-(3), and [PdII{(L*AP)˙2-}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic St (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (St = 0). Density functional theory (DFT) electronic structural calculations at the B3LYP level rationalized the observed experimental results. Time-dependent (TD)-DFT calculations allowed us to identify the nature of the observed absorption spectra.

Phenoxazinyl Zn(II) diradical complex formed via redox-driven cyclization of a 2-aminophenol-based N3O ligand. Isolation of the modified N3 ligand radical and its Ni(II) complex.

Aerobic reaction between the pyridine-2-carboxamide-2-aminophenol N3O ligand (H3L1) and Zn(ClO4)2·6H2O in CH3CN affords an N3 phenoxazinylate coordinated Zn(II) complex; its diradical electronic structure [ZnII{(L1*)˙-}2] has been elucidated from redox, spectroscopic (UV-VIS and EPR), and magnetic measurements and DFT calculations. Isolation and characterization of the metal-assisted redox-driven modified N3 ligand as a radical cation (H2L1*)˙+ and its Ni(II)-diradical complex [NiII{(L1*)˙-}2] have also been achieved.

Phenolate- and acetate (both µ2-1,1 and µ2-1,3 modes)-bridged linear Co(II)3 and Co(II)2Mn(II) trimers: magnetostructural studies.

A full report on the synthesis, crystal structure, spectroscopic characterization, and magnetic properties of two new trinuclear complexes (one homo- and another heterotrinuclear) [(L)2Co(II)3(OAc)4(MeOH)2]·6MeOH (1) and [(L)2Co(II)2Mn(II)(OAc)4(MeOH)2]·6MeOH (2) [HL = N-methyl-N-2-hydroxybenzyl-2-aminoethyl-2-pyridine] is presented. The properties of 1 and 2 are compared to that of three previously communicated complexes [(L)2Ni(II)3(OAc)4(MeOH)2]·6MeOH (3), [(L)2Ni(II)2Mn(II)(OAc)4(H2O)2] (4), and [(L)2Ni(II)2Co(II)(OAc)4(MeOH)2]·6MeOH (5) (Inorg. Chem.2007, 46, 5128-5130). All are centrosymmetric trimers with the central metal ion situated on an inversion center. Adjacent metal ions are triply bridged by a µ2-phenolate and two acetate (one in µ2-1,1 mode and another in µ2-1,3 mode) groups. Magnetic investigations reveal that the complexes are ferromagnetically coupled (J in cm(-1)): +1.20 (1) and +0.71 (2). The reported J values are +1.10 (3), -0.30 (4), and +1.06 (5). It reveals that total spins are as follows: S = 9/2 (1), 11/2 (2), 3 (3), 1/2 (4), 7/2 (5). The role of M-O(µ2-phenoxide)-M', M-O(µ2-1,1-acetate)-M', and M-O(µ2-1,3-acetate)-M' (M = M' = Co(II), Ni(II); M = Co(II), M' = Mn(II); M = Ni(II), M' = Mn(II), Co(II)) bond angles on the observed magnetic behavior has been noted.

A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies.

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(µ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(µ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (µ3-OCH3)- and two carboxylate (µ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; µ-1,1-OCO- and µ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; µ-1-OCO- and µ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two µ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

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.

Modeling tyrosinase and catecholase activity using new m-Xylyl-based ligands with bidentate alkylamine terminal coordination.

Chemical model systems possessing the reactivity aspects of both tyrosinase and catechol oxidase are presented. Using two m-xylyl-based ligands providing bidentate alkylamine terminal coordination, 1,3-bis[(N,N-dimethylaminoethyl)aminomethyl]benzene (L(H,H)) and 1,3-bis[(N,N,N'-trimethylaminoethyl)aminomethyl]benzene (L(Me,Me)), four new dicopper(I) complexes, [Cu(I)(2)(L(H,H))(MeCN)(4)][ClO(4)](2) (1), [Cu(I)(2)(L(H,H))(PPh(3))(2)(MeCN)(2)][ClO(4)](2) (2), [Cu(I)(2)(L(Me,Me))(MeCN)(2)][ClO(4)](2) (3), and [Cu(I)(2)(L(Me,Me))(PPh(3))(2)][ClO(4)](2) (4), have been synthesized and characterized. Complex 2 has been structurally characterized. Reaction of the dicopper(I) complex 3(2+) with dioxygen at 183 K generates putative bis(µ-oxo)dicopper(III) intermediate (absorption spectroscopy). Oxygenation of 1 and 3 brings about m-xylyl-ring hydroxylation (monooxygenase-like activity), with a noticeable color change from pale-yellow to dark green. The presence of phenoxo- and hydroxo-bridges in the end products [Cu(II)(2)(L(H,H)-O)(OH)(MeCN)(2)][ClO(4)](2) (5) and [Cu(II)(2)(L(Me,Me)-O)(OH)(OClO(3))][ClO(4)]·MeCN(6) has been authenticated by structural characterization. Oxygenation of 3 afforded not only the green complex 6 isolation but also a blue complex [Cu(II)(2)(L(Me,Me))(OH)(2)][ClO(4)](2) (7). Variable temperature magnetic susceptibility measurements on 5 and 6 establish that the Cu(II) centers are strongly antiferromagnetically coupled [singlet-triplet energy gap (J) = -528 cm(-1) (5) and -505 cm(-1) (6)]. The abilities of phenoxo- and hydroxo-bridged dicopper(II) complexes 5 and 6, the previously reported complex [Cu(II)(2)(L(1)-O)(OH)(OClO(3))(2)]·1.5H(2)O (8) (L(1)-OH = 1,3-bis[(2-dimethylaminoethyl)iminomethyl]phenol), and [Cu(II)(2)(L(2)-O)(OH)(OClO(3))()][ClO(4)]() (9) (L(2)-OH = 1,3-[(2-dimethylaminoethyl)iminomethyl][(N,N,N'-trimethyl)aminoethyl]-4-methylphenol) have been examined to catalyze the oxidation of catechol to quinone (catecholase activity of tyrosinase and catechol oxidase-like activity) by employing the model substrate 3,5-di-tert-butylcatechol. Saturation kinetic studies have been performed on these systems to arrive at the following reactivity order [k(cat)/K(M) (catalytic efficiency) × 10(-3) (M(-1) h(-1))]: 470 (6) > 367 (5) > 128 (9) > 90 (8).

Ni(II) complexes of a new tetradentate NN'N''O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels.

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H3L2 in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N' and N'' (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L2)] 1 in CH2Cl2 established a three-membered electron-transfer series (oxidative response at E1/2 = 0.57 V and reductive response at -0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L2)]z (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF6 affords [Ni(L2)](SbF6) (2) and reduction of 1 with cobaltocene yields [Co(η5-C5H5)2][Ni(L2)] (3). The molecular structures of 1·CH3CN, 2·0.5CH2Cl2 and 3·C6H6 have been determined by X-ray crystallography at 100 K. Characterization by 1H NMR, X-band EPR (gav = 2.006 (solid); 2.008 (CH2Cl2-C6H5CH3 glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [NiII{(L2)˙2-}], [NiII{(L2)-}]+/1+ and [NiII{(L2)3-}]-/1- electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic St (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state St = 0. Interestingly, the variable-temperature (2-300 K) magnetic measurement reveals that 1 with the St = 1/2 ground state attains the antiferromagnetic St = 0 state at a very low temperature, due to weak noncovalent interactions via π-π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800-1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.

Probing the electronic structure of [Ru(L1)2]Z (z = 0, 1+ and 2+) (H2L1: a tridentate 2-aminophenol derivative) complexes in three ligand redox levels.

Aerobic reaction between [RuII(DMSO)4Cl2], a redox-active 2-aminophenol-based ligand (H2L1: 2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) and Et3N in MeOH under refluxing conditions afforded a purple complex [Ru(L1)2] (S = 0). Structural analysis reveals that the tridentate ligand coordinates in a mer conformation providing a distorted octahedral RuN2O2S2 coordination. Cyclic voltammetry on 1 in CH2Cl2 reveals the accessability of the monocation, dication and monoanion forms. Reddish purple monocation [Ru(L1)2](PF6)·CH2Cl2 ([1OX1](PF6)·CH2Cl2; S = 1/2) and green dication [Ru(L1)2](BF4)2·H2O ([1OX2](BF4)2·H2O; S = 0) have been isolated through the chemical oxidation of 1 in CH2Cl2 by [FeIII(η5-C5H5)2](PF6) and AgBF4, respectively. A structural analysis of the single crystals of the monocation and the dication with the compositions [1OX1](PF6)·CH2Cl2·H2O (2) and [1OX2](BF4)2·1.7H2O (3), respectively, has been done. Metrical (metal-ligand and ligand backbone) parameters, values of metrical oxidation states of coordinated ligands, 1H NMR spectra of 1 and [1OX2](BF4)2·H2O, EPR spectra of [1OX1](PF6)·CH2Cl2, X-ray photoelectron and UV-VIS-NIR spectra of 1-3, spin population analysis from broken-symmetry (BS) density functional theory (DFT) calculations and quasi-restricted orbital (QRO) analysis have allowed us to assign the electronic structure of the complexes. The complexes exhibit highly covalent metal-ligand interactions. The electronic states of 1, [1OX1]1+ and [1OX2]2+ are best described as [RuII{(LISQ)˙-}2] ↔ [RuIII{(LAP)2-}{(LISQ)˙-}] (S = 0), [RuIII{(LISQ)˙-}2]1+ (S = 1/2) and [RuII{(LIBQ)0}2]2+ ↔ [RuIII{(LISQ)˙-}{(LIBQ)0}]2+ (S = 0), respectively. Notably, all redox processes are ligand-centred. Absorption spectral properties have been rationalized based on time-dependent (TD)-DFT calculations. For 1, the appearance of an IVCT band at 1100 nm supports its Class II-III (borderline) ligand-based mixed-valence character.

One-dimensional Co(II) and Cu(II) coordination polymers and a discrete Cu(II)4 complex of carboxylate-appended (2-pyridyl)alkylamine ligands: spin-canting and anti-/ferromagnetic coupling.

Structural characterization of the newly synthesized complexes [M(I)I(L(1)OO)(xH2O)][ClO4] x 2 H2O [M = Co, x = 1 (1); M = Cu, x = 0 (2); L(1)OO- = 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl)methyl}amino]propionate] reveals that 1 and 2 are 1D chainlike coordination polymers. A tridentate variety of this ligand afforded a discrete tetranuclear complex {[Cu(I)I(L(2)OO)(OClO3)]}4 x MeCN (3) [L(2)OO- = 3-[N-methyl-{2-(pyridine-2-yl)ethyl}amino]propionate]. Analysis of the crystal packing diagrams reveals extensive pi-pi stacking in 1 and C-H...O hydrogen bonding interactions in 3, leading to the formation of network structures. For these complexes, absorption spectral properties have been investigated. All three complexes exhibit exchange interaction between the M(II) ions through a syn-anti bridging carboxylate pathway. Magnetic studies on 1 show spontaneous magnetization below 5 K, which corresponds to the presence of spin-canted antiferromagnetism. At T = 2 K, the values of coercive field (H(c)) and remnant magnetization (M(r)) are 200 G and 0.019 muB, respectively. Analysis of the magnetic data through spin Hamiltonians in the form HH = sigma(i

Syntheses, X-ray structures, and physicochemical properties of phenoxo-bridged dinuclear nickel(II) complexes: kinetics of transesterification of 2-hydroxypropyl-p-nitrophenylphosphate.

Four dinuclear nickel(II) complexes [Ni(II)(2)(L(1))(O(2)CMe)(2)(H(2)O)(2)][PF(6)].MeOH.3H(2)O (1), [Ni(II)(2)(L(1))(O(2)CMe)(2)(NCS)] (2), [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (3), and [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][BPh(4)].3MeOH.H(2)O (4) have been synthesized [HL(1): 2,6-bis[N-methyl-N-(2-pyridylethyl)amino]-4-methylphenol; HL(2): 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol]. Complexes 1, 3, and 4 are new while complex 2 was reported previously by Fenton and co-workers (the structure of 2 was presented but no physicochemical properties of this complex were reported; in this work such studies have been completed). X-ray crystallographic analyses of 1 and 4 reveal that each nickel(II) center is six-coordinate, terminally coordinated by two nitrogen donors [(pyridin-2-yl)ethylamine unit in 1 and 3-(pyridin-2-yl)pyrazole moiety in 4], and bridged by an endogenous phenolate ion. Each of the acetate ions in 1 adopts a eta(2)-coordination mode (chelating) whereas in 4 each is coordinated in a mu-eta(1):eta(1) syn-syn bridging mode. In 1 each Ni(II) center has water coordination whereas in 4 one Ni(II) center has a methanol and the other has water coordination. The X-ray structure of 3 could not be determined. The physicochemical properties (electronic spectroscopy and cyclic voltammetry) of the cation of 3 are identical to that of 4. Magnetic susceptibility measurements have revealed the occurrence of ferromagnetic coupling of spins of the nickel(II) centers in 2 [J = +9.80 cm(-1)]. The nickel(II) centers in 1 and 3 are antiferromagnetically coupled, but to different extents [J = -48.4 cm(-1) (1); J = -1.24 cm(-1) (3)]. The magnetic properties are correlated with the nature of bridges between the nickel(II) ions. The two coordinated aqua ligands in 1 and the aqua and methanol ligands in 3 have enabled these dinuclear nickel(II) complexes to function as catalysts in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Complex 1 is more effective in the conversion of substrate to product (p-nitrophenolate ion) than 3, under identical experimental conditions. Pseudo first-order kinetic treatment has been done for complexes 1 and 3. Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification reaction of HPNP and to propose a mechanistic pathway. The activation parameters are DeltaH(++) = 64 kJ mol(-1), DeltaS(++) = -104 J mol(-1) K(-1) for 1 and DeltaH(++) = 68 kJ mol(-1), DeltaS(++) = -109 J mol(-1) K(-1) for 3. A mechanism consistent with the kinetic data is presented.

[CuII{(LISQ)˙-}2] (H2L: thioether-appended o-aminophenol ligand) monocation triggers change in donor site from N2O2 to N2O(2)S and valence-tautomerism.

Using a potentially tridentate o-aminophenol-based redox-active ligand H2L1 (2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) in its deprotonated form, [Cu(L1)2] has been synthesized and crystallized as [CuII(L1)2]·CH2Cl2 (1·CH2Cl2). A cyclic voltammetry experiment (in CH2Cl2; V vs. SCE (saturated calomel electrode)) on 1·CH2Cl2 exhibits two oxidative (E = 0.20 V (peak-to-peak separation, ΔEp = 100 mV) and E = 0.90 V (ΔEp = 140 mV)) and two reductive (E = -0.52 V (ΔEp = 110 mV) and E = -0.92 V (ΔEp = 120 mV)) responses. Upon oxidation using a stoichiometric amount of [FeIII(η5-C5H5)2](PF6), 1·CH2Cl2 yielded [Cu(L1)2](PF6) (2). Structural analysis (100 K) reveals that 1·CH2Cl2 is a four-coordinate bis(iminosemiquinonato)copper(ii) complex (CuN2O2 coordination), and that the thioethers remain uncoordinated. The twisted geometry of 1 (distorted tetrahedral) results in considerable changes in the electronic structure, compared to well-known square-planar analogues. Crystallographic analysis of 2 both at 100 K and at 293 K reveals that it is effectively a four-coordinate complex with a CuN2OS coordination; however, a substantial interaction with the other phenolate O is observed. The metal-ligand bond distances and metric parameters associated with the o-aminophenolate rings indicate a valence-tautomeric (VT) equilibrium involving monocationic (iminosemiquinonato)(iminoquinone)copper(ii) and bis(iminoquinone)copper(i). Complex 1·CH2Cl2 is a three-spin system and a magnetic study (4-300 K) established that it has a S = 1/2 ground-state, owing to the strong antiferromagnetic coupling between the unpaired spin of the copper(ii) and the iminosemiquinonate(1-) π-radical anion. Electron paramagnetic resonance (EPR) spectral studies corroborate this result. Complex 2 is diamagnetic and the existence of VT in 2 was probed using variable-temperature (248-328 K) 1H NMR and EPR (100-298 K) spectral measurements and X-ray photoelectron spectroscopic studies at 298 K. Remarkably, modification of the well-studied 2-anilino-4,6-di-tert-butylphenol by incorporation of a benzylthioether arm leads to the occurrence of VT in 2. The electronic structure of 1·CH2Cl2 and 2 has been assigned using density functional theory (DFT) calculations at the B3LYP-D3 level of theory. Time-dependent (TD)-DFT calculations have been performed to elucidate the origin of the observed UV-VIS-NIR absorptions.

Temperature-dependent interactions and disorder in the spin-transition compound [Fe(II)(L)2][ClO4]2.C7H8 through structural, calorimetric, magnetic, photomagnetic, and diffuse reflectance investigations.

The title compound [Fe (II)(L) 2][ClO 4] 2.C 7H 8 (L = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [Fe (II)(L) 2][ClO 4] 2, published earlier ( Dalton Trans. 2003, 3392-3397). Magnetic susceptibility measurements, as well as Mossbauer and calorimetric investigations on polycrystalline samples of [Fe(L) 2][ClO 4] 2.C 7H 8 revealed the occurrence of an abrupt HS ( (5) T 2) <--> LS ( (1) A 1) transition with steep and narrow (2 K) hysteresis at approximately 232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale.