Deprotonated Schiff Base Research Articles

Zinc (II) complex with a cationic Schiff base ligand: Synthesis, characterization, and biological studies

A cationic Schiff base ligand, TSB (L) and its Zn (II) complex (1) were synthesized and characterized by using CHN, 1H-NMR, FT-IR, UV, LC–MS, and X-ray methods. Their ability to inhibit topoisomerase I, DNA cleavage activities, and cytotoxicity were studied. X-ray diffraction study shows that the mononuclear complex 1 is four coordinated with distorted tetrahedral geometry. The singly deprotonated Schiff base ligand L acts as a bidentate ON-donor ligand. Complexation of L increases the inhibitory strength on topoisomerase I activity. Complex 1 could fully inhibit topoisomerase I activity at 250μM, while L did not show any inhibitory effect on topoisomerase I activity. In addition, L and complex 1 could cleave pBR322 DNA in a concentration and time dependent profile. Surprisingly, L has better DNA cleavage activity than complex 1. The cleavage of DNA by complex 1 is altered in the presence of hydrogen peroxide. Furthermore, L and complex 1 are mildly cytotoxic towards human ovarian cancer A2780 and hepatocellular carcinoma HepG2.

Bis{μ-2-methoxy-6-[(methylimino)methyl]phenolato}bis({2-methoxy-6-[(methylimino)methyl]phenolato}copper(II))

The title compound, [Cu2(C9H10NO2)4], is built of discrete centrosymmetric dimers. The CuII atoms are each five coordinated by two deprotonated Schiff base ligands that are bonded differently to the metal atoms. Of the two phenolate O atoms, one is coordinated to one CuII atom, whereas another bridges the two metal atoms. The basal plane of the square pyramid around CuII atoms is formed by the imino N and phenolate O atoms of the bidentate and the monodentate/bidentate Schiff base ligands. The bridging phenolate oxygen occupies the apical position of the coordination sphere with a considerably longer Cu—O bond length. In the crystal, the dimeric mol­ecules pack relative to each other in such a way that the Cu2O2 planes of adjacent dimers are orthogonal.

Voltage dependence of proton pumping by bacteriorhodopsin mutants with altered lifetime of the M intermediate.

The light-driven proton pump bacteriorhodopsin (BR) from Halobacterium salinarum is tightly regulated by the [H+] gradient and transmembrane potential. BR exhibits optoelectric properties, since spectral changes during the photocycle are kinetically controlled by voltage, which predestines BR for optical storage or processing devices. BR mutants with prolonged lifetime of the blue-shifted M intermediate would be advantageous, but the optoelectric properties of such mutants are still elusive. Using expression in Xenopus oocytes and two-electrode voltage-clamping, we analyzed photocurrents of BR mutants with kinetically destabilized (F171C, F219L) or stabilized (D96N, D96G) M intermediate in response to green light (to probe H+ pumping) and blue laser flashes (to probe accumulation/decay of M). These mutants have divergent M lifetimes. As for BR-WT, this strictly correlates with the voltage dependence of H+ pumping. BR-F171C and BR-F219L showed photocurrents similar to BR-WT. Yet, BR-F171C showed a weaker voltage dependence of proton pumping. For both mutants, blue laser flashes applied during and after green-light illumination showed reduced M accumulation and shorter M lifetime. In contrast, BR-D96G and BR-D96N exhibited small photocurrents, with nonlinear current-voltage curves, which increased strongly in the presence of azide. Blue laser flashes showed heavy M accumulation and prolonged M lifetime, which accounts for the strongly reduced H+ pumping rate. Hyperpolarizing potentials augmented these effects. The combination of M-stabilizing and -destabilizing mutations in BR-D96G/F171C/F219L (BR-tri) shows that disruption of the primary proton donor Asp-96 is fatal for BR as a proton pump. Mechanistically, M destabilizing mutations cannot compensate for the disruption of Asp-96. Accordingly, BR-tri and BR-D96G photocurrents were similar. However, BR-tri showed negative blue laser flash-induced currents even without actinic green light, indicating that Schiff base deprotonation in BR-tri exists in the dark, in line with previous spectroscopic investigations. Thus, M-stabilizing mutations, including the triple mutation, drastically interfere with electrochemical H+ gradient generation.

Heterometallic inorganic–organic frameworks of sodium–nickel(vanen): Cation–π interaction, trigonal dodecahedral Na+ and unprecedented heptadentate coordination mode of vanen2−

Three heterometallic nickel(II)–sodium(I) complexes, [Ni(vanen)Na(NO3)] (1), [Ni(vanen)Na(vanen)Ni]BF4 (2) and [Ni(vanen)Na(ClO4)]n (3), have been synthesized by reacting nickel(II) and H2 vanen with sodium(I), varying the reaction conditions and stoichiometry of the reactants [H2vanen=N,N′-ethylene-bis(3-methoxysalicylideneimine)]. All the three complexes have been characterized by X-ray single-crystal diffraction studies. In all the three complexes, nickel(II) is placed in the N2O2 environment and sodium(I) is placed in the O4 compartment of the compartmental Schiff base. In each complex, nickel(II) is square planar. Sodium(I) is trigonal dodecahedral in complex 2, and pentagonal pyramidal both in complexes 1 and 3. The nitrate ion is coordinated to sodium(I) in a chelating bidentate fashion. The sodium(I) in complex 1 shows strong π interaction with the phenyl ring of a second molecule. In complex 3, a carbon atom from an aryl group of a symmetry-related molecule is connected with sodium(I) in η1 fashion to constitute 1D chain. The structure of complex 3 reveals an unprecedented heptadentate coordination mode of the deprotonated Schiff base.

(Methanol-κO)-cis-dioxido{(4Z,N′E)-N′-[(Z)-4-oxido-4-phenylbut-3-en-2-ylidene]isonicotinohydrazidato}molybdenum(VI)

In the title complex, [Mo(C16H13N3O2)O2(CH3OH)], the deprotonated Schiff base (E)-N′-[(Z)-4-oxido-4-phenyl­but-3-en-2-yl­idene]isonicotinohydrazide coordinates in a meridional fashion through the enolate O-, imine N- and amidate O-atom donors to the Mo atom of a cis-[MoO2]2+ core. The sixth coordination site of molybdenum is occupied by the O atom of a methanol mol­ecule. In this complex, the NO5 coordination sphere adopts a distorted octa­hedral coordination geometry. The metal atom is shifted by 0.335 (1) Å from the square plane defined by the three donor atoms of the Schiff base ligand and one oxide group towards the second oxide group in the cis position. In the crystal, the complex forms inversion dimers through a pair of O—H⋯N hydrogen bonds involving the methanol –OH group and the pyridine N atom. Additional C—H⋯O contacts stack the mol­ecules along the b axis.

Bis{2-[(5-hydroxypentyl)iminomethyl]phenolato-κ2N,O1}copper(II)

In the title compound, [Cu(C12H16NO2)2], the CuII ion, located on a center of inversion, is coordinated by two singly deprotonated Schiff base ligands derived from condensation of salicyldehyde and 1-amino­pentan-5-ol. The imino N and phenol O atoms from both ligands offer a square-planar arrangement around the metal ion. The Cu—N and Cu—O bond lengths are 2.0146 (15) and 1.8870 (12) Å, respectively. Since the Cu—O and Cu—N bond lengths are different, it can be concluded that the resulting geometry of the complex is distorted. The aliphatic –OH group of the ligand is not coordinated and points away from the metal coordination zone and actively participates in hydrogen bonding connecting two other units and thus stabilizing the crystal lattice. This results in a two-dimensional extended array parallel to (201).

Photocycle of Exiguobacterium sibiricum Rhodopsin Characterized by Low-Temperature Trapping in the IR and Time-Resolved Studies in the Visible

The photocycle of the retinal protein from Exiguobacterium sibiricum, which differs from bacteriorhodopsin in both its primary donor and acceptor, is characterized by visible and infrared spectroscopy. At pH above pKa ~6.5, we find a bacteriorhodopsin-like photocycle, which originates from excitation of the all-trans retinal chromophore with K-, L-, M-, and N-like intermediates. At pH below pKa ~6.5, the M state, which reflects Schiff base deprotonation during proton pumping, is not accumulated. However, using the infrared band at ~1760 cm(-1) as a marker for transient protonation of the primary acceptor, we find that Schiff base deprotonation must have occurred at pH not only above but also below the pKa ~6.5. Thus, the M state is formed but not accumulated for kinetic reasons. Further, chromophore reisomerization from the 13-cis to the all-trans conformation occurs very late in the photocycle. The strongly red-shifted states that dominate the second half of the cycle are produced before the reisomerization step, and by this criterion, they are not O-like but rather N-like states. The assignment of photocycle intermediates enables reevaluation of the photocycle; its specific features are discussed in relation to the general mechanism of proton transport in retinal proteins.

Zig-zag [MnIII4] clusters from polydentate Schiff base ligands

Reactions of Schiff base ligands, OH–C6H4–CHNC(R)(CH2OH)2 (R=CH3, H3L1; R=C2H5, H3L2; R=CH2OH, H3L3) with Mn(ClO4)2·6H2O in MeOH solutions afforded MnIII4 complexes, i.e. compounds [Mn4(L1)2(HL1)2(H2O)2(MeOH)2](ClO4)2·yH2O (y=1, 1a·H2O; y=2, 1b·2H2O), [Mn4(L2)2(HL2)2(MeOH)4](ClO4)2 (2a), [Mn4(L2)2(HL2)2(H2O)2(MeOH)2] (ClO4)2·MeOH (2b·MeOH) and [Mn4(L3)2(HL3)2(H2O)4](ClO4)2 (3). The crystal structures of 1a, 1b, 2a and 2b revealed the first examples of MnIII4 complexes consisting of three corner sharing [MnIII2(μ-OR)2] units in trans orientation. The Schiff bases behave as chelating through their phenolato oxygen and imino nitrogen atoms as well as bridging ligands through their deprotonated alkoxo oxygen atoms. In each tetranuclear entity, there are two doubly deprotonated and two triply deprotonated Schiff base ligands which connect two and three metal ions, respectively. Magnetic susceptibility measurements for 2b indicate the interplay of ferromagnetic and antiferromagnetic interactions resulting in a diamagnetic ground state.

Thermodynamic parameters of anion binding to halorhodopsin from Natronomonas pharaonis by isothermal titration calorimetry

Halorhodopsin (HR), an inwardly directed, light-driven anion pump, is a membrane protein in halobacterial cells that contains the chromophore retinal, which binds to a specific lysine residue forming the Schiff base. An anion binds to the extracellular binding site near the Schiff base, and illumination makes this anion go to the intracellular channel, followed by its release from the protein and re-uptake from the opposite side. The thermodynamic properties of the anion binding in the dark, which have not been previously estimated, are determined using isothermal titration calorimetry (ITC). For Cl− as a typical substrate of HR from Natronomonas pharaonis, ΔG=−RT ln(1/Kd)=−15.9kJ/mol, ΔH=−21.3kJ/mol and TΔS=−5.4kJ/mol at 35°C, where Kd represents the dissociation constant. In the dark, Kd values have been determined by the usual spectroscopic methods and are in agreement with the values estimated by ITC here. Opsin showed no Cl− binding ability, and the deprotonated Schiff base showed weak binding affinity, suggesting the importance of the positively charged protonated Schiff base for the anion binding.

Di-μ3-acetato-bis{μ-6,6′-diethoxy-2,2′-[propane-1,2-diylbis(nitrilomethanylylidene)]diphenolato}dicadmiumdisodium ethanol 0.67-solvate

In the crystal of the title compound, [Cd2Na2(C2H3O2)2(C21H24N2O4)2]·0.67C2H6O, the doubly deprotonated Schiff base ligand N,N′,O,O′-chelates to the CdII cation, which is also O,O′-chelated by the acetate ion. Two Cd–Schiff base units are connected to two Na+ atoms to form the tetra­nuclear complex, in which the Cd2+ and Na+ cations show distorted octa­hedral coordinations. The asymmetric unit consists of half a tetra­nuclear mol­ecule (lying on an inversion center) and a full tetra­nuclear mol­ecule (lying on a general position) along with a lattice ethanol mol­ecule, which links to the coordinating acetate ion via an O—H⋯O hydrogen bond. In the crystal, the propyl and ethyl groups of the complex mol­ecule are disordered over two positions in a 1:1 ratio; the ethyl group of the lattice ethanol mol­ecule is also equally disordered over two positions.

Coordination modes of two flexidentate salicylaldimine ligands derived from N-(3-aminopropyl)morpholine toward zinc and copper

Two flexidentate Schiff-base ligands condensed from salicylaldehyde or 5-chlorosalicylaldehyde with N-(3-aminopropyl)morpholine were prepared in situ and reacted with Zn(II) and Cu(II) salts. Upon complexation, the Schiff bases underwent deprotonation at hydroxyl to act as mono-anionic ligands. When a ligand : metal ratio of 2 : 1 was applied, the deprotonated Schiff bases coordinated metal ions through phenolate and imine in a square-planar or tetrahedral geometry. In contrast, 5-chlorosalicylaldimine reacted with the metal ions in a 1 : 1 ratio to form complexes wherein morpholine nitrogen also participates in an N,N,O-tridentate coordination mode. The structures of the complexes were characterized by spectroscopic methods and single-crystal X-ray diffraction.

Di-μ-azido-κ4N1:N1′-bis({1-[(E)-phenyl(pyridin-2-yl-κN)methylidene]thiosemicarbazidato-κ2N1,S}copper(II))

In the title compound, [Cu2(C13H11N4S)2(N3)2], the CuII cation is N,N′,S-chelated by the deprotonated Schiff base ligand and is coordinated by the azide anion, while an N atom from an adjacent azide anion bridges the CuII cation at the apical position with a longer Cu—N distance of 2.533 (3) Å, completing the distorted N4S square-pyramidal coordination geometry. A pair of azide anions bridge the two CuII cations, forming a centrosymmetric binuclear mol­ecule. In the crystal, the binuclear mol­ecules are linked by an N—H⋯N hydrogen bond into a ribbon running along the a axis.

{2-[(2-Carbamoylhydrazin-1-ylidene)methyl-κ2N1,O]-5-methoxyphenolato-κO1}chloridocopper(II)

The asymmetric unit of the title compound, [Cu(C9H10N3O3)Cl], contains two independent mol­ecules with similar structures. The CuII cation is N,O,O′-chelated by the deprotonated Schiff base ligand and is further coordinated by a Cl− anion in a distorted ClNO2 square-planar geometry. In the crystal, adjacent mol­ecules are linked by N—H⋯O and N—H⋯Cl hydrogen bonds, forming a two-dimensional network parallel to [100].

Catena-Poly[[[{1-[(E)-phenyl(pyridin-2-yl-κN)methylidene]semicarbazidato-κ2N1,O}copper(II)]-μ-dicyanamido-κ2N1:N5] monohydrate

The CuII atoms in the title coordination polymer, {[Cu(C13H11N4O)(C2N3)]·H2O}n, are N,N′,O-chelated by the deprotonated Schiff base ligands, and adjacent metal atoms are bridged by the dicyanamide ions, generating a polymeric chain that propagates along the b axis. The two independent metal atoms show a square-pyramidal N4O coordination. The two independent water mol­ecules are disordered over two positions; each water mol­ecule is a hydrogen-bond donor to a carbonyl O atom. Weak N—H⋯N hydrogen bonding is also observed.

(Anilino{(Z)-2-[(E)-5-bromo-3-methoxy-2-oxidobenzylidene]hydrazin-1-ylidene-κ2O2,N2}methanethiolato-κS)(4,4′-dimethyl-2,2′-bipyridine-κ2N,N′)zincN,N-dimethylformamide monosolvate

The asymmetric unit of the title compound, [Zn(C15H12BrN3O2S)(C12H12N2)]·C3H7NO, contains two independent mol­ecules with a similar structure. The doubly deprotonated Schiff base ligand O,N,S-chelates to the metal atom, and the three coordinating atoms along with one N atom of the substituted 2,2′-bipyridine ligand constitute the square plane of the distorted square pyramid surrounding the metal atom. The apical site is occupied by the second N atom of the substituted 2,2′-bipyridine. The secondary amine group of the Schiff base dianion forms a hydrogen bond to the O atom of the dimethyl­formamide solvent. In the crystal, the phenyl ring of one of the two Schiff base anions is disordered over two positions in a 1:1 ratio. The crystal studied is a racemic twin.

Bis[benzyl 3-(3-phenylprop-2-enylidene)dithiocarbazato-κ2N3,S]cadmium

In the title complex, [Cd(C17H15N2S2)2], the CdII ion is located on a twofold rotation axis and exhibits a coordination number of four within a very distorted coordination environment that is best described as bis­phenoidal. The two deprotonated Schiff base ligands chelate the CdII ion through the azomethine N and the thiol­ate S atom. The dihedral angle between the two chelating ligands is 84.01 (9)°. Weak inter­molecular C—H⋯S inter­actions lead to the formation of chains along the c axis.

Syntheses, spectroscopy, and catalysis of (η 4-cod)Rh(I)-complexes with (R or S)-2-(salicylaldimine)-2-phenylethanol or (rac)-2-(salicylaldimine)-1-phenylethanol

Condensation of salicyldehyde with (R or S)-2-amino-2-phenylethanol or rac-2-amino-1-phenylethanol gives enantiopure (R or S)-2-(salicylaldimine)-2-phenylethanol (R- or S-H2L1) or (rac)-2-(salicylaldimine)-1-phenylethanol (rac-H2L2). The Schiff bases coordinate to [Rh(η 4-cod)(μ-O2CCH3)]2 to afford mononuclear [Rh(η 4-cod){(R or S)-2-(salicylaldiminato)-2-phenylethanol-κ 2 N,O}], [Rh(η 4-cod)(R- or S-HL1)] (1 or 2), or [Rh(η 4-cod){(rac)-2-(salicylaldiminato)-1-phenylethanol-κ 2 N,O}], [Rh(η 4-cod)(rac-HL2)] (3). The Schiff base and complexes are characterized by IR-, UV/Vis-, 1H/13C-NMR-, mass-spectroscopy, circular dichroism (CD), and polarimetry. The synthetic and spectroscopic results suggest that deprotonated Schiff base coordinates to [Rh(η 4-cod)] as a six-membered N,O-chelate with distorted square planar geometry at rhodium. CD and polarimetry measurements show the enantiopurity of the Schiff bases as well as the complexes in solution. The in situ system composed of [Rh(η 4-cod)Cl]2 and S-H2L1 has been used as a catalyst for the reduction of acetophenone into rac-1-phenylethanol with 85% conversion in diphenylsilane at 0–5°C.

Heterovalent trinuclear Co(III)-Co(II)-Co(III) complexes with N-(2-Carboxyethyl)salicylaldimines

The heterovalent trinuclear cobalt complexes [Co2IIIL4i · CoII(H2O)4] · nXmY (Li are deprotonated Schiff bases derived from substituted salicylaldehydes and β-alanine; i = 1–3) were obtained and characterized. An X-ray diffraction study of the trinuclear cobalt complex with N-(2-carboxyethyl)salicylaldimine showed that the central Co(II) ion and the terminal Co(III) ions are linked by bridging carboxylate groups. Either terminal Co(III) atom is coordinated to two ligand molecules. They form an octahedral environment consisting of two azomethine N atoms, two phenolate O atoms, and two O atoms of two carboxylate groups. The central Co(II) atom is coordinated to four water molecules and to two O atoms of two bridging carboxylate ligands involved in the coordination sphere of the terminal Co(III) atoms.

Synthesis, spectroscopy, catalysis and crystal structure of [Rh(η4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (Ar = C6H5, 3-/4-MeOC6H4, and 4-BrC6H4)

Condensation of 2-hydroxy-1-naphthaldehyde with (R)-(Ar)ethylamine yields the enantiopure Schiff bases, (R)-N-(Ar)ethyl-2-hydroxy-1-naphthaldimine (Ar=C6H5, 3-/4-MeOC6H4, 4-BrC6H4). These Schiff bases readily react with the dinuclear complex [Rh(η4-cod)(μ-O2CMe)]2 to afford the mononuclear complexes [Rh(η4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (Ar=C6H5 (I); 3-MeOC6H4 (II); 4-BrC6H4 (III)), respectively in C6H6/MeOH (5:1, v/v). The Schiff bases and complexes are characterized by IR, UV–Vis, 1H/13C NMR and mass spectrometry, polarimetry and HPLC. The polarimetric measurements show the enantiopurity of the Schiff bases as well as the complexes. The X-ray structure determination for III demonstrates that the deprotonated Schiff bases, (R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminate, co-ordinate to the [Rh(η4-cod)]-fragment as a six-membered N,O-chelate ligand with distorted square planar geometry at the rhodium metal atom. Reaction of III with O2 leads to the formation of the oxidative aduct [Rh(η4-cod)(μ-O)]2 (IIIa). Compound I or [Rh(η4-cod){(S or R)-N-(phenyl)ethyl-salicylaldiminato}] were used for reduction of acetophenone with diphenylsilane into (±)-1-phenyl-ethanol, and conversions up to 93–97% have been achieved.

Di-μ2-acetato-di-μ2-azido-di-μ3-methanol-tetrakis{μ-2-[(2-methyl-1-oxidopropan-2-yl)iminomethyl]-6-methoxyphenolato}tetranickel(II) methanol disolvate

In the centrosymmetric tetra­nuclear title complex, [Ni4(C12H15NO3)2(CH3COO)2(N3)2(CH3OH)2]·2CH3OH, the asymmetric unit comprises half of a complex mol­ecule and a methanol solvent mol­ecule. The NiII ions display two different coordination environments: (i) two O atoms from the Schiff base ligand, two O atoms from symmetry-related methanol mol­ecules and an O atom from an acetate group, one N atom from the azide group, and (ii) two O atoms and one N atom from the Schiff base, one O atom from methanol, one O atom from the acetate anion, and one N atom from the azide group. Four coplanar NiII ions are connected by two μ2-bridging O atoms from the two deprotonated Schiff bases, two μ3-O atoms from methanol mol­ecules, two μ1,1-N atoms from two azide ions, and four O atoms from acetate groups. The shortest Ni⋯Ni distance in the tetra­nuclear unit is 2.962 (2) Å. O—H⋯O hydrogen bonds between the methanol solvent mol­ecule and an acetate O atom feature in the crystal packing.