Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(III) complex of a L-threonine derivative and a significant red shift of visible spectra with Al3+ salt.

Four salts of an anionic iron(III) bis-complex, [Fe(LL-thr)2]1-, were synthesized from water or methanol. H2LL-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li+ (1), Na+ (2), K+ (3), and Ba2+ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li+ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba2+ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al3+ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.

Characterization of a meso-chiral isomer of a hexanuclear Cu(II) cage from racemization of the L-alanine Schiff base.

We are reporting structural characterization of two new hexanuclear cages (H3O)2[Cu3(µ3-OH)(µ3-NH3)(0.5)(L)3]2·8H2O (1) and (H3O)2[Cu3(µ3-OH)(µ3-H2O)(0.5)(L)3]2·8H2O (1a) where L(2-) is the dianionic form of the Schiff base of L-alanine and salicylaldehyde. The complex 1 has two C3 symmetric hydroxo bridged trinuclear halves joined by an ammonia or water molecule at the center through H-bonding. Each of the trinuclear halves is enantiopure but of opposite chirality to the other half, making the hexanuclear unit a meso isomer. Temperature dependent magnetic measurements showed the presence of ferromagnetic interactions among trinuclear Cu(II) units, a rare occurrence among trinuclear Cu(II) complexes. Characterization of the LiHL showed it to be enantiopure. Addition of a base, monitored using optical rotation, showed that racemization occurs as a result of base addition. The racemization depends on the base as well as the temperature. Base or Cu(II) induced racemization of amino acid derivatives has been indicated in a number of cases in the past but structural characterization of the products or formation of this type of chiral hexanuclear architecture was never reported. Structures of the complex and the ligand have a number of interesting H-bonding situations.

Sodium and potassium ion directed self-assembled multinuclear assembly of divalent nickel or copper and L-leucine derived ligand.

L-leucine derived ligand (H(2)L(L-leu)), KOH, and Ni(II) salt in 2:2:1 ratio self-assembled into a rather large (approximately 13 A) supramolecular assembly with the formula [K{Ni(HL(L-leu))(2)}(3)](+) (1). Structural characterization showed three [Ni(HL(L-leu))(2)] units encapsulated K(+) similar to organic crown ethers/cryptand. Substituting Ni(II) with Cu(II) and K(+) with Na(+) in the above reaction resulted in a set of structurally identical assemblies with the general formula [M'{M(HL(L-leu))(2)}(3)](+), where M' is either K(+) or Na(+) and M is either Cu(II) or Ni(II); [Na{Ni(HL(L-leu))(2)}(3)]ClO(4) (2), Na{Ni(HL(L-leu))(2)}(3)]OTf (3), [K{Cu(HL(L-leu))(2)}(3)]ClO(4) (4), [Na{Cu(HL(L-leu))(2)}(3)]ClO(4) (5), [K{Cu(HL(L-leu))(2)}(3)]NO(3) (6). Electrospray Ionization (ESI)-mass spectra of the assemblies in MeOH showed the retention of assemblies in solution. Visible spectroscopic studies showed retention of assembly 1 in N,N-dimethylformamide (DMF) which is stable even after the addition of 5 equiv of [18]-crown-6. The assemblies in 2-6 show various degrees of dissociation to [M(HL(L-leu))(2)] and M', in stronger H-bonding methanol. The dissociation can be reversed upon addition of excess KNO(3)/NaNO(3) salt. Structural characterization of [Cu(HL(L-leu))(2)(MeCN)] (7) along with its transformation to [K{Cu(HL(L-leu))(2)}(3)](+) in the presence of K(+) salt demonstrated that the assembly formation proceeds through an alkali metal ion induced ligand reorientation within the [Cu(HL(L-leu))(2)] units which is further stabilized by six strong H-bonds holding the assembly. Interestingly, visible spectra of 1 and 2 shows that minor structural changes caused by replacing K(+) with Na(+) is sufficient to shift the d-d transition of Ni(II) by approximately 70 nm, thereby providing an indirect way of distinguishing K(+) and Na(+), none of which have spectroscopic signature in the visible range.

Formation of a narrow chiral cavity in bis-copper(ii) complexes of ferrocenylmethyl-l-tyrosine and its interaction with achiral guests.

Earlier we reported the tendency of ferrocenylmethyl substituted amino acids to organize around Cu(ii) in a C(2) symmetry. In this paper we have utilized this property to form a chiral cavity using a l-tyrosine derivative. Structural characterization of [Cu(II)(S-fTyr)(2)(MeCN)(2)] (), where S-fTyr(1-) is ferrocenylmethyl-l-tyrosinate, showed that the alignment of two tyrosine aromatic rings around the fifth coordination site generates a narrow cavity. Structural characterization of [Cu(S-fTyr)(2)(pyrazine)] (), [Cu(S-fTyr)(2)(pyridine)] (), [Cu(S-fTyr)(2)(bpy)] () and [Cu(S-fTyr)(2)(H(2)O)] () revealed that planar heterocyclic N-donors as guests stabilized the cavity, while polar water molecules destroyed it. Electrochemical properties of the complexes showed a shift of Cu(II)/Cu(I) potentials depending on the strength of the guest binding inside the cavity.

Cu(II)-mediated synthesis of a new fluorescent pyrido[1,2-a]quinoxalin-11-ium derivative.

Quinoxalines are heterocyclic compounds with potential application as drugs or fluorophores. However, few quinoxalinylium salts have been reported in the literature. This manuscript describes the synthesis and structural characterization of a previously unknown quinoxalinylium derivative, 2-aminopyrido[1,2-a]quinoxalin-11-ylium ([1](+)), as perchlorate and thiocyanate salts from the Cu(II)-mediated reaction of a Schiff base. The reaction is most efficient with Cu(II). The formation of a small quantity was observed spectroscopically in the presence of either a strong oxidizer or Mn (II). No product formation was observed with Zn(II), Cd(II), Fe(II), or Ni(II). [1](+) emits at 580 nm, with a quantum yield estimated as 0.23, upon excitation at 470 nm.

Ferrocene substitution in amino acids strengthens the axial binding in Cu(II) complexes and separates the hydrophobic and hydrophilic region in the crystals.

This paper reports on the synthesis, characterization and electrochemical properties of four ferrocenylmethyl substituted L-amino acid ligands and their Cu(II) complexes. Structural characterization of the complexes of L-methionine and L-asparagine derived ligands showed axial coordination of weak thioether and amide respectively to Cu(II). Coordination of the thioether group of L-methionine to copper (2.791 A) is shorter than observed in the electron transfer protein plastocyanin (2.9 A). Methionine thioether does not bind in synthetic Cu(II) complexes. The characterization of bis-complexes (metal : ligand ratio 1 : 2) with L-serine and L-threonine derivatives showed axial coordination of water with a shorter Cu-O bond length compared to that observed in the corresponding amino acid complexes. The structures also revealed separation of the hydrophilic and hydrophobic regions due to amino acid and ferrocene respectively which resulted in the formation of interesting H-bonded networks.

Hexavalent chromium removal from wastewater using aniline formaldehyde condensate coated silica gel.

A resinous polymer, aniline formaldehyde condensate (AFC) coated on silica gel was used as an adsorbent in batch system for removal of hexavalent chromium from aqueous solution by considering the effects of various parameters like reaction pH, dose of AFC coated silica gel, initial Cr(VI) concentration and aniline to formaldehyde ratio in AFC synthesis. The optimum pH for total chromium [Cr(VI) and Cr(III)] adsorption was observed as 3. Total chromium adsorption was second order and equilibrium was achieved within 90-120 min. Aniline to formaldehyde ratio of 1.6:1 during AFC synthesis was ideal for chromium removal. Total chromium adsorption followed Freundlich's isotherm with adsorption capacity of 65 mg/g at initial Cr(VI) 200mg/L. Total chromium removal was explained as combinations of electrostatic attraction of acid chromate ion by protonated AFC, reduction of Cr(VI) to Cr(III) and bond formation of Cr(III) with nitrogen atom of AFC. Almost 40-84% of adsorbed chromium was recovered during desorption by NaOH, EDTA and mineral acids. AFC coated silica gel can be effectively used for treatment of chromium containing wastewaters as an alternative.

Structural characterization of an enantiopure hydroxo-bridged binuclear iron(III) complex with empty one-dimensional helical channels.

A H-bond capable chiral tetradentate ligand, Fe3+, and acetate ion assembles into a hydroxo-bridged binuclear complex with the formula [FeIII2(mu-OH)(mu-OAc)(S-L)2] x 4H2O (1) where H2S-L = S-2-(2-hydroxy-benzylamino)-3-(1H-imidazol-4-yl)-propionic acid. The crystal of 1 contains right-handed one-dimensional (1D) helical channels with 7.3-9.8 A diameter. A similar reaction with a ligand having opposite chirality forms the complex with left-handed helical channels (1a). Heating the crystals of 1 at 95 degrees C under reduced pressure selectively removes three waters from the channel forming an enantiopure porous crystal with empty channels (solvent accessible voids 18% v/v). Intermolecular hydrogen bonding between the imidazole N-H and phenolate oxygen in 1-2 forms a C6 symmetric helix with bridging hydroxo groups pointing inside the channels. All the H-bond capable atoms in the ligand along with one water molecule form an extended H-bonded network throughout the crystal. Exposing the empty channels of 2 to iodine vapor indicates partial filling of the channels with iodine. Crystal data for 1 x 4H2O include the following: hexagonal, P61, a = b = 13.164(3) A, c = 36.305 (11) A, alpha = beta = 90 degrees , gamma = 120 degrees , Z = 6, R1 = 0.0387, wR2 = 0.0842. Crystal data for 1a x 2H2O include the following: hexagonal, P6(5), a = b = 13.151(4) A, c = 36.558(2) A, alpha = beta = 90 degrees , gamma = 120 degrees , Z = 6, R1 = 0.0416, wR2 = 0.1190. Crystal data for 2 x H2O include the following: hexagonal, P61, a = b = 13.160(7) A, c = 36.559 (4) A, alpha = beta = 90 degrees , gamma = 120 degrees , Z = 6, R1 = 0.0574, wR2 = 0.1423.

Role of carboxylate bridges in modulating nonheme diiron(II)/O(2) reactivity.

A series of diiron(II) complexes of the dinucleating ligand HPTP (N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (micro-alkoxo)(micro-carboxylato)diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (micro-alkoxo)bis(micro-carboxylato)diiron(II) center. These complexes react with O(2) in second-order processes to form adducts characterized as (micro-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O(2) binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O(2) adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O(2) adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH(2)Cl(2)/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O(2) adduct of a dicarboxylate complex is even more stable in pure CH(2)Cl(2) and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O(2) adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O(2) adducts of dicarboxylate complexes can be rationalized by the formation of a (micro-alkoxo)(micro-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O(2) adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.