Hexacoordinate Species Research Articles

On the formation of polyol-silicate complexes: DFT study of catechol as model chelator

The pathways of forming catechol-silicate complexes were fabricated based on complexation ratio and examined by DFT calculations. The 1/1 complexation between catechol and silicates cannot obtain both penta- and hexa-coordinated complexes. The 2/1 complexation ratio results in the formation of both penta- ([(HO)Si(Cat)2]− and [OSi(Cat)2]2−) and hexa-coordinated species ([(HO)2Si(Cat)2]2−), and the formation of the hexa-coordinated complex is more energetically favorable. Another hexa-coordinated complex ([Si(Cat)3]2−) can be formed preferentially in energy at the 3/1 complexation ratio. Catechol anions cannot function as chelators to form penta- or hexa-coordinated catechol-silicate complexes.

Comparative (magneto)optical, redox, and photophysical properties of phosphorous octacarbazole- and tetra-tert-butyltetrabenzotriazacorroles

The spectroscopy and electronic structure of the novel phosphorous(V) octacarbazole tetrabenzotriazacorrole (TBC(Cz)8P=O) compound were evaluated using a large array of experimental and theoretical methods, and compared to the reference terta-tert-butyl tetrabenzotriaazacorrole (TBC(tBu)4P=O). The first coordination sphere in TBC(Cz)8P(XY) and TBC(tBu)4P(XY) in a solid state and solution was probed by MALDI-TOF mass spectrometry, 31P NMR spectroscopy, XPS, UV–Vis, MCD, and elemental analysis. Based on the 31P NMR, mass spectrometry, UV–Vis, and MCD data, it was demonstrated that the initial pentacoordinated TBC(tBu)4P=O complex can be easily transformed to the hexa-coordinated species in the presence of water and methanol following TBC(tBu)4P=O ⇌ TBC(tBu)4P(OH)2 ⇌ TBC(tBu)4P(OH)(OMe) ⇌ TBC(tBu)4P(OMe)2 equilibrium. In wet solvents, the UV–Vis and MCD spectroscopies revealed that the two structures possess similar absorbance profiles; however, the TBC(Cz)8P(OH)2 compound had a more intense and red-shifted Q-band than the TBC(tBu)4P(OH)2 compound. The first oxidation under spectroelectrochemical conditions was found to be completely reversible for the TBC(Cz)8P(OH)2 compound; conversely, these experiments were completely irreversible for the TBC(tBu)4P(OH)2 compound. DFT and TDDFT calculations were performed using three different exchange-correlation functionals. The DFT results revealed several (16 in total) carbazole-centered MOs at energies in between the classical Gouterman's a1u and a2u frontier orbitals while the four Gouterman's frontier orbitals were found to be energetically sequential for the TBC(tBu)4P(OH)2 compound. The TDDFT calculations provided a very reasonable agreement between theory and experiment and demonstrated the presence of the inter-ligand (carbazole-to-TBC) charge-transfer transitions in TBC(Cz)8P(OH)2 in ∼500 nm region. Steady-state fluorescence spectroscopy data are indicative of the mid-range fluorescence quantum yields in both compounds. Transient absorption spectroscopy was utilized to probe triplet state formation and lifetimes (τT) which were found to be 112 μs for TBC(Cz)8P(OH)2 and 91 μs for TBC(tBu)4P(OH)2.

Synthesis and Characterization of Monomeric Hexacoordinated Chalcogenonium Salts Bearing 2-(2-Pyridyl)phenyl Ligands

Abstract Consecutive treatment of 2-(2-lithiophenyl)pyridine with chalcogen tetrachlorides (ChCl4; Ch = Se and Te) and KPF6 afforded the hexafluorophosphate salts of tris[2-(2-pyridyl)phenyl]selenonium [(ppy)3Se]+ ([1]+) and tris[2-(2-pyridyl)phenyl]telluronium [(ppy)3Te]+ ([2]+), respectively. The chalcogenonium salts [1][PF6] and [2][PF6] exhibit extremely high thermal stability and high melting points. Their solid-state structures revealed distorted octahedral bonding geometries with a C3-symmetrical facial configuration. Quantum chemical calculations on [1]+ and [2]+ strongly suggest that their HOMO−3s involve lone pairs of electrons (LPs) on the central chalcogen atoms, and that the three Ch–N (Ch = Se and Te) bonds are formed by LP(N)→σ*(Ch–C) donor-acceptor interactions. Based on theoretical calculations and 77Se and 125Te NMR spectral data, the C–Ch–N moieties of [1]+ and [2]+ were characterized as asymmetrical three-center four-electron σ-type bonds [σ(3c–4e)]. Both organochalcogenonium salts represent novel monomeric hexacoordinated species that bear 14 formal valence electrons (14–Se–6 and 14–Te–6, respectively). A quantum theory of atoms-in-molecules dual functional analysis predicted that the C–Ch interaction of the σ(3c–4e) C–Ch–N moiety (Ch = Se and Te) is weakly covalent, while the Se–N and Te–N interactions were predicted to have the typical characteristics of hydrogen bonds and molecular complexes, respectively. The differences between [2][PF6] and the recently reported [2][Br] were also carefully examined.

Crystal structure and NMR study of a bisperoxovanadium complex [NH4][VO(O2)2(ima)

A novel bisperoxovanadium complex, [NH4][VO(O2)2(ima)] (1) (ima = imidazole-4-carboxamide), was synthesized by the reaction of NH4VO3 and ima in the presence of H2O2, and the structure was characterized by single-crystal X-ray technology. The adjacent [NH4][VO(O2)2(ima)] monomers further constructed a 3-D supramolecular framework through intra- and intermolecular hydrogen bonding interactions. The composition of the title complex solution was explored using a combination of multinuclear (1H, 13C and 51V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable temperature NMR in a 0.15 mol L−1 NaCl/D2O solution that mimics physiological conditions. According to NMR experimental results, a pair of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types of coordination modes between the metal center and the ligands; Isomer B (the main product) has the same coordination structure as the crystal structure of [NH4][VO(O2)2(ima)]. The 51V NMR experiment together with single-crystal X-ray diffraction results indicated that Isomer A is the hexa-coordinated peroxovanadium species while Isomer B is the hepta-coordinated species.

Indenyl Compounds with Constrained Hapticity: The Effect of Strong Intramolecular Coordination

A series of cyclopentadienyl and indenyl molybdenum(II) compounds with intramolecularly coordinated pyridine arms, including scorpionate‐like species bearing two irreversibly coordinated arms on the indenyl core, were synthesized and characterized. All presented structural types were confirmed by X‐ray diffraction analysis. Owing to the strong nucleophilicity of pyridine, the intramolecular interaction was found to be considerably stronger than that in analogous species bearing tertiary amines in the side chain. Although the starting compounds for the syntheses were isostructural, the reaction outcomes differed considerably. The cyclopentadienyl precursor gave a pentacoordinate η5:κN‐compound, whereas the indenyl analogue produced a hexacoordinate species with the unprecedented η3:κN‐coordination mode of the indenyl ligand and thus represents an unusual example of the so‐called indenyl effect. The unusually high stability of the η3:κN‐coordination compounds toward η3 to η5 haptotropic rearrangement was clarified by theoretical calculations. As the strong intramolecular interaction prevented rotation of the indenyl moiety, it could not reach the conformation suitable for the η3 to η5 rearrangement. As a result, the low hapticity was effectively locked.

Generation of Zerovalent Metal Core Nanoparticles Using n-(2-aminoethyl)-3-aminosilanetriol.

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of n-(2-aminoethyl)-3-aminosilanetriol is presented. This compound can be used to effectively reduce and complex metal salts into metal core nanoparticles coated with the compound. By controlling the concentrations of salt and silane one is able to control reaction rates, particle size, and nanoparticle coating. The effects of these changes were characterized through transmission electron microscopy (TEM), UV-Vis spectrometry (UV-Vis), Nuclear Magnetic Resonance spectroscopy (NMR) and Fourier Transform Infrared spectroscopy (FTIR). A unique aspect to this reaction is that usually silanes hydrolyze and cross-link in water; however, in this system the silane is water-soluble and stable. It is known that silicon and amino moieties can form complexes with metal salts. The silicon is known to extend its coordination sphere to form penta- or hexa-coordinated species. Furthermore, the silanol group can undergo hydrolysis to form a Si-O-Si silica network, thereby transforming the metal nanoparticles into a functionalized nanocomposites.

Structural flexibility of the heme cavity in the cold-adapted truncated hemoglobin from the Antarctic marine bacterium Pseudoalteromonas haloplanktis TAC125.

Truncated hemoglobins build one of the three branches of the globin protein superfamily. They display a characteristic two-on-two α-helical sandwich fold and are clustered into three groups (I, II and III) based on distinct structural features. Truncated hemoglobins are present in eubacteria, cyanobacteria, protozoa and plants. Here we present a structural, spectroscopic and molecular dynamics characterization of a group-II truncated hemoglobin, encoded by the PSHAa0030 gene from Pseudoalteromonas haloplanktis TAC125 (Ph-2/2HbO), a cold-adapted Antarctic marine bacterium hosting one flavohemoglobin and three distinct truncated hemoglobins. The Ph-2/2HbO aquo-met crystal structure (at 2.21 Å resolution) shows typical features of group-II truncated hemoglobins, namely the two-on-two α-helical sandwich fold, a helix Φ preceding the proximal helix F, and a heme distal-site hydrogen-bonded network that includes water molecules and several distal-site residues, including His(58)CD1. Analysis of Ph-2/2HbO by electron paramagnetic resonance, resonance Raman and electronic absorption spectra, under varied solution conditions, shows that Ph-2/2HbO can access diverse heme ligation states. Among these, detection of a low-spin heme hexa-coordinated species suggests that residue Tyr(42)B10 can undergo large conformational changes in order to act as the sixth heme-Fe ligand. Altogether, the results show that Ph-2/2HbO maintains the general structural features of group-II truncated hemoglobins but displays enhanced conformational flexibility in the proximity of the heme cavity, a property probably related to the functional challenges, such as low temperature, high O2 concentration and low kinetic energy of molecules, experienced by organisms living in the Antarctic environment.

Novel nanohybrids of cobalt(III) Schiff base complexes and clay: Synthesis and structural determinations

The [Co(Me2Salen)(PBu3)(OH2)]BF4 and [Co(Me2Salen)(PPh3)(Solv)]BF4, complexes were synthesized and characterized by FT-IR, UV–Vis, 1H NMR spectroscopy and elemental analysis techniques. The coordination geometry of [Co(Me2Salen)(PPh3)(H2O)]BF4 was determined by X-ray crystallography. It has been found that the complex is containing [Co(Me2Salen)(PPh3)(H2O)]BF4 and [Co(Me2Salen)(PPh3)(EtOH)]BF4 hexacoordinate species in the solid state. Cobalt atom exhibits a distorted octahedral geometry and the Me2Salen ligand has the N2O2 coordinated environment in the equatorial plane. The [Co(Me2Salen)(PPh3)(H2O)]BF4 complex shows a dimeric structure via hydrogen bonding between the phenolate oxygen and hydrogens of coordinated H2O molecule. These complexes were incorporated into Montmorillonite-K10 nanoclay. The modified clays were identified by FT-IR, XRD, EDX, TGA/DTA, SEM and TEM techniques. According to the XRD results of the new nanohybrid materials, the Schiff base complexes are intercalated in the interlayer spaces of the clay. SEM and TEM micrographs show that the resulting hybrid nanomaterials have layer structures. Also, TGA/DTG results show that the intercalation reaction was taken place successfully.

CO Rebinding Kinetics and Molecular Dynamics Simulations Highlight Dynamic Regulation of Internal Cavities in Human Cytoglobin

Cytoglobin (Cygb) was recently discovered in the human genome and localized in different tissues. It was suggested to play tissue-specific protective roles, spanning from scavenging of reactive oxygen species in neurons to supplying oxygen to enzymes in fibroblasts. To shed light on the functioning of such versatile machinery, we have studied the processes supporting transport of gaseous heme ligands in Cygb. Carbon monoxide rebinding shows a complex kinetic pattern with several distinct reaction intermediates, reflecting rebinding from temporary docking sites, second order recombination, and formation (and dissociation) of a bis-histidyl heme hexacoordinated reaction intermediate. Ligand exit to the solvent occurs through distinct pathways, some of which exploit temporary docking sites. The remarkable change in energetic barriers, linked to heme bis-histidyl hexacoordination by HisE7, may be responsible for active regulation of the flux of reactants and products to and from the reaction site on the distal side of the heme. A substantial change in both protein dynamics and inner cavities is observed upon transition from the CO-liganded to the pentacoordinated and bis-histidyl hexacoordinated species, which could be exploited as a signalling state. These findings are consistent with the expected versatility of the molecular activity of this protein.

Formation in aqueous solution of a non-oxido VIV complex with VN6 coordination. Potentiometric, ESI-MS, spectroscopic and computational characterization

The behaviour of the system formed by V(IV)O(2+) ion with all-cis-2,4,6-trimethoxycyclohexane-1,3,5-triamine (tmca) was characterized in aqueous solution through the combined application of electron paramagnetic resonance (EPR) and UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), pH-potentiometry and DFT methods. The formation of an unusual non-oxido [V(tmcaH-2)2] species with VN6 coordination, with the ligand in the bianionic form, was demonstrated. The geometry, EPR and UV-Vis spectra and electronic structure of [V(tmcaH-2)2] were simulated with Gaussian 09 and ORCA software and the results were compared with those of similar oxido and non-oxido vanadium(iv) species formed by other polyamine and polyol related ligands, such as 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci), 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci), cis-inositol (ino) and 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci). The results indicate that V(IV)O(2+) species are formed in acid and weakly basic solution and that [V(tmcaH-2)2] is observed above pH 10. In the non-oxido complex, DFT calculations suggest that the two -NH2 groups are in trans position and that the pre-organization of the ligands favours the metal complexation and the formation of the hexa-coordinated species with VN6 coordination.

An Ab Initio Investigation of Zinc Bromo Complexes

A series of geometry, frequency, and energy calculations of bromoaquazinc(II) complexes were carried out at up to the MP2/6-31+G* level. A thorough examination of all species up to and including hexacoordinate species, and with up to four bromides, was carried out. The structures of the complexes are compared with experimental data where available. The solution chemistry of zinc(II) in the presence of bromide is discussed.

Multireference and Multiconfiguration Ab Initio Methods in Heme-Related Systems: What Have We Learned So Far?

This work reviews the recent applications of ab initio multireference/multiconfiguration (MR/MC) electronic structure methods to heme-related systems, involving tetra-, penta-, and hexa-coordinate species, as well as the high-valent iron-oxo species. The current accuracy of these methods in the various systems is discussed, with special attention to potential sources of systematic errors. Thus, the review summarizes and tries to rationalize the key elements of MR/MC calculations, namely, the choice of the employed active space, especially the so-called double-shell effect that has already been recognized to be important in transition-metal-containing systems, and the impact of these elements on the spin-state energetics of heme species, as well as on the bonding mechanism of small molecules to the heme. It is shown that expansion of the MC wave function into one based on localized orbitals provides a compact and insightful view on some otherwise complex electronic structures. The effects of protein environment on the MR/MC results are summarized for the few available quantum mechanical/molecular mechanical (QM/MM) studies. Comparisons with corresponding DFT results are also made wherever available. Potential future directions are proposed.

Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).

Aluminum Solvate Complexes Forming in Acidic Methanol-Acetone Mixtures Studied by 27Al NMR Spectroscopy

27Al NMR spectroscopy is a powerful tool for the study of coordination and solvation in both aqueous and nonaqueous solutions. In this study, the complexes coexisting upon dissolution of AlCl3 in acidic acetone + methanol solutions are shown to consist essentially of mixed hexacoordinated species of the general formula [Al(CH3OH)6−n(CH3COCH3)n]3+ (n=1,2 and 3), all exhibiting distinctly different 27Al shielding effects. The relative populations of the various mixed species are found to be highly dependent upon the acetone:methanol mole ratio that in the more acetone-rich mixtures with aluminum become appreciably coordinated by acetone. The results demonstrate that the key factor for the formation of acetone-containing species in acidic methanolic solutions is having the CH3COCH3:CH3OH mole ratio at 3:1.

TiIV Exchanged K10-Montmorillonite: Characterisation and Catalytic Properties in Liquid-Phase Sulfide Oxidation

Titanium-containing K10 has been synthesised and characterised by X-ray diffraction, UV-visible reflectance, solid state NMR spectroscopy, N2 nitrogen adsorption at 77K and chemical analysis. Two different titanium species have identified with a high dispersion in K10. The first is in an isolated penta- and hexa-coordinated state and the second is in the form of polymerised hexacoordinated species, which contains Ti-O-Ti bonds. These titanium species were found to be the catalytic sites that readily interact with oxidant to offer an active and selective catalytic system for the oxidation of sulfides. This catalytic system was studied in the asymmetric oxidation of methyl phenyl sulfide, using different types of chiral modifier, and gives excellent conversion and sulfoxide selectivity, together with a significant enantioselectivity in the range of 16–18% ee with (R)-binol.

Iron Chelates of 2,2′‐Bidipyrrin: Stable Analogues of the Labile Iron Bilins

A unique series of halogenidoiron(III) complexes of the open-chain tetrapyrrolic ligand 2,2'-bidipyrrin (bpd) ([FeX(bdp)] X=F, Cl, Br, I) was prepared from simple pyrrolic and bipyrrolic precursors and iron chloride by a one-pot condensation/metalation strategy, followed by salt metathesis with CsF, LiBr, or NaI. Crystallographic analysis revealed that in all cases the 2,2'-bidipyrrin ligand is forced to reside in a helical conformation when bound to the iron atom. Whereas the extremely sensitive fluorido derivative was isolated as a CsF adduct and forms 1D polymeric chains in the solid state, the more stable chlorido, bromido, and iodido derivatives crystallize as discrete monomeric molecules with a distorted pentacoordinate iron(III) ion in an intermediate spin ground state. Magnetic susceptibility measurements and Mössbauer data of the compounds are in agreement with this interpretation. In solution, however, all the compounds are pentacoordinate with the iron atom in the high-spin (S=5/2) state and dynamic with respect to helix inversion. In the presence of air, the iron chelates react stepwise with the nucleophiles methanol and imidazolate at the tetrapyrrole terminal alpha,omega-positions, presumably through the hexacoordinate species [Fe(bdp)(MeOH)2]+ and [Fe(im)2-(bdp)](-), respectively. The successive increase of strain at these positions results in increasingly labile intermediates that spontaneously release the iron ion from the mono- or disubstituted tetrapyrrole ligands.

Discovery and synthetic applications of novel silicon-carbon bond cleavage reactions based on the coordination number change of organosilicon compounds

Some synthetically useful transformations of organosilicon compounds have been developed since the mid 1970s, based on the new concept that the silicon-carbon bonds are activated toward electrophilic cleavage via the formation of penta- and hexa-coordinate species. This review mainly consists of the following aspects: (1) a general concept for the activation of the silicon-carbon bond via penta- and hexa-coordinate species, (2) synthetic application of hexa-coordinate organopentafluorosilicates, and (3) development of the H2O2 oxidation of the silicon-carbon bond and its synthetic applications via the intramolecular hydrosilylation, silicon-tethered intramolecular radical cyclization and Diels-Alder reaction, and some silicon-containing organometallic reagents for nucleophilic hydroxymethylation and hydroxyallylation synthons.

Reversible Inhibition/Activation of Olefin Metathesis: A Kinetic Investigation of ROMP and RCM Reactions with Grubbs' Catalyst

The metathesis activity of Grubbs' catalyst 1 was investigated in the presence of N-donor ligands (1-methylimidazole [MIM], 4-(N,N-dimethylamino)pyridine [DMAP], pyridine, and 1-octylimidazole [OIM]). Ring opening metathesis polymerization (ROMP) reactions of cyclooctene (COE), bulk-ROMP reactions of COE and norbornadiene (NBD), and ring closing metathesis (RCM) reactions of diethyl diallylmalonate (DEDAM) were conducted containing various equivalents of N-donor with respect to catalyst. ROMP reactions could be stopped using MIM (1-5 equiv) and DMAP (2-5 equiv), and slowed with pyridine (1-5 equiv) by factors >100, in benzene solution for 24 h. The stopped reactions could be initiated with excess phosphoric acid (H3PO4), and the reactions proceeded faster than with uninhibited Grubbs' catalyst in the first 4 min after reactivation. Thereafter, the reaction proceeded at the same rate as the reaction with the uninhibited catalyst. ROMP reactions in neat COE and NBD could be inhibited for 72 h using 2 equiv of MIM, DMAP, or OIM and activated with H3PO4 to give polymer gels within minutes or less. RCM reactions could be completely inhibited with MIM (1-5 equiv), but upon treatment with H3PO4, the reaction would proceed at a fraction of the initial rate accomplished by uninhibited Grubbs' catalyst 1. A structural investigation of the inhibited species showed that MIM and DMAP completely or partially transform catalyst 1 into the hexacoordinate species 5a or 5b producing free PCy3, which additionally acts as an inhibitor for the ROMP reaction. Upon reactivation, the PCy3 is protonated along the N-donor ligand; however, over the period of 5 min, the phosphine has been found to coordinate back to the ruthenium catalyst. Therefore, the reaction slows to the same polymerization rate as the reaction using the uninhibited catalyst at this point. Complexes 5a and 5b were isolated, characterized, and employed in ROMP and RCM experiments where they exhibited very low catalytic activity.

An ab initio investigation of zinc chloro complexes

A series of geometry, frequency, and energy calculations of chloroaquazinc(II) complexes were carried out at up to the MP2/6-31+G* level. A thorough examination of all species up to and including hexacoordinate species, and with up to six chlorides, was carried out. The structures of the complexes are compared with experimental data where available. The solution chemistry of zinc(II) in the presence of chloride is discussed, and Raman spectra of zinc perchlorate with increasing amount of chloride are presented.

Understanding of the Mode of Action of FeIII–EDDHA as Iron Chlorosis Corrector Based on Its Photochemical and Redox Behavior

The very low reduction potential of the chelate Fe(III)-EDDHA (EDDHA = ethylenediamine N,N'-bis(2-hydroxy)phenylacetic acid) makes it unreactive in photochemically or chemically induced electron transfer processes. The lack of reactivity of this complex toward light invalidates photodegradation as an alternative mechanism for environmental elimination. However, in spite of its low reduction potential, the biological reduction of Fe(III)-EDDHA is very effective. Based on electrochemical measurements, it is proposed that Fe(III)-EDDHA itself is not the substrate of the enzyme ferric chelate reductase. Likely, at the more acidic pH in the vicinity of the roots, the ferric chelate in a closed form (FeL-) could generate a vacant coordination site that leads to an open hexacoordinate species (FeHL) where the reduction of the metal by the enzyme takes place.