H2O Ligands Research Articles

Density Functional Theory Investigations on the Mechanism of Formation of Pa(V) Ion in Hydrous Solutions.

Due to the enormous threat of protactinium to the environment and human health, its disposal and chemistry have long been important topics in nuclear science. [PaO(H2O)6]3+ is proposed as the predominant species in hydrous and acidic solutions, but little is known about its formation mechanism. In this study, density functional theory (DFT) calculations demonstrate a water coordination-proton transfer-water dissociation mechanism for the formation of PaO3+ in hydrous solutions. First, Pa(V) ion preferentially forms hydrated complexes with a coordination number of 10. Through hydrogen bonding, water molecules in the second coordination sphere easily capture two protons on the same coordinated H2O ligand to form [PaO(H2O)9]3+. Water dissociation then occurs to generate the final [PaO(H2O)6]3+, which is the thermodynamic product of Pa(V) in hydrous solutions.

Nanosheet-like Lithium Borohydride Hydrate with 10 wt % Hydrogen Release at 70 °C as a Chemical Hydrogen Storage Candidate.

A nanosheet-like lithium borohydride hydrate (LiBH4·H2O) measuring 20-30 nm in thickness is successfully synthesized for the first time by a facile, scalable freeze-drying strategy. The prepared LiBH4·H2O nanosheets start releasing hydrogen below 50 °C and release an amount up to approximately 10 wt % at 70 °C because of the strong affinity of H+ in the H2O ligand and H- in the BH4 group. The reported dehydrogenation properties here are superior to those of all known complex hydrides, indicating applicability as an advanced chemical hydrogen storage medium.

Photosubstitution Reaction of cis-[Ru(bpy)2(CH3CN)2]2+ and cis-[Ru(bpy)2(NH3)2]2+ in Aqueous Solution via Monoaqua Intermediate.

The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis-[Ru(bpy)2(CH3CN)2]2+ (bpy = bipyridine), cis-[Ru(bpy)2(NH3)2]2+, and their monoaqua complexes were analyzed. The metal-centered (3MC) structure was lower than the metal-to-ligand charge transfer (3MLCT) structure for cis-[Ru(bpy)2(CH3CN)2]2+, whereas the 3MLCT structure was lower than the 3MC structure for cis-[Ru(bpy)2(NH3)2]2+. Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the 3MC structure, in which the Ru-OH2O and Ru-Nbpy ( trans to the oxygen) bonds were elongated. Therefore, the dissociation of the H2O ligand would be preferred to that of the CH3CN (or NH3) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.

Structural Features of Monomeric Octahedral Monooxo d2-Rhenium(V) Complexes [ReO(Ln)2(Lmono)] with Oxygen Atoms of Bidentate Chelate O,N Ligands (Ln): 1. Complexes with Five-Membered Chelate Rings ReNC2O

The structural features of 24 mononuclear octahedral monooxo d2-Re(V) complexes with single charged oxygen atoms of bidentate chelate (O, N) ligands (Ln) of the general formula [ReO(Ln)2(Lmono)] (where Lmono is a monodentate ligand) containing six-membered chelate rings ReNC3O are discussed. The O(Ln) atoms, with three exception, are in the trans positions to the O(oxo) ligands. In the two complexes, the oxygen atoms of the acido-ligand OMe− occupy the trans positions to the oxo ligands, and in the third, it is occupied by the O atom of the neutral H2O ligand. In the case of complexes [ReO(Ln)2(Lmono)] with five-membered chelate cycles ReNC2O, the structures of three complexes with Lmono ligands in the trans positions to oxo ligands are also known: one with Lmono = Cl− and two with Lmono = OMe−.

Werner clathrates with enhanced hydrogen bonding functionality

The role of hydrogen bonding to generate molecular frameworks for selective enclathration of selected, closely related alcohols, carbonyls and water by three Ni(II) isothiocyanate based Werner hosts was scrutinized and their generation of crystal structures elucidated.Structures of host:guest compounds of H1, Ni(NCS)2(nicotinamide)4, with five alcohols were compared with five guests with carbonyl functionalities. In both cases, host columns formed by interaction via hydrogen bonded amide tetramers. However, the hydrogen bond between the guest alcohol functional group and thiocyanato sulphur showed a more robust structure than those with intercalated carbonyl guests found in layers between sheets of host molecular bands. A predictable framework of H1 host led to architectures with analogous patterns. Physical properties differed according to their hydrogen bonding capabilities. Selectivity was achieved for the separation of a pair of alcohols (1-butanol and 2-pentanol) and a pair of carbonyls (3- and 4-methylcyclohexanone), both difficult to separate due to similar physical properties. The positive discrimination characteristics of host H1 towards one of a pair of alcohols and carbonyl isomers have given possibilities for future industrial prospects.Host H2, Ni(NCS)2(isonicotinamide)4, presented three different tubulate packing arrangements, with the guests (H2O, ethanol and self-included ligand isonicotinamide) occupying channels in the structure. Hydrogen bonding between host carbonyl oxygens and amides formed molecular frameworks into which the guest was included.Mixed-ligand host H3, Ni(NCS)2(nicotinamide)2(isonicotinamide)2 with inclusion of water and methanol, confirmed our findings of hydrogen bonded frameworks with the two different ligands in H1 and H2; nicotinamide ligands formed amide tetramers, whereas the isonicotinamide ligands formed a variety of discrete hydrogen bonds with neighbouring molecules.The host networks formed porous materials for inclusion, giving the advantage of ‘tunability’, and pores with different size and nature available to accommodate guest molecules and to separate isomers.

Tuning Packing, Structural Flexibility, and Porosity in 2D Metal–Organic Frameworks by Metal Node Choice

Understanding the key features that determine structural flexibility in metal–organic frameworks (MOFs) is key to exploiting their dynamic physical and chemical properties. We have previously reported a 2D MOF material, CuL1, comprising five-coordinate metal nodes that displays exceptional CO2/N2 selectively (L1=bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane). Here we examine the effect of utilising six-coordinate metal centres (CoII and NiII) in the synthesis of isostructural MOFs from L1, namely CoL1 and NiL1. The octahedral geometry of the metal centre within the MOF analogues precludes an ideal eclipse of the 2D layers, resulting in an offset stacking, and in certain cases, the formation of 2-fold interpenetrated analogues β-CoL1 and β-NiL1. We used a combination of thermogravimetric analysis (TGA), and powder and single crystal X-ray diffraction (PXRD and SCXRD) to show that desolvation is accompanied by a structural change for NiL1, and complete removal of the coordinated H2O ligands results in a reduction in long-range order. The offset nature of the 2D layers in combination with the structural changes impedes the adsorption of meaningful quantities of gases (N2, CO2), highlighting the importance of a five-coordinate metal centre in achieving optimal pore accessibility for this family of flexible materials.

Manifestations of Weak O-H···F Hydrogen Bonding in M(H2O) n(B12F12) Salt Hydrates: Unusually Sharp Fourier Transform Infrared ν(OH) Bands and Latent Porosity (M = Mg-Ba, Co, Ni, Zn).

Eight M(H2O) n(Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z2- = B12F122-): M = Ca, Sr, n = 7; M = Mg, Co, Ni, Zn, n = 6; M = Ba, n = 4, 5. Weak O-H···F hydrogen bonding between the M(H2O) n2+ cations and Z2- resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp ν(OH) bands, with full widths at half max of 10-30 cm-1, which are much more narrow than ν(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved νasym(OH/OD) and νsym(OH/OD) bands with Δν(OH) as small as 17 cm-1 and Δν(OD) as small as 11 cm-1 were observed (Δν(OX) = νasym(OX) - νsym(OX)). The isomorphic hexahydrates ( R3̅) have two fac-(H2O)3 sets of H2O ligands and nearly octahedral coordination spheres. They exhibited four resolvable ν(OH) bands, one νasym(OH)/νsym(OH) pair for H2O ligands with longer O(H)···F distances and one νasym(OH)/νsym(OH) pair for H2O ligands with shorter O(H)···F distances. The ν(OH) bands for the three H2O molecules with shorter, slightly stronger O(H)···F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm-1 relative to the bands for the three other H2O molecules, the first time that such small differences in relatively weak O(H)···F hydrogen bonds in the same crystalline hexahydrate have resulted in observable IR spectroscopic differences at room temperature. For the first time room temperature ν(OH) values for salt hexahydrates showed the monotonic progression Mg2+ > Co2+ > Ni2+ > Zn2+, essentially the same progression as the p Ka values for these metal ions in aqueous solution. A further manifestation of the weak O-H···F hydrogen bonding in these hydrates is the latent porosity exhibited by Ba(H2O)5,8(Z), Sr(H2O) n,m(Z), and Ca(H2O)4,6(Z). Finally, the H2O/D2O exchange reaction Co(D2O)6(Z) → Co(H2O)6(Z) was ca. 50% complete in 1 h at 50 °C in N2/17 Torr H2O( g).

Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics and NBO analysis

Structural and dynamical properties of Cu+ in 18.6% aqueous ammonia solution at 298.15 K have been investigated via quantum mechanical charge field molecular dynamics (QMCF MD). The QM region was set to a radius of 6.7 Å to include the first and second solvation shell. The Hartree-Fock (HF) level was applied to calculate the ion-ligand and ligand-ligand interactions in the QM region using the LANL2DZ-ECP basis set for the ion and DZP-Dunning for the ligands. The Cu+N and Cu+O radial distribution functions showed maximum first shell probabilities at distances of 2.23 and 2.30 Å, respectively. Predominantly, four NH3 molecules were found to form a tetrahedral [Cu(NH3)4]+ complex, although the formation of a short-lived intermediate [Cu(NH3)3H2O]+ complex was also observed. The mean residence times of NH3 and H2O ligands in the first solvation shell were estimated as 14.6 ps and 1.3 ps, respectively, reflecting the strong interaction between Cu+ and ammonia as well as the occurrence of rapid water exchange. The vibrational power spectrum of the Cu+N vibration in the first solvation shell revealed a wave number of 252 cm−1 with a corresponding force constant of 43.0 Nm−1. In addition, an NBO analysis was carried out, confirming the strong electrostatic character of the Cu+NH3 and Cu+H2O interaction, and highlighting that the presence of H2O ligands may destabilize the first solvation shell.

Unexpected Sequential NH3/H2O Solid/Gas Phase Ligand Exchange and Quasi-Intramolecular Self-Protonation Yield [NH4Cu(OH)MoO4], a Photocatalyst Misidentified before as (NH4)2Cu(MoO4)2.

[NH4Cu(OH)MoO4] as active photocatalyst in the decomposition of Congo Red when irradiated by UV or visible light has been prepared in an unusual ammonia/water ligand exchange reaction of [tetraamminecopper(II)] molybdate, [Cu(NH3)4]MoO4. [Cu(NH3)4]MoO4 was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu(NH3)(H2O)3]MoO4 was formed in 2 days as a result of an unexpected solid/gas phase ammonia-water ligand exchange reaction. This complex does not lose its last ammonia ligand on further standing in open air; however, a slow quasi-intramolecular (self)-protonation reaction takes place in 2-4 weeks, producing a yellowish-green microcrystalline material, which has been identified as a new compound, [NH4Cu(OH)MoO4], ( a = 10,5306 Å, b = 6.0871 Å, c = 8.0148 Å, β = 64,153°, C2, Z = 4). Mechanisms are proposed for both the sequential ligand exchange and the self-protonation reactions supported by ab initio quantum-chemical calculations and deuteration experiments as well. The [Cu(NH3)(H2O)3]MoO4 intermediate transforms into NH4Cu(OH)(H2O)2MoO4, which loses two waters and yields [NH4Cu(OH)MoO4]. Upon heating, both [Cu(NH3)4]MoO4 and [Cu(NH3)(H2O)3]MoO4 decompose, losing three NH3 and three H2O ligands, respectively, and stable [Cu(NH3)MoO4] is formed from both. The latter can partially be hydrated in boiling water into [NH4Cu(OH)MoO4. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu(NH3)4]MoO4. All properties of [NH4Cu(OH)MoO4] match those of the active photocatalyst described earlier in the literature under the formulas (NH4)2[Cu(MoO4)2] and (NH4)2Cu4(NH3)3Mo5O20.

Determination of structures of cupric-chloro complexes in hydrochloric acid solutions by UV-Vis and X-ray absorption spectroscopy

Ultrahigh-purity metals are indispensable to understanding the nature of materials, but the purity of contemporary metals is insufficient for determining their intrinsic properties. The fundamentals of the anion-exchange reaction must be clear in order to increase the refining efficiency of anion-exchange separation for purification of metals. The thermodynamic analysis of the anion-exchange reaction needs to take account of the distributions of metal-chloro complexes in the solution phase in addition to those between the resin and the solution phase. The structures of metal-chloro complexes in the solution phase must be determined as the first step in determining what species is adsorbed on the resin. Copper is one of the base metals most commonly used in modern society, and the anion-exchange behavior of cupric species is representative. The distribution and the molar attenuation coefficients of cupric-chloro complexes in hydrochloric acid solutions were obtained employing factor analysis followed by fitting a thermodynamic model to ultraviolet-visible absorption spectra. The cumulative formation constants were determined as follows: $\log _{10}\beta _{1} = 0.599$ , $\log _{10}\beta _{2} = 0.343$ , $\log _{10}\beta _{3} = -1.88$ , and $\log _{10}\beta _{4} = -5.25$ , and the Setchenow coefficient for the neutral species of [CuIICl2]0 is 0.188. Using the distribution assessed by the above thermodynamic parameters, the X-ray absorption spectra (XAS), consisting of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of individual species, were assessed by factor analysis. EXAFS theoretical models were fitted to experimental spectra of individual species to determine their structures and coordination geometries for the first time, although it is generally very difficult to obtain the spectrum of the individual species in a matrix containing other species simultaneously. The structures of five cupric-chloro complexes were determined to be distorted octahedrons of $\left [\mathrm {Cu^{II}(H_{2}O)_{4}^{eq}(H_{2}O)_{2}^{ax}}\right ]^{2+}, \left [\mathrm {Cu^{II}(H_{2}O)_{4}^{eq}(H_{2}O)^{ax}Cl^{ax}}\right ]^{+}$ , and $\left [\mathrm {Cu^{II}Cl_{2}^{ax}(H_{2}O)_{4}^{eq}}\right ]^{0}$ , a planar triangle of $\left [\mathrm {Cu^{II}Cl_{3}}\right ]^{-}$ , and a tetrahedron of [CuIICl4]2−. The obtained spectra can be used as standards. The XANES spectra of cupric-chloro complexes were interpreted qualitatively. The Cl− ligand in $\left [\mathrm {Cu^{II}(H_{2}O)_{4}^{eq}(H_{2}O)^{ax}Cl^{ax}}\right ]^{+}$ is attracted to the center atom of CuII by electrostatic force, as the H2O ligands are. Contrastingly, the bonding system of the Cl− ligands in the latter three species involves covalency.

The effect of substituents at C2/N1 atoms of salicyladehyde and 2-hydroxyacetophenone based thiosemicarbazones on the nature of nickel(II) complexes with 1,10-phenathroline and terpyridine as co-ligands

Coordination chemistry of nickel(II) with salicyladehyde and 2-hydroxyacetophenone based thiosemicarbazones {(2-OH-5-X-C6H3)-C2(R2)=N3-N2H-C1(=S)-N1HR3, LH2} and 1,10-phenanthroline (phen)/terpyridine (terpy) as a co-ligand has been investigated by varying the substituents at C2 (X = H, Me, MeO; R2 = H, Me) and N1 (R3 = H, Me, Et, Ph) atoms of thiosemicarbazones. Reactions of nickel(II) acetate with various thio-ligands, LH2 {X = H, R2 = H: L1H2 (R3 = H), L2H2 (R3 = Me), L3H2 (R3 = Et), L4H2 (R3 = Ph); X = Me, R2 = H, L5H2 (R3 = Me), L6H2 (R3 = Et); X = H, R2 = Me, L7H2 (R3 = Me), L8H2 (R3 = Et); X = MeO, R2 = H, L9H2 (R3 = Me)} have yielded dinuclear, [Ni2L2(phen)(D)] (D = H2O, L2− = L1–L4, 1–4; MeOH, L5, L6, 5, 6) and tetranuclear [Ni4L4(phen)2] (L2− = L7, 7, L8, 8) complexes in the presence of phen as a co-ligand, while in the presence of terpy as a co-ligand, only mononuclear [NiL(terpy)] (L2− = L2 , 9; L4, 10; L9, 11) complexes were formed. All these complexes have been characterized with the help of analytical data, spectroscopic techniques (IR and UV-visible), magnetic moments and single crystal X-ray crystallography (2, 3, 5, 6, 8–11). Thio-ligands undergo deprotonation of OH and N2H groups during complexation and coordinate to the metal centers as dianions in these complexes. In dinuclear complexes 1–6, one metal center is octahedral and second metal center is square planar (6, 4-coordination pair) and one site is occupied by H2O or MeOH ligands, thus completing six coordination. Tetranuclear complexes, 7 and 8, have 4, 6, 6, 4-coordination pattern and finally mononuclear complexes, 9–11, are octahedral with the thio-ligands forming O, N3, S- chelation and terpyridine coordinating as a terdentate ligand. All complexes are paramagnetic with magnetic moment (μeff) values in the range, 2.92–3.12 BM. Complexes 1–8 represent a less common type of dinuclear coordination pairs/tetranuclear oligomers in the metal-thiosemicarbazone chemistry.

Noninnocent Ligand in Rhodium(III)-Complex-Catalyzed C–H Bond Amination with Tosyl Azide

Six-coordinate rhodium(III) complexes coordinated by a planar trianionic ligand (L3-) are synthesized. One of the axial positions is occupied by chloride (Cl-), bromide (Br-), or iodide (I-), and another axial position is coordinated by a solvent molecule such as acetonitrile (AN), water (H2O), tetrahydrofuran (THF), or pyridine (PY) to complete an octahedral rhodium(III) center; [RhIII(L3-)(X)(Y)]- (1X/Y; X = Cl-, Br-, or I-, Y = AN, H2O, THF, or PY). Coordination of the AN, H2O, and THF ligands to the metal center is rather weak, so that these solvent molecules are easily replaced by PY to give [RhIII(L3-)(Cl)(PY)]-. In the electrochemical measurements, all complexes have two reversible redox couples based on the ligand-centered oxidation L3- to L•2- and to L-, as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium(III) complexes catalyze C-H bond amination of xanthene with tosyl azide (TsN3). Because the yields of the aminated product are nearly the same among the complexes, replacement of the axial solvent ligands with TsN3 readily occurs to give a nitrene-radical-bound rhodium(III) complex, [RhIII(L•2-)(N•Ts)(X)]-, as an active oxidant, which is generated by one-electron transfer from the trianionic L3- to the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1Cl/AN with TsN3 in the absence of the substrate (xanthene). In this case, a rhodium(III) iminosemiquinone complex, 2, was generated by the intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L•2-.

Dinuclear neodymium and lanthanum bis(2,6-diisopropylphenyl) phosphate complexes bearing a hydroxide ligand: catalytic activity of the Nd complex in 1,3-diene polymerization.

The reactions of K[(2,6-iPr2C6H3-O)2POO] either with LaCl3(H2O)7 or with Nd(NO3)3(H2O)6 in a 3:1 molar ratio, followed by vacuum drying and recrystallization from alkanes, have led to the formation of diaquapentakis[bis(2,6-diisopropylphenyl) phosphato]-μ-hydroxido-dilanthanum hexane disolvate, [La2(C24H34O4P)5(OH)(H2O)2]·2C6H14, (1)·2(hexane), and tetraaquatetrakis[bis(2,6-diisopropylphenyl) phosphato]-μ-hydroxido-dineodymium bis(2,6-diisopropylphenyl) phosphate heptane disolvate, [Nd2(C24H34O4P)4(OH)(H2O)4]·2C6H14, (2)·2(heptane). The compounds crystalize in the P21/n and P-1 space groups, respectively. The diaryl-substituted organophosphate ligand exhibits three different coordination modes, viz. κ2O,O'-terminal [in (1) and (2)], κO-terminal [in (1)] and μ2-κ1O:κ1O'-bridging [in (1) and (2)]. Binuclear structures (1) and (2) are similar and have the same unique Ln2(μ-OH)(μ-OPO)2 core. The structure of (2) consists of an [Nd2{(2,6-iPr2C6H3-O)2POO}4(OH)(H2O)4]+ cation and a [(2,6-iPr2C6H3-O)2POO]- anion, which are bound via four intermolecular O-H...O hydrogen bonds. The molecular structure of (1) displays two O-H...O hydrogen bonds between OH/H2O ligands and a κ1O-terminal organophosphate ligand, which resembles, to some extent, the `free' [(2,6-iPr2C6H3-O)2POO]- anion in (2). NMR studies have shown that the formation of (1) undoubtedly occurs due to intramolecular hydrolysis during vacuum drying of the aqueous La tris(phosphate) complex. Catalytic experiments have demonstrated that the presence of the coordinated hydroxide anion and water molecules in precatalyst (2) substantially lowered the catalytic activity of the system prepared from (2) in butadiene and isoprene polymerization compared to the catalytic system based on the neodymium tris[bis(2,6-diisopropylphenyl) phosphate] complex, which contains neither OH nor H2O ligands.

Natural Bond Orbital Analysis of [Fe(H<sub>2</sub>O)<sub>6</sub>]<sup>2+/3+</sup> and [Zn(H<sub>2</sub>O)<sub>6</sub>](H<sub>2</sub>O)<sub>N</sub><sup>2+</sup>;N=0-4

Nature of delocalization of the electrons from the ligands to metals in the first coordination sphere of the high-spin complexes [Fe(H2O)6]2+/3+ and [Zn(H2O)6]2+ are computationally studied using density functional theory. Among the studied complexes, natural charge transfer from H2O ligands to metal ion is found to be maximum of 1.556e in [Fe(H2O)6]3+ and minimum of 0.621e in [Zn(H\2O)6]2+. On the other hand, the interaction between the lone pairs of oxygen with metal ion was found to be stronger in [Zn(H2O)6]2+ than in the complexes with second coordination sphere. Number of such strong interactions in the first coordination sphere was found to be decreased with the addition of H2O ligands in the second coordination sphere.Journal of Institute of Science and Technology Volume 22, Issue 2, January 2018, Page: 148-155

Magnesium Bonds: From Divalent Mg Centres to Trigonal and Tetrahedral Coordination

AbstractωB97XD/aug‐cc‐pVTZ calculations were performed for MgXY complexes (X=Cl, Br; Y=Cl, Br, CH3, C2H5, C6H5) with one or two H2O or (CH3)2O ligands to mimic interactions of Grignard reagents in solvents. The magnesium Lewis acid centre interacts in these complexes with the Lewis base O‐centre and the corresponding Mg…O link may be classified as the magnesium bond (Mg‐bond). This interaction is characterized by properties similar to those of the hydrogen bond; however it also possesses few unique features. The Mg‐centres of isolated MgXY species are characterized by ring regions of a positive electrostatic potential (EP) that determines their acidic properties. The linear geometry of the MgXY unit is disturbed in complexes with H2O or (CH3)2O species where the Mg‐centres are closer to trigonal and to tetrahedral configurations in complexes with one and two ligands, respectively; such centres also possess regions of positive EP.

Structure evolution of mononuclear tungsten and molybdenum species in the protonation process: Insight from FPMD and DFT calculations

In this work, we apply static density functional theory (DFT) calculations, as well as classical and first-principles molecular dynamics (FPMD) simulations, using the free-energy perturbation method to study the protonation ability, active site and structures of W(VI) and Mo(VI) in acidic aqueous solution. Using FPMD simulations, utilizing the pKa's calculation technique, we concluded that the octahedral WO2(OH)2(H2O)2 is the true formula for tungstic acid (H2WO4), and the hydroxyl ligands are the acidic site. This aqueous structure of H2WO4 is analogous to the previously reported structure of molybdic acid (H2MoO4). The FPMD trajectories of the tungstic acid deprotonation show that the mono-protonated monotungstate ion (HWO4−) may partially exist as a five-coordinated WO3(OH)(H2O)− species except for the four-coordinated WO3(OH)− species. This result is supported by DFT calculations, with an isoenergetic point (ΔE = 1.9 kcal·mol−1) for the WO3(OH)(H2O)− and WO3(OH)− species, when explicit solvent molecules are taken into account. In contrast, for the H2MoO4 acid, FPMD trajectories during the deprotonation process show that two H2O ligands immediately escape from the first coordinated sphere of Mo(VI) to form the four-coordinated MoO3(OH)− species. This difference indicates that structural expansion of W(VI) began in the first protonated step, while that of Mo(VI) only occurs in the second step. In addition, our calculated first and second acid constants for tungstic acid are higher than previously reported values for molybdic acid. This result suggests that WO42− is more easily protonated than the MoO42− anion in the same acidic solution, which is further confirmed by DFT calculations of hydrated oxoanions and its protonated species, based upon the hydration energy.

A Water Dimer Shift Activates a Proton Pumping Pathway in the PR → F Transition of ba3 Cytochrome c Oxidase.

Broken-symmetry density functional calculations have been performed on the [Fea34+,CuB2+] state of the dinuclear center (DNC) for the PR → F part of the catalytic cycle of ba3 cytochrome c oxidase (CcO) from Thermus thermophilus (Tt), using the OLYP-D3-BJ functional. The calculations show that the movement of the H2O molecules in the DNC affects the pKa values of the residue side chains of Tyr237 and His376+, which are crucial for proton transfer/pumping in ba3 CcO from Tt. The calculated lowest energy structure of the DNC in the [Fea34+,CuB2+] state (state F) is of the form Fea34+═O2-···CuB2+, in which the H2O ligand that resulted from protonation of the OH- ligand in the PR state is dissociated from the CuB2+ site. The calculated Fea34+═O2- distance in F (1.68 Å) is 0.03 Å longer than that in PR (1.65 Å), which can explain the different Fea34+═O2- stretching modes in P (804 cm-1) and F (785 cm-1) identified by resonance Raman experiments. In this F state, the CuB2+···O2- (ferryl-oxygen) distance is only around 2.4 Å. Hence, the subsequent OH state [Fea33+-OH--CuB2+] with a μ-hydroxo bridge can be easily formed, as shown by our calculations.

Nitrite Reduction Cycle on a Dinuclear Ruthenium Complex Producing Ammonia.

The fundamental biogeochemical cycle of nitrogen includes cytochrome c nitrite reductase, which catalyzes the reduction of nitrite ions to ammonium with eight protons and six electrons (NO2- + 8H+ + 6e- → NH4+ + 2H2O). This reaction has motivated researchers to explore the reduction of nitrite. Although a number of electrochemical reductions of NO2- have been reported, the synthetic nitrite reduction reaction remains limited. To the best of our knowledge, formation of ammonia has not been reported. We report a three-step nitrite reduction cycle on a dinuclear ruthenium platform {(TpRu)2(μ-pz)} (Tp = HB(pyrazol-1-yl)3), producing ammonia. The cycle comprises conversion of a nitrito ligand to a NO ligand using 2H+ and e-, subsequent reduction of the NO ligand to a nitrido and a H2O ligand by consumption of 2H+ and 5e-, and recovery of the parent nitrito ligand. Moreover, release of ammonia was detected.

Effect of magnetic fields on the methyl rotation in a paramagnetic cobalt(ii) complex. Quasielastic neutron scattering studies.

Molecular dynamics is a fundamental property of metal complexes. These dynamic processes, especially for paramagnetic complexes under external magnetic fields, are in general not well understood. Quasielastic neutron scattering (QENS) in 0-4 T magnetic fields has been used to study the dynamics of Co(acac)2(D2O)2 (1-d4, acac = acetylacetonate). At 80-100 K, rotation of the methyl groups on the acac ligands is the dominant dynamical process. This rotation is slowed down by the magnetic field increase. Rotation times at 80 K are 5.6(3) × 10-10 s at 0 T and 2.04(10) × 10-9 s at 4 T. The QENS studies suggest that methyl groups in these paramagnetic Co(ii) molecules do not behave as isolated units, which is consistent with results from earlier magnetic susceptibility studies indicating the presence of intermolecular interactions. DFT calculations show that unpaired electron spin density in 1 is dispersed to the atoms of both acac and H2O ligands. Methyl torsions in 1-d4 have also been observed at 5-100 K in inelastic neutron spectroscopy (INS). The QENS and INS results here help understand the dynamics of the compound in the solid state.

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex.

The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pKa for the bound H2 O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [FeIV (O)(TMC)(OH2 )]2+ (1-OH2 ; TMC=1,4,8,11-tetramethylcyclam) by determining the pKa of the H2 O ligand. We establish that 1-OH2 has a pKa of 6.9±0.5, a value that falls in between those found for [FeIII (OH2 )6 ]3+ and [FeII (OH2 )6 ]2+ . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH2 , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.