Electrochemical probing of the photoreduction of molybdenum and tungsten Dawson-type polyoxometalates in molecular and ionic liquid media using water as an electron donor

The photochemistry of the Dawson-type [Bu(4)N](4)[S(2)Mo(18)O(62)] and [Bu(4)N](4)[S(2)W(18)O(62)] polyoxometalates in molecular solvents and [Bmim][BF(4)] and [Bmim][BF(6)] ionic liquids with water present as the electron donor is reported. Irradiation with UV (275-320 nm) or white (275-750 nm) light leads to reduction of the [S(2)Mo(18)O(62)](4-) anion and concomitant oxidation of water to give dioxygen and protons in all media examined. Oxygen gas also is rapidly evolved when solid [Bu(4)N](4)[S(2)Mo(18)O(62)] in contact with water is irradiated with light. In contrast, photoreduction of [S(2)W(18)O(62)](4-) is observed only in "wet" ionic liquids. Reactants and products associated with the photochemical reactions were monitored by a range of electrochemical methods. Substantial shifts in reversible potentials combined with modified structure of water introduced by water-IL interactions are hypothesised to facilitate photooxidation of water in ionic liquid environments. Intriguingly, whilst the polyoxotungstate is preferable as a photosensitizer, the molybdenum analogue is superior for photooxidation of water to dioxygen.     

Voltammetric Reduction of alpha- and gamma-[S2W18O62]4- and alpha-, beta-, and gamma-[SiW12O40]4-: isomeric dependence of reversible potentials of polyoxometalate anions using data obtained by novel dissolution and conventional solution-phase processes

Comparative studies on the voltammetric reduction of the alpha and gamma isomers of Dawson [S(2)W(18)O(62)](4)(-) and alpha, beta, and gamma forms of Keggin [SiW(12)O(40)](4)(-) polyoxometalate anions have been undertaken. For the six reversible one-electron [S(2)W(18)O(62)](4)(-)(/5)(-)(/6)(-)(/7)(-)(/8)(-)(/9)(-)(/10)(-) processes in acetonitrile, reversible potentials (E(0)(')) were found to be independent of isomeric form within experimental error (+/-5 mV). However, because both the alpha and gamma* isomers of [Bu(4)N](4)[S(2)W(18)O(62)] are insoluble in water, solid-state voltammetric studies with microcrystals adhered to electrode surfaces in contact with aqueous Et(4)NCl and Bu(4)NCl electrolyte media were also possible. Although no isomeric distinction was again detected in the solid-state studies, it was found that reduction of adhered solid by four or more electron equivalents led to rapid dissolution. When Et(4)NCl was the electrolyte, this dissolution process coupled with potential cycling experiments enabled conventional solution-phase data to be obtained in water for the analogous six one-electron reduction steps previously detected in acetonitrile. A strong medium effect attributed to Lewis acidity effects was apparent upon comparison with E(0)(') data obtained in water and acetonitrile. In contrast, with the [SiW(12)O(40)](4)(-) system, E(0)(') values for the [SiW(12)O(40)](4)(-)(/5)(-)(/6)(-)(/7)(-) processes in acetonitrile exhibited a larger (about 70 mV) dependence on isomeric form, and the isomerization step, [gamma-SiW(12)O(40)](6)(-)--> [alpha-SiW(12)O(40)](6)(-), was detected on the voltammetric time scale. The influence of isomeric form on reversible potential data is considered in terms of structural and charge density differences exhibited in the [S(2)W(18)O(62)](4)(-) and [SiW(12)O(40)](4)(-) systems studied in this paper and published data available on the alpha, beta, gamma, and gamma isomers of [As(2)W(18)O(62)](6)(-) and [P(2)W(18)O(62)](6)(-) Dawson anions and Keggin systems.     

Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62]4-: a unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media

The synthesis and characterization of (Bu4N)4[S2W18O62].1.23MeCN.0.27H2O are reported. It crystallizes in the monoclinic space group C2/c with a = 22.389(6) A, b = 22.104(3) A, c = 25.505(5) A, beta = 95.690(15) degrees, V = 12560(5) A3, and Z = 4. The anion exists as the gamma* isomer, the second example of this isomer type to be characterized structurally. The equivalent molybdenum salt occurs as the alpha isomer. gamma*-[S2W18O62]4- in MeCN solution displayed four electrochemically reversible one-electron redox processes at E1/2 values of -0.24, -0.62, -1.18, and -1.57 V versus the Fc+/Fc couple. Upon addition of acid in MeCN/H2O (95/5 v/v), the two most cathodic processes converted to an overall two-electron process at -0.71 V. The total data suggested that this process actually comprises two one-electron transfer processes, occurring at different potentials, with associated proton-transfer reactions. The interpretation is supported by simulation of the effect of acid titration upon the cyclic voltammetry. While multiple pathways for correlated reduction and protonation are present in both the molybdenum and tungsten systems, only a single fast oxidation pathway is available. As the reduced forms of [S2W18O62]4- are much weaker bases than those of [S2Mo18O62]4-, the individual oxidation pathways are not the same. However, their existence determines the highly reversible electrochemical behavior that is characteristic of these anions, and that of polyoxometalate systems in general.     

Synthesis and redox properties of triarylmethane dye cation salts of anions [M6O19]2- (M = Mo, W)

Four salts have been isolated combining the triarylmethane dye cations pararosaniline (PR(+)) and crystal violet (CV(+)) with the hexametalates [M(6)O(19)](2-) (M = Mo, W). A new hexatungstic acid H(2)[W(6)O(19)]·4dma (dma = dimethylacetamide) was isolated and is a useful synthon for hexatungstate salts. Single-crystal X-ray diffraction confirmed the presence of PR(+) and [Mo(6)O(19)](2-) ions in [PR](2)[Mo(6)O(19)]·6dmf (dmf = dimethylformamide). A number of charge-assisted hydrogen bonds N-H···O exist between the cation -NH(2) functions and the anion oxygen atoms. Comparative cyclic voltammetry of salts [A]Cl (A = PR, CV), [Bu(4)N](2)[M(6)O(19)](2-) and A(2)[M(6)O(19)] was established in MeCN and Me(2)SO solutions and of solids in contact with the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [emim][tfsa]. In the molecular solvents, the reversible potential for the process [Mo(6)O(19)](2-/3-) is less negative than the first reduction processes of the dye cations. In contrast, that for [W(6)O(19)](2-/3-) is more negative. Spectro-electrochemistry and bulk electrolysis experiments reveal significantly different pathways in the two cases. In contrast, in the [emim][tfsa] medium, a positive shift in reduction potential of at least 400 mV is seen for the anion processes but relatively little change for the dye cation processes. This means that initial reduction of the anions always precedes that of the dyes, providing significant simplification of the complex voltammetric data. Chemically modified electrodes can be used in the ionic liquid because of slow dissolution kinetics. However, reduced anion salts dissolve rapidly, allowing dissolved phase electrochemistry to be examined. The electrochemistries of the oxidized salts A(2)[M(6)O(19)] are essentially those of the individual ions, although low level interaction of A(+) with reduced anions [M(6)O(19)](3-,4-) is evident. The work establishes protocols for synthesis and handling of intensely absorbing and relatively insoluble salts which can now be applied to systems containing more complex polyoxometalate anions.     

链状三核钼(钨)硫原子簇化合物[M3O2S8]^2^-的合成, 光谱和晶体结构

本文以[NH4]2MOnS4-n(M=Mo, w, n=0-2)为原料, 在酸性和中性条件下, 合成出九个新的三核不对称钼硫和钨硫原子簇状化合物L2[M3O2S8][L=(n-Bu)4N, Ph4P,PyC16H33, (CH3)3NC16H33, Et4N], 用元素分析, 红外光谱, 紫外光谱对簇合物进行了表征, 并测定了其中[(n-Bu)4N]2[Mo3O2S8]和[n-Bu)4N]2[W3O2S8]两个簇合物的晶体结构。本文又一次验证了钼(钨)硫成簇反应是分子内部电子转反应, 并提出了反应历程。     

Synthesis and voltammetry of [bmim]4[alpha-S2W18O62] and related compounds: rapid precipitation and dissolution of reduced surface films

1-n-Butyl-3-methylimidazolium (bmim) salts of [alpha-SiM12O40]4- and [alpha-S2M18O62]4- (M = Mo, W) were synthesized and characterized. In sharp contrast to the multiple one-electron diffusion-controlled reduction processes expected in solution at a glassy carbon electrode, reduction of these salts beyond the one-electron level in CH3CN (0.1 M [bmim][PF6]) led to rapid precipitation of arrays of microdroplets or microcrystals onto the surface. Upon oxidation these arrays dissolved on the electrochemical time scale, resulting in voltammograms displaying both diffusion-controlled and surface-confined behavior. These novel observations appear to be related to the ability of the [bmim]+ cations and polyoxometalate anions to form ion clusters in solution and to the lower solubilities of the reduced salts.     

Sensitization of photo-reduction of the polyoxometalate anions [S(2)M(18)O(62)](4-) (M = Mo, W) in the visible spectral region by the [Ru(bpy)(3)](2+) cation

Voltammetric, photo-physical and photo-electrochemical properties of the Dawson polyoxometalate anions alpha-[S(2)M(18)O(62)](4-) (M = Mo, W) are presented, both in the presence and absence of a series of [Ru(II)L(n)](+/2+) cations [L(n) = (bpy)(3), (bpy)(2)(Im)(2), (bpy)(2)(dpq), (bpy)(2)(box) and (biq)(2)(box)]. Electrochemical processes for both the anion and Ru(II/III) couples were detected in solutions of the salts [Ru(II)L(n)](2)[S(2)M(18)O(62)] in dimethylformamide (0.1 M Bu(4)NPF(6)) by both cyclic and hydrodynamic voltammetries. Responses were also detected when the solid salts were adhered to the surface of a glassy carbon electrode in contact with an electrolyte in which they are insoluble (CH(3)CN; 0.1M Bu(4)NPF(6)). Photolysis experiments were performed on solutions of the salts [R(4)N](4)[S(2)M(18)O(62)] (R = n-butyl or n-hexyl) and [Ru(II)L(n)](2)[S(2)M(18)O(62)] at 355 and 420 nm in dimethylformamide and acetonitrile in the presence and absence of benzyl alcohol (10% v/v). When associated with [Ru(bpy)(3)](2+), the molybdate anion exhibited a large increase in the quantum yield for photo-reduction at 420 nm. The quantum yield for the tungstate analogue was lower but the experiments again provided clear evidence for sensitization of the photo-reduction reaction in the visible spectral region. The origin of this sensitization is ascribed to the new optical transition observed around 480 nm in static ion clusters {[Ru(bpy)(3)][S(2)M(18)O(62)]}(2-) and {[Ru(bpy)(3)](2)[S(2)M(18)O(62)]} present in solution. Measurable photocurrents resulted from irradiation of solutions of the anions with white light in the presence of the electron donor dimethylformamide. Evidence is also presented for possible quencher-fluorophore interactions in the presence of certain [Ru(II)L(n)](+) cations.     

簇合物裂解法合成二硫纶化合物

以[Me₄N]₂[M₄(SPh)₁₀]和[Me₄N]₄[S₄M₁₀(SPh)₁₆](M=Cd,Zn)为前驱体,采用簇合物裂解法合成了配合物(Bu₄N)₂[Cd(mnt)₂]、(Bu₄N)₂[M(dmit)₂](M=Cd,Zn)以及双核配合物(Me₄N)₂[Zn₂(Sph)₂·(S₂TTF(SCH₂)₂)₂].运用PM3计算方法得到了各种二硫纶盐配体中硫负离子的静电荷大小关系:mnt^2->SPh^->dmit^2->[(MeS)₂TTFS₂]^2-≈[(CH₂S)₂TTFS₂]^2-.在理论计算及实验结果的基础上,探讨了配体上硫负离子的静电荷大小与反应产物间的关系.     

Mechanistic analysis of the electrocatalytic properties of dissolved alpha and beta isomers of [SiW12O40]4- and solid [Ru(bipy)3]2[alpha-SiW12O40] on the reduction of nitrite in acidic aqueous media

Voltammetric studies on the reduction of alpha and beta isomers of the Keggin polyoxometalate anion [SiW12O40]4- reveal a series of electrochemically reversible processes in acidic aqueous media. In the presence of NO2-, catalytic current is detected in the potential region of the [SiW12O40]4-/5- process. Electronic spectroscopy and simulation of voltammetric data undertaken at variable [NO2-] and [H+] allow the following mechanism to be postulated, [SiW12O40]4- + e- <-->[SiW12O40]5-, H+ + HNO2 <--> NO+ + H2O, NO+ + [SiW12O40]5- --> NO + [SiW12O40]4-. The second-order rate constant for the rate-determining step is faster for the alpha isomer than for the beta one. This may be attributed to the different reversible potentials of -0.144 V (alpha isomer) and -0.036 V vs Ag/AgCl (beta isomer) and, hence, smaller driving force for an assumed outer sphere electron-transfer reaction in the case of beta isomer. A stable, water-insoluble, thin-film [Ru(bipy)3]2[alpha-SiW12O40] chemically modified electrode was generated electrochemically via ion-exchange of [Ru(bipy)3]2+ with Bu4N+ in the [Bu4N]4[alpha-SiW12O40] solid. The first reduction process with this modified electrode gives rise to the reaction [Ru(bipy)3]2[alpha-SiW12O40](solid) + H+(soln) + e- <--> H[Ru(bipy)3]2[alpha-SiW12O40](solid). The need to transfer a proton from the solution to the solid phase for charge neutralization purposes introduces a hydrogen-ion concentration dependence into this reaction, which is not found in the solution-phase study. Nevertheless, the voltammetric catalytic activity with respect to nitrite reduction is retained with the chemically modified electrode. However, nitrite catalysis with the [Ru(bipy)3]2[alpha-SiW12O40]-modified electrode is now independent of concentration of H+, rather than exhibiting a first-order dependence, and full mechanistic details for this process are unknown.     

Poly(ionic liquid)s: a new material with enhanced and fast CO2 absorption

Novel sorbent and membrane materials for CO2 separation, poly(ionic liquid)s made from ionic liquid monomers, poly[p-vinylbenzyltrimethyl ammonium tetrafluoroborate](P[VBTMA][BF4]) and poly[2-(methacryloyloxy)ethyltrimethylamnonium tetrafluoroborate](P[MATMA][BF4]) have absorption capacities 7.6 and 6.0 times of those of room-temperature ionic liquids, e.g.[bmim][BF4], respectively, with reversible and fast sorption and desorption.     

Structural, electrochemical, and spectroscopic characterization of a redox pair of sulfite-based polyoxotungstates: alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5-

The synthesis, isolation, and structural characterization of the fully oxidized sulfite-based polyoxotungstate cluster (Pr4N)4{alpha-[W18O54(SO3)2]}.2CH3CN and the one-electron reduced form (Pr4N)5{alpha-[W18O54(SO3)2]}.2CH3CN has been achieved. alpha-[W18O54(SO3)2]5- was obtained as a Pr4N+ salt by reducing the "Trojan Horse" [W18O56(SO3)2(H2O)2]8- cluster via a template orientation transformation. Acetonitrile solutions of pure alpha-[W18O54(SO3)2]5- also were prepared electrochemically by one-electron bulk reductive electrolysis of alpha-[W18O54(SO3)2]4-. Cyclic voltammetry of alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5- in CH3CN (0.1 M Hx4NClO4) produces evidence for an extensive series of reversible one-electron redox processes, that are associated with the tungsten-oxo framework of the polyoxometalate cluster. Hydrodynamic voltammograms in CH3CN exhibit the expected sign and magnitude of the steady-state limiting current values for the alpha-[W18O54(SO3)2]4-/5-/6- series and confirm the existence of a stable one-electron reduced species, alpha-[W18O54(SO3)2]5-. Employment of the Randles-Sevcik (cyclic voltammetry) and Levich (rotating disk electrode) equations at a glassy carbon electrode (d=3 mm) enable diffusion coefficient values of 3.7 and 3.8x10(-6) cm2 s-1 to be obtained for alpha-[W18O54(SO3)2]4- and alpha-[W18O54(SO3)2]5-, respectively. The tungsten polyoxometalates are highly photoactive, since measurable photocurrents and color changes are detected for both species upon irradiation with white light. EPR spectra obtained from both acetonitrile solution and solid samples, down to temperatures as low as 2.3 K, of the chemically and electrochemically prepared one-electron reduced species provided evidence that the unpaired electron in alpha-[W18O54(SO3)2]5- is delocalized over a number of atoms in the polyoxometalate structure, even at very low temperatures.     

A series of novel Mo(W)/Cu/S heterobimetallic clusters containing SBu(t-) linkages. Synthesis, structure and spectroscopic characterisation

Six new copper(I) clusters, [Et4N]2[(MOS3)2Cu4(mu-SBu(t))2](1a: M = Mo; 1b: M = W), [Et4N][(MOS3)2Cu6(mu-SBu(t))3](2a: M = Mo; 2b: M = W) and [Bu4N]2[(MOS3)3Cu9(mu-SBu(t))3(mu3-SBu(t))][I](3a: M = Mo; 3b: M = W) have been prepared by the reactions of thiomolybdates and thiotungstates with CuSBu(t) under various conditions. The [(MOS3)2Cu4(mu-SBu(t))2](2-) dianions in 1a and 1b represent the first examples of double butterfly-shaped Mo(W)/Cu/S clusters. Addition of more Cu atoms to 1a or 1b resulted in the formation of incomplete double cubane-like clusters 2a or 2b. Single crystal structural studies showed that the anions of 2a and 2b are formed in a mouth-to-mouth fashion by two incomplete cubanes [MOS3Cu3](M = Mo, W) with three mu-SBu(t-) linkages. In the molecular structure of 3b, the SBu(t-) ligands act as mu- and mu3-bridges which link three WOS3Cu3 incomplete cubane-like fragments. An iodide ion crystallises in the cavity defined by the three incomplete cubanes in 3b. The spectroscopic and electrochemical properties of all the clusters are also studied.     

pH-dependent assembly of hybrids based on Wells-Dawson POM/Ag chemistry

Four new hybrids based on the Wells-Dawson polyoxometalate [P2W18O62](6-) (P2W18), ([Ag(bipy)]2[P2W18O62]).2[H2bipy].4H2O (1), ([Ag(bipy)]4[P2W18O62]).2[Hbipy](2), K[P2W18O62].2.5[H2bipy].2H2O (3), and [P2W18O62] 2.[H2bipy]4.[Hbipy]4 .3H2O (4), were hydrothermally synthesized and structurally characterized by routine techniques and single-crystal X-ray diffraction. Compounds 1 and 2 are isolated at lower pH values. 1 represents a 3D (4,4)-net structure with NbO topology, in which the P2W18 clusters are modified by four Ag-N coordination polymeric chains, and 2 exhibits a 3D (3,4)-net structure with the (9(2).12)(8.10(4).12)(3(2).10(2).11(2))(3.6.10(2).12(2)) topology, in which Ag-bipy layers are intercalated by the dimer of P2W18 clusters in a staggering mode, and the P2W18 clusters show the highest coordination number to date. By increasing the pH value, compounds 3 and 4 are obtained as supramolecular compounds. Their structural differences reveal that the pH value of the reaction system is the key factor influencing the structure and topology of these compounds, which can be explained by the acid-base chemistry of the molecular building units and silver chemistry.     

Investigation of the reactivity of arylamines, organo-hydrazines and tolylisocyanate towards [PW12-xMxO40]n- Keggin anions

Reaction of K7[A,alpha-PW9Mo2O39] with Na2MoO4.2H2O in a mixture of water/dioxane/hydrochloric acid and further precipitation with (Bu4N)Br provided (Bu4N)3[A,alpha-PW9Mo3O40](3). Analogous reaction with K7-xNax[alpha-PW11O39] is an alternative to the synthesis of (Bu4N)3[alpha-PW11O39{MoVIO}]2. Multinuclear NMR and ESI mass spectrometry have been used to interpret the reaction of (Bu4N)x[alpha-PW11O39{ReO}](x=3 1; x=4 1I), (Bu4N)x[alpha-PW11O39{MoO}](x=3 2; x=4 2I) and (Bu4N)3[A,alpha-PW9Mo3O40]3 by organohydrazines, arylamines, tolylisocyanate and tetraphenylphosphine imide.     

Ionic liquid-enhanced photooxidation of water using the polyoxometalate anion [P2W18O62](6-) as the sensitizer

Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K(6)[P(2)W(18)O(62)] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P(2)W(18)O(62)](6-) anion to [P(2)W(18)O(62)](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E(0)(F) for [P(2)W(18)O(62)](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P(2)W(18)O(62)](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H(+) or dioxygen to produce [P(2)W(18)O(62)](7-), rather than reform to [P(2)W(18)O(62)](6-). Thus, under photoirradiation conditions [P(2)W(18)O(62)](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH(2)Cl(2) containing added water, but does occur for solid K(6)[P(2)W(18)O(62)] in contact with water vapor.     

Amidinate-carboxylate complexes of dimolybdenum and ditungsten: M2(O2CR)2((NiPr)2CR')2. Preparations, molecular and electronic structures and reactions.

The compounds M(2)(O(2)CMe)(4) and the lithium amidinates Li[(N(i)Pr)(2)CR] react to give the new compounds trans-M(2)(O(2)CMe)(2)[(N(i)Pr)(2)CR](2) where M = Mo or W and R = Me (M = Mo only), -C[triple bond]C(t)Bu, -C[triple bond]CPh and -C[triple bond]C-Fc where Fc = 1-ferrocenyl. The limitations of this type of reaction are described based on steric considerations together with the preparation and characterization of the compound Mo(2)(micro-O(2)C-9-anthracene)(2)[eta(2)-(N(i)Pr)(2)CMe](2). The electronic structures of the bis-amidinate-bis-carboxylate M(2) complexes are described based on model compounds employing density functional theory and are correlated with the experimental observations of their physicochemical properties and in particular their observed electronic absorption spectra which show intense MLCT absorption bands. Preliminary studies of the reactions of these bis-amidinate-bis-carboxylate complexes in the preparation of 1-D oligomers are also described along with the preparation and molecular structures of the compounds [Li(N(i)Pr)(2)CR.THF](2) where R = 2-thienyl or -C[triple bond]C-Ph. The kinetic lability of these new M(2)-containing compounds toward ligand exchange is also noted.     

Speciation of uranyl complexes in ionic liquids by optical spectroscopy

Uranyl complexes dissolved in room-temperature ionic liquids have diagnostic absorption and emission spectra which reflect the molecular symmetry and geometry. In particular, the characteristic vibrational fine structure of the absorption spectra allows identification of the molecular symmetry of a uranyl complex. The concept of speciation of uranyl complexes is illustrated for the hydrated uranyl ion, the tetrachloro complex [UO2Cl4]2-, the trinitrato complex [UO2(NO3)3]-, the triacetato complex [UO2(CH3COO)3]-, and the crown ether complex [UO2(18-crown-6)]2+ in imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids. The competition between 18-crown-6 and small inorganic ligands for coordination to the uranyl ion was investigated. The crystal structures of the hydrolysis product [(UO2)2(mu2-OH)2(H2O)6] [UO2Br4](18-crown-6)4 and imidazolium salt [C6mim]2[UO2Br4] are described.     

Functionalization of heteropolyanions-osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: synthesis, characterization and multinuclear ((183)W, (15)N) NMR, EPR, IR, and UV/Vis fingerprints

Reaction of K(10)[alpha(2)-P(2)W(17)O(61)] or K(10)[alpha(1)-P(2)W(17)O(61)] or [Bu(4)N][OsCl(4)N] in a water/methanol mixture, and subsequent precipitation with (Bu(4)N)Br provided [alpha(2)-P(2)W(17)O(61){Os(VI)N}](7-) and [alpha(1)-P(2)W(17)O(61){Os(VI)N}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu(4)N)(4)][alpha-H(3)PW(11)O(39)] with either [ReCl(3)(N(2)Ph(2))(PPh(3))(2)] or [Bu(4)N][ReCl(4)N] are alternatives to the synthesis of [(Bu(4)N)(4)][alpha-PW(11)O(39){Re(VI)N}]. (183)W and (15)N NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu(4)N)(4)][alpha-PW(11)O(39){Os(VI)N}] Keggin derivative.     

P2 addition to terminal phosphide M[triple bond]P triple bonds: a rational synthesis of cyclo-P3 complexes

The diphosphaazide complex (Mes*NPP)Nb(N[Np]Ar)3 (Mes* = 2,4,6-tri-tert-butylphenyl, Np = neopentyl, Ar = 3,5-Me2C6H3), 1, has previously been reported to lose the P2 unit upon gentle heating, to form (Mes*N)Nb(N[Np]Ar)3, 2. The first-order activation parameters for this process have been estimated here using an Eyring analysis to have the values Delta H(double dagger) = 19.6(2) kcal/mol and Delta S(double dagger) = -14.2(5) eu. The eliminated P2 unit can be transferred to the terminal phosphide complexes P[triple bond]M(N[(i)Pr]Ar)3, 3-M (M = Mo, W), and [P[triple bond]Nb(N[Np]Ar)3](-), 3-Nb, to give the cyclo-P3 complexes (P3)M(N[(i)Pr]Ar)3 and [(P3)Nb(N[Np]Ar)3](-). These reactions represent the formal addition of a P[triple bond]P triple bond across a M[triple bond]P triple bond and are the first efficient transfers of the P2 unit to substrates present in stoichiometric quantities. The related complex (OC)5W(Mes*NPP)Nb(N[Np]Ar)3, 1-W(CO)5, was used to transfer the (P2)W(CO)5 unit in an analogous manner to the substrates 3-M (M = Mo, W, Nb) as well as to [(OC)5WP[triple bond]Nb(N[Np]Ar)3](-). The rate constants for the fragmentation of 1 and 1-W(CO)5 were unchanged in the presence of the terminal phosphide 3-Mo, supporting the hypothesis that molecular P2 and (P2)W(CO)5, respectively, are reactive intermediates. In a reaction related to the combination of P[triple bond]P and M[triple bond]P triple bonds, the phosphaalkyne AdC[triple bond]P (Ad = 1-adamantyl) was observed to react with 3-Mo to generate the cyclo-CP2 complex (AdCP2)Mo(N[(i)Pr]Ar)3. Reactions of the electrophiles Ph3SnCl, Mes*NPCl, and AdC(O)Cl with the anionic, nucleophilic complexes [(OC)5W(P3)Nb(N[Np]Ar)3](-) and [{(OC)5W}2(P3)Nb(N[Np]Ar)3](-) yielded coordinated eta(2)-triphosphirene ligands. The Mes*NPW(CO)5 group of one such product engages in a fluxional ring-migration process, according to NMR spectroscopic data. The structures of (OC)5W(P3)W(N[(i)Pr]Ar)3, [(Et2O)Na][{(OC)5W}2(P3)Nb(N[Np]Ar)3], (AdCP2)Mo(N[(i)Pr]Ar)3, (OC)5W(Ph3SnP3)Nb(N[Np]Ar)3, Mes*NP(W(CO)5)P3Nb(N[Np]Ar)3, and {(OC)5W}2AdC(O)P3Nb(N[Np]Ar)3, as determined by X-ray crystallography, are discussed in detail.     

The distinct affinity of cyclopentadienyl ligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

The mono and bis(cyclopentadienyl) compounds [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I](M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C5H4Bu t)3] and LnI3 or [UI3(L)4](L = thf or py) in the molar ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI(3)(py)(4)] or LnI(3)(Ln = La, Ce, Nd) with 1 or 2 mol equivalents of LiC5H4Bu t in thf afforded the [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I2]- compounds, respectively. The X-ray crystal structures of [M(C5H4Bu t)I2(py)3](M = U, La, Ce, Nd), [{Ce(C5H4Bu t)2(mu-I)}2] and [M(C5H4Bu t)2I(py)2](M = U, Nd) have been determined; the differences between the average M-C distances in the mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and lanthanide ions while the U-N and U-I bond lengths seem to be smaller than those predicted from a purely ionic bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two series of competing reactions: the ligand exchange reactions between [Ln(C5H4Bu t)(n')I(3-n')](Ln = La, Ce, Nd) and [U(C5H4Bu t)(n')I(3-n')] species (1 < or = n'+n' =n < or = 5), and the addition of n mol equivalents of LiC(5)H(4)Bu(t)(1 [less-than-or-equal]n[less-than-or-equal] 5) to a 1 : 1 mixture of LnI3 and [UI3(thf)4] or [UI3(py)4]. The stability of the [M(C5H4Bu t)I2] species was found to vary in the order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be accounted for by a higher degree of covalency in the U-C5H4Bu t bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.