基于立方烷结构的分子催化剂在光催化水氧化中的研究进展

随着化石燃料大量使用带来的气候变化和环境污染问题日趋严重,寻找清洁高效的可再生能源用做传统化石燃料的替代品,已经成为当前的研究热点。光驱动的水分解反应被认为是太阳能制氢的可行途径。水的全分解包括两个半反应-水的氧化和质子还原。其中水的氧化反应是一个涉及四个电子和四个质子转移的复杂过程,需要很高的活化能,被认为是全分解水反应的瓶颈步骤。因此,开发高效、稳定、廉价丰产的水氧化催化剂是人工光合作用突破的关键因素。立方烷具有类似自然界光合作用酶光系统II(PSII)活性中心Mn_4CaO_5簇的结构,世界各国的科学家受自然界光合作用的启发,开发出了许多基于过渡金属的立方烷结构的催化剂,常见的有锰、钴和铜等立方烷催化剂。本文简要地综述了近年来立方烷分子催化剂在光催化水氧化中的研究进展。首先介绍了立方烷基光催化水氧化反应历程,继而详细介绍了基于有机配体的立方烷配合物和全无机的多金属氧酸盐立方烷水氧化催化剂,其次是半导体(BiVO4或聚合的氮化碳(PCN))为捕光材料复合立方烷分子催化剂的水氧化体系最新研究进展。最后总结并展望了该领域所面临的挑战及其前景。     

基于多金属氧酸盐催化剂的光驱动产氢研究进展

能源和环境问题是21世纪人类面临的两个巨大挑战.鉴于此,为了实现人类社会的可持续发展,寻求能够替代化石能源的安全无污染可再生能源已迫在眉睫.太阳光驱动水分解是实现太阳能转化生产清洁可再生氢能的理想方法,其分解产物氢气和氧气在燃烧释放能量的同时生成洁净无污染的可饮用水,实现了完美的可持续能量循环,对于解决当今全球面临的能源危机与环境污染问题具有巨大的应用价值.然而,长期以来光驱动水分解所面临的巨大难题是半反应动力学非常缓慢,通常需要克服较高的能量势垒,导致整体能量转化效率低.利用非贵金属制备高催化效能、低成本的水分解催化材料成为该领域的研究热点和难点.目前,已报道的光驱动产氢催化剂可以被归纳为两大类:均相催化剂和异相催化剂.均相催化剂通常具备高催化活性、高选择性以及易于进行机理研究等优点,而异相催化剂则具备廉价、易得和高稳定性等优点;然而它们也存在一些不容忽视的问题,如均相催化剂的低稳定性、易分解失活,异相催化剂表面易被毒化失活、低催化转化数及转化频率等.如何设计合成兼具二者优点的产氢催化剂吸引了领域内研究者的广泛关注.作为一类新兴的多电子转移催化剂,多金属氧酸盐因其丰富多样的合成策略以及高度可调的物理化学及光化学性质,已被广泛用于催化水分解制氢气研究.该类多金属氧酸盐催化剂具备了介于均相分子化合物和异相金属氧化物之间的结构,这种独特的结构赋予它们同时具备均相分子催化剂的高活性、高选择性、高可控性、易于进行机理性研究等优点,又具备异相金属氧化物催化剂的廉价易得及稳定性高等优势.随着研究的开展,基于多金属氧酸盐的光催化产氢体系已由当初的贵金属辅助逐渐转变为丰产元素参与,光源的选择方面也从与太阳光谱匹配度低的紫外光转变为可见光.本文对30多年来基于多金属氧酸盐催化剂的光驱动产氢成果进行了综述,主要包括有/无贵金属辅助的多金属氧酸盐,多酸@金属有机框架复合物,多酸-半导体复合材料在紫外光或可见光条件下的光催化产氢研究;同时讨论总结了不同类型催化体系的反应机理;并对该领域的未来发展趋势及研究方向进行了展望.     

基于多金属氧酸盐催化剂的光驱动产氢研究进展（英文）

能源和环境问题是21世纪人类面临的两个巨大挑战.鉴于此,为了实现人类社会的可持续发展,寻求能够替代化石能源的安全无污染可再生能源已迫在眉睫.太阳光驱动水分解是实现太阳能转化生产清洁可再生氢能的理想方法,其分解产物氢气和氧气在燃烧释放能量的同时生成洁净无污染的可饮用水,实现了完美的可持续能量循环,对于解决当今全球面临的能源危机与环境污染问题具有巨大的应用价值.然而,长期以来光驱动水分解所面临的巨大难题是半反应动力学非常缓慢,通常需要克服较高的能量势垒,导致整体能量转化效率低.利用非贵金属制备高催化效能、低成本的水分解催化材料成为该领域的研究热点和难点.目前,已报道的光驱动产氢催化剂可以被归纳为两大类:均相催化剂和异相催化剂.均相催化剂通常具备高催化活性、高选择性以及易于进行机理研究等优点,而异相催化剂则具备廉价、易得和高稳定性等优点;然而它们也存在一些不容忽视的问题,如均相催化剂的低稳定性、易分解失活,异相催化剂表面易被毒化失活、低催化转化数及转化频率等.如何设计合成兼具二者优点的产氢催化剂吸引了领域内研究者的广泛关注.作为一类新兴的多电子转移催化剂,多金属氧酸盐因其丰富多样的合成策略以及高度可调的物理化学及光化学性质,已被广泛用于催化水分解制氢气研究.该类多金属氧酸盐催化剂具备了介于均相分子化合物和异相金属氧化物之间的结构,这种独特的结构赋予它们同时具备均相分子催化剂的高活性、高选择性、高可控性、易于进行机理性研究等优点,又具备异相金属氧化物催化剂的廉价易得及稳定性高等优势.随着研究的开展,基于多金属氧酸盐的光催化产氢体系已由当初的贵金属辅助逐渐转变为丰产元素参与,光源的选择方面也从与太阳光谱匹配度低的紫外光转变为可见光.本文对30多年来基于多金属氧酸盐催化剂的光驱动产氢成果进行了综述,主要包括有/无贵金属辅助的多金属氧酸盐,多酸@金属有机框架复合物,多酸-半导体复合材料在紫外光或可见光条件下的光催化产氢研究;同时讨论总结了不同类型催化体系的反应机理;并对该领域的未来发展趋势及研究方向进行了展望.     

Co_4O_4立方烷分子催化剂用于高效光电催化分解水

正模拟光合作用、利用太阳能分解水制氢是实现太阳能转换的一条理想途径。作为水分解反应的两个半反应之一,高效催化水氧化一直以来是人工光合作用体系的瓶颈所在~1。目前除多相水氧化催化剂外,基于过渡金属配合物的小分子催化剂凭借其高活性、结构明确以及性能易于调控等优势受到越来越多的关注~(2,3)。然而,可见光驱动的水分解对于分子催化剂是一个巨大挑战。尽管分子水氧化催化剂在含有电子牺牲受体的半反应     

光催化全解水助催化剂的设计与构建

能源和环境危机是当今社会面临的两大关键课题,利用太阳光驱动化学反应、将太阳能转化为化学能是解决上述问题的重要措施。通过光催化分解水是直接利用太阳能生产氢燃料的有效策略。光催化水分解过程可以分为三个基元步骤:光吸收、电荷分离与迁移、以及表面氧化还原反应。助催化剂可有效提高电荷分离效率、提供反应活性位点并抑制催化剂光腐蚀的发生,进而提高水分解效率。助催化剂也可以通过活化水分子以提高表面氧化还原动力学,进而提升整体光催化反应的太阳能转换效率。本文综述了助催化剂在光催化反应中的重要作用以及目前常用的助催化剂类型,详细说明了在光催化全解水过程中双助催化剂体系的构建及作用机理,并根据限制全解水的关键因素提出了新型助催化剂的设计策略。     

基于光驱动水氧化的有机底物氧化

模拟光合作用分解水制氢是将太阳能转换为化学能的有效途径,基于光驱动水氧化的有机底物氧化是模拟光合作用体系II释氧中心(OEC)的新思路.该反应过程是通过金属催化剂将水分子活化生成以水为氧源的高价金属-氧中间体,随后将氧原子转移给有机底物,在此过程中水中的氢源得以释放.从催化剂的角度总结了近年来光驱动水分子活化的研究进展,同时对基于光驱动水氧化的有机底物氧化与光致产氢结合进而建立新型光分解水制氢的体系提出展望.     

光催化分解水制氢反应中助催化剂的作用及机理（英文）

近年来,随着一次能源过度消耗所带来的能源和环境问题日益突出,开发廉价、可持续的清洁能源备受关注.光催化分解水制氢可利用太阳能普遍率高和几乎免费等特点制取燃烧热值高、燃烧产物无污染的氢气能源.自从1972年日本的Fujishima教授和Honda教授首次发现TiO2单晶电极光催化分解水可以产生氢气以来,光催化制氢被认为是实现可持续制氢最有潜力的方法之一.有效地将太阳能转换为化学能的关键是设计高效的电荷分离和运输结构.然而,现有的大多数半导体光催化剂因缺少活性位点、光生载流子易复合等缺点而无法达到较高的转换效率.因此,如何提高半导体光催化产氢的转换效率是现阶段面对的重要问题.在众多解决方法中,助催化剂的引入可以为光催化制氢反应增加活性位点,促进光生载流子的有效分离,进而有效地提高半导体光催化产氢速率.本文总结了多种不同类型的助催化剂应用于光催化产氢研究的最新进展,详细讨论了助催化剂在增强光吸收、提供活性位点、增加催化剂稳定性和促进电荷分离等方面的作用,阐明了助催化剂在光催化分解水制氢中的反应机理,同时还提出了光催化制氢的未来研究和预测.本文将助催化剂分为以下几种类别进行讨论:(1)单一助催化剂,包括金属/合金、金属氧化物/氢氧化物、金属磷化物、金属硫化物、碳基材料等助催化剂材料;(2)双助催化剂;(3)Z-Scheme助催化剂;(4)MOFs助催化剂.近年来,助催化剂材料在光催化产氢中应用的发展趋势从当初价格昂贵的贵金属趋于价格相对低廉的非贵金属,从单一体系趋于更复杂的体系.虽然现阶段关于助催化剂与基底之间的匹配还需要进一步研究,但我们相信随着技术的发展,这些问题都可以迎刃而解.希望在不久的将来,可以精确设计和构建出具有高效光催化产氢活性的催化剂体系,开发出更多新的可再生清洁能源,从而缓解能源紧缺和环境恶化等棘手问题.     

多金属氧酸盐催化的水氧化研究进展

氢气以其清洁无污染、燃烧值高等优点成为未来最具潜力的可再生能源之一,而清洁生产氢气的最佳选择之一即为裂解水. 利用太阳能模拟光合作用实现水的全分解产生氢气和氧气是目前最为理想的能源转化方式,并且已经引起了众多研究者的关注. 水分解的半反应之一--水氧化反应由于其过程复杂,一直是制约水分解的瓶颈. 所以寻找高效、稳定的水氧化催化剂便成为了突破该瓶颈的关键. 多金属氧酸盐是一类以前过渡金属氧簇为基本单元形成的多金属氧簇化合物. 由于多金属氧酸盐在物理、化学性质方面具有无法比拟的特性,使得其在催化、药物、纳米科技和材料科学等方面已被广泛地应用. 多金属氧酸盐的全无机配体可很好地抵御水氧化反应的强氧化性环境,故将其作为水氧化催化剂越来越引起研究者们的注意,并且已有多种多金属氧酸盐被设计为水氧化催化剂. 本文详细介绍了各种不同过渡金属取代的多金属氧酸盐水氧化催化剂的研究进展.     

太阳能分解水制氢最近进展:光催化、光电催化及光伏-光电耦合途径

能源是人类生存和发展的物质基础,太阳能作为最丰富的清洁可再生能源之一,其开发利用受到了世界范围内的广泛关注.通过光催化分解水制氢将太阳能以化学能的形式储存起来不仅能利用太阳能制取高燃烧值的氢能,同时氢能可与CO2综合利用结合起来,在减少碳排放的同时,生成高附加值的化学品,实现碳氢资源的优化利用.光催化分解水制氢在过去的几年里取得了长足的进步,本综述从三种研究广泛的太阳能光催化分解水制氢途径(即光催化、光电催化以及光伏-光电耦合途径)入手,分别简要介绍了太阳能分解水制氢在近几年取得的最新研究进展.利用纳米粒子悬浮体系进行光催化分解水制氢成本低廉、易于规模化放大,被认为是未来应用最可行的方式之一,但是太阳能转化利用效率还偏低.最新报道的SrTiO3:La,Rh/Au/BiVO4:Mo光催化剂其太阳能到氢能(STH)转化效率已超过了1.0%,相比之前报道的大多数光催化剂体系有了数量级的飞跃,让人们对太阳能光催化分解水制氢未来的规模化应用看到了希望.高效宽光谱响应的光催化剂、高效电荷分离策略、新型高效助催化剂以及气体分离新方法和新材料等,均是粉末光催化剂体系研究最为关键的问题;光电催化分解水在过去2–3年内发展迅速,在一些典型的光阳极半导体材料(如BiVO4和Ta3N5等)体系上太阳能利用效率超过2.0%以上.最新研究发现,在Ta3N5光阳极的研究中,通过在光电极表面合理设计和构筑空穴传输层和电子阻挡层等策略,光电流和电极稳定性均可得到大幅度提升,光电流大小甚至可接近Ta3N5材料的理论极限电流.如果能进一步在过电位和电极稳定性上取得突破,该体系的STH转化效率还会得到大幅度改进.此外,光阴极的研究也越来越受到了研究者的关注;光伏-光电耦合体系在三种途径里面太阳能制氢效率最高,在多个体系上已超过10%以上,最近报道的利用多结GaInP/GaAs/Ge电池与Ni电催化剂耦合,其太阳能制氢效率可达到22.4%.虽然该种制氢途径的效率已超过其工业化应用的要求,但是光伏电池的成本(尤其是多结GaAs太阳电池)极大限制了其大面积规模化应用,同时还要考虑电催化剂的成本和效率等,光伏-光电耦合制氢是成本最高的太阳能制氢途径.需要指出的是,光伏-光电耦合制氢有望在一些特殊的领域最先取得实际应用,如为外太空航天器、远洋航海以及孤立海岛等传统能源无法满足的地方提供能源供给.总之,太阳能分解水制氢研究取得了一系列重要进展,太阳能制氢效率得到了大幅度提升,也是目前世界范围内关注的研究热点之一,不仅具有强的潜在工业应用背景,更为基础科学提供了诸多新的研究课题.这一极具挑战的研究领域,在先进技术快速发展和基础科学问题认识不断提高的基础上,不久的将来,有望在不久的将来在基础科学和应用研究方面取得重大突破.     

类石墨相C3N4复合光催化剂

利用半导体光催化技术将太阳能转化为化学能或直接降解和矿化有机污染物,是解决能源短缺和环境污染等问题的有效途径。聚合物类石墨相氮化碳(g-C₃N₄)具有类似石墨烯的结构,由于其优异的化学稳定性和独特的电子能带结构,可作为太阳能转化、环境污染物降解的催化剂而得到了广泛关注。g-C₃N₄制备原料便宜易得、制备方法简单,可作为廉价、稳定、不含金属的可见光光催化剂应用于光催化降解污染物、水分解制氢制氧及有机合成领域。然而光生电荷易复合,使得g-C₃N₄的催化活性还不能满足大规模应用的需求。本文针对g-C₃N₄光催化活性的提高,综述了国内外在g-C₃N₄复合改性方面的重要研究进展,如金属/非金属掺杂、半导体复合、表面金属沉积等,并讨论了复合物的催化机理。     

Two wheel-like polytungstates obtained by incorporation of two {Cu(4)} clusters and two molybdenum centers in the {W(48)} wheel

Two new {P(8)W(48)} wheel-based compounds, Na(12)Li(16){[Cu(H(2)O)](2)[Cu(4)(OH)(4)(H(2)O)(8)](2)P(8)W(48)O(184)}·55H(2)O (1), and K(4)Na(24)Li(10){(MoO(2))(2)(P(8)W(48)O(184))}·61H(2)O (2) have been synthesized by a conventional aqueous solution method, and characterized by UV, IR, TG analysis, XPRD, (31)P NMR, XPS, single-crystal X-ray diffraction analyses, magnetic study and electrochemistry study. In compound 1, a wheel-type {P(8)W(48)} containing two {Cu(4)} clusters and two isolated Cu cations results in a 10-Cu-containing polyoxotungstate, which represents the first {P(8)W(48)}-based compound trapping two transition metal (TM) clusters in its inner cavity. Further, the polyoxoanion was connected by Na(+) and Li(+) cations into a 3D framework. Compound 2 is a 2-Mo-containing {P(8)W(48)}-based polyoxotungstate. Magnetic study indicates that antiferromagnetic interactions exist in compound 1.     

Wheel-shaped polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- macroanions form supramolecular "blackberry" structure in aqueous solution

The hydrophilic polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- ({Cu20P8W48}) self-assembles into single-layer, hollow, spherical "blackberry"-type structures in aqueous solutions, as studied by dynamic light scattering (DLS), static light scattering (SLS), zeta potential analysis, and scanning electron microscopy (SEM) techniques. This represents the first report of blackberry formation for a non-Mo-containing polyoxometalate. There is no obvious change in the shape and size of the blackberries during the slow blackberry formation process, neither with macroionic concentration nor with temperature. Our results suggest that the blackberry-type structure formation is most likely a general phenomenon for hydrophilic macroions with suitable size and charge in a polar solvent, and not a specific property of polyoxomolybdates and their derivatives. The {Cu20P8W48} macroions are thus far the smallest type of macroions to date (equivalent radius < 2 nm) showing the unique self-assembly behavior, helping us to move one step closer toward identifying the transition point from simple ions (can be described by the Debye-Hückel theory) to macroions in very dilute solutions. Moreover, by using {Cu20P8W48} blackberry-type structures as the model system, the electrophoretic properties of macroionic supramolecular structures are studied for the first time via zeta-potential analysis. The mobility of blackberry-type structures is determined and used for understanding the state of small cations in solution. We notice that the average charge density on each {Cu20P8W48} macroanion in a blackberry is much lower than that of discrete "free" {Cu20P8W48} macroions. This result suggests that some small alkali counterions are closely associated with, or even incorporated into, the blackberry-type structures and thus do not contribute to solution conductivity. This model is fully consistent with our speculation that monovalent counterions play an important role in the self-assembly of macroions, possibly providing an attractive force contributing to blackberry formation.     

Characterization and electrochemical properties of molecular icosanuclear and bidimensional hexanuclear Cu(II) azido polyoxometalates

Two new Cu(II) azido polyoxometalates compounds have been synthesized, and their structures were determined by X-ray crystallography. The compound Na(14)[SiW(9)O(34)Cu(3)(N(3))(2)(OH)(H(2)O)](2) x 24H(2)O (1) is built from two [SiW(9)O(34)Cu(3)(mu(1,1,3)-N(3))(2)(mu-OH)(H(2)O)](7-) subunits where the copper centers, connected by two azido ligands and one hydroxo group, form a nearly equilateral triangle. The two subunits are related by an inversion center and connected via the two mu(1,1,3)-N(3) ligands in an end-to-end fashion, affording a hexanuclear Cu(II) cluster. Linkage of these fragments via Cu-O=W bonds leads to a bidimensional arrangement of the polyoxometalate units. The complex LiK(14)Na(9)[P(8)W(48)O(184)Cu(20)(N(3))(6)(OH)(18)] x 60H(2)O (2) consists of two {Cu(5)(OH)(4)}(6+) and two {Cu(5)(OH)(2)(mu(1,1,3,3)-N(3))}(7+) subunits connected via four mu-OH and four mu(1,1)-N(3) additional ligands, the 20 copper centers being encapsulated in the [P(8)W(48)O(184)](40-) crown polyoxotungstate ligand. 1 represents the first multidimensional compound based on azido polyoxometalate (POM) units, and 2 represents by far the largest azido POM complex isolated to date. Magnetic measurements revealed an overall antiferromagnetic behavior for both compounds. Nevertheless, the study of the variation of the magnetization with the applied field indicates that 1 possesses a triplet ground state, which can be attributed to weak ferromagnetic interaction between the S = 1/2 triangular subunits. The stability of 1 and 2 evidenced by UV-vis spectroscopy and gel filtration chromatography, in particular at pH 5, has allowed a detailed study of their redox and electrocatalytic properties. For both compounds, the stability of the Cu(II)/Cu(I) couple is remarkable compared with the observations made in other Cu(II)-substituted POMs. Electrochemical quartz crystal microbalance measurements clearly demonstrate that the formation of the Cu(I) species occurs neatly without the formation of Cu(0). The accumulation of such Cu(II) centers within the complexes is a favorable condition to envision applications involving several electrons. The electrocatalytic reduction of dioxygen and hydrogen peroxide was achieved efficiently and has shown that the reactivity increases with the nuclearity and/or the Cu/W ratio of the POM complex. The dioxygen reduction is an overall four-electron process with water as the final product. Finally, the reduction of the W centers triggers a strong electrocatalysis of solvent reduction.     

STM/STS observation of polyoxoanions on HOPG surfaces: the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36-

A combination of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques have been performed on the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36- deposited on highly oriented pyrolytic graphite surfaces. Small, regular molecule clusters, as well as separated single molecules, were observed. The size of the molecules is in agreement with the data determined by X-ray crystallography. In STS measurements, we found a rather large contrast at the expected location of the Cu metal centers in our molecules, i.e., the location of the individual Cu ions in their organic matrix is directly addressable by STS.     

Organoruthenium derivative of the cyclic [H7P8W48O184]33- anion: [{K(H2O)}3{Ru(p-cymene)(H2O)}4P8W49O186(H2O)2]27-

Reaction of [Ru(p-cymene)Cl2]2 with [H7P8W48O184]33- (P8W48) in aqueous acidic medium results in the organometallic derivative [{K(H2O)}3{Ru(p-cymene)(H2O)}4P8W49O186(H2O)2]27- (1); in addition to the four {Ru(p-cymene)(H2O)} units, an unusual WO6 group with four equatorial, terminal ligands is also grafted to the crown-shaped P8W48 precursor.     

Nucleation process in the cavity of a 48-tungstophosphate wheel resulting in a 16-metal-centre iron oxide nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.     

Oxothiomolybdenum derivatives of the superlacunary crown heteropolyanion {P8W48}: structure of [K4{Mo4O4S4(H2O)3(OH)2}2(WO2)(P8W48O184)]30– and studies in solution

Reaction of the cyclic lacunary [H(7)P(8)W(48)O(184)](33-) anion (noted P(8)W(48)) with the [Mo(2)S(2)O(2)(H(2)O)(6)](2+) oxothiocation led to two compounds, namely, [K(4){Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2)(WO(2))(P(8)W(48)O(184))](30-) (denoted 1) and [{Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2)(P(8)W(48)O(184))](36-) (denoted 2), which were characterized in the solid state and solution. In the solid state, the structure of [K(4){Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2)(WO(2))(P(8)W(48)O(184))](30-) reveals the presence of two disordered {Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2+) "handles" connected on both sides of the P(8)W(48) ring. Such a disorder is consistent with the presence of two geometrical isomers where the relative disposition of the two {Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2+) handles are arranged in a perpendicular or parallel mode. Such an interpretation is fully supported by (31)P and (183)W NMR solution studies. The relative stability of both geometrical isomers appears to be dependent upon the nature of the internal alkali cations, i.e., Na(+) vs K(+), and increased lability of the two {Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2+) handles, compared to the oxo analogous, was clearly identified by significant broadening of the (31)P and (183)W NMR lines. Solution studies carried out by UV-vis spectroscopy showed that formation of the adduct [{Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2)(P(8)W(48)O(184))](36-) occurs in the 1.5-4.7 pH range and corresponds to a fast and quantitative condensation process. Furthermore, (31)P NMR titrations in solution reveal formation of the "monohandle" derivative [{Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(P(8)W(48)O(184))](38-) as an intermediate prior to formation of the "bishandle" derivatives. Furthermore, the electrochemical behavior of [{Mo(4)O(4)S(4)(H(2)O)(3)(OH)(2)}(2)(P(8)W(48)O(184))](36-) was studied in aqueous medium and compared with the parent anion P(8)W(48).     

Extended polyoxometalate framework solids: two Mn(II)-linked {P8W48} network arrays

Two polyoxometalate open framework (POMOF) materials have been synthesized using a secondary building unit (SBU) approach that facilitates the convergent assembly of multidimensional framework materials using a preassembled anionic SBU {P(8)W(48)}, with integrated "pore" 1 nm in diameter, and electrophilic manganese {Mn(2+)} linkers. This yields two new POMOFS with augmented hexagonal tiling (2 and 3), related to a known three-dimensional (3D) cubic array K(18)Li(6)[Mn(II)(8)(H(2)O)(48)P(8)W(48)O(184)]·108H(2)O (1), K(12)[Mn(II)(14)(H(2)O)(30)P(8)W(48)O(184)]·111H(2)O (2), and K(8)Li(4)[Mn(II)(14)(H(2)O)(26)P(8)W(48)O(184)]·105H(2)O (3). These frameworks have been crystallized from aqueous Li-buffered solutions of {P(8)W(48)} and Mn(II)(ClO(4))(2)·6H(2)O via careful control of the synthetic strategy akin to a crystal engineering approach using cation and temperature control to isolate different material architectures shown by compounds 1-3.     

Spectroscopy and reactivity of a photogenerated tryptophan radical in a structurally defined protein environment

Near-UV irradiation of structurally characterized [Re(I)(CO)3(1,10-phenanthroline)(Q107H)](W48F/Y72F/H83Q/Y108W)AzM(II) [Az = Pseudomonas aeruginosa azurin, M = Cu, Zn]/[Co(NH3)5Cl]Cl2 produces a tryptophan radical (W108*) with unprecedented kinetic stability. After rapid formation (k = 2.8 x 106 s-1), the radical persists for more than 5 h at room temperature in the folded ReAzM(II) structure. The absorption spectrum of ReAz(W108*)M(II) exhibits maxima at 512 and 536 nm. Oxidation of K4[Mo(CN)8] by ReAz(W108*)Zn(II) places the W108*/W108 reduction potential in the protein above 0.8 V vs NHE.     

Ferromagnetically coupled dimer of Cu(II)-substituted gamma-decatungstosilicate

A novel copper-substituted polyoxoanion [Cu4(H2O)2(OH)4Si2W16O58]8- (1) has been synthesized as its sodium salt, Na(8)- 1 .26H2O (Na-1), potassium salt, K(8)- 1 .28H2O(K-1), and calcium salt, Ca(4)- 1 .48H2O (Ca-1). Compound 1 is the first dimer of dicopper-substituted gamma-decatungstosilicate fused in an unprecedented side-by-side dimerization mode with a ferromagnetic coupling property.