Synthesis of experimental models for molecular inorganic geochemistry—A review with examples

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Geochimica et Cosmochimica Acta 71 (2007) 5590–5604 www.elsevier.com/locate/gca

Synthesis of experimental models for molecular inorganic geochemistry—A review with examples Leone Spiccia a, William H. Casey b

b,c,*

a School of Chemistry, Monash University, Vic. 3800, Australia Department of Chemistry, University of California, Davis, CA 95616, USA c Department of Geology, University of California, Davis, CA 95616, USA

Received 12 September 2006; accepted in revised form 14 March 2007; available online 4 September 2007

Abstract We are observing an sharp evolution within low-temperature geochemistry away from thermodynamics and deep geologic time toward molecular processes, particularly those at mineral or bacterial surfaces, and disequilibria. This evolution has led to a new cooperation between Earth scientists and inorganic chemists who synthesize aqueous metal-(hydr)oxide clusters as models for enzyme centers and hydrolytic products. If geochemists too can embrace these methods, we can create experimental models to answer some of the key questions posed about minerals and their reactions with aqueous solutions. In this paper we lay out the areas where there is overlap in these two fields using particular examples and emphasize how skills from each subdiscipline can benefit the whole. The central point is that this the fusion is extraordinarily healthy to both fields, as inorganic chemistry expands to embrace natural processes and as geochemists embrace methods of molecular synthesis and new useful, yet unnatural materials, that have traditionally been considered exotic. The motivation for this cooperation is the emphasis on reaction mechanisms between surface functional groups on minerals, or cells, in water. By mechanisms is meant the key atoms and molecular motions that cause the reaction to occur. For aqueous reactions, the key variables are: the number and character of innersphere ligands and metals; the Brønsted acidity of key atoms in cleavable bonds and the accessibility of the key atoms to solutes. These variables can be studied systematically in experimental clusters and coupled directly to advances in simulation.  2007 Published by Elsevier Ltd.

1. INTRODUCTION ‘Geochemistry’ is a old word, coined only a few decades after the death of Lavoisier (Scho¨nbein, 1838) and thus immediately after the founding of modern chemistry. In these early days, chemistry was indistinguishable from mineralogy since it was the conversion of Earth substances that yielded new materials, like reagent oxygen, and tested hypotheses about putative substances, like phlogiston. Geochemistry diverged from inorganic chemistry in the early 20th century, when inorganic chemists began to focus on molecular transformations of dyes, coordination com-

*

Corresponding author. Fax: +1 530 752 1552. E-mail address: [email protected] (W.H. Casey).

0016-7037/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.gca.2007.03.041

pounds and proteins, while Earth scientists emphasized mineralization over huge time and length scales. Here we argue that aspects of low-temperature geochemistry are returning to a close relationship with inorganic chemistry to the mutual benefit of both fields. The reason for the fusion is clear—some geochemists are now posing questions about natural reactions that require experimental molecules as models for minerals. The molecular emphasis began most recently with an influential paper by Gibbs (1982), who argued that many reactive properties at minerals and mineral surfaces reflect short-range forces and could thus be captured by well-designed molecular fragments. Correspondingly, many geochemists developed clusters as models for key parts of mineral structures (Fig. 1), but usually as part of a computer simulation, not an experiment. The next step in this evolution is to

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tions and is unavoidable if a predictive capability about the Earth is desired. 2. WHAT INFORMATION IS AVAILABLE?

Fig. 1. Small clusters will be used as experimental models to understand pathways for mineral–fluid reactions. Here a structural fragment of the goethite (FeOOH) structure is shown to resemble a hydrolytic trimer, which isolates the key oxygen coordination environments. To the extent that the reactive properties reflect short-range forces only, experimental models such as these clusters could greatly extend simulation efforts, which are already focused on the reactive properties of putative mineral-like metal-hydroxide clusters.

Hydrolytic reactions are the place to begin. For oxide minerals, geochemists desire information about the protonation states of individual oxygens on mineral surfaces and rate parameters and pathways for polymerizing and dissociating structures. These subjects closely relate to one another because the protonation state of the oxygen is often a key to the kinetics. With this in mind, Rustad (2005) simulated the experimental titration data for two large aluminum hydroxide clusters to examine how electrostatic fields might affect the protonation equilibrium for oxygens at mineral surfaces. He could reproduce, approximately, the experimental titration data in his simulation and was able to show that access of the solvent to the oxygens dramatically affected equilibrium. Furthermore, he showed that proton affinities calculated for gas-phase clusters was not a particularly useful guide to surfaces in water. This information was unattainable without the aqueous clusters because the experimental data and structure are so well-constrained. The protonation reactions intimately relate to kinetics because the polymerization of metal ions into larger structures generally relies on the formation of a coordinated nucleophile (a hydroxyl or oxo group) with sufficient nucleophilicity to attack another metal center, thereby displacing a bound water in the process. In other words, the conjugate bases formed by deprotonation: K a1

½MðOH2 Þ6 3þ  ½MOHðOH2 Þ5 2þ þ Hþ ½MOHðOH2 Þ5 2þ K a2

complement these computational models with real experimental clusters, which requires the help of inorganic chemists. It is now clear that mechanisms on molecules much larger than monomers can rarely be identified from experiment alone, without the tools of simulation, and that simulation alone, absent the experimental data on the same molecule, is misleading. One reason that computation alone is inadequate is because, even for simple minerals, it is unclear which terminations (or surface states) are stable in water (e.g., Henderson, 2002; Trainor et al., 2002), exactly which functional groups are exposed on those terminations, how to assign equilibrium constants (or rate coefficients) to these groups, and how to quantify the interactions of these groups with molecular species present in solution. Inorganic chemists have a long history of synthesizing experimental molecules to isolate key structural environments and we review some of the major features here that might be useful for geochemists. Our focus is almost exclusively on the surface functional groups because low-temperature geochemists are most often working at the molecular scale. The general approach, however, is not limited to surface chemistry. With these models, one can verify predictions of reaction energies, pathways, microscopic protonation constants, and the details of hydrogen bonding to the solvent. This close coupling of simulation to experimental model is the only way to gain confidence about molecular pathways for reac-

 ½MðOHÞ2 ðOH2 Þ4 þ þ Hþ

ð1Þ

may then assemble into larger molecules and colloids. The equilibrium constants for these Brønsted reactions vary considerably for different metals and oxidation states and, for discussion of minerals and oligomers, they differ depending upon the coordination chemistry of the particular oxygen (i.e., the chemical environment in which it is found). Polymerization can, in principle, occur without water release providing that the metal center has a vacant coordination site, but more common is nucleophilic attack by coordinated OH and O2 at a second metal center with concurrent release of one water molecule. The important point is that there is an inherent link between the protonation state of a metal-hydroxide molecule and its tendency to polymerize or its tendency to dissociate. Knowledge of the location of protons would clarify equilibrium properties of a surface, like complexation and charging, and would also elucidate the pathways for mineral growth and dissolution, which relate intimately to these protonations. It is for this reason that there is such interest in assigning protonation equilibrium constants to the individual oxygens on the surfaces of minerals (e.g., Hiemstra et al., 1989a,b, 1996, 1999; Hiemstra and van Riemsdijk, 1990, 1991, 1996, 1999; Rustad et al., 1996a,b, 1999, 2000a,b; Geissler et al., 2001; Rustad, 2001, 2005; Rustad and Felmy, 2005; Bickmore et al., 2006, many others).

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Much of this information already exists for small metalhydroxide oligomers, but it is rarely cited by geochemists and yet could provide a set of key test cases for establishing the accuracy of our simulations. Consider the case of a metal-hydroxide dimer, which has been used widely as a computation model for mineral surface structures. In Scheme 1, the various steps in condensing from a single-bridged dinuclear complex (SBD) into a mixed Cr(III)–Rh(III) doublebridged dinuclear complex (DBD) are shown. The SBD undergoes intramolecular condensation, releasing another water molecule and forming the dihydroxo-bridged dimer (DBD). This bimetallic oligomer (two metals) is highlighted here, in preference to homometallic (single metal analogs of Cr(III) and Rh(III)), as it demonstrates that valuable kinetic and thermodynamic data can also be obtained for these more complex systems, containing more than one metal type. Importantly, most of the various equilibrium constants (Ka1, Ka2, Kp3, Kp4) and rate parameters (k0, k1, k2), including entropies and enthalpies, have been measured (Table 1). This type of detail exists for a considerable number of small hydrolytic molecules (see Springborg, 1988; Spiccia, 2004) so that the role that different Brønsted acidities play in con-

trolling the reactions, both bond formation and dissociation, can be addressed immediately. Brønsted acidities of various oxygens in metal clusters have been compiled and even such things as the strengths of hydrogen-bonded structures has been estimated by careful synthesis of dimers with, and without, ligands that can accept hydrogen bonds. Since polymerization processes generally occur via the deprotonated forms of the metal ion, rate accelerations will result from both labilization of the coordination sphere and the increased nucleophilic character of coordinated OH or O2 (see Crimp et al., 1994; Jolivet et al., 2000). Thus, the removal of protons from nonbridging waters commonly accelerates the rates of ligand substitution at other sites within the oligomer (Richens, 1997, 2005; Helm and Merbach, 2005; Helm et al., 2005). In contrast, the binding of protons to a bridging hydroxyl (converting it to a bridging water) accelerate the rate of bond ruptures in metal-hydroxide oligomers (e.g., Springborg, 1988) since aquo bridges are much weaker than hydroxo or oxo bridges. The power of using small clusters in spectroscopic study is easy to demonstrate. In Fig. 2, we show 17O NMR spectra (adapted from Alam et al., 2004; Black et al., 2006) for the [HxNb6O19](8x) ion (x = 0–3) Lindqvist ion at pH 7.8

Scheme 1. Reaction steps in the cleavage of bonds in a hydrolytic dimer as detailed experimentally (Table 1).

Table 1 Kinetic and thermodynamic data for interconversion between various forms of the binuclear Rh(III)–Cr(III) and Cr(III) ions (Crimp and Spiccia, 1996) Parameter 5

1

10 · k0 (s ) 105 · k1 (s1) 105 · k2 (s1) Ka1 (M1) 102 · Ka2 (M1) 105 · k1 (s1) 105 · k0 · Kp3 (M1 s1) K1

[(H2O)4Rh(l2-OH)2Cr(OH2)4]4+

[(H2O)4Cr(l2-OH)2Cr(OH2)4]4+

k/K (298 K)

DH/0

DS/0

k/K (298 K)

DH/0

DS/0

3.9 20 53 0.5 0.39 2.7 1.1 7.1

92(8) 95(2) 88(15) 45(6) 44(11) 107(2) 60(9) 13(2)

21(26) 1(5) 13(50) 146(22) 100(36) 28(8) 80(30) 28(7)

10.1 41.7 1140. 0.101 0.0054 2.2 5.2 19

88(3) 92(1) 67(2) 48(4) 68(3) 110(5) 59(2) 19.6(5)

26(11) 0(2) 58(6) 141(12) 147(10) 35(16) 130(20) 41(17)

The units of DH are kJ mol1 and for DS JK1 mol1.

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Fig. 2. Oxygen sites in the for the Hx Nb6 O19 ð8xÞ Lindqvist ion at pH 7.8 and 298 K (modified from Alam et al., 2004; see also Black et al., 2006; Balogh et al., 2007a). The molecule, shown above in both ball-and-stick and polyhedral representation, was initially enriched in 17O. Thus the intensity of the peaks decrease progressively with time as structural oxygens exchange between with aqueous solution. There is a small amount of carbonate impurity that gives a peak near +170 ppm. These isotope-exchange reactions proceed without the molecule dissociating.

and 298 K. The 17O NMR spectra distinguish various structural oxygens and the kinetics of oxygen-isotope exchange followed individually. In this case, one need not speculate about the role that protons play in the pathways for bond ruptures—the effect can be observed since potentiometric data exist to assign equilibrium constants (e.g., Etxebarria et al., 1994) and X-ray data have identified the sites of protonation, which in the case of the [HxNb6O19](8x) ion are l2-O bridges (Ozeki et al., 1994; Nyman et al., 2006) not the terminal oxygens. The full rate laws for breaking and

reforming all metal–oxygen bonds can be established and compared with simulations (see Black et al., 2006; Balogh et al., 2007a). While the potentiometric measurements on these clusters are not simple, the results are certainly much better defined than those for the surface of a rock-forming mineral in water where neither the types or concentrations of functional groups can be independently known. Interestingly, the protonation of bridging oxygens in these molecules labilizes all of the exchanging sites, so a view of reactions that is wholly local is inadequate. Such a result

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Scheme 2. Steps in the hydrolysis and inter conversion of trinuclear complexes of Rh (III), as detailed experimentally.

could not have been anticipated, even after endless studies of mineral-oxide colloids. As a further example, in Scheme 2 the steps in forming a Rh3 ðl-OHÞ4 ðH2 OÞ9 4þ trimer could be elucidated by quenching the oligomerization reaction. The intermediate forms of the trimer (Scheme 2), with various stages of condensation of the central l3-OH, were isolated and identified by combining 103Rh NMR with ion-exchange chromatography and simple potentiometry (see Spiccia, 2004). Similar condensation reactions are essential to the formation of minerals containing highly coordinated oxygens, such as the goethite shown in Fig. 1, but cannot be studied directly because the Fe(III) is too labile. With an inert metal like Rh(III), the various reaction steps can be elucidated and the fragment retains structural similarity to the mineral (such as the goethite represented in Fig. 1) at least to a useful degree. Where and how should the information be used? It should be used to calibrate our methods of simulation so that we have faith in the accuracy for treating reactions where experiment is impossible. Although some of the metals (e.g., Nb(V), W(VI), Rh(III), Cr(III)) in these examples are not common in the Earth, the role that the inherent lability of the metal plays in controlling reaction rates is known, at least for simple aqueous monomers. The labilities of different trivalent metals differ by a factor of 1020 in water (Richens, 1997; Helm et al., 2005). Regardless of the actual mechanism of substitution, labile metal centers, such as Fe(III), undergo fast substitution reactions and tend to polymerize or de-polymerize quite rapidly whilst for kinetically inert metal centers, such as Cr(III) and Rh(III), the hydrolytic processes are slower. Certainly, different pathways may prevail for labile metals that are not available to inert metals. The chosen or preferred pathways

will hinge on estimable factors, such as the timescale for proton transfers relative to the rate of dissociating the metal–oxygen bond. But identifying the availability of these pathways is an active area of research that should be just as compelling to geochemists as to inorganic chemists. It is also not inconceivable that the rates of labile functional groups that are particularly relevant to Earth science could be measured. The first step is to construct a model compound where the structure is constrained. The fast-substitution reactions of Fe(III) surface sites, for example, are currently impossible to measure on a material like ferrihydrite or goethite, but they could be measured on material like that shown in Fig. 3. This nanocluster is part of a series synthesized by the Achim Mu¨ller group (e.g., Liu et al., 2006) and consists of a sphere of 72 Mo(VI)-oxide polyhedra that are regularly decorated by 30 Fe(III) oxide octahedra, each of which exposes a >FeIII-OH2 functional group to the aqueous solution. Just like the colloidal iron-hydroxide solid, these functional groups are moderately weak acids and become fully protonated at low pH (pH <2.9). A solution of these clusters can be made stable for months. The rates of exchange of waters bound to these >FeIII-OH2 functional groups can conceivably be measured by the same line-broadening methods that were used for Fe(III) monomers (e.g., Grant and Jordan, 1981; Swaddle and Merbach, 1981). The reasons that these rates do, or do not, differ from the surface functional groups on a mineral is then a well-posed problem for computation.1

1 Since this article was accepted, the rates of such exchanges on the Mo72Fe30 molecule have been measured by Balogh et al. (2007b).

Minerals into molecules  

Fig. 3. A near-spherical aqueous cluster, containing 72 Mo(VI)oxide polyhedra (light blue) and 30 Fe(III) as Fe(O)6 octahedra (brown), is robust in solution (Liu et al., 2006) and could be used to establish the kinetic properties of the coordinated waters. The waters deprotonate at pH > 2.9 (see Balogh et al., 2007b).

Certainly it would be enormously interesting if the rates of exchange of these waters, bound to such a large (2.5 nm diameter) molecule (Fig. 3) do not differ from the aqueous monomer. Think of the answers it could provide about minerals. Does deprotonation of a distal >FeIII-OH2 labilize the other bound waters, as is observed in aqueous monomers? Does reactivity vary with molecular size? Can we estimate the rate coefficients accurately enough from work on aqueous monomers (see Wang et al., 2007)? Work on small experimental clusters has also uncovered features that were not anticipated from simulation. One example is labilization of myriad oxygen sites by protonation of a bridging oxygen in the Lindqvist ions (see above). As a second example, a common precursor in hydrolytic polymerization is the H3 O2  bridge where a bound hydroxyl bonds to a bound water (Ardon and Bino, 1987; see also Rustad et al., 2004; Casey and Rustad, 2007). These bridges can either retard or accelerate the reactions, depending upon the structure of the molecule. Similarly, protonation of a l2-OH to form a weak l2-OH2 is a common pathway for proton-promoted dissociation (see Springborg, 1988). These bridges are discussed in greater detail in the next section because they are useful in isolating metal clusters for purification. 3. METHODS OF ISOLATING EXPERIMENTAL MODELS 3.1. Useful classes of clusters There are two classes of hydrolytic oligomers that can be isolated and that will be particularly useful to geochemists. First are clusters of labile metals, such as the polyoxometallate molecules like the familiar Al13 ion ([Al13 = AlO4Al12(OH)24(H2O)12]7+) and other structures in the Baker–Figgis–Keggin series (see Pope, 2004). These

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molecules are synthesized by adjusting the chemical conditions such that the target molecule is the dominant complex in a solution, which commonly means maintaining such high concentrations that the concentrations of monomer complexes are negligible. Although the metals in these polyoxometallate molecules are labile, the polymerized structures dissociate slowly. Because of the framework of oxy and hydroxo bridges, useful experiments can be conducted. There are literally hundreds of polyoxometallates besides the Al13, most of which are anions of second- and third-row transition metals with d0 electronic configuration (Pope, 2003, 2004), such as the Lindqvist ion shown in Fig. 2. Early transition metals in Periods V–VI form aqueous anions with the Lindqvist structure, as discussed above, having the stoichiometry: [HxM6O19](8x) where M = Nb(V) Ta(V) and 0 6 x 6 3; [M6O19]2 where M = W(VI), Mo(VI), or mixed-metal compounds such as M2 W4 O19 4 (M = Nb(V), V(V)) and others having the general stoichiometry Nbn W6n O19 ð2þnÞ (n = 0–4) (see Dabbabi and Boyer, 1976; Klemperer and Shum, 1977; Pope, 2004). The W(VI) version of the Lindqvist ion is stable to acidic conditions because of the reduced basicity of the oxygens caused by substitution of W(VI) or Mo(VI) for Nb(V) or Ta(V). The most familiar cation to Earth scientists is an Al(III) molecule with a structure of the e-Keggin isomer and the stoichiometry [MAl12 = MO4Al12(OH)24(H2O)12]7/8+ where M = Al(III), Ga(III) or Ge(IV). Included here is the Al13 molecule, which has served as such a useful model in the past for aluminum hydroxide solids (e.g., Wehrli et al., 1990; Furrer et al., 1992, 1999, 2002; Bradley et al., 1993; Amirbahman et al., 2000; Casey, 2006). The MAl12 molecules polymerize to form gels and, ultimately, minerals (e.g., Bradley et al., 1993; Furrer et al., 2002) and it is not an accident that the charge densities (+0.3 to 0.8 C/m2) fall into the familiar range as oxide solid surfaces (see Hiemstra et al., 1999). These polyoxometallate molecules (anions or cations) provide a wealth of information about reactions at individual oxygens. The MAl12e-Keggin and Lindqvist ion molecules are particularly useful as experimental models because oxygens in the center of the molecule are inert to exchange. By tagging with 17O either the l6-O in the Lindqvist ions or the central l4-O in the Al(III) Keggin molecules, one can distinguish reactions that affect individual oxygen sites on the intact molecule from those where the molecule fully dissociates and all oxygens isotopically exchange. It is easy to distinguish reaction pathways where oxygens exchange without dissociating the molecule from those where the molecule dissociates and reforms. The second class of molecules is hydrolytic clusters of inert metals that can be isolated by exchange chromatography and crystallized. The dissolved molecules are inherently unreactive and transform very slowly, although deprotonations are fast. These inert metal clusters are smaller than the polyoxometallate ions, but have been used for decades to understand aqueous reactions (see Richens, 1997; Springborg, 1988; Spiccia, 2004). With metal ions such as Cr3+, Rh3+ and Ir3+, it is possible to generate series of oligomers ranging from dinuclear to hexanuclear (e.g.,

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Drljaca and Spiccia, 1996a,b; Spiccia, 2004). Although experimental tricks can be used to accelerate the synthesis of a target molecule (see below), once formed they dissociate and polymerize so slowly that a study to measure the stability constants for a series of Cr(III) oligomers took 5 years to complete (Stuenzi et al., 1989). 3.2. Methods of separation, isolation and crystallization The formation of oligomers from labile metals, which here includes most of the polyoxometallates, is generally achieved through control of pH and physical parameters so that the target molecule dominates in solution. In the simplest cases, success is met by introducing a suitable counterion that crystallizes only the target molecule. The crystallization of the MAl12 ions by selenate provides a good example. The initial solution contains many aluminum species besides the target, but none form solids from simple addition of selenate. Chromatography is not usually employed with polyoxometallates because the molecules either dissociate too quickly or polymerize on the column. The NaMO4Al12(OH)24(H2O)12(SeO4)4Æ(H2O)x crystals are separated and metathetically dissolved to yield a pure solution of the target molecule. Crystallization of cationic metal-hydroxide clusters is commonly difficult because the positively charged ions are so strongly solvated that their exterior is essentially a sheath of waters that resists crystallization. Choice of counteranions is often key to successful crystallization because they must disrupt this solvation sheath. Aromatic sulfonate counterions, such as those produced on deprotonation of p-toluene sulfonic and mesitylene sulfonic acid, are excellent. Such anions consist of a hydrophilic –SO3  group, which accepts hydrogen bonds from either water or from a coordinated hydroxide and, in addition, is attracted electrostatically to the highly positively charged oligomer. The useful second property of aromatic sulfonates is that orientation of the negatively charged sulfonate group towards the oligomer is aided by supramolecular interactions between adjacent aromatic rings. This results in a supramolecular assembly whose exterior is hydrophobic and aids in the crystallization of the oligomer. This approach has proven successful in crystallizing hydrolytic clusters that previously defied isolation and

structural characterization via X-ray analysis, such as the Cr(III) and Rh(III) dimers and the Rh(III)–Cr(III) dinuclear aqua ion, these have the structures: [(H2O)4 Rh(l-OH)2Rh(OH2)4]4+; [(H2O)4Cr(l-OH)2Cr(OH2)4]4+ and [(H2O)4Rh(l-OH)2Cr(OH2)4]4+], respectively. As an example that highlights the crystallization strategy, the dihydroxo-bridged dinuclear Rh(III)–Cr(III) ion was crystallized using p-toluenesulfonate counterions (Crimp et al., 1992). These counterions balance the charge on the metal center and accept hydrogen bonds from coordinated water molecules and bridging hydroxo groups. The aromatic rings on the p-benzenesulfonate counterions assemble into layers, which expels water from the solvation sphere of the cation in a similar fashion to much larger macrocycle assemblies. The importance of such X-ray structural characterization is brought to the surface by unexpected structural features that are uncovered from time to time. For example, the two scandium centers in the dihydroxo-bridged Sc(III) dimer [(H2O)5Sc(l-OH)2Sc(OH2)5]4+, which was isolated with benzene sulfonate counterions, are seven coordinate and adopt a pentagonal bipyramidal geometry by virtue of the fact that Sc(III) is larger ion than many transition metal ions (Matsumoto et al., 1989). Similarly, it is surprising to many geochemists that so many polyoxometallates (see below) protonate at the bridging oxygens in preference to the terminal oxygens, which are more negative in formal ionic charge, but the X-ray data is irrefutable. The bridging oxygens in these clusters, including the niobate Lindqvist ion shown in Fig. 2, are commonly more basic than the terminal oxygens, which have higher bond orders. In some cases, large macrocycle organic molecules can be used to break-up or trap within the crystal lattice all or part of the solvation sphere that surrounds these highly charged, strongly solvated cations. A good example is isolation of the oxo-bridged iron(III) dimer, [(H2O)5 Fe(l-O)Fe(OH2)5]4+ (Fig. 4) by Junk et al. (2002). In this case crystallization was achieved with relatively simple anion but a crown ether was introduced to accept hydrogen bonds from water ligated to the metal center so that the solvated structure could be crystallized. A simple anion, such as nitrate or sulfate, can be used in combination with the macrocyclic molecule, in this case a crown ether molecule, to break up the solvated structure of the cation. Thus,

Fig. 4. The oxo-bridged iron(III) dimer, [(H2O)5Fe(l-O)Fe(OH2)5]4+ was recently crystallized intact from solution by employing a crown ether ligand that capped the coordinated Fe(III) and bonded to the bound waters (from Junk et al., 2002).

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Cr(III) oligomers have also been isolated as an 18-crown-6 adduct, albeit with a sulfonate counterion (see Drljaca et al., 1997, 1999a,b, 2000; Hardie and Raston, 2000; Schmitt et al., 2001, 2002). This combined approach led to the crystallization of the Cr(III) and Rh(III) trimers and tetramers within a complex lattice consisting of a sulfonated calixarene, a crown ether which hosts an alkali metal ion, and the positively charged oligomer to balance the negatively charge on the calixarene (Fig. 5). Due to the large size of the unit cell and complexity of the crystal lattice within which the cluster crystallized, the atom connectivity of each oligomer could be determined but the fine structural detail, such as accurate metal–ligand bond lengths and angles for the aqua and hydroxo ligands and hydrogen-bonding interactions in which these ligands participate, could not be elucidated. There is a need for alternative crystallization approaches that will allow better definition of the structures of these oligomers. Unwanted hydrolysate molecules must usually be eliminated in order to isolate the target cluster or molecule. One method of purifying the oligomers of inert metals is by controlled polymerization followed by ion-exchange chromatography to separate the various molecules according to size and charge. Although this methods yields pure solutions of individual free of other oligomers, such solution do contain high concentrations of electrolyte used as eluents in the purification procedure. A key development was recognition that the metal-hydroxide clusters of inert metals can be stored as ‘active hydroxide’ solids (see Giovanoli et al., 1973a,b). These active-hydroxide solids form because of the enormous difference between the rates of proton transfer and rates of oxygen transfer for oligomers of metals such as Cr(III), Rh(III) and Ir(III). Some of the waters that are bound to the inert metal deprotonate as pH rises. The newly formed bound hydroxyls accept hydrogen bonds from water molecules on adjacent clusters, resulting in a threedimensional, hydrogen-bonded solid that has discrete clus-

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ters linked on one another by H3 O2  bridges. All of the metal clusters condensed into a solid with their innercoordination spheres intact. Reversing the process is easy—although these bridges can be thermodynamically quite strong (Ardon and Bino, 1987), they dissociate immediately upon acidification and, as the H3 O2  bridges rupture, the desired oligomer is released intact to solution. Precipitation of an oligomer as an active-hydroxide solid allows one to store the target molecule between purification steps and to exclude the unwanted electrolytes. Because the acid-hydroxide solid dissolves by acidification, and because it excludes the electrolytes, dissolution in appropriate proportions of suitable acid yields clean solutions of each oligomer. Active hydroxides have been prepared from the monomers, dimers and trimers of Cr(III) and Rh(III) and characterized in some detail (Spiccia and Marty, 1986; Spiccia et al., 1988, 1987, 1997; Spiccia, 2004). In a typical synthetic approach, solutions of the monomer (e.g., [Cr(H2O)6]3+) are polymerized into larger structures by base hydrolysis at pH (12–13), followed acidification back to near-neutral pH to quench polymerization processes. The solution is then purified, often by cation-exchange chromatography, to eliminate unwanted hydrolysates. The desired molecule can be eluted from the column intact and then precipitated rapidly from solution by partial deprotonation of the bound waters (i.e., neutralization to pH  7) to form the active-hydroxide solid. A solution of [(H2O)4Cr(l-OH)2Cr(OH2)4]4+, for example, could be quantitatively converted into a solid with a hydrogen-bonded array of [(OH)2(H2O)2Cr(l-OH)2Cr(OH2)2(OH)2] that could be quantitatively recovered/regenerated later. It is worth digressing on the crystallization of the oxobridged [(H2O)5Fe(l-O)Fe(OH2)5]4+ dimer because of its geochemical relevance. There was much debate in the literature about whether the two iron(III) centers in this molecule were linked via one oxo or two hydroxo bridges, the latter being most common for other systems. Hydrolysis of Al(III) usually yields hydroxo-bridged structures like

Fig. 5. The Cr(III) tetramer was trapped and crystallized for structural characterization by Drljaca et al. (2000) using a combination of macrocycle supramolecules to disrupt the hydration sheath of water molecules (see Drljaca et al., 1997, 1999a,b, 2000; Hardie and Raston, 2000; Schmitt et al., 2001, 2002 for examples; this figure was adapted from Hardie and Raston, 2000).

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gibbsite (a-Al(OH)3), while the hydrolysis of Fe(III) commonly yields oxo-bridged structures, like hematite (aFe2O3) or goethite (a-FeOOH). The corresponding Al(III) structure, corundum (a-Al2O3), never forms at low temperature. Consistent with the idea the short-range forces are most important, the trapped dimer of Fe(III) (Junk et al., 2002) is oxo-bridged while the Al(III) dimer is hydroxobridged (Johansson, 1962). The real value of these clusters is in computation. Several groups have simulated the exchange of a water molecule from the inner-coordination sphere of a metal ion with bulk solution (see Erras-Hanauer et al., 2003; Helm and Merbach, 2005; Helm et al., 2005; Rotzinger, 2005a,b). The simulations include reasonable transitionstate properties that are within a few percent of the experimental measurements. However, as the focus moves away from the simple processes involving substitution at a single metal center (usually octahedral) to more complex processes, such as adsorbate reorganization on mineral surfaces and mineral growth, we move from simple transition states to ensembles where there exist many pathways and the saddle points are dense. The difficulty lies in identifying the active reaction coordinate among the many possibilities. Even the simple process of auto-ionization of water was only recently solved using computational methods, since it first had to be realized that the transition state for auto-ionization was driven by simultaneous fluctuations in local electric-field and hydrogen-bond connectivity (Geissler et al., 2001). One can argue that these details are not important to geochemistry where the time scales are so long, but this argument misses the point. Verification of simulation by comparison to spectroscopy at the appropriate scale is unavoidable if our goal is to make accurate predictions about molecular transformations (see Casey and Rustad, 2007). 4. EXAMPLES: MEDICINE, MINERALS AND HYDROLYTIC CLUSTERS The overlap of interests between the geochemists and the chemists is enormous and is worth presenting a few examples here. 4.1. Models for metalloproteins Mineral structures are viewed in evolutionary terms as sources of biomolecules (see Sauer and Yachandra, 2002; Zhang et al., 2004) and thus it is not an accident that the experimental models resemble fragments of a mineral, like the iron-sulfide fragments shown in Fig. 6. Nature has been particularly clever in designing hydrolytic enzymes, selecting the metals and associated protein functional groups for optimum performance under the conditions of pH that the enzyme will experience. The formation and cleavage of oxo and hydroxo bridges is particularly important in the enzymes that convert water into oxygen and the reverse. One example is the reduction of dioxygen by the Fe–Cu site in cytochrome c oxidase, the sequence of processes have postulated to include the formation of oxo- and hydroxobridged Fe(III)–Cu(II) intermediates while in the water-

Fig. 6. The core of ferrodoxin (a), and many other iron-sulfur clusters found in metalloenzymes, contains a cubane-like structure of alternating Fe(II) and S(-II) atoms. A synthetic model (b) that captures the essential structure can be synthesized using tert-butyl sulfide ligands to protect the structure from hydrolysis. The cubane-like structure closely resembles a common moiety in iron sulfide minerals, and is easily identified in the structure of greigite [Fe3S4]. In these images, the sulfur atoms are yellow and iron atoms are blue.

oxidation center of photosystem II an tightly bonded oxo-bridged high-valent tetranuclear Mn4 cluster is responsible for firstly converting water into bridging oxo groups, two of which are subjected to a series of electron abstraction steps that result in the formation of oxygen (Fig. 7). Dismukes and co-workers developed an experimental model of the reaction center (four Mn centers and one Ca) that releases O2 in the gas phase (Fig. 7), illustrating the benefits of establishing experimental models. Many metalloproteins contain two or more centers in their active sites, which are often connected via bridging groups that are either present in the protein backbone or are generated through the hydrolysis of water. Many hydrolytic enzymes, and in particular those that carry out the hydrolysis of phosphate ester bonds in biological molecules, fall into this category and there is often debate about whether bridging hydroxo or oxo groups are capable of acting as nucleophiles in such processes, although in all likelihood terminal hydroxo groups would have much greater nucleophilic character. Even when considering single metal enzymes such as carbonic anhydrase or the endopeptidase and carboxypeptidase families of proteins the key hydrolytic process of deprotonation of a terminal water molecule assumes tremendous importance and is often assisted by the presence of weakly basic groups such as carboxylates. Ferritin is one of the most widely studied and intriguing metalloproteins (e.g., Powell, 2004). Such proteins can store up to 4500 iron atoms within its nucleus, roughly one iron per amino acid. Considerable effort has been dedicated to understanding how this metalloprotein functions and to

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5599

Fig. 7. (Top) A simplified structure of the Water Oxidation Center (WOC) of Photosystem II isolated from Thermosynechococcus el. ˚ , adapted from Ferreira et al., 2004). The core of the WOC consists of five metal centers (four Mn centers and one Ca center). (resolution 3.5 A The labels refer to fragments of the protein structure. (Bottom) A model compound developed by Dismukes and co-workers that reproduces many of the structural features of the WOC and which has been shown to release oxygen in the gas phase (Dasgupta et al., 2004).

how the iron enters the protein core and is released. It is clear that redox chemistry plays an important role in the uptake and release of iron from the core. However, the importance of hydrolytic chemistry must not be overlooked. The attachment of additional iron centers, in the trivalent state, to the existing and confined iron oxide core would involve processes that also occur in an open aqueous environment when a metal ion is released or when it is mobilized by changes in conditions. Modeling the attachment of hydrolytically active metal ions to surfaces would therefore benefit efforts to understand the complex mechanism of iron attachment and release from the ferritin core.2 4.2. Molecular magnets The field of nanoscale devices, built from molecular chemical precursors, is growing rapidly due to the range

2 Since this article was accepted, Michel et al. (2007) showed that the core of ferrihydrite, a ubiquitous material in nature, is a nanometer-sized d-Keggin cluster. This result echoes work by Furrer et al. (2002) who showed that the aluminum-hydroxide floc in polluted streams is built around an -Keggin cluster.

of potential applications in the areas of miniaturized electronics and the computer industry. Large molecular clusters, for example the Mn12 carboxylates such as [Mn12O12(CH3CO2)16(H2O)4], have been found to possess magnetic memories that are presently being explored as possible components of molecular (quantum) computers (see King et al., 2005). Similarly, a family of homologous molecules with a similar core but varying numbers of trivalent and divalent [Ga(III), Al(III) and Fe(III)] have been synthesized in a brucite-like structure with edge-shared M(O)6 octahedra, such as those shown in Fig. 8 (e.g., Heath and Powell, 1992; Powell et al., 2004). Although the authors are primarily interested in making Fe(III) clusters with unique magnetic properties, these molecules resemble minerals and might serve as the precursor to minerals of the hydrotalcite class where there is a layer of trivalent metals in a brucite-like structure. The molecular cores of these clusters are similar, indicating that they form in the same structural family, but they differ in detail. Clusters with eight, thirteen, fifteen, nineteen metals have been made (see Goodwin et al., 2004). TheAl15 cluster, for example, has the core structure: Al15 ðl3 -OHÞ6 ðl2 -OHÞ14 ðl3 -OÞ4 17þ . Purely hydrolytic versions of this class of molecules exist (e.g., Seichter et al.,

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Fig. 8. A wide series of oxyhydroxide clusters can be synthesized that are based upon the brucite lattice of edge-shared M(O)6 octahedra (see Heath and Powell, 1992; Powell et al., 1995; Schmitt et al., 2001, 2002 for greater detail). In this figure, two orientations the Fe19 molecule (Fe19 ¼ Fe19 ðheidiÞ10 ðl3 -OHÞ6 ðl2 -OHÞ8 ðl3 -OÞ6 þ (see Goodwin et al., 2000) where heidi is an aminocarboxylate ligand. The structure is shown in polyhedral representation and modified to emphasize the three-dimensional character of the molecule and the ligands have been removed for clarity.

1998; Casey et al., 2005; Casey, 2006) and, again, the clusters are broadly similar to the corresponding mineral. The Fe19 cluster shown in Fig. 8, for example, has a l-OH site density of 2.4 sites/nm2, which is smaller than the density on [0 2 1] goethite (7.5 sites/nm2) but similar enough to be useful for understanding Earth materials. The isolation and detailed study of the hydrolytic clusters, in which all ligands are derived from water, would help to elucidate fundamental physicochemical properties of such molecular clusters and minerals. An understanding of how such clusters are assembled would be of great use in the development of rational synthetic strategies. They also are interesting because of their similarity to the core of the iron storage protein Ferritin (Powell, 2004). In some instances unexpected but interesting products have been produced, for example, ½Fe8 ðl3 -OÞ2 ðl4 -OÞ2 ðl2-OCH2 t BuÞ2 ðl2O2 CC6 H5 Þ12 ðO2 CC6 H5 Þ2 ðHOCH2 t BuÞ2  is an octanuclear Fe(III) complex, which at the time of publication (Ammala et al., 2000) featured a new type of double butterfly iron-oxo core that was of interest to researchers developing molecular magnets or models for ferritin. These molecular magnets are suitable as experimental models for aqueous geochemists if their stability in water can be established. 4.3. Sol–gel synthesis of hydrolytic clusters The sol–gel process has been widely used in the formation of nanosized or nanostructured materials, such as aerogels, xerogels, nano- and micro-porous oxide gels, bulk materials and films of varying thickness. The process generally involves the hydrolysis of precursor solutions to generate small molecular clusters, which through condensation reactions assemble into sols consisting of nanosized particles. Further condensation reactions then lead to the formation of a gel which consists of a cross-linked three-dimensional structure

within which solvent is trapped. Classically the formation of gels through the sol-gel process involved inorganic reagents/salts and the manipulation of parameters such as temperature, pH, and reagent concentration. Particularly useful are metal-alkoxide reagents since the alkoxo group is readily replaced by a hydroxo group in the presence of water. For the alkoxides of more labile metal centers the reaction on exposure to moisture can be so rapid that control of water concentration becomes critical. To slow and control the reaction, precursors are modified to include ligands such as carboxylates and b-diketones that coordinate strongly to the metals and either inhibit or slow down the rate of hydrolysis. Many such clusters have been prepared using this approach in an effort to piece together the processes that are involved in sol and gel formation. Of special significance are the variety of ‘‘TiOx’’ clusters which are stabilized by carboxylate bridges (see below) and a detailed study on controlled Ce(IV)-alkoxide hydrolysis in the presence of acetylacetone which isolated some discrete molecular clusters and moreover established their effect on sol and gel properties. In the latter, an extensive study of the hydrolysis of the alkoxide in the presence of acetylacetone showed that increasing the ratio q = [acac]/[Ce] determines whether the hydrolysate appears as particles, a sol, a turbid gel, a clear gel or as the soluble molecular cluster having the stoichiometry: Ce6(l-O)4(l-OH)4(acac)12 (Ribot et al., 1991). In and Sanchez (2005) highlight the value in carrying out detailed kinetic studies on polymerizing metal-alkoxide solutions. These workers studied the effect of acetylacetone on the relationship between growth and cyclization–condensation reactions occurring in the very early stages of polymerization of zirconium(IV) alkoxide solutions. These investigations mirror efforts to understand the early polymerization of metal aqua ions by Spiccia, Marty and coworkers and more recently by Rustad and Casey (2006) in which the evolution of oligomeric species has been followed experimentally and modeled via molecular-dynamics simulations. The interest in how metal hydrolysis is affected by coordinating alkoxide and aquo ligands derives from the desire to prepare materials (either as films or as bulk materials) with controlled properties. There are many excellent examples of the application of sol–gel approaches in the development of nanostructured oxide surfaces that forms the basis of functional devices. The Graetzel solar cell (see O’Regan and Graetzel, 1991), a device that is attracting the attention of hundreds of research groups around the world, is a major success relies that is worth noting here. The breakthrough in the development of this device was the development of a method that was able to form reproducibly high surface areas titania films via the hydrolysis and hydrothermal treatment of titanium alkoxide precursor solutions that contain additives, such as acetic acid, as a hydrolysis controlling agents. Refinement of the hydrothermal methodology then led to films with improved properties (see Burnside et al., 1998; Scolan and Sanchez, 1998). As a consequence of the many potential applications of titania materials there has been much interest in the study

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of the hydrolysis and condensation reactions that lead to discrete titanium(IV) clusters, which are formed from metal alkoxides alone (for example a titanium-oxo-ethoxo cluster [Ti16O16(OEt)32], Fornasieri et al., 2005) or following addition of chelating agents, such as acetate and citrate ions (e.g., Papiernik et al., 1998; Ammala et al., 2003; Kemmitt et al., 2004; Piszczek et al., 2004; Fornasieri et al., 2005). The structural features of the metal oxide cores found within these molecular clusters contain detail that should be used to inform us in our search for a better understanding of mineral-formation processes and physical and chemical phenomena occurring on these surfaces, not only under ambient but also under extreme conditions. The clusters, of course, are not minerals but they are accessible to spectroscopies in ways that are impossible for mineral suspensions. We can use these cluster to directly test ideas about reactivity. These ideas include the coordination of ligands to metals and their potential labilizing effects on bond ruptures, the relative Brønsted acidities of oxygens and, perhaps, even reaction pathways. Clearly, this hydrolysis chemistry sheds light on the disequilibrium products formed as metals hydrolyze in the presence of ligating organic acids, as one might find in an soil that is rich in oxygen-bearing organic acids. Structures can be synthesized with varying reactivities and mobilities according to the degree of polymerization of the metal.

5. CONCLUSIONS For two decades, geochemists have increased their focus on molecular reactions, but have relied almost exclusively on computational models for complicated mineral structures. Many of the questions posed about molecular pathways can better answered if one is confident in the structure, such as could be achieved by comparing simulation and experiment on the same molecule. The key variables include the effect of different metals, the effect of proximity of reactive ligands and metals, and the Brønsted acidities of the bound waters, the equilibrium protonation/ deprotonation steps, the influence of internal hydrogen bonds on the kinetics and accessibility of water to the reactive site. Cooperation with inorganic chemists will allow Earth scientists to gain skills in making and studying molecular clusters, but will require us to abandon some parochial bias. We will need to work with nonaqueous solvents, exotic inert metals and protecting ligands, to construct a suitable robust model for a mineral. This cooperation would considerably broaden the field. Control of metal hydrolysis, and the construction of cluster-(hydr)oxide clusters, is interesting to many fields, including medicine and materials science. Similarly the functionalization of these clusters is key to device applications, such as nano-engineered photo-electrochemical devices, photosynthetic mimics, energy transducers and molecular wires. In geochemistry, we would call these functionalizations a type of ligand adsorption. What is limiting progress is the fact that so few current geochemists are trained in the skills of synthesis to make molecule models.

5601 ACKNOWLEDGMENTS

The authors particularly thank Jim Rustad for discussions. Support for this research was from the U.S. DOE via Grant DEFG03-02ER15325, from American Chemical Society (PRF 40412-AC2) and from the National Science Foundation (EAR 0515600).

REFERENCES Alam T. M., Nyman M., Cherry B. R., Segall J. M. and Lybarger L. E. (2004) Multinuclear NMR investigations of the oxygen, water, and hydroxyl environments in sodium hexaniobate. J. Amer. Chem. Soc. 126(17), 5610–5620. Amirbahman A., Gfeller M. and Furrer G. (2000) Kinetics and mechanism of ligand-promoted decomposition of the Keggin Al-13 polymer. Geochim. Cosmochim. Acta 64(5), 911–919. Ammala P., Cashion J. D., Kepert C. M., Moubaraki B., Murray K. S., Spiccia L. and West B. O. (2000) An octanuclear Fe(III)compound featuring a new type of double butterfly ironoxo core. Angew. Chem. 39(9), 1688–1690. Ammala P. S., Batten S. T. R., Kepert C. M., Spiccia L., van Den Bergen A. M. and West B. O. (2003) The reaction of iron carboxylates with titanium alkoxides. Isolation and structural characterisation of [Ti6(l3-O)6(O2CPh)6(OCH2C(CH3)3)6]. Inorg. Chim. Acta 353, 75–81. Ardon M. and Bino A. (1987) A new aspect of hydrolysis of metal ions: the hydrogen-oxide bridging ligand ðH3 O2  Þ. Struct. Bond. (Berlin, Germany) 65, 1–28 (Solid State Chem.). Balogh E., Anderson T. M., Rustad J. R., Nyman M. and Casey W. H. (2007a) Rates of oxygen-isotope exchange between sites in the [HxTa6O19]8x(aq) Lindqvist Ion and aqueous solutions– comparisons to [HxNb6O19]8x(aq). Inorg. Chem. 46, 7032–7039. Balogh E., Todea A. M., Muelle A. and Casey W. H. (2007b) Rates of ligand exchange bewtween >FeIII-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution. Inorg. Chem. 46, 7087–7092. Bickmore B. R., Rosso K. M., Tadanier C. J., Bylaska E. J. and Doud D. (2006) Bond-valence methods for pKa prediction. II. Bond-valence, electrostatic, molecular geometry, and solvation effects. Geochim. Cosmochim. Acta 70(16), 4057–4071. Black J. R., Nyman M. and Casey W. H. (2006) Rates of oxygen exchange between the ½Hx Nb6 O19 8x ðaqÞ Lindqvist ion and aqueous solutions. J. Amer. Chem. Soc. 128, 14712–14720. Bradley S. M., Kydd R. A. and Howe R. F. (1993) The structure of aluminum gels formed through the base hydrolysis of Al3+ aqueous solutions. J. Colloid Interf. Sci. 159(2), 405–412. Burnside S. D., Shklover V., Barbe C., Comte P., Arendse F., Brooks K. and Gratzel M. (1998) Self-organization of TiO2 nanoparticles in thin films. Chem. Mater. 10(9), 2419–2425. Casey W. H. (2006) Large aqueous aluminum-hydroxide molecules. Chem. Rev. 106(1), 1–16. Casey W. H. and Rustad J. R. (2007) Reaction dynamics, molecular clusters, and aqueous geochemistry. Annu. Rev. Earth. Pl. Sci. 35, 21–46. Casey W. H., Olmstead M. M., Phillips B. L. and Phillips L. (2005) A new aluminum hydroxide octamer, [Al8(OH)14(H2O)18] (SO4)5.16H2O. Inorg. Chem. 44(14), 4888–4890. Crimp S. J. and Spiccia L. (1996) Kinetic and thermodynamic studies of intramoleculear rearrangement and cleavage of the heterobinucear aqua ion, [(H2O)4Rh(l-OH)2Cr(OH2)4]4+. J. Chem. Soc. Dalton Trans. 1996(6), 1051–1057. Crimp S. J., Fallon G. D. and Spiccia L. (1992) Synthesis and X-ray structure of a chromium(III)–rhodium(III) heterometallic

5602

L. Spiccia, W.H. Casey / Geochimica et Cosmochimica Acta 71 (2007) 5590–5604

hydrolytic dimer: [(H2O)4Rh(l-OH)2Cr(OH2)4](Me3C6H2SO3)4Æ4H2O. J. Chem. Soc. Chem. Commun. 1992(2), 197–198. Crimp S. J., Spiccia L., Krouse H. R. and Swaddle T. W. (1994) Early stages of the hydrolysis of chromium(III) in aqueous solution. 9. Kinetics of water exchange on the hydrolytic dimer. Inorg. Chem. 33(3), 465–470. Dabbabi M. and Boyer M. (1976) Syntheses et pproprietes d’hexa niobo(V)–tungstates(VI). J. Inorg. Nucl. Chem. 18, 1011–1014. Dasgupta J., van Willigen R. T. and Dismukes G. C. (2004) Consequences of structural and biophysical studies for the molecular mechanism of photosynthetic oxygen evolution: functional roles for calcium and bicarbonate. Phys. Chem. Chem. Phys. 6(20), 4793–4802. Drijaca A. and Spiccia L. (1996a) Early stages of the hydrolysis of chromium(III) in aqueous solution. XI. Kinetics of formation of hexamer from trimer and tetramer from monomer and trimer. Polyhedron 15(17), 2875–2886. Drljaca A. and Spiccia L. (1996b) Early stages of the hydrolysis of chromium(III) in aqueous solution. XII. Kinetics of cleavage of the trimer and tetramer in acidic solution. Polyhedron 15(24), 4373–4385. Drljaca A., Hockless D. C. R., Moubaraki B., Murray K. S. and Spiccia L. (1997) A supramolecular approach to the crystallization of polynuclear aqua ions: structure and magnetism of an 18-Crown-6 adduct of bis(l-hydroxo)octaqua dichromium(III) mesitylene-2-sulfonate trihydrate. Inorg. Chem. 36(10), 1988–1989. Drljaca A., Hardie M. J. and Raston C. L. (1999a) Selective isolation of Keggin ions using self-assembled superanion capsules. J. Chem. Soc. Dalton Trans. 1999(20), 3639–3642. Drljaca A., Hardie M. J., Raston C. L. and Spiccia L. (1999b) Self-assembled superanions: ionic capsules stabilized by polynuclear chromium(III) aqua cations. Chem. Eur. J. 5(8), 2295–2299. Drljaca A., Hardie M. J., Ness T. J. and Raston C. L. (2000) Rhodium(III) aqua ion salts of ambivalent self assembled superanion capsules. Eur. J. Inorg. Chem. 10, 2221–2229. Erras-Hanauer H., Clark T. and van Eldik R. (2003) Molecular orbital and DFT studies on water exchange mechanisms of metal ions. Coord. Chem. Rev. 238–239, 233–253. Etxebarria N., Fernandez L. A. and Madariaga J. M. (1994) On the hydrolysis of niobium(V) and tantalum(V) in 3 mol dm3 KCl at 25 C. Part 1. Construction of a thermodynamic model for Nb(V). J. Chem. Soc. Dalton Trans. 1994, 3055–3059. Ferreira K. N., Iverson T. M., Maghlaoui K., Barber J. and Iwata S. (2004) Architecture of the photosynthetic oxygen-evolving center. Science 303(5665), 1831–1838. Fornasieri G., Rozes L., Le Calve S., Alonso B., Massiot D., Rager M. N., Evain M., Boubekeur K. and Sanchez C. (2005) Reactivity of titanium oxo ethoxo cluster [Ti16O16(OEt)32]. Versatile precursor of nanobuilding block-based Hybrid Materials. J. Am. Chem. Soc. 127(13), 4869–4878. Furrer G., Ludwig C. and Schindler P. W. (1992) On the chemistry of the Keggin Al13 polymer. 1. Acid–base properties. J. Colloid Interf. Sci. 149(1), 56–67. Furrer G., Gfeller M. and Wehrli B. (1999) On the chemistry of the Keggin Al-13 polymer: kinetics of proton-promoted decomposition. Geochim. Cosmochim. Acta 63(19–20), 3069–3076. Furrer G., Phillips B. L., Ulrich K. U., Pothig R. and Casey W. H. (2002) The origin of aluminum flocs in polluted streams. Science 297(5590), 2245–2247. Geissler P. L., Dellago C., Chandler D., Hutter J. and Parrinello M. (2001) Autoionization in liquid water. Science 291(5511), 2121–2124. Gibbs G. V. (1982) Molecules as models for bonding in silicates. Amer. Mineral. 67(5–6), 421–450.

Giovanoli R., Stadelmann W. and Feitknecht W. (1973a) Crystalline chromium(III) hydroxide. I. Helv. Chim. Acta 56(3), 839–847. Giovanoli R., Stadelmann W. and Gamsjaeger H. (1973b) Chromium(III)hydroxide hydrate. New hydroxide structure type with crosslinked hydrogen bonding. Chimia 27(3), 170–171. Goodwin J. C., Sessoli R., Gatteschi D., Wernsdorfer W., Powell A. K. and Heath S. L. (2000) Towards nanostructured arrays of single molecule magnets: new Fe19 oxyhydroxide clusters displaying high ground state spins and hysteresis. J. Chem. Soc. Dalton Trans. 2000(12), 1835–1840. Goodwin J. C., Teat S. J. and Heath S. L. (2004) Gallium clusters: how do clusters grow? The synthesis and structure of polynuclear hydroxide gallium(III) clusters. Angew. Chem. 43(31), 4037–4041. Grant M. and Jordan R. B. (1981) Kinetics of solvent water exchange on iron(III). Inorg. Chem. 20, 55–60. Hardie M. J. and Raston C. L. (2000) Russian doll assembled superanion capsule–metal ion complexes: combinatorial supramolecular chemistry in aqueous media. J. Chem. Soc. Dalton Trans. 2000(15), 2483–2492. Heath S. L. and Powell A. K. (1992) Effect of iron hydroxide structural units through ‘‘heidi’’ ligands. Two new oxoiron hydroxo clusters with 19 or 17 iron atoms. Angew. Chem. 104(2), 191–192. Helm L. and Merbach A. E. (2005) Inorganic and bioinorganic solvent exchange mechanisms. Chem. Rev. 105(6), 1923–1959. Helm L., Nicolle G. M. and Merbach A. E. (2005) Water and proton exchange processes on metal ions. Adv. Inorg. Chem. 57, 327–379. Henderson M. A. (2002) The interaction of water with solid surfaces: fundamental aspects revisited. Surf. Sci. Rep. 46(1–8), 1–308. Hiemstra T. and van Riemsdijk W. H. (1990) Multiple activated complex dissolution of metal (hydr)oxides: a thermodynamic approach applied to quartz. J. Colloid Interf. Sci. 136(1), 132–150. Hiemstra T. and van Riemsdijk W. H. (1991) Physical chemical interpretation of primary charging behavior of metal (hydr) oxides. Colloid. Surf. 59, 7–25. Hiemstra T. and van Riemsdijk W. H. (1996) A surface structural approach to ion adsorption: the charge distribution (CD) model. J. Colloid Interf. Sci. 179(2), 488–508. Hiemstra T. and van Riemsdijk W. H. (1999) Surface structural ion adsorption modeling of competitive binding of oxyanions by metal (hydr)oxides. J. Colloid Interf. Sci. 210(1), 182–193. Hiemstra T., De Wit J. C. M. and van Riemsdijk W. H. (1989a) Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: a new approach. II. Application to various important (hydr)oxides. J. Colloid Interf. Sci. 133(1), 105–117. Hiemstra T., van Riemsdijk W. H. and Bolt G. H. (1989b) Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: a new approach. I. Model description and evaluation of intrinsic reaction constants. J. Colloid Interf. Sci. 133(1), 91–104. Hiemstra T., Venema P. and van Riemsdijk W. H. (1996) Intrinsic proton affinity of reactive surface groups of metal (hydr)oxides: the bond valence principle. J. Colloid Interf. Sci. 184(2), 680–692. Hiemstra T., Yong H. and van Riemsdijk W. H. (1999) Interfacial charging phenomena of aluminum (hydr)oxides. Langmuir 15(18), 5942–5955. In M. and Sanchez C. (2005) Growth versus cyclization in the early stages of the polycondensation of metal alkoxides. J. Phys. Chem. B 109(50), 23870–23878.

Minerals into molecules   Johansson G. (1962) The crystal structures of [Al2(OH)2(H2O)8] (SO4)2 2H2O and [Al2(OH)2(H2O)8](SeO4)22H2O. Acta Chem. Scand. 20, 321–342. Jolivet J.-P., Henry M. and Livage J. (2000) Metal oxide chemistry and synthesis from solution to solid state. John Wiley, New York. Junk P. C., Mccool B. J., Moubaraki B., Murray K. S., Spiccia L., Cashion J. D. and Steed J. W. (2002) Utilization of crown ethers to stabilize the dinuclear l-oxo bridged iron(III) aqua ion, [(H2O)5Fe(l-O)Fe(OH2)5]4+. J. Chem. Soc. Dalton Trans. 2002(6), 1024–1029. Kemmitt T., Al-Salim N. I., Gainsford G. J., Bubendorfer A. and Waterland M. (2004) Unprecedented oxo-titanium citrate complex precipitated from aqueous citrate solutions, exhibiting a novel bilayered Ti8O10 structural core. Inorg. Chem. 43(20), 6300–6306. King P., Wernsdorfer W., Abboud K. A. and Christou G. (2005) Single-molecule magnets: a reductive aggregation route to new types of Mn12 complexes. Inorg. Chem. 44(24), 8659–8669. Klemperer W. G. and Shum W. (1977) Charge distribution in large polyoxoanions: determination of protonation sites in vanadate ðV10 O28 6 Þ by oxygen-17 nuclear magnetic resonance. J. Amer. Chem. Soc. 99(10), 3544–3545. Liu T., Imber B., Diemann E., Liu G., Cokleski K., Li H., Chen Z. and Mueller A. (2006) Deprotonations and charges of welldefined (Mo72Fe30) nanoacids simply stepwise tuned by pH allow control/variation of related self-assembly processes. J. Amer. Chem. Soc. 128(49), 15914–15920. Matsumoto F., Ohki Y. I., Suzuki Y. and Ouchi A. (1989) The crystal and molecular structure of di-l-hydroxobis ((H2O)5Sc](OH)2[Sc(H2O)5]](C6H5 SO3)4Æ4H2O: a dimeric complex in pentagonal–bipyramidal heptacoordination). Bull. Chem. Soc. Jpn. 62(6), 2081–2083. Michel F. M., Ehm L., Antao S. M., Lee P. L., Chupas P. J., Liu G., Strongin D. R., Schoonen M. A. A., Phillips B. L. and Parise J. B. (2007) The structure of ferrihydrite, a nanocrystalline material. Science 316, 1726–1729. Nyman M., Alam T. M., Bonhomme F., Rodriquez M. A., Frazer C. S. and Welk M. E. (2006) Solid-state structures and solution behavior of alkali salts of the [Nb6O19] Lindqvist ion. Cluster Sci. 17(2), 197–204. O’Regan B. and Graetzel M. (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films. Nature 353(6346), 737–740. Ozeki T., Yamase T., Naruke H. and Sasaki Y. (1994) X-ray structural characterization of the protonation sites in the dihydrogenhexaniobate anion. Bull. Chem. Soc. Jpn. 67, 3249–3253. Papiernik R., Hubert-Pfalzgraf L. G., Vaissermann J. and Goncalves M. C. H.-B. (1998) Synthesis and characterization of new titanium hexanuclear oxo carboxylato alkoxides. Molecular structure of [Ti6(l3-O)6(l-O2CC6H4OPh)6(OEt)6]. J. Chem. Soc. Dalton Trans. 14, 2285–2288. Piszczek P., Grodzicki A., Richert M. and Wojtczak A. (2004) Structural and thermal stability studies of Ti(IV) hexanuclear oxo trimethylacetato isopropoxide complex. Inorg. Chim. Acta 357(9), 2769–2775. Pope M. T. (2003) Introduction to polyoxometalate chemistry. NATO Science Series, II: Mathematics, Physics and Chemistry 98, 3–31 (Polyoxometalate Molecular Science). Pope M. T. (2004) Polyoxo anions: synthesis and structure. Compr. Coord. Chem. II 4, 635–678. Powell A. K. (2004) Ferritins. Compr. Coord. Chem. II 8, 169–194. Powell A. K., Heath S. L., Gatteschi D., Pardi L., Sessoli R., Spina G., Del Giallo F. and Pieralli F. (1995) Synthesis, structures, and magnetic properties of Fe2, Fe17, and Fe19 oxo-bridged

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iron clusters: the stabilization of high ground state spins by cluster aggregates. J. Amer. Chem. Soc. 117(9), 2491–2502. Powell G. W., Lancashire H. N., Brechin E. K., Collison D., Heath S. L., Mallah T. and Wernsdorfer W. (2004) Building molecular minerals: all ferric pieces of molecular magnetite. Angew. Chem. 43(43), 5772–5775. Ribot F., Toledano P. and Sanchez C. (1991) Hydrolysis–condensation process of b-diketonates-modified cerium(IV) isopropoxide. Chem. Mater 3, 759. Richens D. T. (1997) The Chemistry of Aqua Ions. John Wiley, New York. Richens D. T. (2005) Ligand substitution reactions at inorganic centers. Chem. Rev. 105(6), 1961–2002. Rotzinger F. P. (2005a) Performance of molecular orbital methods and density functional theory in the computation of geometries and energies of metal aqua ions. J. Phys. Chem. B 109(4), 1510–1527. Rotzinger F. P. (2005b) Treatment of substitution and rearrangement mechanisms of transition metal complexes with quantum chemical methods. Chem. Rev. 105(6), 2003–2037. Rustad J. R. (2001) Molecular models of surface relaxation, hydroxylation, and surface charging at oxide–water interfaces. Rev. Geochem. Mineral. 42, 169–197. Rustad J. R. (2005) Molecular dynamics simulation of the titration of polyoxocations in aqueous solution. Geochim. Cosmochim. Acta 69(18), 4397–4410. Rustad J. R. and Casey W. H. (2006) A molecular dynamics investigation of hydrolytic polymerization in a metal-hydroxide gel. J. Phys. Chem. B 110(14), 7107–7112. Rustad J. R. and Felmy A. R. (2005) The influence of edge sites on the development of surface charge on goethite nanoparticles: a molecular dynamics investigation. Geochim. Cosmochim. Acta 69(6), 1405–1411. Rustad J. R., Felmy A. R. and Hay B. P. (1996a) Molecular statics calculations for iron oxide and oxyhydroxide minerals: toward a flexible model of the reactive mineral–water interface. Geochim. Cosmochim. Acta 60(9), 1553–1562. Rustad J. R., Felmy A. R. and Hay B. P. (1996b) Molecular statics calculations of proton binding to goethite surfaces: a new approach to estimation of stability constants for multisite surface complexation models. Geochim. Cosmochim. Acta 60(9), 1563–1576. Rustad J. R., Dixon D. A., Rosso K. M. and Felmy A. R. (1999) Trivalent ion hydrolysis reactions: a linear free-energy relationship based on density functional electronic structure calculations. J. Amer. Chem. Soc. 121(13), 3234–3235. Rustad J. R., Dixon D. A. and Felmy A. R. (2000a) Intrinsic acidity of aluminum, chromium (III) and iron (III) m3-hydroxo functional groups from ab initio electronic structure calculations. Geochim. Cosmochim. Acta 64(10), 1675–1680. Rustad J. R., Dixon D. A., Kubicki J. D. and Felmy A. R. (2000b) Gas-phase acidities of tetrahedral oxyacids from ab initio electronic structure theory. J. Phys. Chem. A 104(17), 4051–4057. Rustad J. R., Loring J. S. and Casey W. H. (2004) Oxygenexchange pathways in aluminum polyoxocations. Geochim. Cosmochim. Acta. 68, 3011–3017. Sauer K. and Yachandra V. K. (2002) A possible evolutionary origin for the Mn4 cluster of the photosynthetic water oxidation complex from natural MnO2 precipitates in the early ocean. Proc. Natl. Acad. Sci. USA 99(13), 8631–8636. Schmitt W., Murugesu M., Goodwin J. C., Hill J. P., Mandel A., Bhalla R., Anson C. E., Heath S. L. and Powell A. K. (2001) Strategies for producing cluster-based magnetic arrays. Polyhedron 20(11–14), 1687–1697. Schmitt W., Jordan P. A., Henderson R. K., Moore G. R., Anson C. E. and Powell A. K. (2002) Synthesis, structures and

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L. Spiccia, W.H. Casey / Geochimica et Cosmochimica Acta 71 (2007) 5590–5604

properties of hydrolytic Al(III) aggregates and Fe(III) analogues formed with iminodiacetate-based chelating ligands. Coord. Chem. Rev. 228(2), 115–126. ¨ ber die Ursache der Farbenvera¨nderung, Scho¨nbein C. F. (1838) U welche manche Ko¨rper unter dem Einflusse der Wa¨rme erleiden. Ann. Phys. Chem. 45, 263–281. Scolan E. and Sanchez C. (1998) Synthesis and characterization of surface-protected nanocrystalline titania particles. Chem. Mater. 10(10), 3217–3223. Seichter W., Mogel H. J., Brand P. and Salah D. (1998) Crystal structure and formation of the aluminium hydroxide chloride [Al13(OH)24 (H2O)24]Cl1513H2O. Eur. J. Inorg. Chem. 6, 795–797. Spiccia L. (2004) Homopolynuclear and heteropolynuclear Rh(III) aqua ions—a review. Inorg. Chim. Acta 357(10), 2799–2817. Spiccia L. and Marty W. (1986) The fate of active chromium hydroxide, Cr(OH)3Æ3H2O, in aqueous suspension. Study of the chemical changes involved in its aging. Inorg. Chem. 25(3), 266–271. Spiccia L., Stoeckli-Evans H., Marty W. and Giovanoli R. (1987) A new active chromium(III) hydroxide: Cr2(l- OH)2 (OH)4(OH2)4. 2H2O. Characterization and use in the preparation of salts of the ðH2 OÞ4 Crðl-OHÞ2 CrðOH2 Þ4 4þ ion. Crystal structure of [(H2O)4Cr(l-OH)2Cr(OH2)4][(H3C)3C6H2SO3]4Æ4H2O. Inorg. Chem. 26(4), 474–482. Spiccia L., Marty W. and Giovanoli R. (1988) Hydrolytic trimer of chromium(III). Synthesis through chromite cleavage and use in the preparation of the active trimer hydroxide. Inorg. Chem. 27(15), 2660–2666. Spiccia L., Aramini J. M., Crimp S. J., Drljaca A., Lawrenz E. T., Tedesco V. and Vogel H. J. (1997) Hydrolytic polymerization of

rhodium(III). Characterization of various forms of a trinuclear aqua ion. J. Chem. Soc. Dalton Trans. 1997(23), 4603–4610. Springborg J. (1988) Hydroxo-bridged complexes of chromium(III), cobalt(III), rhodium(III), and iridium(III). Adv. Inorg. Chem. 32, 55–169. Stuenzi H., Spiccia L., Rotzinger F. P. and Marty W. (1989) Early stages of the hydrolysis of chromium(III) in aqueous solution. 4. The stability constants of the hydrolytic dimer, trimer, and tetramer at 25 C and I = 1.0 M. Inorg. Chem. 28(1), 66–71. Swaddle T. W. and Merbach A. E. (1981) High-pressure O-17 Fourier-transform nuclear magnetic-resonance spectroscopy— mechanism of water exchange on iron(III) in acidic aqueous solution. Inorg. Chem. 20(12), 4212–4216. Trainor T. P., Eng P. J., Brown G. E., Robinson I. K. and De Santis M. (2002) Crystal truncation rod diffraction study of the a-Al2O3 (1 1 0 2) surface. Surf. Sci. 496(3), 238–250. Wang J., Rustad J. R. and Casey W. H. (2007) Calculation of water-exchange rates on aqueous polynuclear clusters and at oxide-water interfaces. Inorg. Chem. 46, 2962–2964. Wehrli B., Wieland E. and Furrer G. (1990) Chemical mechanisms in the dissolution kinetics of minerals—the aspect of activesites. Aquat. Sci. 52(1), 3–31. Zhang X. V., Martin S. T., Friend C. M., Schoonen M. A. A. and Holland H. D. (2004) Mineral-assisted pathways in prebiotic synthesis: photoelectrochemical reduction of carbon(+IV) by manganese sulfide. J. Amer. Chem. Soc. 126(36), 11247–11253. Associate editor: Dimitri Sverjensky