Two Structures Toward Understanding Evolution from Surfactant

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Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates May Nyman,* Mark A. Rodriguez, Travis M. Anderson, and David Ingersoll

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3590–3597

Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185 ReceiVed March 23, 2009; ReVised Manuscript ReceiVed May 27, 2009

ABSTRACT: Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant: POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic #4, P21 a ) 12.636(2) Å, b ) 20.577(4) Å, c ) 22.364(4) Å, β ) 93.394(4)°) holds true to the preference of 4:1 surfactant:POM ratio in well-ordered crystalline phases, whereas [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (triclinic No. 2, P1j, a ) 12.513(7) Å, b ) 23.37(1) Å, c ) 24.44(1) Å, R ) 93.418(8)°, β ) 92.046(8)°, γ ) 99.113(7)°) provides the first example of a surfactant-POM phase with a surfactant:POM ratio >4. This structure provides a glimpse of the structural evolution from ordered lamellar POM-surfactant phases to more disordered phases such as the SECs. Introduction Hybrid organic-inorganic materials composed of cationic surfactants and anionic polyoxometalates (POMs) have attracted interest for their potential use as electrochromic, photochromic, or magnetic thin film devices or coatings,1-11 catalytic materials,12-14 pH probes,15 as well as an avenue to manipulate hydrophilic POMs in hydrophobic environments16 and investigate self-assembly.17-22 The physical characteristics of POMsurfactant materials vary greatly as a function of charge and charge-density of the POM cluster, surfactant tail length or number of tails, and the size, shape, and charge of the surfactant head. Structures can vary from well-ordered layers in singlecrystals or Langmuir-Blodgett films in the case of low-charge clusters23-26 to surfactant-encapsulated clusters (SECs) in the case of larger clusters that possess a high charge.3-7,14,27 Detailed structural information concerning the relative arrangement of clusters and surfactant molecules has been obtained from a few single-crystal X-ray reports including studies on [H2V10O28]4-[DTA]4,24,25 [Mo6O19]2-[DODA]2,23 and [SiMo12O40]4-[CTA]426 (DTA ) dodecyltrimethylammonium, DODA ) dioctadecyldimethylammonium, CTA ) cetyltrimethylammonium). These structures all feature hexagonal or distorted hexagonal packing of the POM clusters in layers that alternate with bilayers of surfactant tails, tilted relative to the normal of the POM plane. The surfactant ammonium heads are located on either side of the POM layer, approximately at the junction of three POM clusters within the hexagonal array. We noted in a preceding publication26 that POM-surfactant phases generally form X-ray quality single crystals only if the ratio of surfactant tail:cluster is 4:1, which translates to a cluster charge of -4 with single-tail surfactants or a cluster charge of -2 with double-tail surfactants. The square-grid or hexagonal packing of the POM clusters and surfactant heads is ideal for a 4:1 ratio of surfactant tail to cluster, as illustrated in the * Corresponding author. E-mail: [email protected].

schematic of Figure 1, and described in detail within the figure caption. This observation also held true for related phases featuring surfactants plus metal-chalcogenido clusters such as [Re6Q8(CN)6]4- (Q ) Se,Te),28 [Ge4S10]4-29-31 or smaller oxoanions such as [Cr2O7]2-.32 In continuing this prior study aimed at obtaining structural information of POM-surfactant phases in which the POM charge exceeds 4-, we have (1) electrochemically reduced solutions of silicomolybdic acid to obtain [SiMo12O40]6- (2-electron reduced) or [SiMo12O40]8- (4electron reduced), (2) precipitated the reduced silicomolybdate clusters with the CTA-surfactant, and (3) recrystallized the precipitated product. Our results largely showed that crystallization of 4:1 phases is still favored with [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic No. 4, P21 a ) 12.636(2) Å, b ) 20.577(4) Å, c ) 22.364(4) Å, β ) 93.394(4)°) as the dominant product from both the 2-electron reduced silicomolybdate and the 4-electron reduced silicomolybdate. However, we have also obtained a partially characterized phase; [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (triclinic No. 2, P1j, a ) 12.513(7) Å, b ) 23.37(1) Å, c ) 24.44(1) Å, R ) 93.418(8)°, β ) 92.046(8)°, γ ) 99.113(7)°) from the 4-electron reduced silicomolybdic acid that clearly has five CTA cations per cluster. This structure thus provides the first glimpse of the evolution of the arrangement of POM-surfactant arrays as the surfactant tail:cluster ratio increases beyond 4. Experimental Section Precipitation and Recrystallization of [SiMo12O40]-CTA Phases. In general, the precipitation and recrystallization of [SiMo12O40]-CTA (CTA ) cetyltrimethylammonium) phases is carried out as follows, using the isolation of (1) as an example. We start with a solution of 2-electron or 4-electron reduced aqueous silicomolybdate solution, prepared as described below. The 2-electron reduced silicomolybdate solution was 0.04 molar in [SiMo12O40]6- and 0.1 molar H2SO4. Twenty milliliters of this solution (containing 0.85 mmol of [SiMo12O40]6-) is stirred at room temperature, while adding via a pipet 6 equiv. of cetyltrimethylammonium chloride (CTACl) as a 2.5 wt % solution (5.1 mmol, 1.6 g of CTACl as 64 g of aqueous solution). The deep blue

10.1021/cg9003356 CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

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Figure 1. Illustration of the two POM-surfactant arrangements with a 4:1 ratio of surfactant tail to cluster. (A) Example of two-tail surfactant with -2 POM cluster.21 The clusters are arranged in a square-grid, each cluster surrounded by 8 other clusters, or four junctions of four clusters. At each junction of four clusters, there are two surfactant heads with +1 charge eachsdark blue is above the cluster plane, light blue is below the cluster plane. Total surfactants per cluster: 4junctions × 2surfactants per junction/4surrounding clusters ) 2. Side view: two rows of surfactant heads between two rows of clusters, one row on each side of the plane. One tail is bent so each tail extends down from apposing sides of the cluster row. (B) Example of single-tail surfactant with -4 POM cluster.24 The clusters are hexagonally arranged, each cluster is surround by 6 other clusters, or six junctions of three clusters. At each junction of three clusters, there are two surfactant heads with +1 charge eachsdark blue is above the cluster plane, light blue is below the cluster plane. Total surfactants per cluster: 6junctions × 2surfactants per junction /3surrounding clusters ) 4. Side view: four rows of surfactant heads between two rows of clusters, two rows on each side of the plane. Like the example in A, there is one surfactant tail extending down from each side of the cluster row. liquid became colorless as a blue precipitate formed. This precipitate was isolated by pressure filtration. We dissolved 0.3 g of this blue solid in 45 mL of acetonitrile at room temperature, and this acetonitrile solution was placed in the freezer. Over several days, platey blue crystals of 1 precipitated, and ∼0.27 g (yield ∼90%) were collected. Elemental analyses were carried out by Galbraith Laboratories, Inc. (Knoxville, TN). Composition of [H2SiMo12O40][CH3CN]2[CTA]4 (1) by weight percent, experimental (calculated): 0.94% Si (0.92%), 37.2% Mo (37.9%), 30.6% C (31.6%), 5.8% H (5.8%), 2.7% N (2.8%). In interest of ionic-liquid applications, we investigated the melting temperature of 1, but up to 220 °C, no melting was observed. A small amount of well-formed crystals of 2 were obtained from a CTA-precipitate of the 4-electron reduced silicomolybdate solution. Two-Electron and Four-Electron Reduction of [SiMo12O40]4-. Bulk electrolysis was performed on a 34 mM solution of Na4[SiMo12O40] (purchased from Pfaltz and Bauer Inc.) in aqueous 0.1 M H2SO4 in a three-electrode cell containing a high-surface-area glassy carbon working electrode. A Pt mesh in aqueous 0.1 M H2SO4 was used as the counter electrode, and the reference electrode was Ag/AgCl in saturated KCl. The counter electrode was located in a separate compartment and was isolated from the working electrode compartment by a fritted glass disk. This compartment was filled with the electrolyte solution, 0.1 M H2SO4. The solution was stirred and deaerated with Ar during the course of the reaction. A potential of 300 mV and 140 mV was applied for the two- and four-electron reduction, respectively, until the initial-to-final current ratio was 1% (see the cyclic voltammogram in the Supporting Information). These potentials are well beyond the peak potentials and sufficiently more positive of the next reduction process. Single-Crystal X-ray Diffraction of 1 and 2. Parallelepipedic crystals of 1 and 2 were mounted with fluorolube on a glass fiber and

transferred in the cold nitrogen stream of a Bruker APEX CCD diffractometer. Data were collected at 173 K. The diffractometer employed an incident-beam graphite monochromator for generation of Mo KR radiation (λ ) 0.71073 Å), and were processed using SAINT+ (version 6.02) and correction for absorption was performed using SADABS v 2.03 within the SAINT+ package. The structure was solved by direct methods (program SHELXTL v. 6.10) and refined by full matrix least-squares (program XSHELL 4.01). Because of the disordering of the CTA molecules, present as solvent cocrystallized with the silicomolybdate, C and N atoms of the CTA were refined as isotropic; hydrogen atoms were not included on the CTA molecules but were included in the final composition to accurately represent the true chemical formula for each compound. In addition, several C-C bond restraints were employed to improve the stability and realistic connectivity of the CTA molecules. Because the purpose of the single crystal analysis was to determine the relative arrangement of surfactant cation heads and silicomolybdate anions, it was deemed useful to refine the lighter elements as isotropic to reduce the number of overall parameters. Hence, the oxygen atoms were also refined as isotropic. These restraints resulted in the improved overall appearance and bond length values for the overall structures. Relevant parameters for the data collection and refinement are given in Table 1. Electrochemical Measurements of 1. Electrochemical data were collected using a BAS100B potentiostat in a three-electrode cell. The working electrode was a freshly polished glassy carbon electrode having a diameter of 3 mm, a Pt coil was used as the counter electrode, and the reference electrode was Ag/AgCl in saturated KCl. The solution was deoxygenated by bubbling Ar for 10 min prior to making any measurements and then blanketed with Ar. The supporting electrolyte was a 90:10 mixture of acetonitrile and aqueous 1 M H2SO4 providing

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Table 1. Summary of Crystallographic Information for 1 and 2

compound formula empirical formula fw space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z temperature (K) density (calcd) (g/cm3) λ (Å) µ (cm-1) 2θ min, max (deg) final R1 [I > 2σ(I)] final wR2 [I > 2σ(I)] GOF

1

2

[SiMo12O40H2][CH3CN]2[C16H33N(CH3)3]4 C80H178N6Mo12O40Si 3043.7 monoclinic P21 (No. 4) 12.636(2) 20.577(4) 22.364(4)

[SiMo12O40Hx][C16H33N(CH3)3]5[CH3CN]4 C103H222N9Mo12O40Si 3406.3 triclinic P1j (No. 2) 12.513(7) 23.372(13) 24.435(14) 93.418(8) 92.046(8) 99.113(7) 7036(7) 6 153 (2) 1.608 0.71073 1.112 2.36, 40.00 0.1241 0.2856 1.017

93.394(4) 5805(2) 2 173 (2) 1.741 0.71073 1.335 2.70, 56.26 0.0681 0.1430 1.042

a final concentration of 0.1 M H2SO4. Bulk electrolysis was performed on a 0.15 mM solution of (1) as described above with a potential of 550 mV.

Results and Discussion Crystal Growth of Reduced Silicomolybdate-CTA Phases. Recrystallization of phase pure products from the CTAprecipitates of [SiMo12O40]6- and [SiMo12O40]8- is challenged by the potential of varied oxidation state of the clusters through either reoxidation (esp. in the case of [SiMo12O40]8-) or protonation of the silicomolybdate Keggin ions. Furthermore, the lability of reduced silicomolybdates may result in reassembly of clusters into different geometric and compositional forms. Crystallization of 1 from both the 2-electron reduced silicomolybdic acid solution as well as the 4-electron reduced silicomolybdic acid solution was the most common and reproducible result. 2 was only obtained once from the 4-electron reduced silicomolybdate solution, and only in very small yield, which frustrated bulk characterization. Description of [H2SiMo12O40][CH3CN]2[CTA]4 Structure 1. By both X-ray crystallography and compositional analysis, 1 has a 4:1 ratio of CTA:[SiMo12O40]. However, to chargebalance a reduced silicomolybdate Keggin ion, we expect more cations than four per cluster. Both electrochemical characterization and the blue hue of the POM-surfactant phase indicate a two-electron reduced [SiMo12O40]6- is the predominant Keggin ion present, rather than the fully oxidized silicomolybdate. Bond valence sum (BVS) calculations reveal that the O6 and O34 oxygens are very likely protonated (see Table 2). Although all other oxygens within the Keggin cluster have BVS values ranging from 2.0 to 2.1, these have values of 1.31 for O6 and 1.35 for O34. These are the only bridging (Ob-Mo2; b ) bridging) oxygen atoms within the cluster that have both Mo-Ob bond lengths >2.0 Å (see Table 2). The arrangement of the Keggin ions and the CTA molecules is very similar to that observed in the fully oxidized phase, [SiMo12O40][CTA]4 (3), illustrated in Figure 1.26 Within the (001) plane, the Keggin ions are arranged in a hexagonal fashion. Figure 2 shows a single [H2SiMo12O40] Keggin ion of 1, emphasizing the protonated oxygens (yellow spheres), the surrounding NR4+ heads of the surfactant ions, and the neighboring acetonitrile molecules. The two protonated oxygens of each cluster sit opposite each other on an “8-ring window” face of the Keggin ion. The central carbon of the acetonitrile is most closely associated with the

cluster, ∼3.2 Å away from a terminal ModOt oxygen. The unit cell volume of 1 (5805 Å3) is a little larger than the equivalent half cell of 3 (5417 Å3),33 to accommodate the two acetonitrile molecules per formula unit. The interdigitated surfactant layers are not normal to the plane of Keggin ions. Rather they are tilted 51° from normal, defined as the tilt-angle. There are four unique surfactant molecules per cluster. As described in Figure 1, each cluster is surrounded by 12 NR4+ surfactant heads, and each head is shared between 3 clusters, give 4 total surfactants per cluster. The NR4+ heads are paired, ∼6 Å apart, across the Keggin ion plane. The pairs are N2-N4 and N1-N3. All four surfactant molecules have unique configurations, and are illustrated in Figure 3: the N3 surfactant is essentially straight, N1 is bent between the first and second carbon, and N2 is bent between the second and third carbon. The N4 surfactant is disordered between C2 to C4, and the hydrogens bonded to these carbons could not be located. The hydrogens of the N-bonded methyls could also not be located, due to disorder. The last carbon of the N2 surfactant is similarly disordered. In the fully oxidized [SiMo12O40][CTA]4 (3), interdigitated surfactants formed pairs with ∼3 Å H · · · H distances.26 This is not the case for 1: all the surfactant molecules are evenly spaced, packed approximately in a hexagonal array with the shortest interchain carbon-carbon distance g4 Å. The surfactant chains bend to accommodate the larger Keggin ions in their hexagonally packed arrays. Electrochemical Measurements of 1. The cyclic voltammograms of two-electron reduced silicomolybdic acid (H6[SiMo12O40]) and 1 in 90:10 acetonitrile and aqueous 1 M H2SO4 (to give a final concentration of 0.1 M H2SO4) are shown in Figure 4A. Both compounds display similar behavior over the range investigated with each having three two-electron reversible waves.34,35 The waves of 1 are slightly shifted to more negative potentials than H6[SiMo12O40] and most likely arises because of association of CTA+ with [SiMo12O40]6- in 1.36 The rest potentials of 1 and H6[SiMo12O40] are 370 and 350 mV, respectively. This is considerably different from the 450 mV rest potential of fully oxidized H4[SiMo12O40] in the same solvent system. We performed bulk electrolysis of 1 at 550 mV to confirm that the Keggin ions are still in the two-electron reduced state. Data points were collected in 3000 ms intervals and the results are shown in Figure 4B. The bulk electrolysis was stopped when the ratio of the initial to final current was

From Surfactant-POM Lamellae to Surfactant-Encapsulated POMs

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Table 2. Mo-O and Si-O Bond Distances of 1 and 2 Mo-O, Si-O distances (Å) in [H2SiMo12O40] in (1)a Si1 Avg Si-O ) 1.632

O3 O1 O2 O4 O10 O11 O5 O12 O6 O2 O8 O7 O14 O13 O6 O3 O9 O7 O15 O5 O16 O4 O24 O12 O19 O18 O30 O2 O25 O20 O31 O13 O19 O3 O26 O21 O33 O14 O20 O3

Mo1

Mo2

Mo3

Mo4

Mo5

Mo6

1.624(9) 1.63 (1) 1.635(9) 1.64(1) 1.65(1) 1.84(1) 1.84(1) 1.984(9) 2.074(9) 2.377(9) 1.68(1) 1.876(9) 1.878(9) 1.973(9) 2.050(8) 2.339(9) 1.66(1) 1.917(9) 1.926(9) 1.94(1) 1.98(1) 2.37(1) 1.68(1) 1.86(1) 1.88(9) 1.932(8) 1.960(9) 2.342(8) 1.68(1) 1.87(1) 1.89(1) 1.93(1) 1.983(9) 2.342(8) 1.669(9) 1.924(9) 1.93 (1) 1.93(1) 1.959(9) 2.353(8)

Mo7

Mo8

Mo9

Mo10

Mo11

Mo12

O27 O21 O15 O22 O34 O4 O28 O16 O17 O22 O32 O4 O23 O29 O17 O18 O11 O2 O39 O36 O33 O37 O34 O1 O40 O37 O32 O35 O29 O1 O38 O30 O35 O31 O36 O1

1.66(1) 1.854(9) 1.88(1) 1.96(1) 2.04(1) 2.312(8) 1.68(1) 1.84(1) 1.911(8) 1.91(1) 2.04(1) 2.340(9) 1.66(1) 1.851(9) 1.899(8) 1.919(8) 2.003(9) 2.337(8) 1.70(1) 1.84(1) 1.87(1) 1.94(1) 2.06(1) 2.36(9) 1.65(1) 1.86(1) 1.87(1) 1.96(1) 1.978(9) 2.354(9) 1.65(1) 1.841(9) 1.93(1) 1.932(2) 2.03(1) 2.347(8)

Figure 2. View of [H2SiMo12O40]4- in 1, with its associated 12 NR4+ surfactant heads and two acetonitrile molecules. Blue is Mo, pink is Si, green is N of NR4+, turquoise is N of acetontrile, red is O, yellow is OH, gray is C, and white is H. The acetonitrile molecules are above and below the Keggin cluster in this view.

Mo-O, Si-O Distances for (Å) [HxSiMo12O40] in 2 Keggin-1b Keggin-2c Si1 avg Si-O ) 1.69 Mo1

Mo2

Mo3

Mo4

Mo5

Mo6

O1 O4 O2 O3 O5 O6 O7 O9 O8 O3 O1 O10 O11 O12 O13 O14 O4 O2 O15 O14 O9 O16 O17 O4 O3 O18 O19 O17 O12 O6 O2 O3 O20 O8 O21 O11 O19 O2 O1 O22 O16 O13 O21 O7 O4 O1

1.64 (4) 1.67 (5) 1.69 (4) 1.74 (5) 1.62(3) 1.82(3) 1.84(3) 1.92(3) 1.99(3) 2.23(5) 2.37(4) 1.65(3) 1.82(3) 1.88(3) 1.92(4) 1.97(3) 2.37(4) 2.45(4) 1.68(3) 1.85(3) 1.88(3) 1.95(3) 2.02(3) 2.39(5) 2.42(5) 1.68(3) 1.82(3) 1.856(5) 1.93(3) 1.97(3) 2.34(4) 2.39(4) 1.71(3) 1.82(3) 1.89(3) 1.92(3) 1.96(3) 2.31(4) 2.41(4) 1.72(3) 1.83(3) 1.907(8) 1.91(3) 1.92(3) 2.37(5) 2.40(4)

Si2 avg Si-O ) 1.66 Mo7

Mo8

Mo9

Mo10

Mo11

Mo12

O26 O23 O24 O25 O27 O28 O29 O31 O30 O23 O26 O32 O31 O34 O35 O33 O25 O23 O36 O37 O35 O38 O28 O25 O26 O39 O30 O41 O40 O37 O26 O24 O42 O43 O34 O38 O41 O25 O24 O44 O33 O40 O29 O43 O23 O24

1.61(5) 1.61(5) 1.67 (4) 1.74 (5) 1.71(3) 1.90(3) 1.93(4) 1.95(3) 1.97(2) 2.40(5) 2.47(5) 1.68(2) 1.89(3) 1.93(3) 1.95(3) 1.99(4) 2.45(5) 2.50(5) 1.69(3) 1.84(3) 1.86(3) 1.93(4) 1.95(3) 2.21(5) 2.53(5) 1.66(3) 1.81(2) 1.87(3) 1.92(3) 1.98(3) 2.24(5) 2.27(4) 1.72(3) 1.83(3) 1.87(3) 1.89(3) 1.96(3) 2.37(5) 2.50(4) 1.65(3) 1.75(4) 1.89(3) 1.97(4) 2.04(3) 2.35(5) 2.37(4)

a Average M-Oterminal bond for 1, 1.67 Å; average M-Obridging bond for 1, 1.93 Å; average M-Ocentral bond for 1, 2.35 Å. b Average M-Oterminal bond for Keggin-1, 1.68 Å; average M-Obridging cbond for Keggin-1, 1.90 Å; average M-Ocentral bond for Keggin-1, 2.37 Å. Average M-Oterminal bond for Keggin-2, 1.69 Å; average M-Obridging bond for Keggin-2, 1.90 Å; average M-Ocentral bond for Keggin-2, 2.39 Å.

Figure 3. View of the four surfactant molecules of 1, showing the different conformations, and disorder in the N4 surfactant (carbons 2-4) and N2 surfactant (carbon 16).

1%. The data clearly confirm that 1 is fully two-electron reduced (theoretical 1.92 C; measured 1.91 C). Finally, we reisolated 1 by evaporation of the solvent and confirmed that 1 was not decomposed during bulk electrolysis by powder X-ray diffraction. Figure 5 shows the calculated and observed powder X-ray diffraction patterns for 1. They are essentially identical with small differences owed to preferred orientation as a result of the predominant lamellar morphology and desolvation (loss of acetonitrile during the measurement). Description of [HxSiMo12O40][C16H33N(CH3)3]5 [CH3CN]4 Structure 2. Keggin Ions and Surfactant Heads. 2 has two crystallographically unique Keggin ions. Like 1 and 3, 2 features a hexagonal arrangement of the Keggin ions in the (001) plane.

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associated with four of these. Along the rows of alternating Keggin-1 and Keggin-2 ions (approximately along the (110) direction), between Keggin-1 and Keggin-2, there is one (N1)R4+ and one (N2)R4+ (Figure 6). There are 4 of these surfactant ions surrounding each Keggin ion. In this way, for structures 1 and 3, we can describe each cluster as surrounded by 12/3, or 4 surfactant heads, which matches the molecular formula. In 2, Keggin-1 is surrounded by 10 surfactant heads and Keggin-2 is surrounded by 14 surfactant heads, for an average of 12 surfactant heads around each Keggin. Using this same mode of description as we do for 1 and 3. Keggin-1 is surrounded by

(4N4 + 2N3)/6 + (2N1 + 2N2)/2 ) 3 surfactant heads And Keggin-2 is surrounded by

(4N3 + 2N4)/6 + (2N1 + 2N2)/2 + 4(N5)/2 ) 5 surfactant heads,

Figure 4. (A) Cyclic voltammograms of 1 (blue, dashed line) and H6[SiMo12O40] (red, dotted line) in 90:10 acetonitrile and aqueous 1 M H2SO4 (to give a final concentration of 0.1 M H2SO4). The scan rate was 100 mV/s, the working electrode was freshly polished glassy carbon having a diameter of 3 mm, and the counter electrode was Pt coil. (B) Bulk electrolysis of 1 at 550 mV.

Figure 5. Calculated and observed XRPD patterns of 1.

There are alternating rows of Keggin-1 (with Si1 in the center) and Keggin-2 (with Si2 in the center), along the a-direction, illustrated in Figure 6. Both Keggin-1 and Keggin-2 are disordered, with eight half-occupied oxygen atoms surrounding the central Si. The arrangement of the NR4+ heads, however, differs significantly from both 1 and 3. 1 and 3 both have a 4:1 ratio of NR4+:Keggin ion; with each Keggin ion surrounded by 12 NR4+, each NR4+ is shared by 3 Keggin ions. Another way to describe this arrangement is there are 2 NR4+ at every junction of 3 Keggin ions. In 2, there is 1 NR4+ at every junction of 3 Keggin ions, for a total of 6 of these surrounding each Keggin ion. Keggin-1 is surrounded by 4 (N4)R4+ and 2 (N3)R4+, and Keggin-2 is surrounded by 4 (N3)R4+ and 2 (N4)R4+. Along the rows of Keggin-2 (parallel the a-direction) between every two Keggin-2 ions there are 2 (N5)R4+; thus each Keggin-2 is

or an average of 4 surfactant heads per cluster. We expected to observe five surfactant heads associated with each Keggin ion in 2, because the Keggin ion charge, to the best of our knowledge, is 5-, with its associated protons. To better understand this discrepancy, we need to look at the distance between the Keggin ions and the surfactant heads. Compiled in Table 3 for structures 1, 2 and 3 is all the Si-N distances for the surrounding surfactant ions (12 per Keggin ion in 1 and 3, 10 and 14 for Keggin-1 and Keggin-2, respectively, in 2). The average Si-N distance for structures 1 and 3 is 7.73 Å and 7.75 Å, respectively. In 2, the average Si-N distance is 7.26 Å and 7.83 Å, respectively for Keggin-1 and Keggin-2, with an average of 7.55 Å. The Keggin-surfactant, or anion-cation distance is less in 2 than 1 and 3, suggesting a stronger electrostatic interaction, and agreeing with the assessment that the Keggin-ions in 2 have a higher charge than those in 1 and 3. Crystal lattices with 5-fold symmetries are rarely observed, so it is not surprising that regular packing of the 5:1 ratio of surfactant:Keggin does not occur. We can consider Keggin-1 plus its 3 surfactant heads to have an excess negative charge over Keggin-2 plus its 5 surfactant heads. Thus this structure can be viewed as an assembly of Keggin-1-NR4+ “anions” and Keggin-2-NR4+ “cations”. Because of the small yield of this reaction, we were unable to carry out the CV experiments to determine the charge of the [HxSiMo12O40] Keggin ions in 2. Unlike 1, oxygen sites for protonation were not obvious by BVS and Mo-O bond lengths. However, disordered protonation seems likely, and we suspect the charge is -6 or -8, which would require 1-3 protons per cluster to achieve charge-balance. Figure 7 shows a view down the a-axis of the alternating Keggin ion layers and surfactant ion layers. In both 1 and 3 (Figure 1), with a 4:1 surfactant:cluster ratio, the surfactant heads lie between the clusters within the Keggin ion plane, flanking both sides of the cluster layer. We see in Figure 7 that in addition to these surfactant head positions of the 1:4 cluster:surfactant phases, the surfactant heads also sit directly above and below the clusters (N5). This observation of the position of N5 is the first clue to understanding the structural evolution of ordered lamellar cluster-surfactant phases to SEC phases as the surfactant:cluster ratio increases. Surfactant Layers. The five crystallographically unique surfactant ions are shown in Figure 8. No H-atoms were located on any of the surfactant chains, because of the quality of the data set, related to first and foremost disorder in the crystal and compounded by small crystal size and large unit cell. Both the N3 and N5 surfactant chains are straight. The N1 surfactant is

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Crystal Growth & Design, Vol. 9, No. 8, 2009 3595

Figure 6. View of 2 along the c-axis, showing the arrangement of rows of Keggin-1 and Keggin-2 ions along the a-direction, and the five ammonium cation heads. Green is N, yellow is Si, blue is Mo, red is O, and white is C. Table 3. Keggin-Amine Distances for 1, 2, and 3 Described as Si-N (Å)

1 2 3 4 5 6 7 8 9 10 11 12

[SiMo12O40H2][CH3CN]2[CTA]4 (1)

[SiMo12O40][CTA]4 (3)26

Si1

Si1

N4 7.014(6) N3 7.020(8) N1 7.027(2) N2 7.167(2) N4 7.479(1) N4 7.613(7) N1 7.66(1) N2 7.67(1) N3 7.739(2) N3 7.785(9) N1 9.07(1) N2 9.47(1) average Si-N distance 7.73

1 2 3 4 5 6 7 8 9 10 11 12

[SiMo12O40][CTA]5 (2) Si2 (Keggin-2)

N4 6.960(1) N2 6.992(1) N3 7.055(2) N1 7.375(2) N4 7.513(2) N1 7.603(1) N1 7.714(2) N2 7.745(2) N3 7.915(2) N3 7.953(1) N2 8.837(3) N4 9.398(3) average Si-N distance 7.75

1 2 3 4 5 6 7 8 9 10 11 12 13 14

N5 N5 N1 N1 N3 N3 N2 N2 N5 N5 N2 N2 N4 N4 average Si-N distance

6.94(3) 6.94(3) 6.96(3) 6.96(3) 7.52(3) 7.52(3) 7.64(2) 7.64(2) 7.84(3) 7.84(3) 8.64(3) 8.64(3) 9.26(2) 9.26(2) 7.83

Si1 (Keggin-1) 1 2 3 4 5 6 7 8 9 10

bent between the second and third carbons, the N2 surfactant is bent between the fifth and sixth carbons, and the N4 is bent between the third and fourth carbon. Like in 3, the surfactant chains are interdigitated in pairs: N5 and N5, N1 and N4, and N2 and N3. However, where the interdigitated pairs are within

N4 6.95(3) N4 6.95(3) N2 7.02(1) N2 7.02(1) N4 7.33(2) N4 7.33(3) N1 7.47(3) N1 7.47(3) N3 7.53(4) N3 7.53(4) average Si-N distance 7.26 average Si-N distance for Keggin-1 and Keggin-2 7.55

H-bonding range in 3 (2.7-3.0 Å),26 they are 3.9-4.1 Å apart in (2). Where the regular layers of surfactants in 3 are all interdigitated at the depth of 14 carbons, they are variable in the irregular layers of 2. The N5 surfactants are completely interdigitated, with the N-bonded methyl ∼4.0 Å from the last

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four acetonitrile molecules are also located within the surfactant chain layers, shown in Figure 7 (acetonitrile N-atoms are orange). Observing this significant increase in disorder of the interdigitated surfactants in 2 as a result of increasing the surfactant: Keggin ion ratio from 4:1 to 5:1 confirms the reason why clustersurfactant phases featuring clusters with high charge (> -4) cannot form well-ordered single-crystals, nor can the related SEC-like phases. The reason lies in the limited ability to interdigitate surfactant tails in an ordered manner, rather than interaction of the surfactant heads with the POMs. The Keggin anions are of cubic symmetry, and thus can associate equally well with NR4+ on any side of the approximately spherical cluster. However, as more NR4+ heads must necessarily penetrate the Keggin-ion layers at different depths, they also interdigitate at different depths, quickly increasing the disorder. Conclusions

Figure 7. View of (2) down the a-axis of the [HxSiMo12O40] Keggin ions and surfactant layers. Some select acetonitrile molecules are shown (acteonitrile N-atoms highlighted in orange.

Figure 8. View of the interdigitated surfactant chain pairs of 2: N5-N5, N2-N3 and N1-N4.

carbon in its interdigitated pair. Recall, the N5 surfactant heads sit above and below the cluster layer, rather than between the clusters within the layer (Figure 7), and it is therefore logical that they exhibit the deepest interdigitation. On the other hand, the N2-N3 pair is interdigitated through only 9 carbon atoms of the chains. The N1-N4 pair is interdigitated through 14 carbon atoms. Similar to 1 and 3, the surfactant chains are tilted from normal to the Keggin ion layers (tilt angle ) 47°). The

With this structure determination of [HxSiMo12O40][C16H33N(CH3)3]5[CH3CN]4 (2), we provide a rare example of a cluster-surfactant phase that has a surfactant:cluster ratio of a value other than 2 or 4. This unique example illustrates the first step of evolution from well-ordered lamellae to the elusive structure of the surfactant-encapsulations clusters (SECs), which do not readily form good-quality single crystals. We have learned that the increase in surfactant:cluster ratio beyond 4 results in significant disorder in the region of the interdigitated surfactant chains. Because the surfactant heads necessarily have different penetration depths within the cluster layers, the interdigitation of the chains and bending of the chains become irregular, even in the small evolution from a 4:1 to a 5:1 surfactant:cluster ratio. Thus given this increase in disorder, it is no surprise that the SECs are not readily crystallized. We also observe that incorporation of solvent molecules in the surfactant-chain region of the crystal lattice is common as the chains become more disordered. Furthermore, protonated clusters are selectively incorporated into these crystalline phases (as illustrated by the ready crystallization of surfactant-cluster phases containing [H2SiMo12O40]4-, such as 1), which presents a challenge to crystallizing phases with higher surfactant:cluster ratios. On the other hand, surfactant molecules can be utilized to selectively precipitate clusters with bound protons (i.e., for catalytic or proton-conducting applications), when a protonated cluster has a charge of -4. From this study, the structure of 2 in particular, we have learned much about the issues and challenges of crystallizing “SEC-like” phases, and have gleaned some potential tricks to forcing crystallization, for instance, utilizing mixed surfactant chain lengths to accommodate the varying surfactant-chain interdigitation and penetration of the heads into the cluster layer. The choice of solvent for crystallization is also paramount in providing the ideal solvent to promote growth of ordered lattices. Acknowledgment. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy under Contract DEAC04-94AL85000. Supporting Information Available: Chemical information files (cif) for 1 and 2 and cyclic voltammogram of the Na4[SiMo12O40] solution used for bulk electrolysis (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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