Synthesis and Ligand-Exchange Reactions of a Tri-Tungsten Cluster

LABORATORY EXPERIMENT pubs.acs.org/jchemeduc

Synthesis and Ligand-Exchange Reactions of a Tri-Tungsten Cluster with Applications in Biomedical Imaging Elizabeth Noey,† Jeff C. Curtis,† Sylvia Tam,‡ David M. Pham,‡ and Ella F. Jones*,‡ † ‡

Department of Chemistry, University of San Francisco, San Francisco, California 94117, United States Center for Molecular and Functional Imaging, Department of Radiology and Biomedical Imaging, University of California, San Francisco, California 94107, United States

bS Supporting Information ABSTRACT: In this experiment students are exposed to concepts in inorganic synthesis and various spectroscopies as applied to a tri-tungsten cluster with applications in biomedical imaging. The tungsten-acetate cluster, Na[W3(μ-O)2(CH3COO)9], 1, was synthesized and characterized by 1H-NMR, UV-vis, FT-IR and Raman spectroscopy. 1H-NMR shows three characteristic chemical shifts of the bridging (δ 2.30 ppm), terminal (δ 2.15 ppm), and free (δ 2.07 ppm) acetate groups in methanol-d4. The UV-vis spectrum shows two distinct absorption peaks at 376 and 455 nm, and the IR and Raman spectra exhibit a number of coincident peaks that verify the non-centrosymmetric D3h or C3h symmetry of the structure. From a 1H-NMR experiment in which 1 is heated in neat acetic-d3 acid-d, students are able to monitor the sequential exchange of the terminal acetate groups followed by the bridging acetate groups with deuterated acetate. Extrapolating from this experiment, students are in a position to adapt the ligand-exchange reaction and replace the acetate groups with acrylate to form a new, functionalized tungsten acrylate cluster 2. UV-vis spectroscopy shows the conversion from 1 to 2 with a distinctive red shift of the absorption peak from 455 nm to 470 nm and the disappearance of the peak at 376 nm. Undergraduate and graduate teaching laboratories can easily adapt this experiment into their curriculum, allowing students to gain deeper experience with both synthetic and analytical aspects of inorganic chemistry. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Drugs/Pharmaceuticals, IR Spectroscopy, NMR Spectroscopy, Raman Spectroscopy, Synthesis, UV-vis Spectroscopy

T

his experiment was introduced to the upper-level inorganic chemistry laboratory course to extend the range of the synthetic inorganic chemistry experience and to teach new methods of molecular characterization and reaction monitoring. To motivate students, a novel tri-tungsten cluster with potential applications in diagnostic X-ray imaging was chosen to synthesize. The current X-ray contrast agents are based on the triiodobenzoic acid platform (Figure 1). They are radio-opaque due to the presence of iodine as a heavy element to absorb X-rays.1 For more than 40 years, advances in iodinated contrast agent research have been limited to the incorporation of watersoluble ligands, changing of the overall molecule from ionic2 to non-ionic forms,3,4 and formation of dimeric molecules (iodixanol) to improve biocompatibility, toxicity, and radio-opacity. Despite these incremental improvements, iodinated contrast agents still lack tissue specificity and may trigger minor to severe adverse effects in some patients.5 Therefore, there is a need of a new approach to address these long overdue clinical issues. Furthermore, under normal X-ray operating energy (80-150 keV), iodine is not the optimum element for X-ray contrast. In fact, other heavy elements such as gadolinium, tungsten, and lead have much higher X-ray attenuation.6 In the past decade, transitionmetal cluster materials have been proposed for use as X-ray contrast agents by us and others.7-14 Through the collective Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

research, tri- and hexanuclear clusters with rhenium, rhodium, tantalum, and tungsten have been synthesized and evaluated. Using X-ray computed tomography (CT) phantoms,a equal molar concentration of these transition-metal clusters exhibit X-ray attenuation from 2 to 6 times better than traditional iodinated contrast agents.6,12 From our experience, we also found that the trinuclear tungsten cluster exhibits desirable In vivo biocompatibility and X-ray imaging efficacy (Figure 2). Therefore, the application of tungsten materials in CT imaging presents a promising new direction in diagnostic imaging. The tungsten materials studied in this experiment are based on a trinuclear tungsten core, [W3(μ3-O)2(carboxylate)6(L)3]m(. This class of compounds has been known since Cotton’s pioneering work in 1978.15 They are formed in the reaction between tungsten hexacarbonyl and carboxylic acid/anhydride mixtures. The core unit comprises a triangular W3 plane, capped above and below the D3h or C3h symmetry plane by two triply bridging oxygen atoms. Each W-W edge is supported by two bridging carboxylate groups and there is a terminal ligand, L, at each tungsten vertex (Figure 1). The most studied derivatives have been the triaquo W3-acetate complexes, [W3(μ3-O)2 (acetate)6(H2O)3]2þ,15 obtained by acid hydrolytic work up of the reaction mixtures. Although deprotonation and substitution Published: March 23, 2011 793

dx.doi.org/10.1021/ed100024f | J. Chem. Educ. 2011, 88, 793–797

Journal of Chemical Education

LABORATORY EXPERIMENT

tungsten-acetate cluster 1; (ii) characterization of 1 using H NMR, UV-vis, FT-IR, and Raman spectroscopic methods; and (iii) ligand-exchange reaction with acetic-d3 acid-d and acrylic acid and characterization of 2. The synthesis of 1 can be carried out by students in lab in about 4 h total in-lab time or in advance by the instructor or teaching assistant (a 10 h reflux step and an overnight waiting period are involved, vide infra). The time required to complete parts (ii) and (iii) is two or three 3 h laboratory periods. 1

Figure 1. A comparison of conventional iodinated contrast agents to tritungsten metal cluster. L is acetate and R is CH3 in cluster 1. The presence of heavy element (iodine and tungsten) is the key to X-ray attenuation in diagnostic imaging. The R group may contribute to the overall charge, solubility, viscosity, and biocompatibility for clinical applications. Replacement of L and the bridging acetates by acrylate yields cluster 2.

Figure 2. In vivo efficacy of the parent tungsten cluster in rats. Image A is the control without contrast agents and B is an image of a rat with the tungsten-acetate cluster 60 min postinjection. Better contrast in the abdominal area and bladder can be observed.

of terminal water molecules have been investigated,16-21 substitution of the bridging carboxylate groups has never been reported.22,23 We have developed a facile synthetic route to modify the entire [W3(μ3-O)2(carboxylate)9]- cluster with different carboxylate ligands.9,24 This new route opens the way to classes of materials that were previously unattainable. These include tri-tungsten clusters functionalized at the terminal and bridging positions shown in Figure 1 with alkyl-, alkenyl-, aromatic-, and bifunctional carboxylates. These functionalized intermediates then provide easy access to bioconjugation reactions and tissue-specific biomedical imaging applications.9,24 Taken together, with the potential in biomedical applications and the simplicity of synthesis and characterization, this tungsten compound class is well suited for an upper-level undergraduate teaching laboratory experiment. In this article, we detail the synthesis of the parent tungsten-acetate cluster 1, its characterization by 1H NMR and UV-vis, and the use of FT-IR and Raman spectroscopic methods to validate the D3h or C3h point group symmetry indicated in previous crystallographic work.15 We then describe the subsequent ligand-exchange reaction with acetate-d3, allowing students to monitor reaction changes in real-time. Finally, the same reaction conditions are adapted to generate the tungsten-acrylate cluster 2. This experiment is designed to be a three-part series: (i) synthesis of the

’ MATERIALS AND EQUIPMENT Solvents and reagents were purchased from Sigma Aldrich and were used without further purifications. NMR spectra were recorded using a Varian Inova 400 MHz spectrometer fitted with a multinuclear probe. UV-vis spectra were recorded on a Hitachi U-2800 spectrophotometer. Vibrational spectra were collected on a Thermo-Nicolet 6700 FT-IR/FT-Raman spectrometer. ’ SYNTHESIS AND CHARACTERIZATION OF THE TUNGSTEN-ACETATE CLUSTER The synthesis and characterization of 1 is well established.9,24 A mixture of Na2WO4 3 2H2O (5.0 g, 15 mmol) and W(CO)6 (6.0 g, 17 mmol) was heated at reflux in acetic anhydride/acetic acid (10:1, 250 mL) at 135-140 °C for 10 h (see footnote b for hazard avoidance info).b The reaction mixture was initially colorless and slowly turned to dark blue when the temperature reached 110 °C. The final solution was a dark brown with yellow precipitate and was left to cool overnight. On the next day, the reaction mixture was heated to 80 °C for 1 h. While the reaction was still warm (35-40 °C), the solution was filtered through a fritted glass funnel, and the filtrate was disposed of as heavy metal waste. The resulting yellow-green solid, collected in the fritted funnel, was dissolved in a minimum volume of warm methanol (35-70 mL). The resulting dark greenish yellow-brown solution was reduced to a small volume (∼3-5 mL of dark greenish-brown oil) by rotary evaporation at 28 °C. The crude product was precipitated by the addition of 4-6 volume equivalents of room temperature acetonitrile followed by chilling the mixture in the refrigerator. The resulting crystalline product was isolated by filtration and dried at 60 °C under vacuum overnight. Purification was achieved by repeating the methanol/acetonitrile dissolution and reprecipitation procedure. Typical yields were 3.7-4.2 g (64-73% based on Na2WO4 3 2H2O).c Analysis for Na[W3(μ3-O)2(OAc)9], C18H27NaO20W3: C, 18.8%; H, 2.7%; Na, 2.1%; W, 49.8% (calc C, 19.00%; H, 2.4%; Na, 2.0%; W, 48.5%). 1 H NMR (400 MHz, CD3OD, δ): 2.15 (s, 9H), 2.30 (s, 18H). UV-vis (H2O) λmax, nm (ε): 455 (1620), 376 (1530). ESI-MS m/z: 1114. FT-IR (KBr pellet) and FT-Raman (pure, microcrystalline sample) spectra of 1 were obtained over the energy range of 4001750 cm-1. Coincident peaks were noted at IR:Raman energies (in cm-1) of 1654.0:1652(sh), 1559.8:1562.3, 1458.4:1454.8, 1420.6:14201.0(sh), 1369.9:1363.3, 1309.8(sh):1307.8, 1294.0: 1295(sh), 1280.6(sh):1280(sh), 966.2(sh):967.7, 944.8:947(sh), 693.7:694.1, 646.0:650.9(sh), 601.3:600(sh), 588.9(sh):588, and 478.6:474.5 (the peaks at ∼1375, 1047.9, 1019.3, and ∼652-682 overlap with regions where acetic anhydride has peaks and are thus considered ambiguous due to the possibility of trace 794

dx.doi.org/10.1021/ed100024f |J. Chem. Educ. 2011, 88, 793–797



Figure 4. An overlay of UV-vis spectra of the tungsten-acetate cluster 1 (red) and the tungsten-acrylate product 2 (blue). Recorded in methanol.

Figure 3. Ligand-exchange reaction of 1 in CD3COOD from 15-80 °C captured by variable temperature 1H NMR.

solvent-entrainment). There were IR-only peaks at 1624.4 (sh, broad) and 423.5 (broad) cm-1. There were Raman-only peaks at 1659.2, 1469.0, 1429.4 (broad), 968.8, 730.1, and 621.4 cm-1 (as well as several Raman peaks in the range of 90-450 cm-1).

’ LIGAND EXCHANGE WITH ACETIC-D3 ACID-D DEMONSTRATED BY 1H NMR The acetate ligand-exchange reaction was demonstrated by first dissolving 5 mg of 1 in 0.5 mL of acetic-d3 acid-d (CD3CO2D, 99.9%) in an ice bath. The chilled tube was then quickly inserted into the NMR spectrometer that was preset at 15 °C. The ligand exchange of 1 at both bridging and terminal positions is evident from the successive 1H NMR spectra taken in neat CD3CO2D at elevated temperatures (Figure 3). Initially, the bridging CH3CO2- signal (2.30 ppm) remained constant, but the free CH3CO2- signal (2.07 ppm) intensified at the expense of the terminal CH3CO2- signal (2.25 ppm) at 20 °C for over 45 min. This shows that the replacement of terminal CH3CO2groups on the cluster by CD3CO2- occurs rapidly even at lower temperatures. Subsequently, the bridging ligands were exchanged as the temperature was heated to 80 °C. The corresponding 1H NMR signal at 2.25 ppm showed both a decrease in intensity and an increase in line width accompanied by splitting. The broadening and splitting of the proton peak at 2.30 ppm probably indicates the loss of the overall D3h symmetry of cluster 1. The final collapse of both bridging and terminal acetate signals indicates complete ligand exchange with deuterated acetate.

’ FUNCTIONALIZATION BY LIGAND EXCHANGE WITH ACRYLIC ACID The ability to activate the tri-tungsten cluster 1 through ligand exchange opens a facile synthetic route to a variety of novel, functionalized materials. Using this reaction, we successfully prepared tri-tungsten clusters with carboxylates containing aliphatic, functional, and aromatic groups. In particular, it was straightforward to synthesize the tri-tungsten acrylate cluster 2 in which all the acetate groups are replaced by acrylate. Tungsten cluster 1 (3.0 g, 2.6 mmol) was heated in 100 mL of acrylic acid at 80 °C for 2 h. The reaction was monitored by UV-vis spectroscopy. Consistent with published data,9 the spectrum of 1 had maximum absorptions at 376 and 455 nm (Figure 4). After complete exchange of acetate with acrylate, the resulting tungsten acrylate cluster 2 showed a distinctive red shift of absorption from 455 to 470 nm and disappearance of the absorption at 376 nm. The red shift of the UV-vis absorption band indicates the incorporation of multiple double bonds onto the cluster and presumably a decrease HOMOLUMO energy gap. Although the purification and isolation of the acrylate cluster 2 has proven to be nontrivial,9 isolation of the product was not necessary to illustrate the process of ligand exchange. Extension of this approach to quantitative measurement of the substitution kinetics could provide a useful challenge to more advanced students. The functionalized cluster presents an open-ended tree of synthetic possibilities for advanced or interested students to explore. ’ HAZARDS Caution: Acetic acid and acetic anhydride are very hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, of inhalation. Very hazardous in case of skin contact (corrosive). Hazardous in case of skin contact (permeator). Liquid or spray mist may produce tissue damage particularly on mucous membranes of eyes, mouth and respiratory tract. Skin contact may produce burns. Inhalation of the spray mist may produce severe irritation of respiratory tract, characterized by coughing, choking, or shortness of breath. Inflammation of the eye is characterized by redness, watering, and itching. Skin inflammation is characterized by itching, scaling, reddening, or, occasionally, blistering. Note: Metal hexacarbonyl is known to clog up condensers when the reaction is performed in smaller scale glassware with 19/22 or 14/20 joints. We therefore strongly recommend the use 795




Table 1. Assignments of Major Vibrational Peaks from the Tungsten-Acetate Cluster Measured by IR and Raman Spectroscopy Observed IR:Raman

Calculated Peak

Peaks (cm-1)

Position (cm-1a)

Irreducible Rep., C3h

Description

—:1659.2

1658.2

in-phase comb asym CdO stretch, terminal acetates

A0

1654.0:1652.0 (sh)

1651.9

doubly degen comb asym CdO stretch, terminal acetates

E0

1559.8:1562.3

1559.3

asym CdO stretch, bridging acetates

E0

1458.4:1454.8

1434

symmetric CdO and CH3 hydrogen stretches on bridging acetates

E0

—:1429.4 (broad)

1422.4

CH3 hydrogen bend, stronger on the terminal acetates

A0

1420.6:1420.0

1421.4

CH3 hydrogen bend, mostly on the terminal acetates

E0

1309.8 (sh):1307.8 1294.0:1295(sh)

1330.6 1250.7

sym CH3 hydrogen bend and C-C stretch, stronger on terminal acetates asym CdO stretch on the terminal acetates

E0 E0

—:968.8

932.6

in-phase sym CdO and C-C stretches on all acetates

A0

—:730.1

696.6

large oxo-anion stretch along C3 axis, small W-W ring stretch, bridging-acetate

A0

693.7:694.1

667.3

out-of-plane bends sym CdO and C-C stretches on bridging acetates, displacement of the

E0

(μ3-O) anions as a unit perpendicular to the C3 axis, no motion of the W ions 646.0:650.9 9(sh) —:621.4

625.3

sym CdO and C-C stretches on terminal acetates, very small displacement of the (μ3-O)

E0

624.2

anions as a unit perpendicular to the C3 axis, even smaller motions of the W ions sym CdO bend and C-C stretch on the terminal acetates, very small oxo-anion

A0

stretch along C3 axis a

To simplify the ab initio calculation, the terminal acetate groups were held at a specific conformation yielding a C3h point group symmetry.

of glassware with 24/40 joints to minimize the potential hazard. If white crystals of tungsten hexacarbonyl form at the neck of the condenser, insert a ∼20 gauge copper wire through a hole in a septum and carefully slide the copper wire into the condenser to push the condensed tungsten hexacarbonyl crystals back into the reaction solution. As the reaction proceeds, tungsten hexacarbonyl will be consumed over time. Detailed descriptions of the hazards associated with the chemicals are available in the Supporting Information.

B3LYP/3-21 g/SDD levels of theory agreed in some cases but showed significantly worse fit overall. The tungsten-tungsten distance arrived at using the B3LYP//6-31þg(d,p)/SDD method employed here was calculated at 2.839 Å, which is in fair agreement with the crystallographic value of 2.78-2.79 Å.15 The B3LYP//3-21 g/SDD calculation predicted 2.906 Å and the HF//3-21 g/SDD optimization predicted 3.096 Å; the lessexpensive methods thus trending away from agreement with experiment.

’ AB INITIO COMPUTATIONAL MODELING The [W3(μ3-O)2(OAc)9]- cluster was assembled and symmetrized in Gaussview 5.0 (Windows version) and optimized in Gaussian 0925 (on the USF computational cluster) using the B3LYP densityfunctional method with the 6-31þg(d,p) basis set on the light atoms and the SDD (Stuttgart-Dresden) effective-core potential contained within Gaussian on the tungsten atoms. The terminal acetates were held to specific rotameric conformation such that the symmetry point group descended from D3h to C3h. The geometry optimizations were followed by IR and Raman frequency calculations at the same level of theory, and a scaling factor of 0.9613 was applied to the calculated frequencies.26 On the basis of the group theory “exclusion rule”,27 the presence of the coincident IR and Raman vibrational peaks requires that the molecule cannot possess a center of inversion. The vibrational spectroscopic data reported here and the 1H NMR data support the D3h or C3h point group found previously by crystallography15 and shown in Figure 1. From the frequency calculations, we are able make to tentative assignments of some of the major peaks based on the combination of their computed peak positions and their relative IR and Raman intensities computed under C3h symmetry. Table 1 lists these assignments along with brief descriptions of the vibrational motions and irreducible representation assignments. Frequency calculations run at the much less expensive Hartree-Fock/3-21 g/SDD and

’ CONCLUSIONS This experiment teaches concepts in inorganic synthesis, molecular characterization, vibrational spectroscopy, and group theory in the context of a novel class of X-ray contrast agents. From the collective evidence using NMR, UV-vis, FT-IR, and Raman spectroscopic methods, the tungsten cluster 1 is a symmetric molecule with a D3h or C3h point group with three terminal and six bridging acetate groups attached. Whereas the terminal acetates are known to be labile, the ligand-exchange reaction illustrates that the bridging acetate groups are also susceptible to ligand-exchange reactions. The entire sequence of ligand-exchange reactions can be followed using a straightforward temperature-dependent 1H NMR experiment. In acetic-d3 acid-d, we observed the exchange of the terminal acetate groups followed by the exchange at the bridging positions at elevated temperatures. Relevant to exploring the range of synthetic possibilities offered by 1, the ligandexchange reaction of 1 with acrylic acid to form the functionalized product 2 was monitored by UV-vis spectroscopy. The distinctive red shift of the absorption peak is indicative of the incorporation of double bonds onto the cluster framework. ’ ASSOCIATED CONTENT

bS

Supporting Information List of chemicals and equipment needed; figure of the assembled apparatus; list of the chemical gazards and toxicities;

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synthetic procedure for the students. This material is available via the Internet at http://pubs.acs.org.

(24) Fung, E. Y.; (Mallinckrodt Medical, Inc., USA). Application: US 5602268, 1997. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J., J. A. ; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K. V.; Zakrzewski, G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; € Foresman, J. B.; Ortiz, J. V.; Dapprich, S. A.; Daniels, D.; Farkas, O.; Cioslowski, J. J.; Fox, D. J. Gaussian 09, Revision A.1 Gaussian, Inc.: Wallingford, CT, 2009. (26) Foresman, J. B., Frisch, A. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996. (27) Atkins, P. W.; Shriver, D. F. Inorganic Chemistry, 4th ed.; W.H. Freeman: New York, 2006.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ADDITIONAL NOTE a Typical computed tomography phantoms are sample vials containing various concentrations of contrast agents for evaluation of X-ray attenuation in clinical settings. b In the initial reflux step, W(CO)6 may sublime and clog-up the inside of the condenser and this may cause pressure build-up much like heating a reaction in a closed system. Please refer to the Supporting Information for the proper experimental setup and the method to alleviate W(CO)6 build-up. c The current procedure is optimized for generating 3 to 4 g of the parent tungsten cluster material. Alternatively, it can be scaled down by 50% to give approximately 2 g of cluster material for subsequent characterizations and ligand-exchange reactions. ’ REFERENCES (1) Morris, T. W. Radiology 1993, 188, 11–16. (2) Grainger, R. G. Br. J. Radiol. 1982, 55, 544. (3) Almen, T. Invest. Radiol. 1994, 29 (Suppl. 1), S37–S45. (4) Siegel, E. L.; Rosenblum, J. D.; Eckard, D. A.; Leef, J.; Bergh, J.; Parsa, M. B.; Redick, M. L. Acad. Radiol. 1996, 3 (Suppl. 3), S507–S513. (5) Morcos, S. K. Br. J. Radiol. 2005, 78, 686–693. (6) Yu, S.-B.; Watson, A. D. Chem. Rev. 1999, 99, 2353–2377. (7) Deutsch, E. A.; Deutsch, K. F.; Nosco, D. L.; (Mallinckrodt Medical, Inc., USA). Application: WO 9302713, 1993. (8) Franolic, J.; Long, J.; Holm, R.; Droege, M.; Downey, S.; (Nycomed Salutar, Inc., USA; Matthews, D. P.; Franolic, J.; Long, J.; Holm, R.; Droege, M.; Downey, S.). Application: WO 9640287, 1996. (9) Fung, E. Y.; Cooper, S. R.; (Mallinckrodt Medical, Inc., USA). Application: WO 9703994, 1997. (10) Krause, W.; Maier, F.-K.; Niedballa, U.; Raduechel, B.; (Schering A.-G., Germany). Application: DE 19631544, 1997. (11) Long, J.; Holm, R.; Sanderson, W.; Yu, S.-b.; Zheng, Z.; (Nycomed Salutar, Inc., USA; Long, Jeffrey). Application: WO 9743293, 1997. (12) Mullan, B. F.; Madsen, M. T.; Messerle, L.; Kolesnichenko, V.; Kruger, J. Acad. Radiol. 2000, 7, 254–259. (13) Yu, S. B.; Droege, M.; Downey, S.; Segal, B.; Newcomb, W.; Sanderson, T.; Crofts, S.; Suravajjala, S.; Bacon, E.; Earley, W.; Delecki, D.; Watson, A. D. Inorg. Chem. 2001, 40, 1576–1581. (14) Yu, S.-B.; Droege, M.; Segal, B.; Kim, S.-H.; Sanderson, T.; Fellmann, J.; Watson, A. Book of Abstracts, 212th ACS National Meeting, Orlando, FL, August 25-29 1996, INOR-267. (15) Bino, A.; Cotton, F. A.; Dori, Z.; Koch, S.; Kueppers, H.; Millar, M.; Sekutowski, J. C. Inorg. Chem. 1978, 17, 3245–3253. (16) Bino, A.; Gibson, D. J. Am. Chem. Soc. 1981, 103, 6741–6742. (17) Bino, A.; Gibson, D. J. Am. Chem. Soc. 1982, 104, 4383–4388. (18) Bino, A.; Gibson, D. Inorg. Chim. Acta 1985, 104, 155–160. (19) Bino, A.; Gibson, D. Inorg. Chim. Acta 1985, 101, L9–L10. (20) Nakata, K.; Nagasawa, A.; Soyama, N.; Sasaki, Y.; Ito, T. Inorg. Chem. 1991, 30, 1575–1579. (21) Powell, G.; Richens, D. T. Inorg. Chem. 1993, 32, 4021–4029. (22) Cotton, F. A.; Dori, Z.; Marler, D. O.; Schwotzer, W. Inorg. Chem. 1984, 23, 4033–4038. (23) Nakata, K.; Yamaguchi, T.; Sasaki, Y.; Ito, T. Chem. Lett. 1992, 983–6. 797


Synthesis and Ligand-Exchange Reactions of a Tri-Tungsten Cluster

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