Inorg. Chem. 1995,34, 767-777
767
Trisubstituted Heteropolytungstates as Soluble Metal Oxide Analogs. Isolation and Characterization of [(CsMes)Rh.P2Wl~NbsO62]~and [(C ~ H ~ ) R U * P ~ W U Including N~~~~~]~-, the First Crystal Structure of a Dawson-Type Polyoxoanion-Supported Organometallic Complex Matthias Pohl,la,bYin Lin,’a Timothy J. R. Weakley,’” Kenji Nomiya,lCMasahiko Kaneko,’c Heiko Weiner,’b,d and Richard G. Finke*Jb Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 12, 1994@
The heteropolyoxoanion-supported complexes [(C5Me5)Rh*P2W15Nb3062]~-, 1, and [(C~H~)RU*P~WI~N~~O~~]~ 2, have been prepared by addition of [(C5Me5)Rh(CH3CN)3I2+or [(C6H6)Ru(CH3cN)3l2+ to a solution of P2Wl~Nb3062~in acetonitrile. Isolation of these complexes as homogeneous, yellow solids as their all-[(nC&,)fl]+ salts (for 1)or as complexes with mixed [(n-C4Hg)a]+/Na+ cation composition (for 1 and 2) was accomplished by repeated reprecipitation from acetonitrile with ethyl acetate. Molecular formulas for these complexes were established by complete elemental analyses, in conjunction with a sedimentation-equilibrium molecular-weight measurement. Further characterization in solution relied heavily on multinuclear NMR spectroscopy. The solution data are in accord with [(C5Me5)RhI2+and [(C6H6)RuI2+being supported on three niobium-bridging oxygens on the “Nb3093-” surface of the heteropolyoxoanion. The structural characterization of 1in the solid state was accomplished by a single-crystal X-ray structural analysis: P63/m; a = 20.544(5), b = 20.544(5), c = 34.648(6) A; 2 = 2; R = 0.096 for 3023 observed independent reflections. The Rh atom in 1 lies on the polyoxoanion’s 3-fold axis within experimental error and is bonded to the three Nb-0-Nb bridging oxygen atoms ( ~ h - 0 ~2.06 2 A) that cap the P2w15Nb30629- Dawson-type polyoxoanion. This crystallographic analysis, although limited by considerable disorder, represents the first solid-state structure of a Dawson-type heteropolyoxoanion-supported organometallic complex.
Introduction One interest in heteropolyoxoanion2 chemistry results from the fact that these compounds resemble discrete fragments of solid metal oxides (Figure 1),3 an important component of Abstract published in Advance ACS Abstracts, December 15, 1994. (1) (a) University of Oregon. (b) Present address: Colorado State @
University, Ft. Collins, CO 80523. (c) Present address: Department of Materials Science, Kanagawa University, Hiratsuka, Kanagawa 25912, Japan. (d) Department of Chemistry, TU Bergakademie Freiberg, D-09596 Freiberg, Germany. (2) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: New York, 1983. (b) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (c) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (d) Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity; Proceedings of the July 15- 17, 1992, Meeting at the Center for Interdisciplinary Research in Bielefeld, Germany; Muller, A., Pope, M. T., Eds.; Kluwer Publishers: Dordrecht, The Netherlands, 1992. (3) This resemblance was first noted by: Baker, L. C. W. In Advances in the Chemistry of Coordination Compounds;Kirschnerr, S., Ed.; Macmillan: New York, 1961; p 604. (4) (a) The case in 1992 is only somewhat advanced from the 1976 conclusion of a workshop of experts that “no definitive proof of any structure of an oxide-supported catalyst has been reported”.“b With modem methods such as EXAFS, some progress has been made in proposing poisoned catalyst structures (e.g. Rh(C0)2*A1203)4c or crude catalyst “structures” constructed from EXAFS distance information only (plus chemical intuition), but even that work has not been able to provide complete structures at the atomic level.4d(b) Proceedings of the 1st lntemationul Workshop on Fundamental Research in Homogeneous Catalysis; Tsutsui, M., Ugo, R., Eds.; Plenum Press:
New York, 1977; p 218. (c) For lead references see: van? Bilk, H. F. J.; van Zon, J. B. A. D.; Huizinag, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. J. Am. Chem. SOC. 1985, 107,3139. Frederick, B. G.; Apai, G.; Rhodin, T. N. J. Am. Chem. SOC.1987,109,4797. Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. SOC.1988, 110,2074. (d) For lead references see: Gates, B. C. CHEMTECH 1989, 173. Lamb, H.; Gates, B. C.; Kniizinger, H. Angew. Chem., Int. Ed. Engl. 1988,27, 1127.
0020- 1669/95/1334-0767$09.00/0
A
B
Figure 1. (A) Polyhedral and (B) space-filling representations of the Dawson-type heteropolyanion a-1,2,3-P2W lsNb30629-.In (A) the three niobiums are represented by hatched octahedra in the 1-3 positions. The W06 octahedra occupy the 4- 18 positions, and the PO4 are shown as the two internal, black tetrahedra. In (B) the open circles represent bridging tungsten oxygens (W20), while the black circles represent terminal tungsten oxygens (WO). Niobium bridging oxygens (Nb2O) are depicted by hatched circles, whereas niobium terminal oxygens (NbO)are shown as gray circles. From the space-filling representation it becomes clear that heteropolyoxoanions are composed of a closepacked array of oxygens, and this representation in turn reveals their potential as soluble metal oxide analogs.
heterogeneous-insoluble, metal oxide-supported catalysts. Given the difficulties4in determining the structures and therefore the mechanisms of oxide-supported catalysts, and thus the subsequent problems in rationally improving such catalysts, the attraction of using polyoxoanion-supported, atomically dispersed transition metals as analogs5 becomes apparent. In addition, 1995 American Chemical Society
768 Inorganic Chemistry, Vol. 34, No, 4, 1995 polyoxoanions having tetraalkylammonium or other organic countercations allow these polyanionic species to be solubilized in nonaqueous solvents, conditions often ideal for organic and organometallic reaction chemistry6 as well as other kinds of reactivity such as oxygen atom- or group-transfer processes.’ Because of these possibilities, polyoxometalate-supported organometallic complexes have attracted considerable interest over including our own efforts.lOsll the past 10 Several years agolOdwe reported the synthesis and characterization of P2w15Nb30629- as well as the preliminary synthesis and solution characterization of the covalently attached, polyoxoanion-supported complexes [(C5Me5)RhgP2W~5Nb3062l7-, 1, and [ ( C ~ H ~ ) R U * P ~ W I ~ 2.12 N ~ However, ~ O ~ ~ ] ~due - ,to the inherent presence of contaminating [(n-C4Hg)a]+BF4-, pure samples of 1 and 2 were not isolated nor could their solid-state
Pohl et al. composition and purity (e.g., by elemental analysis) or solidstate structure be established. Herein we report the isolation and characterization of [(CsMes)~*pzWisNb3O6zl7-,1, and [(c6H6)RU*p2w15Nb3o621~-, 2, as homogeneous, yellow solids as their all-[(n-C&)Y]+ salt (for 1) or as complexes with mixed [(n-C4H9)Y]+/Na+cation composition (for 1 and 2). The composition of these complexes was established by complete elemental analyses and solution molecular-weight measurements. Structural characterization in solution was accomplished by IR, ‘H NMR, 31PNMR, and le3W NMR spectroscopy. In addition, [ ( C ~ M ~ ~ ) R ~5Nb3O62l7.P~WI was characterized by a single-crystal X-ray crystallographic structural analysis, providing the long-sought first solid-state structure of a Dawson-type heteropolyoxoanion-supported organometallic complex.
(5) Obvious disadvantages of polyoxoanion-supported catalysts, relative Results and Discussion to their commercially important and well-studied solid-oxide counterparts, include the limited flat surface-oxide area (6 oxygens) of the Synthesis, Isolation, and Characterization of [(CsMes)present polyoxoanions, the inability (at least presently) to support multimetal clusters or particles, and the lack of high-temperature Rh*PzWlsNb3Oaz]’-. The synthesis of [ ( C ~ M ~ ~ ) R ~ * P Z W I ~ thermal stability of the type commonly found in solid-oxide-supported Nb30a2I7-, 1, parallels those reported earlier for the [(1,5heterogeneous catalysts. Hence, these polyoxoanion-supported analogs COD)MI+ analogs [ ( ~ - C ~ H ~ ) Y J S N ~ ~ [ ( ~ , ~ - C O D ) M * P ~ W I ~ ~ are not intended to replace or even to closely “model” polymetallic (M = Ir, Rh).I3 It follows the conceptually straightforwardroute metal cluster particles supported on solid oxides; this work has not been undertaken in order to “model” oxide-supported catalysts. (Model outlined in eqs la,b. The desired product is obtained as an studies in science generally provide information about the model analytically pure, homogeneous, yellow complex as its all [(nsystem only, rigorously speaking.) Rather, these polyaxoanionC4H9)4Nl+ salt, E(n-C4H9)4Nl7[(C5Me5)Rh.P2W15Nb3O62l1on a supported complexes are novel compositions of matter which are in principle completely characterizable structurally and mechanistically 2.3 g scale (50% yield). (The synthesis of the mixed [(nat the atomic level. In addition, as new and novel materials, they should C‘H&N]+/Na+ salt gives a higher yield (70%); however, this have their own unique chemistry and reactivity. In selected cases they material is slightly impure by elemental analysis and hence is may provide the best available examples of badly needed spectroscopic reported in the supplementary material). models of atomically dispersed metal complexes, for example Rh(CO)2*~olid-oxides,~ but we emphasize that this has never been our primary goal. (6) (a) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (b) Katsoulis, D. E.; Pope, M. T. J. Chem. SOC., Chem. Commun. 1986, 1186. (7) (a) Katsoulis, D. E.; Pope, M. T. J. Chem. Soc., Chem Commun. 1986, 1186. (b) Piepgrass, K.; Pope, M. T. J. Am. Chem. SOC.1989, 1I I, 753. (8) Besecker, C. J.; Klemperer, W. G. J. Am. Chem. SOC.19%0,102,7598. (9) (a) Besecker, C. J.; Day, V. W.; Klemperer, W. G. Organometallics 1985, 4, 564. (b) Day, V. W.; Fredrich, M. F.; Thompson, M. R.; Klemperer, W. G.; Liu, R.-S.; Shum, W. J. Am. Chem. SOC. 1981, 103, 3597. (c) Besecker, C. J.; Day, V. W.; Klemperer, W. G.; Thompson, M. R. J. Am. Chem. SOC. 1984, 106,4125. (d) Besecker, C. J.; Day, V. W.; Klemperer, W. G.; Thompson, M. R. Inorg. Chem. 1985, 24, 44.(e) Besecker, C. J.; Klemperer, W. G.; Day, V. W. J. Am. Chem. SOC. 1982,104,6158. (f) Klemperer, W. G.; Yagasaki, A. Bull. Chem. SOC. Jpn. 1989, 2041. (g) Main, D. J. Dissertation, University o’€ Illinois, Urbana-Champaign, 1987. (h) Klemperer, W. G.; Main, D. J. Inorg. Chem. 1990, 29, 2355. (i) Day, V. W.; The molecular formula of 1 is established by an elemental Klemperer, W. G.; Main, D. J. Inorg. Chem. 1990,29, 2345. (i) Day, analysis (all elements, including oxygen; see the Experimental V. W.; Eberspacher, T. A.; Klemperer, W. G.; Planalp, R. P.; Schiller, Section). Ultracentrifugation sedimentation-equilibrium moP. W.; Yagasalu, A.; Zhong, B. Inorg. Chem. 1993, 32, 1629. lecular-weight experiments using isolated l confirm our previous (10) (a) Finke, R. G.; Droege, M. J. Am. Chem. SOC. 1984, 106,7274. (b) Finke, R. G.; Rapko, B.; Domaille, P. J. Organometallics 1986, 5, findingIw that the compound is an unaggregated monomer under 175. (c) Finke, R. G.; Rapko, B.; Saxton, R. J.; Domaille, P. J. J. Am. the conditions of the experiment %,(talc for the anion) 4328, Chem. SOC.1986, 108, 2947. (d) Edlund, D. J.; Saxton, R. J.; Lyon, k,(found) 4873 f 600 in C H F N containing 0.1 M [(nD. K. Finke, R. G. Organometallics 1988, 7, 1692. (e) Suslick, K. S.; Cook, J. C.; Rapko, B.; Droege, M. W.; Finke, R. G. Inorg. Chem. C~HS)Y]+PF~(supplementary material, Figure A; M, = 1986,25, 241. (f) Rapko, B. Dissertation, University of Oregon, 1986. weight-average molecular weight).I4 (g) Finke, R. G.; Green, C. A.; Rapko, B. Inorg. Synth. 1990, 27, 128. (h) Edlund, D. J. Dissertation, University of Oregon, 1987. (i) Lyon, D. K. Dissertation, University of Oregon, 1990. (12) (a) The first reports of polyoxoanion-supported [(C5Me5)Rhl2+ are (11) (a) Recently we were able to show that [(n-C4Hg)&I]~Na3[(1,5from Klemperer’s laboratories: Besecker, C. J.; Day, V. W.; KlemCOD)IrP2WlsNb3062] is a precatalyst for both hydrogenation“d and perer, W. G.; Thompson, M. R. J. Am. Chem. SOC.1984, 106,4125. oxygenation1Ic reactions. This complex was characterized by a (b) Reports of fully characterized polyoxoanion-supported [(Cs&)complete elemental analysis plus ”P, Is3W, IH, and I3C NMR, IR, RuI2+ from Klemperer’s laboratories (see also ref 12c): Day, V. W.; and sedimentation-equilibrium molecular-weight measurements.’I f ” 0 Ebenpacher, T. A,; Klemperer, W. G.; Planalp, R. P.; Schiller, P. NMR studies demonstrate that [( 1,5-COD)Ir]+ binds in overall C3” W.; Yagasaki, A.; Zhong, B. Inorg. Chem. 1993,32, 1629. Klemperer, symmetry to three Nb2O bridging oxygens of the Nb30g3- ox gen W. G.; Zhong, B. Inorg. Chem. 1993, 32, 5821. (c) Attanasio, D.; surface in the soluble, metal-oxide support s stem, P ~ W I S N ~ ~ O ~ $ . I ’ ~ Bachechi, F.; Suber, L. J. Chem. Soc., Dalton Trans. 1993, 2373. (b) The complex [ ( C ~ H ~ ) R U . P ~ W I ~2,Nhas ~ ~also O ~been ~ ] ~shown , (13) (a) Nomiya, K.; Pohl, M.; Mizuno, N.; Lyon, D. K.; Finke, R. G. to catalyze the oxygenation of cyclohexene with molecular oxygen.’lcJm Inorg. Synrh. in press. (b) Pohl, M.; Lyon,D. K.; Mizuno, N.; Nomiya, (c) Mizuno, N.; Lyon, D. K.; Finke, R. G. J. Catal. 1991, 128.84. (d) K.; Finke, R. G. Inorg. Chem., in press. Lin, Y.; Finke, R. G. J. Am. Chem. SOC. 1994, 116, 8335. (e) Pohl, (14) In the determination of the molecular weight, the absorbance of the M.; Finke, R. G. Organometallics 1993, 12, 1453. (f) Pohl, M. solution is measured in the UV region (335 nm), where both the Dissertation, University of Oregon, 1994; Chapter IV. heteropolyoxoanion and [(CsMe5)Rhl2+absorb.
Inorganic Chemistry, Vol. 34, No. 4, 1995 769
Trisubstituted Heteropolytungstates
I
I -5.0
I -10.0
I -15.0 ppm
Figure 2. 31PNMR spectrum in DMSO-& of the all-[(n-C4Hg)4N]7 complex of 1 showing its high homogeneity and purity (295%).
Infrared measurements (supplementary material, Figure B) confirm that the Dawson-type, “PZMIg06zn-” heteropolytungFigure 3. Structure of [(C5Me5)Rh.P2WlsNb30621’-. The Nb and W(3) state framework remains intact under the conditions of the atoms each are composites, Nbo sW0 5 , because the anion adopts two equally-weighted orientations related by the mirror plane of a 3/m (Dw) synthesis, consistent with formulation of 1 as containing intact crystallographic site. The C5Me5group lies in a plane normal to the P2w15Nb30629- with [(CsMes)RhI2+firmly supported upon its anion C3 axis and is disordered because of the incompatibility of its surface. Moreover, a careful examination of the IR spectrum 5-fold symmetry with that axis. The CsMe5 could not be located by of 1 as a KJ3r disk reveals a lack of bands corresponding to X-ray single-crystal structure analysis but is present by ‘H NMR coordinated CH3CN. The only plausible inner-sphere ligands spectroscopy and elemental analysis. for the otherwise 12e- rhodium fragment [(C5Me5)RhI2+,then, are the surface oxygens of P2w15Nb30629-. The ‘H NMR spectrum shows a single resonance for the C5Me5 group at 6 1.82 in CD3CN (6 1.84 in DMSO-&), providing Further solution characterization relied heavily on multifurther evidence (in addition to the 31PNMR) for existence of nuclear NMR spectroscopy (31P,Ig3W,and ‘H NMR). The 31P and Ig3WNMR spectra are useful in characterizing the heta single species in solution. No resonances are observed at 6 eropolyoxoanion support’s purity and framework, respectively 1.87, indicating the absence of free [Rh(C5Me5)(CD3CN)3I2+ (and provide initial evidence for the binding of the fragment). (in a control experiment 10% of [Rh(C5Me5)(CD3CN)3I2+was added to the solution proving that it could have been easily A 31P NMR spectrum of 1 in DMSO-d6 (Figure 2) shows detected had it been present). primarily two resonances at 6 -8.4 and -14.2 with integrated intensities of 1:l as expected for the two types of phosphorus Evidence for the covalent, inner-sphere bonding of [(Cspresent. These 31PNMR results demonstrate that 1 is obtained Mes)Rhl2+to P2wlSNb30629- (rather than an [(CsMes)Rh(CH3as at least 95% of a single isomer. CN)#+ ion-paired complex) is provided by ion-exchange experiments (as well as IR, ‘H, and 31PNMR spectra; vide Also quite informative are changes in the 31P NMR peak supra), specifically ion (non)-exchange experiments which positions for 1 in DMSO-d6 (-8.4 and -14.2 (f0.2) ppm) in demonstrate the nonexchangeability in acetonitrile of the cationic comparison to the starting material P2w15Nb30629- (-7.2 and [(CsMes)Rhl2+component of [(CsMe~)Rh.PzW1sNb306~]~-. -14.2 (f0.2) ppm), as they show a pronounced upfield shift The IE3WNMR spectrum of 1 in DMSO-& shows three of the phosphorus resonance closest to the “Nb30g3-” cap in peaks, indicating that [(CsMes)Rh*PzW1sNb3062]~has C3” P2w15Nb30629- (recall Figure 1). This observation is in accord pseudosymmetry on the Ig3W NMR time scaleI5 (pseudo, as with, and actually prima facie evidence for, preferential binding the Rh.P2w15Nb3062 part of the molecule has C3” symmetry, of [(CsMes)Rhl2+to the more basic “Nb30g3-” cap in solution. but the CsMe5 moiety does not, having a C5 axis instead). The The observed line widths, AYI/Z= 3-4 Hz for the two integrated intensities are in accord with the presence of two resonances of 1 in the 31PNMR spectrum are comparable to those observed for the unsupported polyoxoanion, P z W I S N ~ ~ O Q ~ - ,tungsten belts consisting of six WO6 octahedra each and a tungsten cap of three WO6 octahedra. which exhibits values for A Y I Rof 3-5 Hz. The ‘H, 31P,and Ig3WNMR spectroscopic data for the mixed Na+/[(n-C4Hg)a]+ salt of 1 are identical to those for the all(15) (a) This argument assumes that any non-C3, isomers would be detected by Is3W NMR spectroscopy.This assumption is supported by the fact [(n-CdHg)a]+ salt. (When 31Pand le3WNMR data for sodiumthat C, symmetry isomers are detected by lS3WNMR for [CpTi3+* containing complexes are collected, addition of Kryptofix 2.2.2. P2w15v30629-]6-, although a more strongly supported CpTi3+trication to the sample solutions is necessary. Otherwise, ion-pairing is involved,15bplus the fact that even Naf ion-pairing is detectable interactionsbetween Na+ and primarily the more basic ‘“b@g3-’’ (in the absence of added Kryptofix 2.2.2.) as excess line width in the Ig3WN M R spectrum of heteropolyoxoanion-supportedcomplexes.lldJZb end of P2w15Nb30f,29- lead to line-broadening and the observa(However, our other work suggests that Is3W NMR 0fP2W,@j&~-tion of additional peaks.’Id) However, note that the isolated supported organometallics becomes relatively insensitive once monocomplex of 1 with mixed Na+/[(n-C4Hg)dN]+ countercation cations such as [Ir(l,S-COD)]+ are supported.”‘) (b) Rapko, B. M.; Pohl, M.; Finke, R. G. Inorg. Chem. 1994, 33, 3625. composition is somewhat less pure than the analytically pure
770 Inorganic Chemistry, Vol. 34, No. 4, I995
Pohl et al. Table 3. Bond Lengths in the Anion
Table 1. Crystallographic Data for
(Ay
N~[(~-C~H~)~NI~[(C~M~~)R~.P~N~~WI~~~~~’~~M~CN.~OM~~CO approx compn C156Hj21Nl$rlaNb3072P2RhW15 fw 6798 space group P6jlm a 20.544(5) 8, b 20.544(5) 8, C 34.648(6) k, V 12664(8)A3 Z 2
1.78 g cm-j T 21 “C 1 0.7 10 69 k, P 71.9 cm-‘ I ) ) re1 transm coeff 0.78-1.00 ( no. of obs rflns 3023 [ I 2 3a(/)] R(Fo)a 0.096 R,(F,)~ 0.147 dmlc
W(1)-0(1) W(1 )-OW W( 1) - 0 ( 5 ) W(1)-0(6) W( 1)-O(7’) W(l)-O( 12) W(2)-0(2) W(2)-0(5) w(2)-0(7) W(2)-0(8) w(2)-0(9)
1.77(3) 1.90(1) 1.95(3) 1.93(3) 1.88(3) 2.33(3) 1.82(3) 1.87(3) 1.94(3) 1.93(1) 1.92(3)
W(2)-0( 12) W(3)-0(3) W(3I-W) W(3)-0(9’) W(3)-O( 10) W(3) -0( 1Oi) W(3)-O( 11) Rh-O( 10) P-O( 11) P-O(12)
2.39(3) 1.68(3) 1.90(3) 1.92(3) 2.02(3) 1.92(3) 2.41(3) 2.06(3) 1.61(6) 1.53(3)
Unrefined bond lengths: Rh-C(15), 2.20; Rh-C(16), 2.14. Table 2. Atomic Coordinates and Isotropic and Thermal Parameters (A2) for Atoms in the Anion
atom
X
Y
Z
W(1) W(2) W(3)” Rho Nb“.b P O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12)
0.48885(12) 0.64120(12) 0.55610(17)
0.17047(12) 0.15105(12) 0.28886(17)
0.19562(6) 0.19557(6) 0.10743(8) 0.0398(4) 0.10744.4 0.1934(7) 0.1854(8) 0.1857(8) 0.0731(9)
211
I13
0.5561
0.2889
213
‘I3
0.3947(17) 0.6429(16) 0.4890(18) 0.4979(27) 0.5376(16) 0.5129(16) 0.7438(15) 0.6183(27) 0.6605(15) 0.621 l(17)
0.1009(17) 0.0651(17) 0.2565(18) 0.1673(27) 0.1 104(16) 0.2090(16) 0.2262(15) 0.1384(28) 0.1856(16) 0.2527(17)
213
‘13
0.6168(14)
0.2518(15)
’14
0.1890(8) 0.1436(8) 0.2078(8) l/4
0.1430(8) 0.0826(9) 0.1469(16) 0.2064(7)
Be,,’
A2
3.9(1) 3.9(1) 4.4(1) 6.1(6) 4.1(5) 4.4(7) 4.37) 5.7(9) 6(1) 4.2(7) 3.8(7) 337) 6(1) 3.4(7) 5.1(8) 5(1) 2.9(6)
anisotropic thermal parameters for non-hydrogen atoms of 1 are given with estimated standard deviations in Table 2. Bond angles and lengths for [ ( C ~ M ~ ~ ) R ~ . P ~ Ware I~ given N~~O~ZI~in Tables 3 and 4;Table 5 lists the distances between heavy atoms. An interesting and important point is that only two [(nC~H~)~N]~N~[(CSM~S)R~*PZN~~WI 50621 units are present in a cell large enough to accommodate five or six in the absence of solvent (supplementary material, Figure G). This is presumably one key reason that it has been very difficult to prepare diffracting crystals of the [(n-CdH&N]+ salts of such large polyoxoanions in non-hydrogen-bonding solvents. Synthesis, Isolation, and Characterization of [(C&)Ru* P2WlsNb3062I7-. The preparation (eqs 2a,b) of [(C&)RuP2w15Nb3062]7-, 2, proceeds analogously to the one described above for the [(CsMe5)Rhl2+ heteropolyoxoanion-supported complex. [Note that we had to use a refined procedure for the
a Site occupancy factor 0.5. W(3) and Nb have the same x , y , z, and U,,parameters. For the metal atoms, B, = (8x2/3)C,C,Ufi*la*p,*a,.
all-[(n-C4H9)4N]+ salt of 1 as judged by elemental analysis and 31PNMR spectroscopy (see the supplementary material for more details). There can be little doubt, therefore (and given the analytical, MW, IR, ion-exchange, and 31PNMR evidence), that even in solution [(C5Me5)RhI2+is firmly attached to a single P2w15Nb30629-, Moreover, the average symmetry of 1 in solution is also (pseudo) C3, (based on the Is3W NMR). Hence, the [(CSMe5)RhI2+ must be attached to the “Nb3093-” cap equally to each of the three NbzO oxygens (i.e., in order to achieve effective C3” symmetry), a conclusion that is independently and unequivocally confirmed by a single-crystal X-ray structure analysis (vide injra) of 1 in the solid state. X-ray Single-Crystal Structure Analysis of 1. An X-ray diffraction structure analysis shows that 1 is composed of discrete, disordered [(C5Me5)Rh0P2W~sNb3062l7anions (Figure 3, Table 1) and [(n-C4H9)4N]+ and Na+ cations. (Due to its lower solubility, the mixed Na+/[(n-C&)&J]+ salt of 1 crystallizes more easily than the all-[(n-C&)4N]+ salt.) The Dawsontype anion has approximate symmetry 3m (C3”), with the Rh atom on or very close to the polyoxoanion’s 3-fold axis and bonded (Rh-ONb2 2.06 A) to three oxygen atoms, each of which bridges a pair of heavy atoms M in a cap of the Dawson unit. The M atoms each are composites, NbO.5w0.5, because the anion adopts two equally-weighted orientations related by the mirror plane of a 3/m (D3h) crystallographic site. The C5Me5 group, which could not be located but is present (by ’H NMR spectroscopy and elemental analysis), must lie in a plane normal to the anion C3 axis; its extreme disorder is due to the incompatibility of its 5-fold symmetry with the 3-fold axis of the P2w15Nb30629- polyoxoanion axis. Atomic coordinates and
2. EtOAc recipitation 3. +3 NaiF;,
preparation of P2w15Nb30629- in its fully deprotonated form, as otherwise persistently low ruthenium analyses were obtained (see the Experimental Section for further details).] The desired product is obtained as a homogeneous, yellow solid in form of the mixed [(n-C4H9)4N]+/Naf cation salt on a 8.4g scale (76% yield). The synthesis of the mixed salt, eq 2b, allows for the isolation of [(n-C4H9)4N]+BF4- free material in high yield. The molecular formula of 2 is consistent with the elemental analysis (all elements, including oxygen; see the Experimental Section and the supplementary material, Table B, where alternative formulations are ruled out). The C, H, N, and Na analysis reveals an average countercation composition of 4.5 [(n-C4H9)4N]+/2.5 Na+ per anion.I6 In two separate control experiments (see the Experimental Section for a detailed account) it was shown that 2 is neither light- nor air-~ensitive,’~ as judged by 31PNMR spectroscopy. The lack of 0 2 sensitivity (16) Note that the given composition of [(n-Cd-I&N]4.5Na2.5 (calc: C, 17.25; H, 3.12; N, 1.30; Na, 1.07) matches the experimental values (found: C, 17.28; H, 3.27; N, 1.44; Na, 1.24) better than the alternative formulations [(n-C4H9)4N]sNaz(calc: C, 18.69;H, 3.39; N, 1.25; Na, 0.83) and [(n-C4H&N]4Na3 (calc: C, 15.80; H, 2.84; N, 1.32; Na, 1.30).
Inorganic Chemistry, Vol. 34, No. 4,1995 771
Trisubstituted Heteropolytungstates
An ultracentrifugation sedimentation-equilibrium molecularweight experiment shows that the compound is an unaggregated monomer under the conditions of the experiment (supplementary material, Figure C). Further solution characterization was accomplished by multinuclear NMR spectroscopy (31P,183W,’H NMR). A 31PNMR spectrum of 2 in CD3CN (Figure 4) acquired after addition (again to remove the ion-pairing1leqf)of 3 equiv of Kryptofix 2.2.2. shows primarily two resonances at 6 -7.7 and -13.3 with integrated intensities of 1:1, as expected for the two types of phosphorus present, and requires that 2 is obtained as at least 94% of a single species and isomer. The ‘H NMR spectrum shows primarily a single resonance for the C6H6 group at 6 5.99 in CD3CN, providing further evidence (in addition to the 31PNMR) for existence of a single supported-(C&)Ru2+ species in solution. [A smaller, unidentified resonance at 6 6.06 (less than 5% by integration and by comparison to the main 6 5.99 resonance) is also observed. However, an elemental analysis for C1- shows the absence of chloride within the detection limits (
