J . Am. Chem. SOC.1986,108, 2947-2960
2947
Trisubstituted Heteropolytungstates as Soluble Metal Oxide Analogues. 3. Synthesis, Characterization, 31P,29Si,51V,and 1- and 2-D 183WNMR, Deprotonation, and H+ Mobility Studies of Organic Solvent Soluble Forms of H,SiW9V3040X-7 Richard G. Fide,*" Brian Rapko," Robert J. Saxton,2aand Peter J. Domaille2b Contribution from the Department of Chemistry, University of Oregon, Eugene, Oregon 97403, and Central Research and Development Department, E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, Delaware 19898. Received August 19, 1985
Abstract: The trivanadium(V)-substituted polyoxoanions A-@-H2iW9V,Oar7 and HxPzW1sV3062X-9 have been synthesized from their lacunary precursors A-@-SiW90341w and P2w1505612-and fully characterized. Specifically, &HSiW,V30m.3H20, x ~=O 0-2), (Me4N)6H,PzW,5V3062.6H20, (BU,N),H~S~W,V~O ( B~ ~U, ~ N ) ~ - , H , S ~ W ~( V ~ ~ K8HP2W15V3062-9H20, ( x = 0-2) (B~~N)SH~P~WISV ( B&U , ~ N ) ~ H ~ P ~ W( BI US ~VN~) ,~K~H~S P, ~ W I S V , O ~ ~ . ~and DMF, have been prepared and characterized by elemental analysis, thermal gravimetric analysis, solution molecular weight measurements or FAB mass spectroscopy (for K6HSiW9V30a.3Hz0),IR, and IH, 29Si, 51V,18,W, r83W(S'VJ, and 2-D '83W(s'V]NMR. The results unambiguouslydemonstrate the A-@structure of the previously unknown A-@-SiW,V30a7-,the adjacent edge-sharing octahedral, or =capnpositions of the three vanadium in P2w~sV306z+,and they provide the first Bu4NCsalt, organic solvent soluble forms of H,SiW9V3040X-7and H,PzW15062*-~. The stepwise deprotonation of ( B U ~ N ) ~ H ~ S ~ Wand ,V~O~~ (Bu4N)5H4P2W,5V3062 by Bu4N+OH-in CH3CN has been studied in detail including documentation of the previously unexplained slV, and lasW NMR. The effects of the degree of protonation, dry vs. wet solvents, and added bases or acids upon 29Si, studies on ( B u ~ N ) ~ H S ~proved W~V especially ~ ~ ~ informative, unequivocally establishing its C, symmetry, A-@-HSiW9V30a6 structure by "V, ISsW,'83W(s'V),and 2-D '83W(SIV) NMR and establishing the HzO, pyridine (py), and pyH+CF3C0< assisted H+ mobility in HSiW9V30a06. These results allow HSiW,V3Om6 to serve as a homogeneous model for H+ mobility on a heterogeneous oxide surface and, when combined with literature studies of reduced heteropolyanions ("heteropoly blues"), a homogeneous model for He (H+ + e-) spillover. The finding of a four-bond vanadium to tungsten (4Jv-0-w) coupling, the problems in the synthesis and characterization of organic solvent soluble R4N+salts of polyoxoanions, and the significance, implications, and future directions of this work are also presented and discussed.
Polyoxoanion~~ consist of a close-packed array of oxide anions and, as such, can be regarded as discrete fragments of extended metal oxide lattices4 (Figure 1). Since 1979 we have pursued a program aimed at exploiting this property by focusing upon the preparation of a series of C,, symmetry, Bu4N+ counterion and, therefore, organic solvent soluble trisubstituted heteropolyanions SiW9M3040J+(Figure 2A) and P2W15M3062L(Figure 2B) ( M = Vs+, Nb5+, Ta5+, Ti4+, Zr4+, Hf"+). One of our goals is to compare these polyoxoanions to their parent oxides M 2 0 S( M = V, Nb, Ta) and M 0 2 ( M = Ti, Zr, Hf).la A second goal is to exploit their anticipated significant surface change density at oxygen for the support of organotransition-metal catalysts or catalyst precursors.' Our third, longer term, and major goal is to use the resultant "solubilized heterogeneous catalysts" to probe the fascinating opportunities that exist for new types of catalysts, for needed spectroscopic models of oxide-supported heterogeneous catalysts and chemisorbed species, and for the insights that should
result from detailed mechanistic studies of such soluble, oxidesupported catalysts. Vanadium (Vs+) containing polyoxoanions with surface-supported transition metals have not been previously but accrue additional interest due to the role of V5+ containing heteropolyanions in Wacker chemistry as Pdo/2+reoxidation cataly~ts.~Prior to presenting the results of the present study, important previous work deserves mention and it will prove useful to follow this by a list of the problem areas that existed when we began these studies. Our approach and focus toward A-SiW9M3040r and P z W I S M 3 O 6trisubstituted p with higher valent metals like M4+ and M5+builds upon the findings of Tbzi, Pope, and c0-workers6a
(1) (a) Part 1: Finke, R. G.; Droege, M. W. J. Am. Chem. Soc. 1984,106, 7274. One conclusion of this paper is that the trisubstituted heteropolytungstate dimer, Si2WI8Nb6o7~-, behaves, in effect, as a solubilized piece of (Nb205)3 sandwiched between two 'SiW9031c' heteropolytungstate fragments. (b) Part 2: Finke, R. G . ;Rapko, B. Organomerallics 1985, 4, 175. (c) Part 4 Finke, R. G.;Rapko, B., manuscript in preparation. d Formation of the disubstituted dimers1' [PW9M2(H20)0,4]21~12~nd1~ )IP2W15M2(H20)056]212occurs when B-PW90349-and P W150s6 are treated with low-valent metals, M"+, such as Co2+ and Zn2+f (e) Finke, R. G.; Droege, M.; Hutchinson, J. R.; Ganzow, 0.J . Am. Chem. Soc. 1981, 103, 1587. ( f ) Finke, R. G.; Droege, M. W. Inorg. Chem. 1983, 22, 1006. (2) (a) Department of Chemistry, University of Oregon, Eugene, OR 97403. (b) Central Research and Development Department, E. I. du Pont de Nemours and Co. Experimental Station, Wilmington, DE 19898. Contribution 3672. (3) Pope, M. T. In 'Heteropoly and Isopoly Oxometallates"; SpringerVerlag: New York, 1983. (4) This feature was first noted by Baker in 1961. Baker, L. C. W. In "Advances in the Chemistry of Coordination Compounds"; Kirschner, S., Ed.; MacMillan: New York, 1961; p 604.
0002-7863/86/1508-2947$01.50/0
(5) (a) Kozhevenikov, I. V.; Matveev, K. I. Appl. Caral. 1983, 5 , 135; Russ. Chem. Reu. (Eng. Transl.) 1982, 51, 1075. (b) Ogawa, H.; Fujinami, H.; Taya, K.; Teratani, S. J . Chem. SOC.,Chem. Commun. 1981, 1274. (e) Taraban'ko, V. E.; Kozhevenikov, I. V.; Matveev, K. I. Kinet. Karal. 1978, 19, 1160. (d) Davidson, S. F.; Mann, B. E.; Maitlis, P. M. J . Chem. SOC., Dalton Trans. 1984, 1223. (6) (a) Mossoba, M. M.; OConnor. C. J.: Pope, M. T.: Sinn, E.: HervC. G.; TtzC, A. J . Am. Chem. Sac. 1980, 102, 6864. (b) Harmalker, S. P.; Leparulo, M. A.; Pope., M. T. J . Am. Chem. SOC.1983, 105, 4286. Harmalker, S. p.; Pope, M. T. J . Am. Chem. SOC.1981, 103, 7381. This earlier synthesis is summarized below: P ~ w ~ ~ o ~+63V1vOS04 12-
W H P
I pH47 NaOAc/HOAc buffer
P2w15vv30629-
NHICl
-
+ P2w~6vv206~~
I NsOH
IO
pH IO 10
deJ1roy the
PPlSVP6l+ I I e-de"Mic
from hot H P
0 1986 American Chemical Society
Finke et al.
2948 J. Am. Chem. SOC.,Vol. 108, No. 11, 1986 A
B
A
Figure 1. Polyhedral (A)and space filling (B) representations of CYSiWi,040e. In the polyhedral representation, A, the central phosphate is represented as the black tetrahedron. Tungsten atoms are located at the center of each octahedra with oxygens at the vertices. The edge- and corner-shared oxygens of the W 0 6 octahedra are easily distinguished in three of the four W,013 triads of edge-shared octahedra (centered at 12 o'clock, 4 o'clock, and 8 o'clock) that are visible here. The space filling model, B, shows the close-packed arrangements of the oxide anions with the shaded circles representing terminal oxygens.
and those of Harmalker, Leparulo, and Pope6bdemonstrating that A-a-SiW9V30407-and P2wIsv30629-, respectively, can be prepared. Also relevant is our work showing that B-PW90349-and ~disubstituted dimers rather than PW9M30.,J P2W1 5 0 5 6 ~ form and P2W1SM3062zwhen lower M2+such as co2+and Zn2+ are used.Id-' In spite of this previous work, a significant number of problems remained at the onset of the present ~ t u d i e seven , ~ for the Vs+ derivatives A-fl-H$iW9V30Mr7 and H,P2W15V3062X-9. Only a one-line sentence in a footnote6a has been published for A-a-SiW9V30M7-and A-fl-SiW9V30M7-has not been previously described. The previous synthesis of P2wIsv30629- was the result of work focused toward the VIv derivative ( P 2 w ~ ~ v 2 v 1 v 0 6 2 ' ~ ) for ESR studies, so that the reported synthesis of P2wl5v,o6?is without isolation in an unknown yield and required unnecessary steps.6b In our work, the proper choice of pH, countercation, the variable number x of H+ for different countercations, and the solvents for the synthesis and recrystallization of H$iW9V30MX-7 and H,PzW1SV3062*-9and their characterization-especially as their previously unknown R4N+ salts-has required a significant amount of effort and extensive 31P,2?Si, 51V,and lE3WN M R time. In the synthesis of ( B U ~ N ) ~ H ~ P ~ Wfor I ~example, V ~ O ~ ~"simple" , metathesis from K+ to (Bu4N)+yielded a product of varying Bu4N+even when the solution pH was carefully controlled. The above illustrates some of the problems and pitfalls that were not well documented in the polyoxoanion area7 which ~~
~~~
Figure 2. Polyhedral representations of (A) a-(1,2,3) SiW9M,040(Atype derivative) and (B) a-(1,2,3)P2WISM,O6,(B-type derivative). The octahedra with hatched lines represent those of the substituted metal vanadium. The A isomer refers to the presence of a triad of cornersharing (Wi, W,, W, in 2A) octahedra and the B isomer refers to the presence of a triad of edge sharing ( W i , W,, W, in 2B) octahedra (see also Figure 6). The M-0-M angle in an A isomer is about 150', as compared to the about 125' M-0-M angle in the B isomer. The a isomerism refers to the arrangement of the W,,, W,,, W,, triad in (A) or the W16, W,,, W,, triad in (B),each of which would be rotated by 7/3 in a p isomer (see also Figure IO). ,**..>*
,
I1.X
-
I'lT6
2600
/'
2.00
""'
' 218C
Figure 3. FAB negative ion mass spectrum of K6HSiW9V,040dissolved in a thioglycerol matrix. A [MH]- = [K&W9V,0,0]- parent ion at m / z = 2712 is observed along with extensive Kt for H + exchange (cationization), peaks due to the loss of 0 ( m / z = 16), and peaks due to the loss of WO, ( m / z = not shown). The envelopes of higher molecular weight (above 2712) are separated by m/z 18,suggesting that they are (H20), adducts.
are addressed by the present studies. Additionally, it was not clear at the beginning of our work whether or not the deprotonation of H$iW9V3040"7 or H,P2W15V3062d could be achieved without decomposition, since SiW,20M4-+ OH- rapidly degrades to Si0;and W042in the range of seconds to minutes depending upon the exact conditions)8 and since P2w,6v20628- is known to be degraded by base.6b Finally, the multiple line and/or broadened slV and IE3WN M R of protonated R4N+ salts of polyoxoanions in dry organic solvents was not previously documented and was a significant impediment to our early studies. Herein we report the full details of our synthesis and characterization by elemental analysis, TGA, solution molecular weight measurements, FAB/mass spectroscopy (FABMS),9 IR, and 31P, 29Si, 51V, and le3W N M R of K6HSiW9V3040 and ( B U ~ N ) ~ H , S ~ W ~ The V ~ Ostepwise ~ ~ . deprotonation and H+ mobility studies starting with ( B U ~ N ) ~ H , S ~ W ~ Vare , Oreported ,~ with an emphasis upon monoprotonated ( B U ~ N ) , H S ~ W ~ V , ~ ~ ~ , including its detailed structure a s elucidated by 1- and 2-D IE3W and lE3W{"V]N M R . Simplified, high-yield syntheses for (cation)9-xH,P2Wi5V3062(cation = K+ ( x = 8), Me4N+ ( x = 6 ) ,
(7) (a) Some previously documented probIems in the synthesis and charactenzation of polyoxoanions include difficulties in obtaining reliable elemental analyses,7bin obtaining X-ray diffraction and in obtaining accurate solution molecular weights.7d Some additional problems, many of which were uncovered by the present studies, are problems in the NMR of protonated anions in dr organic solvents,7cin the (non)crystallization of Keggin (e&, SiW9V,04,') and Dawson (e.g., P,W,5V3062*) anions containing greater and in the thermal than four and six Bu4N+ counterions, re~pectively,~~~' An additional difficulty stems from the isomerism of some polyox~anions.~* absence, to date, of chromatographic methods (except in raresb instances), 29Si, leaving only recrystallization as the purification method and requiring 31P, or s l V or other sensitive NMR methods, rather than chromatographic methods, as the criterion of homogenity. (b) See the results and discussion in: Finke, R. G.; Droege, M. W. Inorg. Chem. 1983, 22, 1006. Smith, D. P.; Pope, M. T. Anal. Chem. 1969,40, 1906. Fernandez, M. A,; Bastiaans, G. J. Anal. Chem. 1979, 51, 1402. A reliable determination of the number of oxygen atoms is a major step toward avoiding misformulated polyoxoanions. For K6HSiW9V3010(see the text) and elsewheregbwe have shown how elemental analysis combined with FABMS can fulfill this need in favorable cases. See also ref 32. (c) Evans, H. T., Jr.; Pope, M. T. Inorg. Chem. 1984, 23, 501. Acerete, R.; Hammer, C. F.; Baker, L. C. W. Inorg. Chem. 1984, 23, 1478 and references therein. (d) See the supplemental materials for ref la and the discussion and references therein. (e) See the results presented in the (8) (a) Kepert, D. L.; Kyle, J. H. J . Chem. Sor., Dalton Trans. 1978, 137; text. (f) For example, ZnWl,0a6 and AlW120msare obtained as crystalline (b) J. Chem. Soc., Dalton Trans. 1978, 1781. ( c ) These papers8a,breport a salts with only four Bu4N+cations: Nomiya, K.; Miwa, M. Polyhedron 1983, significant polyoxoanion stabilizationeffect toward OH- degradation by R4N+ 2,955. We find that (Bu4N),H3SiW9V30m, (Bu4N),[CpTi.SiW,V3Oa], and counterions. are crystalline while ( B U ~ N ) ~ S ~ W ~ V ~ O (9) ~ ~ (a) , Finke, R. G.; Droege, M. W.; Cook, J. C.; Suslick, K. S. J . A m . ( B U ~ N ) ~ [ ( C ~ M ~ ~ ) R ,~ l a . S ~Chem. W ~ NSOC. ~ ~1984, O ~106, ~ ]5750. The K+ salt referred to as "K4H3SiW9V304q" ( B U ~ N )W9Nb3010,1a ~S~ ( B u ~ N ) . $ ' ~ W , ~ Vand, ~ ~ ~(Bu4N)9P2WISNb3,062 ~, are not, in our hands. therein is correctly reformulated as K6HSiW9V304,based on a K+ analysis Crystallization attempts with other, smaller cations should solve this problem (Experimental Section). (b) Suslick, K. S.; Cook, J. C.; Rapko, B.; Droege, and are in progress. (g) See ref le,f and 37. M. W.; Finke, R. G. Inorg. Chem. 1986, 25, 241.
J . Am. Chem. Soc., Vol. 108, No. 11, 1986 2949
Trimbstit uted Heteropoly t ungstates Bu4N+ (x = 5 ) ) , their characterization, and deprotonation studies of are also reported. Results and Discussion ( I ) Synthesis of A-j3-K6HSiW9V3040 and A - 8 (Bu~N)~H,S~W~V The , Otrisubstituted ~~ heteropolytungstate H,SiW9V3040r7 can be obtained as the K+ or Bu4N+ salt from the reaction of solid A-P-Na9HSiW9O3,-23H2Oioadded to a vigorously stirred solution of sodium metavanadate at pH N 1.5, where V02+forms in aqueous solution," according to the following stoichiometry: HSiW90349-+ 3 v 0 3 - + (5 + x)H+ H$iW9V3040X-7+ 3 H 2 0 (1)
-
Following the facile initial synthesis of the H$iW9V3040x-7 anion (indicated by the rapid development of a deep cherry-red solution upon addition of the lacunary heteropolytungstate to the pale yellow vanadate solution), conversion to the potassium salt allows isolation of the trisubstituted heteropolyanion in a form that can be purified rapidly and in good yield (82% from SiW903410on a 37-g scale by recrystallization from hot aqueous solutions and drying at 25 OC under vacuum). Evidence for the composition K6HSiW9V,040.3H20is provided by the elemental analysis for K+, by thermal gravimetric analysis (TGA) (calcd for 3 H 2 02.0%; W found 1.7%), and especially by FABMS. Previously we presented L L U 2 - l 1100 800 600 CM 1100 800 500 c n - l a preliminary report of the FABMS of K6HSiW9V3040as part of the first mass spectra reported for p o l y o x ~ a n i o n s .Figure ~~ 3 Figure 4. IR spectra of a-(Bu,N)4SiWl,040 (top, A and B) and A-j3presents the negative ion FABMS of K,HSiW9V3040obtained ( B U , N ) ~ H ~ S ~ W , (bottom, V ~ O ~ ~C and D), taken as C H 3 C N solutions in thioglycerol. Negative ions for K6HSiW9V3040-and K5H(left-hand sides, A and C ) and as KBr disks (right-hand side, B and D). SiW9V,040-are observed along with extensive exchange of K+ Assignments are based by analogy to the literature assignments12dof XW120,0wand are indicated in the text. and H+ cations (cationization). Loss of 0 ( m / z = 16) and W 0 3 ( m / z = 232) are other dominant features of both this and othergaqb heteropolytungstate FABMS. Significantly, the FABMS firmly [calcd for ( B u , N ) ~ H , S ~ W ~ V ~ O2963; ~ " , ~found, ~ . ~ 3076 (Figure and unequivocally establishes the SiW9V30a7- composition of the 1a. supplemental materials)], but the most compelling evidence trisubstituted Keggin anion including the number of oxygens, a for the SiW9V30407-formulation is the FABMS of the potassium previously unattainable result even by an expensive and relatively salt. The seven minus charge on the SiW9V304:- anion and the With the molecular slow Si, W, V, and 0 elemental analy~is.'~.~ presence of only four Bu4N+but no Na+ or K+ by analysis and composition in hand, the overall C,, symmetry of the SiW9V30407no HzO (or H,O+) by TGA require the presence of three H+ for anion was readily established by the two-line lg3WN M R observed charge balance, ( B U ~ N ) ~ H , S ~ W , V , O The ~ ~ presence . of three in D 2 0 , 6 -110.4 (6 W), -112.9 (3 W). H + is also fully supported by a pH titration and pH vs. 51Vand Metathesis of the K+ salt with Bu4N+Br- in acidic, aqueous Ig3WN M R titrations detailed in section 111. Several attempts solution followed by crystallization of the product from acetoto directly observe these protons by 'H N M R in dry Me2SO-d6 nitrile/dichloromethane yields the desired ( B U ~ N ) ~ H , S ~ W ~ V , Oor~ dry ~ CD,CN failed, however, with only the expected resonances on a 3CF36-g scale (58-70% yield). The success of the K+/Bu4N+ for Bu4N+ being observed. metathesis is critically dependent upon maintaining the solution (B) IR Spectroscopy. In the absence of ,'P, 29Si,51V,and la3W pH at its initial low, pH -1.5 value. Metathesis followed by N M R , the IR of polyoxoanions is useful for comparison to the product precipitation results in an increase in pH and, at pH =7, IR of authentic samples (as well as the assignment of a,p isofor example, t h e more soluble, noncrystalline7' m e r ~ )so , ~that ~ the IR of ( B U ~ N ) ~ H , S ~ W , Vin , OCH3CN ~~ and ( B u ~ N ) ~ H S ~isW formed ~ V ~in ~low ~ ~yield. as KBr disks in comparison to that of (Bu4N)4SiW,2040 has been (11) Characterization of ( B U ~ N ) ~ H , S ~ W ~ V(A) , OMolecular ~. included in Figure 4. By analogy to the assignments for other O , been Formula. The molecular formula ( B u ~ N ) ~ H , S ~ W ~ V ,has XMi2040wKeggin anions,'2d the -960-cm-l band, the broader determined by elemental analysis, TGA, and solution molecular -900-cm-' band, and the very broad -800-cm-' band can be weight measurements. The elemental analysis for C, H, N, Si, assigned to W=O, Si-0 and overlapping corner-sharing ocW, V, and 0 (and Na+, K+ = 0) establishes the empirical formula tahedra M-0-M, and edge-sharing octahedra M-0-M vias [ ( B U ~ N ) ~ S ~ W ~ Vand , O ~accounts ~ , ~ ] ~for 98.8% of the total brations, respectively. These assignments have proven of value mass. Thermal gravimetric analysis (TGA) shows no detectable in establishing the site of CpTi3+attachment to SiW9V,0a7- since (0.1 M ) solutions of ( B U ~ N ) ~ H , S ~ W ~ VOnce , O ~ a~ switch . to the significantly, ca. 3.6 X lo4, more sensitive 5’V N M R was made, it became clear that low symmetry and/or multiple forms of the triprotonated H3SiW9V30a7-(and slow H+ exchange on the N M R time scale) were present and that addition of H 2 0 or bases produces an averaged spectrum as documented below. Figure 5 shows the IS3W, 51V, and 29Si N M R of (Bu,N),H3SiW9V,Om in initially dry CD3CN at 21 OC as a function of added H 2 0 (0, 3, 10, and 100 equiv of H 2 0 ) . The broad, low S / N lS3W resonance in Figure 5A is gradually transformed to the expected two lines of 2:l intensity (six belt W, three cap W; Figure 2A) in the lg3WN M R [6 -108.4 (6 W, A V ~=, 4.4 ~ f 0.2 Hz), -1 10.1 (3 W, AV,,, = 4.0 f 0.4 Hz) with ca. 10 equiv of H 2 0 ] . Concurrently, the three broad 5TVlines of approximate relative intensity 1.0, 5.7, and 9.0 collapse to a single, broad 51Vresonance (6 -579, Avllz = 1358 f 4 Hz) with ca. 10 equiv of H,O. These results require an overall average C,, symmetry for H,SiW9V,040e in the presence of H,O. The 29Si N M R behaves similarly, with the single broadened resonance in dry CD3CN sharpening to one narrow line a t -83.0 ppm ( A v , , ~ = 0.52 0.04 Hz) in CD3CN with ca. 10 equiv of H,O. The data are consistent with a H,O-assisted exchange process such as H3SiW9V30404- H 2 0 == [ H 3 0 + HzSiW9V30405-] H3SiW9V3040e H,O that, in the presence of added H 2 0 , becomes competitive with the 51V and le3W N M R timescales. It’s worth noting that the above results provide a homogeneous model for H+ mobility on a soluble oxide surface. In heterogeneous catalysis, a similar mechanism for H+ e- (H.) migration or spillover has been proposed for H2-reduced WO,, Le., H 2 0 assisted H+ (H30+)mobility on the oxide surface with e- transfer Wv) and “e- mobility” within the oxideT5 to W (Wvl + e
*
+
+
+
+
-
- I10 - 115 - :20 - i 2 5 PPY Figure 7. IS3WNMR spectrum of the free acid H7SiW9V30,,,in D20 (pD 0.9),28a2 g/mL D20, at 21 ‘ C , 10-mmvertical tube, 0.25 LB, 7000 transients, 1.6 h. Central lines are observed at -120.0 (6 W, A v , , ~= 2.41 Hz, S/N = 78) and ca. 115.4 ppm (3 W, A u , , = ~ 1.32 Hz, S / N = 42) with satellites due to the reciprocal 2 J w - ~of - ~16.4 f 1.2 Hz being
readily apparent. (although the mechanism of hydrogen spillover remains controMoreover, since “e- mobility” (mixed valence) in versial) heteropolytungstate is well studied,I6 polyoxyanions can now be said to provide a homogeneous model for both the H+(H30’) and e- mobility involved in one possible mechanism of H- spillover. The coupling or correlation of the two H+ + e- mobilities has not received but deserves study, however. Returning to the N M R studies of H3SiW9V30404-,since our synthesis utilized the crystallographically characterized A isomerEob of the SiW90341(tstarting material, we anticipated that the SiW9V30a7- product would retain the A arrangement of a triad of corner-sharing V 0 6 octahedra, Figure 6A, rather than the B arrangement of a triad of edge-sharing VO, octahedra, Figure 6B. These two possibilities are distinguishable by Is3W NMR 2Jw-e-w couplings since the former, A isomer has a belt of 6 W connected to the 3 W cap via corner-sharing octahedra (Figure ~~~
~
( 1 5) (a) Levy, R. B.; Boudart, M. J . C a r d 1974, 32, 304. (b) Vannice,
M. A.; Boudart, M.; Fripiat, J. J. J . Catal. 1970, 17,359. (c) Benson, J. E.; Kohn, H. W.; Boudart, M. J . Catal. 1966, 5, 307. (d) Pajonk, G. M., Teichner, S. J., Germain, J. E., Eds., “Spillover of Adsorbed Species”;Elsevier: New York, 1983. (e) Dmitriev, R. V.; Steinberg, K.-H.; Detjuk, A. N.; Hofmann, F.; Bremer, H.; Minachev, Kh. M. J . Card. 1980, 65, 105 and references therein. ( I 6) The mixed-valence (“e- mobility“) properties of “heteropoly blues” have received considerable attention. A few leading references include: (a) Pope, M. T. In “Mixed Valence Compounds”; Brown, D. B., Ed.; Reidel Publishing: Dordrecht, Holland, 1980; p 365. (b) Reference 3, p 101-1 18. (c) Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M.; Jeannin, Y. J . Am. Chem. SOC.1982, 104, 3194. (d) Ciabrini, J. P.; Contant, R.; Fruchart, J. M. Polyhedron 1983, 2, 1229.
Trisubstituted Heteropolytungstates
J. Am. Chem. SOC.,Vol. 108, No. 11, 1986 295 1
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6a) which exhibit larger 2Jw+w 13-30 Hz coupling^,^^^^^ while the 6 W belt and 3 W cap of the B isomer have edge-sharing = 5-12 octahedra (Figure 6B) which exhibit smaller 2Jw.+w Hz coupling^.'^^^^ Although the quadrupolar influence of the 51V n ~ c l e i broadens ' ~ ~ ~ ~ (Aul > 4 H z in wet CD3CN) the usually sharp (Av, < 1 Hz) lW-resonances, a resolution-enhanced, ~ VCD3CN ~ O ~ reveals ~ two-line iW of ( B U ~ N ) ~ H ~ S ~inWwet an apparent 2Jw-*w of about 15 Hz. Furthermore, the watersoluble free acid H7SiW9V30a was prepared by ion exchange of the K+ salt and a lS3WN M R of a D 2 0 solution (ca. 2 g/mL of D 2 0 ) , Figure 7, exhibits sharper lines, and it clearly shows reciprocal couplings of 16.4 f 1.2 Hz, confirming the A-type SiW9V,0407-structure. (111) Deprotonation Studies. Preparation of ( B U ~ N ) ~ - ~ H $ ~ W(x~ =V0-2). ~ O ~ One of the primary goals of these studies was to prepare the fully deprotonated ( B U ~ N ) ~ S ~ Was~a Vsynthetic ~ O ~ ~precursor for the support of transition-metal and organometallic catalysts. The mono- and (x = 1, 2) and especially their diprotonated HxSiW9V3040X-7 overall symmetry and sites of H + attachment (and their H+ mobility with added H 2 0or base) are also of interest since one might expect catioflic metals to behave similarly. There is also the question of whether or not any SiW9V30407-degradation by OH- occurs along with removal of one, two, and three H+. Initially, the deprotonation of (Bu4N),H3SiW9V30a in CH3CN, Me2S0, or D M F was surveyed by a potentiometric titration using Bu4N+OH- in MeOH or H 2 0as titrant and a pH electrode and meter (millivolt scale) to monitor the titrations. In each solvent, sharp break points at 1.0 and 2.0 equiv of Bu4N+OHwere observed (Figure 2, supplementary material), but control experiments indicated that under our conditions, the electrode was unable to monitor the third equivalence point.20 Monitoring the IR vs. equiv of Bu4N+OH- revealed a general decrease in energy for all of the heteropolytungstate vibrations upon deprotonation due to the buildup and delocalization21 of negative charge. For example, the M=O vibration in CH3CN decreased by approximately 20 cm-I for ( B U , N ) ~ S ~ W ~ V compared ~O,~ to ( B U ~ N ) , H ~ S ~ W(Figure ~ V ~ O3,~supplemental ~ material). The diprotonated (Bu4N),H2SiW9V30a was prepared by addition of 1 equiv of Bu,N+OH-/MeOH to (Bu4N),H3SiW9V30a in CH3CN followed by evaporation of the solvent to dryness under vacuum. Is3W and ,'V N M R studies in dry CD3CN revealed broadened/multiple-line spectra that sharpened to averaged two (IE3W)and one (51V) line spectra with added H 2 0 , as anticipated on the basis of the earlier results for H3SiW9V30ao". A summary of the IS3Wand ,'V N M R data is provided in the Experimental Section. (A) ( B U ~ N ) ~ H S ~ W ~ VSolid, ~ O , ~dry . samples of the monoprotonated salt ( B U ~ N ) ~ H S ~ Wwere ~ Vprepared ~ O ~ ~ by addition of 2.0 equiv of Bu,N+OH-/MeOH to a CH3CN solution of the triprotonated salt followed by evaporation of the mixture to dryness (17) (a) Lefebrve, J.; Chauveau, F.; Doppelt, P.; Brevard, C. J. Am. Chem. SOC.1981, 103,4589. (b) Knoth, W. H.; Domaille, P. J.; Roe, D. C. Inorg. Chem. 1983, 22, 198. (c) Domaille, P. J.; Knoth, W. H. Inorg. Chem. 1983, 22,818. (d) Brevard, C.; Schimpf, R.; Tournt, G.; Tournt, C. J. Am. Chem. SOC.1983, 105, 7059. (18) Domaille. P. J. J. Am. Chem. SOC.1984. 106. 7677. (19) Acerete, R.;Hammer, C. F.; Baker, L. C. W. J . Am. Chem. SOC. 1982. 104. 5384. (20)(a) Control experiments indicated that the limit of the electrode's ~
response under our experimental conditions was near the potential observed following 2.0 equiv of Bu,N+OH- (as shown on Figure 3, supplementary material). Some degradation of SiW9V30a7- (see text) also may have generated an interference as weILzob (b) Skoog, D. A.; West, D. M. 'Principles of Instrumental Analysis"; Holt, Rinehart, Winston: New York, 1971; pp 450-459. (21) Charge delocalization in polyoxoanions via a long M-O, short M=O,
long M-O, and so on trans bond alternation ("cooperative trans influences") has been observed: (a) Besecker, C. J ; Day, V. W.; Klemperer, W. G.; Thompson, M. R. Inorg. Chem. 1985, 24,44. (b) Besecker, C. J.; Day, V. W.; Klemperer, W G.; Thompson, M. R. J. Am. Chem. Soc. 1984,106,4125. (c) Flynn, C. M., Jr.; Stucky, G. D. Inorg. Chem. 1969,8, 335. (d) Day, V. W.; Fredrick, M. F.; Thompson, M. R.; Klemperer, W. G.; Liu, R.-S.; Shum, W. J. Am. Chem. SOC.1981,103,3597. ( e ) Pope, M. T. Inorg. Chem. 1976, 15, 2008. (f) Pope, M. T.; Garvey, J. F. Inorg. Chem. 1978, 17, 1115.
A
B
-80
-8s
-90
-95
-100
-105
-110
-115
PPM
Figure 8. slV (A) and lS3W(B) N M R of HSiW9V30,06-. Conditions: s l V NMR, 1 g / 3 mL of CD3CN, 12-mm vertical tube, LB = 10, 200 transients, 0.04 min, 21 "C); Is3W N M R , 1 g/mL of CD3CN, 10-mm vertical tube, LB = 1, 70000 transients, 15.9 h. The inset peaks of (B) show the peak sharpening that results from 1.9-W decoupling of the -545 ppm s l V resonance. H+
6-
[.slWs;v3eo]
Figure 9. Polyhedral representation of A-P-HSiW9V,0,,6- illustrating the probable VI-O(H)-V, protonation site.
under vacuum at room temperature. The 'IV and lS3WN M R at 21 "C of the product dissolved in dry CH3CN results in strikingly clean and informative spectra, Figure 8. The 51VN M R exhibits two resonances, one sharp resonance, 6 -545 (1 V, AuIl2 = 106 Hz), and one broad resonance of relative intensity of approximately two, 6 -578 (2 V, AulI2 = 1871 Hz), requiring that HSiW9V30406-possess a single mirror plane of symmetry (C, symmetry). The substantially different line widths of the two 51V resonances reflect the large difference in field gradient of the distinct vanadium sites upon protonation and suggest protonation of an oxygen bridging the two V responsible for the broad resonance, e&, VI-O(H+)-V2, Figure 9. The Ig3WN M R confirms the finding of overall C, symmetry, since a five-line spectrum with resonances at -83.3, -88.1, -93.3,
2952 J. Am. Chem. Soc., Vol. 108, No. 11, 1986
Finke et al. I H'
a isomer
p
isomer
Figure 10. Polyhedral representations of a and 0 isomers of AHSiW,V3ON6 for illustrating the different connectivities resulting from imposition of C, symmetry due to, for example, protonation of the VI0-V, bridging oxygen.
-99 (very broad), and -1 12.9 ppm of relative intensity 1:2:2:2:2 is observed. The very broad line at ca. -99 ppm and the partially broadened resonance at ca. -83 ppm are two additional interesting features of the la3WNMR. Such broadened resonances have been observed previously for vanadotungstates and are attributed to the scalar relaxation of Is3W caused by quadrupolar relaxation of 51V.18319 The broad 51Vline has little effect on the Is3W N M R spectrum because the rapid relaxation self-decouples it from the adjacent tungstens. On the other hand, the more slowly relaxing, sharper 51Vline broadens both the Ia3Wresonance of the adjacent pair of tungstens (-99 ppm) and the unique tungsten which isfour bonds removed (-83 ppm). Consistent with the above interpretation, decoupling of the sharper W resonance results in a marked narrowing of the broadened Is3W resonance centered at -99 ppm (Figure 8B). We have also quantified the vanadium-tungsten couplings from fitting the line shapes using established theory.Is The measured TI value of the sharper 51V resonance is 9.5 ms in the same solution used to obtain the Ia3WN M R spectrum. Line shape calculations, which are quite sensitive to the magnitude of the J values, give *JVaw= 10.5 f 0.5 Hz for the broad Is3W = 4.0 f 0.5 H z for the line at -83 line at -99 ppm and 4Jv-0-w ppm. The large magnitude of the latter four-bond coupling is a surprising and potentially important result, one that issues a warning that tungsten-tungsten couplings of a similar magnitude may be observable over such large distances. If more general, the observation of four-bond couplings might limit the use of tungsten-tungsten couplings in establishing connectivities, since adjacent two-bond couplings for edge-shared tungsten are typically 5-12 Hz. Why the four-bond vanadium-tungsten coupling is observed for HSiW9V30406 is not obvious in the absence of an X-ray crystallographic structure determination. Since *JWew and 2Jv-o-w couplings increase as the M-0-M angle increases toward 180°,17318a possible explanation is that protonation causes a flattening of one or more M-0-M ( M = V, W) angles. The use of 2-D lS3W N M R allows us to answer the final structural question about A-SiW9V30407-,namely, whether the /3 isomerismlo of the A-j3-SiW90341*starting material is retained in the A-SiW9V30407-product. As evident in Figure 10, differences in the mode of W-W connectivity are apparent for the a vs. /3 isomer of Cs symmetry SiW9V30,7-. The 2-D INADEQUATE pulse sequence2*allows detection of each pair of low abundance, coupled satellite doublets and has already proven powerful in unambiguously establishing the atomic framework of heterop o l y t ~ n g s t a t e s . ~ ~ ~The - l ~51V-decoupled ~-~~ 2-D INADEQUATE Ia3WNMR of ( B U ~ N ) , H S ~ W ~ V in, dry O ~ CD3CN ~ is shown in Figure 11, with the data presented as a contour plot; the 1-D 183W(51V) spectrum is presented below it on the same scale. The (22) (a) Bax, A. 'Two-Dimensional NMR in Liquids"; Delft University Press: 1982. (b) Bax, A,; Kempsell, S. P.;Freeman, R. J . Map.Reson. 1980, 41, 349. (c) Bax, A.; Freeman, R.; Frenkiel, T. A,; Levitt, M. H. Ibid. 1981, 43, 478. (23) In fact, it is already quite clear that chemical shift assignments made in the absence of the connectivity data obtainable by 2-D Is3W NMR and 2 J ~ couplings + ~ are likely to be in error. See, for example, the discussion and references in ref 18.
I N
1
400
"
'
1
~
'
'
200
1
0
"
~
/
-200
"
'
I -400
'
~ HZ
hi -70
-80
-90
-100
-110
-120
-130
PPM
Figure 11. Contour plot of the 183W(J1V) NMR 2-D INADEQUATE spectrum for A-@-HSiW9V3Oa6 using a 20-mm sideways-spinning probe at 30 O C . The bottom spectrum is of the I-D IS3W NMR with 1.9-W decoupling of the -545 ppm resonance in the 51VNMR. Connectivities between lS3Wresonances are indicated by dashed lines. Corner couplings appear as "dumbbells" while edge couplings appear as "single ovals". For a discussion of the tungsten octahedral connectivities (according to the labeling scheme indicated) see the text.
complete connectivity pattern can be accounted for only with the
/3 isomer and the assignment -82.5 (W,,), -87.7 (w6, W9), -92.8 (W4, W5), -98.3 (W7, W8), and -1 11.7 ppm (W,,, Wll).24 This assignment is made, step-by-step, from the contour 2-D plot, Figure 11, starting with the unique tungsten (W 12 for the /3 isomer). The spectrum reveals both a W12-W7,8corner and a W12-WIo,11edge connectivity. Similarly, a W7,8-W6,9corner, a w629-w5,4edge and a w6,9-wlO,l~ corner, and a W5,4-Wlo,llcorner coupling are apparent, thereby unequivocally establishing the structure as /3-SiW9V304,, Figure 10. Although the Cs symmetry, A-/3 structure of HSiW9V30406is unequivocally established by the 5'V and 1- and 2-D Ia3WNMR, the site of protonation is not. However, the 51VN M R line widths (vide supra) are most consistent with H+ addition to an oxygen-bridging VI and V,, Le. V,-(OH)+-V2 (Figure 10). Furthermore, the ESR studies by Pope et a1.6bof the l-e--reduced HSiW9V2V1V0407also favored protonation of a V 2 0 site and Klemperer et al. determined by I7O N M R that V 2 0 protonation occurs in25aHV2W40,93-and that V 2 0 plus some V 3 0 protonation occurs in25b,c Vlo0286-. The IR of ( B U , N ) ~ H S ~ W , V , vs. O ~that ~ (24) If one tries to assign the 2-D ls3WNMR in Figure 11 to the a isomer using its tungsten numbering scheme as shown in Figure IO, the intensity-one resonance would have to be W,, and its corner coupling to the peak at -99 ppm would assign the latter resonance to W4,5. Next, a W4,5-W6,9edge coupling would be required. However, one is not present in the 2-D spectrum which rapidly illustrates, in part, how the a isomer was ruled out. Additionally, the four-bond vanadium-tungsten coupling to the unique, intensityone tungsten assigned to V,-W,, (4Jv,-0-w,2)in the @ isomer would be an in the a isomer. untenable, >six bond 6Jv3-wlo (25) (a) Klemperer, W. G.; Shum, W. J . Am. Chem. SOC.1978,100,4891; (b) J . Am. Chem. SOC.1977, 99, 3544. (c) Evans, H. T., Jr.; Pope, M. T. Inorg. Chem. 1984, 23, 501. See also: Harrison, A. T.; Howarth. 0. W. J .
Chem. SOC.,Trans. 1985, 1953.
J . Am. Chem. SOC.,Vol. 108, No. 11, 1986 2953
Trisubstituted Heteropolytungstates
T B A ~ H S YqV3040 I t
1
EO P Y R I D I N E
+
2
EO P Y R I D I N I U M TRIFLUOROACETATE
C
LL
TBA4H3S iW9V3Oq0 t
3
EQ P Y R I D I N E
B
+ EXCESS
I)
H20
/I
TBA6HS~WgV3040
h
t
B
1 EO
PYRIDINE
A
I -80
+
t
EQ
C
PYRROLIDINE
I,
/I
j/ 1
1
"
"
I
-20
"
I " -100
"
' - '
I
-110
'
PPW
Figure 13. Effects of pyridine upon protonated A-@-SiW9V30407as monitored by lS3WN M R . From bottom to top the spectra are of (A) A-~-(BLI~N)~HS~W in ~CD$N V ~ O ~alone (2 g / 3 mL of 3:l D M F / CD,CN, IO-" vertical tube, LB = 1,40000 transients, 9.1 h, 21 "C), (B) sample (A) following the addition of 1 equiv of pyridine (41 pL) (LB = 2, 40000 transients, 9.1 h, 21 "C), (C) ( B u ~ N ) ~ H , S ~ W , V , O plus ~, 3 equiv of pyridine (2 g of sample/3 mL of 3:l DMF/CD,CN plus 141 pL of pyridine, LB = 1, 70000 transients, 15.9 h, 21 "C),and (D) sample (B) following the addition of 2 equiv, 197 mg, of pyridinium trifluoroacetate (LB = 1, 70000 transients, 15.9 h, 21 "C).
equiv of pyridine to H3SiW9V30a4- causes full collapse to a sharp, two-line spectrum. Both these experiments were expected to generate HSiWgV30406 in the presence of 1 equiv of pyridine, except that the latter reaction, H3SiW9V30404-+ 3 equiv of py, also produces 2 equiv of p y w . As a check, 2 equiv of pyH+CF3COZ-along with 1 equiv of py was added to HSiWgV,0406and, indeed, a sharp two-line I8,W N M R spectrum was observed, Figure 13D. These results require that in addition to a deof (Bu4N),SiWgV3Oa in CH3CN (and ( B U ~ N ) ~ H ~ S ~ W and ~ V , O ~protonation/reprotonation mechanism, HSiW9V3O4
