170
NMR Spectroscopy of Polyoxometalates
Inorganic Chemistry, Vol. 18, No.
(27) A. Abragam and B. Bleaney, “Electron Paramagnetic Resonance of Transition Ions”, Clarendon Press, Oxford, 1970. (28) R. L. Carlin, C. J. O’Connor, and S. N. Bathia, J. Am. Chem. Soc., 98, 685 (1976).
1979 93
1,
R, J. Kurland and B. R. McGarvey, J. Magn. Reson., 2, 286 (1970). W. D. W. Horrocks, Jr., Inorg. Chem., 9, 690 (1970). B. J. Hathaway and D. E. Billing, Coord. Chem. Rev., 5, 143 (1970). W. B. Lewis and L. O. Morgan, Transition Met. Chem., 4, 33 (1968).
(29) (30) (31) (32)
Contribution from the Department of Chemistry, Columbia University, New York, New York 10027
Nuclear Magnetic Resonance Spectroscopy of Polyoxometalates. and Resolution 170
1.
Sensitivity
M. FILOWITZ, R. K. C. HO, W. G. KLEMPERER,*1 and W. SHUM
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Received August 23, 1978
Over 100 I70 NMR chemical shifts are reported for 27 diamagnetic polyoxoanions of the early transition metals. Efficient procedures for obtaining 170-enriched compounds are described, and the factors which control sensitivity and spectral resolution are examined and discussed in detail. Comparisons of chemical shift values with structural data show that chemical shifts are determined largely by metal-oxygen bond strengths.
concentration of X, T for temperature in °C, enr for l70 content in atom percent 170, np for number of pulses, pr for pulse repetition frequency in hertz, bdw for spectral bandwidth in hertz, and exp for exponential line broadening in hertz. All pH measurements were made at 25 °C. 1: cpd = [(n-C4H9)4N]2Mo6019, sol = (CH3)2NCHO, |Mo| = 1.4, T = 25, enr = 0.9, np = 5729, pr = 2.4, bdw = 15152, exp = 9. 2: cpd = [(n-C4H9)4N]2Mo6019, sol = CH3CN, |Mo| = 0.1, T = 80, enr = 20, np = 4096, pr = 7.1, bdw = 15152, exp = 2. 3: cpd = [(/z-C4H9)4N]2W6019, sol = (CH3)2NCHO, IWI = 0.52, T = 25, enr = 2, np = 9926, pr = 1.5, bdw = 15152, exp = 24. 4: cpd = K7HNb6019-13H20, sol = H2G (pH 14), |Nb| = 5, T = 100, enr = 3, np = 1046, pr = 5.0, bdw = 15152, exp = 24. 5: cpd = K8Ta6019-17H20, sol = H2G (pH 14), |Ta| = 1.2, T = 25, enr = 3, np = 7000, pr = 2.2, bdw = 15152, exp = 24. 6: cpd = [(n-C4H9)4N]3VMo5019, sol = CH3CN, |Mo| = 0.4, T = 80, enr = 10, np = 16384, pr = 6.7, bdw = 20000, exp = 11. 7: cpd = [(/i-C4H9)4N]3VW5019, sol = CH3CN, |W| = 0.3, T = 30, enr = 18, np = 14 926, pr = 8.3, bdw = 20000, exp = 8. 8: cpd = a-[(n-C4H9)4N]4SiMo12O40, sol = CH3CN, |Mo| = 0.4, T = 85, enr = 9, np = 32768, pr = 8.3, bdw = 20000, exp = 31. 9: cpd = a-[(n-C4H9)4N]4SiW12O40, sol = CH3CN, |W| = 0.4, T 90, enr = 10, np = 18 204, pr = 6.7, bdw = 20000, exp = 31. 10: cpd = a-[(«-C4H9)4N]4SiMoW, ,O40, sol = CH3CN, |W| = 0.3, T = 90, enr = 25, np = 65 536, pr = 3.1, bdw = 20000, exp =
Introduction The early transition metals vanadium, tiiobium, tantalum, molybdenum, and tungsten in their highest oxidation states are known to form a wide variety of polynuclear metal-oxygen complexes.2 X-ray crystallographic studies have yielded solid-state structures for several of these species, but solution structures have in many cases remained ambiguous due to the rapid and complex structural equilibria which often prevail in solution. The 170 NMR technique was first successfully applied to polyoxoanion chemistry in 1965 by two groups in independent investigations of the aqueous Cr2072™ anion.3 Due to sensitivity problems, however, little progress was possible until the advent of Fourier transform NMR (FT NMR) spectroscopy. Since 1975, this technique has enabled 170 NMR spectra of polyvanadates, -niobates, -tantalates, -molybdates, and -tungstates to be obtained.4™16 In this paper, we attempt to delineate the scope and limitations of the 170 NMR technique as a structural and dynamic probe in polyoxoanion chemistry. First, the factors which influence spectral resolution and sensitivity are discussed, and experimental procedures for optimizing spectral quality are outlined. Next, the 13.5-MHz nO NMR spectra of a variety of diamagnetic polyoxometalates measured in solution under various conditions are presented and compared. Finally, these results are briefly summarized and areas for future investigation defined.
—
31.
T
CH3CN, |Mo| = 0.5, (remaining oxygens),
= 184 978, pr = 3.2, bdw = 20000, exp = 31. 12: cpd = a-[(/t-C4H9)4N]3PW12O40, sol = CH3CN, |W| = 0.6, T = 80, enr = 2, np = 32 768, pr = 7.7, bdw = 15 152, exp = 24. 13: cpd = a-[(C4H9)4N]uH(P2W!80 62)2, sol = CH3CN, IWI =
np
Experimental Section A. Spectral Measurements. 170 NMR spectra were recorded at MHz using the pulse FT NMR technique on a Jeol PFT/PS-100 NMR spectrometer interfaced with a Nicolet 1080 data system. All spectra were digitized using 4096 data points such that digital spectrometer resolution varied between 3.7 and 4.9 Hz/data point, depending on the spectral bandwidth employed. The spectrometer was locked on the *H resonance of an external H2Q sample. Spectra were obtained using cylindrical 10-mm o.d. sample tubes (1.1-mL sample volume) and referenced externally to pure H20 at 25 °C by the sample replacement method. Chemical shifts were calculated in parts per million, with positive values in the low field direction relative to H20. The error associated with chemical shift values is ±3 ppm for line widths 200 Hz and 400 Hz. All line widths reported below in hertz have been corrected for exponential line broadening. The error associated with line-width values is ±20 Hz for line widths 100 Hz and 400 Hz. Unless otherwise indicated, samples were neither rotated nor degassed since such precautions were found to have no measurable effect on line widths. B. Spectral Parameters. In this section, spectra are numbered as indicated in column one of Tables II and III. The following abbreviations are used: cpd for compound, sol for solvent, |X| for molar
0.5, T 24.
13.51
0020-1669/79/1318-0093S01.00/0
11: cpd = a-[(n-C4H9)4N]3PMo12O40, sol = 80, enr = 30 (phosphate oxygens), enr = 3
=
= =
85, enr
=
=
4, np
=
131 072,
pr
=
7.7, bdw
=
15 152,
exp
=
14: cpd = /3-[(n-C4H9)4N]nH(P2W180 62)2, sol = CH3CN, |W| 0.4, T = 90, enr = 4, np = 262 000, pr = 7.7, bdw = 15 152, exp
24. 15: cpd =
|Mo| exp
=
= [(n-C4H9)4N]4(C6H5As)4Mo12046-H20, sol = CH3CN, 0.5, T = 94, enr = 25, np = 9915, pr = 3.1, bdw = 20000,
47.
16: cpd
=
[(n-C4H9)4N]4H4As4Mo12O50'5H2O, sol = CH3CN, |Mo| 80, enr = 25, np = 1119, pr = 3.1, bdw = 20000, exp
=
0.6, T
=
31.
=
17: cpd = [(«-C4H9)4N]2(CH3)2AsMo4015H, sol = CH3CN, |Mo| 0.9, T = 80, enr = 25, np = 4734, pr = 3.2, bdw = 20000, exp 47.
=
=
=
=
18: cpd = [(n-C4H9)4N]2(C6H5)2AsMo4015H, sol = CH3CN, |Mo| 0.9, T = 80, enr = 25, np = 16 000, pr = 3.2, bdw = 20000, exp
31.
19: cpd = [(n-C4H9)4N]4W10O32, sol = CH3CN, 25, enr = 2, np = 16 384, pr = 2.9, bdw = 15 152, 20: cpd = d-[(n-C4H9)4N]3KM08O26-2H2O, sol = = 0.05, T = 25, enr = 34, np = 131 072, pr = 3.2, exp = 23. ©
1979 American Chemical Society
|W|
=
exp
=
2.5, T
=
12.
CH3CN, |Mo| = 20000,
bdw
Inorganic Chemistry, Vol. 18, No.
94
1,
1979
21: cpd
= (NH4)6Mo70244H20, sol = H20 (pH 5.4), |Mo| = 2.5, 25, enr = 2, np = 8192, pr = 3.0, bdw = 15 152, exp = 24. 22: cpd = Na6Mo7024-4H20, sol = H20 (pH 5.5), |Mo| = 2.8, T = 25, enr = 3, np = 12 844, pr = 3.0, bdw = 15 152, exp = 24. 23: cpd = H8CeMo12042-18H20, sol = HzO (pH 1), |Mo| = 3.1, T = 25, enr =¡ 15 (see below), np = 12288, pr = 6.7, bdw = 20000, exp = 24. 24: cpd = a-[(n-C4H9)4N]4Mo8026, sol = hydrated CH3CN, |Mo| = 0.15, T = 4, enr = 25, np = 8192, pr = 3.2, bdw = 20000, exp
T
=
=
11.
25: cpd = Na6TeMo6024-2H20, sol = H20 (pH 5.8), |Mo| = 8.6, T = 82, enr = 0.4, np = 4465, pr = 3.0, bdw = 15 152, exp = 24. 26: cpd = Na5IMo6024-3H20, sol = H20 (pH 5.0), |Mo| = 2.8, T = 100, enr = 2, np = 1029, pr = 1.8, bdw = 15 152, exp = 8. 27: cpd = Na3H6AlMo6024-4H20, sol = H20 (pH 4.1), |Mo| = 1.0, T = 70, enr = 3, np = 19707, pr = 3.1, bdw = 15 152, exp = 24. 28: cpd = Na3H6CoMo6024-5H20, sol = H20 (pH 2.6), |Mo| =
1.2, T = 93, enr = 3, np = 10 320, pr = 3.0, bdw = 15 152, exp = 24. 29: cpd = [(/í-C4H9)4N]2Mo207, sol = CH3CN, |Mo| = 0.8, T = 25, enr = 9, np = 9504, pr = 6.3, bdw = 15 152, exp = 24. C. Sample Preparations. For spectral measurements, CH3CN (Aldrich Gold Label) solvent was distilled from CaH2, and (CH3)2NCHO (Pierce silylation grade) solvent was used without further purification. 170-enriched water was purchased from Miles Laboratories and Monsanto Research Corp. Enriched water recovered from aqueous syntheses was purified by distillation and analyzed for 170 content using procedures described elsewhere.17 All preparative procedures involving l70-enriched water were carried out in closed systems in order to prevent isotopic dilution by atmospheric water. (n-C4H9)4NCl (Eastman Organic Chemical) was recrystallized18 and C6H5As03H2 was filtered through activated charcoal and recrystallized use. Fisher certified reagent recrystallized by cooling an aqueous solution, saturated at 25 °C, to 0 °C. Anal. Caled: N, 6.80; H, 2.61; Mo, 54.34. Found: N, 6.83; H, 2.67; Mo, 54.18. All other reagents employed were the best commercial grades available and were not purified before use. Elemental analyses were performed by Galbraith Laboratories on unenriched samples. The literature references accompanying several of the compounds listed below refer to the original preparations upon which our preparations were based. In each case, preparative conditions were adjusted in order to achieve optimum product purity, minimum degree of product hydration, and/or maximum yield of product per quantity of 170-enriched water employed. Product identity was confirmed by comparison of IR and/or Raman spectra with literature spectra. A solution of 0.40 g of a-[(n[(h-C4H9)4N]2Mo6019. C4H9)4N]4Mo8026 and 0.4 mL of 170-enriched water in 20 mL of CH3CN was stored for 1 h. After addition of 0.05 mL of 12 N aqueous HC1, the solution volume was reduced to ca. 8 mL. Cooling of the mixture to 0 °C yielded 0.25 g of product as yellow needles which were air-dried. Anal. Caled: C, 28.17; , 5.31; N, 2.05; Mo, 42.19. Found: C, 28.38; H, 5.44; N, 1.99; Mo, 40.96.
from
1:1
(v/v) ethanol-water before
( 4)6 7024·4 20
was
[(«-C4H9)4N]2W6019. 170-enriched [(«-C4H9)4N]4W10O32 (1.5 g) refluxed in a mixture of 8 mL of CH3CN and 30 mL of CH3OH for 24 h. Upon cooling of the mixture to 0 °C, a precipitate formed which was filtered and air-dried. Crystallization of this precipitate from a 60 °C, saturated acetone solution cooled to 25 °C yielded 0.21 g of product as clear and colorless, block-shaped crystals. Anal. Caled: C, 20.31; H, 3.84; N, 1.48; W, 58.30. Found: C, 20.52; , 3.90; N, 1.43; W, 58.21. K7HNb3019-13H20.19 Nb205 (13.3 g) was added slowly to a melt of 26 g of KOH pellets (85% KOH) in a nickel crucible. After 30 min of heating, the melt was cooled to room temperature and dissolved in 100 mL of degassed water. After filtering of the mixture, the solution volume was reduced to ca. 50 mL. Needle-shaped crystals formed after 12 h at 0 °C. These crystals were filtered off, washed with 1:1 (v/v) ethanol-water and absolute ethanol, and dried in vacuo to yield 12.4 g of product. Anal. Caled: K, 19.97; Nb, 40.68; H20, 17.73. Found: K, 20.13; Nb, 40.55; H2Q, 17.60. was
Enriched compound was obtained by allowing a solution in 170-enriched water to stand for 12 h at 25 °C. K8Ta6019-17H20.20 Ta205 (10 g) was added slowly to a melt of 40 g of KOH pellets (85% KOH) in a nickel crucible. After 30 min of heating, the melt was cooled to room temperature and dissolved
Filowitz et al. in 100 mL of degassed water. After filtering of the mixture, the solution volume was reduced to ca. 25 mL. Crystals which formed after 12 h at 0 °C were filtered off, washed with 1:1 (v/v) ethanol-water and absolute ethanol, and dried in vacuo to yield 9.3 g of product. Anal. Caled: K, 15.57; Ta, 54.05; HzO, 15.39. Found: K, 15.68; Ta, 53.60; F120, 15.30. Enriched compound was obtained by heating an aqueous solution in 170-enriched water to 80 °C for 12 h. [(n-C4H9)4N]3VMo5019. V205 (0.11 g) and 0.08 mL of 170-enriched water were added to 30 mL of a 0.1 M (n-C4H9)4NOH solution in CH3OH. After the mixture was stirred for 24 h at 25 °C, the solution added to a solution of 1.5 g of 170-enriched a-[(nwas C4H9)4N]4Mo8 0 26 in 20 mL of CH3CN, and the resulting solution was refluxed for 6 h. Ca. 0.5 g of crude product obtained by slow addition of ether was crystallized from a saturated, 60 °C CH3CN solution cooled to 25 °C, yielding 0.38 g of orange crystals. Anal. Caled: C, 36.91; H, 6.97: N. 2.69; V, 3.26; Mo, 30.71. Found: C, 37.01; . 6.90; N, 2.80; V, 3.38; Mo, 30.92. [(«-C4H9)4N]3VW5019. V205 (0.08 g) and 0.06 mL of 170-enriched water were added to 20 mL of a 0.1 M solution of (n-C4H9)4NOH in CH3OH. After stirring of the mixture for 24 h at 25 °C, the solution was added to a solution of 1.3 g of 170-enriched [(«-C4H9)4N]4W10O32 in 8 mL of CH3CN, and the resulting solution was refluxed for 24 h. Crude product, obtained by slow addition of ether, was crystallized from a saturated, 80 °C CH3CN solution cooled to 25 °C, yielding 0.47 g of bright lemon yellow crystals. Anal. Caled: C, 28.80: H, 5.44; N. 2.10; V, 2.55; W, 45.93. 'Found: C, 28.94; H, 5.47; N, 2.17: V, 2.63; W, 45.70. Na2Si03-9H20 (0.04 g) and a-[(zt-C4H9)4N]4SiMo12O40.21 Na2Mo04-2H20 (0.40 g) were dissolved in 1.3 mL of 170-enriched water and stored at 25 °C for 12 h. Aqueous F1C1 (6 N) was then added dropwise with stirring until the solution pH was 2.4 Á) and resulting weakness of the Oa-M bonds. These weak bonds apparently isolate the central X04”~ unit from the surrounding M12036 cage to a significant extent and prevent extensive delocalization of increased negative charge to the OM and OM2 oxygens. We have measured the nO NMR spectrum of the aSiMoWnO404“ anion (see Figure 3d and Table II) in order to observe the effect of metal atom substitution which does not alter net anionic charge. Although substitution of a WVI by MoVI in the SiW12O404” structure lowers the anion symmetry drastically, no measurable change in chemical shift is observed for the OW and OW2 resonances, reflecting the localized nature of the perturbation. In addition, the OMo oxygen chemical shifts for «-SiMoW11O404“ and a-SiMo^O^4™ are identical within the experimental error. Note also that each of the chemical shift values for the two OMoW resonances from a-SiMoWnO404~, 504 and 469 ppm, compares well with the average value of the corresponding OMo2 and OW2 chemical shift values from a-SiMo12O404~ and aSiW12O404~, 504 and 480 ppm. Other Iso- and Heteropolyanions. nO NMR spectra of other iso- and heteropolyanions are shown in Figures 4-11, and spectral data are given in Table III. The a3 factor in eq 2 implies that the rate of molecular tumbling decreases rapidly as the ionic radius, a, increases. The spectra of large species shown in Figure 4 display broad resonances as a result of their slow tumbling in solution.68 In the a-P2W]80626- ion, only one OW2 resonance is observed (see
100
Inorganic Chemistry, Vol. 18, No.
Table III.
a-P2W18062 i3-P2W1BOf26-
(C6H,)2AsMo4015H2“
6a27 7a60
20
(3-Mo,0264
8a61
, , ·7
8b62 8b62
4q25,56 .
(C6H5As)4Mo,2 0464H4As4Mo12OS04~
(CH3)2AsMo40]5H2-
o
21
22
Mo70246"6
23 24 25 26 27 28 29
CeMo12042
s"
5a58 5a59 6a27
9a63 10a13-64
°-MOj0264" TeMo6 0 246" 5~ IMo6024
10b65 10b65 10b66 10b66 11a15'67
H6A1Mo60243~
H6CoMo60243Mo2072"
chemical shifts, ppm (assignments6*) [line widths6 in Hz] 759 (B) [200], 738 (A) [h], 418 (f)s [290] 757 [180], 736 \h], 451 [/ ], 418* [280] 946 (E) [500], 366 (D) [390], 225 (C) [340], 103 (B) [>1000] 951 (E) [420], 364 (D) [560], 225 (C) [350], 90 (A, B) [>1000] 867, 855 (E, F) [230‘], 389, 375 (C, D) [340*], 80 (B) [330], -15 (A) [320] 868, 861 (E, F) [200*], 390 (C, D) [180], 68 (B) [440],-10 (A) [130] 762 (E, F) [240], 430, 416 (B, C, D) [120] [h\, -6(A) [110] 900 (I) [h], 866 (E-H) [250], 743 (D) [200], 425 (C) [250], 296 (B) [200], 56 (A) [140] 824 (F-I) [430], 759 (E) [120], 398 (C, D) [170], 340 (B) [150], 123 (A) [90] 814 (F-I) [530], 757 (E) [150], 395 (C, D) [250], 335 (B) [230], 123 (A) [140] 898 (C) [650], 214 (A or B) [690] 866 (D) [350], 775 (C) [200], 495 (B) [320], 396 (A) [370] 807 (C) [170], 383 (B) [170], 180 (A) [480] 825 (C) [50], 387 (B) [90], 255 (A) [540] 833 (C) [100], 378 (BV [50] 838 (C) [60], 382 (B)* [90] 715 (B) [40], 248 (A) [200]
? 8 The central notes a-e. Assigned to OW2 resonances. OPW2 and OPW3 oxygens were not enriched in this compound * were not observed. 6 Shoulder. * Combined line width of two resonances. Ammonium salt. 6 Sodium salt. *0a not observed.
See Table
resonances
Filowitz et al.
structure6
6“
136 14 15 16 17 18 19
and their
1979
13.51-MHz 170 NMR Spectral Data for Selected Iso- and Heteropoly anions0 anion6
0-6
1,
II,
resonances
1000
PPM
0
Figure 5. Polyhedral model of the tetrahedral H4As4Mo1205o4~ structure is shown in (a). Octahedra outline the octahedral molybdenum coordination polyhedra and tetrahedra outline the tetrahedral arsenic coordination polyhedra. One member from each set of symmetry-equivalent oxygen atoms is labeled, and this labeling scheme is used in (b) and (c). In the H4As4Mo12O504~ structure, the Oa oxygens are protonated. In the (C6H5As)4Mo120 464“ structure, the 0A sites are occupied by C6H5 groups. The l70 NMR spectra shown in (b) and (c) correspond to spectra 15 and 16, respectively, in Table III.
Figure 4b), even though there are five nonequivalent types of OW2 oxygens in the structure. As a result of this low resolution, 170 NMR is unable to provide any information regarding the structure of the isomeric ß- 2\ 18 0 626~ ion (see Figure 4c).57 Although the Fl4As4Mo12O504~ ion shown in Figure 5a has approximately the same ionic radius as the Keggin ions (see above), it yields much broader 170 resonances, presumably because its less spherically shaped surface interacts
Figure 6. Idealized bond structure of (CH3)2AsMo4015H2~ is shown in (a). Large open circles represent oxygen atoms, small open circles represent molybdenum atoms, large shaded circles represent methyl groups, and the small shaded circle represents the arsenic atom. One member from each set of symmetry-equivalent oxygen atoms is labeled, and this labeling scheme is used in (b). The 0A oxygen is protonated. The 170 NMR spectrum shown in (b) corresponds to spectrum 17 in Table III.
with solvent molecules, thus inhibiting molecular tumbling.69 As expected for isostructural species having the same net charge, the Fr4As4Mo12O504~ and (C6H5As)4Mo120464- anions exhibit chemical shift values for OMo, OMo2, and OMo3 oxygens which are identical within experimental error. The completely resolved spectrum of (CH3)2AsMo4015H2(see Figure 6) demonstrates the high resolution which may be obtained from small polyoxoanions. This spectrum also illustrates the sensitivity of 170 NMR chemical shifts to structural environment: the nonequivalent OMo and OMo2 oxygen resonances are resolved even though the average Mo-0 bond lengths for the nonequivalent OMo and the nonequivalent OMo2 oxygens differ by less than 0.003 Á and 0.001 Á, respectively.27·70 Clearly, other factors besides bond lengths must more strongly
170
NMR Spectroscopy of Polyoxometalates
Figure 7. Idealized bond structure of Wio0324™ is shown in (a). Small circles represent tungsten atoms and large circles represent oxygen atoms. One member from each set of symmetry-equivalent oxygen atoms is labeled, and this labeling scheme is used in (b). The l70 NMR spectrum shown in (b) corresponds to spectrum 19 in Table
III.
Figure 8. Idealized bond structures of /3-Mo80264~ (a) and Mo70246~ (b). Small circles represent molybdenum atoms and large circles represent oxygen atoms. Within each structure, one member from each set of symmetry-equivalent oxygen atoms is labeled, and these labeling schemes are used in (c) and (d). The 170 NMR spectra shown in (c) and (d) correspond to spectra 20 and 21, respectively, in Table III. The asterisk in (c) labels the resonance due to a-Mo80264~ (see
ref 13).
be taken into account if resonances with similar chemical shift values are to be assigned unambiguously. Similar ambiguities
exist in the spectrum of W10O324" (see Figure 7). Here only one OW resonance and two OW2 resonances are observed although the structure contains two types of OW oxygens and three types of OW2 oxygens. Comparison of W-0 bond
Inorganic Chemistry, Vol. 18, No.
1,
1979
101
Figure 9. Polyhedral model of the tetrahedral CeMo120428~ structure is shown in (a). Octahedra outline the octahedral molybdenum coordination polyhedra, two of which are not visible. Only the forward portion of the icosahedral cerium coordination polyhedron is shown, and portions obscured by octahedra are indicated with dashed lines. One member of each set of symmetry-equivalent oxygen atoms is labeled, and this labeling scheme is used in (b). The 170 NMR spectrum shown in (b) corresponds to spectrum 23 in Table III.
Figure 10. Idealized bond structures of -MosO^4™ (a) and XMo6024"~ (b). Small open circles represent molybdenum atoms, large open circles represent oxygen atoms, and the shaded circle in (b) represents the X atom. In each structure, one member from each set of symmetry-equivalent oxygen atoms is labeled, and this labeling scheme is used in (c) and (d). In the H6XMo6024”~ structure, the Oa atoms in (b) are protonated. The 17Ó NMR spectra shown in (c) and (d) correspond to spectra 24 and 25, respectively, in Table III. The asterisks in (c) label resonances due to /3-Mo80264” (see ref 13).
102
Inorganic Chemistry, Vol. 18, No.
)
O
1,
O
I_,_ _1 Figure 11. Idealized bond structure for Mo2072~ is shown in (a). Small circles represent molybdenum atoms and large circles represent oxygen atoms. The labels “A” and “B” given in (a) and used in (b) refer to bridging and terminal oxygens, respectively. The spectrum shown in (b) corresponds to spectrum 29 in Table III.
lengths for the OW oxygens in W10O324~ reveals no significant variations, and it is even possible that one of the OW2 oxygen resonances
is not observed.
The reactivity of water toward certain polyoxoanions is illustrated by the aqueous polymolybdate system. When aqueous Mo70246" is acidified stoichiometrically to Mo80 264~, the 170 NMR spectrum of the resulting solution displays no polyoxoanion resonances. Only a broadened water resonance is observed, indicating rapid oxygen exchange between solvent and solute. This rapid exchange process may be eliminated by measuring the spectrum of the ß- 80264~ ion as a mixed K+/(«-C4H9)4N+ salt in CFI3CN (see Figure 8c). Resonances in this spectrum are assigned as follows. The #3-Mo8 0 264~ ion (see Figure 8a) contains nine types of symmetry-equivalent oxygens: 4 types of cis dioxo OMo oxygens (0E~0H), he., terminal oxygens bonded to molybdenums having two terminal oxygens; 1 type of monooxo OMo oxygens (03), i.e., terminal oxygens bonded to molybdenums having only one terminal oxygen; 2 types of OMo2 oxygens (Oc, 0D); type of OMo3 oxygens (Ob); type of OMo5 oxygens (0A). The 56 ppm resonance is assigned to the OMo5 oxygens since all 0A-Mo distances are greater than 2.1 Á. In the terminal oxygen region, the larger 866 ppm resonance is assigned to the 12 cis dioxo terminal oxygens and the smaller 900 ppm resonance is assigned to the two monoxo terminal oxygens. Comparison of the -Mo bond lengths for the two types of OMo2 oxygens shows that the 0D oxygens have one very short bond (i/Mo_0 = 1.75, 2.32 Á) whereas the Oc oxygens have “normal” bond lengths for doubly bridging oxygens (
