183W NMR of polyoxotungstates. Correlation of chemical shifts and 2Jw—w couplings with local geometry

24 June 1994

CHEMICAL

PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 223 (1994) 289-296

ls3W NMR of polyoxotungstates. Correlation of chemical shifts and 2Jw_w couplings with local geometry Leonid P. I&an&y

*

Laboratoirede Chimie de Metaux de Transition,UniversitkPierre et Marie Curie, 4, place Jussieu, 75252 Paris Cedex 05, France

Received 7 October 1993;in final form 15 April 1994

The ls3W NMR chemical shifts for a large number of polyoxotungstates are analyzed and compared with mean W-O bond lengths. The dependence of the ls3W NMR shift on the mean tungsten-oxygen distance is an increased shielding with eompression of the W06 octahedron ( CdVsymmetry) which constitutes the polymetalate. The influence of the vertex and edge sharing oxygens on the chemical shifts and the 25w_wcouplings is discussed.

1. Introduction NMR spectroscopy for the structural identification of polyoxotungstates has become a routine method [ 11, and X-ray crystal structure determinations are now substantiated by ‘*‘W NMR. The chemical shifts are sensitive to structural and electronic changes in the polyoxometalate framework. The connectivity pattern revealed by the 2Jw_w coupling allows an unambiguous assignment of the observed lines in the NMR spectrum to distinct tungsten atoms in the complex anions. However, the origins of the NMR shifts are incompletely understood. According to the generally accepted view the chemical shift of a nucleus is determined by two shielding terms: diamagnetic cd and paramagnetic a, with the latter being dominant [ 21. The diamagnetic term is effectively constant for closely related mole’ Ou leave from the Institute of Physical Chemistry, Russian Academy of Sciences, 31, Leninski prospekt, Moscow 117071, Russian Federation.

cules. The paramagnetic term arises from the distortion of the spherical distribution of the electrostatic potential around the nucleus, and involves mixing of the ground and excited states by the applied magnetic field. The paramagnetic term is determined by [ 21

where P, and D, represent the unbalance of the valence electrons in the p and d orbitals, r is the mean radius of these orbitals and AE is the average energy of the excitations to states of the correct symmetry, connected to the ground state by magnetic-dipole allowed transitions. The ‘83WNMR chemical shifts (a= - 82 to - 13 1 ppm) for a limited number of Keggin anions XW12010 were found to correlate with the wavelength (2 257-267 nm) of the first charge transfer (CT) band [ 31. However, in many cases the data (a= + 59 to - 290 ppm) for other polytungstates having octahedra of similar symmetry do not have

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L.P. Kazansky /Chemical Physics Letters 223 (1994) 289-296

this dependence, so we cannot link the first CT excitation energy (HOMO-LUMO separation) with the observed chemical shift in any simple way. The splitting of the d orbitals in the octahedral field depends on the bond lengths and angles. The position of the orbitals from which the excitations occur (or the ground state electron distribution) will contribute greatly to the chemical shift and, therefore, it is expected that a dependence of the chemical shifts on the distance may be found. Earlier we had assumed that a correlation between the is3W NMR chemical shifts and the mean distance Rw4 for individual octahedra may exist [ 41. Recently Mason [ 5 ] found a dependence of “C and i5N NMR chemical shifts on the compression of the polyhedron formed by metallic cations around the nucleus under study. It should be kept in mind that polyoxotungstates, especially large ones, are not rigid molecules, some bonds may relax upon dissolution, so slight deviations from a general trend may be observed. Here we consider crystallographic data for ordered (crystalline) structures in which the distance are determined with high accuracy and have reasonable and realistic values. For these polytungstates the ls3W NMR spectra are measured and reliable assignments are made from the ‘Jw_w connectivity patterns or from 2D experiments. Based on these data the 183WNMR chemical shifts (6) and ‘Jw_w spin couplings are correlated with the geometry. Polyoxotungstates are based on the combination of the distorted W06 octahedra linked to each other by vertices and edges (Fig. 1). In most cases the symmetry of W06 may be considered as C& with one short terminal bond W=04 (R (bond length) x 1.69 A), one long bond W-O1 trans to O4 (Rz2.12-2.45 A) with Oi linking several tungstens, and four bridging bonds (Rq 1.92 A) with O2 (vertex) and O3 (edge) oxygens each linking two tungstens. Different angles (0) for vertex (zz 150’) and edge (z 120”) W-O-W bonds result in different chemical shifts and 2Jw_w internuclear couplings.

2. The chemical shifts. ( 1) The W,O$j anion (structure I, Fig. 1a ) is compact since it has only edge shared octahedra [61

Fig. 1. The structures of the polyoxoanions and fragments with oxygens shown as open circles: (a) the structure of W,Ok and its fragment (bold line) WSO1s; (b) the structure of the Keggin anion XW,20!& and its fragment (II-A) (bold line) A-XW90M; (c) fragment (II-B) B-XW90M; (d) the structure of the Keggin anion and l/5 fragment (II-C) (bold line) of Naps Ws,O~:{ anion; (e) the lacunary anion XW, , O&i.

and has the largest chemical shift for anions having octahedra of CdVsymmetry [ 71. Two W50i8 fragments formed by the removal of one octahedron from structure I can give rise to two polyoxoanions W,,O‘$y (Ia) [8] and XW,,O&- (Ib) [9] where X is a lanthanide or actinide, both of which reveal a noticeable shielding for tungsten, especially in the case ofIa [4] (Table 1). In the case of Ia almost linear bonds linking two fragments WSOi8 substantially decrease the CT energy. However, despite the increased wavelength of the first CT band (A=330 nm) compared with A=275 nm for I, two lines in the ls3W NMR spectrum are remarkably shifted to negative values. The larger shielding for W, in the Ia and Ih complexes corresponds to decreased R,, values (Table 1) .

L.P. K&a&v Table 1 Mean bond lengths

(A) and ls3W shifts NMR

Allion

IWb@F Ia wloo:l Ib CeW,,Ofl II II II II

H2W120& SiW&z PW,20& ~SilzWIzO~

III a-P2W,sO& IV (SiW,O,),(ZrOH):‘V M4(PW,0,):o-

VI NaPsWmO:$y

Wl w2 Wl w2

Wl a w2 w3 Wl w2 Wl w2 Wl Wla W2a W2b w2c Wl Wla w2 W2a

/Chemical Physics Letters 223 (1994) 289-296

for polytungstates

R

6 (ppm)

1.954 1.933 1.956 1.942 1.960 1.945 1.954 1.964 1.971 1.965 1.950 1.963 1.950 1.952 1.943 1.980 1.975 1.969 1.963 1.957 1.948 1.939 1.918 1.926

+59 -174 -43 -15 +2 -113 -104 -99 -112 - 127 -136 -128 -174 -140 -189 -91 -106 -118 -131 -136 -210 -212 -290 -277

’ Wl - rotated triplet, W2 - six-membered ring, W3 -triad.

(2) The well-known polyoxotungstates a-XW,,Oz (II) with a-Keggin structure (Figs. lb1d ) are formed by four W90, 3 triplets, combined by vertex sharing, with an internal tetrahedron occupied by the heteroatom X. The number ratio of the vertex O2 and edge O3 shared oxygen atoms is 12 : 12. According to crystal structure determinations [ lo- 12 1, the mean bond length R is the octahedra decreases from X - P5+ to Hz+ (Table 1) . At the same time the ls3W NMR line shows a more negative shift [ 3 1. The structure of p-SiW,,O& (11-B) differs [ 131 from the u-structure in having one triplet rotated by 60”, so that the single line in the ‘*‘W NMR spectrum observed for II is split into three with relative intensities 3 : 6 : 3 corresponding to three sets of nonequivalent W atoms (rotated triplet, ring and triad) [ 141. The mean distances change as well, giving decreasing 6 values with the contraction of the octahedra (Table 1). It is worth noting that decreasing the symmetry of the whole anion results in decreased weight averaged S values despite the low-energy side

291

of the first CT band in the W spectrum being slightly red-shifted. ( 3 ) The removal of three W06 units from adjacent triplets of the Keggin anion gives an A-XW903,, fragment (II-A) (Fig. lb) which has a chelating sixmembered ring. Two such fragments combine by vertex oxygen atoms located in the mirror plane to form the Dawson anion [ 15,161 a-P2Wi80$F (III) with a vertex-to-edge sharing ratio 12 : 6. The two different tungstens give two lines in the le3W NMR spectrum [ 3 1, with the smaller 6 corresponding to the more compressed octahedron (Table 1). Despite a substantial decrease in the energy of the first CT band (A 308 nm) compared with the parent PW120k (A 263 nm) the W nuclei are strongly shielded, especially the ring tungstens linked by vertex sharing (the angle m between the two bonds in the bridge W-O;?W is x 162 ’ ) . Once again the first CT excitation does not contribute to the a,, and a substantial stabilization of the deep o and Abonding MOs resulting from the contraction of each octahedron, is responsible for the large negative chemical shift. It should be noted that W06 in the ‘polar’ triplet has two edge and two vertex oxygens but each WOs in the six-membered rings has one edge and three vertex oxygen atoms. This results in a large shielding for tungsten. Two anions XW9034 (II-A) may also act as ligands chelating three cations Zr4+ which are placed between equatorial tungsten atoms [ 17 1. However, this time two p-A-SiW9034 (polar triplets are rotated by 60”) fragments form the (SiW9034)2(ZrOH):‘complex (IV); nevertheless, the decreased R for the six ring octahedra results in the expected decrease in the 6 value. The crystal structure determinations of complexes M4(PW9034)2 (V) where M is divalent Co, Cu, Zn showed [ 181 that they are formed by two B-type PW9034 anions (structure II-B) (Fig. 1c) (with one triplet removed) attached to both sides of four octahedra in an M4016 array. The calculated R,,, (Table 1) for each type of WOs gives the expected correlation with the observed 6 for all assigned lines. (4) The structure of NaP, W300~:~ (VI) may be represented as five fused fragments (II-C) of the Keggin anion (Fig. 1d) combined by vertex sharing [ 19 1. The displacement of Na+ from the center of a cavity of this anion results in four types of symmetry-

L.P. Kazandy /Chemical Physics Letters 223 (1994) 289-296

292

equivalent W atoms with ratio 5 : 5 : 10: 10, and the slightly different radii of the WOs octahedra give rise to the different chemical shifts (Table 1) [ 19 1. It is worth noting that the substantial contraction of each octahedron (with ratios of vertex to edge shared oxygen atoms 3 : 1 for 20 W06 and with 4 : 0 for 10 W06 and the decreased W-0, bonds) leads to a high shielding for tungsten. At the same time this anion exhibits a substantial decrease in CT energy (2 349 nm with a shoulder at 278 nm) [ 191 with no increase in a,,. We observe the same for u-P2Wi80& and Wi,,O$,-. A markedly higher shielding is observed, showing the important contributions fo transitions other than the first CT. (5) Removal of one W06 from the Keggin anion results in the lacunary anion XW,, 0;~ (II-lacunary) (Fig. le) which may act as a tetra- or pentadentate ligand towards many metal cations M. Usually in the crystalline state the position of M is disordered over 12 sites. In the case of GaPbW, ,O:q this statistical disordering was not observed [ 201 and it is possible to estimate the bond lengths for each W06 (Table 2). Unfortunately, there is no 183WNMR spectrum for this complex, but similar variations in the R values may be expected for PMW,,O;< and PPbWi, 0:~ for which an assignment has been made from the 2Jw_wcoupling connectivities pattern [ 2 11. Moreover, an analysis of the chemical shift pattern for a number of complexes PMW, i 0;; (where M is Li+, Pb2+, Zn2+, Ti4+, V5+) [21-241 shows once again that for the C4” octahedra in a given complex, decreasing R corresponds to an increased shielding for tungsten (Table 2 ) . Data presented in Table 2 show two important regularities: (a) A strong effect of substitution is observed for W2, linked by the vertex with W4, adjacent to an introduced M. The difference A(dW5 -6w2)

may be a measure of the distortion of the polyoxoanion as a whole, depending on the size and notably the charge of M”+: (b ) Generally, edge sharing oxygens in M-03-W bridge induces deshielding of the adjacent tungstens and vertex sharing oxygens in M-02W results in a larger shielding. This is especially notable for the Zn complex. Possible asymmetry in the bridge makes effective the cis-8 %W configuration, resulting in a larger shielding. As can be seen from Tables 1 and 2 for different types of polyoxotungstates there is a regular shielding with decreasing R. Analyses of known structures and measured 183WNMR shifts shows the same general trend of increasing shielding with compression of the W06 octahedron, though a large scatter is observed as a result of uncertainties in the bond-length determination and influence of the central atom. In Fig. 2a the *83WNMR chemical shifts are plotted against the mean bond length for the octahedra constituting different polyanions of phosphorous with vertex to edge oxygen ratios larger than 1: 1. As can be seen there is a general trend in the shielding of W atoms with a compression of octahedra. This may be explained by a stronger interaction of the W and 0 atomic orbitals resulting in a greater stabilization of the bonding MO from which the excitations occur. Therefore, the paramagnetic contributions (see equation) should be reduced. As was noted above, vertex oxygen binding strongly affects the chemical shift, showing substantial interaction in the W-02-W bridge. In general, the ranges of chemical shifts for individual W06 in polyoxotungstates depending on the number of vertex oxygen atoms may be represented as shown in Fig. 2b.

Table 2 The chemical shifts for PMW,,O$i anions Anion PLiW, ,049 PPbW, , 0:~

PZUW,, 0:; PTiW,, O:< Pvw,,O~ (=‘bW,,O:,

(&,(A))

w5

w6

w3

WZ

-104 -103 -107 -93 -99

-121 -112 -130 -102 -104

-132 -127 -131 -107 -101

-152 -146 -140 -118 -109

1.958

1.958

1.955

Ref. 48 43 33 25 10 1.942

-98 -74 -73 -57 -72

-99 -83 - 157 -109 -107

1211 1211 1221 ~231 ~241

L.P. Kazanshy /Chemical Physics Letters 223 (1994) 289-296

/saO 0

00

‘c I

’ d)

/0

P

1.92

111

1.96

1.94

w NMR ctlemic,,

Ihd,,

1.98

PP”)

Fig. 2. (a) The correlation of ls3W shifts with the mean bondlengths Rw_,, for P-centered polyanions; (b) the plot of ‘*‘W NMR shift (C, symmetry against the number of vertex-shared oxygens for polyoxotunptates).

3. The zJw_w internuclear couplings It is well recognized that spin-spin coupling through two bonds depends on the orbital hybridization and the geometry of the bonds, and on the excited states [ 2 ]. According to Lefebvre et al. [ 14 ] the vertex and edge sharing the octahedra results in different *J,,,_,,,values, 20 and 8 *Hz, respectively. Later, Thouvenot et al. [ 1,25-271 found larger variations in the *Jw_w values (37-4.9 Hz) for vertex sharing in and p,w,,0::7 @iW,,O& NH~As~W~O:&. The interaction of two W nuclei strongly depends on the ligand properties of oxygen forming the bridge W-O-W. The more bent bridge (wx 120” ) results in less interaction of orbitals of the bridging oxygen and, therefore, the spread of 2Jw_wvalues is not large ( 1O-5 Hz) despite the similar bond-lengths. A larger variation of the coupling values is observed for ver-

293

tex W-0,-W bridges (Table 3). If the *Jw_wvalues are plotted against the sum of both bond lengths in the bridge W-0*-W, a general trend of decreasing 2Jw_w with increasing RI +R2 can be seen (Fig. 3a). This corresponds to a decreasing interaction via these bonds, which may sometimes be lower than in edge sharing W-0,-W, for which 2Jw_w are within IO-5 Hz for a similar range of RI + R2. It may be noted also that the asymmetry of the bridge W-0*-W becomes larger with an increase in the sum RI + R2. The same decrease in *Jw_wwith decreasing angle w is also seen in Fig. 3b, giving the lowest *Jw+ value for 140’. It may be expected that a smaller angle should result in a small spin-spin coupling and there may be some contribution from ‘Jw+ coupling (if there is metalmetal bonding at a distance of ~3.3 A or from through-space interaction) or from the interaction of two tungstens through a common edge (common O3 and Or ). At present it is not possible to say anything definitely on the W-W interactions. Some uncertainties in the determination of the bond lengths and angles and the different contributions from the excited states for different polyoxoanions may be responsible for the scatter in Fig. 3.

4. General remarks From Table 1 and Fig. 2 three dependences of the ‘*‘W NMR chemical shift for three different types of polyoxotungstates may be noted: with only edge sharing octahedra, a one-to-one ratio of vertex and edge oxygens atoms and mostly by vertex sharing. For identical R a decreasing 6 with increasing number of vertex shared oxygen atoms is also observed. However, the influence of Oi which is trans to W=04 may also be important. One of three tungstens in the As~W~,O~~(H~O)~- complex [29] is ‘squeezed’ between two II-B fragments by vertex sharing oxygens with 0, from a water molecule and shows a large shielding (CL - 209 ppm). The other two tungstens (pentacoordinate W) lacking water molecules reveal a marked increase in 6 ( - 40 ppm ) . The lack of real terminal bonds in the unique octahedron WOs of the WZn3( H20) (ZnW,O,,), complex [30] results in a substantial deshielding (CL + 20 ppm) of tungsten, as for the unique W in the W,Oqb polyanion [ 281, which reveals the largest

L.P. Kazansky/ ChemicalPhysicsLetters 223 (I 994) 289-296

294

Table3 The distancesW-O,the anglesW-O-Wand 2Jw_win polyoxotungstates Anion

RI (A)

& (A)

w

*JWV

Ref.

&SiW,,O& P,W,,O%M.(H20)2(PW,0d:“-

1.87 1.89 1.86 1.79 1.79 1.90 1.92 1.89 1.89 1.90 1.84 1.89 1.92 1.88 1.88 1.85 1.82 1.88 1.89 1.89

1.97 1.91 1.95 2.12 2.18 1.95 1.94 1.96 1.94 1.95 2.2 1.96 1.93 1.93 1.94 1.95 2.19 1.95 1.95 1.94

145 157 144 147 141 149 152 144 158 148 151 143 150 162 152 150 143 155 153 150

20.0 37.0 23.7 10.3 7.0 25.0 19.6 14.0 27.9 19.6 16.9 22.0 22.0 31.0 21.1 23.8 4.9 26.5 23.8 15.4

1141 [251 [I81

ZndH20)2(AsW&&o(ZrOH),(SiW,O,,)j’XPbW, , 0;; b

SiW9034(Me2Si):SiW9034(EtSi)3 l3-P2W18@~

c

WW10%2

WZndH20)2(ZnW~Od:*-

*

[181 [I71

WI

1271 [271 1271 [261 [281

a The structure is that of the Co complex, NMR parameters are those of the Zn complex. b The structure for GaPbW, ,O:; , NMR for PPbW, ,O:,-. c The structure for the a-complex.

( +269 ppm) chemical shift for octahedral W6+. In addition the influence of the heteroatom as in the ZnS complex mentioned above (A6 300 ppm) should also be considered. As we have shown, the position of the first CT does not vary in a consistent way and, therefore, the deep MOs (both o and 7cbonding MOs) may play a dominant role. As Nakatsuji et al. [ 3 1 ] have shown, the magnetic dipole-allowed excitations from bonding IS and K MOs (so-called d-d* transitions) for Mo04_,X:(where X is S2- or Se’- ) which contribute to the paramagnetic term of the 95Mo NMR chemical shift, should occur in the far UV region. It should be noted that “MO NMR chemical shifts for these tetrahedral anions are linearly correlated with the mean RMwx showing large deshielding with increasing R. The nature of the chemical shifts and 2Jw_w couplings for polyoxotungstates where only the interaction of oxygen and tungsten varies, may be considered in detail only when MO calculations on the excited states are made. An analysis of the distances

W-O in the individual W06 octahedra also shows that: (a) the sum of RW+ + Rw4, is usually larger than the sum of the two bonds 02-W-09, so the octahedra are slightly elongated along the C, axis, despite substantial shortening of Rwa,. Different sizes of heteroatoms will result in substantial deviations of the angle 04-W-O{. It should be noted that the combination of two octahedra by vertex sharing eliminates one oxygen atom and by edge sharing eliminates two oxygens, so the different charge on the tungsten atom may affect the chemical shift. Taking into account distortions of the W06 octahedron and types of oxygen atoms, our molecular orbital calculations show substantial stabilization of bonding and greater destabilization of antibonding o orbitals with decreasing distances. The observed “‘W NMR shifts are linearly correlated with the average energy separation between occupied and vacant molecular orbitals for W06 octahedra, even for a symmetry close to Czv and with the calculated electron population on tungsten. We could remark that the chemical shifts may correlate with atomic populations, as Clayton

L.P. Kazansky /Chemical Physics Letters 223 (1994) 28%296

a)

3.80

3,85

3.90

sum of bondlengths

3.95 R ,+R

295

which determines the chemical shift. Analysis of the available data for the 95Mo and s’V NMR chemical shifts and the corresponding structural parameters shows the same trend: increase in shielding of the metal nucleus with decreasing R,, in MO6 octahedra having a symmetry close to C4”. This correlation is in contrast to one found by Mason [ 5 ] for nonmetal chemical shifts which increase with decreasing bond lengths. This may be explained by a different electronic mechanism for the chemical shifts of metals with unfilled d shells [ 34,35 1.

4.00 2

A

Acknowledgement The financial support of CNRS (France) and unpublished NMR data of Dr. R. Thouvenot are highly appreciated.

References I

I

140

I

145 W-O-W

I

I

150 bond

angle.

155

L60°

w

Fig. 3. (a) The plot of the 2Jw_wvalues against the sum of bondlengths in the W-O-W bridge; (b) the plot of the 2Jw_wvalues against the angle W-G-W.

and Bursten [ 321 showed for the 55Mn NMR chemical shifts, as the spin-spin couplings correlate reasonably well with the bond order (also a ground state property) even when calculated by the simple Pauling equation [ 33 1.

5. Conclusions It has been shown that the 183WNMR chemical shifts for polyoxotungstates build from W06 octahedra of a symmetry close to Cdv greatly depend on the bond length W-O and the number of vertex sharing oxygen atoms. The most noticeable changes for closely related polyoxotungstates occur in the bond which is trans to the terminal W=O bond and it is possible that the CT from the orbital involving dz2 may greatly contribute to the paramagnetic term

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