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  J. CHEM. SOC. DALTON TRANS. 1995 252 1 Reduced Polyoxomolybdates with the Keggin and Dawson Structures: Preparation and Crystal Structures of Two- electron Reduced [ K 18-crown-6)1,[N PPh,)2]2[ HPMo120,]. 8MeCN.18-crown-6 and Four-electron Reduced [ N BunA],- [ H,S,Mol,0,,]~4MeCN 1 8-crown-6 = 4,7,10,13,16-hexaoxa- cyclooctadecane) Ralf Neier, Christa Trojanowski and Rainer Mattes* Anorganisch Chemisches lnstitut der Westfalischen Wilhelms- Universitat, Withelm Klemm - Strasse 8 0 48149 Miinster, Germany By reaction of [K(18-crown-6)] N(PPh3),],[PMo,,O,].2MeCN 1 and [NBu ,],[S,Mo,,O,,] with PPh, in acetonitrile the new compounds [K(1 8-crown-6)],[N PPh3),],[HPMo1,0,]~8MeCN~1 8- crown-6 2 (1 8-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) and [ NBu ,],[H,S,M~,,~,,]~ 4MeCN 3 were prepared. Compound 2 contains the two-electron reduced [ H PMO,,O,]~- ion having the a-Keggin structure, 3 the four-electron reduced [H,S,MO,,O,]~- ion having the CC Dawson structure. Compounds 1-3 were characterised by means of IR and NMR spectroscopy and X-ray diffraction. The structures of 1 and 2 re disordered in space group P1. The heteropolyanions show apparent T,, symmetry; their actual symmetry is T. Upon reduction the Mo Mo distances increase slightly. The strongly alternating 'short' (mean 1.814 A) and 'long' (1.990 A) Mo-0-Mo bonds in the unreduced compound 1 become more equal in 2. The structural changes upon reduction observed in 3 were analysed in detail. The most significant alterations are an increase of the Mo Mo distances between corner-sharing MOO, octahedra in the equatorial belt by 0.066 A, and a decrease of the Mo-0-Mo bond lengths connecting the two halves of the anion. The alternation of the Mo-O(bridge) bonds is nearly evened out after reduction. All structural changes observed in 2 nd 3 are consistent with the description that the additional electrons occupy delocalised molecular orbitals extending over the Mo-0, framework. Reduced heteropolyanions ('heteropoly blues') are of continu- ing interest in important areas of chemistry like analytical and medical applications, catalysis, electron transfer and delocalis- ation of electrons in mixed-valence compounds. Despite significant evidence obtained by electrochemical studies, and by UV, ESR and multinuclear NMR spectroscopy, structural data are still scarce. Pope and co-workers5'6 reported the structure of four-electron reduced P-[H,PMo, 2040]4-, iiller et al. he structures of four-electron reduced P-[H,AsMo,,- 040]3 and of two-electron reduced or-[H,AsM0,,0,,]~-. The crystal structure of two-electron reduced a-[Co - W 20 ]8 was elucidated by Baker and co-workers.8 Some of these structural studies were hampered by the limited quality of the diffraction data and by disorder. We prepared two-electron reduced a-[HPMo, 20401~- and four- electron reduced ~~-[H,S,MO,~O,,]~- y chemical reduction in acetonitrile using PPh, as reductant, a method recently described by Kawafune and co-worker~.~-' We present here the results of analytical, spectroscopic and low- temperature single-crystal X-ray studies of these species. The X-ray data allow a more complete description of the structural changes occurring upon reduction of heteropoly- molybdates. To our knowledge, reduced heteropolymetalates with Dawson structures have not been structurally charac- terised so far. t Supplementary data available: see Instructions for Authors, J. Chem. SOC., alton Trans., 1995, Issue 1, xxv-xxx. Results and Discussion Preparation.-The reduction of polyoxomolybdates with Keggin and Dawson structure by triphenylphosphine in non- aqueous solutions under anaerobic conditions is a very convenient method. The ion [PMo, 2040] undergoes stoichiometric reduction by PPh,, accompanied by oxygen transfer to PPh3.10v11 However, in contrast to Kawafune, we suggest that the overall structure of the anion remains unchanged. The transferred oxygen atoms are immediately replaced in the polyanion according to equation I). [PMoI2O4,l3- + PPh, + H20- [HPMo,,O,,]~- + OPPh, + H+ Applying large organic counter ions like [N(PPh,),] +, [K( 18-crown-6)I (1 8-crown-6 = 1,4,7,10,13,16-hexaoxocy- clooctadecane) or WBu ,] we obtained well defined compounds in good yields as X-ray-quality crystals from acetonitrile. Thus reaction of [K( 18-~rown-6)]~(PPh,),]~[P- Mo, 20,0]*2MeCN 1 yields the two-electron reduced hetero- polyoxomolybdate [K( 18-~rown-6)],[N PPh,)~]~[HPMo, - 04,]-8MeCN- 18-crown-6 2 having the Keggin structure, and that of [NBu ,],[S,Mo , gives the four-electron reduced heteropoly oxomolybdate ~Bu ,],[H3S2M~18062]~ 4MeCN 3 having the Dawson structure. The degree of re- duction was determined by several independent methods. Volumetric titration with cerium(1v) sulfate showed that for [HPMo 2040]4- two-electron reduction had taken place per formula unit and for [H3S2M0,80,2]5 - a four-electron reduction. In the 31P NMR spectrum of [HPMo,,O,,]~- (1)    P  u   b   l   i  s   h  e   d  o  n   0   1   J  a  n  u  a  r  y   1   9   9   5 .   D  o  w  n   l  o  a   d  e   d   b  y   U  n   i  v  e  r  s   i   t  y  o   f   M   i  s  s  o  u  r   i  a   t   C  o   l  u  m   b   i  a  o  n   0   6   /   0   8   /   2   0   1   3   1   1  :   0   7  :   2   1 . View Article Online / Journal Homepage / Table of Contents for this issue  2522 J. CHEM. SOC. DALTON TRANS. 1995 the signal of the heteroatom phosphorus shifts from 6 0.89 for [PMO,~O~~]~- o 6 -2.02, with the signal for the [N(PPh,),]+ ion constant at 6 25.52 as internal standard for both compounds. Similar upfield shifts have been observed earlier and assigned to a two-electron reduced species [H,PMo, 2040](5 n -. The UV/VIS spectrum of [PMo,,O,,]~- shows no absorption in the range of d-d transitions. However, that of [HPMo,,O,,]~- displays a wide absorption band with the maximum at 727 nm and an absorption coefficient E = 380 dm3 mol-I cm-'. The intensity of this intervalence charge- transfer band has been reported to be proportional to the number of electrons introduced into the polyanion.' An absorption coefficient of this magnitude corresponds to a reduction by 1.8 to 2 electrons. Independently from the volumetric redox titration, the four-electron reduction of [H,S,MO,,O,,]~ - was corroborated by the structural data (see below). Unlike all other heteropolyanions, Dawson-type heteropolymolybdates have long been believed to undergo only even-numbered electron reductions. ' However, very recently the first odd oxidation level Dawson heteropolymolybdates [P,M018O6,]'- and [S,MO,,O,,]~- have been identified. 12 ~ No 95Mo or 170 NMR data could be obtained for 2 and 3 due to the limited solubility of these compounds and the relative sensitivity of these nuclei. Infrared Spectra.-The IR spectra of [PMo 20,0]3 , are shown in Figs. 1 and 2. In the first pair of compounds the spectrum of the reduced species is significantly different from that of the unreduced species. The relative intensities of the peaks at 809, 881 and 1062 cm--' decrease considerably after reduction. The first two absorptions have to be assigned to the antisymmetric Mo-0-Mo stretching vibrations of corner- and edge-sharing MOO, octahedra, the last one to the three-fold degenerate P-0(-Mo) vibration. ' According to Kawafune and co-workers9- the decrease in intensity is caused by elimination of oxygen atoms from the MoV1-O-MoV' bonds to produce two neighbouring molybdenum(v) ions in an incomplete Keggin ion. We do not agree with this conclusion. All chemical and structural evidence shows that the Keggin structure PMo, 2040 remains intact after reduction. 1-8 The loss in intensity is rather due to the smaller transition moment for the vibrations under discussion. The transition moment decreases because the molybdenum-oxygen bridge bonds, which are alternating 'short' and 'long' in [PMo,,O,,]~-, become more equal in [HPMO,~O,~]~- nd because the additional electrons occupy delocalised orbitals. The Moat stretching vibration (0, s a terminally bonded oxygen atom) at 956 cm-' is not affected by the reduction. intensity of the absorption bands in the region of the Mo-O- Mo bridging vibrations (700-900 cm-') changes even more dramatically for the reasons just mentioned. Owing to the lower symmetry of [S2M01806,]4- the S-O vibration is split into two components. In [H3S2M01 8062]5 further splitting is observed. The most intense band is shifted from 1 170 to 1 156 cm-'. At the same time the broad bands of the Mo-O-Mo bridging vibrations move to lower frequencies. These observations will be discussed below in the context of the structural results. CHPMol 20401~- [SZMol806ZI4- and [H3S2Mo18062I5 - For the species [SzMO1806z]4- and [H3SzMO1806z]5- he The Crystal Structures.---(a) The unit cell of compound 1 contains two independent, nearly identical [PMo,,O,,]~- ions, located at the inversion centres 000 and &@, four [N(PPh,),] ions, and two [K( 18-crown)(MeCN),] ions with the potassium atoms located at 08 and $0. The polymolybdate ions form a centred arrangement in the ab plane together with the potassium ions. The site symmetry is incompatible with the tetrahedral symmetry of the Keggin structure. Therefore the structure of 1 shows the central P atom surrounded by a cube of eight oxygen atoms and the Mo atoms L 1200 1000 800 600 Wavenu m ber/cm- Infrared spectra of [PMO~~O,~]~- top) and [HPMo,,O,,]~- ig. 1 (bottom) I 1 1200 1000 aoo 600 Fig. 2 Infrared spectra of [S,Mol,0,,]4- and [H3S2Mo,8062]5- (bottom) W avenu m be r/cm- situated at the corners of a regular cubooctahedron. All Mo Mo distances are equal, although they are different between edge- and comer-sharing octahedra in a single Keggin anion. The vibrational ellipsoids of all oxygen atoms have very characteristic shapes: those of bridging oxygen atoms are elongated radially, whereas those of terminal oxygen atoms are either flattened or tangentially extended (see Fig. 3). Evans and Pope l4 have pointed out that a crystallographic result like this can be explained by disorder: a single Keggin anion is self- superimposed by a second one, which is rotated 90 about a 3    P  u   b   l   i  s   h  e   d  o  n   0   1   J  a  n  u  a  r  y   1   9   9   5 .   D  o  w  n   l  o  a   d  e   d   b  y   U  n   i  v  e  r  s   i   t  y  o   f   M   i  s  s  o  u  r   i  a   t   C  o   l  u  m   b   i  a  o  n   0   6   /   0   8   /   2   0   1   3   1   1  :   0   7  :   2   1 . View Article Online  J. CHEM. SOC. DALTON TRANS. 1995 2523 axis. However, this type of disorder does not explain completely the present structufal results. Polyoxomolybdates with Keggin structures lack Td(43m) symmetry; the symmetry is lowered to T(23), a chiral point group.’.’’ This arises from small displacements of the Mo atoms within their MOO, octahedra, which result in alternating ‘long’ and ‘short’ Mo-0-Mo bonds. The symmetry operation mentioned above would average these ‘long’ and ‘short’ bonds, contrary to our results, where the Mo-0-Mo bonds fall into two well resolved categories: 1.790(9)-1.843(9) (mean 1.814) and 1.962(9)-2.019(7) (mean 1.990) A. We propose the following model to explain these observations. The single anion [PMo,,O,,]~- is chiral. Both enantiomers, which are related to each other by a mirror operation, exist in equal amounts. If they are located at the same site of the unit cell and crystallographically superimposed, they appear as a pseudo-molecule which belongs to the centrosymmetric point group Th(2/m3). The alternating ‘short’ and ‘long’ Mo-O-Mo bonds are then retained, but edge- and corner-shared MOO, octahedra are superimposed. The recently published structure of [Fe C5Me5)2]4[SiMol,040]~dmf (dmf = dimethylformamide) shows exactly the same type of disorder with retention of alternating Mo-0-Mo bonds.* Table contains a summary of the most significant bond distances and angles of [PMO~,~~~]~ . Despite the disorder the structure is discussed in some detail in order to evaluate the obvious structural changes upon reduction. Chemically identical bond distances and angles vary only slightly with the exception of Mo-O(P) and P-O Mo) bonds. Literature data give Mo Mo distances of 3.41 8, for bonds within a triplet of edge-shared MOO, octahedra and of 3.71 8, for bonds between triplets (corner-shared MOO, octahedra). The average value is 3.56 A. A recent publication, where 33m symmetry has been proposed for WMo, 2040]3 gives the following distances: Mo... Mo 3.324(2) and 3.6;4(2) A (mean 3.499 A), Mo-0, 1.665(9) A, Mo-0, 1.902(9) within the triplets and 1.928(5) A between the triplets. In [Ee C5Me,)2]4[SiMo,2040] he mean Mo Mo distance is 3.522 A.4 In com arison to these values the mean Mo Mo distance in 1,3.569 1, s slightly larger. The Mo=O, bonds tend also to be larger than in the reference data {with the exception of [Fe C5Me5)2]4[SiMo12040]). e ascribe this to the predominant hydrophobic environment of the Keggin ion in the crystal lattice of 1. The Mo-0-Mo bonds are strongly asymmetric as already mentioned. In the reference structures the bridging oxygen atoms participate in hydrogen bonds. Therefore the asymmetry of the Mo-O-Mo bonds, ranging from 1.85 to 1.97 A, is less expressed. (h) [HPMol,040]4-. The unit cell of compound 2 contains one reduced Keggin ion, located at the inversion centre O-@ two [K( 18-crown-6)(MeCN)] ions and two m(PPh,),] + ions; six unco-ordinated MeCN molecules and one empty 18- crown-6 molecule at complete the contents of the unit cell. According to the analytical results, 2 is a two-electron reduced species. It then carries the charge 5 To reach electroneutrality within the structure, the presence of one proton has to be inferred. The most probable location is at one of the 24 bridging oxygen atoms. -’ he Keggin anion of 2 shows exactly the same disorder as described previously. The magnitude of the thermal displacement parameters of the Mo and the three groups of 0 atoms, and the orientation of the vibrational ellipsoids, are very similar in both structures (see Fig. 4). Baker and co-workers8 noticed, on the contrary, a marked decrease of the displacement parameters upon reduction of a- [Co“W 2040]4 . Despite the disorder the structural data show a high degree of precision for both compounds. It is legitimate, therefore, to compare the structures of [PMo,,04,13 - and [HPMo,,O,,]~- in order to evaluate the changes caused by reduction. * The authors of ref. 4 describe the disorder as caused by the mechanism reported in ref. 14. G- O 3) P Fig. 3 vibrational ellipsoids (50% probability) Structure of [PMo12040]3- isplaying atomic labelling and The anion [HPMo, 2040]4- isplays the a-Keggin structure like the starting material. Fruchart and Souchay ’ ave shown that reduction of a-[Mol 20,0]3- n aqueous solutions gives heteropoly blues possessing the P-Keggin structure. This has been confirmed by Pope and co-workers’ for Cao,5H,P- MO~~O~~~CU. 8H20 by X-ray analysis. Obviously reduction in non-aqueous solutions, as in our case, leads to the more symmetrical a-type anions. The following significant structural changes in [HPMO,~O,,]“- can be detected (see Table 1): the metal atom framework expands slightly from the anion’s centre; the alternating ‘long’ and ‘short’ Mo-0, bonds become more equal, but the deviation from equivalence is still significant. In this connection the size of the Ob-Mo-Ob(Ci. ?) bond angle varies much less in [HPMO~~O~~]~-. he Ma, bond lengths remain unaffected. In the disordered structures of 1 and 2 four least-squares planes can be calculated, which comprise six metal atoms each. The average mean deviation from these planes is much smaller in [HPMo, 2040]4 (0.024 A) than in [PMo,,O,,]~- (0.155 A). All changes observed are consistent with the description that the two additional electrons occupy tangentially oriented    P  u   b   l   i  s   h  e   d  o  n   0   1   J  a  n  u  a  r  y   1   9   9   5 .   D  o  w  n   l  o  a   d  e   d   b  y   U  n   i  v  e  r  s   i   t  y  o   f   M   i  s  s  o  u  r   i  a   t   C  o   l  u  m   b   i  a  o  n   0   6   /   0   8   /   2   0   1   3   1   1  :   0   7  :   2   1 . View Article Online  2524 J. CHEM. SOC. DALTON TRANS. 1995 Table 1 Comparison of bond distances A) and angles ( ) in [PMO,~O~~]~- nd [HPMO,~O~~] - mean values in square brackets) MO MO M0.e.P Mdb 'short') Mo-0, M0-0, ('long') Mo-O(P) P-0 Ob-Mo-Ob (Cis) 0,-MO-0, (Cis) Mo-Ob-Mo a Between 'short' bonds. Between 'long' bonds. [PMo 204033 3.553(2)-3.587(2) 3.549(2)-3.576(2) 1.641(8)-1.663(8) 1.790(9 -1.843(9) 1.962(9)-2.019(7) 1.482(9)-1.578(14) 2.403( 10)-2.520(9) 94.9(4)-97.6(4) 78.1(4)-81.2(4) 1 37.8(6)- 14 1.1(7) [3.569( l)] [3.558(1)] [ .654(2)] [1.814(2)] [ .990(2)] [ .47 (3)] [ .535(3)] C9642)I 179 %2)1 [ 39.5( )] CHPMOI @4014 - 3.548( 2)-3.594(2) [3.573( 1 )] 3.564(2)-3.586(2) [3.573( l)] 1.651(9)-1.669(7) [1.659(4)] 1.859(9)-l.910(6) [1.888(3)] 1.897(13)-l.936( 10) [1.920(3)] I .443( 14)-1.608( 16) [ .526(4)] 2.397( 14)-2.576( 15) [2.491(4)] 87.6(4)-91.7(5) [89.6(4)] 84.7(4)-87.0(4) [85.1(4)] 137.7(4)-140.5(6) [139.6(2)] do(,, O 18) Fig. 4 Structure of [HPMo, 2040]4 isplaying atomic labelling and vibrational ellipsoids (50% probability) delocalised and non-bonding molecular orbitals. 1-3*8 Owing to the disorder no differentiation can be made between the bonds within the triplets (edge-sharing octahedra) and the bonds connecting the triplets (corner-sharing octahedra). These two kinds of distances may behave differently upon reduction. However, the observed similarity of the atomic displacement parameters in [PMO~,~~~]~ and limits such distinction strongly. (c) [H,S,Mo, 8062]5-. ompound 3 crystallises in space group C2/c with four formula units; besides the polyanion, which is located upon a two-fold axis, the unit cell contains 20 [NBu ,]' ions and 16 MeCN molecules. The anion is not disordered. Considering the analytical results three protons are afforded for electroneutrality. Their locations were determined (see below). The crystal structure of the oxidised S2Mo18 ion (in mEt4]4[S,Mol ,O,,]*MeCN) has been reported recently. 8 So the alterations occurring upon the four-electron reduction can be analysed in detail. The anions [S,MO~~O,~]~- nd [H3S,Mol8o,,]' - both display the a-Dawson structure (see Fig. 5). In Table 2 the range and/or mean values for chemically identical interatomic distances are given. The following discussion refers in general to the mean values. The overall symmetry of [H,S,MO,~~,,]~ approaches closely D3h. The strongest changes upon reduction take place in the two equatorial belts of edge- and corner-sharing MOO, octahedra. The Mo Mo distance between corner-sharing MOO, octahedra increases from 3.693(2) to 3.759(1) A, whereas the distance between edge-sharing octahedra remains nearly constant at 3.45 A. The puckering of the Mo atoms in these belts is much less pronounced than in [S2MO18062]4-. The mean deviation from the least-squares planes decreases from 0.18 to 0.007 A. The lengths of the Mo-O, bonds within the belts, which alternate considerably in [S2M0180,2]4-, are completely evened out upon reduction and increase slightly. The lateral Mo S and Mo-O(S) distances also grow with the expansion of the belts. Within the triplets of edge-sharing Mo, octahedra at the top and the bottom of the Dawson structure the following changes occur: the alternation of the Mo-0, bond lengths, which is very distinct in [S,MO,,O,~]~- with 1.827(5) and 2.000(5) 8, for the 'long' and 'short' bonds, respectively, nearly disappears; this contrasts two-eletron reduced [HPMO,,O~~]~- here the Mo-0-Mo bonds remain significantly unequal. The mean Mo-0, bond length and the Mo . Mo distance increase by 0.028 A to .930(3) 8, and by 0.014 8 o 3.450( 1) A, respectively. The Mo 9 Mo distances along the bridges between triplets and equatorial belts vary in [S2Mo18062]4- from 3.681(3) to 3.785(3)~andin[H,S,M0,,0,,]~- from3.726 l)to3.836 1)A. The spread in [S,MO,,~,,]~- s caused by severe puckering in the equatorial belt, but in [H$,MO180~2]5 - predominantly by a rotation of the triplets uersus the belt. The Mo-O, distances in this part of the anion, which alternate in [S,MO~,~,~]~- s usual, show interesting details in [H3S,Mo180,,]5- (see Table 3): the two bonds srcinating at each of the Mo atoms in the triplets differ by0.06 A. All bonds srcinating at the Mo atoms in the central belt are larger than those srcinating at the triplets; the difference of 0.102 8, is highly significant. This may signal that the additional electrons preferentially populate orbitals    P  u   b   l   i  s   h  e   d  o  n   0   1   J  a  n  u  a  r  y   1   9   9   5 .   D  o  w  n   l  o  a   d  e   d   b  y   U  n   i  v  e  r  s   i   t  y  o   f   M   i  s  s  o  u  r   i  a   t   C  o   l  u  m   b   i  a  o  n   0   6   /   0   8   /   2   0   1   3   1   1  :   0   7  :   2   1 . View Article Online
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