10 V, Nb, and Ta

By H. SLOAN Consulting Chemist, 40 Wendan Road, Newbury, Berkshire RG 14 7AF, UK

1 Introduction

Oxoniobates containing metal clusters,' hydrides,, solid state NMR of coordination compound^,^ polyvanadates4 and their coordination and solution ~hernistry,~ and halogen exchange between main group metal compounds and vanadium (and molybdenum) compounds as a synthetic technique6 have been reviewed. Major sections on the vanadium group are also included in a review of di-, tri- and polyphosphine complexes of transition metals;' niobium and tantalum compounds are referred to in a review of the protonation of coordinated dinitrogen.' Bond-stretch isomerism in niobium compounds9 has been considered (see also Chapter 11, Section 2) and a publication on the catalysis of hydrogen peroxide oxidations includes" the early transition polyoxmetalates.

This report is arranged on the basis of the most significant other atom in the compounds discussed; there is a separate section on metalates.

2 Hydrides

Dihydrogen reacts' ' with [Ta(OR),R;] (R = C,H,Pri-2,6, R' = CH2C,H,-4-Me) in the presence of a phosphine, L, to give [Ta(H),(OR),L]. The crystal structure of the dihydride (L = PMe,Ph) showed a severely distorted octahedron with the trans- hydride ligand bonds bent towards the phosphine ligand (H-Ta-H = 13.5") and a long Ta-P bond (2.650 A). NMR and IR spectroscopy support the trans-dihydride disposition. Phosphine exchange occurs readily, but not rapidly, on the NMR timescale; in benzene-d, the hydride ligands are scrambled with deuteride over several days at room temperature. Hydrogenation of the aryl substituent of the phosphine ligand occurs over 24 hours, e.y. PMe,Ph is converted to PMe,Cy. Use of the

J. Kohler, G. Svennson, and A. Simon, .4nqew. Chem.. l n t . E d . Engl., 1992, 31, 1421. A. Y. Esayed and D. 0. Northwood, Inr. J . Hydrogen Energy, 1992, 17, 41.

W . G . Klemperer, T. A. Marquart, and 0 M. Yaghi, Anyew. Chem., Int. E d . Enyl., 1992,31,49; H. Reuter. .4nqew. Chem., Int . Ed. Engl., 1992, 31, 1185. Q. Chen and J. Zubieta, Coord. Chem. Rev. , 1992, 114, 107.

A.-M. Caminade, J.-P. Majoral, and R. Mathieu, Chem. Rev. , 1991, 91, 575.

' J . A . Davies and S. Dutremez, Coord. Chem. Rev. , 1992, 114, 201.

' P. Sobota, Polyhedron, 1992, 11, 715. '

' G. J . Leigh, Acc. Chem. Res. , 1992. 25, 177. ' V.C. Gibson and M. McPartlin, J . Chern. Soc., Dalton Trans., 1992, 947.

l o 'Metal catalysis in Hydrogen Peroxide Oxidations'. ed. G. Strukul, Reidel. London, 1991, Chapter 8. ' I V. M. Visciglio, P . E. Fanwick, and I . P. Rothwell, J . Chem. Soc., Chem. Commun.. 1992, 1505.

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118 H . Sloan

phosphine PMePh, gave only products resulting from sequential hydrogenation of the phenyl groups, with no products from partial hydrogenation of one or both rings. Free condensed aromatic hydrocarbons are also hydrogenated though free phosphine inhibits the reaction, indicating dissociation to [Ta(H),(OR),] is a key step.

3 Carbonyls

The compounds [MCI(CO),(dmpe),] (M = Nb, Ta) are important precursors and a convenient, high yield synthesis has been developed,12 involving the catalytic reduction of [MCl,(dmpe),] with Mg(anthracene) under CO. These compounds have a capped trigonal prismatic structure with an acute OC-M-CO angle; the close non-bonded C-C contact is consistent with the previously observed reductive coupling of the carbonyl ligands.

4 Nitrides and Nitrogen-donor Ligands

Lithium niobium nitride, Li,NbN,, has' an anti-fluorite superstructure with regularly-ordered distorted Li,Nb cubes, each Nb atom being tetrahedrally co- ordinated by nitrogen. Marked differences from similar Ta and V compounds suggest some doubts over the latter structures.

Reduction of [VCl,(thf),] with NaBHEt,, followed by addition of a cold suspension of Li(dfm) in thf, gavel4 the complex [V,(dfm),] as the first structurally characterized V" complex with a triple bond between vanadium(i1) atoms. As well as the short V-V bond, the V-N bonds are also short, consistent with the high basicity of the dfm ligand. The methine proton has a very large chemical shift. In the presence of [PPh,CH,Ph]- C1, [VCl,(thf),] reacted' with o-phenylenediamine, or its 4,Sdimethyl derivative, to give the octahedral anions [VCl,(diamine)] - . The dimethyl complex is isostructural with the analogous chromium anion and evidence points to the metal-N-C angle being diagnostic of the nature of the coordinating nitrogen. The chloro groups trans to N and trans to another chloro ligand show greater differences than in the chromium analogue. In the absence of a phosphonium salt, trans-[VCl,(diarnine),]Cl is obtained, and with the vanadium(i1) salt [V, (p-Cl),(thf )6] 2 [Zn,CI,], trans-[VCI , (o-phenylene- diamine),] was obtained.

Reaction of [VOCl,(tmu),] with vinylimidazole in CH,CI, readily gave' trans- [VOCl( 1-vinylimidazole),]Cl with N-coordination of the heterocycles. X-Ray diffrac- tion studies of the air-stable compound were complicated by disorder in the 0-V-Cl moiety; the vanadium K-edge EXAFS was used to further refine the structure.

Few first row nitride complexes are known as models for nitrogen fixation. In contrast to the high oxidation state of other known nitrides of vanadium, the Viil(p-N)ViV core was obtained' in [V1i'Br,(N,)(dmpe)(p-N)VivBr(dmpe),l, formed by the reductive decomposition of Me,SiN, in the presence of truns-[VBr,(dmpe),]

J . D. Protasiewicz. P. A. Bianconi, I . D. Williams. S. Liu, Ch. P. Rao,and S. J . Lippard. I n o r y . Chrm.. 1992. 31, 4134.

F.A. Cotton, L. M. Daniels, and C.A. Murillo, A n y c w . Chem.. I n t . E d . Enyl., 1992. 31, 737.

1803.

654.

1 2

l 3 D. A. Vennos and F. J . DiSalvo. Acru C r y u a l l o y r . . Srcr. C. 1992. 48. 610.

'' C. Redshaw, G . Wilkinson. B. Hussain-Bates, and M . B. Hursthouse. J . Chrm. Sot,.. Dal ton Trans . , 1992.

'' L. J . Calviou. J . M. Arber. D. Collison. C. D. Garner. and W. Clegp. J . Clirm. Soc .. Chrm. Comrnun., 1992,

" D. B. Sable and W. H . Armstrong. I n o r y . Chrm.. 1992. 31. 161.

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V , N b , and Ta 119

(1). The V-N-V bonds are non-linear (165”) with very different V--N bond lengths (VI’I-N = 1.972 A, VIV-N = 1.652 A). Notable spectral features included a 19-line EPR spectrum in thf solution at room temperature; complex (1) was obtained in situ from trans-[VBr,(tht),] and two equivalents of dmpe. The asymmetric vanadium(v)-vanadiurn(tI1) complex (2), with a nitride bridge, was formed’ rapidly via various sequences (Scheme 1). The crystal structure shows a short V-N bond (1.588 A), involving five-coordinate vanadium(v), typical of a triple bond with the nitrido ligand in the axial position of the square pyramid. The six-coordinate vanadium-nitrido bond is somewhat shorter (2.092 A) than those found for VII1-Namine single bonds.

\ c1

thf---. v= / -

Cl

Reagents: i, tmeda, with lossof SiClMe,;ii, [VCl,(thf),] with loss ofSiCIMe,, iii, tmeda. [VCl,(thf),] with loss of SiCIMe,: iv, tmeda, [VCl,(thf),]; v, tmeda. with loss of (3)

Scheme 1

A series of seven-coordinate complexes, [MC1(CNR)(CO)(dmpe)2] (M = Nb, R = Me, Cy, Bu‘; M = Ta, R = Me, Et, Bu‘), has been synthesized” by photolysis of [MCl(CO),(dmpe),] in the presence of CNR, and reduction of [TaCl,(dmpe),] in the presence of CNR gave [TaCl(CNMe),(drnpe),]; all these complexes are fluxional in solution, having NMR spectra consistent with capped trigonal prismatic or pentagonal bipyramidal geometry. Crystal structure determinations showed a distorted capped trigonal prism as the common structural form, with significantly bent C-N-C bonds.

Niobium pentachloride reacted” with Me,SiNPPh, in acetonitrile to form [NbCI3(NPPh,),] which has a distorted trigonal bipyramidal structure; the two phosphorane inimato ligands and one of the chlorides are in the equatorial positions. The Nb-N and P-N bond lengths indicate double bonds, and the Nb-Cl bonds are unusually long. Reaction of [Nb(NEt,),Cl,] in thf/py with two equivalents of LiNHmes gave2 the trigonal bipyramidal [NbCl(=Nmes),py,], analogous to known

K. L. Sorensen, M . E. Lerchen. J . W. Ziller, and N. M. Doherty. Inory. Chrm., 1992, 31, 2678 E. M. Carnahan, R. L. Rardin, S. G. Bott. and S. J . Lippard. Znory. Chem.. 1992, 31, 5193. F. Weller. D. NuBhar, and K . Dehnicke. Z . Anorg . Allg. Chem., 1992. 615, 7. D.P. Smith. K.D. Allen, M . D . Carducci. and D E. Wigley. Inorg. Chrm., 1992. 31. 1319.

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120 H. Sloan

tantalum compounds. Using six equivalents of LiNHmes gave the salt [Li(thf),] [Nb(=Nmes),(NHmes)], though the thf is slowly replaced by pyridine giving a product with a less labile solvate. The thf-solvated salt reacted with LiBu" giving [Li(thf),][Nb(=Nmes),Bu] where the n-butyl group in the tetrahedral anion is disordered.

A study of the 14N and 15N NMR spectra of complexes of tantalum (as well as complexes of Mo, W, Re, and 0 s ) has shown" little variation of chemical shift with linear and bent metal-N-R moieties for a wide range of R, coordination number, and other ligand type. This contrasts with other nitrogen donors such as diazenides (-N=NR) and nitrosyls. There is also a closer parallel between the nitrogen and metal shielding.

Reaction of TaC1, with Me,SiNPPh, gave2, [Ta(NPPh,),][TaCl,]. The cations are distorted tetrahedrons with Ta-N and N-P bond lengths indicating double bonds. There are two independent cations with differing Ta-N-P angles, relating to the disposition of the phenyl substituents. The octahedral anions are symmetrical.

Electron diffraction studies of [Ta(NMe,),] indicated2, a square pyramidal structure with the apical Ta-N bond (1.937 A) shorter than the basal Ta-N (2.040 A). Electron diffraction data for [V(NMe,),] and [V(OBu'),] indicatedz5 distorted structures, the distortions being in the form of variations from the regular tetrahedral angle and not of bond distances. In [V(NMe,),], angles of 100.6' and 114.1' were obtained; V-N is 1.879 A. In the butoxide the angles are 106.7' and 115.1 O and the V-0 distance is 1.779 A.

Reduction of [VCl,(thf),] by sodium, in the presence of diphosphines and dinitrogen, yielded26 trans-[V(N,),(L-L),] - . The crystal structure (for L-L = dppe) of the anion is asymmetric, because of the phenyl group dispositions, and very like that of the neutral molybdenum analogue, but with a close interaction, V-NEN . . . Na, between the sodium ion and terminal nitrogen. In solution, 51V NMR showed that the dmpe analogue retains the same anionic structure. Excess HBr converts one in four of the nitrogen atoms to ammonia, and a small amount of hydrazine, but no hydrogen, is formed.

5 Oxygen-donor Ligands

A simple, high yield synthesis of vanadium(v) compounds [VO(O,)QL] (Q = quinolinate, L = EtOH, dmf, dmso, or thf) is provided27 by the reaction of V,O, with hydrogen peroxide and a solution of quinoline in the chosen solvent at pH = 2. The salts A3[VO(02)(HP0,)]~2Hz0 (A = K or NH,) are similarly produced from V,O,, H 2 0 2 , and phosphoric acid at pH = 6. Evidence for an q2-bonded dioxygen group is provided from spectral studies. A similar reaction with bipy or phen gave28 the seven-coordinate complexes [VO(Oz)( L-L)z][CIO,] (L-L = bipy or phen). An X-ray

2 2 D. C. Bradley, S. R. Hodge, J. D. Runnacles, M . Hughes. J . Mason,and R. L. Richards. J . Chem. Soc., Dalton

2 3 D. NuBhar, F. Weller, A. Neuhaus, G . Frenking, and K. Dehnicke, Z . Anorg. Al ly . Chem., 1992, 615, 86. 24 K. Hagen, C . J. Holwill, D. A. Rice. and J . D. Runnacles, Inorg. Chrm., 1992. 31, 4733. 2 5 A. Haaland, K. Rypdal, H.V. Volden, and R. A. Andersen, J . Chrm. So t . . Dalton Trans., 1992, 891. 26 D. Rehder. C. Woitha, W. Priebsch, and H. Gailus, J . Chem. Soc., Chrm. Commun.. 1992. 364. 2 7 M. Bhattacharjee, M . K. Chaudhuri, and P. C. Paul, Can. J. Chem., 1992, 70, 2245. 2 8 V. S. Sergienko, V. K. Borzunov, and M . A. Porai-Koshits, Russ. J . Inorg. Chem. ( E n g l . Transl.), 1992.37.

Trans., 1992, 1663.

534 (1062).

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V, N b , and T a 121

structural determination showed the 0x0 ligand and one of the nitrogen atoms in the axial positions of a pentagonal bipyramid around vanadium; the v2-dioxygen group is almost symmetrically bound to vanadium and lies in the equatorial plane. The V-N bond opposite the dioxygen is significantly longer (bipy, 2.247; phen, 2.24 A) than other equatorial V-N bonds (bipy, 2.126,2.136; phen, 2.12,2.13 A), and are similar to the axial V-N bonds (bipy 2.243; phen 2.24A) which are trans to the 0x0 ligand. Reaction of VOCI, with bulky biphenolic ligands (4) gave29 monomeric tetrahedral [VOCl(biphenate)] with a labile chloride.

R

(4) R = H, R' = Me; R = Me, R' = But (6)

The complex [V(acac)Cl,(thf ),I (5) was prepared,' from [VO(acac),] by reaction with [TiCl,(thf),], or from [VCl,(thf),] and Hacac. Reduction of (5) with zinc in thf gave (6), the crystal structure of which showed meridional disposition of the thf ligands and bridging between vanadium and zinc by chlorine and by an oxygen from the acac ligand.

There are few simple non-oxo vanadium(1v) or vanadium(v) species. [VO(acac),] in CH,Cl, reacted,' with phen and dtbc to give [V'v(dtbc),(phen)]CH2C12 which was oxidized with AgSbF, to [VV(dtbc),(phen)][SbF,]CH,C1,. The removal of the 0x0 group by dtbc is of particular interest. X-Ray diffraction showed very little difference between V-0 and V-N bond lengths, or between the bite angles of the ligands of the two compounds. However, other coordination angles change significantly and the vanadium(1v) compound has a marked distortion towards the trigonal prismatic form, contrasting with the slightly distorted octahedral geometry of the vanadium(v) compound. The bipy analogue [V'v(dtbc),(bpy)]CH,CI, was ,similarly prepared3, and has an analogous structure. For all three compounds, the most sterically demanding isomer, with the 3,St-butyl groups syn, is observed.

The equilibrium in Equation 1 shows an expected variation with concentration, but the exchange is sufficiently slow at room temperature to allow the two species to be detected3, by NMR spectroscopy. The dimer shows short V-Oalkoxy bonds, with vanadium in near trigonal pyramidal coordination.

n[VO(OR)J [{VO(OR),),] (R = cyclopentyl) ( 1 1 The unusual all-oxygen donor, dimeric vanadium(v) complex (7) has 34 the 0x0

ligands in the axial positions of trigonal bipyramidal disposition around each

29 P. J . Toscano, E. J . Schemerhorn, C. Dertelbacher, D. Macherone, and J. Zubieta, J . Chem. SOL'., Chem. Commun., 1991, 933.

30 E. Solari, S. De Angelis, C. Floriani, A. Chiesi-Villa, and C . Guastini. Inory. Chem., 1992, 31, 141. T. A. Kabanes, A. J . P. White, D. J. Williams, and J. D. Woollins, J . Chem. Soc., Chem. Commun., 1992, 17.

3 2 T. A. Kabanos, A. M. Z. Slawin. D. J. Williams, and J. D. Woollins, J . Chem. Soc., Dalron Trans., 1992, 1423. 3 3 F. Hillerns, F. Olbrich, U. Behrens, and D. Rehder. Anyew. Chem., Int . E d . Engl., 1992, 31, 447.

3 1

T. W. Hambley, R. J. Judd, and P. A. Lay, Inory Chem.. 1992, 31, 343. 3 4

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122

Et

H . Sloan

Et (7)

vanadium atom. The V=O double bonds are long compared with those of other alkanoate complexes. NMR spectroscopy showed that the plane of symmetry is retained in solution, with limited rotation about the C-C bond of the methylene to alkanoate link. The stability of this complex probably results from the ethyl groups providing steric protection of the metal site.

In attempting to form monomeric vanadium(Ir1) chelated complexes of dppb in an inert atmosphere, binuclear [(VOCl,(thf)),(dppb)] was formed;35 the oxygen prob- ably comes from adventitious air or water in the Celite filter. Each vanadium(1v) is trigonal bipyramidal with oxygen and two chlorine atoms in the equatorial plane. The EPR spectrum shows one unpaired electron on each vanadium atom, with no hyperfine splitting from ,lP.

The four-coordinate complexes [NbO(O-2,6-R2C,H,),] (R = Me, But) were ~yn thes i zed~~ by the reaction of [NbOCI,(NCMe),] and Li0-2,6-R2C6H3. The Nb-0 bond lengths and Nb-0-C angles37 in tetrahedral [NbO(OC6H,Ph2-2,6),] are typical of mono-oxo niobium compounds, even though these often take distorted octahedral form. The complex [TaCI(OC,H,Bu'-4),] was synthesized from TaCI, and the free phenol in refluxing benzene. It forms 1 : 1 adducts with aliphatic amines, and the salts [Ta(OC,H,Bu'-4),][MC14] with MCI, (M = Al, Fe, Sb).

6 Other Chalcogen-donor Ligands

Addition of two equivalents of Na(pyt) to [VOCl,(thf),] in thf gave 3 8 [V,O,(pyt),], where each vanadium atom is octahedral with a terminal 0x0 group, one pyt as a simple chelate, and two pyt groups with the sulfur bridging to the other vanadium atom, Reaction of Na(pyt) with [VCl,(thf),] in thf gave [Na(thf),V(pyt),], a rare example of seven-coordinate vanadium(rI1). In the crystal, vanadium is bound by three chelated pyt ligands and a fourth by sulfur only; the nitrogen of this fourth pyt ligand, and three sulfur atoms, bridge to the sodium, the coordination sphere of which is completed by thf ligands. Na(pyt) reacted with [VCl,(tmeda),] to give the unusual thiolatovanadium(i1) complex [V(pyt),(tmeda)], and with VC12-4MeOH in acetonit- rile to give39 the [V(pyt),]- anion. Under similar conditions, the use of sodium aniline-2-thiolate gave the oxidation product [V(NH2C,H,S-2),(NHC,H,S-2)] -, with loss of hydrogen from one ligand. The neutral complex [V(quinoIine-8-thiolate),] was similarly formed.

35 A . K . Fazlur-Rahman, V.G. Young, J r , and J . G . Verkade, Inorg. Chem., 1992. 31, 2285. 36 T. P. Kee and V.C. Gibson, Polyhedron, 1992. 11, 587. 3 7 J.S. Yu, P.E. Fanwick, and 1. P. Rothwell, Actu Crystullogr., Sect. C , 1992, 48, 1759.

J . G . Reynolds. S .C. Sendlinger. A . M . Murray, J . C . Huffman, and G. Christou. Angew. Chem., Inr. Ed. Engl . , 1992, 31. 1253. G . Henkel, B. Krebs, and W . Schmidt, Angew. Chem., I n t . Ed. Engl., 1992, 31. 1366.

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V , N b , and Ta 123

A novel niobium-sulfur cluster stabilizes the tetrathioborato ligand in diamagnetic complex (8), formed4' by the reaction of [Nb,Cp,f (p-B2H6),] (Cp,' = (q-C,Me,Et),) with sulfur in decane at 170°C. The Nb-S, bonds are similar to those in other, non-bridging, situations. The Nb-S(Nb) bond lengths are shorter than in other cyclopentadienylsulfidoniobium complexes. The Nb-Nb distances are consistent with partial single bonds.

The reaction of NbOI, with niobium, iodine, and tellurium at high temperature gave41 the cluster cation (9). The starred(*) atoms in the structure of (9) nearly define a plane of symmetry -the oxygen atom is just off this plane in the otherwise symmetric molecule. The four Te atoms form a square plane.

HS

I---. I'

The reaction of NbCI, with NaSBu' in MeCN gave4, the complex anions [Nb(S),(SBu'),]- (10) and [Nb(S)(SBu'),]- (1 l ) , containing Nb=S units formed by cleavage of a C-S bond. The tetrahedral (10) was also formed by oxidation of (1 1 ) with sulfur. Complex (1 1 ) is distorted trigonal bipyramidal, apparently showing a strong trans influence due to the axial sulfide ligand; the axial Nb-SBu' bond length is longer, 2.567,&, than the equatorial average of 2.38,&. The reaction of NaS,CNEt, with [M,Cl,(tht),] (M = Nb, Ta) gave43 [MS(SCNEt,)(S,CNEt,),].;C6H6 which has a pentagonal bipyramidal structure; the terminal sulfide and a sulfur from one of the dithiocarbamate ligands occupy the axial positions. The thiocarbamyl ligand is bound through both S and C.

A new synthesis of [MS4I3- (M = Nb, Ta), previously only known in intractable solids, has enabled44 a structural study to be made. Reaction of [M(OEt),], S(SiMe,), and LiOMe (1 : 4 : 3) in MeCN gave Li3[MS,]-4MeCN, which gave the stable, pure compound Li,[MS4].2tmeda on recrystallization in the presence of tmeda. The vanadium compound was prepared similarly, from [VO(OMe),]. All three com- pounds are isomorphous and have tetrahedral anions with M-S bond lengths longer than those generally found in terminally bound sulfides. The differences in colour (V, dark red; Nb, light yellow; Ta, colourless) arise from a progressive high energy shift of the first ligand-to-metal charge-transfer band.

40 H . Brunner, G . Gehart. B. Nuber, J . Wachter, and M. L. Ziegler, Angew. Chem., I n t . Ed. Engl.. 1992. 31,

4 1

43 P. F. Gilletti, D.A. Femec. F. I . Keen, and T. M. Brown, Inorg. Chem., 1992, 31, 4008.

1021. W. Tremel, J . Chrm. SOC. , Chem. Commun, 1992. 126. D. Coucouvanis, S. Al-Ahmad, C.G. Kim. and S.-M. Koo, Inorg. Chem.. 1992, 31, 2996.

S.C. Lee, J. Li, J .C. Mitchell, and R . H. Holm, Inorg. Chem.. 1992, 31, 4333.

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124 H . Sloan

A mixture of [NH,][VS,], CuCl, Na[S,CN(CH,CH,),O] (Nadtc' ), and NaSPh in dmf gave4, the distorted cubane-type anion [V,CU,S, (~~C' ) , (SP~) , ]~ ~ where each vanadium atom has a distorted square pyramidal coordination sphere comprising the two sulfur atoms of a dtc' anion and three sulfur atoms of the (V,Cu,S,) cubane core. A vanadium-silver analogue was obtained similarly. Relatively shorter bonding to the external ligands at both vanadium and copper support the presence of charge transfer from copper(]) to vanadium(1v).

7 Halide Ligands

The electronic and EPR spectra of VF, have shown4, the presence of two short axial V-F bonds with longer bridging equatorial V-F-V bonds. The anion of [PMePh,]- [TaF,] has 47 a regular octahedral structure with a Ta-F bond length of 1.84(6) A. The anions in Na[MCI,] (M = Nb, Ta) have48 a significant off-centre displacement of the metal atom within the octahedral C1, coordination sphere. The overall structure is similar to that in K,[PtCI,] with half the cation sites vacant; the off-centre displacement relates to the cation location in alternate channels along [OOl]. The isomorphic 1,2,3-tris(dirnethylamino)cyclopropenium hexachlorometalates [C,(NMe,),][MCI,] (M = Nb, Ta) are known to exhibit strong outersphere charge transfer interactions. Each salt contains4' two crystallographically different anions in one of which three chlorine atoms have close contacts with the three nitrogens of the cation. This may be the reason for the colour and stability of these compounds compared with those of the individual colourless ions.

The methanol solvated clusters in [Ta,C1, 2(MeOH),]Br3 and [Ta,CI, , (MeOH),]- Br3.4H,O have5' an octahedral arrangement of tantalum atoms with a chloride bridge associated with every Ta-Ta bond; each Ta has one terminally bound methanol ligand. Similar cluster ions with chloride or phosphine terminal ligands are known; shorter Ta-Ta bonds are found with the terminal oxygen donor.

8 Metalates

Niobic acid, formed by in situ hydrolysis of aqueous K,Nb04 or K8Nb6Ol9, catalyses' the photochemical decomposition of MeOH to give hydrogen. Calcining the acid at up to 380 K increases the catalytic activity. Orthorhombic NbPO,, grown at high temperature (1 375 K) in a K-Nb-P-0 system of composition KNb, 2P6033, has a framework built5, from corner-sharing NbO, octahedra and PO, tetrahedra. There is no suggestion of oxygen deficiency in this product. Preparation by chemical vapour transport at 1000"-900 "C, using I, or NH,Cl as the transport agent, gave53 known phases; the orthorhombic phase required reducing conditions by the addition of NbP. However, this preparation gave NbPO, - b , a blue orthorhombic form, deficient in

Y. Yang, Q. Liu, L. Huang, B. Kang, and J . Lu, J . Chem. Soc.. Chem. Commun., 1992, 1512.

M.A. McLoughlin. N. 1,. Keder, and W.C. Kaska. Acta Crystalloyr., Sect. C, 1992, 48, 1098.

4 5

46 H. Stratemeier, M.A. Hitchman, R. J. Deeth, and R. Hoppe, J . Chem. Soc., Dalton Trans., 1992, 3419.

48 H. Henke, Z. Kristallogr., 1992, 198, 1. 49 H.N. Schafer, H. Burzlaff, A.M. H. Grimmeiss. and R . Weiss, Acta Crystalloyr., Sect. C, 1992, 48, 795.

N. Brnicevic, D. Nothig-Hus, B. Kojic-Prodic, Z . Ruzic-Toros, 2. Danilovic, and R . E. McCarley, Inorg. Chem., 1992, 31, 3924.

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5 1 A. Kudo, A. Tanaka, K. Domen, K. Maruya, and T. Onishi, Bull. Chem. SOC. J p n . , 1992,65, 1202. 5 2 D. L. Serra and S.-J. Hwu. Acta Crystal lop. , Sect. C. 1992, 48, 733.

U . Kaiser, G . Schmidt, R. Glaum, and R. Gruehn, Z . Anorg. Allg. Chem., 1992. 607, 113. 53

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V , N h , and Ta 125

oxygen, with a similar structure to the colourless form observed at > 340 "C. Heating the oxygen-deficient phase to 340 "C gave a reversible transformation between monoclinic and orthorhombic phases.

Identification of the localized or delocalized nature of vanadium centres in polyoxovanadates has been shown54 to be possible using valence bond summation. The M,O,, core is a common structural type in polyoxometalate chemistry. The V,O, core can remain intact while undergoing55 considerable distortion on reduction or protonation.

The simplicity of the hydrothermal technique has led to its frequent use for the production of polyoxometalates. Two equivalents of [NH,][VO,] with one equivalent of EtC(CH,OH),, in water at 200°C for four days, gave56 crystals of [NH4]4[V,,0,6(EtC(CH,0),)4]~4H20 (12). The structure consists of a reduced

(Reproduced from J . Chem. SOC., Chem Commun., 1992, 305)

{Vlo028) core where ten doubly bridging and two triply bridging 0x0 groups are replaced by the twelve alkoxy donors from the four ligands. Of the six potential triangular ligand sites, four are symmetrically occupied. Refluxing [NBu,],- [H,V,,O,,] in MeCN for several hours under nitrogen gave57 the tridecavanadate [NBu,],[V,,O~~] with an anion built from VO, octahedra, in contrast to the VO, square pyramids previously known. This isopolyvanadate has the lowest charge density of any known polyvanadate. Reactions of [NBu,],[H,V,,O,,] with RC(CH,OH), yield5, hexavanadate clusters in the anion [V,O, 3{(OCH2)3CR),]2 - [R = Me, NO,, NHC(O)CHCH,]. These vanadium(v) clusters are readily reduced by organohydrazines to the mixed-valence species [VTVYO, ,(OH),{ (OCH,),- CNO,),]* - and the completely reduced [V',"O,(OH),{ (OCH2),CMe),l2- anions. While the common hexametalate core (M,O,,) is present, this is distorted by substituting doubly bridging 0x0 groups by the alkoxy donor oxygen atoms and by the presence of reduced sites.

5 4 A.-L. Barra, D. Gatteschi, B. S. Tsukerblatt. J. Doring, A. Muller,and L.-C. Brunel, lnorg. Chem., 1992,31,

5 5 Q. Chen, D. P. Goshorn, C. P. Scholes, X . L. Tan, and J . Zubieta, J . Am. Chem. Sot.. 1992, 114, 4667. 5 6 M.I . Khan, Q. Chen, and J . Zubieta, J . Chem. SOC., Chem. Commun., 1992, 305. 5 7 D. Hou, K.S. Hagen, and C.L. Hill, J . A m . Chem. Sot., 1992, 114, 5864. '' Q. Chen and J . Zubieta, lnorg. Chim. Actu, 1992, 198-200, 95.

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126 H . SIoan

Diammonium hydrogen phosphate reacted59 with [NH,],Na,K,[V 100,s] to give, after addition of [NHMe,]CI, [NHMe,],[NH,][H,PV,,O,,], which has an 2- Keggin type framework capped by two VO units. The protonated sites are at doubly bridging oxygen atoms. Reaction of ammonium metavanadate with phenylphosphonic acid and hydrazine hydrate, in the presence of dimethylammonium chloride, gave,’ the host/guest compound [NH,Me2][2NH,+ ,2C1-] c [V,,O,z(OH),(H,O),(PhPO,),l .5H20-4Me2NCH. The anion is a ring of two { V,O,}, + (one localized Vv) units and two {V202(0H),(H20)(PhP03),)6- (two V”) units. The two aqua ligands partition the cavity within the anion.

The reaction of NaVO,, As,O,, KSCN, and sodium rhodizonate in water at pH 4.4 gave6’ K , N ~ [ A s , V ~ V ~ O , ~ H , ] . ~ H , O . The structure consists of an E-Keggin core with three VO vertices removed and capped by three VO and three AsOH units. The open ‘pole’ of the anion is capped by a potassium cation. Magnetic susceptibility studies gave an effective magnetic moment of 3.59 pB, corresponding to 1.69 pB per vanadium(1v) centre, so all five vanadium(1v) centres are practically uncoupled. The analysis of the magnetic susceptibility data and EPR spectra of the vanadium(1v) ions [V, 5AS6042(H,0)]6 - and [V,,As80,,(S0,)]6 -, and extended Huckel calculations thereon, has provided62 a detailed understanding of the preferred spin arrangement and energies of the spin levels; qualitative considerations of exchange pathways are still needed to reduce the number of independent parameters. The anion of [NBu,], [NH,] 2[V lo0,,(0,AsC6H,-4-NH,)3] consists63 of a VO, group encapsulated by a [V,02 (O,ASC,H,-~-NH,),]~ - toroid; the latter has three (V,O, ,} moieties linked by bridging 0x0 and arsenato ligands. The central octahedral VO, unit has vanadium displaced toward one face of the octahedron. There are thus four different 0x0 groups within the ion. An unusual twisted 24-membered ring { V&6012} with intraring 0 x 0 bridges occurs in [ ~ ~ u , ] , [ ~ , { ~ , ~ , , ( ~ , ~ s C , H , ) , ) I ~ ~ ~ , ~ .

s9 M . I . Khan, J. Zubieta, and P. Toscano, Inorg. Chim. Acta, 1992, 193. 17. A. Miiller, K. Hovemeier, and R. Rohlfing, Angew. Chem., Inr. E d . E n y l . . 1992, 31, 1192. M.I . Khan, Q. Chen, J. Zubieta. D. Goshorn, and R.C. Haushalter, Inorg. Chim. Acfa, 1992, 191, 97. 6 1

6 2 A.-L. Barra, D. Gatteschi, L. Pardi, A. Miiller, and J. Doring, J . A m . Chem. Soc.. 1992, 114. 8509. 6 3 M . I . Khan, Y . Chang, Q. Chen. H . Hope, S. Parking, D. P. Goshorn, and .I. Zubieta. Angew. Chem.. I n t . Ed .

Engl . . 1992. 31. 1197.

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