Molybdenum(V) oxidetribromide complexes and the oxidation of molybdenum(III) halides by triphenylphosphine oxide and triphenylarsine oxide

J. inorg, nucl.Chem., 1968,Vol. 30, pp. 2641 to 2646. PergamonPress. Printed in Great Britain

MOLYBDENUM(V) OXIDETRIBROMIDE COMPLEXES AND THE OXIDATION OF MOLYBDENUM(III) HALIDES BY TRIPHENYLPHOSPHINE OXIDE AND TRIPHENYLARSINE OXIDE W. M. C A R M I C H A E L

and D. A. E D W A R D S

School of C h e m i s t r y , Bath University of T e c h n o l o g y . Claverton Down. Bath, Somerset

(Received 8 January 19681 A b s t r a c t - C o m p l e x e s of m o l y b d e n u m ( V ) oxidetribromide of the types MoOBr3,2L (L = methyl cyanide or triphenylarsine oxide) and MoOBr3,B (B = 2,2'-bipyridyl or 1,10-phenanthroline) have been prepared by direct reaction. M o l y b d e n u m ( I l l ) chloride or bromide and potassium hexachlorom o l y b d a t e ( I l l ) are oxidised by molten triphenylphosphine oxide to MovOX3,2L (X = C1, Br) complexes, whereas the same species react with molten triphenylarsine oxide with greater oxidation to give MoviO2X2.2L (X = CI, Br) complexes. INTRODUCTION

MANY COMPLEXES of molybdenum(V) oxidetrichloride with neutral donor ligands[1,2] have been prepared and characterised, but few derived from the oxidetribromide are known. MoOBr3,2Ph3PO and MoOBr3[Ph2P(O)CH2CH2 P(O)Ph2] have been obtained[3] by oxidation of various phosphine-substituted molybdenum carbonyl compounds with bromine followed by exposure to moist air. MoOBr3, bipy has similarly been obtained [4] from Mo(CO)4bipy. We have prepared and characterised complexes of molybdenum(V) oxidetribromide by the direct reaction of the oxyhalide with some nitrogen and oxygen donor ligands and also investigated the solvolytic reaction with anhydrous liquid ammonia. The complexes prepared, together with their conductivities in methyl cyanide and nitrobenzene and significant infrared data are given in Table 1, and Table 2 summarises their visible and u.v. spectra. It is well established that the complex chemistry of molybdenum is dominated by the formation and stability of oxo-species in the + 5 and + 6 oxidation states. We can now present some further reactions which illustrate this feature well. The reaction of molybdenum(Ill) bromide with an excess of molten triphenylphosphine oxide in a sealed tube at 160° results in the formation of MoOBr~, 2Ph3PO. In this reaction not only must the polymeric structure of the tribromide[5] be broken down but also the phosphorus-oxygen bond of 1 mole of ligand per mole of bromide must be broken to provide the oxygen necessary to form the oxomolybdenum(V) compound. Presumably the reduced product of the ligand is triphenylphosphine which will be a weaker donor than the phosphine oxide with I. 2. 3. 4. 5.

P. C. H. Mitchell, Q. Rev. 20, 103 (1966). K. F e e n a n and G. W. A. Fowles. lnorg. Chem. 4, 310(1965). J. Lewis and R. W h y m a n , J . chem. Soc. 6027 (1965). C. G. Hull and M. H. B. Stiddard,J. chem. Soc. (A), 1633 (1966). D. Babel and W. Riidorff. Naturwissenschaften 51, 85 (1964). 2641

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W. M. C A R M I C H A E L

and D. A. E D W A R D S

Table 1 MeCN Concn. (104M)

MoOBrn,bipy MoOBr3,phen MoOBr~,2MeCN

PhNO2 AM

Concn. (104M)

Ar~

7'0 3.4 --

24-6 25.0 --

4-3 -5-7

3" I -6-6

965 968 988

MoOBr3,2PhaAsO

--

--

2.7

7-1

979

MoOBr3,2Ph3PO

2.7

24.6

3.9

7-0

972

MoOCI3,2Ph3PO

7-2

17-0

8.3

5.7

968

[MoOBr(NH2)2]~

Insol.

--

lnsol.

--

740-840* v. br.

Complex

Mo==O str. Other i.r. cm -1 bands (cm -1) --2286 C-=N str. 848, 876 A~--~O str. 1153 P = O str. 1157 P = O str. 3136 br. N - - H str.

* - - M o - - - O - - M o - - - O - - chain vibration.

reference to oxomolybdenum(V), so it is the oxide which finally complexes. Similarly, the reaction of either molybdenum(IIl) chloride or potassium hexachioromolybdate(III) with triphenylphosphine oxide at 160° results in the formation of MoOC13,2Ph3PO, previously isolated[6] from the reaction of molybdenum(V) chloride with the ligand. It is of interest that similar reactions using molten triphenylarsine oxide at 200 ° as the oxidising ligand lead to the dioxomolybdenum(Vl) species, MoO2Xz,2PhaAsO, although in these reactions great care is necessary to avoid the presence of moisture at any stage otherwise hydrolysed species result. Thus the arsine oxide is capable of effecting a greater amount of oxidation of the molybdenum. Also relevant to the oxidation reactions is that the tribromide can be obtained unchanged after attempted reaction with either triphenylphosphine or triphenylarsine in sealed tubes at 150° for several days. Oxidation of molybdenum(Ill) species solely by oxygen-donor ligands has not previously been reported, although we have already reported[7] upon the MoO2X2,2Ph3AsO complexes having prepared them by direct reaction with the dioxidedihalides. The magnetic susceptibilities, conductivities and infrared and ultraviolet spectra of the complexes prepared by oxidation were in accord with the results already published. X-ray powder photography has shown that the complexes prepared from either the dioxidedibromide and triphenylarsine oxide in me@yi cyanide or molybdenum(Ill) bromide with molten triphenylarsine oxide were identical. Solutions of the oxidetribromide complexes in either methyl cyanide or nitrobenzene are very weakly conducting, the molar conductances being well below those for typical uni-univalent electrolytes in these solvents, the results probably indicating slight solvolysis. 6. S. M. H o m e r and S. Y. Tyree, Jr., lnorg. Chem. 1, 122 (1962). 7. W. M. Carmichael, D. A. Edwards, G. W. A. Fowles and P. R. Marshall, lnorg, chim. Acta 1, 93 (1967).

Molybdenum(V) oxidetribromide complexes

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Table 2 Peak Complex MoOBrs.bipy

Method (10 -3 c m - ' ) D.R:: MeCN

MoOBr3,phen

D.R. MeCN

MoOBr.~,2MeCN

MeCN

MoOBrz,2Ph3AsO D.R. MeCN

MoOBr3,2Ph3PO

D.R. CHeCle

MoOCI:~2Ph3PO

D.R. CHeCI2

[MoOBr(NHe)2]x D.R.

Other bands Emax. A s s i g n m e n t

13.1 21.0 13.1 20.6

--40 750

be b, be b,

--~ --~ --~ --"

e~* be e~* be

12.6 21.0

be bt be b,

--~ --~ --~ ~

e,~* be

21.5

--30 700

13-0 20.7

l0 900

b.e ~ e~* b, ~ b~

13.3 20.6 13-7 21.8

--25 450

b._, -," bt --~ be - * b, --~

e~-* be e~* be

13.3 20.0 13-3 20.5

--

--~ --* --~ ~

e=*

1000

be b, be b,

13-5 22-3 13.9 22.4 29.1

--18 21 1000

be be be b~ b,

--~ ~ --~ --~ --~

e,* b,* e,~* b,* be

13.0

--

11

e. * be

be e,* b~

(I 0 -3 c m - ' )

25-0;29-6(sh); 32.9:42-0: 45.9.

25.6;29.1: 32-9;37.5: 45-0 24.3;26.3(sh) 34.3;39.7: 45.8

24-0:26'2; 30-0(sh): 31.5(sh): 34-7; 38.5: 45.0

26.6; 31.6(sh) 34-6(sh); 38"5; 43.3

33.5;38-5: 43.2 14.3; 15.7: 20.0; 27.4.

* D. R. = diffuse reflectance.

Where oxidation state titrations could be carried out the molybdenum was shown to be in the +5 oxidation state and the magnetic moments, all being close to the 'spin-only' moment, are also in agreement with this state. Much deviation from the 'spin-only' value would not be expected[8] since the free ion spin-orbit coupling constant of Mo(V), 1030cm-', will be reduced and the asymmetric ligand field will remove most of the orbital degeneracy of the 2T,,g ground term. The i.r. spectra show the terminal Mo(V) = 0 stretching frequencies in the range 965-988 cm-' and the lowering of the phosphorus-oxygen or arsenic-oxygen stretching frequencies[6, 9] on co-ordination. The electronic spectra are interpreted on the basis of the strong tetragonal 8. B. J. B r i s d o n , D. A. E d w a r d s , D. J. M a c h i n . K. S. Murray and R. A. W a l t o n , J. chem. Soc. (A), 1825 (1967). 9. F. A. C o t t o n , R. D. Barnes and E. Bannister, J. chem. Soc. 2 1 9 9 (1960).

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distortion of the octahedral species, using the molecular orbital scheme devised [I0] for MoOCI52-. although in the present complexes, where the ligands are combinations of three halide ions and two other donor species the ligand field symmetry will be Czv or Cs rather than C4v. It is justifiable to discuss the results in terms of C4v symmetry since other deviations will be small compared with the tetragonal distortion due to the MoO group. Three ligand field transitions are possible (b2 ~ e~, *" ~ b*; ~ la*) but only the first two are observed for the chlorocomplexes and only one for the bromo-complexes. The remainder must be hidden under the complex charge-transfer and internal ligand transition spectrum. The original molecular orbital scheme[10] assigns the first charge-transfer band to an e ~ b2 transition (Mo---O 7r orbital ~ Mo dx~ orbital) but a further investigation[11] of the electronic spectra and ESR of MoOX~ 2- species leads to the conclusion that the transition involved is a bl* ~ b2 (Mo--Br o--orbital to Mo dxu orbital), so a re-ordering of the e~ levels below the b~ is necessary in the original scheme. As the latter assignment involves a symmetry-forbidden transition it is favoured here as the band found for the oxidetribromide complexes at about 20,000 cm -1 has a relatively low intensity compared with the remaining charge transfer bands. The reaction between anhydrous liquid ammonia and the oxidetribromide at -33 ° formally leads to two of the three Mo--Br bonds being broken leaving [MoOBr(NHz)z]~ as an ammonia-insoluble product. However, 33 per cent of the total starting molybdenum is found in the ammonia-soluble product suggesting that partial breakdown of the polymer occurs to give a complex anion in the presence of the ammonium bromide formed in the solvolysis reaction. xMoOBr2 + 4xNH~ ~ [MoOBr(NH2)~]x+ 2xNH4Br ~ z(NH4)2[MoOBr3(NH2)2]. Under our experimental conditions most of the ammonium bromide would be removed in the first filtration so breakdown of the polymer is incomplete. The i.r. spectrum of the ammonobasic oxybromide shows no evidence for a terminal Mo(V) = O stretching frequency but a very strong broad band at 740-840 cm -~ supports the presence of bridging oxygens. As there is no reason to suppose the complex to be other than octahedral, a reasonable formulation would also involve amino-bridging. The i.r. spectrum shows a broad N - H stretch centred at 3136 cm -a, an N H2bending mode at 1608 cm -a and an N Hz-wagging mode at 917 cm -~. Titration shows the molybdenum still to be in the +5 oxidation state but the low magnetic moment indicates that the complex is magnetically concentrated presumably either through magnetic exchange via oxygen- and amino-bridging or spin-spin interaction due to the close approach of molybdenum atoms in the doubly-bridged species. EXPERIMENTAL

Materials. Molybdenum(V) oxidetribromide was prepared as previously described[12]. Methyl cyanide was dried by repeated distillation in vacuo from phosphoric oxide and ammonia was similarly dried with sodium. Solid ligands were recrystallised and dried by pumping in vacuo, together with vacuum sublimation where possible. 10. H. B. Gray and C. R. Hare, lnorg. Chem. 1,363 (1962). 11. H. Kon and N. E. Sharpless, J. phys. Chem. 70, 105 (1966). 12. R. Colton and I. B. Tomkins,Aust.J. Chem. 18,447 (1965).

Molybdenum(V) oxidetribromide complexes

2645

Analysis. Molybdenum was determined as the 'oxinate' and halogen by potentiometric titration with silver nitrate. Phosphorus was estimated as quinolinium phosphomolybdate and arsenic by the iodometric method of I ngram[l 3], both after peroxide fusion of the complexes. Oxidation state titralions were carried out by oxidation of a weighed sample with a known excess of cerium(IV) sulphate or dichromate, the excess being titrated with standard iron(l 1) solution. Physical measurements, Ultraviolet and visible spectra were determined on solutions in 1 cm. sealed silica cells, using Unicam SP 500 or Perkin-Elmer 137 spectrophotometers. The Unicam SP 500 with a standard attachment was used for reflectance spectra, the samples being mounted between silica plates. Infrared spectra were measured for Nujol mulls on a Perkin-Elmer 237 spectrophotometer. Magnetic moments at room temperature were measured by the Gouy method. Conductance measurements were made in nitrobenzene or methyl cyanide at 25 ° using a Wayne-Kerr Autobalance universal bridge. X-ray powder measurements were made with a Nonius Guinier focussing camera using filtered Cu K~ radiation. Line intensities were visually estimated. Reactions. All reactions were carried out using a vacuum system but several methods differing in detail were used. Reaction of molybdenum(V) oxidetribromide with methyl cyanide. Excess of anhydrous ligand was distilled on to the oxyhalide contained in an ampoule which was then sealed. After 1 day the ampoule was opened, attached to the vacuum line, excess of ligand distilled out and the product pumped. Found C, 10.9; H, 1.5: Br, 54.8: Mo, 22-3; N, 6.3% oxidation state, 4.98; p, 1.70 B.M. C4H~BraMoN20 requires C, 11.1 : H, 1.4; Br, 55-2; Mo, 22.1 : N, 6.5%.

Reactions of molybdenum(V) oxidetribromide with 2,2'-bipyridyl, I,lO-phenanthroline or triphenylarsine oxide. An excess of anhydrous methyl cyanide was distilled on to the oxybromide in vacuo and warmed gently to give a solution. To this solution was added an excess of a methyl cyanide solution of the ligand. The precipitate formed was filtered, washed with anhydrous ether and pumped. Found for 2,2'-bipyridyl product: C, 24.0; H, 1.8: Br, 46.7; Mo, 18.5; N, 5.7%, oxidation state 4.96:/x, 1.75 B.M, C.oH~BraMoN20 requires C, 23.6; H, 1-6: Br, 47.2; Mo, 18.9: N, 5.5%. Found for 1,10-phenanthroline product: C, 26.7; H, 1.7: Br, 44,9; Mo, 17.8; N, 5-4% C~2HsBr:~MoN20 requires C, 27.1; H,I.5; 13r, 45.1; Mo, 18.0; N, 5.3%. Found for triphenylarsine oxide product: As, 14.9; Br, 23.8: Mo, 9.5%. C:j6HaoAsBr3MoO requires As, 15,0; Br, 24.1; Mo, 9.6%.

Reaction of molybdenum( l l l ) bromide with triphenylphosphine oxide. The bromide and a four molar excess of ligand were sealed in a 'Pyrex" tube and heated for 8 days at 160°. After cooling the tube contained a red-brown resin which was soluble in anhydrous dichloromethane leaving a very small residue of unchanged bromide. Addition of anhydrous benzene to the dichloromethane solution resulted in the slow precipitation of a yellow-green solid which was filtered, washed with benzene and pumped. Found C, 47-0: H, 3.6; Br, 27.0; Mo, 10-6; P, 6-7%, oxidation slate, 4.95;/x, 1.73 B.M. Ca~H30Br.~MoOP requires C, 47-6: H, 3-3: Br, 26.4: Mo, 10.6: P, 6.8%. Reaction of molybdenum(Ill) chloride with triphenylphosphine oxide. The method was similar to that above but using a reaction time of 3.5 days. The excess of ligand was extracted from the product with several portions of anhydrous benzene. The insoluble part was then extracted with anhydrous dichloromethane, evaporation of the solvent leaving a pale green solid, which was further washed with benzene and pumped. Found CI, 13-8; Mo, 12.4; P, 7.9%, oxidation state 5.00: /x, 1-71 B.M. C:~nH:~)CI:~MoOPrequires CI, 13.7; Mo, 12.4: P, 8.0%. Reaction of potassium hexachloromolybdate(lll) with triphenylphosphine oxide. After sealeo tube reaction at 160° for 4 days some unreacted K3M oCI~ remained. The tube contents were repeatedly extracted with benzene, leaving an insoluble residue of K:~MoCI~, potassium chloride and a green solid. This solid was extracted with dichloromethane, evaporation of the solvent leaving a pale green solid, which was washed with benzene and pumped. Found CI. 14.0: Mo, 12.2; P, 7.9%, oxidation state 5.00:/z, 1.72 B.M. C36H3oCI3MoOP requires CI, 13-7; Mo, 12-4: P, 8-0%. Reaction of molybdenum(Ill) chloride or bromide with triphe nylarsine oxide. The molybdenum( 1i 1) halide and a four molar excess of ligand were sealed in an evacuated tube and heated for four days at 200 °. After cooling and opening the tube the contents were extracted with anhydrous dichloromethane. Removal of the solvent by vacuum distillation left a solid which was washed with warm benzene several times, to remove excess of ligand, before pumping. The products formed in non-evacuated tubes and bench working-up were hydrolysed molybdenum blue species. Found C, 51.1; H, 3.6; As, 17.6: 13. G. I ngram, Methods of Organic Elemental Microanalysis, p. 297. Chapman-Hall, London (1962).

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C A R M I C H A E L and D. A. E D W A R D S

CI, 8.5; Mo, 11.8%. C3eH30AsCIzMoO2 requires C, 51.3; H, 3.6; As, 17.8; CI, 8-4; Mo, 11.4%. Found C, 46.8; H, 3.8; As, 16.2; Br, 17.5; Mo, 10-1%. C36H3oAsBrzMoO2 requires C, 46.4; H, 3.2; As, 16.1; Br, 17.2; Mo, 10.3%. Reaction of potassium hexachloromolybdate(ill) with triphenylarsine oxide. A similar reaction to the above was carried out using K3MoC10 when extraction with anhydrous dichloromethane leaves a residue of potassium chloride (Found CI, 47.~ KCI requires C1, 47.5%). Found C, 50.1; H, 3.4; As, 17.6; CI, 8.5; Mo, 11.2%. C3eH3oAsCl2MoOz requires C, 51.3; H, 3.6; As, 17'8; C1, 8.4; Mo, 11-4%. Reaction of molybdenum(V) oxidetribromide with liquid ammonia. Anhydrous ammonia was distilled on to the oxybromide in vacuo and allowed to warm to its boiling point. The ammonia was filtered off leaving a brown solid product. Ammonia was condensed back on to the solid, allowed to warm to its boiling point and the solution filtered again. Nine such washes were carried out before the ammonia was evaporated from the combined filtrates leaving a heterogeneous pale brown solid. Both the ammonia-soluble and -insoluble products were then pumped at room temperature. Found for NH3-insoluble product: Br, 35.3 ; Mo, 43.0; N, 12.0~, oxidation state, 5.08; p, 0.9 B.M. H4BrMoN20 requires Br, 35-7; Mo, 42.8; N, 12-5%. Found for NH3-soluble product: Br, 67.4; Mo, 9.9; N, 12.4%. Mo: Br: N = 1.0 : 8.2 : 8.6. NH3 soluble Mo/total Mo = 0,33.

Acknowledgement - W e thank Bath University of Technology for a maintenance grfint to W.M.C.