Study of a new type of transition metal peroxo complexes as oxidants

Jomnal of Molecular Cafalysis. 7 (1980) 141148 @ Ebevier Sequoia SA. - Printed in the Netherlands

STUDY OF A NEW TYPE OF TRANSFITON COMPLEXES AS OXIDANTS

JEAN

SALA-PALA,

SEAN

ROUE

and JACQUES

L-aboratoire de chimie inorganique moI&uloire, Occidentale. Facult& des Sciences et Techniques. Cedex (France)

141

METAL PEROXO

E. GUERCHAJS

ERA CNRS 6, avenue

822. UniwrsitP de Bretagne V. Le Gorgeu, 29283 Brest-

Summary

The (RCp)zNbC12 complexes (R-Cp = r15-CsH4R; R = H,CH3) react with hydrogen peroxide affording the peroxocomponnds (R-Cp),Nb(O-0)Cl (I). These complexes, having two q5 bonded cyclcpentadienyl rings ald a sideon coordinated O2 group, present an electronic configuration with 18 electrons and a coordination number equal to nine. The complexes (I) react with SO1 giving the sulfato derivative (RCp),Nb(SO,)Cl, while Pe3 in excess affords the oxocompound (RCp),NbOCI together with 03P0. In the presence of Ha02, compounds (Ij catalytically react with cyclohexene to give the corresponding epoxide_

Introduction Although peroxides are of considerable and growing importance in relation to the catalytic oxidations involving O2 or H202 [I, 21, relatively little effort has been made to synthesize new types of transition metel peroxocomplexes in order to obtain 2 new reactivity. Up to now, peroxides v&h one or more linkages of the tVpe /o M,A

can be grouped into two main classes:

(a) those formed by metals on the right hand side of the Periodic Table which usually have the following features: preparation from molecular oxygen, reversibilility, a low oxidation state for the metal, an 184ectron confignration (sometimes 16), the presence in the coordination sphere of n-acceptor hgands such as CO or PPh,, which are soft in the Pearson sense, (b) those formed by metals on the left hand side of the Periodic Table which generally have the following characteristic properties: preparation from hydrogen peroxide, irreversibility, a higher oxidation state for tie metal (usually the highest oxidation state), an electronic confignration with

142

12,14,16 or 17 electrons (hardly ever 18), the presence of hard ligands, such as fkorinc, o-phenanthroline or oxzlate in the coordination sphere.

For this reason, we thought it would be particularly interesting to look for a new type of peroxo complex. To this end, we have tried to prepare some organome+dic peroxo compcunds containing early transition metalcarbon a-bonds. We report here the most conclusive results which have been obtained with niobium.

Experimental Elemental analyses were performed by the Service Central de IMicroanalyses du CNRS. Infrared spectra were recorded on a Rye-Unicam spectrophotometer from 200 to 4 000 cm-‘. The molecular weights were measured by osmometry, using a Knauer osmometer and chloroform solutions at 27 “C. The starting materials (R-Cp),NbCl, (R = H,CHa) were prepared as described in the literature 13 - 53. (i) Peroxocompiexes

(R
(I)

(R

= H,Me)-

syntheses

To a solution of Cp,NbClz (2 g; 6-8 X lo-3 mole) in deoxygenated dichloromethane (cu. 300 ml), hydrogen peroxide (3090, ca. 15 ml) is added. After vigorous stirring for a few minutes, the solution becomes yellow and After Zltration on a paper, the the organic phase is separate d by decant&ion. solution is concentrated (to cu. 50 ml) and hexane (cu. 50 ml) is added. These last two operations are repeated until the solution becomes colourless. The yellow precipitate is filtered, washed with hexane and dried under vacuum at room temperature for a few minutes (yield up to 80%). (CHs-CSH,)aNb(O-0)Cl is prepard by the Same procedure_ Caution These operations were repeated 40 - 50 tunes, however on two occasions, the product exploded violently when we scraped the walls of the Schlenk tube before weighing. Analyses R = H % found: C, 41.6; H, 3.7; Cl, 13.1. % calcd. (CIoHleNb07_C1): C, 41.3; H, 3.4; Cl, 12.2. R = Me % found: C, 44.8; H, 4.4; Cl, 11.3. % calcd. (CrzH14Nb02Cl): C, 45.2; H, 4.4; Cl, 11.1. Molecular

weight

for R = H found:

277 g; calcd.:

290.5

g.

Infraredspectra(200 - 1300cn~-~l R =H 27O(m)(vNhC1),375(m) (ring tilt), 525(s) and 545(s) (2. and vahO,), 615(vw), 630(s) and 845(m) (6CH). R = CHs (7CH), 870(s) (YOD), 98O(vw), 1 OlO( m ) and 1020(w) 27qs) (vNbCl), 32@w), 356(w), 380(m) (ring tilt), 527(s) and 550(s) (v, and va.NbOs), 615(m), 820(s) (YCH), 87O(vs) (WOO), 940(w), 97O(vw), 1040

and 1055(w) (5 CH), 1080(w). No Raman spectrrrm couldberecorded,the products explade violently when aposed to a laser besm, even at -SO ?I!. Physicul properties. The peroxocomplexes (I) are yellow _microcryst.aRine products, stable in air for several weeks. They are soluble in some organic solvents (CHCla, CH2C12, THF, CHaCN, DMF . . .) from which they cm be recrystalhsed. They are not soluble in al-es. They are relatively stable in solutions, even under ultraviolet irradiation: they can be recovered v&h a good yield from CH2C12 solutions after a long irradiation (LIZ. 4 days).

(ii) Reactiuity

of the peroxocomplexes (I) (a) reaction with triphenylphosphim

To a solution of Cp,Nb(O_O)Cl(O_75 g; 2.6 X IO-” mole) in dichloromethane (cu. 300 ml), a solution of PQs (0.76 g; 2.9 X 10e3 mole) in the same solvent (cc. 50 ml) is added. After four days of stirring, the resulting solution is filtered. The filtrate is the-r concentzsted (to ca. 50 ml). By addition of benzene (cc. 100 ml), a rare powder precipitates (0.4 g)_ It is identified as the previously known c*-coderivative Cp,NbOCl by comparison of its infraredspszctra with that of an authentic sample prepared by the action of dmso with Cp,NbClc according to Broussier el al. [S] _ Concentration of the benzene solution affords a precipitate in which Po3 and @sPO may be identified by i-r. (b) reaction Nb(SO_,)Cl (II)

with

SO,.

Formuti~n

of

the sulfato

complexes

(R-Cp),

Liquid SOa (cu. 50 ml) is poured at +O “c into a Schlenk tube contaming Cp,Nb(O+)C1(0.8 g; 2.7 X LOP3 mole)_ After stirring (for oz. 30 mm), the suspension is filtered. Evaporation of the liquid SO2 from the ftitrate affords a pale precipitate which is washed with hexane and dried under vacuum (yield ca. 49%). The yield can be improved using SO2 gas_ Sulphur dioxide is bubbled (for cc. 1 h) through a solution of Cp,Nb(O+)Cl (1 g; 3.4 X IO-” mole) in dichlorometbane (ca. 100 mi). The expected complex (U) is obtzined by filtration of the resulting suspension (yield 66%). Concentration of the filtrate gives another sample of the same compound (total yield: 99%). (CH3-CsH4)1Nb(S04)CI is prepared following the same procedure_ Analyses. R = H % found: C, 33.8; H, 3.0; Cl, 10.6; S, 8.6. % c&d. (CIoHIoNbS’3&l): C, 33.8; H, 2.8; Cl, 9.7; S, 9.0. R = CHa % found: C, 36.5; H, 3.6; Cl, 9.2; S, 8.1_ % calcd. (C,.H,.NbSO,Cl): C, 37.6; H, 3.7, Cl, 9.3, S, 8.4.

Infrared spectm. R = H 270(m) and 2SO(sh) (uNbCl), 305(w), 395(w) (ring tilt), 530(w) (YzSO& 578(m) (cqSOq), 625(m) (urS04), 57O(vs) (y&C& 769(m), 850(s) (rCW, 910(s) (+SO& 935(s) (+SO& 1026(w) (6 CH), 1039(w), 1080(w), 1140(w), 1175(vs) (v3S04), 1295(vs) (vsso+).

144

R=CHs 270(w) (vNbCl), 300(w), 330(w), 376(w), 385(w) (ring tilt), 540(w) (QSOC), 570(m) (~4=4), 620(m) (y.GOa), 67OWs) (Y&W, 750(m), 855(s) (rCH). 900(s), 910(s) (v,SO4), 935(s) (vsS04), 98O(vw), 104O(sh) and 1050(w) (6 CH), lOgO( 117O(vs) (u,S04), 1285(vs) (u$W4). Physical properties. The sulfato complexes (II) are pale yellow products. They are stable in air and only slightly soluble in common organic solvents. (c)

reaction

of the sulfato

complexes

(II)

with potassium

thiocyanate

To a solution of Cp,Nb(S04)CI (0.16 g; 4.5 X lo-” mole) in dichloromethane (cu. 106 mlj is added KNCS (0.18 g; 1.8 X lop3 mole) in acetone (cu. 100 ml). After stirring (for C(I. 72 h), the solution is concentrated. Addition of hexane gives a precipitate. Extraction of this residue with dichloromethane affords the tithiocyanatocomplex Cp,Nb(NCS)3 as yellow microcrystals. R = H % found: C, 39.0; C, 39.3; H, 2.5; N, 10.6.

AnaZyses.

NbN,S3).

H, 2.7; N, 9.3; 95 calcd. (C13H1e

Infrared spectrum. 26O(vw), 27O(vw), 35O(vw), 370(w) (ring tilt), 385(sh), 495(m) (6 NCS), 575(m), 590(m), 650(w), 700(m), 740(s), 855(vs) (yCH), 86O(sb) (;ICS), lOlO and 1020(m) (6CH), 1075(w), 1135(w), 122O(vw), 2 OSO(ws) (YCN). (iii) Study

of H-e catalytic

properties

of the peroxocomplexes

iI)

The catalytic properties of the peroxocomplexes (I) for epoxidation have been tested with cyclohexene. In a typical experiment, a mixture of Cp,Nb(O-O)Cl(50 mg; 0.17 X lop3 mole), cyclohexene (0.5 mole) and hydrogen peroxide (70%; 0.05 mole) in acetonitrile (60 g) was heated at 70 “C for 4 h. Analyses of the mixture after heating showed the % H20a conversion (100 X number of consumed H,Oz moles/number of introduced HsOs moles) to be equal to 76% whiIe the epoxyde selectivity (100 X number of formed epoxide mo:es/number of consumed Hz02 moles) was equal to 36%.

Results and discussion (i) Synthesis

and structural

study

of the peroxocomplexes

In solution in an organic solvent (CHCl, or CH,CI,), the biscyclopentadienyl dichloroniobium(IV) complexes, hereafter abbreviated (R-Cp), NbClz (RCp = 1)5-CSH4R; R = H, CHa) [3 - 51, react rapidly with hydrogen peroxide giving yellow solutions which afford, after further work-up, yellow microcrystals (I) in yield up to 80%. The formulation of (I) as the new moncmeric peroxocomplexes (R-Cp),Nb(O-0)Cl is based on the analytical data, on the detzmination of the molecular weight and on the i-r. spectra

145

The ix. spectrum.of (I) is not very different from that of the correqmnding (RCp)axbCla complex. They both exhibit the characteristic absorptions of the (q5-C5H5)aNt moiety ]7], the main differences being the presence on the s,pectrum of the peroxoderivatives of three strong bands: the first at 870 cm-l and the two others near 525 and 550 cm-l_ These absorptions are, respectively, assignable to the I, V. and uBe(MOIj frequencies of a sideon bonded O2 group ES, 9]_ Although the presence of a peroxoligand in complexes (I) was clearly shown by infrared spectroscopy and by chemical evidence (see below), no satisfactory method was found for the titration of the O$- group_ Addition of ar’Wied iodide solutions to a solution of compound(f) gave an immediate coloration due to liberated iodine, but in a few seconds, the coiour faded (presumably due to a reaction of the niobhnn complex with iodine) precluding the titration with S20z-. The structure of the Cp,ML, complexes (n = 1,2,3) has been discussed by Lauher and Hoffmann [lo] _In such compounds, the kvo cyclopentadienyl rings are not parallel and in this geometry, after M-Cp TTbond formation, the three first eontier orbitals which play a prime role in coordinating further ligands, all significantly ext-snd into the x-y plane which bisects the normals to the planes of the cyciopentadienyl rings (see scheme). Thus, in the absence of crystal structure determiration (all the crystals we obtairred seemed to be twin crystals), sugge&ed gecmetq for complexes (I) is shown in the scheme_

A similar structure was found by an X-ray study of the corresponding disulphur derivatives CpaNbSaX [Ill, 121. Complexes (I) seem to be the frost wellcharacterized peroxocompounds containing q5 bonded cyclopenkdienyl rings. They presumably show an electronic cotimtion with 18 electrons and a coordination number equal to nine. Up to the present, all attempts to synthesize, in the Same way, peroxides with the other elements of Group V (V and Ta, starkg from the dichlorides Cp,MCla) and elements of Groups IV (Ti and Zr, from Cp2MCl,) and VI (MO, from Cp,MoCl, or Cp,MoMa) were unsucce=ful_ (ii) Reactivity

of

tke peroxocomplexes

(a) In sohkion in dichloromethane, the complexes (I) react with P@, affording the previously known oxocompound (RCp),NbOCl[4] together with 9sPO according to the reaction (RCp),Nb(O+)(=E

f

W3

-

(R-Cp)aN’bOCl

+ .@sPO

(1)

146

Such oxygen atom Mer from the metal to the phosphine has aheady been reported. ENi(O*)(t-BuNC),] reacts with 03P affording [Ni(&3P),(t-BuNC)z] and @$O [13] _With the niobium derivative, only one oxygen. can be tranzferred inti the substrate;this result is not surprising, the niobium having a strong tendency to give oxocompounds even in organometalk chemistry [S, 14]_ (b) The peroxocomplexes (I) react with sulfur dioxide either as a neat liquid or in dichloromethane solution to yield pale yellow products (IQ. The i-r. spectra of these complexes do not show the characteristic absorptions of the peroxo group but exhibit the characteristic bands of a sulfato ligand. In agreement with these results, the elemental analysis confm the formation of the sulfato complexes (R+p)2Nb(S04)Cl according to the following reaction: (RZp)2Nb(O-O)Cl

f- SO2 -

(R-CP)ZN~(SO~)G

(2)

Ionic, monodentate, bide&ate and bridging sulfates belong to the Td, c Ju, Czv and Cav point groups, respectively, and may be distinguished by in&ared and Raman analyses [ 15]_ The fact that for complexes (li), the v3 and vq give rise ti three infrared bands indicates the presence in these compounds of a bidentate sulfato group. Reactions such as eqn. (2), i.e., the formation of a sulfato complex from a peroxoderikative and sulfur dioxide, ha_Je been described for several compounds containing electron-rich metals, and we believe that the mechanism (absence of the oxygen-oxygen bond cleavage hut formation of a peroxosulfite inknnediate and then re arrangement giving the sulfato complex) [ 151 is similar with niobium and these metals. (c) Attempts to substitute the chloroligand, in the peroxocomplexes (I) and in the sulfato derivatives (II), by other halogens or pseudohalogens have been made. With potassium thiocyanate, complexes (I) afford a mixture of the starting chlorocomplex and the corresponding thiocyaF_ati compound, but the two products could not be eEfectively sepakd. Surprisingly, CpzNb (SC*)Cl reacts with KNCS ;-n excess to give the unknown CP,N~(NCS)~ derivative_ The position of the infrared absorptions of the NCS group [ 161, and particularly a single band at 475 cm-I a&gnable ta 6 N,-s, clearly shows the coordination to the niobium uia the nitrogen atoms. Up to the present, all attempts to prepare the fluoro analog of (I) using the following pmcedures: (I) + AgF in acetone or (I) f Et4NF in acetonitrile or even CpzNbC12 in an H202-HF mixture, were unsuccessful. Reactions of Pt(O-O)L2 complexes with various reagents such as CO, COz, NO, NOz . . _[lb] have been examined. In order to learn more about the rezctivity of the niobium peroxocomplexes, we have tried to examine the reaction of (I) with various small molecules: CS2, XzS, COz, NO and CH3Li_ With the first three molecules, ‘the starting material was recovered with a good yield, while with NO and CHBLi some precipitites were isolated but,

?_47 to date, elemental unclear.

analyses aktd infkared spectra of these products

remain

(iii) Shaiy of the cutulyhk properhmesof the peroxocomptexes The cata&ic okfin epoxidation .using peroxo derivatives & catalysts has already been reported [17] _ Owing to the importance of epoxydes, we t41cught it of interest to study the reaction of complexes (I) with akenes in the presence of hyckgen peroxide. In such conditions, C&&(CEO)c=2 does react catiyticalIy with cyclohexene affording the corresponding epoxide derivatie. However, the percent. H,& conversion (76%) and the epoxyde sekctivi~ (36%) do not seem to indica+~ cab&tic properties higher than those found for other complexes

ClSl-

Acknowledgements The authors thank Mr R. P. Martin and LMme M. Pralus (Ugke Kuhlmann, Pierre Eenite, France) for the study of the catalytic properties of the peroxocomplexes.

References 1 For recent general reviews see (a) G. HenriciGfive and S. Olive, Angew. Chem. Ink Ed. Engl.. 13 (1974) 29. (b) J. Valentine, Chem. Rev.. 73 (1973) 235. J. Chem. Sot., Chem. Commun., (1978) 888. 2 (a) S. E. Jacobson, R. Tang and F-Mares, (6) S. E. Jacobson, R. Tang and F. Mares, Ir10a-g~Chemr., 17 (1978) 3 055. (c) D. A. Mueigrm, S. E. Jacobson, P. A. Adgar and F. Mares, 6. Am_ Chem. Sot.. 100 (197c’) 7 063_ (d) D. Westlake, R. Kergoat and J. E. Guercbais, C. R. Acad. Sci., Sect. C, 280 (1975) 113_ (e) A. Sen and J. Halpem, J. Am_ Chem. Sot.. 99 (1977) 8 337. 3 C. R. LUCZS,in F. Basolo (cd.), [nor-g onic Synthesis. Vol. XVI. Me Graw-Hill, New York, 1st Edn, 1976, p_ 107. (1977) 59. 4 M. Bunker, A. de cian and M. L. H. Green, J. Chem. Sot., Chem. Commun.. 5 R. Brother, H. Normand and B. Gautheron, OganomeL Chem.. 155 (1978) 337. 6 R. Broussier, H. No rrnand and B_ Gauthemn, OrgcnomeL Chem., 155 (1978) 347. 7 H. P. Fritz, in F. G. A. Stone and R. West (eds), Advances in OGarrometallic Chemise. Academic Press, Nem York, 1st Edn, 1964, p_ 240. 175. S L. Vaska, Act. Chem_ Ren_. 9 (1976) 9 (a) W. P. Griffith, b. Chem. Sot.. (1964) 5 248. (b) W. P. Griffitb,J. Chem. Sot., (1963) 5 345. (c)W. P_ Griffith and T. D. Wickins, J. Chem. Sot. (.4). (1968) 397. J. Am. Chem_ Sot.. 98 (1976) 1729. 10 J_ W_ Lauher and R. Hoffmann, 11 P. M. Treichel and G. P_ Werber, J- ilm. Chem. Sot.. 90 (1968) 1753 12 R. M_ Roder, Ph.D.. Wiind~, 1973; Chem. Absh. 81(1974) 30 727 g. 18 S. O&&a, A. Nakamura and Y. Tatsuno, J. Am. Chem. Sot., 91 (1969) 6 994.

148 14

15 16

17 18

(a) D. A. Lemenowkii. T. V. Baukow. E. G. Perevdova and A_ N. Neameyanov, IN. Akad. Nauk SSSR, Ser. Khim.. 10 (1976) 2 404. (b) N. I. Kirillova, D. A. Lemenovskii, T. V. Raukom and Yu. T. Struchkov, KOOK. Khim.. 3 (1977) 1600. (c) D_ A. Lemenowkii and V. P. Fedin, Koord. Khim.; 4 (1978) 394. R. W. Horn, E. Wtiberger and J. P. Collman, Inorg. Chem.. 9 (1970) 2367 and referencea therein. (a) D. M. Adarns, &Vetal-Ligand and Related Vibmtions. E. Arnold, London. 1967. p_ 318. (b) M. E. Fwqo end J. M. Jam-, Irzorg. Chem.. 4 (1965) 1706. (c) J. Lewis, R. NyhoLm and P. W. Smith, J. Chem. Sec.. (1961) 4 590. H. Mimoun, I. Seree de Roth. L. Sajus end P. Menguy, French Patent /Z968) 1549184; Chem. Abstr., 72 (1970) 3 345~. R. P. Martin and M. Pmlus, personal communication.