MÖSSBAUER STUDIES ON SOME π-CYCLOPENTADIENYLIRON CARBONYL COMPLEXES

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MÖSSBAUER STUDIES ON SOME

π-CYCLOPENTADIENYLIRON CARBONYL COMPLEXES

T. Sawai, J. Martin, I. Butler, D. Simkin

To cite this version:

T. Sawai, J. Martin, I. Butler, D. Simkin. MÖSSBAUER STUDIES ON SOME π- CYCLOPENTADIENYLIRON CARBONYL COMPLEXES. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-247-C6-250. �10.1051/jphyscol:1974632�. �jpa-00215789�

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JOURNAL DE PHYSIQUE Colloque C6, suppKment au no 12, Tome 35, Dkcembre 1974, page C6-247

MOSSBAUER STUDIES

ON SOME x-CYCLOPENTADIENYLIRON CARBONYL COMPLEXES

T. SAWAI (*), J. P. MARTIN, I. S. BUTLER and D. SIMKIN (**)

Department of Chemistry, McGill University, P. 0 . Box 6070, Montreal H3C 3G1, Quebec, Canada

RBsumB. - Les spectres Mossbauer d'un nombre de complexes n-cyclopentadienyl carbonyl de fer ont ete mesures

a

une temperature de 96 K. La formule gknkrale des complexes etudiks est CpFe(C0)zL ou L = SMe2, SEt2, SPr;, SBu," et aussi CpFe(C0)zSiClzMe. A 96 K on a pu observer une ligne d'absorption extrsmement large pour le complexe CpFe(C0)2SiC12Me. Ce complexe (et la plupart des autres inclus dans cette etude) a une structure qui permet des isomhres de rotation autour du ligand Fe-Si. L'orientation desordonnee des groupes autour de Si (ou S) par rapport aux groupes de l'autour de Fe donne comme effet une garnrne de gradients de champ Blectrique ou de dkplace- ment isomerique. Nous interpr6tons donc l'elargissement des lignes d'absorption comme dii a une rotation ralentie autour du ligand Fe-Si. Pour verifier cela, des spectres des deux complexes CpFe(C0)zSiClzMe et CpFe(C0)zTHT furent mesures a une temperature de 5K. Nous avons estime une frequence maximum de rotation d'a peu pr6s 15 MHz et une barritre de rotation d'environ 30 calories en considkrant la dependance de la largeur de la ligne d'absorption du complexe CpFe(C0)zTHT par rapport A la temperature. La barrikre de rotation du complexe CpFe(C0)zSiClzMe est plus grande (env. 800 cal), ce qui explique l'klargissement de la ligne spectrale observe aussi B 96 K.

Abstract. - The Mossbauer spectra of a number of n-cyclopentadienyliron carbonyl complexes were measured at 96 K. The general formula of the complexes studied is CpFe(C0)zL where L ⁼SMe2, SEt2, SPr;, SBu;, PP43, etc., and SiClZMe. An extremely broad line was observed at 96 K for CpFe(C0)~SiClzMe. This complex (and most of the others included in this work) has a structure which permits rotational isomers about the Fe-Si bond. The line broadening is interpreted in terms of hindered rotation about the Fe-Si bond giving rise to a range of slightly different field gradients or isomer shifts due to the random orientation of the groups on the Si (or S) with respect to the groups on the Fe atom. To check this idea, spectra were measured to 5 K on both the CpFe(C0)zSiClzMe and CpFe(C0)zTHT samples. From the temperature dependence of the line width of the CpFe(C0)zTHT complex, a maximum rotation frequency of about 15 MHz and a barrier to rotation of about 30 calories were estimated. A somewhat larger barrier to rotation is expected for the CpFe(C0)BiClzMe complex (about 800 cal) which accounts for the broadening which was observed in this complex even at 96 K.

1. Introduction. - Evidence for conformational isomerism has been presented recently based on the i. r. spectra of n-cyclopentadienylmetal car- bony1 complexes such as CpFe(CO),SiCl,Me [I] ('), TC-M~C~H,M~(CO),[P(OM~),] [ 2 ] , and others 131.

However, because the iron analogues of these complexes are often ionic, the i. r. spectra in the CO stretching region are so broad that the small splittings expected as a result of this isomerism have not been observed. Both 57Fe and '19sn Mossbauer spectra have been published for complexes which have rotational isomers [4, 51. Splittings of the '"Sn resonance due to rotational isomers were observed in frozen solutions but not in the neat solid ; no splittings in the 57Fe reso-

(*) Present address : Department of Chemistry, Iowa State University, Ames, Iowa 50010, U. S. A.

(**) To whom correspondance should be addressed.

nance were reported. This is not too surprising as the quadrupole interaction in 57Fe is relatively less sensitive to the environment than it is in '19Sn. The present Mossbauer investigation was undertaken primarily in the hope that for some of these complexes the quadrupole splittings of the two rotational forms might be sufficiently different to enable their identification.

2. Experimental. - 2.1 MATERIALS. - Bis(z-cyclo- pentadienydicarbonyliron), [CpFe(CO),], purchased from Strem Chemicals Inc., was used without further purification for the preparation of the derivatives, but was recrystallized from dichloromethane-pentane [6]

prior to the Mossbauer measurements. All operations involving anhydrous ferric perchlorate, Fe(ClO,), (Alfa Inorganics), were carried out under nitrogen in a dry glove-bag. The ligands were obtained from the sources indicated : SMe,, SEt,, SPrk SBuL and PPh, (Aldrich Chemical Co.) ; S(CH2), (tetrahydrotiophene

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974632

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C6-248 T. SAWAI, J. F!. MARTIN, I. S. BUTLER A N D D. SINKIM

THT) (J. T. Baker Chemical Co.) ; P(OBun), (Eastman Organic Chemicals). All of these ligands were used without further purification.

The complexes, CpFe(CO),Cl [7], [CpFe(CO),]

BPh, [8], [C~F~(CO),PP~,]C~O, [9] and [CpFe(CO), SEt,]PF6 [9] were prepared according to the published methods.

The new n-cyclopentadienyliron dicarbonyl derivatives, [CpFe(CO),L]PF, [L ⁼SMe, ('), SPrL SBu;, and P(OBun),], were prepared according to Meyer's method for the diethyl sulphide complex [9]. Two typical preparations are described below.

2.1.1 [c~F~(CO),SM~,]PF,. - The compounds, [Cp~e(cO),l, (1.05 g, 3.0 mmole) and Fe(ClO,), (4.34 g, 12.3 mmole), were dissolved in deoxygenated acetone (5 ml) separately and then mixed together.

After 5 min, the mixture was chromatographed on a Florisil column (60-100 mesh, 3 x 30 cm) using acetone as an eluant. The dark-red band eluted from the column was collected and evaporated to approximately 120 mi. Dimethyl sulphide (15 ml, 12.7 g) and NH4PF6 (6.45 g) were added to this and the mixture was stirred for about 30 min. The resulting solution was then run through a fresh Florisil column (3 x 30 cm) with acetone eluent. Next, NH4PF6 (5.14 g) in 100 ml H,O was added to the orange yellow eluate and the mixture was concentrated on a rotary evaporator, resulting in the formation of dark-yellow crystals. The product was filtered, washed with n-hexane, and dried overnight in vacuo at room temperature. The product was recrystallized by first dissolving the crystals in about 15 ml acetone followed by the slow addition of benzene.

2 . 2 . 2 [CpFe(CO),P(OBun),]PF6. -The procedure was quite similar to that. described above for the dimethyl s6lphide derivative except for a few minor modifications. The amounts of the two starting materials used were [CpFe(CO),], (0.99 g, 2.8 mmole) and Fe(ClO,), (6.88 g, 19.4 mmole). The mixture of P(OBun), (15 ml, 13 g) NH,PF6 (6.54 g) and the eluate of the first chromatographic separation was stirred for 30 min and the colour of the slurry mixture became dark-green. The mixture was then concentrated to

ca. 50 ml. The excess NH,PF, which precipitated out and the dark-grey-green deposit were removed by filtia- tion. The filtrate was placed on a Florisil column (3 x 30 cm) and eluted with acetone. A golden-yellow fraction (150 ml) was collected and to this, NH4PF6 (5.0 g) in 100 ml H 2 0 was added, resulting in the for- mation of a pale-yellow flakey precipitate contaminated by excess P(OBun),. The mixture was filtered, washed with water and dried in vacuo. The product was recrystallized by the step-wise addition of 12 ml acetone, 10 ml benzene and 40 ml n-hexane.

(2) The dimethyl sulphide complex, [CpFe(CO)zSMen]I, has been prepared ,previously from the reaction of CpFe(C0)zSMe and MeI.

All the products were characterized by their elemen- tal analyses, melting points and i. r. spectra in the C - 0 stretching region.

2.2 MOSSBAUER ^SPECTRA. - Mossbauer spectra were measured for this series of related n-cyclopentadienyliron carbonyl complexes in powdered form. The spectra were taken on a constant acceleration spectro- meter with a 10 mC source of 57Co on CU. The temperature at the sample was determined by a calibrated copper-constantan thermocouple to be 96 K, except for the two samples which were studied at lower temperatures, using an Oxford Instruments continuous flow He transfer cryostat.

3. Results and discussion. - The complexes included in this study can be classified into three categories according to the possibilities for rotational isomerism.

First of all, complexes of the type : [CpFe(CO),L]PF6, where L ⁼SMe,, SEt,, SPr;,, SBuZ, or THT ; CpFe(CO),SiCI,Me is also in this group. These complexes can all have conformational isomers about the Fe-S or Fe-Si bond, as a result of the two possible orientations of the lone pair of electrons on the S atom, or of the CH, groups on the Si atom, with respect to the two CO groups on the Fe atom. This behaviour has been observed for the complexes CpFe(CO),SiCl,Me [I] and CpMn(CO),L (L= SMe,, SEt,, SBuk SPr",nd THT) 131, all of which show distinct splittings in the CO stretching regions of their i. r. spectra. The second category consists of [CpFe(CO),P(OBu) ,]PF, and [CpFe(~0),PPh,]C10,.

Unlike the first group, these complexes cannot have rotational isomers about the iron - ligand bond.

However, their manganese analogues [2] do show splittings in the i. r. spectra, which are interpreted as being due to either staggered or eclipsed conformations along the metal-ligand bond or to rotational isomers about bonds other than the metal-ligand bond. The final category is [CpFe(CO),]BPh,, whose structure does not permit conformational differences at all.

The results of the Mossbauer measurements at 96 K are given in Table I. These data indicate that neither the isomer shifts nor the quadrupole splittings are sensitive to conformational isomerism. This is consistent with earlier observations on similar complexes [4, 51. In fact the only clue to unusual behaviour is to be found in the line-width data in Table I. All of the observed line-widths are somewhat broader than usual, but only CpFe(CO),SiCl,Me, with a line-width of 1.23 mm/s is really startling.

One way to explain such a large line-width that is consistent with the known facts about CpFe(CO), SiCI,Me, is to suppose that the electric field gradient (and therefore the quadrupole splitting) and/or the isomer shift is slightly sensitive to the rotational confi- guration about the Fe-Si bond. Then if the rotation about the Fe-Si bond were sufficiently free so that on the time scale of a Mossbauer experiment (i. e. -- s),

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MOSSBAUER STUDIES O N SOME TC-CYCLOPENTADIENYLIRON CARBONYL COMPLEXES C6-249

Complex - :C~F~(CO),P(OBU"),]PF, 'CpFe(CO),PPh,]ClO,

;CpFe(CO),SMe,]PF, 'CpFe(CO),SEt,]PF,

~CpFe(CO),SPr~PF, 'CpFe(CO),SBu~PF, -CpFe(CO),(THT)]PF, 'CpFe(CO),]BPh, ZpFe(CO),SiCl,Me

Mossbauer Data at 96 K for the n-Cyclopentadienyliron Carbonyl Complexes

I. S. (mm/s) Rel. to SNP

k

0.02

-

- 0.09

- 0.04

+

0.02

+

0.03

+

0.03

+

0.04

+

0.03 - 0.06

- 0.23

the 57Fe nuclei in the sample would experience a random distribution of

-

rotational configurations, the resulting resonance would be broad. If the time scale for rotation about the bond were much longer than s, perhaps separate lines due to the two different stable rotamers could be resolved. The latter con- dition is apparently the case in the carbonyl region of the infrared, where the experimental time scale is

-

² ^x ^lo-', S, and a conformational splitting is observed [2]. On the other hand, if the hindered rotation about the Fe-Si bond could occur much faster than s one would not expect the Mossbauer resonance to be broadened at all. This would account for the fact that of all the complexes studied only CpFe (CO),SiCI,Me showed a very broad line at 96 K : the barrier to rotation about the Fe-S bond in the other complexes would be expected on steric grounds to be much smaller than in the Fe-Si case (about 800 cal. [12]).

To check the above explanation one of the Fe-S complexes, [CpFe(CO),THT], and CpFe(CO),SiCI,Me were measured at several different temperatures. The line widths at different temperatures are given in Table I1 and some of the spectra ale shown in figures 1 and 2. The line width of the CpFe(CO),SiCl,Me does not change in a very consistent way with temperature, that of the CpFe(CO),THT does. According to the

Line width vs. temperature for two of the complexes T (K) :pFe(CO),SiCl,Me CpFe(CO),THT

+

¹⁰ L. W. (mm/s) L. W. (mm/s)

- - -

5 1.55 1.54

35 0.94 0.94

96 0.50

105 1.23

Line width (L. W.) (mm/s>

-

0.50 0.50 0.50 0.50 0.50 0.51 0.50 0.36 1.23

FIG. 1 . - Spectrum of CpFe(C0)zSiClzMe at 5 K.

above argument one expects that the line width would broaden at low temperatures since the rotation about the Fe-X bond will be slower. If the barrier to rotation is high enough, as it may be in the CpFe(CO),SiCl,Me case, then the rotation may be slow enough even at room temperature to give rise to a broad line, as is seen in that complex. On the other hand, if the barrier to rotation is smaller, the rotation about the bond may be fast enough to cause an observable narrowing at higher temperatures (as in the CpFe(CO),THT case).

Although the data presented here are not sufficient for a detailed calculation, one can nevertheless obtain estimates for the rotation frequency and the barrier to rotation. Hindered rotation about a bond has a temperature dependent frequency given by

where AE,,, is the barrier to rotation caused presumably by steric hindrance. We assume the Mossbauer line width to be narrowed by the rotation, so that

298 0.86 where

r

is the observed line width and T , is the maxi-

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C6-250 T. SAWAI, J. P. MARTIN, I. S. BUTLER AND D. SIMKIM

VELOCITY (MMIS)

FIG. 2. - Spectrum of CpFe(C0)zTHT at A. 5 K and B. 96 K.

mum line width (obtained by extrapolation to T=O K).

The data for CpFe(CO),THT give E,,, ⁼30 cal and a value of w, = 15 MHz. At 10 K the rotation frequency

is about 30 MHz. These calculated values must be considered very approximate, being based on so few data points ; they are given here only to lend some support to the proposed explanation for the unusual line widths observed. Undoubtedly a preferable treatment would be based on analysis of the line shapes, and the temperature dependence would have to be studied much more carefully. Values fo'r the rotation frequency and barrier were also calculated using the relationship for relaxation broadening given by Wignall [ 1 3 ] . This gave a rotation frequency about an order of magnitude slower, but about the same barrier height.

4. Conclusions. - The broadening observed in the Mossbauer spectra of [CpFe(CO),THT] and CpFe(CO),SiCl,Me can reasonably be ascribed to a relaxation process involving hindered rotation about the iron-ligand bond. Although other possible relaxation processes cannot be ruled out on the basis of the data included here, the hindered rotation treatment leads to a barrier height which is smaller for the Fe-S bond than for the Fe-Si bond which is consistent with the molecular geometries. A more detailed study of the temperature dependances of the line widths of these complexes is in progress in the hope of obtaining more quantitative barrier heights and more conclusive evidence for the proposed processes.

Acknowledgments. - The authors gratefully acknowledge the financial support of the National Research Council of Canada. One of us (5. P. M.) is also indebted to this Council for a graduate fellowship.

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