Proton N .M.R. Spectra of some Cyclopentadienyltitanium Compounds

Depnrtmetzt of Chetnistry, Qrleen's University, Kingston, O~ztnrio

Received March 26, 1973

N.m.r. spectra obtained preliminary to studies of compounds of the form CpzTiRCl and CpzTiRz raise questions about some previous interpretations of related spectra.

Les spectres r.m.n., obtenus anterieurement B I'etude des composes de la forme CpzTiRCI et CpzTiRz, ont soulev6 certaines questions quant aux interpretations prkctdantes des spectres se rattachant a ceux-ci.

[Traduit par le journal] Can. J . Chem.. 51, 2609 (1973)

Introduction Preliminary to a study of the stability and re-

activity of cyclopentadienyl titanium, a catalogue of 'H n.m.r. spectra was developed for reference, These spectra have been compared with those already reported (1-8) and any significant dif- ferences examined.

Procedures Since many of the subject compounds are air sensitive,

all manipulations were either within a nitrogen-filled glove box or within a vacuum line.

The glove box atmosphere was dry nitrogen (99.9% purity supplied by Matheson of Canada Limited and Canox Limited) which entered the box through a column packed with Linde molecular sieves No. 5A and was con- tinually circulated, once within the box, through a Vac Corporation HE-493 Dri-Train. Glassware used within the box had been previously dried within high vacuum vessels that could be placed, sealed, within the interchange. The interchange itself could be evacuated to 6 Nm-2 before being flushed with nitrogen.

In the glove box, samples were prepared to known concentrations and component ratios in n.m.r. tubes fitted with tapers. They were degassed three times while frozen to liquid nitrogen temperatures on a vacuum line outside of the glove box and sealed at 10- Nm-'. Spectra were obtained using frequency sweep on a Bruker HX-60 spectrometer. Tetramethyl silane was employed both as reference and as lock. All the samples were measured a t 294 & 1 OK. All shifts are reported relative to tetra- methylsilane (T = 10). The accuracy of the reported .r values is better than f 0.027.

Cp2Ti(C6H5)2 was prepared by the method of Summers et al. (9); Cp2Ti(C6H5CH2)2 by the method of Razuvaev et al. (10); and CpTi(C,H,CH,), by the method of Cannell (11). Except for Cp2Ti(C6H,CH2)C1, other organo- metallic materials were obtained from Alfa Inorganics.

The preparation of Cp2Ti(C6H5CH2)C1 is based on a

'Present address: Research Department, Imperial Oil Limited, Sarnia, Ontario.

method by Dubsky and Jacot-Guillarmod (12), and is recommended for preparation of related compounds.

Dioxane (5.1 mmol) was added slowly with constant stirring to a 50 rnl flask containing a 1.02 M solution of C6H5CH2MgCI (5.1 mmol) in tetrahydrofuran. After stirring for 15 min, the mixture was filtered and the filtrate evaporated to dryness. The white residue of (C6H5CH2),Mg was washed three times with pentane, dried for + h at reduced pressure, and finally dissolved in 15 ml of benzene.

(C6H,CH2)2Mg (2.5 mmol) so prepared was added slowly, in benzene solution, to a 50 ml flask containing Cp,TiClz (1.54 g, 6.2 mmol), benzene (20 ml) a n d a mag- netic stirring bar. After stirring for an additional 10 min, the resulting violet mixture was filtered through a fine frit. Addition of a two fold excess of 11-pentane t o the violet filtrate precipitated crystals. After approximately 20 h, the liquid was decanted and the remaining violet black crystals were dried at reduced pressure. Elemental analysis was by Alfred Bemhardt (Elbach).

Anal calcd. for Cl,Hl,TiCI: C, 67.02; H, 5.62; C1, 11.64; Ti, 15.72. Found: C, 66.72; H, 5.61; C , 11.82; and Ti, 15.79.

It should be noted that the sample used in the above analysis was deliberately exposed to air for 6 h before being sealed in an evacuated tube and sent for analysis. The resulting analysis would tend to disprove the recent suggestion (6) that decomposition is rapid in a i r at room temperature.

The "melting point" was 365 "K with decomposition, as determined in an evacuated capillary. A value of 380 "K has been reported in an argon atmosphere (6).

Results and Discussion

N.m.r. data are presented in Tables 1 to 7. Chemical shifts are nearly independent of con- centration. On the other hand, solvent influence is marked. The reported values can be explained in terms of the shielding contributions of the solvent (13, 14), as can most values reported in the literature (2-8), but some anomalies exist among the literature values. These are discussed below.

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CAN. J. CHEM. VOL. 51, 1973

TABLE 1. N.m.r. shifts of CpZTi(C6H5), in different solvents

7 values Concentration

Solvent (M x lo3) Cp protons* Ring protons?

CCI4 CzCI4 CHzClz CDC13 C6Hiz C6D6 C6D.5 + CHzClz C6F6 Acetone CHaOH

'Sharp singlet. i s = singlet: mc = center of the multiplet. $Peak of 2.987 was tentatively assigned to o-protons and that at 3.08~ to m- and p-protons.

TABLE 2. N.m.r. shifts of CpzTi(C6H5)CI in different solvents

7 values Concentration

Solvent (M x lo3) Cp protons* Ring protons?

'Sharp singlet. t s = singlet; mc = center o f the multiplet.

TABLE 3. N.m.r. shifts of CpzTi(C6H5CHZ)z in different solvents

7 values

Ring protons? Concentration CP CHz

Solvent (M x lo3) protons* protons* m-, p- o-

CC14 CzCI4 CHzCIz CDC13 CDCI3(216K) C6H12 C6D6 C6Fs THF Acetone

- - 3.18 rnc 2.85 mc 3.18 mc 2.85 mc - -

3.12mc 2.77mc 3.33rnc 3.00mc 2.90 mc 3.25 mc

3.02 mc

'Sharp singlet. t s = singlet; mc = center of the multiplet.

The Results of Waters and Mortimer [I] 2CpzTi(C6H5CHz)Cl + 2CDC13 +

Waters and Mortimer (6) comment on the 2CpZTiClz + bibenzyl "abnormal" chemical shift observed for the + combination products of solvent radicals cyclopentadienyl protons of Cp2Ti(C6H5CH2)C1 in chloroform. However, in the present work, it Some CpTiC1, was also identified. was found that dissolving Cp2Ti(C6H5CH2)C1 in In order to verify that the reported spectra (6) CDCl, led rapidly to the formation of an in- were indeed those of the decomposition products, tensely red solution by the following process: the chemical shifts of Cp2TiC12 and of bibenzyl

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GLlVICKY A N D McCOWAN: SOME PROTON N.M.R. SPECTRA

TABLE 4. N.m.r. shifts of Cp2Ti(C6HSCH2)CI in different solvents

s values

Solvent

c2c14 CHzC12 C6H12 C6D6 C6F6 Acetone-de

Concentration CP (M x lo3) protons*

10 3.94 200 3.87

5 4.17 - 4.35

100 3.88 - (3.73)

CH2 protons*

7.63 7.54

Ring protonst

'Sharp singlet. ts = singlet; mc = center of the multiplet.

TABLE 5. N.m.r. shifts of CpTi(C6HsCH2)3 in different solvents

Solvent

CzCJ4 CzCI4+ CHzCIz(3 :I) C ~ H I Z C6H6 THF Pentane

Concentration (M x 1 0 9

100 - - - 20 20

s values

CP CH2 Ring protons* protons* protonst

4.23 7.03 2.97 mc 4.17 7.00 2.97 mc 4.27 - 3.00 mc - 7.20 -

4.10 6.97 2.92 mc 4.17 6.93 2.95 mc

'Sharp singlet. ts = singlet; mc = center of the multiplet.

TABLE 6. N.m.r. shifts of Cp2TiCI2 in different solvents

Concentration Solvent (M x lo3) r value*

CH2Clz Saturated solution 3.45 THF Saturated solution 3.45 Acetone 50 3.38 CDC13 - 3.41 C6D6 10 4.08 Toluene 60 4.12 Nitrobenzene Saturated solution 3.32 CCI4 Saturated solution 3.52

'Sharp singlet.

The Results of Nesmeyanov et al. The singlet resonance arising from the cyclo-

pentadienyl protons of Cp,TiCl, dissolved in tetrahydrofuran (Table 6) is inconsistent with the report of Nesmeyanov et al. (15) but agrees with reports from other investigators (1, 2, 16). Nesmeyanov observed two resonances, one close to that reported here and the other 0.252 upfield, which coalesced into a singlet when the

TABLE 7. N.m.r. shifts of CpTiCla in different solvents

Concentration in CDCl, were measured. They correspond ex- Solvent (M x lo3) r value* actly to those shifts reported (6) for the mono- benzyl compound. Probably the same situation c2c14 20 3.10

CH2C12 - exists for the neopentyl compound. It seems 2.95 CDCI, - 2.94

certain that the reported values (6) are for the C6D6 10 3.77 decomposition products, and that the sub- THF - 2 .82 seauent discussion of "abnormal" behavior is cc14 Saturated solution 3.02 without meaning. *Sharp singlet.

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2612 CAN. J . CHEM. VOL. 51, 1973

system was heated to 408 OK. He interpreted the results in terms of nonequivalent protons.

A more plausible explanation is the presence of (Cp2TiC1),0 in Nesmeyanov's sample. An n.m.r. spectrum of this material showed its reso- nance to be 0.252 upfield from that of Cp,TiCl, in tetrahydrofuran. Moreover, this material dis- proportionates (1 7) with polymerization of one of the products

This reaction, which proceeds visibly at room temperature, is very rapid at 408 OK. Since the fine yellow polymeric solid produces no obser- vable proton resonance, all of Nesmeyanov's observations are explained.

The Results of Beachell and Butter Beachell and Butter (2) have examined the in-

fluence of the electronegativity and electronic character of ligands R on the chemical shifts of the cyclopentadienyl protons in the compounds (CSHs),TiR'R".

In one case, cyclopentadienyl proton chemical shifts of four compounds, Cp,Ti(CH,),, Cp,Ti- (CH,)Cl, Cp,TiCl,, and CpTiCl, are plotted against the summed electronegativities of the "first atom" of the R groups and an excellent linear relationship is obtained. On this basis it is concluded that methyl, chlorine, and cyclopenta- dienyl ligands contribute an equal electronic effect. While the observation that the data for Cp,Ti(CH,)Cl lie intermediate between those of the dimethyl and dichloro compounds is not unexpected, the apparent equivalence of the effects of the n bonded cyclopentadienyl and the methyl and chlorine is more surprising.

It is also fortuitous, since three of the values are in doubt. Both the value for Cp,Ti(CH,), and that for CpTiC1, differ from those reported in Table 1 of the same paper, with the further difference that the CpTiCl, value was obtained in tetrahydrofuran (18). The CpTiCl, and Cp,TiCl, values both differ from those reported here, the latter substantially.

Taking the tabulated values for Cp,Ti(CH,), and Cp,Ti(CH,)Cl and the value from Table 6 for Cp,TiCl,, a constant increment for ring pro-

ton chemical shift in the series Cp,Ti(CH,),Cl, _, is still obtained, but with F/n equal to 0.29. While these data could still be fitted to the correlation with electronegativity attempted by Beachell and Butter, the value for CpTiCl, could not. This fact, taken with the lack of linearity in some analogous series, suggests that there is little justification for simple correlations of cyclopen- tadienyl ring proton chemical shift with the electronegativity of the "first atom".

T h e authors thank the National Research Council of Canada, the Ontario Department of University Affairs, and Queen's University for financial support of this re- search. The award o f fellowships to o n e of us (A.G.) by the Ontario Government and by Queen's University is gratefully acknowIedged.

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