COORDINATION COMPLEXES OF GROUP (IV) HALIDES PREPARATION AND INFRARED SPECTRA OF COORDINATION COMPOUNDS OF TITANIUM TETRACHLORIDE, TITANIUM TETRABROMIDE, ZIRCONIUM TETRACHLORIDE, AND TIN

TETRACHLORIDE WITH a,o-DINITRILES AS LIGANDS1

SUMER CHAND JAIN AND ROI,AP;D KIVEST Department of Chenzistry, L7niversity of Montreal, Montreal, Qne.

Received hlarch 25, 1963

ABSTRACT

Coordination complexes between dinitriles of dicarboxylic acids and tetrahalides of titanium, zirconium, or tin have been prepared. The analytical results and the infrared spectra indicate that on working under different experimental conditions three types of compounds are ob- tained: 2MX4.L-L, MX4.L-L, and MX1.2L-L where M X 4 is a Lewis acid and L-L a bidentate ligand. The compounds 2MXd.L-L are explained by the formation of halogen bridging between the two metal atoms. The compounds MX4.L-L are considered to be either coordination polymers or chelates of variable ring size and the compounds MX4.2L-L, ordinary addition compounds where no chelation takes place because of the mutual interaction of the two CN groups in a dinitrile of a short chain length.

INTRODUCTION

The compounds TiC14, TiBr4, ZrC14, and SnCI4 are all strong Lewis acids. The reactions of Tic14 with donor molecules have been summarized in a review paper (I) . Very little work has been published on the reactions of nitriles with the above mentioned Lewis acids. Emel6us and Rao (2) reported that Tic14 reacted with two mononitrile molecules to give the 1:2 addition compound. Physicochemical measurements carried out on benzene solutions of these addition compounds (3) led to the conclusion that in dilute solution the 1:2 con~plexes are dissociated into the 1 : l complexes and the free nitriles. Coerver and Curran (4j, reporting on the infrared absorption of the C-N group in addition corn- pounds, indicated that coordination involving the nitrile group could be evidenced by infrared spectra measurements because of the positive shift of the C=N stretching fre- quency on addition-compound formation. I t has been recently established (6) that halogen bridging is possible in multinuclear complexes of titanium tetrachloride.

The present work was undertaken to study the possibility of chelate foriuation and halogen bridging, when dinitriles of increasing chain length react with TiC14, TiBr4, ZrC11, and SnC14. With a short carbon chain the possible interaction between the two CN groups was expected to reduce the donor ability of the second CN group involved in coordination. In such cases compounds of the general formulae RIX4.2L-L are expected.

EXPERINIENTAL

Material and ,Vanipulations Reagent grade titanium tetrachloride was purified by the method of Gilchrist (6) and the middle fraction

of the distillate used in complex formation. Tin tetrachloride was purified by the method patented by Pittsburg Plate Glass Compa~ly (7). Co~nniercial zircolliurn tetrachloride was sublirned in nitrogen a t~nos- phere a t 200' C. The compound so obtained was a white powder. Titanium tetrabromide was purified by distilling the commercial product a t low pressure. The dinitriles were obtained from Fisher Scientific Company and used as such. The solvents used were purified and made anhydrous by suitable methods. The experimental methods and the methods of analysis for metal and halogens are essentially the same as described previously (8, 9). All the analytical results are included in Table I. The setup used to prepare the complexes was the same as the one used in previous experiments (10).

'Presented at the X X X t h Congr6s de l 'dssociation Canadienne Fran~aise pour l 'daancement des Sciences, Montrkal, Nov. 3, 1962.

Canadian Journal of Chemistry. Volume 41 (1963)

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JAIN .4ND RIVEST: COORDINATION COMPLEXES 2131

The spectrophotometric measurements were carried out on a Perkin-Elmer Infrared spectrophotometer, model 21, using sodium chloride prism in nujol mulls.

The molecular weights of the complexes mere determined by cryoscopic method using anhydrous acetic acid as solvent.

P~epara t ion of the Conzplexes The preparation of these complexes was performed in methylene chloride. Three types of compound

were prepared, The MX4.L-L compounds were prepared with solutions of the reactants in 1:1 molar ratio.

TABLE I

Analytical results, Infrared spectra results found and calculated (yo) (cm-I)

Complex Metal Halogen M.p. ( O C ) VC* 2% t Av

* v c : C S stretching frequency in the compound. i v ~ : C N stretching frequency in the pure nitrile. TD: decomposed by turning black without melting.

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2132 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963

In all cases powders were obtained which were washed with dry methylene chloride and dried in vacuum a t room temperature. The Tic14 complexes yielded yellow powders; TiBra, dark red powders; and SnC14, white powders. Zirconium tetrachloride being insoluble in common solvents, its complexes were prepared by adding excess dinitrile to a suspension of zirconium tetrachloride in the same solvent and the reaction allowed to proceed for about a week. The excess ligand could then be washed out and the white precipitates so obtained dried under vacuum as usual. The 2PIX4. L-L complexes could be obtained only with TiC11, and were prepared by adding a dinitrile solution to an excess of the Lewis acid. The yellow powders so obtained were filtered on a sintered-glass filter, washed with the solvent, and dried under vacuum. The MX1 2L-L complexes were obtained only with malononitrile when reacted either with titanium tetra- chloride, tin tetrachloride, or zirconium tetrachloride.

The TiCl4.2Cn'--CHz-CN complex is obtained when excess malononitrile is reacted with TiC14 in solution. The yellow powder is then washed and dried as usual. The SnC14. 2CNCH2CN crystalline white compound is always obtained even if the reactants are made to react in a 1 : l inolar ratio.

The results of elementary analysis and of infrared spectra measurements are included in Table I.

IXFRARED SPECTRA AiYD STRUCTURAL DISCCSSION

The infrared absorption frequencies of the pure dinitriles and their complexes are included in Table I and illustrated in Fig. 1. As it was observed by Coerver and Curran

2230- 0 2 4 6 8

NO. OF G H 2 G R O U P S

FIG. 1. Variation of the C-4 stretching frequency with respect to the number of CHs groups in the dinitrile: . . . 2: 1 TiC14NC(CH2),CN compounds; o- 1: 1 TiC14NC(CH2),CN compounds; - - 1:1 TiBr4- NC(CH2),CP\T compounds; -. . 1:l SnCl4NC(CH?),CX compounds; - pure dinitriles.

(4), Kharitonov, Chiang, and Babaeva (11), as a result of complex formation the C s S stretching frequency increases instead of decreases as in the complexes of carbon>-1 donors (12). The fact that coordination in nitrile complexes takes place through the nitrogen atom and not through the triple bond of CIY group has been confirmed by X-ray examina- tion (13, 14). In all the dinitrile complexes an increase in -C=N stretching frequency varying from 12 to 56 cm-I has been observed. This suggests that the shift of free electron pair from nitrogen atom of CN group towards metal ion would decrease the mutual repulsion and favor an increase in electron density between the carbon and the nitrogen of the triple bond. The above discussion explains the reason for an increase in -C=Y stretching frequency on coordination.

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JAIN AND RIVEST: COORDINATION COMPLEXES 2133

In Fig. 1 is represented the -C=N stretching frequencies of the pure dinitriles as a function of the number of CH2 groups in the chain. The decrease in - C N stretching frequency with increase in chain length shows that with short chain length one C=IV group markedly influences the spectroscopic and chemical behavior of the second C=N group in the same molecule. But as the chain length increases this mutual influence fades away and in the later inembers of the series the two C=N groups ill the molecule co- ordinate independently without influencing each other. This can be observed from the plots of C=X stretching frequencies of the coinplexes as a function of CH2 groups (Fig. 1). The shape of the curves is similar to the pure dinitrile one.

In Fig. 2 the shift in C=N stretching frequencies has been plotted as a function of the number of CIl2 groups in the dinitrile complexes of the 1:l composition. From these

0- NO. OF C H 2 G R O U P S

FIG. 2. \'ariatioil of the bathochromeeffect as a functian of the number of CHB groups in the dinitrile: . . 1 : 1 TiCI4NC(CH2),CN compounds; - - 1: 1 TiBr4NC(CH2),CN compounds; - 1: 1 SnCI,NC(CH~),CN

compounds.

results i t is observed that after the nuinber of CH2 group reaches three and four the curve becomes almost flat, showing that after the chain has reached a certain length, the inter- action between the two CN of the same dinitrile is small. From previous experience (8) it seems that if these 1: 1 complexes were chelates of variable ring size, it should be evident from the curves of Fig. 2. The fact that compounds of the 1: I composition are obtained with inalononitrile and succinonitrile where ring closure, on the same titanium atom, is almost impossible due to rigid nature of C-C=N group led us to believe that a t least for these specific cases we have polynuclear complexes, which could be cyclic dimers, trimers, or linear coordination polymers.

The formation of MX4.2L-L complex with malononitrile with one CH2 group is favored where one CN group is coordinated, leaving another C=N group free. This interpretation is further supported by the infrared spectra of such complexes, where both the shifted peak due to coordination and the unshifted original peak are obtained (Fig. 3).

In the complexes 2MX4.L-L we assume that the two inetal ions are held together through halogen bridging (Fig. 4).

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CANADIAN JOURNAL O F CHEMISTRY. VOL. 41, 1963

3"0-- 2000 F R E Q U E N C Y CM-I

FIG. 3. Infrared spectra of the malollonitrile complexes with TiCI,: . 9 1:2 TiCI4SCCH2CN compounds; - 1: l TiC1,NCCHlCS compounds; - - pure SCCH2CN.

FIG. 4. Chloride bridging in 2TiC14NC(CHg),CN compounds.

An attempt to determine the molecular weight of these conlpounds was made. The 1 : l TiC14, TiBr4, and SnC14 con~plexes gave yellow, dark red, and colorless solutions respectively in dry acetic acid. The infrared spectra of some of these solutions indicated that there was dissociation leading to some uncoordinated C=N group. The amount of dissociation was found to increase with dilution. Conductivity measurements on these solutions indicate about 100-fold increase in conductivity of the solutions as compared to the conductivity of the pure solvent. The increase in conductivity shows that ionization is not very important. The dissociation effect is much harder to assess since the infrared absorption of C d X frequency is much more intense when this group is coordinated. We have attempted to recover the complex from the solution by evaporating the solvent and observed the formation of a white precipitate the analysis of which confirms the com- position TiC12(CH ,Coo) ,.

The molecular weight determinations by cryoscopy on these solutions are included in Table 11. They were found to be about half that of the theoretical values. I t is to be noticed that the molecular weight found increased with concentration and this is in con- formity with the dissociation hypothesis. The fact that the acetate complex mentioned above is very slightly soluble in CHBCOOH and the fact that the observed molecular weight depends upon the dinitrile used strongly suggest that if the complexes are polymers they are probably decoinposed by the very polar solvent.

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JAIN A S D RIVEST: COORDIN.ZTIOX COMPLEXES

TABLE I1

Molecular weight Molal

Complex conceiltration Obtained Calculated

CONCLUSION

Froin the previous discussion one can draw the following conclusions: (1) The interaction of Lewis acids with dinitriles can lead to two main tqpes of com-

pounds: 31X4.L--L and 2lIX4.L-L. (2) The compounds 2AIX4.L-L cannot be polynleric in nature, but can be best ex-

plained by assuming halogen bridging between the two titanium atoms of the binuclear con~plex. I t is also possible that in these co~llpounds titanium exhibits a coordination number of 5. To prove this we have prepared the compound TiC14.CH3CN, which is very stable and can even be sublimed under low pressure without decomposition. The infrared spectrum of this last complex does not indicate that the triple bond is involved in co- ordination since the CX stretching frequency is increased by about 40 cm-I through co- ordination. Attempts to determine the molecular weight of this compound have failed due to the lack of suitable solvents. So there are only two possibilities left for this com- pound, it is either a single addition co~npound where the titanium atom has a coordination number of 5,_or it is a dinuclear complex where halogen bridging takes place.

(3) The compounds MX4.1,-L can be either coordination polymers or chelated com- plexes. LT7e have no definite experimental evidence to prove any one of these two possi- bilities. However, the observations made on solutions in dry acetic acid ~vould give a slight indication that in solutions we are not dealing with coordination polymers.

ACKNOWLEDGMENT

The authors gratefully acknowledge the continued financial support from the National Research Council of Canada.

SOhlMAIRE

Des complexes de coordination entre les dinitriles dkrivks des acides a,w-dicarboxyliques e t des tktrahalogknures de titane, de zirconium ou d'6tain ont 6tk pr6par6s. Les rksultats analytiques e t les spectres infrarouges montrent que suivant les conditions expkrimentales, on peut obtenir les trois sortes de cornposhs suivants: 2MX4.L--L, R/IX4.L-L e t RIX4.2L-L od MX4 est un acide de Lewis et L-L un ligand bidentate. On explique la formation des composks 2R4X4.1d-L par la prksence de ponts halog6ni.s entre les deux atomes de mktal. On considPre que les compos6s h4X4.L-L sont ou bien des polym6res de coordination ou des chklates de dimension variable et les composhs MX4.2L-L des composks d'addition ordinaire oh il n'y a pas chelation, ?I cause de I'itlteraction mutuelle des deux groupes C S du m$me dinitrile.

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2136 CANADIAN JOURXAL OF CHEMISTRY. VOL. 41, 1963

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