Induced optical rotation — the Pfeiffer Effect and C.D. of some racemic group(IV) metal complexes

INORG.

NUCL.

CHEM.

LETTERS

Vol.

7,

pp.

91-98,

1971.

Pergamon P r e s s .

Printed in Great Britain.

INDUCED OPTICAL ROTATION - THE PFEIFFER EFFECT AND C.D. OF S O M E R A C ~ I C GROUP(IV) METAL COMPLEXES V. Doron~ W. Durham and D. Frazier Department of Chemistry, Rutgers The State University, Newark, New Jersey 07102 (Reeeiw~d Jl August 1970~

Recently the observation of the Pfeiffer Effect was reported (1'2) for some labile hexacoordinate racemic species of tin(IV). In the present work these studies are extended to include two analogous series of compounds, namely the dihalobisacetylacetonato complexes of tin(IV) and titanium(IV). These complexes were of special interest due to their lability (1'2) and their established cis configuration (3'S'5'6'7). The titanium(IV) series seems to be kineticallymore

labile as evi -

~enced from N.M.R. studies~ 7) At around -150C the proton N.M.R. spectrum shows only one peak for the two unequivalent methyl groups. Only at considerably lower temperatures the exchange is slowed down sufficiently to show the two methyl resonances. In case of the tin(IV) series, proton N.M.R. studies show the splitting of the methyl resonances at 250C. In the latter case (S) this exchange process seems to be slower, also temperature dependent, c~mplete coalescence occurring well above lO0°~ On the basis of the above data one would expect (1) that the more labile, or the stereochemically less rigid species would tend to give a more instantaneous and stronger Pfeiffer Effect. Apparently this seems to be the case and these points will be ~ 3 c u ~ s , d ~AII

in the next section.

Imquiries to be addressed to this author. 91

SOME RACEMlC GROUP(IV) METAL COMPLEXES.

92

VoL 7, No. 1

Furthermore since the mechanism of the Pfeiffer Effect (8,9) spurred considerable interest in recent literature an effort was made to evaluate the factors contributing to these asymmetric perturbations for the above systems. It is important to note that though there may be one factor common to all Pfeiffer active systems, the widely accepted perturbation of a 50-50 4-~ I equilibrium, which was introduced by Pfeiffer h~m~elf, supported in a number of works by Dwyer and co-workers (lO'll'12) and recently confirmed by Kirschner (8'13) for the racemic ~Ni(dipy)3] ++

and [Ni(o-phen)3] ++

ions, using 1-malic acid

@s environment compound, this is b,y no means the only contributin~ factor in all systems, and every Pfeiffer active system requires its own analysis.° Not all Pfeiffer active systems (9) including the ones discussed in the present study, can be explained adequately only by a configurational d a-~ 1 shift. For the systems investigated in the present work there are at least two superimposed effects contributing to the induced Pfei~fer rotations. In support of this, the molar Pfeiffer rotations, C.D. data and spectral evidence are presented. DISCUSSIONS AND RESULTS The molar Pfeiffer rotations (1) of ~Sn(acac)2C12] , [S,(dbzm)2C1 ~

and [(C6~)2Sn(dbzm)2 ] "~ employing d- and 1-ma-

lic acid as environment compounds are 3500 for the latter and 600 ° for the first two compounds, using identical concentrations of racemic species and environment compound. It is evident that ~he first tw~ species are quite analogous with the two halides, and the ~-diketones acting as bidentate ligands. The third spe~ A paper is currently in preparation dealing with induced rotations in symmetric molecules° The factors responsible for the induced rotations in these systems differ from case to case • ~acac = acetylacetonate; dbzm = dibenzoylmethanate

J

(0.168M)

(O.160M)

d-tartaric acid

(Q.16OM)

d-tartaric acid (0.160~) d-~artaric acid (O.160M) d-~ar%aric acid (O.160M) d-tartaric acid

d-tartaric acid

Environment compound

+ 1.208

+ 1.018

+ 0.69@

+ 0.580

+ 0.496

+ 0.488

+1.646

+1.473

+1.132

+0.970

+0.886

+0.925

" ~g)

~

+0.~38

+0.455

+0.458

+0.390

+0.390

+0.435

* edeg)

and ~ ( a c a o ) 2 X 9 1 8erios

Pfeiffe~ rotation ~ P(obsd) (deg)

= ~or definition of terms see reference i .

(o.o2o~)

Ti(acac)oBr21

i(acac)20121 (Oo020M)

~i(acac)~1 (o.oao~)~a

(o.o3o~)

~n(acac)912 ]

(o.o31)

(o.o35M)

~m(acac)2Cl2]

ii

Initial racemic complex

in DMF Solutions.

The Pfeiffem Effect of the fSn(acac~2X~

TABLE. i.

+3,600

+3,180

+1,980

+I,22C

+i, 000

+ 830

m

m

i

m

|

o

Z

o_

94

SOME RACEMIC GROUP(IV) METAL COMPLEXES

Vol. 7, No. 1

cies has two phenyl groups, containing no halides in the coordination sphere. If the molar Pfeiffer rotations of the [Sn(acac)2X ~

series are considered (Table 1), a steady increase

in their values from the dichloro, to the dibromo, to the diiodo species is observed. Table 1 shows the same trend for the [Ti(acac)2X ~

series going from the difluoro, to the dichlo-

ro and dibromo species respectively.

It seems to be evident that

the halide has an important contribution to the value of the observed Pfeiffer rotation. The more covalent the nature of the Sn-halide bond, or the less electronegative the halide, the more effective seems to be the contribution to the observed induced rotation. This contribution is probably the result of hydrogen bonding of the hydroxyl groups on the tartrate to the halides on the metal. The less electronegative the halide,tne greater its polari2;ibility,

which would favor hydrogen bon-

ding. That the hydroxyl groups of the environment compound are significant is evident also when the results obtained employing d- and 1-malic acids as environment compounds are compared with those of tartaric acid. The latter having one more hydroxyl group available for hydrogen bonding, proved to be a more effective environment compound in all cases (14) tested involving halide containing species. For example the molar Pfeiffer rotation of [Sn(acac)2C1 ~

is 830 ° (Table 1), emp-

loying d-tartaric acid whereas only 600 ° employing d- or 1-malic acid (1) under similar concentrations of racemic species and environment compound. On the other hand when comparing the value of the molar Ffeiffer rotation obtained for [(C6Hs)2Sn(dbzm)2J and

~Sn(dbzm)2Cl~

, with those obtained for [Sn( acsc)2C12 , 350 ° for the former and 600 ° for each

of the two latter species, it seems that the former value is

Vol. 7, No. 1

SOME RACEMIC GROUP(IV) METAL COMPLEXES

95

lower because of the absence of this type of hydrogen bonding. That more than just a shift in the d = = 1 equilibrium is occurring is strongly supported by the absorption spectra of the [Ti(acac)2X ~

series in DMF solution. All the dihalobisacetyl-

acetonato complexes of titanium(IV) absorb around 320 m ~

in DMF

solution (see Table 2). This absorption disappears upon addition of the environment compound. Figure 1 shows the spectrum of the dibromo species. Since tartaric acid itself does not absorb in the region considered, a shift in a d 4-~ 1 equilibrium alone without any bonding type of interaction would not alter the absorption spectrum. Though the 320 m ~

absorption disappears in

the absorption spectrum upon addition of environment compound, the C.D. spectrum shows the peak, which suggests a shift in the absorption which is resolvable by C.D. The values of [~] , the molecular ellipticity also seem to follow the general trend of the Pfeiffer Effect, namely an increase in the values going from the more electronegative halogeno species to the less elec(15) _ tronegative ones. A study oz the spectrum of titanium(IV) chloride in hexane and DMF solutions revealed an absorption band around 270 m ~

. This region coincides with the u.v. absorption

of acetylacetonates (6'16). The C.D. data for the

[Ti(acac)2X2 ]

series (see Table 2) though strongly suggest as being the result of the halide-h~drogen bonding interaction which increases from the fluoi'ide to the bromide. As expected (1) both the tin(IV) series as well as the ti. tanium(IV) series being labile compounds give an instantaneous Pfeiffer Effect*, however the more labile titanium(IV)

compounds

gave their maximum reading as soon as the solution could be read whereas the tin(IV) compounds needed some time to reach their T-~-bou't two minutes are required from the time the solution i~ prepared till St can be read in the polsrimeter.

Absorption m axima(m~)

320 270

320 270 320 270

Molarity

9.68~i0-~ 1.82mlO-) 8.7K 10-4

1.45Ji0-3

Initial raoemic complex

i(aoao)~2]

i(acao)2Cl~

~i(acac)2Br2]

I

d-tartarlc acid

d-tartaric acid

acid

d-tartarlc

III

Environment compound

0.160

0.155

0.160

Molarlty

285

55o 285

335 280

i

--3,610 +85,500

+88,800

-3,55o

-3,2oo +59,000

Induced C.D. BamdB

Induced C.D. Bands and Absorption Maxima of the ~i(aeac)2X~ Series in DMF Solutions at Room Temperatuze.

TABLE 2.

O C

E

Vol. 7, No. 1

0.7

SOME RACEMIC GROUP(IV) METAL COMPLEXES

97

A

O.6

O,g

o o.3

/.

,

270

,

29,o

,

~].o

.

Wavelength Q m ~ )

.

,

330

350

FIG. 1

Absorption Spectrum of ~i(acac)2Br21in O ~ A - with environment compound B -without

Solution.

envirou~ent com~ound

constant maxim~imreading°

As expected the molar Pfei~fer rota-

tion3 of the more labile titanium(IV) compounds are considerably higher than the corresponding tin(IV) compounds for identical concentratious of racemic species and environment compound.

98

SOME RACEMIC GROUP(IV) METAL COMPLEXES

Vol. 7, No. 1

Some of these data are given in Table 1. ACKNO~WLEDG~/'~EI~T. We wish to thank the Rutgers University Research OouncLl for financial support and the National Science Foundation for a Summer(1970) undergr~:duate research participancy for W.D. We also wish to thank Professor George Bird of New Brunswick for his helpfulness in connection with the use of the C.D. equipment, which he made available to us. RE~ERE~TCES 1.

V. DORON, W. DURHAM, Inolg. and ~hcl. Chem. Letters, 6, 285 (1970)

2.

V. DORON, We DURHAM, presented in part at Metrochem 70, Hoboken, New Jersey (March 1970)

3.

J.A. SMITH, E.ff. WILKINS, Chem. Comm., 381 (1965)

g.

J.W. F~TT.~R, A. DAVISON, Inorg.Chem., 6, 182 (1967)

5-

V. DORON, C.FISCHER, Inorg. Chem., 6, 1917 (1967)

6.

W.Ho NELSON, Incrg.Chem., 6, 1509 (1967)

7.

RoC. FAY, R.N. LOWRY, Inorg. Chem., 6, 1512 (1967)

8.

S. KTRSCHNER, N. AH~L~D, "Coordination Chemistry", Plenum Press, New York (1969), S. Kirschmer Editor

9.

R.C. BRASTED, V.J. LANDIS, E.J. KUHAJ~, P.E.R. NORDQUIST, L. MAYER, "Coordination Chemistry", Plenum Press, New York (1969), S.Kirschner Editor

lO.

N. DAVIES, F.P. DWYER, J. Prec. Roy. Soc. N.S. Wales, 86, 6~ (1953)

ll.

E.G. GYARFAS, F.P. DWYER, Rev. Pure Appl. Chem., _6, (1) 73 (195~)

12.

E.C. GYARFAS, F.P. DWYER, J. Am° Chem. Soc., 73, 2332 (1951)

13-

So KIRSCHNER~ No AHMAD, J. Am. Chem. Soc., 90, 1910 (1968)

14.

V. DORON, W. DURHAM, to be published

15.

V. PORON, W. DURHAM, J. Am. Chem. Soc., ( in press)

16.

S.K. DHAR, V. DORON, S. KIRoCILN~ZR, J. Am. Chem. Soc., 81 , 637P (1959)