The absolute configurations of trisdiamine complexes of chromium (III)

J. lnorg. Nucl. Chem.. 1965. Vol. 27. pp. 361 to 363, Pergamon Press Ltd. Printed in Northern Ireland

THE ABSOLUTE CONFIGURATIONS OF TRISDIAMINE COMPLEXES OF CHROMIUM (Ill) J. H. DUNLOP and R. D. GILLARD Department of Chemistry, The University, Sheffield, 10

(Received 11 July 1964) Abstract--The tris complex of (--) propylenediamine with chromium (III) is formed stereospecifically; the absolute configuration of the complex is fixed as L, A, or R(Ca)S(C~)by the steric requirements of the ligand. The circular dichroism of the complex is opposite to that of (÷) trisethylenediaminechromium (III) cation, which is thereby shown to have the D configuration. The magnetic-dipoleselection rule is illustrated, and a correlation made between absolute configurations of trisdiamine complexes of cobalt (III), chromium (III) and rhodium (III). STEREOSPECIFIC induction of configuration in a metal complex by optically active diamine ligands is well known, m If the absolute configuration of the diamine is known, conformational considerations '2) give the preferred configuration of the complex ion. It is knownC~,a) that the preferred configuration of tris ( ÷ ) propylenediamine cobalt (III) cation is the same as that found t4) for the (-I-) trisethylenediaminecobalt (III) cation by X-ray diffraction; this may be called D,t3, 4) A, C5) or S(C3)R(Cz). By comparing the signs of the Cotton effects of the complex containing the active diamine with those of a resolved complex of an inactive diamine, the configuration of the latter complex may be deduced. We now find that, when chromium (III) forms a complex ion with three molecules of (--) propylenediamine,* the stereospecific product is ( + ) [Cr(--pn)a] a+, whose absolute configuration is known from the steric requirements of the (--) diamine to be L, A, or R(Ca)S(Ce). The circular dichroism of this complex ion is shown in the Figure, with that of (-t-) [Crena] a+. Considering the circular dichroism spectra arising from the longer wavelength Band I of the absorption spectrum (4E~ I- 4A1 ~ 4A2, arising from 4Tz, ~-- 4A~ in Oh), it is clear that the complexes have enantiomorphous configurations. This gives the configuration D(+)[Cren3] a+. This enantiomer therefore has the same configuration as ( + ) [Coen3] 3÷, in agreement with our earlier conclusion 16~ drawn from the similar rotatory dispersion curves of D(--)[Cr(~-chxr03] 3÷ and (-~)[Crenz] 3+. The same result is indirectly obtained ~6) for the relative configurations of (÷)[Coen3] z+ and (+)[Cren3] a+ from the method of least soluble diastereoisomers. * A b b r e v i a t i o n s used are : en, e t h y l e n e d i a m i n e ; pn, p r o p y l e n e d i a m i n e ; tn, t r i m e t h y l e n e d i a m i n e ; c h x n , trans-1,2-cyclohexanediamine. m F. BASOLO a n d R. G. PEARSON, Mechanisms of Inorganic Reactions, p. 289. Wiley, N e w Y o r k (1958). ~2~ E. J. COREr a n d J. C. BAILAR, J. Amer. Chem. Soc. 81, 2620 (1959). '~' R. D . GILLARD a n d G. WILKINSON, J. Chem. Soc. 1368 (1964). ~4~ y . SAITO, K . NAKATStJ, M. SI~mO a n d H, KUROVA, Acta Cryst. 8, 729 (1955). ~'~' T. S. P[PEg, J. Amer. Chem. Sac. !i3, 3908 (1961). ,6~ j. H. DUNLOP, R. D. GILLARO a n d G. WILKINSON, J. Chem. Soc. 3160 (1964). 361

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FiG. 1. - Circular dichroism of (+)[Cren3~]+. - - - - - - Circular dichroism of (+)[Cr(+pn)d a+. - - - - A b s o r p t i o n spectrum of [Crensp+ (that of [Crpn~p+ is extremely similar). In dihedral complexes with the d a spin-free, or d 6 spin-paired ground state, with D a symmetry, the lowest energy spin-allowed excited state splits into two components, E + A. If the complex has the D(A) configuration, the E tranr:~ion has a positive rotational strength. In D(+)[Cren3] 3+, the transition responsible for the circular dichroism under the longest wavelength absorption band is therefore aE ~- 4A. It has been said 'v' that in cases where only a single circular dichroism band appears under Band I, owing to the small splitting of E and A, the Cotton effect, as manifested by circ~.llar dichroism, has the sign of the rotational strength of the E transition. That 41" +-4A do~"s, in fact, dominate the circular dichroism is now shown to be true for ( + ) [ C r e % P " and (+)[Cr(+pn)a] a~. The magnitudes of the Cotton effects of [Cr(-kpn)a] 3+ provide a vc,) striking demonstration of the selection rule of MOFFITT.Is) Using the notation appropriate to O~, symmetry, the rotational strength of the magnetic-dipole-allowed 4T~ + - - 4 A 2 , J transition is an order of magnitude larger than that of tire 4T1~ , - 4A2~, transition, which is not magnetic-dipole-allowed. In terms of the g-factor (e~ -- eSe ) of KUHN, 19)for Band I (4Tlo*-- 4A2g), g = 1"3 × 10-2, and for Band II (4T1~,-~---4A2~), g 0'08 x 10 .o. It has been pointed out previously, ~74°) that a g-factor of 10 2 or more is characteristic of a magnetic-dipole transition. The enantiomers (+)[Cren3]a+, ( _ ) [ C r t % l ~ , (_)[Cr(+chxn)a].~ t-and ( "-)[Cr(+pn):~] a+ are now known, from this and our earlier w o r k f ;I to have the D configuration. (~) S. F. MASON, Quart. Rev. 17, 20 (1963). (81W. MOFFITT, J. Chem. Ph)'s. 25, 1389 (1956). (91 W. KUHN, Amt. Rev. Phys. Chem. 9, 417 (1958). a0) R. D. GILLARO, J. Chem. Soc. 2092 (1963).

The absolute configurations of trisdiamine complexes of chromium (Ill)

363

TABLE 1.---CIRCULAR DICHROISM SPECTRA OF TRIS(+)PROPYLENEDIAMINE COMPLEXES OF CoIII~ C r III AND Rh m

(2 in m/*) Band 1 Complex

( + ) [Co(+pn)a] ~+ ( + ) [Cr(+pn)a] a+ ( - ) [Rh(+pn)3] 3+

470 457 301

0 Signifies not observable.

Band 11

E a -,- A

A +- A

acDt (el -- ca)

ZcD (ex -- ta)

496 +1"9 470 +0.9 312 +0-5

441 0 290

--0"5 0 -0.8

* Absorption maximum.

Ref.

Eb "--- A

;t~b, 2c~ 350 346 350 350 250 0

(~1 -- ed) +0-27 +0-3 0

6 This work 6

1 Circular dichroism maximum.

T h e c i r c u l a r d i c h r o i s m s p e c t r a o f the t r i s ( + ) p r o p y l e n e d i a m i n c c o m p l e x e s o f c o b a l t (III), c h r o m i u m ( I I I ) , a n d r h o d i u m ( I I I ) are c o m p a r e d in T a b l e 1; it is clear t h a t splittings o c c u r i n the s a m e sense, a n d t h a t a g e n e r a l c o r r e s p o n d e n c e exists b e t w e e n the d a c o m p l e x a n d the d e c o m p l e x e s . EXPERIMENTAL Microanalyses were by the Microanalytical Laboratory, Imperial College. Electronic spectra were determined using a Perkin-Elmer 350 spectrophotometer. Circular dichroism spectra were observed using monochromated light from a Unicam SP 500 spectrophotometer, which passed successively through a converging lens, focussing on the window of the photocell, a Rochon prism mounted in a ball race rotatable through 90 ° in the plane perpendicular to the light path, a Babinet compensator, and the sample cell. After establishing the wavelength of the monochromated light, the Babinet compenstor was adjusted manually to retain circular polarization of the light, and the percentage difference of transmittance readings for left and right circularly polarized light determined. Polarized light of opposite hands was obtained by settings of the Rochon prism 90 ~. apart. An automatic instrument using the same optical components has recently been described. ~u)

Tris(- )propylenediaminechromium (111) chloride A modification of the method ~1~ of RoUinson and Bailar was used. Anhydrous chromic sulphate (2 g) was heated with (--)propylenediamine (3 ml). The reaction product was extracted with methanol containing a little dilute hydrochloric acid. Tetrahydrofuran was added slowly to the yellow-orange extract, precipitating a product which contained sulphate ions (revealed by the infra-red spectrum). The product was therefore dissolved in dilute hydrochloric acid and the solution treated with barium chloride. The filtered solution was then treated with tetrahydrofuran, the precipitate was collected and extracted with methanol (25 ml) containing concentrated hydrochloric acid (0-5 ml). The complex was reprecipitated from the methanolic solution with ether. This reprecipitation was repeated twice, when tris(--)propylenediaminechromium(111) chloride was obtained as a yellow solid. [Found : CI, 27.95. Calc. for CgH30CI~CrN6: C1, 28'18%]. The visible spectrum (ca. 10 3 M in water) was identical with, that given in the literature. ~la~

Acknowledgement--We wish to thank the Department of Scientific and Industrial Research for a maintenance grant (J. H. D.), Professor W. L. S. GARTONfor the loan of components for the circular dichroism measurements, and Mr. L. HAILES for drawing our attention to reference (I I). Illl S. KOZAWA,N. OKAZAKIand A. WADA, Oyo Buturi 33, 110 (1964). tt~' C. L. ROLLINSONand J. C. BAILAR,Inorg. Synth. 2, 198 (1942). ~ ' K. G. POULSENand C. S. GARNER,J. Amer. Chem. Soc. 81, 2615 (1959).