The use of proton magnetic resonance spectroscopy in assigning the absolute configuration of coordination complexes containing optically active propylenediamine ligands

INORG.

NUCL.

CHEM.

LETTERS

¥*1.

3,

pp.

525-529,

1967.

Pergamon

Press

Ltd.

Printed

In

GMmt Brlfaln.

THE USE OF PROTON MAGNETIC RESONANCE SPECTROSCOPY IN ASSIGNING THE ABSOLUTE

CONFIGURATION OF C00RDINATION COMPLEXES CONTAINING 0PrICALLY ACTIVE PROPYLENEDIAMINE LIGANDS John G. Brushmiller and Leon G. Stadtherr Department of Chemistry, The University of North Dakota Grand Forks, North Dakota 58201 (Received 17 August 1967)

Proton magnetic resonance spectroscopy (pmr) can be a valuable tool in studying the conformations of chelate rings in coordination complexes 1'2~3 and in assigning the absolute configurations of cobalt (lll) 4 and platinum (IV) coordination compounds.

In this communication we wish to show how the pmr

spectra of (+)589 and (-)589-Na/Co(-)pn ox2 7can be used to assign the absolute configurations of the optical isomers. Recently the absolute configurations of (+)589 and (-)589-Na/Co(en)(ox)2J have been inferred by comparing the ORD spectra of the optical isomers to the ORD spectrum of (+)589/Co(en)3 7 ~15, ~ 3 6whose absolute configuration is known to be

~

l~e ORD spectrum of (+)589Na/Co(-)pn(ox)27

is practically identical

to that of (+)589Na/Co(en)(ox)2_77 and on this basis the isomers of Na/Co(en) (ox) 2 7 and Na/-Co(-)pn(ox)2Y may be assigned the The in Fig. i.

~/~ and In the

~ ~

~

absolute configuration.

configurations of the ~-Co(-)pn(ox)2_7-anion are shown absolute configuration of the complex~ the C_C bond

axis of the (-)pn chelate ring is parallel to the pseudo C 3 axls of the complex and the CH 3 group of (-)pn occupies an equitoral position on the chelate

* The nomenclature used in this communication is that proposed by Prof. B.E. Douglas to the IUPAC for the notation of absolute configurations and chelate ring conformations in coordination compounds. 525

526

PROTON MAGNETIC RESONAMCE SPECTROSCOPY

Vol. 3, No. 11

fo

~0

0..

I

/ O/

"Cd

i~ I

..N..~

0

:~

~'

,bO

c--CH3 N/~

j~. (+)589Na_/-Co (-)pn ox2~_~

N

H CH3 .~..

A (-)589Na_/-Co (-)pn ox2 ~._7

Figure 1. Optical isomers of Nal-co (-)p~ ox2J.

ring 8

This is the

~

* conformation of the chelate ring.

In the

~

abso-

lute configuration of the complex~ the C-C bond axis of the (-)pn ring is oblique to the pseudo C 3 axis of the complex ion and the CH 3 group of (-)pn occupies an eq~itoral position on the chelate ring.

This is tse

X

confor-

mation of the chelate ring in tk~e complex. Froebegand Brushmiller I0 have studied the pmr spectra of J ~

and

A

configurations of Pt(IV) complexes containing (-) and (+)pn as ligands. They have observed that the chemical shift of a CH 3 group which is attached to a C-C bond axis which ks oblique to the pseudo C 3 of the complex ion is greater than the chemical shift of a CH 3 group which is attached to a C-C bond axis which is parallel to the pseudo C 3 of the complex ion. The chemical sl~ift difference between the two types of CH 3 group is independent of the isomeric form of pn which is coordinated and depends only upon the orientation of the C-C bond axis in the coordination compound.

This observation provides

a basis for using pmr spectra to asslgn the absolute configurations to optically active coordination complexes containing optically active pn ligands. In Fig. 2 is shown the pmr spectra of the racemic and active forms of Na/-Co(-)pn(ox)2_7.

In the pmr spectrum of the racemic mixture, two different

CH 3 resonance signals (a doublet with J=6cps) seen.

separated by 1.5 cps can be

These two resonance signals are due to having the CH 3 group of (-)pn

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PROTON MAGNETIC RESONANCE SPECTROSCOPY

(±) ~89

( - )

5s9 ~/9.5 cps

( J589 81 cps

Figure 2.

Pmr spectra of racemic and optically active NaG-Co C-) pn ox2_ ~ in D20 with NaTPS internal standard.

527

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PROTON MAGNETIC RESONANCE SPECTROSCOPY

Vol. 3, Fie. 11

attached to both parallel and oblique C-C bond axes in the racemic mixture. In the pmr spectra of the optical isomers, only a single CH 3 resonance signal is seen for the CH 3 group.

The chemical shift of the CH 3 resonance signal

in the (-)589Na_/-Co(-)pn(ox)2 7 isomer is slightly less (from the NaTPS standard used as an internal reference) than the chemical shift of the CI{3 group in (+)589- Na_/-Co(-)pn(ox)2 J. assigned to the

-~

On this basis the (+)589 isomer can be

absolute configuration and the (-)589 antipode to the

absolute configuration. In Fig. 2 the pmr patterns in the C ~

region of the spectra of both

racemic and active forms of Na_/-Co(-)pn(ox)2 J are also shown.

The pmr

patterns in the CH2 region of the (-)589 and (+)589 antipodes are distinctly different as would be expected for a C-C bond axis parallel and oblique to the pseudo C 3 axis of the complex ion.

If the pmr patterns in the C ~

region of the optical isomers are summed, the pmr pattern in the C ~ of the racemic mixture is obtained.

region

The stabilization of the chelate ring

by keeping the CH 3 group of (-)pn in an equitoral position in both configurations of the complex has been estimated at 2 kcal/mole/ring by BailarS~ The pmr spectra of the optical isomers indicate that the (-)pn chelate ring system in these complex ions is not rapidly inverting but remains fixed in the optical antipodes.

A temperature dependent pmr study was

carried out up to lO0°C and the pmr patterns of the racemic CH 3 signals failed to merge~ indicating that the ring remains locked in the conformation. The pmr specta of optically active coordination complexes containing optically active propylenediamine as a ligand provides a method of assigning the absolute configurations of the complexes independent of the ORD method. The two techniques both predict that the (+)589 isomer of Na_/Co(-)pn(ox)2! has the ~

absolute

configuration.

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PROTON MAGNETIC RESONANCE SPECTROSCOPY

529

Acknowledgements This work was supported in part by a Faculty Research Grant from the National Science Foundation to J.G.B. and an N°D.E.A. Fellowship (Grant no. 65-2158) award to L.G.S.

Reference s I. D.A. BUCHINGHAM~ L. DURHAM and A.M. SARGESON, Aust. J.Chem. 20, 257, (1967) 2. S.T.SPEES, L.J. DURHAM and A.M. SARGESON, Inorg. Chem., 5, 2013, (1966). 3. A.M. SARGESON, in Transition Metal Chemistry; edited by R.L. Carlin, Vol. 3, Marcel Dekker, Inc., New York, (1966). 4. R.G. ASPERGER and C.F. LIU, J.Am. Chem. Soc., 89, 708, (1967). 5. T.E. MACDERMOI~ and A.M. SARGESON, Aust. J.Chem., 16, 334, (1963). 6. A.M. SARGESON in Chelating Agents and Metal Chelates, edited by F.P.Dwyer and D.P. Mellor~ Chap. 5, Academic Press, New York and London, (1964). 7. F.P. I~gYER, T.E. MACDERMOTT and A.M. SARGESON, J. Am. Chem. Soc., 85, 661,

(1963). 8. E.J. COREY and J.C. BAILAR Jr., J.Am. Chem. Soc., 8~i, 2620, (1959). 9. L.R. FROEBE, M.S. Dissertation, The University of North Dakota, 1967. i0. J.G.BRUSHMILLER and L.R. FROEBE, to be published.