ISOMERS OF BIS-((-
Oct. 5 , 1963 [COSTRIBUTIOS FROM
THE
2913
)-PROPYLENEDIAMINE)-ETHYLENEDIAMINECOBALT(~~~)
JOHNCURTINSCHOOL OF MEDICALRESEARCH, AUSTRALIASXATIONAL UNIVERSITY, CASBERRACITY, AUSTRALIA]
Stereospecific Influences in Metal Complexes Containing Optically Active Ligands. VII. The Isolation of the D- and L-Isomers of Bis-( ( - ) -propylenediamine)-ethylenediaminecobalt(111) and Bis-(ethylenediamine)-( - )-propylenediaminecobalt (111) Ions BY F. P. D W Y E R ,T. ’ ~ E. MACDERMOTT,’~ A K D A. M. SARGESOX RECEIVED FEBRUARY 25, 1963 The optical isomers L-[CO(-)pns]Cla, L-[CO(-)pnnen]Cls, L-[CO(-)pn(en)2]C13, D-ICO(-)pn3]C13, D[ Co! )pn,en] (C101)3, and D- [CO(- )pn(en )*I,( CIO& have been isolated by a chromatographlc separatlon The l l 0 133, equilibrium constants between the D- and L-isomers were found to be K = DI/LI = 0 47, K = ~ l l / ~ = K = ~ l / l / ~ l= / l 0 0685, and the stabilities of the isomers have been rationalized with respect t o the possible conformations which may be adopted by the en and ( - )-pn rings (en = ethylenediamine, pn propylenediamine)
Introduction Two principal tenets have been associated with stereospecificity in metal complexes containing three optically active bidentate ligands, namely, that only one configuration of the complex occurred with one antipode of the base”;‘ and that complexes containing both ligand antipodes “seem to be unstable from stereometric causes.’’2b Both were disproved by the isolation of Land ~ a - [ C o ( f ) ( a l a n i n e ) s ]of~ ,L~ and D-[CO(-)pn3]13,4 and L- and D- [Pt(+)pn2(-)pn]C145 (in denoting the optical isomers ( -) = / is the sign of rotation of the base in the N a D line.). The last two isomers were quite stable even in hot acid or alkali solutions and it has been proposed6that the inability of earlier to isolate tris complexes containing “mixed” d- and 1-bidentate amines was not due to the “inherent instability” of the complexes but to the establishment of equilibrium, among all the possible species, during the preparative procedures. Evidence is already available to support this view and more will be presented in a subsequent communication. In the attempt to clarify the preceding issues the quantitative aspect of stereospecificity has been neglected except for the conformational analysis of the [Co(- - ) ~ n ! ] ~ +ionlo and the measurement of its ~ l l l eLL// equilibrium c o n ~ t a n t .The ~ former work had the merit of accounting for the stability of the ~ l l isomer l over the ~ l l and l also gave an equilibrium constant in reasonable agreement with the measured value (revised). Further it gave an explanation for the isolation of roughly equal amounts of both D- and ~ a - [ C o ( f ) ( a l a n i ~ ~ e )and ~ ] ~implied , or suggested that appreciable quantities of both D-and ~ - [ c(o- )pn2C03]+,1 , 1 2 [Co(en)z(+)tartrate]+,13 [Co(-)pn2(CZO4)]+,and [Co(-)pn( C 2 0 & - l 4 ions should occur, a prediction which agreed with the experimental observations. It is now understood that the degree of stereospecificity in complexes is largely a property of the ligand and
may vary from practically zero as in the [Co(-)pn( C z O & - l 4 system to the almost total specificity of the [Co(-)PDTA]-15 ion. The failure of the initial a t tempts6 to determine the stereospecific effect of optically active propylenediamine by successively replacing ethylenediamine in [ C ~ ( e n ) ~has ] ~now + been overcome and the present paper records the equilibrium constants for the D- and L-cobalt(II1) complexes containing both en and (- )-pn. Results and Discussion The equilibrium concentrations of the isomers in the reaction mixtures [ C ~ ( e n ) ~ )pn]C13, ([Co(en)(-)pn2]C13, and [Co(-)pn3]CI3 are recorded in Table 1. The mixtures were prepared by oxidizing solutions of cobalt(11) chloride (1 mole), hydrochloric acid (1 mole), and the appropriate amount of ethylenediamine and /or propylenediamine, with air (3 hr.) in the presence of charcoal at 25’. The concentration of cobalt(I1) remaining in these solutions is negligible (
