Circular dichroism studies of some dissymmetric coordination compounds

Abstract

The Pfeiffer effect is a change in optical rotation of a chiral organic compound or complex ion (the environment substance) in solution upon the addition of certain racemic metal complexes. .............................................................................

The Pfeiffer effect is a change in optical rotation of a chiral organic compound or complex ion (the environment subs tanco) in solution upon the addition of certain racemic metal complexes. The Pfeiffer effect was investigated in a series of racemic complexes in presence of various chiral environment substances. A number of mixed ligand complexes [Cr(phen)2 CI 2 J+, [Cr(b;::>Y)2 9Iy ]2+ + [Cr(phen)2 0x ] , and such as cis- [M(EDTA)]- , [I'!(DT'PA) ] Cis-[Cr(bpY)2(H20)]3+ 1M = Cr3+, v 3+), [Co(BPA) 3]2+, [Cu(2-ampY)2] 2+ and [Nci(EDTA)] show the Pfeiffer effect in presence of (+)- cinchonine HCl (-)-cinchonidine HCl, (+)-tartaric acid, (+)- <lscorbie acid, (+) -a-methylbenzylamine HCl, (-) -ephedrine HCl and/or (-) -brucine Hel. Most of t.hese complexes show a positive Pfeiffer effect in presence of various environment substances. However (+) -cinchonine HCI and (-) -cinchonidine HCI induced a ive Pfeiffer effect In mixed ligand complexes of chromium(III), Na[V(EDTA)] and Na 2 [V(DTPA)]. A few new environmon~ substances like (+)-a-methylbenzylamine HCI, (-)-ephedrine HCI, (-)-cinchonidine HCI and (+)-ascorbic acid have been found to induce the Pfeiffer effect in some of these complexes. -ii- CD spectra of the Pfeiffer systems in aqueous solutions were lTIeasured. The mixed ligands chromium(III) complexes show 1-2 cott.on bands in visible region. The band I is observed between 16,666-16949 cm -1 and a higher energy band I I between 19,607-20,000 em -1 These bands are assigned to electronic transition to Al and B1 upper states respectively. The CD spectra of [Cr(EDTA)H2 0] ion in presence of (+)-tartaric acid and (+)-ascorbic acis has a b road b and at 17,857 em -1 wh 1' l e that in presence of (+)-cinchonine HCl and (-)-cinchonidine HCI consist of an additional weak Cotton band of "opposite sign at 20,408 -1 [Cr(DTPA) ]2- shows stronger positive Cotton band at (;1\1 a 19,230 em -1 and a weaker negative CD band at 23,809 em -1 in presence of (+)-tartaric acid and (+)-a-methylbenzylamine HCl. In presence of (+)-ascorbic acid a mirror image spectrum is obtai.ned. I'hese tVIO Cotton bands are assigned to electroni.c CD spectra of [V(EDTA)] in presence of (-) -cinchonidine Hel! (+)-a-methylbenzylamine HCl, (-)-brucine HCl and (+)- tartaric acid has a positive Cotton band ~t 19,607 cm-1 and a weaker band at 23,255 cm-~ [V(DTPA)]2 shows Cotton at 15,873 -1 em and at 21,276 em -1 wn'fi;h are assigned to ---4 3A (3T1g (P) and 3A 3B 3 2 1 ------'> 1 (T Ig(P» transition ly. CD spectra of [CO(BPA) 3]2+ in presence of (+) -a-bromoeamphorsulph"bnate and (-) -1 O-camphorsulph- "ollie acid has a couplet or~ opposJ.. t e 1 y sl.g ne d Cotton b ands ce:lt:ered at 18,348 em 1 and 21,505 cm- 1 These are assigned to 4A 1 an d 4A 2 ---"-) 4E'1 ~c ..ranS..l j :.. lon respectively. [Nd(EDTA) ] shows a negative CD band at 17,871 --1 em in presence of (+)-tartaric acid and (-)-cinchonidine HCl which is due to f --, f transition. The binuclear complexes of Vanadium (III) like [V (llA) (J.1-0H) )+4 (AA ~ phen or bpy) exhibited 2 4 '1. strong charge transfer CD bands in presence of (+) -tartaric acid. These two binuclear species were precipitated out as perchlorate salts. The elemental analysis and IR spectra of the solid perchlorate complexes indicated that tartarate ion has been coordinated to Vanadium (III) replacing hydroxo bridges. The complex ions [V2 (phen)4(J.1-(+)-tart)] +4 and [V2 (bPY)4(J.1-0H)(J.1-(+)-tart))3+ slowly oxidized to a green mononuclear species which could also be precipitated as perchlorate salts. Elemental analysis of this green compound fits well in cis-[VO(bpY)2Cl)(Cl04). It is indicated from these studies that anasymmetric synthesis of has been achieved during oxidation of its binuclear analog. The absolute configuration of ligands around the metal ions in these complexes have been assigned on the basis of the sign of Pfeiffer effect in presence of various environment substanceS and CD spectra of the Pfeiffer systems. The l0\!lO • hi enantloers of the complexes cis-[Cr(phen)2C12)+' 2+ 3+ [Cr(bpY)291y) cis-[Cr(bpY)2(H20)2] and ct~ enantiomer of + [Cr(phen)20x] are assigned "6" absolute configuration. Similarly (+)-Na(Cr(EDTA)H 2 0], (+)-Na 2 [Cr(DTPA) j 2+ and (·-)-Na(V(EDTA) j, (--)Na 2 [V(DTPA)] and (+)--[CO(BPA)3] have also been assigned If IJ. H absolute configuration. The binucleaI~ complexes and (+)- [V 2 (bpY)4(!l-OH) (Il-(+)-tart) J (Cl04 )3 are assigned "/:;" absolute configuration while the mononuclear (+)-cis-[VO(bPY)2CljCl04 is assigned "At' absolute configuration.