Tris Diamine Complexes

In 1966, [Co(en)3] Bfj was separated into its optically active enantiomers on a colunm of anion-exchange resin, which was in advance loaded with tartrate or anti-monyltartrate ions. The resolution was partial. In the same year, Brubaker et al. achieved the total resolution of a trinuclear cobalt(III) complex, hexakis(2-amino-ethanethiolato)tricobalt(III) bromide on a column of a cation-exchange cellulose (Bio-Rad Cellex CM) by eluting with 0.1 mol/dm NaCl. 

After several attempts with unsufticient resolutiontotal resolution was achieved for [Co(en)3] Br3 dissolved in water on a column (1.1 x 120 cm) filled with an ion-exchange dextran, SE-Sephadex C-25 in Na form. The elution with 0.15 mol/ dm sodium (-i-)-tartrate showed two completely separated peaks corresponding to the A(—)s89- and A( -t-)589-isomer, respectively. Since then column chromatography on Sepluidex became a powerful tool for the resolution of various complexes or for the paration of geometrical isomers and diastereoisomers. 

Yoshikawa and Yamasaki have reviewed the resolution of optical isomers and the separation of geometrical isomers of cobalt(III) complexes on Sephadex ion-exchangers , including the experimental techniques. 

For the tris[( )-propylenediamine]cobalt(III) ion, there are 24 possible isomers (Table 3.1, where symbols A and A refer to the absolute configurations of the complex ion, lei and ob to the conformations of the chelate ring, and fac and mer to the isomerism with respect to the methyl groups of the chelated pn molecules). Dwyer et al. studied this system by column chromatography with cellulose powder and paper chromatography (see Section 3.4). MacDermott separated the fac isomer 

Conformation optii isomers isomers with respect to CHj groups total isomers 

---

The first serious attempt to measure the separation (/IK) of E and A for tris-diamine complexes was that of Yamada and Tsuchida (106). Piper (81) calculated from their results that dK =45 cm-1. Dingle (103) studied the polarized absorption of 2 [Co(en)3]Cls NaCl 6 H2O at temperatures of 5 °, 77 0 and 300 °K and found that AK = 0 2 cm-1, and that the D3 selection rules are not rigorously obeyed. This, while... 

The addition of polarizable anions such as selenite to the tris diamine complexes gives specific ion pairs (100, 101) and enhances the 1A 2 component of the ig circular dichroism band, and diminishes the 1Ea component... 

Tris(diamine) complexes of chromium(III) have been important from several viewpoints. First, the tris(ethylenediamine) complex is valuable as a synthetic intermediate,1 the action of heat on the chloride salt giving the cis-dichlorobis(ethylenediamine) complex and on the thiocyanate salt giving the irans-bis(iso-thiocyanato) complex. Second, the cations are resolvable, and studies2 of their optical activity have been fruitful in establishing relations between signs of Cotton effects and absolute stereochemistries. A large number of other studies, including kinetic and equilibrium measurements, have shown these complexes to be readily hydrolyzed to bis(ethylenediamine) complexes.3... 

Most studies deal with cobalt clathrochelates. In several cases, nonmacrocyclic tris-diamine complexes have also been investigated to elucidate the specificity of interactions brought about by the formation of cage structures. 

As seen from the BCD spectra, the tartrate ion interacts with tris-diamine complexes along the Cs axis. In this case, however, steric hindrances for a A-[Co(en)3]. ..tart-- associate are drastically enhanced and Kd and Ki decrease compared with the values for associates with [Sb2(tart)2] dianions and other tartrate dianions. 

In the dihedral tris-diamine complexes of cobalt(III) the upper state of the octahedral Ai - Ti d—d transition is broken down into components with Aj and... 

The CD curves of [Co(-)-pn(NH3)4]Cl3, S9 trans-[Co -)-pn2(NH3)2]Cl3,s and the vicinal or conformational effects of ( —)pn Ss in tris(diamine) complexes are strikingly similar. Such similarities through the series do not lead one to expect significant changes in the frequency intervals for tris-(diamine) complexes as chelate ring conformations change. 

The small contributions of the fixed conformations of trien are inconsistent with the explanations of the appearance of one or two CD peaks in the Tig band region for tris(diamine) complexes given by Woldbye/ Dingle and Ballhausen/ and Mason/ vide ante. These small effects do not lead one to expect that the two peaks observed for [Co(en)3] could arise from the presence of two conformational isomers. Nor should one expect that there would be a significant change in frequency interval... 

Many tris(diamine)- and cw-diacidobis(diamine)chromium(III) complexes have been resolved into their enantiomers, thus providing proof of structure. Absolute configurations are frequently inferred from ORD and CD measurements, which have become of great importance because they provide quick structural information. The spectra-structure correlations are... 

Very recently, time-dependent density functional theory (TD-DFT) calculations have been applied to bis-diamine as well as tris-diamine Co(III) or Rh(III) complexes spanning the entire experimental spectral region including the charge transfer... 

The compounds that we will study are tris-bidentate complexes with trans-cyclohexane-1,2-diamine (see Section 17.6). We start from [Co(NH3)6]3+ (see Section 17.1). Using the usual tools in HyperChem, edit that complex so that it has six identical Co-N distances of 1.955 A and valence angles involving Co of 90° or 180°. Select the cobalt and the six nitrogen atoms with the select tool and save this structure make sure that the same name also appears as the MOMEC input structure (Setup/Files/Input). Open the module that allows you to compute the structure with the metal center and the six donor atoms fixed Execute/Rigid Geo-metry/Fixed Coordinates. Read File will mark the coordinates to be fixed, i.e., the x,y,z coordinates of the atoms that you have selected in HyperChem. The window shown in Fig. 17.20.1 will appear. 

A great number of papers deal with the stereospecificity of the interaction of optically active [CoNe] + complex cations with optically active anions and the stereospecificity depending on the structures of these cations and anions. A variety of tris-diamine cobalt(HI) complexes with different structures, clathrochelates with diverse capping groups, and the semiclathrochelate [Co(sen)]3+ cation have been investigated. Many investigators have employed d- and Z-tart -[Sb2(d-,Z-tart)2]2, and [As2(d-,Z-tart)2] dianions as optically active reagents [307, 310-312]. 

Fig. 12.29 The tel conformer of the A or o enantiomer of tris(diamine) metal complexes. The hatched circles represent the positions of the methyl groups in the propylene-diamine complex. For propylenediamine, this represents the ASSS or d6S5 isomer. [Modified from Corey, E. J. Bailar, J. C., Jr. J. Am. Chem. Soc.

Among the transition metal complexes, the tris(diamine)metal system, particularly tris(ethylenediamine)cobalt(III) and its analogues, has been studied most extensively from both experimental and theoretical sides. 

The circular dichroism spectra in the region of the first absorption band of the tris-bidentate complex ions having six-membered chelate rings sure known to be particularly sensitive to experimental conditions. For example, the CD spectrum of A-lel -[Co(R,R-ptn) ]C1, in an aqueous solution shows two peaks A = -O.589, 522 nm A = - 0.104, 462.5 nm, whereas that of A-lel,-[Co(R,R-ptn) ](CIO,) in an aqueous solution gives a negative peak (A , = -O.587T at 518 nm (41). The solid state CD differ from the solution CD and the solution CD are sensitive to the temperature of measurement and are affected by the presence of oxo anions (42, 43, 44). Table V lists the lowest frequency CD spectra of tris-diamine cobalt(III) complexes in the CT region. 

Using all of the recorded CD data for tris-diamine cobalt-(III) complexes of known absolute configuration, an empirical rule relating the absolute configuration to CD spectra of tris-diamine cobalt(III) complexes in the charge-transfer region was estab-... 

Table V The lowest frequency CD band of tris-diamine cobalt(III) complexes in the CT region...

---