Chelate rings puckered

Five-membered chelate ring of meso-2,3-diaminobutane is puckered with one methyl group equatorial and one axial. 

In a metal complex (102), five-membered chelate rings fonr d by 1,2-dia-mines exist in two puckered conformations, the enantiomeric X and 5 forms 36 and 37. For an achiral ligand in a bis or tris complex, steric interactions between rings are present, and the two conformations are no longer equivalent energetically. 

It is known that five-membered Cr(V) chelates are favored over six-membered ones.19,50,61 For Cr(V)-diolato complexes formed with linear diols, it was observed that all of the protons are equivalent in the isotropic EPR spectra,62 although the strain of a six-membered ring imparts inequivalence to the magnetic environment of the protons in the second coordination sphere.63 This observation at room temperature again points to rotational flexibility in the chelate ring (namely, puckering in the 8 or X configuration),20 similar to that observed for 1. 

In comparison to 1,2-diamines the five-membered chelate rings of amino-acids are much less puckered. Whereas dihedral angles of diamine chelate rings lie between... 

The reaction of mercuric iodide with the chelating diphosphine bis(diphenyl-phosphinoethyl)sulphide gives the complex [(Ph2PC2H4)2S]HgI2.184 Structural analysis shows the mercury to be in a distorted tetrahedral co-ordination, with formation of an eight-membered puckered chelate ring. 

A second example relates to the complex bis(l,5-diazacyclooctane)nickel(II) (Fig. 7.7). A crystal structure analysis of this complex revealed apparently planar NCCCN moieties with unusual bond lengths and angles[218]. A molecular mechanics analysis of the same complex showed that these chelate rings are puckered and concluded that the crystal structure result was a consequence of conformational disorder, the refined geometry corresponding to an average of two puckered conformations (see Fig. 7.7)[219l... 

An important feature of five-membered chelate rings is the amount of puckering. You can check this by computing the angle between the plane defined by the Co center and two N donors of one of the chelate rings, and the line defined by the two C atoms of this ring. For /e/3-[Co(cn)3]3+ this angle is 27.3°. 

Figure 1-18 Different ways of viewing the puckering of ethyl-enediamine chelate rings. The ab-S solute configurations A and S are defined. [Reproduced by permission from C. J. Hawkins, Absolute Configurations of Metal Complexes, Wiley, New York, 1971.]...

Conformation of Ethylenediamine. Whether the ethylenediamine chelate ring is completely planar or puckered has also been a subject of controversy. The conformation of ethylenediamine may be cis if planar and gauche if puckered. Infrared studies have proved to be very useful in distinguishing cis, trans, and gauche conformation of 1,2-substituted... 

Fig. 16. Schematic projection along the N—Sn bond in 54 showing the two diastereoisomers as a consequence of the a- of -puckering of the six-membered chelate ring.

A 1H NMR study (6 ) found that the chemical shifts of the N-H protons were very sensitive to solvent with the shifts being related to the donor number (j) of the solvent, i.e. the ability of the solvent to donate electrons to form a hydrogen bond with the N-H protons. Further, the N-H protons that were essentially equatorial in the puckered five-membered chelate ring showed larger variations in their 6 values than the axial protons. The differences in the 6 values for the equatorial and axial protons for the NH2 groups were in the order ... 

For the puckered diaminoethane, there are some further observations to make. The chelate ring is more rigid than the freely-rotating unbound ligand, so that the protons on each carbon are nonequivalent, as one points essentially vertically (axial, Hax), the other sideways approximately parallel to the N—M—N plane (equatorial, Heq). Nevertheless it is sufficiently flexible that it can invert - one carbon moving upwards while the other moves downward to yield the other form. These two forms are examples of two different conformations one is called X, the other 8, by convention they are mirror images of each other. Any chelate ring that is not flat may have such conformers. 

---