Conformational analysis of coordination compounds. XII. N- and C-Substituted five-membered diamine chelate rings

Abstract

An analysis of the ³J_H,H coupling constants for the CH₂CH₂ segment in the chelate rings formed by N-methylethane-1,2-diamine, and a crystal structure analysis of [Co(NH₃)₄{(±)-pn}] (S₂O₆)_1.5,H₂ (pn, propane-1,2-diamine) have shown that in cobalt(III) five-membered diamine chelate rings the NCCN torsional angle is approximately 51°. For platinum(II) complexes of N-methylethane-1,2- diamine, the ³J_H,H technique yielded a value of 52.5° for the NCCN torsional angle. Problems involved with the refinement of the structure prevented the determination of a reliable value of this torsional angle in the crystal structure analysis of [Co(NH₃),{(-)-dmbn}] (NO₃)₃,1.5H₂O where dmbn is 3,3-dimethylbutane-1,2-diamine. However, this analysis showed that (-)-dmbn has the (R)-absolute configuration. The ³J _H,H values for the CH₂CH segment in complexes of dmbn show that the NCCN torsional angle for the dmbn chelates in solution is significantly greater than for the other diamines. The conformer populations for the N-methylethane-1,2-diamine complexes were also determined by the ³J_H,H technique. For the tetraamminecobalt(III) and tetracyanocobaltate(III) complexes the mole fractions of the conformer with the methyl group equatorial were found to be 0.92 and 0.78, respectively. For the bipyridineplatinum(II) complex, one conformation was found to be populated exclusively. This was assumed to be the axial conformer because of the unfavourable interaction of the equatorial N-methyl group with the proton on C6 of the 2,2'-bipyridine group. The J_{pt, H} values for an equatorial C-H proton in square-planar platinum(II) complexes of dmbn show that the t-butyl group is restricted to the equatorial orientation even in this stereochemistry. The conformational-energy minimization technique was applied to the two complexes studied by crystal structure analysis.