Effect of a methyl group on the spontaneous resolution of a square-pyramidal coordination compound: crystal packing and conglomerate formation

Abstract

Reaction of [Cu(pic)₂]•2H₂O (where pic stands for 2-picolinato) with 2-({[2-(dimethylamino)ethyl]amino}methyl)phenol (HL¹) produces the square-pyramidal complex [CuL¹(pic)] (1), which crystallizes as a conglomerate (namely a mixture of optically pure crystals) in the Sohncke space group P2₁2₁2₁. The use of the methylated ligand at the benzylic position, i.e. (±)-2-(1-{[2-(dimethylamino)ethyl]amino}ethyl)phenol (HL²), yields the analogous five-coordinate complex [CuL²(pic)] (2) that crystallizes as a true racemate (namely the crystals contain both enantiomers) in the centrosymmetric space group P2₁/c. Density Functional Theory (DFT) calculations indicate that the presence of the methyl group indeed leads to a distinct crystallization behavior, not only by intramolecular steric effects, but also because its involvement in non-covalent C–H⋯π and hydrophobic intermolecular contacts appears to be an important factor contributing to the crystal-lattice (stabilizing) energy of 2.