Coordination chemistry effects of the space-demanding solvent molecule N,N'-dimethylpropyleneurea

Crystallographic investigations of eight homoleptic N,N'-dimethylpropyleneurea (dmpu) coordinated metal ions in the solid state, [Mg(dmpu)₅]I₂ (1), [Ca(dmpu)₆]I₂ (2), [Ca(dmpu)₆](ClO₄)₂ (3), [Ca(dmpu)₆](CF₃SO₃)₂ (4), [Sr(dmpu)₆](CF₃SO₃)₂ (5), [Ba(dmpu)₆](CF₃SO₃)₂ (6), [Sc(dmpu)₆]I₃ (7), and [Pr(dmpu)₆]I(I₃)₂ (8), and the complex [CoBr₂(dmpu)₂] (9) as well as the structures of the dmpu coordinated calcium, strontium, barium, scandium(III) and cobalt(II) ions and the cobalt(II) bromide complex in dmpu solution as determined by EXAFS are reported. The methyl groups in the dmpu molecule are close to the oxygen donor atom, causing steric restrictions, and making dmpu space-demanding at coordination to metal ions. The large volume required by the dmpu ligand at coordination contributes to crowdedness around the metal ion with often lower coordination numbers than for oxygen donor ligands without such steric restrictions. The crowdedness is seen in M⋯H distances equal to or close to the sum of the van der Waals radii. To counteract the space-demand at coordination, the dmpu molecule has an unusual ability to increase the M-O-C bond angle to facilitate as large coordination numbers as possible. M-O-C bond angles in the range of 125-170° are reported depending on the crowdedness caused by the coordination figure and the M-O bond distance. All reported structures of dmpu coordinated metal ions in both the solid state and dmpu solution are summarized to study the relationship between the M-O-C bond angle and the crowdedness around the metal ion. However, highly symmetric complexes seem to be favoured in the solid state due to favourable lattice energies. As a result, the dmpu coordinated lanthanoid(III) ions are octahedral in the solid state, while they, except lutetium, are seven-coordinate in the dmpu solution.