Influence of 2-methyl substitution on the geometry and complexing ability of sparteine: Packing of chiral vs. racemic building blocks

2-Methylsparteine (1) and its complexes with ZnL2 of the general formula [(C16H28N2)ZnL2] [L = CN− (2), Br− (3), CH3COO− (4), C4H5O2- (5)] have been prepared and the 1:1 (metal:alkaloid) stoichiometry was confirmed by elemental analysis. The complexes have been characterised by IR and NMR spectroscopy. Crystal structures are reported for racemic compounds 1, 2 and 3, and compared with the structures of chiral (-)-sparteine analogues. For the sake of comparison, synthesis and crystal structure of 1:1 (-)-sparteine complex with Zn(CN)2 (6) is also reported. The crystal structures of 2, 3 and 6 reveal the zinc center four coordinated by two nitrogen atoms from the alkaloid and two halides or cyanide groups in the form of distorted tetrahedral. Comparison with the crystal structure of 1 reveals that coordination to metal center in 2 and 3 brings about an inversion at the N16 nitrogen and consequent change in configuration at the ring junction from trans to cis, accompanied by a change in the C-ring conformation from boat to chair. The racemic crystals of 2 and 3 display only moderate packing similarity, but 3 exhibits very similar supramolecular architecture to that of the previously reported chiral C16H28N2ZnCl2, albeit in space groups P1¯ and P21, respectively. Packing of 2-methylsparteine ZnL2 metal complexes in racemic crystals is more efficient and diverse than in chiral crystals of the analogous (-)-sparteine and α-sparteine complexes which, in turn, tend to form an isomorphic series.