Synthesis and Crystal Packing of trans‐Bis(2‐aminotroponato)palladium(II) Complexes Bearing Linear Alkyl Chains – Hard Lamellar Structures Self‐Locked by Cross‐Shaped Molecular Units

The synthesis, structure, and three-dimensional lamellar array of a series of trans-bis(2-aminotroponato-κN,κO)palladium(II) complexes bearing linear alkyl chains (1a: n = 5; 1b: n = 8; 1c: n = 14; 1d: n = 16; 1e: n = 18, where n is the number of carbon atoms in the chain) attached to trans nitrogen donor atoms are described. The trans-coordination of the ligands, the cross-shaped molecular structures, and the crystal packing of 1b and 1c have been unequivocally established from single-crystal XRD studies. Highly regulated multilayered lamellar structures are observed for 1b and 1c, where every lamellar layer in the crystallographic ab plane, formed by π-stacking interactions of metal cores and van der Waals interactions between alkyl chains, is laminated alternately on the c axis with an orthogonal lamellar array of the identical unit. Powder XRD analysis showed that 1b–1e exhibit diffraction peaks attributed to periodic (n00) reflections of a typical lamellar structure. A linear correlation between the d-spacing and chain lengths in these compounds indicates that they form the same type of three-dimensional layer-by-layer structure of lamellar aggregates. The correlation between the cross-shaped molecular structures of 1b–1e and the dimensionality of molecular constraint in the crystal packing is discussed by comparison with those of rod-shaped analogues that exhibit high liquid crystallinity.