Design and Crystal Structures of Triple Helicates with Crystallographic Idealized D[sub 3] Symmetry: The Role of Side Chain Effect on Crystal Packing.

Novel crystallographic D[sub 3]-symmetric binuclear triple molecular helices [C0[sub 2]L[sup 1, sub 3]][BF[sub 4]][sub 4] (1), [Zn[sub 2]L[sup 1, sub 3]][BF[sub 4]][sub 4] (2), [Mn[sub 2]L[sup 1, sub 3]][BF[sub 4]][sub 4] (3), [C0[sub 2]L[sup 2, sub 3]][BF[sub 4]][sub 4] (4), [Zn[sub 2]L[sup 2, sub 3]][BF[sub 4]][sub 4] (5), and [Mn[sub 2]L[sup 2, sub 3]][BF[sub 4]][sub 4] (6) have been achieved to establish the side chain effect on molecular packing, where L¹ is [(C[sub 5]H[sub 4]N)C(CH[sub 3])=N-(C[sub 6]H[sub 4])-][sub 2]CH[sub 2] and L² is [(C[sub 5]H[sub 4]N)C(CH[sub 3])= N-(C[sub 6]H[sub 4])-][sub 2]O, respectively. Crystal structure analyses show that each helix crystallizes in a hexagonal crystal system with space group P&3maacr;c1 and the general axis of the helix occupies the crystallographic 3-fold axial position with the other three crystallographic 2-fold symmetries perpendicular to it. Each metal center is bound to three pyridylimine units to attain C[sub 3] pseudooctahedral coordination geometry with respective equivalent metal-N (CH= N) and metal-N (pyridyl) bonds. It is speculated that the existence of the methyl group might minimize the potential intermolecular interactions, which would be the essential factor controlling the helices formed in idealized crystallographic D[sub 3] symmetry. Moreover, crystallographic idealized C[sub 3]-symmetric helicates [C0[sub 2]L[sup 3, sub 3]][BF[sub 4]][sub 4] (7), [Zn[sub 2]L[sup 3, sub 3]][BF[sub 4]][sub 4] (8), [Ni[sub 2]L[sup 3, sub 3]][BF[sub 4]][sub 4] (9), and [Cu[sub 2]L[sup 3, sub 3]][BF[sub 4]][sub 4] (10) were also structurally characterized for comparison, where L³ is [(C[sub 5]H[sub 4]N)C(CH[sub 3])=N-][sub 2]. All the results indicate that the existence of the methyl group in the side chain of aromatic ligands could effectively reduce the potential π-π intermolecular interactions and the side chain effect of the methyl group in crystal packing is robust enough to be... [ABSTRACT FROM AUTHOR]