Versatility of Two Diimidazole Building Blocks in Coordination-Driven Self-Assembly

A series of discrete
molecular architectures was synthesized via self-assembly of two “linear”
diimidazole donors [L1 = 1,4-di­(1H-imidazol-1-yl)­benzene
and L2 = 1,1′-(2,5-dimethyl-1,4-phenylene)­bis­(1H-imidazole)] independently with cis-[(tmeda)­Pd­(NO3)2] [tmeda = N,N,N′,N′-tetramethylethane-1,2-diamine], cis-[(tmeda)­Pt­(NO3)2], a diplatinum­(II)
acceptor (bisPt), and Pd­(NO3)2.
The donors L1 and L2 are isostructural with
four hydrogens in the benzene ring of L1, whereas in L2 two of such hydrogens are substituted by methyl groups.
The assemblies were characterized by NMR spectroscopy and electrospray
ionization mass spectrometry analyses along with single-crystal structure
determination of three of them. While the self-assembly of L1 with cis-[(tmeda)­Pd­(NO3)2] solely formed a [3 + 3] self-assembled molecular triangle (1), L2 with the same acceptor predominantly resulted
in the formation of a [4 + 4] molecular square (3). Such
a dramatic change in the final outcome in the coordination-driven
self-assembly by simple alkyl substitution of isostructural donors
is remarkable. Interestingly, self-assembly of L1 and L2 with analogous Pt­(II) acceptor cis-[(tmeda)­Pt­(NO3)2] yielded mixtures of [3 + 3] triangle and [4
+ 4] square, where the molecular triangles (4 and 6) were the predominant products in both the cases. The same
donors in combination with a 0° acceptor bisPt independently
formed the expected [2 + 2] metallo-macrocycles (8 and 9). Surprisingly, the macrocycle (9) involving L2 is found to exist in more than one conformation at room
temperature. Moreover, the diimidazole donors formed unprecedented
Pd6L12 molecular spheres (10 and 11) when they were separately treated with Pd­(NO3)2. The imidazole moieties in the ligands are found to
appear in versatile orientations in the synthesized molecules due
to their rotational flexibility to produce required bite angles for
the particular architecture.