Supramolecular Structures and Columnar Mesophase Induction in Nondiscoid Pyrazoles by Complexation to Rhodium(I)

Several new cis-[RhCl(CO)2(Ln)] complexes have been prepared using different polycatenar pyrazole ligands (Ln)
in order to obtain columnar liquid crystalline arrangements. The topology of the ligand plays an essential role, and
a mesophase is induced at room temperature from a nonmesogenic pyrazole only when it is symmetrically substituted
with six decyloxy chains. The single-crystal structure of a methoxy-substituted analogue, 3,5-bis(3,4,5-trimethoxyphenyl)pyrazole, is formed by globular tetrameric structures held together by H-bonding. However, parallel
dimers are present in the corresponding cis-chlorodicarbonylrhodium(I) complex, a situation that explains the induction
of a columnar mesophase in the decyloxy-substituted complex. The XRD pattern of the mesophase is consistent
with a hexagonal symmetry in which the columns are formed by molecules assembled in an antiparallel mode. The
crystal-to-mesophase transition was also detected by spectroscopic techniques as a shift in the IR carbonyl stretching
bands and the appearance of a charge-transfer band in the absorption spectrum with thermochromic behavior.