Structural questions on crystalline, columnar and cubic phases of mesogenic bimetallic carboxylates answered by single-crystal crystallography

MARIA ANA CASTRO; MARCIA RUSJAN; RICARDO BAGGIO; DANIEL VEGA; OSCAR PIRO; ANDRES ZELCER; ZULEMA D. CHAIA; ROSANA L. GOMEZ; AUGUSTO FAGNOLA; MA. ANGELES PERSELLO; BENOIT HEINRICH; BERTRAND DONNIO; DANIEL GUILLON; FABIO D. CUKIERNIK

Lake Arrowhead, California, EEUU

Simposio; 9th International Symposium on Metallomesogens; 2005

Syngle crystal crystallography of mesogens or their non-mesogenic analogs have been used in the past in order to provide structural information useful for suggesting suitable models of molecular structure and intermolecular organization in the mesophase. These studies allowed to know molecular lengths (correlated to characteristic distances in N, S or D mesophases), molecular conformations (including monomeric vs dimeric alternatives), interatomic distances and angles (used sometimes as reference for EXAFS studies), etc. We report herein three recent crystallographic studies we performed on bimetallic carboxylates, and explain the way these results helped us to understand some structural features of such mesogenic compounds, either by validating the models previously suggested or by showing completely different pictures.Case 1) Coordination polymers based on diruthenium tri(alkoxy)benzoates of general forumla [Ru2(3,4,5‑(CnH2n+1O)3C6H2CO2)4Cl]x exhibit ColH mesophases even below room temperature. We proposed a model aimed to describe the supramoleclar organization in the mesophase based on information coming from both local probes (RR, SQUID, EXAFS, IR) and ?macro-structural? dtereminations (XRD, dilatometry). One of its central aspects is the ?zig-zag? conformation of the polymeric ...Ru2-Cl-Ru2-Cl-... strands, with a Ru-Cl-Ru angle esatimated as 112 ? 120º. We succeeded now to crystallize and solve the structure of the n = 2 homologue (non mesogenic); it confirms the zig-zag nature of the polymeric strands, and exhibits a Ru-Cl-Ru angle of 116º.Case 2) Compounds of general formula [Ru2(O2C(CH2)n-2CH3)4Y]x  (Y- = long chain axial anion: octylsulfonate, dodecylsulfate or aliphatic carboxylate) exhibit lamellar crystalline phases where the bimetallic centers are separated by two layers of aliphatic chains. Interlamellar distances, as measured by powder XRD experiments, suggest some degree of interpenetration of these two layers for all the cases in which the lenghts of equatorial carboxylates and axial anions differ, and no interpenetration when their lengths are similar The crystalline structure of a mid-chain derivative confirmed this hypothesis.Case 3) Long-chain copper tri(alkxoy)benzoates exhibited columnar or cubic mesophases, depending upon the aliphatic chain length (and thermal history!). Some controversies took place in the past about the micellar or bicontinuous nature of the cubic phase. The model we suggested a few years ago, based on some magnetic evidence, involved the formation of mononuclear species, geometrically similar to the well known phasmids. However, the crystalline structure of the non-mesogenic n = 2 homolog revealed the presence of completely unexpected hexameric units of formula [Cu6(O2CC6H2(OC2H5)3)12] and globular geometry, supporting a micellar model for the cubic mesophases of long chain derivatives.