Various compounds of formula M(2)(O(2)CR)(4), where M = Cr, Mo, and W, have been synthesized, and their thermal behavior has been investigated. In the solid state they share a common laddered structure wherein one M(2)(O-2-CR)(4) unit is weakly associated with its neighbors through the agency of intermolecular M...O bonds. For M = W and R = n-Pr, the cell parameters at -171 degrees C are a = 8.867(9) Angstrom, b = 10.934(14) Angstrom, c = 5.563(6) Angstrom, alpha = 100.01(4)degrees, beta = 108.02(3)degrees, gamma = 79.58(4)degrees, Z = 1, d(calcd) = 2.38 g cm(-3), and space group P (1) over bar. Upon heating, the tungsten-containing compounds either undergo a simple crystal-to-isotropic liquid phase transition or decompose. The Mo-2(O2C(CH2)(n)CH3)(4) compounds have crystalline, liquid crystalline (mesomorphic), and isotropic liquid phases when n = 3-9. The related Cr-2(O2C(CH2)(n)CH3)(4) compounds show crystal-to-liquid crystal transitions but do not clear to isotropic liquids before decomposing at around 300 OC. The different thermotropic behavior of the Cr-, Mo-, and W-containing compounds is traced to the relative strength of the M...O intermolecular interactions that are preserved in the mesophase. The introduction of a branched chain lowers the crystal-to-liquid crystal phase transition, while the introduction of perfluorinated n-alkyl or para-n-alkyl or -alkoxy C6H4 groups raises the crystal-to-liquid crystal transition temperature. The phase changes (for several compounds) are reversible (enantiotropic) and have been characterized by differential scanning calorimetry. The mesophases of the Mo-2(O(2)CR)(4) compounds reveal fan-shaped birefringent textures characteristic of discotic hexagonal disordered (D-hd) liquid crystalline arrangements. Variable temperature X-ray diffraction (XRD) studies were performed on Mo-2(O2C(CH2)(n)CR)(4) for n = 3, 4, 6, and 7. The polycrystalline XRD patterns revealed triclinic cells with very similar values of c = ca. 5.6 Angstrom, corresponding to the repeat unit of the polymer [Mo-2(O(2)CR)(4)](infinity). In the mesophase the XRD patterns are dominated by a relatively intense peak at an angle below 10 degrees in 2 theta (Cu K alpha). With increasing n, the d-spacing corresponding to this peak increases and is in all cases between the a and b lattice constants. For n = 6, two very much weaker peaks were found at 8.15 and 7.05 Angstrom, corresponding to 1/root 3 and 1/2 of the main peak at 14.1 Angstrom. These peaks confirm the hexagonal packing of the discotic columns. In addition, a broad band corresponding to 4.7 Angstrom was seen, which is consistent with the expected average layer spacing along the columnar axis. An estimation of the coherence length within a column is ca. 30 Angstrom, and thus the XRD pattern of the mesophase confirms the assignment D-hd. C-13 NMR studies employing Mo-2((O2C)-C-13(CH2)(6)CH3)(4) allowed the determination of the chemical shift principal tensor elements of the carboxylate carbon by use of cross-polarization followed by high-powered proton decoupling. Comparison of the C-13 NMR spectra obtained for the isotropic liquid, the liquid crystalline phase, and the polycrystalline sample shows that the Mo-Mo axis is aligned perpendicularly to the magnetic field in the mesophase, consistent with the known large negative anisotropy of the magnetic susceptibility associated with the Mo-4-Ma bond.
