MULTIPLE BONDS BETWEEN METAL ATOMS IN ORDERED ASSEMBLIES .2. QUADRUPLE BONDS IN THE MESOMORPHIC STATE

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 investigat ed. 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 throu gh the agency of intermolecular M...O bonds. For M = W and R = n-Pr, t he 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, be ta = 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 liqu id phase transition or decompose. The Mo-2(O2C(CH2)(n)CH3)(4) compound s have crystalline, liquid crystalline (mesomorphic), and isotropic li quid phases when n = 3-9. The related Cr-2(O2C(CH2)(n)CH3)(4) compound s show crystal-to-liquid crystal transitions but do not clear to isotr opic liquids before decomposing at around 300 OC. The different thermo tropic 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 intro duction of perfluorinated n-alkyl or para-n-alkyl or -alkoxy C6H4 grou ps raises the crystal-to-liquid crystal transition temperature. The ph ase changes (for several compounds) are reversible (enantiotropic) and have been characterized by differential scanning calorimetry. The mes ophases of the Mo-2(O(2)CR)(4) compounds reveal fan-shaped birefringen t textures characteristic of discotic hexagonal disordered (D-hd) liqu id crystalline arrangements. Variable temperature X-ray diffraction (X RD) 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 repe at unit of the polymer [Mo-2(O(2)CR)(4)](infinity). In the mesophase t he 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-sp acing 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 correspond ing to 4.7 Angstrom was seen, which is consistent with the expected av erage layer spacing along the columnar axis. An estimation of the cohe rence length within a column is ca. 30 Angstrom, and thus the XRD patt ern of the mesophase confirms the assignment D-hd. C-13 NMR studies em ploying Mo-2((O2C)-C-13(CH2)(6)CH3)(4) allowed the determination of th e chemical shift principal tensor elements of the carboxylate carbon b y 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 t hat 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.