DYNAMIC PROCESSES IN THE SOLID-STATE - PROTON RELAXATION STUDIES AND POTENTIAL-ENERGY BARRIER CALCULATIONS FOR (ARENE)M(CO)3 SPECIES - X-RAY CRYSTAL-STRUCTURES OF (1,2,3-C6H3ME3)CR(CO)3 AND (1,2,4,5-C6H2ME4)CR(CO)3

The dynamic behavior in the solid state of (C6Me6)Cr(CO)3 (1), (1,2,3-C6H3Me3)Cr(CO)3 (2), and (1,2,4,5-C6H2Me4)Cr(CO)3 (3) has been investigated by means of variable-temperature H-1 spin-lattice relaxation time T1 measurements and potential energy barrier calculations. Structural characterization of 2 and 3 at room temperature by single-crystal X-ray diffraction has been carried out. Crystal data: 2, space group P2(1)/n, a = 7.289 (4) angstrom, b = 13.114 (4) angstrom, c = 12.526 (3) angstrom, beta = 97.64 (4)-degrees, V = 1186.7 angstrom-3, Z = 4; 3, space group P2(1)/n, a = 8.985 (1) angstrom, b = 12.527 (2) angstrom, c = 9.011 (1) angstrom, beta = 118.22 (1)-degrees, V = 1275.7 angstrom-3, Z = 4. It is found that, in all cases, the lowest energy dynamic process involves rotation of the methyl groups (E(a) = 2-6 kJ/mol). Facile reorientation of the C6Me6 fragment in 1 is also detected (E(a) = 25.9 kJ/mol) in good agreement with the results of potential energy barrier calculations. For 2 and 3, it is shown that reorientation, at room temperature, of the arene fragments is forbidden by the crystal packing, while large amplitude motions appear to be responsible for the additional modulation detected in the log T1 versus 10(3)/T plots. The drastic reduction of T1 observed for 2 and 3, as the temperature is increased to ca. 333 K, is interpreted in terms of an order/disorder phase transition associated with full rotational freedom of the arenes or of the entire molecules. Support for this observation comes from the behavior of the diffraction patterns of the two species with temperature and from DSC measurements.