SOLID-STATE NMR AND X-RAY-DIFFRACTION STUDIES OF STRUCTURE AND MOLECULAR-MOTION IN ANSA-TRITANOCENES

The structures and dynamics of a family of crystalline ansa-metallocen es, [(C5H4)(2)-C(2)Me(4)]TiX(2) (X = F (1), Cl (2), Br (3), and I (4)) , have been elucidated by the joint application of solid state NMR and single crystal X-ray diffraction techniques. X-ray crystal structures at ambient temperature indicate a single molecule with noncrystallogr aphic 2-fold symmetry in the asymmetric unit of 1. The structure of 2 comprises two distinct layers which are each formed from only one of t he two crystallographically distinct molecules found in the asymmetric unit. These two molecules have the same conformation, but do not have C-2 point symmetry. The structure of 3 has four molecules per asymmet ric unit; one has axial symmetry, but the others deviate from this to a greater extent than do the molecules of 2. A layer structure with br omine atoms at the layer surfaces is discernible in 3, but in contrast to 2 each layer contains all of the crystallographically distinct mol ecules. The structure of 4 has a single molecule in its asymmetric uni t, of similar nonaxial conformation to those of 2. The most noticeable feature is channels of iodine atoms parallel to the c-axis. The eight distinct molecules of these structures are placed into three conforma tional categories based on their deviation from axial symmetry. The su rprisingly different structures for different halogens appear to be du e to increasing halogen-halogen interaction, I > Br > Cl > F, in the p referred intermolecular contacts. C-13 CP/MAS NMR spectra in the tempe rature range 200-380 K show resonance multiplicities consistent with t he crystallographically determined asymmetric units. Spectra for 2, 3, and 4 show significant changes with temperature, indicating the occur rence of activated exchange processes. 1D and 2D magnetization transfe r experiments below 300 K and line shape simulations of exchange-broad ening phenomena above 300 K indicate that for 4 and for one of the mol ecules in 2 a pairwise exchange process occurs. The other molecule in 2 remains rigid, indicating that packing interactions between the laye rs of the structure of 2 do not influence the barriers to the dynamic process. The measured Arrhenius activation barriers, E(a) = 87 kJ mol( -1) for 4 and 86 kT mol(-1) for the dynamic molecule of 2, are remarka bly similar given the dissimilarity of the molecular packing in crysta ls of 2 and 4. All of the molecules of 3 show dynamic behavior; the re sonances of one molecule are broadened even at 200 K, those for two of the others are in. the coalescence regime at ca, 250 K, and the reson ances of the fourth only broaden above 300 K; H-2 NMR quadrupole echo line shapes of a perdeuterated sample of 4 indicate only small changes between 296 and 480 K. The X-ray structures of all four compounds sho w no suggestion of disorder, the atomic displacement parameters are un remarkable, all the electron density is accounted for by the structure solutions, and there is no significant diffuse scatter in the X-ray p hotographs. The joint interpretation of the NMR and X-ray data indicat es that the dynamic processes detected for 2-4 by NMR are not the expe cted enantiomeric exchange process, which is observed to be facile in solution. Both the NMR and X-ray data are, however; consistent with a hypothesis that a 180 degrees reorientation about the pseudo-C-2 axis, bisecting the X-Ti-X interbond angle, occurs in the crystal concomita nt with a small polytopal relaxation about the metal center. This proc ess averages carbon atoms in a pairwise manner, but leads to no atomic positional disorder, and it averages the H-2 efg and C-13 CSA tensors only slightly. This motion constitutes an interesting dynamic mode of a molecular crystal, which is intermediate between the well establish ed examples of the effectively isotropic overall molecular reorientati onal disorder characteristic of a plastically crystalline phase and th e disorder or fluxionality of specific groups or moieties within a mol ecule. Crystal data: (1) triclinic, P1, a = 11.934(2), b = 8.5057(7), and c = 7.718(1) Angstrom, alpha = 108.091(1), beta = 110.71(1), and g amma = 90.916(9)degrees, Z = 2, R = 0.033, R(w) = 0.039 for 253 parame ters and 2488 reflections I > 3 sigma I. (2) monoclinic, P2(1)/a, a 13 .014(2), b = 18.129(2), and c = 14.547(1) Angstrom, beta = 2 116.416(8 )degrees, Z = 8, R = 0.046, R(w) = 0.061 for 344 parameters, and 4635 reflections I > 3 sigma I. (3) triclinic, P ($) over bar 1, a = 13.597 (2), b = 14.406(2), and c = 20.101(2) Angstrom, alpha = 124.97(1), bet a = 91.46(1), and gamma = 93.37(1)degrees, Z = 8, R = 0.039, R(w) = 0. 042 for 687 parameters and 5141 reflections I > 3 sigma I. (4) rhomboh edral (hexagonal axes), R3c, a = 24.608(7) and c = 14.741(1) Angstrom. Z = 18, R = 0.032, R(w) = 0.035 for 172 parameters and 765 reflection s I > 3 sigma I.