Solid State Energy Barriers to Olefin Ligand Rotation of Rhodium Compounds and Single Crystal X-Ray Structure Determination of Synthetic and Natural Products.

Abstract

Atomic displacement parameters (ADPs) were derived from X-Ray diffraction data collected at two different temperatures (294K and 115K) from a single crystal of Rh(acac)(C\rm\sb2H\sb4)\sb2. Since the thermal ellipsoids of atoms in the one crystallographically independent ethylene group displayed distinctive elongation normal to the C=C bond, a TLS rigid body analysis of the \rm C\sb2H\sb4 ligand rotation was undertaken. Using a locally modified version of Trueblood\u27s THMA14 computer program, it was found that at 115K the root-mean-square amplitude of libration normal to the Rh-(CC centroid) axis is 7(4)\sp\circ, while at 294K it is 10(6)\sp\circ (estimated standard deviations in parentheses). The rotational barrier height of the ethylene group was found to be the same at both temperatures (39(10) kJ/mol at 115K, 39(12) kJ/mol at 294K), slightly below but statistically consistent with a barrier height of 54 kJ/mol (at 265K) derived from solid state NMR analysis of Rh(acac)(\rm C\sb2H\sb4)\sb2. In a related rhodium-ethylene complex, (Rh-$\mu$-Cl-(\rm C\sb2H\sb4)\sb2\rbrack\sb2, TLS analysis showed that the ethylene groups are highly hindered rotors and that the conditions for TLS analysis are not met. In addition, eighteen single crystal structures of synthetic and natural compounds were analyzed. (Abstract shortened by UMI.)