Full Conformational Analyses of the Ultrafast Isomerization in Penta-coordinated Ru(S₂C₂(CF₃)₂)(CO)(PPh₃)₂: One Compound, Two Crystal Structures, Three CO Frequencies, 24 Stereoisomers, and 48 Transition States

Abstract

The stereodynamics of an ultrafast (picosecond) isomerization in a penta-coordinated ruthenium complex, Ru­(S2C2(CF3)2)­(CO)­(PPh3)2, were characterized by density functional theory (DFT). The ruthenium complex crystallizes in two almost-square pyramidal (SP) forms. The violet form has an apical PPh3 ligand, the orange form has an apical CO ligand, and their solution displays three CO stretching frequencies. With 4 possible centers of chirality (1 ruthenium, 2 phosphines, and 1 dithiolate), there are 24 stereoisomers. DFT calculations of these stereoisomers show structures ranging from almost-perfect SP (τ5 ≈ 0) to structures significantly distorted toward trigonal bipyramidal (TBP) (τ5 ≈ 0.6). The stereoisomers fall neatly into three groups, with νCO ≈ 1960 cm–1, 1940 cm–1, and 1980 cm–1. These isomers were found to interconvert over relatively small barriers via Ru–S bond twisting, CF3 rotation, phenyl twisting, PPh3 rotation, and, in some cases, by coupled motions. The composite energy surface for each CO frequency group shows that interconversions among the low-energy structures are possible via both the direct and indirect pathways, while the indirect pathway via isomers in the νCO ≈ 1980 cm–1 group is more favorable, which is a result consistent with recent experimental work. This work provides the first complete mechanistic picture of the ultrafast isomerization of penta-coordinated, distorted SP, d6-transition-metal complexes.