Abstract
The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)3) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)3 (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)3 were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)3 complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)3 were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)3. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol−1. Notably, PBE0/BS1 describes the (CO)3 rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)3 complexes (except for (TMCOT)W(CO)3), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)3 complex (computed with the PBE0 functional) has a ΔG‡ of 12.6, 12.8, and 13.2 kcal mol−1 for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed 1H-NMR and 13C-NMR chemical shifts for (TMCOT)Cr(CO)3 and (TMCOT)Mo(CO)3 at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures.
Highlights
Fluxional molecules are dynamic compounds in which magnetically or chemically distinct groups can readily interchange positions
TS-2 (1,3-shift) is a Cs-symmetric complex where the considered in the fluxional processes of (COT) ligand is η4-bound to Cr
Good agreement was observed in the relative ∆Ee‡ values computed using PBE0/BS1 and those derived using consistent Composite Approach for transition metals (ccCA-TM)
Summary
Fluxional molecules are dynamic compounds in which magnetically or chemically distinct groups can readily interchange positions. The stereochemically fluid nature of such molecules at room temperature is illustrated by the fluxional shifts that they undergo [1–3]. These systems perform a significant role in increasing enantioselectivity in asymmetric synthesis [4–10]. CpRu((R)-BINOP-F)(H2O)][SbF6] has been used as a catalyst in the Diels– Alder reaction of methacrolein and cyclopentadiene to produce a [4+2] cycloadduct with enantioselectivity of 92% ee (exo) [9]. During this reaction, the catalyst was shown to exhibit a fluxional pendular motion of the BINOP-F ligand, thereby creating chemically equivalent environments about the two phosphorus substituents. Previous studies incorporating DFT on fluxional systems range from biochemical applications, such as Cu (II)· · · GlyHisLys peptide
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
