Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

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 (1)H-NMR and (13)C-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.