Ligand exchange processes in transition metal alkane and alkene complexes

When (isopropylcyclopentadienyl)rhenium tricarbonyl [(i-PrCp)Re(CO)3], i-Pr = isopropyl] is photolysed in pentane at low temperature in situ in an NMR probe, three short half-life isomers of i-PrCpRe(CO)2(pentane) were yielded as described previously. The decay rate and half-life of the isomers were measured in kinetics experiments. The chemical shifts of the i-PrCp part and pentane part of three isomers were assigned with TOCSY NMR experiments with different mixing times. Exchange processes and rates among the isomers were measured with EXSY NMR at 160 – 176 K. Possible intramolecular exchanges among pentane parts of isomers include [1,2], [1,3], [1,4] and [1,5] shifts. [1,3] shifts were found to be the fastest exchange process and rates were measured with EXSY NMR occurring with a rate of 2.8 - 4.0 s-1 at 176 K. [1,2] shifts were also observed to be occurring at an approximately 20 times slower rate and difficult to quantify. [1,4] and [1,5] shifts may be occurring slowly but could not be quantified. The lowest free energy of activation barrier for a [1,3] shift was ΔG++ = 40.3 kJ/mol, calculated with the Eyring equation. The soluble final product (i-PrCpRe)2(CO)5 was observed in NMR spectra; the minor insoluble final product (i-PrCpRe)(CO)2O(i- PrCpRe)(CO)2 was identified with mass spectrometry. Cyclohepta-1,3,5-triene molybdenum tricarbonyl (η6-C7H8)Mo(CO)3 undergoes a fluxional process. There are two protons on carbon 7 of the cyclohepta-1,3,5-triene ring ; One is Hexo, the other is Hendo. Analysis of the pattern of exchanging protons in EXSY NMR experiments at +150 °C showed directly that an intramolecular [1,5] shift of Hendo is occurring at a rate of 0.0139 ± 0.054 1/s. Exchange peaks and NOE peaks were confirmed with EXSY experiments at variable temperature, 120 -150 °C. The barrier to exchange from the EXSY NMR spectra (ΔG++ = 28.7 ± 0.4 kcal/mol) is similar to the barrier calculated using DFT. An intermolecular exchange between (η6-C7H8)Mo(CO)3 and free cycloheptatriene was also observed