59Co-NMR spectroscopy of the tetrahedral cluster Co4(CO)12: relaxation and exchange

Selective 59Co-NMR inversion–recovery experiments combined with non-selective experiments on a solution of the cluster Co4(CO)12 at various temperatures (from 220 to 330 K) yield information about 59Co relaxation data and carbonyl ligand exchange. Relaxation data characterise the motional behaviour of Co4(CO)12 in solution, but also provide the value of the quadrupole coupling constant of the basal cobalt nucleus (χ=100 MHz) and thus complete previous solid state NMR results. Simulations based on relaxation data confirm that the difference between the two quadrupole coupling constants accounts for the anomalous features observed in 13C-NMR spectra via 59Co13C scalar coupling. As far as the ligand exchange is concerned, the measurements show that all four metal centres are involved, which implies that carbonyl scrambling is complete. Experimental and simulated 2D-EXSY 59Co-NMR spectra corroborate this result. Rate constants and activation parameters (ΔG≠(298 K)=50.4±2.0 kJ mol−1; ΔH≠=46.5±2.5 kJ mol−1; ΔS≠=−13±9 J K−1 mol−1) are found in the range of values reported for other tetranuclear carbonyl clusters.