The duration of the saturation pulse in the 1 H NMR CEST approach needs to be optimized experimentally. Moreover, the dependence of CEST signal intensity on the saturation pulse length can be used to obtain an approximate values of the hydrogen exchange rates active in the system under investigation. Figure shows the CEST reduction of the free H2 line intensity (normalized to the value without irradiation) for irradiation applied on the resonances of complexes 1 (-22.7 ppm), 2 (-23.5 or -25.5 ppm), and 3 (-27ppm) at a temperature of 280 K. For the spectra reported in the main text we choose a duration of 3 seconds for time efficiency (relative saturation value is 85-95%). In order to obtain an approximated values of the exchange rates in the system, we used a simple two-site exchange model. In this case, the evolution of the magnetization of the free H2 pool can be described by a simple differential equation: