Slow Evaporation of Water from Hydrated Salen Transition Metal Complexes in the Gas Phase Reveals Details of Metal Ligand Interactions

Abstract

Water clusters of salen [N,N‘-ethylenebis(salicylideneaminato)] transition metal complexes [(salen)M, M = Cr^(3+), Mn^(3+), Co^(3+)] formed by electrospray source have been investigated using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Kinetics of water evaporation from the cluster ions is observed to be highly dependent on the central metal ion. For example, the evaporation rate of water from solvated salen chromium ion ([SCr + nH_2O]^+) is significantly slower than that from solvated salen manganese ([SMn + nH_2O]+) and solvated salen cobalt ([SCo + nH_2O]^+) ions. Furthermore, the clusters of salen chromium ions with two waters attached exhibit special stability, indicated by their prominence in the overall cluster distribution. In contrast, no specific solvation is observed for the manganese and cobalt complexes. The lability observed for the hydrated salen cobalt complex suggests that the high-spin state is likely to be involved in the evaporation process. These results are in accordance with observations in solution-phase chemistry and can be explained by ligand field theory.