Lanthanide Complexes of the Kläui Metalloligand, CpCo(PO(OR)2)3: An Examination of Ligand Exchange Kinetics between Isotopomers by Electrospray Mass Spectrometry

A series of lanthanide complexes, {[CpCo­(PO­(OR)2)3]2Ln­(H2O)x}+Cl– (Ln = Nd, 3; Eu, 4; Tb, 5; Yb, 6; R = Et, a; R = Ph, b) bearing two cobalt metalloligands were prepared. Electrospray mass spectrometry and thermogravimetric analysis suggest that the cations are either solvent-free or contain very weakly bound water molecules. The related complex {[CpCo­(PO­(OPh)2)3]2Yb}+ [CoCl3(THF)]−, 7, was crystallographically characterized, and the cation in this case was confirmed to be 6-coordinate and solvent-free. Ligand exchange rates between the d0- and d60-isotopomers of 3a–6a and 5b were determined in acetonitrile by electrospray mass spectrometry. The ligand exchange rate was found to increase by almost 4 orders of magnitude from the smallest (Yb, 6a, k = 0.3 M–1 s–1) to largest ion (Nd, 3a, >2500 M–1 s–1) in acetonitrile. Additionally, the ligand exchange rate increased rapidly for 5a (Tb) with increasing water concentration from 30 M–1 s–1 in pure acetonitrile to 268 M–1 s–1 in 50:50 (v/v) acetonitrile/water. Changing the phosphite substituent had no significant impact on the rate of ligand exchange for 5b (R = Ph) relative to 5a (R = Et).