Controlled Hydrolysis of Lanthanide Complexes of the N-Donor Tripod Tris(2-pyridylmethyl)amine versus Bisligand Complex Formation

The reaction of the lanthanide salts LnI3(thf)4 and Ln(OTf)3 with tris(2-pyridylmethyl)amine (tpa) was studied in
rigorously anhydrous conditions and in the presence of water. Under rigorously anhydrous conditions the successive
formation of mono- and bis(tpa) complexes was observed on addition of 1 and 2 equiv of ligand, respectively.
Addition of a third ligand equivalent did not yield additional complexes. The mono(tpa) complex [Ce(tpa)I3] (1) and
the bis(tpa) complexes [Ln(tpa)2]X3 (X = I, Ln = La(III) (2), Ln = Ce(III) (3), Ln = Nd(III) (4), Ln = Lu(III) (5); X
= OTf, Ln = Eu(III) (6)) were isolated under rigorously anhydrous conditions and their solid-state and solution
structures determined. In the presence of water, 1H NMR spectroscopy and ES-MS show that the successive
addition of 1−3 equiv of tpa to triflate or iodide salts of the lanthanides results in the formation of mono(tpa) aqua
complexes followed by formation of protonated tpa and hydroxo complexes. The solid-state structures of the complexes
[Eu(tpa)(H2O)2(OTf)3] (7), [Eu(tpa)(μ-OH)(OTf)2]2 (8), and [Ce(tpa)(μ-OH)(MeCN)(H2O)]2I4 (9) have been determined.
The reaction of the bis(tpa) lanthanide complexes with stoichiometric amounts of water yields a facile synthetic
route to a family of discrete dimeric hydroxide-bridged lanthanide complexes prepared in a controlled manner. The
suggested mechanism for this reaction involves the displacement of one tpa ligand by two water molecules to form
the mono(tpa) complex, which subsequently reacts with the noncoordinated tpa to form the dimeric hydroxo species.