Accessing homoleptic neutral and anionic five-coordinate Pr(iv) siloxide complexes

Abstract

Structurally characterized Pr(IV) complexes are limited to four examples because the ligands and reaction conditions capable of stabilizing Pr(IV) remain elusive. Here we identify reaction conditions allowing the synthesis of Pr(IV) complexes that were originally thought difficult to isolate. The Pr(IV) complexes of the tris(tert-butoxy)siloxide (–OSi(O^tBu)₃) and triphenylsiloxide (–OSiPh₃) ligands, [Pr^IV(OSi(OtBu)₃)₄] (2-Pr^OtBu), [MPr^IV(OSiPh₃)₅] (5M-Pr^Ph) (M = K, Cs), and [KDB18C6][Pr^IV(OSiPh₃)₅], (5[KDB18C6-Pr^Ph]) were isolated and fully characterized upon the oxidation of the tetrakis and pentakis(siloxide)praseodymium(III) ate complexes, [KPr^III(OSi(OtBu)₃)₄] (1-Pr^OtBu) and [M₂Pr^III(OSiPh₃)₅] (4M-Pr^Ph) (M = K, Cs), using the thianthrene radical cation tetrafluoroborate oxidant, thiaBF₄. The crucial role of reagents and reaction conditions, like thiaBF₄ over the magic blue oxidant and non-coordinating over coordinating solvents, are demonstrated for the isolation of high valent Pr(IV) complexes. The solid state structural and electrochemical properties were studied and further augmented with theoretical calculations. The Pr(IV) oxidation state was further confirmed by electron paramagnetic resonance (EPR) and SQUID magnetometry measurements. Complexes 5M-Pr^Ph and 5[KDB18C6]-Pr^Ph provide the first example of anionic Ln(IV) complexes demonstrating the possibility of accessing charged Pr(IV) complexes as a tool to manipulate the redox potential and therefore access to more stable complexes with the same ligand.