Widely contrasting outcomes from the use of tris(pentafluorophenyl)bismuth or pentafluorophenylsilver as oxidants in the reactions of lanthanoid metals with N,N′-diarylformamidines

Abstract

Reactions of lanthanoid metals with tris(pentafluorophenyl)bismuth or pentafluorophenylsilver and two widely disparate formamidines, N,N′-bis(2,6-difluorophenyl)formamidine (DFFormH) and N,N′-bis(2,6-diisopropylphenyl)formamidine (DippFormH) have been investigated as possible redox transmetallation/protolysis (RTP) syntheses of lanthanoid formamidinates. Thus, [Ln(DFForm)₃(thf)] (Ln = Lu, 1, Yb, 2, Tm, 3, Er, 4, Ho, 5, Dy, 6; thf = tetrahydrofuran), [Ln(DFForm)₃(thf)₂] (Ln = Tb, 7, Gd, 8, Sm, 9, Nd, 10), and [Yb(DippForm)₂(thf)₂]·2thf (11) complexes were obtained from an excess of lanthanoid metals, [Bi(C₆F₅)₃]·0.5diox (diox = 1,4-dioxane) and the appropriate formamidine. Reaction of neodymium and [Bi(C₆F₅)₃]·0.5diox with the bulkier DippFormH in thf resulted in C–F activation and formation of [Nd(DippForm)₂F(thf)₂] (12) and o-HC₆F₄O(CH₂)₄N(Dipp)CHN(Dipp) (Dipp = 2,6-di-isopropylphenyl). Although the reaction of erbium and [Bi(C₆F₅)₃]·0.5diox with DippFormH was not complete after one week (by ¹⁹F NMR), use of [Hg(C₆F₅)₂] instead of [Bi(C₆F₅)₃]·0.5diox resulted in C–F activation and formation of [Er(DippForm)₂F(thf)] (13) and o-HC₆F₄O(CH₂)₄N(Dipp)CHN(Dipp) in two days. Attempted RTP reactions between lanthanoid metals, AgC₆F₅ and DFFormH in thf gave [Ag₂(DFForm)₂]·3thf (14), except in the case of ytterbium. All other lanthanoid metals required activation by I₂ (2%) before lanthanoid complex formation could be achieved. From these reactions, lanthanoid formamidinates, [Ln(DFForm)₂(solv)I] (Ln = Lu, 15, Tm, 16, Er, 17, Tb, 18, Gd, 19), [Ln(DFForm)₃(py)] (Ln = Er, 17a, Tb, 18a; py = pyridine), and [Nd(DFForm)₃(thf)₂]·thf (10) were obtained. Where two types of lanthanoid formamidinates [Ln(DFForm)₃(py)] and [Ln(DFForm)₂(solv)I] were obtained from the Ln metals activated by I₂, they could not be separated on preparative scale. From Yb, the peroxide-bridged complex, [Yb₂(DippForm)₂(O₂)₂(py)₄] (20) was surprisingly isolated. Thus [Bi(C₆F₅)₃]·0.5diox has potential as an oxidative replacement for diarylmercurials in RTP syntheses of lanthanoid formamidinates but [AgC₆F₅(py)] does not.