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)CH[double bond, length as m-dash]N(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)CH[double bond, length as m-dash]N(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.