Steric Modulation of Coordination Number and Reactivity in the Synthesis of Lanthanoid(III) Formamidinates

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Abstract
Reactions of a range of the readily prepared and sterically tunable N,N′‐bis(aryl)formamidines with lanthanoid metals and bis(pentafluorophenyl)mercury (Hg(C6F5)2) in THF have given an extensive series of tris(formamidinato)lanthanoid(III) complexes, [Ln(Form)3(thf)n], namely [La(o‐TolForm)3(thf)2], [Er(o‐TolForm)3(thf)], [La(XylForm)3(thf)], [Sm(XylForm)3], [Ln(MesForm)3] (Ln=La, Nd, Sm and Yb), [Ln(EtForm)3] (Ln=La, Nd, Sm, Ho and Yb), and [Ln(o‐PhPhForm)3] (Ln=La, Nd, Sm and Er). [For an explanation of the N,N′‐bis(aryl)formamidinate abbreviations used see Scheme 1.] Analogous attempts to prepare [Yb(o‐TolForm)3] by this method invariably yielded [{Yb(o‐TolForm)2(μ‐OH)(thf)}2], but [Yb(o‐TolForm)3] was isolated from a metathesis synthesis. X‐ray crystal structures show exclusively N,N′‐chelation of the Form ligands and a gradation in coordination number with Ln3+ size and with Form ligand bulk. The largest ligands, MesForm, EtForm and o‐PhPhForm give solely homoleptic complexes, the first two being six‐coordinate, the last having an η1‐π‐ArLn interaction. Reaction of lanthanoid elements and Hg(C6F5)2 with the still bulkier DippFormH in THF resulted in CF activation and formation of [Ln(DippForm)2F(thf)] (Ln=La, Ce, Nd, Sm and Tm) complexes, and o‐HC6F4O(CH2)4DippForm in which the formamidine is functionalised by a ring‐opened THF that has trapped tetrafluorobenzyne. Analogous reactions between Ln metals, Hg(o‐HC6F4)2 and DippFormH yielded [Ln(DippForm)2F(thf)] (Ln=La, Sm and Nd) and 3,4,5‐F3C6H2O(CH2)4DippForm. X‐ray crystal structures of the heteroleptic fluorides show six‐coordinate monomers with two chelating DippForm ligands and cisoid fluoride and THF ligands in a trigonal prismatic array. The organometallic species [Ln(DippForm)2(CCPh)(thf)] (Ln=Nd or Sm) are obtained from reaction of Nd metal, bis(phenylethynyl)mercury (Hg(CCPh)2) and DippFormH, and the oxidation of [Sm(DippForm)2(thf)2] with Hg(CCPh)2, respectively. The monomeric, six‐coordinate, cisoid [Ln(DippForm)2(CCPh)(thf)] complexes have trigonal prismatic geometries and rare (for Ln) terminal CCPh groups with contrasting LnCC angles (Ln=Nd, 170.9(4)°; Ln=Sm, 142.9(7)°). Their formation lends support to the view that [Ln(DippForm)2F(thf)] complexes arise from oxidative formation and CF activation of [Ln(DippForm)2(C6F5)] intermediates.

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