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

摘要

Reactions of a range of the readily prepared and sterically tunable N,N'-bis(aryl)formamidmes with lan-thanoid metals and bis(pentafluorophe-nyl)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-Tol-Form)_3] by this method invariably yielded [{Yb(oTolForm)_2(mu-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 anda gradation in coordination number with Ln~(3+) 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 eta~1-pi-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 functional-ised 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 andNd) and 3,4,5-F3C6H2O(CH2)4Dipp-Form. 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 organometalhc species [Ln-(DippForm)_2(C ident to CPh)(thf)] (Ln = Nd or Sm) are obtained from reaction of Nd metal, bis(phenylethynyl)mercury (Hg(C ident to CPh)_2) and DippFormH, and the oxidation of [Sm(DippForm)_2(thf)_2] with Hg(C ident to CPh)_2, respectively. The monomeric, six-coordinate, cisoid [Ln-(DippForm)_2(C ident to CPh)(thf)] complexes have trigonal prismatic geometries and rare (for Ln) terminal C ident to CPh groups with contrasting Ln-C ident to 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.