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

Reactions of a range of the readily prepared and sterically tunable N,N ′‐bis(aryl)formamidines with lanthanoid metals and bis(pentafluorophenyl)mercury (Hg(C 6 F 5 ) 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 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 η 1 ‐π‐ArLn interaction. Reaction of lanthanoid elements and Hg(C 6 F 5 ) 2 with the still bulkier DippFormH in THF resulted in CF activation and formation of [Ln(DippForm) 2 F(thf)] (Ln=La, Ce, Nd, Sm and Tm) complexes, and o ‐HC 6 F 4 O(CH 2 ) 4 DippForm in which the formamidine is functionalised by a ring‐opened THF that has trapped tetrafluorobenzyne. Analogous reactions between Ln metals, Hg( o ‐HC 6 F 4 ) 2 and DippFormH yielded [Ln(DippForm) 2 F(thf)] (Ln=La, Sm and Nd) and 3,4,5‐F 3 C 6 H 2 O(CH 2 ) 4 DippForm. 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 (CCPh)(thf)] (Ln=Nd or Sm) are obtained from reaction of Nd metal, bis(phenylethynyl)mercury (Hg(CCPh) 2 ) and DippFormH, and the oxidation of [Sm(DippForm) 2 (thf) 2 ] with Hg(CCPh) 2 , respectively . The monomeric, six‐coordinate, cisoid [Ln(DippForm) 2 (CCPh)(thf)] complexes have trigonal prismatic geometries and rare (for Ln) terminal CCPh groups with contrasting LnCC angles (Ln=Nd, 170.9(4)°; Ln=Sm, 142.9(7)°). Their formation lends support to the view that [Ln(DippForm) 2 F(thf)] complexes arise from oxidative formation and CF activation of [Ln(DippForm) 2 (C 6 F 5 )] intermediates.