Mini review
Aspects of non-classical organolanthanide chemistry

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Abstract

This paper provides a survey of our studies on: (i) subvalent compounds of lanthanum and early 4f elements; (ii) cationic mononuclear samarium(II) and ytterbium(II) organic complexes; (iii) low valent samarium and ytterbium β-diketiminates; and (iv) mononuclear cerium(IV) amides and a mixed valence trinuclear Ce(IV)/Ce(III) cluster. A brief introductory section points to our earlier (1973–1995) studies on organo-4f-element chemistry.

This paper provides a survey of our studies on: (i) subvalent compounds of lanthanum and early 4f elements; (ii) cationic mononuclear samarium(II) and ytterbium(II) organic complexes; (iii) low valent samarium and ytterbium β-diketiminates; and (iv) mononuclear cerium(IV) amides and a mixed valence trinuclear Ce(IV)/Ce(III) cluster. A brief introductory section points to our earlier (1973–1995) studies on organo-4f-element chemistry.

Introduction

This survey will focus on four areas of organolanthanide chemistry which we regard as ‘non-classical’ (the ‘classical’ organo-Ln(II) compounds are neutral or anionic Sm, Eu or Yb derivatives). These are: (i) subvalent compounds of lanthanum and the early 4f elements; (ii) cationic mononuclear samarium(II) and ytterbium(II) organic complexes; (iii) low valent samarium and ytterbium β-diketiminates; and (iv) mononuclear cerium(IV) amides and a mixed valence trinuclear Ce(IV)/Ce(III) cluster. These studies date from 1995.

Our earlier work on organolanthanide chemistry (which we take to encompass Sc, Y, La, as well as the 4f elements, collectively abbreviated as Ln) began in the early 1970s (for the first publication, see Ref. [1]) and was carried out by (in chronological order): (a) postdoctoral fellows R. Pearce, G.K. Barker, A. Singh, G.A. Lawless and X. Cai; and (b) graduate students J. Holton (1976), P.I.W. Yarrow (1979), R.G. Smith (1987), S. Prashar (1991), S.R. Holmes (1992), S. Tian (1994) and J.R. van den Hende (1995); the dates in parentheses refer to their DPhil (Sussex) Theses. Some highlights of their contributions relate to a number of ‘firsts’ (R=SiMe3): (a) Ln(III) alkyls: the neutral [Ln(CH2R)3(thf)2] (Ln=Sc, Y) [1], [Ln(CHR2)3] (Ln=La, Sm) [2], and the heterobinuclear [Li(thf)4][Ln(CHR2)3Cl] and [Li(thf)4][Ln(CH2R)4] (Ln=Er, Yb) [3] and [Ln(CHR2)3(μ-X)Li(pmdeta)] (Ln=La, Sm; X=Cl, Me) [4], [4](a), [4](b); (b) Ln(III) bridging alkyls: [LnCp2(μ-Me)2AlMe2] and [{LnCp2(μ-Me)}2] [5], [5](a), [5](b), [5](c); (c) Ln(II) alkyls and cyclopentadienyls: [{Yb(CR3)(μ-OEt)(OEt2)}2] [6], [6](a) (see also Ref. [7]), [Yb(CHR2)2(OEt2)2] [6b] and [YbCp′2(thf)2] [8] (see also Ref. [9], [9](a), [9](b)); (d) Ln(III) 1,3-bis(trimethylsilyl)cyclopentadienyls: [{PrCp″2(μ-Cl)}2] [10], [NdCp″2(μ-Cl)2Li(thf)2] [11], and [AsPh4][NdCp″2Cl2] [12]; (e) Ln(III) mononuclear aryloxides and thiolates: [Ln(OArMe)3] (Ln=Sc, Y) [13], [13](a), [13](b) and [Sm(SArtBu)3] [14] (OArMedouble bondOC6H2 tBu2-2,6-Me-4; SAr tBudouble bondC6H2 tBu3-2,4,6); (f) Ln(II) alkoxides and aryloxides: [Yb(OCtBu3)2(thf)2] [15] and [Yb(OArMe)2(thf)2] [16], [16](a), [16](b); (g) homoleptic Ln(II) cyclopentadienyls: [(LnCp″2)] (Ln=Eu, Yb) [17]; (h) mononuclear Ln(III) β-diketiminates (LL) and 1-azaallyls (LL′): [Ce(LL)(CHR2)2] and [Nd(LL)2Cl] [18] (see also Ref. [19], [19](a), [19](b)) and [Sm(LL′)2I(thf)] [20]; (i) observation of 171Yb-NMR spectra [21]; and (j) silylene complexes [LnCp3{Si(NCH2tBu)2C6H4-1,2}] (Ln=Y, Yb) [22].

Section snippets

Cationic mononuclear samarium(II) and ytterbium(II) organic complexes

Although cationic inorganic Ln(II) complexes (Ln=Sm, Eu, Yb) are known, such as [Yb(hmpa)4(thf)2]I2[59] or [Sm(thf)7][Zn42-SePh)6(SePh)4] [60], as are heterobimetallic complexes such as [Eu(thf)43-SePh)3Zn(SePh)] [60], cationic mononuclear organic Ln(II) complexes were unknown prior to our work in 1998 [61]. Our objective was to gain access to salts in which the cation contained a Lnsingle bondX moiety, X being a monoanionic organic ligand. Such a species would feature a highly electropositive Ln(II)

Low valent samarium and ytterbium β-diketiminates

The β-diketiminato ligands are of structure J. Those relevant to the present study have the substituents R1=SiMe3=R5 (i.e. R) and R3=H. They are readily synthesised as lithium derivatives; for example for R2=Ph=R4 (J′), from LiCHR2+2PhCN [63]. An alternative precursor is the potassium β-diketiminate available from the lithium salt Li(J′) and KOtBu [64]. Two unusual heterobimetallic, paramagnetic, crystalline compounds 26 and 27 were obtained in modest yield from the reagents shown in 9, 10,

Mononuclear cerium(IV) amides and a mixed valence trinuclear Ce(IV)/Ce(III) cluster

Although the inorganic Ce(IV) salt [NH4]2[Ce(NO3)6] is widely used as a reagent in organic synthesis (under the acronym CAN, cf. Ref. [66]), well authenticated organic Ce(IV) compounds, excluding O-donor complexes, such as the X-ray-characterised [Ce(OtBu)2(NO3)2(HOtBu)2] [67], [Ce(OtBu)2(μ-OtBu)23-OtBu)2{Na(dme)}2] [67], [{Ce(OCtBu3)2}3(μ-OC6H4O-1,4)] [68], [Ce{(C6H11-c)8Si8O13}2(py)3] [69] and [CeCp3(OR′)] (R′=tBu [70], iPr [71]), are rare. A major breakthrough was the report that the

Acknowledgements

We are grateful for valuable contributions made by Professors F. Laschi (EPR; Refs. [42], [46]) and H. Nöth (X-ray; Ref. [51]), Drs D.J. Duncalf (X-ray; Ref. [47]), C.J. Pickett and S.K. Ibrahim (cyclic voltammetry), Professors O. Eisenstein and L. Maron (DFT, Ref. [74]), Dr. S.D. Elliott (DFT, Ref. [76]) and A.G. Avent (NMR). We also acknowledge with thanks the support received from EPSRC and The Royal Society.

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