2 Scandium, Yttrium, and the Lanthanide and Actinide Elements, Excluding their Zero Oxidation ...

202 Pages·2004·19.92 MB·English

2
Scandium, Yttrium, and the
Lanthanide and Actinide
Elements, Excluding their Zero
Oxidation State Complexes
FRANK T. EDELMANN
Otto-von-Guericke-Universitat Magdeburg, Germany
2.1 INTRODUCTION 12
2.2 SCANDIUM, YTTRIUM AND THE LANTHANIDES 13
2.2.1 Carbonyls 13
2.2.2 Hydrocarbyls 13
2.2.2.1 Neutral homoleptic compounds 13
2.2.2.2 Anionic homoleptic compounds 20
2.2.2.3 Heteroleptic compounds 21
2.2.3 Alkenyl and Alkynyl Compounds 26
2.2.4 Allyls 27
2.2.5 Cyclopentadienyl Compounds 28
2.2.5.1 M(Cp)X compounds 28
2.2.5.2 M(Cp) compounds 28
2
2.2.5.3 M(Cp)X compounds 32
2
2.2.5.4 M{Cp)^i compounds 35
2.2.5.5 M(Cp)i compounds 51
2.2.5.6 [M(Cp)]L] compounds 55
2.2.5.7 [M(Cp)iL] compounds 57
2
2.2.5.8 [M(Cp)^X] compounds 57
2.2.5.9 [M(Cp)]~ compounds 58
4
2.2.6 Modified Cyclopentadienyl Ligands 59
2.2.6.1 Ring-bridged cyclopentadienyls 59
2.2.6.2 Indenyl and related compounds 63
2.2.6.3 Pentamethylcyclopentadienyls 64
2.2.7 Cyclopentadienyl-like Compounds 95
2.2.7.1 Pentadienyl compounds 95
2.2.7.2 Heteroatom five-membered ring ligands 96
2.2.8 Arene Complexes 97
2.2.9 Cyclooctatetraenyl Compounds 98
2.2.9.1 M{CJi ) compounds 98
H
2.2.9.2 MiC^^X compounds 99
2.2.9.3 [M(C H )]~ compounds 102
H H2
2.2.9.4 [M{C^ti )] compounds 104
H2
2.2.9.5 Other cyclooctatetraenyl compounds 104
2.2.10 Fullerenes 106
2.2.77 Heterobimetallic Complexes 107
2.2.11.1 Metal-metal bonded complexes 107
2.2.11.2 Heterobimetallic complexes without metal-metal bonds 108
2.2.12 Heteronuclear NMR Spectroscopy 113
n
12 Scandium, Yttrium, and the Lanthanide and Actinide Elements
2.2.13 Homogeneous Catalysis 113
2.2.14 Organic Synthesis 122
2.2.15 Organolanthanides in Materials Science 129
2.3 ACTINIDES 130
2.3.1 Carbonyls 131
2.3.2 Hydrocarbyls 131
2.3.2.1 Homoleptic compounds 131
2.3.2.2 Heteroleptic compounds 133
2.3.3 Allyls 136
2.3.4 Cyclopentadienyl Compounds 137
2.3.4.1 M(Cp)^C compounds 137
2.3.4.2 M(Cp) compounds 139
3
2.3.4.3 M(Cp)X compounds 141
3
2.3.4.4 M{Cp)JL compounds 145
2
2.3.4.5 [M(Cp)J(] compounds 147
2.3.4.6 [MX(L)(Cp)] compounds 159
3
2.3.4.7 [M(Cp)] compounds 160
4
2.3.5 Modified Cyclopentadienyl Ligands 161
2.3.5.1 Ring-bridged cyclopentadienyls 161
2.3.5.2 Indenyl compounds 162
2.3.5.3 Pentamethylcyclopentadienyls 164
2.3.6 Cyclopentadienyl'like Compounds 111
2.3.6.1 Pentadienyl and cyclohexadienyl compounds 177
2.3.6.2 Heteroatom five-membered ring ligands 178
2.3.7 Arene Complexes 178
2.3.8 Cyclooctatetraenyl Compounds 181
2.3.8.1 [M(C H)]~ compounds 181
H 82
2.3.8.2 [M(CH)] compounds 182
8 S2
2.3.8.3 [MX(C H )J compounds 184
2 H H
2.3.9 Heterobimetallic Compounds 186
2.3.9.1 Metal-metal bonded complexes 186
2.3.9.2 Heterobimetallic complexes without direct metal-metal bonds 188
2.3.10 Homogeneous Catalysis 190
2.3.11 Organic Synthesis 192
2.4 REFERENCES 192
2.1 INTRODUCTION
The year 1954 marks the beginning of organolanthanide chemistry, when Birmingham and Wilkinson
described the tris(cyclopentadienyl)lanthanide complexes Ln(Cp) . Only two years later, [UCl(Cp) ],
3 3
the first fully characterized organoactinide complex, was isolated by Reynolds and Wilkinson.2
However, rapid development of this new area of organometallic chemistry was hampered by the intrinsic
instability of organo-/-element compounds towards moisture and oxygen. The highly oxophilic character
of the /-elements requires rigorous exclusion of traces of air and moisture during preparation and
characterization of organometallic compounds of these metals. With today's more sophisticated
experimental and analytical techniques, organolanthanides and -actinides are routinely synthesized and
handled and their unique properties are easily studied. Historically, the early discovery of Ln(Cp) and
3
[UCl(Cp)] was followed by a period of relative stagnation that lasted about two decades. The 1970s and
3
1980s, however, have witnessed an enormous and exciting development of organo-/-element chemistry.
This spectacular growth in research activities is mainly due to the fact that many organo-/-element
complexes exhibit unique structural and physical properties and many of them are highly active in
various catalytic processes. In fact, the catalytic activity of certain organolanthanide hydrocarbyls and
hydrides is often dramatically higher than that of comparable ^/-transition metal catalysts.
This chapter summarizes the present knowledge about the chemistry of organo-/-element complexes.
Synthetic methods, structural investigations, and the chemical reactivity of these compounds will be
discussed. The prelanthanides scandium, yttrium, and lanthanum will be included because of their
similarity in size, charge, and chemical behavior as compared to the lanthanide elements. Generally, this
chapter has been restricted to compounds of the lanthanide and actinide elements containing M-C
bonds. A brief summary of the general chemical and physicochemical characteristics of the metal ions
can be found in COMC-I. Thus, in the second part of this chapter we will discuss the chemistry of
scandium, yttrium, and lanthanide organometallics, while the third section is devoted to organoactinide
chemistry. Each of these sections is subdivided into ligand types and concluded by a discussion of the
Scandium, Yttrium, and the Lanthanide and Actinide Elements 13
applications of the compounds in organic synthesis and homogeneous catalysis. For additional
information on topics discussed in this chapter, the reader is referred to a number of recent monographs
and detailed review articles that have appeared since the previous edition. Annual reviews covering the
lanthanides and actinides have been published regularly in the Journal of Organometallic Chemistry3'1
and other periodicals.8"15 In addition, a number of review articles have appeared since 1982 covering
both organolanthanides and organoactinides.16 Organolanthanide chemistry has been reviewed by
Schumann,17"20 McCleverty,21 Deacon,22 Bochkarev,23 Shen,24 Bercaw,25 Schaverien,26 and Evans.27*2*
Some review articles have been devoted to organoactinide chemistry,29*30 more recently those by
Marks,31"3 Takats,34 Pires de Matos,35 and Ephritikhine.36 The /-elements have also been included in
several other review articles covering more specific topics such as certain ligand types37*38 and
metal-carbon a-bonds.39^13 A NATO Advanced Study Institute monograph, Fundamental and
Technological Aspects of Organo-f-Element Chemistry, was published in 1985.44 Some representative
synthetic procedures in organolanthanide and -actinide chemistry have been collected in a chapter of
Inorganic Syntheses.45
In addition to review articles devoted to all areas of organo-/-element chemistry, numerous reviews
covering specific synthetic, spectroscopic, and technological aspects of organo-/-element compounds
have appeared. Among these are articles on low-oxidation-state organolanthanide chemistry,28'46"8 early
lanthanide organometallics,49*50 /-element cyclooctatetraenyl complexes,51*52 organoactinide chemistry
with phosphoylide,53 and polypyrazolylborate ligands,54 reactions of cyclopentadienyluranium(IV)
complexes with Lewis acids and bases,55 neptunium and plutonium complexes with monodentate
ligands,56 the organometallic chemistry of neptunium, the magnetochemical properties of
organolanthanide complexes,58'59 thermochemistry of organo-/-element complexes, bonding
properties and electronic structure,64'65 C-H activation reactions,66"8 catalytic properties,69'70 the
cryosynthesis of organometallic compounds,71 the chemical vapor deposition of rare-earth compounds in
superconductor synthesis,72 and structural trends in Ln(Cp) and Ln(Cp*) complexes.73"6 Furthermore
3 2
the crystal structures of over two hundred known cyclopentadienyl lanthanide and actinide complexes
have been analyzed in order to find some evidence for covalent bonding in these compounds.77 Various
other articles of both theoretical and experimental nature have been devoted to the question of covalency
in organolanthanide78"87 and -actinide64'88'89 complexes.
Other reviews dealing with specific synthetic aspects of organo-/-element chemistry will be
mentioned in the introductions to the particular subsections.
2.2 SCANDIUM, YTTRIUM AND THE LANTHANIDES
2.2.1 Carbonyls
The first and so far only example of a lanthanide(III) complex involving ^-coordination of a CO
ligand is the structurally characterized erbium complex [Er(EtOH)(HO){Mo(CO)3(Cp)}] (Figure 1).
2 4 3
This compound exhibits semi-bridging carbonyl ligands between molybdenum and erbium as well as
weak metal-metal bonding.90
2.2.2 Hydrocarbyls
2.2.2,1 Neutral homoleptic compounds
In this section homoleptic lanthanide hydrocarbyls as well as their THF adducts will be discussed,
although the latter are formally heteroleptic complexes. First, low-valent lanthanide hydrocarbyls will be
mentioned, followed by a discussion of lanthanide(III) complexes with a-bonded hydrocarbyl ligands.
Considerable research efforts have been directed in the past towards the synthesis and
characterization of lanthanide complexes involving coordination to neutral alkenes, dienes, allenes, or
alkynes. However, the tendency of the rare-earth elements to form complexes of this type is extremely
low as compared to the corresponding ^/-transition metal chemistry. This can be traced back to the
limited radial extension of the 4/ orbitals and to the highly ionic nature of the bonding in
organolanthanide complexes. Metal vapor synthesis has been the main preparative tool to study the
reactivity of lanthanide elements towards unsaturated hydrocarbons. The products were generally
difficult to characterize and very little is known about their molecular structures. However, an
14 Scandium, Yttrium, and the Lanthanide and Actinide Elements
Er • M o ©O OC
Figure 1 The molecular structure of [Er(EtOH)(HO){Mo(CO)(Cp)}].90
2 4 3 3
investigation of the hydrocarbons formed upon hydrolysis indicated in most cases that the products
contained direct lanthanide-carbon bonds and the oxidation state of the lanthanide elements was 2+ or
3+. Thus, these compounds will be briefly discussed in this section. Metal vapors of samarium, erbium,
and ytterbium have been reacted at -196 °C with various unsaturated hydrocarbons such as ethylene,
propylene, and allene. Due to their low solubility the resulting deeply colored products were difficult to
analyze. Only Er(MeCH=CH ) was isolated and characterized.91 Hydrolysis of the products gave
2 3
mixtures of saturated and unsaturated hydrocarbons like methane, ethane, propane, butane, propylene,
butene, allene, and methylacetylene. Similar metal-vapor reactions between samarium, erbium, and
ytterbium with 3-hexyne yielded dark brown solids which have been characterized mainly by elemental
analysis. The structure of these materials is still unknown, although it can be anticipated that they
contain lanthanide-hydrogen bonds as they show some activity as hydrogenation catalysts.92*93 Similar
results were obtained in cocondensation reactions of vaporized lanthanum, neodymium, samarium, and
erbium with butadiene or 2,3-dimethylbutadiene at -196 °C, which afforded dark brown paramagnetic
solids.94 Various types of bonding have been discussed for the coordinated diene ligands, but the real
nature of the products remains uncertain. The same is true for diene complexes of the type
[(diene)^LnCl _ M(L)J (Ln = La, Pr, Nd, Sm, Gd, Dy; M = MgCl, LiCl; x, n=0-3; L = THF,
3 x 2
DMSO).95
Several lanthanide(II) complexes containing direct lanthanide-carbon bonds with no cyclopen-
tadienyl auxiliary ligands have been mentioned in the literature. In most cases such complexes have not
been unambiguously characterized and virtually no structural information is available with the exception
of the complexes shown in Scheme 1. Single-crystal x-ray data reported for the 1-aza-allyl complex
[Yb{N(R)C(Bul)CHR} ] (R = TMS) revealed a monomeric structure in which the rac-diastereoisomer
2
predominated with each 1-aza-allyl ligand bound r|3-fashion to the ytterbium atom.96 Certainly the most
thoroughly investigated representative of this class of compounds is bis(pentafluorophenyl)ytterbium(II).
This interesting species is easily accessible through a transmetallation route involving treatment of
bis(pentafluorophenyl)mercury with elemental ytterbium (Equation (1)). The corresponding europium
diaryl has been made analogously.97"9 Complexes with partially fluorinated phenyl ligands such as
Yb(CFH-(?) or Yb(CFH-/?) are significantly less stable than the pentafluorophenyl derivative. No
6 4 2 6 4 2
samarium(II) complex of this type has ever been isolated in a pure state.98'99 The a-bonded
pentafluorophenyl groups in Yb(C F ) can be transferred to transition metals.99'100 In addition, Ln(C F )
6 5 2 6 5 2
(Ln = Sm, Eu) have been demonstrated to be highly versatile precursors for the synthesis of a variety
of divalent lanthanide phenoxides and amides.101
Substituted carboranes have been used to stabilize divalent organolanthanides containing Ln-C a-
bonds. Several solvated ytterbium(II) derivatives have been prepared by a transmetallation reaction
between mercury(II) carboranides and ytterbium powder (Equations (2) and (3)).102"4
The homoleptic amide complex [{Yb(NR )(u-NR )} ]105 has served as the starting material for the
2 2 2
preparation of a series of neutral ligand-free homoleptic ytterbium(II) and unique heteroleptic amido
ytterbium(II) alkoxides and aryloxides [{YbX(u-X)} ] (X = OAr or OCBu1,) or [{Yb(NR )(u-X)} ]
2 2 2
(X = OAr or OCBul ) (R = TMS, Ar = C H Bul -2,6-Me-4), Scheme 2. X-ray structures of [{YbX(u-
3 6 2 2
OAr)} ] and [{Yb(NR)(ji-OCBut3)}] were reported.106
2 2 2
Scandium, Yttrium, and the Lanthanide and Actinide Elements 15
Ph R R Ph
\ \
H Yb H
\
N N
/ \ \
Ph R R Ph
r 1
2 K{N(R)C(Ph)C(H)C(Ph)NR}
4PhCN
Ybl2 2 NaCHR2, Et2O [Yb(CHR2)2(OEt2)2]
r \ 2 Bu'CN
3 NaCHR2 2K{N(R)C(But) CHR} dmpe
R H R
[Yb(CHR)(dmpe)]
22
[Yb(CHR2)3Na] Bul — cf Yb :c— Bul
N
R
(R = TMS)
Scheme 1
THF
Ln + Hg(C6F5)2 Ln(C6F5)2 + Hg (1)
Ln = Sm, Eu
THF
Yb+ Hg(o-C2H2B10H9)2 [Yb(o-C2H2B10H9)2(THF)] + Hg (2)
THF
Yb+ Hg(O-RC2Bi0H10)2 [Yb(o-RC2B10H10)2(THF)M] + Hg (3)
[{Yb(OAr)(n-OAr)}]
2
2 or 4 ArOH
[{Yb(OAr)(|i-OAr)}2] [{Yb(NR)(^i-NR)}] 2 Bu^COH
2 2 2
[{Yb(NR2)(|Li-OAr)}2] 4 Bul3COH
R = TMS, Ar = CHBut-2,6-Me-4106
6 2 2
Scheme 2
The preparation of isolable homoleptic lanthanide(III) compounds containing a-bonded hydrocarbyl
ligands was probably one of the most difficult tasks in organolanthanide chemistry. For many years this
area of research was dominated by failures, although the first attempts to isolate such simple lanthanide
alkyls were made about 50 years ago. The first indication for the possible formation of lanthanide alkyls
came from the observation that lanthanum reacts with methyl radicals.107 However, no simple compound
of the type LnMe has ever been isolated. Coordinative unsaturation is clearly the main reason for the
3
inherent instability of simple lanthanide trialkyls or triaryls. All early attempts to isolate such
compounds failed. For example, an early report on the synthesis of ScEt and YEt could never be
3 3
reproduced.108'109 A related study showed the inaccessibility of triphenyllanthanum. Treatment of
anhydrous LaCl with three equivalents of phenyllithium gave biphenyl as the only isolable reaction
3
product.110 The same result was obtained when lanthanum metal was reacted with diphenylmercury. In
the late 1960s the syntheses of triphenylscandium, triphenylyttrium111'112 and lithium tetraphenyllan-
thanate, Li[LaPh], were published. The products were characterized only on the basis of their IR spectra
4
and elemental analyses as well as quenching reactions with mercury dichloride, CO, and benzophenone.
2
16 Scandium, Yttrium, and the Lanthanide and Actinide Elements
The structures of these materials still await confirmation by x-ray crystallography. In view of the
pronounced tendency of the lanthanide elements to adopt high coordination numbers it seems most
likely that the homoleptic triphenylmetal derivatives are polymeric materials.113'114
By now four different synthetic approaches to well-defined lanthanide hydrocarbyls have been
developed. Such compounds can be successfully prepared (a) by formation of stoichiometric adducts
with donating solvents such as THF, DME, or TMEDA, (b) by incorporation of donor functions into the
organic ligands, (c) by using sterically demanding hydrocarbyl ligands, or (d) by formation of anionic
"ate" complexes. Early reports showed that hydrocarbyl ligands such as f-butyl, neopentyl, or
trimethylsilylmethyl are not sufficiently bulky to permit the isolation of base-free homoleptic
organolanthanides. The bis(THF) adducts [Ln(CHTMS)(THF)] are the final products when lanthanide
2 3 2
trichlorides are reacted with LiCHTMS in THF solution (Equation (4)).115"18 In the presence of sodium
2
naphthalide the ytterbium derivative has been found to activate N.119
2
THF
LnCl + 3 LiCHTMS • [Ln(CHTMS )(THF)] + 3 LiCl (4)
3 2 2 3 2
Ln = Er, Tm, Yb, Lu
According to NMR spectral investigations these lanthanide alkyls are trigonal-bipyramidal with the
THF ligands occupying the trans positions.117'118 The products are thermally unstable and decompose
upon prolonged standing by elimination of THF and tetramethylsilane to give pyrophoric materials of
polymeric nature (Equation (5)). These materials have been postulated to be lanthanide carbene
complexes, although no structural information is available and the true nature of the products remains
unclear.117
[Ln(CHTMS )(THF)] • \ln [{Ln(CH-TMS)(CHTMS)}]+ SiMe (5)
2 3 2 2 n 4
Ln = Er, Lu
Similar decomposition reactions and formation of polymeric "carbene" materials and/or hydride
species have been observed when LnCl (Ln = Y, Nd) was reacted with BunLi, LiCH TMS, LiBz, or
3 2
LiCH CMe Ph, respectively. These reactions gave no indications of the formation of isolable homoleptic
2 2
lanthanide hydrocarbyls.120"6
An effective synthetic route leading to monomeric lanthanide(III) hydrocarbyls is the intramolecular
saturation of coordination sites by "in-built" donor functions. The earliest reports using this concept
described the preparation of homoleptic organolanthanides containing dimethylamino-substituted phenyl
or benzyl ligands. The resulting six-coordinated organolanthanides [Sc{CH(SiMe C H OMe-o) } ],
2 6 4 2 3
[Ln(CH C H NMe -0) ], and [Ln(CHCHNMe-o)] (Ln = Sc, Y, La, Nd, Er, Yb, Lu) are stable
2 6 4 2 3 6 4 2 2 3
without additional THF ligands.127"30 More recently two other series of lanthanide aryls, [Ln(CHMe-
6 4
o) ] (Ln = Ce, Pr, Nd, Sm, Gd, Er)127 and [Ln(CHOMe-o)] (Ln = Ce, Pr, Nd),1 as well as the
3 6 4 3
binuclear Yb27Yb3+ pentaphenyl complex [Yb(THF)(u-Ph)PhYb(THF)]132'133 have been described.
3 2 3
The latter is a truly remarkable species. It has been prepared by reacting naphthaleneytterbium,
[Yb(THF)C H] with either PhHg or PhBi in THF solution and isolated in the form of red
2 10 8 2 3
paramagnetic crystals. A crystal structure determination (Figure 2) revealed the presence of an
asymmetrical binuclear complex, in which both ytterbium atoms adopt a distorted octahedral
coordination geometry. The two ytterbium atoms are bridged by three phenyl groups, which are linked
with the first ytterbium atom by an r^-bond and with the second one by an unsymmetrical r|2-bond. In
addition the first ytterbium atom is T|'-bonded to two terminal phenyl groups and one THF ligand,
whereas the second ytterbium atom is linked to three THF molecules.133
An interesting class of homoleptic lanthanide hydrocarbyls has become available through the use of
ally Hike diphosphinomethanide ligands. The first member of the series was the lanthanum derivative,
[La{CH(PPh ) } ] (1), which was characterized by x-ray crystallography.134
2 2 3
R P' ^ * PR
2 2
(l)
Scandium, Yttrium, and the Lanthanide and Actinide Elements 17
C-69
C-68
C-83
C-67
C-73
C-78
Figure 2 The molecular structure of [Yb(THF)(^-Ph)PhYb(THF)].133
3 2 3
Mono- and dinuclear diphosphinomethanides have been reported for samarium(III) (Equations (6)
and (7)). Both compounds have been structurally characterized. The disamarium complex is a
centrosymmetric dimer in a chair conformation with two diphosphinomethanide ligands bridging via the
P-2 and C-l atoms.135'136
SmCl(THF) + 3 LiCH(PPh) - [Sm{CH(PPh)}] + 3 LiCl + 3 THF (6)
3 3 22 22 3
2Sm(O SCF ) + 6 LiCH(PMe) — • [(Sm{CH(PMe)}){|i-ri2-CH(PMe)}H] + 6LiO SCF (7)
3 3 3 22 22 2 22 3 3
Several lanthanide(III) derivatives containing cr-bonded carboranyl ligands have also appeared in the
literature. These materials have been prepared by reacting mercury(II) carboranides with elemental
lanthanides in THF. This transmetallation reaction directly yields THF adducts of the corresponding
lanthanide(III) carboranides (Equation (8)).107'137
2 Ln + 3 Hg(0-RCB H ) 2 [Ln(o-RCB H )(THF)] + 3 Hg (8)
2 1O 1()2 2 10 103 n
R = Me; Ln = Tm (n = 3), Yb (n = 2)
R = Ph; Ln = La (n = 1), Tm (n = 3), Yb (n = 2)
Yet another approach to neutral, homoleptic lanthanide hydrocarbyls is the use of bidentate anionic
phosphoylide anions of the type RP(CH)~. However, early attempts to obtain monomeric lanthanide
2 22
complexes with these ligands met with little success. The first such compounds were prepared by a two-
step procedure according to Scheme 3.18
+3 BunLi
LnCl + 3Me P=CH [Ln{CHPMeCl}] [Ln{(CH)PMe}]
3 3 2 2 3 3 22 2 3
-3 C4H10
-3 LiCl
Ln = La, Pr, Nd, Sm, Gd, Ho, Er, Lu
Scheme 3
The lutetium complex [Lu{(CH ) PBu! } ] was made similarly by treating LuCl with three
2 2 2 3 3
equivalents of Li(CH ) PBul . The *H NMR spectra of all these complexes indicated a temperature-
2 2 2
dependent equilibrium between the monomer and oligomeric and polymeric species in solution. For
[Lu{(CH ) PBut } ] this equilibrium was investigated by *H, 13C, and 3IP NMR spectral studies. The
2 2 2 3
results clearly demonstrated that even the chelating phosphoylide ligand [Bul P(CH ) ]" is not
2 2 2
sufficiently bulky to stabilize low-coordinate homoleptic lanthanide hydrocarbyls.138
On summarizing the results discussed above it becomes quite clear that base-free monomeric
lanthanide hydrocarbyls can only be synthesized by using very bulky organic ligands. Also, the chances
to successfully isolate such complexes should be higher for scandium, yttrium, and the heavier (i.e.,
smaller) lanthanide ions. In fact, the earliest reports on base-free homoleptic lanthanide hydrocarbyls
described the synthesis of scandium and yttrium compounds containing highly sterically demanding
disilylated methyl groups.118 The unsolvated compounds [Y{CH(TMS) } ]139 and
2 3
[Sc(CH SiMe C H OMe-p) ] were synthesized by conventional salt elimination reactions between the
2 2 6 4 3
metal trichlorides and three equivalents of the corresponding lithium reagents. Obviously in these two
18 Scandium, Yttrium, and the Lanthanide and Actinide Elements
cases the ligands were sufficiently bulky to prevent the products from adding solvent molecules or
retaining lithium halide.
The first synthesis of base-free lanthanide alkyls by Lappert et al. was achieved by combining two
useful synthetic strategies. The bis(trimethylsilyl)methyl ligand was employed as a very bulky
hydrocarbyl group and the reaction was conducted in the absence of alkali halides by using the 2,4,6-
tri-f-butylphenoxide as a leaving group (Equation (9)).140"2
pentane
[Ln{OCH(But)3-2,4,6}3 + 3 LiCH(TMS) - [Ln{CH(TMS)}] + 3 LiOCH(But)-2,4,6 (9)
6 2 3 2 2 3 6 2 3
Ln = Y, La, Sm, Lu
Under the chosen reaction conditions (pentane solution) the lithium phenoxide by-product
precipitates, thus facilitating the isolation of the highly soluble lanthanide hydrocarbyls. Like the
homoleptic silylamides [Ln{N(TMS) } ] the tris-alkyls adopt a flat pyramidal coordination geometry as
2 3
has been determined by x-ray crystallography for the lanthanum and samarium derivatives (Figure 3).
The average metal-carbon distances are La-C 0.2515(9) nm and Sm-C 0.233(2) nm.
H-4a .
pH 4c
C-2'
C-5'
C-7
C-7'
C-6
C-61 C-6"
Figure 3 The molecular structure of [La{CH(TMS)}].140
2 3
Several explanations have been given for the deviation from planarity in [Ln{CH(TMS) } ]
2 3
compounds. Interligand repulsion is thought to be minimized in the pyramidal structure, but more
important is the possibility of forming Ln-H-C agostic interactions. In fact, short y-agostic Ln- -Me
interactions were found in both structurally characterized compounds (La-C 0.3121 nm, Sm- -C 0.285
nm) which are a result of the extreme electronic and coordinative unsaturation of the central lanthanide
atoms.
In marked contrast, the more obvious metathetical reaction between lanthanide trichlorides and
bis(trimethylsilyl)methyllithium does not afford the homoleptic lanthanide alkyls due to retention of
lithium halide (Equation (10)), La-C 0.260(3) nm):140
pmdeta
LaCl + 3 LiCH(TMS) [La{CH(TMS)}(|i-Cl)Li(pmdeta)] + 2 LiCl (10)
3 2 2 3
pmdeta = Af,Af,AVN",N"-pentamethyldiethylenetriamine
The LiCl adduct contains an almost linear La-Cl-Li unit (La-Cl-Li 165.1°, La-Cl 0.2762 nm, Li-Cl
0.228 nm) and the [La{CH(TMS) } ] fragment remains virtually unchanged as compared to the free
2 3
homoleptic alkyls.140
When ytterbium trichloride is used in a similar preparation, the saltlike compound
[Li(THF) ][YbCl{CH(TMS) } ] is obtained, which consists of separated ions.115 Obviously the
4 2 3
formation of one or the other type of product depends on a subtle balance between various factors, for
example nature of the alkali metal, presence of halide ions, solvent, and ionic radius of the lanthanide
ion. A similar situation was recently observed in the chemistry of low-coordinate lanthanide alkoxides
and amides. The chloride-bridged neodymium compounds [Nd{N(TMS)}(fi-Cl)Li(THF)] and
2 3 3
[Nd{(But) CO} (u-Cl)Li(THF) ] correspond to the lanthanum alkyl [La{CH(TMS) } (u-Cl)Li(pmdeta)],
3 3 3 2 3
whereas [Li(THF) ][Nd0-TMS{N(TMS) } ]143 can be compared to [Li(THF) ][YbCl{CH(TMS) } ].
4 2 3 4 2 3
The following example may further illustrate how slight changes in the reaction conditions or the
choice of reagents can cause interesting differences in the product formation. When the more reactive
potassium alkyl KCH(TMS) is used instead of LiCH(TMS) , the reaction with lanthanide trichlorides
2 2
Scandium, Yttrium, and the Lanthanide and Actinide Elements 19
(in this case LuCl ) can be carried out in diethyl ether instead of THF and affords the solvated KC1
3
adduct [Lu{CH(TMS) } (u-Cl)K(Et O)] (Scheme 4). In contrast to the corresponding LiCl adduct the
2 3 2
coordinated diethyl ether can easily be removed in vacuo to give unsolvated [Lu{CH(TMS) } (u-Cl)K].
2 3
In this compound the potassium is coordinatively unsaturated and readily adds various solvent
molecules such as diethyl ether or even toluene (Scheme 4):144'145
vacuum
ether -ether
LuCl + 3 KCH(TMS) [Lu { CH(TMS)}(u-Cl)K(EtO)] [Lu{CH(TMS)}(u-Cl)K]
3 2 2 3 2 23
ether
vacuum toluene
[Lu {CH(TMS)}(^-Cl)K(Ti6-toluene)]
2 3 2
Scheme 4
Especially remarkable is the hydrocarbon-soluble toluene solvate, which has been structurally
characterized by x-ray diffraction. As compared to the almost linear Ln-Cl-M (M = Li or K) bridging
in [Lu{CH(TMS) } (u-Cl)K(Et O)] and [Nd{N(TMS) } (u-Cl)Li(THF) ] the Lu-Cl-K unit deviates
2 3 2 2 3 3
significantly from linearity (145.9°). Two toluene molecules are T|6-coordinated to the potassium ion
(Figure 4).
C-75
C-74
C-83 C-76
C-82
C-71
C-70
C-81
Figure 4 The molecular structure of [Lu{CH(TMS)}(^-Cl)K(r|6-toluene)].144
2 3 2
Very little has been published on heavier homologues of homoleptic lanthanide hydrocarbyls (i.e.,
silyls, germyls, stannyls, and plumbyls). Organosilicon and organogermanium complexes of divalent
ytterbium have been prepared as shown in Equation (II).146
THF
2Yb + 2Ph3ECl [Yb(EPh3)2(THF)4] + [YbCl2(THF)2] (11)
E = Si, Ge
X-ray structural analyses revealed that both compounds have similar centrosymmetrical octahedral
structures with the EPh ligands in axial positions.
3
A crystal structure determination has also been reported for [Yb{Sn(CH But) } (THF) ]. The stannyl
2 3 2 2
derivative was made by reacting Ybl with two equivalents of K[Sn(CH Bul) ].147 The compounds
2 2 3
[Ln{Sn(CH TMS) } (DME)] have been prepared and characterized by spectroscopic methods.148'149
2 3 3
These compounds can formally be regarded as heavier homologues of lanthanide hydrocarbyls. In this
context the homoleptic phosphides and arsenides of the type Ln{P(Bul)}3 and Ln{As(Bul)}3 should
2 2
also be mentioned, although these are not organometallic compounds in a strict sense. So far no
structural information is available on these materials.150
20 Scandium, Yttrium, and the Lanthanide and Actinide Elements
2.2.2.2 Anionic homoleptic compounds
Simple lanthanide(III) hydrocarbyls show a strong tendency to add anionic ligands and form "ate"
complexes, thereby increasing the formal coordination number around the lanthanide atoms. This
phenomenon is frequently encountered when lanthanide hydrocarbyls are prepared from anhydrous
lanthanide trichloride and the corresponding lithium alkyl or aryl. Formation of anionic hydrocarbyls is
especially favored when the organic ligands are not sufficiently bulky to permit the stabilization of low
coordination numbers around the lanthanide atoms. Of special interest are reactions of lanthanide
trichlorides with methyllithium, which have been studied in detail by Schumann et al. Neutral
homoleptic LnMe derivatives are coordinatively highly unsaturated and have not been isolated for any
3
member of the lanthanide series. However, treatment of LnCl with six equivalents of methyllithium in
3
the presence of a coordinating solvent yields stable, six-coordinated hexamethyllanthanidates,
[LnMe]3~, which can be isolated for all rare-earth elements except promethium and europium.151
6
Whereas the promethium reaction has not been investigated, the europium(III) derivative cannot be
isolated, because EuCl is reduced to an uncharacterized divalent europium species upon treatment with
3
methyllithium.151 Suitable coligands capable of saturating the coordination sphere of the lithium atoms
are AWA^AT-tetramethylethylenediamine (TMEDA) and 1,2-dimethoxyethane (DME).151 The DME
derivatives are thermally less stable than the TMEDA adducts (Equations (12) and (13)).
Et2O
LnCl3 + 6 MeLi + 3 TMEDA [Li(TMEDA)]3[LnMe6] + 3 LiCl (12)
Ln = Sc, Y, La-Nd, Sm-Lu
Et2O
LnCl3 + 6 MeLi + 3 DME [Li(DME)]3[LnMe6] + 3 LiCl (13)
Ln = Ce, Pr, Gd, Tb, Dy, Er, Lu
The crystalline compounds display the characteristic colors of the lanthanide cations. The molecular
structures of [Li(TMEDA)] [LnMe ] (Ln = Ho, Er) and [Li(DME)] [LuMe ] have been determined by x-
3 6 3 6
ray crystallography.151'152 Figure 5 shows the molecular structure of [Li(TMEDA)] [HoMe ]. In these
3 6
compounds the central lanthanide ion is surrounded by six methyl groups in a slightly distorted
octahedral fashion. All Ln-C distances are equal. Each of the three lithium ions is connected to the
central lanthanide by two bridging methyl ligands and the tetrahedral coordination sphere around lithium
is completed by the chelating TMEDA or DME ligand.153
C206 C20
C40 C40
C40
C20
C202
CC44U0 3 CC44U0-
Figure 5 The molecular structure of [Li(TMEDA)] [HoMe ].151'152
3 6
The derivative chemistry of the anionic hexamethyllanthanidate complexes remains relatively little
explored. Besides some reactivity studies toward organic reagents (cf. Section 2.2.14), the samarium
derivative [Li(TMEDA)] [SmMe ] has recently been used to prepare the mixed-metal alkoxide
3 6
[Li Sm(OBut) ] via treatment with r-butanol in diethyl ether.154
5 8
With the chelating dimethylaminopropyl ligand a related anionic cerium complex,
Li [Ce{(CH ) NMe } ], has been isolated.155 An interesting binuclear derivative was obtained by
3 2 3 2 6
reacting LuCl with excess methyllithium in the presence of Af,Af,Af',Af'-tetraethylethylenediamine
3
(TEEDA). In this structurally characterized compound there are methyl bridges between the two
lutetium atoms as well as between lutetium and lithium.152