Synthesis and reactions of polynuclear polyhydrido rare earth metal complexes containing (C5Me4SiMe3)LnH 2 units: A new frontier in rare earth metal hydride chemistry

Article abstract of DOI:10.1002/ejic.200700085

A series of tetranuclear octahydrido rare earth metal complexes of general formula [(C₅Me₄SiMe₃)Ln(μ-H) ₂]₄(THF)_n (Ln = Sc, Y, Gd, Dy, Ho, Er, Tm, Lu; n = 0, 1, or 2) that contain C₅Me₄SiMe₃ as an ancillary ligand have been prepared and structurally characterized. These hydride clusters are soluble in common organic solvents such as THF, toluene, and hexane, and maintain their tetranuclear framework in solution. Such polynuclear polyhydrido complexes exhibit extremely high and unique reactivity toward a variety of unsaturated substrates including CO, CO₂, and nitriles. The reaction of these neutral polyhydrides with one equivalent of [Ph₃C][B(C₆F₅)₄] affords the corresponding cationic hydride clusters [(C₅Me₄SiMe ₃)₄Ln₄H₇(THF)_n][B(C ₆F₅)₄], which can act as catalysts for the syndiospecific polymerization of styrene and regio- and stereospecific cis-1,4-polymerization of 1,3-cyclohexadiene. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700085

MICROREVIEW
DOI: 10.1002/ejic.200700085
Synthesis and Reactions of Polynuclear Polyhydrido Rare Earth Metal
Complexes Containing “(C5Me4SiMe3)LnH2” Units: A New Frontier in Rare
Earth Metal Hydride Chemistry
Zhaomin Hou,*[a] Masayoshi Nishiura,[a] and Takanori Shima[a]
Keywords: Rare earths / Hydride ligands / Cyclopentadienyl ligands / Cluster compounds / Hydrogenation
A series of tetranuclear octahydrido rare earth metal com-
plexes of general formula [(C5Me4SiMe3)Ln(µ-H)2]4(THF)n
(Ln = Sc, Y, Gd, Dy, Ho, Er, Tm, Lu; n = 0, 1, or 2) that contain
C5Me4SiMe3 as an ancillary ligand have been prepared and
structurally characterized. These hydride clusters are soluble
substrates including CO, CO2, and nitriles. The reaction of
these neutral polyhydrides with one equivalent of
[Ph3C][B(C6F5)4] affords the corresponding cationic hydride
clusters [(C5Me4SiMe3)4Ln4H7(THF)n][B(C6F5)4], which can
act as catalysts for the syndiospecific polymerization of sty-
in common organic solvents such as THF, toluene, and hex- rene and regio- and stereospecific cis-1,4-polymerization of
ane, and maintain their tetranuclear framework in solution.
Such polynuclear polyhydrido complexes exhibit extremely
high and unique reactivity toward a variety of unsaturated
1,3-cyclohexadiene.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
[{(C5Me5)Sm(µ-H)2}6{KH(THF)2}3], which formally con-
sists of six “(C5Me5)SmH2” units and three “KH(THF)2”
units.[6a] This complex was serendipitously obtained by the
reaction (σ-bond metathesis and subsequent oxidation) of
the SmII alkyl complex [{(C5Me5)SmCH(SiMe3)2[(C5Me5)-
K(THF)2]}n][6] with an excess of PhSiH3. Very recently, a
series of salt-free polynuclear polyhydrido rare earth metal
complexes that contain “(C5Me4SiMe3)LnH2” units has
been synthesized from the C5Me4SiMe3-ligated dialkyl
complexes [(C5Me4SiMe3)Ln(CH2SiMe3)2(THF)] (Ln = Sc,
Y, Gd, Dy, Ho, Er, Tm, Lu). It has been found that these
polyhydrido rare earth metal complexes show a unique re-
activity that differs from that of conventional monohydride
complexes. This article summarizes recent studies on this
new class of rare earth metal hydride complexes.
1. Introduction
Metal hydrides are fundamental components in a wide
range of stoichiometric and catalytic reactions, and their
importance in modern inorganic and organic chemistry
cannot be overemphasized.[1,2] A large number of monohy-
dride complexes of the rare earth (group 3 and lanthanide)
metals of the general type “L2MH” or “(L)(LЈ)MH”, which
are supported by two anionic ancillary ligands per metal,
have been synthesized and studied[2] since the first rare
earth metallocene hydride complex was reported in the
early 1980s.[3] In contrast, dihydrido rare earth metal com-
plexes of the type “LMH2”, which bear one anionic ancil-
lary ligand per metal, have been much less extensively inves-
tigated even though such complexes are of great interest
both structurally and chemically. A major factor that has
hampered the investigation of rare earth metal dihydride or
polyhydride complexes is the difficulty in isolating structur-
ally well defined complexes. Early attempts to prepare a di-
hydrido rare earth metal complex by reaction of dialkyl
complexes such as [(C5Me5)Lu(CH2CMe3)2(THF)][4] or
[(C5Me4SiMe3)Y(CH2SiMe3)2(THF)][5] with H2 did not af-
ford a structurally characterized hydride species. The first
structurally characterized dihydrido rare earth metal species
was isolated as the SmIII/K heteropolymetallic form
2. Synthesis and Structures of Tetranuclear
Octahydrido Rare Earth Metal Complexes
Hydrogenolysis of the C5Me4SiMe3-ligated bis(trimethyl-
silylmethyl)scandium complex [(C5Me4SiMe3)Sc(CH2-
SiMe3)2(THF)] with H2 in toluene at room temperature
gives the THF-free tetranuclear octahydride complex
[{(C5Me4SiMe3)Sc(µ-H)2}4] (1) (Scheme 1).[7] This complex
can be viewed as being the result of a self-assembly of four
“(C5Me4SiMe3)ScH2” units in a tetrahedron through
“intermolecular” Sc–H interactions. There are a total of
eight hydride ligands in the molecule, one of which is body-
centered in a µ4-H–Sc4 fashion, one is face-capped in a µ3-
[a] Organometallic Chemistry Laboratory, RIKEN (The Institute
of Physical and Chemical Research), and PRESTO, Japan
Science and Technology Agency (JST),
Hirosawa 2-1, Wako, Saitama 351-0198, Japan
Fax: +81-48-462-4665
E-mail: houz@riken.jp
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2535

Z. Hou, M. Nishiura, T. Shima
MICROREVIEW
Scheme 1. Synthesis of a tetranuclear octahydrido scandium complex.
H–Sc3 form, and six are edge-bridged in a µ2-H–Sc2 style. is also observed, probably due to the greater steric unsatu-
The THF ligand bonded originally to the scandium metal ration of these larger metal centers (Scheme 2).[9,10] These
center of the alkyl precursor is squeezed out in the hydride results show that the fine structures of the tetranuclear oc-
complex due to the formation of “intermolecular” Sc–H tahydride complexes are, to some extent, dependent on the
bonds.
Hydrogenolysis of the bis(trimethylsilylmethyl) com-
ionic radius of the metal centers.
In the case of lutetium, the reaction of the bisalkyl com-
plexes of larger rare earth metals such as Y, Er, and Tm in plex [(C5Me4SiMe3)Lu(CH2SiMe3)2(THF)] with H2 did not
toluene affords the corresponding tetranuclear octahydride give a single hydride complex.[11] However, hydrogenolysis
complexes [{(C5Me4SiMe3)Ln(µ-H)2}4(THF)] (2: Ln = Y;[8] of the hydride-bridged dimeric mixed alkyl/hydride complex
3: Ln = Er;[9] 4: Ln = Tm[9]) where one of the four metal [{(C5Me4SiMe3)Lu(CH2SiMe3)(µ-H)(THF)}2] (8), which
centers in these complexes bears an additional THF ligand was prepared by reaction of [(C5Me4SiMe3)Lu(CH2SiMe3)2-
(Scheme 2). The overall structure of the mono(THF)-con- (THF)] with one equivalent of PhSiH3, affords the tetranu-
taining polyhydride complexes 2–4 is almost the same as clear mono(THF)-coordinated lutetium octahydride
that of the THF-free Sc complex 1. In the case of the analo- complex [{(C5Me4SiMe3)Lu(µ-H)2}4(THF)] (9) in THF
gous complexes of larger metals such as Gd (5),[9] Dy (6),[9] (Scheme 3).[12] The structure of 9 is identical with those of
and Ho (7),[9] an agostic interaction between the metal cen- the analogous mono(THF)-containing complexes 2–4.
ter and a methyl group of an adjacent C5Me4SiMe3 ligand When the reaction of the alkyl/hydride complex 8 with H2
Zhaomin Hou was born in Shandong Province, China, in 1961. He received his B.Sc. from the China University of
Petroleum (Shandong, China) in 1982, and his M.Sc. and Ph.D. from Kyushu University (Fukuoka, Japan) in 1986 and
1989, respectively. He joined RIKEN (The Institute of Physical and Chemical Research) in 1989, where he is now Chief
Scientist and Director of the Organometallic Chemistry Laboratory. The current research activity in his group includes
development of new catalysts for regio- and stereospecific olefin polymerization/copolymerization, new catalysts for ef-
ficient organic synthesis, and rare earth metal complexes with novel structures and reactivities, as well as synthesis of new
functional materials. He is a recipient of the Japan Society of Coordination Chemistry Award for Young Scientists
(1996), the Rare Earth Society of Japan Award for Young Scientists (1997), the Progress Award in Synthetic Organic
Chemistry, Japan (2000), the JSPS Prize (2007), the Chemical Society of Japan Award for Creative Work for 2007,
and the Mitsui Chemicals Catalysis Science Award (2007).
Masayoshi Nishiura was born in Hokkaido, Japan in 1971. He did his graduate studies at Chiba University under the
supervision of Professor Tsuneo Imamoto, and received his Ph.D. in 2000 from the same University. He moved to RIKEN
as a special postdoctoral researcher in 2000 and was promoted to Research Scientist at RIKEN in 2003. His research
focuses on development of organometallic catalysts for organic synthesis and olefin polymerization.
Takanori Shima was born in Osaka, Japan, in 1972. He obtained his Ph.D. degree in 2001 from Tokyo Institute of
Technology on the study of multinuclear polyhydrido complexes, under the supervision of Prof. Hiroharu Suzuki. He
undertook postdoctoral research in the group of Prof. John A. Gladysz in the Institute für Organische Chemie of Friedrich-
Alexander Universität (Erlangen, Germany) as an Alexander von Humboldt fellow in 2002. He joined RIKEN in 2004.
His research interests include the synthesis and development of new rare earth metal polyhydrido complexes.
2536
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2535–2545

Rare Earth Metal Hydride Chemistry
MICROREVIEW
Scheme 3. Synthesis of THF-free and mono(THF)-coordinated lu-
tetium polyhydride complexes.
The octahydride complexes 1–7, 9, and 10 are all soluble
in common organic solvents such as THF, toluene, and hex-
ane, and their tetranuclear frameworks are maintained in
solution. In the case of yttrium, recrystallization of the
Scheme 2. Synthesis of mono(THF)-coordinated tetranuclear oc-
tahydrido rare earth metal complexes.
was carried out in toluene, the corresponding THF-free tet- mono(THF) complex 2 from THF gives the structurally
ranuclear lutetium octahydride complex 10 was obtained characterizable bis(THF)-coordinated complex 11
(Scheme 3).[12] (Scheme 4);[9,12,13] formation of an analogous bis(THF)
Scheme 4. Formation of yttrium polyhydride complexes with and without THF ligands.
Figure 1. ORTEP drawings of the Y4H8 core structures of 12 (left, THF-free), 2 (middle, with one THF ligand), and 11 (right, with two
THF ligands).
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2537

Z. Hou, M. Nishiura, T. Shima
MICROREVIEW
complex was not observed under the same conditions for ethylbenzene and the THF-free yttrium octahydride com-
the smaller Sc and Lu complexes 1, 9, and 10. The plex [{(C5Me4SiMe3)Y(µ-H)2}4] (12).[14] Under 1 atm of
bis(THF)-coordinated yttrium polyhydride 11 also adopts H2, styrene can be catalytically hydrogenated into ethylben-
a tetrahedral metal skeleton, but it has four µ3-H and four zene by 2 or 12.
µ2-H ligands whereas the mono(THF) analogue 2 possesses
one µ4-H, one µ3-H, and six µ2-H ligands (Figure 1).
Recrystallization of the bis(THF) complex 11 from hexane
regenerates the mono(THF) complex 2, while treatment of
the mono(THF) yttrium complex 2 with styrene followed
by hydrogenolysis of the resultant product in benzene yields
the corresponding THF-free octahydride complex 12
(Schemes 4 and 5).[8,14] ORTEP drawings of the Y4H8 core
structures of 2, 11, and 12 are shown in Figure 1.
Scheme 5. Formation of a tetranuclear yttrium benzylic allyl hep-
tahydride complex and its reaction with H2.
3. Reaction of Rare Earth Metal Polyhydrides
with Unsaturated C–C Bonds
Reaction of the mono(THF)-coordinated tetranuclear yt-
trium octahydride complex 2 with styrene yields a product
formed by insertion of a single molecule of styrene Ϫ the
benzylic allyl heptahydride complex 13 Ϫ in which the allyl
part is bonded to one Y atom in an η3-fashion and the
phenyl part is bonded to another Y atom in an η2-form
(Scheme 5).[8] No further reaction was observed even when
an excess of styrene was present. Hydrogenolysis of the ben-
Scheme 6. Reaction of an yttrium polyhydride complex with 1,3-
zylic allyl complex 13 with H2 (1 atm) in benzene affords cyclohexadiene. CpЈ = C5Me4SiMe3.
Scheme 7. Reaction of an yttrium polyhydride complex with 1,4-bis(trimethylsilyl)-1,3-butadiyne. CpЈ = C5Me4SiMe3.
2538
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2535–2545

Rare Earth Metal Hydride Chemistry
MICROREVIEW
The reaction of 2 with 1,3-cyclohexadiene (CHD) yields drido complex [(C5Me4SiMe3Y)4(µ3-O)2(µ-H)4(THF)] (20) is
the 1,4-addition product 14 in which the resulting allyl unit obtained with simultaneous release of ethylene. Complex 20
interacts with two Y atoms through the two “terminal” car- can also be isolated directly from the reaction of 2 with CO
1
bon atoms, each of which bonds to one metal center in an without isolating 17. H NMR monitoring of the reaction
η1-fashion (Scheme 6).[9] As in the case of styrene, the use of 17 with CO in [D8]THF at low temperatures revealed the
of an excess amount of CHD does not lead to further reac- formation of an enolate intermediate such as [(C5Me4Si-
tion.
Me3Y)4(OCH=CH2)(µ-O)(µ-H)5(THF)] (19). Further reac-
The reaction of 2 with 1,4-bis(trimethylsilyl)-1,3-buta- tion of the yttrium tetrahydrido/dioxo complex 20 with CO
diyne affords the tetranuclear yttrium tetrahydride complex gives the corresponding tetraoxo complex 21 and ethylene.
15, which formally consists of a [(C5Me4SiMe3)YH]44+ unit
and a butene-tetraanion species (Scheme 7).[8] This butene-
tetraanion moiety is bonded to two yttrium atoms in a π,η4-
“inverse-sandwich” fashion and to the other two yttrium
atoms in a σ1,µ1-terminal form at the two terminal carbon
atoms. The formation of 15 can be explained as follows.
Addition of two Y–H units of 2 to the two CϵC units of
1,4-bis(trimethylsilyl)-1,3-butadiyne in
a “2,1” fashion
yields the 1,3-butadien-1,4-diyl (or butadiene-dianion)/
hexahydride species A (Scheme 7). Subsequent 1,4-Y–H ad-
dition to the 1,3-butadiene unit in A could give the 2-buten-
1,1Ј4-triyl/pentahydride species B. Deprotonation at the C4
position of the butene species by a Y–H unit in B should
afford the 2-buten-1,1Ј,4,4Ј-tetrayl/tetrahydride species C.
This species can be regarded as a resonance structure (a
localized form) of 15 (cf. alo D and E).
4. Reaction of Rare Earth Metal Polyhydrides
with Unsaturated C–O Bonds
Reaction of the lutetium octahydride complex 9 with γ-
butyrolactone gives the tetranuclear mixed alkoxido/dihy-
drido complex 16 (Scheme 8).[8] γ-Butyrolactone is a stable,
non-polymerizable cyclic ester. In this reaction, three mole-
cules of γ-butyrolactone are completely reductively ring-
opened by six of the eight hydride ligands in 9 to give the
linear diolate species “-O(CH2)4O-”, thus demonstrating
the unusually high reactivity of this rare earth metal polyhy-
dride complex. Further reaction between 16 and γ-butyrol-
Scheme 9. Selective formation of ethylene by deoxygenative C–C
actone was not observed, even when an excess amount of coupling of carbon monoxide. CpЈ = C5Me4SiMe3.
γ-butyrolactone was present. However, both 9 and 16 show
high activity for the ring-opening polymerization of ε-cap-
Reaction of the lutetium octahydride complex 9 with CO
rolactone.
(1 atm) affords the analogous lutetium hexahydrido/oxyme-
thylene complex 18 and the tetraoxo lutetium cubane com-
plex 22 with release of ethylene (Scheme 9); formation of a
tetrahydrido/dioxo Lu complex intermediate analogous to
20 was not observed. The solid-state structure of 22 is
shown in Figure 2.
Although the reactions of CO with various transition
metal hydrides, including lanthanide and actinide metallo-
cene hydrides, have been reported,[16] the selective formation
Scheme 8. Reaction of a lutetium polyhydride complex with γ-bu-
of ethylene by reduction of CO is unprecedented. These re-
tyrolactone. CpЈ = C5Me4SiMe3.
sults demonstrate again the unique reactivity of polyhy-
The reaction of 2 with CO in toluene at –10 °C occurs drido rare earth metal complexes.
rapidly to give the hexahydrido/oxymethylene yttrium com-
A possible reaction mechanism for the formation of eth-
plex 17 (Scheme 9).[15] When 17 is stirred at –10 °C in the ylene is shown in Scheme 10. Addition of two Y–H bonds
presence of CO for a few minutes and then at room tem- in 2 across the C–O triple bond of one molecule of CO
perature in the absence of CO for 1 h, the dioxo/tetrahy- could give the oxymethylene complex 17. Subsequent inser-
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2539

Z. Hou, M. Nishiura, T. Shima
MICROREVIEW
Figure 2. ORTEP drawing of 22.
tion of another molecule of CO into the Y–CH2 bond in
17 would afford an α-oxyacetyl species such as F (or its
carbenoid isomer). Y–H bond addition across the acyl C–O
double bond in F could yield the oxametallacyclopropane
species G, which would produce the enolate species 19 by
rupture of a C–O and the C–Y bond. Addition of another
Y–H bond across the C–C double bond of the enolate unit
might give the β-oxyethylene species H, which, after cleav-
age of the C–Y and the C–O bond, yields the dioxo com-
plex 20 with release of ethylene. Formation of the tetraoxo
complex 21 and ethylene from the reaction of 20 with CO
could follow an analogous process, although intermediate
species analogous to the oxymethylene 17 or an enolate spe-
cies 19 have not been observed.
Scheme 10. A possible mechanism for the reaction of an yttrium
polyhydride complex with carbon monoxide. CpЈ = C5Me4SiMe3.
Reaction of the octahydride complex 2 with CO2 also
takes place very rapidly, although a characterizable product
could not be isolated from this reaction. However, the struc-
turally characterizable bis(methylene diolate) complex 23
was obtained in high yield from the reaction of the butene-
tetraanion/tetrahydrido
complex
15
with
CO2,
(Scheme 11).[17] In this reaction, the two C–O double bonds
of CO2 are completely reduced to C–O single bonds. This
reaction takes place only at the hydride sites and the but-
ene-tetraanion unit remains unchanged. Although a large
number of studies on the hydrogenation of CO2 with both
homogeneous and heterogeneous systems have been re-
ported, the observation of formation of a methylene diolate
species remains rare.[18] The present formation of the meth-
ylene diolate species is in sharp contrast with what was ob-
served previously in the case of other transition metal po-
lyhydride complexes such as [{(C5H5)Ti(µ-H)}2(C10H8)],[19]
[(C5H5)2Zr(H)(µ-H)(µ-NtBu)Ir(C5Me5)],[20] [H2Ru(dmpe)2],[21]
and [H4Mo(dmpe)2],[22] [dmpe = 1,2-bis(dimethylphos-
phanyl)ethane] which yield either bis(formate),[18,21] mixed
hydride/formate,[19,20] or mixed hydride/carbonate com-
plexes[21] upon reaction with CO2. Further insertion of CO2
into the metal-diolate bonds in 23 could also take place to
give the mono- and dicarbonate complexes 24 and 25 in
a stepwise manner, with possible release of formaldehyde
(HCHO; Scheme 11).
Scheme 11. Reduction of CO2 into methylene diolate species and
subsequent CO2 insertion reactions. CpЈ = C5Me4SiMe3.
2540
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2535–2545

Rare Earth Metal Hydride Chemistry
MICROREVIEW
To see how complex 15 would behave toward an isocya- = 95–99%] with a TOF of up to 2000 gpolymer/(mol
nate, which is a commonly accepted model for CO2 with Lnh)–1 (Scheme 13).[28] The first step in the copolymeriza-
respect to reaction patterns,[23] the reactions of 15 with aryl tion could be the formation of a methylene diolate species
isocyanates have been studied.[17] When 15 was allowed to by insertion of one molecule of CO2 into two Ln–H bonds.
react with two equivalents of phenyl isocyanate or 1-naph-
thyl isocyanate, the dioxo complex 26 was formed together
with the corresponding aromatic imine compounds, proba-
bly via the bis(methylene olate/amido) intermediate I
(Scheme 12). Exposure of the di-µ3-oxo complex 26 to an
atmosphere of CO2 at room temperature led to immediate
formation of the dicarbonate complex 25 (Scheme 12). Al-
though formation of metal oxo complexes, such as
[{Cp2ZrCl}2(µ-O)],[24] [{Cp2Zr(µ-O)}3],[25] and [{(C5H4-
SiMe3)3U}2(µ-O)],[26] has been observed in various CO2 re-
duction reactions, the insertion of CO2 into such a metal
oxo species has not been reported previously. The present
reaction of 26 with CO2 may provide, at a molecular level,
a nice piece of evidence for interactions of CO2 with metal
oxides.[27]
5. Reaction of Rare Earth Metal Polyhydrides
with Nitriles
The reaction of the yttrium or lutetium octahydride com-
plex (2, 9) with four equivalents of benzonitrile or acetoni-
trile affords the corresponding tetranuclear cubane-like
imido complexes [{(C5Me4SiMe3)Ln(µ3-NCH2Ph)}4] (Ln =
Y (27), Lu (28)] or [{(C5Me4SiMe3)Y(µ3-NCH2Ph)}4] (29),
respectively (Scheme 14).[8] The solid-state structure of the
yttrium complex 27 is shown in Figure 3. In these reactions,
the C–N triple bond of a nitrile compound is completely
reduced to a C–N single bond by double Ln–H addition.
This is in sharp contrast to the previously reported reac-
tions of rare earth metallocene hydride complexes[29] or zir-
conocene dihydride complexes[30] with nitriles, which
yielded only single-insertion products such as [{(C5H5)2-
Y(µ-N=CHtBu)}2] or [(C5Me5)2Zr(H)(N=CHR)] (R =
C6H4Me-p). The reaction of polyhydrido rare earth metal
complexes with nitriles may offer a convenient salt-free
route to the corresponding rare earth metal imido com-
plexes, a class of compounds that are of considerable cur-
rent interest but still remain very limited in number.[31]
Scheme 12. Formation of a µ3-oxo yttrium complex and its reaction
with CO2. CpЈ = C5Me4SiMe3.
Polyhydride complexes such as 2, 6, and 9 can also act
as single-component catalysts for the ring-opening homo-
polymerization of cyclohexene oxide (CHO) and the alter-
nating copolymerization of CHO and CO2. The homopoly-
merization of CHO in bulk takes place much more rapidly
than in solution and affords the corresponding polyethers
[Mn = (60–80)ϫ103 and Mw/Mn ≈ 2] in high yields in most
cases. The copolymerization of CHO and CO2 at 70–110 °C
under 12 atm of CO2 yields the corresponding polycarbon-
ates [Mn = (20–40)ϫ103, Mw/Mn = 4–5; carbonate linkages
Scheme 14. Formation of cubane-type imido complexes by re-
duction of nitriles. CpЈ = C5Me4SiMe3.
Further reactions of the rare earth metal imido com-
plexes with nitriles can also take place. The reaction of the
benzylimidolutetium
complex
[{(C5Me4SiMe3)Lu(µ3-
NCH2Ph)}4] (28) with four equivalents of benzonitrile at
room temperature, for instance, gives the benzamidinate-di-
anion complex 30 (Scheme 15).[32] An X-ray analysis
showed that 30 possesses a dimeric structure in which the
two Lu atoms are bridged by the ketimido nitrogen atom
and the amido nitrogen atom is bonded terminally to one
Lu atom. The formation of 30 apparently proceeds by nu-
cleophilic addition of the imido unit in 28 to the CN group
of benzonitrile, which demonstrates that a rare earth metal
imido bond, even in a bridging form, is very reactive. This
Scheme 13. Homopolymerization of cyclohexene oxide (CHO) and
copolymerization of CHO with CO2.
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2541

Z. Hou, M. Nishiura, T. Shima
MICROREVIEW
NCH2CH3)}4] (29) with benzonitrile gives the mixed
benzaldimido/ethylimido complex 31 rather than an inser-
tion product (Scheme 16). An X-ray analysis established
that 31 adopts a tetranuclear crown-like core structure in
which the four Y atoms are unsymmetrically bridged by
four µ2-benzaldimido and two µ3-ethylimido ligands. Four
molecules of benzonitrile are hydrogenated in this reaction
to give four benzaldimido species (PhCH=N–) by abstrac-
tion of four methylene hydrogen atoms from two of the four
ethylimido units (CH3CH2N2–) in 29 with the release of two
molecules of acetonitrile (CH3CN); the other two eth-
ylimido units remain unchanged (Scheme 16). This forma-
tion of an aldimido species through hydrogen transfer from
an alkyl imido species to a nitrile compound is, as far as we
are aware, unprecedented.
Figure 3. ORTEP drawing of 27. The Me3Si and Me groups in
C5Me4SiMe3 have been omitted for clarity.
The reaction of 10 equivalents of benzonitrile with the
benzylimido complexes 27 or 28 gives the benzonitrile tetra-
merization products 32 or 33, respectively (Scheme 17). The
reaction of the benzamidinate-dianion complex 30 with
three equivalents of benzonitrile also affords 33 in high
yield. These results suggest that 30 should be an intermedi-
ate in the formation of 33 from 28. The reaction of the
polyhydride complex 2 with 14 equivalents of benzonitrile
also affords 32. A possible reaction mechanism for the for-
mation of the benzonitrile tetramerization product 32 or 33
is shown in Scheme 18. The coordination of one molecule
of benzonitrile to each metal atom in 30 should yield M.
Hydrogen transfer from a methylene group to a coordinated
PhCN in M (path a) should then afford N. Nucleophilic
contrasts with what was observed previously for transition
metal imido bridging bonds, which are usually robust and
unreactive.[33]
Scheme 15. Nucleophilic addition of an imido complex to benzoni-
trile. CpЈ = C5Me4SiMe3.
In sharp contrast to the above-mentioned nucleophilic addition of the newly formed benzaldimido unit to the ald-
addition reaction of the benzylimido complex 28, the reac- imine group (path b) could yield O, which gives P upon
tion of the ethylimido complex [{(C5Me4SiMe3)Y(µ3- coordination of one molecule of PhCN. Migration of the
Scheme 16. Unique hydrogen transfer from ethyl imido species to benzonitrile. CpЈ = C5Me4SiMe3.
2542
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2535–2545

Rare Earth Metal Hydride Chemistry
MICROREVIEW
methine hydrogen to the PhCN unit in P (path c), followed newly formed imine moiety (step f) could give T, and aro-
by addition of the resulting aldimido unit to the aldimine matization of the resultant six-membered ring in T (step g,
group in the resultant Q (path d), should give 33.
Ln–N bond cleavage and C=N bond formation and step h,
C–N bond cleavage and Ln–N bond formation) should re-
lease the triazine and generate U. Insertion of benzonitrile
into U would then give V, which resembles intermediate N
in Scheme 18 and could regenerate 32 (or 33) as described
above. Thus, the combination of C–N bond formation, hy-
drogen transfer, and C–N bond cleavage steps constitutes a
unique catalytic cycle for the cyclotrimerization of benzoni-
trile.[34]
Scheme 19. Catalytic cyclotrimerization of benzonitrile. CpЈ =
C5Me4SiMe3.
Scheme 17. Tetramerization of benzonitrile. CpЈ = C5Me4SiMe3.
Scheme 20. A possible mechanism for the catalytic cyclotrimeri-
zation of benzonitrile. CpЈ = C5Me4SiMe3.
Scheme 18. Possible mechanism for the formation of 33. CpЈ =
C5Me4SiMe3.
When an excess amount of benzonitrile was added to a
toluene solution of 32 or 33, the benzonitrile-cyclotrimeri-
zation product, namely triphenyl triazine (PhCN)3, is formed
selectively (Scheme 19).[32] The recovery of 32 or 33 from
6. Cationic Rare Earth Metal Polyhydride
Complexes
In contrast to cationic rare earth metal alkyl complexes,
these reactions confirms that these benzonitrile-tetra- which have received much interest as novel polymerization
merized complexes are true catalysts. The polyhydride com- catalysts,[35,36] cationic hydrido rare earth metal complexes
plexes 2 and 9, the imido complexes 27 and 28, and the remain almost unexplored. It has recently been found that
benzamidinate-dianion complex 30 are also active for the tetranuclear rare earth metal octahydride complexes such as
catalytic trimerization of benzonitrile. A possible mecha- 1–7 and 9–12 are suitable precursors for the preparation of
nism for the catalytic cyclotrimerization of benzonitrile by cationic hydrido rare earth metal species.[9] For example, the
32 or 33 is shown in Scheme 20. The coordination of one reactions of the yttrium octahydride complexes 2, 11, and
molecule of benzonitrile to a metal center in 32 (or 33) 12 with one equivalent of [Ph3C][B(C6F5)4] afford the corre-
could give R, and methine hydrogen transfer (step e) to co- sponding cationic heptahydride complexes 35, 36, and 34,
ordinated benzonitrile would afford S. Subsequent intramo- respectively, in high yields (Scheme 21). An direct bonding
lecular nucleophilic addition of the ketimido unit to the interaction between the [(C5Me4SiMe3)4Y4H7]+ cation and
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2543

Z. Hou, M. Nishiura, T. Shima
MICROREVIEW
the [B(C6F5)4]– anion via a Y–F bond is observed in the scribed above, the THF-free or mono(THF)-coordinated
THF-free complex 34, while complexes 35 and 36 are sol- cationic hydride clusters such as 34 or 35 (either isolated or
vent (THF)-separated ion pairs. The X-ray structures of 34 generated in situ) are active catalysts for the syndiospecific
and 36 are shown in Figures 4 and 5, respectively. These polymerization of styrene and the regio- and stereoselective
complexes are the first examples of well-defined cationic hy- cis-1,4-polymerization of CHD (Scheme 22).[9] The
drido rare earth metal complexes.
bis(THF)-coordinated cationic complex 36, however, is not
catalytically activity under the same conditions, probably
due to the occupation of the active metal centers by two
strongly coordinative THF ligands.
Scheme 22. cis-1,4-Selective polymerization of 1,3-cyclohexadiene
by cationic yttrium polyhydride complexes.
7. Conclusion and Perspective
Hydrogenolysis of the bisalkyl complexes [(C5Me4SiMe3)-
Ln(CH2SiMe3)2(THF)] leads to the formation of a series
of structurally characterizable polyhydrido polynuclear rare
earth metal complexes. These polyhydride complexes show
a unique reactivity towards various substrates, such as hy-
drogenation of the C–N triple bond of a nitrile compound
into a C–N single bond, reduction of carbon dioxide into a
methylene diolate species, and deoxygenative C–C coupling
of CO into ethylene. These reactions are in sharp contrast
with what has been observed for the analogous monohy-
drido metallocene complexes. A new class of polynuclear
rare earth metal complexes, such as cubane-type imido and
oxo complexes, has also been derived from the polyhydride
complexes. Abstraction of a hydride ligand from the neutral
hydride clusters leads to the formation of structurally char-
acterizable cationic polyhydrido rare earth metal complexes.
It should also be noted, however, that the synthesis and
reactivity studies of polyhydrido rare earth metal complexes
is still in its infancy, and well defined polyhydrido rare earth
metal complexes reported in the literature are limited essen-
tially to those bearing a C5Me4SiMe3 ligand. As the expert-
ise in this field increases and more complexes with new li-
gand systems are prepared,[37] the diverse chemistry of po-
lyhydrido rare earth metal complexes will certainly be fur-
ther extended. An exciting and prosperous future in this
area can therefore be expected.
Scheme 21. Synthesis of cationic yttrium polyhydride complexes.
CpЈ = C5Me4SiMe3.
Figure 4. ORTEP drawing of 34. The Me3Si and Me groups in
C5Me4SiMe3 have been omitted for clarity.
[1] Selected reviews on d-block transition metal hydride com-
plexes: a) G. S. McGrady, G. Guilera, Chem. Soc. Rev. 2003,
32, 383–392; b) H. Suzuki, Eur. J. Inorg. Chem. 2002, 1009–
1023; c) F. Maseras, A. Lledο´s, E. Clot, O. Eisenstein, Chem.
Rev. 2000, 100, 601–636; d) S. Sabo-Etienne, B. Chaudret,
Chem. Rev. 1998, 98, 2077–2092; e) R. Bau, M. H. Drabnis,
Inorg. Chim. Acta 1997, 259, 27–50; f) Z. Lin, M. B. Hall, Co-
ord. Chem. Rev. 1994, 135/136, 845–879; g) R. H. Crabtree, in
Comprehensive Coordination Chemistry (Eds.: G. Wilkinson, R.
D. Gillard, J. A. McCleverty), Pergamon, Oxford, 1987, vol. 2,
p. 689; h) G. G. Hlatky, R. H. Crabtree, Coord. Chem. Rev.
Figure 5. ORTEP drawing of 36. The Me3Si and Me groups in
C5Me4SiMe3 and the [B(C6F5)4] ion have been omitted for clarity.
In contrast to the neutral hydride cluster 2, which yields
a single Y–H addition product 13 or 14 on reaction with
styrene or 1,3-cyclohexadiene (CHD), respectively, as de-
2544
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2535–2545

Rare Earth Metal Hydride Chemistry
MICROREVIEW
1985, 65, 1–48; i) L. M. Venanzi, Coord. Chem. Rev. 1982, 43,
251–274.
[22] L. K. Fong, J. R. Fox, N. J. Cooper, Organometallics 1987, 6,
223–231.
[2] Selected reviews on rare earth metal hydride complexes: a) Z.
Hou, Bull. Chem. Soc. Jpn. 2003, 76, 2253–2266; b) J. Okuda,
Dalton Trans. 2003, 2367–2378; c) S. Arndt, J. Okuda, Chem.
Rev. 2002, 102, 1953–1976; d) G. A. Molander, J. A. C. Ro-
mero, Chem. Rev. 2002, 102, 2161–2186; e) Z. Hou, Y. Wakat-
suki, Coord. Chem. Rev. 2002, 231, 1–22; f) Z. Hou, Y. Wakat-
suki, in Science of Synthesis (Eds.: T. Imamoto, R. Noyori),
Thieme, Stuttgart, 2002, vol. 2, p. 849–942; g) A. J. Hoskin,
D. W. Stephan, Coord. Chem. Rev. 2002, 233–234, 107–129; h)
M. Ephritikhine, Chem. Rev. 1997, 97, 2193–2242; i) H. Schum-
ann, J. A. Meese-Marktscheffel, L. Esser, Chem. Rev. 1995, 95,
865–986; j) F. T. Edelmann, in Comprehensive Organometallic
Chemistry II (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson,
M. F. Lappert), Pergamon, Oxford, 1995, vol. 4, p. 11–212; k)
C. J. Schaverien, Adv. Organomet. Chem. 1994, 36, 283–362.
[3] W. J. Evans, J. H. Meadows, A. L. Wayda, W. E. Hunter, J. L.
Atwood, J. Am. Chem. Soc. 1982, 104, 2008–2014.
[4] H. van der Heijden, P. Pasman, E. J. M. de Boer, C. J. Schaver-
ien, Organometallics 1989, 8, 1459–1467.
[23] P. Braunstein, D. Nobel, Chem. Rev. 1989, 89, 1927–1945.
[24] a) N. E. Schlörer, E. J. Cabrita, S. Berger, Angew. Chem. Int.
Ed. 2002, 41, 107–109; b) N. E. Schlörer, S. Berger, Organome-
tallics 2001, 20, 1703–1704; c) A. Cutler, M. Raja, A. Todaro,
Inorg. Chem. 1987, 26, 2877–2881; d) S. Gambarotta, S. Strol-
ogo, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Am. Chem.
Soc. 1985, 107, 6278–6282; e) G. Fachinetti, C. Floriani, A.
Roselli, S. Pucci, J. Chem. Soc., Chem. Commun. 1978, 269–
270.
[25] G. Fachinetti, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Am.
Chem. Soc. 1979, 101, 1767–1775.
[26] J. C. Berthet, M. Ephritikhine, New J. Chem. 1992, 16, 767–
768.
[27] a) I. A. Fisher, A. T. Bell, J. Catal. 1997, 172, 222–237; b)
J. C. J. Bart, R. P. A. Sneeden, Catal. Today 1987, 2, 1–124; c)
J. B. Peri, J. Phys. Chem. 1966, 70, 3168–3179; d) N. D. Par-
kyns, J. Chem. Soc. A 1969, 410–417; e) L. Fan, K. Fujimoto,
J. Catal. 1997, 172, 238–242.
[28] D. Cui, M. Nishiura, Z. Hou, Macromolecules 2005, 38, 4089–
4095.
[29] W. J. Evans, J. H. Meadows, W. E. Hunter, J. L. Atwood, J. Am.
Chem. Soc. 1984, 106, 1291–1300.
[30] J. E. Bercaw, D. L. Davies, P. T. Wolczanski, Organometallics
1986, 5, 443–450.
[5] K. C. Hultzsch, T. P. Spaniol, J. Okuda, Angew. Chem. Int. Ed.
1999, 38, 227–230.
[6] a) Z. Hou, Y. Zhang, O. Tardif, Y. Wakatsuki, J. Am. Chem.
Soc. 2001, 123, 9216–9217; b) Z. Hou, Y. Zhang, M. Nishiura,
Y. Wakatsuki, Organometallics 2003, 22, 129–135.
[7] M. Nishiura, J. Baldamus, T. Shima, Z. Hou, manuscript in
preparation.
[31] a) H.-S. Chan, H.-W. Li, Z. Xie, Chem. Commun. 2002, 652–
653; b) J. C. Gordon, G. R. Giesbrecht, D. L. Clark, P. J. Hay,
D. W. Keogh, R. Poli, B. L. Scott, J. G. Watkin, Organometal-
lics 2002, 21, 4726–4734; c) G. R. Giesbrecht, J. C. Gordon,
Dalton Trans. 2004, 2387–2393.
[8] D. Cui, O. Tardif, Z. Hou, J. Am. Chem. Soc. 2004, 126, 1312–
1313.
[9] X. Li, J. Baldamus, M. Nishiura, O. Tardif, Z. Hou, Angew.
Chem. Int. Ed. 2006, 45, 8184–8188.
[10] R. D. Shannon, Acta Crystallogr., Sect. A 1976, 32, 751–767.
[11] A mixture of trinuclear and tetranuclear polyhydride com-
plexes was obtained (M. Nishiura, J. Baldamus, T. Shima, Z.
Hou, unpublished results).
[32] D. Cui, M. Nishiura, Z. Hou, Angew. Chem. Int. Ed. 2005, 44,
959–962.
[33] a) J. R. Fulton, T. A. Hanna, R. G. Bergman, Organometallics
2000, 19, 602–614; b) D. A. Dobbs, R. G. Bergman, Organome-
tallics 1994, 13, 4594–4605; c) S. Takemoto, T. Kobayashi, H.
Matsuzaka, J. Am. Chem. Soc. 2004, 126, 10802–10803; d) J.-
S. Song, S.-H. Han, S. T. Nguyen, G. L. Geoffroy, Organome-
tallics 1990, 9, 2386–2395.
[34] Although benzonitrile cyclotrimerization is known, the mech-
anistic aspects of this process are not well understood. For ex-
amples, see: a) J. H. Forsberg, V. T. Spaziano, T. M. Balasubra-
manian, G. K. Liu, S. A. Kinsley, C. A. Duckworth, J. J. Pot-
eruca, P. S. Brown, J. L. Miller, J. Org. Chem. 1987, 52, 1017–
1021; b) F. Xu, J. H. Sun, H. B. Yan, Q. Shen, Synth. Commun.
2000, 30, 1017–1022.
[35] Selected reviews on cationic rare earth metal alkyl complexes:
a) P. M. Zeimentz, S. Arndt, B. R. Elvidge, J. Okuda, Chem.
Rev. 2006, 106, 2404–2433; b) Z. Hou, Y. Luo, X. Li, J. Or-
ganomet. Chem. 2006, 691, 3114–3121; c) S. Arndt, J. Okuda,
Adv. Synth. Catal. 2005, 347, 339–354; d) J. Gromada, J. F.
Carpentier, A. Mortreux, Coord. Chem. Rev. 2004, 248, 397–
410; e) W. E. Piers, D. J. H. Emslie, Coord. Chem. Rev. 2002,
233–234, 131–135; f) Z. Hou, Y. Wakatsuki, Coord. Chem. Rev.
2002, 231, 1–22.
[12] O. Tardif, M. Nishiura, Z. Hou, Organometallics 2003, 22,
1171–1173.
[13] K. C. Hultzsch, P. Voth, T. P. Spaniol, J. Okuda, Z. Anorg. Allg.
Chem. 2003, 629, 1272–1276.
[14] Y. Luo, J. Baldamus, O. Tardif, Z. Hou, Organometallics 2005,
24, 4362–4366.
[15] T. Shima, Z. Hou, J. Am. Chem. Soc. 2006, 128, 8124–8125.
[16] For examples, see: a) T. Matsuo, H. Kawaguchi, J. Am. Chem.
Soc. 2005, 127, 17198–17199; b) W. J. Evans, J. W. Grate, R. J.
Doedens, J. Am. Chem. Soc. 1985, 107, 1671–1679; c) K. G.
Moloy, T. J. Marks, J. Am. Chem. Soc. 1984, 106, 7051–7064;
d) S. Gambarotta, C. Floriani, A. Chiesi-Villa, C. Guastini, J.
Am. Chem. Soc. 1983, 105, 1690–1691; e) D. A. Katahira, K. G.
Moloy, T. J. Marks, Organometallics 1982, 1, 1723–1726; f) P. J.
Fagan, K. G. Moloy, T. J. Marks, J. Am. Chem. Soc. 1981, 103,
6959–6962; g) P. T. Wolczanski, R. S. Threlkel, J. E. Bercaw, J.
Am. Chem. Soc. 1979, 101, 218–220; h) J. M. Manriquez, D. R.
McAlister, R. D. Sanner, J. E. Bercaw, J. Am. Chem. Soc. 1978,
100, 2716–2724.
[17] O. Tardif, D. Hashizume, Z. Hou, J. Am. Chem. Soc. 2004, 126, [36] a) Y. Luo, J. Baldamus, Z. Hou, J. Am. Chem. Soc. 2004, 126,
8080–8081.
13910–13911; b) X. Li, Z. Hou, Macromolecules 2005, 38,
6767–6769; c) X. Li, J. Baldamus, Z. Hou, Angew. Chem. Int.
Ed. 2005, 44, 962–964; d) L. Zhang, Y. Luo, Z. Hou, J. Am.
Chem. Soc. 2005, 127, 14562–14563; e) L. Zhang, T. Suzuki, Y.
Luo, M. Nishiura, Z. Hou, Angew. Chem. Int. Ed. 2007, 46,
1909–1913.
[18] a) N. E. Schlörer, S. Berger, Organometallics 2001, 20, 1703–
1704; b) N. E. Schlörer, E. J. Cabrita, S. Berger, Angew. Chem.
Int. Ed. 2002, 41, 107–109; c) J. C. Berthet, M. Ephritikhine,
New J. Chem. 1992, 16, 767–768; d) A. Cutler, M. Raja, A.
Todaro, Inorg. Chem. 1987, 26, 2877–2881.
[19] J. Ni, Y. Qiu, T. M. Cox, C. A. Jones, C. Berry, L. Melon, S.
Bott, Organometallics 1996, 15, 4669–4671.
[20] T. A. Hanna, A. M. Baranger, R. G. Bergman, J. Am. Chem.
Soc. 1995, 117, 11363–11364.
[21] M. K. Whittlesey, R. N. Perutz, M. H. Moore, Organometallics
1996, 15, 5166–5169.
[37] We recently found that the use of C5Me5 instead of C5Me4-
SiMe3 as an ancillary ligand led to formation of the corre-
sponding octahedral dodecahydride complexes (M. Nishiura,
K. Mori, T. Mashiko, Z. Hou, unpublished results).
Received: January 23, 2007
Published Online: April 17, 2007
Eur. J. Inorg. Chem. 2007, 2535–2545
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2545