Dihydrogen addition in a dinuclear rare-earth metal hydride complex supported by a metalated TREN ligand

Article abstract of DOI:10.1021/ja207180x

The dinuclear lutetium dihydride dication supported by metalated tripodal ligands undergoes facile hydrogenolysis with H₂ to form a trihydride dication. Molecular orbital analysis shows that the LUMO is a bonding Lu???Lu orbital that is poised to activate dihydrogen.

Full text of DOI:10.1021/ja207180x

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Dihydrogen Addition in a Dinuclear Rare-Earth Metal Hydride
Complex Supported by a Metalated TREN Ligand
Ajay Venugopal,† Waldemar Fegler,† Thomas P. Spaniol,† Laurent Maron,*,‡ and Jun Okuda*,†
†Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany
‡Universitꢀe de Toulouse, INSA, UPS, LPCNO, and CNRS, LPCNO UMR 5215, 31077 Toulouse, France
S Supporting Information
b
is coordinated by the (Me5TRENCH2)À ligand in a k4N:kC
ABSTRACT: The dinuclear lutetium dihydride dication
supported by metalated tripodal ligands undergoes facile
hydrogenolysis with H2 to form a trihydride dication. Molecular
orbital analysis shows that the LUMO is a bonding Lu Lu
orbital that is poised to activate dihydrogen.
manner (Figure 1). The overall geometry around the metal
center can be considered as distorted trigonal bipyramidal. The
LuÀCH2SiMe3 distance in 2a-Lu is comparable to those found
in other monocationic lutetium alkyls.14,15
3 3 3
When an Et2O solution of 2-Lu was treated with a 4-fold
excess of PhSiH3 and allowed to stand for 24 h at room tem-
perature, a colorless crystalline precipitate of [Lu2(Me5TRENCH2)-
H2][X]2 (3-Lu) was obtained in 76% yield (Scheme 1). 3-Lu was
characterized by elemental analysis, multinuclear NMR spectros-
copy, and single-crystal XRD. 3-Lu is the first example of a
cationic rare-earth metal hydride featuring a Ln2(μ-H)2 core.3
Each of the metal centers is additionally coordinated by the
anionic (Me5TRENCH2)À ligand in a k4N:kC manner (Scheme 1).
The dicationic charge is balanced by two organoborate anions,
nterstitial rare-earth compounds such as LaNi5H6 have been
recognized as hydrogen storage materials.1 Molecular hydrides
I
could act as model compounds toward understanding the me-
chanism of dihydrogen uptake and release.2 Metallocene hy-
drides of the type [Ln(η5-C5R5)2H]3 exist as monomers and
exhibit high reactivity4 but cannot change the number of hydride
ligands. Half-sandwich rare-earth metal hydrides of varied nucle-
arity, [Ln(η5-C5R5)H2]x,5 as well as neutral rare-earth metal
hydrides of the type [Ln(LnX)H2]6 bearing a non-cyclopenta-
dienyl monoanionic ligand (LnX), have also been shown to be
active in a wide range of stoichiometric and catalytic reactions4À6
but fail to undergo dihydrogen addition and/or release.7
We report here that a dinuclear rare-earth metal hydrido cation
supported by the anionic ligand (Me5TRENCH2)À (L4X-type
ligand derived from the neutral ligand tris{2-(dimethylamino)-
ethyl}amine, Me6TREN)8,9 adds dihydrogen to give another
hydride cation. Cationic hydrides can be expected to be less
aggregated and to exhibit different reactivity patterns, but they
are still rare.10,11
1
[B{C6H3-3,5-(CF3)2}4]À. A characteristic feature in the H
NMR spectrum of 3-Lu in THF-d8 recorded at 24 °C is the
appearance of two broad signals at δ 11.65 and 11.69 ppm for the
bridging hydride ligands and two AB patterns at δ 1.13 and 1.76
ppm for the methylene protons of the (Me5TRENCH2)À ligand.
1
1
Variable-temperature H NMR and H COSY studies indicate
that 3-Lu is present in solution as a diastereomeric mixture of
centrosymmetric heterochiral (meso) and C2-symmetric homo-
chiral (rac) dimers (see SI). Similar observations have previously
been made in the case of metallocene and half-sandwich rare-
earth metal hydrides.16 The solid-state structure of 3-Lu 2Et2O
3
was elucidated by single-crystal XRD. The dication consists of
two Lu centers of opposite chirality bridged by two μ2-hydride
ligands (Figure 2). The two lutetium centers are separated by a
Me6TREN reacts with [Lu(CH2SiMe3)2(THF)x][X]12 to form
the monocationic lutetium alkyl [Lu(Me6TREN)(CH2SiMe3)2]-
[X] (X=B{C6H3-3,5-(CF3)2}4, 1-Lu, Scheme1). 1-Lu is thermally
sensitive and decomposes at temperatures above 0 °C. The
identity and purity of 1-Lu were established by 1H NMR spectro-
scopy. At room temperature, 1-Lu is converted over a period of
3 days, yielding 91% of [Lu(Me5TRENCH2)(CH2SiMe3)][X]
(2-Lu) with concomitant elimination of SiMe4. This transforma-
tion results from the metalation of one of the six methyl groups in
Me6TREN, forming the anionic (Me5TRENCH2)À ligand of
L4X-type. Similar metalations of NMe2 groups by alkyl derivatives of
rare-earth metals have been previously observed.13 2-Lu is soluble
in Et2O, THF, and CH2Cl2, is stable at room temperature for
several days both in the isolated state and in solution, and was
characterized by elemental analysis and multinuclear NMR
spectroscopy.
distance of 3.5147(2) Å, which is comparable to the Lu Lu
3 3 3
distances in the neutral dimeric μ2-bridged lutetium complexes.17
Each of the Lu atoms is coordinated by the (Me5TRENCH2)À
ligand. An inversion center exists between the two lutetium
atoms, and hence the atoms are symmetry related. The LuÀN
distances lie in the range of 2.397(2)À2.484(2) Å, and the
Lu1ÀC1A distance is 2.328(5) Å; both these distances are
slightly larger than the corresponding distances found in 2-Lu.
When a THF-d8 solution of 3-Lu was treated with 1 bar of H2
at room temperature for 24 h, one of the two (Me5TRENCH2)À
ligands in 3-Lu reverted to Me6TREN, resulting in the formation
of a dicationic tri(μ2-hydride) complex, [Lu(Me5TRENCH2)-
(μ2-H)3Lu(Me6TREN)][X]2 (4-Lu, Scheme 1). This reaction
was observed also in the absence of any solvent in the solid state.
Notably, further hydrogenation of 4-Lu was not observed even
Single crystals suitable for X-ray diffraction (XRD) experiments
were obtained for [Lu(Me5TRENCH2)(CH2SiMe3)][B(C6F5)4]
(2a-Lu), synthesized in a method similar to the preparation of
2-Lu (see Supporting Information (SI)). The Lu atom in 2a-Lu
Received: August 4, 2011
Published: October 07, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja207180x J. Am. Chem. Soc. 2011, 133, 17574–17577
|

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COMMUNICATION
Scheme 1. Synthesis and Reactivity of Cationic Lutetium
Hydrides
Figure 2. ORTEP diagram of the cationic part of 3-Lu 2Et2O. Thermal
3
ellipsoids are drawn at 50% probability level. Hydrogen atoms bound to
carbon atoms are omitted for clarity. Selected bond lengths [Å] and
angles [°]: Lu1 Lu1 3.5147(2), Lu1ÀC1A 2.328(5), Lu1ÀN1
3 3 3
2.397(2), Lu1ÀN2 2.398(2), Lu1ÀN3 2.459(2), Lu1ÀN4 2.484(2);
C1AÀLu1ÀN1 36.23(14), N1ÀLu1ÀN2 125.71(8), N1ÀLu1ÀN3
103.26(10), N1ÀLu1ÀN4 72.89(8), N2ÀLu1ÀN3 104.28(9),
N2ÀLu1ÀN4 72.76(7), N3ÀLu1ÀN4 71.79(9).
the first example of a dicationic rare-earth metal trihydride. The
signal for the three hydrides in the 1H NMR spectrum of 4-Lu in
THF-d8 appears as a singlet and is upfield shifted to δ 8.27 ppm as
compared to those in 3-Lu. The two signals for the diastereotopic
methylene protons of the (Me5TRENCH2)À unit appear at δ
1.44 and 1.53 ppm.
In separate experiments, treatment of either 1-Lu or 2-Lu with
1 bar of H2 in Et2O for 24 h also resulted in the precipitation of
4-Lu as colorless crystals in 70% yield. The pathway from 1-Lu to
give 4-Lu possibly involves [(Me6TREN)LuH2]+ as an inter-
mediate, which, during dimerization, loses dihydrogen to form
4-Lu. However, there is no experimental evidence for the forma-
tion of this type of mononuclear intermediate with terminal
hydrides. The transformation from 2-Lu to 4-Lu can occur either
via this intermediate or via 3-Lu. An isostructural yttrium complex
4-Y has also been obtained in 69% yield using an analogous
synthetic procedure (see SI), but a hydride cation analogous to
3-Lu was not observed. The solid-state structure of 4-Lu 2Et2O
3
was confirmed by single-crystal XRD. The dication consists of
two lutetium metal atoms which are μ2-bridged by three hydride
ligands (Figure 3). The Lu2H3 core in 4-Lu resembles those found
in the monocations [Ln2(Me-PNPiPr)2H3(THF)2]+ (Ln =
Y, Lu).11 The Lu Lu distance of 3.2775(4) Å in 4-Lu is shorter
Figure 1. ORTEP diagram of the cationic part of 2a-Lu. Thermal
ellipsoids are drawn at 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [°]: Lu1ÀC1
2.298(5), Lu1ÀC13 2.347(5), N1ÀLu1 2.376(4), N2ÀLu1 2.420(4),
N3ÀLu1 2.405(4), N4ÀLu1 2.464(4); C13ÀLu1ÀN1 130.30(16),
C13ÀLu1ÀN4 151.62(16), N3ÀLu1ÀN2 120.04(14).
3 3 3
by 0.24 Å than the Lu Lu distance in 3-Lu and comparable to
3 3 3
that in [Lu2(Me-PNPiPr)2H3(THF)2][BPh4] (3.2174(4) Å).11
While one of the Lu atoms in 4-Lu is coordinated by the
neutral Me6TREN, the other is coordinated by the anionic
(Me5TRENCH2)À ligand. The LuÀN and LuÀC distances
are slightly larger than corresponding distances found in 2-Lu
and 3-Lu.
with 7 bar of H2 at 70 °C. 4-Lu did not convert back to 3-Lu
when it was heated in THF-d8 in a sealed NMR tube under
vacuum for 3 days, but H2 evolution was observed, accompanied
by the formation of intractable Lu compounds. The hydrogeno-
lysis from 3-Lu to 4-Lu resembles the reaction between [Ln(η5-
C5Me4SiMe3)(η5:η1-C5Me4SiMe2CH2)(THF)] (Ln = Y, Nd,
Sm, Dy, Lu) and H2, wherein the “tucked-in” chelating methy-
lene bridge undergoes σ-bond metathesis to form the metallo-
cene hydride [Ln(η5-C5Me4SiMe3)2H(THF)].4a 4-Lu was char-
acterized by NMR spectroscopy and elemental analysis and is
In order to gain further insight into the bonding as well as the
relative stability of each of the complexes, DFT calculations on
both the lutetium and yttrium compounds were carried out (see
SI). The geometry of the complexes 3-Lu and 4-Lu as well as 3-Y
and 4-Y was optimized. In 3-Lu, the optimized Lu Lu and
3 3 3
LuÀCH2N distances are 3.57 and 2.29 Å, in excellent agreement
with the experimental values. The LuÀH distances are found to
be 2.13 Å, indicating a symmetrical coordination of the bridging
hydrides. A close analysis of the molecular orbitals shows that
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COMMUNICATION
Figure 3. ORTEP diagram of the cationic part of 4-Lu 2Et2O. Thermal
3
ellipsoids are drawn at 50% probability level. Hydrogen atoms bound to
carbon atoms are omitted for clarity. Selected bond lengths [Å] and
angles [°]: Lu1 Lu1 3.2775(4), Lu1ÀC1A 2.441(12), Lu1ÀN1
3 3 3
2.448(2), Lu1ÀN2 2.448(2), Lu1ÀN3 2.482(2), Lu1ÀN4 2.539(2);
C1AÀLu1ÀN1 33.9(3), N2ÀLu1ÀN1 117.75(8), N1ÀLu1ÀN3
107.05(9), N1ÀLu1ÀN4 71.52(8), N2ÀLu1ÀN3 105.68(9), N2À
Lu1ÀN4 71.61(8), N3ÀLu1ÀN4 70.75(8).
Figure 5. Proposed activation of dihydrogen in 3-Lu.
of [Lu(Me5TRENCH2CPh2O)(OCHPh2)][X] (5-Lu). The reac-
tion of 4-Lu with 4 equiv of benzophenone led to CO insertion
between three LuÀH bonds and one LuÀCH2N bond to give
a 1:1 mixture of 5-Lu and [Lu(Me6TREN)(OCHPh2)2][X]
(6-Lu). Similar reactivity has been observed in the case of 4-Y.
Both insertion reactions resulted in quantitative yields of the
corresponding products, which have been fully characterized
(see SI).
In conclusion, facile dihydrogen addition to a cationic hydrido
lanthanide complex 3-Lu has been found. Although clean
dihydrogen release has not been observed, dihydrogen evolution
is observed upon heating 4-Lu under vacuum. Currently, we are
investigating other ligand systems in order to achieve reversible
dihydrogen activation.
Figure 4. LUMO of 3-Lu.
’ ASSOCIATED CONTENT
the LUMO is a bonding Lu Lu orbital that is poised to activate
dihydrogen prior to σ-bond metathesis (Figure 4).
This has further been demonstrated by computing the entire
3 3 3
S
Supporting Information. Synthesis and characterization
b
of all new compounds; 1H NMR spectra of 1-Lu, 2-Lu, 2a-Lu,
1
free energy pathway (Figure 5). The reaction of 3-Lu with H2 to
3-Lu, 4-Lu, and 4-Y; variable-temperature H NMR spectra of
give 4-Lu is predicted to be exergonic by 4.9 kcal molÀ1. The
3
3-Lu in THF-d8; tables of crystallographic data; data on computa-
tional studies; and X-ray crystallographic data, in CIF format, for
2a-Lu, 3-Lu, 4-Lu, 4-Y, and 6-Y. This material is available free of
charge via the Internet at http://pubs.acs.org.
activation barrier is predicted to be 23.4 kcal molÀ1. In the
3
transition state, the two hydrogen and two lutetium atoms are no
longer coplanar. This bending allows the d-orbital of Lu, involved
in a bonding interaction in the LUMO, to interact with the σ-
orbital of H2. The observed H H distance of 0.96 Å is typically
found in a σ-bond metathesis transition state. In 4-Lu, the
3 3 3
’ AUTHOR INFORMATION
Corresponding Author
jun.okuda@ac.rwth-aachen.de
Lu Lu distance is reduced to 3.28 Å (experimentally observed
3 3 3
3.278 Å) as compared to that in 3-Lu. The LuÀCH2N distance
of 2.31 Å is in agreement with the experimental value. The
three hydrogen atoms in 4-Lu are no longer symmetrically
bonded to the lutetium atoms. LuÀH distances vary between
2.15 and 2.27 Å.
Reactivity studies of 3-Lu and 4-Lu toward benzophenone
were performed to confirm the nucleophilic property of the
hydride ligands despite the overall positive charge.18 3-Lu reacted
with 4 equiv of benzophenone, leading to CO insertion between
two LuÀH and two LuÀCH2N bonds, resulting in the formation
’ ACKNOWLEDGMENT
We are grateful to the Deutsche Forschungsgemeinschaft for
financial support and to the Alexander von Humboldt Founda-
tion for a postdoctoral fellowship to A.V. We thank Toni Gossen
for assistance with the NMR spectroscopy experiments. L.M.
thanks the Institut Universitaire de France, CINES, and CalMip
for granting computational time.
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