Unusual reactivity of lanthanide borohydride complexes leading to a borane complex

Article abstract of DOI:10.1039/b907193h

Depending on the ionic radius of the central metal atom the BH ₄^- group, which usually reacts in lanthanide chemistry like a pseudo halide, can be involved in redox chemistry and the resulting product contains an N-BH₃ uni

Full text of DOI:10.1039/b907193h

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Unusual reactivity of lanthanide borohydride complexes leading
to a borane complexw
Nils Meyer,a Jelena Jenter,b Peter W. Roesky,*b Georg Eickerlingc and Wolfgang Scherer*c
Received (in Cambridge, UK) 8th April 2009, Accepted 9th June 2009
First published as an Advance Article on the web 29th June 2009
DOI: 10.1039/b907193h
Depending on the ionic radius of the central metal atom the
BH4ꢀ group, which usually reacts in lanthanide chemistry like a
pseudo halide, can be involved in redox chemistry and the
resulting product contains an N–BH3 unit, which binds in an
unusual g2-fashion onto the metal atom.
are neutral compounds, which cannot react via a salt
metathesis route, we and others recently introduced the com-
pound 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole
(DIP2-pyr)H as a ligand in lanthanide chemistry.14,15
We now report on the reaction of the Schiff-base ligand
(DIP2-pyr)H with lanthanide trisborohydrides [Ln(BH4)3(THF)3].
(DIP2-pyr)H can easily be deprotonated with suitable reagents
such as nBuLi or KH to form the corresponding alkali metal
compounds [(DIP2-pyr)Li]16 and [(DIP2-pyr)K] (1).15 Transmetal-
lation of 1 with [La(BH4)3(THF)3] in THF at elevated temperature
leads to the expected product [(DIP2-pyr)La(BH4)2(THF)2]
(2)z (Scheme 1). A similar product, [(DIP2-pyr)LnCl2(THF)2],
(Ln ¼ Y, Lu), was observed by the reaction of 1 with anhydrous
yttrium and lutetium trichloride in THF.15 Clearly in the forma-
tion of compound 2 the trisborohydride [La(BH4)3(THF)3] reacts
like a pseudo halide compound.
The 1H and 13C{1H} NMR spectra of compound 2 show the
expected sets of signals. A characteristic splitting of the iso-
propyl CH3 signals into two sets of doublets is observed, which
is interpreted as a consequence of restricted rꢀotation about the
N–Cipso bond. For the protons of the BH4 group a broad
signal is observed in the 1H NMR spectrum only, whereas in the
11B NMR spectrum a well resolved quintet is seen at d ꢀ21.3 ppm.
In the IR spectrum of compound 2, two characteristic peaks at
2222 cmꢀ1 and 2422 cmꢀ1 are observed, which can be assigned
to a terminal coordinated La(Z3-H3B-H) unit.1
Lanthanide trisborohydrides [Ln(BH4)3(THF)3]1 are efficient
catalysts for the polymerization of isoprene2 and some polar
monomers such as e-caprolactone3 and trimethylene carbonate.4
Lanthanide trisborohydrides were originally prepared in the
early 1950s by the reaction of lanthanide alkoxides with
B2H6.5 A more convenient approach to [Ln(BH4)3(THF)3] is
the reaction of LnCl3 (Ln ¼ La, Ce, Pr, Nd, Sm) with NaBH4,
which was developed by Mirsaidov et al. in the 1970s.6
Currently borohydrides are being investigated as potential
hydrogen storage material.7 Besides their broad application
in catalysis lanthanide trisborohydrides [Ln(BH4)3(THF)3]
have also been used as valuable starting materials to prepare
a large number of lanthanide borohydride derivatives, which
also have a high catalytic potential. To obtain these derivatives
[Ln(BH4)3(THF)3] were reacted in a metathetic approach with
alkali metal reagents, e.g. the reaction of [Ln(BH4)3(THF)3]
with alkali metal cyclopentadienyl derivatives resulted
in different mono cyclopentadienyl complexes8 such as
[(Z5-C5HiPr4)Ln(BH4)2(THF)] (Ln ¼ Sm, Nd)9 and metallo-
cenes such as [(Z5-C5Me5)2Ln(BH4)].10,11 Also post-metallocene
complexes12 and cyclooctatetraene derivatives could be
obtained by salt metathesis.11 To the beꢀst of our knowledge
in all these metathetic reactions the BH4 groups react like a
pseudo halide showing no redox reactions. Thus, in the
The solid-state structure of compound 2 was also estab-
lished by single crystal X-ray crystallography (Fig. 1).
The BH4-hydrogen atoms were freely refined, showing a
ꢀ
ꢀ
Z3-coordination of the BH4 group. If the BH4 group is
considered monodentate the structure reveals a seven-fold
coordination sphere of the ligands around the lanthanum atom
resulting in a distorted pentagonal bipyramidal coordination
ꢀ
described salt metathesis reactions the BH4 groups react as
innocent leaving groups forming MBH4 (M ¼ Li, Na, K) as a
byproduct. Based on these considerations we were interested
in using ligands which potentially may alter the reactivity of
the BH4ꢀ group. It is well known that Schiff-base ligands such
as bis(imino)pyridines are non-innocent ligands, which can be
alkylated at the imino function.13 Since bis(imino)pyridines
ꢀ
polyhedron. The two BH4 groups are located in the apices in
an almost linear setup forming a B1–La–B2 angle of 169.1(2)1,
a Institut fur Chemie und Biochemie, Freie Universita¨t Berlin,
¨
Fabeckstraße 34-36, 14195 Berlin, Germany
b Institut fur Anorganische Chemie, Universitat Karlsruhe (TH),
¨
¨
Engesserstr. 15, Geb. 30.45, 76128 Karlsruhe, Germany.
E-mail: roesky@chemie.uni-karlsruhe.de; Fax: þ49 721-608-4854;
Tel: þ49 721-608-6117
c Institut fur Physik, Universitat Augsburg, Universitatsstr. 1, 86159
¨
¨
¨
Augsburg, Germany. E-mail: wolfgang.scherer@physik.uni-augsburg.de;
Fax: þ49 821-598-3227; Tel: þ49 821-598-3351-3350
w Electronic supplementary information (ESI) available: Crystallo-
graphic data of 2 and 3 and experimental preparations for 2 and 3.
CCDC 689078 (2) and 689232 (3). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b907193h
Scheme 1
ꢁc
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Fig. 1 ORTEP representations of the solid-state structure of 2 at 200 K
(50% probability) showing the atom labeling scheme, omitting hydrogen
atoms. Selected bond lengths [A] or angles [1]: La–N1 2.852(3), La–N2
2.453(3), La–N3 2.890(3), La–O1 2.616(3), La–O2 2.604(3), La–B1
2.714(5), La–B2 2.711(6); N1–La–N2 61.37(9), N1–La–N3 122.40(8),
N2–La–N3 61.15(9), N1–La–O1 77.43(9), N1–La–O2 160.32(10),
N2–La–O1 138.51(9), N2–La–O2 138.30(10), N3–La–O1 160.14(9),
N3–La–O2 77.20(10), N1–La–B1 91.21(14), N1–La–B2 96.07(15),
N2–La–B1 96.2(2), N2–La–B2 94.40(15), N3–La–B1 91.45(15),
N3–La–B2 91.4(2), O1–La–O2 83.03(10), O1–La–B1 89.0(2), O1–La–B2
84.8(2), O2–La–B1 86.11(15), O2–La–B2 84.3(2), B1–La–B2 169.1(2).
Fig. 2 ORTEP representations of the solid-state structure of 3 (50%
probability) at 6 K. Two additional THF solvent molecules and the
hydrogen atoms except for the freely refined B–H atoms are omitted for
clarity. Selected bond lengths [A] and angles [1]: Lu1–N2 2.284(3),
Lu1–N3 2.168(3), Lu1–O1 2.356(3), Lu1–O2 2.358(3), Lu1–B1
2.486(5), Lu1–B2 2.853(5), N1–C1 1.317(5), N3–C6 1.465(5),
N2–Lu1–N3 76.03(12), N2–Lu1–O1 88.38(10), N2–Lu1–O2 82.85(10),
O1–Lu1–B1 93.50(13), O1–Lu1–B2 88.06(12), O2–Lu1–B1 94.96(13),
O2–Lu1–B2 86.45(12), N2–Lu1–B1 174.67(13), N2–Lu1–B2 74.26(12),
N3–Lu1–B1 108.86(14), N3–Lu1–B2 150.27(13), B1–Lu1–B2 100.79(14).
whereas the (DIP2-pyr)ꢀ ligand and both THF molecules are
located in the molecular plane. The almost planar arrangement
of the LaN3O2 fragment is reflected by the sum of the corres-
ponding five valence angles of 360.181. The two phenyl rings are
oriented perpendicular to the plane of the pyrrolyl moiety. The
structural model of compound 2 is very similar to the one of
[(DIP2-pyr)YCl2(THF)2],15 which also shows a pentagonal
bipyramidal coordination with two chlorine atoms located in
apical positions. This again clearly supports the pseudo halide
maps (Fig. 2). The obtained solid-state model is consistent
with the NMR data of 3 in solution and as a result of the
reduction process the ligand displays one imino and one amido
functionality. Accordingly, the C–N bond lengths of both
groups differ significantly (N1–C1 1.317(5) A vs. N3–C6
1.465(5) A) and clearly reveal the single bond character of
the amino C–N moiety. Furthermore, as a result of this ligand
reduction, the remaining BH3 group is coordinated to the
imino function featuring a covalent B–N bond length of
1.562(6) A. The distorted pentagonal bipyramidal coordina-
tion polyhedron of 3 is completed by tꢀwo THF ligands in axial
positions and a Z3-coordinated BH4 group. In contrast to
complex 2 both borohydride groups are now situated in the
molecular plane spanning a B1–Lu–B2 angle of 100.79(14)1.
The different chemical nature of the coordinating BH4ꢀ ligand
and the QN–BH3 moiety is reflected in significantly different
Lu–B contacts of 2.486(5) and 2.853(5) A, respectively.
ꢀ
character of the BH4 groups in lanthanide chemistry.
Reacting compound 1 with [Lu(BH4)3(THF)3] under the
same reaction conditions used for the preparation of compound
2 does not result in an analogous complex. Instead, a reduction
ꢀ
of one imino function by one BH4 group takes place and
compound 3 is formedz (Scheme 1). The resulting BH3 unit is
bound to the nitrogen atom of the remaining imino function of
the ligand. Thus, the resulting formally dinegatively charged
ligand is coordinated to a Lu–BH4 unit via the nitrogen atoms
of the amido and the pyrrolyl function and the two hydrogen
atoms of the newly formedQN–BH3 group (Scheme 1). To thꢀe
best of our knowledge a comparable reactivity of the BH4
group was not observed previously in lanthanide chemistry. It
Moreover, the coordination mode of both borohydride
moieties differs (Fig. 2). Accordingly, the QN–BH3 moiety
coordinates in a Z2-fashion to the lutetium atom resulting in
Lu–H distances of 2.38(6) A and 2.47(7) A. The B–H bond
distances of these two hydrogen atoms (1.24(7) and 1.18(7) A,
respectively) are significantly longer than that one of the non-
coorꢀdinated B–H hydrogen atom (1.01(6) A). In contrast, the
BH4 ligand binds in an asymmetric Z3-fashion and displays
one shorter and two longer Lu–H bond lengths of 2.24(6) A,
2.33(6) A, and 2.35(7) A, respectively. The observed reactivity
1
can be clearly seen from the H and 13C{1H} NMR spectra of
compound 3 that the symmetry of the ligand is broken due to
the reduction process. Thus, the four isopropyl CH3 groups
are all in chemically different environments and display four
doublets and two septets in the 1H NMR spectrum while
the hydrogen atoms of the pyrrolyl ring show two doublets.
The 13C{1H} NMR is consistent with these observations. In the
11B NMR a quintet for the BH4ꢀ group (d ¼ ꢀ25.2 ppm) and a
broad signal for theQN–BH3 moiety (d ¼ ꢀ14.9 ppm) is seen.
The solid-state structural model of compound 3 was estab-
lished by single crystal X-ray diffraction at low temperatures
(6 K) to minimize the smearing of the electron density due to
thermal motion and to overcome the structural disorder
observed above 200 K. Subsequent structural refinements
allowed us to resolve the position of the B–H hydrogen atoms
in close proximity to the lutetium atom in difference Fourier
ꢀ
and the simultaneous coordination of a BH4 and a BH3
unit are to the best of our knowledge unique in lanthanide
chemistry. The only other structurally characterized rare-
earth complex, featuring a N–BH3 moiety in the ligand sphere,
is a scandium compound, which is coordinated by four anionic
3-borane-1-alkylimidazol-2-ylidene ligands.17 This ligand
is an N-heterocyclic carbene having one BH3 group attached
onto one of the two nitrogen atoms. In contrast to compound
3, this complex was obtained by a standard salt metathesis
reaction starting from ScCl3. In the scandium complex one
hydrogen atom of each N–BH3 coordinates to the metal
center.
ꢁc
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ꢀ
7100 independent reflections (Rint ¼ 0.0573). The structure was solved by
In conclusion, we have shown that the BH4 group, which
Patterson methods and refined by full-matrix least-square techniques
usually reacts in lanthanide chemistry like a pseudo halide, can be
involved in a redox reaction. By reacting compound 1 with
[Lu(BH4)3(THF)3] one of the two Schiff-base functions of the
ligand is reduced.18 The byproduct BH3 of this reduction was
trapped by the remaining imino nitrogen atom. The resulting
N–BH3 unit binds in a Z2-fashion via two three-center-two-
electron bonds onto the lutetium atom. To the best of our know-
ledge this kind of coordination has not been observed previously in
lanthanide chemistry. It should also be noted that under the same
reaction conditions the BH4ꢀ groups in [La(BH4)3(THF)3] do not
show any redox reaction. Maybe as a result of the smaller ion
radius of lutetium a compound of composition [(DIP2-pyr)-
Lu(BH4)2(THF)2] cannot be formed because of steric reasons.
The resulting steric strain may cause the unusual reactivity.
This work was supported by the Deutsche Forschungsge-
meinschaft (SPP 1166).
using 4451 reflections with I 4 2s(I) to R1 ¼ 0.0340 and wR2
¼
0.0723. 3: MAR345 imaging plate detector system (MARRESEARCH)
mounted on a four-circle goniometer with Eulerian geometry (HUBER)
with a rotating anode generator (Bruker FR591, MoKa radiation l ¼
0.71073 A). The crystal was cooled to 6 K employing a closed-cycle helium
cryostat (ARS-4K). Details of the data reduction and refinement:
space group Pn (no. 7) with a ¼ 9.5587(9) A, b ¼ 15.1757(17) A, c ¼
16.1030(19) A, b ¼ 100.420(9)1 at 6(2) K, Z ¼ 2, V ¼ 2297.4(4) A3, m ¼
2.192 mmꢀ1, 2ymax ¼ 60.841, 13 834 reflections collected, 8265 indepen-
dent reflections (Rint ¼ 0.0282). The data set was corrected for beam
inhomogeneity and absorption effects (Tmin/Tmax ¼ 0.688(4)/0.814(4)).
The structure was solved by Patterson methods and refined by full-matrix
least-square techniques using 8047 reflections with I 4 2s(I) to R1
¼
0.0271, wR2 ¼ 0.0618, goodness of fit 1.141, largest diff. peak and hole
1.091 and ꢀ0.848 e Aꢀ3. All hydrogenatom positions could be located in
the difference fourier map but were put to calculated positions (except for
the BH3 and the BH4 group) in the final structural model to achieve faster
convergence of the least-square refinement.
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Notes and references
z Preparation of 2–3: All manipulations were carried out under
anaerobic and anhydrous conditions. 2: THF (10 ml) was condensed
at ꢀ78 1C onto a mixture of [La(BH4)3(THF)3] (0.40 g, 1.0 mmol) and
(0.48 g, 1.0 mmol) [(DIP2-pyr)K] (1) and the resulting yellow reaction
mixture was stirred for 16 h at 60 1C. The yellow solution was filtered
off and concentrated until a white precipitate appears. The mixture
was heated carefully until the solution became clear. The solution was
allowed to stand at ambient temperature to obtain the product as
yellow crystals after 16 h. Yield: 0.55 g, 0.7 mmol, 70%. 1H-NMR
(THF-d8, 400 MHz, 25 1C): d ¼ 0.62–0.84 (br, 8H, BH4), 1.03 (d, 12H,
CH(CH3)2, JH,H ¼ 6.7 Hz), 1.20 (d, 12H, CH(CH3)2, JH,H ¼ 6.7 Hz),
3.57 (sept, 4H, CH(CH3)2, JH,H ¼ 6.7 Hz), 6.62 (s, 2H, 3,4-pyr),
7.07–7.14 (m, 6H, Ph), 8.05 (s, 2H, NQCH). 13C{1H} NMR (THF-d8,
100.4 MHz, 25 1C): d ¼ 22.1 (CH(CH3)2), 25.6 (CH(CH3)2), 27.4
(CH(CH3)2), 117.1 (3,4-pyr), 123.2 (Ph), 126.4 (Ph), 141.0 (2,5-pyr),
142.9 (Ph), 148.7 (Ph), 163.3 (NQCH). 11B NMR (THF-d8,
128.15 MHz, 25 1C): d ¼ ꢀ21.3 (br qt, JH,B ¼ 89.1 Hz). IR
(KBr, n/cmꢀ1): 871(m), 1049(m), 1099(m), 1161(s), 1327(s), 1450(m),
1566(vs), 2171(w), 2222(s), 2330(w), 2422(m), 2874(s), 2962(vs).
C38H62B2N3O2La (753.45): calcd C, 60.58, H, 8.29, N, 5.58%;
found C, 59.63, H, 8.84, N, 5.40%. 3: THF (10 ml) was condensed
at ꢀ78 1C onto a mixture of [Lu(BH4)3(THF)3] (0.63 g, 1.4 mmol) and
[(DIP2-pyr)K] (1) (0.67 g, 1.4 mmol) and the resulting yellow reaction
mixture was stirred for 16 h at 60 1C. The yellow solution was filtered
off and concentrated until a white precipitate appears. The mixture
was heated carefully until the solution became clear. The solution was
allowed to stand at ambient temperature to obtain the product as
yellow crystals after several hours. Yield 0.45 g, 0.6 mmol, 43%.
1H NMR (THF-d8, 400 MHz, 25 1C): d ¼ 0.82–0.90 (br, 4H, BH4), 1.12
11 (a) S. M. Cendrowski-Guillaume, M. Nierlich, M. Lance and
786–788;
M.
Ephritikhine,
Organometallics,
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17,
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I. Korobkov and P. H. M. Budzelaar, J. Am. Chem. Soc., 2005,
(d, 6H, CH(CH3)2, JH,H
¼
7.0 Hz), 1.14 (d, 6H, CH(CH3)2,
JH,H ¼ 7.0 Hz), 1.15 (d, 6H, CH(CH3)2, JH,H ¼ 7.0 Hz), 1.21 (d, 6H,
CH(CH3)2, JH,H ¼ 7.0 Hz), 2.42 (br, 3H, BH3), 3.05 (sept, 2H,
CH(CH3)2, JH,H ¼ 7.0 Hz), 3.73 (sept, 2H, CH(CH3)2, JH,H
¼
7.0 Hz), 4.51 (s, 2H, N–CH2), 6.21 (d, 1H, pyr, JH,H ¼ 3.8 Hz), 6.85
(t, 1H, Ph, JH,H ¼ 7.3 Hz), 6.95 (d, 2H, Ph, JH,H ¼ 7.3 Hz), 7.04
(d, 1H, pyr, JH,H ¼ 3.8 Hz), 7.14–7.22 (m, 3H, Ph), 7.56 (s, 1H, NQCH).
13C{1H} NMR (THF-d8, 100.4 MHz, 25 1C): d ¼ 23.2 (CH(CH3)2), 23.5
(CH(CH3)2), 24.8 (CH(CH3)2), 25.7 (CH(CH3)2), 26.4 (CH(CH3)2), 28.4
(CH(CH3)2), 58.6 (N–CH2), 110.3 (pyr), 122.6, 122.9, 123.6, 127.0, 129.6,
133.2, 142.6, 146.0, 149.6, 153.0, 153.3, 165.7 (NQCH). 11B NMR
127, 13019–13029; (d) M. Zimmermann, K. W. Tornroos and
¨
R. Anwander, Angew. Chem., 2007, 119, 3187–3191 (Angew.
Chem., Int. Ed., 2007, 46, 3126–3130).
14 Y. Matsuo, K. Mashima and K. Tani, Organometallics, 2001, 20,
3510–3518.
(THF-d8, 128.15 MHz, 25 1C): d ¼ ꢀ25.2 (br qt, BH4, JH,B
¼
15 N. Meyer, M. Kuzdrowska and P. W. Roesky, Eur. J. Inorg.
Chem., 2008, 1475–1479.
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J. Chem. Soc., Dalton Trans., 2000, 459–466.
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R. D. Ernst, H. Pritzkow and W. Siebert, J. Organomet. Chem.,
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18 Reduction chemistry of U-BH4 compounds were reported earlier;
e.g. T. Arliguie, P. Thuery, M. Fourmigue and M. Ephritikhine,
´ ´
Organometallics, 2003, 22, 3000–3003.
71.3 Hz), ꢀ14.8 (br, BH3). IR (KBr, n/cmꢀ1): 865(m), 906(w), 1056(s),
1103(m), 1139(m), 1198(s), 1246(s), 1289(s), 1322(s), 1382(m), 1462(s),
1538(w), 1584(s), 1607(vs), 2180(w), 2230(m), 2295(m), 2423(m),
2467(m), 2867(s), 2963(vs). C38H62B2N3O2Lu (789.51): calcd C, 57.81,
H, 7.92, N, 5.32%; found C, 57.62, H, 8.09, N, 5.38%.
Crystal data for 2–3: 2: Stoe IPDS2T diffractometer, MoKa radiation
(l ¼ 0.71073 A): space group Pbca (no. 61) with a ¼ 16.0927(8) A, b ¼
25.137(2) A,
c
¼
19.9083(10) A, at 200(1) K,
Z
¼
8,
V
¼
8053.4(8) A3, m ¼ 1.094 mmꢀ1, 2ymax ¼ 54.31, 28 300 reflections collected,
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4693–4695 | 4695