[Ln(BH4)2(THF)2](Ln = Eu, Yb)-A highly luminescent material. Synthesis, properties, reactivity, and NMR studies

Article abstract of DOI:10.1021/ja308077t

The divalent lanthanide borohydrides [Ln(BH₄) ₂(THF)₂] (Ln = Eu, Yb) have been prepared in a straightforward approach. The europium compound shows blue luminescence in the solid state, having a quantum yield of 75%. Nonradiative deactivation of C-H and B-H oscillator groups could be excluded in the perdeuterated complex [Eu(BD₄)₂(d₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln(BH₄)(THF) ₅][BPh₄] and the bis(phosphinimino)methanides [{(Me ₃SiNPPh₂)₂CH}Ln(BH₄)(THF) ₂] have been prepared from [Ln(BH₄)₂(THF) ₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH} Yb(BH₄)(THF)₂], we performed a 2D ³¹P/ ¹⁷¹Yb HMQC experiment.

Full text of DOI:10.1021/ja308077t

Communication
pubs.acs.org/JACS
[Ln(BH4)2(THF)2] (Ln = Eu, Yb)A Highly Luminescent Material.
Synthesis, Properties, Reactivity, and NMR Studies
Sebastian Marks, Joachim G. Heck, Marija H. Habicht, Pascual Ona-Burgos, Claus Feldmann,
̃
and Peter W. Roesky*
Institut fur Anorganische Chemie, Karlsruher Institut fur Technologie (KIT), Engesserstraße 15, 76131 Karlsruhe, Germany
̈
̈
S
* Supporting Information
potential of 1a,b and to apply, to the best of our knowledge,
for the first time a 31P/171Yb HMQC NMR experiment.
The europium borohydride compound [Eu(BH4)2(THF)2]
(1a) is easily accessible by two synthetic pathways: either via a
reductive pathway from EuCl3 and NaBH4 in high yield or by a
salt metathesis from [EuI2(THF)2] and NaBH4 (Scheme 1). In
ABSTRACT: The divalent lanthanide borohydrides [Ln-
(BH4)2(THF)2] (Ln = Eu, Yb) have been prepared in a
straightforward approach. The europium compound shows
blue luminescence in the solid state, having a quantum
yield of 75%. Nonradiative deactivation of C−H and B−H
oscillator groups could be excluded in the perdeuterated
complex [Eu(BD4)2(d8-THF)2], which showed a quantum
yield of 93%. The monocationic species [Ln(BH4)-
(THF)5][BPh4] and the bis(phosphinimino)methanides
[{(Me3SiNPPh2)2CH}Ln(BH4)(THF)2] have been pre-
pared from [Ln(BH4)2(THF)2]. They show significantly
lower or no luminescence. Using the diamagnetic
compound [{(Me3SiNPPh2)2CH}Yb(BH4)(THF)2], we
performed a 2D 31P/171Yb HMQC experiment.
Scheme 1. Syntheses of 1a,b
contrast, the corresponding ytterbium compound [Yb-
(BH4)2(THF)2] (1b) was synthesized, analogous to [Sm-
(BH4)2(THF)2], from [Yb(BH4)3(THF)3] and Yb metal
(Scheme 1).16 1b could not be obtained directly from YbCl3
and NaBH4.
anthanide borohydrides [Ln(BH4)3(THF)3] have attracted
Lincreasing research interest over the past two decades.
They have been used as starting materials to prepare a large
number of lanthanide borohydride coordination compounds
and organometallic complexes.1−4 Borohydride complexes
exhibit some hydridic character through single or multiple
Ln(μ-H)B bonds.1−4 The hydridic character makes them useful
as initiators for the ring-opening polymerization of cyclic esters
and carbonates.4−12 For the polymerization of nonpolar
monomers, an alkylating agent as cocatalyst is required.4,13,14
The developments of lanthanide borohydrides in synthesis and
catalysis have been summarized in some comprehensive
reviews.1−4 The divalent bisborohydrides [Ln(BH4)2(THF)2]
(Ln = Sm, Eu, Yb) have been known since 1999.15 They were
originally prepared by thermal reduction of Na[Ln(BH4)4-
(DME)4] (Ln = Sm, Eu, Yb) at 150−200 °C in a vacuum.
Unfortunately, the characterization was poor. Recently, Nief
and Visseaux reported a more convenient approach to
[Sm(BH4)2(THF)2], which was obtained from [Sm(BH4)3-
(THF)3] and Sm metal.16 The divalent thulium compound
[Tm(BH4)2(DME)2] was prepared recently by reduction of
[Tm(BH4)3(THF)3] with C8K or via salt metathesis from TmI2
and KBH4 in DME.17
The new complexes were characterized by standard
analytical/spectroscopic techniques, and the solid-state struc-
ture of 1a was established by single crystal X-ray diffraction.
Although the X-ray data collected from 1a were poor, the
connectivity of 1a and its composition were deduced.
Moreover, the lattice parameters of 1a show that it is
isostructrual to [Sm(BH4)2(THF)2]16 and [Sr(BH4)2-
(THF)2].18,19 In the solid state, 1a forms an infinite one-
dimensional chain in which each Eu atom is surrounded by four
−
borohydrides and two THF molecules (Figure 1). If the BH4
Herein we report an efficient synthesis of [Ln(BH4)2-
(THF)2] (Ln = Eu (1a), Yb (1b)) as well as of the
perdeuterated species [Eu(BD4)2(d8-THF)2] (1a′). We studied
the luminescence properties of all three compounds. Moreover,
some cationic species and some bis(phosphinimino)methanide
derivatives were prepared to demonstrate the synthetic
Figure 1. Cutout of the polymeric solid-state structure of 1a, omitting
hydrogen atoms.
Received: August 14, 2012
Published: October 4, 2012
© 2012 American Chemical Society
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Communication
group is considered as one ligand, the Eu atoms are six-fold-
coordinated in a distorted octahedral fashion. Although we
could not localize the hydrogen atoms in the difference Fourier
transfer between the Eu2+ sites and thereby allows an excellent
quantum yield. Moreover, blue emission of Eu2+ is quite
uncommon and typically only observed for highly ionic
compounds such as BaMgAl10O17:Eu or CaF2:Eu.21,22 This
finding therefore also reflects the predominately ionic binding
situation of Eu2+ in the borohydride compounds that entails a
small Stokes shift and d→f emission in the blue spectral range.
Since it is known that high-energy X−H oscillators close to
the emissive metal center (X = O, N, C, B) efficiently quench
lanthanide excited states,23 deuteration is an efficient pathway
to encourage the luminescence efficiency.24,25 Whereas acid
protons easily can be exchanged in D2O by deuterium, the
elimination of C−H oscillators is more difficult.26,27 In contrast,
a deuterated analogue of 1a, [Eu(BD4)2(d8-THF)2] (1a′),
could easily be obtained by the reaction of EuCl3 and NaBD4 in
d8-THF. As a consequence of the deuteration, 1a′ with about
90% efficiency shows a significantly increased quantum yield as
compared with 1a (Table 1). Notably, the quantum yield is
even higher than for BaMgAl10O17:Eu (i.e., 80% quantum yield
with 10 mol% Eu2+)a standard industrial blue-emitting
phosphor utilized in fluorescent lamps.21
−
map, we anticipate that the bridging BH4 groups are
tridendate, as observed in [Sm(BH4)2(THF)2]16 and [Sr-
(BH4)2(THF)2].18,19 The NMR data of the diamagnetic
compound 1b show the expected signals. Thus, in the 11B
NMR spectrum we observe one quintet (1JB−H = 83 Hz) of the
BH4− groups at δ −34.6 ppm and in the 171Yb{1H} spectrum of
1b a singlet at δ 319.2 ppm. Beside these analyses, we
synthesized some derivatives of 1a,b to prove indirectly their
successful synthesis (see below).
The europium compound 1a shows bright blue luminescence
at room temperature (Figure 2). Interestingly, there is, to the
We then investigated the reactivity of compounds 1a,b.
Protonation of 1a,b with 1 equiv of Me3NHBPh4 in THF gave
the monocationic species [Ln(BH4)(THF)5][BPh4] (Ln = Eu
(2a), Yb (2b)) (Scheme 2).28
Scheme 2. Syntheses of 2a,b
Figure 2. (Top) Excitation/emission spectra of complex hydrides.
(Bottom) Photographs of 1a (a,b) and 1a′ (c,d) under 366 nm
excitation (a,c) and daylight (b,d).
Both complexes have been characterized by standard
analytical/spectroscopic techniques and their solid-state
structures established by single crystal X-ray diffraction (Figure
3). Although both compounds have the same sum formula,
best of our knowledge, only one report on the luminescence of
Eu(II) hydrides addressing doped alkaline earth hydrides
MH2:Eu (M = Ca, Sr, Ba) with Eu2+ doping ranging from 5
to 10%.20 These metal hydrides show comparably weak
emission in the yellow to red spectral range (720−770 nm).
The luminescence of complex hydrides such as [Eu-
(BH4)2(THF)2] is here first observed. Surprisingly, the title
compound exhibits a very high quantum yield of about 75%
(Table 1). Such a quantum yield is even more remarkable since
compounds with >10% Eu2+ content typically are accompanied
by significant concentration quenching.21 Separation of the
−
Figure 3. Cutout of the polymeric solid-state structure of [Eu(μ-
BH4)(THF)5]+ (left) and the monomeric cation [Yb(BH4)(THF)5]+
(right), omitting some hydrogen atoms.
luminescent centers by BH4 , however, suppresses energy
Table 1. Luminescence Properties of Complex Hydrides
quantum yield/%
their solid-state structures are significantly different. The
cationic part of 2a forms a linear infinite one-dimensional
(λexc = 360 nm)
−
maximum of emission/
nm (λexc = 360 nm)
polymer, in which the hydrogen atoms of each BH4 group
compound
absolute relative
coordinate to two Eu atoms in a μ-(κ2(H):κ2(H)) mode. The
−
[Eu(BH4)2(THF)2]
490
490
519
458
75(1)
93(1)
10(2)
5(1)
79(3)
88(3)
11(3)
11(2)
BPh4 counterions are localized in between these chains. The
−
[Eu(BD4)2(d8-THF)2]
[Yb(BH4)2(THF)2]
Eu atom is seven-fold-coordinated if the BH4 groups are
considered as one ligand. Thus, a pentagonal bipyramid is
formed in which the five THF molecules are in equatorial
[Eu(BH4)(THF)5] [BPh4]
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Communication
positions. In contrast, the cation of 2b is a monomer. As a result
of the smaller ionic radius of Yb(II), only the five THF and the
BH4 group are coordinated around the ytterbium atom,
bered metallacycle (N1−P1−C1−P2−N2−Ln) upon chelation
of the two trimethylsilylimine groups to the metal atom. The
ligand is almost symmetrically coordinated to the metal center.
As observed earlier for bis(phosphinimino)methanides of the
lanthanides, the metallacycle adopts a twist boat conformation
in which both the central carbon and lanthanide atoms are
displaced from the N2P2 least-squares plane. The interaction
between the methine carbon atom (C1) and the lanthanide
atom (2.8664(7) Å (3a), 2.7403(7) Å (3b)), which is longer
than common Ln−C distances,32 is shorter than in the dimeric
iodine complex [{(Me3SiNPPh2)2CH}EuI(THF)]2 (2.945(2)
Å) but longer than in the monomeric compound
−
forming a distorted octahedral coordination polyhedron. In the
11B NMR spectrum we see a singulet for the BPh4 group (δ
−
−
5.1 ppm) and a quintet for the BH4 group (δ = −32.9 ppm;
1JB−H = 83 Hz). In the 171Yb{1H} spectrum, a significant upfield
shift (δ 271.7 ppm) is observed in comparison with 1a (δ 319.2
ppm).
The transformation of 1a,b to 2a,b has a dramatic influence
on the luminescence properties. Thus, [Eu(BH4)2(THF)2]
shows a quantum yield of 75%, whereas [Eu(BH4)(THF)5]-
[BPh4] exhibits an absolute quantum yield of only 5% (Table
1). This dramatic decrease can be rationalized on the basis of
three aspects: (1) The higher number of THF coordination
leads to increased vibronic quenching. (2) The higher
coordination is accompanied by lengthening of the Eu−H
and Eu−O bond distances, which also favors vibronic
[{(Me3SiNPPh2)2CH}YbI(THF)2] (2.700(4) Å).33 The BH4
−
group binds in a terminal κ3(H)-coordination mode to the
metal center.
The diamagnetic compound 3b was also fully characterized
by 1H, 13C{1H}, 31P{1H}, 11B, and 171Yb{1H} NMR. In the 11
B
NMR the expected quintet at δ −32.2 ppm is observed. In the
31P{1H} NMR a coupling of the 171Yb nucleus could be seen at
δ 14.6 (t, 2JP−Yb = 58.5 Hz) ppm (Figure S1). As expected, the
−
quenching. (3) The BPh4 anion shows a certain absorption
at 360 nm as well that does not lead to any emissive energy
1
2JP−Yb coupling is significantly smaller than the JP−Yb coupling
transfer to Eu2+.
observed in [{([Me3Si]2CH)(C6H4-2-CH2NMe2)P}2Yb] (680
Hz).34 The 171Yb chemical shift was determined by direct
method, 171Yb{1H} NMR (Figure S2),35 and indirect methods,
1H/171Yb gHMQC (Figure S3)36 and 31P/171Yb HMQC
(Figure 5). To the best of our knowledge, we have performed
Beside the formation of a cationic species, we were also
interested in attaching an organic ligand to study the reactivity
of 1a,b and the luminescence properties of the resulting
product. As ligand we chose the bis(phosphinimino)methanide,
{(Me3SiNPPh2)2CH}−, which is well established in lanthanide
chemistry.29,30 The ligand has several NMR-active nuclei, which
are useful for studying the behavior of the resulting complexes
in solution. Reaction of 1a,b with K{(Me3SiNPPh2)2CH}31
resulted in the corresponding heteroleptic monoborohydride
derivatives [{(Me3SiNPPh2)2CH}Ln(BH4)(THF)2] (Ln = Eu
(3a), Yb (3b)) (Scheme 3).
Scheme 3. Synthesis of 3a,b
Figure 5. 31P/171Yb HMQC spectrum of complex 3b in d8-THF at 293
K.
−
the first 2D 31P/171Yb HMQC experiment. The resonance of
the phosphorus atoms was unambiguously correlated to the
171Yb NMR resonance at δ 744.2 ppm. The 171Yb NMR signal
is shifted significantly downfield compared with those of
compounds 1b and 2b. This method allows the determination
of 171Yb NMR data in a short time, together with connectivity
The BH4 group behaves like a pseudo-halide; thus, KBH4
was formed as byproduct. The solid-state structures of both
compounds could be established by single-crystal X-ray
diffraction (Figure 4); they are isostructural to each other.
The bis(phosphinimino)methanide ligand forms a six-mem-
1
between both nuclei. The H/171Yb gHMQC 2D map of 3b
shows scalar interactions with the methine group of the
bis(phosphinimino)methanide ligand (Figure S3). Unfortu-
nately, no coupling was observed to the BH4− hydrogen atoms
due to the influence of the quadrupole moment of the boron
atom.
Neither 3a nor 3b show any luminescence. Obviously the
coordination of the bis(phoshinimino)methanide ligand leads
to complete quenching. The strong difference in the
luminescence properties of compounds 1a,b and 3a,b inspired
us to follow the reaction of 1a to 3a by measuring the decay of
the luminescence. In a THF solution, 1a shows a yellow
emission at 561 nm upon exitation at 423 nm. To this solution
Figure 4. Solid-state structures of 3a,b, omitting some hydrogen
atoms.
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the divalent lanthanide borohydrides [Ln(BH4)2(THF)2] (Ln
= Eu, Yb) in high yields. The europium compound shows a
very bright blue luminescence, which inspired us to prepare the
perdeuterated species [Eu(BD4)2(d8-THF)2]. This compound
gave quantum yields of 93%. To probe the application of
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* Supporting Information
Experimental details, NMR spectra, and CIF files for 2a,b and
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The authors declare no competing financial interest.
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