A hydrocarbon-soluble lithium hydride complex

Article abstract of DOI:10.1002/anie.201108134

LiH-ghtweight: The title complex having a central (LiH)₄ cube has been prepared and structurally characterized (see picture). The compound is stable towards LiH elimination at room temperature in solution, and can be employed for hydrolithiatio

Full text of DOI:10.1002/anie.201108134

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Angewandte
Communications
DOI: 10.1002/anie.201108134
Cluster Compounds
A Hydrocarbon-Soluble Lithium Hydride Complex**
Andreas Stasch*
Lithium hydride is the lightest metal hydride and is consid-
ered to be the simplest ionic compound.[1,2] With a low
molecular weight of just under 8 gmolÀ1 and a relatively high
hydrogen content of 12.7% it is of interest for numerous
applications including hydrogen storage technologies[3] and
uses in organic and inorganic synthesis.[4] LiH crystallizes with
a rock salt structure and shows a high lattice energy of
approximately 220 kcalmolÀ1.[1] This renders the material
stable and relatively easy to handle, but also makes it too
insoluble and unreactive for many applications.
In recent years, a number of well-defined and structurally
characterized molecular s-block metal hydride complexes
have been forthcoming and show significant differences to the
properties of the parent bulk metal hydride, that is, MH for
the group 1 metals and MH2 for the group 2 metals. The
majority of these achievements concentrate on the molecular
Scheme 1. Preparation of 2 and 3.
compounds of the group 2 metals Be,[5] Mg,[6] and Ca.[7] Some
of these hydride complexes have already been employed in
hydrometalation reactions[8] and have even been successfully
Reaction of the new phosphinoamine DipNHPPh2 (1;
Dip = 2,6-iPr2C6H3; Scheme 1), prepared from DipNHLi and
ClPPh2,[17] with two equivalents of nBuLi in n-hexane
afforded the mixed alkyl phosphinoamido lithium complex
[(DipNPPh2)2Li4nBu2] (2).[17] For comparison, the reaction of
equimolar amounts of DipNHPPh2 and nBuLi in a hydro-
carbon solvent affords the poorly soluble phosphinoamido
lithium salt [(DipNPPh2)Li], which is presumably oligo- or
polymeric in structure. Addition of donor solvents to
[(DipNPPh2)Li] afford the soluble complexes [(DipNP-
Ph2)Li(Do)2] [(Do)2 = (THF)2 or (TMEDA)], which have
three-coordinate lithium cations showing N,O,O and N,N,N
coordination modes, respectively, and no Li···P contacts.[17,18]
A single-crystal X-ray structure analysis of compound 2[17]
revealed a mixed dialkyl bisphosphinoamido lithium complex
composition (Figure 1). Compound 2 crystallizes with a full
molecule in the asymmetric unit and features a severely
distorted Li4 tetrahedron (dashed lines in Figure 1) with
interatomic Li···Li distances ranging from 2.407(4) ꢀ (Li1-
used as a hydrogenation catalyst on alkenes.[9]
The majority of hydride complexes involving group 1
metals are mixed metal systems, and can be considered ate
complexes, for example LiAlH4 and its derivatives.[2,10] Some
remarkable examples include mixed metal systems such as
[[{(Me3Si)2N}AlH2Li]3(LiH)] having a central distorted (LiH)4
cube featuring one hydride moiety solely bound to Li
cations.[11] There are also a number of complexes having an
interstitial hydride surrounded by seven or eight lithium ions,
as in cationic [L6HLi8]+ (L = a chelating monoanionic N-
ligand) and the related neutral complex [L6HLi7].[12,13] Active
forms of alkali metal hydrides MH (M = Li, Na, K) have been
obtained from mixtures of metal alkoxides and alkyl metal
compounds in the presence or absence of dihydrogen or from
alkyl metal compounds and silanes.[4,14] Mixed lithium alk-
oxide/lithium hydride aggregates have been generated ther-
mally or photolytically in solution,[15] and the remarkable
’superaggregate’ [(tBuOLi)16(LiH)17] was, in one instance,
obtained in an undetermined yield and structurally charac-
terized.[16]
Li3) to 3.218(4) ꢀ (Li1-Li2).[19] The P N bond lengths in the
À
phosphinoamide moiety are only marginally shorter than the
comparable bond length in the parent phosphinoamine 1
(1.699(5) ꢀ).[17] Two n-butyl groups in 2 cap two of the four
Li3 faces with Li-C distances ranging from 2.202(3) ꢀ to
2.270(3) ꢀ. Both n-butyl groups in 2 are disordered and have
been modeled with two positions for the outer three carbon
atoms, and only the major butyl components are shown in
Figure 1. Two Li cations (Li1, Li2) each coordinate to one N
and one P atom from different phosphinoamide ligands and
have one contact each to one nBu carbon atom. In addition,
each of those two Li cations show Li···H contacts to alkyl
groups of one iPr group of the Dip moiety; the shortest Li···H
distances being approximately 2.06–2.09 ꢀ. The other two Li
cations (Li3, Li4) bind to one N atom each and to two nBu
[*] Dr. A. Stasch
School of Chemistry, Monash University
PO Box 23, Melbourne, VIC, 3800 (Australia)
E-mail: andreas.stasch@monash.edu
Homepage: http://www.chem.monash.edu.au/staff/stasch/
index.html
[**] We are grateful to the Australian Research Council for a fellowship
and the Monash Research Accelerator Program. Part of this
research was undertaken on the MX1 beamline at the Australian
Synchrotron, Victoria, Australia.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108134.
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Figure 1. Crystal structure of [(DipNPPh2)2Li4nBu2] (2). The thermal
ellipsoids are shown at 30% probability. Hydrogen atoms have been
omitted for clarity. Only the main components of the disordered n-
butyl groups are shown. Selected bond lengths [ꢀ] and angles [8]: P1–
N1 1.6786(13), P2–N2 1.6785(13), Li1–N1 2.034(3), Li1–P2 2.560(3),
Li2–N2 2.072(3), Li2–P1 2.591(3), Li3–N1 2.023(2), Li3···P1 2.962(3),
Li4–N2 2.018(3), Li4–P2 2.884(3), Li2–C49 2.256(3), Li3–C49 2.218(3),
Li4–C49 2.202(3), Li1–C53 2.270(3), Li3–C53 2.217(3), Li4–C53
2.245(3), Li1···Li2 3.218(4), Li1···Li3 2.407(4), Li1···Li4 2.856(4); N1-P1-
Li2 94.89(7), P1-N1-Li1 112.26(10), P1-N1-Li3 105.92(10), Li3···Li1···Li4
57.04(10), Li3···Li1···Li2 58.67(10), Li4···Li1···Li2 46.57(8), Li4···Li2···Li3
57.07(10), Li4···Li2···Li1 58.84(10), Li3···Li2···Li1 46.29(8), Li1···Li3···Li4
70.40(12), Li1···Li3···Li2 75.04(11), Li4···Li3···Li2 53.11(10),
Figure 2. Crystal structure of [{(DipNPPh2)Li}4(LiH)4]·C6H6, (3·C6H6).
The thermal ellipsoids are shown at 30% probability. The solvent
molecule and hydrogen atoms except hydride ligands have been
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: P1–N1
1.666(2), P2–N2 1.673(2), P1–Li2 2.530(4), N1–Li3 2.037(4), N1–Li1
2.042(4), Li1–P2 2.530(4), N2–Li4 2.032(4), N2–Li2 2.038(4), Li1–H1
1.83(4), Li2–H2 1.81(4), Li3–H2 1.98(4), Li3–H1 1.96(4), Li4–H1
1.99(4), Li4–H2 1.97(4), Li1···Li2 3.131(6), Li1···Li3 2.509(5), Li1···Li4
3.051(6), Li2···Li3 3.061(6), Li3···Li3’ 2.668(8), Li3···Li4 2.821(6),
Li3’···Li4 2.691(5), Li4···Li4’ 2.682(8); H2-Li4-H1 88.3(18), Li3-H2-Li4
ca. 91.0, Li4-H1-Li4’ ca. 87.8, Li4-H1’-Li3’ ca. 89.2. Symmetry oper-
ation: #1Àx, y, Àz + 1= ; #2 Àx, Ày+1, Àz.
2
Li2···Li4···Li1 74.58(11), Li3···Li4···Li1 52.56(10).
structural determination), a hydrocarbon-soluble compound
having a central distorted (LiH)4 cube. As with other s-block
metal precursor complexes,[6a,c–e,7,8a–c] phenylsilane readily
substitutes alkyl groups with hydride ligands. When the
reaction is carried out in deuterated benzene and monitored
by multinuclear (1H, 7Li and 31P{1H}) NMR spectroscopy it is
evident that compound 2 is formed in a rapid reaction in
approximately 90% yield with PhSiH2nBu as the by-product.
[{(DipNPPh2)Li}4(LiH)4]·C6H6 (3·C6H6) crystallizes with
half a molecule in the asymmetric unit. During the course of
this study, we characterized two additional solvates,[17] each
with two full molecules of 3 in the asymmetric unit, which
have very comparable metrical features. Only the structure of
3·C6H6 will be discussed herein. The structure of 3 derives
from that of 2 by retaining the overall geometry, substituting
the nBu groups with hydride ligands, and dimerization of the
fragments through the newly formed hydride ligands. This
process leads to an Li8 cluster compound with a central
distorted (LiH)4 cube. The hydride ligands have been located
from difference maps and refined in all solvates. The Li
cations form an array of three connected edge-sharing
tetrahedrons,[21] with the middle one containing the (LiH)4
cube. All Li···Li distances fall into the range found for those of
carbon atoms. In addition, they show short Li···H contacts to
one b-methylene unit in each of the n-butyl groups (Li3 to
C50 and Li4 to C54); the shortest contacts being around
2.0 ꢀ. Some related complexes having the general formula
L2Li4nBu2 (L = anionic ligand) and featuring a distorted Li4
tetrahedron have been structurally characterized and show
similar overall features compared with those of 2.[20]
The 1H NMR spectrum of compound 2 in deuterated
benzene at 308C displays one broadened septet resonance
and two very broad doublets corresponding to the protons of
the iPr substituents. At elevated temperatures of approx-
imately 60–708C, only one sharp septet and one broadened
doublet resonance is found for those protons. Cooling a
sample of 2 in deuterated toluene leads to splitting of the
broad septet resonance at a coalescence temperature of
approximately 198C (equates to a DG° of ca. 13 kcalmolÀ1),
and at low temperatures of approximately À30 to À508C two
sharp septet and four doublet resonances are found. This
situation remains largely unchanged down to À858C, albeit
with significantly broadened resonances. At room temper-
ature, one major singlet is observed in the 7Li NMR spectrum
and the 31P{1H } NMR spectrum shows one broad resonance,
which resolves to a multiplet at low temperatures.[17] This
behavior suggests an exchange of ligands around the Li4 core
in 2, which is rapid on the NMR timescale at elevated
temperatures. Similar fluxional processes have been
observed, especially at elevated temperatures for several of
the related L2Li4nBu2 molecules.[20]
À
2. The lengths of the determined Li H bonds within the
central (LiH)4 cube (1.96(4)–1.99(4) ꢀ) are close to the
shortest Li-H distances in cubic LiH (ca. 2.03-2.04 ꢀ),[1] close
to those of [[{(Me3Si)2N}AlH2Li]3(LiH)],[11] and comparable
to those of the interstitial Li-H compounds [L6HLi8]+ and
[L6HLi7].[12,13] In addition, comparable short Li···Haliphatic
contacts are found in 3, the shortest one being around
2.0 ꢀ, and similar interactions are observed in 2. Four shorter
Li-H contacts of 1.81(4)–1.83(4) ꢀ are measured to Li1 and
Li2 of the outer tetrahedra. The hydrides, like the nBu groups
Treating [(DipNPPh2)2Li4nBu2] (2) with two equivalents
of phenylsilane at room temperature in benzene or toluene
affords the crystalline complex [{(DipNPPh2)Li}4(LiH)4] (3;
see Figure 2 for the molecular structure from a single-crystal
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Angewandte
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in 2, can be considered capping two of the four Li3 faces of the
outer two Li4 tetrahedra.
Ph2CH resonance shifts to d = 6.08 ppm, which compares well
with the reported resonance in deuterated THF (d =
6.03 ppm).[22] On a preparative scale, [{(Ph2CHO)Li}6] could
be obtained from 3 and benzophenone in good yield.[17]
In summary, we have prepared and structurally charac-
terized a hydrocarbon-soluble LiH complex having a central
(LiH)4 cube, and it is a promising reagent for synthesis as has
been demonstrated by a hydrolithiation reaction with benzo-
phenone.
In solution, the 1H NMR spectrum of 3 in deuterated
benzene or toluene at 308C shows two septets and three
doublets, having the relative intensities of 1:1:2, for the
protons of the isopropyl groups, and one broad resonance at
d = 4.18 ppm with an integration of four for the hydride
ligands. This chemical shift represents a typical value for
main-group hydride compounds including most of the char-
acterized s-block metal hydride complexes.[2,5–7] Previously,
IGLO calculations for LiH containing cluster compounds
Received: November 19, 2011
Revised: December 15, 2011
Published online: January 13, 2012
1
have suggested an H NMR chemical shift range of approx-
imately 3.5–4.0 ppm.[15] The 31P{1H} NMR spectrum displays a
multiplet centered at d = 33.7 ppm,[17] and the 7Li NMR
spectrum shows two broad resonances at d = 3.0 and 2.6 ppm.
The former resonance appears considerably sharper in the 1H
decoupled 7Li NMR spectrum and it is therefore likely
associated with the central (LiH)4 core. The NMR spectra are
consistent with an idealized D2 symmetry as obtained from
the crystals structure. At elevated temperatures, the septets
and doublets broaden (ca. 408C) and the two broadened
septets merge into one at around 558C (DG° of ca.
16 kcalmolÀ1). Above this temperature, one broad resonance
is observed for the methine septets, and sharpens at approx-
imately 90–1008C, and one broad doublet is found for the
isopropyl methyl groups. The broad resonance assigned to the
LiH moieties remains essentially unchanged across the
investigated temperature range. The high-temperature
NMR spectra are consistent with fluxional behavior of the
Keywords: cluster compounds · hydrides · lithium · N,P ligands ·
structure elucidation
.
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(Scheme 2). When this reaction is carried out in deuterated
Scheme 2. Schematic hydrolithiation of benzophenone using 3.
benzene at room temperature and monitored by 1H, 7Li, and
31P{1H} NMR spectroscopy, a rapid reaction is observed. The
1
broad singlet assigned to the LiH moieties in the H NMR
spectrum of 3 disappeared upon treatment with benzophe-
none and new sharp resonances at d = 5.41 ppm emerged.
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