A cationic calcium hydride cluster stabilized by cyclen-derived macrocyclic N,N,N,N ligands

Article abstract of DOI:10.1002/anie.201200690

Cationic calcium hydride complexes with a [Ca₃(μ₃- H)₂] core were prepared from different organocalcium precursors and Ph₂SiH₂. The hydride complexes were found to be active in the catalytic hydrosily

Full text of DOI:10.1002/anie.201200690

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DOI: 10.1002/anie.201200690
Calcium Hydrides
A Cationic Calcium Hydride Cluster Stabilized by Cyclen-Derived
Macrocyclic N,N,N,N Ligands**
Phillip Jochmann, Julien P. Davin, Thomas P. Spaniol, Laurent Maron,* and Jun Okuda*
Metal hydride complexes of the d and f block metals are
precatalysts in many important homogeneous transforma-
tions.[1,2] Much less is known about molecular main group
metal, in particular s block metal hydrides.[3] Recently, a few
well-defined examples of magnesium hydride complexes were
reported in the context of hydrogen storage and for pyridine
functionalization.[4,5] Since the discovery of the homogeneous
calcium-based hydrogenation catalysts by Harder et al.,
[{Ca(DIPP-nacnac)(H)(thf)}2]
(DIPP = 2,6-diisopropyl-
phenyl; DIPP-nacnac = CH{(CMe)(2,6-iPr2C6H3N)}2)
remained the only fully characterized molecular calcium
hydride to date.[6,7] [{Ca(DIPP-nacnac)(H)(thf)}2] was exten-
sively tested in homogeneous catalysis and various ligand
exchange reactions. A crucial issue is to stabilize heteroleptic
hydride species by an appropriate ligand set to prevent
formation of insoluble [CaH2]1. The latter saline hydride
forms easily in a Schlenk type equilibrium.[6a] We report
herein the isolation and characterization of three ion pairs,
each containing a monocationic trinuclear calcium hydride.
Starting from calcium precursors [CaR2(thf)x] (1a: R =
N(SiMe3)2, x = 2;[8] 1b: R = C3H5, x = 0;[9] 1c: R = CH2Ph, x =
0.25[10]), reactions with the tetradentate proligand
(Me3TACD)H ((Me3TACD)H = Me3[12]aneN4: 1,4,7-tri-
methyl-1,4,7,10-tetraazacyclododecane) afforded the mono-
(ligand) complexes [(Me3TACD)CaR]n (2a, 2c: n = 1; 2b: n =
2) (Scheme 1). The amide 2a was reported previously.[11]
Subsequent reactions of the precursors 2a–c with Ph2SiH2 in
THF at 258C led to the formation of the trinuclear cationic
calcium hydride [(Me3TACD)3Ca3(m3-H)2]+, with different
silicon-containing counteranions [A]ꢀ. These anions are
either an amide [N(SiMe3)(SiPh3)]ꢀ (3a), a hydridosilicate
[Ph3SiH2]ꢀ (3b), or a carbanion [(Ph2SiH)CHPh]ꢀ (3c).
Hitherto, only very few examples of trinuclear compounds
of calcium have been structurally characterized.[12] The
hydrides 3a–c were isolated by crystallization from the
Scheme 1. Synthesis of Me3TACD-stabilized calcium hydride complexes
3a–c from 2a–c.
reaction mixtures in yields up to 32%. Attempts to increase
the yield by precipitation with pentane or drying of the crude
reaction mixtures gave product mixtures. These consisted of
the cationic part of 3 and a broad variety of aryl-substituted
anions, as evident from 1H NMR spectroscopy.
X-ray crystal structure determination and NMR spectros-
copy of hydrides 3a–c indicate the formation of identical
cationic hydride units [(Me3TACD)3Ca3(m3-H)2]+, irrespective
of the starting compound 2a–c. Compound 3a gave the best
refinement parameters, and its C3-symmetric cation is
depicted in Figure 1. The central Ca3H2 core forms a trigonal
bipyramid. The metal atoms are bridged by amido nitrogen
atoms, resulting in an almost planar Ca3N3 ring. Further
coordination to the nitrogen atoms of the macrocyclic ligand
leads to a coordination number of seven for the metal atoms.
[*] Dr. P. Jochmann, J. P. Davin, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
ꢀ
Ca Ca distances of 3.3551(10) ꢀ (3a) were found in the
crystal structure, and DFT calculations indicate direct inter-
action between these atoms. This formally increases the
coordination number of the metal centers to nine. As
expected, shorter bonds are observed between calcium and
the bridging amido nitrogen atoms N1 (2.438(2) and
2.471(3) ꢀ) compared to the amine nitrogen atoms N2–N4
E-mail: jun.okuda@ac.rwth-aachen.de
Prof. Dr. L. Maron
Universitꢀ de Toulouse et CNRS, INSA, UPS, CNRS; UMR 5215
LPCNO, 135 avenue de Rangueil, 31077 Toulouse (France)
E-mail: laurent.maron@irsamc.ups-tlse.fr
[**] Financial support by the DFG and the Cluster of Excellence “Tailor-
Made Fuels from Biomass” is gratefully acknowledged. P.J. thanks
the NRW Forschungsschule “BrenaRo” for a scholarship. L.M. is
member of the Institut Universitaire de France. Cyclen=1,4,7,10-
tetraazacyclododecane.
ꢀ
(2.578(3) to 2.627(3) ꢀ). The Ca H bond lengths of 2.36(4) ꢀ
ꢀ
ꢀ
(Ca H1) and 2.37(4) ꢀ (Ca H2) in 3a are virtually equal but
longer than reported for [{Ca(DIPP-nacnac)(H)(thf)}2].[7]
ꢀ
Ca H bond lengths observed for 3b and 3c show larger
variations (2.03(8) to 2.50(9) ꢀ), resulting in an overall
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200690.
ꢀ
average Ca Hav value of 2.33(9) ꢀ. The structural motif is
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of trinuclear [(Me3TACD)3Ca3(m3-H)2]+ in THF solution.
1H NMR spectra ([D8]THF, 248C) show a singlet for the
hydrido ligands at 3.99 ppm. This is 0.46 ppm upfield of the
hydride chemical shift reported for [{Ca(DIPP-nacnac)(H)-
(thf)}2] (C6D6, 208C).[7] The methyl groups of the Me3TACD
ligands show singlets at 2.22 and 2.55 ppm with an integration
ratio of 1:2. The CH2 protons of the macrocycle appear as
broad signals in the range of 1.9–3.3 ppm. This suggests
dynamic behavior of the CH2CH2 groups in Me3TACD.
The anionic part of 3a, [N(SiMe3)(SiPh3)]ꢀ, shows unex-
ceptional proton resonances,[16] whereas the anion in 3b
[Ph3SiH2]ꢀ shows a singlet at 5.95 ppm, which is consistent
with the chemical shift reported for the hydridic hydrogen
atoms of the anion in [K([18]crown-6)][Ph3SiH2].[14a] This
finding for 3b suggests separated ion pairs in THF solution.
The anionic part of 3c, [(Ph2SiH)CHPh]ꢀ, shows a doublet for
the silicon-bound hydrogen atom at 5.31 ppm (3JHH = 3.5 Hz,
1JSiH = 171.4 Hz).
The formation of 3 was investigated by in situ NMR
spectroscopic reaction monitoring, using different silanes
(Ph3SiH, Ph2SiH2, PhSiH3, Et3SiH) as hydride transfer agents.
Common observations for reactions of 2a–c with these silanes
were: 1) the cationic hydride cluster [(Me3TACD)3Ca3-
(m3-H)2]+ is formed at early stages in high yield and appears
to be favored; 2) complex mixtures of silanes PhxSiHxꢀ4 in
varying ratios are formed owing to facile substituent scram-
bling; and 3) orange to red reaction solutions and H2
evolution were observed. Relative reaction rates for different
silanes were Ph2SiH2 ꢁ PhSiH3 > Ph3SiH @ Et3SiH. For
a given silane, 2a and 2c react faster than the allyl 2b. The
scrambling process is crucial for the production of 3. Such
exchange processes are known to be catalyzed by metal
hydrides.[17]
Figure 1. Molecular structure of the cationic calcium hydride cluster
[(Me3TACD)3Ca3(m3-H)2]+ in 3a. Ellipsoids are set at 50% probability;
hydrogen atoms except for those of the Ca3H2 core are omitted for
clarity.
reminiscent of the one reported for the lanthanide hydrides
[{Ln(Me3TACD)(m2-H)2}3] (Ln = Y, Ho, Lu).[13] Up to four
non-coordinating THF molecules were found in the crystal
packing of 3a–c. These THF molecules can be removed by
drying of 3a–c in vacuo.
Compound 3b contains a rare example of an isolated
organosilicate with a five-coordinate silicon atom.[14] Bonding
parameters compare well to the those reported for
[K([18]crown-6)][Ph3SiH2], which is the only known example
of
anion.[14a]
The new precursors 2b and c are sparingly soluble in THF
a
structurally characterized dihydridotriphenylsilicate
Although complex mixtures and side reactions prevent
assessment of the detailed mechanism, we propose that the
formation of products 3 is induced by the initial formation of
neutral [(Me3TACD)CaH] according to the silane route.[7]
The energetically favored formation of a cationic trimer
requires a hydride transfer to an acceptor molecule. The latter
is provided by different species resulting from groups R (from
starting compounds 2), the corresponding silane, and ligand
exchange products thereof. Only one anion–cation match
with high lattice energy crystallizes from the reaction mixture
to give products 3a–c. Evolution of H2 is proposed to arise
from silane coupling side reactions to give di- or oligosilanes.
This reaction has precedence and is usually catalyzed by
transition and lanthanide complexes.[18] The fate of the
calcium-bound group R is obscured by hydrosilylation reac-
tions. Indeed, reactions of 2b with phenylsilanes showed
proton resonances, which were attributed to propane-1,2-
diylbis(triphenylsilane).
and were obtained by slow diffusion of a THF solution of 1b
or 1c into that of (Me3TACD)H. Compound 2b was obtained
as single crystals at ambient temperature and single-crystal
X-ray analysis established a dimeric solid-state structure.
Although disorder of the Me3TACD ligands and poor crystal
quality prevent discussion of bonding parameters, connectiv-
ity is unambiguous. m2-N bridges provided by the tetradentate
macrocyclic ligands are observed, whereas the allyl ligands
coordinate to the calcium centres in an h3-fashion. This
contrasts with bridging allyl ligands observed for other
multinuclear allyl complexes of Group 2 metals.[15] In solu-
tion, compound 2c shows the expected h1 coordination mode
for the benzyl ligand, and signals characteristic for the
Me3TACD ligand (see the Supporting Information).
1
Although H NMR resonances ([D8]THF, 248C) match the
formulae
[{(Me3TACD)Ca(h3-C3H5)}2]
(2b)
and
To investigate the stability of complexes 3, a bonding
analysis was carried out using DFT methods on 3a. The
geometry of the ion pair was optimized at the DFT (B3PW91/
6-311G(d,p)) level of theory. The geometrical parameters are
[(Me3TACD)Ca(CH2Ph)] (2c), low solubility prevented
detailed assignment of the observed signals.
Compounds 3a–c have low solubility in THF and are
insoluble in hydrocarbons. In more polar solvents (DMSO,
CH3CN) they undergo immediate decomposition. NMR
spectroscopic analysis of compounds 3a–c is in agreement
with the structure in the solid state and indicate the presence
ꢀ
in excellent agreement with the experimental data. The Ca
Ca distances are found to be 3.35 ꢀ, in perfect agreement
with the experimental value (3.35 ꢀ). Similarly, the distance
between the calcium atoms and the bridging nitrogen atoms
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Angewandte
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are reproduced (2.47 and 2.43 ꢀ vs. 2.47 and 2.44 ꢀ) as well as
the distance to the other nitrogen atoms of the ring (2.61 vs.
noted, but conditions need to be optimized. The role of
silicon-based anions is currently under investigation in our
laboratories.
ꢀ
2.57–2.62 ꢀ). The Ca H distances are also well-reproduced
(2.34 vs. average value of 2.35 ꢀ). The bonding situation was
analyzed using NBO analysis: three three-center bonds were
found, each involving two sp hybrid orbitals of Ca and one sp2
hybrid orbital of the bridging nitrogen. At the second order,
these bonds are delocalized into Ca d orbitals (up to 0.15
electrons per Ca). These d orbitals overlap, leading to a direct
Experimental Section
All operations were performed under an inert atmosphere of argon
using standard Schlenk-line or glovebox techniques. Deuterated
solvents were distilled under argon from sodium/benzophenone ketyl
prior to use and stored in a glovebox. All other solvents were purified
using a MB SPS-800 solvent purification system. Glassware and vials
were dried in an oven at 1208C overnight and exposed to a vacuum-
argon cycle three times. (Me3TACD)H,[19] 1a,[8] 1b,[9] 1c,[10] and 2a[11]
were synthesized following literature procedures. NMR spectra were
recorded on a Bruker DRX 400 spectrometer (1H: 400.1 MHz) at
248C unless otherwise stated. Chemical shifts for 1H NMR spectra
were referenced internally using the residual solvent resonances and
reported relative to tetramethylsilane. Owing to low solubility, no
13C NMR data were obtained.
Ca Ca interaction (50.71 kcalmolꢀ1 at the second-order NBO
ꢀ
analysis). Both hydrogen atoms also interact with the three
calcium atoms, as demonstrated by the formation of strongly
ꢀ
delocalized Ca H interactions at the NBO level (interaction
of more than 85 kcalmolꢀ1 at the second order NBO level).
No direct interaction between the two hydrogen atoms were
located at the NBO level. Thus, based on this analysis, we
conclude that the cohesion of the trinuclear core is ensured by
ꢀ
strong Ca Ca direct interactions, supported by the presence
of the bridging amido nitrogen atoms. The three Ca atoms
form a triangle, where each edge is capped by a bridging
nitrogen and each face is capped by an hydrogen atom.
Compound 3b was tested in a catalytic hydrosilylation and
hydrogenation of 1,1-diphenylethene (DPE) using Ph2SiH2
and H2, respectively (Scheme 2). Preliminary results show
that 3b catalyzes the hydrosilylation of DPE with Ph2SiH2
(5 mol%, 258C). After 18 h, 95% of DPE was selectively
3a: Neat Ph2SiH2 (18.4 mg, 18.5 mL, 0.1 mmol) was added via
microliter syringe to a solution of 2a (41.4 mg, 0.1 mmol) in 0.5 mL of
THF. The mixture was allowed to stand at 258C. The color of the
solution changed to yellow after 5 min and colorless crystals formed
within 1 h. The mixture was stored at ꢀ308C overnight to complete
crystallization. The supernatant was decanted and the crystals
(4.0 mg, 0.004 mmol, 4%) were dried under reduced pressure.
1H NMR (400.1 MHz, [D8]THF): d = ꢀ0.14 (s, 9H, N(SiMe3)(SiPh3)),
2.23 (s, 9H, NCH3), 2.45–2.90 (br m, 24H, CH2), 2.56 (s, 18H, NCH3),
3.00–3.15 (br, 6H, CH2), 3.99 (s, 2H, CaH), 6.95–7.10 (m, 9H,
N(SiMe3)(SiPh3)), 7.76–7.71 ppm (m, 6H, N(SiMe3)(SiPh3)).
C,H,N analysis calcd (%) for C54H101Ca3N13Si2 (1108.87 gmolꢀ1):
C 58.49, H 9.18, N 16.42; found: C 57.78, H 8.80, N 17.45.
3b: A solution of Ph2SiH2 (21 mg, 0.11 mmol) in 0.5 mL of THF
was added to a suspension of 2b (30 mg, 0.05 mmol) in 0.5 mL of
THF. A color change to red and gas evolution were observed. After
3 h, all 2b was consumed and the reaction mixture was placed in the
freezer (ꢀ358C). The product was isolated as light yellow, plate-like
single crystals by decantation, washing with pentane, and drying
under reduced pressure (11 mg, 0.011 mmol, 32%). 1H NMR
(400.1 MHz, [D8]THF): d = 1.90–2.45 (br m, 18H, CH2), 2.20 (s, 9H,
NCH3), 2.45–2.90 (br m, 24H, CH2), 2.54 (s, 18H, NCH3), 2.95–3.20
(br, 6H, CH2), 3.99 (s, 2H, CaH), 5.95 (s, 2H, Ph3SiH2), 6.91 (tt, 3H,
3JHH = 7.1 Hz, 4JHH = 1.6 Hz, p-C6H5), 6.99 (t, 6H, 3JHH = 7.2 Hz, m-
C6H5), 8.12 ppm (dd, 6H, 3JHH = 6.8 Hz, 4JHH = 1.5 Hz, o-C6H5).
C,H,N analysis calcd (%) for C51H94Ca3N12Si (1023.96 gmolꢀ1):
C 59.84, H 9.26, N 16.42; found: C 59.53, H 8.90, N 15.92.
Scheme 2. Catalytic hydrogenation and hydrosilylation of DPE.
converted into the anti-Markovnikov addition product, as
1
determined by in situ H NMR spectroscopy. The catalytic
hydrogenation of DPE with 3b (19 mol%) proceeded under
mild conditions (608C, 1 bar H2) but is slow (13 d, > 99%
conversion). It is noteworthy that the reaction mixture turned
dark red as soon as 3b and DPE were reacted. NMR
spectroscopic data suggested the formation of 1,1-diphenyl-
ethane and [Ca3(Me3TACD)3(H)2][C(Ph)2Me]. A related
complex was isolated by Harder et al. from the reaction of
DPE with [{Ca(DIPP-nacnac)(H)(thf)}2], and found to be
highly active in catalytic hydrogenation of DPE under
somewhat more forcing conditions (20 bar H2).[6b,c]
3c: A solution of Ph2SiH2 (8.0 mg, 0.044 mmol) in 0.5 mL of THF
was added to a suspension of 2c (15 mg, 0.044 mmol) in 0.5 mL of
THF. The mixture was shaken, the color immediately turned to red,
and the solution became homogeneous. After filtration, 0.5 mL of
toluene were added and the mixture was stored at ꢀ308C overnight.
The supernatant was decanted and the red crystals (7.5 mg, 21.5%)
were dried under reduced pressure. 1H NMR (400.1 MHz, [D8]THF):
3
d = 1.90–2.45 (br m, 18H, CH2), 2.20 (s, 9H, NCH3), 2.28 (d, JHH
=
3.5 Hz, 1H, CHC6H5), 2.45–2.90 (br m, 24H, CH2), 2.54 (s, 18H,
3
NCH3), 2.95–3.20 (br, 6H, CH2), 3.99 (s, 2H, CaH), 5.28 (tt, JHH
=
=
4
3
1
6.7 Hz, JHH = 1.4 Hz, 1H, p-CHC6H5), 5.31 (d, JHH = 3.5 Hz, JHSi
In summary, we have presented the preparation and full
characterization of the first cationic calcium hydrides. Their
preparation utilizes different silanes as hydride sources,
resulting in silicon-containing anions. The macrocyclic
ligand was found suitable for the stabilization of the
thermodynamically favored cationic Ca3H2 core in
[(Me3TACD)3Ca3(m3-H)2]+. Activity in the catalytic hydro-
silylation and hydrogenation of 1,1-diphenylethene was
171.4 Hz, 1H, SiH), 6.10 and 6.17 (2 br, 4H, o- and m-CHC6H5), 7.05
(m, 6H, m- and p-C6H5), 7.63 (dd, 3JHH = 7.8 Hz, 4JHH = 1.8 Hz, 4H, o-
C6H5).
C,H,N analysis
calcd (%)
for
C52H94Ca3N12Si
(1035.70 gmolꢀ1): C 60.30, H 9.15, N 16.23; found: C 59.78, H 7.79,
N 12.40.
CCDC 861124, 861125, 861126 (3a–c) contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
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Published online: March 23, 2012
Keywords: alkaline earth metals · calcium ·
.
homogeneous catalysis · hydrides · hydrogen storage
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