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Rational Design of a Well-Defined Soluble Calcium Hydride Complex.

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Zuschriften
Main-Group Hydrides
DOI: 10.1002/ange.200601013
Rational Design of a Well-Defined Soluble
Calcium Hydride Complex**
Sjoerd Harder* and Julie Brettar
There is a wealth of information on transition-metal hydride
complexes, a field driven by numerous applications in
catalytic processes.[1] Recently, significant progress has also
been made in the syntheses of lanthanide hydride complexes.[2] The area of main-group metal hydride complexes is
biased in the sense that p-block metal hydrides are much
better documented[3] than s-block metal hydrides, for which
only few examples have been reported.[4] There are two
important reasons for this imbalance: 1) relatively small bond
energies in these predominantly ionic complexes allow for
facile ligand exchange and thus formation of ligand-free metal
hydride (this holds especially for the larger metals) and
2) very high lattice energies for early-main-group metal
hydrides result in immediate precipitation of [MHx]1. Consequently, the few examples of early-main-group metal
hydrides all show the hydride encapsulated in mixed aggregates. A well-defined Li-Al-hydride[4a] cluster, a lithium
amide-hydride aggregate[4b,c] and the alkoxide-hydride cluster
(tBuOLi)16(LiH)17[4d] have been described. A magnesium
hydride was found in an “inverse crown ether”, that is,
encapsulated in a ring that also contains metal ions (1).[4e,f]
Also a beryllium hydride stabilized with a scorpionate ligand
(TptBu) has been reported (2).[4g] Although soluble arylcalcium
hydrides (prepared by activation of the CH bond with
calcium vapor) have been reported,[4h,i] no NMR spectroscopic or structural proof was available. Herein we describe the
synthesis and characterization of a well-defined soluble
calcium hydride complex.
[*] Prof. Dr. S. Harder, Dr. J. Brettar
Anorganische Chemie
Universit't Duisburg-Essen
Universit'tsstrasse 5, 45117 Essen (Germany)
Fax: (+ 49) 201-183-2621
E-mail: sjoerd.harder@uni-due.de
[**] We kindly acknowledge H. Bandmann for measurement of 500 MHz
NMR spectra. We are grateful to Prof. Dr. R. Boese and D. Bl'ser for
measuring some of the X-ray diffraction data sets.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3554 –3558
Angewandte
Chemie
Our first approach was based on the synthesis of 1, a
mixed-metal hydride that was obtained by b-hydrogen
elimination in an iPr2N-complex (iPr2N !iPrN=CMe2 +
H). Since preliminary results indicated that such an elimination is also possible in calcium chemistry,[5] we converted
the heteroleptic benzylcalcium complex 3 into the heteroleptic amide 4 according to Scheme 1. This calcium amide
Scheme 1.
compound crystallizes as a dimer with terminal 9-Me3Sifluorenyl ligands and bridging iPr2N ions (Figure 1). There
are no precedents for iPr2N–Ca complexes, but the CaN
bond lengths in 4 (average: 2.399(5) :) are significantly
shorter than those for the bridging (Me3Si)2N ions in
Figure 1. Crystal structure of 4; hydrogen atoms omitted for clarity.
Selected bond lengths [D] and angles [8]: Ca1–ring 2.716(6)–2.806(6),
Ca2–ring 2.724(6)–2.814(7), Ca1–N1 2.385(5), Ca1–N2 2.420(5), Ca2–
N1 2.395(5), Ca2–N2 2.394(5); N1-Ca1-N2 86.3(2), N1-Ca2-N2
86.7(2). Agostic Me···Ca interactions are indicated by dashed lines:
Ca···H distances range from 2.57–2.80 D.
Angew. Chem. 2006, 118, 3554 –3558
[((Me3Si)2N)2Ca]2 (average: 2.475(6) :).[6] This situation is
in line with the higher basicity and smaller steric bulk of the
iPr2N ion. The crystal structure reveals several short agostic
Me···Ca contacts (indicated in Figure 1) similar to those
observed in the dimer [iPr2NK·TMEDA]2 (TMEDA =
N,N,N’,N’-tetramethyl-1,2-ethanediamine).[7] Heating benzene or THF solutions of 4 in closed NMR-tubes for several
days at 100 8C only showed small amounts of decomposition
products which could be neither identified nor isolated. The
high stability of 4 is remarkable and strongly contrasts with
that of iPr2NK.[7]
A second approach to generate metal hydride complexes
is the reaction of a nucleophilic metal complex with phenylsilane (Scheme 1). Although this method is well-established
in lanthanide chemistry,[2] it surprisingly has not been applied
in early-main-group chemistry. The validity of this metathesis
reaction is suggested by a new class of alkene hydrosilylation
catalysts based on early-main-group metals.[8] Reaction of the
heteroleptic complex 3 with phenylsilane was monitored by
1
H NMR spectroscopy: clean conversion into the expected
benzylsilane (Scheme 1) was observed and broad signals for a
fluorenyl species appeared in the spectrum. A crystalline
product could only be obtained by addition of small amounts
of THF. The product was analyzed as the homoleptic complex
[9-trimethylsilylfluorenyl]2 [Ca2+·(THF)6].[9] The additional
observation of small amounts of a white precipitate during the
reaction suggests that the hydride precipitated in the form of
CaH2.
Heteroleptic calcium hydrides will possibly only be stable
if the Schlenk equilibrium is steered exclusively to the
heteroleptic side. Strongly bonding bulky ligands are therefore an absolute prerequisite. The bulky scorpionate ligand
TptBu (as in 2) seemed promising for two reasons: 1) the
attempted synthesis of its homoleptic Ca complex was not
possible even under extreme conditions: reaction of two
equivalents of TptBuTl with CaI2 only produced TptBuCaI[10]
and 2) stable monomeric TptBuZnH has been reported;[11] the
Ca2+ ion is approximately 0.15 : larger than the Zn2+ ion.
Heteroleptic TptBuCaN(SiMe3)2, earlier reported by Chisholm,[10] gave, after reaction with phenylsilane, clean conversion into PhH2SiN(SiMe3)2 which indicated concommitant
formation of TptBuCaH. However, crystallization of the
presumed product gave a batch of large, colorless crystals
that were determined to be (TptBu)2Ca (5) by X-ray diffraction
(Figure 2 a). Although a space-filling model of the TptBuCa
unit (Figure 2 b) suggests that it is impossible that two ligands
can coordinate at one metal center, both TptBu units coordinate as tridentate ligands: one is coordinated as a (k3
N,N’,N’’)-ligand whereas the other is bonded through two
pyrazole nitrogen atoms and a B-H unit. Two rings in the (k3
N,N’,N’’)-ligand are tilted towards each other creating space
for the two bulky pyrazole rings of the (k3 N,N’,BH)-ligand
(Figure 2 b). Steric crowding in (TptBu)2Ca is underscored by
the relatively long CaN bonds: the CaN bonds for the (k3
N,N’,N’’)-ligand (average: 2.481(1) :) are significantly longer
than those in TptBuCa(O-2,6-iPr2C6H3) which range from
2.412(1) to 2.437(1) :.[10] The CaN bonds for the (k3
N,N’,BH)-ligand (average: 2.521(1) :) are even longer. The
BH···Ca contact of 2.47(2) : is also considerably longer than
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
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Zuschriften
Scheme 2.
Figure 2. a) Crystal structure of (TptBu)2Ca (5); hydrogen atoms omitted
for clarity. Selected interatomic distances [D] and angles [8]: Ca–N2
2.510(1), Ca–N6 2.531(1), Ca–N8 2.493(1), Ca–N10 2.483(1), Ca–N12
2.468(1), Ca···H1 2.47(2), B1–H1 1.14(2), B2–H2 1.12(2); N2-Ca-N6
92.73(4), N2-Ca-H1 64.6(4), N6-Ca-H1 63.5(4), Ca···H1-B1 109(1), N8Ca-N10 69.99(4), N8-Ca-N12 92.53(4), N10-Ca-N12 91.32(4). b) Spacefilling models of part of the structure showing the (k3 N,N’,N’’)TptBu
ligand bonded to Ca.
that of 2.21(4) : in (1,2,4-Me3Si-Cp)Ca(HBEt3).[12] Few cases
of tridentate coordination using a polarized Bd+–Hd unit
instead of a pyrazole nitrogen have been reported.[13] This
coordination mode is a good alternative in ionic complexes,
since the negative charge in the scorpionate anion is largely
on the atoms directly attached to boron. The b-diketiminate
anion, in which the charge is mainly on the nitrogen atoms,
could, despite its lower denticity, be a stronger ligand than a
scorpionate ligand.
Reaction of (DIPP-nacnac)CaN(SiMe3)2(thf) (6)[10]
((DIPP-nacnac = (2,6-iPr2C6H3)NC(Me)C(H)C(Me)N(2,6iPr2C6H3)) with phenylsilane in hexane (Scheme 2) gave, after
cooling, a batch of large colorless crystals. The crystal
structure shows the heteroleptic dimer: [(DIPP-nacnac)CaH(thf)]2 (7; Figure 3). The hydride atoms, which could be
located in the difference Fourier map and were refined
isotropically, bridge symmetrically between the two Ca2+ ions
which are separated by 3.524(4) :. The CaH bonds are in
the range of 2.09(4)–2.21(3) : and thus considerably longer
than the MgH bonds in 1 (1.94(2) :)[4e] and the BeH bond
in 2 (1.23(7) :).[4g]
The hydride complex 7 dissolves in benzene and shows a
remarkable stability. Even under reflux conditions no disproportionation to (DIPP-nacnac)2Ca and insoluble [CaH2]8
is observed. This result is the more remarkable since we have
shown that homoleptic (DIPP-nacnac)2Ca is a rather stable
complex on account of Me2C-H···p(aryl)-bonding.[14] NMR
spectroscopic investigations suggest that the structure of 7 is
3556
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Figure 3. Crystal structure of 7; hydrogen atoms omitted for clarity.
Selected bond lengths [D] and angles [8]: Ca1–H1 2.10(3) Ca1–H2
2.09(4), Ca2–H1 2.21(3), Ca2–H2 2.20(4), Ca1–O1 2.391(1), Ca2–O2
2.355(1), Ca1–N1 2.384(1), Ca1–N2 2.372(1), Ca2–N3 2.396(1), Ca2–
N4 2.390(1); Ca1-H1-Ca2 109.7(11), Ca1-H2-Ca2 110.4(17), H1-Ca1-H2
71.8(13), H1-Ca2-H2 67.8(13).
retained in benzene solution. The iPr-methyl groups are
inequivalent as a result of hindered rotation around the N
C(aryl) bond and give two doublets. The outer methyl groups
give strong NOE cross peaks with the methyl groups in the
backbone, whereas for the inner methyl groups strong NOE
cross peaks are observed with the a-CH2 of THF and with the
hydride. Although the 1H NMR signals for the interstitial
hydride in mixed metal complexes[4a–c] have not been
observed, we find a relatively sharp singlet at d = 4.45 ppm.
This is downfield of the 1H resonance for the hydride in the
inverse crown ether 1 (d = 3.70 ppm)[4e] but upfield of that in 2
(d = 5.00 ppm).[4g] 1H NMR chemical shifts for the bridging
hydride in numerous dimeric yttrium hydride complexes are
spread over a large range: d = 1.85–8.31 ppm.[15]
In summary, we have prepared the first well-defined
soluble calcium hydride by metathesis of a calcium amide
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 3554 –3558
Angewandte
Chemie
with phenylsilane. This method, which was shown to function
equally well with benzylic calcium, strontium, and potassium
precursors,[8] could give impetus to the hydride chemistry of
the early-main-group metals in general. We are currently
exploring the reactivity and applications of soluble calcium
hydride complexes.
Experimental Section
All experiments were carried out using standard Schlenk techniques
and freshly dried solvents. Diisopropylamine was distilled from CaH2
prior to use. Phenylsilane was used as received. The following
compounds were prepared according to literature methods: 3,[16]
TptBuCaN(SiMe3)2[10] and (DIPP-nacnac)CaN(SiMe3)2(thf) (6).[10]
4: Dry iPr2NH (86 mg, 0.85 mmol) was added to a solution of 3
(400 mg, 0.72 mmol) in toluene (5.0 mL). The resulting solution was
heated to 60 8C for 30 min, concentrated to 2.5 mL, and slowly cooled
to 30 8C. A crop of orange plate-like crystals was isolated (185 mg,
68 %). Elemental analysis (%) calcd for C22H31CaNSi (Mr = 377.67):
C 69.97, H 8.27; found: C 69.58, H 8.21. 1H NMR (250 MHz, [C6D6],
20 8C): d = 0.35 (d, 3J(H,H) = 6.0 Hz, 12 H; iPr), 0.46 (s, 9 H; Me3Si),
1.86 (sept, 3J(H,H) = 6.0 Hz, 2 H; iPr), 7.00 (t, 3J(H,H) = 7.6 Hz, 2 H;
fluorenyl), 7.22 (t, 3J(H,H) = 7.5 Hz, 2 H; fluorenyl), 7.86 (d, 3J(H,H) = 7.6 Hz, 2 H; fluorenyl), 7.95 ppm (d, 3J(H,H) = 7.5 Hz, 2 H;
fluorenyl). 13C NMR (75 MHz, [C6D6], 20 8C): d = 2.3 (Me3Si), 26.3
(iPr), 48.1 (iPr), fluorenyl: 87.0, 117.3, 121.5, 122.0, 123.7, 124.8,
141.6 ppm.
5: Phenylsilane (30 mg, 0.28 mmol) was added to a solution of
TptBuCaN(SiMe3)2 (163 mg, 0.28 mmol) in benzene (3.0 mL). After
stirring for 3 h at 60 8C, all solvent was removed. Crystallization of the
oily product from pentane at 30 8C gave a crop of large colorless
blocks (51 mg, 45 %). Elemental analysis (%) calcd for
C42H68B2CaN12 (Mr = 802.79): C 62.84, H 8.54; found: C 62.49, H
8.43. 1H NMR (250 MHz, [C6D6], 20 8C): d = 1.12 (s broad, 54 H; tBu),
5.82 (s broad, 3 H; pyrazole), 5.91 (s broad, 3 H; pyrazole), 7.46 (s
broad, 3 H; pyrazole), 8.19 ppm (s broad, 3 H; pyrazole). 13C NMR
data could not be obtained owing to severe line-broadening.
7: Phenylsilane (140 mg, 1.29 mmol) was added to a solution of
(DIPP-nacnac)CaN(SiMe3)2(thf) (6; 900 mg, 1.30 mmol) in hexane
(5.0 mL). The solution was stirred for 1 h at 60 8C and then slowly
cooled to 30 8C giving 7 in the form of colorless crystals (419 mg,
61 %). Elemental analysis (%) calcd for C33H50CaN2O (Mr = 530.86):
C 74.67, H 9.49; found: C 74.32, H 9.58. 1H NMR (250 MHz, [C6D6],
20 8C): d = 1.06 (d, 3J(H,H) = 6.8 Hz, 12 H; iPr), 1.24 (d, 3J(H,H) =
6.9 Hz, 12 H; iPr), 1.40 (m, 4 H; THF), 1.65 (s, 6 H; Me), 3.14 (sept,
3
J(H,H) = 6.8 Hz, 4 H; iPr), 3.57 (m, 4 H; THF), 4.45 (s, 1 H, Ca-H),
4.72 (s, 1 H, H-backbone), 7.05–7.15 ppm (m, 6 H, aryl). 13C NMR
(75 MHz, [C6D6], 20 8C): 24.5 (iPr-Me), 24.6 (Me-backbone), 25.2
(THF), 25.7 (iPr-Me), 28.0 (iPr-CH), 69.3 (THF), 93.6 (backbone),
123.5 (Ar), 123.9 (Ar), 141.9 (Ar), 146.6 (Ar), 164.9 ppm (backbone).
Crystal data for 4.[17, 18] Monoclinic, space group P21/c, a =
10.235(9), b = 18.714(13), c = 33.958(10) :, b = 97.70(8), V =
6446(8) :3, formula C44H62Ca2N2Si2, Z = 6, R = 0.0738, wR2 =
0.2614, GOF = 0.98, 1max = 0.86 e :3, 1min = 0.89 e :3. Measurement on an Enraf Nonius CAD4 at 90 8C, MoKa, 2qmax = 508, 12 019
reflections measured, 11 333 independent reflections (Rint = 0.046),
6643 reflections observed with I > 2s(I). All hydrogen atoms were
placed on calculated positions and refined in a riding mode. The
asymmetric unit comprises a dimer with no overall symmetry in a
general position, and a centrosymmetric dimer severely disordered in
the amide part. Since no effective disorder model could be found,
these atoms have high anisotropic displacement factors. The discussion on bond lengths relates only to the ordered dimer, which is
structurally similar to the disordered dimer.
Crystal data for 5.[17, 18] Orthorhombic, space group P212121, a =
11.2161(2), b = 18.9172(4), c = 22.1466(5) :, V = 4699.0(2) :3, forAngew. Chem. 2006, 118, 3554 –3558
mula C42H68B2CaN12, Z = 4, R = 0.0416, wR2 = 0.0975, GOF = 1.04,
1max = 0.40 e :3, 1min = 0.26 e :3. The Flack parameter refined to
0.005(19). Measurement on a Siemens SMART CCD diffractomer
at 90 8C, MoKa, 2qmax = 58.88, 185 322 reflections measured, 13 005
independent reflections (Rint = 0.051), 11 354 reflections observed
with I > 2s(I). All hydrogen atoms were placed on calculated
positions and refined in a riding mode.
Crystal data for 7.[17, 18] Monoclinic, space group C2/c, a =
47.9796(17), b = 12.5912(4), c = 22.2426(8) :, b = 103.069(2)8, V =
13089.2(8) :3, formula C66H100Ca2N4O2, Z = 8, R = 0.0455, wR2 =
0.1354, GOF = 1.02, 1max = 0.70 e :3, 1min = 0.38 e :3. Measurement on a Siemens SMART CCD diffractomer at 90 8C, MoKa,
2qmax = 628, 433851 reflections measured, 20 768 independent reflections (Rint = 0.038), 15 482 reflections observed with I > 2s(I). The
hydride atoms, like nearly all other H atoms, were located and refined
isotropically; few iPr-hydrogen atoms were placed on calculated
positions and refined in a riding fashion. Ring puckering disorder in
THF was resolved. Slight disorder in one of the iPr groups was treated
by anisotropic refinement.
CCDC-601651–601653 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.
Received: March 14, 2006
Published online: April 25, 2006
.
Keywords: alkaline-earth metals · amides · calcium · hydrides ·
scorpionate ligands
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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