Stable monomeric germanium(II) and tin(II) compounds with terminal hydrides

Article abstract of DOI:10.1002/anie.200504337

Going solo: The use of AlH₃·NMe₃ as a hydrogen-transfer agent with β-diketiminatogermanium(II) and β-diketiminatotin(II) halides gives the divalent hydrides [{HC(CMeNAr) ₂}GeH] and [{HC(CMeNAr)₂}SnH] (Ar = 2,6-iPr ₂C₆H₃). In the solid state, the compounds exhibit monomeric and dimeric arrangements, respectively, with weak intermolecular Sn-H...Sn contacts for the Sn analogue (see picture). (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200504337

Communications
Main-Group Hydrides
A
(3,
Scheme 1)[11]
and
[{HC-
AHCTREUNG
DOI: 10.1002/anie.200504337
volatile by-product was observed during the course of the
reactions of 1 and 2 with AlH3·NMe3. The color of the
reaction mixture changed from yellow to orange-red in the
synthesis of 3, and from pale yellow to green in the synthesis
of 4. No further color change was observed for either reaction
mixture upon warming to ambient temperature.
Stable Monomeric Germanium(ii) and Tin(ii)
Compounds with Terminal Hydrides**
Leslie W. Pineda, Vojtech Jancik, Kerstin Starke,
Rainer B. Oswald, and Herbert W. Roesky*
Metal hydrides and their complexes are considered valuable
synthons in chemistry. It was demonstrated that main-group
and transition-metal hydrides are important intermediates in
some industrial processes and also function as catalysts.[1]
Furthermore, in the quest for alternative energy sources,
metal hydrides have been considered as potential feedstocks
for hydrogen storage.[2] Group 14 hydrides such as R3SiH,
R3GeH, and R3SnH are important reagents for some key
reactions in organic synthesis. The preparation of these
species is commonly carried out by reduction of the corre-
sponding chloride compounds with lithium aluminum hy-
dride.[3] However, the isolation and structural characteriza-
tion of monomeric, terminal low-valent Group 14 hydrides
seem to be difficult as a result of the potential reactivity and
Scheme 1. Formation of 3 and 4.
Compound 3 is thermally stable over a long period of time
when stored in an inert atmosphere in a glove box, whereas 4
decomposes slowly under the same conditions to form an
insoluble gray solid within four days. However, 4 is stable at
À328C over a longer period of time. Both 3 and 4 are air- and
moisture-sensitive and were characterized by IR spectrosco-
py, multinuclear NMR spectroscopy, single-crystal X-ray
structural analysis, and DFT calculations. In the IR spectrum
of 3, a strong absorption was observed at 1733 cmÀ1, which can
instability of these species. So far, only [{2,6-Trip2C6H3Sn(m-
A
H)}2] (Trip = 2,4,6-iPr3C6H2) has been structurally character-
ized and exhibits a dimeric structure with the tin(ii) atoms
connected by bridging hydride ligands,[4] although the exis-
tence of MH2 (M = Si to Pb) has been anticipated by
theoretical calculations.[5] Previously, we reported on the
preparation of the germanium(ii) and tin(ii) precursors [{HC-
À
be attributed to the Ge H stretching mode and compares well
N
with that reported for Ar’(H)GeGe(H)Ar’[13] (1785 cmÀ1
;
A
Ar’ = 2,6-Dipp2C6H3, Dipp = 2,6-iPr2C6H3), but is found at a
lower frequency than that for [{HC(CMeNAr)2}Ge(H)BH3]
U
(1928 cmÀ1).[7] Hydride complexes of germanium(iv) show
À1 [3a,14–16]
À
A
Ge H absorptions in the range from 1953 to 2175 cm .
À1
À
prepare a terminal tin(ii) hydride from 2 in reactions with
reducing reagents such as NaBH4, KBH4, and KH were
unsuccessful. In contrast, the reactions of 1 and 2 with
AlH3·NMe3[8–10] in toluene at À48C yield the first monomeric
and terminal germanium(ii) and tin(ii) hydrides [{HC-
The Sn H stretching mode of 4 (1849 cm ) is comparable to
those in the hydrogen-bridged tin compound [{2,6-
Trip2C6H3Sn
(m-H)}2] (1828, 1771 cmÀ1).[4] In the 1H NMR
A
À
spectrum of 3, the Ge H hydrogen atom resonates at lower
field (d = 8.08 ppm) than those in Ar’(H)GeGe(H)Ar’ (d =
3.48 ppm) and Ar’(H)2GeGeAr’·PMe3 (d = 3.81 ppm).[13]
À
Interestingly, the Ge H proton resonance of 3 is in the
[*] Dipl.-Chem. L. W. Pineda, Dr. V. Jancik, Dipl.-Chem. K. Starke,
Prof. Dr. H. W. Roesky
range of germanium(iv) hydrides.[17] The Sn H proton
À
resonance of 4 is found at a surprisingly low-field shift (d =
Institut für Anorganische Chemie
Universität Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
Fax: (+49)551-39-3373
13.83 ppm) relative to that in [{2,6-Trip2C6H3Sn
ACHTREUNG
7.87 ppm), and is flanked by Sn satellites (1J
(
N
64 Hz).[4] The Sn H resonance of 4 is the lowest-field
À
a
tin hydride.[3a,18–20] The
E-mail: hroesky@gwdg.de
chemical shift observed for
119Sn NMR resonance of 4 (d = À224.7 ppm) is in accordance
Dr. R. B. Oswald
Institut für Physikalische Chemie
Universität Göttingen
with
a
three-coordinate tin(ii) environment ([{HC-
AHCTREUNG
Tammannstrasse 6, 37077 Göttingen (Germany)
[**] This work was supported by the Fonds der Chemischen Industrie
and the Göttinger Akademie der Wissenschaften. L.W.P. thanks the
Deutscher Akademischer Austauschdienst (DAAD) for a predoc-
toral fellowship.
AHCTREUNG
Orange-red single crystals of 3 and green crystals of 4
suitable for X-ray structural analysis[22] were grown from
saturated hexane solutions at À328C within two days (3) or
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2602
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Angew. Chem. Int. Ed. 2006, 45, 2602 –2605

Angewandte
Chemie
after one week (4). Compound 3 crystallizes in the monoclinic
space group P21/n with two independent isostructural mono-
À
mers in the asymmetric unit. Although the Ge H hydrogen
atom in 3 could not be localized in the difference electron-
À
density map, the Ge H linkage was unequivocally confirmed
by IR and 1H NMR spectroscopy (see above). In 3, the
germanium atom is tetrahedrally coordinated by the b-
diketiminato ligand, a hydrogen atom, and, we assume, a
lone pair of electrons at the fourth coordination site
À
(Figure 1). Compound 3 exhibits Ge N bond lengths of
1.989(2) Š (molecule 1) as well as 1.994(2) and 1.988(2) Š
(molecule 2), and these lengths are very similar to those of 1
(1.988(2), 1.997(2) Š).[6]
Figure 2. Thermal-ellipsoid plot of 4 at the 50% probability level.
Carbon-bound H atoms are omitted for clarity. Selected bond lengths
o
À
À
À
[Š] and angles [ ]: Sn(1) H(1) 1.74(3), Sn(1) N(1) 2.195(2), Sn(1)
N(2) 2.198(2), Sn(1)···H(1A) 4.01(3); H(1)-Sn(1)-N(1) 93.4(8), H(1)-
Sn(1)-N(2) 92.6(8), N(1)-Sn(1)-N(2) 85.1(1).
31G[31,32] basis set (for the remaining atoms) with additional
double-diffuse functions were used. The structures of both
complexes were determined by full geometry optimization.
The obtained equilibrium geometries were in good agreement
with the X-ray data. Since the structural parameters are the
result of the calculated molecular orbitals, an analysis of the
electronic structure was perfomed with the natural bond
order (NBO)[33] procedure, which makes it possible to
describe and quantify the contribution of the atomic orbitals
to the molecular orbitals. The calculations showed that the
electron density of the lone pair on the central atom in each of
3 and 4 contributes to the bonding in the complex. The NBO
Figure 1. Thermal-ellipsoid plot of one molecule of 3 at the 50%
probability level. Hydrogen atoms are omitted for clarity. The GeÀH
hydrogen atom could not be localized. Only the numbering scheme of
molecule 1 is depicted. Selected bond lengths [Š] and angles [o]:
À
À
molecule 1: Ge(1) N(1) 1.989(2), Ge(1) N(2) 1.989(2), N(1)-Ge(1)-
À
À
N(2) 90.3(1); molecule 2: Ge(2) N(1A) 1.994(2), Ge(2) N(2A)
1.988(2), N(1A)-Ge(2)–N(2A) 90.5(1).
Compound 4 crystallizes in the monoclinic space group
C2/c with one monomer in the asymmetric unit. Weak
intermolecular contacts between the lone pair of electrons
À
analysis of the M H bond shows that the contribution of the
lone pair of electrons on germanium can best be described as
an sp0.32 hybrid, while the hybrid for tin is of the type sp0.25 (see
the Supporting Information). The larger p character at the Ge
center in compound 3 (20% greater than that at the Sn center
in 4) leads to an enhanced delocalization of p electrons over
the neighboring p orbitals of the nitrogen atoms. Remarkably,
in the case of 4, there is no participation of the hydrogen
s orbital in the lone pair of electrons.
À
on the tin atom and the Sn H hydrogen atom from another
molecule generate hydrogen bridges and formation of a dimer
(d(Sn(1)···H(1A)): 4.01(3) Š; d(Sn···Sn): 3.71 Š). Compound
E
4 comprises a distorted tetrahedral geometry at the tin atom,
which coordinates to a monoanionic b-diketiminato ligand, a
hydrogen atom, and, we assume, a lone pair of electrons at the
À
fourth coordination site (Figure 2). The Sn H hydrogen atom
À
was localized from the residual electron density. The most
Analysis of the metal hydrogen bond by using the natural
À
prominent structural feature of 4 is the Sn H bond length of
localized molecular orbitals (NLMO) reveals significant
À
À
1.74(3) Š, which is in good agreement with Sn H bond
differences between 3 and 4. For the Ge H bond in 3, the
lengths predicted by theoretical calculations (1.77 Š).[5,25]
However, this length is shorter than those of [{2,6-
hydrogen atom contributes 65%, and the corresponding
À
hydrogen atom in the Sn H bond contributes 70%. Analysis
Trip2C6H3Sn
A
of the molecular orbitals of 4 also shows that there is no
participation of the hydrogen s orbital in the wave function
describing the lone pair of electrons. Another factor that
influences the electron density at the hydrogen atom is the
amount of donor–acceptor interaction with the nitrogen
atoms of the ring system. This interaction occurs mainly
through two-electron stabilization in a donor–acceptor sit-
[4]
À
bonds (d(Sn H): 1.89(3), 1.95(3) Š).
A
To investigate the electronic properties of 3 and 4,
ab initio calculations were carried out by using the Gaussian
program package[26] with the well-established DFT-variant
B3LYP.[27–29] Two different basis sets were employed to
achieve a suitable description of the electronic structure.
The LANL2DZ[30] basis set (for Ge and Sn) and the 6-
uation[34] involving the bonding Ge H orbital and nonbond-
À
Angew. Chem. Int. Ed. 2006, 45, 2602 –2605
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 2603

A
N
Communications
ing orbitals of the ring system, which transfers electron
Me4Sn): d = À224.7 ppm; EI-MS (70 eV): m/z (%): 537 (5) [MÀH]+.
Elemental analysis (%) calcd for C29H42N2Sn (537.36): C 64.82, H
7.88, N 5.21; found: C 64.20, H 7.45, N 5.33.
À
density from the nitrogen atoms into the Ge H bond. This
effect is 50% larger for 3 than for 4. Taking these findings into
account, the low-field chemical shift of the hydrogen atom in
4 might be deduced. The lack of further stabilization from
delocalization causes the tin atom to pull out electron density
strongly from the hydride ligand to compensate for its
electron-deficient character. Hence, the “naked” nature of
Received: December 6, 2005
Revised: January 25, 2006
Published online: March 14, 2006
À
the hydrogen atom in the Sn H bond is a reflection of
extreme deshielding and leads to such a low-field chemical
shift.
Keywords: density functional calculations · germanium ·
Group 14 elements · hydrides · tin
.
In summary, we were able to isolate and structurally
characterize
compounds
of
composition
[{HC-
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A
ACHTREUNG
tions of AlH3·NMe3 with the appropriate chloride precursors.
These compounds represent the first examples of terminal,
monomeric hydrides of germanium(ii) and tin(ii).
Experimental Section
All manipulations were performed in
atmosphere (N2 or Ar) by using Schlenk-line and glove-box
techniques. Solvents were purified prior to use by distillation over
appropriate drying agents under nitrogen.
a dry and oxygen-free
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´
3: A solution of AlH3·NMe3 (1.18 mL, 1.0m in toluene) was
slowly added to a solution of 1 (0.62 g, 1.18 mmol) in toluene (25 mL)
at À48C, and the yellow solution immediately turned to orange-red.
The cooling bath was removed after 20 min, and stirring of the
solution was continued until the elimination of NMe3 had ceased. All
volatiles were removed in vacuum, and the remaining orange-red
residue was extracted with n-hexane (15 mL). The solvent was
removed in vacuo to yield 3 as an orange-red powder. Yield: 0.32 g
(60%); m.p. 1708C (decomp); IR (KBr pellet): n˜ = 1733 cmÀ1 (s,
Chem. Rev. 2004, 104, 1283 – 1315; d) F. Schüth, B. Bogdanovic,
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GeH); 1H NMR (300 MHz, C6D6, 258C, TMS): d = 8.08 (s, 1H, GeH),
3
7.15–7.07 (m, 6H, ArH), 4.92 (s, 1H, g-CH), 3.58 (sept, J
ACHTREUNG
6.8 Hz, 2H, CH
1.55 (s, 6H, CH3), 1.37 (d, 3J
3J (CH3)2), 1.19 (d, 3J
(H,H) = 6.8 Hz, 6H, CH
CH
C
G
ACHTREUNG
G
U
G
U
A
(CH
A
G
G
G
23.7 (CH
A
ACHTREUNG
491 (100) [MÀH]+, 449 (40) [MÀiPr]+. Elemental analysis (%) calcd
for C29H42GeN2 (491.26): C 70.69, H 8.60, N 5.70; found: C 70.20, H
8.53, N 5.36.
[4] B. E. Eichler, P. P. Power, J. Am. Chem. Soc. 2000, 122, 8785 –
8786.
[5] G. Trinquier, J. Am. Chem. Soc. 1990, 112, 2130 – 2137.
[6] Y. Ding, H. Hao, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt,
Organometallics 2001, 20, 4806 – 4811.
4: A solution of 2 (1.85 g, 3.24 mmol) in toluene (30 mL) was
cooled to À48C and added slowly to a solution of AlH3·NMe3
(3.24 mL, 1.0m in toluene). The cooling bath was removed after
15 min, and the solution was stirred further until NMe3 elimination
had ceased. The color of the solution changed from pale yellow to
green. The solvent was removed in vacuo, and the green residue was
extracted with n-hexane. The solvent was removed to afford 4 as a
green powder. Yield: 1.58 g (91%); m.p. 1258C (decomp); IR (KBr
pellet): n˜ = 1849 cmÀ1 (m, SnH); 1H NMR (500 MHz, C6D6, 258C,
[7] Y. Ding, H. W. Roesky, M. Noltemeyer, H.-G. Schmidt, Organo-
metallics 2001, 20, 1190 – 1194.
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[9] H. W. Roesky, Aldrichimica Acta 2004, 37, 103 – 108.
[10] S. S. Kumar, H. W. Roesky, Dalton Trans. 2004, 3927 – 3937.
[11] Alternatively, compound 3 could also be prepared in good yield
TMS): d = 13.83 (s, 1J
ArH), 4.89 (s, 1H, g-CH), 3.52 (sept, 3J
(CH3)2), 3.46 (sept, 3J
(H,H) = 6.8 Hz, 2H, CH
(H,H) = 6.8 Hz, 6H, CH(CH3)2), 1.27 (d, J
(H,H) = 6.8 Hz, 6H, CH
(H,H) = 6.8 Hz, 6H, CH
(CH3)2); 13C NMR
(125.8 MHz, C6D6, 258C, TMS): d = 167.7 (CN), 145.5, 143.0, 142.8,
126.8, 124.7, 124.3 (Ar), 98.3 (g-CH), 29.0 (CH3), 27.8 (CH(CH3)2),
26.8 (CH(CH3)2), 24.8 (CH(CH3)2), 24.4 (CH(CH3)2), 24.0 (CH-
A
119Sn,1H) = 64 Hz, 1H, SnH), 7.15–7.06 (m, 6H,
(H,H) = 6.8 Hz, 2H, CH-
(CH3)2), 1.62 (s, 6H,
G
by the reaction of [{HC
(CMeNAr)2}GeOH][12] and AlH3·NMe3.
A
U
U
N
[12] L. W. Pineda, V. Jancik, H. W. Roesky, D. Neculai, A. M.
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[13] A. F. Richards, A. D. Phillips, M. M. Olmstead, P. P. Power, J.
Am. Chem. Soc. 2003, 125, 3204 – 3205.
3
3
ACHTREUNG
CH3), 1.34 (d, J
6.8 Hz, 6H, CH(CH3)2), 1.19 (d, J
1.17 ppm (d, 3J
A
T
3
A
N
ACHTREUNG
G
N
U
[14] A. Castel, P. Rivi›re, J. SatgØ, H. Y. Ko, Organometallics 1990, 9,
205 – 210.
A
N
ACHTREUNG
2604 www.angewandte.org
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Angew. Chem. Int. Ed. 2006, 45, 2602 –2605

Angewandte
Chemie
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W. A. Shirley, J. Mantzaris, J. Chem. Phys. 1988, 89, 2193 – 2218.
[33] a) J. E. Carpenter, F. Weinhold, J. Mol. Struct. 1988, 169, 41 – 62;
b) J. P. Foster, F. Weinhold, J. Am. Chem. Soc. 1980, 102, 7211 –
7218; c) A. E. Reed, F. Weinhold, J. Chem. Phys. 1985, 83, 1736 –
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[34] F. Weinhold, C. Landis, Valency and Bonding, University Press,
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[22] a) Crystal data for 3: C29H42GeN2, Mr = 491.24, crystal dimen-
sions 0.10 ” 0.05 ” 0.02 mm3, monoclinic, space group P21/n, a =
15.238(3), b = 18.201(4), c = 20.485(4) Š, b = 99.11(3)8, V=
5610(2) Š3, Z = 8, 1calcd = 1.163 gcmÀ3
1.54178 Š, T= 100(2) K, m
(CuKa) = 1.593 mmÀ1. Of the 35990
measured reflections, 7983 were independent (R(int) = 0.0418).
,
F
A
AHCTREUNG
AHCTREUNG
The final refinements converged to R1 = 0.0388 for I > 2s(I),
wR2 = 0.1035 for all data. The final difference Fourier synthesis
À3
gave a min/max residual electron density of À0.476/ + 1.47 eŠ
;
b) crystal data for 4: C29H42SnN2, Mr = 537.34, crystal dimensions
0.10 ” 0.05 ” 0.05 mm3, monoclinic, space group C2/c, a =
25.209(5), b = 15.139(3), c = 14.682(3) Š, b = 94.92(3)8, V=
5583(2) Š3, Z = 8, 1calcd = 1.279 gcmÀ3
,
F
ACHTREUNG
1.54178 Š, T= 100(2) K, m
ACHTREUNG
AHCTREUNG
The final refinements converged to R1 = 0.0216 for I > 2s(I),
wR2 = 0.0548 for all data. The final difference Fourier synthesis
gave
a
min/max residual electron density of À0.745/
+ 0.749 eŠÀ3; c) data for the structures were collected on a
Bruker three-circle diffractometer, which was equipped with a
SMART 6000 CCD detector. Intensity measurements were
performed on a rapidly cooled crystal in the ranges 6.54 ꢀ 2q ꢀ
117.948 (3) and 6.82 ꢀ 2q ꢀ 117.808 (4). The structures were
solved by direct methods with SHELXS-97[23] and refined for all
data by full-matrix least squares on F2.[24] All non-H atoms for 3
and 4 were refined anisotropically, and all H atoms for 3 and 4,
À
except for the g-C H hydrogen atoms H(3A) and H(3AA) in 3
A
(the packing diagram shows chain formation through weak
intermolecular interactions between the lone pair of electrons at
À
the Ge atom and the g-C H hydrogen atom of another
molecule), were included in geometrically idealized positions
and refined with the riding model. Localization of the H(3A)
and H(3AA) atoms from the electron-density maps in 3 proved
G
to be more accurate than fixing the atoms in the idealized
positions and led to lower R1 and wR2 values. The SnÀH
hydrogen atom in 4 was localized from the difference electron-
density map and refined isotropically with Uij tied to the parent
atom. CCDC-291156 and CCDC-291157 contain the supple-
mentary 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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