H2MBH2 and M(μ-H)2BH2 Molecules Isolated in Solid Argon: Interelement M-B and M-H-B Bonds (M = Ge, Sn)

Article abstract of DOI:10.1021/acs.inorgchem.7b03109

Laser-ablated boron atoms react with GeH₄ molecules to form novel germylidene borane H₂GeBH₂, which undergoes a photochemical rearrangement to the germanium tetrahydroborate Ge(μ-H)₂BH₂ upon irradiation with light of λ = 405 nm. For comparison, the boron atom reactions with SnH₄ only gave the tin tetrahydroborate Sn(μ-H)₂BH₂. Infrared matrix-isolation spectroscopy with deuterium substitution and the state-of-the-art quantum-chemical calculations are used to identify these species in solid argon. A planar structure of H₂GeBH₂ with an electron-deficient B-Ge bond with a partial multiple bond character (bond order = 1.5) is predicted by quantum-chemical calculations. In the case of M(μ-H)₂BH₂ (M = Ge, Sn) two 3c-2e B-H-M hydrogen bridged bonds are formed by donation of electrons from the B-H σ-bonds into empty p-orbitals of M.

Full text of DOI:10.1021/acs.inorgchem.7b03109

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
H2MBH2 and M(μ-H)2BH2 Molecules Isolated in Solid Argon:
Interelement M−B and M−H−B Bonds (M = Ge, Sn)
Jie Zhao,† Helmut Beckers,‡ Tengfei Huang,† Xuefeng Wang,*,† and Sebastian Riedel*,‡
†School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China
‡Institut fur Chemie und Biochemie−Anorganische Chemie, Freie Universitat Berlin, Fabeckstrase 34/36, D-14195 Berlin, Germany
S
* Supporting Information
ABSTRACT: Laser-ablated boron atoms react with GeH4 molecules to
form novel germylidene borane H2GeBH2, which undergoes a
photochemical rearrangement to the germanium tetrahydroborate
Ge(μ-H)2BH2 upon irradiation with light of λ = 405 nm. For
comparison, the boron atom reactions with SnH4 only gave the tin
tetrahydroborate Sn(μ-H)2BH2. Infrared matrix-isolation spectroscopy
with deuterium substitution and the state-of-the-art quantum-chemical
calculations are used to identify these species in solid argon. A planar
structure of H2GeBH2 with an electron-deficient B−Ge bond with a
partial multiple bond character (bond order = 1.5) is predicted by quantum-chemical calculations. In the case of M(μ-H)2BH2
(M = Ge, Sn) two 3c−2e B−H−M hydrogen bridged bonds are formed by donation of electrons from the B−H σ-bonds into
empty p-orbitals of M.
identified based on experimental infrared and microwave
spectroscopic studies. This result prompted us to investigate
the reactions of laser ablated boron atoms with GeH4 and
SnH4, and bonding characters for boron with group 14
hydrides. Here, we report experimental and computational
evidence of both H2GeBH2 and Ge(μ-H)2BH2 molecules
isolated in cryogenic conditions in argon matrix. Interestingly,
only Sn(μ-H)2BH2 was observed in the boron atom reaction
with SnH4.
INTRODUCTION
■
Group 14 hydrides, such as R3SiH, R3GeH, and R3SnH, are of
great importance due to their industrial applications, synthetic
chemistry, and the interesting nature of the chemical
bonding.1−4 Subvalent germanium and tin hydrides show a
high reactivity and attracted considerable attention after the
preparation of divalent tin(II)hydride (Ar·SnH)2 in 2000.5
Several germanium(II) hydrides featuring a central GeH2
moiety flanked by electron-donating, electron-accepting, or
both groups have been prepared based on kinetic or
thermodynamic stabilization.6−9 In principle, these species
representing chemical analogues to the well-known carbenes
provide considerable insights into the nature of the chemical
structure and bonding in subvalent group 14 compounds.
Moreover, akin to transition metal hydrides, some low-valent
group 14 compounds have shown to readily activate small
molecules such as ammonia, dihydrogen, CO2, ketones, as well
as CC, C−H, and CC bonds.10−14 Compared to transition
metal complexes the low-valent oxidation state group 14
element hydride complexes need further investigations, because
of their still scarcely explored potential in many areas of
synthetic chemistry and catalysis.
On the other hand, the heavier ethylene analogues like
H2EEH2 (E = Si, Ge, Sn, Pb) are characterized by “strained”
structures which show trans-bent bonding.15−18 However, in
our previous work we have shown that the H2BSiH2 molecule
adopts a planar structure. It was prepared under cryogenic
conditions in argon matrices from the reaction of laser ablated
boron atoms with silane, SiH4.19 By this matrix-isolation
technique numerous novel molecules containing BH2 subunits
have been isolated and characterized in the recent years.20−25 In
these compounds significant π-bond character has been
EXPERIMENTAL AND COMPUTATIONAL
METHODS
■
Matrix-isolation experiments based on laser-ablated boron atoms in
the presence of small reactant molecules seeded in a large excess of a
noble gas have been described earlier.19 The 1064 nm fundamental of
a Nd:YAG laser (continuum, Minilite II; 10 Hz repetition rate) was
focused on a rotating boron target. Laser-ablated enriched 10B (Eagle
Pitcher, 93.8% 10B, 6.2% 11B), and enriched 11B (Eagle Pitcher, 97.5%
n
11B, 2.5% 10B), or natural boron B (Aldrich, 80.4% 11B, 19.6% 10B)
atoms have been codeposited with GeH4 or SnH4 (typically 0.3−0.5%)
under excess of argon onto a 5 K CsI window. This window was
mounted onto the cold tip of a closed-cycle helium refrigerator
(Sumitomo Heavy Industrious, Model RDK408D). The sample
preparation by laser ablation was done for 1 h at a rate of 2−4
mmol/h of the GeH4/Ar or SnH4/Ar mixture. After deposition, IR
spectra have been recorded in the middle infrared region from 4000 to
450 cm−1 on a Bruker Vertex 80 V spectrometer at a resolution of 0.5
cm−1 using a liquid nitrogen cooled broad band MCT detector.
Afterward, the matrices were annealed to allow further product
formation. In the annealing process, the heat system was turned off
immediately after reaching the desired temperature. Selected matrices
Received: December 11, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03109
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Article
were also subjected to λ = 455 and 405 nm photolysis using LED light
bands appeared at 2529.0, 2459.8, 1902.2, 1357.9, and 1113.8
cm−1.
(half-wave width 5 nm, 5 W). The GeH4 and GeD4 samples have been
prepared by treating GeO2 with NaBH4 using Schlenk techniques.26
A
solution of NaBH4 in H2O (1 M) or NaBD4 in D2O (1 M) was slowly
added to the GeO2 slurry. The germane product was passed through a
trap cooled to −120 °C and collected in two traps held at liquid
nitrogen temperature at −196 °C. No further purification has been
applied. SnH4 and SnD4 were prepared from SnCl2 and NaBD4 in the
same method.26
All structures have been fully optimized at the DFT level of theory
using the B3LYP27,28 functional in conjunction with the def2-TZVPP
basis sets.29 For tin, quasi-relativistic effective core potential (ECP-28)
of the Stuttgart type was used to describe 28 inner electrons.30
Subsequent optimizations at the CCSD(T) level were carried out for
the ground states of the BGeH4 and BSnH4 isomers. Harmonic and
anharmonic frequency calculations were performed for all optimized
structures. The DFT calculations were performed using GAUSSIAN
0931 while the CCSD(T) calculations have been performed using the
CFOUR program package.32 The Multiwfn code was applied for an
atoms-in-molecule (AIM) analysis and for the evaluation of the
electron localization function (ELF).33−35
Figure 2. Infrared spectra obtained from the reaction of laser-ablated
enriched boron 11B atoms with GeH4 in excess argon: (a) 11B + 0.3%
GeH4 in argon codeposited for 1 h, (b) after annealing to 25 K, (c)
after λ = 405 nm LED photolysis, and (d) difference spectrum
between traces c and b.
RESULTS
■
A series of matrix-isolation experiments have been undertaken
using different GeH4 (GeD4) or SnH4 (SnD4) concentrations
ranging from 0.2% to 1.0% in argon together with laser-ablated
boron atoms. To control the boron atom concentration
different laser energies have been applied (typically 20−30
mJ/pulse).
Experiments using boron in natural abundance (19.8% 10B,
80.2% 11B) have been performed as well. Selected regions of
the corresponding spectra are shown in Figure 3. Several modes
of group m and group h clearly reveal the expected 10/11B
isotopic splitting and the relative intensities between the high-
and low-frequency absorptions for each pair match the natural
abundance of boron. Figure 4 illustrates difference spectra
obtained for a 0.3% GeD4/Ar mixture and boron atoms after λ
= 405 nm LED irradiation and subsequent annealing of the
matrix to 25 K, as well as a comparison of the spectra obtained
from experiments using 10B-enriched and natural abundant
boron atoms.
B + GeH4. IR spectra of codeposited 10B-enriched atoms
using a 0.3% GeH4/Ar mixture are shown in Figure 1. Two sets
In addition absorptions at 878.7 and 748.7 cm−1 assigned to
Ge2H6 appeared as strong absorptions in our experiment.36
Also other weak absorptions are observed in all laser-ablation
experiments using GeH4 due to precursor fragments and
reactive species such as GeH (1813.5 cm−1), GeH2 (1839.2;
913.6 cm−1), GeH3 (2074.1; 852.4 cm−1), and Ge2H4 (787
cm−1), which have been reported previously.36 Besides these
absorptions other bands are observed owing to traces of
impurities, such as H2O, CO2, and CO.
B + SnH4. Figure 5 presents the IR spectra of codeposited
10B enriched boron atoms using a 0.3% SnH4/Ar mixture. One
set of new absorptions at 2521.1, 2446.3, 1283.3, and 1109.4
cm−1 (labeled h in figures) were observed in the experiment,
which appeared immediately after codeposition and slightly
increased (about 20%) on the 455 nm LED irradiation. The
infrared spectra in selected regions from codeposition of laser-
ablated 11B atoms and 0.3% SnH4 in argon are shown in the
same figure. The infrared spectra in selected regions from
codeposition of laser ablated 10/11B and 0.3% SnD4 in argon
matrix are shown in Figure 6 and all observed bands of the
different isotopic substitution are summarized in Table 1.
Figure 1. Infrared spectra obtained from the reaction of laser-ablated
enriched boron 10B atoms with GeH4 in excess argon: (a) 10B + 0.3%
GeH4 in argon codeposited for 1 h, (b) after annealing to 25 K, (c)
after λ = 455 nm LED photolysis, (d) after λ = 405 nm LED
photolysis, and (e) difference spectrum between traces d and c.
of absorptions have been identified based on their growth and/
or decay characteristics. The first set of absorptions at 2611.4,
2520.2, 2112.7, 1145.4, and 873.6 cm−1 (labeled m) appeared
during the sample deposition at 5 K, and increased by about
20% with subsequent annealing up to 25 K. Upon λ = 405 nm
LED photolysis, this set of bands (m) nearly disappeared along
with increasing bands at 2543.4, 2464.8, 1905.8, 1359.3, and
1120.1 cm−1 (labeled h). In experiments using 11B enriched
boron atoms and a mixture of 0.3% GeH4 in argon the group m
bands have been observed at 2597.6, 2516.9, 2112.7, 1138.6,
and 873.6 cm−1 (Figure 2). Again these bands vanished upon λ
= 405 nm LED photolysis and the corresponding group h
DISCUSSION
■
H2GeBH2. As shown in Figure 1 the reaction of laser ablated
10B atoms with germane leads in the formation of five bands at
2611.4, 2520.2, 2112.7, 1145.4, and 873.6 cm−1 (labeled m).
These bands tracked together in the whole experimental
process, which appeared on codeposition, increased by about
B
DOI: 10.1021/acs.inorgchem.7b03109
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Figure 3. Infrared spectra obtained from the reaction of laser-ablated natural boron atoms with GeH4 in excess argon: (a) nB + 0.3% GeH4 in argon
codeposited for 1 h, (b) after annealing to 25 K, (c) after λ = 455 nm LED photolysis, (d) after λ = 405 nm LED photolysis, and (e) difference
spectrum between traces d and c. The 10/11B isotope splitting of bands of the two boron compounds marked by h and m is indicated.
Figure 6. (a) 11B + 0.3% SnD4 in argon codeposited for 1 h, (b) after λ
= 500 nm LED photolysis, (c) 10B + 0.3% SnD4 in argon codeposited
Figure 4. Difference spectrum obtained after λ = 405 nm LED
photolysis of the products of the reaction between laser-ablated boron
atoms and GeH4 in excess argon and subsequent annealing to 25 K:
for 1 h, and (d) after λ = 500 nm LED photolysis.
(a) 10B + 0.3% GeD4 in argon codeposited for 1 h and (b) natural
(Figure 2) these bands appeared at 2597.6, 2516.9, 2112.7,
boron atoms +0.3% GeD4 in argon codeposited for 1 h.
1138.6, and 873.6 cm−1 with similar thermal and photochemical
behavior, resulting in a 10B/11B isotopic ratio of 1.0053, 1.0013,
20% on annealing to 25 K, decreased slightly on λ = 455 nm
LED photolysis, but nearly vanished upon λ = 405 nm LED
photolysis. In experiments with the 11B enriched isotope
1.0000, 1.0060, and 1.0000. In the case of a target with boron in
natural abundance, a series of doublet isotopic patterns at
2611.2/2597.6, 2520.7/2516.9 and 1145.4/1138.6 cm−1 are
Figure 5. Infrared spectra obtained from the reaction of laser-ablated enriched boron atoms with SnH4 in excess argon: (a) 10B + 0.3% SnH4 in argon
codeposited for 1 h, (b) after annealing to 25 K, (c) after λ = 500 nm LED photolysis, (d) after λ = 455 nm LED photolysis, (e) 11B + 0.3% SnH4 in
argon codeposited for 1 h, and (f) after λ = 500 nm LED photolysis.
C
DOI: 10.1021/acs.inorgchem.7b03109
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Table 1. Observed Infrared Band Positions (cm−1) in Solid
Argon for Different Isotopologues of H2GeBH2, Ge(μ-
H)2BH2, and Sn(μ-H)2BH2
of 1.3213 and 1.3642 are typical for symmetric and
antisymmetric stretching vibrations of a terminal BH2
group.19 This assignment is in agreement with the two B-D
stretching modes observed in the 10B + GeD4 experiments at
1986.5 and 1856.4 cm−1, which reveal nearly identical H/D
isotopic frequency ratios. The weak absorptions at 1138.6 cm−1
(11B + GeH4) and 1145.4 cm−1 (10B + GeH4) show up in the
usual terminal B−H bending vibration region. Furthermore, the
isotopic rations of H/D (1.2862) and 10B/11B (1.0060) of these
absorptions are appropriate to their assignment to H−B−H
bending vibration, which is 19.4 cm−1 lower than the terminal
B−H bending mode of the known H2BSiH2 molecule.19 The
2112.7 and 873.6 cm−1 bands show no 10B/11B isotopic shift,
but shifted to 1523.1 and 625.9 cm−1, respectively, using GeD4.
The corresponding H/D isotopic ratios are 1.3871 and 1.3958,
which are appropriate to Ge−H stretching and bending
vibrations, respectively. Notice the 873.6 cm−1 band appeared
as a shoulder of an absorption assigned to a GeH2 bending
mode of the Ge2H6 molecule.36 The 2112.7 cm−1 band is
masked by the Ge−H stretching absorption of GeH4 (strong
broad absorptions at 2093-2143 cm−1), which can be observed
clearly in the difference spectra obtained before and after λ =
405 nm LED irradiation. So this group bands are appropriate
for H2GeBH2 molecule.
H/10B
H11B
D/10B
D/11B
assignment
2611.4
2520.2
2112.7
1145.4
873.6
2597.6
2516.9
2112.7
1138.6
873.6
1986.5
1856.4
1523.1
890.5
1965.9
1844.9
1523.1
876.3
H2GeBH2
H2GeBH2
H2GeBH2
H2GeBH2
625.9
625.9
H2GeBH2
2543.4
2464.8
1905.8
1359.3
1120.1
2521.1
2446.3
1283.3
1109.4
2529
1927.5
1805.5
1396.2
996.3
1910.0
1798.1
1389.8
993.9
Ge(μ-H)2BH2
Ge(μ-H)2BH2
Ge(μ-H)2BH2
Ge(μ-H)2BH2
Ge(μ-H)2BH2
Sn(μ-H)2BH2
Sn(μ-H)2BH2
Sn(μ-H)2BH2
Sn(μ-H)2BH2
2459.8
1902.2
1357.9
1113.8
2508.2
2440.7
1280.3
1102.6
846.0
839.0
1947.6
1793.5
948.6
1930.6
1784.7
945.6
839.1
830.7
produced with relative intensities of approximately 1:4 (Figure
3). This clearly indicates a carrier with only one boron atom
involved. The 2597.6 and 2520.7 (11B + GeH4) absorptions
shifted to 1965.9 and 1844.9 cm−1 in the 11B + GeD4
experiments. The corresponding H/D isotopic frequency ratios
Table 2. Calculated and Observed Vibrational Frequencies (cm−1) and Predicted Intensities (km/mol, in Parentheses) of
H2GeBH2 at the CCSD(T)/def2-TZVPP and B3LYP/def2-TZVPP Levels
H/10B
H/11B
B3LYP
harmonic
CCSD(T)
B3LYP
harmonic
CCSD(T)
anharmonic
2609.4
harmonic
anharmonic
2624.7
obsd
anharmonic
2594.6
harmonic
anharmonic
2609.9
obsd
assignment
2719.1 (70)
2732.9 (69)
2611.4
2702.9 (71)
2716.7 (69)
2597.6 B−H asym
str
2609.7 (84)
2521.5
2161.1
2628.1 (94)
2540.6
2100.7
2520.2
2112.7
2604.2 (82)
2515.6
2161.3
2622.3 (91)
2534.3
2100.6
2516.9 B−H sym str
2129.2 (118)
2171.4 (103)
2129.2 (118)
2171.4 (104)
2112.7 Ge−H asym
str
2114.2 (59)
2159.3
2161.9 (48)
2091.5
2114.2 (59)
2159.5
2161.9 (48)
2091.5
Ge−H sym
str
1160.9 (33)
880.5 (60)
819.7 (11)
731.1 (15)
640.2 (2)
398.6 (0)
393.7 (13)
341.4 (2)
1138.0
874.8
805.8
718.0
619.0
363.2
400.5
249.2
1183.2 (33)
886.6 (55)
844.2 (14)
728.2 (14)
654.8 (4)
419.0 (0)
384 (12)
334.3 (1)
D/10B
1161.2
893.9
832.0
720.6
639.8
409.9
386.6
333.2
1145.4
873.6
1153.3 (32)
880.1 (60)
810.9 (11)
724.4 (14)
619.6 (2)
398.6 (0)
393.7 (13)
340.9 (2)
1130.5
874.5
797.3
712.1
600.4
363.2
400.6
260
1175.0 (31)
886.1 (55)
835.1 (14)
721.5 (13)
634.1 (5)
419.0 (0)
384.0 (12)
333.8 (1)
1153.2
893.7
1138.6 BH2 scissor
873.6 GeH2 scissor
BH2 wag
823.1
714.0
H2GeBH2 def
B−Ge str
H2GeBH2 def
H2GeBH2 def
GeH2 wag
620.3
410.0
386.6
333.0
D/11B
B3LYP
CCSD(T)
B3LYP
CCSD(T)
harmonic
anharmonic
harmonic
anharmonic
obsd
harmonic
anharmonic
harmonic
anharmonic
obsd
assignment
2047.4 (33)
1892 (54)
1984.1
1875.9
1533.7
2056.9 (32)
1907.7 (62)
1548.7 (54)
1994.5
1904.5
1512.5
1986.5
1856.4
1523.1
2025.3 (33)
1883.2 (52)
1518.4 (62)
1963.6
1860.4
1533.8
2034.8 (33)
1898.4 (59)
1548.7 (54)
1973.9
1882.2
1512.6
1965.9 B−D asym str
1844.9 B−D sym str
1518.4 (62)
1523.1 Ge−D asym
str
1501.9 (30)
894.1 (25)
645.0 (6)
1523.6
896.9
636.5
626.3
572.6
560.2
263.5
284.4
209.7
1535.7 (24)
915.4 (27)
665.1 (7)
633.9 (27)
587.8 (0)
565.4 (10)
296.4 (0)
273.8 (6)
245.2 (1)
1499.9
923.0
657.2
631.1
598.2
560.3
291.4
277.6
246.3
1501.9 (30)
879.7 (24)
633.7 (6)
628.5 (29)
566.8 (0)
560.3 (11)
282 (0)
1523.0
880.3
625.6
625.8
561.0
553.8
262.8
283.5
209.1
1535.7 (24)
900.1 (25)
653.3 (7)
633.1 (27)
576.8 (1)
558.7 (10)
296.4 (0)
273.8 (6)
244.8 (1)
1500.4
903.0
645.8
630.8
574.5
553.9
291.5
265.6
246.0
Ge−D sym str
876.3 BD2 scissor
625.9 GeD2 scissor
BD2 wag
890.5
625.9
629.1 (29)
578.3 (0)
D2BGeD2 def
B−Ge str
D2BGeD2 def
D2BGeD2 def
GeD2 wag
567.0 (11)
282.0 (0)
280.8 (7)
280.8 (7)
249.5 (1)
249.9 (1.1493)
D
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Table 3. Calculated and Observed Vibrational Frequencies (cm−1) and Predicted Intensities (km/mol, in Parentheses) of Ge(μ-
H)2BH2 at the B3LYP/def2-TZVPP and CCSD(t)/def2-TZVPP levels
H/10B
H/11B
B3LYP
harmonic
CCSD(T)
B3LYP
harmonic anharmonic
CCSD(T)
anharmonic
harmonic
anharmonic
obsd
harmonic
anharmonic
obsd
assignment
2655 (86)
2560.7 (88)
2007.5 (351)
1877 (1)
2535.8
2461.7
1863.2
1709.4
1154.6
1272.4
1110.2
948.3
2682.3 (79)
2589.5 (96)
2017.4 (352)
1904.5 (0)
1512.1 (418)
1478.3 (28)
1169.7 (98)
1018.2 (0)
934.4 (2)
2567.6
2493.3
1896.2
1747.2
1384.4
1431.6
1139.7
938.1
2543.4
2464.8
1905.8
2639.7 (85)
2555.5 (84)
2005.2 (349)
1870.6 (1)
1444.7 (366)
1414.5 (27)
1139.9 (115)
981.1 (0)
2522.6
2457.0
1860.8
1704.7
1150.5
1266.5
1103.7
948.1
2666.8 (79)
2584.0 (92)
2015.2 (350)
1897.8 (0)
1511.0 (418)
1475.0 (29)
1163.1 (106)
1018.2 (0)
927.0 (2)
2553.9
2488.2
1894.1
1742.3
1381.1
1428.1
1133.5
937.6
2529
B−Ht asym str
B−Ht sym str
B−Hb sym str
B−Hb asym str
Ge−Hb sym str
Ge−Hb asym str
B(Ht)2 bend
2459.8
1902.2
1445.7 (367)
1418 (26)
1146.2 (108)
981.1 (0)
1359.3
1120.1
1357.9
1113.8
Ge(μ-H)2BH2 def
Ge(μ-H)2BH2 def
Ge(μ-H)2BH2 def
Ge(μ-H)2BH2 def
Ge(μ-H)2BH2 def
918.1 (1)
882.7
913.4
910.8 (1)
875.5
906.2
778.7 (2)
663.5
807.8 (2)
758.8
772.3 (2)
656.6
801.1 (2)
751.3
468.6 (48)
197.6 (1)
478.9
493.7 (51)
204.0 (2)
D/10B
474.3
453.7 (44)
197.5 (1)
474.9
478.1 (46)
203.9 (2)
D/11B
467.6
163.5
262.1
170.7
266.5
B3LYP
CCSD(T)
B3LYP
CCSD(T)
harmonic
anharmonic
harmonic
anharmonic
obsd
harmonic
anharmonic
harmonic
anharmonic
obsd
assignment
1995.7 (47)
1855.7 (68)
1437.8 (181)
1382.8 (1)
1034.3 (186)
1029.0 (12)
857.4 (19)
696.5 (0)
1921.1
1801.1
1351.2
1289.2
903.7
983.5
839.3
689.1
687.1
541.0
414.2
105.3
2016.8 (44)
1879.1 (74)
1444.2 (183)
1404.2 (0)
1082.5 (206)
1070.0 (13)
876.0 (16)
720.3 (0)
1946.7
1826.3
1409.2
1315.3
1028.1
1063.2
857.9
1927.5
1805.5
1396.2
1974.6 (46)
1847.2 (63)
1434.6 (180)
1372.3 (1)
1032.4 (187)
1025.1 (12)
849.5 (24)
694.0 (0)
1903.8
1793.8
1382.2
1280.4
889.4
968.7
831.7
683.5
679.3
524.1
401.1
105.3
1995.4 (44)
1869.9 (69)
1441.2 (182)
1393.3 (0)
1080.5 (209)
1066.6 (13)
867.7 (20)
720.3 (0)
1928.1
1818.2
1386.4
1306.1
1025.7
1054.3
849.9
1910.0
1798.1
1389.8
B−Dt asym str
B−Dt sym str
B−Db sym str
B−Db asym str
Ge−Db sym str
Ge−Db asym str
B(Dt)2 bend
996.3
846.0
993.9
839.0
682.1
681.3
Ge(μ-D)2BD2 def
Ge(μ-D)2BD2 def
Ge(μ-D)2BD2 def
Ge(μ-D)2BD2 def
Ge(μ-D)2BD2 def
694.0 (0)
708.7 (1)
705.9
689.2 (0)
701.2 (1)
689.5
592.4 (2)
615.5 (2)
596.0
586.3 (2)
609.0 (2)
590.0
431.9 (50)
141.5 (1)
453.8 (53)
145.5 (1)
446.3
421.6 (46)
141.4 (1)
443.2 (49)
145.4 (1)
436.3
160.9
160.8
Quantum-chemical calculations support the assignment.
Harmonic and anharmonic frequencies calculated at the
CCSD(T)/def2-TZVPP and B3LYP/def2-TZVPP levels of
theory are summarized in Table 2. The 10BH2 antisymmetric-
and symmetric-stretching as well as bending modes are
predicted at 2732.9, 2628.1, and 1183.2 cm−1 at the CCSD(T)
level and therefore overestimated the experimental bands by
4.7%, 4.3% and 3.3%, respectively. The predicted Ge−H
stretching and bending modes show 2.7% and 1.5% over-
estimation. As expected, the anharmonic corrected frequencies
match the observed group m absorptions very well to only 1%
of deviation for all modes. Notice this holds also for the
isotopologues of H2GeBH2 (Table 2). For completeness, also
the B3LYP functional was used leading in very similar
predictions and the agreement (Table 2).
Assignment of closed-shell species H3GeBH2 can be ruled
out based on our theoretical frequency calculations. Similarly
H3SiBH2 was not observed in boron atom reactions with
SiH4.19 As shown in Table S1 the strongest Ge−H stretching
and bending modes for H3Ge10BH2 were predicted at 2119.1
and 812.3 cm−1, respectively, at B3LYP/Def2-TZVPP level. At
the same level the similar Ge−H stretching and bending modes
for H2GeBH2 were predicted at 2129.2 and 880.5 cm−1 (Table
2). Notice for germanium hydrides the B3LYP frequency
calculations gave 0−5% overestimations,36 and a strong
absorption should appear around 770−820 cm−1 if H3GeBH2
is produced; however, there is no any band was observed in this
region.
Ge(μ-H)2BH2. The absorptions at 2543.4, 2464.8, 1905.8,
1359.3, and 1120.1 cm−1 (labeled h) with 10B isotopes
appeared as very weak absorptions after codeposition. The
bands doubled on λ = 455 nm LED irradiation and sharply
increased by λ = 405 nm LED photolysis. The 11B + GeH4
reaction (Figure 2), showed red-shifted bands at 2529.0,
2459.8, 1902.2, 1357.9, and 1113.8 cm−1. If naturally abundant
boron was used, five pairs of bands at 2543.4/2529.0, 2464.8/
2459.8, 1905.8/1902.2, 1359.3/1357.9, 1120.1/1113.8 cm−1
(labeled h) appeared with relative intensities of approximately
1:4. This isotope pattern again confirms the involvement of a
single boron atom in the carrier molecule. The deuterated
counterparts of the 2543.4 and 2464.8 cm−1 bands (10B +
GeH4) appeared at 1927.5 and 1805.5 cm−1 for 10B atoms with
GeD4 and at 1910.0 and 1798.1 cm−1 in 11B + GeD4. These
bands show typical H/D ratios for the terminal antisymmetric
and symmetric BH2 stretching vibrations. The 1120.1 cm−1
band in the 10B + GeH4 reaction with 11B counterpart at 1113.8
shifted to 846.0 cm−1 in the reaction of 10B + GeD4, and to
839.0 cm−1 in the reaction of 11B + GeD4. The band position as
well as the H/D isotopic ratio (1.3240 for 10B) and 10B/11B
isotopic ratio (1.0057) are appropriate to a terminal BH2
bending mode. The absorptions at 1905.8 and 1359.3 cm−1
(10B + GeH4) with their corresponding deuterium counterparts
at 1396.2 and 996.3 cm−1 (10B + GeD4) show large H/D ratios
(1.3650 and 1.3643) but little 10B/11B isotopic shift (ratio =
1.0019 and 1.0010), respectively. The band positions of these
two modes are located much lower than terminal B−H or Ge−
H stretching modes and much higher than terminal B−H or
E
DOI: 10.1021/acs.inorgchem.7b03109
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Inorganic Chemistry
Article
Ge−H bending modes. These modes indicate the presence of
bridged hydrogen atoms and the influence of two heavy atoms
on these vibrational modes. In fact this set of absorptions are
typical for bidentate tetrahydroborate complexes. The sym-
metric and antisymmetric terminal B−Ht (Ht = terminal
hydrogen atom) stretches usually appear as strong sharp,
doublets at 2400−2600 cm−1 and the symmetric and
antisymmetric B−Hb (Hb = bridged hydrogen atom) stretches
coupled with ring deformation usually seen as overlapped bands
near 2000 cm−1. In addition, M−Hb symmetric and
antisymmetric stretches couple with ring deformation at
1300−1500 cm−1. One additional band at 1050−1150 cm−1
(BH2 deformation) is regularly observed in such species.37
Accordingly, this group of absorptions are assigned to the
Ge(μ-H)2BH2 molecule.
The assignment to the Ge(μ-H)2BH2 molecules is confirmed
by the excellent agreement between observed and calculated
frequencies (Table 3) of five fundamental vibrations in three
different spectral regions, that is, terminal B−Ht stretching,
bridged B−Hb and Ge−Hb stretching modes coupled with ring
deformation, and the B(Ht)2 bending mode. As shown in Table
5, the Ge(μ-H)2BH2 molecule is predicted to have a 2B2 ground
state with C2v symmetry. The calculated anharmonic
frequencies at the CCSD(T)/def2-TZVPP level of theory
(Table 3) are in good agreement with the experimental values,
with a maximum deviation of 30 cm−1. The calculated isotope
shifts are also in excellent agreement with the experimental
values, which give additional support for its assignment. As can
be seen in Table 3, the harmonic frequencies at the B3LYP/
def2-TZVPP level also agree with the experimental observa-
tions. However, the bridged B−Hb or Ge−Hb stretching modes
are underestimated (maximum deviation of 200 cm−1) in the
anharmonic frequency calculation, suggesting B3LYP calculated
anharmonic frequencies cannot describe these modes correctly,
although the B3LYP calculated B−Ht stretching frequencies
match the experimental values very well.
Assignment of one hydrogen bridge species HGe(μ-H)BH2
must be ruled out because the calculated vibrational modes
(Table S3) cannot reproduce the observed frequencies. For
example, the calculated strongest B−Ht bending mode is
calculated at 1062.7 cm−1 (B3LYP), but no such a band was
found in our experiments. In addition the HGe(μ-H)BH2 is not
stabilized in matrix due to 8 kcal/mol higher in energy than
both H2GeBH2 and Ge(μ-H)2BH2 (Figure 7).
Sn(μ-H)2BH2. The absorptions at 2521.1, 2446.3, 1283.3,
and 1109.4 cm−1 (labeled h) appeared after codepostion of
laser ablated 10B atoms with SnH4 (Figure 5). This group of
absorptions show red-shifts to 2508.2, 2440.7, 1280.3, and
1102.6 cm−1 in the reaction of 11B + SnH4 in argon matrix
(Figure 6), and split into four pair of absorptions at 2521.1/
2508.2, 2446.3/2440.7, 1355.6/1321.8 and 1109.4/1102.6 cm−1
with relative intensity ratios of about 1:4 using natural
abundance boron as reagent. The corresponding deuterium
counterparts of these four absorptions appeared at 1947.6,
1793.5, 948.6, and 839.1 cm−1 in the reaction of 10B + SnD4,
and at 1930.6, 1784.7, 945.6, and 830.7 cm−1 in the reaction of
11B + SnD4 in solid argon. Analogues to the absorptions
assigned to Ge(μ-H)2BH2, this group of absorptions are typical
for the bidentate tetrahydroborate complex,38 and are
appropriate for the Sn(μ-H)2BH2 molecule. Unfortunately,
the strong B−Hb symmetric stretching vibration calculated at
1885.3 cm−1 (predicted intensity = 526 km/mol) was not
Figure 7. Potential energy surface calculated at the CCSD(T)/def2-
TZVPP level of theory for the reaction of boron atoms and MH4 (E =
Ge, Sn) leading to the final product M(μ-H)2BH2. Transition states
are calculated at the CCSD(t)//B3LYP/def2-TZVPP level.
observed in our experiment, which may be covered by very
strong Sn−H stretching mode centered at 1893.8 cm−1.
We also failed to identify this mode by difference spectra as
the yield of Sn(μ-H)2BH2 is much less than Ge(μ-H)2BH2
obtained from B + GeH4 reaction (Figure 3). Our CCSD(t)
calculations gave anharmonic frequencies (Table 4) of the
terminal B−Ht antisymmetric and symmetric stretching
vibration of Sn(μ-H)2BH2 at 2553.2 and 2478.1 cm−1, the
Sn−Hb symmetric stretching vibration at 1357.8 cm−1 and the
B−Ht bending vibration at 1133.8 cm−1, which match the
experimental value very well (maximum deviation of 55 cm−1)
and support the assignment. But similar to Ge(μ-H)2BH2, the
anharmonic frequencies calculated at B3LYP level cannot
describe the Sn−Hb stretching accurately.
The calculated frequencies for closed-shell species H3SnBH2
are shown in Table S2, but no observed absorptions can match
this molecule. In addition, we optimized HSn(μ-H)BH2 and
H2SnBH2 molecules as shown in Figure 7, which are much
higher in energy than Sn(μ-H)2BH2 and unlikely isolated in
matrix. For comparison the calculated frequencies are listed in
Tables S4 and S5. The calculated much higher B−Ht stretching
vibration and much lower strongest B−Ht bending vibration
compared with the experimental values rule out the formation
of HSn(μ-H)BH2 molecule.
REACTION MECHANISM AND ELECTRONIC
STRUCTURE
■
A series of work about the reaction of laser ablated boron atoms
with EH4 (E = C, Si, Ge, and Sn) were carried out to have an
insight into the periodical trends of the boron bonding with
group 14 hydrides.19,39 In former matrix-isolation investigations
the major products have been identified to be H2BCH2 in
boron atom reaction with CH4,39 and H2BSiH2 and Si(μ-
H)2BH2 with SiH4.19 The reaction of boron atoms with GeH4
in argon matrix was similar to the reaction of boron with SiH4.
The H2GeBH2 molecules were produced in the annealing
process, suggesting that the formation of H2GeBH2 is
thermodynamically driven. Upon 405 nm LED photolysis,
one electron is excited from the B−Ge π orbital to B−Ge π*
orbital (calculated at 407 nm at MCQDPT level by GAMESS-
US program package),40 leading to the cleavage of the B−Ge
F
DOI: 10.1021/acs.inorgchem.7b03109
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. Calculated and Observed Vibrational Frequencies (cm−1) and Predicted Intensities (km/mol, in Parentheses) of Sn(μ-
H)2BH2 at the B3LYP/def2-TZVPP and CCSD(t)/def2-TZVPP Levels
H/10B
H/11B
B3LYP
harmonic
CCSD(t)
B3LYP
harmonic
CCSD(t)
anharmonic
harmonic
anharmonic
obsd
anharmonic
harmonic
anharmonic
obsd
assignment
2634.2 (93)
2543.9 (99)
2016.5 (504)
1928.8 (2)
1391.0 (220)
1318.4 (1)
1138.5 (134)
1034.3 (0)
937.6 (2)
2517.4
2445.2
1903.0
1751.0
1104.1
1159.5
1099.3
1013.3
918.1
2667.3 (86)
2576.6 (117)
2033.8 (526)
1954.6 (4)
1451 (290)
1372.4 (29)
1166.9 (134)
1070.9 (0)
966.9 (3)
2553.2
2478.1
1885.3
1790.5
1338.6
1342.8
1133.8
1052.2
942.5
2521.1
2446.3
2619.3 (92)
2538.7 (94)
2013.8 (503)
1922 (8)
2504.3
2440.7
1899.0
1745.8
1059.4
1156.1
1092.0
1011.5
911.1
2652 (86)
2571 (112)
2031.2 (526)
1947.6 (4)
1450.1 (290)
1367.6 (30)
1160.1 (144)
1070.9 (0)
959.3 (3)
2539.6
2472.6
1875.1
1785.5
1334.9
1339.4
1127.3
1050.6
935.4
2508.2
2440.7
B−Ht asym str
B−Ht sym str
B−Hb sym str
B−Hb asym str
Sn−Hb sym str
Sn−Hb asym str
B(Ht)2 bend
1283.3
1109.4
1390.3 (219)
1313.4 (26)
1131.8 (143)
1034.3 (0)
930.2 (2)
1280.3
1102.6
Sn(μ-H)2BH2 def
Sn(μ-H)2BH2 def
Sn(μ-H)2BH2 def
Sn(μ-H)2BH2 def
Sn(μ-H)2BH2 def
712.4 (5)
502.5
749.6 (5)
652.3
707.7 (4)
462.8
744.4 (4)
635.1
426 (68)
372.3
450.4 (80)
196.3 (0)
D/10B
415.8
411.8 (62)
171.3 (0)
368.8
435.5 (74)
196.2 (0)
411.0
171.4 (0)
209.1
236.2
213.0
239.2
D/11B
B3LYP
CCSD(t)
B3LYP
CCSD(t)
harmonic
anharmonic
harmonic
anharmonic
obsd
harmonic
anharmonic
harmonic
anharmonic
obsd
assignment
1978.9 (49)
1843.9 (77)
1447.3 (250)
1421.6 (4)
992.3 (112)
968.2 (9)
1908.8
1788.9
1397.9
1325.7
844.5
912.6
834.6
726.4
725.9
460.3
360.6
125.7
2004.5 (46)
1870.9 (90)
1458.9 (259)
1441.6 (2)
1035.8 (144)
1004.3 (12)
874.8 (22)
757.6 (0)
1936.7
1816.9
1393.9
1350.7
965.2
988.9
856.5
754.5
741.3
527.6
403.6
148.8
1947.6
1793.5
1958.2 (48.4)
1835.3 (71.8)
1443.6 (250.7)
1410.8 (4.3)
990.8 (112.8)
962.5 (9.9)
845.7 (31.6)
731.6 (0)
1890.7
1780.9
1382.6
1313.4
838.6
906.7
826.8
725.6
709.5
454.7
352.2
125.4
1983.4 (46)
1861.5 (84)
1455.3 (261)
1430.4 (2)
1034.1 (146)
999.0 (12)
866.6 (29)
757.6 (0)
1917.4
1809.4
1378.7
1341.4
961.6
982.6
848.5
753.5
726.9
522.3
393.9
148.7
1930.6
1784.7
B−Dt asym str
B−Dt sym str
B−Db sym str
B−Db asym str
Sn−Db sym str
Sn−Db asym str
B(Dt)2 bend
948.6
839.1
945.6
830.7
853.7 (25)
731.6 (0)
Sn(μ-D)2BD2 def
Sn(μ-D)2BD2 def
Sn(μ-D)2BD2 def
Sn(μ-D)2BD2 def
Sn(μ-D)2BD2 def
711.1 (0)
733.2 (1)
703.6 (0.3)
529 (4.1)
725.5 (1)
533.2 (5)
563.2 (5)
558.4 (4)
389.1 (66)
121.9 (0)
410.7 (78)
139.6 (0)
379.3 (61.5)
121.9 (0)
400.5 (73)
139.5 (0)
Table 5. Structural Parameters and Electronic Structure Analysis of H2GeBH2, Ge(μ-H)2BH2, and Sn(μ-H)2BH2 Obtained at
a
CCSD(T)/def2-TZVPP Level of Theory
a
Structural parameters calculated at the B3LYP/def2-TZVPP level are also given in parentheses (units in ångstrom and deg).
G
DOI: 10.1021/acs.inorgchem.7b03109
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Inorganic Chemistry
Article
bond and the formation of Ge(μ-H)2BH2, which is only 1.1
kcal/mol lower in energy than H2GeBH2. However, only Sn(μ-
H)2BH2 was identified in the reaction of boron atoms with
SnH4 from our experiment. The CCSD(t) calculations indicate
that the Sn(μ-H)2BH2 molecule is 23.6 kcal/mol lower in
energy than H2BSnH2 molecule. The isomerization from
H2SnBH2 to Sn(μ-H)2BH2 take place in a two-step process
by the rearrangement of two hydrogen atoms with correspond-
ing barriers of 15.4 and 9.8 kcal/mol (Figure 7). Both steps are
thermodynamically favored, and the energy needed to
surmount the reaction barrier can be supplied by the internal
energy of hot [H2BSnH2]* molecules produced by the reaction
of B + SnH4 before being quenched by the matrix.
and a ring critical point enclosed by the two hydrogen bridges
have been found (Table 5). The bond critical points between
Ge/Sn and bridged H atoms possess positive value of Laplacian
value 0.099/0.118 and negative value of electron energy density
−0.026/-0.012, which is common for a dative covalent bond.47
In the picture of ELF, two trisynaptic basins occupied by about
two electrons between B, H and E (E = Ge and Sn)
corresponding to two 3c-2e hydrogen bridged bonds were
identified. Consistent with both, the AIM and ELF analysis also
the NBO analysis indicates a dative bond, where the B−H σ
bonds donate electrons into the empty 4p orbitals of the Ge
atom. The computed MP2 energy for this interaction accounts
to E2 = 73.2 kcal/mol in Ge(μ-H)2BH2 and to 47.6 kcal/mol in
Sn(μ-H)2BH2, while the back-donation into the B−H σ*
orbital is almost negligible.48 In conclusion, dual 3c-2e bonds
were formed in both Ge(μ-H)2BH2 and Sn(μ-H)2BH2
complexes through two hydrogen bridged bonds.
It is well-known that for the structure of heavier main group
ethylene analogues (e.g., H2GeGeH2 H2SiSiH2, H2SiGeH2) a
trans-pyramidalized configuration is favored against the planar
ethylene structure due to pseudo-Jahn−Teller mixing of the π
bonding and σ* levels.41−43 This molecular orbital mixing takes
place more readily in the heavier main group element
derivatives. However, for our H2GeBH2 molecule quantum-
chemical calculations at the B3LYP and CCSD(T) levels
CONCLUSION
■
Our investigation clearly shows that ground state boron atoms
insert spontaneously into the Ge−H bonds of germane to form
H2GeBH2 on annealing in solid argon. This molecule further
isomerize under λ = 405 nm irradiation to the slightly more
stable Ge(μ-H)2BH2 isomer. In the reaction of boron with
SnH4 only Sn(μ-H)2BH2 molecule was produced. The
electronic structure analysis for E(μ-H)2BH2 (E = Ge and
Sn) support two 3c−2e hydrogen bridged E−H−B bonds.
H2GeBH2 possesses a planar structure with an electron
deficient B−Ge bond with a partial multiple bond character
(bond order = 1.5). The bond orbital analysis indicates a
polarized (32% B + 68% Ge) singly occupied π orbital.
Furthermore, a substantial spin density on B at the expense of
Ge substantiates this π bonding interaction. Thus, a new class
of electron deficient germylidene complex is reported.
2
predict a planar structure with B1 ground state. This planar
structure is also expected based on the Goddard−Malrieu−
Trinquier model,44 as the singlet−triplet gap of GeH2 (18.5
kcal/mol) is much less than half of the interaction energy (96.8
kcal/mol) of BH2 and GeH2. Similar structure was predicted for
H2BSiH2 that was trapped in low temperature matrix;19
however, the H2SnBH2 molecule was not isolated in our
experiment although the planar structure is predicted from our
theoretical calculations.
A topological analysis of the electron density of H2GeBH2,
Ge(μ-H)2BH2, and Sn(μ-H)2BH2 have been carried out at the
B3LYP level of theory to gain more insight into the nature of
these interactions. The bond critical point in the B−Ge
interaction region of H2GeBH2 possesses negative values of
energy density (−0.074) and the Laplacian (−0.170) at the
bond critical point in the AIM analysis (see Table 5), which is
typical for a covalent bond.35 An ELF analysis34 revealed that
the disynaptic basin V(B,Ge) between the boron and
germanium atoms is occupied by 2.93 e. This indicates the
formation of a three electron multiple bond between boron and
germanium. Further NBO analysis suggests σ-bond formation
by the interaction of a sp2 hybrid orbital of boron with a sp2
hybrid orbital of germanium. The singly occupied orbital of π
character is perpendicular to the molecular plane (Table 5) and
built by the Ge 4pz orbital and an empty 2pz orbital of boron.
The orbital overlap results in a donation of 0.32 e from 4pz-AO
of germanium to the empty 2pz-AO of boron in H2GeBH2,
which stabilizes the molecule even more, due to the unfavored
empty 2p-orbital of boron.45
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03109.
Calculated vibrational frequencies and predicted inten-
sities and atomic coordinates (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: xfwang@tongji.edu.cn.
*E-mail: s.riedel@fu-berlin.de.
ORCID
Beyond this structural motif heavier group 14 element
hydrides show also the tendency to form hydrogen bridged
bonds. Tetrahydroborate complexes E(μ-H)2BH2 were ob-
served in argon matrix for E = Si, Ge and Sn, while no bridged
structure were found for BCH4 in argon matrices, which is not
stable predicted by quantum chemical calculations. The [BH4]−
forms covalent complexes with several transition metals,
lanthanides and actinides, in which the B−Ht stretches usually
appear in the region of 2400−2600 cm−1.38 An ionic character
of these species tends to lower the B−Ht stretching vibrations
Xuefeng Wang: 0000-0001-6588-997X
Sebastian Riedel: 0000-0003-4552-5719
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (No. 21373152) and the International
Exchange Program for Graduate Students, Tongji University
(No. 2015020029). We thank X. Q. Zeng for GeH4 and GeD4
sample preparation.
46
(e.g., the B−Ht stretching of NaBH4 appeared at 2274 cm−1).
From the AIM analysis, four bond critical point linking the
bridged H atoms with B and E (E = Ge and Sn) by bond path
H
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Inorganic Chemistry
Article
(23) Magnusson, E. Substituent effects in second row molecules.
Tetrahedron 1985, 41 (14), 2939−2943.
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