Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition

Article abstract of DOI:10.1002/ange.202012218

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, ⁱPr₂P(C₆H₄)BCy₂ (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH₂}] and [PB{GeH₂}] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH₂}] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe₂}] was also prepared, and shown to release poly(dimethylsilane) [SiMe₂]_n upon heating. Overall, this study introduces a “closed loop” deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Full text of DOI:10.1002/ange.202012218

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Akzeptierter Artikel
Titel: Frustrated Lewis Pair Chelation as a Vehicle for Low
Temperature Semiconductor Element and Polymer Deposition
Autoren: Alvaro Omaña, Rachel Green, Ryo Kobayashi, Yingjie He,
Evan Antoniuk, Michael Ferguson, Yuqiao Zhou, Jonathan
Veinot, Takeaki Iwamoto, Alex Brown, and Eric Rivard
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Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.202012218
Link zur VoR: https://doi.org/10.1002/anie.202012218

10.1002/ange.202012218
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Frustrated Lewis Pair Chelation as a Vehicle for Low Temperature
Semiconductor Element and Polymer Deposition
Alvaro A. Omaña,[a] Rachel K. Green,[a] Ryo Kobayashi,[b] Yingjie He,[a] Evan R. Antoniuk,[a] Michael J.
Ferguson,[a] Yuqiao Zhou,[a] Jonathan G. C. Veinot,[a] Takeaki Iwamoto,[b] Alex Brown,[a] Eric Rivard*,[a]
Abstract: The stabilization of silicon(II) and germanium(II)
H2 (via free EH2) at much lower temperatures, while enabling the
eventual recycling of the PB ligand. This Communication
documents our success with this approach, leading to the low
temperature (≤ 110 °C) deposition of semiconducting Si and Ge
films onto substrates from solution. We also prepare the
dialkylsilicon(II) adduct[7] [PB{SiMe2}] and use this storable
species to generate poly(dimethylsilane) [SiMe2]n, an inorganic
polymer precursor to silicon carbide.[8]
dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand,
i
PB, Pr2P(C6H4)BCy2 (Cy = cyclohexyl) is reported. The resulting
hydride complexes [PB{SiH2}] and [PB{GeH2}] are indefinitely stable
at room temperature, yet can deposit films of silicon and germanium,
respectively, upon mild thermolysis in solution. Hallmarks of this
work include: (1) the ability to recycle the FLP phosphine-borane
ligand (PB) after element deposition, and (2) the single-source
precursor [PB{SiH2}] deposits Si films at a record low temperature
from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2}] was
also prepared, and shown to release poly(dimethylsilane) [SiMe2]n
R
“FLP-
E
trapped”
R
upon heating. Overall, this study introduces
a “closed loop”
deposition strategy for semiconductors that steers materials science
away from the use of harsh reagents or high temperatures.
iPr iPr
E (films) + H2
iPr
2P
P
E
heat
R
R
or
Main group chemistry represents one of the pillars of molecular
synthesis, with the ongoing discovery of new inorganic multiple
bonds, non-metal mediated catalytic transformations, and the
recent fixation of nitrogen by the light element boron receiving
widespread attention.[1] Despite such breakthroughs, it is rare
that modern synthetic chemistry makes a deep impact in the
realm of materials science, a field that often relies upon
decades-to-centuries old methods and harsh reaction conditions
for synthesis. In this Communication, we provide a general,
solution-based route to semiconducting Si or Ge films and a σ-
conjugated polysilane by harnessing labile Frustrated Lewis Pair
(FLP) chelation[2] to access the storable precursors [FLP{ER2}]
(E = Si or Ge; R = H or Me).
R
B
Cy2B
- PB
Cy Cy
PB
E
n
E = Si and Ge
R
Polymers
Ligand
recycling!
Scheme 1. General concept of FLP-assisted semiconductor (E) and polymer
[ER2]n deposition (E = Si, Ge).
i
The requisite FLP for this study Pr2P(C6H4)BCy2 (PB) was
obtained via a modification of a procedure by Bourissou and
coworkers.[4,9] Addition of Cl2Ge•dioxane to PB in toluene
afforded the FLP adduct [PB{GeCl2}] (1) in a 94 % yield,[9] which
was then converted to the target Ge(II) dihydride [PB{GeH2}] (2)
(59 % yield) upon treatment of 1 with two equiv. of Li[HBEt3] in
Et2O (Scheme 2); 2 can be also synthesized in one-pot starting
from PB.[9,10]
Frustrated Lewis Pairs (FLPs) contain unquenched Lewis
basic and acidic sites, leading to the cooperative activation of
typically inert substrates, such as H2, CO or CO2.[2] Furthermore,
FLPs are being explored as next generation main group
catalysts.[3] As outlined in Scheme 1, we sought to use the
i
intramolecular FLP Pr2P(C6H4)BCy2 (PB; Cy = cyclohexyl)[4] to
stabilize silicon(II) and germanium(II) dihydrides.[5] Herein, the
singlet EH2 units (E = Si or Ge) engage in synergistic donor-
acceptor interactions with PB. Knowing that the Ge(II) dihydride
donor-acceptor complex Ph3PCMe2•GeH2•BH3 can afford
luminescent Ge nanoparticles from solution, albeit at a high
temperature of 190 °C under microwave irradiation,[6] we hoped
that the new [PB{EH2}] chelates could release bulk Si or Ge and
As expected, [PB{GeH2}] (2) affords
a
11B{1H} NMR
resonance at 2.8 ppm in C6D6, consistent with a four-coordinate
boron environment. For comparison, the three-coordinate boron
center in free PB resonates at 75.6 ppm in C6D6. The GeH2 unit
1
in 2 yields a H NMR signal at 3.81 ppm and a ν(Ge-H) IR band
at 1990 cm-1.[11] The solid state structures of 1 and 2 have been
determined by single-crystal X-ray crystallography and are
depicted as Figures S5[9] and 1 (left), respectively. The Ge–P
distance in [PB{GeH2}] (2) [2.3447(3) Å] is similar in length as
the coordinative Ge–P length in Marschner’s disilylgermylene
Me3P•[Ge{Si(SiMe3)3}2] [2.3484(6) Å].[12] The Ge–B distance in 2
[2.1490(15) Å] is slightly longer than in the corresponding Ge–B
[a]
A. A. Omaña, R. K. Green, R. Kobayashi, Y. He, E. R. Antoniuk, Dr.
M. J. Ferguson, Dr. Y. Zhou, Prof. Dr. J. G. C. Veinot, Prof. Dr. A.
Brown, Prof. Dr. E. Rivard*
Department of Chemistry
University of Alberta
11227 Saskatchewan Dr., Edmonton, Alberta, T6G 2G2 (Canada)
E-mail: erivard@ualberta.ca
bond
in
Kinjo’s
germylene
adduct
Homepage: http://www.chem.ualberta.ca/~erivard/index.html
R. Kobayashi, Prof. Dr. T. Iwamoto
Department of Chemistry, Graduate School of Science
Tohoku University, Aoba-ku, Sendai 980-8578 (Japan)
Me3P•[GeB(Mes)N(Ad)CH=CHC(SiMe3)2] [2.121(2) Å] (Ad
adamantyl; Mes = 2,4,6-Me3C6H2).[13]
=
[b]
Two routes to the Si(II)-FLP complex [PB{SiCl2}] (3) were
developed, each involving the in situ generation of SiCl2
[14]
Supporting information for this article can be found under: DOI xxxxxx
(Scheme 2). The highest isolated yield of pure 3 (colorless
1
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10.1002/ange.202012218
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crystals) was 35
%
starting from iPr2P(H)(C6H4)BCy2(Cl)
hydride
adduct
IMe4•Si(H){(N(Boryl)CH2)2}
(Boryl
=
[PB{HCl}] and 100 equiv. of both Et3N and HSiCl3. The solid
state structure of 3 was confirmed by X-ray crystallography
(Figure S6).[9] Conversion of 3 into the Si(II) dihydride [PB{SiH2}]
(4) was accomplished in a 86 % yield via the addition of two
equiv. of Li[HBEt3] in Et2O (Scheme 2).
[B(NDippCH2)2]; IMe4 = (MeCNMe)2C:) [2.014(5) Å].[16]
A major goal of this study was to use the newly prepared
FLP-EH2 complexes (2 and 4) as single-source precursors for
the deposition of the bulk semiconductors Ge and Si. To start,
[PB{GeH2}] (2) was subjected to thermal gravimetric analysis
(TGA), which showed an onset of decomposition at 115 °C, with
a residual mass after heating to 550 °C of 15 % (Figure S29);[9]
this value corresponds very well with the expected wt. % Ge of
16 % in 2. For [PB{SiH2}] (4), the onset of decomposition was
significantly lower (67 °C), with 14 wt. % remaining after heating
to 550 °C (Figure S30).[9] The expected weight % of Si after the
H
Cl
H
PiPr2
BCy2
Cl
Cl2Ge•dioxane
Li[HBEt3] (2 eq,)
Et2O
iPr2
iPr2
Ge
Ge
P
P
BCy2
BCy2
toluene
PB
1
2
Ligand Recycling
toluene, 110 °C
Ge + H2
Element Deposition
decomposition of
4 is ca. 7 %, suggesting incomplete
iPr
Dipp
degradation of 4 in the solid state and/or partial oxidation of the
reactive Si(0) surface.
iPr
H
Me
Cy
Cy
N
P
HSiCl3
1) 100 eq. HSiCl3
2) 100 eq. NEt3
+
B
N
Me
Cl
Encouraged by the low decomposition temperatures for the
[PB{EH2}] adducts 2 and 4, and by our computations that show
decomposition of these hydrides into PB, E (solid) and H2 to be
exoergic (ΔG) in toluene by -25.5 kcal/mol (Ge) and -13.7
kcal/mol (Si) (Figure S76),[9,17] we explored the release of Ge
and Si from solution. Heating solutions of 2 and 4 in toluene-D8
to 110 °C (8 hrs) deposited Ge and Si, respectively, with the sole
species detected in solution being dissolved H2 and the starting
ligand, PB.[9] When element deposition was conducted on a
preparative scale, we were able to recover/re-use up to ca. 80 %
of PB, thus forming a complete “closed loop” cycle for element
deposition (Scheme 2). Notably, the solution-phase deposition of
Si from 4 at 110 °C represents, to our knowledge, the lowest
temperature deposition of this ubiquitous semiconductor from a
storable single-source molecular precursor.[18]
[
MeIPrH]Cl
(
MeIPr)
Dipp
toluene/Et2O
PB{HCl}
H
Cl
Cl
“SiCl2”
toluene
H
PiPr2
BCy2
iPr2P
iPr2P
Si
Si
Li[HBEt3] (2 eq.)
Et2O
BCy2
BCy2
PB
3
4
Ligand Recycling
toluene, 110 °C
Si + H2
Element Deposition
Scheme 2. Routes to [PB{GeH2}] (2) and [PB{SiH2}] (4) and thermolysis to
yield recyclable PB and Ge/Si.
To characterize the morphology and elemental composition
of the deposited Si and Ge films, SEM imaging and EDX maps
were obtained (Figures
2
and S7-S25).[9] When element
deposition was repeated onto Si or Ge wafers from solution,
SEM revealed the presence of Si and Ge films with thicknesses
of 110(15) nm and 14(4) µm, respectively (Figures 2a/b, S8 and
S17-20).[9] Si or Ge deposition in the absence of substrates gave
fused nanoparticle clusters (Figures S7, S9-S11, and S13-
S15).[9] Furthermore, EDX showed negligible P atom content in
all cases,[9] thus sample contamination with PB or starting
PB[{EH2}] complex (E = Si or Ge) was minimal. The oxidation
state of the deposited Si and Ge was probed by XPS, which
showed one Si(0) environment for the Si films (Figure 2c), while
both Ge(0) and Ge(II) environments could be found for the Ge
films (Figure S23).[9] Oxidation of deposited Ge is likely due to
brief exposure to air during sample transfer to the XPS stage.[19]
Raman spectroscopy confirmed the presence of amorphous Si
and Ge, with broad Si-Si (485 cm-1) and Ge-Ge (280 cm-1) peaks
present (Figures S26 and S27).[9]
Figure 1. Molecular structures of [PB{GeH2}] (2) (left) and [PB{SiH2}] (4) (right)
with thermal ellipsoids at a 30 % probability level. All carbon-bound hydrogen
atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]:
2: Ge1–P1 2.3447(3), Ge1–B1 2.1490(15), Ge1–H1A 1.49(2), Ge1–H1B
1.58(2); P1–Ge1–B1 92.32(4), H1A–Ge1–H1B 102.99(11). 4: Si–P 2.2787(5),
Si–B 2.0769(15), Si–H1 1.414(19), Si–H2 1.415(19); P–Si–B 94.09(4), H1–Si–
H2 102.9(11).
The NMR spectra of [PB{SiH2}] (4) are consistent with the
assigned structure, including a diagnostic triplet 29Si NMR
resonance at -84.0 ppm [1JSiH = 158 Hz] (Figure S64).[9] The
SiH2 unit appears as a ν(Si-H) IR band at 2107 cm-1.[15] The X-
ray structure of 4 is depicted in Figure 1 (right), which shows a
similar heterocyclic structure as in [PB{GeH2}] (2). The
coordinative Si–P bond in 4 [2.2787(5) Å] is slightly shorter than
Our strategy also enables the formation of FLP-
diorganosilylene complexes, as demonstrated by the synthesis
of [PB{SiMe2}] (5) (Figure 3a) in a 70 % yield from the reaction
between 3 and two equiv. of MeLi. The X-ray structure of 5 is
presented in Figure 3b. Heating 5 in toluene at 110 °C for 15 hrs
yields predominantly free PB (by 31P{1H} NMR) and an orange
mixture. Dissolution of the products in THF and precipitation
into cold (-30 °C) pentane afforded poly(dimethylsilane) [SiMe2]n
as a white solid, which was identified using FT-IR (Figure S31),[9]
1H-29Si HMBC (Figure S71),[9] and gel permeation
in
Baceiredo’s
silicon(II)
hydride
[(Me2Si(NtBu)2)P](C8H10)N(Dipp)]SiH [2.318(15) Å].[1b] While the
Si–B distance in 4 [2.0769(15) Å] is longer than in Cui’s Si(II)-
2
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chromatography (Figure S32).[9] 1H-29Si HMBC analysis is
consistent with a predominantly linear polymer structure,[9,20]
while GPC data shows unimodal distribution with a number
average molecular weight (Mn) of 2.7 kDa (45 repeat units), and
a polydispersity index (PDI) of 2.2.[9] Zybill and coworkers have
previously observed the extrusion of oligomeric [SiMe2]x (mol.
weight = 550±10 Da, by cryoscopy) upon heating their silicon-
iron adduct [(Me2N)3PO•Me2Si•Fe(CO)4] to 120 °C, releasing
toxic OP(NMe2)3 and metal carbonyls in the process.[21] Our
formation of [SiMe2]n, a demonstrated precursor to silicon
carbide,[8] is a promising new direction for FLP chemistry. The
synthetic protocol introduced here should provide future access
to a wide range of poly(diorganotetrelanes) [GeR2]n and [SiR2]n
of tuneable composition (R = alkyl or aryl groups).
This Communication introduces the FLP-stabilized adducts
[PB{GeH2}] (2) and [PB{SiH2}] (4), and the diorganosilylene
complex [PB{SiMe2}] (5). The E(II) dihydride complexes
represent bottleable single-source precursors for the deposition
of bulk Ge and Si upon mild heating in solution. Another
hallmark of this work is the ability to isolate/recycle the FLP
ligand (PB) after element deposition. To our knowledge, the
deposition of Si at 110 °C from the single-source molecular Si(II)
dihydride precursor
4 in solution represents the lowest
temperature for such a process. We also show that [SiMe2]n can
be deposited from the diorganosilylene adduct [PB{SiMe2}] (5).
Future work will involve expanding FLP-assisted deposition to
include other optoelectronically active elements and polymers.
Acknowledgements
E.R., A.B., J.G.C.V., and E.R.A. thank NSERC of Canada for
Discovery, Accelerator Supplement (E.R.) and/or CREATE
(J.G.C.V.) grants. E.R. also thanks the Canada Foundation for
Innovation and the Faculty of Science at the University of
Alberta. The authors thank Compute/Calcul Canada for access
to computational resources. A.A.O. thanks Dr. Emanuel Hupf
and Dr. Matthew M. D. Roy for helpful comments and Taylor
Lynk for Raman data.
Keywords: Frustrated Lewis Pairs • Hydrides • Element
Deposition • Inorganic Bonding • Group 14 Elements
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Figure 2. (a) Secondary-electron SEM image of deposited Si film where the
intersection was carved with a needle on a Ge wafer; (b) EDX map (Si) of the
imaged section in Figure 2a; (c) High-resolution XPS spectrum of Si (black)
with fits to 2p1/2 (red) and 2p3/2 peaks (cyan) shown. Related data for deposited
Ge can be found in the Supporting Information.[9]
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Figure 3. (a) Synthesis of [PB{SiMe2}] (5) and the subsequent extrusion of
[SiMe2]n; (b) Molecular structure of [PB{SiMe2}] (5) with thermal ellipsoids at a
30 % probability level. All hydrogen atoms have been omitted for clarity.
Selected bond lengths [Å] and angles [°]: Si1–P1 2.3055(6), Si1–B1
2.0746(17), Si1–C1 1.890(2), Si1–C2 1.8902(19); P1–Si1–B1 93.63(5), C1–
Si1–C2 104.21(10); (c) Photograph of deposited crude [SiMe2]n in C6D6 after
heating 5 in toluene at 110 °C for 15 hrs.
3
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For full synthetic, crystallographic (CCDC 2011820-2011828) and
computational details, see the ESI. The X-ray structures of
iPr2P(C6H4)BCy2 (PB), [iPr2P(C6H4)Li(OEt2)]2, iPr2P(C6H4)BMes2 (PBMes),
[PB{GeCl2}] (1) and [PB{SiCl2}] (2) are provided in the ESI.
[10] We also explored the coordination chemistry of iPr2P(C6H4)BMes2
(PBMes
(Mes 2,4,6-Me3C6H2), but observed no reaction with
)
=
Cl2Ge•dioxane, presumably due to the greater steric bulk at boron in
this ligand (vs. in PB).
[11] K. C. Thimer, S. M. I. Al-Rafia, M. J. Ferguson, R. McDonald, E. Rivard,
Chem. Commun. 2009, 7119–7121.
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[17] Computations were performed at the M06-2X/cc-pVTZ level with a
toluene polarizable continuum model (PCM).
[18] Oligosilanes also extrude H2 upon strong UV-irradiation at room
temperature to yield amorphous hydride-terminated Si: M. Hass, V.
Christopoulos, J. Radebner, M. Holthausen, T. Lainer, L. Schuh, H.
Fitzek, G. Kothleitner, A. Torvisco, R. Fischer, O. Wunnicke, H. Stueger,
Angew. Chem. Int. Ed. 2017, 56, 14071–14074.
[19] Similar surface oxidation of Ge nanofilms deposited from the precursor
ImMe2•GeH2•BH3 (ImMe2
= (HCNMe2)2C:]) was found. While the
temperature of deposition (100 °C) was similar to that used in the
current study, this approach could not be applied for the deposition of
Si, nor could the donor- and acceptor-ligands involved be recycled (as
in this study): J. M. Sinclair, G. Dai, R. McDonald, M. J. Ferguson, A.
Brown, E. Rivard, Inorg. Chem. 2020, 59, 10996–11008.
[20] a) H. Kaji, F. Horii, Macromolecules 2007, 40, 5420–5423; b) K. Deller,
B. Rieger, RSC Adv. 2015, 5, 87445–87455.
[21] C. Zybill, D. L. Wilkinson, C. Leis, G. Müller, Angew. Chem. Int. Ed.
Engl. 1989, 28, 203–205.
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10.1002/ange.202012218
Angewandte Chemie
COMMUNICATION
Entry for the Table of Contents
Putting Frustration to Work: We
use a phosphinoborane Frustrated
Lewis Pair (FLP) ligand to stabilize
ER2 species (E = Si, Ge; R = H or
Me) for the subsequent deposition of
Alvaro A. Omaña, Rachel K. Green,
Ryo Kobayashi, Yingjie He, Evan R.
Antoniuk, Michael J. Ferguson, Yuqiao
Zhou, Jonathan G. C. Veinot, Takeaki
Iwamoto, Alex Brown, and Eric Rivard*
semiconducting
polymers from solution, with ligand
recycling possible.
elements
and
[Page No. – Page No.]
Frustrated Lewis Pair Chelation as a
Vehicle for Low Temperature
Semiconductor Element and
Polymer Deposition
Institute and/or researcher Twitter usernames: @RivardGroupUofA
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