Selective hydrosiloxane synthesis: Via dehydrogenative coupling of silanols with hydrosilanes catalysed by Fe complexes bearing a tetradentate PNNP ligand

Article abstract of DOI:10.1039/c8dt04168g

A well-defined iron complex system was established using PNNP-R (R = Ph and Cy) as a strong σ-donating ligand with a rigid meridional tetradentate structure. Reactive Fe(0) complexes [{Fe(PNNP-R)}₂(μ-N₂)] were synthesized by a reaction of the corresponding iron dihalide with NaBEt₃H and structurally characterized. The reaction proceeded via the iron dihydride intermediate [Fe(H)₂(PNNP-R)], which underwent H₂ reductive elimination, supporting the hemilabile behavior of PNNP-R. [{Fe(PNNP-R)}₂(μ-N₂)] catalyzed the dehydrogenative coupling of silanols with silanes to selectively form various hydrosiloxanes, which are important building blocks for the synthesis of a range of siloxane compounds. This system exhibited higher catalytic efficiency than the previously reported precious-metal-catalyzed systems.

Full text of DOI:10.1039/c8dt04168g

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PAPER
Selective hydrosiloxane synthesis via
dehydrogenative coupling of silanols with
hydrosilanes catalysed by Fe complexes bearing a
tetradentate PNNP ligand†
Cite this: Dalton Trans., 2018, 47,
17004
a,b
Tomohiro Takeshita,a,b Kazuhiko Satoa and Yumiko Nakajima
*
A well-defined iron complex system was established using PNNP-R (R = Ph and Cy) as a strong σ-donat-
ing ligand with a rigid meridional tetradentate structure. Reactive Fe(0) complexes [{Fe(PNNP-R)}2(μ-N2)]
were synthesized by a reaction of the corresponding iron dihalide with NaBEt3H and structurally charac-
terized. The reaction proceeded via the iron dihydride intermediate [Fe(H)2(PNNP-R)], which underwent
H2 reductive elimination, supporting the hemilabile behavior of PNNP-R. [{Fe(PNNP-R)}2(μ-N2)] catalyzed
the dehydrogenative coupling of silanols with silanes to selectively form various hydrosiloxanes, which are
important building blocks for the synthesis of a range of siloxane compounds. This system exhibited
higher catalytic efficiency than the previously reported precious-metal-catalyzed systems.
Received 17th October 2018,
Accepted 9th November 2018
DOI: 10.1039/c8dt04168g
rsc.li/dalton
stable diamagnetic species and are potentially useful as in-
expensive and nontoxic surrogates for precious-metal catalysts.
Introduction
Iron complexes with low toxicity and high terrestrial abun-
In this study, various PNNP-R-supported-Fe complexes
dance have recently drawn increasing attention as a new class including reactive Fe(0) complexes coordinated with a labile
of catalyst precursors in an attempt to develop environmentally ligand were synthesized and structurally characterized. Using
benign and sustainable methods for organic synthesis.1 this complex system as a catalyst precursor, selective hydrosi-
Intensive studies on Fe complexes have been conducted to loxane formation by catalytic dehydrogenative coupling of sila-
date; however, examples of well-defined reactions remain nols and silanes was achieved with high efficiency.
scarce compared with those of 4d or 5d metal complexes. A
main reason for this is the diverse and changeable reactivity of
Fe complexes, which is often attributed to the paramagnetism
Results and discussion
Synthesis and characterization of PNNP-R-supported Fe
of the Fe metal with a high spin state.1d
In this context, we focus on tetradentate PNNP-R ligands,2
2,9-bis((diphenylphosphino)methyl)-1,10-phenanthroline (R =
Ph)2a and 2,9-bis((dicyclohexylphosphino)methyl)-1,10-phe-
nanthroline (R = Cy),2c,d which connect with a metal centre
through a rigid meridional coordination mode with four σ-
donating moieties. The strong electron-donating ability of
PNNP-R allows it to act as a strong-field ligand that effectively
stabilizes Fe complexes with a low-spin state. Thus, we
expected that PNNP-R supported-Fe complexes behave as
complexes
Complexation of an Fe precursor with a PNNP-R ligand was
performed by heating a solution of FeCl2 and PNNP-Ph ligand
in THF under reflux. After 6 h, PNNP-Ph-supported iron
dichloride [FeCl2(PNNP-Ph)] (1a) was obtained as a red solid
(82% yield) (Scheme 1). The reaction of PNNP-Cy and FeCl2
was similarly performed; however this resulted in the for-
mation of a complicated mixture. Thus, the reaction of FeBr2
with PNNP-Cy was performed following the previous report2d
to selectively form [FeBr2(PNNP-Cy)] (1b). Single-crystal X-ray
diffraction studies revealed two different solid-state structures:
a pentacoordinate distorted trigonal–bipyramidal structure of
1a (τ = 0.50)3 and a hexacoordinate octahedral structure of 1b
with the more donating PNNP-Cy ligand, both of which exhibit
paramagnetic broad signals in the 1H NMR spectra.
aInterdisciplinary Research Centre for Catalytic Chemistry (IRC3),
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
E-mail: yumiko-nakajima@aist.go.jp
bFaculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai,
Tsukuba, Ibaraki 305–8571, Japan
†Electronic supplementary information (ESI) available. CCDC 1866672–1866679.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt04168g
Several Fe(II) complexes were reported as being convertible
into catalytically active Fe(0) species by treatment with a suit-
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Scheme 3 Synthesis of 4a.
crystal X-ray analysis under a cold N2 stream, which supports
the N2-bridged dinuclear Fe structure of 2a and 2b (vide infra).
Since 2a and 2b are supported by the electron-donating
PNNP-R ligand, they are expected to possess a highly electron-
Scheme 1 Synthesis of 1a and 1b.
rich Fe(0) center. However, the νN was not detected either
2
by IR or Raman.6 To evaluate the π-back-bonding ability
of the Fe(0)(PNNP-Ph) moiety, the Fe(0) carbonyl complex
[Fe(CO)(PNNP-Ph)] (4a) was synthesized by the ligand exchange
of 2a with CO. Reaction of 2a with CO (1 atm) immediately
proceeded at room temperature to form 4a quantitatively
(Scheme 3). The latter exhibits a strong νCO band at a rather
lower field of 1866 cm−1 than structurally similar Fe(0) com-
plexes bearing a bipyridine or a phosphinomethyl(bipyridine)
ligand.7
Complexes 2a, 2b, and 4a adopt a square-pyramidal geome-
try [τ = 0.099 (2a), 0.143 (2b), and 0.143 (4a)],3 in which the N2
or CO ligands are located at the axial position. The core struc-
ture around the Fe atom is comparable to those of previously
reported Fe(0) analogous complexes (Fig. 1).8 The bond
lengths of N–N bonds in 2a and 2b of 1.135(4) and 1.134(6) Å,
respectively, are in the range of normal N–N triple bond
lengths. The phenanthroline ligands are reported to act as
redox-active ligands and exhibit shortened C11–C12 and
elongated N–C11/C12 bond lengths in their reduced form.9
Indeed, the C11–C12 bonds in 2a, 2b, and 4a (1.390–1.396 Å)
are slightly shorter than those in Fe(II) complexes 1a and 1b
(1.41–1.44 Å), whereas N–C11/C12 bonds are slightly longer in
2a, 2b, and 4a (1.382–1.391 Å) than in 1a and 1b
(1.354–1.367 Å) (Table 1). Thus, the occurrence of the electron
transfer from the Fe(0) atom to the PNNP-R ligand is indicated
in 2a, 2b, and 4a.
able reducing agent.4,5 Motivated by these observations, we
aimed to demonstrate the reduction pathway of Fe(II) com-
plexes in an attempt to isolate active Fe(0) species by utilizing
a rigid PNNP-R ligand. Treatment of 1a with NaBEt3H
(2 equiv.) under an Ar atmosphere resulted in the formation of
a complex mixture. On the other hand, when the reaction was
performed under a N2 atmosphere, Fe(0) dinitrogen complex
2a was obtained as the sole product (Scheme 2). In contrast,
the reaction of 1b with NaBEt3H resulted in the formation of a
dihydrido complex [Fe(H)2(PNNP-Cy)] (3b) in 82% yield.
Complex 3b was confirmed to be gradually converted into 2b,
which was obtained as a mixture containing several un-
identified compounds. Therefore, 2a is likely formed via inter-
mediary dihydrido complex 3b, which undergoes H2 reductive
elimination.
Notably, two hydride ligands in 3b, which locate in trans
positions, underwent reductive elimination reaction.
Therefore, the occurrence of phosphine side-arm dissociation
is strongly indicated in this step. This is also supported by the
reluctant step-wise reduction of 1b via 3b, which has the more
donating PNNP-Cy ligand than the PNNP-Ph ligand.
Complexes 2a and 2b easily decomposed under 1 atm of N2
pressure even in the solid state because of the facile liberation
of the N2 ligand. Thus, it was difficult to isolate 2a and 2b, and
they were prepared in situ and identified by NMR spectroscopic
analysis. Fortunately, when the THF/hexane solution of the
reaction mixture was cooled to −40 °C, single deep-red crystals
of 2a or 2b were obtained and quickly subjected to a single-
Fig. 1 ORTEP diagram of 2a with 50% probability ellipsoids. Hydrogen
Scheme 2 Synthesis of 2a, 2b and 3b.
atoms are omitted for clarity.
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Table 1 Selected bond lengths and angles of 1a, 1b, 2a, 2b, 4a and 5a
loxane (Me3SiO)2(SiHPh) was selectively obtained in the reac-
tion of PhSiH3 with 3 equiv. of Me3SiOH as the sole product
even at a higher catalyst loading, 1a (1 mol%) and NaBEt3H
(2 mol%).
Since complex 1a was partially converted to 2a upon treat-
ment with 1 equiv. of NaBEt3H, the combination of 1a
(0.1 mol%) and NaBEt3H (0.1 mol%) exhibited a lower catalytic
performance.11
Complex
C11–C12
N2–C12
N1–C13
1a
1b
2a
2b
4a
5a
1.438(2)
1.426(9)
1.392(4)
1.390 (av.)
1.390(4)
1.407(5)
1.364(2)
1.354(8)
1.382(3)
1.387 (av.)
1.375(4)
1.387(4)
1.360(2)
1.367(8)
1.387(3)
1.384 (av.)
1.384(4)
1.381(4)
The catalyst loading could be reduced to 0.01 mol% to give
(Me3SiO)2(SiHPh) in a highly selective manner (89% yield)
accompanied by the formation of disiloxane (Me3SiO)(SiH2Ph)
(8% yield) as a minor product (Table 2, entry 2). The reaction
suggests a much higher turnover number (TON) of >18 000
than those of the previous reports, which are catalysed by pre-
cious-metal complexes (TON 1000–4000).7a,12a,b Complex 1b
reacts with NaBEt3H at a slower rate to form active 2b
(vide supra) and thus exhibited less activity, resulting in the
formation of (Me3SiO)2(SiH2Ph) (28%) and (Me3SiO)(SiH2Ph)
(28%) in the presence of 1b (0.01 mol%) and NaBEt3H
(0.02 mol%) (Table 2, entry 3). It was alternatively confirmed
that 1a did not catalyse the reaction in the absence of NaBEt3H
(Table 2, entry 4). The reaction catalysed by NaBEt3H was also
performed, leading to the formation of a mixture of disiloxane
(Me3SiO)(SiH2Ph) (55%) and trisiloxane (Me3SiO)3(SiPh) (30%),
accompanied by the homo-coupling product Me3SiOSiMe3
(4%) (Table 2, entry 5).
With the optimized conditions using 0.1 mol% of the 1a/
2NaBEt3H combination, we next examined the substrate scope
of the dehydrogenative coupling of silanols with primary
silanes (Scheme 4). PhSiH3 reacted with various aliphatic sila-
nols. When using iPr3SiOH and tBuMe2SiOH, the corres-
ponding trisiloxanes (R3SiO)2(SiHPh) were formed in excellent
yields. Reaction of PhSiH3 with bulky (tBuO)3SiOH resulted in
the selective formation of hydrodisiloxane ((tBuO)3SiO)
(SiH2Ph) in 92%. Interestingly, the silanol with a coordinating
thiophenyl substituent, Me2(2-thiophenyl)SiOH was also appli-
cable in the reaction to furnish the corresponding hydrotrisi-
loxane in 92% yield. 4-MeC6H4SiH3 and nHexSiH3 were also
Dehydrogenative coupling of silanols with hydrosilanes
Interestingly, complexes 1a and 1b can be successfully applied
as a catalyst to the dehydrogenative coupling reaction of hydro-
silanes with silanols to form various hydrotrisiloxanes.
Hydrosiloxanes containing a reactive Si–H terminus are impor-
tant building blocks for the precise synthesis of various silox-
ane compounds (silicones),10 and thus their efficient syntheses
have recently received much attention. However, the number
of examples that afford high reaction selectivity is extremely
limited.11 One reason for this is the complicated reactivity of
silanols; i.e. silanols easily undergo dehydrative condensation
to form a homo-coupling product. In addition, the by-product,
H2O further reacts with unreacted hydrosilanes to generate
various silanol molecules, which are also quickly converted to
homo-coupled-siloxanes. In this study, selective formation of
various hydrotrisiloxanes has been achieved with high
efficiency using 1a as a catalyst precursor. In the presence of
1a (0.1 mol%) and NaBEt3H (0.2 mol%), PhSiH3 reacted with
2 equiv. of Me3SiOH to form hydrotrisiloxane (Me3SiO)2(SiHPh)
in an excellent yield (Table 2, entry 1). Notably, neither disilox-
ane (Me3SiO)(SiH2Ph) nor tetrasiloxane (Me3SiO)3(SiPh) was
obtained in the reaction. It was also confirmed that hydrotrisi-
Table 2 Dehydogenative coupling of Me3SiOH with PhSiH3 catalysed
by 1a and 1b a
Entry
1
Cat. (× mol%)
Product yields by NMRb
1a + 2NaBEt3H (0.1 mol%)
(Me3SiO)2(SiHPh) >99
(Me3SiO)(SiH2Ph) —
(Me3SiO)2(SiHPh) 89
(Me3SiO)(SiH2Ph) 8
(Me3SiO)2(SiHPh) 28
(Me3SiO)(SiH2Ph) 28
(Me3SiO)2(SiHPh) —
(Me3SiO)(SiH2Ph) 5
(Me3SiO)2(SiHPh) —
(Me3SiO)(SiH2Ph) 55
(Me3SiO)3SiPh 30
2
3
4
5
1a + 2NaBEt3H (0.01 mol%)
1b + 2NaBEt3H (0.01 mol%)
1a (1.0 mol%)
NaBEt3H (0.1 mol%)
Me3SiOSiMe3 4
a Reaction conditions: PhSiH3 (32.4 mg, 0.30 mmol) and Me3SiOH
(56.8 mg, 0.63 mmol) in THF (5 mL) at room temperature for 12 h.
b Yields are determined by 1H NMR with mesitylene as an internal
standard.
Scheme 4 Reaction of silanols with primary silanes catalyzed by 1a/
NaBEt3H.
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subjected to the reaction, leading to the quantitative formation
of the corresponding hydrotrisiloxane (Me3SiO)2(SiHR) (R =
4-MeC6H4, nHex).
The combination of 1a/2NaBEt3H also catalysed the coup-
ling of Me3SiOH with secondary silanes. In the presence of 1a
(0.1 mol%) and NaBEt3H (0.2 mol%), Me3SiOH reacted with
Ph2SiH2 and PhMeSiH2 to selectively form the corresponding
hydrotrisiloxanes in good yields, (Me3SiO)SiHPh2 (77%) and
(Me3SiO)SiHPhMe (84%), respectively (Scheme 5).
Stoichiometric reactions were conducted to shed light on
the reaction mechanism. Complex 2a slowly reacted with
Me3SiOH at ambient temperature to give a complicated
mixture, which exhibited extremely suppressed catalytic
activity.13 In contrast, complex 2a smoothly underwent ligand
exchange with PhSiH3 to form the Fe(0) silane complex 5a as
the sole product in 76% yield (Scheme 6). In the 1H NMR spec-
trum, the agostic Si–H signal was observed in a high-field
region, δ = –8.06 ppm as a triplet (2JPH = 36.8 Hz) and SiH2
protons are equivalently observed at δ = 4.75 ppm as a singlet
signal. The silyl signal was observed at δ = 12.2 ppm in the 29Si
{1H} NMR spectrum.
X-ray diffraction studies revealed a highly distorted square-
pyramidal structure of 5a (τ = 0.11) (Fig. 2).3 The agostic
proton H1, which was detected by difference Fourier synthesis,
locates at the bridging position between the Si and the Fe
atoms.
The Si1–H1 bond length is slightly elongated (1.711 Å),
which is indicative of the agostic coordination mode of the
Si1–H1 moiety.14 The C11–C12 and N–C11/C12 bond lengths
exhibit similar values to those in 2a, 2b, and 4a, again indicat-
ing the electron transfer from the Fe(0) centre to the PNNP-Ph
ligand in 5a.
It was confirmed that 5a catalysed the dehydrogenative
coupling of Me3SiOH with PhSiH3 to the same level as 1a/
2NaBEt3H.15 Thus, we postulate that 5a is a potential catalytic
intermediate. Although a mechanistic study is still underway
in our laboratory, one possible mechanism including an Fe(0)/
Fig. 2 ORTEP diagram of 5a with 50% probability elipsoids. Hydrogen
atoms except those attached to the Si atom are omitted for clarity.
Fe(II) cycle is postulated following the dehydrogenative coup-
ling reactions of silanes with alcohols;16 i.e. the reaction
initiates with the oxidative addition of PhSiH3 to form a silyl
hydrido intermediate, which successively reacts with silanol,
resulting in the formation of siloxanes, H2 and the active Fe(0)
species. Alternatively, the activated PhSiH3 of 5a is directly
attacked by silanol to form siloxane, in which step, the Si–H
bond cleavage and the Si–O bond formation proceeded in a
concerted fashion. This mechanism is also demonstrated in
the transition-metal-catalysed dehydrogenative coupling of
hydrosilanes with alcohols.17
Conclusions
In summary, the PNNP-R ligands with strong σ-donating
ability successfully enabled us to isolate various Fe complexes
as stable low-spin species. Notably, efficient hydride reduction
of Fe(II) complexes to reactive Fe(0) complexes 2a and 2b was
demonstrated to support the formation of dihydrido com-
plexes 3a and 3b as intermediates. It was also confirmed that
3b underwent H2 reductive elimination reaction, which poss-
ibly initiated with the dissociation of the phosphine side-arm
of the PNNP-Cy ligand. Thus, the hemilabile behaviour of the
PNNP-R ligands was indicated.
Selective formation of hydrotrisiloxanes was achieved
through dehydrogenative coupling of silanols with silanes
using 1a as the catalyst precursor. The reaction proceeded with
high efficiency with good reaction selectivity.
Scheme 5 Reaction of Me3SiOH with secondary silanes catalysed by
1a/NaBEt3H.
Experimental
General considerations
All experiments were performed under a nitrogen atmosphere
using Schlenk techniques or a glove box unless otherwise
noted. n-Hexane, tetrahydrofuran, toluene and dichloro-
methane were purified by a solvent purification system
(MBraun SPS-800 or a Glass Contour Ultimate Solvent System).
C6D6 was dried over sodium benzophenone ketyl and distilled.
Scheme 6 Synthesis of 5a.
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CD2Cl2 was dried over CaH2 and distilled. Neocuproine was Synthesis of [FeCl2(PNNP-Ph)] (1a)
recrystallized by C6H6 from neocuproine hemihydrate. All
A Schlenk tube equipped with a Teflon valve was charged with
a THF solution (10 mL) of PNNP-Ph (0.200 g, 0.34 mmol) and
FeCl2 (0.045 g, 0.35 mmol). The solution was stirred at 60 °C
overnight. After cooling to room temperature, the red solids
were filtered and evaporated. The residue was extracted with
CH2Cl2 and dried in vacuo to give 1a as a red solid (0.201 g,
82.3%). Single crystals of 1a were obtained from a cold CH2Cl2/
Et2O solution. 1H NMR (600 MHz, C6D6): δ 51.36 (br), 24.81
(br), 15.49 (br), 11.07 (br), 7.32 (s), 4.95 (br), 2.58 (br), −8.78
(br).
other reagents were purchased from commercial suppliers and
1
used without further purification unless otherwise noted. H,
13C{1H}, 29Si{1H}, and 31P{1H} NMR spectra (1H, 600 MHz; 13C,
150 MHz; 29Si, 119 MHz; 31P, 243 MHz) were recorded on a
Bruker AVANCE III HD 600 spectrometer. Chemical shifts are
reported in δ (ppm) and referenced to the residual solvent
1
signals for H and 13C, 1,4-bis(trimethylsilyl)benzene for 29Si,
and 85% H3PO4 as an external standard for 31P. The high-
resolution ESI mass spectra were obtained on a Bruker
microTOF II. Column chromatography was performed with
silica gel (Kanto Chemical CO., INC. Silica gel 60 N,
100–210 μm).
Anal. calcd for C38H30Cl2FeN2P2: C 64.89; H 4.30; N 3.98%.
Found: C 64.91; H 4.48; N 4.07%.
Synthesis of [FeBr2(PNNP-Cy)] (1b)
Synthesis of 2,9-bis((diphenylphosphino)methyl)-1,10
phenanthroline (PNNP-Ph)2a
Complex 1b was synthesized by the modified procedure of the
previous report.2d A Schlenk tube equipped with a Teflon valve
was charged with a THF solution (10 mL) of PNNP-Cy (0.500 g,
0.83 mmol) and FeBr2 (0.180 g, 0.84 mmol). The solution was
stirred at room temperature overnight. After removal of the
volatiles, the residue was washed with toluene (10 ml × 3) and
dried. The resulting red solids were extracted with THF and fil-
tered. Volatiles were removed to give 1b as a red solid (0.598 g,
88.1%). Single crystals of 1b were obtained from CH2Cl2/Et2O
solution at room temperature. 1H NMR (600 MHz, C6D6):
δ 64.02 (br), 30.77 (br), 26.80 (br), 9.85 (br), 8.68 (br), 7.32 (s),
4.86 (br), 1.89–0.85, −2.54 (br), −12.84 (br) ppm.
PNNP-Ph was prepared according to the modified procedure of
the reported mehod.2b To a THF solution of neocuproine
(0.500 g, 2.4 mmol) was added a THF solution of LDA (1 M in
THF, 14.4 ml, 9.84 mmol) at −30 °C. PPh2Cl (1.05 g,
4.81 mmol) was added to the solution at 0 °C and the solution
was slowly warmed to room temperature and stirred overnight.
The solution was quenched with H2O (0.5 mL) and stirred for
1 h. After removal of solvent in vacuo, the residue was dissolved
in CH2Cl2 (10 mL) and washed with H2O (5 mL). The organic
layer was corrected, dried over Na2SO4 and filtered. After evap-
oration, the residue was dissolved in CH2Cl2 (10 mL) and sub-
jected to silica gel column chromatography (THF/hexane =
1/2). Crystallization of the obtained orange solid from CH2Cl2/
Et2O (1 : 50 v/v) afforded PNNP-Ph as a white crystalline solid
(1.24 g, 82.4%). 1H NMR (600 MHz, C6D6): δ 7.67 (t, 8H, J =
16.2 Hz, PPh-H), 7.40 (d, 2H, J = 8.16 Hz, Phen-H), 7.14–7.11
(m, 10H, Ar–H), 7.07 (d, 2H, J = 8.16 Hz, Phen-H), 7.04 (t, 4H,
J = 7.85 Hz, PPh-H), 4.00 (s, 4H, PCH2) ppm. 13C{1H} NMR
(150 MHz, C6D6): δ 159.05 (d, 2JP = 8.7 Hz), 146.6, 139.7, 135.7,
133.7 (1JP = 18.7 Hz), 128.8, 128.7, 128.7, 128.2, 125.6, 123.3,
123.2, 40.4 (1JP = 17.9 Hz) ppm. 31P{1H} NMR (243 MHz, C6D6):
δ −10.2 ppm.
Synthesis of [Fe(PNNP-Ph)]2(μ-N2) (2a)
1a (5.0 mg, 7.1 μmol) was suspended in C6D6 (0.4 mL) and
NaBEt3H (1 M in THF, 14.2 μl, 14.2 μmol) was added. The
1
resulting deep red solution was filtered and H and 31P NMR
spectra were obtained to confirm the quantitative formation of
2a as the sole product. Alternatively, preparation of a single
crystal of 2a is as follows. To a THF suspension (5 mL) of 1a
(0.030 g, 43 μmol) was added NaBEt3H (1 M in THF, 86 ml,
86 μmol) under a N2 atmosphere. The solution was stirred at
room temperature for 30 minutes and filtered. After adding
hexane (5 ml), the solution was kept at −40 °C for 2 days to
give 2a as deep red crystals, which easily decompose at room
temperature. 1H NMR (600 MHz, C6D6): δ 7.51 (d, 2H, J =
6.72 Hz, Phen-H), 7.37 (d, 2H, J = 6.28 Hz, Phen-H), 7.23 (s, 2H,
Phen-H), 6.94 (t, 2H, J = 6.78 Hz, PPh-H), 6.61–6.66 (m, 8H,
PPh-H), 6.57 (t, 2H, J = 13.7 Hz, PPh-H), 6.41–6.38 (br, 8H, PPh-
H), 4.27 (d, 2H, J = 15.61 Hz, PCH2), 4.16 (d, 2H, J = 14.58 Hz,
PCH2) ppm. 31P{1H} NMR (243 MHz, C6D6): δ 93.0 ppm. The
13C NMR spectrum of 2a was obscured due to the significant
overlap with NaBEt3H/THF signals.
Synthesis of 2,9-bis((dicyclohexylphosphino)methyl)-1,10
phenanthroline (PNNP-Cy)
PNNP-Cy (1.31 g, 2.18 mmol, 78.0%) was prepared following
the same procedure as that for PNNP-Ph using neocuproine
(0.600 g, 2.8 mmol), LDA (1 M in THF, 11.5 ml, 11.5 mmol)
and PClCy2 (1.34 g, 5.8 mmol). 1H NMR (600 MHz, C6D6):
δ 7.62 (d, 2H, J = 8.22, Phen-H), 7.57 (d, 2H, J = 8.28, Phen-H),
7.26 (s, 2H, Phen-H), 3.44 (s, 4H, PCH2), 1.94–1.92 (m, 8H, Cy-
H), 1.80–1.70 (m, 12H, Cy-H), 1.62–1.61 (m, 4H, Cy-H), 1.46 (qt,
4H, J = 12.30, 7.23, Cy-H), 1.33–1.16 (m, 16H, Cy-H) ppm. 13C
Synthesis of [Fe(PNNP-Cy)]2(μ-N2) (2b)
{1H} NMR (150 MHz, C6D6): δ 161.7 (d, 2JP = 9.9 Hz), 146.5, 2b was prepared following the same procedure as that for 2a.
3
1
135.7, 127.1, 125.5, 123.5 (d, JP = 7.3), 34.2 (d, JP = 16.2 Hz), 1H NMR (600 MHz, C6D6): δ 7.61 (d, 2H, J = 6.48 Hz, Phen-H),
33.8 (d, 1JP = 23.1 Hz), 30.5 (d, 2JP = 14.3 Hz), 29.6 (d, 3JP
=
7.50 (d, 2H, J = 5.30 Hz, Phen-H), 7.16 (s, 2H, Phen-H), 3.68 (d,
9.1 Hz), 27.8 (d, JP = 17.6 Hz), 27.7, 27.0 ppm. 31P{1H} NMR 2H, J = 14.71 Hz, PCH2), 3.41 (d, 2H, J = 15.03 Hz, PCH2),
2
(243 MHz, C6D6): δ 2.6 ppm.
1.90–0.31 (44H, PCy-H) ppm. 31P{1H} NMR (243 MHz, C6D6):
17008 | Dalton Trans., 2018, 47, 17004–17010
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δ 94.1 ppm. The 13C NMR spectrum of 2b was obscured due to −8.06 ppm (t, 1H, J = 36.8 Hz, agostic-H). 13C{1H} NMR
the significant overlap with NaBEt3H/THF signals.
(150 MHz, C6D6): δ 155.9, 140.8, 140.6, 133.1, 132.9, 132.8,
131.3, 131.2, 129.5, 128.3, 126.7, 126.0, 118.8, 114.6, 47.1 ppm.
31P{1H} NMR (243 MHz, C6D6): δ 106.22 ppm.29Si{1H} NMR
Synthesis of [Fe(H)2(PNNP-Cy)] (3b)
To a C6H6 suspension (5 mL) of 1b (0.030 g, 37 μmol) was (119 MHz, C6D6): δ −12.2 ppm. Complex 5a easily decomposes
added NaBEt3H (1 M in THF, 74 μl, 74 μmol) under a N2 atmo- via PhSiH3 dissociation even at −35 °C and elementary analysis
sphere. The solution was stirred at room temperature for is not available.
30 minutes and filtered. Removal of the solvent gave 3b as a
green solid (0.0201 g, 31 μmol, 82.4%). 3b was crystallized
Catalytic dehydrogenation of silanes with silanols
from THF/n-hexane in −40 °C for 1 day to form green block A typical procedure (Table 1, entry 1) is as follows. All reactions
crystals. 1H NMR (600 MHz, C6D6): δ 7.36 (s, 2H, Phen-H), 7.32 were carried out under a nitrogen atmosphere. To a vial were
(d, 2H, J = 7.32 Hz, Phen-H), 7.16 (d, H, J = 7.31 Hz, Phen-H), added
a THF solution (0.5 mL) of PhSiH3 (32.4 mg,
3.63–3.62 (m, 4H, PCH2), 2.26 (d, 4H, J = 7.26 PCH), 1.87–1.64 0.30 mmol) and a 0.1 M stock solution of 1a/2NaBEt3H (30 μl,
(m, 24H, PCy-H), 1.37–1.16 (m, 16H, PCy-H), −8.45 ppm (t, 2H, 0.030 μmol).18 After stirring for 5 minutes, Me3SiOH (56.0 mg,
2JP = 68.9 Hz, FeH2) ppm. 13C{1H} NMR (150 MHz, C6D6): 0.62 mmol) was added, and the mixture was stirred at room
1
δ 160.0, 142.2, 129.5, 125.3, 116.9, 111.6, 38.8 (d, JP = 23.8 Hz), temperature for 12 h. Mesitylene (36.0 mg, 0.30 mmol) as an
1
37.8 (d, JP = 14.0 Hz), 31.6, 28.1, 27.5, 27.3, 22.6 ppm. 31P{1H} internal standard was added to the reaction mixture, and 1H
NMR (243 MHz, C6D6): δ 93.0 ppm. Complex 3b easily decom- NMR spectra were recorded to determine the NMR yield of
posed and its elemental analysis was not available.
(Me3SiO)2(SiHPh) (>99%).
For compound characterization data, please see the ESI.†
Synthesis of [Fe(PNNP-Ph)(CO)] (4a)
In a Schlenk tube, 2a was in situ generated by the reaction of
1a (0.050 g, 0.071 mmol) with 2 equiv. of NaBEt3H (1 M in Conflicts of interest
THF, 86 μl, 86 μmol). After freeze–thaw–pump cycles (×3), CO
(1 atm) was introduced into the reaction vessel. After stirring
There are no conflicts to declare.
at room temperature for 30 minutes, all the volatiles were evap-
orated. The residue was dissolved in C6H6 (5 mL) and filtered
with an alumina pad. After evaporation, 4a was obtained as a
Acknowledgements
deep green solid (0.038 g, 82.2%). Single crystals of 4a were
obtained from cold THF/hexane solution. H NMR (600 MHz,
This work was supported by the “Development of Innovative
1
Catalytic Processes for Organosilicon Functional Materials”
project (PL: K. S.) from the New Energy and Industrial
Technology Development Organization (NEDO).
C6D6): δ 7.60–7.51 (m, 8H, PPh-H), 7.15–7.12 (m, 10H, Phen-H
+ PPh-H), 6.84 (d, 2H, J = 6.84 Hz, Phen-H), 6.78–6.76 (m, 4H,
PPh-H), 6.65 (t, 2H, J = 7.02 Hz, Phen-H), 6.49 (t, J = 7.62 Hz,
PPh-H), 4.32 (d, 2H, J = 14.58 Hz, PCH2), 4.08–4.03 (m, 2H,
PCH2) ppm. 13C{1H} NMR (150 MHz, C6D6): δ 216.3, 156.4,
139.3, 139.0, 132.7, 130.8, 130.5, 129.2, 125.8, 117.5, 112.1,
46.5 ppm. 31P{1H} NMR (243 MHz, C6D6): δ 108.6 ppm. IR
(ATR) νCO 1866 cm−1. Anal. calcd for C39H30FeN2OP2: C 70.92;
H 4.58; N 4.24%. Found: C 70.99; H 4.69; N 4.07%.
Notes and references
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17010 | Dalton Trans., 2018, 47, 17004–17010
This journal is © The Royal Society of Chemistry 2018