Synthesis of a bis(silanolate) complex from a bis(silyl) complex by air oxidation: Evidence for the participation of silanolate complexes in the transition-metal-catalyzed formation of disiloxanes from hydridosilanes

Article abstract of DOI:10.1021/om990947d

In a novel type of reaction, the bis(silyl) complex (κ²-P,N)-(PN)Pt[o-(Me₂Si)₂C ₆H₄] (PN = 2-(diphenylphosphino)-N,N-dimethylaniline) was quantitatively converted to the bis(silanolate) analogue (κ²-P,N)-(PN)-Pt[o-(OMe₂Si)₂C ₆H₄]. Reaction of the bis(silanolate) complex with 1,2-bis(dimethylsilyl)benzene under argon leads to the starting bis(silyl) complex along with 1,1,3,3-tetramethyl-4,5-benzo-2-oxa-1,3-disilacyclopent-4-ene. In air the bis(silane) is quantitatively converted to the siloxane by catalytic amounts of the bis(silanolate) complex.

Full text of DOI:10.1021/om990947d

Organometallics 2000, 19, 957-959
957
Syn th esis of a Bis(sila n ola te) Com p lex fr om a Bis(silyl)
Com p lex by Air Oxid a tion : Evid en ce for th e
P a r ticip a tion of Sila n ola te Com p lexes in th e
Tr a n sition -Meta l-Ca ta lyzed F or m a tion of Disiloxa n es
fr om Hyd r id osila n es
J u¨rgen Pfeiffer, Guido Kickelbick, and Ulrich Schubert*
Institut fu¨r Anorganische Chemie der Technischen Universita¨t Wien, Getreidemarkt 9,
A-1060 Wien, Austria
Received December 1, 1999
Summary: In a novel type of reaction, the bis(silyl)
complex (κ2-P,N)-(PN)Pt[o-(Me2Si)2C6H4] (PN ) 2-(diphe-
nylphosphino)-N,N-dimethylaniline) was quantitatively
converted to the bis(silanolate) analogue (κ2-P,N)-(PN)-
Pt[o-(OMe2Si)2C6H4]. Reaction of the bis(silanolate) com-
plex with 1,2-bis(dimethylsilyl)benzene under argon
leads to the starting bis(silyl) complex along with 1,1,3,3-
tetramethyl-4,5-benzo-2-oxa-1,3-disilacyclopent-4-ene. In
air the bis(silane) is quantitatively converted to the
siloxane by catalytic amounts of the bis(silanolate)
complex.
29Si NMR spectroscopic identification of the organosili-
con products, and therefore there is a high chance that
siloxanes were formed instead of the reported di- or
oligosilanes. Almost nothing is known about mecha-
nisms or potential intermediates leading to the oxida-
tion products. We report here on the quantitative con-
version of a metal-silicon bond into a stable M-O-Si
entity when the metal silyl complex was reacted with
air under controlled conditions and on the subsequent
formation of a disiloxane from a bis(silanolate) complex
by treatment with a hydridosilane.
A frequently encountered problem in reactions involv-
ing metal silyl complexes1 is the easy formation of
organic compounds containing Si-O bonds whenever
oxygen sources (moisture, oxygen, Si-OH groups of the
glassware2) are not strictly excluded. Silanoles or silox-
anes are often formed as major byproducts or even as
the exclusive products in transition-metal-catalyzed
reactions of hydridosilanes in the presence of oxygen or
in the thermal decomposition of silyl complexes. For
example, despite the presence of a nitrogen atmosphere
with dry solvents, (HMe2Si)2O was formed among other
organosilicon products upon thermolysis of cis-(HMe2-
Si)2Pt(PEt3)2.3 In experiments directed toward the
metal-catalyzed dehydrogenative coupling of hydridosi-
lanes, disiloxanes were the major products instead of
disilanes when the reaction was conducted in air.4a
Oxygen was shown to be necessary to activate platinum
catalysts for hydrosilylation reactions, but an excess of
oxygen led to the formation of siloxanes.4b
During our studies on the chemistry of bis(silyl)
complexes bearing P,N-chelating ligands,5 we found that
(κ2-P,N)-(PN)Pt[o-(Me2Si)2C6H4] (1;6 PN ) 2-(diphe-
nylphosphino)-N,N-dimethylaniline) gave (κ2-P,N)-(PN)-
Pt{o-[O(Me2)Si]2C6H4} (2) in nearly quantitative yield
when exposed to air for 6 h at 60 °C in benzene (Scheme
1).8 Complex 2 is surprisingly stable: No decomposition
was observed when a solution in toluene-d8 was heated
(5) (a) Pfeiffer, J .; Schubert, U. Organometallics 1999, 18, 3245. (b)
Pfeiffer, J .; Kickelbick, G.; Schubert, U. Organometallics 2000, 19, 62.
7
(6) Synthesis of 1: To a solution of 0.5 mmol (0.265 g) of (PN)PtMe2
in 2 mL benzene was added 1.0 mmol (0.196 g) of 1,2-bis(dimethylsilyl)-
benzene dropwise. After the reaction mixture was stirred for 60 h at
60 °C, the solution was cooled to room temperature and transferred
into 20 mL of petroleum ether (30/50) by syringe. After storage at -30
°C for 24 h to complete precipitation, the products were filtered off,
washed with three portions of 2 mL petroleum ether each, and dried
under reduced pressure: pale yellow powder, yield 84%. Anal. Calcd.
for C30H36NPPtSi2‚C3H6O (Mr 750.93): C, 52.78; H, 5.64; N, 1.87.
Found: C, 52.64; H, 5.66; N, 1.81. 31P{1H} NMR (101.25 MHz, acetone-
d6): δ 51.37 (s with Pt and Si satellites, 1J PtP ) 1481.90 Hz, 2J SiPtP
trans
) 161.13 Hz). 29Si NMR (79.49 MHz, INEPT, acetone-d6): δ -1.38 (d
These arbitrarily selected examples show that the
oxygen incorporation is a major issue in the chemistry
of metal silyl complexes, although the extent of this
problem is not always realized. For example, early
reports on the supposed reductive elimination of disi-
lanes from bis(silyl) complexes often do not include the
1
2
with Pt satellites, J PtSi ) 1510.31 Hz, J PPtSi ) 3.60 Hz, cis-Si), 30.04
1
2
(d with Pt satellites, J PtSi ) 1490.44 Hz, J PPtSi ) 163.99 Hz, trans-
3
Si). 13C{1H} NMR (62.5 MHz, acetone-d6): δ 4.30 (d, J PPtSiC ) 3.61
3
Hz, cis-Si(CH3)2), 4.99 (d, J PPtSiC ) 8.27 Hz, trans-Si(CH3)2), 54.12 (s,
N(CH3)2), 121.67 (d, J PC ) 8.28 Hz, Ar Ct), 127.10 (s, Ar Ct), 127.30 (s,
1
Ar Ct), 127.99 (d, J PC ) 23.9 Hz, Ph Cq), 128.40 (d, J PC ) 10.11 Hz,
Ar Ct), 128.95 (d, J PC ) 9.19 Hz, Ph Ct), 130.20 (s, Ar Ct), 130.81 (s,
Ph Ct), 131.29 (d, J PC ) 24.82 Hz, Ar Cq), 133.06 (d, J PC ) 9.19 Hz, Ar
Ct), 133.53 (d, J PC ) 11.03 Hz, Ar-Ct), 134.45 (d, J PC ) 14.71 Hz, Ph
Ct), 135.30 (s, Ar Ct), 152.80 (s, SiCAr), 157.60 (s, SiCAr), 160.05 (d,
2J PC ) 21.14 Hz, Ar Cq). 1H NMR (250 MHz, acetone-d6): δ -0.04 (s
(1) For recent reviews see: (a) Tilley, T. D. In The Chemistry of
Organo Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New
York, 1989; p 1415. (b) Schubert, U. Angew. Chem., Int. Ed. Engl. 1994,
33, 419. (c) Schubert, U. In Progress in Organosilicon Chemistry;
Marciniec, B., Chojnowski, J ., Eds.; Gordon and Breach: New York,
1995; p 287. (d) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999,
99, 175.
(2) See, for example: van der Boom, M. E.; Ott, J .; Milstein, D.
Organometallics 1998, 17, 4263.
(3) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1992, 11,
3227.
3
with Pt satellites, J PtSiCH ) 32.51 Hz, 6 H, cis-Si(CH3)2), 0.46 (d with
3
4
Pt satellites, J PtSiCH ) 22.54 Hz, J PPtSiCH ) 2.85 Hz, 6 H, trans-Si-
3
(CH3)2), 3.41 (s with Pt satellites, J PtNCH ) 15.36 Hz, 6 H, N(CH3)2),
7.12 (t, J HH ) 7.04 Hz, 1 H, Ar H), 7.17 (t, J HH ) 7.04, 1 H, Ar H), 7.32
(d, J HH ) 6.91, 1 H, Ar H), 7.36 (t, J HH ) 7.55 Hz, 1 H, Ar H), 7.45-
7.58 (m, 8 H, Ar H), 7.64 (t, J HH ) 7.68, 1 H, Ar H), 7.75-7.85 (m, 4
H, Ar H), 7.90 (dd, J ) 4.35, 8.19 Hz, 1 H, Ar H).
(7) (PN)PtMe2 was prepared from (η4-2,5-norbornadiene)Pt(CH3)2
and 2-(diphenylphosphino)-N,N-dimethylaniline as described for the
corresponding 2-(diphenylphosphino)ethyldimethylamine complex in
ref 5a.
(4) (a) Brown-Wensley, K. A. Organometallics 1987, 6, 1590. (b)
Onopchenko, A.; Sabourin, E. T. J . Org. Chem. 1987, 52, 4118.
10.1021/om990947d CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/15/2000

958 Organometallics, Vol. 19, No. 6, 2000
Communications
Sch em e 1. Syn th esis of 2
to 100 °C in air for 24 h. The 31P{1H} NMR spectrum of
2 consists of a singlet with platinum satellites at 8.70
F igu r e 1. Molecular structure of 2. Selected bond lengths
(pm) and angles (deg): Pt-O(23) ) 198.8(2), Pt-O(36) )
201.6(2), Pt-N(20) ) 207.5(3), Pt-P(1) ) 215.9(1); O(23)-
Pt-O(36) ) 89.17(9), O(23)-Pt-N(20) ) 177.49(8), O(36)-
Pt-N(20) ) 88.42(9), O(23)-Pt-P(1) ) 94.91(7), O(36)-
Pt-P(1) ) 174.84(6), N(20)-Pt-P(1) ) 87.53(7).
1
ppm with the coupling constant J PtP ) 4128.42 Hz,
typical for platinum complexes bearing a ligand with a
weak trans influence trans to the phosphorus nucleus.9
1
In contrast, the J PtP value of 1 was much smaller
(1481.9 Hz), which is attributed to the strong trans
influence of the silyl ligand.10 The resonances of the
silicon atoms in the 29Si NMR (INEPT) spectrum at
-1.57 and 2.68 ppm are in the characteristic region for
Me2PhSiO- groups (for example, Me2PhSi-O-SiPhMe2
δ -0.88 ppm).
Crystals of 2 containing 1 equiv of C6D6 suitable for
a X-ray crystal structure analysis were obtained from
a saturated benzene-d6 solution by slow evaporation of
the solvent.11 The solid-state structure of 2 (Figure 1)
is characterized by the typical square-planar coordina-
tion around platinum (sum of angles 360.03°). The angle
P(1)-Pt-O(23) (94.91°) is slightly widened, presumably
due to the steric strain caused by the phenyl substitu-
ents of the Ph2P group. The five-membered chelate ring
formed by the PN ligand is almost planar, whereas the
F igu r e 2. Atom labeling for 2 (NMR spectra).
seven-membered ring containing the bis(silanolate)
ligand adopts a puckered configuration with the oxygen
atoms below and above the plane formed by Si(24),
C(27), C(32), and Si(33). In comparison to the molecular
structure of 1,5b the Pt-N and Pt-P bonds of 2 are
shortened (1: Pt-N ) 236.8(5) pm, Pt-P ) 230.3(2)
pm) by 29.3 (!) and 14.3 pm, respectively, revealing the
strong trans influence of silicon compared to oxygen. The
Pt-O bond lengths of 2 are within the range expected
from other complexes.12
To the best of our knowledge, the synthesis of bis-
(silanolate) complex 2 from the bis(silyl) complex 1 is
the first example of the deliberate formation of a stable
silanolate complex from a metal silyl complex.
Analogues of 2 may be involved in the formation of
siloxane byproducts in metal-complex-catalyzed reac-
tions of hydridosilanes as well as in the air-induced
(8) Synthesis of 2: 69.2 mg (0.1 mmol) of 1 was dissolved in 0.6 mL
of C6D6 in
a
NMR tube. Monitoring by 1H and 31P{1H} NMR
spectroscopy revealed that 1 was completely consumed after heating
the solution to 60 °C for 6 h in air. During this period a finely divided
white powder started to precipitate. After addition of 3 mL of petroleum
ether (30/50) to complete precipitation, the product was filtered off,
washed with three 2 mL portions of petroleum ether, and dried under
reduced pressure: colorless powder, yield 65.1 mg (0.09 mmol, 90%).
31P{1H} NMR (101.25 MHz, acetone-d6): δ 8.70 (s with Pt satellites,
1J PtP ) 4128.42 Hz). 29Si NMR (79.49 MHz, INEPT, acetone-d6):
δ
-1.57, 2.68. 13C{1H} NMR (62.5 MHz, acetone-d6): δ 4.09 (s, 2 C, Si-
4
(CH3)2), 4.48 (s, Si(CH3)2), 54.98 (s, N(CH3)2), 122.67 (d, J PH ) 10.11
Hz, PN C3), 126.37 (s, ArSi Ct), 126.48 (s, ArSi Ct), 129.02 (d, J PC
)
1
4
11.04 Hz, ArP Ct), 130.35 (d, J PC ) 46.88 Hz, ArP C1), 131.43 (d, J PC
) 1.84 Hz, ArP C4), 133.00 (s, 2 × ArSi Ct), 133.52 (d, J PC ) 11.03 Hz,
ArP Ct and PN C6), 133.80 (d, J PC ) 5.52 Hz, PN C4), 152.71 (s, ArSi
4
Cq), 152.75 (s, ArSi Cq), 164.84 (d, J PC ) 15.62 Hz, PN C1). 1H NMR
1
(400 MHz, acetone-d6): δ 0.05 (s, 6 H, Si(CH3)2), 0.42 (s, 6 H, Si(CH3)2),
3.61 (s, 6 H, N(CH3)2), 7.13 (t, J HH ) 7.54 Hz, 1 H, ArSi H), 7.16 (t, J HH
) 7.31, 1 H, ArSi H), 7.39 (d, J HH ) 7.01, 1 H, ArSi H), 7.50-7.65 (m,
8 H, 6 × ArP H, 1 ArSi H, PN H5), 7.74 (t, 3J HH ) 8.18 Hz, 1 H, PN H4),
(11) Crystal data for 2: C30H36NO2PPtSi2‚C6D6 (Mr 806.91), colorless
crystal, size 0.40 × 0.20 × 0.14 mm; triclinic, space group P1h; a )
970.0(5) pm, b ) 1075.2(6) pm, c ) 1689.2(9) pm, R ) 107.736(14)°,
â ) 100.47(2)°, γ ) 95.66(2)°, V ) 1627(2) × 106 pm3, Z ) 2, Dcalcd
)
3
3
3
7.89 (dd, J HH ) 7.10, J PH ) 8.98 Hz, 1 H, PN H6), 7.95 (dd, J HH
)
)
1.559 g/cm3; Siemens SMART diffractometer with a CCD area detector;
293(2) K; Mo KR radiation (λ ) 71.073 pm); data collection range 2.01-
30.51°; number of independent reflections 9219 (Rint ) 0.0183). The
structure was solved by the Patterson method (SHELXS86) and refined
by the full-matrix least-squares method based on F2 (SHELXL97).
Hydrogen atoms were added in ideal positions and refined by a riding
model: R ) 0.026 and Rw ) 0.0623 for reflections with I > 2σ(I);
goodness of fit 1.094. The crystallographic data for the reported
structure has been deposited as Supplementary Publication No. CCDC
135994 at the Cambridge Crystallographic Data Centre. Copies are
available free of charge at CCDC, 12 Union Road, Cambridge CB21EZ,
U.K. (Fax, (+44)1223-336-033; E-mail, deposit@ccdc.cam.ac.uk).
(12) (a) Kim, K. W.; Sohn, Y. S. Inorg. Chem. 1998, 37, 6109. (b)
Kuroda, R.; Neidle, S.; Ismail, I. M.; Sadler, P. J . Inorg. Chem. 1983,
22, 3620. (c) Goto, M.; Hirose, J .; Noji, M.; Lee, K. I.; Saito, R.; Kidani,
Y. Chem. Pharm. Bull. 1992, 40, 1022.
3
3
7.01 Hz, J PH ) 12.32 Hz, 4 H, ArP H2 and ArP H6), 8.08 (dd, J HH
8.18, J PH ) 4.27 Hz, 1 H, PN H3). Assignments according to Figure 2
have been made on the basis of 1H{31P}, gs-HH-HMQC, gs-CH-HMQC,
and gs-SiH-HMBC experiments. The resonances of the carbon atoms
PN C1 and PN C5 could not be unambigiously assigned in the 13C NMR
spectrum due to the low solubility of 2 and overlap of the signals.
(9) The data known from other platinum complexes bearing silano-
late ligands are consistent with the found coupling constant. (a) (dppe)-
3
1
Pt(OSiMe3)2, J PtP ) 3595 Hz: Andrews, M. A.; Gould, G. L. Organo-
metallics 1991, 10, 387. (b) (Me3P)2Pt(OSiMe3)2, J PtP ) 3400 Hz:
1
Schmidbauer, H.; Adlkofer, J . Chem. Ber. 1974, 107, 3680.
(10) Pregosin, P. S.; Kunz, R. W. In 31P and 13C NMR of Transition
Metal Phosphine Complexes; Diehl, P., Fluck, E., Kosfeld, R., Eds.;
NMR Basic Principles and Progress 16; Springer: Berlin and Heidel-
berg, 1979.

Communications
Sch em e 2. Rea ction of 2 w ith
Organometallics, Vol. 19, No. 6, 2000 959
Sch em e 3. P r op osed Mech a n ism for th e
P la tin u m -Ca ta lyzed F or m a tion of Siloxa n es fr om
Hyd r id osila n es
1,2-Bis(d im eth ylsilyl)ben zen e
basis of these results, a catalytic cycle for the formation
of disiloxanes from hydridosilanes can be proposed
(Scheme 3): Reaction of a bis(silyl) complex (I; formed
from a hydridosilane and a suitable metal precursor)
with oxygen leads to the formation of a bis(silanolate)
complex (II). A ligand exchange reaction with a hydri-
dosilane results in the formation of a siloxane and
regenerates the bis(silyl) complex.
decomposition of transition-metal bis(silyl) com-
plexes.9b,13,14 To prove these assumptions, bis(silanolate)
complex 2 was reacted with 2 equiv of 1,2-bis(dimeth-
ylsilyl)benzene in the absence of air. Heating to 60 °C
leads to complete consumption of 2 within 20 h.15 The
only products (according to the 1H, 31P, 29Si, and 1H-
29Si-HMBC NMR spectra) were the bis(silyl) complex 1
and the cyclic siloxane 1,1,3,3-tetramethyl-4,5-benzo-
2-oxa-1,3-disilacyclopent-4-ene (3) in a 1:2 ratio (Scheme
2). The reaction is indeed catalytic: heating of a benzene
solution of 1,2-bis(dimethylsilyl)benzene and bis(silano-
late) complex 2 (5 mol %) in air to 60 °C leads to
complete consumption of the hydridosilane and forma-
tion of 3 within 16 h. 31P NMR spectra showed the
presence of both 1 and 2 during the reaction.16 On the
Ack n ow led gm en t. Financial support by the Fonds
zur Fo¨rderung der wissenschaftlichen Forschung (FWF)
of Austria is gratefully acknowledged. We also thank
Wacker-Chemie GmbH for gifts of chemicals.
Su p p or tin g In for m a tion Ava ila ble: Figures giving NMR
spectra of 1 and 2 and tables giving X-ray crystallographic data
for 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM990947D
(13) The elimination of Et3SiOSiMe3 from (COD)Ru(H)(SiEt3)-
(OSiMe3) has been reported: Marciniec, B.; Krzyzanowski, P. J .
Organomet. Chem. 1995, 493, 261.
(14) (a) Hornbaker, E. D.; Conrad, F. J . Org. Chem. 1959, 24, 1858.
(b) Schindler, F.; Schmidbaur, H. Angew. Chem., Int. Ed. Engl. 1967,
6, 683 and references therein.
(15) A 0.05 mmol amount (36.2 mg) of 2 and 0.1 mmol (19.4 mg,
21.4 µL) of 1,2-bis(dimethylsilyl)benzene were dissolved in 0.6 mL of
benzene-d6 under argon and heated to 60 °C for 20 h.
(16) A 0.05 mmol amount (36.2 mg) of 2 and 1 mmol (194 mg, 0.21
mL) of 1,2-bis(dimethylsilyl)benzene were dissolved in 0.5 mL of
toluene-d8 and heated to 60 °C for 16 h to give 1,1,3,3-tetramethyl-
4,5-benzo-2-oxa-1,3-disilacyclopent-4-ene as the only silicon-containing
product. Spectroscopic data: 1H NMR (toluene-d8, 300 MHz) δ 0.35
(s, 12 H, 2 × Si(CH3)2), 7.23 (dd, J HH ) 3.14, 5.35 Hz, 2 H, Ar H), 7.44
(dd, J HH ) 3.14, 5.35 Hz, 2 H, Ar H); 29Si INEPT (toluene-d8, 59.6 MHz)
δ 14.40.