Transition metal silyl complexes. 53.1 magnitude of the chelate effect in the oxidative addition of Si-H bonds

Article abstract of DOI:10.1021/om9600598

The chelate complexes (CO)₄W(Ph₂PCH₂CH₂SiHR₂) (R = Me, Ph), containing W,H,Si three-center two-electron bonds, were photochemically prepared from W(CO)₆ and Ph₂PCH₂CH<

Full text of DOI:10.1021/om9600598

Organometallics 1996, 15, 2373-2375
2373
Tr a n sition Meta l Silyl Com p lexes. 53.1 Ma gn itu d e of th e
Ch ela te Effect in th e Oxid a tive Ad d ition of Si-H Bon d s
Ulrich Schubert* and Hilmar Gilges
Institut fu¨r Anorganische Chemie, Technische Universita¨t Wien,
Getreidemarkt 9, A-1060 Wien, Austria
Received J anuary 30, 1996X
The chelate complexes (CO)4W(Ph2PCH2CH2SiHR2) (R ) Me, Ph), containing W,H,Si three-
center two-electron bonds, were photochemically prepared from W(CO)6 and Ph2PCH2CH2-
SiR2H. The phosphine complexes (CO)5W(Ph2PCH2CH2SiR2H) are intermediates in this
reaction. The high NMR coupling constants J SiWH in the chelate complexes (98.1 and 95.2
Hz) indicate that due to the chelating situation an earlier stage of the oxidative addition of
the Si-H bond is frozen than in the corresponding nonchelated complexes. The stabilization
brought about by the chelating (phosphinoethyl)silyl ligand is slightly larger than the
electronic effect caused by substituting another CO ligand by PR3 or by replacing SiR3 (R )
alkyl, aryl) for SiCl3 in nonchelated complexes.
In tr od u ction
effect compared with other factors promoting oxidative
addition reactions.
In complexes of a given type, there is a good correla-
tion of the stage of the oxidative addition reaction with
Oxidative addition of an A-B bond to a transition-
metal center can be promoted by its incorporation into
a chelate system.2 “Chelate assistance” sometimes
allows addition of A-B bonds to metal centers which
otherwise would not undergo this type of reaction. For
example, while the Sn-C bonds of tetraalkyl- or tetra-
arylstannanes cannot be added to iron carbonyl moieties
to give stable complexes, one of the Sn-R bonds (R )
methyl, phenyl) in (CO)4Fe(R′2PCH2CH2SnR3) readily
adds to the iron center.3 The resulting complex
1
the NMR coupling constants, which are between J EH
2
and J EMH
.
1J SiH is typically in the range of 200 Hz,
2
while J SiMH in hydrido silyl complexes without a Si,H
interaction is below 20 Hz. The SiMH coupling con-
stants in the complexes of the type (π-arene)L2M(H)-
SiR3, in which the three-center bonds were confirmed
by structure analyses, are in the range 40-70 Hz.4
We selected complexes of the type (CO)5-n
-
(PR′3)nW(H)SiR3 for the present study, because the
deliberate tuning of the W,H,Si three-center interaction
by the electronic influence of the coligands L and the
substituents at silicon was previously demonstrated by
us for nonchelated complexes of this type (including the
molybdenum and stannyl derivatives).5,6 In the present
study we prepared the corresponding complexes in
which PR′3 and SiR3 are part of an undistorted chelate
ligand. By comparison of the relevant spectroscopic
data we were able to quantify the stabilizing effect of
the chelating situation relative to the previously inves-
tigated electronic factors.
(CO)3(R)Fe(R′2PCH2CH2SnR2) is stabilized by formation
of the five-membered Fe-Sn-C-C-P ring.
Oxidative addition/reductive elimination reactions are
very much influenced by the electronic and steric
properties of the reactants. For example, small and
electron-donating coligands at the metal or electro-
negative substituents at the A-B bond promote the
oxidative addition. We have previously shown that
these factors can be studied in much detail by investi-
gating the E‚‚‚H interaction (E ) Si, Sn) in complexes
containing M,E,H three-center two-electron bonds.4
Complexes of this type can be considered “frozen inter-
mediates” in the oxidative addition (or reductive elimi-
nation) of E-H bonds. We have shown that the
contribution of the various factors influencing oxidative
addition (reductive elimination) can be correlated with
the magnitude of the E‚‚‚H interaction, which can be
determined by monitoring the changes in structural or
spectroscopic parameters.5 The goal of the present work
was to get an estimate on the magnitude of the chelate
Exp er im en ta l Section
All manipulations were carried out under an atmosphere
of dry and oxygen-free argon, using standard Schlenk tube
techniques. All solvents were dried by standard procedures.
Silica was dried in vacuo for 4 h and flushed with Ar.
A
medium-pressure mercury lamp type Heraeus TQ150 (150 W)
was used for UV irradiation.
Infrared spectra were recorded on a Perkin-Elmer IR 1310
spectrometer using CaF2 cuvettes. NMR spectra were re-
corded on a Bruker AC 250 spectrometer. 31P NMR spectra
are referenced against external H3PO4. DSC measurements
were performed on a Shimazu DSC-50 analyzer in an Ar
atmosphere.
X Abstract published in Advance ACS Abstracts, April 1, 1996.
(1) Part 52: Schubert, U.; Mayer, B.; Russ, C. Chem. Ber. 1994, 127,
2189.
(2) For example: Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T.
S.; J ochem, K. J . Chem. Soc., Chem. Commun. 1981, 937. Auburn,
M. J .; Stobart, S. R. Inorg. Chem. 1985, 24, 318. Grundy, S. L.;
Holmes-Smith, R. D.; Stobart, S. R.; Williams, M. A. Inorg. Chem. 1991,
30, 3333.
P r ep a r a tion of (CO)5W(P h 2P CH2CH2SiP h 2H) (1a ).
A
0.41 g (1.15 mmol) amount of W(CO)6 was dissolved in 110
(3) Schubert, U.; Grubert, S.; Schulz, U.; Mock, S. Organometallics
1992, 11, 3165. Grubert, S. Ph.D. Thesis, University Wu¨rzburg, 1994.
(4) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151.
(5) Schubert, U. In Progress in Organosilicon Chemistry; Marciniec,
B., Chojnowski, J ., Eds.; Gordon and Breach: Basel, 1995; p 287.
(6) Piana, H.; Schubert, U. J . Organomet. Chem. 1991, 411, 303.
S0276-7333(96)00059-3 CCC: $12.00 © 1996 American Chemical Society

2374 Organometallics, Vol. 15, No. 9, 1996
Schubert and Gilges
mL of Et2O. The solution was irradiated at -20 °C until the
IR spectrum showed complete conversion to (CO)5W(OEt2)
[bands at 1930 cm-1 (vs) and 1890 cm-1 (s)] after about 1 h.
Then 0.43 g (1.09 mmol) of Ph2PCH2CH2SiPh2H7 was added
in the dark to the deep yellow solution. After being warmed
to room temperature and stirred for an additional 2 h, the
solution was filtered through glass wool/Celite. Then all
volatiles were removed in vacuo. The light brown residue was
dissolved in 10 mL of petroleum ether and the solution filtered
through 10 cm of silica. During this procedure, the product
is adsorbed on silica (brown zone). After washing with 50 mL
of petroleum ether, the product was eluated with 60 mL
toluene/petroleum ether (1:4). After removal of the solvent,
the remaining yellow oil was washed three times with 8 mL
of petroleum ether each at -70 °C. After drying in vacuo 1a
is obtained as a yellow powder: Yield 0.33 g (42%); mp 86 °C;
IR (petroleum ether) ν(SiH) ) 2120 (w, br), ν(CO) ) 2070 (m),
1985 (w), 1940 (vs) cm-1; 1H NMR (250.13 MHz, C6D6) δ 1.19-
1.28 (m, 2 H, SiCH2), 2.48-2.58 (m, 2 H, PCH2), 5.03 (t, 1 H,
3J HCSiH ) 3.10 Hz, 1J SiH ) 196.5 Hz, SiH), 6.89-7.62 (m, 20 H,
Ph); 31P NMR (101.25 MHz, C6D6) δ 16.40 (1J WP ) 239.2 Hz);
13C NMR (62.90 MHz, C6D6) δ 6.76 (s br, SiCH2), 28.47 (d, 1J PC
) 22.2 Hz, PCH2), 127.0-137.5 (Ph), 197.4 (d, cis-2J PWC ) 6.92
Hz, CO), 199.4 (d, trans-2J PWC ) 21.3 Hz, CO). Anal. Calc
for C31H25O5PSiW: C, 51.86; H. 3.49. Found: C, 51.24; H, 3.61.
P r ep a r a tion of (CO)5W(P h 2P CH2CH2SiMe2H) (1b). A
0.89 g (2.53 mmol) amount of W(CO)6 was irradiated as
described for 1a , and then 0.60 g (2.53 mmol) of Ph2PCH2CH2-
SiMe2H7 was added. The mixture was warmed to room
temperature in 1 h and stirred for an additional 2 h, during
which the color faded to pale yellow and the IR band at 1890
cm-1 disappeared. After filtration of the solution through glass
wool, all volatiles were removed in vacuo. The light brown
oil was extracted 3 times with 20 mL of petroleum ether,
leaving a brown residue. The solvent was removed from the
combined solution. The residue was purified by column
chromatography on silica. Some (phosphinoalkyl)silane and
traces of W(CO)6 were first extracted with petroleum ether,
and then the complex was eluated with 1:1 toluene/petroleum
ether. After removal of all volatiles in vacuo, the light yellow
powder was twice washed with 5 mL of pentane at -30 °C
and dried in vacuo: Yield 0.49 g (37%); mp 74 °C; IR (toluene)
254.0 Hz); 13C NMR (62.90 MHz, C6D6) δ 16.3 (d, 2J PCC ) 22.4
1
Hz, SiCH2), 28.7 (d, J PC ) 27.1 Hz, PCH2), 128-137 (Ph),
200.2 (d, cis-2J PWC ) 6.92 Hz, CO), 202.8 (d, trans-2J PWC ) 22.6
Hz, CO), 205.9 (d, cis-2J PWC ) 6.51 Hz, CO); 29Si NMR (49.7
MHz, C6D6) δ 11.4 (s, br). Anal. Calc for C30H25O4PSiW: C,
52.04; H, 3.64. Found: C, 52.61; H, 3.80.
P r ep a r a tion of (CO)4W(P h 2P CH2CH2SiHMe2) (2b). A
1.24 g (2.07 mmol) amount of 1b was irradiated in 120 mL of
toluene at -20 °C, bubbling Ar through the solution. The
reaction was monitored by IR spectroscopy. After 1.5 h of
irradiation, all 1b was consumed, and there were only ν(CO)
bands of 2b in the IR spectrum. Then the brown reaction
mixture was filtered through glasswool/Celite at -20 °C to
separate unsoluble decomposition products. After removal of
all volatiles in vacuo, the remaining brown oil was dissolved
in 25 mL of petroleum ether/toluene (4:1) and filtered through
10 cm of silica. The adsorbed complexes (brown zone) were
washed with 20 mL of petroleum ether to remove organic
byproducts (particularly Ph2PCH2CH2SiR2H). Then a yellow
solution was eluated with 40 mL of toluene. Their concentra-
tion in vacuo yielded a yellow oil, consisting of a mixture of
mostly 2b and some 1b. During the workup procedure 1b is
regenerated from 2b, even at low temperatures. Attempts to
separate both complexes by chromatography were unsuccess-
ful; pure 1b can be eluted, but the zone of 2b always contains
some 1b due to decomposition during chromatography. Frac-
tional crystallization was unsuccessful due to the very similar
solubility of both compounds. IR (toluene): ν(CO) ) 2020 (m)
1985 (w) 1930 (vs) 1885 (s) cm-1
.
1H NMR (250.13 MHz, C6D6):
δ -8.18 (s, br, 1 H, J SiWH ) 95.2 Hz, J WH ) 36.4 Hz, WH), 0.35
(s, br, 6 H, SiCH3), 0.85-0.96 (m, 2 H, SiCH2), 2.52-2.60 (m,
2 H, PCH2), 7.19-7.80 (m, 20 H, Ph). 31P NMR (101.25 MHz,
C6D6): δ 40.0 (1J WP ) 256.7 Hz).
Resu lts a n d Discu ssion
The complexes 2 were prepared by UV-irradiating a
toluene solution of W(CO)6 and Ph2PCH2CH2SiR2H at
-20 °C (eq 1). IR spectroscopic monitoring of the
reaction showed the phosphine complexes 1 to be
intermediates.
ν(SiH) ) 2105 (br), ν(CO) ) 2060 (m), 1980 (w), 1935 (vs) cm-1
;
3
1H NMR (250.13 MHz, C6D6) δ 0.13 (d, 3H, J HSiCH ) 3.80 Hz,
Si-CH3), 0.50-0.65 (m, 2 H, SiCH2), 2.35-2.42 (m, 2 H,
PCH2), 3.97 (sep, 1 H, J HCSiH ) 3.85 Hz, SiH), 7.25-7.69 (m,
3
10 H, Ph); 31P NMR (101.25 MHz, C6D6) δ 16.1 (1J WP ) 239.2
Hz). Anal. Calc for C21H21O5PSiW: C, 42.30; H, 3.55. Found:
C, 42.53; H, 3.77.
P r ep a r a tion of (CO)4W(P h 2P CH2CH2SiHP h 2) (2a ).
A
0.67 g (1.91 mmol) amount of W(CO)6 and 0.76 g (1.91 mmol)
of Ph2PCH2CH2SiPh2H were dissolved in 120 mL of toluene,
and the solution was irradiated at -20 °C. Ar was bubbled
through the solution during irradiation. The reaction was
monitored by IR spectroscopy. After the IR band at 2070 cm-1
typical for the intermediate (CO)5W(Ph2PCH2CH2SiPh2H) has
almost completely disappeared (after about 2 h), the yellow
solution was warmed to room temperature and filtered through
glass wool. After removal of all volatiles in vacuo, the
remaining light brown oil was treated with 10 mL of Et2O and
stirred for about 0.5 h. The brown solution was separated from
the pale yellow precipitate, which was washed twice with 3
mL of ether each. After drying in vacuo, 0.61 g (0.88 mmol)
of 2a (46%) was obtained as a pale yellow powder: Mp 47 °C
(dec); IR (toluene) ν(CO) ) 2035 (m), 1980 (s), 1920 (vs), 1890
The phosphine complexes 1 were independently pre-
pared by thermal reaction of the corresponding (phos-
phinoalkyl)silane with the photochemically generated
solvent complex (CO)5W(OEt2). The complexes 1 can
be converted to 2 by UV irradiation, although this
procedure has no preparative advantage.
The phenyl derivative 2a was obtained analytically
pure, while 2b could not be isolated free of the phos-
phine complex 1b. Attempts to purify 2b always
resulted in partial decomposition regenerating 1b. The
fact that 2b readily decomposes by elimination of the
Si-H bond indicates that the complexes 2 are at the
borderline of stability at room temperature with regard
to reductive elimination.
2
(s) cm-1; 1H NMR (250.13 MHz, C6D6) δ -6.90 (d) (1 H, J PWH
1
) 2.15 Hz, J WH ) 37.0 Hz, J SiWH ) 98.1 Hz, W-H), 1.43-
1.60 (m, 2 H, SiCH2), 2.19-2.28 (m, 2 H, PCH2), 7.03-7.73
(m, 20 H, Ph); 31P NMR (101.25 MHz, C6D6) δ 42.8 (1J WP
)
(7) Holmes-Smith, R. D.; Osei, R. D.; Stobart, S. R. J . Chem. Soc.,
Perkin Trans. 1983, 861.

Transition Metal Silyl Complexes
Organometallics, Vol. 15, No. 9, 1996 2375
Ta ble 1. Com p a r ison of NMR Cou p lin g Con sta n ts
J SiH in Com p lexes of th e Typ e (CO)5-n (L)W(SiHR3)
As a matter of fact, a few complexes were prepared
in the meantime with J SiMH coupling constants larger
than 90 Hz. In all of them the silicon atom involved in
three-center bonding has an additional link to the
metal.9-12 However, the complexes 2 are the first where
the corresponding nonchelated complexes are available
for a direct comparison.
compd
J SiH (Hz)
Uncoordinated
(CO)5W(Ph2PCH2CH2SiPh2H) (1a )
196.5
Chelating
98.1
95.2
(CO)4W(Ph2PCH2CH2SiHPh2) (2a )
(CO)4W(Ph2PCH2CH2SiHMe2) (2b)
Con clu sion s
Nonchelatinga
(CO)4(Ph3P)W(SiHPh3)
(CO)3(dppe)W(SiHTolCl2)
(CO)3(dppe)W(SiHCl3)
n.s.
34.8
19.9
We have previously shown that addition of HSiR3 to
the photochemically generated 16-electron complexes
[(CO)5M] or [(CO)4(R3P)M] does not result in hydrido
silyl complexes stable at ambient conditions, because
the complexes readily decompose by reductive elimina-
tion of the silane. When the parameters favoring
oxidative addition were successively changed, complexes
not undergoing reductive elimination of the silane at
room temperature were only obtained with at least two
electron-donating ligands and two electronegative sub-
stituents at silicon.5,6 The stable complex with the least
electronic stabilization was (CO)3(dppe)W(H)SiTolCl2,
for which the coupling constant J SiWH of 34.8 Hz clearly
showed the presence of a W,H,Si three-center bond. In
the complex (CO)3(dppe)W(H)SiCl3, with a third chloride
substituent (i.e. where oxidative addition was pushed
one step further) the Si-H bond was fully added to the
tungsten center (J SiWH ) 19.9 Hz) (Table 1).
While the complexes 2 are stable at room tempera-
ture, the electronically equivalent nonchelated com-
plexes (CO)4(R′3P)W(H)SiR3 (R, R′ ) alkyl, aryl) instan-
taneously decompose by reductive elimination of the
silane. Since the chelating Ph2PCH2CH2SiR2 ligand
does not distort an octahedral complex significantly,
geometric distortion of the metal fragment, observed in
other types of complexes,13 cannot be the source of the
stabilization in 2. The sterically less demanding situ-
ation (the Ph2PCH2CH2SiR2 ligand requires less space
than the electronic equivalent Ph2PCH3 + CH3SiR2)
probably has only a minor influence on the stabilization.
The major source of stabilization therefore is the chelate
effect. By comparison with nonchelated complexes of
this type, we find that the stabilization brought about
by the chelating (phosphinoethyl)silyl ligand is slightly
larger then the electronic effect caused by substituting
a CO ligand by PR3 or by replacing SiR3 (R ) alkyl, aryl)
for SiCl3 in nonchelated complexes.
a
From ref 5. “n.s.” ) not stable at room temperature due to
reductive elimination. See text for the discussion of the electronic
influence of the substituents at silicon.
The ν(CO) bands in the IR spectrum of 2a are typical
for an octahedral complex of the type cis-(CO)4WLL′.
The three chemically nonequivalent CO ligands cause
three separate doublets in the 13C NMR spectrum with
the corresponding cis- or trans-2J PWC couplings. Due
to coupling with the phosphorus atom, the hydride
resonance at -6.90 ppm is also a doublet with the
typical cis-2J PWH of 2.5 Hz. The singlet resonance in
the 31P{1H} NMR spectrum shows a significant down-
field shift of about 25 ppm, compared to 1. This is a
well-documented phenomena for phosphorus atoms
incorporated into a five-membered ring.8 Combining
these data, we conclude that the structures of the
complexes 2 are almost undistorted octahedral. This
is typical for complexes containing M,H,Si three-center
bonds, where the Si-H bond occupies one coordination
site.
The presence of a W,H,Si three-center bond was
confirmed by the J SiWH coupling constants (Table 1).
While the Si,H coupling constant in 1a (196.5 Hz) is in
1
the normal range for J SiH, the absolute values of J SiWH
for 2a (98.1 Hz) and 2b (95.2 Hz) are typical for three-
center bonds. Their magnitude indicates that the
oxidative addition of the Si-H bond in 2 is arrested in
an earlier stage than in the nonchelated complexes
(CO)5-n(PR′3)nM(H)SiR3 (M ) Mo, W).
A comparison of complexes containing M,H,C three-
center bonds with those containing M,H,Si or M,H,Sn
three-center bonds shows that in most organyl com-
plexes oxidative addition of the C-H bond is in an
earlier stage than that of E-H in the corresponding silyl
or stannyl complexes. We have pointed out earlier that
this may be a coincidence, because in nearly all organyl
complexes the relevant C-H bond is tethered to the
metal center by another interaction (i.e. there is a
chelating situation).4 We have postulated that incor-
porating Si-H bonds into chelate systems allows one
to arrest the oxidative addition of the Si-H bond in an
earlier stage than in corresponding nonchelated com-
plexes.
OM9600598
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