Activation of water and of dioxygen by a bis(diphenylphosphinopropyl)silyl (biPSi) complex of ruthenium(II): Formation of bis(diphenylphosphinopropyl)siloxo cage complexes. Concomitant oxygen atom insertion into a silicon-carbon bond

Article abstract of DOI:10.1021/om010463t

The coordinatively saturated hydrido complex RuH(biPSi)(CO)₂ (1), biPSi = -Si(Me)(CH₂CH₂CH₂PPh₂)₂, reacts slowly with either water/piperidine or dioxygen to afford the novel anchored siloxo complexes RuH[OSiMe(CH₂CH₂CH₂ PPh₂)₂](CO)L (3, L = CO) and RuH[OSi(OMe)(CH₂CH₂CH₂ PPh₂)₂](CO)L (6, L = CO), respectively, which undergo carbonyl displacement to yield analogues in which L = P(OMe)₃ that are crystalline (5 or 7, respectively) and for which anti stereochemistry (i.e., H at Ru vs Me or OMe at Si) has been identified by using X-ray crystallography (i.e., products 5-a, 7-a respectively). Complex 1 catalyzes autoxidation of cyclohexene (mainly to cyclohexenone, cyclohexenol: TTO ≈ 300, 4 h).

Full text of DOI:10.1021/om010463t

4766
Organometallics 2001, 20, 4766-4768
Activa tion of Wa ter a n d of Dioxygen by a
Bis(d ip h en ylp h osp h in op r op yl)silyl (biP Si) Com p lex of
Ru th en iu m (II): F or m a tion of
Bis(d ip h en ylp h osp h in op r op yl)siloxo Ca ge Com p lexes.
Con com ita n t Oxygen Atom In ser tion in to a
Silicon -Ca r bon Bon d
Stephen R. Stobart,*,† Xiaobing (J oe) Zhou,†,‡ Raymundo Cea-Olivares,§ and
Alfredo Toscano§
Department of Chemistry, University of Victoria, British Columbia, Canada V8W 2Y2, and
Instituto de Quimica, Universidad Nacional Autonoma de Mexico, 04510 Mexico, D.F.
Received J une 4, 2001
Summary: The coordinatively saturated hydrido complex
RuH(biPSi)(CO)2 (1), biPSi ) -Si(Me)(CH2CH2CH2-
PPh2)2, reacts slowly with either water/ piperidine or
dioxygen to afford the novel “anchored” siloxo complexes
ized.4 The mechanism of homogeneous dioxygen activa-
tion, particularly by biomimetic systems, has been delib-
erated5 in great detail, but use of the gaseous element
directly for selective oxidation of unsaturated organic
compounds continues to be regarded6 as an elusive goal.
RuH[OSiMe(CH2CH2CH2PPh2)2](CO)L (3, L ) CO) and
We have recently found that both water and dioxygen
react slowly along separate and unexpected manifolds
RuH[OSi(OMe)(CH2CH2CH2PPh2)2](CO)L (6, L ) CO),
respectively, which undergo carbonyl displacement to
yield analogues in which L ) P(OMe)3 that are crystal-
line (5 or 7, respectively) and for which anti stereochem-
istry (i.e., H at Ru vs Me or OMe at Si) has been
identified by using X-ray crystallography (i.e., products
5-a , 7-a respectively). Complex 1 catalyzes autoxidation
of cyclohexene (mainly to cyclohexenone, cyclohexenol:
TTO ≈ 300, 4 h).
with
a
coordinatively saturated hydrido-complex,
RuH(biPSi)(CO)2 (1), a ruthenium(II) silyl in which7 the
Ru-Si bond is stabilized by a tridentate framework
derived from the silane8 SiH(Me)(CH2CH2CH2PPh2)2
(i.e.,7 biPSiH). The products so obtained share a novel
ligand connectivity, in which the Ru-biPSi unit is
elaborated into a chelate-supported siloxo cage, nomi-
nally by insertion of an oxygen atom into the Ru-Si
bond. Most remarkably, the uptake of dioxygen leads
to insertion of a second oxygen atom, between silicon
and its methyl substituent, thereby generating a meth-
oxysilyl analogue by unprecedented partial oxidation of
an inert Si-C bond. We have also observed that in the
presence of complex 1 liquid cyclohexene reacts briskly
with dioxygen gas (alone, and under ambient condi-
tions), forming autoxidation products in catalytic con-
version based on Ru.
Chemical cycles in which either water or oxygen is a
primary substrate carry special strategic implications.1
The industrial requirement for each one as a reagent
in olefin functionalization has led to a protracted search
for catalyst improvements that may allow the relevant
technologies to be optimized. Nevertheless, efficient
management of alkene hydration2 or of related oxidation
steps that use elemental oxygen directly3 is not yet prac-
tical under conditions that are fully environmentally
acceptable, and indeed the mechanistic fundamentals
of either type of chemistry are still imprecisely under-
stood. Oxidative addition of water at a low-valent tran-
sition-metal center has repeatedly been suggested as a
propagative event in the first of the two contexts, but
its immediate products have not often been character-
Under 13CO gas (1 atm; 25 °C), the hydrido-dicar-
bonyl7,9 RuH(biPSi)(CO)2 (diastereomeric mixture:9 1-s,
(4) Burn, M. J .; Fickes, M. G.; Hartwig, J . F.; Hollander, F. J .;
Bergman, R. G. J . Am. Chem. Soc. 1993, 115, 5875. Yoon, M.; Tyler,
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A. Organometallics 1998, 17, 3423. Morales-Morales, D.; Lee, D. W.;
Wang, Z.; J ensen, C. M. Organometallics 2001, 20, 1144.
† University of Victoria.
‡ Present address: Dow Corning Corporation, Midland, Michigan.
§ Universidad Nacional Autonoma de Mexico.
(1) Ort, D.; Yocum, C. Eds. Oxygenic Photosynthesis: The Light
Reactions; Kluwer Academic Publishers: Dordrecht, 1996. Ruttinger,
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Crabtree R. H. J . Am. Chem. Soc. 1999, 121, 117. Ruettinger, W.; Yagi,
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10.1021/om010463t CCC: $20.00 © 2001 American Chemical Society
Publication on Web 10/17/2001

Communications
Organometallics, Vol. 20, No. 23, 2001 4767
1-a ) incorporates label only extremely slowly,9 and
accordingly ligand replacement at the d6 Ru(II) center
requires forcing conditions. Thus substitution by P(OMe)3
is effected only after 2 h in refluxing toluene; as
expected, it occurs10 trans to the strongly labilizing silyl
group, to afford a single diastereomer that is11 syn (2-
s). Although it is unaffected either by water (100 °C; 17
h) or when heated in carefully dried piperidine, com-
pound 1 also reacts with wet piperidine (100 °C; 17 h).
The product (3) is again detected as a single isomer,12
the 29Si{1H} NMR signal for which is shifted to high
frequency vs that of 1 and is observed as a singlet (i.e.,
unlike those8,9 at δ -2.5, -1.1 for 1-s, 1-a , which are
triplets, J Si-P 19.7, 11.2 Hz, respectively); it decomposes
on attempted isolation, but undergoes facile further
reaction with CO (22 °C; 10 min) or P(OMe)3 (78 °C; 20
min), affording analogues 4 and 5, respectively.13 Isola-
tion of the phosphite complex 5 as suitable crystals14
allowed it to be characterized by X-ray crystallography.
The molecular geometry, which is shown in Figure 1,
immediately explains why the 29Si NMR data13 for the
family 3-5 are different from those for 1-s, 1-a , and
2-s: an oxygen atom has entered between Ru and Si of
the bound biPSi unit, transforming the latter into a
novel tridentate cage that supports a siloxo-ruthenium
interaction. The Ru-O and Si-O bond lengths (2.115,
1.605 Å) are both longer and the Ru-O-Si angle
(121.7°) is much less obtuse than those15 in the five-
coordinate complex RuH(OSiPh3)(CO)(PtBu2Me)2, while
the methyl group on silicon and the hydride occupy
opposite faces of the molecule, i.e., the latter is anti,7
5-a .
F igu r e 1. Molecular geometry of complex 5. Selected bond
lengths (Å) and angles (deg): Ru-H(1), 1.62(9); Ru-P(1),
2.366(2); Ru-P(2), 2.353(2); Ru-P(3), 2.348(2); Ru-O(1),
2.115(3); Ru-C(2), 1.823(5); Si-O(1), 1.605(4); Si-C(1),
1.865(7); H(1)-Ru-P(1), 169 (2); H(1)-Ru-P(2), 87 (2);
H(1)-Ru-P(3), 77 (2); P(1)-Ru-P(2), 100.6(1); P(1)-Ru-
P(3), 95.2(1); P(2)-Ru-P(3), 163.3(1); H(1)-Ru-O(1), 91(2);
P(1)-Ru-O(1), 81.0(1); P(2)-Ru-O(1), 88.5(1); P(3)-Ru-
O(1), 88.8(1); H(1)-Ru-C(2), 92 (2); P(1)-Ru-C(2), 96.1(1);
P(2)-Ru-C(2), 91.1(2); P(3)-Ru-C(2), 92.5(2); O(1)-Ru-
C(2), 176.9(2); Ru-O(1)-Si, 121.7(2).
3-d 1 and 5-a -d 1 respectively, with the latter showing a
2
broad signal (δ -5.4; 2J P-D 29.5 Hz) in the H NMR and
both products showing a very obvious diminution in
relative integrated intensity of 1H NMR signals due to
Ru-H hydrogens. This observation was checked by
examining the effect of H2O/piperidine on the 2H-
isotopomer9 1-d 1 of 1, which led to identification (again
To elucidate the mechanism of formation of complex
3, the effect of isotope incorporation was investigated.
2
Allowing precursor 1 to react in H2O-saturated piperi-
1
by H NMR integration) of hydrido-isotopomers 3 and
dine, followed by treatment with P(OMe)3, gave rise to
sequential formation of the monodeuterio-isotopomers
5-a only, with no 2H NMR signal detectable for the latter
and no 2H incorporation into either during heating (100
°C/3 h) in 2H2O-saturated toluene. In a third type of
experiment conducted in toluene-d8 solution in sealed,
evacuated NMR tubes, compound 1 was heated with
H217O (20% label) and excess piperidine; after 48 h/105
°C, the 1H NMR spectrum (high-field range,12 Ru-H)
(9) Zhou, X.; Stobart, S. R. Organometallics 2001, 20, 1898.
2
(10) As is established by the magnitude of J Si-P (112.4 Hz) in the
29Si{1H} NMR spectrum, which with cis coupling (21.3 Hz) to equiva-
lent biPSi nuclei generates a doublet of triplets pattern centered at δ
-2.2.
(11) For compound 2: Anal. Calcd for C35H45O4P3RuSi: C, 55.91;
H, 5.99. Found: C, 55.56; H, 6.06. In NOE difference experiments, on
irradiation at the frequency of the 1H NMR signal attributed to Ru-H
(δ -6.57, dt), significant intensity enhancement at the frequency of
the signal due to Si-CH3 hydrogens (δ 0.46) was evident, and vice versa.
Irradiation into Ru-H also caused similar enhancement at the P(OCH3)3
frequency.
indicated ca. 30% conversion to product, with a new 17
O
NMR signal (δ 347.0) attributable to label incorporation
into 3, i.e., confirming its formation as a 17O-isotopomer.
While air appeared to be without effect on solid 1, very
slow (>7 days) oxidation was evident in refluxing
benzene solution, affording a new compound 6 es-
sentially quantitatively (>80% yield after recrystalli-
zation). The reaction is accelerated by bubbling in
oxygen gas, but this leads to formation of coproducts
that have not yet been identified. Spectroscopic data16
for 6 were indicative of a close structural relationship
with the prototype 1, but included unaccountable peaks
at δ 3.75 (1H NMR) and 49.64 (13C); also, as was found
for 3, the 29Si{1H} NMR signal is a singlet. Addition of
P(OMe)3 yielded (>90%) a diastereomeric derivative 7,
from which the major component17 was selectively
crystallized from diethyl ether, then subsequently shown
(12) Selected spectroscopic data for 3 are as follows: 1H NMR (C6D6,
2
360.1 MHz): δ 0.36 (SiCH3), -12.65 (t, RuH, J PH 21.8 Hz). 13C{1H}
2
NMR (C6D6, 90.6 MHz): δ 203.4 (t, RuCO, J PC 14.5 Hz). 29Si{1H}
NMR (C7D8, 49.7 MHz): δ 8.1. 31P{1H} NMR (C6D6, 145.8 MHz):
δ
43.6. The shift of the high-field proton signal is typical for Ru-H trans
to coordinated heterocyclic N; see for example: Christ, M. L.; Sabo-
Etienne, S.; Chung, G.; Chaudret, B. Inorg. Chem. 1994, 33, 5316.
(13) Selected data for compounds 4 and 5 are as follows. Compound
2
4: 1H NMR: δ 0.46 (SiCH3), -4.17 (t, RuH, J PH 23.5 Hz). 29Si{1H}
NMR (C6D6): δ 7.9. 31P{1H} NMR: δ 33.9. IR: v(CO) 2048 (s), 1948
(s) cm-1. This product was always contaminated by small amounts of
impurity due to decomposition of residual 3 as volatiles were removed
from the reaction mixture. Compound 5: Anal. Calcd for C35H45O5P3-
RuSi: C, 54.75; H, 5.87. Found: C, 54.53; H, 5.54. 1H NMR: δ 0.35
2
2
(SiCH3), -5.40 (dt, RuH, J PHtrans 185.4, J PHcis 21.5 Hz). 13C{1H}
NMR: δ 203.1 (q, RuCO, both J PC 13.2 Hz). 29Si{1H} NMR: δ 4.6.
2
31P{1H} NMR: δ 135.7 (t, 2J P-P 24.4 Hz), 40.0 (d). IR: v(CO) 1904 cm-1
.
(14) Crystal data for 5: C35H45O5P3RuSi, M ) 767.8, triclinic, P1h, a
) 9.042(1) Å, b ) 12.040(1) Å, c ) 18.046(2) Å, R ) 95.11(1)°, â )
101.65(1)°, γ ) 106.57(1)°, V ) 1821.6(3) Å3, Z ) 2, Fcalcd ) 1.400 g
(16) Selected data for compound 6 (major isomer) are as follows:
1H NMR: δ 3.75 (SiOCH3), -4.22 (t, RuH, 2J PH 20.3 Hz). 13C{1H} NMR:
δ 199.7 (t, RuCO, 2 J PC 11.9 Hz), 193.1 (t, RuCO, 2 J PC 7.1 Hz). 29Si{1H}
cm-3, F(000) ) 796, λ ) 0.71073 Å, T ) 293 K, µ(Mo KR) ) 0.634 mm-1
,
8362 unique reflections, 5544 observed reflections refined to a final R
) 0.0512, Rw ) 0.0564.
NMR: δ -8.2, 31P{1H} NMR: δ 33.8. IR: v(CO) 2046 (s), 1954 (s) cm-1
.
(15) Poulton, J . T.; Sigalas, M. P.; Folting, K.; Strieb, W. E.;
Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1994, 33, 1476.
The minor isomer, which accounted for ca. 15% of the mixture, showed
2
δ RuH at -4.15 (t, J PH 20.3 Hz) in the 1H NMR.

4768 Organometallics, Vol. 20, No. 23, 2001
Communications
sion occurred, typically 90% to 2-cyclohexen-1-one and
2-cyclohexen-1-ol with ca. 10% cyclohexene oxide and a
trace of trans-1,2-cyclohexanediol (TTO19 ≈ 300). Very
similar results were obtained at 5 bar O2 applied
pressure, but significantly at higher catalyst concentra-
tion (2 × 10-3 mol equiv) the conversion is actually lower
(TTO ≈ 85), and the reaction is shut down by addition
of 2,6-di-tert-butyl-4-methylphenol. All of these observa-
tions are consistent with a catalyzed autoxidation that
while reasonably efficient does not appear to be sustain-
able beyond ca. 20% conversion, at least under the
conditions so far applied. Addition of tert-butyl hydro-
peroxide (a familiar co-oxidant20 in olefin oxidation) to
solutions of 1 caused instant formation of black, in-
soluble material.
The mechanism by which the six-coordinate precursor
1 either undergoes hydrolysis (to 3) or is oxidized (to 5)
has not yet been ascertained. The requirement for base
(i.e., piperidine: with pyridine, a weaker base, parallel
chemistry generates an analogue of 3 ca. 4 times more
slowly) in its reaction with water is consistent with a
transformation to 3 that begins by attack of OH- (at
Si). The fact that the hydrogen isotope bound to Ru in
the ultimate product originates exclusively from water
(i.e., is not carried through from 1) militates for H2 (or
H2H) elimination from Ru as a constituent event, but
further detail of a sequence into which this fits remains
to be determined. The effect of isotope incorporation into
compound 6 (i.e., from labeled dioxygen) has not yet
been investigated. In view of the saturated state of
precursor 1, however, and in the light of a relevant
recent report by Duncan and Hill,21 initiation of its
conversion to 6 by outer sphere electron transfer to
F igu r e 2. Molecular geometry of complex 7. Selected bond
lengths (Å) and angles (deg): Ru-P(1), 2.351(2); Ru-P(2),
2.352(2); Ru-P(3), 2.364(2); Ru-O(1), 2.111(6); Ru-C(31),
1.82(1); Si-O(1), 1.591(6); Si-O(3), 1.685(9); O(3)-C(32),
1.22(2); P(1)-Ru-P(2), 162.7(1); P(1)-Ru-P(3), 94.4(1);
P(2)-Ru-P(3), 102.2(1); P(1)-Ru-O(1), 89.0(2); P(2)-Ru-
O(1), 88.6(2); P(3)-Ru-O(1), 80.9(1); P(1)-Ru-C(31),
92.1(3); P(2)-Ru-C(31), 91.4(3); P(3)-Ru-C(31), 95.4(3);
O(1)-Ru-C(31), 176.2(3); Ru-O(1)-Si, 127.9(3); O(1)-Si-
O(3), 108.3(4); Si-O(3)-C(32), 137 (1).
-
give22 Ru(III)+ and O2 (rather than by complexation
of molecular dioxygen at the metal center) must be
admitted as a serious mechanistic possibility. Ensuing
attack by superoxide at Si (or its addition across the
Ru-Si bond) would resemble the silyl hydroperoxide-
mediated oxidation of alkyl-silicon bonds reported by
Tamao et al.23,24 and could effect the double insertion
(into Ru-Si and Si-C) that is required to complete the
observed connectivity.
by using X-ray crystallography18 to possess the molec-
ular geometry depicted in Figure 2. With Ru-O, O-Si
) 2.111, 1.591 Å, respectively, and Ru-O-Si ) 127.9°,
coordination about Ru closely resembles that in 5-a .
Like that of the latter, the stereochemistry is anti (i.e.,
7-a ): a vacancy revealing the position of the hydride
ligand is centered on the molecular face opposite of that
occupied by the substituent attached to silicon. Unex-
pectedly, however, the latter contains an oxygen atom,
O(3) (with Si-O ) 1.69, O-C ) 1.22 Å; Si-O-C )
137°): it is a methoxysilyl group, a discovery that
explains the otherwise puzzling NMR features observed
for17 7-a and16 6. Separate experiments have established
that bubbling oxygen gas through solutions of 4 or 5-a
(i.e., rather than 1, q.v.) converts neither one of these
to its Si-O-C-bonded analogue 6 or 7-a ; this might be
taken as circumstantial evidence to suggest that both
inserted oxygen atoms in 6 originate from the same
dioxygen molecule.
The remarkable transformation of coordinatively
saturated 1 into its oxygenated derivative 6 led us to
screen the system for activity in alkene oxidation.
Oxygen gas was bubbled through a frit into mixtures
of cyclohexene and 1,2-dichloroethane at ambient tem-
perature. On addition of solid 1 (5 × 10-4 molar equiv
vs C6H10), an immediate color change was evident, to
yellow and then deep red. During 4 h, ca. 15% conver-
Ack n ow led gm en t. We thank the NSERC, Canada,
for financial support and the Dow Corning Corp. for a
generous gift of H217O.
OM010463T
(17) Selected data for compound 7 are as follows: Anal. Calcd for
C
35H45O6P3RuSi: C, 53.63; H, 5.75. Found: C, 53.21; H, 5.62. Major
isomer (i.e.,18 7-a ): 1H NMR: δ 3.73 (SiOCH3), -5.44 (dt, RuH, 2J PHtrans
2
2
184.7, J PHcis 21.3 Hz). 13C{1H} NMR: δ 202.8 (q, RuCO, both J PC
13.1 Hz). 29Si{1H} NMR: δ -8.8. 31P{1H} NMR: δ 135.9 (t, 2J P-P 25.0
Hz), 41.6 (d). IR: v(CO) 1906 cm-1. Minor isomer (ca 15%):16 δ RuH
2
-5.40 (t, J PH 21.5 Hz).
(18) Crystal data for 7:
C35H45O6P3RuSi, M ) 777.8, monoclinic,
P21/n, a ) 9.045(2) Å, b ) 22.524(2) Å, c ) 18.356(2) Å, â ) 99.63(2)°,
V ) 3687.0(8) Å3, Z ) 4, Fcalcd ) 1.401 g cm-3, F(000) ) 1612, λ )
0.71073 Å, T ) 293 K, µ(Mo KR) ) 0.629 mm-1, 8475 unique reflections,
3910 observed reflections refined to a final R ) 0.0616, Rw ) 0.0638.
(19) Total turnovers, i.e., here corresponding to a minimum TON
of 75 h-1
.
(20) Minisci, F.; Fontana, F.; Araneo, S.; Recupero, F.; Banfi, S.;
Quici, S. J . Am. Chem. Soc. 1995, 117, 226. Snelgrove, D. W.; MacFaul,
P. A.; Ingold, K. U.; Wayner, D. D. M. Tetrahedron Lett. 1996, 37, 823.
(21) Duncan, D. C.; Hill, C. J . J . Am. Chem. Soc. 1997, 119, 243.
(22) Pacheco, A.; J ames, B. R.; Rettig S. J . Inorg. Chem. 1999, 38,
5579.
(23) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M. Organometallics
1983, 2, 1694.
(24) Tamao, K.; Ishida, N.; Ito, Y.; Kumada, M. Org. Synth. 1990,
69, 102.