Using NMR and ESI-MS to probe the mechanism of silane dehydrocoupling catalyzed by wilkinson's catalyst

Article abstract of DOI:10.1002/ejic.201001165

The reaction of Wilkinson's catalyst Rh(PPh₃)₃Cl (1) with 1-2 equiv. of di-n-hexylsilane gives rise to a complex mixture of products, which has been analyzed by ³¹P{¹H} NMR and shown to include the hydrido complex Rh(PPh₃)₃H (3). Continuous sampling of the 1:1 reaction mixture by ESI-MS provided time-dependent speciation that tracks the formation of 3 at the expense of the initial silane oxidative addition product [Rh(PPh₃)₂(Cl)(H){Si(nHex) ₂H}] (2), the subsequent 1st order disappearance of 3, and the formation of other, minor side-products. Our results provide insight into established activity of 1 for catalytic dehydrocoupling of di-n-hexylsilane, and implicate complex 3 as the active species in this catalysis. The combination of ³¹P NMR and ESI-MS provides detailed insight into the dynamics of the activation of a secondary silaneby Rh(PPh₃)₃Cl, with strong evidence pointing to the role of Rh(PPh₃)₃H as the active catalyst for silane dehydrocoupling and to the deleterious effect of trace water in this system. Copyright

Full text of DOI:10.1002/ejic.201001165

SHORT COMMUNICATION
DOI: 10.1002/ejic.201001165
Using NMR and ESI-MS to Probe the Mechanism of Silane Dehydrocoupling
Catalyzed by Wilkinson’s Catalyst
Sarah M. Jackson,[a] Danielle M. Chisholm,[a] J. Scott McIndoe,*[a] and Lisa Rosenberg*[a]
Keywords: Silanes / Reaction mechanisms / NMR spectroscopy / Electrospray ionization mass spectrometry
The reaction of Wilkinson’s catalyst Rh(PPh3)3Cl (1) with 1–
2 equiv. of di-n-hexylsilane gives rise to a complex mixture
of products, which has been analyzed by 31P{1H} NMR and
shown to include the hydrido complex Rh(PPh3)3H (3). Con-
uct [Rh(PPh3)2(Cl)(H){Si(nHex)2H}] (2), the subsequent 1st or-
der disappearance of 3, and the formation of other, minor
side-products. Our results provide insight into established
activity of 1 for catalytic dehydrocoupling of di-n-hexylsil-
tinuous sampling of the 1:1 reaction mixture by ESI-MS pro- ane, and implicate complex 3 as the active species in this
vided time-dependent speciation that tracks the formation of
catalysis.
3 at the expense of the initial silane oxidative addition prod-
The stoichiometric activation of Si–H bonds in tertiary
We have carried out a number of NMR experiments to
silanes by Wilkinson’s catalyst, Rh(PPh3)3Cl (1) was first identify the Rh-containing products resulting from the ad-
reported in the late 1960s,[1] and not long afterwards, the dition of di-n-hexylsilane to 1. Although the anticipated 1:1
activity of this RhI phosphane chloride complex for the de- oxidative addition product, [Rh(PPh3)2(Cl)(H){Si(nHex)2-
hydrogenative coupling of secondary silanes to give di- and H}] (2)[5c] does form, it is unstable and rapidly decomposes
trisilanes was discovered.[2] This initial communication, and to give a mixture of products. For example, when the 1:1
most subsequent reports of the activity of rhodium com- reaction of 1 and di-n-hexylsilane is carried out in [D6]-
plexes for silane dehydrocoupling,[3] noted the propensity of benzene in an NMR tube, monitoring by 31P{1H} NMR
the system to give competing redistribution of substituents shows that signals due to 2 diminish (over ca. 1 h) relative
at silicon, which fueled a debate on the nature of both the to a very broad signal consistent with one or more species
Si–Si bond-forming step and the apparent Si–C bond acti- undergoing phosphane exchange, and the corresponding 1H
vation by late-metal complexes.[4] Our previous examina- NMR spectra indicate the formation of several different hy-
tions of this system demonstrated the high selectivity for dride-containing species (see Supporting Information).
coupling over redistribution reactions of diphenylsilane un- More detailed NMR assignments have come from 1:1 or
der conditions in which hydrogen is removed rapidly,[5a] as 1:2 reactions carried out in toluene, from which we have
well as the redistribution-free coupling of primary alkyl- attempted to precipitate and isolate the Rh-containing
silanes to longer oligosilane chains,[5b] and a survey of the products. These reactions consistently yield a bright yellow
relative activities of a series of related RhI bis(phosphane) solid that, when dissolved in deuterated benzene or toluene,
1
catalyst precursors for the redistribution-free coupling of shows the presence of multiple species by H and 31P{1H}
di-n-hexylsilane.[5c] However, the actual mechanism of cou- NMR spectroscopy. Figure 1 shows the variable tempera-
pling mediated by this system is still not well understood. ture 31P{1H} NMR of one such solid, along with assign-
We present here NMR and electrospray ionization mass ments that were confirmed by low-temperature 31P{1H}
spectrometry (ESI-MS) analysis of the reaction of Wilkin- COSY and 1H experiments. The room temperature 31P{1H}
son’s catalyst with stoichiometric amounts of di-n-hexyl- NMR spectrum (lower trace) shows peaks due to 1,[6] 2,[5c]
silane. These results shed light on the catalyst initation step [Rh(PPh3)3H] (3)[7] and [Rh(PPh3)3(Cl)(H)2] (4)[8] along
and point to a rhodium hydride complex as the active spe- with signals due to several other, unidentified products, in-
cies in the coupling reaction. They also highlight the com- cluding a broad signal almost lost in the baseline that corre-
plementarity of these two analytical techniques and demon- sponds to one or more species undergoing phosphane ex-
strate highly resolved time-dependent speciation possible change. At 210 K (upper trace) the exchange is sufficiently
using ESI-MS.
slow to allow identification of [Rh(PPh3)4H] (5),[9,10] the
PPh3 adduct of 3; and the peaks due to the inequivalent
phosphanes in 3 and 4 are better resolved. This “snapshot”
of the reaction mixture shows that the major Rh containing
products are the hydrido complexes 3 and 5, which are in
equilibrium in the presence of excess PPh3.[11] The obvious
source of the hydride ligand in 3 and 5 is the silane: in
[a] Department of Chemistry, University of Victoria,
P. O. Box 3065, Victoria, BC V8W3V6, Canada
Fax: +1-250-721-7147
E-mail: mcindoe@uvic.ca
lisarose@uvic.ca
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001165.
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327

J. S. McIndoe, L. Rosenberg et al.
SHORT COMMUNICATION
particular, 3 probably results from the reductive elimination sons.[14] Here we extend this approach to tracking the dy-
of (nHex)2Si(H)(Cl) from 2, along with re-association of the namics of the solution reaction of 1 with di-n-hexylsilane,
third phosphane ligand (Scheme 1, below).[12] 29Si{1H} by monitoring the stoichiometric reaction continuously
NMR of the supernatant from these reactions showed the over a period of 48 minutes. The cationic dopant ligand
–
product chlorosilane at δ = 14.1 ppm, along with the dis- used was [Ph2P(CH2)4PPh2(CH2Ph)]+BF4 (6). A typical
iloxane {(nHex)2SiH}2O (δ = –1.6 ppm),[5c] a condensation spectrum of a mixture of 1 + 30% 6 + (nHex)2SiH2 appears
product associated with hydrolysis of the Si–Cl bond in in Figure 2.
(nHex)2Si(H)(Cl), and a third species tentatively assigned as
the corresponding disilanol (nHex)2Si(OH)2 at
δ =
–6.9 ppm.
Figure 2. Positive-ion ESI-MS of Rh(PPh3)3Cl + 0.36 + (nHex)2-
SiH2 in fluorobenzene, obtained at t = five min.
The apparent complexity of the spectrum is made tracta-
ble by the fact that essentially the same species is repre-
sented by more than one ion. Wilkinson’s catalyst itself,
Rh(PPh3)3Cl (1), is represented by the ions [Rh(PPh3)2Cl-
(6)]+ (1179.2 m/z, 1a), [Rh(PPh3)Cl(6)2]2+ (717.2 m/z, 1b),
[Rh(PPh3)Cl(6)]+ (917.2 m/z, 1c), [RhCl(6)2]2+ (586.2 m/z,
1d) and even [RhCl(6)2(BF4)]+ (1260.3 m/z, 1e), [RhCl(6)3
+ BF4]2+ (888.8 m/z, 1f) and [Rh(6)3 + 2(BF4)]+ (1865.6
m/z, 1g).[15] By combining these species together as a single
entity “Rh(PR3)3Cl” and repeating for all similarly-dupli-
cated ions, a seeming forest of peaks is rendered down to
10 chemically distinct compounds. The identities of each
can be established by a combination of m/z value, isotope
pattern and product ion MS/MS (see Supporting Infor-
mation). Key species observed by ESI-MS that were also
Figure 1. Variable temperature 145.8 MHz 31P{1H} NMR spectra
of the yellow solid isolated from a 1:2 reaction of Wilkinson’s cata-
lyst with di-n-hexylsilane, in [D8]toluene. The lower trace is typical
of the room temperature spectra obtained for solids isolated from
either 1:1 or 1:2 reactions.
Scheme 1. Principal mode of decomposition of 2 following the oxi-
dative addition of silane to 1, based on NMR analysis.
The NMR spectra left us with a number of unanswered observed by NMR include: Rh(PR3)3Cl (1), [Rh(PR3)2-
questions. In particular, we observed small peaks due to (Cl)(H){Si(nHex)2H}] (2), Rh(PR3)3H (3), and Rh(PPh3)3-
unidentified rhodium complexes. Also, we observed (appar- (Cl)(H)2 (4), along with PR3, and O=PR3. We did not ob-
ently unreacted) 1 and its dihydrogen adduct 4, which sug- serve peaks due to Rh(PR3)4H (5), even when this com-
gested that Si–Si bond formation (producing hydrogen) was pound was prepared and analyzed individually by ESI-MS
occurring far more rapidly than the initial Si–H oxidative under identical conditions: this complex loses PPh3 even in
addition reaction, even though we had not detected any of extremely soft ionization conditions to show only peaks due
the product disilane or trisilane by 29Si NMR spectroscopy. to 3. The complex is similarly labile in solutions studied by
We turned to ESI-MS to gain more information on the ele- NMR, as evidenced by the phosphane exchange occurring
mental composition of the species being formed in this reac- on the NMR timescale, but evidently the equilibrium be-
tion, both to confirm our NMR assignments and to shed tween (3 + PPh3) and 5 lies toward 5 at NMR concentra-
light on the structures of the unidentified species. Catalytic tions. Species observed by ESI-MS that had not previously
reactions involving complexes with labile phosphane li- been identified by NMR include [Rh(PR3)2(Cl)(H){Si-
gands can be monitored by ESI-MS[13] by the simple expe- (nHex)2(OH)}] (7), [Rh(PR3)3]+ (8), [Rh(PR3)2{Si(nHex)2-
dient of doping in a charged phosphane ligand: we pre- H}] (9), and [Rh(PR3)2(H)2{Si(nHex)2H}] (10).
viously demonstrated the validity of this ESI-MS approach
As shown in Figure 3 for the six most abundant com-
in the study of olefin hydrogenation catalyzed by 1, for pounds, tracking the evolution of each species over time
which we were able to spot simultaneously intermediates, generates a revealing reaction profile.[16] Similar to what
catalyst resting states, off-cycle species and catalyst poi- was observed by NMR, the Si–H oxidative addition prod-
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Mechanism of Silane Dehydrocoupling
uct [Rh(PR3)2(Cl)(H){Si(nHex)2H}] (2) is 25% abundant pate in the overall reaction with silane.[18] The relative abun-
initially but decays to 0% after 20 min. Rh(PR3)3H (3) dance of complexes 9 and 10 (not shown in Figure 3) is low
grows in rapidly at the start of the reaction at the expense throughout (Յ 1%, see Supporting Information), but these
of 2 to peak at 27%, then decays away following first order Si-containing species are relevant: 9 represents the loss of
kinetics (t1/2 = 8.8 min, see inset in Figure 3). This behav- HCl, as opposed to (nHex)2Si(H)(Cl), from complex 2, or
iour is consistent with the production of 3 directly from 2, the loss of H2 from 10, while 10 would result from the ad-
by reductive elimination of (nHex)2Si(H)(Cl), followed by dition of silane to complex 3. Also, preliminary ESI-MS
the consumption of 3 in some slower reaction step. Rh(PR3)3- data collected for a mixture of independently synthesized 3
Cl (1) drops from 13% to 5% after 5 minutes, then slowly and the silane show compounds 9 and 10 in low abundance.
grows back in over the next 40 minutes to about 8%. This Highlighted in the dotted box in Scheme 2 is a proposed
unexpected re-emergence of 1 is mimicked almost perfectly catalytic cycle for silane dehydrocoupling that relies on the
by Rh(PPh3)3(Cl)(H)2 (4) and [Rh(PR3)2(Cl)(H){Si(nHex)2- formation and loss of a silylene ligand. There is literature
(OH)}] (7), though with different starting values (4% and precedent for the α-elimination reaction at Rh-silyl com-
0%, respectively). Qualitatively, the analogous slopes of plexes such as 9 to generate reactive silylene intermediates
these traces suggest that increasing concentrations of 1, 4 (9Ј, for instance),[3e,19] and these have long been postulated
and 7 derive from a common reaction step. A straightfor- as key intermediates in these Rh-catalyzed coupling reac-
ward explanation is that the chlorosilane (nHex)2Si(H)(Cl) tions.[2] Some evidence for alpha-elimination chemistry in
is slowly being hydrolysed by trace water to produce HCl this system comes from MS/MS experiments on 2, which
and (nHex)2Si(H)(OH). The reaction of 3 with HCl would show the loss of H2 (see Supporting Information). However,
generate complex 4, which could then reductively eliminate it is certainly possible to replace this portion of the cycle
H2 to give 1. Observation of complex 7, resulting from the with the oxidative addition of silane to 9, followed by re-
oxidative addition of (nHex)2Si(H)(OH) to 1, represents the ductive elimination of a Si–Si bond.[20]
first evidence we have obtained for the interaction of rho-
dium in this system with silane hydrolysis products, despite
the fact that the disiloxane {(nHex)2SiH}2O was prepared
from the reaction of (nHex)2SiH2 with H2O in the presence
of 1.[5c,17]
Scheme 2. Reactions illustrating the activation of Rh(PPh3)3Cl (1)
by dialkylsilane, the possible silane dehydrocoupling activity of the
resulting rhodium hydride complex 3, and the competing decompo-
sition resulting from the action of trace water in the reaction mix-
ture.
In conclusion, we have obtained new insight into the
early time regime of the reaction of Wilkinson’s catalyst
with a dialkylsilane, using time-averaged structural infor-
mation from NMR and fast sampling and rapid data collec-
tion by ESI-MS. These results demonstrate the sensitivity
of this late metal halide catalyst system to steady, indirect
degradation by trace water,[21] and the probable importance
Figure 3. Reaction profile showing the evolution of the six most
abundant Rh-containing species observed over time by ESI-MS.
Each trace is generated by summing the intensities of all related
entities and normalizing to the total ion current. Inset: ln[RhHP3]
vs. time for the decay of this species, demonstrating first-order be-
havior.
Scheme 2 presents a series of reactions that rationalize of rhodium hydride intermediates in the dehydrocoupling
the intermediates we see both by NMR and by MS, and reaction. In keeping with this latter result, we find that the
that correlate well with the time-dependent speciation we hydride complexes 3 and 5 do indeed catalyze the dehy-
have been able to observe by ESI-MS. The cation 8 is a drocoupling of di-n-hexylsilane, with activities comparable
prominent peak in all the mass spectra collected, although to and higher than 1 (see Supporting Information). Cur-
its intensity relative to the total ion current changes little rently we are extending these studies to the examination
over time (see Figure 3), suggesting that it does not partici- and correlation of NMR and ESI-MS data for actual cata-
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J. S. McIndoe, L. Rosenberg et al.
SHORT COMMUNICATION
[4] For relevant reviews see a) M. D. Curtis, P. S. Epstein, Adv.
Organomet. Chem. 1981, 19, 213; b) T. D. Tilley, Comments In-
org. Chem. 1990, 10, 37; c) J. Y. Corey, Adv. Silicon Chem. 1991,
1, 327; d) F. Gauvin, J. F. Harrod, H. G. Woo, Adv. Organomet.
Chem. 1998, 42, 363; e) J. A. Reichl, D. H. Berry, Adv. Or-
ganomet. Chem. 1999, 43, 197.
[5] a) L. Rosenberg, C. W. Davis, J. Z. Yao, J. Am. Chem. Soc.
2001, 123, 5120; b) L. Rosenberg, D. N. Kobus, J. Organomet.
Chem. 2003, 685, 107; c) S. M. Jackson, C. E. Hughes, S. Mon-
fette, L. Rosenberg, Inorg. Chim. Acta 2006, 359, 2966.
[6] T. H. Brown, P. J. Green, J. Am. Chem. Soc. 1970, 92, 2359.
[7] S. H. Strauss, S. E. Diamond, F. Mares, D. F. Shriver, Inorg.
Chem. 1978, 17, 3064.
lytic mixtures, and we will report on this work in due
course.
Experimental Section
NMR analysis of the stoichiometric reaction of 1 with silane: (a)
To solid complex 1 (0.026 g, 0.028 mmol) in an NMR tube was
added di-n-hexylsilane as a standard solution (20 mg/mL) in [D6]
benzene (0.3 mL, 0.006 g, 0.03 mmol). Another 0.2 mL of [D6]ben-
zene was added, and the tube was capped and sealed with parafilm.
The solid 1 took about 15 min to completely dissolve, giving a light
orange solution. (b) Complex 1 (101 mg, 0.109 mmol) was dis-
solved in toluene (18 mL). Di-n-hexylsilane (0.050 g, 0.249 mmol)
in toluene was added, and the solution was stirred for 1 h (solid
red complex took ca. 5 min to dissolve completely, giving a clear,
light orange-yellow solution). The toluene was removed under vac-
uum, leaving a yellow residue. The addition of pentane (10 mL)
gave a suspension of a fine yellow powder and a yellow supernatant
solution. The yellow solid was isolated by cannula filtration, dried
under vacuum, and 19 mg were used to prepare a sealed NMR
sample in [D8]toluene.
[8] a) M. A. Esteruelas, J. Herrero, M. Olivan, Organometallics
2004, 23, 3891; b) A. A. Peterson, K. A. Thoreson, K. McNeill,
Organometallics 2009, 28, 5982.
[9] S. H. Strauss, D. F. Shriver, Inorg. Chem. 1978, 17, 3069.
[10] Orange crystals of 5 were obtained independently via slow
evaporation from a [D6]benzene solution of the yellow solid
isolated from a 1:1 reaction of 1 with di-n-hexylsilane, as deter-
mined by comparison of X-ray crystallographic data with that
previously reported; a) R. McDonald, personal communica-
tion; b) R. W. Baker, P. Pauling, J. Chem. Soc. C 1969, 1495.
[11] The free PPh3 results from the initial oxidative addition reac-
tion generating the bis(phosphane) complex 2 from the tris-
(phosphane) complex 1. We presume the mass balance between
residual 2 and complex 5 is skewed in Figure 1 because of the
selective precipitation used to obtain the sample: either PPh3
and/or complex 5 is less soluble than complex 2.
ESI-MS analysis of the stoichiometric reaction of 1 with silane: All
mass spectrometric analyses were performed on a Micromass QToF
micro hybrid quadrupole/time-of-flight mass spectrometer in posi-
tive ion mode using pneumatically-assisted electrospray ionization
with a cone voltage of 10 V, capillary voltage of 3100 V, source
temperature of 120 °C and desolvation temperature of 150 °C.
Solutions were run in fluorobenzene and introduced to the mass
spectrometer by a syringe pump at a rate of 10 μLmin–1. ESI-MS
spectra of the catalyst mixture were obtained by adding to 12 mL
of fluorobenzene RhCl(PPh3)3 (11 mg, 0.012 mmol, 0.96 mmolL–1)
[12] Precedent for the transformation of RhI phosphane chloride
complexes into rhodium hydride complexes in the presence of
hydrosilane reagents includes ref.[8a,8b] and a) K. Osakada, J.
Organomet. Chem. 2000, 611, 323.
[13] K. L. Vikse, M. A. Henderson, A. G. Oliver, J. S. McIndoe,
Chem. Commun. 2010, 46, 7412.
[14] D. M. Chisholm, A. G. Oliver, J. S. McIndoe, Dalton Trans.
2010, 39, 364.
–
and [6]+BF4 (2 mg, 0.003 mmol, 0.28 mmolL–1) to give a bright
[15] The bis(phosphane) ions 1c–e are included in the speciation of
“P3RhCl” because previous studies (ref.[14]) showed them to
result directly from 1 as a consequence of the cone voltage
required to obtain acceptable spectra of the solution.
[16] The first 30 seconds or so of reactivity is lost because it takes
approximately that long to mix the reactants, transfer to a gas-
tight syringe and inject into the mass spectrometer.
[17] Neither di-n-hexylsilane, nor the di- and trisilanes resulting
from its homodehydrocoupling reactions, react with water in
the absence of Rh.
orange solution. To this solution was added (nHex)2SiH2 (2 mg,
0.012 mmol) to give a bright yellow solution within seconds.
Supporting Information (see also the footnote on the first page of
this article): additional NMR and ESI-MS data, experimental de-
tails for dehydrocoupling catalysed by 1, 3, and 5.
Acknowledgments
[18] Loss of halide ligands to generate such ions is commonly ob-
served in ESI-MS of metal halide complexes. See, for example:
a) A. O. Aliprantis, J. W. Canary, J. Am. Chem. Soc. 1994, 116,
6985; b) C. Decker, W. Henderson, B. K. Nicholson, J. Chem.
Soc., Dalton Trans. 1999, 3507; c) W. Henderson, C. Evans,
Inorg. Chim. Acta 1999, 294, 183; d) C. Vicent, M. Viciano, E.
Mas-Marza, M. Sanau, E. Peris, Organometallics 2006, 25,
3713; e) L. S. Santos, G. B. Rosso, R. A. Pilli, M. N. Eberlin,
J. Org. Chem. 2007, 72, 5809.
[19] a) G. P. Mitchell, T. D. Tilley, Organometallics 1998, 17, 2912;
b) N. Schneider, M. Finger, C. Haferkemper, S. Bellemin-La-
ponnaz, P. Hofmann, L. H. Gade, Angew. Chem. 2009, 121,
1637; Angew. Chem. Int. Ed. 2009, 48, 1609.
[20] If the catalytic cycle relies on reductive elimination of disilane
from a bis(silyl) complex, the fact that we do not observe a
Rh(Si)2 species by MS suggests that this elimination step is not
rate-determining.
[21] Preliminary ESI-MS data for the mixture of complex 3 with
silane do not show any O-containing Rh complexes similar to
complex 7.
J. S. M. thanks Canada Foundation for Innovation (CFI) and BC
Knowledge Development Fund (BCKDF) for infrastructure sup-
port, and Natural Sciences and Engineering Research Council of
Canada (NSERC) for operational funding (Discovery Grant and
Discovery Accelerator Supplement). L. R. thanks NSERC for op-
erational funding (Discovery Grant).
[1] a) F. de Charentenay, J. A. Osborn, G. Wilkinson, J. Chem.
Soc. A 1968, 787; b) R. N. Haszeldine, R. V. Parish, D. J. Parry,
J. Chem. Soc. A 1969, 683.
[2] I. Ojima, S. Inaba, T. Kogure, Y. Nagai, J. Organomet. Chem.
1973, 55, C7.
[3] a) M. F. Lappert, R. K. Maskell, J. Organomet. Chem. 1984,
264, 217; b) K. A. Brown-Wensley, Organometallics 1987, 6,
1590; c) L. S. Chang, J. Y. Corey, Organometallics 1989, 8,
1885; d) L. Rosenberg, M. D. Fryzuk, S. J. Rettig, Organome-
tallics 1999, 18, 958; e) R. Goikhman, D. Milstein, Chem. Eur.
J. 2005, 11, 2983; f) S. J. Toal, H. Sohn, L. N. Zakarov, W. S.
Kassel, J. A. Golen, A. L. Rheingold, W. C. Trogler, Organome-
tallics 2005, 24, 3081.
Received: November 3, 2010
Published Online: December 9, 2010
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