Benchmarked Intrinsic Olefin Metathesis Activity: Mo vs. W

Article abstract of DOI:10.1002/hlca.201700302

Combining Surface Organometallic Chemistry with rigorous olefin purification protocol allows evaluating and comparing the intrinsic activities of Mo and W olefin metathesis catalysts towards different types of olefin substrates. While well-defined silica-supported Mo and W imido-alkylidenes show very similar activities in metathesis of internal olefins, Mo catalysts systematically outperform their W analogs in metathesis of terminal olefins, consistent with the formation of stable unsubstituted W metallacyclobutanes in the presence of ethylene. However, Mo catalysts are more prone to induce olefin isomerization, in particular when ethylene is present, probably because of their propensity to undergo more easily reduction processes.

Full text of DOI:10.1002/hlca.201700302

Article Type: Full Paper
1
2
Pavel A. Zhizhko, Victor Mougel, Jordan De Jesus Silva, and Christophe Copéret*
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1–5, 8093
Zürich, Switzerland
e-mail: ccoperet@inorg.chem.ethz.ch
1
On leave from A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of
Sciences, Vavilov str. 28, 119991 Moscow, Russia.
2
Current address: Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de
France, Université Pierre et Marie Curie, 11 Place Marcelin Berthelot, 75005 Paris, France.
Combining Surface Organometallic Chemistry with rigorous olefin purification protocol allows
evaluating and comparing the intrinsic activities of Mo and W olefin metathesis catalysts towards
different types of olefin substrates. While well-defined silica-supported Mo and W imido-alkylidenes
show very similar activities in metathesis of internal olefins, Mo catalysts systematically outperform
their W analogs in metathesis of terminal olefins, consistent with the formation of stable
unsubstituted W metallacyclobutanes in the presence of ethylene. However, Mo catalysts are more
prone to induce olefin isomerization, in particular when ethylene is present, probably because of
their propensity to undergo more easily reduction processes.
Keywords: Olefin Metathesis, Surface Organometallic Chemistry, Benchmark, Activity, Mo vs. W
Introduction
Molybdenum and tungsten are the two key elements for the industrial heterogeneous olefin
[
1]
metathesis catalysts (SHOP, Phillips Triolefin Process, Lummus’ OCT, etc.). Both metals also provide
[
2]
[3-5]
the most active homogeneous and well-defined silica-supported catalysts based on the Schrock-
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1002/hlca.201700302
This article is protected by copyright. All rights reserved.

type alkylidene complexes. However, despite years of research and industrial application, it is still
difficult to evaluate the intrinsic activity of Mo vs. W. Indeed, the marked difference in operating
conditions for industrial heterogeneous catalysts based on supported MO
400 °C for W) is likely associated with the initiation steps and the number of active sites, rather
than with the intrinsic activity of Mo and W active species. Well-defined silica-supported Mo
3
(100–200 °C for Mo vs.
[
6]
≥
[
7-9]
[10-12]
alkylidenes are more active than their W counterparts in propene metathesis,
however,
[
13]
molecular W alkylidenes are reported to outperform Mo analogs in metathesis of cis-2-pentene.
The difficulty to compare catalyst activities (rates) is however not surprising considering that these
catalysts are both highly active and sensitive to poisons, hence requiring low catalyst loadings and
very pure substrates to obtain reliable data (kinetic regime and no deactivation by poisoning). DFT
0
calculations have shown that the main differences between isostructural Mo and W d metathesis
catalysts arise from: the difference in the stability of the metallacycles and the difference in energy
barriers for the TBP/SP interconversion. In particular, W leads to more stable TBP and SP
metallacycles; while the former is on the metathesis pathway, the more stable SP structure is an off-
[
14-18]
cycle intermediate that contributes to an overall decrease of activity of the catalyst.
Recent investigations have shown that well-defined silica-supported alkylidenes, prepared by
[
3-5]
Surface Organometallic Chemistry (SOMC), do not suffer from bimolecular decomposition
[
19, 20]
pathways due to site isolation in contrast to their molecular analogs.
In addition, it has been
shown in the context of alkyne metathesis that using highly purified substrates is essential for
[
21]
reliable measurement of rates. We thus reasoned that combining SOMC on a series of silica-
supported Mo and W imido-alkylidene metathesis catalysts bearing identical ligand sets (Scheme 1)
with a standardized olefin purification protocol would warranty reproducible metathesis activity
measurement, allowing in fine the comparison of the intrinsic activities between Mo and W. This
benchmark measurement of activity will be carried out on two prototypical substrates: cis-4-nonene
and 1-nonene.
Scheme 1. Catalysts and substrates investigated.
This article is protected by copyright. All rights reserved.

Results and Discussion
In order to compare Mo and W catalysts, we have prepared a series of silica-supported Mo imido-
alkylidenes (≡SiO)Mo(=NAr)(=CHCMe
ranging from electron-donating Me Pyr to highly electron-withdrawing OtBuF9 (Scheme 1; Ar = 2,6-
iPr ; X = Me Pyr (MoPyr), OtBuF3 (MoF3), OtBuF6 (MoF6), and OtBuF9 (MoF9)), as well as the
corresponding W analogs reported previously (Scheme 1; WPyr, WF3, WF6, WF9; see ESI for details).
2
Ph)(X) bearing X ligands with different σ-donating ability
2
2
C
H
6 3
2
[
22,
23]
We have then evaluated the performance of these supported complexes in the self-metathesis of
cis-4-nonene and 1-nonene in batch conditions. The olefins are purified by successive distillation
from Na, degassing, and passing through activated neutral alumina prior to a treatment with
[
24]
Selexsorb CD (see ESI for details), the latter reagent being key for reaching high and reproducible
activity at such low loadings.
Metathesis of cis-4-nonene results in the formation of an equilibrium mixture of cis/trans 4-octenes,
4
-nonenes, and 5-decenes, corresponding to ca. 50% conversion of nonene (Scheme 1a). Since
metathesis occurs without initiation, turnover frequency measured at time = 3 min (TOF3 min) is used
as a descriptor of the catalyst activity, while time required to reach equilibrium (τequil) depends on
both activity and stability. As shown in Table 1, the performance of certain W catalysts (in particular,
the least active ones, WPyr and WF3) is significantly improved after Selexsorb pretreatment of the
[
22, 23]
olefin as compared to the previously reported data,
although the general trend within the
series stays unchanged. The results summarized in Table 1 show that there is hardly any difference
in activity between Mo and W catalysts for the same ligand set, the greatest distinction being
observed for MoPyr and WPyr. Interestingly, the activity of Mo catalysts within the series follows the
same trend upon changing the σ-donating ability of the X ligand as observed before for supported
[
22, 23]
[13]
[25]
W
and molecular Mo and W catalysts (Me Pyr < OtBuF3 < OtBuF6 < OtBuF9).
2
Table 1. Catalytic activities of silica-supported Mo and W imido-alkylidenes in metathesis of cis-4-nonene (Scheme 1a).
a,b
a
b
Cat.
TOF3 min
,
τ
equil
,
Cat.
TOF3 min
,
equil
τ ,
min
−1
−1
min
min
min
For 1-nonene, metathesis yields cis/trans 8-hexadecenes and ethylene (Scheme 1b). In this case, full
conversion can be reached if ethylene is removed from the reaction mixture (see ESI for details). The
results are summarized in Table 2 and Figure 1. All Mo catalysts demonstrate high activities and
This article is protected by copyright. All rights reserved.

perform very similarly with little influence of the pendant X-group. However, in contrast to what is
found for cis-4-nonene, the initial activity of W catalysts is ca. an order of magnitude lower than that
of Mo. The effect of the X ligand on the initial activity is also not strongly pronounced for W.
However, it has a significant effect on the overall catalytic performance (number of turnovers)
(Figure 1), σ-donating X ligands clearly yielding more efficient catalysts. Thus, WPyr catalyst
containing electron-donating Me Pyr ligand shows remarkably better performance than other W
2
catalysts albeit still worse than its Mo analog MoPyr. It should be also noted that, while being more
active, Mo catalysts display lower selectivity. While selectivity in metathesis product (C16) remains
≥99.5% for all the W catalysts over the course of the reaction, 2–3% of isomeric products (C15, C14…)
are formed after 8 h of the reaction for MoF3, MoF6 and MoF9, and up to 7% for MoPyr. In addition, if
ethylene is not efficiently released from the reaction mixture (e.g. using a closed vessel), the
selectivity can drop even further to reach values as low as 80–85% (MoPyr), probably indicating that
ethylene is responsible (at least in part) for the lower selectivity. The reaction also appears to be
slower in this case for both Mo and W.
Table 2. Catalytic activities of silica-supported Mo and W imido-alkylidenes in metathesis of 1-nonene (Scheme 1b).
a
−1
a
−1
Cat.
TOF3 min, min (conv. after 8 h)
Cat.
TOF3 min, min (conv. after 8 h)
Figure 1. Catalytic performance of silica-supported Mo and W imido-alkylidenes in metathesis of 1-nonene.
The observed difference in activity towards 1-nonene metathesis between Mo and W is in line with
[
7-9]
[10, 11]
the previous findings on catalytic behavior of silica-supported Mo and W
alkylidenes in the
metathesis of propene under flow conditions, where W catalysts are also found to be significantly
less efficient than their Mo analogs. We reasoned that the main difference between metathesis of
This article is protected by copyright. All rights reserved.

internal (cis-4-nonene) and terminal olefins (propene and 1-nonene) arises from the presence of
ethylene and the formation of unsubstituted metallacyclobutanes in the latter case. For W, these
metallacyclobutanes have been previously shown to be rather stable and reluctant to participate in
[
22]
metathesis, as evidenced by pronounced induction periods. Their stability is also consistent with
[
17]
DFT calculations and with the fact that most isolated and fully characterized metallacyclobutanes
[
26]
[11, 22, 23, 27, 28]
are based on W, in both molecular and surface chemistry.
In contrast, the stability of
[
13, 29, 30]
Mo metallacyclobutanes is significantly lower,
only few cases with molecular catalysts
Mo metallacycles to form d hydride/alkyl or d species is also likely the reason of lower selectivity
observed in this case, as these species can promote double bond migration. Earlier studies have
shown that ethylene is particularly prone to induce deactivation of metathesis catalysts.
and so far these species have been isolated in
and not reported on surfaces. Easier decomposition of
[
13, 29]
0
2
[
15, 31]
In fact,
we have so far failed to observed the formation of metallacyclobutanes in the case of supported Mo
catalysts.
Overall, based on the catalytic data discussed above, we conclude that there is no dramatic
difference in intrinsic activity between Mo and W alkylidenes towards metathesis of internal olefins
and that the lower activity of W catalysts in metathesis of terminal olefins is mainly due to the
relatively high stability of unsubstituted metallacyclobutane intermediates that serve as a catalytic
off-cycle resting state. Moreover, the better performance of WPyr can also be explained using the
same rationale, as the presence of σ-donating ligands leads to destabilization of the
[
17]
metallacyclobutane intermediates. This is also in line with the data on the previously reported W
[
32]
[33]
catalysts bearing strongly σ-donating ligands, e.g. thiolates and N-heterocyclic carbenes, that
displayed remarkably high activities in metathesis of terminal olefins.
Conclusions
Using SOMC and careful olefin purification protocol has allowed us to measure the intrinsic activities
of Mo and W olefin metathesis catalysts and to compare their relative performance. The set of data
reported here can serve as a perfect illustration to the general principles of olefin metathesis
0
[14-17, 31, 34]
catalyzed by group 6–7 d metal alkylidenes found in DFT studies.
While there is no
essential difference in activity between Mo and W in metathesis of internal olefins (given the
identical ligand environment and rigorous purification of the substrate), Mo catalysts systematically
outperform their W analogs by ca. one order of magnitude in metathesis of terminal olefins, albeit at
the expense of selectivity. This differences are rationalized by the formation of stable unsubstituted
metallacyclobutanes in the presence of ethylene for W and presumably the easier reduction of Mo.
The activity of both Mo and W systems towards internal olefins increases with decreasing the
σ-donating ability of the X ligand, which favors TBP over SP. In the case of terminal olefins,
destabilization of unsubstituted metallacyclobutane intermediates by introducing σ-donating ligands
appears to be a more important factor for W: increasing σ-donating ability of the X ligand leads to
the improved overall catalytic performance. The trends identified in this work provide a benchmark
in comparing Mo and W and pave the way towards a more rational development of Mo and W-
based alkene metathesis catalysts.
This article is protected by copyright. All rights reserved.

Supplementary Material
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/MS-number.
Acknowledgements
The authors are grateful to the Scientific Equipment Program of ETH Zürich and the SNSF (R’Equip
grant 206021_150709/1) for financial support of the high-throughput catalyst-screening facility
(HTE@ETH). We thank Dr. Alexey Fedorov for assistance with high-throughput experimentation.
Author Contribution Statement
PZ and VM synthesized the catalysts and developed the olefin purification protocol and procedures
for testing the catalytic activity. JJS designed and performed the high-throughput experiments with
1
-nonene. The manuscript was written by CC, PZ and VM.
References
[
[
1]
2]
J. C. Mol, ‘Industrial Applications of Olefin Metathesis’, J. Mol. Catal. A: Chem. 2004, 213, 39-45.
R. R. Schrock, A. H. Hoveyda, ‘Molybdenum and Tungsten Imido Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts’,
Angew. Chem. Int. Ed. 2003, 42, 4592-4633.
[
[
3]
4]
C. Copéret, F. Allouche, K. W. Chan, M. P. Conley, M. F. Delley, A. Fedorov, I. B. Moroz, V. Mougel, M. Pucino, K. Searles, K.
Yamamoto, P. A. Zhizhko, ‘Bridging the Gap between Industrial and Well-Defined Supported Catalysts’, Angew. Chem. Int. Ed.
2017. DOI: 10.1002/anie.201702387.
C. Copéret, A. Comas-Vives, M. P. Conley, D. P. Estes, A. Fedorov, V. Mougel, H. Nagae, F. Núñez-Zarur, P. A. Zhizhko, ‘Surface
Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and
Activities’, Chem. Rev. 2016, 116, 323-421.
[
[
[
5]
6]
7]
C. Coperet, ‘Design and Understanding of Heterogeneous Alkene Metathesis Catalysts’, Dalton Trans. 2007, 5498-5504.
S. Lwin, I. E. Wachs, ‘Olefin Metathesis by Supported Metal Oxide Catalysts’, ACS Catal. 2014, 4, 2505-2520.
F. Blanc, A. Salameh, J. Thivolle-Cazat, J. M. Basset, C. Coperet, A. Sinha, R. R. Schrock, ‘Grafting Mechanism and Olefin
Metathesis Activity of Well-Defined Silica-Supported Mo Imido Alkyl Alkylidene Complexes’, C. R. Chim. 2008, 11, 137-146.
[
[
[
8]
F. Blanc, R. Berthoud, A. Salameh, J. M. Basset, C. Coperet, R. Singh, R. R. Schrock, ‘Dramatic Improvements of Well-Defined
Silica Supported Mo-Based Olefin Metathesis Catalysts by Tuning the N-Containing Ligands’, J. Am. Chem. Soc. 2007, 129, 8434-
8435.
9]
N. Rendon, R. Berthoud, F. Blanc, D. Gajan, T. Maishal, J. M. Basset, C. Coperet, A. Lesage, L. Emsley, S. C. Marinescu, R. Singh,
R. R. Schrock, ‘Well-Defined Silica-Supported Mo-Alkylidene Catalyst Precursors Containing One OR Substituent: Methods of
Preparation and Structure-Reactivity Relationship in Alkene Metathesis’, Chem. Eur. J. 2009, 15, 5083-5089.
10]
B. Rhers, E. A. Quadrelli, A. Baudouin, M. Taoufik, C. Coperet, F. Lefebvre, J. M. Basset, B. Fenet, A. Sinha, R. R. Schrock,
‘
Understanding the Reactivity of W=NAr(CH
Temperatures through a Combined Use of Molecular and Surface Organometallic Chemistry’, J. Organomet. Chem. 2006, 691,
448-5455.
2 2 6 3
tBu) (=CHtBu) (Ar=2,6-iPrC H ) with Silica Partially Dehydroxylated at Low
5
[
11]
F. Blanc, R. Berthoud, C. Coperet, A. Lesage, L. Emsley, R. Singh, T. Kreickmann, R. R. Schrock, ‘Direct Observation of Reaction
Intermediates for a Well Defined Heterogeneous Alkene Metathesis Catalyst’, Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 12123-
12127.
This article is protected by copyright. All rights reserved.

[
12]
N. Merle, F. Le Quéméner, Y. Bouhoute, K. C. Szeto, A. De Mallmann, S. Barman, M. K. Samantaray, L. Delevoye, R. M. Gauvin,
M. Taoufik, J. M. Basset, ‘Well Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic
Chemistry: A Highly Active Olefin Metathesis Pre-Catalyst’, J. Am. Chem. Soc. 2017, 139, 2144-2147.
[
[
[
13]
14]
15]
R. R. Schrock, J. S. Murdzek, G. C. Bazan, J. Robbins, M. DiMare, M. O'Regan, ‘Synthesis of Molybdenum Imido Alkylidene
Complexes and Some Reactions Involving Acyclic Olefins’, J. Am. Chem. Soc. 1990, 112, 3875-3886.
0
A. Poater, X. Solans-Monfort, E. Clot, C. Coperet, O. Eisenstein, ‘Understanding d -Olefin Metathesis Catalysts: Which Metal,
Which Ligands?’, J. Am. Chem. Soc. 2007, 129, 8207-8216.
X. Solans-Monfort, C. Coperet, O. Eisenstein, ‘Shutting Down Secondary Reaction Pathways: The Essential Role of the Pyrrolyl
0
Ligand in Improving Silica Supported d -ML
4
Alkene Metathesis Catalysts from DFT Calculations’, J. Am. Chem. Soc. 2010, 132,
7750-7757.
0
[
[
[
[
[
[
[
[
16]
17]
18]
19]
20]
21]
22]
23]
X. Solans-Monfort, C. Coperet, O. Eisenstein, ‘Oxo vs Imido Alkylidene d -Metal Species: How and Why Do They Differ in
Structure, Activity, and Efficiency in Alkene Metathesis?’, Organometallics 2012, 31, 6812-6822.
0
X. Solans-Monfort, C. Copéret, O. Eisenstein, ‘Metallacyclobutanes from Schrock-Type d Metal Alkylidene Catalysts: Structural
Preferences and Consequences in Alkene Metathesis’, Organometallics 2015, 34, 1668-1680.
C. P. Gordon, K. Yamamoto, W.-C. Liao, F. Allouche, R. A. Andersen, C. Copéret, C. Raynaud, O. Eisenstein, ‘Metathesis Activity
Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors’, ACS Cent. Sci. 2017, 3, 759-768.
R. R. Schrock, C. Copéret, ‘Formation of High-Oxidation-State Metal–Carbon Double Bonds’, Organometallics 2017, 36, 1884-
1892.
L. P. H. Lopez, R. R. Schrock, P. Muller, ‘Dimers that Contain Unbridged W(IV)/W(IV) Double Bonds’, Organometallics 2006, 25,
978-1986.
1
D. P. Estes, C. Bittner, Ò. Àrias, M. Casey, A. Fedorov, M. Tamm, C. Copéret, ‘Alkyne Metathesis with Silica-Supported and
Molecular Catalysts at Parts-per-Million Loadings’, Angew. Chem. Int. Ed. 2016, 55, 13960-13964.
0
V. Mougel, C. Copéret, ‘Magnitude and Consequences of OR Ligand σ-Donation on Alkene Metathesis Activity in d Silica
Supported (≡SiO)W(NAr)(=CHtBu)(OR) Catalysts’, Chem. Sci. 2014, 5, 2475-2481.
V. Mougel, C. B. Santiago, P. A. Zhizhko, E. N. Bess, J. Varga, G. Frater, M. S. Sigman, C. Coperet, ‘Quantitatively Analyzing
Metathesis Catalyst Activity and Structural Features in Silica-Supported Tungsten Imido-Alkylidene Complexes’, J. Am. Chem.
Soc. 2015, 137, 6699-7604.
[
[
24]
25]
BASF Selexsorb® is a doped alumina-based adsorbent widely used in industry for purification of olefin feeds from COS, H
CO , H O, NH , mercaptans, sulfides, disulfides, thiophenes, oxygenates (alcohols, aldehydes, ketones, etc.), amines, amides,
nitriles, organosilicates, and arsine.
2 2
S, CS ,
2
2
3
R. R. Schrock, R. T. Depue, J. Feldman, C. J. Schaverien, J. C. Dewan, A. H. Liu, ‘Preparation and Reactivity of Several Alkylidene
Complexes of the Type W(CHR')(N-2,6-C -i-Pr )(OR) and Related Tungstacyclobutane Complexes - Controlling Metathesis
6
H
3
2
2
Activity through the Choice of Alkoxide Ligand’, J. Am. Chem. Soc. 1988, 110, 1423-1435.
[
[
[
[
[
[
26]
27]
28]
29]
30]
31]
J. Feldman, W. M. Davis, J. K. Thomas, R. R. Schrock, ‘Preparation and Reactivity of Tungsten(VI) Metallacyclobutane Complexes
-
Square Pyramids Versus Trigonal Bipyramids’, Organometallics 1990, 9, 2535-2548.
M. P. Conley, W. P. Forrest, V. Mougel, C. Coperet, R. R. Schrock, ‘Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis
Catalyst’, Angew. Chem. Int. Ed. 2014, 53, 14221-14224.
V. Mougel, K.-W. Chan, G. Siddiqi, K. Kawakita, H. Nagae, H. Tsurugi, K. Mashima, O. Safonova, C. Copéret, ‘Low Temperature
Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents’, ACS Cent. Sci. 2016, 2, 569-576.
J. Robbins, G. C. Bazan, J. S. Murdzek, M. B. Oregan, R. R. Schrock, ‘Reduction of Molybdenum Imido Alkylidene Complexes in
the Presence of Olefins to Give Molybdenum(IV) Complexes’, Organometallics 1991, 10, 2902-2907.
S. C. Marinescu, A. J. King, R. R. Schrock, R. Singh, P. Muller, M. K. Takase, ‘Simple Molybdenum(IV) Olefin Complexes of the
Type Mo(NR)(X)(Y)(olefin)’, Organometallics 2010, 29, 6816-6828.
A. M. Leduc, A. Salameh, D. Soulivong, M. Chabanas, J. M. Basset, C. Coperet, X. Solans-Monfort, E. Clot, O. Eisenstein, V. P. W.
Bohm, M. Roper, ‘β-H Transfer from the Metallacyclobutane: A Key Step in the Deactivation and Byproduct Formation for the
Well-Defined Silica-Supported Rhenium Alkylidene Alkene Metathesis Catalyst’, J. Am. Chem. Soc. 2008, 130, 6288-6297.
[
32]
V. Mougel, M. Pucino, C. Copéret, ‘Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved
1-Alkene Metathesis Efficiency’, Organometallics 2015, 34, 551-554.
This article is protected by copyright. All rights reserved.

[
[
33]
34]
M. Pucino, V. Mougel, R. Schowner, A. Fedorov, M. R. Buchmeiser, C. Copéret, ‘Cationic Silica-Supported N-Heterocyclic
Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis’, Angew. Chem. Int. Ed. 2016,
55, 4300-4302.
0
X. Solans-Monfort, E. Clot, C. Coperet, O. Eisenstein, ‘d Re-Based Olefin Metathesis Catalysts, Re(=CR)(=CHR)(X)(Y): The Key
Role of X and Y Ligands for Efficient Active Sites’, J. Am. Chem. Soc. 2005, 127, 14015-14025.
This article is protected by copyright. All rights reserved.