Pyridine as trigger for chloride isomerisation in chelated ruthenium benzylidene complexes: Implications for olefin metathesis

Article abstract of DOI:10.1039/c0cc04897f

The cationic pyridine adduct of a ruthenium complex bearing a chelating benzylidene and an N-heterocyclic carbene was identified as an intermediate during the activation of cis dichloro species and a novel triggering concept for olefin metathesis catalyst

Full text of DOI:10.1039/c0cc04897f

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COMMUNICATION
Pyridine as trigger for chloride isomerisation in chelated ruthenium
benzylidene complexes: implications for olefin metathesisw
¨
Michaela Zirngast, Eva Pump, Anita Leitgeb, Jorg H. Albering and Christian Slugovc*
Received 10th November 2010, Accepted 5th January 2011
DOI: 10.1039/c0cc04897f
The cationic pyridine adduct of a ruthenium complex bearing a
chelating benzylidene and an N-heterocyclic carbene was identified
as an intermediate during the activation of cis dichloro species
and a novel triggering concept for olefin metathesis catalysts
based on cationic species was disclosed.
initiating6 or latent7 (i.e. slowly initiating) complexes can be
obtained upon properly designing this leaving co-ligand. For
the design of latent catalysts a different, less commonly observed
coordination geometry is particularly useful. In this case the base
of the square pyramidal complex structures is composed by the
NHC-ligand and the neutral co-ligand in cis configuration and
likewise two chloride ligands in cis configuration. The apex is
again formed by the carbene ligand.8,9 It is believed that
cis dichloro complexes are inactive in olefin metathesis but can
rearrange to their trans dichloro isomers which then are
active catalysts.9 Not much is known about the mechanism of
this preceding isomerisation reaction, which is assumed to be
accountable for the more pronounced latency of the corres-
ponding cis-dichloro compounds. Theoretical studies of Poater
et al. and Benitez and Goddard support this assumption and
suggest either a concerted mechanism or a mechanism based on
the hemilability of the chelating benzylidene. Both possibilities
are associated with considerably high activation energies.10
Herein we aim at unravelling mechanistic aspects of the
isomerisation reaction using novel cis dichloro complexes bearing
a chelating ester substituted benzylidene ligand. For the
assessment of the catalytic activity ring opening metathesis
polymerisation (ROMP)2,11 of a norbornene derivative was used.
The first use of N-heterocyclic carbenes (NHC) as co-ligands in
ruthenium-based carbene complexes1 was maybe the most
important breakthrough for using this kind of complexes as
catalysts for olefin metathesis reactions. Owing to the paramount
functional group tolerance and high activity of these catalysts
olefin metathesis in its various implementations has become a
powerful carbon–carbon double-bond-forming tool in organic,
pharmaceutical and macromolecular chemistry.2
Commonly used and commercially available catalysts
generally exhibit a square pyramidal coordination geometry with
the carbene ligand forming the apex. The base is formed by the
NHC-ligand3 and a neutral co-ligand in trans configuration, and
two chloride ligands in trans configuration (cf. Fig. 1).4 For the
initiation of the catalytic cycle the dissociation of the neutral
co-ligand trans to the NHC ligand is required.5 Hence fast
Fig. 1 Coordination geometries of typical and exemplary commercial
ruthenium based olefin metathesis catalysts [for Caz1 see ref. 9i].
Graz University of Technology, Institute for Chemistry and
Technology of Materials, Stremayrgasse 9, A 8010 Graz, Austria.
E-mail: slugovc@tugraz.at
w Electronic supplementary information (ESI) available: Experimental
part including complete characterization of 2, 3, 4 and 6 and description
of all experiments. CCDC 799955 (2) and 799945 (3). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c0cc04897f
Scheme 1 Preparation of compounds 2–6. (a) CH2Cl2, 3 eq. pyridine,
18 h, 20 1C, 29% 2, 37% 3; (b) 25 eq. pyridine, CH2Cl2, 1 h, 20 1C,
25%; (c) 1.1 eq. triethyl phosphite, CH2Cl2, 1 h, 20 1C, 78%; (d) 1.1 eq.
NH4PF6, MeOH, 0.5 h, 20 1C, 65%.
c
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Chem. Commun., 2011, 47, 2261–2263 2261

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A mixture of the commercially available complex M316c and
1.2 eq. iso-propyl 5-methoxy-2-vinylbenzoate (1) in CH2Cl2
was stirred at room temperature for 24 h (see Scheme 1).
Evaluation of the 1H NMR of the crude reaction mixture
suggested the presence of two benzylidene complexes in a 4 : 1
ratio. Column chromatography allowed for their separation
and compounds were identified as the cis dichloro complex (2)
Table 1 Results of the polymerisation of 5 initiated by 2–4a
Initiator Solvent Reaction time/h Yield [%] Mndg molÀ1 PDId
b
CH2Cl2 0.25
M31
97
90
93
89
94
96
91
54 600
482 000
315 000
133 000
147 000
126 000
95 400
1.07
2.7
1.5
1.4
1.6
1.8
1.8
b
2
3
4
2
3
4
CH2Cl2b
CH2Cl2b
CH2Cl2
4
1
7
1
1
1
Toluenec
Toluenec
Toluenec
1
and the cationic pyridine adduct (3) as established by H and
13C NMR analysis as well as single crystal X-ray diffraction
measurements. The ratio 2 : 3 can be shifted in favour of 3 by
adding additional 3 eq. of pyridine to the reaction mixture. In
this way 2 and 3 can be isolated in pure form in 29% and 37%
yield. Furthermore, 3 can be prepared upon addition of excess
pyridine to 2 in 25% yield. With the latter reaction it is proven
that 3 can be accessed directly from 2 and is not (only) a
competing product during the preparation of 2. The neutral
complex 2 is characterised by a chemical shift of the carbene
proton of 18.90 ppm (in CDCl3 as the solvent) whereas
the cationic compound 3 is characterized by a carbene
proton resonance at 17.92 ppm. In this case, a coordinated
pyridine can be observed, which is characterized by a doublet
at 8.51 ppm, a triplet at 7.77 ppm and a doublet of doublet at
7.33 ppm. While in the case of 2, the aromatic protons of the
mesityl groups are split into 4 distinct signals indicating
impeded rotation of the NHC ligand, the rotation of the
NHC ligand in 3 takes place under the same conditions and
the aromatic protons of the mesityl groups give rise to
two signals. Because the ruthenium centre in cis dichloro
complexes is chiral, the two mesityl groups and the two methyl
groups of the iso-propoxy substituent are diastereotopic in
both compounds providing an additional decision guidance
for the assessment whether cis or trans dichloro species are
present in solution. Finally, structural proof of 2 and 3 was
obtained from single crystal X-ray diffraction measurements
(cf. Fig. 2 and ESIw). Similarly, a cationic triethyl phosphite
adduct (4) was synthesized using 2 as the starting material
(cf. ESIw).
a
Reaction conditions: [5] : [1]
c
=
300 : 1; [5] = .
0.1 mol LÀ1
b
d
40 1C. 80 1C. Determined by GPC, relative to polystyrene
standards in THF.
Fig. 3 Comparison of the polymerisation progress of 5 initiated by
2–4. Reaction conditions: [5] : [I] = 70 : 1, CDCl3, 21.5 1C.
polymers of 5 are not degraded by the initiators.12 Thus,
determination of the molecular weight (e.g. the number
average molecular weight (Mn)) will allow for an indirect,
qualitative comparison of the ratios of initiation rate to
propagation rate constants (ki/kp) for the initiators under
investigation.
Initiator M31 was used for comparison because M31
provides fast and complete initiation and thus every initiator
starts an individual polymer chain (ki/kp c 10). Polymers with
a narrow polymer weight distribution and effective molecular
weights close to the calculated ones are obtained. In this case,
the overall polymerisation rate (rp) is, in a first approximation,
uninfluenced by ki. In contrast, compounds 2–4 exhibit ki/kp
values distinctly less than 1 as expressed by considerably high
Mn values of the corresponding polymers (see Table 1).
These results are characteristic for initiators with a cis-
dichloro structure.7–9 The ki/kp values increase (as revealed
by the corresponding Mn values) in the order of ki/kp(2) o ki/
kp(3) o ki/kp(4). Since it cannot be assumed that kp is equal
for 2, 3 and 4, because the presence of pyridine or triethyl
phosphite will change (i.e. decrease) kp in the case of 3 and
4,12b it is necessary to study the kinetics of the polymerisations.
As can be seen in Fig. 3, the cationic pyridine adduct 3 is the
Separation and pure isolation of the complexes rendered a
comparative reaction study possible. Accordingly, 2, 3 and 4
were used as initiators in the ROMP of dimethyl bicyclo-
[2.2.1]hept-5-ene-2,3-dicarboxylate (5). 5 was selected because
1
2
1
2
most active initiator (t = 130 min) followed by 2 (t = 275 min)
Fig. 2 Diamond plot of 3 (thermal ellipsoids at 50% probability
level; chloride and co-crystallized CHCl3 omitted for clarity); selected
bond lengths (A) and angles (1): Ru–C(22) = 1.827(3), Ru–C(1) =
2.034(3), Ru–O(1) = 2.074(2), Ru–N(3) = 2.092(2), Ru–Cl(1) =
2.376(1), C(1)–Ru–C(22) = 98.03(11), O(1)–Ru–Cl(1) = 85.63(6),
and the triethyl phosphite adduct 4 being the least active
1
initiator (t = 900 min). The low rp of 4 can be explained by
2
the strong binding of phosphites to such types of complexes9i
and consequently both, ki and kp, are decreased in a way
that ki/kp is increased. However, the difference in reactivity
between 2 and 3 is striking and deserves further attention.
C(22)–Ru–O(1)
=
90.57(10), C(22)–Ru–Cl(1)
=
104.82(8),
C(22)–Ru–N(3) = 94.98(11).
c
2262 Chem. Commun., 2011, 47, 2261–2263
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is indispensable for the activity of the initiator. Changing the
counterion for hexafluorophosphate produced an almost
inactive initiator under tested reaction conditions. This
inactive species can be activated upon addition of
a
chloride source. Thus a novel concept for triggering latent
initiators/catalysts has been disclosed.
Financial support of this work by the European Community
(CP-FP 211468-2 EUMET) is gratefully acknowledged.
Notes and references
Scheme 2 Mechanisms for the formation of the proposed catalytically
active forms of 2.
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W. A. Herrmann, Angew. Chem., Int. Ed., 1998, 37, 2490;
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C. W. Lee and R. H. Grubbs, Org. Lett., 1999, 1, 953.
2 Handbook of Metathesis, ed. R. H. Grubbs, Wiley-VCH,
Weinheim, 2003.
3 C. Samoj"owicz, M. Bieniek and K. Grela, Chem. Rev., 2009, 109,
3708.
4 G. C. Vougioukalakis and R. H. Grubbs, Chem. Rev., 2010, 110,
1746.
For the explanation two possible mechanistic scenarios were
considered: (i) 3 is an intermediate in the conversion of 2 to its
corresponding trans-dichloro complex (7, Path A) or (ii) the
polymerisation proceeds via a cationic species upon liberation
of pyridine (8, Path B) (see Scheme 2). In order to differentiate
between the two possibilities, the chloride counterion in 3 was
exchanged for PF6À (complex 6, see Scheme 1). In case 8 being
the putative key-intermediate, the back reaction to 2 should be
blocked when using 6 instead of 3. Moreover, 6 should be a
more active initiator than 3.
5 (a) M. S. Sanford, J. A. Love and R. H. Grubbs, J. Am. Chem.
Soc., 2001, 123, 6543; (b) T. Vorfalt, K.-W. Wannowius and
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1743; (b) C. Slugovc, S. Riegler, G. Hayn, R. Saf and F. Stelzer,
Macromol. Rapid Commun., 2003, 24, 435; (c) D. Burtscher,
C. Lexer, K. Mereiter, R. Winde, R. Karch and C. Slugovc,
J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 4630.
7 (a) S. Monsaert, A. Lozano Vila, R. Drozdzak, P. Van Der Voort
and F. Verpoort, Chem. Soc. Rev., 2009, 38, 3360;
(b) A. Szadkowska, X. Gstrein, D. Burtscher, K. Jarzembska,
K. Wozniak, C. Slugovc and K. Grela, Organometallics, 2010,
29, 117.
Accordingly, 6 was tested in the polymerisation of 5 under
the same reaction conditions as quoted in Table 1. After a
reaction time of 19 h at 40 1C only a minimal conversion of
10% could be found. This result clearly discards Path B and
Path A occurs, i.e. 3 is an intermediate on the way to the active
trans-dichloro species 7. Further evidence for Path A could be
retrieved as follows: upon addition of 4.5 eq. benzyltriethyl-
ammonium chloride to the above mentioned polymerisation
mixture of 6 and 5, the conversion reached completeness after
19 h at 40 1C (yield: 86%, Mn = 93 000 g molÀ1, PDI = 1.9).
Monitoring this reaction under different reaction conditions
8 C. Slugovc, B. Perner, F. Stelzer and K. Mereiter, Organometallics,
2004, 23, 3622.
9 (a) T. Ung, A. Hejl and R. H. Grubbs, Organometallics, 2004, 23,
5339; (b) M. Barbasiewicz, A. Szadkowska, R. Bujok and
K. Grela, Organometallics, 2006, 25, 3599; (c) X. Gstrein,
D. Burtscher, A. Szadkowska, M. Barbasiewicz, F. Stelzer,
K. Grela and C. Slugovc, J. Polym. Sci., Part A: Polym. Chem.,
2007, 45, 3494; (d) A. Ben-Asuly, E. Tzur, C. E. Diesendruck,
M. Sigalov, I. Goldberg and N. G. Lemcoff, Organometallics,
2008, 27, 811; (e) C. E. Diesendruck, Y. Vidavsky, A. Ben-Asuly
and N. G. Lemcoff, J. Polym. Sci., Part A: Polym. Chem., 2009, 47,
4209; (f) A. Ben-Asuly, A. Aharoni, C. E. Diesendruck,
Y. Vidavsky, I. Goldberg and N. G. Lemcoff, Organometallics,
2009, 28, 4652; (g) C. E. Diesendruck, E. Tzur, A. Ben-Asuly,
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48, 10819; (h) E. Tzur, A. Szadkowska, A. Ben-Asuly, A. Makal,
1
([5] : [6] = 5 : 1, CDCl3, 21.5 1C) via H NMR spectroscopy
revealed about 20% conversion of 5 after 70 h and upon
addition of 3 eq. benzyltriethylammonium chloride completion
of the polymerisation after further 26 h. Evaluation of the
carbene region of the corresponding 1H NMR spectra showed
neither the appearance of an additional carbene peak in the
first 70 h nor any decomposition of 6, but after having
added benzyltriethylammonium chloride, a second singlet at
18.71 ppm emerged. This signal is tentatively assigned to the
trans-dichloro species 7 on the basis of related complexes
(see ESIw).13
I. Goldberg, K. Woz´ niak, K. Grela and N. G. Lemcoff,
Chem.–Eur. J., 2010, 16, 8726; (i) X. Bantreil, T. E. Schmid,
R. A. M. Randall, A. M. Z. Slawin and C. S. J. Cazin, Chem.
Commun., 2010, 46, 7115.
In conclusion, a cationic intermediate in the rearrangement
of cis dichloro ruthenium benzylidenes to their catalytically
active trans dichloro counterparts has been identified. Pyridines,
and presumably donor ligands in general, facilitate the
dissociation of the chloride ligand trans to the s-donor ligand
of the chelating benzylidene ligand. This observation is
consistent with reports on the lability of halide ligands in
ruthenium–benzylidene complexes,14 but this is the first report
on a cationic complex of this type. The results suggest that
cationic species must not be neglected as intermediates when
considering the overall isomerisation process.10
10 (a) A. Poater, F. Ragone, A. Correa, A. Szadkowska,
M. Barbasiewicz, K. Grela and L. Cavallo, Chem.–Eur. J., 2010,
16, 14354; (b) D. Benitez and W. A. Goddard III, J. Am. Chem.
Soc., 2005, 127, 12218.
11 A. Leitgeb, J. Wappel and C. Slugovc, Polymer, 2010, 51, 2927.
12 (a) S. Riegler, S. Demel, G. Trimmel, C. Slugovc and F. Stelzer,
J. Mol. Catal. A, 2006, 257, 53; (b) C. Slugovc, S. Demel,
S. Riegler, J. Hobisch and F. Stelzer, J. Mol. Catal. A, 2004,
213, 107.
13 A. Furstner, O. R. Thiel and C. W. Lehmann, Organometallics,
2002, 21, 331.
14 (a) K. Tanaka, V. P. W. Bohm, D. Chadwick, M. Roeper and
¨
D. C. Braddock, Organometallics, 2006, 25, 5696; (b) J. Wappel,
C. A. Urbina-Blanco, M. Abbas, J. H. Albering, R. Saf,
S. P. Nolan and C. Slugovc, Beilstein J. Org. Chem., 2010, 6,
1091.
Most striking is the fact that the chloride counterion
(or more general, a counterion which can coordinate to ruthenium)
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2261–2263 2263