Improved metathesis lifetime: Chelating pyridinyl-alcoholato ligands in the second generation grubbs precatalyst

Article abstract of DOI:10.3390/molecules19055522

Hemilabile ligands can release a free coordination site "on demand" of an incoming nucleophilic substrate while occupying it otherwise. This is believed to increase the thermal stability and activity of catalytic systems and therefore prevent decomposition via free coordination sites. In this investigation chelating pyridinyl-alcoholato ligands were identified as possible hemilabile ligands for incorporation into the second generation Grubbs precatalyst. The O,N-alcoholato ligands with different steric bulk could be successfully incorporated into the precatalysts. The incorporation of the sterically hindered, hemilabile O,N-ligands improved the thermal stability, activity, selectivity and lifetime of these complexes towards the metathesis of 1-octene. A decrease in the activity of the second generation Grubbs precatalyst was additionally observed after incorporating a hemilabile O,N-ligand with two phenyl groups into the system, while increasing their lifetime.

Full text of DOI:10.3390/molecules19055522

Molecules 2014, 19, 5522-5537; doi:10.3390/molecules19055522
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Improved Metathesis Lifetime: Chelating Pyridinyl-Alcoholato
Ligands in the Second Generation Grubbs Precatalyst
Jean I. du Toit, Margaritha Jordaan, Carlijn A. A. Huijsmans, Johannes H. L. Jordaan,
Cornelia G. C. E. van Sittert and Hermanus C. M. Vosloo *
Research Focus Area for Chemical Resource Beneficiation, Catalysis and Synthesis Research Group,
North-West University, Potchefstroom 2520, South Africa; E-Mails: 12317624@nwu.ac.za (J.I.T.);
rita.jordaan@telkomsa.net (M.J.); 10063552@nwu.ac.za (C.A.A.H.);
johan.jordaan@nwu.ac.za (J.H.L.J.); cornie.vansittert@nwu.ac.za (C.G.C.E.S.)
*
Author to whom correspondence should be addressed; E-Mail: manie.vosloo@nwu.ac.za;
Tel.: +27-18-299-1669; Fax: +27-18-299-1667.
Received: 16 March 2014; in revised form: 22 April 2014 / Accepted: 24 April 2014 /
Published: 29 April 2014
Abstract: Hemilabile ligands can release a free coordination site “on demand” of an
incoming nucleophilic substrate while occupying it otherwise. This is believed to increase
the thermal stability and activity of catalytic systems and therefore prevent decomposition
via free coordination sites. In this investigation chelating pyridinyl-alcoholato ligands were
identified as possible hemilabile ligands for incorporation into the second generation
Grubbs precatalyst. The O,N-alcoholato ligands with different steric bulk could be
successfully incorporated into the precatalysts. The incorporation of the sterically hindered,
hemilabile O,N-ligands improved the thermal stability, activity, selectivity and lifetime of
these complexes towards the metathesis of 1-octene. A decrease in the activity of the
second generation Grubbs precatalyst was additionally observed after incorporating a
hemilabile O,N-ligand with two phenyl groups into the system, while increasing their lifetime.
Keywords: second generation grubbs precatalyst; pyridinyl-alcoholato ligand;
1
-octene metathesis; lifetime

Molecules 2014, 19
. Introduction
In the last decade the development of alkene metathesis catalysts has made significant progress with
5523
1
the design of catalytic systems to improve one or all of the following aspects: (i) catalyst efficiency
and activity, (ii) substrate scope and selectivity and (iii) the removal of metal impurities and catalyst
recycling.[1] The ruthenium carbene complexes developed by the Grubbs group, 1 and 2 (Figure 1),
remain of great importance due to their high activity and tolerance towards polar functional groups [2,3].
Figure 1. Grubbs-type precatalysts.
The development of new classes of ligands bearing donor atoms other than phosphorus was inspired
by the fact that phosphine ancillary ligands undergo degradation (e.g., P-C bond cleavage, oxidation to
phosphine oxides, etc.) during catalysis, especially under thermal conditions, which limits the lifetime
of 1 [4–6]. The incorporation of an N-heterocyclic carbene (NHC) into 1, provided precatalyst 2 with
improved activity and lifetime, despite its low selectivity at elevated temperatures. The limited thermal
stability of 1 and 2 is believed to be due to the fact that PCy is very labile at room temperature
3
resulting in fast decomposition of the precatalyst. This has motivated several groups to design new
2
thermally switchable initiators (Figure 2) with chelating ligands, where L is either attached to the
carbene (motif B, Hoveyda-type precatalysts [7–10]) or via X (motif C, [11–13] where X is for
example an oxygen) to the central ruthenium atom.

Molecules 2014, 19
Figure 2. Design concepts for thermally switchable initiators [7–13].
5524
For motif C Grubbs [12] and later Verpoort [14,15] introduced bidentate O,N-chelated Schiff base
ligands on 1 and 2 to give 3a and 3b (Figure 1), respectively. The combination of a Schiff-base ligand
with an NHC-ligand to produce 3b, increased the thermal stability and activity of these complexes
towards ROMP and RCM [11,13,14]. Another example for motif C is the first and second generation
ruthenium benzylidene complexes synthesized by Herrmann and coworkers [11] bearing a hemilabile
pyridinyl alcoholato ligand (4) (Figure 1).
The NHC catalytic systems showed low activity for ROMP at room temperature due to their resting
state stabilization [11]. The catalytic activity of these systems increased with an increase in
temperature, which was comparable to 2. The first generation system 4a has been shown to be active
for 1-octene metathesis at 60 °C, with a 1-octene/Ru molar ratio of 9000 in a neat solution [22]. Van
der Schaaf et al. [16] have patented a number of symmetrical and unsymmetrical O,N-chelated
ruthenium alkylidene systems with a tri-isopropylphosphine ligand with no reported catalytic activity.
O,N-chelated pyridinyl alcoholato second generation ruthenium alkylidene precatalysts 6 and 10 [17–19],
showing an extraordinary stickiness to silica, were reported for the cross-metathesis reaction of
5
-decene with 5-hexenyl acetate [19]. The use of these complexes on a TLC plate for catalyst
screening were also reported [18]. Slugovc and Wappel [20] have patented a number of O,N-chelated
-quinolinolate ruthenium alkylidene systems (12). We have reported the O,N-chelated pyridinyl
8
alcoholato second generation ruthenium alkylidene precatalyst 5 which was shown to be active
towards the metathesis of 1-octene at higher temperatures in a neat solution at low Ru loadings [22].
The diphenyl substituted precatalyst 6 also showed very good catalytic lifetime in separation studies [21].
Here we report on the synthesis and characterization of a number of new pyridinyl alcoholato second
generation ruthenium alkylidene complexes (7–9). The activity and lifetime in the metathesis reaction
with 1-octene of these precatalysts were compared with that of known complexes (2,5,6,10–12)
(
Figure 1). Apart from complex 12, all the complexes showed high activity, temperature stability and a
long lifetime towards the self-metathesis of 1-octene, relative to 2.
2
. Results and Discussion
2.1. Synthesis and NMR Characterization of Second Generation O,N-Chelated Ruthenium
Carbene Complexes
The procedure followed for the synthesis of precatalysts 5–12, consisted of stirring 2 with the
lithium salt of the corresponding alcohols at 35–40 °C for 2 days in THF [22].

Molecules 2014, 19
5525
As previously reported for 5 [22], the high solubility of LiCl in THF, which drove the reaction, may
have caused the reactions to take longer than that reported by Denk et al. [11] for similar complexes.
The complexes were obtained in moderate yields as microcrystalline powders, but no suitable
crystals for X-ray crystallographic analysis could be obtained through the slow diffusion of pentane
into a saturated solution of the complexes in THF. For 7, 8 and 9, it was necessary to repeat the
washing procedure twice to obtain a pure product. These complexes were obtained in lower yields
(
7 = 16%, 8 = 27%, 9 = 49%) which are probably a result of the intensive purification procedure used.
It was however noted that 12 was soluble in most of the solvents used for recrystallization, and was
therefore obtained in low yield as an orange powder through slow vacuum condensation of pentane
after sonificating the solution for 10 min.
1
The H-NMR chemical shifts of the carbene α-H signal (denoted H* in the structures) and pyridine
†
α-H signal (denoted as H in the structures) for precatalysts 5–12 are summarized in Table 1.
1
Table 1. Selected H-NMR signals of the second generation hemilabile complexes as
compared to the free O,N-ligands and 2.
†
a
† b
Catalyst
δ
H*
δ
H
δ
H
2
5
6
7
8
9
19.19, s
17.96, s
17.18, s
17.32, s
17.31, s
17.33, s
17.82, s
18.52, s
18.25, s
-
-
9.55, d
9.67, d
9.75, d
9.87, d
9.39, d
9.15, d
9.65, d
7.75, d
8.48, d
8.50, d
8.59, d
8.54, d
8.44, d
8.49, d
8.52, d
8.77, d
1
1
1
0
1
2
a
†
carbene α-H signal (denoted H*) and pyridine α-H signal (denoted as H ) of the hemilabile
b
†
complexes; pyridine α-H signal (denoted as H ) of the pyridinyl carbinol ligands.
Similar to the first generation hemilabile complexes [22], a strong upfield shift is observed in the H
α
signals of the second generation hemilabile complexes. Relative to 2, the H signals also appeared as
α
3
1
singlets. As expected no P-NMR resonances were observed, since no phosphorous group should be
coordinated to the Ru-centre after substitution with the O,N-chelated ligands. Additionally a downfield
shift in the pyridine α-H signal was observed for complexes 5–12, indicating that the electronic
environment of the ligand has changed. This implies that the N-atom has coordinated to the Ru-centre,
since a downfield shift of a proton resonance signal is indicative of reduced electron density of the
attached C-atom [23].
2.2. Metathesis of 1-Octene with Different Precatalysts
During the metathesis reaction of 1-octene a mixture of products can form (Table 2). This mixture
is obtained due to alkene isomerisation and the continued formation of new metathesis products via
self- and cross-metathesis [24].

Molecules 2014, 19
5526
Table 2. Possible reactions of 1-octene in the presence of metathesis catalysts [24].
a
a
Reaction
Substrate
Products
Primary metathesis
b
d
-
-
Self-metathesis
Isomerisation
C=C
C=C
7
C=C + C
=C + C =C
7
=C
7
(PMP)
c
7
C
2
6
3
5
+ C
4
=C
4
(IP)
Secondary metathesis
-
-
Cross-metathesis
C=C + C =C
C =C + C=C + C=C + C =C
(SMP)
7
2
6
2
7
6
2
6
7
Self-metathesis
a
C
2
=C
6
C
2
=C
2
+ C
6
=C
b
6
Hydrogens are omitted and geometrical isomers not shown for simplicity; Primary metathesis products
c
(
PMP) refers to the self-metathesis products of 1-octene, i.e., C =C and C=C; Isomerisation products (IP)
7
7
d
refers to the double bond isomerisation reaction of terminal to internal alkenes; Secondary metathesis
products (SMP) refers to the metathesis of the isomerisation products of 1-octene.
The optimum working temperature whereby 5 gives high activity while retaining a high degree of
selectivity towards PMP formation with a limited amount of SMP and IP formation was reported to be
6
1
0 °C [22]. It was therefore decided to investigate the catalytic activity of 6–12 at 60 °C, with
-octene/Ru molar ratio = 9000 in the absence of any solvent.
In Table 3 the 1-octene metathesis activity and selectivity of the second generation hemilabile
precatalysts are compared to 2 after 420 min and in Table 4 after 10 h up to 1.5 days. For the
calculation of the percentage of 1-octene, PMP, IP or SMP the following formula was used:
%
n
x
= (n
-octene, PMP, IP or SMP and ntot = sum of mol of 1-octene, PMP, IP and SMP. The turnover number
TON) in this study was calculated using the formula (%PMP × (oct/Ru))/100 with oct/Ru the
x x x
/ntot) × 100. Where %n = mol percentage of 1-octene, PMP, IP or SMP; n = mol of
1
(
−1
1
-octene/Ru molar ratio at t = 0 and the turnover frequency (TOF) as TON h .
Table 3. Catalytic activity and selectivity of the precatalysts at 60 °C (1-octene/Ru = 9000,
no solvent) after 420 min.
a
b
c
d
Precatalyst %PMP
%SMP
3.7
%IP
0
%S
TON
7254
7236
3204
1800
7254
4320
6660
7785
TOF
1036
1034
458
257
1036
617
951
1,112
5
2
5
6
7
8
9
80.6
80.4
35.6
20
80.6
48
96.3
85.5
99.1
95.3
80
92.7
78.2
86.1
99.8
13
0.9
3.7
18.6
6.8
21.8
12.4
0
0
0
0.9
0.1
0.5
0
1
1
1
0
1
2
74
86.5
0.4
0
0.2
36
d
a
b
c
C=C + C
7
=C
7
; C
2
=C
7
+ C=C
6
+ C=C
2
+ C
6
=C
7
; C
2
=C
6
+ C
3
=C
5
+ C
4
=C
4
; Selectivity towards PMP.
The lifetime and activity of the second generation precatalysts have been improved regardless of the
slow initiation rate of these systems compared to 2 at 60 °C. However, an approximate 10%–20%
decrease in the selectivity of 5, 8, 10 and 11 towards the formation of PMPs was evident after 420 min
compared to 2, while the selectivity of 6 and 12 slightly increased, and 7 and 9 showed similar
selectivity. After 20 h (Table 4) all the precatalysts still showed activity towards the metathesis of
1
-octene, i.e., between 80%–90% PMP for 5, 8, 9 and 10, and 86.5% and 94% PMP for 11 and 7

Molecules 2014, 19
5527
respectively, while 2 (82% PMP) is inactive after ca. 7 h. Therefore a 4%–10% increase in PMP
formation for 5 and 10 was observed after 20 h compared to a 35%, 40% and 74% increase for 6, 9 and
7
respectively, indicating the high stability and activity of these complexes. The SMP formation for all
the precatalysts remained almost unchanged with only a 1%–5% increase leading to a ca. 2% decrease
in the selectivity of 5, 6 and 10, and a 4% decrease for 9 towards the formation of PMPs. Furthermore,
a 5%–10% increase in TON of 5 and 10 was observed after 20 h. However, an 84%, 97% and 370%
increase in cumulative TON of 9, 6 and 7 respectively, was observed after 20 h. Although the
observable IPs for all the complexes remained below 0.2%, an increase to a maximum within the first
5
–10 min was followed by a sharp decrease to ca. 0% within 10–30 min as the reaction proceeded,
most probably due to cross-metathesis of the IPs to form SMPs. The major SMP product was found to
be tridecene (C ) for all the systems, which is mainly as a result of the cross-metathesis of 1- and
1
3
2
-octene. However, 12 showed very low activity towards the formation of PMPs, only achieving 0.4%
PMP formation and were not investigated further. The formation of PMPs during the metathesis
reactions after 420 min is shown in Figure 3.
Table 4. Catalytic activity and selectivity of the precatalysts at 60 °C (1-octene/Ru = 9000,
no solvent) after 10 h up to 1.5 days.
a
b
c
d
e
Precatalyst %PMP
%SMP
3.6
%IP
%S
t (min)
1200
1246
1200
2098
630
1121
1246
1246
1200
TON
7344
7902
6327
8460
7200
7938
7407
7785
36
TOF
367
381
316
242
686
425
357
375
2
2
5
6
7
8
9
81.6
87.8
70.3
94
0
0
0
0
0
0
0
0
0.2
96.4
84.4
97.2
94.5
80.1
88.5
75.8
86.1
99.8
12.2
2.8
5.5
19.9
11.5
17
12.4
0
80
88.2
82.3
86.5
1
1
1
0
1
2
0.4
a
b
c
d
C=C + C
7
=C
7
; C
2
=C
7
+ C=C
6
+ C=C
2
+ C
6
=C
7
; C
2
=C
6
+ C
3
=C
5
+ C
4
4
=C ; Selectivity towards
e
PMP; Reaction time to maximum conversion.
Figure 3. The formation of PMPs during the metathesis of 1-octene in the presence of 2,
–11 at 60 °C and a 1-octene/complex molar ratio of 9000.
5

Molecules 2014, 19
.3. Stability of Different Precatalysts
To obtain an idea of the stability of the different precatalysts the definition and approach of Grubbs
5528
2
and coworkers [25] were used. The stability of the Ru precatalysts is defined as the lifetime of the
catalytic species during the course of the reaction and can be monitored by the loss of activity. They
used plots of ln([starting material]) versus time as an illustration of the relative stabilities of Ru
precatalysts. A linear plot indicates a reaction with pseudo-first-order rate kinetics while a curved plot
points towards catalyst decomposition. The relative stability of the precatalysts is compared in
Figure 4 by plotting the ln(n% substrate) versus time.
Figure 4. Logarithmic plots for the precatalysts at 60 °C and a 1-octene/Ru molar ratio = 9000.
The linear plots of 5, 6, 7, 8 and 9 compared to the curved plots of 2, 10 and 11, indicates that the
aforementioned systems are more stable [25]. Precatalyst 2 was not active upon further addition of
1-octene after 3 h while 6 showed reduced activity after 3 further additions of 1-octene [21]. However,
despite the high thermal stability of 5 and 8 low selectivities are displayed towards PMP formation. On
the other hand, high selectivity and stability is observed for 6, 7 and 9 despite its slow activity.
2
.4. Optimization of Reaction Conditions for 7, 8 and 9
Optimization reactions of the newly synthesized precatalysts 7, 8 and 9, in comparison with 6, were
performed by changing the 1-octene/Ru molar ratio, as well as the reaction temperature. When the
complexes are used at different molar ratios at 60 °C the same overall trends are observed for the
reaction time to maximum conversion and TON (Table 5), i.e., it increases with an increase in
1
-octene/Ru molar ratio. There is however a difference in the trends observed for PMP and SMP
formation with an increase in molar ratio. Precatalyst 6 shows an increase in both PMP and SMP.
However, 7 and 8 show an increase in PMP formation (decrease in SMP) while 9 shows a decrease in
PMP (increase in SMP).

Molecules 2014, 19
5529
Table 5. Catalytic activity and selectivity of 7, 8 and 9 at 60 °C and different 1-octene/Ru
molar ratios.
a
b
c
Precatalyst 1-octene/Ru %PMP
%SMP
1.1
t (min)
420
TON
2165
6327
6058
8460
3465
7200
8731
10,560
7938
9720
11,466
TOF
309
316
194
242
664
726
1,089
653
377
469
393
6
7
8
7000
000
6500
000
4500
000
30.9
70.3
93.2
94.0
77.0
80.0
81.6
88.0
88.2
81.0
81.9
9
2.8
1200
1875
2098
313
6.7
9
5.5
20.6
19.9
17.1
11.1
11.5
18.1
9
595
1
1
0,700
2,000
481
970
9
9000
1265
1244
1752
1
2,000
4,000
1
18.0
a
b
c
C=C + C
7
=C
7
; C
2
=C
7
+ C=C
6
+ C=C
2
+ C
6
=C
7
; Reaction time to maximum conversion.
When the precatalysts are used at different reaction temperatures and the same 1-octene/Ru molar
ratio = 9000 the same overall trends are observed (Table 6). For all the precatalysts an increase in
reaction temperature leads to a faster conversion (Figure 5), a decrease in PMP formation and TON,
while an increase in SMPs and TOF is observed. The initial increase of PMPs follows a linear trend
with increasing slopes as the temperature increases which is similar for all the precatalysts except for 9
at 40 °C. For 9 a slow initiation of about 1400 min is observed at 40 °C (Figure 5).
Table 6. Catalytic activity and selectivity of 6, 7, 8 and 9 at different temperatures and an
1
-octene/Ru molar ratio = 9,000.
a
b
c
Precatalyst
T (°C)
%PMP
-
%SMP
-
t (min)
420
TON
-
TOF
-
6
40
6
8
0
0
70.3
74.0
10.7
95.4
94.0
64.9
85.2
80.0
63.1
90.2
88.2
64.4
2.8
1200
420
6327
6661
963
316
952
46
11.5
77.8
3.6
1
20
1256
3224
2098
203
7
8
9
40
8586
8460
5841
7668
7200
5679
8118
7938
5796
160
242
1726
187
726
1055
202
377
354
6
0
0
5.5
8
34.4
10.5
19.9
35.6
9.4
40
2463
595
6
8
0
0
323
40
2410
1265
981
6
8
0
0
11.5
35.4
a
b
c
7 7 2 7 6 2 6 7
C=C + C =C ; C =C + C=C + C=C + C =C ; Reaction time to maximum conversion.

Molecules 2014, 19
5530
Figure 5. The formation of PMPs during 1-octene metathesis in the presence of 6, 7, 8 and
at 40 °C and 80 °C and a 1-octene/Ru molar ratio = 9000.
9
An increase in temperature results in a 20% average decrease in PMP formation, with a
simultaneous 20% average increase in SMP formation within 60 min for all four precatalysts. This
indicates that the isomerisation activity of the precatalysts increased with an increase in temperature,
which led to an increase in SMP formation due to the cross-metathesis of the IPs. The isomerisation
can probably occur either by reversible allyl-hydride formation or due to the decomposition of the
precatalyst to form a Ru-hydride intermediate [26]. These mechanisms have been previously explained
in much detail for the Grubbs carbenes and are also applicable to our system [26].
2
.5. Catalytic Lifetime Studies of Precatalysts 7, 8 and 9
To obtain an idea of the extent of the catalytic lifetime of 7, 8 and 9 1-octene (20 mL) was added
consecutively when most of the 1-octene was converted in the previous step. Table 7 shows the time it
takes for the precatalysts to convert the maximum amount of 1-octene to PMPs and SMPs.
In general it was found that the activity, selectivity and TON decreases after each new addition of
1
-octene. With each addition the reaction time to maximum conversion increases. This may mainly be
due to the 1-octene/Ru molar ratio increasing, i.e., the precatalyst loading decreasing, with each
addition. Furthermore, the average % PMP formation for 8 and 9 was 73% and 81% respectively after
a fifth addition of 1-octene and that of 7 was 92% after a fourth addition. A cumulative TON of the
three precatalysts with an initial precatalyst loading of 1:9000 with consecutive additions of 1-octene is
3
3,039 for 7, 32,634 for 8 and 36,576 for 9. The formation of the PMPs for the different precatalysts is
shown in Figure 6. Similar trends with a steady decrease in PMP formation for 8 and 9 are observed.
The precatalysts remained active for days after the final addition of the 1-octene. Precatalyst 7 was
active for a further 7 days (10,325 min), 8 for 4 days (5,976 min) and 9 for 8 days (11,827 min).
Precatalyst 6 was active for about 2 days according to literature [21]. It is clear that all three
precatalysts show a significant improvement in lifetime when compared to 2.

Molecules 2014, 19
Table 7. Catalytic activity and selectivity of 7, 8 and 9 with consecutive additions of
5531
1-octene at 60 °C and an initial 1-octene/Ru molar ratio = 9000.
a
b
c
d
Precatalyst Addition %PMP
%SMP
5.6
%S
t (min)
2190
2067
2246
3822
630
TON
8487
8262
7992
8298
7200
6606
6795
6219
5814
7938
8055
7038
6453
TOF
233
240
213
130
686
381
398
183
280
367
296
385
172
82
7
1
2
3
4
1
2
3
4
5
1
2
3
4
5
94.3
91.8
88.8
92.2
80.0
73.4
75.5
69.1
64.6
88.2
89.5
78.2
71.7
78.8
94.4
91.8
88.9
92.4
80.1
73.8
76.0
69.3
65.8
88.5
90.5
85.0
92.0
88.9
8.2
11.1
7.6
8
19.9
26.1
23.8
30.6
33.6
11.5
9.4
1039
1024
2036
1247
1298
1632
1484
2246
5167
9
13.8
6.2
9.8
7092
d
a
b
c
C=C + C
7
=C
7
;
C
2
=C
7
+ C=C
6
+ C=C
2
+ C
6
=C
7
; Selectivity towards PMP, Reaction time to
maximum conversion.
Figure 6. The formation of PMPs during metathesis in the presence of 7, 8 and 9 with consecutive
additions of 1-octene at 60 °C and an initial 1-octene/Ru molar ratio of 9000.
3. Experimental Section
3.1. Reagents and Solvents
Diethyl ether (b.p. 34.6 °C, Labchem, Johannesburg, South Africa), THF (b.p. 66 °C, Saarchem,
Johannesburg, South Africa) and toluene (b.p. 110–111 °C, Merck, Johannesburg, South Africa) were
distilled under N from sodium with benzophenone as indicator. Pentane (b.p. 35–36 °C, Labchem)
was distilled under N from CaH . THF and toluene were stored over 4 Å mol sieves. Diethyl ether and
2
2
2
pentane were used directly from the distillation. The 1-octene (98%, Sigma-Aldrich, Johannesburg,
South Africa) was purified by filtering the solution through an alumina column after the alumina was dried
overnight at 70 °C. The solution was stored over 3 Å mol sieves. n-Butyl lithium, 2-bromopyridine (99%),

Molecules 2014, 19
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isobutyrophenone, 2-methylbenzophenone, 2'-methylacetophenone, cyclohexanone, benzophenone and
the 2nd generation Grubbs precatalyst were purchased from Aldrich and used as received. tert-Butyl
hydroperoxide (5.5 M in decane) and nonane (99%) were purchased from Fluka (Johannesburg, South
Africa) from which the first mentioned was stored over 4Å mol sieves. All gases used during this
study were supplied by Afrox (Johannesburg, South Africa).
3.2. Characterization Techniques
For the characterization of the synthesized ligands and complexes, the following apparatus were
used. The melting points of the ligands were determined on a Büchi B-540 melting point apparatus.
Infra-red spectra of all the compounds were obtained on a Nicolet FTIR 550 infrared
spectrophotometer (IR) [a KBr pellet was prepared of all samples (0.005 g sample mixed with 0.28 g
−
1
dry KBr) and analyzed using 10 scans over a wave number range of 400–4000 cm ]. The molecular
masses of the ligands were obtained on a Micromass Autospec mass spectrometer and the molecular
masses of the complexes on a Bruker Ultraflex III TOF/TOF (TOFMS) [samples were prepared by
mixing 2 µL (1 mg/mL in CHCl ) with 20 µL DCTB (20 mg/mL in THF); 1 µL was placed on a
3
Bruker ground steel target plate before analysis]. A Bruker MicroTOF-Q II mass spectrometer was
used for the determination of high resolution masses. For the determination of the purity as well as the
mass of the alcohols an Agilent Technologies 6890N Gas Chromatograph equipped with a
Phenomenex ZB-1 column (30 m × 320 μm × 1 μm), a 7683B autoinjector and 5973 Mass Selective
1
13
31
Detector was used. Nuclear magnetic resonance spectra ( H, C and P) of all compounds were
obtained on a Varian 300MHz and a Bruker Avance II 600MHz spectrometer (NMR) [20 mg sample
was dissolved in 1.5 mL CDCl
using freeze dried CDCl ].
3
; the samples of the complexes were prepared under inert conditions
3
3
.3. General Procedure for the Preparation of Lithium Salts
For the preparation of lithium salts, the alcohol (4 mmol) was dissolved in THF (40 mL) and
n-butyl lithium (1.8 mL, 4 mmol, 2.5 M) was added dropwise to the reaction mixture under inert (Ar)
and very dry conditions. This reaction was carried out at room temperature, so it was important to add
the n-butyl lithium slowly due to the exothermal character of the reaction. After 2 h of stirring the
solution became slightly yellow and no precipitate formed. When the THF was removed under reduced
pressure a yellow solid was obtained. The solid was washed with dry pentane (5 × 20 mL) forming a
white powder. The white powder was dried under vacuum and used without further characterization or
purification, due to its sensitivity to air.
3
.4. General Procedure for the Synthesis of the Second Generation Precatalysts [22]
A solution of lithium alcoholate (0.63 mmol, 0.090 g) in THF (5 mL) was added dropwise to a
solution of 2 (0.55 mmol, 0.470 g) in THF (5–10 mL) under inert (Ar) and very dry conditions. The
reaction was stirred at 30–40 °C for 60min with a colour change from brown to various shades of
green. The progress of the reaction was monitored by TLC (Merck Silica gel 60 F254; eluent hexane–
ethyl acetate = 1:2) to determine if the starting complex was being consumed. The reaction mixture

Molecules 2014, 19
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was stirred until the TLC indicated that the starting complex was not present any more. After removing
the solvent under vacuum the residue was dissolved in a minimal amount of toluene. The lithium
chloride was then removed by filtration via a syringe filter and the solution concentrated to a volume
of ca. 1 mL. The complexes were obtained by adding 10–15 mL cold pentane onto the filtrate and
placing it in the fridge for 8 h or by immediate sonification for 10 min resulting in the immediate
formation of a precipitate. After removal of the pentane via syringe or filtration, the desired complex
was washed with cold pentane to produce analytically pure microcrystalline powders.
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-1,1-
1
diphenylmethanolato]ruthenium (6). Light green microcrystalline powder (91% yield). H-NMR (300 MHz,
CDCl ): δ = 17.18 (s, 1H, Ru=CHPh), 6.43 (d, 2H, ortho H of C H ), 7.19 (dd, 1H, para H of C H ),
3
6
5
6
5
6
6 5 5 4 5 4
.77 (dd, 2H, meta H of C H ), 9.67 (d, 1H, ortho H of C H N), 7.19 (dd, 1H, para H of C H N), 7.04
(
dd, 1H, meta H of C H N), 6.64 (d, 1H, meta H of C H N), 6.71 and 6.99 (s, 4H, meta H mesityl),
5 4 5 4
4
.05 (bs, 4H, CH of NHC), 2.20, 2.30 and 2.65 (3s, 18H, CH mesityl), 7.12–7.19 (m, 10H, phenyl Hs
2
3
of O,N-ligand); anal. C46
.33, N 5.16.
H47ClN
3
ORu (794.42g/mol): C 69.55, H 5.96, N 5.29; calcd. for C 69.97, H
6
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-1-phenyl-
1
1
-(2-tolyl)-methanolato]ruthenium (7). Yield: 16%; H-NMR (600 MHz, CDCl ): δ 17.33 (s, 1H,
3
H
Ru=CHPh), 9.76–9.53 (d, 1H, ortho of C
4
H
5
N), 7.10–7.08 (dd, 2H, para Hs of C
6
H
5
and C
5
H
4
N),
6 5
7.08–6.99 (m, 13H, aromatic Hs of O,N ligand and mesityl), 6.68–6.55 (t, 2H, meta Hs of C H
and
C
5
H
4
N), 6.44–6.36 (d, 1H, meta H of C
5
H
4
N), 6.36–6.16 (d, 1H, ortho of C ), 3.73–3.47 (m, 4H, m,
6
H
5
Hs of NHC), 2.49–1.83 (3s, 18H, CH
3
mesityl), 1.24 (s, 3H, tolyl CH of N,O ligand) ppm,
3
+
Maldi-TOF: 807 m/z, [M ], HRESIMS m/z 807.2498 (calcd for C47
H48ClN
3
ORu, 807.2533).
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-1-(2-
1
tolyl)-1-methylmethanolato]ruthenium (8). Yield: 27%; H-NMR (600 MHz, CDCl
3
): δH 17.32 (s, 1H,
Ru=CHPh), 9.87–9.46 (d, 1H, ortho of C H N), 7.44–7.26 (d, 1H, ortho H of C H ), 7.13–6.99 (m,
4
5
6
5
2
6
4
3
H, para Hs of C
.70–6.57 (t, 2H, meta Hs of C
H, Hs of NHC), 2.60–2.36 (m, 18H, CH mesityl), 1.57 (s, 3H, CH of N,O ligand), 1.41–1.09 (m,
6
H
5
and C
5
H
4
N), 6.97–6.72 (m, 8H, aromatic Hs of O,N ligand and mesityl),
6
H
5
and C N), 6.33–6.00 (d, 1H, meta H of C N), 4.24–3.78 (t,
5
H
4
5 4
H
3
3
H, tolyl CH3 of N,O ligand) ppm, Maldi-TOF: 745 m/z [M+], HRESIMS m/z 745.2500 (calcd for
ORu, 745.2498).
43 3
C H46ClN
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-1-phenyl-
1
1
-isopropylmethanolato]ruthenium (9). Yield: 49%; H-NMR (600 MHz, CDCl
3
): δ
H
17.33 (s, 1H,
), 7.03 (m, 3H,
aromatic Hs of O,N ligand), 6.94 (m, 2H, para Hs of C H and C H N), 6.87–6.845 (m, 2H, meta Hs
Ru=CHPh), 9.39–9.38 (d, 1H, ortho H of C H N), 7.29–7.27 (d, 1H, ortho H of C H
4 5 6 5
6
5
5
4
of mesityl), 6.59–6.57 (t, 2H, meta Hs of C H and C H N), 4.08–4.00 (t, 4H, Hs of NHC), 2.72–2.12
6
5
5
4
(
m, 18H, CH
3
3
mesityl), 1.28–1.24 (m, 3H, iso-propyl CH of O,N-ligand), 0.65–0.64 (m, 3H, iso-
propyl CH
3
of O,N-ligand), 1.55–1.34 (t, 1H, iso-propyl CH of O,N-ligand) ppm, Maldi-TOF: 758 m/z
+
[M ], HRESIMS m/z 758.2463 (calcd for C H ClN ORu, 758.2454).
4
3
47
3

Molecules 2014, 19
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Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-propane-
1
2
-olato]ruthenium (10). Green microcrystalline powder (55% yield). H-NMR (300 MHz, CDCl ):
3
δ = 17.82 (s, 1H, Ru=CHPh), 7.15 (d, 2H, ortho H of C
6
H
5
6 5
), 7.05 (dd, 1H, para H of C H ), 6.85 (dd,
2
H, meta H of C H ), 9.15 (d, 1H, ortho H of C H N), 7.10 (dd, 1H, para H of C H N), 6.75 (dd, 1H,
6
5
5
4
5
4
meta H of C
5
H
4
N), 6.60 (d, 1H, meta H of C
5
H
4
N), 6.70 and 6.85 (bs, 4H, meta H mesityl), 4.02 (m,
mesityl), 1.20 (s, 6H, CH of O,N-ligand); anal.
C H ClN ORu (670.28 g/mol): C 64.51, H 6.47, N 6.27; calcd. for C 64.03, H 5.99, N 5.81.
4
H, CH
2
of NHC), 2.15, 2.60 and 2.80 (s, CH
3
3
3
6
43
3
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[1-(2′-pyridinyl)-2,4-
1
dimethylpentan-3-olato]ruthenium (11). Green microcrystalline powder (57% yield). H-NMR
(
300 MHz, CDCl ): δ = 18.52 (s, 1H, Ru=CHPh), 7.65 (d, 2H, ortho H of C H ), 7.55 (dd, 1H, para H
3
6
5
of C
6
H
5
6 5 5 4
), 7.35 (dd, 2H, meta H of C H ), 9.65 (d, 1H, ortho H of C H N), 7.55 (dd, 1H, para H of
C H N), 7.15 (dd, 1H, meta H of C H N), 7.05 (d, 1H, meta H of C H N), 7.15 and 7.35 (s, 4H, meta
5
4
5
4
5
4
H Mesityl), 4.45 (bs, 4H, CH of NHC), 2.59, 2.89 and 3.05 (3s, 18H, CH mesityl), 2.15 (dt, 2H,
2
3
iso-propyl CH of O,N-ligand), 2.15 (dt, 2H, iso-propyl CH of O,N-ligand), 1.01–1.35 (2d, 6H,
iso-propyl CH of O,N-ligand), 0.10–0.50 (2d, 6H, iso-propyl CH of O,N-ligand); anal.
ORu (726.39 g/mol): C 66.14, H 7.08, N 5.78; calcd. for C 65.68, H 6.73, N 5.08.
3
3
40 3
C H51ClN
Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)-[8-quinolinolate]ruthenium
12). A solution of the lithium salt of quinoline (0.99 mmol, 0.150 g) in THF (5 mL) was added
(
dropwise to a solution of 2 (0.24 mmol, 0.200 g) in THF (5–10 mL). Three carbene complexes formed
during the synthesis of 12 of which two were completely soluble in pentane, with the third only
partially soluble. As a result of the solubility differences of the three carbenes in pentane, two product
layers were formed upon slow vacuum condensation. The bottom analytically pure orange-brown layer
contained the partially soluble 12 with a H
α
resonance signal at δ 18.25 ppm (42 mg, 26% yield).
), 7.35 (t, 1H,
), 7.75 (d, 1H, H-1 of naphthyl), 6.40 (d, 1H, H-3
of naphthyl), 6.72–6.65 (m, H-4 of naphthyl), 7.05–6.89 (m, H-5 of naphthyl), 8.85 (d, H-6 of
naphthyl), 6.52 and 6.33 (2 bs, 4H, meta H of mesityl), 3.95 (bs, 4H, CH of NHC), 1.95, 2.05 and
.25 (3s, 18H, CH of mesityl); anal. C46 ORu (678.26 g/mol): C 65.52, H 5.80, N 6.20; calcd.
for C 65.04, H 5.40, N 6.13.
1
H-NMR (300 MHz, CDCl
3 6 5
): δ = 18.25 (s, 1H, Ru=CHPh), 6.85 (d, 2H, ortho H of C H
para H of C
6
H
5
), 6.72–6.65 (m, 2H, meta H of C H
6 5
2
2
3
H47ClN
3
3
.5. General Procedure for the 1-Octene Metathesis Reactions
The metathesis reactions of 1-octene were carried out in a 250 mL three-necked round-bottomed
flask fitted with a condenser, thermometer and septum as reported previously [24]. The 1-octene (20 mL)
was transferred to the reaction flask and heated to the desired reaction temperature using an oil bath on
a controlled hotplate magnetic stirrer. Thereafter the precatalyst amount was added to the flask and the
reaction mixture was continuously stirred with a magnetic stirrer bar until the formation of the PMPs
was completed. Samples (0.3 mL) were withdrawn with a gastight syringe during the reaction at
different time intervals and added to a solution of nonane (0.1 mL), toluene (0.3 mL) and 2 drops of
tert-butyl hydrogen peroxide for analysis by GC/FID. Nonane was used as an external standard,
toluene to increase sample volume and tert-butyl hydroperoxide as a quencher. The experiments were

Molecules 2014, 19
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repeated at least 3 times to ensure good reproducibility. Typically the average deviation of the %
observed products were 5%. An Agilent 6890 GC equipped with an Agilent 7683 auto-injector, HP-5
capillary column (30 m × 320 µm × 0.25 m) and a FID was used for analysis. The GC analysis
conditions were: inlet temperature 200 °C; N carrier gas flow rate 94 mL/min; injection volume
2
0
1
.2 mL (auto injection); split ratio 50:1; oven programming 60 °C for 5min, 60–110 °C at 25 °C/min,
10 °C for 5 min, 110–290 °C at 25 °C/min, 290 °C for 5 min; FID detector temperature 300 °C; H
2
flow rate 40 mL/min and air flow rate 450 mL/min.
. Conclusions
A number of chelating pyridinyl-alcoholato ligands have been successfully incorporated into 2, to
4
give a selection of second generation hemilabile precatalysts. Apart from complex 12, all the
complexes showed high activity, temperature stability and a long lifetime towards the self-metathesis
of 1-octene, relative to 2, since after 20 h PMP formation was still visible. Furthermore, 5, 8, 10 and 11
showed a 10%–20% higher SMP formation relative to 2. However, relative to 2, 9 showed only a 3%
increase, 7 showed similar % SMP formation, while a decrease was visible for 6, demonstrating the
improved activity and stability of these complexes. On the other hand, at temperatures ≥ 80 °C, an
increase in SMP formation was visible for 6, 7, 8 and 9 due to possible precatalyst decomposition or
reversible allyl-hydride formation, which is being investigated further.
Acknowledgments
The authors wish to thank the South African Department of Science and Technology – National
Research Foundation’s Centre of Excellence in Catalysis (c*change) and the North-West University
for financial support and Andre Joubert for acquiring the NMR data.
Author Contributions
Conceptualization of the research, interpretation of the results and cooperation in preparation of the
manuscript: H.C.M.V., J.H.L.J. and C.G.C.E.S. Preparation of the manuscript and interpretation of the
results: J.I.T. Experimental work and interpretation of the results: M.J. and C.A.A.H.
Conflicts of Interest
The authors declare no conflict of interest.
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2014 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/3.0/).
(