Non-innocent character of oxyanions in ruthenium metathesis catalysts

Article abstract of DOI:10.1021/om200124z

The synthesis, characterization, reactivity, and selectivity of six novel ruthenium metathesis catalysts containing oxyanions as ligands are described. A drop in chemoselectivity and/or reactivity in alternating ring-opening metathesis polymerization of norbornene and cyclooctene for catalysts with nitrate, acetate, and benzoate ligands (i.e., catalysts 7, 8, and 10a) compared to the parent complex bearing chloride as a ligand (5b) is observed, while catalysts with trifluoroacetate, methylbenzoate, and triisopropylbenzoate ligands (i.e., catalysts 9, 10b, and 10c) exhibit the expected activities and chemoselectivities. A model accounting for the aforementioned observations is based on a chelating effect of oxyanion ligands in these complexes and is supported by NMR data, crystal structures of the new complexes, and DFT calculations. Through comparison of selectivity and reactivity in copolymerizations with NMR and X-ray structures, we have uncovered correlations that serve as predictive tools for catalyst design.

Full text of DOI:10.1021/om200124z

ARTICLE
pubs.acs.org/Organometallics
Non-innocent Character of Oxyanions in Ruthenium
Metathesis Catalysts
Marija Joviꢀc, Sebastian Torker, and Peter Chen*
Laboratorium f€ur Organische Chemie, Eidgen€ossische Technische Hochschule (ETH), Wolfgang-Pauli-Strasse 10,
CH-8093 Z€urich, Switzerland
S Supporting Information
b
ABSTRACT: The synthesis, characterization, reactivity, and
selectivity of six novel ruthenium metathesis catalysts contain-
ing oxyanions as ligands are described. A drop in chemoselec-
tivity and/or reactivity in alternating ring-opening metathesis
polymerization of norbornene and cyclooctene for catalysts
with nitrate, acetate, and benzoate ligands (i.e., catalysts 7, 8,
and 10a) compared to the parent complex bearing chloride as a
ligand (5b) is observed, while catalysts with trifluoroacetate,
methylbenzoate, and triisopropylbenzoate ligands (i.e., catalysts
9, 10b, and 10c) exhibit the expected activities and chemos-
electivities. A model accounting for the aforementioned ob-
servations is based on a chelating effect of oxyanion ligands in these complexes and is supported by NMR data, crystal structures of
the new complexes, and DFT calculations. Through comparison of selectivity and reactivity in copolymerizations with NMR and
X-ray structures, we have uncovered correlations that serve as predictive tools for catalyst design.
’ INTRODUCTION
possibility of steering the cis/trans ratio, a diminished reactivity as
compared to the parent complex 5b was observed.23
Since the discovery of the first metathesis catalysts, olefin
metathesis rapidly gained an important position in organic synth-
esis as well as in polymer chemistry.1À5 This was achieved by the
development of well-defined, highly active catalysts by optimizing
the ligand sphere around the corresponding metal center.6À10 For
instance, replacement of one of the phosphines in first-generation
Grubbs’ catalyst (1) by N-heterocyclic carbenes (NHCs) resulted
in second-generation Grubbs’ catalyst (2), which exhibits en-
hanced reactivity by several orders of magnitude compared to
the former (Chart 1).11,12 Similarly, the introduction of the
“Hoveyda-type unit”,13,14 i.e., the [(dCH-2-(2-PrOC6H4)] car-
bene ligand, resulted in olefin metathesis catalysts with increased
stability and recyclability. Further variations and improvements of
ligand sphere are numerous and have been published by many
groups.4À17,21À44,46À51,57À59
To guide further refinement of our catalyst system, we have
discovered additional structural parameters that effect selectivity
and reactivity and are presented through a series of complexes
with carboxylate ligands (8À10), as well as the nitrate ligand (7).
There have been reports in the literature25À39 about replacement
of the chloride ligands by other electron-withdrawing groups
(Figure 1), although, in general, not too much attention has been
dedicated to this topic. The reason might be that replacement of
the halide has an effect on reactivity, although it is hard to predict
whether it will be an increase or decrease.40À42 Here we
document the negative effect of a chelating oxyanion ligand on
the rate and selectivity in alternating ring-opening metathesis
polymerization (AROMP) and introduce NMR and crystallo-
graphic parameters that correlate with the degree to which the
negative effects actually appear.
Through mechanistic studies18À20 in the field of ROMP by
our group, and altering the ligand sphere around the ruthenium,
the first example of a rationally designed ruthenium catalyst that
can produce alternating copolymers was developed (3).21
Further structural work and correlation of the structures to
selectivity have produced a catalyst structure whose complete
chemoselectivity in alternating copolymerization was demon-
strated even at room temperature with modest ratios of cyclooc-
tene to norbornene (5a,b).22À24
Subsequent replacement of the chloride anion in 5b by various
sulfonates (11aÀd) of increasing steric bulk allowed us then to
tune the cis/trans ratio of the produced copolymer, while leaving
the chemoselectivity unchanged (6aÀd).23 However, despite the
’ EXPERIMENTAL SECTION
General Remarks. Unless otherwise stated, all manipulations were
carried out under an argon atmosphere on a vacuum line using standard
Schlenk techniques. The solvents were dried by distillation from the
following drying agents prior to use and were transferred under N2:
diethyl ether (Na/K), n-hexane (Na/K), CH2Cl2 (CaH2). Flash chro-
matography was performed using Fluka silica gel 60, type 60752
(230À400 mesh). NMR measurements were either performed on 300
Received: February 9, 2011
Published: July 05, 2011
r
2011 American Chemical Society
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|

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Chart 1. Grubbs' First-Generation Catalyst 1, Grubbs' Second-Generation Catalyst 2, Chemoselective AROMP Catalysts
(prototype: 3; previously reported optimized versions: 4, 5a, 5b), Sulfonate Catalysts with Additional Stereocontrol (6aÀd),
Novel Nitrate Catalyst 7, Acetate Catalyst 8, Trifluoroacetate Catalyst 9, and Benzoate Catalysts (10aÀc)
(1H: 300 MHz, 13C: 75 MHz, 31P: 121 MHz) or 600 MHz instruments
(1H: 600 MHz, 13C: 150 Hz). Chemical shifts (δ-values) are reported in
ppm and calibrated with respect to the residual solvent signal for 1H and
13C NMR (CD2Cl2: 5.32 and 53.80 ppm; CDCl3: 7.26 and 77.00 ppm).
Synthesis of Nitrate Complex 7. Catalyst 6a (10 mg, 13.9 μmol)
and tetrabutylammonium nitrate (4.7 mg, 15.3 μmol) in 2 mL of
benzene were stirred for 2 h at room temperature. After filtration, the
complex was purified by crystallization, affording complex 7 in 53%
yield. Suitable crystals for X-ray analysis were obtained by vapor
diffusion of Et2O into a concentrated solution of the complex in CH2Cl2.
1H NMR (600 MHz, CD2Cl2): δ 15.63 (d, 1H, CH(dRu), JH,P = 9.3
Hz), 8.29 (very br s, 2H, Ar(P)Hortho), 7.90 (td, 1H, Ar(PO)H, JH,H = 1.5
and 7.9 Hz), 7.61 (tm, 1H, Ar(O)H, JH,H = 7.8 Hz), 7.57 (td, 1H,
Ar(P)Hpara, JH,H = 1.3 and 7.5 Hz), 7.49 (br s, 2H, Ar(P)Hmeta), 7.15 (d,
1H, Ar(O)H, JH,H = 8.5 Hz), 7.11 (tm, 1H, Ar(PO)H, JH,H = 7.7 Hz), 7.03
(td, 1H, Ar(O)H, JH,H = 7.6 and 1.7 Hz), 6.98 (td, 1H, Ar(O)H, JH,H = 7.4
and 0.8 Hz), 6.73 (tm, 1H, Ar(PO)H, JH,H = 7.4 Hz), 6.62 (ddd, 1H,
Ar(PO)H, JH,H = 8.4, 4.4, and 1.0 Hz), 5.25 (m, 1H, CH(isopropoxy)),
1.76 (d, 3H, CH3(isopropoxy), JH,H = 6.3 Hz), 1.60 (d, 3H, CH3-
(isopropoxy), JH,H = 6.2 Hz), 1.17 (d, 3H, CH3(TMP), JH,P = 18.3 Hz),
1.10 (d, 3H, CH3(TMP), JH,P = 13.6 Hz), 0.96 (s, 9H, tert-butyl(TMP)).
13C NMR (150 MHz, CD2Cl2): δ 295.88 (dd, 1C, C(dRu), JC,P = 35.4
and 13.5 Hz), 179.55 (d, 1C, CAr(PO)(ORu), JC,P = 14.0 Hz), 156.33 (s, 1C,
CAr(O)(OR)), 144.18 (s, 1C, CAr(O)(CHdRu)), ∼136.3 and 132.8 (2 very
br s almost in the baseline, 2C, CAr(P)(ortho)), 133.72 (s, 1C, CAr(PO)(H)),
132.46 (d, 1C, CAr(PO)(H), JC,P = 1.7 Hz), 131.31 (d, 1C, CAr(P)(ipso), JC,P
= 46.8 Hz), 130.62 (d, 1C, CAr(P)(para), JC,P = 2.3 Hz), 130.40 (s, 1C,
An 85% aqueous H3PO4 solution is used as an external standard for 31
P
NMR. Coupling constants (J) are given in Hz. 13C NMR and 31P NMR
spectra were proton broad-band-decoupled. The multiplicities of peaks
are denoted by the following abbreviations: s: singlet, d: doublet, dd:
doublet of doublets, t: triplet, tm: triplet with an additional unresolved
m, m: multiplet, br: broad. Elemental analyses were performed by the
Microanalytical Laboratory of the Laboratorium f€ur Organische Che-
mie, ETH-Z€urich. Syntheses of complexes 5b and 6a have been reported
previously.23,24
General Polymerization Procedure. A 150À200 mg amount of
norbornene (NBE) was polymerized in the presence of 20 equiv of
cyclooctene (COE) under argon. Prior to polymerization, the reaction
volume was filled up to 20 mL with CH2Cl2. The catalyst (1:2000 with
respect to NBE) was then added, and the reaction temperature kept at
room temperature. The reaction was stopped by precipitation with
100 mL of MeOH after 40 min. The coagulated polymer was dried under
high vacuum for 2 h and analyzed by NMR in CDCl3. The NMR
measurements were conducted with 30À40 mg of polymer left over-
night in the NMR solvent.
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Figure 1. Examples of ruthenium metathesis catalysts with carboxylate ligands by Buchmeiser (13, 2003; 14, 2009)25,32 and Grubbs (15, 2008)39 and a
catalyst with a potential FÀRu interaction by Grubbs (16, 2006).36
CAr(O)(H)), 128.37 (br s, 2C, CAr(P)(meta)), 123.86 (s, 1C, CAr(O)(H)),
123.17 (s, 1C, CAr(O)(H)), 119.63 (d, 1C, CAr(PO)(PRu), JC,P = 42.9 Hz),
119.20 (d, 1C, CAr(PO)(H), JC,P = 9.9 Hz), 117.43 (d, 1C, CAr(PO)(H), JC,P
= 6.5 Hz), 114.12 (s, 1C, CAr(O)(H)), 77.97 (s, 1C, Cisopropoxy), 46.80 (d,
1C, CTMP, JC,P = 23.7 Hz), 38.45 (d, 1C, CTMP, JC,P = 2.6 Hz), 27.78 (d, 3C,
CTMP, JC,P = 4.9 Hz), 22.83 (d, 1C, CTMP, JC,P = 1.3 Hz), 21.34 (s, 1C,
Cisopropoxy), 21.29 (d, 1C, CTMP, JC,P = 7.2 Hz), 21.20 (s, 1C, Cisopropoxy).
31P NMR (121 MHz, CD2Cl2): δ 78.38 (s). No elemental analysis was
obtained due to traces of the tetrabutylammonium salt in the sample.
Synthesis of Acetate Complex 8. Catalyst 6a (10 mg, 13.9
μmol) and ammonium acetate (1.2 mg, 15.3 μmol) in 2 mL of benzene
were stirred for 2 h at room temperature. The complex was filtered and
used without further purification.
JC,P = 4.9 Hz), 23.87 (s, 1C, Cisopropoxy), 22.98 (s, 1C, Cisopropoxy), 21.67
(d, 1C, CTMP, JC,P = 7.2 Hz), 21.62 (d, 1C, CTMP, JC,P = 2.9 Hz).
31P NMR (121 MHz, CD2Cl2): δ 78.97 (s). HRMS (ESI+): calcd
for C31H39NaO4PRu [M + Na+] 631.1530, found 631.1515. No
elemental analysis was obtained due to partial decomposition of the
complex.
Synthesis of Trifluoroacetate Complex 9. Catalyst 5b (10 mg,
13.9 μmol) and silver trifluoroacetate (2.5 mg, 15.3 μmol) in 2 mL of
CH2Cl2 were stirred for 2 h at room temperature. After filtration, the
complex was purified by crystallization, affording complex 9 in 40%
yield. Suitable crystals for X-ray analysis were obtained by evaporation of
Et2O from a solution of the complex in Et2O and hexane (Et2O/hexane,
0.3 mL:0.3 mL) into hexane (0.1 mL in the outer flask, setup similar to
vapor diffusion).
1H NMR (600 MHz, CD2Cl2): δ 15.42 (d, 1H, CH(dRu), JH,P = 9.4
Hz), 8.15 (very br s, 2H, Ar(P)Hortho), 7.89 (td, 1H, Ar(PO)H, JH,H
=
1H NMR (600 MHz, CD2Cl2): δ 16.29 (d, 1H, CH(dRu), JH,P = 7.8
Hz), 8.05 (br s, 2H, Ar(P)Hortho), 7.86 (td, 1H, Ar(PO)H, JH,H = 7.7, 1.5
Hz), 7.60 (tm, 1H, Ar(O)H, JH,H = 5.2 Hz), 7.59 (td, 1H, Ar(P)Hpara, JH,
H = 8.1, 1.6 Hz), 7.55 (tm, 2H, Ar(P)Hmeta, JH,H = 7.0 Hz), 7.25 (dd, 1H,
Ar(O)H, JH,H = 7.6, 1.6 Hz), 7.13 (tm, 2H, Ar(O)H, JH,H = 7.8 Hz), 7.04
7.8, 1.5 Hz), 7.54 (tm, 1H, Ar(O)H, JH,H = 7.2 Hz), 7.51 (td, 1H,
Ar(P)Hpara, JH,H = 8.3, 1.3 Hz), 7.45 (br s, 2H, Ar(P)Hmeta), 7.09 (d, 1H,
Ar(O)H, JH,H = 8.4 Hz), 7.05 (td, 1H, Ar(PO)H, JH,H = 8.4, 7.0 Hz),
6.99 (dd, 1H, Ar(O)H, JH,H = 7.6, 1.7 Hz), 6.92 (td, 1H, Ar(O)H, JH,H
=
7.4, 0.8 Hz), 6.66 (tm, 1H, Ar(PO)H, JH,H = 7.0 Hz), 6.55 (ddd, 1H,
Ar(PO)H, JH,H = 8.4, 4.4, 0.9 Hz), 5.17 (m, 1H, CH(isopropoxy)), 1.98
(s, 3H, CH3(acetate ligand)), 1.77 (d, 3H, CH3(isopropoxy), JH,H = 6.3
Hz), 1.59 (d, 3H, CH3(isopropoxy), JH,H = 6.2 Hz), 1.16 (d, 3H,
CH3(TMP), JH,P = 17.8 Hz), 1.08 (d, 3H, CH3(TMP), JH,P = 13.4 Hz),
0.95 (s, 9H, tert-butyl(TMP)). 13C NMR (150 MHz, CD2Cl2): δ 289.27
(d, 1C, C(dRu), JC,P = 23.9 Hz), 183.31 (s, 1C, COO, acetate ligand),
(td, 1H, Ar(PO)H, JH,H = 7.5, 0.8 Hz), 6.76 (tm, 1H, Ar(PO)H, JH,H =
7.8 Hz), 6.65 (ddd, 1H, Ar(PO)H, JH,H = 8.4, 4.4, and 1.0 Hz), 5.25 (m,
1H, CH3(isopropoxy)), 1.76 (d, 3H, CH(isopropoxy), JH,H = 6.1 Hz),
1.63 (d, 3H, CH(isopropoxy), JH,H = 6.1 Hz), 1.25 (d, 3H, CH3(TMP),
JH,P = 12.1 Hz), 1.23 (s, 9H, tert-butyl(TMP)), 1.08 (d, 3H, CH3(TMP),
JH,P = 17.2 Hz). 13C NMR (150 MHz, CD2Cl2): δ 294.86 (dd, 1C,
C(dRu), JC,P = 18.0, 12.9 Hz), 178.78 (d, 1C, CAr(PO)(ORu), JC,P
=
180.08 (d, 1C, CAr(PO)(ORu), JC,P = 14.3 Hz), 156.13 (s, 1C, CAr(O)
-
14.3 Hz), 161.02 (q, 1C, CF3, JC,F = 36.4 Hz), 155.31 (s, 1C, CAr(O)-
(OR)), 144.20 (s, 1C, CAr(O)(CHdRu)), ∼136.3 and 131.5 (2 very br s
almost in the baseline, 2C, CAr(P)(ortho)), 133.78 (s, 1C, CAr(PO)(H)),
132.61 (d, 1C, CAr(P)(ipso), JC,P = 45.6 Hz), 132.26 (d, 1C, CAr(PO)(H),
JC,P = 1.5 Hz), 130.39 (d, 1C, CAr(P)(para), JC,P = 2.2 Hz), 129.00 (s, 1C,
(OR)), 143.63 (s, 1C, CAr(O)(CHdRu)), ∼136.3 and 133.8 (2 very br s
almost in the baseline, 2C, CAr(P)(ortho)), 133.60 (s, 1C, CAr(PO)(H)),
132.97 (d, 1C, CAr(PO)(H), JC,P = 1.6 Hz), 131.16 (d, 1C, CAr(P)(para),
JC,P = 2.4 Hz), 130.26 (s, 1C, CAr(O)(H)), 128.67 (d, 2C, CAr(P)(meta),
JC,P = 10.2 Hz), 126.64 (d, 1C, CAr(P)(ipso), JC,P = 48.6 Hz), 123.71 (s,
1C, CAr(O)(H)), 123.07 (s, 1C, CAr(O)(H)), 119.81 (d, 1C, CAr(PO)(H),
JC,P = 10.1 Hz), 119.49 (d, 1C, CAr(PO)(PRu), JC,P = 43.5 Hz), 118.06 (d,
1C, CAr(PO)(H), JC,P = 6.4 Hz), 116.39 (br s, 1C, COO), 113.25 (s, 1C,
CAr(O)(H)), 75.19 (s, 1C, Cisopropoxy), 47.61 (d, 1C, CTMP, JC,P = 24.3
Hz), 37.48 (d, 1C, CTMP, JC,P = 5.2 Hz), 29.03 (d, 3C, CTMP, JC,P = 5.6
CAr(O)(H)), 128.16 (br s, 2C, CAr(P)(meta)), 123.82 (s, 1C, CAr(O)
(H)), 122.84 (s, 1C, CAr(O)(H)), 121.02 (d, 1C, CAr(PO)(PRu), JC,P
-
=
42.2 Hz), 119.24 (d, 1C, CAr(PO)(H), JC,P = 9.8 Hz), 116.71 (d, 1C,
CAr(PO)(H), JC,P = 6.4 Hz), 113.95 (s, 1C, CAr(O)(H)), 77.73 (s, 1C,
Cisopropoxy), 47.00 (d, 1C, CTMP, JC,P = 23.6 Hz), 38.48 (d, 1C, CTMP
JC,P = 2.7 Hz), 30.26 (s, 1C, CH3, acetate ligand), 28.16 (d, 3C, CTMP
,
,
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Hz), 22.26 (s, 1C, Cisopropoxy), 21.93 (s, 1C, Cisopropoxy), 20.64 (d, 1C,
CTMP, JC,P = 12.9 Hz), 20.60 (d, 1C, CTMP, JC,P = 7.0 Hz). 31P NMR
(121 MHz, CD2Cl2): δ 81.61 (s). Anal. Calcd (%) for C31H36O4F3PRu
(661.66 g/mol): C 56.27, H 5.48. Found: C 56.15, H 5.56.
CH3(isopropoxy), JH,H = 6.2 Hz), 1.67 (d, 3H, CH3(isopropoxy), JH,
= 6.1 Hz), 1.18 (d, 3H, CH3(TMP), JH,P = 18.1 Hz), 1.15 (d, 3H,
H
CH3(TMP), JH,P = 12.6 Hz), 1.09 (s, 9H, tert-butyl(TMP)). 13C NMR
(150 MHz, CD2Cl2): δ 290.69 (dd, 1C, C(dRu), JC,P = 42.3, 13.6 Hz),
179.32 (d, 1C, CAr(PO)(ORu), JC,P = 14.1 Hz), 178.15 (s, 1C, COO
(benzoate ligand)), 155.51 (s, 1C, CAr(O)(OR)), 144.25 (s, 1C, CAr-
(O)(CHdRu)), 137.96 (s, 1C, Cpara (benzoate ligand)), ∼136.3 and
132.8 (2 very br s almost in the baseline, 2C, CAr(P)(ortho)), 136.03 (s,
2C, Cortho (benzoate ligand)), 134.76 (s, 1C, CCOO (benzoate ligand)),
133.60 (s, 1C, CAr(PO)(H)), 132.40 (s, 1C, CAr(PO)(H)), 130.54 (s, 1C,
CAr(P)(para)), 130.02 (d, 1C, CAr(P)(ipso), JC,P = 46.5 Hz), 129.12 (s,
1C, CAr(O)(H)), 129.00 (s, 2C, Cmeta (benzoate ligand)), 128.16 (d, 2C,
CAr(P)(meta), JC,P = 9.9 Hz), 123.79 (s, 1C, CAr(O)(H)), 122.90 (s, 1C,
CAr(O)(H)), 120.47 (d, CAr(PO)(PRu), JC,P = 42.5 Hz), 119.41 (d, 1C,
CAr(PO)(H), JC,P = 9.8 Hz), 117.06 (d, 1C, CAr(PO)(H), JC,P = 6.3 Hz),
114.50 (s, 1C, CAr(O)(H)), 77.27 (s, 1C, Cisopropoxy), 46.64 (d, 1C,
CTMP, JC,P = 23.8 Hz), 37.79 (d, 1C, CTMP, JC,P = 3.2 Hz), 28.56 (d, 3C,
CTMP, JC,P = 5.0 Hz), 22.15 (s, 1C, Cisopropoxy), 22.06 (d, 1C, CTMP, JC,P
= 7.5 Hz), 21.66 (d, 1C, CTMP, JC,P = 2.1 Hz), 21.32 (s, 1C, Cisopropoxy),
21.20 (s, 1C, CH3,para (benzoate ligand)), 20.95 (s, 2C, CH3,ortho
(benzoate ligand)). 31P NMR (121 MHz, CD2Cl2): δ 78.93 (s). Anal.
Calcd (%) for C39H47O4PRu (711.84 g/mol): C 65.81, H 6.65. Found:
C 65.61, H 6.65.
Synthesis of Benzoate Complex 10a. Catalyst 6a (10 mg,
13.9 μmol) and sodium benzoate (2.2 mg, 15.3 μmol) in 2 mL of
CH2Cl2 were stirred for 2 h at room temperature. After filtration, the
complex was purified by crystallization, affording complex 10a in 60%
yield. Suitable crystals for X-ray analysis were obtained by evaporation
of Et2O from a solution of the complex in Et2O and pentane (Et2O/
pentane, 0.3 mL:0.3 mL) into pentane (0.1 mL in the outer flask,
setup similar to vapor diffusion).
1H NMR (600 MHz, CD2Cl2): δ 15.68 (d, 1H, CH(dRu), JH,P
=
9.1 Hz), 8.05 (very br s, 2H, Ar(P)Hortho), 8.04À8.02 (m, 1H, CH
(benzoate ligand)), 8.02 (t, 1H, CH (benzoate ligand), JH,H = 1.7
Hz), 7.92 (td, 1H, Ar(PO)H, JH,H = 7.8, 1.5 Hz), 7.57À7.55 (m, 1H,
Ar(O)H), 7.53 (td, 1H, Ar(P)Hpara, JH,H = 6.9, 1.7 Hz), 7.51 (br s,
2H, Ar(P)Hmeta), 7.47À7.43 (m, 1H, CH (benzoate ligand)), 7.39
(tt, 2H, CH (benzoate ligand), JH,H = 6.7, 1.3 Hz), 7.10 (ddd, 2H,
Ar(O)H, JH,H = 8.2, 4.4, 1.5 Hz), 7.07 (d, 1H, Ar(O)H, JH,H = 8.4 Hz),
6.97 (td, 1H, Ar(PO)H, JH,H = 7.5, 0.8 Hz), 6.70 (tm, 1H, Ar(PO)H,
JH,H = 8.1 Hz), 6.62 (ddd, 1H, Ar(PO)H, JH,H = 8.4, 4.4, 0.9 Hz), 5.18
(m, 1H, CH(isopropoxy)), 1.85 (d, 3H, CH3(isopropoxy), JH,H
6.3 Hz), 1.46 (d, 3H, CH3(isopropoxy), JH,H = 6.2 Hz), 1.10 (d, 3H,
CH3(TMP), JH,P = 13.0 Hz), 1.08 (d, 3H, CH3(TMP), JH,P
=
Synthesis of Triisopropylbenzoate Complex 10c. Catalyst
6a (10 mg, 13.9 μmol) and sodium 2,4,6-triisopropylbenzoate
(4.1 mg, 15.3 μmol) in 2 mL of CH2Cl2 were stirred for 2 h at room
temperature. The complex was filtered and used without further
purification.
=
17.5 Hz), 1.03 (s, 9H, tert-butyl(TMP)). 13C NMR (150 MHz,
CD2Cl2): δ 289.83 (dd, 1C, C(dRu), JC,P = 37.7, 13.6 Hz), 179.94
(d, 1C, CAr(PO)(ORu), JC,P = 14.2 Hz), 176.33 (s, 1C, COO
(benzoate ligand)), 155.99 (s, 1C, CAr(O)(OR)), 144.11 (s, 1C,
CAr(O)(CHdRu)), ∼136.3 and 132.8 (2 very br s almost in the
baseline, 2C, CAr(P)(ortho)), 134.56 (s, 1C, CCOO (benzoate
ligand)), 133.79 (s, 1C, CAr(PO)(H)), 132.40 (d, 1C, CAr(PO)(H),
JC,P = 1.4 Hz), 131.62 (s, 1C, Cpara (benzoate ligand)), 131.33 (d, 1C,
CAr(P)(ipso), JC,P = 46.8 Hz), 130.56 (d, 1C, CAr(P)(para), JC,P = 2.2
1H NMR (600 MHz, CD2Cl2): δ 16.12 (d, 1H, CH(dRu), JH,P = 8.0
Hz), 8.24 (t, 2H, Ar(P)Hortho, JH,H = 8.5 Hz), 7.92 (td, 1H, Ar(PO)H,
JH,H = 7.7, 1.5 Hz), 7.59À7.56 (m, 1H, Ar(O)H,), 7.55 (td, 1H,
Ar(P)Hpara, JH,H = 3.6, 1.6 Hz), 7.51 (t, 2H, Ar(P)Hmeta, JH,H = 7.6
Hz), 7.23 (dd, 1H, Ar(O)H, JH,H = 7.6, 1.6 Hz), 7.18 (d, 1H, Ar(O)H,
JH,H = 8.4 Hz), 7.14À7.10 (m, 1H, Ar(O)H), 7.01 (td, 1H, Ar(PO)H,
JH,H = 7.5, 0.8 Hz), 6.87 (s, 2H, CHAr (benzoate ligand)), 6.73 (tm, 1H,
Ar(PO)H, JH,H = 8.0 Hz), 6.64 (ddd, 1H, Ar(PO)H, JH,H = 8.4, 4.4, 0.9 Hz),
5.23 (m, 1H, CH(isopropoxy)), 2.83 (m, 3H, CHisopropyl,ortho and para
(benzoate ligand)), 1.88 (d, 3H, CH3(isopropoxy), JH,H = 6.2 Hz),
1.65 (d, 3H, CH3(isopropoxy), JH,H = 6.0 Hz), 1.23 (d, 3H, CH3(TMP),
JH,P = 11.9 Hz), 1.20 (two d, 6H, CH3,para (benzoate ligand), JH,H = 6.9
Hz), 1.18 (s, 9H, tert-butyl(TMP)), 1.17 (d, 3H, CH3(TMP), JH,P = 16.1
Hz), 1.12 (d, 6H, CH3,ortho (benzoate ligand), JH,H = 6.9 Hz), 0.87 (d,
6H, CH3,ortho (benzoate ligand), JH,H = 6.8 Hz). 13C NMR (150 MHz,
CD2Cl2): δ 291.85 (d, 1C, C(dRu), JC,P = 12.9 Hz), 179.19 (d, 1C,
CAr(PO)(ORu), JC,P = 14.2 Hz), 176.64 (s, 1C, COO (benzoate ligand)),
155.21 (s, 1C, CAr(O)(OR)), 148.37 (s, 1C, Cpara (benzoate ligand)),
Hz), 129.56 (s, 2C, Cortho (benzoate ligand)), 129.16 (s, 1C, CAr(O)
-
(H)), 128.48 (s, 2C, Cmeta (benzoate ligand)), 128.31 (br d, 2C,
CAr(P)(meta), J = 9.4 Hz), 123.77 (s, 1C, CAr(O)(H)), 122.97 (s, 1C,
CAr(O)(H)), 120.87 (d, CAr(PO)(PRu), JC,P = 42.3 Hz), 119.39 (d, 1C,
CAr(PO)(PRu), JC,P = 9.9 Hz), 116.96 (d, 1C, CAr(PO)(H), JC,P = 6.4
Hz), 113.80 (s, 1C, CAr(O)(H)), 77.13 (s, 1C, Cisopropoxy), 47.15 (d,
1C, CTMP, JC,P = 23.6 Hz), 38.30 (d, 1C, CTMP, JC,P = 3.2 Hz), 28.43
(d, 3C, CTMP, JC,P = 5.0 Hz), 22.54 (d, 1C, CTMP, JC,P = 1.5 Hz),
21.77 (s, 1C, Cisopropoxy), 21.63 (s, 1C, Cisopropoxy), 21.60 (d, 1C,
CTMP, JC,P = 8.4 Hz). 31P NMR (121 MHz, CD2Cl2): δ 79.47 (s).
Anal. Calcd (%) for C36H41O4PRu (669.76 g/mol): C 64.56, H 6.17.
Found: C 64.27, H 6.20.
144.80 (s, 2C, Cortho (benzoate ligand)), 144.26 (s, 1C, CAr(O)
-
Synthesis of Trimethylbenzoate Complex 10b. Catalyst 6a
(10 mg, 13.9 μmol) and sodium 2,4,6-trimethylbenzoate (2.8 mg,
15.3 μmol) in 2 mL of CH2Cl2 were stirred for 2 h at room temperature.
After filtration, the complex was purified by crystallization, affording
complex 10a in 42% yield. Suitable crystals for X-ray analysis were
obtained by evaporation of Et2O from a solution of the complex in Et2O
and pentane (Et2O/pentane, 0.3 mL:0.3 mL) into pentane (0.1 mL in
the outer flask, setup similar to vapor diffusion).
(CHdRu)), 136.59 (s, 1C, CCOO (benzoate ligand)), ∼136.3 and
132.8 (2 very br s almost in the baseline, 2C, CAr(P)(ortho)), 133.43 (s,
1C, CAr(PO)(H)), 132.56 (s, 1C, CAr(PO)(H)), 130.83 (d, 1C, CAr(P)
-
(para), JC,P = 2.2 Hz), 129.31 (s, 1C, CAr(O)(H)), 128.67 (d, 1C,
CAr(P)(ipso), JC,P = 47.1 Hz), 128.28 (d, 2C, CAr(P)(meta), JC,P = 10.1
Hz), 123.91 (s, 1C, CAr(O)(H)), 123.10 (s, 1C, CAr(O)(H)), 120.99 (s,
2C, Cmeta (benzoate ligand)), 120.58 (d, CAr(PO)(PRu), JC,P = 43.0 Hz),
119.66 (d, 1C, CAr(PO)(H), JC,P = 9.8 Hz), 117.35 (d, 1C, CAr(PO)(H),
JC,P = 6.3 Hz), 114.56 (s, 1C, CAr(O)(H)), 76.70 (s, 1C, Cisopropoxy),
46.80 (d, 1C, CTMP, JC,P = 23.9 Hz), 37.47 (d, 1C, CTMP, JC,P = 4.6 Hz),
34.82 (s, 1C, CH(CH3)2,para), 31.22 (s, 2C, CH(CH3)2,ortho), 28.79 (d,
3C, CTMP, JC,P = 5.4 Hz), 24.61 (s, 4C, CH(CH3)2,ortho), 24.35 (s, 2C,
CH(CH3)2,para), 24.33 (s, 1C, CH(CH3)2,para), 22.67 (s, 1C,
Cisopropoxy), 22.05 (d, 1C, CTMP, JC,P = 7.2 Hz), 21.10 (s, 1C,
Cisopropoxy), 20.71 (d, 1C, CTMP, JC,P = 4.0 Hz). 31P NMR (121 MHz,
CD2Cl2): δ 81.64 (s). Anal. Calcd (%) for C45H59O4PRu (796.00 g/mol):
C 67.90, H 7.47. Found: C 67.61, H 7.66.
1H NMR (600 MHz, CD2Cl2): δ 15.84 (d, 1H, CH(dRu), JH,P = 8.8
Hz), 8.19 (br s, 2H, Ar(P)Hortho), 7.92 (td, 1H, Ar(PO)H, JH,H = 7.7, 1.5
Hz), 7.57À7.54 (m, 2H, Ar(O)H), 7.53 (td, 1H, Ar(P)Hpara, JH,H = 6.9,
1.7 Hz), 7.50 (br t, 2H, Ar(P)Hmeta, JH,H = 6.7 Hz), 7.15 (dd, 1H,
Ar(O)H, JH,H = 7.6, 1.6 Hz), 7.12 (d, 1H, Ar(O)H, JH,H = 8.4 Hz),
7.12À7.09 (m, 1H, Ar(O)H), 6.98 (td, 1H, Ar(PO)H, JH,H = 7.5, 0.8
Hz), 6.74 (d, 2H, CHAr (benzoate ligand), JH,H = 0.6 Hz), 6.72 (tm, 1H,
Ar(PO)H, JH,H = 8.1 Hz), 6.63 (ddd, 1H, Ar(PO)H, JH,H = 8.4, 4.3, 0.9
Hz), 5.19 (m, 1H, CH(isopropoxy)), 2.21 (s, 3H, CH3,para (benzoate
ligand)), 2.18 (s, 6H, CH3,ortho (benzoate ligand)), 1.70 (d, 3H,
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Table 1. Copolymerization Experiments with Catalysts 5a, 5b, 6a, 7À9, and 10aÀc Carried out at Room Temperature with
150À200 mg of Norbornene (NBE), and 20 equiv of Cyclooctene (COE) with the Reaction Volume Filled up to 20 mL with
a
CH2Cl2
catalyst
X
t [min]
yield [%]b
alternating linkages [%]c
cis content [%]d
5a
5b
6a
7
ClÀ
ClÀ
15
30
40
40
40
40
40
40
40
88
90
82
31
27
77
26
78
71
97
13
13
25
20
13
18
14
14
15
97
CH3C6H4 SO3À
NO3À
95 (1.3)
77 (5.2)
92 (2.5)
92 (1.7)
90 (1.8)
96 (0.7)
96 (0.7)
8
CH3COOÀ
9
CF3COOÀ
10a
10b
10c
C6H5COOÀ
2,4,6-Me3C6H2COOÀ
2,4,6-iPr3C6H2COOÀ
a NBE/catalyst ratio was 2000:1. b Yields were determined after drying the coagulated copolymer under high vacuum for 2 h (relative to 100% totally
alternating copolymer). c The percentage of alternating linkages has been determined by integration of the olefinic region of the 13C NMR spectrum;
standard deviation is given in parentheses. d The cis content was estimated by integration of the CH (CHdCHR) protons on the norbornene terminus of
the double bond (see Figures S6 and S8).
’ COMPUTATIONAL DETAILS
Geometry optimizations were performed with the Amsterdam Den-
sity Functional (ADF) package 2010.02 using the BP86 density func-
tional with an all-electron triple-ζ quality basis set with added
polarization functions (TZP) and an integration accuracy of 5.0.
Relativistic effects were treated with the spinÀorbit coupled zeroth-
order regular approximation (ZORA).43 For simplicity reasons, a
truncated model was used replacing 1,1,2,2-tetramethylpropyl (TMP)
with a tert-butyl substituent and the benzylidene with a methylidene
carbene. All structures were fully optimized without constraints and
checked with frequency calculations to ensure that they were minima.
Optimized geometries and absolute energies are given in the Supporting
Information.
’ RESULTS
A series of ruthenium catalysts with oxy ligands were synthe-
sized (7À10) by direct ligand exchange starting from catalyst 6a
(5b in the case of trifluoroacetate ligand) with the corresponding
salt. Conversions to the desired complexes were fast, showing
complete conversion of the starting material within 2 h.
Figure 2. 13C NMR spectra of copolymers produced with catalysts 5a
and 7.
In screening studies of copolymerization of norbornene and
cyclooctene (Table 1) catalysts 7 (the nitrate complex), 8 (the
acetate complex), and 10a (the benzoate complex) gave the
desired polymers in low conversions (in the range 26À31%).
The lower reactivity exhibited by 7 is concomitant with the
highest drop in chemoselectivity for alternating copolymeriza-
tion (among the catalysts tested). The selectivity dropped from
95À97% alternating units in the copolymer produced with 6a
and 5b to 77% for nitrate catalyst 7 (Figure 2). A smaller decrease
in chemoselectivity is observed in the series of benzoate ligands
(Figure 3), going from 96% to 90% alternating units. At a first
glance, this drop in selectivity might not appear as significant;
however, this drop in selectivity is reproducible (all polymeriza-
tion results were repeated at least three times; Table 1, standard
deviations of polymerization results). Thus, it is apparent that the
aforementioned changes are significant and do not result from
experimental errors.
The trifluoroacetate catalyst 9, the trimethylbenzoate catalyst
10b, and the triisopropylbenzoate catalyst 10c are intermediate
in their performance. The drop in chemoselectivity was espe-
cially surprising since we expected the chemoselectivity to be
Figure 3. 13C NMR spectra of copolymers produced with catalysts
10aÀc.
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Figure 6. Aromatic region of the 1H NMR spectra of catalysts 10aÀc in
CD2Cl2 at rt (600 MHz).
Figure 4. Aromatic region of the 1H NMR spectra of catalysts 4, 5a, and
7 in CD2Cl2 at rt (600 MHz).
Table 2. Comparison of the Most Significant Structural
Parameters for Chelating Interactions in Complexes 4, 5a, 7, 9,
10a,b, and Buchmeiser’s Structure 1432
OÀCÀO or RuÀOÀN or
RuÀOshort RuÀOlong OÀRuÀO OÀNÀO
RuÀOÀC
[deg]
complex
[Å]
[Å]
[deg]
[deg]
4
153.13 (8)
5a
158.68 (10)
7
2.128 (2) 2.358 (2) 163.22 (9)
2.068 (2) 3.385 (2) 160.42 (9)
2.142 (2) 2.333 (2) 164.75 (3)
2.115 (2) 2.406 (2) 168.52 (9)
114.82 (2)
131.00 (3)
119.79 (3)
123.75 (3)
124.55
98.89 (17)
126.95 (2)
93.43 (10)
97.85 (19)
98.06
9
10a
10b
14chelating
2.094
2.493
3.430
14nonchelating 2.022
130.59
130.87
Figure 5. Aromatic region of the 1H NMR spectra of catalysts 8 and 9 in
CD2Cl2 at rt (600 MHz).
interaction with a second oxygen atom of the nitrate ligand
(RuÀO34 and RuÀO36 distances are 2.128(2) and 2.358(2) Å,
respectively). The angle O14ÀRu1ÀO34 (163.22(9)°) is sig-
nificantly larger than the corresponding angles for the ruthenium
catalyst with a chloride ligand (4 (153.13(8)°, Figure S4) and 5a
(158.68(10)°, Figure S5)), as a result of the additional chelating
interaction. The opening of this angle has an effect on the
selectivity: it pushes the whole phosphine ligand closer to the
carbene and constrains the space of the phenyl substituent,
thereby leading to its hindered rotations, which is the presumed
origin of the broader signals in the NMR spectra. The same
increased steric encumbrance that hinders the phenyl group
rotation would lessen the energetic difference between the two
diastereomeric carbenes in the catalytic cycle. A reduced energy
difference would manifest itself as a reduced chemoselectivity
between norbornene and cyclooctene in the alternating copo-
lymerization, as we have observed. This hypothesis was checked
by quantum chemical calculations (Figure 8). Indeed, whereas
the energetic difference (ADFÀBP86/ZORAÀTZP) between
the two diastereomeric carbene states for the truncated tert-butyl
model is 3.4 kcal/mol with a chloride substituent (C1 and C2),24
the value decreases to 2.7 kcal/mol with a nitrate ligand (N1 and
N2). This is due to a stronger chelating interaction of the nitrate
ligand (by 0.7 kcal/mol) when the methylidene carbene is on
the side of the tert-butyl group (N2). The effect is not very
dependent on the size difference of the two substituents on the
bidentate phosphine ligand only.
The drop in selectivity and/or reactivity correlates to the
observed peak broadening in the 1H NMR spectra of the signals
for the ortho and meta protons of the phenyl ring on the bidentate
phosphine/phenolate ligand. While the proton signals are well
resolved in the case of catalyst 4, the ortho and meta protons of
the phenyl ring are broadened for catalyst 5a (Figure 4). For
catalyst 7 the signal for the meta protons is further broadened and
the signal for the ortho protons disappears almost completely in
the baseline, which indicates that rotation of the phenyl ring is
slow, presumably due to steric hindrance. A similar behavior
can be observed for the ortho and meta protons of the 1H NMR
spectra of 8 and 9 (Figure 5) and 10aÀc (Figure 6). Further-
more, broadening of ortho and meta carbon signals is seen in
the 13C NMR spectra (see Figures S1, S2, and S3), and no
cross-peak is seen between the ortho protons and ortho carbons
of the phenyl ring in the HSQC of complex 7 in CD2Cl2 (see
Figure S13).
Crystal structures of the catalyst complexes give us additional
information and help connect catalyst structure to performance
(see Table 2). For the ruthenium catalyst with a nitrate ligand (7)
(Figure 7) a hexacoordinated complex is formed by a chelating
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Figure 7. Crystal structure of complex 7 (ORTEP plot, 30% probability
ellipsoids). Hydrogen atoms are omitted for clarity. Selected distances
(Å) and angles (deg): Ru1ÀC3 1.856(3), Ru1ÀO14 1.988(2), Ru1ÀP2
2.2404(8), Ru1ÀO10 2.278(2), C3ÀRu1ÀO14 100.00(12), C3À
Ru1ÀO34 95.45(11), C3ÀRu1ÀP2 89.69(10), O14ÀRu1ÀP2
84.21(6), O34ÀRu1ÀP2 102.47(7), C3ÀRu1ÀO10 79.75(11), O14À
Ru1ÀO10 89.28(8), O34ÀRu1ÀO10 87.06(8), P2ÀRu1ÀO10
166.47(6), C3ÀRu1ÀO36 150.25(11), O14ÀRu1ÀO36 106.29(8),
O34ÀRu1ÀO36 57.17(8), P2ÀRu1ÀO36 106.38(6), O10ÀRu1À
O36 86.82(8).
Figure 9. Crystal structure of complex 9 (ORTEP plot, 30% probability
ellipsoids). Hydrogenatoms are omitted for clarity. Selecteddistances (Å)
and angles (deg): Ru1ÀC30 1.838(3), Ru1ÀO3 2.0018(18), Ru1ÀP2
2.2374(7), Ru1ÀO23 2.2760(19), C30ÀRu1ÀO3 98.09(10),
C30ÀRu1ÀO36 99.68(10), C30ÀRu1ÀP2 95.64(9), O3ÀRu1ÀP2
84.65(6), O36ÀRu1ÀP2 101.65(6), C30ÀRu1ÀO23 79.49(10),
O3ÀRu1ÀO23 89.21(8), O36ÀRu1ÀO23 86.03(8).
pronounced but large enough to produce considerable amounts
of polynorbornene linkages during copolymerization of NBE
with COE. Gel permeation chromatography (GPC analysis) of
the obtained copolymers shows that the sequences of polynor-
bornene produced are part of the same chain as alternating
linkages, which is to be expected from the reaction mechanism
(tables in SI). Calculations were done in accordance and agree-
ment with our previous reports, in which a combined experi-
mental and computational investigation indicated that the origin
of chemoselectivity in our system is attributable to diastereo-
meric site control.24
Catalyst 9 (Figure 9) does not show a chelation of the
trifluoroacetate ligand, with the RuÀO37 bond length of
3.385(2) Å. The O3ÀRu1ÀO36 angle, at 160.42(9)°, is slightly
larger than the corresponding angle for catalyst 5a (158.68(10)°).
This effect can be explained by the larger size of the anion itself.
Still, theangle issmaller than inthe case of complex7. The factthat
the trifluoroacetate ligand does not show chelation, while the
acetate one does, could be explained by both steric and electronic
factors (CF3 is strongly electron-withdrawing). One may venture
to suggest that chelation influences reactivity more than it does
chemoselectivity (Table 1). The lower activity of the catalysts 7, 8,
and 10a could be explained by the stabilizing effect of chelating
ligands on the Ru center.
A less pronounced chelation is seen in the crystal structure of
10b (Figure 11) as compared to 10a (Figure 10). The difference
between RuÀO bonds in 10a (2.142(2) and 2.333(2) Å) is
smaller compared to those in 10b (2.115(2) and 2.406(2) Å),
which indicates stronger chelation in the case of the former, most
probably due to sterics. Although the angle O9ÀRuÀO45 in 10b
(168.52(9)°) is significantly larger than the corresponding angle
in 10a (164.75(3)°), less broadening of the NMR signals was
observed. Due to the weaker chelation in complex 10b, there is a
better chance of opening of the chelate ring, allowing for a more
facile rotation in solution and observing it on the NMR time
scale. It can also be noted that the cis content of obtained
polymers with benzoate catalysts is rather low compared to
sulfonate ones (14À15% for catalysts 10aÀc), which may be
Figure 8. Explanation of the lower chemoselectivity reached with
catalyst 7 versus 5a. The nitrate chelation counteracts the steric bulk
of the two substituents on the phosphorus ligand. Calculated structures
for the nitrate complex are shown at the bottom; hydrogen atoms are
omitted for clarity.
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’ DISCUSSION
On our way toward the development of a chemo- and
stereoselective ruthenium-based catalyst for the alternating co-
polymerization of norbornene and cyclooctene, which is based
on two diastereomeric carbene states in the catalytic cycle,18À24
we uncovered interesting steric features, which, in hindsight,
allow for a more detailed understanding of this system.
The solid-state structure of our original prototype 3 was
somewhat unexpected since the carbene was on the side of the
more bulky tert-butyl group rather than below the smaller phenyl
ring, an effect that was caused by a CÀH agostic interaction
between one cyclohexyl ring and the ruthenium center (Chart 1).
The agostic interaction rendered the cyclohexyl ring sterically
more demanding than the carbene.22
We then improved our system by introducing a Hoveyda-
type13,14,46À49 carbene unit (4),22 which shows the reverse and
expected configuration around the metal center. In this complex
the chelating carbene unit with the hydrogen atom pointing “up”
(toward the substituents on the phosphorus) is on the side of the
smaller phenyl ring. This was followed by the optimization of the
bidentate phosphine/phenolate ligand to yield a totally chemo-
selective AROMP catalyst (5a, 5b).24 Other examples of AR-
OMP catalysts can be found in the literature.50À56
Replacement of the chloride anion in 5b with various sulfo-
nates (11aÀd) of increasing steric bulk allowed us to influence
the cis/trans ratio of the produced copolymer, leaving the
chemoselectivity unchanged (6aÀd).23 Schrock and Hoveyda
also published Z-selective molybdenum catalysts.57,58
Figure 10. Crystal structure of complex 10a (ORTEP plot, 30% prob-
ability ellipsoids). Hydrogen atoms are omitted for clarity. Selected
distances (Å) and angles (deg): Ru(1)ÀC(7) 1.845(3), Ru(1)ÀO(4)
2.0046(19), Ru(1)ÀP(2) 2.2358(7), Ru(1)ÀO(1) 2.2643(19), C-
(7)ÀRu(1)ÀO(4) 100.64(10), C(7)ÀRu(1)ÀO(2) 93.99(10), C(7)À
Ru(1)ÀP(2) 91.38(9), O(4)ÀRu(1)ÀP(2) 84.45(6), O(2)ÀRu-
(1)ÀP(2) 101.29(6), C(7)ÀRu(1)ÀO(1) 79.66(10), O(4)ÀRu-
(1)ÀO(1) 89.65(7), O(2)ÀRu(1)ÀO(1) 87.08(7), P(2)ÀRu(1)À
O(1) 168.20(5), C(7)ÀRu(1)ÀO(3) 150.45(10), O(4)ÀRu(1)ÀO(3)
106.39(7), O(2)ÀRu(1)ÀO(3) 58.10(7), P(2)ÀRu(1)ÀO(3) 102.79(5),
O(1)ÀRu(1)ÀO(3) 88.66(7).
From these results, we considered that chemo- and stereoselectiv-
ity in ruthenium-catalyzed olefin metathesis have their origin in
defined steric parameters that we have been optimizing by careful
tuning of the substituents on the ligands.22À24 Reactivity, on the other
hand, is usually believed to be determined primarily by electronic
properties of ligands. A more electron-donating NHC ligand44 (i.e.,
the second-generation Grubbs catalyst 2) in general gives a more
reactive catalyst compared to the first-generation Grubbs systems
featuring phosphines (1). In the case of the phosphine ligands larger
and more electron-donating ones are favored.40 Among the anions,
chloride-containing catalysts exhibit the highest activity, albeit at the
expense of the possibility for steric modifications that would allow
tuning of the cis/trans ratio.40 The aforementioned strongly electron-
withdrawing sulfonate ligand-containing complexes 6aÀd present an
alternative to the chloride. However, despite the possibility of tuning
the cis/trans ratio, they already induce a significantly diminished
reactivity as compared to the parent complex 5b.23 This stands in
contrast to the results of a computational study of Straub,45 where
calculations for NHCÀRu complexes predicted sulfonates to be
equally efficient or even superior as compared to chloride.
Further alternatives to the chloride ligand are carboxylate
ligands, againdue totheir electron-withdrawingcharacter. Recently
Ru-carboxylate olefin metathesis catalysts were prepared as an
alternative for Ru-chlorides (Figure 1).25À39 Numerous supported
(i.e., 13) and unsupported (i.e., 14, 15) Grubbs-, Hovey-
daÀGrubbs-, or GrubbsÀHerrmann-type catalysts have been
reported. Fluorinatedcarboxylate(e.g., trifluoroacetate, pentafluor-
opropionate, pentafluorobenzoate) ligands are usually used be-
cause of their strong electron-withdrawing effect, which is expected
to produce high activities in metathesis reactions. Examples of
chelate effects are reported as well, giving catalysts with low (15) or
high reactivities (16, see discussion below), although comparisons
are difficult, as multiple factors are varied at the same time.
Figure 11. Crystal structure of complex 10b (ORTEP plot, 30%
probability ellipsoids). Hydrogen atoms are omitted for clarity. Selected
distances (Å) and angles (deg): Ru1ÀC16 1.852(3), Ru1ÀO9
2.019(2), Ru1ÀP2 2.2321(9), Ru1ÀO23 2.239(2), C16ÀRu1ÀO9
101.20(11), C16ÀRu1ÀO45 89.05(11), C16ÀRu1ÀP2 92.75(12),
O9ÀRu1ÀP2 84.35(7), O45ÀRu1ÀP2 100.44(7), C16ÀRu1ÀO23
80.76(13), O9ÀRu1ÀO23 89.23(9), O45ÀRu1ÀO23 87.33(9), P2À
Ru1ÀO23 169.85(6), C16ÀRu1ÀO44 142.27(10), O9ÀRu1ÀO44
111.44(9), O45ÀRu1ÀO44 57.19(9), P2ÀRu1ÀO44 108.39(6),
O23ÀRu1ÀO44 81.28(8).
explained by the fact that the oxygen atoms of the SO3 group (in
contrast to a carboxylate) are pointing more toward the active
site, as has been structurally proven by X-ray crystallography.23
The incoming norbornene would then coordinate in such a way
to avoid steric interaction with the bridgehead.
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Blechert and Buchmeiser, for example, have used triflate and
carboxylate anions with electron-poor alkyl chains (for example
trifluoroacetate).25À33 Buchmeiser and co-workers disclosed one
crystal structure with two trifluoroacetate anions, which, in
contrast to our example, shows a possible chelating interaction
involving one of the anions. One could hypothesize that this
could also have an impact on selectivity (catalyst 14, Figure 1).32
In that instance, the chelating trifluoroacetate anion shows
RuÀO distances of 2.094 and 2.493 Å. The latter distance is
longer as compared to nitrate complex 7 or benzoate complexes
10a,b, indicating that nitrate and benzoate form stronger chelates
(Table 2). Evidence that there is, in fact, a chelating interaction in
Buchmeiser’s complex is given by the larger OÀCÀO angle of
the second, nonchelating trifluoroacetate anion (130.59° com-
pared to 124.55° for the chelating one) and from the very small
RuÀOÀC angle of the chelating one (98.06°).
observed by the Grubbs group for complex 16,36 where interac-
tion of one of the ortho fluorine substituents on the NHC ligand
with the metal center promotes phosphine dissociation, hence
yielding a more active catalyst.
Regarding the crystal structures, one should take into con-
sideration that the real situation in solution might be different
than in the solid state. The NMR data indicate the dynamics of
the molecule, i.e., in real time the constrained space for the
phenyl substituent. Upon comparison of the NMR data of the
complexes to the polymerization results, it can be concluded that
a significant broadening of the phenyl signals also correlates with
diminished chemoselectivity and/or reactivity. We attribute the
broadening of NMR signals to hindered phenyl rotation due to
structural distortions induced by chelation by the oxyanion
ligands. Most likely this involves a fluctional behavior of the
chelating anion in solution. Rotation of the phenyl ring will be
the faster, the easier the anion can switch from a bidentate to a
monodentate coordination mode. The same increased steric
hindrance of the phenyl rotation diminishes the steric difference
between the two diastereomeric sites in the catalytic cycle, thus
diminishing the selectivity for alternating copolymerization.
To compare nitrate and trifluoroacetate anions directly, one
might consider the following literature: Ruthenium nitrate com-
plexes [Ru(NO3)x(O2CCF3)2Àx(CO)(PPh3)2] have been re-
ported by Robinson et al. for the catalytic dehydrogenation of
primary and secondary alcohols to aldehydes and ketones,
respectively. The bis-trifluoroacetate complex shows much great-
er catalytic efficiency relative to the bis-nitrate complex. In the
latter complex the alcohol has to compete with the nitrate anion,
a stronger chelating ligand than the trifluoroacetate anion, in
order to enter the coordination sphere.59
’ CONCLUSION
We have shown another interesting structural effect caused by
chelating ligands that can reduce selectivity and/or reactivity in
AROMP of norbornene and cyclooctene. Given the recent
interest in more versatile substitutes for chloride ligands in
metathesis catalysts, one should consider the possibility that a
given oxyanion substitute for chloride could chelate and ad-
versely affect the desired reactivity or selectivity. The NMR and
X-ray-based indicators should give advanced guidance for the
cases where one might expect these adverse effects.
For our complexes, we can conclude from the crystallographic
data (Table 2) that the nitrate ligand in complex 7 is the strongest
chelating ligand among the oxyanions examined in this paper.
Important structural features are RuÀO distances and OÀCÀO
and RuÀOÀC angles. In the nitrate complex, the OÀNÀO
angle (114.82(2)°) is significantly smaller compared to others:
131.00(3)°, 119.79(3)°, and 123.75°(3) for 9, 10a, and 10b,
respectively. Here we can also see the agreement with the angles
for chelating and nonchelating ligands in Buchmeiser’s complex
(123.75(3)° in 10a compared to 124.55° for chelating and
131.00(3)° in 9 compared to 130.59° for nonchelating ligand
in catalyst 14). Comparing the RuÀOÀN angle with the
RuÀOÀC angle, the same effect can be seen: 98.89(17)° in
the nitrate complex compared to 93.43(10)° and 97.85 (19)° in
10a and 10b, which are significantly smaller compared to the
126.95(2)° of complex 9. The conclusion that the nitrate ligand
is the strongest chelating agent is supported by the polymeriza-
tion results, since nitrate complex 7 gives the biggest drop in
chemoselectivity (77%).
The results shown in this paper might be discussed considering
pKa values of the conjugate acids of the anionic ligands. From the
literature,60,61 the order of increasing pKa's would be as follows:
ClÀ < NO3À < CF3COOÀ < C6H5COOÀ ∼ Me3C6H5COOÀ <
CH3COOÀ. Accordingly, one would expect the following reactiv-
ity trend (7 > 9 > 8) and that reactivities for benzoate complexes
10aÀc are similar. However, this assumption holds only for the
comparison of acetate and trifluoroacetate, but not for the other
cases. Nitrate complex 7 is more than 2-fold less reactive than the
trifluoroacetate complex (9), while benzoate complex 10a is as
much as 3-fold less reactive than the trimethyl and triisopropyl-
benzoate complexes 10b and 10c. Again, we attribute this behavior
to the chelating effect of nitrate and benzoate ligands, which
stabilize the Ru center, rendering it less reactive.
’ ASSOCIATED CONTENT
S
Supporting Information.
Comparison of 13C NMR
b
spectra of complexes 5a, 7À9, and 10aÀc (Figures S1, S2, and
S3), crystal structures of complexes 4 and 5a (Figures S4 and S5),
crystallographic data for complexes 7, 8, and 10a,b (Table S1À4),
1H NMR spectra of copolymers (Figures S6 and S8), 13C NMR
spectra of copolymers with catalysts 8 and 9 (Figure S7), 1H and
13C NMR spectra of complexes 5a, 7À9, and 10aÀc (Figures
S9ÀS12, S14ÀS23), HSQC experiment of complex 7 (Figure
S13), details of the DFT calculations, and GPC results of polymers
obtained with catalysts 7 and 10b. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: (+41) 44 632 2898. Fax: (+41) 44 632 1280. E-mail: peter.
chen@org.chem.ethz.ch.
’ ACKNOWLEDGMENT
We thank Dr. Bernd Schweizer and Dr. Michael Solar from the
X-ray crystallographic service of the ETH and Dr. Philip Zum-
brunnen and Dr. Rainer Frankensteiner from the NMR service of
the Laboratorium f€ur Organische Chemie (ETH) for performing
the corresponding experiments.
In other words, the more electron-rich Ru center has a lower
tendency for olefin binding, since the chelating interaction would
actually favor its dissociation. A similar chelate effect has been
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