Rational design of highly active "hybrid" phosphine-phosphinite pincer iridium catalysts for alkane metathesis

Article abstract of DOI:10.1021/cs400624c

Both the bisphosphine and bisphosphinite pincer complexes ( ^tBu4PCP)IrH₂ and (^tBu4POCOP)IrH₂ can cocatalyze alkane metathesis in tandem with olefin metathesis catalysts, but the two complexes have different resting states during catalysis, suggesting that different steps are turnover-limiting in each case. This led to the hypothesis that a complex with intermediate properties would be catalytically more active than either of these two species. Accordingly, "hybrid" phosphine-phosphinite pincer ligands (PCOP) and the corresponding iridium complexes were synthesized (3c-e). In tandem with olefin-metathesis catalyst MoF12, (^tBu4PCOP)IrH₂ displays significantly higher activity for the metathesis of n-hexane than does (^tBu4PCP)IrH ₂ or (^tBu4POCOP)IrH₂. (^tBu2PCOP ^iPr2)IrH₄ (3d) is even more active (>30-fold more active than (^tBu4POCOP)IrH₂) and affords nearly 4.6 M alkane products after 8 h at 125 C.

Full text of DOI:10.1021/cs400624c

Research Article
pubs.acs.org/acscatalysis
Rational Design of Highly Active “Hybrid” Phosphine−Phosphinite
Pincer Iridium Catalysts for Alkane Metathesis
Agnieszka J. Nawara-Hultzsch,† Jason D. Hackenberg,† Benudhar Punji,† Carolyn Supplee,†,⊥
Thomas J. Emge,† Brad C. Bailey,‡ Richard R. Schrock,‡ Maurice Brookhart,§ and Alan S. Goldman*,†
†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903,
United States
‡Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
§Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
S
* Supporting Information
ABSTRACT: Both the bisphosphine and bisphosphinite
pincer complexes (tBu4PCP)IrH2 and (tBu4POCOP)IrH2 can
cocatalyze alkane metathesis in tandem with olefin metathesis
catalysts, but the two complexes have different resting states
during catalysis, suggesting that different steps are turnover-
limiting in each case. This led to the hypothesis that a complex
with intermediate properties would be catalytically more active
than either of these two species. Accordingly, “hybrid”
phosphine−phosphinite pincer ligands (PCOP) and the
corresponding iridium complexes were synthesized (3c−e).
In tandem with olefin-metathesis catalyst MoF12, (tBu4PCOP)-
IrH2 displays significantly higher activity for the metathesis of n-hexane than does (tBu4PCP)IrH2 or (tBu4POCOP)IrH2.
(
tBu2PCOPiPr2)IrH4 (3d) is even more active (>30-fold more active than (tBu4POCOP)IrH2) and affords nearly 4.6 M alkane
products after 8 h at 125 °C.
KEYWORDS: pincer complexes, alkane metathesis, homogeneous catalysis, dehydrogenation, iridium
amounts. Further, natural gas liquids and condensates are
INTRODUCTION
■
composed exclusively of alkanes with molecular weights well
below this desirable range. Thus, we can anticipate a critical
need for the development of practical methods for the large-
scale catalytic conversion of light alkanes to heavier-molecular-
weight species.
The only general method reported to date for the direct
conversion of light alkanes to heavier alkanes is alkane
metathesis. Alkane metathesis with heterogeneous catalysts
has been reported by Burnett and Hughes18 and by Basset and
Coperet et al.19 These systems represent major breakthroughs,
but they suffer from limitations, including the need for severe
reaction conditions, relatively low reaction rates, and low
selectivity with respect to the formation of higher molecular
weight and unbranched species.
We have previously reported the catalytic metathesis of n-
alkanes based on a tandem process that utilizes an iridium-
based catalyst for alkane dehydrogenation and an olefin
metathesis catalyst to metathesize the alkenes generated in
situ (Figure 1).20−22 The hydrogen removed from the alkanes is
then used to hydrogenate the metathesized olefin products.
The gap between global consumption of liquid fuels and
production of conventional crude oil is expected to widen
dramatically over the next several decades.1−4 Although
conventional crude is the source of >90% of liquid fuel
consumed today, Brandt et al. have estimated that fraction will
decline to roughly 50% by 2050 and perhaps 15% by 2080;1
although the rate of this change has a high degree of
uncertainty, the direction and the eventual magnitude does
not. The gap will be met by large increases in the use of
“natural gas liquids” and condensates (low-molecular-weight
hydrocarbons from geologic sources) and, especially, “uncon-
ventional liquids” of origin that is yet to be determined.1,4
Fischer−Tropsch (F−T) catalysis is likely to play an important
role in the context of unconventional liquid fuel production.5−7
The feedstock for F−T may be derived from coal or natural gas
(the current sources for commercial reactors), from bio-
mass,8−12 or from CO2/H2O with energy input from carbon-
free sources.13−16
The increased demand for liquid fuel is driven specifically by
increased demand for diesel and jet fuel,4,17 both of which are
generally of high molecular weight (typically ca. C9−C19 and
C8−C16, respectively). Although F−T catalysis followed by
hydrocracking of the heavier products can give high yields in
this range, lighter n-alkanes are still produced in substantial
Received: July 30, 2013
Revised: September 19, 2013
© XXXX American Chemical Society
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particular, “hybrid” or “PCOP” pincer catalysts in which X1 =
CH2, and X2 = O). These hybrid PCOP complexes are, indeed,
found to be highly effective, yielding the highest catalytic
activities reported to date for this class of alkane metathesis
catalysts.
RESULTS AND DISCUSSION
■
The pincer ligands that have been most commonly used for
alkane metathesis and alkane dehydrogenation more generally26
Figure 1. Alkane metathesis via tandem dehydrogenation−hydro-
genation and olefin metathesis catalysis.
t
are tBu4PCP (R1, R2, R3, R4 = Bu; X1, X2 = CH2; 1a) and
t
tBu4POCOP (R1, R2, R3, R4 = Bu; X1, X2 = O; 1b). At the
Since the olefin metathesis catalysts afford high reaction rates
even at ambient temperatures, whereas alkane metathesis and
simple transfer−dehydrogenation require temperatures >100
°C to achieve even modest rates, it is presumed that the
transfer−dehydrogenation component limits the rate of alkane
metathesis; however, it is not obvious which segment of the
transfer−dehydrogenation cycle (hydrogenation or dehydro-
genation) is rate-limiting.
In the case of dehydrogenation at the alkane terminus and
metathesis of the resulting α-olefins, the product of metathesis
of n-alkane CnH2n+2 is C2n−2H4n−2 plus ethane, as illustrated in
Figure 1.20,21
outset of our studies, we found that complexes of these ligands
afforded relatively similar rates of alkane metathesis. This
similarity notwithstanding, the resting states of the complexes
were quite different. Specifically, the resting states were found
to be (tBu4PCP)IrH2 and (tBu4POCOP)Ir(olefin), respectively,
with no evidence of the “converse” species, (tBu4PCP)Ir(olefin)
and (tBu4POCOP)IrH2, detected under catalytic conditions in
situ.
It might be presumed that catalytic transfer−dehydrogen-
ation by tBu4PCP and tBu4POCOP catalysts proceeds by
analogous pathways, and indeed, there is a considerable body
of evidence that supports this presumption.27−30 The observed
difference in resting states therefore implies that although the
pathways may be analogous, the respective rate-determining
steps in the catalytic cycles are different for the two catalysts.
Specifically, the (tBu4PCP)IrH2 resting state implies that olefin
hydrogenation is rate-determining, whereas the (tBu4POCOP)-
Ir(olefin) resting state implies that the rate-determining step
involves loss of olefin (possibly reversible), followed by alkane
dehydrogenation. Moreover, this suggests that the correspond-
ing step for each catalyst that is not rate-determining is
relatively fast; for example, olefin hydrogenation by
Our previous reports have focused on PCP- and POCOP-
ligated iridium catalysts (Figure 2) for dehydrogenation20,21 in
Figure 2. Pincer ligands of the type used in this work (X = CH2 or O;
R = alkyl).
(
tBu4POCOP)IrH2.31 Thus, for the tBu4PCP catalyst, the
combination with various types of olefin metathesis catalysts,
including molecular species,23 such as Mo(NAr)(CHCMe2Ph)-
[OCMe(CF3)2]2 (Ar = 2,6-iPrC6H3) (MoF12), as well as
traditional heterogeneous olefin metathesis catalysts, such as
Re2O7 on γ-alumina.24
In our initial work, we observed that similar rates of alkane
metathesis were obtained with PCP (X1, X2 = CH2) and
POCOP (X1, X2 = O) iridium species (operating in tandem
with MoF12).19,20 We therefore focused, in our initial attempts
to improve rates, on varying the phosphinoalkyl groups, the
nature of which presumably dominated steric effects; this
approach has, indeed, yielded faster catalytic systems.23−25
Herein, however, we report that mechanistic considerations
have led us to consider variation of the linker groups X, in
alkene-hydrogenation segment of the cycle is relatively slow,
and the alkane-dehydrogenation segment (including dissocia-
tion of olefin to give the active fragment, (pincer)Ir32) is
relatively fast, whereas the converse is true for the tBu4POCOP
catalyst.
We considered the possibility that if the catalytic cycles of the
tBu4PCP and tBu4POCOP catalysts include converse sets of one
slow and one fast segment, then the catalytic cycle of a species
whose properties are intermediate between tBu4PCP and
tBu4POCOP could contain two segments with rates that are
each intermediate between the two extremes. If, indeed, the two
segments have intermediate rates, rather than one fast and one
slow, the overall catalytic rate of such a species would be faster
Figure 3. Schematic diagram indicating relative overall free energy barriers, under alkane metathesis conditions, for the alkane dehydrogenation and
olefin hydrogenation segments of the transfer−dehydrogenation catalytic cycle for (a) (tBu4PCP)Ir, (b) (tBu4POCOP)Ir, and (c) a hypothetical
species with properties intermediate between (tBu4PCP)Ir and (tBu4POCOP)Ir.
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Scheme 1. Synthesis of “PCOP-H” Ligand Precursors
Scheme 2. Metalation of PCOP-H Ligand Precursors To Form (PCOP)IrHCl and Subsequent Reduction to Catalyst Precursors
than that of either tBu4PCP or tBu4POCOP catalysts. This is
illustrated, in a completely schematic fashion, with the free
energy diagram in Figure 3.
The most obvious candidate for a species with properties
(including reactivity) intermediate between that of (tBu4PCP)Ir
and (tBu4POCOP)Ir catalysts would be a “hybrid” phosphine−
phosphinite species (tBu4PCOP)Ir (Figure 2; X1 = CH2, X2 = O,
R = t-Bu).
Synthesis and Characterization of PCOP Ligands and
Iridium Complexes. The tBu4PCOP-H ligand precursor (1c-
H) was prepared following the method used by Eberhard and
Jensen33 (Scheme 1) to synthesize tBu2PCOPiPr2-H (Figure 2;
X1 = CH2; X2 = O; R1, R2 = t-Bu; R3, R4 = i-Pr). A pale yellow
viscous oil was obtained in 84% yield. The 31P NMR spectrum
displayed a peak at 152.7 ppm corresponding to the O−PtBu2
moiety (cf. tBu4POCOP-H, δ 153.1 ppm) and a peak at 34.1
Figure 4. Thermal ellipsoid plot of (tBu4PCOP)IrHCl (2c). Hydrogen
atoms other than the hydride ligand omitted for clarity.
ppm corresponding to the CH2−PtBu2 moiety (cf. tBu4PCP-H,
δ 33.0 ppm). The crude oil was used without further
purification.
Table 1. Selected Bond Distances (Å) and Bond Angles (°)
Metalation of the crude ligand precursor with [Ir(COD)Cl]2
in refluxing toluene occurred cleanly to yield the corresponding
for (tBu4PCOP)IrHCl (2c)
(
tBu4PCOP)IrHCl species (2c) in 68% isolated yield as
bond distances (Å)
Ir(1)−C(1)
bond angles (deg)
C(1)−Ir(1)−P(1)
analytically pure, air-stable, red microcrystalline solids after
recrystallization from pentane (Scheme 2). The 31P{1H} NMR
spectrum of 2c displayed a doublet of doublets at 168.6 ppm
2.016(3)
2.2685(9)
2.3194(8)
2.4012(10)
1.662(3)
1.848(4)
1.852(4)
1.814(4)
1.868(4)
1.865(4)
81.27(10)
82.47(10)
163.55(3)
177.71(9)
97.03(3)
Ir(1)−P(1)
Ir(1)−P(2)
Ir(1)−Cl(1)
P(1)−O(1)
P(1)−C(8)
P(1)−C(12)
P(2)−C(7)
P(2)−C(16)
P(2)−C(20)
C(1)−Ir(1)−P(2)
P(1)−Ir(1)−P(2)
C(1)−Ir(1)−Cl(1)
P(1)−Ir(1)−Cl(1)
P(2)−Ir(1)−Cl(1)
O(1)−P(1)−C(8)
O(1)−P(1)−Ir(1)
C(7)−P(2)−Ir(1)
C(2)−O(1)−P(1)
C(6)−C(7)−P(2)
2
(2JPP = 345.0 Hz, JPH = 12.3 Hz) attributable to the O−PtBu2
group (cf. (tBu4POCOP)IrHCl,30 δ 175.8 ppm) and 70.6 ppm
2
(2JPP = 345.0 Hz, JPH = 11.0 Hz) attributable to the CH2−
99.16(3)
2
PtBu2 group (cf. δ 67.3 ppm, JPH = 12.4 Hz for (tBu4PCP)-
101.01(18)
104.63(10)
102.67(13)
115.2(2)
IrHCl34,35). The large value of JPP (345.0 Hz) is characteristic
2
of mutually trans-coordinated inequivalent 31P nuclei. In the 1H
NMR spectrum, the hydride is manifest with a doublet of
doublets at δ −41.38 ppm (2JPH = 13.3 Hz, 2JPH = 12.3 Hz) that
is nearly identical to the corresponding shifts of the nonhybrid
species and is attributable to an apical hydride trans to a vacant
coordination site in a 5-coordinate square pyramidal (pincer)Ir
complex.30,35
111.0(3)
that of (tBu4POCOP)IrHCl (2b, 160.1°).37 The C−Ir−P(CH2)
bond angles (82.5°) are similar to those reported for 2a (82.0°,
82.3°), and the C−Ir−P(O) angle (81.3°) is slightly greater
than the corresponding angles in (tBu4POCOP)IrHCl (80.0°,
80.1°).30
Compound 2c was further characterized by X-ray crystallog-
raphy (Figure 4). The coordination geometry is approximately
square pyramidal with the hydride located in the apical
position. Selected bond lengths and angles are summarized in
Table 1. The Ir−P(O) bond length is 2.269 Å, slightly shorter
than the Ir−P bond lengths 2.295( 2) in (tBu4POCOP)IrHCl.
The Ir−P(CH2) bond length is 2.319 Å, slightly longer than
that reported36 for (tBu4PCP)IrHCl (2.305( 1) Å). These
values all seem consistent with a greater trans influence exerted
by the phosphinite vs the phosphine groups.
A solution of 2c was treated with LiEt3BH under H2
atmosphere, resulting in 93% conversion to (tBu4PCOP)IrH2
(3c). Complex 3c displayed 31P{1H} NMR signals consistent
with those of its symmetrical analogues, exhibiting a doublet at
δ 200.3 ppm (2JPP = 330 Hz) attributable to the O−P(tBu)2
group (cf. (tBu4PCOP)IrH2, δ 204.9 ppm) and a doublet at 87.3
ppm (2JPP = 330 Hz) due to the CH2−P(tBu)2 group (cf.
1
(
tBu4PCP)IrH2, δ 86.1 ppm). The hydride signal in the H
The P(1)−Ir-P(2) bond angle of 2c (163.6°) is slightly less
than that of (tBu4PCP)IrHCl34 (2a, 164.3°)36 and greater than
NMR spectrum appeared as a triplet at δ −18.12 ppm with 2JPH
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Table 2. Product Concentrations (mM) of Products Formed by Metathesis of n-Hexane (7.5 M) Catalyzed by MoF12 (16 mM)
and Catalysts 3a, 3b, 3c (10 mM) at 125 °C
Catalyst 3b, (tBu2POCOPtBu2)IrH2
t (h)
C2
C3
C4
C5
C7
C8
C9
trans-5-decene
C10
C11−19
[total product]
selectivity C10/(C7−10)
1
3
2
4
12
38
11
34
10
34
7
4
14
40
98
3
9
2
2
1
1
2
6
1
7
54
171
0.22
0.15
0.13
0.12
22
64
8
7
93
81
94
28
19
47
14
33
442
24
15
232
194
249
168
76
1111
Catalyst 3a, (tBu2PCPtBu2)IrH2
t (h)
C2
C3
C4
C5
C7
C8
C9
trans-5-decene
C10
C11−19
[total product]
selectivity C10/(C7−10)
1
3
21
63
14
52
21
30
22
100
200
303
11
57
1
6
4
10
19
32
11
14
12
7
7
38
0
1
113
370
0.55
0.42
0.43
0.48
8
165
295
96
62
121
167
14
26
106
202
10
25
805
24
158
119
1335
Catalyst 3c, (tBu2PCOPtBu2)IrH2
t (h)
C2
C3
C4
C5
C7
C8
C9
trans-5-decene
C10
C11−19
[total product]
selectivity C10/(C7−10)
1
3
12
33
82
317
708
896
53
215
495
639
103
339
727
948
67
233
477
576
28
115
234
267
20
74
2
2
2
2
19
55
15
53
401
1437
3111
3997
0.16
0.12
0.11
0.14
8
85
145
183
104
162
134
168
24
156
= 8.6 Hz. Comparison with the corresponding values for the
symmetrical analogues supports the assumption that 3c should
have properties intermediate between (tBu4POCOP)IrH2 (δ
2
−17.4 ppm, JPH = 8.2 Hz) and (tBu4PCP)IrH2 (δ −19.2 ppm,
2JPH = 8.8 Hz).
Catalytic Activity of (tBu4PCP)Ir, (tBu4POCOP)Ir and
(
tBu4PCOP)Ir for Alkane Metathesis. Alkane metathesis
reactions catalyzed by (tBu4PCOP)IrH2 (3c) as well as the
previously reported symmetrical complexes (tBu4POCOP)IrH2
(3b) and (tBu4PCP)IrH2 (3a) were performed with 10 mM of
the (pincer)Ir catalyst, 16 mM of the olefin metathesis catalyst,
Mo(NAr)(CHCMe2Ph)[OCMe(CF3)2] (Ar = 2,6-i-PrC6H3)
(MoF12), 30.0 mM mesitylene (internal GC standard), and n-
hexane as the solvent/substrate (∼7.6 M). 3,3-Dimethyl-1-
butene (TBE; 20 mM) was added as a hydrogen acceptor to
dehydrogenate the active catalyst and to generate a steady-state
concentration of olefin (see Figure 1).
All experiments were performed under an argon atmosphere.
Reactions were conducted at 125 °C in flame-sealed tubes, and
the progress of the reaction was monitored by gas
chromatography at intervals of 1, 3, 8, and 24 h, with three
tubes opened at each time. Reported values represent the
average of the three runs.
Figure 5. Total alkane products formed in the metathesis of n-hexane
catalyzed by MoF12 and catalysts 3a−c.
After 8 h, the reaction rates decrease. This presumably
reflects decomposition of the MoF12 catalyst,20,23,38,39
decreased concentration of hexane (particularly in the case of
catalyst 3c), and possibly some decomposition of (pincer)Ir.
The concentration of total products obtained with 3c at 24 h is
4.0 M (52% conversion).
Our initial hypothesis was based on the premise that the
Complex 3c is found to be a remarkably active catalyst
(Table 2 and Figure 5), strongly supporting the hypothesis that
a “hybrid” species with properties intermediate between those
of (tBu4POCOP)Ir and (tBu4PCP)Ir would display activity
greater than that of either of the symmetrical complexes. For
the first 8 h, all three catalysts display roughly constant turnover
frequencies. The relative conversions for (tBu4POCOP)Ir,
transfer dehydrogenation cycles of (tBu4PCP)Ir and
(
tBu4POCOP)Ir each had one fast segment and one slow
segment (for (tBu4PCP)Ir alkane dehydrogenation is fast and
olefin hydrogenation is slow, but the reverse is true for
(
tBu4POCOP)Ir); a hybrid species would approach a limit
whereby the rates of the two segments would be intermediate
between the two extremes and the overall rate would therefore
be greater.
(
tBu4PCP)Ir, and (tBu4PCOP)Ir are approximately (1 h:
1:2.1:7.4), (3 h: 1:2.2:8.4), and (8 h: 1:1.8:7.0). Since a larger
fraction of the hexane has undergone conversion in the
To test the validity of this reasoning, we investigated the
nature of the resting state in the (tBu4PCOP)Ir-catalyzed
reaction. An n-hexane solution of (tBu4PCOP)IrH2 (10 mM),
MoF12 (16 mM), and TBE (40 mM) was prepared and
monitored by 31P NMR at 90 °C. After 60 min, it exhibited two
major sets of signals: a smaller set attributable to (tBu4PCOP)-
IrH2 and a larger set (a doublet at δ 171.2 ppm and doublet at δ
(
tBu4PCOP)Ir-catalyzed reactions (e.g., 41% after 8 h), the
values indicated for this catalyst are somewhat lower than the
actual relative rates because of increased back-reactions that
produce hexane as well as the decreased rate of reaction with
hexane due to its lowered concentration.
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67.6 ppm) that appeared to be attributable to the olefin-bound
complex. In a separate NMR tube, 40 mM 1-hexene was added
to a p-xylene-d10 solution of 10 mM tBu4PCOPIrH2; this yielded
the doublets at 171.2 and 67.6 ppm in the 31P NMR spectrum,
indicating that the larger set of peaks is, indeed, attributable to
an olefin-bound species. Thus, the mixture of hydride and
olefin complex in the alkane solutions confirms that the rates of
alkane dehydrogenation and olefin hydrogenation are, indeed,
comparable with this catalyst.
quite similar to that of 2c. The Ir−P(O) distance is slightly
shorter in 2d (2.245 Å vs 2.269 Å in 2c), presumably allowed
by the less bulky i-Pr groups, whereas the Ir−P(CH2) distance
is slightly longer in 2d (2.329 Å vs 2.319 Å), probably as a result
of the greater trans influence due to the shorter Ir−P(O)
distance. The P−Ir−P angle of 2d is 163.76(4)°, nearly
identical to that in 2c, 163.55(3)°.
The hydride complex 3d was prepared from (tBu2PCOPiPr2)-
IrHCl (2d) in analogy with the synthesis of 3c; this relatively
uncrowded complex was isolated as an 85:15 mixture of the
corresponding tetrahydride and dihydride. (Note that whereas
Other PCOP-Based Complexes. Encouraged by our
success with (tBu4PCOP)Ir, we investigated the effect of varying
the alkyl groups while maintaining the PCOP motif. We
prepared, in analogy to the preparation of tBu4PCOP-H
(Scheme 1), iPr4PCOP-H and the previously reported33
tBu2PCOPiPr2-H. The 31P NMR spectra of these PCOP ligand
precursors were consistent with the spectra of the respective
(
tBu4PCP)IrH4 easily loses H2 to give the isolable (tBu4PCP)-
IrH2, the less crowded (iPr4PCP)IrH4 does not so readily lose
H2,40 and the corresponding (iPr4PCP)IrH2 has not been
reported.) Difficulty in synthesizing the analogous iPr4PCOP
iridium hydrides led us to conduct the reduction of 2e with
LiEt3BH under ethylene atmosphere instead of dihydrogen,
affording (iPr4PCOP)Ir(C2H4) as a brown solid in 95% yield;
(pincer)Ir olefin complexes are generally assumed to be
functionally equivalent to the hydrides as catalyst precursors.20
iPr4-
“non-hybrid” species, tBu4PCP, tBu4POCOP, iPr4PCP, and
POCOP; that is, all signals in the 31P NMR spectra of the
hybrids had chemical shifts similar to the tBu2PCH2, tBu2PO,
iPr2PCH2, and iPr2PO groups in the respective symmetrical
ligand precursors. The crude oils were used without further
purification, and metalation with [Ir(COD)Cl]2 yielded the
corresponding (PCOP)IrHCl species in good yield (Scheme
2). Crystals of (tBu2PCOPiPr2)IrHCl were obtained, and the
molecular structure (Figure 6) was determined by X-ray
(
tBu2PCOPiPr2)IrH4 (3d) afforded catalytic rates even greater
than those of (tBu4PCOP)IrH2 (3c) (Table 4 and Figure 7).
After 1 h (under the same conditions as the catalyzes discussed
above), 1.75 M metathesis products were obtained, represent-
ing a rate 4.4 times that of 3c or ∼32 times that of
(
tBu4POCOP)IrH2 (3b). After 3 and 8 h, 3.2 and 4.6 M
product was observed. 3d is the most effective dehydrogenation
catalyst for alkane metathesis reported to date.
(
iPr4PCOP)Ir(C2H4) (3e)41 was also found to give a higher
turnover number, after 1 h, than the (tBu4PCOP)Ir precursor
3c, but not as great as obtained with 3d. Moreover, productivity
leveled off with 3e earlier than with catalysts 3a−d; it seems
likely this is due to bimolecular degradation of the sterically
very open complex, possibly dimerization or perhaps a reaction
with MoF12. In any case, 3e seems less promising than 3d.
A brief summary of results obtained with all five catalysts is
given in Table 5.
Selectivity. The molecular-weight or chain-length selectiv-
ity obtained with all three (PCOP)Ir catalysts was similar to
that obtained with (tBu4POCOP)Ir and not intermediate
between (tBu4POCOP)Ir and (tBu4PCP)Ir (the latter is much
more selective for the formation of C2n−2 alkanes and ethane
from Cn alkanes, in this case n-decane and ethane from n-
hexane). We believe that the large selectivity difference
observed for the (tBu4POCOP)Ir and (tBu4PCP)Ir-based
catalysts in alkane metathesis reflects a different regioselectivity
of alkane dehydrogenation, which results from the two catalysts
having a different rate- and product-determining step (e.g., C−
H addition, β-H elimination, or olefin extrusion) in the overall
alkane dehydrogenation.42 The similar selectivity (or lack
thereof) in alkane metathesis observed for the (PCOP)Ir
complexes and (tBu4POCOP)Ir suggests that these complexes
have the same rate- and product-determining step within the
alkane dehydrogenation sequence (or at least that they do not
share the same rate- and product-determining step as
Figure 6. Thermal ellipsoid plot of (tBu2PCOPiPr2)IrHCl (2d).
Hydrogen atoms other than the hydride ligand omitted for clarity.
crystallography. Selected bond lengths and angles are
summarized in Table 3. In general, the structure of 2d is
Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for
(
tBu2PCOPiPr2)IrHCl (2d)
bond lengths (Å)
bond angles (deg)
C(1)−Ir(1)−P(1)
Ir(1)−C(1)
Ir(1)−P(1)
Ir(1)−P(2)
Ir(1)−Cl(1)
P(1)−O(1)
P(1)−C(8)
P(1)−C(11)
P(2)−C(7)
P(2)−C(14)
O(1)−C(2)
2.011(4)
2.2446(12)
2.3292(12)
2.3888(12)
1.643(4)
1.800(6)
1.848(7)
1.843(5)
1.866(5)
1.396(6)
80.99(13)
83.05(13)
163.76(4)
173.52(13)
97.03(3)
C(1)−Ir(1)−P(2)
P(1)−Ir(1)−P(2)
C(1)−Ir(1)−Cl(1)
P(2)−Ir(1)−Cl(1)
P(1)−Ir(1)−Cl(1)
O(1)−P(1)−C(8)
O(1)−P(1)−Ir(1)
C(7)−P(2)−Ir(1)
C(2)−O(1)−P(1)
C(6)−C(7)−P(2)
(
tBu4PCP)Ir). Further results of selectivity studies will be
reported in due course.
Summary. Bisphosphine complex (tBu4PCP)Ir and bi-
sphosphinite complex (tBu4POCOP)Ir, previously reported to
catalyze alkane metathesis in tandem with MoF12, catalyze
transfer dehydrogenation with similar rates under typical alkane
metathesis conditions. Different resting states observed for each
catalyst, however, indicate that different segments of the
99.16(3)
101.9(3)
105.64(14)
102.59(18)
114.7(3)
110.6(3)
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ACS Catalysis
Research Article
Table 4. Product Concentrations (mM) of Products Formed by Metathesis of n-Hexane (7.5 M) Catalyzed by MoF12 (16 mM)
and Catalysts 3d or 3e (10 mM) at 125 °C
Catalyst 3d, (tBu2PCOPiPr2)IrH4
t (h)
C2
C3
C4
C5
C7
C8
C9
trans-5-decene
C10
C11−19
[total product]
selectivity C10/(C7−10)
1
3
133
220
450
344
339
595
986
974
248
509
764
792
385
754
224
441
535
572
105
218
278
293
120
204
234
242
4
2
1
1
143
193
190
189
48
99
1750
3234
4594
4619
0.25
0.18
0.15
0.15
8
969
187
183
24
1029
Catalyst 3e, (iPr4PCOP)Ir(C2H4)
t (h)
C2
C3
C4
C5
C7
C8
C9
trans-5-decene
C10
C11−19
[total product]
selectivity C10/(C7−10)
1
3
25
55
119
307
398
717
78
202
261
463
179
424
521
771
118
256
298
368
46
102
117
146
69
138
153
167
1
3
4
0
67
169
200
197
10
31
712
1687
2066
3076
0.23
0.26
0.26
0.22
8
70
44
24
127
120
cycle are comparably fast under catalytic conditions, the resting
state is found to be a mixture of dihydride and dehydrogenated
(olefin-bound) complex.
Decreasing the steric bulk of the phosphine−phosphinite
pincer led to the synthesis of catalyst precursor (tBu2PCOPiPr2)-
IrH4 (3d), which exhibits rates of catalysis about a factor of 4
faster than (tBu4PCOP)IrH2, that is, ∼32 times faster than
(
(
tBu4POCOP)Ir. Further decreasing steric crowding led to a
iPr4PCOP)Ir precursor (the corresponding ethylene complex)
that gave rates faster than (tBu4PCOP)Ir but slower than
(
tBu2PCOPiPr2)IrH4 and appeared to undergo decomposition
more readily than the other catalysts studied.
EXPERIMENTAL SECTION
■
General Information. All manipulations were conducted
under an argon atmosphere either in a glovebox or using
standard Schlenk techniques. All anhydrous solvents were
purchased from Aldrich, flushed with argon, and stored in an
argon atmosphere in a glovebox. Mesitylene, n-hexane
(anhydrous, 99%+), tert-butylethylene (TBE; 3,3-dimethyl-1-
butene, 97%) were purchased from Aldrich and were distilled
over Na/benzophenone and stored in a glovebox. (tBu4PCP)-
Figure 7. Total alkane products formed in the metathesis of n-hexane
catalyzed by MoF12 and PCOP catalysts 3c−e.
Table 5. Summary of Results with All Iridium Catalysts
a
Investigated
c
initial TOF
TON
selectivity C10/
b
iridium catalyst
3a, (tBu2PCPtBu2)IrH2
(h−1
11
)
(24 h)
(C7−10)
111
134
400
462
308
0.48
0.12
0.14
0.15
0.22
IrH2,35
(
tBu4POCOP)IrH2,27,34 3-(bromomethyl)phenol,43
3b, (tBu2POCOPtBu2)IrH2
3c, (tBu2PCOPtBu2)IrH2
3d, (tBu2PCOPiPr2)IrH4
3e, (iPr4PCOP)Ir(C2H4)
5.4
40
HPiPr2,44 and Mo(NAr(CHCMe2Ph)(ORF6)2 (Ar = 2,6-i-
PrC6H3; ORF6 = OCMe(CF3)2)45 were prepared as described
175
71
i
t
previously. Pr2PCl and Bu2PCl were used as purchased from
Strem Chemicals, Inc. All other reagents were obtained from
commercial sources and used as received. All glassware was
placed in a vacuum oven at least 24 h prior to use. Stock
solution vials and glass tubes for reaction vessels (5 mm ×120
mm) were flame-dried before being placed in the vacuum oven.
NMR spectra were recorded on 400- or 500-MHz Varian
a
Conditions as described in text: 16 mM MoF12, 10 mM iridium
catalyst in n-hexane (7.6 M), TBE (2.0 equiv per mole of catalyst for
dihydrides 3a−c, 4.0 equiv per mole for tetrahydride 3d, none for
b
ethene complex 3e). TOF = turnover frequency = moles of product
(total alkane products) per mole of iridium catalyst observed after 1 h.
TON = turnover frequency = moles of product (total alkanes) per
mole of iridium catalyst observed after 24 h.
c
1
VNMRS spectrometers. H NMR spectra are referenced to
residual protio signal in the deuterated solvent. 31P{1H} NMR
chemical shifts are referenced to an external standard consisting
of PMe3 (δ −62.4 ppm) in mesitylene-d12 solvent inside a
flame-sealed capillary tube. Elemental analyses were performed
by Roberston Microlit Laboratories, Ledgewood, New Jersey.
GC Method. Gas chromatography was performed on a
Varian 430 gas chromatograph utilizing flame ionization
detection with the following parameters:
transfer dehydrogenation cycle are rate-determining in each
case. This observation led to the hypothesis that for a species
with intermediate properties, neither segment of its catalytic
cycle would be as slow as the respective rate-determining
segments for (tBu4PCP)Ir and (tBu4POCOP)Ir, and thus, the
overall rate of catalysis would be faster. Indeed, the “hybrid”
phosphine−phosphinite catalyst (tBu4PCOP)Ir is found to
cocatalyze alkane metathesis ∼4 and 8 times faster than
Column: 30 m × 0.25 mm × 0.5 μm (Supelco) fused
silica capillary column, Petrocol, HD
Starting temperature: 38 °C
Time at starting temp: 1.4 min
Ramp: 20 °C/min to 250 °C, hold time 3 min
(
tBu4PCP)Ir and (tBu4POCOP)Ir, respectively, and affords
higher total turnover numbers. In accord with the idea that
the hybrid catalyst approaches a balance whereby the
hydrogenation and dehydrogenation segments of the catalytic
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ACS Catalysis
Ramp: 280 °C at 30 °C/min, hold time 35 min
Research Article
precipitate was treated with a saturated solution of aqueous
NaHCO3 (40 mL) and stirred at 80 °C for 6 h. After cooling to
room temperature, the mother liquor was cannulated out, and
the white precipitate was dried under vacuum. Product was
extracted with Et2O or THF (30 mL × 3) and the combined
solution was evaporated under vacuum to obtain the waxy
orange products.
Flow rate: 1 mL/min (He)
Split ratio: 90
Ending temp: 280 °C
Injector temp: 300 °C
Detector temp: 310 °C
3-(iso-Propylphosphinomethyl)phenol. Yield: 3.120 g,
13.91 mmol, 65%. (NMR δ, CDCl3) 1H: 7.11 (t, JHH = 7.75 Hz,
1H, Ar−H), 6.82 (d, JHH = 7.5 Hz, 1H, Ar−H), 6.77 (s, 1H,
Ar−H), 6.63 (d, JHH = 9.0 Hz, 1H, Ar−H), 4.73 (s, 1H, OH),
GC Response Factors. The GC response factors for the
selected n-alkanes (C6, C8, C10, C12, and C15) were obtained
experimentally. A known concentration of the alkane and a
mesitylene standard was prepared and injected into the GC.
The response factor of each alkane was calculated as an average
of three independent runs. A plot of response factor versus
alkane carbon number was generated and found to be
essentially linear; the response factors for alkanes not directly
measured were extrapolated from this plot. The Rf of trans-5-
decene was measured independently.
General Procedure for Alkane Metathesis Catalysis. In
an argon-filled glovebox, a 20-mL vial was charged with the
(pincer)Ir catalysts (3a−e, 0.050 mmol) and the MoF12
catalyst (0.080 mmol). A 5.0 mL portion of an n-hexane
solution containing mesitylene (0.162 mmol as an internal
standard) and TBE (0.100 mmol for dihydrides 3a−c, 0.200
mmol for tetrahydride 3d, none for ethene complex 3e) was
added, and the solution was thoroughly mixed. Twelve aliquots
of this stock solution (0.400 mL each) were then syringed into
glass tubes (5 mm × 120 mm). Vacuum adapters were fixed
onto the tubes via plastic tubing to allow for flame-sealing.
Samples were frozen in liquid nitrogen, and the headspace was
evacuated on a high vacuum line, after which the glass tubes
were flame-sealed so that the ratio between liquid phase and
headspace was approximately 1:1. The samples were placed in a
temperature-calibrated GC oven at 125 °C. Tubes were taken
from the oven at appropriate intervals and frozen in liquid
nitrogen to minimize the escape of volatile alkanes. The seal of
each tube was then broken and quickly capped with a 5-mm
rubber septum. The sample was then brought back to room
temperature and promptly analyzed by GC.
3
2.74 (d, 2JPH = 1.5 Hz, 2H, CH2), 1.74 (d of septet, JHH = 7.0
2
Hz, JPH = 2.5 Hz, 2H, CH(CH3)2), 1.03−1.09 (m, 12H,
CH(CH3)2). 13C{1H}: 155.5 (s, Ar−C), 141.9 (d, JPC = 8.2 Hz,
Ar−C), 129.5 (s, Ar−C), 121.8 (d, JPC = 6.5 Hz, Ar−C), 116.1
(d, JPC = 7.4 Hz, Ar−C), 112.5 (d, JPC = 2.0 Hz, Ar−C), 29.5
1
1
(d, JPC = 20.1 Hz, CH2), 23.3 (d, JPC = 14.1 Hz, 2C,
2
CH(CH3)2), 19.6 (d, JPC = 13.4 Hz, 2C, C(CH3)2), 19.2 (d,
2JPC = 10.7 Hz, 2C, CH(CH3)2). 31P{1H}: 12.1 (s).
3-(tert-Butylphosphinomethyl)phenol. Yield: 4.807 g,
19.05 mmol, 89%. (NMR δ, CDCl3) 1H: 7.01 (t, JHH = 7.8 Hz,
1H, Ar−H), 6.82 (d, JHH = 7.5 Hz, 1H, Ar−H), 6.75 (s, 1H,
Ar−H), 6.47 (d, JHH = 8.0 Hz, 1H, Ar−H), 4.66 (s, 1H, OH),
2.71 (d, 2JPH = 3.5 Hz, 2H, CH2), 1.06 (d, 3JPH = 11.0 Hz, 18H,
2 C(CH3)3). 13C{1H}: 155.8 (s, Ar−C), 143.8 (d, JPC = 12.4
Hz, Ar−C), 129.7 (s, Ar−C), 122.3 (d, JPC = 8.0 Hz, Ar−C),
116.7 (d, JPC = 9.2 Hz, Ar−C), 112.7 (d, JPC = 2.1 Hz, Ar−C),
1
2
32.1 (d, JPC = 21.7 Hz, CH2), 30.1 (d, JPC = 13.1 Hz, 6C,
C(CH3)3), 28.6 (d, JPC = 23.2 Hz, 2C, C(CH3)3). 31P{1H}:
1
33.2 (s).
General Procedure for Syntheses of PCOP-H Ligands.
A solution of the appropriate 3-(dialkylphosphinomethyl)-
t
phenol (for compound 1c, R = Bu, 2.657 g, 10.53 mmol; for
compound 1e, R = iPr, 0.480 g, 2.14 mmol; for compound 1d,
t
R = Bu, 0.540 g, 2.14 mmol) in THF (10 mL; 40 mL in the
case of 1c) was added dropwise to the suspension of NaH
(0.2779 g, 11.58 mmol in the case of 1c; 0.057 g, 2.37 mmol for
1d and 1e) in THF (20 mL), and the reaction mixture was
heated to reflux for 1.5 h. After cooling to room temperature,
the solution of dialkylchlorophosphine, R′2PCl (for compound
Resting State of (tBu4PCOP)Ir in Alkane Metathesis. A
solution of 10 mM tBu4PCOPIr(H)2 (3c, 0.005 mmol), 16 mM
MoF12 catalyst (0.008 mmol), 40 mM TBE (0.020 mmol), and
roughly 7.6 M n-hexane (total volume of solution: 0.5 mL) was
added to an NMR tube along with an external PMe3 in p-
xylene-d10 capillary insert, and the NMR tube was subsequently
flame-sealed. This mixture was heated to 90 °C in the NMR
spectrometer for ∼60 min. After 60 min, 31P NMR exhibited
two main sets of peaks of differing intensity: a smaller set of
peaks indicative of the dihydride species (3c, 24%) and larger
set of peaks consisting of a doublet at 171.2 ppm and a doublet
at 61.6 ppm (76%) indicative of an olefin-bound complex. To
confirm the identity of this putative olefin-bound species, in a
separate NMR tube, 10 mM tBu4PCOPIrH2 and 40 mM 1-
hexene (in p-xylene-d10 solution, 0.5 mL total sample volume)
were mixed, reproducing the sets of doublets at 171.2 and 61.6
ppm in the 31P NMR and confirming that the larger set of
peaks is attributable to an olefin-bound species.
t
1c, R′ = Bu, 2.00 mL, 10.50 mmol; for compound 1e and 1d,
i
R′ = Pr, 0.326 g, 2.14 mmol) in THF (10 mL) was added
dropwise through a cannula, and the resultant reaction mixture
was heated to reflux for 2 h. The solvent was then removed
under vacuum, and the product was extracted with pentane (20
mL × 2). The combined pentane solution was removed under
vacuum to obtain a pale yellow viscous liquid product.
tBu4PCOP-H (1c). Yield: 3.514 g, 8.862 mmol, 84%. (NMR, δ,
C6D6) 1H: 7.62 (s, 1H, Ar−Hipso), 7.10−7.18 (m, 3 H, Ar−H),
2.80 (d, 2JPH = 2.25 Hz, 2H, CH2), 1.21 (d, 3JPH = 11.6 Hz, 18
3
H, C(CH3)3), 1.11 (d, JPH = 10.5 Hz, 18H, C(CH3)3).
3
13C{1H}: 160.3 (d, JPC = 9.4 Hz, Ar−Cipso), 143.7 (dd, JPC
=
12.7 Hz, JPC = 0.7 Hz, Ar−C), 129.5 (s, Ar−C), 123.3 (dd, JPC
= 8.7 Hz, JPC = 1.2 Hz, Ar−C), 120.1 (dd, JPC = 11.3 Hz, JPC
9.1 Hz, Ar−C), 115.9 (dd, JPC = 10.5 Hz, JPC = 1.9 Hz, Ar−C),
=
1
1
35.7 (d, JPC = 26.8 Hz, CH2), 31.8 (d, JPC = 24.5, C(CH3)3),
2
1
Synthesis of 3-(Alkylphosphinomethyl)phenol. A
30.0 (d, JPC = 13.5 Hz, C(CH3)3), 29.2 (d, JPC = 25.8 Hz,
2
mixture of 3-(bromomethyl)phenol (4.003 g, 21.40 mmol)
C(CH3)3), 27.6 (d, JPC = 15.8 Hz, C(CH3)3). 31P{1H}: 152.7
i
t
and HPR2 (R = Pr, 2.554 g, 21.61 mmol; R = Bu, 3.161 g,
21.62 mmol) in degassed acetone (30 mL) was heated to reflux
for 12 h and then stirred at room temperature overnight. The
mother liquor was decanted from the waxy white precipitate
that formed, and the precipitate was dried under vacuum. The
(s, O−P), 34.1 (s, CH2−P).
tBu2PCOPiPr2-H (1d). Yield: 0.591 g, 1.604 mmol, 75%. (NMR
δ, C6D6) 1H: 7.47 (s, 1H, Ar−Hipso), 7.08 (d, JHH = 4.0 Hz, 2H,
2
Ar−H), 7.03 (t, JHH = 4.0 Hz, 1H, Ar−H), 2.71 (d, JPH = 2.0
3
2
Hz, 2H, CH2), 1.77 (d of septet, JHH = 5.0 Hz, JPH = 2.5 Hz,
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ACS Catalysis
Research Article
3
3
2H, CH(CH3)2), 1.15 (dd, JHH = 10.3 Hz, JPH = 6.6 Hz, 6H,
(
tBu2PCOPiPr2)IrHCl (2d). Yield: 0.662 g, 1.11 mmol, 86%.
(NMR, δ, C6D6) 1H: 6.84−6.89 (m, 3H, Ar−H), 3.10
2 2
3
2CH(CH3), 1.03 (d, JPH = 10.6 Hz, 18H, C(CH3)3, 0.99 (dd,
3JHH = 10.2 Hz, 3JPH = 7.4 Hz, 6H, 2CH(CH3). 13C{1H}: 160.4
(d, JPC = 9.0 Hz, Ar−C), 144.3 (d, JPC = 12.4 Hz, Ar−C), 137.6
(s, Ar−C), 124.0 (dd, JPC = 9.0 Hz, JPC = 1.1 Hz, Ar−C), 120.9
(dd, JPC = 10.7 Hz, JPC = 9.4 Hz), 116.6 (dd, JPC = 10.5 Hz, JPC
(apparent septet, JHaHb = 18.3 Hz, JHP = 9.0 Hz, CHaHb),
2.69 (septet, 3JHH = 7.0 Hz, 1H, CH(CH3)2), 2.28 (septet, 3JHH
= 6.0 Hz, 1H, CH(CH3)2), 1.27 (dd partially obscured by
another signal, JHH = 7.2 Hz, CH(CH3)2), 1.21 (dd, JHH = 13.1
Hz, 18H, C(CH3)3), 1.16 (dd partially obscured by another
signal, JHH = 6.5 Hz, CH(CH3)2), 1.12 (dd partially obscured
= 1.9 Hz, Ar−C), 32.3 (d, 1JPC = 24.4 Hz, CH2), 30.5 (d, 2JPC
=
13.4 Hz, C(CH3)3), 29.6 (d, 1JPC = 25.8 Hz, C(CH3)3), 29.2 (d,
1JPC = 18.6 Hz, CH(CH3)3), 18.5 (d, 2JPC = 20.5 Hz, C(CH3)2),
by another signal, JHH = 6.0 Hz, CH(CH3)2), 0.99 (dd, JHH
=
2
2
17.8 (d, JPC = 8.6 Hz, C(CH3)2). 31P{1H}: 147.1 (s, O−P),
2
7.0 Hz, CH(CH3)2), −39.45 (dd, JPH = 15.7 Hz, JPH = 11.0
Hz). 13C{1H}: 137.0 (s, Ar−C), 127.3 (d, JPC = 18.2 Hz, Ar−
C), 126.7 (d, JPC = 23.4 Hz, Ar−C), 124.4 (s, Ar−C), 118.3 (d,
JPC = 15.3 Hz, Ar−C), 108.6 (d, JPC = 12.1 Hz, Ar−C), 38.3
33.5 (s, CH2−P).
iPr4PCOP-H (1e). Yield: 0.561 g, 1.65 mmol, 77%. (NMR, δ,
1
C6D6) H: 7.43 (br s, 1H, Ar−Hipso), 7.12−7.07 (m, 2H, Ar−
(dd, JPC = 16.0 Hz, JPC = 3.3 Hz CH(CH3)2), 35.1 (dd, JPC
=
H), 6.96 (d, JHH = 7.0 Hz, 1H, Ar−H), 2.64 (s, 2H, CH2), 1.78
1
(d of septet, 3JHH = 7.0 Hz, 2JPH = 2.5 Hz, 2H, OPCH(CH3)2),
17.3 Hz, JPC = 3.3 Hz CH(CH3)2), 34.9 (d, JPC = 30.4 Hz,
CH2), 30.6 (dd, JPC = 25.7 Hz, JPC = 4.3 Hz C(CH3)3), 29.1
(dd, JPC = 29.8 Hz, JPC = 5.1 Hz C(CH3)3), 17.4 (br s,
3
2
1.57 (d of septet, JHH = 7.0 Hz, JP3H = 2.0 Hz, 2H,
3
PCH(CH3)2), 1.16 (dd, JHH = 10.5 Hz, JPH = 7.0 Hz, 6H,
CH(CH3), 1.02−0.99 (m, 18H, CH(CH3)2). 13C{1H}46: 160.4
C(CH3)3), 16.8 (d, JPC = 7.4 Hz, CH(CH3)2), 16.7 (dd, JPC
=
1
3.3 Hz, JPC = 1.9 Hz C(CH3)2), 16.6 (d, JPC = 11.9 Hz,
C(CH3)3), 16.4 (d, 1JPC = 13.3 Hz, CH(CH3)3). 31P{1H}: 165.4
(dd, 2JPP = 350 Hz, 2JPH = 12.0 Hz O−P), 72.9 (dd, 2JPP = 350
Hz, 2JPH = 8.0 Hz, CH2−P). Anal. Calcd. for C19H34OIrP2Cl: C,
42.31; H, 6.24; Cl, 5.95. Found: C, 42.59; H, 6.37; Cl, 5.81.
4
(d, JPC = 8.5 Hz, Ar−Cipso), 142.5 (d, JPC = 8.23 Hz, Ar−C),
129.6 (s, Ar−C), 123.2 (dd, JPC = 5.8 Hz, JPC = 1.1 Hz, Ar−C),
120.0 (dd, JPC = 10.8 Hz, JPC = 7.4 Hz), 116.2 (dd, JPC = 10.5
1
Hz, JPC = 2.0 Hz, Ar−C), 30.2 (d, JPC = 22.2 Hz, CH2), 28.6
1
1
(d, JPC = 18.5 Hz, 2C, CH(CH3)2), 23.8 (d, JPC = 16.2 Hz,
(
iPr4PCOP)IrHCl (2e). Yield: 0.462 g, 0.814 mmol, 63%.
2C, CH(CH3)2), 19.8 (d, 2JPC = 14.4 Hz, CH(CH3)2), 19.3 (d,
(NMR, δ, C6D6) 1H: 6.89−6.85 (m, 2H, Ar−H), 6.78 (d, JHH
=
=
2JPC = 11.2 Hz, CH(CH3)2), 17.9 (d, JPC = 20.6 Hz,
2
2.0 Hz, 1H, Ar−H), 2.75 (apparent qd, 2JHaHb = 18.0 Hz, 2JPH
CH(CH3)2), 17.2 (d, JPC = 8.7 Hz, CH(CH3)2). 31P{1H}:
2
9.3 Hz, 2H, CHaHb), 2.65−2.69 (m, 1H, CH(CH3)2), 2.27 (d
of septet, 3JHH = 4.5 Hz, 2JPH = 2.5 Hz, 1H, CH(CH3)2), 2.05−
2.08 (m, 1H, CH(CH3)2), 1.95 (d of septet, 3JHH = 6.5 Hz, 2JPH
= 1.5 Hz, 1H, CH(CH3)2), 1.29 (dd partially obscured by
another signal, 3JHH = 17.0 Hz, 3JPH = 7.0 Hz, 3H, CH(CH3)2),
1.17 (dd, JHH = 17.0 Hz, JPH = 7.5 Hz, 3H, CH(CH3)2),
1.16−1.13 (m, 6H, CH(CH3)2), 1.11 (dd partially obscured by
another signal, 3JHH = 11.0 Hz, 3JPH = 7.5 Hz, 3H, CH(CH3)2)
147.4 (s, O−P), 10.2 (s, CH2−P).
General Procedure for Syntheses of (PCOP)IrHCl
Complexes. A mixture of the appropriate (RPCOPR′-H)
(3.0 mL of 0.367 M solution in toluene for 1c, 1.1 mmol; 0.462
g of 1e, 1.36 mmol; 0.500 g of 1d, 1.36 mmol) and
[Ir(COD)Cl]2 (0.335 g, 0.499 mmol for 1c; 0.434 g, 0.646
mmol for 1e and 1d) in toluene (15 mL) was heated to reflux
(72 h for 1c, 3 h for 1e, and 12 h for 1d) under H2 atmosphere.
After cooling the reaction mixture to room temperature, the
mother liquor was evaporated under vacuum. The product was
extracted with pentane (60 mL × 3) and the combined pentane
solution was evaporated to obtain the orange-red crystalline
products of 2c−e.
3
3
3
3
1.03 (dd, JHH = 15.5 Hz, JPH = 7.0 Hz, 3H, CH(CH3)2),
2
0.87−0.82 (m, 6H, CH(CH3)2), −36.54 (apparent t, JPH
=
13.8 Hz). 13C{1H}46: 166.3 (s, Ar−Cipso), 149.7 (s, Ar−C),
2
124.2 (s, Ar−C), 118.5 (d, JPC = 16.3 Hz, Ar−C), 109.2 (d,
2JPC = 11.7 Hz, Ar−C), 33.3 (d, 1JPC = 32.0 Hz, CH2), 31.3 (dd,
1JPC = 25.4 Hz, 3JPC = 5.0 Hz, CH(CH3)2), 29.3 (dd, 1JPC = 29.0
(
tBu4PCOP)IrHCl (2c). Yield: 0.470 g, 0.753 mmol, 68%.
Hz, 3JPC = 5.1 Hz, CH(CH3)2), 25.1 (dd, 2JPC = 23.6 Hz, 4JPC
=
(NMR, δ, C6D6) 1H: 6.93−6.83 (m, 3H, Ar−H), 3.10 (dd, 2JHH
2
4
2.4 Hz, CH(CH3)2), 24.6 (dd, JPC = 25.9 Hz, JPC = 2.6 Hz,
CH(CH3)2), 18.3 (dd, JPC = 12.4 Hz, JPC = 4.0 Hz, 4C,
= 17.6 Hz, 2JPH = 9.5 Hz, 1H, CH2), 3.00 (dd, JHH = 17.6 Hz,
2
2
4
2JPH = 8.9 Hz, 1H, CH2), 1.34 (d, JPH = 14.0 Hz, 9H,
3
2
4
CH(CH3)2), 17.1 (dd, JPC = 11.0 Hz, JPC = 3.8 Hz, 4C,
C(CH3)3), 1.29 (d, 3JPH = 14.2 Hz, 9H, C(CH3)3), 1.22 (d, 3JPH
CH(CH3)2). 31P{1H}: 168.2 (dd, JPP = 355 Hz, JPH = 8.5 Hz,
2
3
= 13.0 Hz, 9H, C(CH3)3), 1.19 (d, JPH = 13.4 Hz, 9H,
O−P), 60.7 (dd, JPP = 356 Hz, JPH = 3.2 Hz, CH2−P).
Unknown species (12% based upon integrals from H NMR),
2
2
C(CH3)3), −41.38 (dd, JPH = 13.3 Hz, JPH = 12.3 Hz).
1
46
13C{1H} : 168.1 (apparent t, JPC = 6.1 Hz, Ar−Cipso), 152.0
3
1
(NMR, δ, C6D6): H −25.2, (JHH = 17.0 Hz, JPH = 11.0 Hz),
31P{1H}: 150.5 (dd, JPP = 361 Hz, JPH = 8.1 Hz), 42.0 (dd,
2JPP = 363 Hz, 2JPH = 5.4 Hz). Anal. Calcd. for C19H34OIrP2Cl:
C, 40.17; H, 6.03; Cl, 6.24. Found: C, 42.15; H, 6.17; Cl,
6.40.47
2
2
(dd, JPC = 11.3 Hz, JPC = 5.5 Hz, Ar−C), 132.2 (dd, JPC = 5.6
Hz, JPC = 3.0 Hz Ar−C), 124.6 (s, Ar−C), 118.1 (d, JPC = 15.5
1
Hz, Ar−C), 108.5 (d, JPC = 11.7 Hz, Ar−C), 43.6 (dd, JPC
=
3
1
19.3 Hz, JPC = 4.9 Hz, C(CH3)3), 39.4 (dd, JPC = 20.9 Hz,
3JPC = 5.9 Hz, C(CH3)3), 37.5 (dd, JPC = 15.9 Hz, JPC = 3.2
1
3
General Procedure for Syntheses of (PCOP)Ir(L). The
(PCOP)IrHCl (0.150 g, 0.247 mmol of 2c; 0.200 g, 0.352
mmol of 2e; 0.210 g, 0.352 mmol of 2d) was dissolved in
pentane (60 mL), and a 1.0 M solution (in THF) of LiBEt3H
(0.25 mL, 0.25 mmol for 2c; 0.35 mL, 0.35 mmol for 2e and
2d) was added dropwise via syringe under hydrogen or
ethylene atmosphere, causing the orange solution to turn a pale
yellow (brownish in the case of 2e) and resulting in the
precipitation of a white solid. The reaction mixture was stirred
for 30 min prior to cannula filtration of the solution. The
1
3
Hz, C(CH3)3), 35.3 (dd, JPC = 29.8 Hz, JPC = 1.0 Hz, CH2),
1
3
35.0 (dd, JPC = 17.7 Hz, JPC = 3.3 Hz, C(CH3)3), 29.9 (dd,
2JPC = 3.8 Hz, 4JPC = 1.3 Hz C(CH3)3), 29.3 (dd, 2JPC = 3.8 Hz,
4JPC = 1.3 Hz C(CH3)3), 27.8 (d, 2JPC = 4.9 Hz, C(CH3)3), 27.7
(dd, JPC = 4.9 Hz, JPC = 1.0 Hz, C(CH3)3). 31P{1H}: 168.6
2
4
2
2
2
(dd, JPP = 345.0 Hz, JPH = 12.3 Hz O−P), 70.6 (dd, JPP
=
2
345.0 Hz, JPH = 11.0 Hz, CH2−P). Anal. Calcd. for
C23H42OIrP2Cl: C, 44.26; H, 6.78; Cl, 5.68. Found: C, 44.03;
H, 6.59; Cl, 5.67.
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ACS Catalysis
Research Article
pentane solution was evaporated under vacuum to obtain a red
crystalline solid of 3c, orange solid of 3d, and brownish solid of
3e. In the case of complex 3d, the product was obtained as a
mixture of (tBu2PCOPiPr2)IrH4 and (tBu2PCOPiPr2)IrH2 in an
ASSOCIATED CONTENT
* Supporting Information
Crystal structure data of complexes 2c and 2d, NMR spectra of
complexes 3c and 3d, and 31P NMR spectrum of (tBu4PCOP)Ir
during alkane metathesis illustrating the catalyst resting state.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
1
85:15 ratio based upon H NMR spectroscopy. The calculated
yield is based on (tBu2PCOPiPr2)IrH4.
(
tBu4PCOP)IrH2 (3c). Yield: 0.136 g, 0.231 mmol, 93%.
1
2
(NMR, δ, C6D6) H: 7.23−7.13 (m, 3H, Ar−H), 3.50 (d, JPH
= 8.4 Hz, 2H, CH2), 1.34 (d, JPH = 13.8 Hz, 18H, C(CH3)3),
3
AUTHOR INFORMATION
Corresponding Author
*E-mail: alan.goldman@rutgers.edu.
■
3
1.19 (d, JPH = 12.9 Hz, 18H, C(CH3)3), −18.12 (apparent t,
2JPH = 8.6 Hz, 2H, IrH2). 13C{1H}: 171.0 (dd, JPC = 5.7 Hz,
3
3JPC = 3.1 Hz, Ar−Cipso) 170.7 (dd, JPC = 8.5 Hz, JPC = 7.0 Hz,
Ar−C), 157.6 (dd, JPC = 14.2 Hz, JPC = 5.8 Hz, Ar−C), 129.5
(dd, JPC = 0.6 Hz, Ar−C), 116.3 (d, JPC = 16.1 Hz, Ar−C),
107.7 (d, JPC = 12.5 Hz, Ar−C), 40.3 (d, 1JPC = 28.6 Hz, CH2),
Present Address
⊥(C.S.) Department of Chemistry & Biochemistry, Rowan
University, Glassboro, NJ 08028, United States
Notes
1
3
40.1 (dd, JPC = 20.7 Hz, JPC = 4.6 Hz, C(CH3)3), 35.1 (dd,
The authors declare no competing financial interest.
1JPC = 17.6 Hz, JPC = 2.4 Hz, C(CH3)3), 29.9 (dd, JPC = 5.3
3
2
Hz, 4JPC = 0.8 Hz, C(CH3)3), 29.0 (d, 2JPC = 6.3 Hz, C(CH3)3).
ACKNOWLEDGMENTS
■
31P{1H}: 200.3 (d, JPP = 330 Hz, O−P), 87.3 (d, JPP = 330
Hz, CH2−P). Anal. Calcd. for C23H43OIrP2: C, 46.84; H, 7.35.
Found: C, 41.89; H, 6.55.47
2
2
This work was supported by NSF through the CCI Center for
Enabling New Technologies through Catalysis (CENTC),
CHE-1205189 and CHE-0650456. J.D.H. is supported by an
NSF-IGERT fellowship (NSF DGE 0903675).
(
tBu2PCOPiPr2)IrH4 (3d). Yield: 0.185 g, 0.329 mmol, 94%.
(NMR, δ, p-Xyl-d10) 1H: 6.91 (t, JPH = 7.5 Hz, 1H, Ar−H), 6.82
(d, JPH = 7.4 Hz, 1H, Ar−H), 6.79 (d, JPH = 7.8 Hz, 1H, Ar−
REFERENCES
■
3
H), 3.23 (d, JPH = 9.0 Hz, 2H, CH2), 2.19 (septet, JHH = 2.2
(1) Brandt, A. R.; Millard-Ball, A.; Ganser, M.; Gorelick, S. M.
Environ. Sci. Technol. 2013, 47, 8031.
(2) Kerr, R. A. Science 2011, 331, 1510−1511.
(3) Murray, J.; King, D. Nature 2012, 481, 433−435.
(4) International Energy Agency (IAE) World Energy Outlook 2010;
http://www.worldenergyoutlook.org/docs/weo2010/WEO2010_es_
english.pdf
(5) (a) Dry, M. E. J. Chem. Technol. Biotechnol. 2001, 77, 43−50.
(b) Dry, M. E. Catal. Today 2002, 71, 227−241. (c) Dry, M. E. Appl.
Catal., A 2004, 276, 1−3.
Hz, 2H, CH(CH3)2), 1.16 (d, 3JPH = 12.8 Hz, 18H, C(CH3)3),
1.07 (dd, 3JHH = 15.0 Hz, 3JPH = 7.0 Hz, 6H, 2CH(CH3)), 0.98
(dd, 3JHH = 17.7 Hz, 3JPH = 7.0 Hz, 6H, 2CH(CH3)), −8.74 (t,
2JPH = 9.7 Hz, 4H, Ir−H). 13C{1H}: 163.4 (apparent t, JPC = 9.5
Hz, Ar−C), 148.5 (Ar−C), 135.1 (br s, Ar−C), 124.7 (s, Ar−
C), 117.2 (apparent t, JPC = 23.7 Hz, Ar−C), 107.6 (apparent t,
1
JPC = 19.6 Hz, Ar−C), 40.7 (d, JPC = 31.8 Hz, CH2), 32.5 (d,
2
1JPC = 18.5 Hz, C(CH3)2), 30.4 (d, JPC = 3.5 Hz, C(CH3)2),
2
(6) Leckel, D. Energy Fuels 2009, 23, 2342−2358.
(7) Holdebrandt, D.; Glasser, D.; Hausberger, B.; Patel, B.; Glasser,
B. J. Science 2009, 323, 1680−1681.
18.5 (d, JPC = 10.5 Hz, HC(CH3)2), 17.9 (br s, C(CH3)2).
2
2
31P{1H}: 171.2 (d, JPP = 327 Hz, O−P), 69.8 (d, JPP = 327
Hz, CH2−P). (tBu2PCOPiPr2)IrH2 1H: −17.2 (br s 2H, Ir−H).
(8) Zwart, R.; van Ree, R. Biofuels 2009, 95−116.
(9) Digman, B.; Joo, H. S.; Kim, D.-S. Environ. Prog. 2009, 28, 47−51.
(10) Demirbas, A. Prog. Energy Combust. 2007, 33, 1−18.
(11) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106,
4044−4098.
2
2
31P{1H}: 188.9 (d, JPP = 331, O−P, 87.2 (d, JPP = 330 Hz,
CH2−P). Anal. Calcd. for C21H41OIrP2: C, 44.74; H, 7.33.
Found: C, 41.83; H, 6.35.47
(
iPr4PCOP)Ir(C2H4) (3e). Yield: 0.187 g, 0.334 mmol, 95%.
1
3
(12) Demirbas, A.; Dincer, K. Energy Sources, Part A 2008, 30, 1233−
1241.
(NMR, δ, p-Xyl-d10) H: 7.09 (d, JHH = 7.5 Hz, 1H, Ar−H),
2
7.02−6.90 (m, 2H, Ar−H), 2.98 (d, JPH = 9.5 Hz, 2H, CH2),
(13) Centi, G.; Perathoner, S. Catal. Today 2009, 148, 191−205.
(14) Wang, W.; Wang, S. P.; Ma, X. B.; Gong, J. L. Chem. Soc. Rev.
2011, 40, 3703−3727.
2
2.67 (apparent t, JHH = 2.5 Hz, 4H, C2H4), 2.35 (d of septet,
3JHH = 7.0 Hz, JPH = 2.0 Hz, 2H, CH(CH3)2), 2.09 (d of
2
septet, 3JHH = 7.0 Hz, 2JPH = 2.0 Hz, 2H, CH(CH3)2), 1.18 (dd,
3JHH = 11.5 Hz, 3JPH = 7.0 Hz, 6H, CH(CH3)2), 1.10 (dd, 3JHH
(15) Borodko, Y.; Somorjai, G. A. Appl. Catal., A 1999, 186, 355−
362.
3
3
(16) Graves, C.; Ebbesen, S. D.; Mogensen, M.; Lackner, K. S. Renew.
Sust. Energy Rev. 2011, 15, 1−23.
(17) The Outlook for Energy: A View to 2040, ExxonMobil report,
2012, p 19; http://www.exxonmobil.com/Corporate/files/news_pub_
eo.pdf.
(18) Burnett, R. L.; Hughes, T. R. J. Catal. 1973, 31, 55−64.
(19) (a) Basset, J. M.; Coperet, C.; Lefort, L.; Maunders, B. M.;
Maury, O.; Le Roux, E.; Saggio, G.; Soignier, S.; Soulivong, D.; Sunley,
G. J.; Taoufik, M.; Thivolle-Cazat, J. J. Am. Chem. Soc. 2005, 127,
8604−8605. (b) Soulivong, D.; Coperet, C.; Thivolle-Cazat, J.; Basset,
J.-M.; Maunders, B. M.; Pardy, R. B. A.; Sunley, G. J. Angew. Chem., Int.
Ed. 2004, 43, 5366−5369. (c) Vidal, V.; Theolier, A.; Thivolle-Cazat,
J.; Basset, J.-M. Science 1997, 276, 99−102.
= 16.0 Hz, JPH = 7.0 Hz, 6H, CH(CH3)2), 1.03 (dd, JHH
=
15.5 Hz, 3JPH = 7.0 Hz, 6H, CH(CH3)2), 0.92 (dd, 3JHH = 12.5
Hz, 3JPH = 7.0 Hz, 6H, CH(CH3)2). 13C{1H}: 165.9 (dd, 3JPC
12.8 Hz, 3JPC = 7.8 Hz, Ar−Cipso), 157.8 (dd, JPC = 8.5 Hz, JPC
=
=
6.3 Hz, Ar−C), 149.3 (dd, JPC = 17.7 Hz, JPC = 5.3 Hz, Ar−C),
123.9 (s, Ar−C), 116.3 (d, JPC = 18.0 Hz, Ar−C), 107.0 (d, JPC
1
= 14.3 Hz, Ar−C), 37.3 (d, JPC = 31.5 Hz, 1C, CH2), 34.1
(apparent t, 2JPC = 1.5 Hz, 2C, C2H4), 29.8 (dd, 1JPC = 29.4 Hz,
3JPC = 3.7 Hz, 2C, CH(CH3)2), 25.2 (dd, 1JPC = 25.4 Hz, 3JPC
=
2
1.5 Hz, 2C, CH(CH3)2), 17.8 (d, JPC = 4.2 Hz, 2C,
2
CH(CH3)2), 17.5 (br s, 2C, CH(CH3)2), 17.1 (d, JPC = 5.3
Hz, 2C, CH(CH3)2), 16.6 (br s, 2C, CH(CH3)2). 31P{1H}:
(20) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.;
Brookhart, M. Science 2006, 312, 257−261.
(21) Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc.
Chem. Res. 2012, 45, 947−958.
2
2
175.0 (d, JPP = 286 Hz, O−P), 55.6 (d, JPP = 286 Hz, CH2−
P). Anal. Calcd. for C21H37OIrP2: C, 45.07; H, 6.66. Found: C,
43.76; H, 6.35.47
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Research Article
(22) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110, 681−
703.
(23) Bailey, B. C.; Schrock, R. R.; Kundu, S.; Goldman, A. S.; Huang,
Z.; Brookhart, M. Organometallics 2009, 28, 355−360.
(24) (a) Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray,
A.; Scott, S. L.; Vicente, B. C. Adv. Synth. Catal. 2009, 351, 188−206.
(b) Huang, Z.; Rolfe, E.; Carson, E. C.; Brookhart, M.; Goldman, A.
S.; El-Khalafy, S. H.; MacArthur, A. H. R. Adv. Synth. Catal. 2010, 352,
125−135.
(25) Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.;
Warmuth, R.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S.
Organometallics 2009, 28, 5432−5444.
(26) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S.
Chem. Rev. 2011, 111, 1761−1779.
(27) (a) Krogh-Jespersen, K.; Czerw, M.; Kanzelberger, M.;
Goldman, A. S. J. Chem. Inf. Comput. Sci. 2001, 41, 56−63.
(b) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc.
2003, 125, 7770−7771.
(28) Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004,
̈
126, 9330−9338.
(29) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. Organo-
̈
metallics 2004, 23, 1766−1776.
(30) Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
̈
Soc. 2004, 126, 1804−1811.
(31) Kundu, S. K. Ph. D. Thesis, Rutgers University, 2010.
(32) Renkema, K. B.; Kissin, Y. V.; Goldman, A. S. J. Am. Chem. Soc.
2003, 125, 7770−7771.
(33) Eberhard, M. R.; Matsukawa, S.; Yamamoto, Y.; Jensen, C. M. J.
Organomet. Chem. 2003, 687, 185−189.
(34) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020−1024.
(35) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C.
M. J. Am. Chem. Soc. 1997, 119, 840−841.
(36) Punji, B.; Emge, T. J.; Goldman, A. S. Organometallics 2010, 29,
2702−2709.
(37) Huang, Z. Ph. D. thesis, University of North Carolina at Chapel
Hill, 2009.
(38) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145−179. (b) Schrock,
R. R.; Czekelius, C. Adv. Synth. Catal. 2007, 349, 55−77. (c) Schrock,
R. R. Adv. Synth. Catal. 2007, 349, 41−53.
(39) Dobereiner, G. E.; Yuan, J.; Schrock, R. R.; Goldman, A. S.;
Hackenberg, J. D. J. Am. Chem. Soc. 2013, 135, 12572−12575.
(40) Liu, F.; Goldman, A. S. Chem. Commun. 1999, 655−656.
(41) Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.;
Brookhart, M.; Goldman, A. S. Nat. Chem. 2011, 3, 167−171.
(42) Biswas, S.; Zhou, T.; Wang, D. Y.; Hackenberg, J.; Nawara-
Hultzsch, A.; Schrock, R. R.; Brookhart, M.; Krogh-Jespersen, K.;
Goldman, A. S. Abstracts of Papers, 245th ACS National Meeting &
Exposition, New Orleans, LA, April 7−11, 2013, INOR-681.
(43) Przybilla, K. J.; Vogtle, F. Chem. Ber. 1989, 122, 347−355.
(44) Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.;
Goldman, A. S. J. Am. Chem. Soc. 2004, 126, 13044−13053.
(45) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.;
DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875−3886.
(46) In principle, six aromatic signals should be present in the
spectrum, but only five are observed, most likely due to overlap of one
peak with the C6D6 solvent peak.
(47) A satisfactory microanalysis was not obtained for this sample.
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