Investigation of iridium CF3PCP pincer catalytic dehydrogenation and decarbonylation chemistry

Article abstract of DOI:10.1021/om2011886

The iridium fluorinated pincer complex (CF₃PCP)Ir(cod) (CF ₃PCP = 2,6-C₆H₃(CH₂P(CF ₃)₂)₂) catalyzes hydrogen transfer from cyclooctane (coa) to tert-butylethylene (tbe) in 1/1 coa/tbe at 200 °C to give cyclooctene (coe) and neohexane (tba) at an initial rate of 40 TO h ^-1. In 5/1 coa/tbe, higher initial activity (155 TO h^-1) and higher turnovers (2580 TON's after 1450 min) are found. Samples of 95% tbe contain significant amounts of isoprene (2-methyl-1,3-butadiene), which reacts with (CF₃PCP)Ir(cod) to initially form (CF₃PCP) Ir(isoprene). Alkene inhibition studies show that (CF₃PCP)Ir is only modestly inhibited (67% reduced initial activity) in the presence of 800 equiv of added coe. Unlike donor pincer systems, no decrease in activity is noted under 1 atm of N₂ or in the presence of excess water. Hydrogenation of (CF₃PCP)Ir(L) (L = cod, isoprene) did not produce (CF ₃PCP)Ir(H)_x but instead afforded the first example of the unusual aryl-bridged bimetallic complex [(μ-1κ²(P,C), 2κ²(P′,C)-CF₃PCP)Ir(H)₂] ₂(μ-CF₃PCPH)(μ-H), which has been isolated and crystallographically characterized. Ir(I) pincer complexes (CF ₃PCP)Ir(L) (L = MeP(C₂F₅)₂, CO, dfepe (dfepe = (C₂F₅)₂PCH₂CH ₂P(C₂F₅)₂)) also serve as moderately active aldehyde decarbonylation catalyst precursors for 2-naphthaldehyde with similar activities in diglyme (1.7 TO h^-1, 152 °C) and in 1,4-dioxane (0.052 TO h^-1, 94 °C). The catalyst resting states are the corresponding five-coordinate carbonyl complexes (CF₃PCP) Ir(MeP(C₂F₅)₂)(CO), (CF₃PCP)Ir(CO) ₂, and [(CF₃PCP)Ir(CO)]₂(μ-dfepe). DFT studies indicate that the preferred catalyst resting state for alkane dehydrogenation, (CF₃PCP)Ir(cod), can be ascribed to the lower steric requirements of the CF₃-substituted pincer ligand.

Full text of DOI:10.1021/om2011886

Article
pubs.acs.org/Organometallics
Investigation of Iridium CF3PCP Pincer Catalytic Dehydrogenation and
Decarbonylation Chemistry
Jeramie J. Adams, Navamoney Arulsamy, and Dean M. Roddick*
Department of Chemistry, Box 3838, University of Wyoming, Laramie, Wyoming 82071, United States
S
* Supporting Information
ABSTRACT: The iridium fluorinated pincer complex (CF PCP)Ir-
3
(cod) (CF PCP = 2,6-C6H3(CH2P(CF3)2)2) catalyzes hydrogen
3
transfer from cyclooctane (coa) to tert-butylethylene (tbe) in 1/1
coa/tbe at 200 °C to give cyclooctene (coe) and neohexane (tba) at
an initial rate of 40 TO h−1. In 5/1 coa/tbe, higher initial activity
(155 TO h−1) and higher turnovers (2580 TON’s after 1450 min)
are found. Samples of 95% tbe contain significant amounts of
CF3
isoprene (2-methyl-1,3-butadiene), which reacts with (CF PCP)Ir(cod) to initially form ( PCP)Ir(isoprene). Alkene inhibition
3
studies show that (CF PCP)Ir is only modestly inhibited (67% reduced initial activity) in the presence of 800 equiv of added coe.
3
Unlike donor pincer systems, no decrease in activity is noted under 1 atm of N2 or in the presence of excess water.
CF3
Hydrogenation of (CF PCP)Ir(L) (L = cod, isoprene) did not produce ( PCP)Ir(H)x but instead afforded the first example of
3
the unusual aryl-bridged bimetallic complex [(μ-1κ2(P,C),2κ2(P′,C)-CF PCP)Ir(H)2]2(μ- PCPH)(μ-H), which has been isolated
CF3
3
and crystallographically characterized. Ir(I) pincer complexes (CF PCP)Ir(L) (L = MeP(C2F5)2, CO, dfepe (dfepe =
3
(C2F5)2PCH2CH2P(C2F5)2)) also serve as moderately active aldehyde decarbonylation catalyst precursors for 2-naphthaldehyde
with similar activities in diglyme (1.7 TO h−1, 152 °C) and in 1,4-dioxane (0.052 TO h−1, 94 °C). The catalyst resting states are
CF3
CF3
the corresponding five-coordinate carbonyl complexes (CF PCP)Ir(MeP(C2F5)2)(CO), ( PCP)Ir(CO)2, and [( PCP)Ir-
3
(CO)]2(μ-dfepe). DFT studies indicate that the preferred catalyst resting state for alkane dehydrogenation, (CF PCP)Ir(cod), can
3
be ascribed to the lower steric requirements of the CF3-substituted pincer ligand.
sensitive to the metal coordination environment and have been
shown to vary between Ir(III) hydrido vinyl complexes, which
are favored for PCP catalysts, to Ir(I) alkene complexes, which
are favored for more electron-deficient POCOP systems.4a
INTRODUCTION
■
There is continuing interest in stoichiometric and catalytic
transformations mediated by pincer transition-metal com-
plexes.1 A specific application that has garnered considerable
attention is alkane dehydrogenation and hydrogen transfer
using PCP iridium phosphine pincer systems [2,6-
C6H3(EPR2)2]Ir(H)x (E = CH2 (“RPCP”), E = O
(“RPOCOP”)).2 A number of studies have examined the
variation of reactivity with ancillary ligand effects focusing on
pincer linking groups (RPCP ligands versus resorcinol-based
RPOCOP systems),3−5 the central bridging unit,6 pincer ligand
sterics,7,8 and arene ring substituents.4b,5 Direct correlations
between ligand steric and electronic factors and catalyst activity
are not straightforward. Less sterically encumbering (iPrPCP)Ir
catalysts are more active relative to (tBuPCP)Ir,7a,9 and
We have recently reported catalytic alkane dehydrogenation
10
by the first group 8 system, (CF PCP)Ru(cod)(H). High
3
initial rates for acceptorless and tert-butylethylene (tbe)-
mediated cyclooctane (coa) dehydrogenation were observed,
though catalyst thermal stability was limited. Catalytic rates
were remarkably insensitive to the presence of N2, O2, or water.
It is notable that N2 effectively poisons dehydrogenation
activity for (RPCP)Ir11 and to a lesser extent (RPOCOP)Ir;4c
no explicit compatibility studies of iridium donor pincer
systems with water and oxygen have been reported, though
the air sensitivity of (tBuPCP)Ir(H)2 has been noted.4b
Enhanced catalyst stability toward air and moisture may be
t
due to the reduced electron density of (CF PCP)Ru centers
substitution of one methyl for a Bu phosphine substituent
II
3
increases dehydrogenation activity.8a Recently Brookhart and
Goldman have noted that substrate access to the metal center
in three-coordinate (tBuPOCOP)Ir is greater than in (tBuPCP)Ir,
and they ascribe the increased dehydrogenation activity for
POCOP systems to this difference.2a
imparted by perfluorinated phosphine substituents.
Recent studies from our group have detailed the synthesis
and coordination properties of Ir(I) and Ir(III) pincer systems,
CF3
CF3
CF3
(
(
PCP)Ir(L)x (x = 1, 2),12
(
PCP)Ir(X)(H)(L), and
13
+
PCP)Ir(H)(L)2 . This class of iridium pincer complexes
There is no clear evidence for pincer electronic influence on
hydrocarbon dehydrogenation activity. Very modest differences
in activity for para ring-substituted (tBuPOCOP)Ir systems were
found.4b However, calculations suggest that electron-with-
drawing phosphine substituents may favor the kinetics of
alkane addition to (RPCP)IrI.5 Catalyst resting states are
exhibits enhanced ligand binding affinities and favors
Special Issue: Fluorine in Organometallic Chemistry
Received: November 28, 2011
Published: February 1, 2012
© 2012 American Chemical Society
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We have reported that (CF PCP)Ru(cod)(H) initial catalytic
activity is decreased by 80% in the presence of 800 equiv of
added coe.10 1 is also inhibited by added coe but is remarkably
insensitive to product inhibition: in the presence of 800 equiv
of added coe, the initial activity (24 TO h−1) is 67% of that in
the absence of added coe, and TON’s after 19 h are decreased
by only 23% (Figure 2).
3
coordinative saturation. The unusual stability of Ir(I) five-
coordinate complexes suggests that (CF PCP)Ir(L)2 coordina-
3
tion may play a role in alkane dehydrogenation catalysis by
these systems.
In this paper we report a survey of catalytic alkane/alkene
hydrogen transfer activity as well as aldehyde decarbonylation
I
for (CF PCP)Ir compounds. Distinct differences in reactivity
3
patterns are revealed which may be ascribed to the very
different steric and electronic influence of trifluoromethyl
phosphine substituents on the supporting pincer ancillary
ligand.
RESULTS AND DISCUSSION
■
Hydrogen Transfer Catalysis. Catalytic coa/tbe hydrogen
transfer was surveyed for (CF PCP)Ir(cod) (1; 1.30 mM) at 200
3
°C in equimolar coa/tbe (∼3030 equiv, 95% tbe (Aldrich))
solutions, conditions which correspond to Brookhart’s prior
studies with (tBuPOCOP)Ir(H)x and also our recent report of
coa dehydrogenation by the ruthenium acceptor pincer system
4b,10
CF3
(
PCP)Ru(cod)(H) (Figure 1).
Figure 2. Cyclooctene production by 1.23 mM (CF PCP)Ir(cod) in 1/
3
●
1 coa/tbe at 200 °C in the absence of added coe (brown ) and in the
■
◆
●
presence of 100 (blue ), 200 (green ), 400 (black ), and 800
▲
equiv (purple ) of added coe.
The dehydrogenation activity of 1 toward linear alkanes was
briefly examined: reaction of 1 mM 1 in n-octane with 0.2 M
tbe at 150 °C gave ca. 45% conversion (81 turnovers) of tbe to
tba after 48 h, as judged by 1H NMR. This activity is
significantly less than reported for donor systems after only 20
min at this temperature: (tBuPCP)IrH4, (iPrPCP)IrH4, and
8a
(
tBu MePCP)IrH4 (57, 125, and 198 turnovers, respectively).
3
Figure 1. Comparison of cyclooctene production at 200 °C by
No dehydroaromatization products which are found in donor
pincer systems (o-xylene or ethylbenzene) were detected under
these conditions.14
CF3
CF3
●
●
(
(
PCP)Ir(cod) (brown ), ( PCP)Ru(cod)(H) (black ), and
CF3
tBuPOCOP)Ir(H)2 (blue ) in 1/1 coa/tbe and ( PCP)Ir(cod)
■
◆
(green ) in 5/1 coa/tbe.
As was found for the ruthenium system (CF PCP)Ru(cod)-
3
(H),10 carrying out hydrogen transfer runs under 1 atm of N2
or in the presence of added H2O does not significantly affect
catalytic hydrogen transfer activity. While complete inhibition
by N2 for (tBuPCP)Ir(H)x and significant inhibition for
The initial rate for 1 was 40 TO h−1, much slower than the
initial activity reported for (tBuPOCOP)IrH2 (6900 TO h−1)
−1
and (CF PCP)Ru(cod)(H) (1,000 TO h ); however, the total
3
turnovers after 58 h (660 TON’s) are greater than that found
(
tBuPOCOP)Ir(H)x has been ascribed to the formation of
for (tBuPCP)IrH2 (230 TON’s, 40 h) and ∼40% that of
stable dinitrogen adducts,11 the formation of iridium dinitrogen
CF3
(
tBuPOCOP)IrH2 (1609 TON’s after 103 h).4b
(
PCP)Ru-
adducts with (CF PCP)Ir is not observed. Addition of excess
3
(cod)(H) produces only 186 TON’s due to limited catalyst
lifetime. Thus, (CF PCP)Ir(cod) is initially less active but can be
3
H2O (0.5 μL, 45 equiv.) to catalytic runs with 1 at 200 °C
showed no significant changes in initial rate and TON after 18
h. Monitoring the reaction by 19F NMR showed pincer
complex speciation similar to that in anhydrous catalytic runs,
driven to reasonable total turnovers. Mercury doping experi-
ments gave essentially identical catalytic activities and suggest
that heterogeneous iridium species are not involved.
and no oxidative addition products (CF PCP)Ir(OH)(H)(L)
3
Previously reported iridium dehydrogenation catalysts are
inhibited by the presence of excess tert-butylethylene as well as
the buildup of alkene dehydrogenation products, which is
responsible for the rapid drop-off in activity during the course
of cycloalkene production. Catalyst inhibition by tbe for 1 is
indicated by a significant increase in initial activity (155 TO
h−1) and a dramatic increase in TON’s (85% conversion of tbe
to tba, 2580 TON’s after 1450 min at 200 °C; 3020 TON’s
after 3850 min.) in 5/1 coa/tbe solutions which maintain a
3030/1 ratio of tbe to catalyst (15150/1 coa to catalyst)
(Figure 1).
were observed. The lack of water reactivity found for
CF3
(
PCP)Ir(cod) contrasts with the donor system (tBuPCP)Ir-
(H)2, which readily adds water in the presence of tbe at
ambient temperature to give (tBuPCP)Ir(H)(OH).15
Catalyst Resting State and Stability Studies. As stated
in the Introduction, catalyst resting states for (tBuPCP)Ir and
(
tBuPOCOP)Ir systems (in the presence of tbe) depend on the
relative affinities for C−H oxidative addition and alkene
adducts as a hydrogen acceptor.4a For (CF PCP)Ir(cod), a
3
single new iridium pincer species was formed quantitatively
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within 20 min in 1/1 coa/tbe at 20 °C. Dissolution of 1 in neat
tert-butylethylene at ambient temperature similarly resulted in
the formation of this product, which was initially believed to be
the alkene adduct (CF PCP)Ir(tbe). However, this product was
3
identified by X-ray diffraction as the isoprene (2-methyl-1,3-
4
butadiene) adduct (CF PCP)Ir(η -C5H8) (2) (Figure 3).
3
NMR spectrum. The coordinated isoprene exhibits five
1
separate H vinylic resonances and a methyl doublet at δ
1.68 (4JHH = 8 Hz).
The isolated isoprene adduct 2 also serves as a catalyst
precursor: during the course of reaction in 1/1 coa/tbe at 200
°C, 2 is replaced by the cod complex 1 as the primary catalyst
resting state after 6 h (Figure 5). The appearance of 1 correlates
with the production of significant amounts of the double
dehydrogenation product 1,3-cyclooctadiene. Monitoring coa/
tbe hydrogen transfer reactions by NMR using (CF PCP)Ir-
3
(dfmp) (dfmp = MeP(C2F5)2) and (CF PCP)Ir(nbe) as
3
alternative Ir(I) catalyst precursors similarly showed conversion
to (CF PCP)Ir(cod) as the catalyst resting state. Complex 1
3
slowly decomposes to unidentified side products and free
CF3
PCPH.
Decreased catalytic activity correlated with percent decom-
position: removal of volatiles after an initial catalyst run and
restarting the thermolysis at 200 °C with fresh 1/1 coa/tbe
gave decreased initial rates which roughly correspond to the
percent decomposition, as determined by 19F NMR. From
these observations it is apparent that a significant loss in catalyst
activity over the course of time is due to catalyst decomposition
rather than product coe inhibition.
4
Figure 3. Molecular structure of (CF PCP)Ir(η -C5H8) (2) with
3
thermal ellipsoids at the 50% level. Hydrogen atoms are omitted for
clarity. Selected metrical data (bond lengths in Å and angles in deg):
Ir(1)−C(1), 2.099(2); Ir(1)−C(13), 2.274(2); Ir(1)−C(14),
2.202(2); Ir(1)−C(15), 2.170(2); Ir(1)−C(16), 2.182(2); Ir(1)−
P(1), 2.2268(5); Ir(1)−P(2), 2.2346(4); C(13)−C(14), 1.403(3);
C(14)−C(15), 1.438(3), C(15)−C(16), 1.429(3); C(1)−Ir(1)−P(1),
78.08(5); C(1)−Ir(1)−P(2), 77.62(5); P(1)−Ir(1)−P(2), 118.02(2).
The formation of 1 from 1,3-cyclooctadiene was tested:
reaction of the norbornene complex (CF PCP)Ir(nbe) with 1
3
Isoprene was subsequently found to be a major impurity in
purchased tert-butylethylene, and subsequent control reactions
using 99.5−99.8% purity tert-butylethylene (Aldrich) showed
no initial buildup of the isoprene adduct and higher initial
activity (136 TO h−1; Figure 4).
equiv of 1,3-cod at 100 °C for 12 h in cyclooctane under an N2
atmosphere quantitatively produced 1, 1 equiv of free
norbornene, and 1.5 equiv of cyclooctene. No free or
1
coordinated 1,3-cod was detected by H NMR at this point.
Acceptorless Cyclooctane Dehydrogenation. A 0.2
mM coa solution of 1 containing 5 equiv of mesitylene as an
internal standard was heated at reflux (∼140 °C at 590 Torr)
under a N2 atmosphere. The reaction was stopped periodically
1
and quantified by H NMR, which showed the buildup of 2.7,
30, and 60 equiv of coe after 4, 44, and 120 h, respectively, with
an initial rate of 0.68 TO h−1. The onset of catalyst
decomposition was indicated by 19F spectra after 3 h, and
after 120 h only 10% of 1 remained. The resulting complex
mixture of fluorine-containing products was not further
characterized. Acceptorless dehydrogenation using the phos-
phine complex (CF PCP)Ir(dfmp) resulted in formation of the
3
previously reported H2 addition product fac-cis-(CF PCP)Ir-
3
(dfmp)(H)2 after 15 min at 200 °C (80% yield, 4 turnovers).12
No change in catalyst activity was observed under an argon
atmosphere.
Acceptorless dehydrogenation of cyclodecane (cda) with 1
was also examined. Refluxing (190 °C, 590 Torr) a 1 mM
solution of 1 in cda for 24 h gave 92 TON of a ∼1:3 trans-/cis-
cyclodecene mixture (initial rate of 3.8 TO h−1). This activity is
significantly less than that reported for (tBuPCP)Ir(H)2 (267
TON, 102 TO h−1), (iPrPCP)IrH4 (364 TON, 136 TO h−1),
and (AdPCP)Ir(H)2 (509 TON, 74 TO h−1).8b
Aldehyde Decarbonylation Studies. Stoichiometric and
catalytic decarbonylation of aldehydes is well established for
group 9 metal systems. Following the early demonstration of
stoichiometric aldehyde dehydrogenation by Wilkinson’s
Figure 4. Cyclooctene production by 1.30 mM (CF PCP)Ir(cod) in 1/
3
●
1 coa/tbe at 200 °C using 95% tert-butylethylene (Aldrich) (brown
and 99.5+% tert-butylethylene (Aldrich) (blue ).
)
■
Complex 2 was independently synthesized in good yield by
2
dehydrohalogenation of (CF PCP)Ir(η -C2H4)(H)Cl with Et3N
3
in the presence of excess isoprene (eq 1). 31P and 19F NMR
spectra reflect the asymmetry of isoprene coordination to the
CF3
(
PCP)Ir unit, with inequivalent 31P resonances, four CF3 19F
1
doublets, and four diasterotopic benzylic resonances in the H
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Figure 5. 19F NMR spectra of 1.30 mM (CF PCP)Ir(isoprene) (2) in 1/1 coa/tbe at 200 °C as a function of time. Resonances due to 2 (brown ),
(
3
■
CF3
CF3
▲
PCP)Ir(cod) (1) (brown *), and PCPH (brown ) are labeled.
Scheme 1
complex Rh(PPh3)3Cl,16 the catalytic systems (dppp)RhCl,
[(Me3P)(CO)RhCl]2, and more recently (cod)Ir(PPh3)Cl have
been developed and comprise the most active neutral
homogeneous catalysts for aldehyde decarbonylation under
mild conditions.17 For the PCP ligand motif stoichiometric
decarbonylation of alkyl formates and alkynyl aldehydes using
and (cod)Ir(PPh3)Cl (3.07 TO h−1).17b Very similar activities
for 4 and 5 were observed.
The catalyst resting states for complexes 3−5 are the
corresponding 18-electron CO adducts (CF PCP)Ir(MeP-
3
CF3
(C2F5)2)(CO) (6), (CF PCP)Ir(CO)2 (7), and [( PCP)Ir-
3
(CO)]2(μ-dfepe) (8) (Scheme 1). Complexes 6 and 8 were
independently prepared by the addition of MeP(C2F5)2 or
dfepe to 7 or by the addition of CO to 3 or 5. Complexes 6 and
8 were structurally characterized by X-ray diffraction (Figures 6
and 7). As anticipated, the ν(CO) values for 6 (2051 cm−1) and
for 8 (2052 cm−1) are significantly higher than the 2011 cm−1
(
tBuPCP)Rh(N2) has also been reported.18 We recently
reported catalytic decarbonylation of 2-naphthaldehyde in
refluxing diglyme using (CF PCP)Ir(PPh3)(CO) (0.25 TO
3
h−1, 152 °C, 590 Torr).19 The low catalytic activity for
CF3
(
PCP)Ir(PPh3)(CO) may be due to donor−acceptor
value for (CF PCP)Ir(CO)(PPh3). In our previous paper the
3
synergism between Ph3P and CO and the accompanying
lowered CO lability. Hence, we expected that more electron-
CO ligand dissociation energy for (CF PCP)Ir(CO)2 was
3
deficient derivatives (CF PCP)Ir(L) (L = MeP(C2F5)2 (3), CO
estimated to be 9.7 kcal mol−1.12 Since decarbonylation rates
3
(4)) and (CF PCP)Ir(dfepe) (5) (dfepe = (C2F5)2PCH2CH2P-
by 6−8 are all essentially identical, CO dissociation energies for
3
(C2F5)2)) would be more active catalysts for decarbonylation.
Reaction of 5 mol % of 3 with 2-naphthylaldehyde in refluxing
digyme (152 °C at 590 Torr) for 6 h gave 10 equiv of
naphthalene (1.67 TO h−1), and complete conversion required
22 h. Under milder conditions (dioxane reflux, 94 °C) 3 had an
6 and 8 are probably comparable.
4
Hydrogenation of (CF PCP)Ir(L) (L = cod, η -C5H8)
3
Complexes. (RPCP)Ir(H)x and (RPOCOP)Ir(H)x complexes
provide an effective entry, via dihydrogen loss, to catalytic
alkane dehydrogenation based on a 14-electron Ir(I)
activity of 0.052 TO h−1. The catalytic activity of (CF PCP)Ir(L)
intermediate. In the case of the catalyst precursor (CF PCP)-
3
3
in diglyme falls between those of (dppp)RhCl (0.75 TO h−1)
Ir(cod), dissociation of one or both Ir−alkene bonds is
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and crystallographically characterized (Figure 8). Examples of
analogous NCN pincer bimetallic-bridging bonding modes with
Figure 6. Molecular structure of (CF PCP)Ir(MeP(C2F5)2)(CO) (6)
with 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
Selected metrical data (bond lengths in Å and angles in deg): Ir(1)−
P(1), 2.2366(5); Ir(1)−P(2), 2.2394(5); Ir(1)−P(3), 2.2901(5);
Ir(1)−C(13), 1.924(2); Ir(1)−C(1), 2.126(2); C(13)−O(1),
1.133(2); C(13)−Ir(1)−C(1), 172.96(8); P(1)−Ir(1)−P(2),
126.29(2); P(1)−Ir(1)−P(3), 114.14(2); P(2)−Ir(1)−P(3),
111.84(2).
3
Figure 8. Molecular structure of [(μ-1κ2(P,C),2κ2(P′,C)-CF PCP)Ir-
3
(H)2]2(μ-CF PCPH)(μ-H) (9) with 50% thermal ellipsoids. CF3
3
groups and hydrogen atoms are omitted for clarity, except for hydride
ligands. Selected metrical data (bond lengths in Å and angles in deg):
Ir(1)−H(1), 1.76(5); Ir(1)−H(1A), 1.56(5); Ir(1)−H(1B), 1.51(4);
Ir(1)−C(1), 2.227(3); Ir(1)−P(1), 2.2844(7); Ir(1)−P(4),
2.2105(7); Ir(2)−H(1), 1.70(5); Ir(2)−H(2A), 1.43(5); Ir(2)−
H(2B), 1.45(5); Ir(2)−C(1), 2.323(3); Ir(2)−P(2), 2.3214(8);
Ir(2)−P(3), 2.1779(8); Ir(1)−Ir(2), 2.7943(1). P(4)−Ir(1)−C(1),
171.98(8); P(4)−Ir(1)−P(1), 106.55(3); P(1)−Ir(1)−H(1B),
168.9(15); H(1)−Ir(1)−H(1A), 176(2); P(3)−Ir(2)−P(2),
105.67(3); P(3)−Ir(2)−C(1), 171.95(7); P(2)−Ir(2)−H(2A),
167(2); H(1)−Ir(2)−H(2B), 174(3); Ir(1)−C(1)−Ir(2), 75.75(8).
Ta, Zr, Li, and Cu are known.20 However, to our knowledge the
μ-1κ2(P,C),2κ2(P′,C)-CF PCP coordination mode in complex 9,
3
with the central aryl participating in three-center two-electron
bonding between two group 8 metal centers, is unprecedented
and is unique in PCP pincer chemistry.
1H NMR data for 9 are consistent with the solid-state
structure: a dd hydride resonance at δ −9.78 is assigned to the
Figure 7. Molecular structure of [(CF PCP)Ir(CO)]2(μ-dfepe) (8)
3
terminal hydrides trans to μ-κ2,κ2-PCP (2JHP = 188 Hz) and cis
with 50% thermal ellipsoids. Hydrogens and perfluoroalkyl groups are
omitted for clarity. One of three independent molecules is shown.
Selected metrical data (bond lengths in Å and angles in deg): Ir(1)−
C(13), 1.915(4); Ir(1)−C(1), 2.130(4); Ir(1)−P(1), 2.2464(10);
Ir(1)−P(2), 2.2490(10); Ir(1)−P(3); 2.2964(10); Ir(2)−C(36),
1.926(4); Ir(2)−C(24), 2.120(4); Ir(2)−P(6), 2.2466(9); Ir(2)−
P(5), 2.2466(10); Ir(2)−P(4), 2.2876(10); C(13)−Ir(1)−C(1),
168.8(2); P(1)−Ir(1)−P(2), 123.38(4); P(1)−Ir(1)−P(3),
117.85(4); P(2)−Ir(1)−P(3), 113.87(4); C(36)−Ir(2)−C(24),
170.1(2); P(6)−Ir(2)−P(5), 124.65(4); P(6)−Ir(2)−P(4),
118.02(4); P(5)−Ir(2)−P(4), 111.36(4).
2
to μ-CF PCPH ( JHP = 27 Hz) phosphines. No additional
3
coupling to the remaining hydride ligands is observed. The μ-H
resonance appears as a singlet at δ −12.96, and the remaining
terminal hydrides appear as a complex multiplet at δ −15.86.
CF3
31P spectra show two distinct sets of PCP phosphines at δ
56.6 and 12.52 which integrate 1:1, and the 19F spectra show
four 1:1:1:1 CF3 resonances.
The unusual ligand arrangement in complex 9 may be simply
formulated as (PCP)(PCPH)Ir2H5, which has the same atom
balance as a 1/1 mixture of (PCP)Ir(H)2 and (PCP)Ir(H)4.
This conceptual equivalence of 9 with “(PCP)Ir(H)x”
suggested the potential for regenerating monomeric iridium
centers. Reaction of 9 with excess (15 equiv) cod in C6D6 at
130 °C for 30 min cleanly generated the cod complex 1.
Hydrogenation of 1 in C6D6 with 1000 psi of H2 in a sapphire
NMR tube at 130 °C also gave 9. The reversible formation of 9
prompted us to investigate its reactivity as a hydrogen transfer
catalyst, and under analogous conditions the catalytic activity
was similar, with 1 once again being formed as the catalyst
resting state.
required prior to reaction with hydrocarbon C−H bonds.
Accordingly, we were interested in accessing the corresponding
CF3
(
PCP)Ir(H)x systems in order to have a more direct
comparison of intrinsic (RPCP)Ir activities. Reaction of
CF3
(
PCP)Ir(C5H8) with 3 atm of H2 in CH2Cl2 gave an orange
solution and quantitative formation of the unusual aryl-bridged
bimetallic complex [(μ-1κ2(P,C),2κ2(P′,C)-CF PCP)Ir-
3
(H)2]2(μ-CF PCPH)(μ-H) (9) (eq 2), which has been isolated
3
1,5-Cyclooctadiene DFT Binding Energies. The ability
4
of (CF PCP)Ir systems to bind 1,5-cyclooctadiene in an η
3
fashion is a critical difference between (CF PCP)Ir chemistry
3
and the well-established (tBuPCP)Ir and (tBuPOCOP)Ir catalyst
systems. As discussed in our previous paper,10 the preference
for coordinatively saturated complex products in (CF PCP)M
3
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Table 1. Calculated Bond Lengths (Å) and Angles (deg) and Relative 1,5-cod Dissociation Energies (kcal mol−1) for
CF3
(
PCP)Ir(η4-cod), (CF3PCP)Ir(η2-cod), (MePCP)Ir(η4-cod), (MePCP)Ir(η2-cod), (tBuPCP)Ir(η4-cod), and (tBuPOCOP)Ir(η4-
a
cod)
b
Ir−Ceq
Ir−Cax
CCeq
CCax
P−Ir−P
ΔGdiss
CF3
(
PCP)Ir(η4-cod)
2.161, 2.155 (2.175, 2.162)
2.299, 2.299 (2.295, 2.299)
2.209, 2.276
1.446 (1.435)
1.399 (1.375)
1.405
116.8 (119.02)
152.5
19.4
6.8
(
PCP)Ir(η2-cod)
CF3
(
(
(
(
MePCP)Ir(η4-cod)
MePCP)Ir(η2-cod)
tBuPCP)Ir(η4-cod)
tBuPOCOP)Ir(η4-cod)
2.145, 2.133
2.218, 2.194
2.233, 2.234
1.460
1.413
111.8
22.8
1.420
159.9
11.9
−12.7
−24.6
a
12 b
Crystallographic data for (CF PCP)Ir(η -cod) are given in parentheses. Free energies of dissociation are defined relative to (RPCP)Ir and free 1,5-
4
3
cyclooctadiene.
systems can be ascribed partially to the electron-withdrawing
distorted-trigonal-bipyramidal geometry;12 we have recently
CF3
nature of the
PCP moiety but also to reduced steric
reported the structure of the analogous triphenylphosphine
19
derivative (CF PCP)Ir(Ph3P)(CO), and (CF PCP)Ir(MeP-
3
3
crowding about the metal center, since pendant P(CF3)2
groups (θ((CF3)3P) = 137°) are significantly smaller than
P(tBu)2 (θ(tBu3P) = 182°) or P(iPr)2 (θ(iPr3P) = 160°). We
(C2F5)2)(CO) (6) completes the structurally characterized
substitution series (CF PCP)Ir(MeP(C2F5)2)x(CO)2−x. All three
3
have examined the energetics of 1,5-cod binding for (CF PCP)Ir
3
mixed phosphine/carbonyl derivatives have axial CO coordi-
nation trans to the pincer aryl group.
as well as (MePCP)Ir, (tBuPCP)Ir, and (tBuPOCOP)Ir metal
centers using DFT methods at the TPSSh/cc-pVDZ level of
CF3
The PCP bonding mode in the dinuclear Ir−Ir hydride
theory in order to further probe this concept. Metrical
complex 9 is unprecedented for the PCP ligand motif. We are
also not aware of any other structurally characterized diiridium
system with a μ-η1:η1 bridging aryl unit. The dinuclear Ir−Ir
distance in 9, 2.7943(1) Å, is longer than that observed in the
hydride-bridged complex [(dfepe)Ir(H)2](μ-H)2 (2.703(2)
Å).23 The bridging aryl C−Ir bonds differ in length (Ir(1)−
C(1), 2.227(3) Å; Ir(2)−C(1), 2.323(3) Å) and are
significantly longer than nonbridging C(aryl)−Ir bond lengths
4
parameters for the optimized geometry for (CF PCP)Ir(η -
3
cod), in particular the asymmetric binding of the axial and
equatorial alkene groups, are in good agreement with published
crystal structure data (Table 1). The asymmetric alkene
bonding parameters in (MePCP)Ir(η4-cod) are comparable.
4
Both (CF PCP)Ir(η -cod) and (MePCP)Ir(η4-cod) have
3
comparable 1,5-cod total binding energies (19.4 vs 22.8 kcal
mol−1). However, the dissociation energy of η2-cod from
in (CF PCP)Ir complexes (2.05−2.15 Å). The constraints
́
3
CF3
(
PCP)Ir(η2-cod) (6.8 kcal mol−1) is significantly less than
imposed by coordination of the pincer phosphine arms to
Ir(1) and Ir(2) results in some aryl bridging asymmetry: the
aryl ring is tilted 15° toward Ir(2) and is also slightly twisted
with respect to the Ir−Ir axis. The bridging and terminal
hydride ligands in 9 were located satisfactorily, though with
lower precision: the terminal Ir−H lengths, 1.43(5)−1.56(5) Å,
that found for (MePCP)Ir (11.9 kcal mol−1); therefore, the
association of the second alkene group in (CF PCP)Ir(η -cod)
2
3
to form (CF PCP)Ir(η -cod) is more favorable by 1.7 kcal
4
3
mol−1. Attempts to optimize η4-cod complexes with the more
sterically restrictive (tBuPCP)Ir and (tBuPOCOP)Ir metal
centers resulted in structures where one of the pendant PtBu2
arms is fully dissociated and which are at significantly higher
are within the range of other reported (CF PCP)Ir complexes,
3
and the bridging hydride Ir−(μ-H) lengths are essentially the
same (1.70(5) and 1.76(5) Å) and are within the range of other
bridging hydrides of dinuclear iridium centers.24
energies (12.7 and 24.6 kcal mol−1, respectively). We conclude
that the stability of (CF PCP)Ir(η -cod) is primarily due to a
4
3
relaxation of steric restrictions about the iridium center rather
than an increase in electrophilicity imparted by the CF3
substituents.
SUMMARY
■
CF3
Iridium(I) pincer systems with the fluoroalkylated
PCP
Crystallographic Studies. We have structurally charac-
terized a number of (CF PCP)Ir four- and five-coordinate
3
ligands promote alkane dehydrogenation and exhibit features
which are in marked contrast with well-studied donor pincer
RPCP and RPOCOP catalysts. One key difference is in the
catalyst resting state: in the presence of excess tbe, (tBuPCP)-
Ir(H)2 favors the Ir(III) vinylic C−H bond oxidative addition
Ir(I),12 six-coordinate Ir(III),13 and bridging μ-CF PCPH Ir(I)
3
and Ir(III) complexes which provide a useful basis for
comparison with these new systems. Ir−P bond lengths for
Ir(I) complexes 3, 6, and 8 average 2.241 Å and Ir−C(aryl)
adduct
(
t B u PCP)Ir(H(C(H)C(H)t Bu), whereas
I
bonds average 2.13 Å, in accord with previous (CF PCP)Ir
3
tBuPOCOP)Ir(H)2 favors Ir(I) alkene adducts.4a
(
PCP)Ir
CF3
(
systems. It has been previously noted that electron-withdrawing
phosphine CF3 pincer substituents induce significantly
shortened M−P bonds.21
follows the preference previously noted for coordinative
saturation12,13 and favors the formation of the 18-electron
1,5-cyclooctadiene adduct (CF PCP)Ir(η -cod) in cyclooctane
4
3
While complex 2 is the first structurally characterized
isoprene adduct of iridium, the structure of the Rh(I) bis-
isoprene complex (η4-C5H8)2Rh(OTf) has been reported.22
The coordination geometry of complex 2 is effectively trigonal
bipyramidal but is much more distorted than that of
dehydrogenation. The lower catalytic activity of (CF PCP)Ir
I
3
systems is not surprising, considering that alkane substrates
must compete for binding not only with η2-alkene products but
with η4-bound 1,5-cod as well. Another interesting feature of
12
I
the (CF PCP)Ir catalyst system is its relative insensitivity to
CF3
3
(
PCP)Ir(cod) due to the more restricted 1,3-disposition
of double bonds.
product inhibition: 67% activity is maintained in the presence
of 800 equiv of added coe, and in 5/1 coa/tbe mixtures very
high TON’s and essentially complete conversion to cyclooctene
Complexes 6 and 8 are additional examples of five-
coordinate (CF PCP)Ir(L1)(L2) complexes which adopt a
3
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Organometallics
Article
of hydrogen transfer was accomplished by integrating the ratio of the
tert-butylethylene terminal CH2 group with the coe vinylic proton
resonances.
Acceptorless coa Dehydrogenation using 1. Solutions of 1 (2
mM, 7.4 mg) in coa (5 mL) containing 5 equiv (7.0 μL) of mesitylene
as an internal standard were heated to reflux with stirring under 1 atm
of N2. The reaction mixture was cooled periodically, and a 0.5 mL
aliquot was syringed into a NMR tube containing an acetone-d6
capillary. Integration of mesitylene aryl protons and vinylic coe
protons was used to determine TONs.
Catalytic Cyclodecane Dehydrogenation using 1. Solutions of
1 (1 mM, 1.5 mg) in cda (2 mL) containing 5 equiv (1.4 μL) of
mesitylene as an internal standard were heated to reflux with stirring
under 1 atm of N2. The reaction mixture was cooled periodically, and a
0.5 mL aliquot was syringed into a NMR tube containing an acetone-
d6 capillary. Integration of mesitylene aryl protons and vinylic cis/trans
protons was used to determine TONs.
are observed. Alkane dehydrogenation is also not inhibited by
the presence of N2 or H2O.
DFT calculations indicate that η4-cod binding is accessible
for both Me and CF3-substituted pincer complexes of iridium
t
but not for Bu-substituted pincer catalysts. Decreased sterics
may also play a role in the formation of the unusual bimetallic
pincer bridging mode in complex 9; it is notable that this
bridging mode is well-established by van Koten in NCN pincer
systems, which have relatively small NMe2 pendant groups.20
We expect that additional examples of this novel PCP pincer
bimetallic bridging mode will be found when the chemistry of
R
less sterically encumbering PCP is further developed.
Catalytic Decarbonylation of 2-Naphthaldehyde with
CF3
(
PCP)Ir(L) (L = MeP(C2F5)2 (3), CO (4), dfepe (5), (CO)2 (7)).
In a typical experiment 0.025 mmol of (CF PCP)Ir(L) and 0.50 mmol
(0.078 g) of 2-naphthaldehyde were placed in a 10 mL round-bottom
flask fitted with a Teflon stir bar and dissolved in 1 mL of unpurified
degassed diglyme or dioxane. The solutions were brought to reflux
under N2 for 6 h, and the reaction mixtures were then cooled and
transferred into an NMR tube containing a capillary of acetone-d6 for
3
Catalysts based on (CF PCP)Ir appear to be less stable than
3
tBu-substituted iridium pincer catalysts. In Goldman’s study of
t
t
mixed Bu/Me pincer systems (R4PCP)Ir (R = Bu, Me),
substitution of more than one methyl group resulted in lowered
activity and the formation of dimeric products derived from
oxidative addition of phosphine methyl C−H bonds.8a While
analysis.
(
CF3
PCP)Ir(η4-C5H8) (2). Isoprene (0.500 mL, 0.341 g, 5.00 mmol)
we believe that the lower stability of (CF PCP)Ir(cod) and
3
and Et3N (0.250 mL, 0.182 g, 1.79 mmol) were added to a solution of
CF3
(
PCP)Ru(cod)(H) may be attributed, at least in part, to
CF3
(
PCP)Ir(C2H4)(H)Cl (0.300 g, 0.435 mmol) dissolved in 20 mL of
steric accessibility of the reactive metal centers and the
potential for bimolecular deactivation, the inertness of
perfluoroalkyl C−F bonds may allow for efficient and long-
lived hydrogen transfer chemistry if a suitable balance in the
steric tuning of pincer perfluoroalkyl sterics is found.
C6H6. The reaction mixture was stirred at ambient temperature for 18
h, Et3NH+Cl− was filtered away from the reaction mixture, and the
volatiles were removed to give a white solid. The solid was triturated
with hexane and cooled to −78 °C, giving a white solid that was
collected via filtration (0.265 g, 88% yield). Crystals suitable for single-
crystal X-ray diffraction were grown by the slow evaporation of a
saturated hexane solution of 2. Anal. Calcd for C12H15F12P2Ir: H, 2.16;
C, 29.11. Found. H: 2.40, C: 29.33. 1H NMR (C6D6, 400.13 MHz, 20
°C): δ 6.82 (s, 2H; m-C6H3(CH2P(CF3)2)2), 6.77 (s, 1H; p-
C6H3(CH2P(CF3)2)2), 5.04 (m, 1H, vinylic CH), 3.76 (virtual t, JPH
= 14 Hz, 1H; C6H3(CH2P(CF3)2)2), 3.46 (virtual t, JPH = 15 Hz, 1H;
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were conducted under N2
or vacuum using high-vacuum-line and glovebox techniques unless
otherwise noted. All ambient-pressure chemistry was carried out under
a pressure of approximately 590 Torr (elevation ∼2195 m). All
solvents were dried using standard procedures and stored under
vacuum. Aprotic deuterated solvents used in NMR experiments were
2
C6H3(CH2P(CF3)2)2), 3.08 (d, JHH = 17 Hz, 1H; C6H3(CH2P-
(CF3)2)2), 3.01 (d, 2JHH = 16 Hz, 1H; C6H3(CH2P(CF3)2)2), 2.56 (d,
4
JHH = 7 Hz, vinylic CH), 1.685 (d, JHH = 8 Hz, 3H; CH3), 1.52 (d,
́
JHH = 4 Hz, 1H; vinylic CH), 0.80 (dd, JHH = 9 Hz, JHH = 7 Hz, 1H;
vinylic CH), 0.59 (br m, 1H; vinylic CH). 13C NMR (C6D6, 100.61
MHz, 20 °C): 147.6 (m; CAr−Ir), 141.3 (m; o-C6H3(CH2P(CF3)2)2),
dried over activated 3 Å molecular sieves. Elemental analyses were
performed by Columbia Analytical Services. NMR spectra were
obtained with a Bruker DRX-400 instrument using 5 mm NMR tubes
fitted with Teflon valves (Chemglass, CG-512) or in 5 mm sealed
tubes containing acetone-d6 capillaries as an internal standard. 31P
spectra were referenced to an 85% H3PO4 external standard. 19F
spectra were referenced to CF3CO2CH2CH3 (δ −75.32) and CF3C6H5
external standards. The gas H2 (UHP, Praxair, Inc.) was purchased and
used without further purification. tert-Butylethylene (99.5 and 95%,
Aldrich) was used as received. Cyclooctane, n-octane, and cyclodecane
(Pfaltz and Bauer) were stirred over concentrated H2SO4 for 24 h and
distilled from Na/benzophenone; all other reagents, unless otherwise
noted, were purchased from Aldrich and were used without further
1
140.1 (m; o-C6H3(CH2P(CF3)2)2), 127.9 (d, JCH = 158 Hz; p-
1
C6H3(CH2P(CF3)2)2), 124.9 (d, JCH 160 Hz; m-C6H3(CH2P-
1
(CF3)2)2), 122.3 (ps tm, JCH = 160 Hz; isoprene CH2), 91.9 (d,
1JCH = 167 Hz; isoprene CH), 87.7 (m, isoprene C), 42.2 (tdd, 1JCH
=
134 Hz, 2JCP = 37 Hz, 3JCP = 12 Hz; C6H3(CH2P(CF3)2)2), 38.2 (tdm,
1JCH = 157 Hz, 2JCP = 32 Hz), 29.0 (t, 1JCH = 162 Hz; isoprene CH2),
1
19.1 (quartet, JCH = 128 Hz; isoprene CH3). 31P{1H} NMR (C6D6,
161.97 MHz, 20 °C): δ 56.2 (m, 1P; P(CF3)2), 43.0 (m, 1P; P(CF3)2).
19F NMR (C6D6, 376.50 MHz, 20 °C): δ −54.5 (dd, 2JPF = 64 Hz, JPF
2
= 8 Hz, 3F; PCF3), −56.5 (dd, JPF = 64 Hz, JPF = 8 Hz, 3F; PCF3),
CF3
CF3
2
2
purification. (CF PCP)Ir(cod), ( PCP)Ir(nbe), ( PCP)Ir(MeP-
−60.0 (dt, JPF = 68 Hz, JPF = 8 Hz, 3F; PCF3), −61.2 (dt, JPF = 72
3
(C2F5)2), (CF PCP)Ir(CO), ( PCP)Ir(CO)2, and ( PCP)Ir(dfepe)
CF3
CF3
3
Hz, JPF = 8 Hz, 3F; PCF3).
were prepared following published procedures.12
CF3
(
PCP)Ir(MeP(C2F5)2)(CO) (6). MeP(C2F5)2 (50 μL, 0.079 g,
0.276 mmol) was added to a solution of (CF PCP)Ir(CO) (0.050 g,
0.076 mmol) dissolved in 5 mL of hexane. Upon addition of
MeP(C2F5)2 the orange solution immediately became colorless. The
reaction mixture was placed in a freezer at −28 °C, and slow
evaporation of the solution gave a quantitative yield (0.71 g) of
colorless crystals which were suitable for single-crystal X-ray
diffraction. Anal. Calcd for C18H10F22P3OIr: H, 1.07; C, 22.87.
Catalytic coa/tbe Hydrogen Transfer Studies with (CF PCP)-
3
3
Ir(L) (L = η4-cod (1), MeP(C2F5)2, nbe, Isoprene). For 1/1 coa/tbe
catalyst runs, 250 μL of a stock solution of 2.543 mM (CF PCP)Ir(L)
3
(corresponding to 3030 equiv of coa versus catalyst) in coa, and 240
μL (3030 equiv) of tbe were syringed into a 5 mm glass tube
containing an acetone-d6 capillary and sealed under vacuum. For 5/1
coa/tbe catalyst runs, the concentration of catalyst and the amount of
tbe were divided by 5 (0.85 mM catalyst, 48 μL of tbe). Thermolyses
were carried out in an isothermal oven at 200 °C. NMR tubes were
periodically removed from the oven and allowed to cool for at least 15
min and mixed thoroughly before taking NMR spectra. Quantification
1
Found. H: 1.29, C: 23.24. H NMR (C6D6, 400.13 MHz, 20 °C): δ
6.71 (ps t, 3JHH = 8 Hz, 1H; p-C6H3(CH2P(CF3)2)2), 6.61 (ps d, 3JHH
= 8 Hz, 2H; m-C6H3(CH2P(CF3)2)2), 3.47 (m, 2H; C6H3(CH2P-
2
(CF3)2)2), 3.00 (ps. d, JHP = 17 Hz, 2H; C6H3(CH2P(CF3)2)2), 0.83
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Organometallics
Article
(d, 2JHP = 6 Hz, 3H; CH3P(CF2CF3)2). 31P{1H} NMR (C6D6, 161.97
MHz, 20 °C): δ 53.2 (m, 2P; P(CF3)2), 19.8 (m, 1P; MeP(CF2CF3)2).
19F NMR (C6D6, 376.50 MHz, 20 °C): δ −57.9 (d, 2JPF = 75 Hz, 6F;
maps and refined anisotropically. Hydrogen atoms for complexes 2, 6,
and 9 were located on difference Fourier maps and refined
isotropically. Hydrogen atoms for complex 8 were placed in idealized
positions. The fluorine atoms of one of the CF3 groups of the dfmp
ligand in 6 were disordered and were modeled as two sets of sites
2
PCF3), −63.1 (ps d, JPF = 75 Hz, 6F; PCF3), −76.3 (s, 6F;
2
MeP(CF2CF3)2), −110.4 (d, JPF = 94 Hz, 4F; MeP(CF2CF3)2). IR
ν(CO) CH2Cl2: 2051 cm−1.
which refined to occupancies of 58.5% and 41.5%. The P1 asymmetric
̅
unit of 8 consists of three independent [(CF PCP)Ir(CO)]2(μ-dfepe)
3
[(CF PCP)Ir(CO)]2(μ-dfepe) (8). One atmosphere of CO was
3
introduced into a flask containing (CF PCP)Ir(dfepe) (0.350 g, 0.292
3
m o l e c u l e s . T h e t w o i r i d i u m c e n t e r s i n [ ( μ -
CF3
1κ2(P,C),2κ2(P′,C)-CF PCP)Ir(H)2]2(μ- PCPH)(μ-H) are in essen-
3
mmol) in 20 mL of toluene. The pale yellow solution immediately
turned colorless upon stirring. A small amount of solid was filtered
away, and the volatiles were removed from the filtrate to give a white
powder. The white residue was triturated with 10 mL of hexane and
collected via filtration (0.128 g, 74% yield). Crystals suitable for single-
crystal X-ray diffraction were grown by the slow evaporation of a 1/4
benzene/hexane solution. Anal. Calcd for C36H18F33P6O2Ir2: H, 0.96;
C, 22.89. Found: H, 1.29; C, 22.74. IR ν(CO) (CH2Cl2): 2052 cm−1.
1H NMR (C6D6, 400.13 MHz, 20 °C): δ 6.70 (ps t, 3JHH = 8 Hz, 2H;
tially identical coordination environments but are not related by
crystallographic symmetry.
Computational Details. All calculations were carried out using
Gaussian 09 Rev. A.02.26 The hybrid exchange-correlation functional
TPSSh was used for geometry optimization and frequencies.27 The
Dunning cc-pVDZ basis set was used for all main-group elements; the
diffuse basis set AUG-cc-pVDZ was used for phosphorus and
fluorine.28 Figgen et al. energy-consistent pseudopotentials and a
correlation-consistent basis set for iridium were used.29 Geometry
optimizations and frequency calculations for (RPCP)Ir(η2-cod),
(RPCP)Ir(η4-cod), (tBuPOCOP)Ir(η4-cod), and free 1,5-cod were
carried out without any symmetry constraints; the absence of any
imaginary frequencies confirmed the optimized structures as energy
minima. All optimizations employed the polarizable continuum model
(IEPCM) with dichloromethane as the solvent.30
3
p-C6H3(CH2P(CF3)2)2), 6.60 (ps. d, JHH = 8 Hz, 4H; m-
C6H3(CH2P(CF3)2)2), 3.44 (br s, 4H; C6H3(CH2P(CF3)2)2), 3.17
(br s, 4H; C6H3(CH2P(CF3)2)2), 2.00 (br s, fwhm = 46 Hz, 4H;
(C2F5)2PCH2CH2P(C2F5)2). 31P{1H} NMR (C6D6, 161.97 MHz, 20
° C ) :
δ
5 3 . 1 ( m , 4 P ; P ( C F 3 ) 2 ) , 2 5 . 8 ( m , 2 P ;
(C2F5)2PCH2CH2P(C2F5)2). 19F NMR (C6D6, 376.50 MHz, 20 °C):
δ −57.6 (br s, 12F; PCF3), −62.2 (br s, 12F; PCF3), −75.9 (br s, 12F;
PCF2CF3), −106.8 (br s, 8F; CH3P(CF2CF3)2).
ASSOCIATED CONTENT
* Supporting Information
[(μ-1κ2(P,C),2κ2(P′,C)-CF PCP)Ir(H)2]2(μ- PCPH)(μ-H) (9). The
CF3
3
■
S
isoprene complex 2 (0.400 g, 0.570 mmol) was dissolved in 8 mL of
CH2Cl2 in a 16 mL medium-walled storage tube, and 3 atm of H2 was
introduced. The reaction mixture was stirred at ambient temperature
for 2 weeks, during which time the headspace was recharged with 3
atm of H2 three times and the colorless solution became orange. The
solution was transferred to a 50 mL round-bottom flask, and the
volatiles were removed. The residue was taken up in ca. 35 mL of
hexane, the volume was reduced to 10 mL, and the orange product was
collected via filtration (0.196 g, 54% yield). A second crop was
collected by dissolving the filtrate in 2 mL of CH2Cl2, layering with 4
mL of MeOH, and allowing slow evaporation to give orange crystals
which were suitable for single-crystal X-ray diffraction (0.113 g, 85%
overall yield). Anal. Calcd for C24H20F24P4Ir2: H, 1.58; C, 22.65.
CIF files giving crystallographic data for complexes 2, 6, 8, and
9 and tables giving DFT optimization details and coordinates.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dmr@uwyo.edu.
■
Notes
The authors declare no competing financial interest.
1
Found: H, 1.76; C, 22.66. H NMR (C6D6, 400.13 MHz, 20 °C): δ
ACKNOWLEDGMENTS
We thank The National Science Foundation (Grant No. CHE-
0911739) for financial support.
3
■
7.47 (t, JHH = 7 Hz, 1H; C6H3(CH2P(CF3)2)2), 7.38 (s, 1H;
CF3
3
PCPH), 6.94 (t, JHH = 8 Hz, 1H; C6H3(CH2P(CF3)2)2), 6.81 (d,
3JHH = 7 Hz, 2H; C6H3(CH2P(CF3)2)2), 6.20 (d, JHH = 7 Hz, 2H;
3
2
2
C6H3(CH2P(CF3)2)2), 4.03 (dd, JHH = 17 Hz, JHP = 11 Hz, 2H;
CH2P), 3.42 (ps t, 2JHP = 12 Hz, 2H; CH2P), 3.29 (ps t, 2JHP = 12 Hz,
2H; CH2P), 2.51 (dd, 2JHH = 18 Hz, 2JHP = 12 Hz, 2H; CH2P); −9.78
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