Reactivity of iPrPCPIrH4 with para-benzoquinones

Article abstract of DOI:10.1016/j.poly.2017.10.026

In the interest of investigating new hydrogen acceptors for pincer–iridium catalyzed dehydrogenations with the ability to be catalytically recycled, a series of para-benzoquinones have been reacted with ^iPrPCPIrH₄ in various solvents and conditions. Preliminary results indicate that a wide range of quinones are capable of dehydrogenating ^iPrPCPIrH₄, and that several turn-overs in alcohol dehydrogenation by ^iPrPCPIr are possible at room temperature using benzoquinone acceptors. However, strong acceptor–catalyst interactions are inhibitory toward catalysis when the acceptor is used in excess. A new class of (bis)-η² pi-adducts, formed between ^iPrPCPIr and benzoquinones, nicknamed “barber-chairs”, has been identified and 3 examples have been characterized.

Full text of DOI:10.1016/j.poly.2017.10.026

Accepted Manuscript
Reactivity of iPrPCPIrH4 with para-Benzoquinones
Miles Wilklow-Marnell, William W. Brennessel, William D. Jones
PII:
S0277-5387(17)30687-3
https://doi.org/10.1016/j.poly.2017.10.026
POLY 12889
DOI:
Reference:
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24 October 2017
Please cite this article as: M. Wilklow-Marnell, W.W. Brennessel, W.D. Jones, Reactivity of iPrPCPIrH4 with
para-Benzoquinones, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.10.026
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Reactivity of iPrPCPIrH4 with para-Benzoquinones
Miles Wilklow-Marnell, William W. Brennessel, William D. Jones*
Department of Chemistry, University of Rochester; Rochester, NY. 14627 USA
Graphical Abstract
Synopsis:
A series of para-benzoquinones have been reacted with iPrPCPIrH4 in various solvents and conditions. A
wide range of quinones are capable of dehydrogenating iPrPCPIrH4, and several turnovers in alcohol de-
hydrogenation are possible at room temperature using benzoquinone acceptors. Strong acceptor-catalyst
interactions are inhibitory toward catalysis when the acceptor is used in excess.

Reactivity of iPrPCPIrH4 with para-Benzoquinones
Miles Wilklow-Marnell, William W. Brennessel, William D. Jones*
Department of Chemistry, University of Rochester; Rochester, NY. 14627 USA
KEYWORDS: iridium, PCP ligand, pincer complex, benzoquinone, hydrogenation, dehydrogenation, catalysis
Abstract: In the interest of investigating new hydrogen acceptors for pincer-iridium catalyzed dehydrogenations with the ability to
be catalytically recycled, a series of para-benzoquinones have been reacted with iPrPCPIrH4 in various solvents and conditions. Pre-
liminary results indicate that a wide range of quinones are capable of dehydrogenating iPrPCPIrH4, and that several turn-overs in
alcohol dehydrogenation by iPrPCPIr are possible at room temperature using benzoquinone acceptors. However, strong acceptor-
catalyst interactions are inhibitory toward catalysis when the acceptor is used in excess. A new class of (bis)-η2 pi-adducts, formed
between iPrPCPIr and benzoquinones, nicknamed “barber-chairs”, has been identified and 3 examples have been characterized.
possibility of low temperature dehydrogenations by iPrPCPIr
1. Introduction
with quinones as catalytic acceptors.
RPCP pincer ligated iridium complexes (RPCP = κ3-2,6-
C6H3(CH2PR2)2) are highly efficient homogenous catalysts for
Scheme 1. A) alkane dehydrogenation by an iridium PCP
complex using tert-butylethylene as acceptor. B) Chemical
and electrochemical redox couple of 1,4-benzoquinone and
1,4-hydroquinone.
alkane dehydrogenation [1,2]. However, the need for stoichi-
ometric quantities of a sacrificial olefin hydrogen acceptor is a
limiting factor in the practical application of this methodology.
Acceptorless dehydrogenation is possible in systems where
hydrogen is efficiently expelled from the system. However, a
relatively high barrier to the rate determining step of hydrogen
elimination, and highly positive reaction enthalpy (~30
kcal/mol) [3], prevents high conversion [4]. As such, catalytic
hydrogen acceptors are of great interest to the improvement of
this system. Para-benzoquinones are readily hydrogenated,
and the hydroquinone formed upon hydrogenation of a benzo-
quinone may be oxidized chemically, or electrochemically, to
regenerate the parent benzoquinone, making these species
ideal candidates for use as catalytic hydrogen acceptors[59]. A
traditional transfer dehydrogenation catalytic cycle with
RPCPIr, and redox systems of benzoquinone, are depicted in
Scheme 1.
A)
PR2
PR2
H
H
Ir
Ir
PR2
PR2
R
R
PR2
Ir
H
PR2
PR2
Ir
R
R
H
H
PR2
In the present work, we assess the reactivity of a number of
substituted para-benzoquinones with iPrPCPIrH4. All those
tested react with iPrPCPIrH4, and some are found to form stable
iridium-quinone species at room temperature, wherein the
quinone moiety may be reduced by addition of hydrogen, or
made labile at elevated temperature as evidenced by trapping
experiments. The catalytic hydrogenation of 9 quinones at
reasonably low catalyst loadings, low temperatures, and under
only 1 atm of hydrogen (representing only 1.2 equivalents
with respect to the quinone) is demonstrated, as well as the
transfer hydrogenation of 2,6-di-tert-butylbenzoquinone (2,6-
tBu2BQ) using isopropanol or 2,3-dihydrobenzofuran as hy-
drogen donors. In the case of isopropanol and 2,6-tBu2BQ,
transfer hydrogenation can occur at room temperature when
≤6 equivalents of the quinone are used, giving promise to the
B)
H2 + [cat]
or
+2 e-, 2 H+)
O
O
OH
- (2 e- , 2 H+)
or
OH
H2O2
O2
2. Results and Discussion
Firstly, to assess the activity of benzoquinones as hydrogen
acceptors and potential interactions between iPrPCPIr and ben-
zoquinones or hydroquinones, iPrPCPIrH4 was reacted with 3
equivalents of various quinones at room temperature. As envi-
2

sioned, two equivalents of quinone would be reduced to form
hydroquinones, leaving one equivalent of free benzoquinone
(Scheme 2). In this way, the propensity of the iPrPCPIr frag-
ment to interact with hydroquinones and benzoquinones, as
well as any preference between them, could be determined.
Scheme 2. Expected reactivity of iPrPCPIrH4 with benzo-
quinones.
The fac binding mode of the pincer ligand in these com-
plexes is rather rare, though examples have been reported for
iridium complexes containing the strongly electron accepting
CF3PCP ligand. Notably the closely related CF3PCPIr(COD)
displays a P-Ir-P angle of 119° (COD = cyclooctadiene) [10].
Substituted cyclopentadienyl complexes of PhPCPOs and
PhPCPOs(Cl) have also been reported, with P-Ir-P angles
around 113° [11]. The P-Ir-P angles of 1, 1-tBu, and 1-tBu2
were determined by X-ray crystallography to be approximate-
ly 112°, 115° and 113° respectively. In comparison, we have
recently reported the complexes iPrPCPIr(η2-styrene) and
iPrPCPIr(κ-C,C-CH2=C(C6H5)2) which display P-Ir-P angles of
163° and 160°, respectively [12]. This would suggest that a
deviation of around 50° from a more “ideal” geometry is as-
sumed in complexes 1, 1-tBu, and 1-tBu2.
iPr
iPr iPr
P
iPr
+
O
O
OH
OH
P
Rn
Rn
IrH4
+
xs
Ir
P
2
P
iPr iPr
iPr
iPr
Calculations have predicted that a bend of 40° from the ide-
al geometry comes with a cost of roughly 24 kcal/mol for
iPrPCPIr(CO) [13]. Clearly, the bis-η2-binding of certain ben-
zoquinones to iPrPCPIr is highly favorable energetically, even
more so than O-H oxidative addition of the corresponding
hydroquinones.
Both unsubstituted 1,4-benzoquinone (BQ), and 2-tert-
butylbenzoquinone (2-tBuBQ) readily reacted with iPrPCPIrH4
at room temperature. In toluene or benzene reaction with BQ
led to a rapid (<1 minute) color change to opaque purple, fol-
lowed by a change to brown and precipitation of brownish
solid leaving a brown-tinted orange supernatant. When more
soluble 2- tBuBQ was used, the same changes were noted, but
the brownish solids which formed quickly re-dissolved. The
reactivity of highly sterically hindered 2,5-di-tert-
Scheme 3. Formation of “barber-chair” products 1, 1-tBu,
and 1-tBu2 by the reaction of iPrPCPIrH4 and the corre-
sponding benzoquinones.
t
butylbenzoquinone (2,5- Bu2BQ) was much slower, turning
iPrPCPIrH4 +
O
PiPr
reddish-brown over a ten minute period, changing to greenish-
brown by 5.5 hours, and eventually purely brown after 24
hours. At this time, the only product observed by 31P-NMR
spectroscopy was represented by an apparent AB patterned
doublet of doublets centered at approximately δ 20.1, though a
marked asymmetry between the left and right sides of this AB
pattern was noted. The 31P-NMR spectrum of the solution
from reaction between iPrPCPIrH4 and 2-tBuBQ also showed a
single product, but manifest as a singlet at δ ~30.0. A singlet
with similar chemical shift (approx. δ 31.6) was noted in the
31P-NMR spectrum of the reaction solution with BQ, though
only representing about 12% of total products, with the re-
mainder attributable to a singlet at δ 44.2. However, upon
removal of solvent by vacuum and re-dissolution of the resi-
due in THF-d8, only a singlet at δ 30.7 was observed. Crystal-
lization of the residue from these reactions in toluene, or mix-
tures of THF and toluene or benzene, provided in each case a
product wherein the iPrPCPIr fragment is bound (bis)-η2 to the
C-C double bonds of a benzoquinone to give products 1, 1-
tBu, and 1-tBu2 (Scheme 3, Figure 1). These unique structures
have been given the moniker “barber-chairs” due to their re-
semblance to the seating found in most barber-shops. In the
solid state, the iridium bound benzoquinone engages in hydro-
gen bonding with hydroquinone formed during the initial reac-
tion, though in the product formed with BQ this is confined to
dimetallic units wherein two barber-chairs hydrogen bond to
one hydroquinone, while an infinite-chain network forms with
2
PiPr
Ir
R
2
room temp
xs
O
THF or toluene
OH
R
R'
R
O
O
R
- 2
1, R,R' = H,H
1-tBu, R,R' = tBu,H
1-tBu2, R,R' = tBu,tBu
R
OH
A)
B)
C)
2- BuBQ or 2,5-tBu2BQ. No indication of bonding between
t
iridium and the benzoquinone carbonyl carbons is seen and, in
fact, they are actually pushed away from the metal center in
comparison to the C=C carbons. This is not surprising, how-
ever, in light of the fact that these are 18 electron complexes,
and inclusion of the carbonyl carbons would make them 20
electron.
3

Scheme 5. Reactions of iPrPCPIrH4 with 1-tBu2 and 2,5-
tBu2BQ in the presence of excess biphenyl to form 2.
Figure 1. Crystal structures of A) 1, B) 1-tBu, and C) 1-tBu2.
These results indicate that there is thermodynamic prefer-
ence for pi-bonding to quinones over similar bonding to, or
RO-H activation of, hydroquinones with BQ, 2-tBuBQ, and
iPr iPr
P
PiPr2
t
2,5- Bu2BQ. This is further corroborated by the exclusive
PiPr2
excess
formation of 1 in the reaction of iPrPCPIr, generated by activat-
ing iPrPCPIrH4 with tert-butylethylene (TBE), and
quinhydrone, which is a hydrogen bonded pairing of benzo-
quinone and hydroquinone in a 1:1 ratio (Scheme 4). Interest-
ingly, when conducted in alkane solvent (octane, hexane, etc.)
no brown color or solids were noted and only precipitation of
yellow microcrystals was observed, which were
spectroscopically identical to 1 when re-dissolved in toluene
of THF. In toluene or benzene under otherwise identical con-
ditions, 31P-NMR spectroscopy showed only formation of 1,
though the solution was noticeably brown tinted.
+ 2
IrH4
Ir
O
2 hr, 120 °C
toluene
80%
tBu
tBu
P
iPr iPr
O
1-tBu2
iPrPCPIr
98%
iPr iPr
P
O
2
tBu
excess
+ 2
tBu
IrH4
< 20 min, 120 °C
toluene
P
O
iPr iPr
Scheme 4. Reaction of iPrPCPIr(TBE) and quinhydrone at
room temperature to form 1.
2,5-tBu2BQ
When 2,6-di-tert-butylbenzoquinone (2,6-tBu2BQ) is used
in place of 2,5-tBu2BQ, reactivity is much faster, and divergent
from that of BQ, 2-tBuBQ, and 2,5-tBu2BQ. The reaction of
iPrPCPIrH4 and 3 equivalents 2,6-tBu2BQ in toluene quickly
provides a clear red solution, and 31P-NMR spectroscopy re-
vealed only a singlet at δ 52.8 after 10 minutes at room tem-
iPr
iPr
H
OH
OH
O
O
O
O
PiPr2
PiPr2
P
Ir(TBE) +
+
Ir
O
1
perature. Inspection of the H NMR spectrum at this time re-
P
vealed only a triplet at δ –32.1, consistent with a 5-coordinate
RO-H addition product [10,15,16], likely that of the hydroxyl
moiety meta to the tert-butyl substituents (3). In the presence
of 6 equivalents biphenyl, 46% conversion of 3 to 2 was
achieved after 10 minutes at 100 °C as determined by 31P-
NMR spectroscopy. After 2 hours at 100 °C ≥98% conversion
to 2 was found (Scheme 6). No evidence of a barber-chair type
complex was found at any time with 2,6-tBu2BQ. Apparently,
RO-H activation of 2,6-di-tertbutylhydroquinone is preferred
by iPrPCPIr over pi-bonding with 2,6-tBu2BQ, likely due to a
combination of steric and electronic factors.
H
O
iPr
iPr
Exposure of toluene solutions of 1, 1-tBu, or 1-tBu2 to an
atmosphere of hydrogen gas readily regenerates iPrPCPIrH4 at
room temperature. Experiments with isolated 1-tBu2 also
showed that the quinone moiety can be reduced
intermolecularly by added iPrPCPIrH4. In previous work we
have shown that the iPrPCPIr fragment will cyclometalate bi-
phenyl within 5-10 minutes at room temperature by double C-
H activation [14]. This species (2) is very stable, and biphenyl
may serve as a trapping agent to detect the generation of
iPrPCPIr. When a toluene solution of iPrPCPIrH4, 2 equivalents
1-tBu2, and 6 equivalents biphenyl was heated for 2 hours at
120 °C, 2 was formed in about 80% yield as determined by
31P-NMR spectroscopy. When a similar experiment was con-
ducted with iPrPCPIrH4 and 2 equivalents of 2,5-tBu2BQ in the
presence of excess biphenyl, 2 formed in ≥98% yield in under
20 minutes (Scheme 5). These results indicate that the affinity
for O-H activation of 2,5-di-tert-butyl hydroquinone is mini-
mal for iPrPCPIr (at least at 120 °C), and that formation of bar-
ber-chair complexes is not a dead-end thermodynamically.
Scheme 6. Reactivity of iPrPCPIrH4 with 2,6-tBu2BQ and
trapping experiment with biphenyl.
iPr iPr
iPr iPr
O
O
toluene
room temp
P
P
Ir
P
tBu
+ 3
tBu
H
O
tBu
OH
tBu
IrH4
OH
P
tBu
tBu
iPr iPr
iPr iPr
2,6-tBu2BQ
3
OH
excess
100 °C
toluene
iPrPCPIr
2
We next investigated the catalytic hydrogenation of para-
benzoquinones by iPrPCPIrH4 under 1 atm of hydrogen gas.
We first found optimal conditions using BQ as model sub-
strate. Initially THF was used as solvent for enhanced solubili-
ty, however, the reaction was found to run much quicker in
toluene. This is likely due to the stabilization of quinhydrone
formed in THF, while in toluene the majority of hydroquinone
formed crashes out of solution, limiting quinhydrone for-
mation. A catalyst loading of 0.5% and 60 °C reaction temper-
ature was found to give optimal results, though a loading as
4

low as 0.01% could be used achieving 9240 turn-overs after
at 150 °C for 39 hours (Scheme 7, see SI for GC chromato-
grams). When 0.03% iPrPCPIrH4 was used, 2455 turn-overs
were achieved within 4 hours. Under optimized conditions,
using 0.05% iPrPCPIrH4 with respect to 2,6-tBu2BQ at 120 °C
in neat isopropanol led to complete conversion of 2,6-tBu2BQ
within 20 hours, and a 96% isolated yield of 2,6-di-tert-
butylhydroquinone in excellent purity. GC analysis of the re-
action volatiles showed good mass balance for isopropanol
consumed and acetone formed.
1
24 hours. Yields of hydroquinones were determined by H-
NMR, and results are given in Table 1.
Table 1. Catalytic hydrogenation of para-benzoquinone
(BQ) by iPrPCPIrH4.a
Catalyst loading
Time
% Yieldb
1%
0.5%
0.01%
15 min
1 hour
24 hours
>99 (TON = 100)
>99 (TON = 200)
92.4 (TON = 9240)
1
Unfortunately, attempts to extend this methodology to the
transfer hydrogenation of other para-benzoquinones in isopro-
panol at 120 °C was largely unsuccessful. All other benzoqui-
nones tested gave poor yields, and isolation was complicated
by large amounts of hard to remove brown to red-brown mate-
rial. We believe this is partly due to stabilization of
quinhydrones in isopropanol, and also promotion of an unex-
pected coupling reaction. The brown side product from the
reaction of 2,5-dimethylbenzoquinone (2,5-Me2BQ) was found
to crystalize from toluene, and X-ray diffraction of these crys-
tals revealed that C-C coupling of both 2,5-Me2BQ and 2,5-
dimethylhydroquinone (2,5-Me2HQ) had occurred forming
dimeric units which are linked in a quinhydrone type fashion
in the solid state (4, Figure 3). Interestingly, when illuminated
from behind, the crystals took on a bluish hue. This type of
coupling is known with benzoquinones, usually occurring
during the oxidation of 1,4-dimethoxybenzenes to para-
benzoquinones by ceric ammonium nitrate (CAN) [17-19].
a) 60 °C, 1 atm H2, 0.25 mL toluene, 0.66 M BQ. b) H-NMR
yield.
A series of para-benzoquinones was screened as well, and
results are given in Figure 2 (yields determined by H-NMR
1
spectroscopy). In general, relatively low temperatures and
catalyst loadings could be used. The highly sterically encum-
bered 2,5-tBu2BQ was resistant to hydrogenation, even more
so than duroquinone (2,3,5,6-tetramethylbenzoquinone). By
contrast, hydrogenation of 2,6-tBu2BQ was markedly more
facile, showing that easy approach to only one end of the ben-
zoquinone unit is necessary for hydrogenation. Substrates
which
dihydroxybenzoquinone and tetra-tert-butyldiphenoquinone
performed especially poorly. As well, 2,5-
are
likely
to
chelate,
such
as
2,5-
dichlorobenzoquinone displayed reduced yields, which is be-
lieved to be the result of C-Cl activation and possible H/Cl
scrambling. Ubiquinone (also known as Co-enzyme Q-10) was
hydrogenated in good yield, though an elevated temperature
and catalyst loading of 1% was necessary. Notably, no evi-
dence of hydrogenation of the 10-unit isoprene chain was
found by 1H-NMR spectroscopy.
Scheme 7: Attempts at iridium catalyzed dehydrogenations
with para-benzoquinones as acceptors. Percent catalyst
loading relative to benzoquinone, [Ir] = iPrPCPIrH4, neat
octane or isopropanol solutions in sealed vessel.
O
Figure 2. Catalytic hydrogenation of substituted para-
benzoquinones by iPrPCPIrH4.
OH
Rn 1% [Ir]
Rn
+
+
O
OH
OH
150 °C
O
iPrPCPIr]
OH
[
R
R
R
R
R
R
R
R
O
O
OH
OH
tBu
tBu
tBu
+
tBu
TON
1 atm H2
toluene, 60-120 °C
OH
0.008% [Ir]
150 °C
O
O
+
> 5000
O
O
O
O
O
tBu
0.5% [Ir]
60°C, 2 h
>99%
tBu
tBu
0.5% [Ir]
60°C, 45 m
98%
0.5% [Ir]
60°C, 1 h
98%
0.5% [Ir]
60°C, 72 h
65%
O
O
O
O
O
O
Cl
tBu
OMe
MeO
0.5% [Ir]
60°C, 72 h
46%
1% [Ir]
120°C, 24 h
65%
0.5% [Ir]
100°C, 2 h
51%
0.5% [Ir]
100°C, 30 m
82%
Cl
tBu
O
O
O
O
O
tBu
tBu
MeO
MeO
OH
H
10
O
O
0.5% [Ir]
100°C, 2 h
~0.5%
0.5% [Ir]
100°C, 2 h
~5%(THF)
HO
1% [Ir]
120°C, 24 h
98%
tBu
tBu
O
O
When used in a 100-fold excess, with respect to iridium, in
attempted transfer dehydrogenation of n-octane (neat) at 150
°C, GC analysis did not show evidence of octane dehydro-
genation with any of the benzoquinones tested. However,
when isopropanol was used as a substrate instead, dehydro-
genation occurred rather readily and 5238 turn-overs for con-
version of isopropanol to acetone were achieved in neat iso-
propanol with 0.008% iPrPCPIrH4 with respect to 2,6-tBu2BQ
5

Scheme 8. Room temperature dehydrogenation of isopro-
panol by iPrPCPIr.
toluene
room temp
OH
P
P
2
4
O-H, C-H,
or pi
adduct
IrH4
IrH4
excess
O
O
O
P
P
tBu
4
O
tBu
Lastly, the dehydrogenation of 2,3-dihydrobenzofuran
(DHBF) was also attempted with limited 2,6-tBu2BQ as accep-
tor. Although octane dehydrogenation was not realized, we
anticipated that this substrate might benefit from the presence
of an oxygen atom adjacent to the C-C bond to be dehydro-
genated. Accordingly,
a
toluene solution containing
iPrPCPIrH4, 10 equivalents DHBF, and 12 equivalents 2,6-
tBu2BQ was heated at 120 °C for 3 hours. GC analysis of the
resultant solution confirmed complete consumption of DHBF
and conversion to benzofuran (Scheme 9, see SI for GC chro-
matograms). These results are quite promising, and demon-
strate that para-benzoquinones can act as acceptors in dehy-
drogenations of C-C bonds catalyzed by iPrPCPIr.
3. Conclusions
Figure 3. A) Crystal structure of dimerized side product 4
from attempted transfer hydrogenation of 2,5-Me2BQ in iso-
propanol, highlighting hydrogen bonding interactions. B)
Skeletal diagram of dimer units for clarity. C) Extended hy-
drogen bonding network of dimerized 2,5-Me2HQ and 2,5-
Me2BQ units.
This brief study has revealed that para-benzoquinones are
quite capable of dehydrogenating iPrPCPIrH4 and are, in gen-
eral, readily hydrogenated under only 1 atm H2 with relatively
low catalyst loadings. Strong catalyst-acceptor interactions are
inhibitory when used in excess as acceptors for transfer dehy-
drogenation, other than in the seemingly special case of iso-
propanol dehydrogenation with 2,6-tBu2BQ as acceptor. A
new class of pincer-iridium complexes in a “barber-chair”
conformation has been discovered and characterized with
three different benzoquinones. The ability of iPrPCPIr to effect
transfer dehydrogenation of isopropanol at room temperature
when limited 2,6-tBu2BQ is used as acceptor is promising for
the potential extension of this chemistry to electrocatalysis. As
well, the demonstrated 10 turnovers in the transfer dehydro-
genation of 2,3-dihydrobenzofuran by iPrPCPIr shows that
benzoquinone acceptors are not limited to alcohol dehydro-
genations.
Despite the generally poor performance in the transfer hy-
drogenations of benzoquinones in isopropanol, results with
2,6-tBu2BQ show that para-benzoquinones can indeed act as
acceptors for dehydrogenations catalyzed by iPrPCPIr when
used in excess. However, we are more interested in the use of
para-benzoquinones as catalytic acceptors. Ideally, this would
be achieved electrochemically by oxidizing any hydroquinone
formed back to the corresponding benzoquinone under a con-
stant applied positive potential. However, conducting electro-
chemistry at >100 °C under inert atmosphere is quite a chal-
lenge in its own right. Therefore, the ability of 2,6-tBu2BQ to
act as an acceptor at room temperature, when used in limited
amounts, was investigated.
Scheme
9.
Transfer
dehydrogenation
of
2,3-
First, the ability of iPrPCPIr to dehydrogenate isopropanol at
room temperature was assessed by reacting iPrPCPIrH4 with
TBE in toluene, removing volatiles by vacuum, and then add-
ing isopropanol. By the time an NMR spectrum could be col-
lected (<10 minutes), complete reversion to iPrPCPIrH4 was
dihydrobenzofuran (DHBF) to benzofuran by iPrPCPIr
using 2,6- tBu2BQ as acceptor in toluene.
P
O
O
1
achieved, as determined by 31P and H-NMR spectroscopy,
10
10
IrH4
indicating at least 2 turn-overs for isopropanol dehydrogena-
tion can occur rapidly at room temperature. Similar experi-
ments were conducted with 4 equivalents of 2,6-tBu2BQ or
BQ, yielding identical results, and demonstrating 4 turnovers
for isopropanol dehydrogenation relatively rapidly at room
temperature. As well, identical results were obtained when
these acceptors were added to iPrPCPIrH4 in benzene or tolu-
ene and >40 equivalents isopropanol was then added (Scheme
8).
O
O
P
OH
OH
tBu
But
tBu
But
+ 12
+ 12
toluene, 120 °C
3 hours
6

ra-PCP), 6.64 (d, JHH = 2.8 Hz, 1 H, CH [2-tBuBQ]), 6.48 (d, J
= 8.4 Hz, 1 H, CH2-PCP), 6.37 (app dd, J1 = 8.4 Hz, J2 = 2.8
Hz, 1 H, CH2-PCP), 5.19 (d, J = 8.0 Hz, 1 H, CH2-PCP), 5.15
(app dd, J1 = 8.0 Hz, J2 = 2.4 Hz, 1H, CH2-PCP), 1.37 (ap-
parently the CH-PCP, CH3-PCP, and 2-tBuHQ-CH3 protons
overlap to give one large peak representing 32.5 H), 0.85 (s, 9
H, CH3 [2-tBuBQ]).
4. Experimental
All manipulations were carried out under argon atmosphere
either in a Vacuum Atmospheres glove box, or by modified
Schlenk techniques. All NMR spectra were collected on a
Bruker AMX 400 MHz spectrometer. All 31P-NMR spectra
were referenced to external H3PO4. Proton NMR spectra were
referenced to residual deuterated solvent signal. All aromatic
and alkane solvents were dried over sodium/benzophenone,
distilled from the resultant purple solution prior to use, and
stored over 3 Ǻ molecular sieves. All other reagents were used
as received from commercial sources without further purifica-
tion, except for para-benzoquinone which was sublimed 3
times at room temperature under reduced atmosphere prior to
use. X-ray structure collection was conducted on a Bruker
SMART APEX II CCD platform diffractometer. The iPrPCPH
ligand and iPrPCPIrH4 complex were synthesized by literature
methods [20,21], or slight variations thereof. GC was carried
out on a Shimadzu GC-2010 with a DB-WAXetr column (30
m x 0.25 mm ID, 0.50 um film) at 50-200 °C, 4.76 mL/min
flow.
4.3 Synthesis of 1-tBu2
To a small vial, 20 mg (37 µmol), 30 mg 2,5-di-tert-
butylbenzoquinone, and 1 mL toluene were added. This is
mixed, sealed, and left to sit at room temperature for about 1
week, after which the supernatant was decanted leaving be-
hind a crop of yellow-orange crystals. These were washed
with hexane and dried in vacuo yielding 30 mg of crystals
suitable for X-ray diffraction. The asymmetric unit contains
one co-crystallized 2,5-di-tert-butylhydroquinone and one
toluene molecule per iridium complex. 31P{1H} NMR (C6D6):
δ 20.74 (d, left side AB pattern, J = 24 Hz, 1 P) 19.22 (d, right
side AB pattern, J = 25.6 Hz, 1 P). 1H-NMR (C6D6): all peaks
of interest are found to significantly overlap making analysis
difficult, however, there are peaks in “the right places” (see
SI). Anal. Calcd for 1-tBu2 + 2,5-tBu2HQ + toluene: C =
62.06%, H = 8.05%. Found: C = 63.26%, H = 8.05%.
4.1 Synthesis of 1
This compound has been generated by several means, the
most efficient synthesis is given here: To a small vial, 10 mg
(18.7 µmol) iPrPCPIrH4, 6 µL TBE (46.6 µmol), and 0.25 mL
n-octane were added, providing a deep red solution. Then, 8
mg (37.4 µmol) quinhydrone in approximately 0.6 mL 50:50
octane/THF was added and a yellow solid separated from so-
lution. This was collected by filtration, and washed with oc-
tane, followed once by benzene. This was then suspended in
~0.25 mL benzene and THF added dropwise until complete
dissolution. This solution was stored at –15 °C, and yielded
9.5 mg of yellow crystals after about one week. This was
found to be C33H50IrO4P2 (1). One THF and one half of a hy-
droquinone molecule per iridium complex was found to co-
4.4 General Procedure for hydrogenation of benzoquinones
To a Teflon stoppered tube shaped glass ampoule ~4.8 mL
in volume, 0.166 mmol of the desired para-benzoquinone, a
stir bar, and 0.25 mL toluene containing 0.5 mg iPrPCPIrH4 are
added. Then the solution is freeze-pump-thaw degassed twice,
and 1 atm H2 applied. The sealed vessel is heated at a given
temperature in an aluminum heat block with stirring for a pre-
scribed period of time. When the reaction is ceased, volatiles
are removed until only several µL toluene remain, and then the
ampoule is quickly sealed. After this, 0.5 mL THF-d8 is added
and the solution transferred to an NMR tube for analysis.
crystallize. 31P{1H} NMR (THF-d8): δ 30.73 (s). H-NMR
1
4.5 Transfer Hydrogenation of 2,6-di-tert-butylbenzoquinone
in isopropanol
(THF-d8): It is somewhat unclear whether benzoquinone re-
mains tightly bound in THF solution, peaks attributable to
iPrPCPIr given: δ 7.00 (d, JHH = 7.2 Hz, 2 H, meta-PCP), 6.82
(t, JHH = 7.4 Hz, 1 H, para-PCP), 3.42 (m, 2 H, CH2-PCP),
3.17 (vdd, 2 H, CH2-PCP), 1.58 (m, 4 H, CH-PCP), 1.42 (m,
12 H, CH3-PCP), 1.16 (m, 12 H, CH3-PCP). Anal. Calcd for
C33H50IrO4P2: C = 51.82%, H = 6.59%. Found: C = 45.85%, H
= 5.60%.
To a Teflon stoppered ampoule approx. 6 mL in volume,
100 mg 2,6-di-tertbutylbenzoquinone (0.454 mmol) and 3.3
µL toluene stock containing 0.229 µmol iPrPCPIrH4 are added,
along with a stir bar. Then, under positive argon pressure 0.3
mL isopropanol is added, and the sealed vessel heated at 120
°C for 20 hrs. After this time, all volatiles were vacuum trans-
ferred to another flask for later analysis, leaving behind a
white slightly green tinted residue. This was washed with cold
pentane, and the washings stored at –15 °C. The remaining
solids were dried in vacuo. After storing overnight, further
crystals had formed in the pentane washings and these were
combined with the earlier obtained product to give 97 mg
4.2 Synthesis of 1-tBu
To a small vial, 10 mg (18.7 µmol) iPrPCPIrH4, 12 mg (73
µmol) 2-tertbutylbenzoquinone, and 0.5 mL toluene are added.
An opaque purple mixing is obtained, and the vial is then
sealed and shaken. After 2 to 3 minutes a clear brown solution
is obtained. Pentane is then added causing a yellow powder to
precipitate, which is collected by filtration, washed with fur-
ther pentane, and re-dissolved in 1:1 benzene/THF. The result-
ant solution was stored at -15 °C, and provided 10 mg of small
yellow crystals after several days, some of which were found
suitable for X-ray diffraction. The structure was found to con-
tain a molecule of co-crystallized benzene as well as another
molecule of toluene or benzene which was badly disordered.
One half of a 2-tert-butylhydroquinone molecule is also pre-
sent in the asymmetric unit. 31P{1H} NMR (THF-d8): δ 28.28
1
(96% yield) 2,6-di-tertbutylhydroquinone. H-NMR (C6D6): δ
6.59 (s, 2 H, CH), 4.46 (s, 1 H, OH), 3.88 (bs, 1 H, OH), 1.32
(s, 18 H, CH3). 13C-NMR (C6D6) δ 149.04, 147.45, 137.28,
111.89, 34.2, 29.99.
4.6 Transfer Hydrogenation of 2,6-di-tert-butylbenzoquinone
with 2,3-dihydrobenzofuran
To a J-Young NMR tube, 5 mg (9.4 µmol) iPrPCPIrH4, 0.35
mL toluene, and 10.6 µL (94 µmol) 2,3-dihydrobenzofuran
were added and mixed, followed by 24.8 mg (112.4 µmol) 2,6-
di-tert-butylbenzoquinone. The sealed reaction vessel was
then heated at 120 °C in an aluminum heating block for 3
hours. After this time the reaction was allowed to cool to room
temperature and a sample prepared in toluene for GC analysis,
1
(s). H-NMR (THF-d8): 7.69 (s, 1 H, CH [2-tBuBQ]), 7.49 (s,
1 H, CH [2- tBuBQ]), 7.32 (s, 1.5 H, CH [2-tBuHQ]), 6.99 (d,
JHH = 7.6 Hz, 2 H, meta-PCP), 6.84 (t, JHH = 7.4 Hz, 1 H, pa-
7

which revealed nearly quantitative conversion of 2,3-
dihydrobenzofuran to benzofuran.
Acknowledgments
This work was supported by the NSF under the CCI Center for
Enabling New Technology through Catalysis (CENTC), CHE-
1205189.
[20] M. Montag, L. Schwartsburd, R. Cohen, G. Leitus, Y. Ben-
David, J.M.L. Martin, D. Milstein, D. Angew. Chem. Int. Ed. 46
(2007) 1901-1904.
[21] A.S. Goldman, R. Ghosh, Handbook of C-H Transformations 2
(2005) 616.
Appendix A. Supplementary Data
CCDC 1565308-1565311 contain the supplementary crystal-
lographic data for complexes 1, 1-tBu, 1-tBu2, and 4. These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +441223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with
1
this article includes selected H and 31P-NMR spectra and X-
ray data. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
Corresponding Author Information
*E-mail: jones@chem.rochester.edu
ORCID
William D. Jones: 0000-0003-1932-0963
Miles Wilklow-Marnell: 0000-0002-2678-8700
William W. Brennessel: 0000-0001-5461-1825
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