The Ferraquinone-Ferrahydroquinone Couple: Combining Quinonic and Metal-Based Reactivity

Article abstract of DOI:10.1021/jacs.6b13050

A ferraquinone-ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal-ligand cooperation.

Full text of DOI:10.1021/jacs.6b13050

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Article
The ferraquinone – ferrahydroquinone couple:
combining quinonic and metal-based reactivity
Alexander Dauth, Urs Gellrich, Yael Diskin-Posner, Yehoshoa Ben-David, and David Milstein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b13050 • Publication Date (Web): 31 Jan 2017
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Journal of the American Chemical Society
Theꢀferraquinoneꢀ–ꢀferrahydroquinoneꢀcouple:ꢀcombiningꢀquinonicꢀ
andꢀmetal-basedꢀreactivityꢀꢀ
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,
†, δ
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‡
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,†
Alexander Dauth Urs Gellrich, Yael Diskin-Posner, Yehoshoa Ben-David, David Milstein*
†
||
Department of Organic Chemistry and Department of Chemical Research Support, Weizmann Institute of Science, Re-
hovot, 76100, Israel
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ABSTRACT: A ferraquinone – ferrahydroquinone organometallic redox-couple was prepared and characterized. Intricate coopera-
tivity of the metal is observed with different positions on the ligand. This allowed cooperative activation of small molecules like
molecular hydrogen, oxygen and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal-ligand co-
operation.
general reactivity of the resulting redox couple towards the
activation of small molecules (Scheme 2).
Introductionꢀꢀ
Quinones are prevalent in biological and chemical redox
1
Scheme 2. Envisioned ferraquinone-hydroferraquinone
redox couple.
processes. For example, the production of hydrogen peroxide
utilizing anthraquinone is performed yearly on a multi-ton
2
scale. Quinones like DDQ or chloranil are routinely used for
3
–9
oxidation reactions in academic laboratories.
Likewise,
quinonic cofactors like ubiquinone (coenzyme Q10) play an
essential role in the respiratory chain of almost all aerobic
10,11
organisms and act as antioxidants in the body.
Recently,
quinones have also found utilization in transdisciplinary appli-
12
cations like enzymatic fuel cells. We sought to combine the
redox properties of quinonic compounds with suitable transi-
tion metals in the anticipation to observe new cooperative
patterns which can in turn lead to novel chemical transfor-
mations. To this end we envisaged the employment of iron
which has experienced a renaissance in its use in transition
metal catalysis due to its obvious ecological and economic
Specifically, the role and mode of metal ligand-cooperation
in this system was explored in the various transformations
facilitated by the system. In recent years cooperation between
the metal and the supporting ligands has led to unprecedented
reactivity which harvests both the reactivity of the metal as
3
6–40
1
3–25
well as the ligand.
benefits.
Scheme 1. The only metallaquinone reported to date.
Resultsꢀandꢀdiscussionꢀ
The phenolic PCP pincer ligand 3,5-bis- (di-iso-
propylphoshinomethylene)phenol 3 was prepared (see Sup-
plementary Information, SI) and reacted with Fe(CO) in THF
5
under UVB irradiation (Scheme 3). After 4 days, ligand meta-
lation was complete and a highly air sensitive, green crystal-
line solid was obtained. The product was identified by NMR
analysis as ferrahydroquinone-hydride 2 exhibiting a clear
1
triplet in the H NMR spectrum at -8.83 ppm in C
6
D
6
charac-
Several years ago, our group reported the formation of the
first and only metallaquinone, in which an oxygen atom of a
quinone is replaced by a metal, namely the ruthenaquinone
depicted in Scheme 1. Spectroscopic and computational
studies of this new compound class were performed but no
further reactivity studies were undertaken. Specifically, the
spectral properties were solvent-dependent, which was as-
cribed to the presence of an overall quinonic structure in non-
polar solvents, while a zwitterionic structure was stabilized in
more polar solvents.
teristic for a hydride cis to two equivalent P ligands and trans
to CO as a strong π-acceptor ligand. The ring proton of the
pincer ligand was found at 6.48 ppm which is typical for the
aromatic protons in organic hydroquinones.
2
6
X-ray quality crystals were obtained by slow diffusion of
4
1
pentane into an ether/pentane solution of 2 (Figure 1). The
unusually expansive unit cell (see SI) contains 12 molecules in
the asymmetric unit cell of 2 and confirms the formation of the
ferrahydroquinone-hydride di-carbonyl complex as a distorted
octahedron. The bond distance between the iron center and the
ipso carbon of the ligand (C1) is 2.070(4) Å. This is slightly
longer than the Fe-C(ipso) distance in the related POCOP iron
Other reports from our group described phenoxonium cati-
ons as well as quinone methides stabilized by transition met-
2
7–29
als.
contain ortho- and para-quinonic moieties
framework but in no other case is the metal center an integral
A number of organometallic species are known which
30–32
pincer bis-carbonyl hydride complex reported by Guan et al.
in their ligand
42
(
1.995 Å). In the ligand scaffold, the C5 – O1 bond is
3
3–35
1.440(6) Å, slightly longer than the C–O single bond in organ-
ic hydroquinone (1.392 Å). The P1 – Fe – P2 and C1 – Fe –
C21 bond angles are smaller than 180° with 159.10(6)° and
part of the quinonic system.
Based on our earlier find-
2
6
ings we intended to prepare a corresponding complex of
earth-abundant iron, ferraquinone 1, with its hydrogenated
counterpart – a ferrahydroquinone 2 – and to investigate the
1
70.8(3)° respectively, while the angle between the CO lig-
ands is almost a proper right angle at 88.8(3)°.
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was also prepared independently via a two-step sequence in
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order to confirm its formation (Scheme 3). In a first step, a
benzene solution of 2 was treated with aqueous hydrochloric
acid. The corresponding ferrahydroquinone chloride complex
1
Scheme 3. Preparation of 1 and 2.
4 was formed as a light yellow solid in 87% yield. The H
NMR spectrum of 4 confirmed the elimination of the hydride
ligand and the solid-state structure also showed the replace-
ment of the hydride by a chloride ligand (Figure 2).
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Figure 2. Solid state structure of 4 (thermal ellipsoids set at 50%
probability level; isopropyl groups presented as wireframe and
hydrogens omitted for clarity). Selected bond lengths [Å] and
angles [°] for 4: Fe1 – C21 1.801(5), Fe1 – C22 1.660(1), Fe1 –
Cl1 2.402(3), Fe1 – C1 2.041(4), Fe1 – P1 2.265(2), Fe1 – P2
2.274(2), C5 – O1 1.381(6), C1 – C7 1.402(8), C6 – C7 1.400(7),
C5 – C6 1.384(8), P1 – Fe1 – P2 165.14(5), C1 – Fe1 – C21
179.6(4), C1 – Fe1 – Cl1 87.5(2), C21 – Fe1 – C22 93.6(4).
Again, the complex adopts a slightly distorted octahedral geome-
try with the Fe1 – C1 bond length being 2.041(4) Å and the Fe1 –
Cl1 distance 2.402(3) Å. The ligand is aromatic with C – C bond
lengths varying slightly between 1.384 and 1.403 Å and the C – O
bond being a typical hydroquinonic single bond at 1.381(6) Å.
Consequently, the phenolic position of complex 4 was deproto-
nated with KHMDS in benzene which led to formation of ferra-
quinone 1 in 60% yield with identical spectral properties as after
reaction of 2 with benzoquinone. In contrast to the previously
2
6
reported ruthenaquinone, the spectral properties of 1 were not
dependent on the polarity of the solvent and it was found soluble
in a wide spectrum of organic solvents ranging from methanol,
acetonitrile, tetrahydrofuran and dichloromethane to diethyl ether
and pentane. The addition of external ligands like CO, acetonitrile
Figure 1. Solid state structure of 2 (thermal ellipsoids set at 50%
probability level; isopropyl groups presented as wireframe and
hydrogens omitted for clarity). Selected bond lengths [Å] and
angles [°] for 2: Fe1 – C21 1.747(8), Fe1 – C22 1.640(6), Fe1 –
P1 2.239(2), Fe1 – P2 2.276(2), Fe1 – C1 2.070(4), C5 – O1
3
or PPh did not affect the spectral properties of 1, signifying a
coordinative saturated complex. Indeed, calculations at the BP86-
1
.440(6), C1 – C7 1.394(6), C7 – C6 1.427(6), C6 – C5 1.340(7),
P1 – Fe1 – P2 159.10(6), C1 – Fe1 – C21 170.8(3), C1 – Fe1 –
C22 99.5(3), C21 – Fe1 – C22 88.8(3). Data collected at ESRF
D3/def2–TZVP level of theory clearly minimize 1 in a quinonic
4
5
4
3
geometry as a trigonal bipyramid. The C–O bond in the ligand
was calculated to be a double bond at 1.25 Å and the C–C bond
lengths in the aryl moiety varied from 1.46 Å for the single bonds
to 1.37 Å for the double bonds in conjugation with the quinonic
C=O double bond (Figure 3). The angle between the CO ligands
is 96° which is corroborated by the FT-IR spectrum of 1 where
the CO ligands appear as two bands of almost equal intensity at
ID-29.
Ferrahydroquinone-hydride 2 was treated with two equiv.
benzoquinone in order to transform it to the ferraquinone 1.
High resolution mass spectrometry confirmed the formal loss
2
of H by the dehydrogenation of 2 with benzoquinone. Further
support for the formation of a quinonic structure comes from
NMR data which show the signal of the ring-proton in the
phenyl moiety of the pincer ligand at 6.87 ppm which is also
typical for organic quinones. Likewise, the carbonyl-carbon
-1
1978 and 1919 cm . These values are in turn in good agreement
with the data reported for the ruthenaquinone system (1983, 1926
-
1
cm ). The quinonic C=O bond stretch falls at relatively low
-
1
1
3
wavenumbers (1563 cm ) suggesting a strong contribution of the
metal center to the electronic structure of the ligand. This observa-
tion is qualitatively confirmed by frequency calculations at the
gives a signal at 170 ppm in the C NMR, slightly up-field
from organic quinones. The carbenoid ipso-carbon could not
be detected, possibly due to fast relaxation through the neigh-
-1
4
4
BP86-D3/def2–TZVP, rendering this band at 1583 cm .
boring Fe center or due to trace paramagnetic impurities. As
X-ray quality crystals could not be obtained, ferraquinone 1
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Having established synthetic routes to both members of the
2
Figure 4. Free energy pathway for the H activation by the ferra-
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quinone 1, calculated at the TPSS-D3BJ/def2-TZVPP//BP86-
D3/def2-SV(P) level of theory. Solvent effects are implicit taken
into account by using the SMD model.
ferraquinone–ferrahydroquinone couple, 1 and 2, we set out to
investigate their reactivity with special attention to cooperativ-
ity between metal and ligand.
The net-reaction comprises a formal 1,6 type cooperation of
the Fe center with the carbonyl oxygen in the para-position of
the ligand. Ferrahydroquinone 2 is resistant to acceptorless H
loss when heated in boiling toluene. Likewise, heating 2 to
00°C as a solid under vacuum left the starting material un-
2
2
46
changed. These findings are in agreement with the calculated
ΔG for the H activation (Figure 4). When 2 was reacted with
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a large excess (at least 10 equivalents) diphenylacetylene as
hydrogen acceptor under UVB irradiation, disappearance of 2
and formation of 1 was confirmed by IR spectroscopy. Con-
comitant formation of a 3:1 mixture of Z and E-stilbene was
detected by GCMS. The hydrogen transfer could also be ob-
served with phenylacetylene as substrate, yielding styrene and
ethylbenzene in a 15:1 mixture as determined by GC-MS. Treat-
ing 1 with dilute mineral acid like HCl yielded the ferrahydro-
quinone-chloride complex 4 (Scheme 5). As can be seen, the
proton was transferred to the para-oxygen on the ligand while
the chloride is bound to the metal center.
Figure 3. DFT optimized structure of 1 (BP86-D3/def2-TZVP).
Initially, we were interested whether 1 could be directly trans-
formed into 2 by activation of molecular hydrogen.When a
solution of 1 in C D was pressurized with two bar H , for-
6 6 2
mation of 2 was observed after 18 hours (Scheme 4). Under
UVB irradiation, the reaction was complete after only 2 hours.
Scheme 5. Reaction of 1 with HCl via a formal 1,6-
addition.
2
Scheme 4. Reaction of 1 with H via a formal 1,6-addition.
Organic quinones are known to undergo hydrogen-halogen
exchange at the ring via halogen (X2) addition followed by
1
2
HX elimination. We were interested to see whether a similar
behavior could be observed with ferraquinone 1. Indeed, reac-
The mechanism of the thermal hydrogen activation by the
ferraquinone 1 was examined by DFT calculations at the
SMD(benzene)-TPSS-D3BJ/def2-TZVPP//BP86-D3/def2-
tion with 2.2 equivalents of a 1wt% Br benzene solution
2
proceeded rapidly to form a new organometallic species. This
compound was identified by NMR, FT-IR, MS and X-ray
crystallography to be ferrahydroquinone-bromide 6 with a
brominated aryl moiety of the PCP pincer ligand in both meta-
positions to the metal (Scheme 6, Figure 5).
2
SV(P) level of theory (Figure 4). Direct addition of H to the
Fe-Cipso is possible according to the calculations via a low
barrier of 21.4 kcal/mol. A subsequent keto-enol tautomeriza-
tion yields the Ferrahydroquinone-hydride 2 in an overall exer-
gonic reaction.
Scheme 6. Formation of 5 by treatment of 1 with ele-
mental bromine.
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The solid state structure shows a slightly distorted octahe-
dral geometry around the metal center with a Fe1 – C1 bond
length of 2.031(4) Å and a Fe1 – Br3 distance of 2.4900(7) Å
(Figure 5). In the ring moiety of the pincer ligand C – C bond
lengths vary slightly between 1.392 and 1.405 Å and the C – O
bond is a single bond at 1.356(4) Å, demonstrating aromaticity
of the ring. C – Br bond lengths are essentially equal 1.904(3)
and 1.906(4) Å.
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Using a deuterated alcohol like CD
was evident from the absence of a hydride signal in the H
NMR spectrum while the P NMR spectrum displayed a 1:1:1
triplet at 110.5 ppm for the coordinated ligand, showing the
coupling of the two equivalent phosphorus atoms with a deu-
3
OD afforded d-2 which
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1
2
teride (Scheme 7). As in the reaction with H and HCl, a for-
mal 1,6-type metal-ligand cooperation is involved in the oxi-
dation of alcohols.
Figure 5. Solid state structure of 5 (thermal ellipsoids set at 50%
probability level; isopropyl groups presented as wireframe and
hydrogens omitted for clarity). Selected bond lengths [Å] and
angles [°] for 5: Fe1 – C21 1.813(4), Fe1 – C22 1.765(4), Fe1 –
Br3 2.4900(7), Fe1 – C1 2.031(4), Fe1 – P1 2.268(1), Fe1 – P2
2.279(1), ¬C5 – O1 1.356(4), C4 – Br1 1.906(4), C6 – Br2
.904(3), C1 – C3 1.404(5), C3 – C4 1.392(5), C4 – C5 1.396(5),
P1 – Fe1 – P2 168.23(4), C1 – Fe1 – C21 177.3(2), C1 – Fe1 –
Br3 89.1(1), C21 – Fe1 – C22 98.5.
When a THF or benzene solution of 2 was exposed to air, an
instantaneous color change to deep orange was observed. X-
ray quality crystals were obtained from a cooled toluene solu-
tion layered with diethylether and the solid-state structure
revealed the formation of oxyferraquinone 6. One oxygen
atom has been incorporated into the complex and is bridging
the metal center and the pincer ligand in a 1,2 fashion in the
ipso position (Scheme 8).
1
A likely mechanism for this transformation is analogous to
hydrogen-halogen substitution in organic quinones, namely,
Br addition to the quinonic double bonds, followed by HBr
2
Scheme 8. Formation of the oxaferraquinone 6 by expo-
sure of 2 to oxygen.
elimination. The HBr liberated during this step is immediately
incorporated into the complex, akin to the reaction of 1 with
HCl (Scheme 6).
Furthermore, it was found that when standing in alcoholic
solutions for 18 hours at room temperature, ferraquinone 1
was also quantitatively converted into ferrahydroquinone 2.
The reaction time could be significantly decreased to 2 hours
by UVB irradiation. At the same time, formation of the corre-
sponding aldehyde and no other organic products was ob-
served by GCMS. Short and medium chain aliphatic alcohols
as well as benzyl alcohol were stochiometrically dehydrogen-
ated to their corresponding aldehydes (Scheme 7).
The solid state bond lengths as well as computational analy-
sis (NBO, Mayer bond order and bond critical point anal-
4
8
yses) confirm that the best description of the bonding is that
of a true metallaoxirane rather than a π-bonded carbonyl
group, in contrast to our earlier observations with Ir stabilized
Scheme 7. Dehydrogenation of alcohols mediated by 1.
3
8
phenoxonium cations. The character of the C(ipso)-O bond
(C1 – O4) is that of a single bond at 1.341(2) Å while the
C(para)-O bond retains clear double bond character with a
1
2
.251(2) Å (C5 – O1) bond length. The Fe1 – C1 distance is
.1111(9) Å, only slightly longer than in complexes 2, 4 and 5.
The bond lengths in the ring alternate from double bonds for
C3 – C4 (1.359(2) Å) to single bonds for C1 – C3 (1.458(2)
Å) and C4 – C5 (1.459(2) Å). The C1 – O4 bond is 16.6°
above – and the C1 – Fe1 bond 55.6° below the pincer ring
plane, giving C1 the resemblance of a spiro-junction of the
six-membered and the three membered rings (Figure 6).
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A recent report from our group describes the activation of
dioxygen by metal-ligand cooperation involving the pincer
sidearms of a pyridine-based PNP-Iridium complex. Like-
1
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wise, Goldberg
formation of complexes in which an oxygen atom originating
from O or N O, respectively is bridging the metal center and
the ligand in a fashion similar to that observed in 7. However,
we are not aware of a reported observed O a1ctivation process
and more recently Piers reported the
2
2
2
5
involving O insertion into a metal-aryl bond.
The mechanism of the activation of dioxygen by 2 was in-
vestigated by DFT calculations at the SMD(THF)-TPSS-
D3BJ/def2-TZVPP//BP86-D3/def2-SV(P) level of theory.
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Initial insertion of O2 via the long range adduct 2 O2 and
.
Figure 6. Solid state structure of 6 (thermal ellipsoids set at 50%
probability level; isopropyl groups presented as wireframe and
hydrogens omitted for clarity). Selected bond lengths [Å] and
bond and torsion angles [°] for 6: Fe1 – C21 1.780(1), Fe1 – C22
1.764(1), Fe1 – C1 2.1111(9), Fe1 – O4 1.9707(8), Fe1 – P1
2.2633(4), Fe1 – P2 2.2676(4), C1 – O4 1.341(2), C5 – O1
TS2 O2-7 into the Fe-hydride yields the hydroperoxo complex
7
(Figure 7). A double cross-over pathway for this insertion
can be found with a barrier of 19.7 kcal/mol, which would
allow this step to occur at room temperature. Exergonic dimer-
ization of 7 leads to the Ci symmetric dimer 8. Protonation of
the hydroperoxy ligand by the hydroxyl group of the second
molecule and concomitant C(ipso)-oxygen bond formation
yields the oxaferraquinone 6 and one equivalent water as
byproduct in an overall strongly exergonic reaction.
1
.251(2), C1 – C3 1.458(2), C3 – C4 1.359(2), C4 – C5 1.459 (2),
P1 – Fe1 – P2 166.23(1), C1 – Fe1 – C21 147.80(5), Fe1 – C1 –
O4 65.25(5), C3 – C1 – O4 119.39(9), C1 – Fe1 – O4 38.15(4),
C1 – O4 – Fe1 76.60(6), C22 – Fe1 – C21 96.08(6), C1 – Fe1 –
C22 116.11(5), C4 –C3 – C1 – O4 -163.4(1), C4 – C3 – C1 – Fe1
1
24.4(1).
Figure 7. Free energy pathways of oxygen insertion into 2, calculated at the TPSS-D3BJ/def2-TZVPP//BP86-D3/def2-SV(P) level of
3
theory. Solvent effects are implicit taken into account by using the SMD model. All free energies are given with respect to 2 and O
7 in the case of 8 and TS8-6).
2
(and
The experimental observation that a full equivalent of oxy-
gen is necessary for the reaction to proceed to completion
supports the proposed mechanism. Essential for the reaction is
the formation of the interesting dimeric structure 8. The DFT
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Page 6 of 10
optimized structure reveals intermolecular short hydrogen over activated 3Å molecular sieves. Dichloromethane was
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
bonds between the para-hydroxyl group and the hydroperoxy
ligand bound to the iron center of a second molecule (Figure
8). An analysis of the potential energy density at the bond
critical point of these hydrogen bonds revealed that each sin-
gle hydrogen bond stabilizies the dimer 8 by – 12.9 kcal/mol.
These hydrogen bonds facilitate the protonation and weaken
the O-O bond. Furthermore, the orientation of the hydroperoxy
ligand in 8 allows for an interaction of the Fe-C(ipso) σ-bond
with the antibonding O-O σ*-bond of the hydroperoxy ligand,
necessary for the SN2 - like C-O bond formation to occur
dried and stored over activated 3Å molecular sieves. All
commercially available reagents were used as received. UVB
(280-315 nm) irradiation was performed in a Luzchem LZC-
ORG photoreactor equipped with 10 lamps operated at 60Hz
(3A, 220V). The vessels used during irradiation were regular
Pyrex glass Schlenk flasks. Experiments with quartz glass
vessels yielded the same results and reaction times. NMR
spectra were recorded using Bruker Avance III 300, Avance
III 400, and Avance 500 spectrometers at 298K. Chemical
1
13
shifts were referenced to the residual solvent peaks ( H, C),2
as well as to an external standard of phosphoric acid (85%
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Figure 8).
31
solution in D2O) at 0.0 ppm ( P). Chemical shifts are reported
in parts per million, and coupling constants (J) are reported in
1
1
hertz. NMR assignments were assisted by H− H-COSY,
1
1
1
1
13
1
13
H− H-NOESY, H− C-HSQC and H− C-HMBC. In the
3
C-DEPTQ NMR spectra, primary and tertiary carbon signals
are phased down (d), secondary and quaternary carbon signals
are phased up (u). IR spectra were recorded on a Thermo
Scientific Nicolet 6700 FT-IR spectrophotometer as thin films
on NaCl or CaF2 disks. Electrospray ionization mass spec-
trometry (ESIMS) spectra were recorded on a Micromass ZQ
V4.1 by the Chemical Research Support Unit of the Weiz-
mann Institute of Science. Crystal data for complexes 4,5 and
6
were measured at 100 K on a Bruker Kappa Apex-II CCD
diffractometer equipped with [λ(Mo Kα) = 0.710 73 Å] radia-
tion, a graphite monochromator, and MiraCol optics. The data
were processed with APEX2 collect package programs. Struc-
tures were solved by the AUTOSTRUCTURE module and
refined with full-matrix least squares refinement based on F2
with SHELXL-2013.
Figure 8. DFT optimized structure of 8 with a schematic repre-
sentation of the Fe-C(ipso) σ-bond - O-O σ*-bond interaction.
The computed O--HO hydrogen bond length is shown.
Data for complex 2 was collected on APEX2 and then in the
synchrotron ESRF ID-29. Data were processed with HKL2000
keeping the Friedel Pairs separate. The structure was solved
with SHELXT. The best suggested solution was a non-
centrosymmetric space group Pc with 12 molecules in the
asymmetric unit cell and Flack parameter 0.49 and most of the
iso-propyl side chains were missing. Data was processed again
with CrysAlisPro keeping the FP separate and the initial solu-
tion with SHELXT contained almost all the atoms (including
iso-propyl side chains). The structure was further refined with
SHELXL-2013 with full-matrix least squares refinement based
on F2. The refinement was extremely complex due to the high
level of the molecules disorder and twinning. Many constrains
were applied in order to bring the refinement to completion.
The hydride was calculated and placed at 1.5 Å from the metal
center in direct extension of the Fe-CO bond.
In summary, we have developed a conceptually new metal-
ligand cooperation pathway through an unprecedented ferra-
quinone – ferrahydroquinone couple. The metal center shows
cooperativity with three different positions of the ligand in a
formal 1,2 and 1,6 fashion depending on the reaction condi-
tions, which leads to a number of unique reactivity patterns.
The ferraquinone reacts with alcohols to form aldehydes and
lactones, thereby regenerating the ferrahydroquinone. In an
unprecedented reaction for any metal complex, and analogous
2
to reactions of organic quinones, it activates Br and selective-
ly incorporates it into the complex. The ferraquinone can be
transferred to its hydrogenated form, the ferrahydroquinone by
activation of molecular hydrogen or alcohols. This compound
instantaneously selectively activates molecular oxygen at
room temperature by metal-ligand cooperation resulting in O-
insertion into the aryl-Fe bond and formation of the corre-
sponding oxyferraquinone. This novel mode of metal-ligand
cooperation harnesses the reactivity of metal center and the
ligand in three different positions alike and the described
diversity in transformations and reactivity modes promises
further fruitful studies with this motif.
Synthesis of 1. Method A: In a 5 mL vial equipped with a
Teflon stirbar was placed 2 (10 mg, 0.021 mmol) in dry ben-
zene (1 mL) and stirred. A solution of benzoquinone (4.5 mg,
0
.042 mmol) was added and the reaction mixture turned from
yellow to deep orange-brown. The mixture was stirred for 30
min. at room temperature and then filtered and the solvent was
removed in vacuo. The crude residue was extracted with pen-
tane (5x0.5 mL) and the extracts combined and dried to give a
dark orange powder (8 mg, 0.018 mmol, 86%). Method B: In a
ExperimentalꢀDetailsꢀ
2
0 mL scintillation vial equipped with a Teflon stirbar was
General specifications. All reactions were performed under
a nitrogen atmosphere in a Vacuum Atmospheres Co. Model
Nexus glovebox or using standard Schlenk techniques unless
otherwise noted. All solvents were reagent grade or better.
THF, diethyl ether, benzene, and pentane were refluxed over
sodium and distilled under a nitrogen atmosphere and stored
placed 4 (150 mg, 0.300 mmol) in dry benzene (6 mL) and
stirred. A solution of KHMDS (72 mg, 0.360 mmol) was
added dropwise and the reaction mixture turned from yellow
to deep orange-brown with some precipitate forming. The
mixture was stirred for 30 min. at room temperature and then
filtered and the solvent was removed in vacuo. The crude was
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Journal of the American Chemical Society
extracted with pentane (5x3 mL) and the extracts pooled and
dried to give a dark orange powder (83 mg, 0.179 mmol,
with some gas evolution. The mixture was stirred for 1 hr at
room temperature and the volatiles were removed in vacuo.
The crude was extracted with THF (3 x 2 mL) and the filtered
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
6
0%). H NMR (300 MHz, C
3.10 (m, 4H, Ar-CH -P), 2.44 (br.s., 2H, CH(iPr)), 2.07 (br.s.,
2H, CH(iPr)), 1.61-0.62 (m, 24H, iPr). C{ H} NMR (126
MHz, C ) δ ppm 217.8(u) (t, JC-P=27.5 Hz, CO), 214.8(u) (t,
C-P=11.8 Hz, CO), 170.1(u) (s, C=O), 148.4(u) (s, Ar),
117.1(d) (br. s., Ar), 37.2.4(u) (br. s., Ar-CH -P), 26.5(d) (t, JC-
=7.6 Hz, CH(iPr)), 25.2(d) (t, JC-P=25.1 Hz, CH(iPr)), 19.9(d) (s,
6
D
6
) δ ppm 6.87 (s, 2H, Ar), 3.77-
extracts combined and dried to give 4 as a light yellow powder
2
1
3
1
1
(93 mg, 0.186 mmol, 87%). H NMR (400 MHz, CDCl
3
) δ
D
6
ppm 6.63 (s, 2 H, Ar), 4.47 (br. s., 1 H, ArOH), 3.57 (dt,
J=15.6, 4.6 Hz, 2H, Ar-CH -P), 3.33 (dt, J=15.6, 3.7 Hz, 2H,
Ar-CH -P), 3.03-2.89 (m, 2H, CH(iPr)), 2.51-2.35 (m, 2H,
CH(iPr)), 1.45-1.32 (m, 12H, iPr) 1.30-1.20 (m, 12H, iPr).
6
J
2
2
2
P
3
1
1
13
1
iPr), 19.8(d) (s, iPr), 19.5(d) (s, iPr), 19.4(d) (s, iPr). P{ H}
3
C{ H} NMR, DEPT-Q (100.7 MHz, CDCl ) δ ppm 216.1(u)
-
NMR (121 MHz, C
6
D
6
) δ ppm 94.5. IR 1983, 1921, 1563 cm
(t, JC-P=25.6 Hz, CO), 212.1(u) (t, JC-P=13.6 Hz, CO), 160.0(u)
(t, JC-P=12.8 Hz, ArC-Fe), 154.2(u) (s, ArC-OH), 148.3(u) (t,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
+
.
HRMS (ESI) calc. 465.1411 (C22
65.1409.
Synthesis of 2. In a 100 mL Schlenk flask equipped with a
Teflon stirbar was placed 3 (400 mg, 1.13 mmol) in dry THF
15 mL). Fe(CO) (200 mg, 1.02 mmol) was added, the flask
34 3 2
H O P Fe + H ), found
4
J
C-P=8.5 Hz, Ar) 110.9(d) (t, JC-P=7.4 Hz, Ar), 37.7(u) (t, JC-
=15.0 Hz, Ar-CH -P), 26.1(d) (t, JC-P=9.9 Hz, CH(iPr)) 24.9 (t,
C-P=9.9 Hz, CH(iPr)), 19.7(d) (s, iPr), 19.6(d) (s, iPr), 19.5(d)
P
2
J
31
1
(
s, iPr), 19.3(d) (s, iPr). P{ H} NMR (162.1 MHz, CDCl ) δ
3
(
5
-1
ppm 94.1 (s). IR 1995, 1932 cm .
was sealed and put in a UVB reactor with stirring at room
temperature. CO gas forming in the course of the reaction was
vented after 6 h and again after 18 h and 48 h. After 4 days the
reaction mixture was emerald green and the volatiles were
removed in vacuo. The residue was washed with pentane (3 x
Synthesis of 5. In a 20 mL scintillation vial equipped with a
Teflon stirbar was placed 1 (20 mg, 0.043 mmol) in dry ben-
zene (3 mL), sealed with a septum screw-cap and stirred. A
2
solution of Br in dry benzene (1 wt%, 0.5 mL, 0.097 mmol)
5
mL) and dried to yield 2 as a green powder (390 mg, 0.837
was added dropwise and the reaction mixture turned from dark
brown-orange to a vibrant bright orange. The mixture was
stirred for 1 hr at room temperature, filtered and the volatiles
were removed in vacuo. The crude was extracted with a mix-
ture of pentane/Et O (1:1, 3 x 2 mL) and the filtered extracts
2
were combined and dried to give 5 as a bright orange powder
mmol, 82%). Crystals (fine needles) suitable for X-ray analy-
sis were obtained by slow diffusion of pentane into a solution
1
of 2 in Et
2
O/pentane (1:1). H NMR (300 MHz, C
6
D
6
) δ ppm
6.48 (s, 2H, Ar), 3.86 (br. s., 1H, ArOH), 3.15-2.93 (m, 2H,
Ar-CH -P), 2.79 (dt, J=16.48, 4.58 Hz, 2H, Ar-CH -P), 2.10-
1.74 (m, 4H, CH(iPr)) 1.32-0.98 (m, 18H, iPr), 0.92-0.76 (m,
2
2
(28 mg, 0.040 mmol, 93%). Crystals (prisms) suitable for X-
1
3
1
6H, iPr) -8.81 (t, JH-P=50.4 Hz, 1H, Fe-H). C{ H} NMR,
DEPT-Q (126 MHz, C ) δ ppm 217.2(u) (t, J=15.5 Hz,
ray analysis were obtained by slow diffusion of pentane into a
1
D
6
solution of 5 in Et
2
O/pentane (1:1). H NMR (400 MHz, C
6
D
6
)
6
CO), 216.5(u) (t, J=12.0 Hz, CO), 159.3(u) (t, JC-P=13.4 Hz,
ArC-Fe), 153.6(u) (s, ArC-OH), 147.1(u) (t, JC-P=10.2 Hz, Ar),
δ ppm 5.60 (br. s., 1H, PhOH), 3.89 (dt, J=16.5, 4.3 Hz, 2H,
Ar-CH -P), 3.48 (dt, J=16.5, 3.6 Hz, 2H, Ar-CH -P), 3.12-2.99
(m, 2H, CH(iPr)), 2.02-1.89 (m, 2H, CH(iPr)), 1.22-1.09 (m, 6H,
2
2
1
09.9(d) (s, Ar), 39.4(u) (t, JC-P =13.6 Hz, Ar-CH
2
-P), 28.4(d)
1
3
(
t, JC-P=9.2 Hz, CH(iPr)), 26.9(d) (t, JC-P=14.3 Hz, CH(iPr)),
iPr) 1.07-0.85 (m, 18H, iPr). C NMR, DEPT-Q (100.7 MHz,
) δ ppm 217.5(u) (t, JC-P=25.7 Hz, CO), 212.7(u) (t, JC-
=14.5 Hz, CO), 160.5(u) (t, JC-P=13.6, ArC-Fe), 147.1(u) (s,
ArC-OH), 146.2(u) (t, JC-P=8.2 Hz, Ar) 107.4(u) (t, JC-P=7.0
Hz, Ar), 40.8(u) (t, JC-P=15.8 Hz, Ar-CH -P), 26.5(d) (t, JC-
P
=10.7 Hz, CH(iPr)), 26.4(d) (t, JC-P=9.8 Hz, CH(iPr)), 19.4(d) (s,
1
8.9(d) (s, iPr), 18.7(d) (s, iPr) ,18.5(d) (s, iPr), 18.1(d) (s,
6 6
C D
3
1
1
iPr). P{ H} NMR (121 MHz, C
967, 1920, 1888 (Fe-H), 1590 cm . Elemental Analysis,
Calc’d for C22 : C, 56.67; H, 7.78; Fe, 11.98; O,
0.29; P, 13.28. Found: C, 56.46; H, 8.26
Synthesis of 3. Di(bromomethyl)phenol was prepared ac-
6
D
6
) δ ppm 111.5 (s) IR
P
-
1
1
H
36FeO
3
P
2
2
1
31
iPr), 19.3(d) (s, iPr), 19.1(d) (s, iPr), 19.0(d) (s, iPr).
P
-1
1
4
NMR (162.1 MHz, C D ) δ ppm 84.7 (s). IR 2003, 1941 cm .
6
6
cording to a literature procedure. In a thick walled 250 mL
round-bottom flask equipped with a Teflon stirbar was placed
a solution of the dibromide (2.82 g, 10 mmol) and
di(isopropyl)phosphine (5.88 g, 50 mmol) in methanol (30
mL). The flask was sealed with a Teflon screwcap and the
clear, amber solution was heated to 50°C with stirring for 2
days. Triethylamine (8.3 mL, 60 mmol) was added under inert
atmosphere and the resulting solution was stirred for 30 min.
at room temperature. The volatiles were removed in vacuo and
the residue was taken up repeatedly in diethylether and the
volatiles were removed again in vacuo (4 x 20 mL). The lig-
and was crystallized from a cooled and highly concentrated
LRMS (ESI) 724.89 (highest abundance peak, 1:3:3:1 isotope
+
3 3 2
pattern for 3 Br) (C22H33Br O P Fe + Na ).
Synthesis of 6. In a 20 mL scintillation vial equipped with a
Teflon stirbar was placed 2 (60 mg, 0.129 mmol) in dry ben-
zene (5 mL). The vial was removed from the glovebox and a
2
stream of O was bubbled through the solution for 10 sec. The
dark mixture was filtered, yielding a black filter residue and a
bright orange filtrate. The volatiles of the filtrate were re-
moved in vacuo to give 6 as a bright orange powder (32 mg,
0.067 mmol, 52%). Crystals (prisms) suitable for X-ray analy-
sis were obtained by slow diffusion of pentane into a solution
of 5 in toluene.
pentane solution to give colorless crystals (2.93 g, 8.26 mmol,
1
8
(
1
3%). H NMR (300 MHz, C
6
D
6
) δ ppm 6.96 (s, 1H, Ar), 6.67
-P),
.60 (h, 4H, CH(iPr)), 0.99 (m, 24H, iPr). P{ H} NMR (121
) δ ppm 9.5 (s).
To test the stoichiometry of the reaction, the reaction was
repeated under identical conditions with the addition of only
s, 2H, Ar), 4.80 (br. s., 1H, ArOH), 2.63 (s, 4H, Ar-CH
2
3
1
1
0
2
.5 equiv. and 1 equiv. of O , respectively, added via a gas-
6 6
MHz, C D
tight syringe through a septum. The work-up was performed in
an analogous fashion inside the glovebox and it was found that
Synthesis of 4. In a 20 mL scintillation vial equipped with a
Teflon stirbar was placed 2 (100 mg, 0.215 mmol) in dry
benzene (5 mL), sealed with a septum screw-cap and stirred.
Concentrated aqueous HCl (50 µL, 0.54 mmol) was added
dropwise and the reaction mixture turned from green to yellow
in the case of the addition of only 0.5 equiv. of O
conversion of the starting material was not complete. In con-
trast, in the case of addition of one full equivalent of O , the
starting material had been fully consumed. H NMR (400
2
that the
2
1
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Journal of the American Chemical Society
Page 8 of 10
MHz, C
CH -P), 1.89-1.73 (m, 2H, CH(iPr)), 1.72-1.53 (m, 2H, CH(iPr)),
.44 - 1.24 (m, 6 H, iPr), 1.19-0.75 (m, 14H, Ar-CH -P and
iPr), 0.74-0.59 (m, 6 H, iPr). Residual toluene at 2.1 ppm
6
D
6
) δ ppm 6.53 (s, 2H, Ar), 2.58-2.38 (m, 2H, Ar-
This research was supported by the European Research Council
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(ERC AdG 692775) and by the Kimmel Center for Molecular
Design. AD thanks the Austrian Science Fund for an Erwin
Schroedinger Postdoctoral Fellowship and the Azrieli Foundation
for a Research and Materials Grant. UG thanks the DAAD for a
Post Doctoral Fellowship and the Feinberg Graduate School for a
Senior Post-Doctoral Award. DM holds the Israel Matz Professo-
rial Chair of Organic Chemistry. We thank Linda J. W. Shimon
for collecting data in the ERSF ID-29 and the staff members of
ERSF ID-29 are acknowledged for their support.
2
1
2
1
3
1
6 6
C{ H} NMR, DEPT-Q (100.7 MHz, C D ) δ ppm 217.0(u)
(t, J=21.1 Hz, CO), 215.60(u) (t, J=22.7 Hz, CO) 183.6(u) (s,
ArC=O), 157.4(u) (s, Ar), 124.6(d) (t, J=5.1 Hz, Ar), 96.7(u)
(t, J=2.2 Hz, ArC-Fe), 26.4(d) (t, J=10.2 Hz, CH(iPr)), 20.2(u)
(
1
C
(
t, J=9.6 Hz, Ar-CH
7.0(d) (s, iPr), 16.7(d) (s, iPr). P{ H} NMR (162.1 MHz,
2
-P), 17.9(d) (s, iPr), 17.4(d) (s, iPr),
3
1
1
-
1
6 6
D ) δ ppm 69.4 (s).IR 1967, 1904 cm . LRMS (ESI) 503.19
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
C
22
H
34
O
4
P
2
Fe + Na ). Elemental Analysis, Calc’d for C22
H
34
REFERENCESꢀ
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(
1)
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(
Ed.: S. Patai). Wiley, London, 1974; The Chemistry of the Quinoid
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was used for the geometry optimizations. Thermodynamic
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Supporting Information containing experimental details of syn-
thetic procedures, X-ray data and computational details. This
material is available free of charge via the Internet at
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(
19) Zell, T.; Butschke, B.; Ben-David, Y.; Milstein, D. Chem.
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AUTHORꢀINFORMATIONꢀ
(
Corresponding Author
D. Angew. Chem. Int. Ed. 2013, 52, 14131–14134.
(21) Rivada-Wheelaghan, O.; Dauth, A.; Leitus, G.; Diskin-
Posner, Y.; Milstein, D. Inorg. Chem. 2015, 54 (9), 4526–4538.
(22) Butschke, B.; Fillman, K. L.; Bendikov, T.; Shimon, L. J.
W.; Diskin-Posner, Y.; Leitus, G.; Gorelsky, S. I.; Neidig, M. L.;
Milstein, D. Inorg. Chem. 2015, 54, 4909–4926.
*
david.milstein@weizmann.ac.il
Author Contributions
δ
These authors contributed equally
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23) Zell, T.; Milstein, D. Acc. Chem. Res. 2015, 48 (7), 1979–
1994.
(24) Garg, J. A.; Chakraborty, S.; Ben-David, Y.; Milstein, D.
Chem. Commun. 2016, 52, 5285–5288.
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the manu-
script. /
ACKNOWLEDGMENTꢀꢀ
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41) Due to the extreme air-sensitivity of the crystals, the resul-
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44) Complexes in which a metal-coordinated carbene cannot be
13
detected by C NMR were reported: a) Alcarazo, M.; Roseblade, S.
J.; Cowley, A. R.; Fernández, R.; Brown, J. M.; Lassaletta, J. M. J.
Am. Chem. Soc. 2005, 127, 3290–3291. b) Mayr, M.; Wurst, K.;
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(
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characterization.
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