Ruthenium hydride complexes with zwitterionic quinonoid ligands-isomer separation, structural properties, electrochemistry, and catalysis

Article abstract of DOI:10.1002/ejic.201101091

Reactions of [Ru(PPh₃)₃(CO)(H)Cl] with the zwitterionic p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6-(n- butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q¹), 4-(isopropylamino)-6-(isopropylimino)-3-oxocyclohexa-1,4

Full text of DOI:10.1002/ejic.201101091

FULL PAPER
DOI: 10.1002/ejic.201101091
Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands – Isomer
Separation, Structural Properties, Electrochemistry, and Catalysis
Stephan Hohloch,[a] Pierre Braunstein,*[b] and Biprajit Sarkar*[a]
Keywords: Quinones / Ruthenium / Hydrides / Noninnocent ligands / Redox chemistry / Transfer hydrogenation catalysis
Reactions of [Ru(PPh3)3(CO)(H)Cl] with the zwitterionic p-
benzoquinonemonoimine-type ligands 4-(n-butylamino)-6-
and established the distorted octahedral coordination geom-
etry around the ruthenium center. The bond lengths in the
(n-butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q1), 4-(iso- complexes are consistent with localization of the double
propylamino)-6-(isopropylimino)-3-oxocyclohexa-1,4-dien-1-
olate (Q2), and 4-(benzylamino)-6-(benzylimino)-3-oxocy-
clohexa-1,4-dien-1-olate (Q3) in the presence of a base led to
the formation of mononuclear complexes [Ru(PPh3)2(CO)(H)-
(Q1–H)] (1a and 1b), [Ru(PPh3)2(CO)(H)(Q2–H)] (2a and 2b),
and [Ru(PPh3)2(CO)(H)(Q3–H)] (3a and 3b), respectively. The
positional isomers (a and b) that were formed in each case
bonds in Q2 and Q3 in both their monodeprotonated and
–H –H
metal-coordinated forms. The Ru–C–O(carbonyl) bond angle
is almost linear. Cyclic voltammetry of the complexes showed
one oxidation and one reduction process. These are predomi-
nantly centered on the quinonoid ligands, which shows their
redox-noninnocent character. Studies of transfer hydrogena-
tion with 2a as a precatalyst showed that, in the presence of
were separated by preparative TLC. The structural charac- KOH, acetophenone could be converted to 1-phenylethanol
terization of 2a and 3a·MeCN helped to identify the isomers,
within 10 h in over 90% yield.
Introduction
form is more stable than the canonical forms, have found
use in homogeneous catalysis,[45,46] redox[47–49] and supra-
molecular chemistry,[44] and as spacers for “metal–metal
coupling”.[50,51]
Quinones have fascinated chemists for decades,[1] and
their interaction with transition metals has relevance to bio-
logical systems.[2–4] The redox noninnocence of such mole-
cules imparts many interesting properties to them.[5–12]
Thus, metal complexes of quinonoid ligands have been ex-
tensively investigated due to their valence ambiguity and
captivating electronic structures,[13–18] their engrossing mag-
netic properties,[19–22] their use as bridges for molecular and
supramolecular systems,[19,23–36] and in homogeneous catal-
ysis.[37–40] In recent years, we have developed the chemistry
of the potentially antiaromatic zwitterionic quinonoid li-
gands Q[41–44] (Scheme 1) and their metal complexes. Metal
complexes of these ligands, where the 6π + 6π zwitterionic
In this work we have extended the chemistry of these
zwitterionic ligands to new ruthenium hydride complexes
and probed their use as catalysts for transfer hydrogenation
reactions. Hydride complexes of transition metals are inter-
mediates in a variety of useful chemical transformations.[52]
The syntheses of [Ru(PPh3)2(CO)(H)(Q1–H)] (1a and 1b),
[Ru(PPh3)2(CO)(H)(Q2–H)] (2a and 2b), and [Ru(PPh3)2-
(CO)(H)(Q3–H)] (3a and 3b), where Q1, Q2, and Q3 are
4-(n-butylamino)-6-(n-butylimino)-3-oxocyclohexa-1,4-
dien-1-olate, 4-(isopropylamino)-6-(isopropylimino)-3-oxo-
cyclohexa-1,4-dien-1-olate, and 4-(benzylamino)-6-(benz-
ylimino)-3-oxocyclohexa-1,4-dien-1-olate, respectively, are
presented. The separation of the positional isomers (a and
b in each case) of these complexes is reported and discussed.
Results obtained from elemental analysis, mass spectrome-
1
try, H NMR and IR spectroscopy, and structural analysis
have been used to establish the formulation of these metal
complexes. Cyclic voltammetry studies have been carried
out to elucidate the redox properties of these complexes.
Finally, the use of these complexes as precatalysts for trans-
fer hydrogenation is presented and explained.
Scheme 1. Zwitterionic quinonoid ligands.
[a] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany
Fax: +49-711-68564165
E-mail: sarkar@iac.uni-stuttgart.de
Results and Discussion
Synthesis, Spectroscopy, and Structures
Q1–Q3 were deprotonated by using KOtBu. Reactions of
these deprotonated ligands with [Ru(PPh3)3(Cl)(CO)(H)]
[b] Laboratoire de Chimie de Coordination, Institut de Chimie
(UMR 7177 CNRS), Université de Strasbourg,
4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101091.
546
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Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands
Scheme 2. Synthesis of the complexes.
under reflux conditions led to the formation of the metal
complexes shown in Scheme 2.
1
Initial H NMR spectroscopy (particularly the hydride
signals) of the crude products indicated the formation of
two different products in each case. Preparative TLC was
used to separate these two fractions to obtain pure forms
of 1a–3a, 1b, and 3b. Unfortunately, the yield of 2b was not
high enough to warrant further investigation. During the
reaction, one PPh3 ligand dissociated from the precursor as
a result of its high steric demand. Additionally, the chlorido
ligand was abstracted as KCl with the K+ ion associated
with the deprotonated ligand. This process afforded six-co-
ordinate, neutral ruthenium(II) complexes.
Complexes with the formula [Ru(PPh3)2(CO)(H)(Q–H)]
can exist as various positional isomers. The 31P NMR spec-
tra of all the isolated complexes showed only one signal.
Hence, the two PPh3 ligands must be trans to each other,
which also makes sense on steric grounds. With the two
PPh3 ligands trans to each other, [Ru(PPh3)2(CO)(H)(Q–H)]
can exist as two different positional isomers, because the Q–
Figure 1. ORTEP plot of 2a. Ellipsoids are drawn at 50% prob-
ability.
ligands bind to the ruthenium center through an O and
H
an N atom. The final identity of such isomers was estab- probably by the steric bulk of the two PPh3 groups. Thus,
lished by single-crystal X-ray diffraction studies. the O1–Ru–N1 angle is 75.7(2) and 75.2(1)°, and the P2–
Complex 2a crystallized at –10 °C from a dichlorometh- Ru–P1 angle is 164.1(1) and 158.0(1)° in 2a and 3a·MeCN,
ane solution layered with n-hexane (1:4), and 3a crystallized respectively. The CO ligand in 2a and 3a·MeCN is trans to
by slow concentration of a CD3CN solution under ambient the O atom of Q–H. Hence, the identity of the red isomers
conditions (Figures 1 and 2). Complex 2a crystallized in the 2a and 3a·MeCN was unambiguously established from the
monoclinic P21/n space group, and 3a·MeCN crystallized structural data. Extrapolation of these results to the red 1a
¯
in the triclinic P1 space group. Crystallographic details are and the yellow 1b–3b also helped to determine their ident-
given in Table 4, and selected bond lengths and bond angles ity.
are listed in Table 1.
Inspection of the bond lengths within Q–H in 2a and
In both complexes, the ruthenium center is in a distorted 3a·MeCN shows that the C1–O1 distances of 1.276(8) and
octahedral environment and is coordinated by the O and N 1.290(3) Å are slightly longer than the C3–O2 distances of
atoms of Q–H, two P atoms from PPh3, a C atom from 1.263(9) and 1.244(3) Å, respectively. Accordingly, the C1–
CO, and a hydrido ligand. The distortion from octahedral C2 distances of 1.398(10) and 1.382(3) Å for 2a and
geometry is imposed by the chelating nature of Q–H and 3a·MeCN are slightly shorter than the C2–C3 distances of
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547

S. Hohloch, P. Braunstein, B. Sarkar
FULL PAPER
which is in contrast to the free ligands where the double
bonds are delocalized within the “upper” and “lower” parts
of the molecule (Scheme 2). The C1–C6 distances of
1.490(9) and 1.504(3) Å and the C3–C4 distances of
1.528(12) and 1.510(3) Å for 2a and 3a·MeCN, respectively,
correspond to typical C–C single bonds and are comparable
to the values found in the free ligands. These results are
consistent with trends found in reported mononuclear com-
plexes with these zwitterion-derived ligands.[47,49,51] The C–
O distances of the carbonyl ligand of 1.148(9) and
1.157(3) Å for 2a and 3a·MeCN, respectively, are consistent
with a CϵO triple bond. This is also reflected in the nearly
linear Ru–C–O(carbonyl ligand) angle of 178.2 and 176.8°
for 2a and 3a·MeCN, respectively.
In 3a·MeCN, the phenyl rings of the benzyl groups are
perpendicular to the plane of the benzoquinonemonoimine
ring as well as perpendicular to each other. If we consider
a plane that passes through C2 and C5 that is perpendicular
to the plane containing the benzoquinonemonoimine ring,
then the phenyl ring on the noncoordinated side of the li-
gand is parallel to this plane and that on the coordinated
side is perpendicular to it. This probably happens to reduce
the steric hindrance between the phenyl ring of the benzyl
substituents and those of the PPh3 group.
Figure 2. ORTEP plot of 3a·MeCN. Ellipsoids are drawn at 50%
probability.
Table 1. Selected bond lengths [Å] and angles [°].
2a
3a·MeCN
The positional isomers show significant differences in
their H NMR and IR spectroscopic signatures. The C–O
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
C1–O1
C3–O2
C6–N1
C4–N2
C13–O3
C7–O3
Ru–O1
Ru–N1
Ru–H
Ru–P1
Ru–P2
1.398(10)
1.420(13)
1.528(12)
1.372(9)
1.427(9)
1.490(9)
1.276(8)
1.263(9)
1.298(8)
1.347(9)
1.148(9)
–
2.107(5)
2.185(5)
1.73(9)
2.366(2)
2.345(2)
1.832(8)
–
75.7(2)
102.0(3)
77.0(3)
–
105.4(3)
–
164.1(1)
116.6(4)
114.6(4)
1.382(3)
1.420(3)
1.510(3)
1.368(3)
1.428(3)
1.504(3)
1.290(3)
1.244(3)
1.307(3)
1.340(3)
–
1.157(3)
2.138(2)
2.153(2)
1.54(3)
2.3634(6)
2.3428(5)
–
1.817(2)
75.2(1)
99.0(1)
–
85.0(1)
–
99.8(1)
158.0(1)
116.4(1)
117.5(1)
1
stretching vibration for the carbonyl ligand attached to the
ruthenium center appears at 1911 cm–1 for 1a and at
1920 cm–1 for 1b (Figure S1). A similar trend is also seen in
3a/3b and 2a (see Exp. Sect.). The CO ligand is trans to the
O donor of Q1
in 1a and to the N donor in 1b
–H
(Scheme 1). Structural analyses have shown that the donor
atoms in the quinonoid ligands in their monometalated
forms are best described as an O– and a neutral imine N
atom. A negatively charged O– atom is expected to be a
better π donor than a neutral imine N atom. Hence, in the
case of 1a, backbonding from the ruthenium center to the
carbonyl ligand is larger than that in 1b, which results in a
lower carbonyl stretching frequency in 1a than 1b.
Ru–C13
Ru–C7
N1–Ru–O1
O1–Ru–H
H–Ru–C13
H–Ru–C7
C13–Ru–N1
C7–Ru–N1
P1–Ru–P2
Ru–O1–C1
Ru–N1–C6
1.420(13) and 1.420(3) Å, respectively. A look at the C–N
distances shows a related trend. The C6–N1 distances of
1.298(8) and 1.307(3) Å for 2a and 3a·MeCN, respectively,
are shorter than the C4–N2 distances of 1.347(9) and
1.340(3) Å. This pattern is also reflected in the C5–C6 dis-
tances of 1.427(9) and 1.428(3) Å for 2a and 3a·MeCN,
respectively, which are longer than the C4–C5 distances of
1.372(9) and 1.368(3) Å. Thus, the double bonds within the
Figure 3. 1H NMR (hydride region) spectra of 3a (bottom) and 3b
Q–H ligands became localized upon metal coordination, (top).
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Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands
The influence of the different local electronic structures cesses were not reversible at room temperature, our instru-
in the two positional isomers is also seen in the chemical mental set up did not allow us to carry out detailed spectro-
1
shifts of the hydride in their H NMR spectra. Thus, the electrochemical measurements on these systems. In the ab-
hydride resonance appears at δ = –10 and –14.53 ppm in sence of such data we can only speculate on the site of elec-
the spectra of 3a and 3b, respectively (Figure 3). Both sig- tron transfer in comparison to systems reported in the lit-
nals are triplets because of coupling with the two equivalent erature. The reduction step can be safely assigned to a re-
P nuclei of PPh3. In the case of 3b, the hydrido ligand is duction that predominantly takes place on Q–H. Metal
trans to the better π-donating O– donor. Because of the complexes reported with such ligands usually show re-
reasons stated above, its resonance shifts to higher fields duction steps at relatively high negative potentials.[47–49,51]
compared to that of 3a. Similar trends are also observed Additionally, the free forms of such ligands also display
for the isomers of the other complexes (see Exp. Sect.).
such reduction processes.[47] Assignment of the oxidation
step is more tricky because of the presence of the redox-
active ruthenium(II) center and the noninnocent Q–H li-
gands. Both these entities are known to undergo oxidation
processes at comparatively low potentials.[51] As the oxi-
dation step is irreversible even at low temperatures, one is
tempted to assign it to an oxidation of the Q–H ligand.
However, we cannot totally rule out the oxidation of the
ruthenium(II) center or a mixed situation.
Cyclic Voltammetry
The presence of a redox-active ruthenium center as well
as a redox-active noninnocent quinonoid ligand provides
an opportunity to probe the electron-transfer properties of
the complexes. Cyclic voltammetry was carried out on 1a–
3a, 1b, and 3b in CH2Cl2/Bu4NPF6 (0.1 m). All the com-
plexes show one oxidation and one reduction step, both of
which are electrochemically and chemically irreversible at
295 K at all studied scan rates (50–1000 mV/s). The irre-
versibility of the processes is probably related to the pres-
ence of the hydrido ligand on the ruthenium center and
the acidic N–H group in the Q–H ligands. On lowering the
temperature to 223 K, the reduction step became reversible,
which can be seen from the peak separation of the forward
and reverse waves and their heights (Figure 4). However,
the oxidation step remained irreversible, and one can see
further follow-up peaks on the anodic side (not shown in
Figure 4), which are probably an effect of the first oxidation
step.
Table 2. Redox potentials [V] obtained from cyclic voltammetry ex-
periments.[a]
Complex
Epa (ox)
E1/2(red)
1a
1b
2a
3a
3b
0.32
0.35
0.40
0.55
0.57
–1.90
–1.85
–1.85
–1.70
–1.72
[a] Potentials vs. Fc/Fc+. From measurements in CH2Cl2/Bu4NPF6
(0.1 m).
Catalytic Transfer Hydrogenation
Ruthenium hydride complexes are intermediates in many
catalytic processes,[52] and transfer hydrogenation is an im-
portant example of such a process.[53] This reaction contin-
ues to attract considerable interest for both academic and
industrial applications, because it allows hydrogenation
without the use of H2 gas.[54–64] In order to test the utility
of our ruthenium hydride complexes as catalysts in the
transfer hydrogenation reaction, we chose 2a because of its
relatively high yield compared to that of the other com-
plexes. Acetophenone was chosen as the substrate and 2-
propanol as the hydride source. The reaction is shown in
Scheme 3.
Figure 4. Cyclic voltammogram of 2a in CH2Cl2/Bu4NPF6 (0.1 m)
at 223 K. Scan rate: 100 mV/s.
The redox potentials for 1a, 1b, and 2a are comparable,
which is to be expected because of the similar electronic
properties of the n-butyl and isopropyl groups. Positional
isomers are also known to show virtually identical re-
sponses in their cyclic voltammograms. In contrast, the re-
Scheme 3. Transfer hydrogenation reaction catalyzed by 2a.
Initial attempts to perform the reaction without a base
dox potentials of 3a and 3b are positively shifted compared failed at room temperature as well as at a higher tempera-
to those of the other complexes (Table 2), which is expected ture. However, in the presence of excess base, 93% conver-
because of the larger +I effect of n-butyl and isopropyl sion to 1-phenylethanol was observed when the reaction
groups compared to the benzyl group. As the redox pro- was carried out at 70 °C (Table 3).
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549

S. Hohloch, P. Braunstein, B. Sarkar
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Table 3. Results of catalytic experiments.
sphere mechanism for this reaction. However, detailed de-
scription of the reaction steps will have to await elaborate
mechanistic studies.
Catalyst
Base
Temperature
Time
Conversion
2a
2a
2a
2a
2a
–
–
r.t.
80 °C
r.t.
70 °C
70 °C
15 h
15 h
15 h
5 h
–
2%
0%
82 %
93%
KOH
KOH
KOH
Experimental Section
10 h
General Considerations: All commercially available reagents were
used as received. Solvents were dried and distilled according to
standard procedures. The syntheses of the organic compounds were
The question arises as to which mechanism this reaction carried out under air, and the ruthenium complexes were synthe-
sized under argon. Q1–Q3[47]and [Ru(PPh3)3(CO)(H)(Cl)][69] were
prepared according to literature procedures.
follows. The outer-sphere and the inner-sphere mechanisms
are both well established for transfer hydrogenation cataly-
sis.[65–67] It is also known that the distinction between the
Instrumentation: The 1H and 31P{1H} NMR spectra were recorded
two mechanisms is sometimes not straightforward.[68] Be- with a Bruker AC 250 spectrometer. Elemental analysis was per-
formed with a Perkin–Elmer Analyzer 240. Mass spectrometry ex-
periments were carried out with a Bruker Daltronics Mictrotof-Q
mass spectrometer. IR spectra were obtained with a Nicolet 6700
FTIR instrument. Cyclic voltammetry was carried out in Bu4NPF6
(0.1 m) solutions by using a three-electrode configuration (glassy
carbon working electrode, Pt counter electrode, Ag wire as psuedo-
reference) and a PAR 273 potentiostat and function generator. The
ferrocene/ferrocenium couple served as the internal reference. For
the catalytic conversions and yields, a gas chromatograph 7890A
from Agilent technologies was used.
cause of the presence of an additional N–H group in Q–H
,
it is tempting to propose an outer-sphere mechanism that
follows a Noyori-type metal–ligand bifunctional route.[53]
The required amide could be generated by an internal tau-
tomeric transformation from a predominant p-benzoqui-
nonemonoimine form to an o-benzoquinonemonoimine
form. However, the experimental conditions that were used
here point to an inner-sphere mechanism because of the
high temperature that is required for the catalysis to pro-
ceed. This is probably related to the generation of a free
coordination site at the ruthenium center by the dissoci-
ation of a PPh3 ligand, for example. Addtionally, excess
KOH is required for the catalysis to work well, which points
to the requirement of a certain concentration of 2-propa-
nolate for the catalysis to occur. Finally, the fact that the
hydrido ligand is trans to the possible amide site in 2a is
not in favor of an outer-sphere metal–ligand bifunctional
mechanism. Although the mechanism remains to be clari-
fied in detail, with the present data we would like to pro-
pose an inner-sphere mechanism for the transfer hydrogen-
ation reaction.
Syntheses
[RuH(CO)(PPh3)2(Q1–H)] (1a and 1b): Q1 (1 equiv., 0.1 mmol,
0.025 g) and KOtBu (1 equiv., 0.1 mmol, 0.011 g) were mixed in a
Schlenk flask under argon. To the mixture was added dry tetra-
hydrofuran (THF, 10 mL), and it was stirred at room temperature
for 12 h. The violet solution turned orange, and an orange precipi-
tate was formed. After 12 h, the THF was evaporated under high
vacuum, and the solid dried under high vacuum for 30 min.
[Ru(PPh3)3(CO)(H)(Cl)] (1 equiv., 0.1 mmol, 0.095 g) was added to
the orange solid followed by dry ethanol (20 mL). The mixture was
heated to reflux under Ar overnight. The ethanol was evaporated
under high vacuum. The solid was dissolved in CH2Cl2 (2 mL) in
air and purified by preparative alumina TLC using 25% MeCN in
CH2Cl2 as eluent. After purification, a red and a yellow fraction
were obtained, which were identified as the positional isomers in a
red/yellow ratio of 7:3. The overall yield was 30%. 1a (red fraction):
1H NMR (250 MHz, CD3CN): δ = 7.64–7.57 (m, 12 H), 7.45–7.33
(m, 18 H), 5.97 (t, J = 5.75 Hz, 1 H), 4.79 (s, 1 H), 4.75 (s, 1 H),
3.04 (q, J = 6.5 Hz, 2 H), 2.90 (t, J = 8.5 Hz, 2 H), 1.62–1.51 (m,
3 H), 1.42–1.29 (m, 3 H), 1.07–0.85 (m, 7 H), 0.68 (t, J = 6 Hz, 3
H), –10.01 (t, J = 21.25 Hz, 1 H) ppm. 31P{1H} NMR (120 MHz,
CDCl3): δ = 43.5 ppm. MS: m/z = 905.2 [M]+. C51H53N2O3P2Ru
(905.26): calcd. C 67.76, H 5.80, N 3.10; found C 68.33, H 5.50, N
Conclusions
We have reported the mononuclear ruthenium hydride
complexes of noninnocent quinonoid ligands derived from
zwitterions that are characterized by 6π + 6π delocalized
systems. The positional isomers of these complexes were
separated for each of the complexes. Structural data helped
to unambiguously establish isomer identity and showed the
relocalization of the π systems within the quinonoid li-
gands, which resulted in the alternation of single and
double bonds. Correlations could be made between local
electronic effects and the carbonyl stretching frequencies in
the IR spectra as well as the hydride chemical shifts ob-
3.24. IR: ν = 3054 (w), 2958 (w), 2923 (w), 2362 (br.), 1911 (vs),
˜
1735 (s), 1555 (s), 1497 (s), 1480 (s), 1090 (s) cm–1. 1b (yellow frac-
tion): 1H NMR (250 MHz, CD3CN): δ = 7.75–6.95 (m, 70 H), 6.22
(t, J = 6.25 Hz, 1 H), 4.88 (s, 1 H), 4.5 (s, 1 H), 3.00 (q, J = 6.75 Hz,
2 H), 2.32 (t, J = 7.38 Hz, 2 H), 1.60–1.47 (m, 4 H), 1.41–1.31 (m,
4 H) 1.01–0.82 (m, 8 H), 0.59 (t, J = 7.25 Hz, 3 H), –14.32 (t, J =
17.5 Hz, 1 H) ppm. 31P{1H}NMR (120 MHz, CD3CN): δ = 43.6
ppm. MS: m/z = 905.2 [M]+. C51H53N2O3P2Ru (905.26): calcd. C
1
served in the H NMR spectra of the isomers. The com-
plexes show one oxidation and one reduction processes in
their cyclic voltammograms, both of which are tentatively
assigned to electron-transfer steps on the quinonoid li-
gands. One of the complexes was tested for its activity as a
catalyst precursor in the transfer hydrogenation reaction of
acetophenone, for which it shows good activity. Initial stud-
67.76, H 5.80, N 3.10; found C 66.95, H 5.94, N 2.99. IR: ν = 3055
˜
(w), 2954 (s), 2924 (s), 2853 (br.), 1921 (vs), 1735 (s), 1495 (s), 1481
(s), 1433 (s), 1091 (s) cm–1.
[RuH(CO)(PPh3)2(Q2–H)] (2a and 2b): Q2 (1 equiv., 0.1 mmol,
0.022 g) and KOtBu (1 equiv., 0.1 mmol, 0.011 g) were mixed in a
ies of the reaction conditions point to a possible inner- Schlenk flask under argon. To the mixture was added dry THF
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Ruthenium Hydride Complexes with Zwitterionic Quinonoid Ligands
(10 mL), and it was stirred at room temperature for 12 h. The violet
solution turned orange, and an orange precipitate was obtained.
After 12 h, the THF was evaporated under high vacuum, and the
residue was dried under high vacuum for 30 min. [Ru(PPh3)3-
(CO)(H)(Cl)] (1 equiv., 0.1 mmol, 0.095 g) was added to the
orange solid followed by dry ethanol (20 mL). The mixture was
heated to reflux overnight under Ar. The ethanol was evaporated
under high vacuum. The reddish solid was dissolved in CH2Cl2
(2 mL) in air and purified by preparative alumina TLC using 25%
MeCN in CH2Cl2 as eluent. Two fractions were obtained, a red
and a yellow fraction, which were identified as positional isomers
in a ratio of 100:1. Because of the extremely low yield of the yellow
fraction (2b) it was not possible to characterize it. The overall yield
a red/yellow ratio of 7:3. The overall yield was 37%. 3a (red frac-
tion): 1H NMR (250 MHz, CD3CN): δ = 7.42–7.25 (m, 36 H), 7.07
(t, J = 7.5 Hz, 2 H), 6.84 (t, J = 7.5 Hz, 2 H), 6.63 (d, J = 7.5 Hz,
2 H), 5.12 (s, 1 H), 4.84 (s, 1 H), 4.36 (d, J = 6.5 Hz, 2 H), 4.05 (s,
2 H), –10.00 (t, J = 22.5 Hz, 1 H) ppm. 31P{1H} (120 MHz,
CD3CN): δ = 41.5 ppm. MS: m/z = 973.2 [M]+. C57H49N2O3P2Ru
(973.26): calcd. C 70.43, H 4.98, N 2.88; found C 70.13, H 5.05, N
2.77. IR: ν = 3057 (w), 2955 (s), 2924 (s), 2853 (s), 2360 (s), 2342
˜
(s), 1915 (vs), 1735 (s), 1556 (s), 1258 (s), 1091 (br.), 1009 (br.) cm–1.
1
3b (yellow fraction): H NMR (250 MHz, CD3CN): δ = 7.70–7.24
(m, 35 H), 6.92 (d, J = 7.25 Hz, 3 H), 6.54 (t, J = 7.25 Hz, 3 H),
4.80 (s, 1 H), 4.78 (s, 1 H), 4.38 (d, J = 6.25 Hz, 2 H), 3.8 (s, 2
H), –14.53 (t, J = 17.25 Hz, 1 H) ppm. 31P{1H} NMR (120 MHz,
CD3CN): δ = 43.1 ppm. MS: m/z = 973.2 [M]+. C57H49N2O3P2Ru
1
of both isomers was 35%. 2a (red fraction): H NMR (250 MHz,
CD3CN): δ = 7.53–7.45 (m, 13 H), 7.33–7.25 (m, 17 H), 5.83 (d, J (973.26): C 70.43, H 4.98, N 2.88; found C 70.18, H 4.86, N 2.75.
= 7.5 Hz, 1 H), 5.02 (s, 1 H), 4.86 (s, 1 H), 3.59 (sept, J = 6.25 Hz,
1 H), 3.42 (sept, J = 6.5 Hz, 1 H), 1.19 (d, J = 6.25 Hz, 6 H), 0.77
(d, J = 6.5 Hz, 6 H), –10.75 (t, J = 22.5 Hz, 1 H) ppm. 31P{1H}
NMR (120 MHz, CD3CN): δ = 40.4 ppm. MS: m/z = 876.2 [M]+.
C49H49N2O3P2Ru (876.23): calcd. C 67.19, H 5.52, N 3.20; found
IR: ν = 2959 (s), 2925 (s), 2216 (w), 2127 (w), 1926 (vs), 1736 (vs),
˜
1436 (s), 1258 (s), 1091 (br.), 1006 (br.) cm–1.
General Procedure for the Catalysis: The catalyst (0.03 equiv.,
0.015 mmol, 0.013 g) was dissolved in 2-propanol (5 mL) and the
mixture stirred for 5 min. KOH (0.33 equiv., 0.17 mmol, 0.009 g)
was added, and the mixture was stirred for 5 min. Acetophenone
(1 equiv., 0.5 mmol, 0.061 g), dissolved in 2-propanol (5 mL), was
added. The mixture was and stirred under Ar at 70 °C for 5 h or
overnight. After completion of the reaction, the mixture was fil-
tered through cotton wool, and the conversion was analyzed by gas
chromatography.
C 66.76, H 5.35, N 3.07. IR: ν = 3054 (w), 2968 (w), 2928 (w),
˜
2361 (br.), 1909 (vs), 1732 (s), 1579 (s), 1493 (s), 1433 (s), 1277 (s),
1090 (s) cm–1. 2b (yellow fraction, not isolated): 1H NMR
(250 MHz, CD3CN): δ = 7.55–7.42 (m, 13 H), 7.33–7.28 (m, 17 H),
5.83 (d, J = 7.5 Hz, 1 H), 5.15 (s, 1 H), 4.78 (s, 1 H), 3.53 (sept, J
= 6.25 Hz, 1 H), 3.38 (sept, J = 6.5 Hz, 1 H), 1.19 (d, J = 6.25 Hz,
6 H), 0.77 (d, J = 6.5 Hz, 6 H), –14.52 (t, J = 17.5 Hz, 1 H) ppm.
31P{1H} NMR (120 MHz, CD3CN): δ = 41.5 ppm.
X-ray Crystallography: Data collection was performed with a
Kappa CCD diffractometer. The measurements were carried out at
173 K by using Mo-Kα radiation (graphite monochromator). The
structures were solved and refined by full-matrix least-squares tech-
niques on F2 with SHELX-97.[70] A severe disorder of the solvent
was observed in 2a; attempts to model the disorder failed. A
SQUEEZE procedure[71] was applied on 586 Å3 of solvent-access-
ible voids and 286 e/cell. The acetonitrile molecules in 3a are disor-
dered (Table 4). CCDC-804975 (for 2a) and -804977 (for 3a) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[RuH(CO)(PPh3)2(Q3–H)] (3a and 3b): Q3 (1 equiv., 0.1 mmol,
0.032 g) and KOtBu (1 equiv., 0.1 mmol, 0.011 g) were mixed in a
Schlenk flask under argon. To the mixture was added dry THF
(10 mL), and it was stirred at room temperature for 12 h. The violet
solution turned orange, and an orange precipitate was obtained.
After 12 h, the THF was evaporated under high vacuum, and the
residue was dried under high vacuum for 30 min. [Ru(PPh3)3-
(CO)(H)(Cl)] (1 equiv., 0.1 mmol, 0.095 g) was added to the
orange solid followed by dry ethanol (20 mL). The mixture was
heated to reflux under Ar overnight. The ethanol was evaporated
under high vacuum. The reddish solid was dissolved in CH2Cl2
(2 mL) in air and purified by preparative alumina TLC using 25%
MeCN in CH2Cl2 as eluent. Two fractions were obtained, a red
and a yellow one, which were identified to be positional isomers in
Supporting Information (see footnote on the first page of this arti-
cle): IR spectra of the complexes.
Table 4. Crystallographic details.
2a
3a·MeCN
Empirical formula
Mr
Crystal system, space group
a, b, c [Å]
C49H48N2O3P2Ru
C60H49N3O3P2Ru
1013.03 gmol–1
triclinic, P1
875.90 gmol–1
¯
monoclinic, P21/n
12.2122(7), 15.6578(5), 13.9000(8)
13.1210(3), 13.5752(2), 15.7258(4)
α, β, γ [°]
90, 108.663(3), 90
2518.1(2)
2
1.155
908
112.9880(10), 98.2500(10), 103.0220(10)
2427.20(9)
2
1.386
1048
V [Å3]
Z
Density [gcm–3]
F(000)
Radiation type
Mo-Kα
0.412
Mo-Kα
0.439
μ [mm–1]
Crystal size
0.25ϫ0.15ϫ0.15 mm
6433
3524
4857
0.0789
0.0545, 0.1483, 1.070
0.589, –1.047
0.25ϫ0.10ϫ0.15
20522
11556
10301
0.0380
0.0296, 0.1092, 1.177
0.573, –1.272
Measured reflections
Independent reflections
Observed [IϾ2σ(I)] reflections
Rint
R[F2 Ͼ 2σ(F2)], wR(F2), S
Δρmax, Δρmin [eÅ–3]
Eur. J. Inorg. Chem. 2012, 546–553
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
551

S. Hohloch, P. Braunstein, B. Sarkar
FULL PAPER
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for financial support of this work. We thank Dr. Roberto Pattacini
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Received: October 10, 2011
Published Online: December 13, 2011
Eur. J. Inorg. Chem. 2012, 546–553
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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