Abnormal N-heterocyclic carbene-phosphine ruthenium(II) complexes as active catalysts for transfer hydrogenation

Article abstract of DOI:10.1021/om400400w

The bifunctional phosphine-abnormal N-heterocyclic carbene ruthenium(II) complex RuBr(OAc)(PPh₃)(P-aNHC) (1) has been synthesized in high yield by reaction of Ru(OAc)₂(PPh₃)₂ with a phosphine imidazolium bromide (P-NHC × HBr) and characterized by X-ray diffraction. This compound shows high catalytic activity for the transfer hydrogenation of ketones to alcohols in 2-propanol. Rate and efficiency of 1 can be enhanced by the addition of ethylenediamine or benzylamine, affording TOFs up to 140 000 h^-1. Reaction of 1 with ethylenediamine leads to the Ru carbene/amine complex [Ru(OAc)(PPh₃)(P-aNHC)(H₂NCH ₂CH₂NH₂)]Br (2), which displays the same activity of the in situ generated species.

Full text of DOI:10.1021/om400400w

Communication
pubs.acs.org/Organometallics
Abnormal N‑Heterocyclic Carbene-Phosphine Ruthenium(II)
Complexes as Active Catalysts for Transfer Hydrogenation
Julia Witt,† Alexander Pothig,† Fritz E. Kuhn,*,† and Walter Baratta*,‡
̈
̈
†Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische Universitat Munchen, Ernst-Otto-Fischer-Straße 1,
̈
̈
D-85747 Garching b. Munchen, Germany
̈
‡Dipartimento di Chimica, Fisica e Ambiente, Universita
̀
di Udine, Via Cotonificio 108, I-33100 Udine, Italy
S
* Supporting Information
ABSTRACT: The bifunctional phosphine−abnormal N-heterocyclic carbene
ruthenium(II) complex RuBr(OAc)(PPh3)(P−aNHC) (1) has been synthesized in
high yield by reaction of Ru(OAc)2(PPh3)2 with a phosphine imidazolium bromide (P−
NHC × HBr) and characterized by X-ray diffraction. This compound shows high
catalytic activity for the transfer hydrogenation of ketones to alcohols in 2-propanol.
Rate and efficiency of 1 can be enhanced by the addition of ethylenediamine or
benzylamine, affording TOFs up to 140 000 h−1. Reaction of 1 with ethylenediamine
leads to the Ru carbene/amine complex [Ru(OAc)(PPh3)(P−aNHC)-
(H2NCH2CH2NH2)]Br (2), which displays the same activity of the in situ generated
species.
he reduction of the carbonyl compounds with hydrogen1
described to exhibit superior catalytic activity compared to their
2
Tor hydrogen donor molecules
normal analogues, and this can be ascribed to their stronger σ-
donating property. Although several metal aNHC−complexes
with Ir, Rh, and Pd have been reported,13 only a few derivatives
with Ru have been isolated.14 The strategy for the preparation
of aNHC complexes includes the blocking of the C2 carbon
atom, the modification of the bulkiness of N-moieties, and the
nature of the anion.
mediated by defined
homogeneous catalysts is a well-established core process for the
preparation of fine chemicals.3 Development of new catalysts
that operate with high chemoselectivity and productivity is
crucial for the improvement of industrial processes. Out-
standing catalytic systems for the hydrogenation (HY) of C
O bonds are the Noyori ruthenium catalysts trans-RuCl2(PP)-
(1,2-diamine)4 (PP = diphosphine) displaying the N−H
function.5 Conversely, the 2-aminomethylpyridine (ampy)
complexes cis-RuCl2(PP)(ampy)6 and the related CNN pincer
RuCl(CNN)(PP)7 (HCNN = 1-(6-arylpyridin-2-yl)-
methanamine) are the most active systems for ketone transfer
hydrogenation (TH) and HY of bulky substrates and catalyze
the dehydrogenation, racemization, and deuteration of
alcohols.8
Herein we report the synthesis and characterization of a
bifunctional phosphine−abnormal NHC ruthenium complex
and its application in TH of ketones. Addition of primary
amines has been proven to enhance the catalytic activity of the
ruthenium catalyst via formation of a carbene/amine species.
The ruthenium complex 1 was prepared by treatment of
15
Ru(OAc)2(PPh3)2 with 1.2 equivalents of the phosphine
imidazolium bromide P−NHC × HBr16 and sodium acetate in
THF at refluxing conditions (Scheme 1).
N-Heterocyclic carbenes (NHC) have been proven to be
efficient alternatives to phosphine ligands for transition metal
complexes.9 On account of the strong metal carbon bond,
carbene complexes are expected to give robust catalytic
systems. Accordingly, several ruthenium complexes bearing
NHC ligands were successfully applied for the TH of carbonyl
compounds.10 Recently, Morris and co-workers described
highly active ruthenium HY catalysts displaying chelating
NHC−primary amine ligands.11 Although a few NHC−
phosphine ruthenium complexes have been isolated,12 no
example of TH ruthenium catalysts containing a bidentate
NHC−phosphine has been reported to date. It is worth noting
that when the imidazole-based NHC ligand coordinates
through the C4 or C5 atom, instead of C2, an abnormal
NHC (aNHC) complex forms. Metal−aNHC compounds are
The progress of the reaction has been observed by the
changing of the color of the precipitate from reddish-brown to
yellow, leading to RuBr(OAc)(PPh3)(P−aNHC) (1), which
was isolated in 80% yield. Complex 1 is thermally stable in the
solid state, and it is air sensitive in solution. The 31P{1H} NMR
spectrum of 1 in CD2Cl2 shows two doublets at δ 62.4 and 53.7
2
with a JPP of 43.1 Hz, indicating that the two phosphorus
atoms are in cis-position. The ethylene bridge of the phosphine
carbene ligand gives rise to a 1H NMR doublet of doublets at δ
4.33 with JHH = 13.4 Hz and JHP = 29.0 Hz and a pseudoquartet
at 4.01 (JHH = JHP = 13.4 Hz) for the NCH2 moiety, whereas
Received: May 8, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400400w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
2.2640(4) Å), whereas the acetate is symmetrically coordinated
to the metal center (Ru1−O1 2.2118(11) Å, Ru1−O2
2.2168(11) Å). It is worth noting that the normal coordination
mode of the P−NHC ligand has been observed for Pd, Ir, and
Ru complexes,18 whereas the abnormal one has been proven by
solid-state studies only for two examples of Ir species.19 On the
basis of NMR studies, abnormal and normal coordination of
P−NHC has been reported by Lavigne et al., for six- and five-
coordinate Ru(II) and Ru(0) complexes, respectively.12c The
abnormal NHC coordination observed for the six-coordinate
Ru(II) complex 1 can be ascribed to steric factors,13 which lead
to the bulky mesityl group pointing away from the metal center,
reducing the strain with the PPh3 phenyls.
Scheme 1. Synthesis of the Ruthenium Complex 1
the two signals of the PCH2 unit appear as multiplets at δ 2.33
1
1
and 1.57, as inferred from H{31P} NMR and H−1H COSY
experiments. The three methyl signals of the mesityl group
appear as three singlets at δ 2.31, 2.13, and 1.86, whereas the
signal at δ 1.72 is for the acetate ligand. The imidazolylidene
ring shows two doublets at significantly different positions (δ
The phosphine−aNHC ruthenium complex 1 is an efficient
catalyst for the TH of ketones to alcohols with 2-propanol as
hydrogen donor and in the presence of sodium isopropoxide
(eq 1).
4
7.89 and 5.82) with a small JHH of 1.9 Hz. In the 13C{1H}
NMR spectrum, the carbene is relatively upfield shifted (δ
161.2), consistent with an abnormal NHC binding to the
ruthenium center.14
The reaction conditions were optimized in a 0.1 M solution
of ketone in 2-propanol with 0.05 mol% loading of 1 in the
presence of 2 mol% of NaOiPr at refluxing conditions. Complex
1 catalyzes the transfer hydrogenation of acetophenone (K1)
with high rate (TOF = 38 000 h−1 at 50% conversion), but
requires 2 h for the quantitative conversion of the substrate,
indicating that 1 undergoes deactivation after a few minutes
(Table 1, entry 1, and Figure 2).
To establish the coordination mode of the phosphine−
carbene ligand and the arrangement of the ligands at the
ruthenium center, a single-crystal X-ray diffraction study was
carried out on 1. Crystals suitable for the solid-state analysis
were obtained by slow diffusion of diethyl ether into a CH2Cl2
solution of 1. The complex crystallizes in a distorted octahedral
geometry with the two cis phosphorus atoms trans to the
bidentate acetate and the carbene trans to the bromide (Figure
1).17 This structure unambiguously confirms that the bidentate
phosphine−NHC ligand adopts the abnormal carbene
coordination with a typical carbene Ru−C distance
(2.0228(16) Å). The Ru−P length of the chelating phosphine
is slightly shorter with respect to that of PPh3 (2.2294(4) vs.
Table 1. Catalytic Transfer Hydrogenation of Ketones Using
a
Complex 1 with the in situ Ligands L
b
c
conversion
(time)
%
TOF
[h−1
entry ketone catalyst [mol%] ligand
]
1
2
3
4
5
6
7
8
K1
K1
K1
K1
K1
K2
K3
K4
0.05
0.05
0.05
0.05
0.01
0.1
97 (2 h)
38 000
58 000
15 000
34 000
140 000
11 000
39 000
6400
L1
L2
L3
L1
L3
L3
L3
97 (20 min)
85 (1 h)
95 (8 min)
86 (1 h)
95 (8 min)
99 (2 min)
95 (1 h)
0.1
0.1
a
Conditions: reactions were carried out at reflux, 0.1 M ketone in 2-
b
propanol, 2 mol% sodium isopropoxide. The conversion was
determined by GC. Turnover frequency is given at 50% conversion.
c
To increase the catalytic activity of 1, several nitrogen ligands
containing the N−H function have been added to 1 in situ.
Reaction of complex 1 with ethylenediamine (L1) (2 equiv) in
refluxing 2-propanol (30 min) gives a more active and
productive transfer hydrogenation catalyst. Thus, K1 is
converted quantitatively into 1-phenylethanol in 20 min,
leading to a TOF value of 58 000 h−1 (0.05 mol% of Ru).
Surprisingly, in the presence of 2-aminomethylpyridine (L2)
Figure 1. ORTEP representation of complex 1. Ellipsoids are shown at
the 50% probability level. The phenyl rings at P2 as well as the
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Ru1−Br1 2.6299(2), Ru1−C2 2.0228(16), Ru1−O1
2.2118(11), Ru1−O2 2.2168(11), Ru1−P1 2.2294(4), Ru1−P2
2.2640(4), C2−Ru1−Br1 168.58(4).
B
dx.doi.org/10.1021/om400400w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 2. Synthesis of the Ruthenium Carbene/Amine
Complex 2
nonequivalent N−H protons at δ 8.66, 4.57, 2.40, and 1.38,
as inferred from 1H−1H COSY experiments, the low-field
resonance being consistent with an intramolecular NH···OC
hydrogen bond interaction.20 Addition of basic D2O (KOH) to
2 in CD2Cl2 leads to a fast proton exchange of the four NH and
the carbene NCHN (δ 8.72) protons, which may play an active
role in bifunctional catalysis.5 Finally, the isolated complex 2
(0.05 mol %) shows the same catalytic activity of the in situ
generated system 1/L1 (Table 1, entry 2), affording
quantitative reduction of acetophenone in 20 min with TOF
= 55 000 h−1, indicating that 2 is the catalytic precursor.
In conclusion, we have reported the synthesis and character-
ization of the ruthenium complex RuBr(OAc)(PPh3)(P−
aNHC) (1), which displays a phosphine NHC ligand
coordinated in an abnormal mode, as proven by an X-ray
study. To the best of our knowledge, this is the first solid-state
structure of an abnormally coordinated phosphine−NHC Ru
complex. Complex 1 shows high catalytic activity in the transfer
hydrogenation of ketones, and its rate and productivity increase
by addition of ethylenediamine or benzylamine, achieving TOF
values up to 140 000 h−1. The carbene/amine complex
[Ru(OAc)(PPh3)(P−aNHC)(H2NCH2CH2NH2)]Br (2) has
been isolated and displays the same catalytic activity with
respect to the in situ generated catalyst. Studies are under way
to characterize new Ru carbene/amine complexes and to
extend this protocol in asymmetric catalysis.
Figure 2. Transfer hydrogenation of acetophenone (K1) with 1 (0.05
mol%) in the presence of nitrogen ligands (ethylenediamine, L1; 2-
aminomethylpyridine, L2; and benzylamine, L3).
the rate is lower with respect to that of 1, affording 86%
conversion of K1 in 1 h. Although the in situ prepared catalyst
with benzylamine (L3) provides much of the same rate as 1,
quantitative conversion of the ketone is achieved in 8 min
(Table 1, entry 4).
Complex 1 in the presence of L1 has been proven to catalyze
the reduction of K1 at 0.01 mol% loading, affording 1-
phenylethanol (86% conv. in 1 h) and achieving a TOF of
140 000 h−1 (Table 1, entry 5). These results should be
compared with those reported for the phosphine/amine
complexes RuCl2(PP)(NN),4,6 which show high catalytic
activity in TH when NN = ampy and poor performances for
NN = 1,2-diamine.2b Using the system 1/L3, dialkyl, cyclic, and
diaryl ketones have been quantitatively reduced with 0.1 mol%
of 1. Thus, hex-5-en-2-one (K2), 2-methylcyclohexanone (K3)
and benzophenone (K4) were used as model substrates (Table
1, entries 6−8). The unsaturated ketone K2 is chemoselectively
reduced at the CO bond without hydrogenation and
isomerization of the CC bond, whereas the diaryl ketone
K4 is reduced to benzhydrol in 1 h (95%). TH of K3 leads
quantitatively to 2-methylcyclohexanol in 2 min (TOF =
39 000 h−1) with a cis/trans ratio of 3, whereas after 30 min the
two alcohols are in about 1:1 ratio, as a result of a catalytic
epimerization process.
ASSOCIATED CONTENT
* Supporting Information
■
S
Details on experimental procedures, H−1H COSY and H-
{31P} NMR spectra of 1 and 2, details on single-crystal X-ray
structure determinations of compound 1, tables of crystal and
data collection parameters, atomic coordinates, bond lengths,
and bond angles for 1 in .cif format. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
1
To characterize the ruthenium carbene/amine species that
forms in situ, we studied the reaction of 1 with ethylenediamine
(L1). The thermally stable complex 2 is promptly obtained as a
pale green solid (91% yield) by treatment of 1 with L1 in
CH2Cl2 at room temperature (Scheme 2).
AUTHOR INFORMATION
Corresponding Author
*E-mail: walter.baratta@uniud.it (W.B.), fritz.kuehn@ch.tum.
de (F.E.K.).
■
The derivative 2 shows two 31P NMR doublets at δ 53.9 and
2
Notes
29.1 with a JPP of 32.6 Hz (CD2Cl2), lower than that of 1. In
the 13C{1H} NMR spectrum the abnormal carbene gives rise to
a triplet at δ 155.6 (2JCP = 13.3 Hz), while the NCH2CH2P
bridge affords two resonances at δ 46.7 and 21.0 (2JCP = 30.3
Hz). The diamine ligand leads to a broad singlet at δ 44.1 for
the two carbon atoms (free L1 δ 44.9), which does not
significantly change at −60 °C (δ 43.6). A doublet at δ 181.5
(3JCP = 1.3 Hz) is for the CO acetate, indicating that OAc− is
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ministero dell’Universita
■
̀
e
della Ricerca (MIUR) and the European Graduate School on
Sustainable Energy by a PhD grant for J.W. DAAD, CRUI
(Vigoni Program), and the TUM Graduate School are
acknowledged for financial support of the personnel exchange
between the two involved institutions. We thank Johnson-
1
coordinated to Ru with Br− as counterion. In the H NMR
spectrum the ethylenediamine ligand gives rise to four
C
dx.doi.org/10.1021/om400400w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
(15) (a) Mitchell, R. W.; Spencer, A.; Wilkinson, G. J. Chem. Soc.,
Dalton Trans. 1973, 846. (b) Ding, N.; Hor, T. S. A. J. Chem. Soc.,
Dalton Trans. 2010, 39, 10179.
Matthey/Alfa Aesar for a generous loan of ruthenium and Dr.
P. Martinuzzi for NMR assistance.
(16) Wolf, J.; Labande, A.; Daran, J.-C.; Poli, R. J. Organomet. Chem.
2006, 691, 433.
(17) See Supporting Information.
(18) (a) Tsoureas, M.; Danopoulos, A. A.; Tulloch, A. A. D.; Light,
M. E. Organometallics 2003, 22, 4750. (b) Lee, H. M.; Chiu, P. L. Acta
Crystallogr. 2004, E60, m1473. (c) Nanchen, S.; Pfaltz, A. Helv. Chim.
Acta 2006, 89, 1559. (d) Cabeza, J. A.; Damonte, M.; Guadalupe
REFERENCES
■
(1) (a) The Handbook of Homogeneous Hydrogenation, Vols. 1−3; de
Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, 2007.
(b) Transition Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm,
C., Eds.; Wiley-VCH: Weinheim, 2004; p 29.
(2) (a) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282. (b) Baratta, W.;
Rigo, P. Eur. J. Inorg. Chem. 2008, 4041. (c) Wang, C.; Wu, X.; Xiao, J.
Chem. Asian J. 2008, 3, 1750. (e) Ikariya, T.; Blacker, A. J. Acc. Chem.
Res. 2007, 40, 1300. (f) Samec, J. S. M.; Backvall, J. E.; Andersson, P.
G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237.
(3) Magano, J.; Dunetz, J. R. Org. Process Res. Dev. 2012, 16, 1156.
(4) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.;
Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int.
Ed. 1998, 37, 1703.
́
Hernandez-Cruz, M. J. Organomet. Chem. 2012, 711, 68.
(19) (a) Stylianides, N.; Danopoulos, A. A.; Tsoureas, N. J.
Organomet. Chem. 2005, 690, 5948. (b) Wolf, J.; Labande, A.;
Daran, J.-C.; Poli, R. Eur. J. Inorg. Chem. 2008, 3024.
̈
(20) Baratta, W.; Ballico, M.; Del Zotto, A.; Herdtweck, E.; Magnolia,
S.; Peloso, R.; Siega, K.; Toniutti, M.; Zangrando, E.; Rigo, P.
Organometallics 2009, 28, 4421.
(5) (a) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000,
122, 1466. (b) Petra, D. G. I.; Reek, J. N. H.; Handgraaf, J. W.; Meijer,
E. J.; Dierkes, P.; Kamer, P. C. J.; Brussee, J.; Schoemaker, H. E.; van
Leeuwen, P. W. N. M. Chem.Eur. J. 2000, 6, 2818. (c) Clapham, S.
E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201.
(6) (a) Baratta, W.; Herdtweck, E.; Siega, K.; Toniutti, M.; Rigo, P.
Organometallics 2005, 24, 1660. (b) Baratta, W.; Chelucci, G.;
Herdtweck, E.; Magnolia, S.; Siega, K.; Rigo, P. Angew. Chem., Int.
Ed. 2007, 46, 7651. (c) Ohkuma, T.; Sandoval, C. A.; Srinivasan, R.;
Lin, Q.; Wei, Y.; Muniz, K.; Noyori, R. J. Am. Chem. Soc. 2005, 127,
̃
8288. (d) Sandoval, C. A.; Li, Y.; Ding, K.; Noyori, R. Chem. Asian J.
2008, 3, 1801.
(7) (a) Baratta, W.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti, M.;
Zanette, M.; Zangrando, E.; Rigo, P. Angew. Chem., Int. Ed. 2005, 44,
6214. (b) Baratta, W.; Siega, K.; Rigo, P. Adv. Synth. Catal. 2007, 349,
1633. (c) Baratta, W.; Chelucci, G.; Magnolia, S.; Siega, K.; Rigo, P.
Chem.Eur. J. 2009, 15, 726.
(8) (a) Putignano, E.; Bossi, G.; Rigo, P.; Baratta, W. Organometallics
2012, 31, 1133. (b) Baratta, W.; Bossi, G.; Putignano, E.; Rigo, P.
Chem.Eur. J. 2011, 17, 3474. (c) Bossi, G.; Putignano, E.; Rigo, P.;
Baratta, W. Dalton Trans. 2011, 40, 8986.
(9) (a) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2010, 110,
1746. (b) Díez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009,
́
109, 3612. (c) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008,
47, 3122. (d) N-Heterocyclic Carbenes in Synthesis; Nolan, S. P., Ed.;
Wiley-VCH: Weinheim, Germany, 2006. (e) Cesar, V.; Bellemin-
Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004, 33, 619. (f) Herrmann,
W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(10) (a) Cheng, Y.; Lu, X.-Y.; Xu, H.-J.; Li, Y.-Z.; Chen, X.-T; Xue,
Z.-L. Inorg. Chim. Acta 2010, 363, 430. (b) Burling, S.; Whittlesey, M.
K.; Williams, J. M. H. Adv. Synth. Catal. 2005, 347, 591. (c) Poyatos,
M.; Mata, J. A.; Falomir, E.; Crabtree, R. H.; Peris, E. Organometallics
2003, 22, 1110. (d) Danopoulos, A. A.; Winston, S.; Motherwell, W. B.
Chem. Commun. 2002, 13. (e) Baratta, W.; Schutz, J.; Herdtweck, E.;
̈
Herrmann, W. A.; Rigo, P. J. Organomet. Chem. 2005, 690, 5570.
́
(f) Fernandez, F. E.; Puerta, M. C.; Valerga, P. Organometallics 2012,
31, 6868. (g) Ding, N.; Hor, T. S. A. Dalton Trans. 2010, 39, 10179.
(11) (a) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics
2011, 30, 1236. (b) O, W. W. N.; Lough, A. J.; Morris, R. H. Chem.
Commun 2010, 46, 8240.
́
(12) (a) Cabeza, J. A.; Damonte, M.; García-Alvarez, P.; Kennedy, A.
R.; Perez-Carreno, E. Organometallics 2011, 30, 826. (b) Hahn, F. E.;
Naziruddin, A. R.; Hepp, A.; Pape, T. Organometallics 2010, 29, 5283.
(c) Benhamou, L.; Wolf, J.; Cesar, V.; Labande, A.; Poli, R.; Lugan, N.;
́
̃
́
Lavigne, G. Organometallics 2009, 28, 6981.
(13) (a) Crabtree, R. H. Coord. Chem. Rev. 2013, 257, 755.
(b) Arnold, P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596.
(14) (a) Saha, S.; Ghatak, T.; Saha, B.; Doucet, H.; Bera, J. K.
Organometallics 2012, 31, 5500. (b) Ellul, C. E.; Mahon, M. F.; Saker,
O.; Whittlesey, M. K. Angew. Chem., Int. Ed. 2007, 46, 6343.
D
dx.doi.org/10.1021/om400400w | Organometallics XXXX, XXX, XXX−XXX