Bifunctional rhenium complexes for the catalytic transfer-hydrogenation reactions of ketones and imines

Article abstract of DOI:10.1002/chem.201103685

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR ₃)(C₅H₄OSiMe₂tBu)] (R=iPr (3a), Cy (3b)) were obtained by the reaction of [Re(H)(Br)(NO)(PR₃) ₂] (R=iPr, Cy) with Li[C₅H₄OSiMe ₂tBu]. The ligand-metal bifunctional rhenium catalysts [Re(H)(NO)(PR₃)(C₅H₄OH)] (R=iPr (5a), Cy (5b)) were prepared from compounds 3a and 3b by silyl deprotection with TBAF and subsequent acidification of the intermediate salts [Re(H)(NO)(PR ₃)(C₅H₄O)][NBu₄] (R=iPr (4a), Cy (4b)) with NH₄Br. In nonpolar solvents, compounds 5a and 5b formed an equilibrium with the isomerized trans-dihydride cyclopentadienone species [Re(H)₂(NO)(PR₃)(C₅H₄O)] (6a,b). Deuterium-labeling studies of compounds 5a and 5b with D₂ and D ₂O showed H/D exchange at the H_Re and H_O positions. Compounds 5a and 5b were active catalysts in the transfer hydrogenation reactions of ketones and imines with 2-propanol as both the solvent and H₂ source. The mechanism of the transfer hydrogenation and isomerization reactions was supported by DFT calculations, which suggested a secondary-coordination-sphere mechanism for the transfer hydrogenation of ketones. The Re-al deal: Bifunctional rhenium complexes [Re(H)(NO)(PR ₃)(C₅H₄OH)] (R=Cy, iPr) of Shvo-type were prepared and used as catalysts for the transfer hydrogenation of ketones and imines. TOFs up to 1164 h^-1 were obtained for ketones and up to 79 h^-1 for imines. DFT calculations suggested a secondary-coordination- sphere mechanism for the transfer hydrogenation of ketones.

Full text of DOI:10.1002/chem.201103685

FULL PAPER
DOI: 10.1002/chem.201103685
Bifunctional Rhenium Complexes for the Catalytic Transfer-Hydrogenation
Reactions of Ketones and Imines
Anne Landwehr, Balz Dudle, Thomas Fox, Olivier Blacque, and Heinz Berke*[a]
Abstract: The silyloxycyclopentadienyl
hydride complexes [Re(H)(NO)-
(PR3)(C5H4OSiMe2tBu)] (R=iPr (3a),
Cy (3b)) were obtained by the reaction
of [Re(H)(Br)(NO)(PR3)2] (R=iPr,
(R=iPr (4a), Cy (4b)) with NH4Br. In
nonpolar solvents, compounds 5a and
5b formed an equilibrium with the iso-
merized trans-dihydride cyclopentadie-
the HRe and HO positions. Compounds
5a and 5b were active catalysts in the
transfer hydrogenation reactions of ke-
tones and imines with 2-propanol as
both the solvent and H2 source. The
mechanism of the transfer hydrogena-
tion and isomerization reactions was
supported by DFT calculations, which
ACHTUNGTRENNUNG
G
none
species
[Re(H)2(NO)ACTHUNGTERNNU(G PR3)-
Cy) with Li[C5H4OSiMe2tBu]. The
ligand–metal bifunctional rhenium cat-
ACHTUNGTRENNUNG
studies of compounds 5a and 5b with
D2 and D2O showed H/D exchange at
alysts
[Re(H)(NO)
(PR3)
N
(R=iPr (5a), Cy (5b)) were prepared
from compounds 3a and 3b by silyl de-
protection with TBAF and subsequent
acidification of the intermediate salts
suggested
a
secondary-coordination-
sphere mechanism for the transfer hy-
drogenation of ketones.
Keywords: bifunctional catalysts ·
homogeneous catalysis · hydrides ·
rhenium · transfer hydrogenation
[Re(H)(NO)ACHTUNGTRENNUNG(PR3)ACHTUNGTRENNUNG(C5H4O)]ACHTNUGERTN[NUGN NBu4]
Introduction
It was interesting to see whether the bifunctional property
of heterolytically split H2 (formally into Hꢀ and H+)[8] could
be retained by isoelectronic substitution at the ruthenium
center, thereby leaving the hydroxy cyclopentadienyl group
as a proton donor but replacing the hydridic moiety of H-
Ru-CO with the isoelectronic H-Re-NO unit. Realization of
this goal seemed feasible, because the acidic hydrogen atom
appeared to be grossly transition-metal independent and the
hydridic hydrogen atom was anticipated to be tunable by
the ligand sphere and adjustable in the hydridic character
required for proper transfer reactivity. Certain geometric re-
straints were expected to arise for “bifunctional” H transfer,
but principally these restrictions seemed to also be tunable
within a certain range. Recently, complexes with rhodium,[9]
iridium,[9b,10] osmium,[11] iron,[7a,c,12] tungsten,[13] and rheni-
um[14] centers have been reported to accomplish related bi-
Ligand–metal bifunctional catalysis is an efficient method
for the hydrogenation of various unsaturated organic com-
pounds. Since Shvo and Czarkie reported that dinuclear pre-
catalyst [({2,3,4,5-Ph4ACHTNUTRGENNUG 4HCATUNGTRENNUGN
(h5-C4CO)}2H)Ru2(CO) (m-H)][1] cata-
lyzed the hydrogenation reactions of alkenes, alkynes,[2]
imines,[3] and carbonyl compounds,[4] a number of catalysts
with similar ligand environments have been demonstrated to
accomplish related transformations.[5] In 1995, Noyori and
co-workers discovered a highly efficient catalyst with excel-
lent chemo- and enantioselectivities that exhibited “bifunc-
tionality” of the transferred hydrogen atoms with hydridic
HRu atoms and acidic HN atoms on the chiral diamine li-
gands.[6] The mechanism of the hydrogen transfers of Shvo-
type catalysts is a matter of controversy. Two plausible
mechanisms have been suggested based on mechanistic ex-
periments and DFT calculations.[3b,4a,7] Casey et al. proposed
hydrogen transfer without pre-coordination of the substrate,
the so-called “secondary-coordination-sphere mechanism”,[3-
functional reactivity. [Re(NO)(L)(H)ACHTUNRTGNEUNG(C5H4OH)] complexes,
which are analogous to Shvo systems, were sought as targets
where L was envisaged to be a 2eꢀ donor, such as CO or
a monodentate phosphine group.
b,4a]
whereas Bꢀckvall suggested a pre-coordination of the
4
3
ꢀ
substrate to the metal center that was initiated by (h h )
ring-slippage of a p ligand before the hydrogen-transfers oc-
curred in the primary coordination sphere.[7b]
[a] A. Landwehr, Dr. B. Dudle, Dr. T. Fox, Dr. O. Blacque,
Prof. Dr. H. Berke
Institute of Inorganic Chemistry, University of Zurich
Winterthurerstrasse 190, 8057 Zurich (Switzerland)
Fax : (+41)446354806
The only reported bifunctional rhenium complex to oper-
ate along the lines of bifunctional catalysis was described by
E-mail: hberke@aci.uzh.ch
Gusev
and
co-workers.
However,
the
[Re-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103685.
H(NO){N(C2H4PiPr2)2}] system, which is a Noyori-type cata-
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5701

lyst with an acidic amine group, revealed low catalytic per-
formance.[14] Several rhenium–nitrosyl complexes have
shown much-better catalytic performance in hydrosilyla-
tion[15] and hydrogenation reactions,[15a,16] which confirmed
the high affinity of rhenium towards hydrogen and its gener-
al ability to drive catalytic hydrogenation cycles. Herein, we
report the synthesis and reactivity of the first rhenium-based
Shvo-type bifunctional [Re(H)(NO)ACTHNURGTENN(UG PR3)ACHTUGNTRNEN(UGN C5H4OH)] com-
pounds and we probe their catalytic performance in the
transfer hydrogenation of ketones and imines by using 2-
propanol as a hydrogen donor. Such transfer-hydrogenation
catalysis seemed feasible on the basis of the existence of hy-
drogen-enriched and hydrogen-depleted forms.
Scheme 1. Synthesis of [Re(H)(NO)ACTHNUTRGENNU(G PR3)ACHUTGNTRENN(NGU C5H4OH)] complexes 5a and
5b.
pentadienyl lithium (2) to accomplish the substitution of
one phosphine ligand and the bromide ligand. At room tem-
perature in THF, these reactions afforded the desired
[Re(H)(NO)ACHTUNTRGNEUNG(PR3)(C5H4OSiMe2tBu)] products (3a (R=
iPr), 3b (R=Cy)) as orange oils in 92% (3a) and 86% (3b)
yield.
Similar to the Shvo system, the hydrogen-depleted form
was envisaged to be a 16eꢀ cyclopentadienone–rhenium
complex, which was perhaps further stabilized by weak OCO
coordination. Both forms differed in the rhenium redox
states: rhenium(I) versus rhenium(ꢀI). The accessibility of
these states seemed feasible in view of the generally high
disposition of rhenium towards redox changes, such as the
ReI/ReIII changes in the already mentioned hydrogenation
and hydrosilylation reactions.[15b,16a,c] In addition, provided
that the protonic/hydridic polarization of the hydrogen-en-
riched form and the secondary-coordination-sphere geome-
try for the H(dꢀ)/H(d+) atom-transfers are not too different
for ruthenium and rhenium, effective bifunctional catalysis
Compounds 3a and 3b were fully characterized by IR
and NMR spectroscopy, MS, as well as by elemental analysis
1
and X-ray diffraction. The H NMR spectra exhibited char-
acteristic doublets for the hydride ligands of compound 3a
(d=ꢀ9.01 ppm, 2J
ACHTUNGTRNE(NUNG H,P)=28.1 Hz) and compound 3b (d=
ACHTUNGRTEN(UNGN H,P)=27.9 Hz). The C5H4 signals were ob-
ꢀ8.97 ppm, 2J
served as multiplets at d=4.64 and 4.70 ppm (3a) and at d=
4.83, 4.79, 4.64, and 4.57 ppm (3b). In the 31P{1H} NMR
spectra, resonances for the PiPr3 and PCy3 ligands appeared
at d=47.1 and 37.8 ppm, respectively. In the 13C{1H} NMR
spectra, the signals for the Cipso atoms were found at d=
136.9 (3a) and 138.1 ppm (3b). In the IR spectra, bands for
ꢀ
could indeed be expected to result from this simple Ru CO/
ꢀ
Re NO replacement.
the n˜ACTHNURTGNEU(GN ReH) stretching vibrations were detected at 1976 (3a)
Results and Discussion
and 1975 cmꢀ1 (3b) and strong bands at 1629 (3a) and
1620 cmꢀ1 (3b) were attributed to the n˜(NO) vibrations. The
structures of compounds 3a and 3b were established by
single-crystal X-ray diffraction, which showed quite similar
bond-lengths and angles in the two structures. The structure
of compound 3b is shown in Figure 1 (for compound 3a, see
the Supporting Information). The half-sandwich complexes
adopted a pseudo-tetrahedral geometry with planar h5-
We wanted to synthesize the target [Re(H)(NO)
ACHTUNGTNER(NUGN PR3)-
ACHTUNGTRENNUNG
of the hydroxycyclopentadienyl ligand, once all of the other
ligands (hydride, phosphine, and NO ligands) had been in-
troduced into the coordination sphere of rhenium. The syn-
thesis of suitable starting complexes of the type
ꢀ
[Re(H)(Br)(NO)
G
C5H4OSiMe2tBu rings. The Re1 N1 bond lengths (1.749(5)
“fixed” ligands and other replaceable moieties, has been re-
ported by ourselves previously.[15b] The hydroxycyclopenta-
dienyl moiety needed to be introduced into the rhenium co-
ordination sphere in its silyl-protected form to prevent
oxygen-coordination to the rhenium center (Scheme 1).
(3a) and 1.740(3) ꢂ (3b)) were within the expected range
for rhenium–nitrosyl complexes (1.734–1.766 ꢂ)[17] and the
Re-N-O angles (178.4(6)8 (3a) and 177.9(4)8 (3b)) indicated
linear NO-ligand binding in both cases. In contrast to the
fact that only slight differences were seen in the bond-
lengths of both compounds, significant deviations were ob-
served between the angles, with a maximum difference of
7.28 for the N1-Re1-H angle (90(3)8 (3a) and 97.2(19)8
(3b)), which was presumably due to different Tolman cone-
angles of PiPr3 (1608) and PCy3 (1708).[18]
Preparation
(PR3)(C5H4OSiMe2tBu)] (3a and 3b): The 16eꢀ rhenium–
hydride complexes [Re(H)(Br)(NO)(PR3)2] (R=iPr (1a),
Cy (1b)) were reacted with (tert-butyldimethylsiloxy)cyclo-
ACHTUNGTRENNUNG
5702
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
using column chromatography on silica gel at ꢀ308C under
a nitrogen atmosphere (Scheme 1). Compounds 5a and 5b
were obtained as yellow solids in 50% yield, which showed
high sensitivity towards oxygen; both compounds were fully
characterized by elemental analyses and by various spectro-
1
scopic methods. The H NMR spectra in [D8]THF exhibited
broad singlets at d=8.03 ppm (5a/5b) for the acidic proton.
The HRe resonances were observed as doublets (d=
2
2
ꢀ9.42 ppm, J
G
AHCTUNGTREG(NNNU H,P)=27.4 Hz for 5b). In the IR spectra, the characteristic
n˜(NO) and n˜(OH) vibrations were found at 1607 and
3118 cmꢀ1 for compound 5a and at 1620 and 3144 cmꢀ1 for
compound 5b. The structure of compound 5a was confirmed
by single-crystal X-ray diffraction. It crystallized as a hydro-
gen-bonded dimer (Figure 2) with an OH···Re interaction.
Figure 1. Molecular structure of compound 3b. Thermal ellipsoids are set
at 50% probability. All hydrogen atoms are omitted for clarity, except
for the hydride atom. Selected bond lengths [ꢂ] and angles [8]: Re1–H
1.63(6), Re1–N1 1.740(3), Re1–P1 2.3752(9), N1–O1 1.209(5), C5–O2
1.355(7); Re1-N1-O1 177.9(4), N1-Re1-P1 91.85(13), N1-Re1-H 97.2(19),
P1-Re1-H 81(2).
[Re(H)(NO)
ACHTUNGTRENNUNG(PR3)ACHTUNGTRENNUNG(C5H4O)]ACUTHNGTRENNNUG
anionic complexes [Re(H)(NO)
N
N
ACHTUNGTRENNUNG
iPr3 (4a), Cy (4b)) were formed through deprotection of the
ꢀ
siloxy group. Cleavage of the Si O bond of the tert-butyldi-
methylsiloxy group with tetrabutylammonium fluoride
(TBAF) afforded, after purification, the [Re(H)(NO)
ACHTUNGTNER(NUGN PR3)-
A
ACHTUNGTRENNUNG
1
(4a: 64%, 4b: 85%). In the H NMR spectra, the hydride
signals shifted to lower field (d=ꢀ8.01 (4a), ꢀ8.11 ppm
(4b)) with respect to the complexes of type 3. In both cases,
the HCp resonances appeared as four multiplets in the range
d=5.56–3.25 ppm. In the 31P{1H} NMR spectra, the signal of
the PiPr3 ligand was observed at d=43.3 ppm and the signal
of the PCy3 ligand was observed at d=31.7 ppm. In compar-
ison with the 13C NMR chemical shift of the Cipso carbon
atom in compounds 3a and 3b (d=136.9 and 138.1 ppm, re-
spectively) and the shifts for the CO group of the cyclopen-
tadienone moiety in compounds 10a and 10b (d=171.4 and
170.6 ppm, respectively; see below), the corresponding sig-
nals of the Cipso ring atoms were observed at d=167.5 (4a)
and 167.2 ppm (4b), which indicated a reduced CO double-
bond character with significant delocalization of the nega-
tive charge over the cyclopentadienyl ring and the metal
center. The IR spectra of compounds 4a and 4b showed
strong n˜(NO) bands at low wavenumber (1533 cmꢀ1) in com-
parison with those of compounds 3a and 3b (1629 and
1620 cmꢀ1, respectively), presumably owing to strong back-
bonding from the rhenium center to NO.
Figure 2. Molecular structure of the dimeric form of compound 5a. Ther-
mal ellipsoids are set at 50% probability. All hydrogen atoms are omitted
for clarity, except for H and H1. Selected distances [ꢂ] and angles [8]:
Re1–H 1.57(4), Re1–N1 1.754(3), Re1–P1 2.397(10), N1–O1 1.212(4),
Re1···H1 2.73(5), H···H1 2.03(6); Re1-N1-O1 179.5(3), N1-Re1-P1
91.33(11), N1-Re1-H 100.3(14), P1-Re1-H 76.5(14), O2-H1···Re1 169(4).
In the dimer, the acidic proton H1 pointed towards the
other rhenium center, with a Re···H non-bonding distance
of 2.73(5) ꢂ. The distance between the acidic proton and
the hydride atom was 2.03(6) ꢂ, which was within the range
of dihydrogen bonding.[19] The rhenium centers possessed
pseudo-tetrahedral geometries and the h5-C5H4OH rings on
top of the metal were planar. The nitrosyl ligand was bound
in an almost-linear geometry to the metal center (Re-N1-O1
179.5(3)8) and showed a N1-Re-H angle of 100.3(14)8, which
was 108 wider than for compound 3a.
[Re(H)(NO)
complexes [Re(H)(NO)
(R=Cy)) were obtained by acidification of compounds 4a
and 4b with ammonium bromide followed by purification
(PR3)
E
NMR spectroscopy of the isomerization of compounds 5a
and 5b in nonpolar solvents: NMR spectroscopy of com-
pounds 5a and 5b revealed the presence of a second, struc-
turally quite-different species, which preferentially appeared
T
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5703

H. Berke et al.
in non-polar solvents (toluene, benzene, CH2Cl2). We attrib-
uted these signals to trans-dihydride cyclopentadienone
complexes 6a and 6b, which were formed from compounds
5a and 5b, respectively, in equilibrium reactions (Scheme 2).
the signals overlapped with those of the cyclopentadienone
ligand or of the decomposition products. The structures of
the molecules shown in Scheme 2 were calculated by using
DFT (Figure 3 and Figure 5). The calculated energy for the
dissociation of the H2 ligand from complex D to form com-
plex E (Figure 5) was 76 kJmolꢀ1. Taking this value as
a benchmark for the analogous reaction of compounds 5a
and 5b, this result confirmed the thermal accessibility of H2-
liberation from these molecules at 608C. Furthermore, for
compounds 5a and 5b, we anticipated that hydrogen bond-
ing to hydrogen-bond acceptors, such as polar solvent mole-
cules (DMSO, THF, MeCN, and 2-propanol), would stabilize
the hydroxy cyclopentadienyl structure and enforce the ther-
modynamic preference for these isomers over the trans-di-
hydride structures. In [D8]THF, as an exemplary more-polar
1
solvent, the H NMR spectra of compounds 5a and 5b re-
vealed the presence of only hydroxy-cyclopentadienyl–mon-
ohydride–rhenium complexes in the temperature range
ꢀ90–+608C.
Scheme 2. Isomerization and decomposition pathway for compounds 5a
and 5b.
1D NOE/EXSY of [Re(H)(NO)ACTHNUTRGENNU(G PR3)ACHTUGNTRNE(NUGN C5H4OH)] (5a and
1
Compounds 6a and 6b could not be isolated. The H NMR
spectra in [D8]toluene showed signals for the cyclopentadie-
5b): The proton-exchange equilibria of complexes 5a and
5b via 6a and 6b were investigated by using 1D NOE/
EXSY experiments in [D8]THF and [D8]toluene over the
temperature range 20–608C. Irradiation of the HRe signal
did not lead to any response from the acidic OH proton at
228C, which was interpreted as no exchange between the
HRe and OH protons on the NMR timescale. This exchange-
reaction became visible for compound 5a at 428C, with the
appearance of a negative HO signal in [D8]THF (d=
8.15 ppm) and at 328C in [D8]toluene (d=7.00 ppm). In
[D8]toluene, an additional response was seen at 528C, with
the enhancement of the trans-dihydride signal (d=
ꢀ3.05 ppm). The related proton-exchange reaction of com-
pound 5b was observed at 528C in MeCN, but not in
[D8]THF. In [D8]toluene, the response signals became ob-
servable at temperatures 108C higher than for compound
5a. The reversible OH proton-exchanges, which presumably
occurred via the trans-dihydrides 6a and 6b, were also ex-
pected to lead to racemization at the rhenium centers of
compounds 5a and 5b.
none ring that were split into two CH multiplets. The dou-
blets at d=ꢀ3.50 ppm (2J
ACHTUNGERTN(NUNG H,P)=47.3 Hz, 6a) and d=
ACHTUNGTRENNUNG(H,P)=47.0 Hz, 6b) were assigned to the two
ꢀ3.18 ppm (2J
magnetically and chemically equivalent hydride ligands. This
coupling pattern was in agreement with their trans position
and a freely rotating or symmetrically bound C5H4O ring. In
the 13C{1H} NMR spectra, the carbonyl signals of the cyclo-
pentadienone rings appeared at d=175.0 ppm for compound
6a and at d=181.6 ppm for compound 6b. These signals
were strongly upfield-shifted in comparison with those of
compounds 4a and 4b (d=167.5 (4a) and 167.2 ppm (4b))
and were slightly upfield of the corresponding signals in
compounds 10a and 10b (d=171.4 (10a) and 170.6 ppm
(10b)). The T1 relaxation times, as measured by the inver-
sion-recovery method, of compounds 6a and 6b (562 and
317 ms, respectively) at 293 K confirmed the dihydride struc-
ture.[20] In the presence of cis-dihydride or dihydrogen com-
1
plexes of type 7 or 8, the H NMR spectra would require
the appearance of AB- or AX-type patterns for the hydride
ligands in compounds 7a and 7b or a broad signal that was
typical for the fast-rotating H2 ligand in compounds 8a and
8b; however, such signals could not be detected. Further-
more, the IR spectra supported the presence of one new
species, because only one new n˜(NO) band (at 1713 cmꢀ1
for 6a and at 1712 cmꢀ1 for 6b) was observed, both of
which were shifted to higher wavenumber with respect to
compounds 5a and 5b.
In non-coordinating solvents, compounds 5a and 5b de-
composed at 608C, presumably owing to H2 evolution from
the activated cyclopentadienone–dihydrogen complex that
was in equilibrium at this temperature (Scheme 2). Howev-
er, a signal that corresponded to the H2 moiety could not be
traced with confidence in the spectra of the decomposed
complexes, because, in the expected region (dꢁ4.2 ppm),
Reaction of compounds 5a and 5b with D2O and D2: Be-
cause the proton-exchange reactions described above were
already noticeable in the range 40–508C, we also followed
the H/D exchanges by using D2O and D2 at room tempera-
ture. The H2-exchange reactions were anticipated not to pro-
ceed via trans-dihydrides 6a/6b, but rather via an equilibri-
um between cis-dihydrides (7) and their related dihydrogen
complexes (8; Scheme 2). H/D-exchange reactions with D2
1
were studied in [D8]THF under 1.5 bar D2. H NMR spec-
troscopy of the reactions of compounds 5a/5b at room tem-
perature showed a hydride signal that decreased in intensity
over time. After 77 h, the exchange of the HRe atom
amounted to 85% for compound 5a and 55% for com-
pound 5b, and the exchange of the HO signal to 100% for
compound 5a and 77% for compound 5b, which clearly in-
5704
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
dicated ongoing H2/D2 exchange at room temperature that
could not be derived from the 1D NOE/EXSY spectra. By
applying excess D2O, 1H NMR spectroscopy showed 94%
H/D exchange of the hydridic hydrogen atom for compound
5a and 92% for compound 5b within 5 h. In all cases, the
acidic proton exchanged faster than the HRe proton. For the
deuterium exchange of compounds 5a/5b with D2, we antici-
pated that a cis-dihydride (7) compound was formed first
(Scheme 2). H2-loss was proposed to occur in these experi-
ments through an h2-H2 structure (type 8) to afford the 16eꢀ
species of type 9, which, for complete exchange, had to be
reloaded with H2 or D2 (see the DFT calculations, Figure 3
and Figure 5). Furthermore, we monitored the exchange re-
action of the HRe hydride of compounds 3a and 3b in [D8]2-
propanol, as well as under 1.5 bar D2 in [D8]THF at room
1
temperature by H NMR spectroscopy. In both cases, the in-
tensity of the hydride signal did not change over 48 h, there-
by indicating that the [Re(H)(NO)ACHTNURTGNEUNG(PR3)(C5H4OSiMe2tBu)]
complexes were not susceptible to exchange, because, for
these compounds, no pathway existed to form H2 complexes.
The Shvo-type ruthenium–hydride complex [({2,5-Ph2-3,4-
Tol2ACHTUNGTRENNUNG 2ACHTUNTGERN(NUGN m-H)] was found to display
(h5-C4CO)H})Ru(CO)
faster exchange and, in either case, hydrogen-atom-selective
RuH/RuD exchange with D2 (4 atm); moreover, the ex-
change of the C5H4OH proton exclusively occurred in
D2O.[21] A related iron system reported by Casey and
Guan[7c] showed 18% exchange of both types of hydrogen
atoms within 27 h, thus indicating much-lower activity in
these H-exchange reactions than compounds 5a/5b.
Figure 3. The lowest-energy rotamers of complexes A–D and selected
bond lengths [ꢂ]. The methyl groups of the PMe3 ligands and the pro-
tons of the C5H4 rings are omitted.
nation sphere (H···H distance 3.06 ꢂ). For the trans-H dihy-
dride complex (B), three stable rotamers were calculated as
the local minima on the energy hypersurface. As in the case
of the rotamers of type A, the C=O bond of either structure
of the cyclopentadienone conformers was eclipsed by either
DFT study of the equilibrium between tautomers 5a/5b and
6a/6b: The formation of the trans-dihydrides of type 6 was
further studied by DFT calculations on freely optimized
PMe3-substituted model compounds (A–E) and on the tran-
sition states (TS1–TS5) by using the B3LYP[22] functional
and the 6-31G (d,p)[23] basis set for H-P and the
LANL2DZ[24] basis set in connection with the associated ef-
fective core potential for Re with the GAMESS program
package.[25] All free energies were calculated at 298.15 K
and referenced to the energy of structure A1.
ꢀ1
ꢀ1
ꢀ
ꢀ
the Re H (2.7 kJmol , B1ꢀ)1, Re P (5.4 kJmol , B2), or the
ꢀ
Re NO bond (24.4 kJmol , B3). The gas-phase free ener-
gies of the most-favored trans-H dihydride structure (B1, 2.7
kJmolꢀ1) was close to the energy of the ground-state struc-
ture (A1, 0.0 kJmolꢀ1). By qualitatively including the poten-
tial for hydrogen bonding to the A and B PMe3 model struc-
tures, we estimated that structure A1 would be the preferred
isomer in polar solvents, whilst structure B1 would be more
favored in non-polar solvents, which reflected the experi-
mental equilibrium positions quite well. In the case of the
cis-dihydride species (C), three stable rotamers were opti-
Three stable rotational isomers were found for tautomer
A, with A1 as the local minimum. In all of these cases, the
Cipso atom of the hydroxy cyclopentadienyl ring (C5H4OH)
ꢀ
possessed the longest Re C bond. In the local minima, the
ꢀ
Cipso atom eclipsed one of the tripod axes (Re H
mized as local minima. For which the C=O bond was either
ꢀ1
ꢀ1
ꢀ1
ꢀ
ꢀ
ꢀ
ꢀ
0.0 kJmol , A1; Re PMe3 0.1 kJmol , A2; or Re NO
8.9 kJmol , A3; Figure 3). The long Re Cipso bond was in-
terpreted in terms of an early outline of an incipient buta-
eclipsed by the Re H (C1, 20.1 kJmol ), the Re P (C2,
ꢀ1
ꢀ1
ꢀ1
ꢀ
ꢀ
ꢀ20.2 kJmol ), or by the Re NO bond (C3, 29.1 kJmol )
for the same reasons as for the A and B structures. In addi-
tion, in comparison with the A and B isomers, the cyclopen-
tadienone ring showed a pronounced butadiene structure
with a “leg in the hole” conformational preference for the
cis-diene.[26] The free-energy span for these C-type rotamers
was larger, with the most-favorable conformer (C2) at lower
energy than conformers A1 or B1. Next, the dihydrogen
complex (D), which was related to the cis-dihydride struc-
tures of type C1, was structurally optimized, thereby reveal-
ing only the D1 conformer to be a local energetic minimum
ꢀ
diene character in the Re C bonds that was not yet reflect-
ꢀ
ed in the C C bond lengths of the C5H4OH ring. Because
the NMR spectra suggested a free rotation of the C5H4OH
ring in compounds 5a/5b, we assumed that rotamers A1–A3
(energy span of less than 9 kJmolꢀ1) were populated accord-
ing to the relative energies of the A1, A2, and A3 models,
with an equilibrium ratio of 1:0.95:0.3. In rotamer A1, the
hydride and the proton were in a favorable arrangement for
concerted bifunctional H2-transfers in the secondary coordi-
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5705

H. Berke et al.
(Figure 3). Conformer D1 was energetically less-favored
than conformer A1 by 43.0 kJmolꢀ1 and by 20.1 kJmolꢀ1 rel-
ative to the cis-dihydride structure (C1). Although conform-
ers C1 and D1 were structurally very similar, they were con-
nected by a transition state (TS5) at relatively high energy:
78.0 kJmolꢀ1 above conformer C2, 35.0 kJmolꢀ1 above con-
former D1, and 57.9 kJmolꢀ1 above conformer C1. Releasing
H2 from conformer D1 to form the unsaturated 16eꢀ cyclo-
etry. This latter process resembled the frequently observed
protonation of hydride complexes, which almost exclusively
occurs in a kinetically controlled fashion at the hydride
site.[27] An additional observation was that the proton-trans-
ꢀ
fer in TS1 was accompanied by a short Re Cipso bond
(2.319 ꢂ), which was much-shorter than the same process
via TS2 (2.427 ꢂ) and also shorter than in the A-type struc-
tures (2.418 ꢂ). Moreover the OCO atom was bent towards
the Re center (TS1: 11.58, TS2: 14.78). This bending was an-
ticipated to impose strain on these structures and to be sup-
ported by an “inserted” hydrogen-bonding auxiliary that
mediated the H+ transfer. Ito and Ikarya attributed similar
auxiliary functions to acidic HX groups in bifunctional
ruthenium catalysts,[28] which enabled the acceleration of
transfer-hydrogenation reactions owing to the role of hydro-
gen-bonding reagents in organocatalysis.[29] Therefore, we
calculated transition states that included a water molecule
as potential proton donor or -acceptor. The calculations re-
vealed the existence of a TS3···H2O state at an energy of
96.2 kJmolꢀ1 relative to an A1···H2O “hydrate”, thereby ena-
bling connection of the trans-hydride B1···H2O with
A1···H2O. Although the hydrogen-bonded structure
C1···H2O, which was relevant for the HO proton-transfer to
the rhenium center, could be traced as a local minimum in
which a single H2O molecule bridged the C5H4OH group
and the rhenium center of A1, no transition state was found
for the transition of A1···H2O into C1···H2O. An alternative
pathway from A-type to C-type structures was imagined to
proceed via the initial formation of structure B, which was
then transformed into C. Therefore transition state TS4 was
optimized, in which one of the hydrides of structure B1 was
pushed toward the PMe3-Re-NO plane. Transition state TS4
(144.4 kJmolꢀ1) was 11.6 kJmolꢀ1 more favorable in free
energy than TS2. The computational results agreed with the
existence of pathways for the equilibrium between com-
pounds 5a/5b and 6a/6b. Mechanistic paths that proceeded
via the dissociation of the PMe3 ligand or via ring-slippage
were also tested but were excluded owing to the high bind-
pentadienone complex [Re(NO)ACHTNUGTRNENUG(PMe3)ACHTUGNTERN(NUGN C5H4O)] (E,
119.0 kJmolꢀ1 with respect to C2) revealed that the H2
ligand was strongly bound to the Re center in conformer D1
(dissociation energy D(H2)=76.0 kJmolꢀ1). Overall, these
DFT-calculated energies suggested that, in solutions of com-
pounds 5a/5b, a cis-dihydride structure that was analogous
to conformer C2 was the main constituent. However, this
result was, to some extent, in contrast to the experimental
data; therefore, we tried to further analyze the kinetic re-
sults. A reaction path that connected the A and C-type
structures seemed to be inaccessible under ambient condi-
tions. To find a kinetically meaningful mechanism for the
equilibrium between compounds 5a/5b and 6a/6b, exclud-
ing formation of the cis-dihydrides, we optimized all of the
possible transition states for the interconversion between
complexes A–D. In the first step, the transition states of the
direct transfer of the HO proton to the metal center and its
reverse were calculated. However, the transition states that
connected complex A1 with B1 (TS1, 174.4 kJmolꢀ1; see
Figure 4) and complex A1 with D1 (TS2, 156.0 kJmolꢀ1),
through proton transfers to the rhenium and HRe atoms, re-
spectively, were found at high energies; hence, they were
not considered to be operative in the exchange processes.
Proton transfer from the HO atom to the rhenium center
in transition state TS1 proceeded via a strained four-mem-
bered ring, whilst proton transfer to the HRe atom via TS2
proceeded via a five-membered ring in a less-strained geom-
ing energy of the PMe3 ligand in structure
A
(198.5 kJmolꢀ1) and the inability to find local energetic
minima for an unsaturated h3-haptotropic [ReH(h3-
C5H4OH)(NO)ACHTUNRGTNEUNG(PMe3)] species. Furthermore, although dihy-
dride structure C2 would be preferred over A and B, a high
kinetic barrier strictly separated this isomer from the AÐB
equilibrium. This result had two important consequences for
the reactivity of the system (Figure 5): 1) The hydrogen re-
lease/addition
AHCTUNGTRENNUNG
equilibrium
AÐE+H2
(E=[Re(NO)-
barriers, which were even higher than the H2-affinity of E
(119.0 kJmolꢀ1). 2) The regeneration of structure A from E
with H2 was envisaged to proceed via a C-type species, but
this was also not a facile process, with a relatively high ener-
getic barrier for the conversion of a dihydride into structures
A or B. These findings made it clear that hydrogenation re-
actions with catalysts 5a/5b would be associated with rela-
tively harsh conditions.
Figure 4. Transition states TS1–TS4 and selected bond lengths [ꢂ]. PMe3
methyl groups and protons of the Cp rings are omitted.
5706
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
Table 1. Transfer hydrogenation of ketones and imines catalyzed by
[Re(H)(NO)
(C5H4OH)] (5a and 5b).[a]
ACHUTGTNERN(NUG PR3)ACHTUNGTRENNUGN
Entry
Substrate
Catalyst
t
Yield TOF
[%][b] [hꢀ1
N
]
5a
5b
10 min 97
30 min 95
1164
380
1
2
3
4
5a
5b
1.5 h
1.5 h
98
96
131
128
5a
5b
45 min 98
45 min 99
261
264
Figure 5. Free-energy diagram [kJmolꢀ1] of compounds A–E and their
connecting transition states TS1–TS5 at 298.15 K.
5a
5b
1 h
3 h
98
98
196
65
Catalytic activity of compounds 5a and 5b in the transfer-
hydrogenation reactions of ketones and imines: We explored
the catalytic potential of compounds 5a and 5b in the trans-
fer-hydrogenation reactions of various ketones and imines
with 2-propanol as both a solvent and a hydrogen donor.
The comparably low hydrogenation enthalpy of acetone
(ꢀ69.7 kJmolꢀ1)[30] made 2-propanol a versatile reactant in
such reactions. The 5a/5bÐ6a/6b tautomerization was an-
other reason to choose a polar solvent, because it was ex-
pected to favor higher effective concentrations of the cata-
lyst (5a/5b). In addition, it seemed appropriate to stabilize
the highly activated intermediates 9a and 9b through hydro-
gen-bonding interactions with 2-propanol (see below).
The addition of 0.5 mol% of compound 5a/5b efficiently
catalyzed the reduction of ketones and imines in 2-propanol
(30 equiv with respect to the substrates) at 1208C (Table 1).
For example, we obtained the highest TOF for acetophe-
none by using catalyst 5a (TOF=1164 hꢀ1, 97% conversion
after 10 min), as determined by GCMS. Aryl ketones were
hydrogenated more-rapidly than, for instance, the alkyl
ketone pinacoline (Table 1, entry 5). The nonpolar olefin
double bond (Table 1, entry 8) was not hydrogenated by cat-
alysts 5a/5b and ketones were hydrogenated much faster
than imines (Table 1, entries 9–11). Except for the transfer-
hydrogenation reactions of acetophenone and p-diacetylben-
zene, the catalytic performances of compounds 5a and 5b
were comparable.
5a
5b
2.5 h
2.5 h
97
92
78
74
5
6
5a
5b
1 h
35 min 94
98
196
322
5a
5b
2 h
2 h
81[c]
0
81
0
7
8
5a
5b
1.5 h
1.5 h
0
0
0
0
5a
5b
3 h
2.5 h
97
99
65
79
9
5a
5b
3 h
3 h
73
86
49
57
10
11
5a
5b
5 h
5 h
77
90
31
36
[a] Conditions: substrate (0.436 mmol), catalyst (5a/5b; 0.00218 mmol,
0.5 mol%), 2-propanol (1 mL), 1208C. [b] Yield determined by GCMS
analysis. [c] 19% mono-hydrogenated product obtained.
Imines were reduced more-efficiently with compound 5b
than with compound 5a. To confirm that the combination of
the hydridic HRe and the acidic HO atoms was required for
catalytic activity, we tested compounds 3a and 3b, which
only contained the hydridic functionality. Using 1.2 mol%
of compound 3a or 3b, the transfer-hydrogenation reaction
of acetophenone in [D8]2-propanol was monitored at 808C
by using 1H NMR spectroscopy. After 24 h, we detected
21.2% (3a, TOF=0.7 hꢀ1) and 2.5% (3b, TOF=0.1 hꢀ1) re-
duction of acetophenone, which indicated a much-lower cat-
alytic activity compared to compounds 5a and 5b. These
very low activities were attributed to the presence of traces
of compounds 5a and 5b that were generated by silyl depro-
tection of compounds 3a and 3b, either owing to the pres-
ence of traces of water or to an as-yet-unknown reaction
pathway.
Transfer hydrogenation of benzaldehyde: The reductions of
benzaldehyde with compounds 5a/5b (1 mol%) in [D8]2-
1
propanol at 350 K were monitored by H NMR spectrosco-
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5707

H. Berke et al.
py. Quite unexpectedly, these reactions were found to be
slow. After 4 h, 75% (5a, TOF=19 hꢀ1) and 72% conver-
sion (5b, TOF=18 hꢀ1) of the aldehyde were observed,
much lower than the analogous reductions of acetophenone
after 30 min in the presence of 1.2 mol% of compounds 5a/
5b (5a: 80%, TOF=133 hꢀ1; 5b: 90%, TOF=151 hꢀ1).
Moreover, the reaction solution became green during the
catalysis and the typical signal for the catalyst in the
31P NMR spectra disappeared, with the concomitant forma-
tion of various unidentified signals. Presumably, the formed
benzyl alcohol reacted with the catalyst, thus blocking the
catalytic system. To confirm this idea, we monitored the re-
duction of acetophenone in [D8]2-propanol in the present of
excess benzyl alcohol at 778C by 31P NMR spectroscopy.
After 30 min, two new signals appeared at d=22.8 ppm
(12%) and d=24.3 ppm (13%), which were not detected in
reactions without benzyl alcohol. Changing the solvent and
hydrogen donor from [D8]2-propanol to [D4]MeOH in the
versions of acetophenone became approximated. We envis-
aged two ways in which water could interact with the cata-
lysts to have the observed effects on the rate of transfer-hy-
drogenation (Scheme 3): 1) water might stabilize and acti-
Scheme 3. Possible modes of HOR interaction with compounds 5a and
5b.
vate the substrate during the transfer-hydrogenation reac-
tion, thereby leading to an increase in the catalytic activity,
or 2) water could inhibit the catalysis by forming hydrogen-
bonding interactions with the acidic proton and the hydridic
hydrogen atom of compounds 5a/5b, or fully coordinate to
the rhenium center of unsaturated species 9, thereby form-
ing catalytically inactive or less-active water complexes [Re-
1
reduction of acetophenone, the H NMR spectra indicated
about 43% conversion (TOF=3.9 hꢀ1) after 22 h at 778C. In
addition, the 31P NMR spectra indicated a decrease in the
catalyst signal and the appearance of new unidentified sig-
nals. The solution also turned green, as in the reaction of
benzaldehyde. From this result, we concluded that primary
alcohols could react with active species 5a/5b, or even, most
likely, with 16eꢀ intermediates of type 9. Related investiga-
tions on the formation of alcohol complexes were carried
out by Casey and Guan by using the iron complex [{2,5-
AHCUTNGERT(GUNNN C5H4O)(NO)ACHTUNRTGEN(GUNN OH2)ACHTNUGERTN(NUGN PR3)]. To find out whether hydrogen
bonding or direct interaction to the Re center with water
caused the decrease in catalytic activity, we monitored relat-
ed experiments by 1H NMR spectroscopy by using the
strong hydrogen-bond-donor hexafluoro-2-propanol. After
30 min at 350 K, we observed a steady decrease in the cata-
lytic activity, with 81% (0 equiv), 77% (20 equiv), 39%
(40 equiv), and 22% (60 equiv) conversion for compound
5a. The same trend was observed for compound 5b, with
90% (0 equiv), 77% (20 equiv), 42% (40 equiv), and 24%
(60 equiv) in the reduction of acetophenone. These observa-
tions supported the idea of a decrease in the catalytic activi-
ty in presence of strong hydrogen-bond donors.
(SiMe3)2-3,4-(CH2)4ACHTUNGTRENNUNG
(h5-C4COH)}Fe(CO)2(H)] as a transfer-
hydrogenation catalyst. Indeed, iron–alcohol complexes
were observed in toluene in the present of equivalent
amounts of benzaldehyde, and were stable at ꢀ308C, but
underwent decomposition within 1 h at room temperature.[7c]
DFT calculations reported by Chen et al. described these al-
cohol complexes as the most-stable species in the cycle.[7a]
The effect of water in the transfer-hydrogenation reactions:
It has been reported that water had an influence on the cat-
alytic performance of Shvo-type ruthenium complexes in
transfer-hydrogenation reactions; however, these effects
were difficult to rationalize on the basis of a mechanistic
scheme.[3a,4a] Therefore, we wanted to test the influence of
water in rhenium-based systems, and chose the transfer-hy-
drogenation reactions of acetophenone catalyzed by com-
pounds 5a/5b in [D8]2-propanol in the presence of various
amounts of water. The reactions were monitored by
1H NMR spectroscopy. Catalyst 5a (1.2 mol%) showed a de-
crease in catalytic activity with increasing amounts of water:
After 5 min, 27% (0 equiv), 23% (40 equiv), and 20%
(80 equiv) reduction of acetophenone were observed at
350 K. However, in the presence of compound 5b
(1.2 mol%), an increase in the catalytic activity was ob-
served with the addition of up to 40 equiv H2O relative to
the catalyst, but more water led to a decrease in activity.
After 5 min, 39% (0 equiv), 72% (40 equiv), and 25%
(80 equiv) conversion into 1-phenylethanol was detected.
With longer reaction times, the differences between the con-
DFT modeling of the transfer hydrogenation of ketones
with [Re(H)(NO)ACHTUNGTRNENUG(PMe3)AHCTUNGTERN(NUGN C5H4OH)] (A): With regards to
Shvoꢃs catalyst,[7d] primary- and secondary-coordination-
sphere mechanisms were envisaged to be operative in the
transfer-hydrogenation reactions with compounds 5a/5b.
Therefore, we modeled these processes by using DFT calcu-
lations with the PMe3 analogues of type A as starting points.
H2CO and CH3CHO were selected as substrates because
their gas-phase hydrogenation enthalpies (DHhydr. =ꢀ92.4
and ꢀ68.6 kJmolꢀ1, respectively[30]) were thought to be rep-
resentative of a wide range of carbonyl compounds. Key to
an efficient primary-coordination-sphere mechanism would
be the binding of the substrate to the Re center. Because
compound A was an 18eꢀ complex, this binding would re-
quire either 1) the replacement of the phosphine ligand, 2) a
hapticity change of the C5H4OH ligand, or 3) bending of the
NO ligand. Substitution of the PMe3 ligand following a disso-
ciative mechanism would proceed through the generation of
the [ReH(h5-C5H4OH)(NO)] intermediate, which was
198.5 kJmolꢀ1 higher in free energy than compound A.
Under real catalytic conditions with compound 5a/5b, either
5708
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
at room temperature or at 1008C, this barrier would be in-
surmountable. However, an interchange mechanism in
which the PMe3 ligand was replaced directly, could lower
the barrier to access [ReHACHTNUGRTNEUNG
(CH2O)(h5-C5H4OH)(NO)] to
93.0 kJmolꢀ1, which marked the lower limit of the thermo-
dynamics of this process.
However, this activation barrier would still be too high
compared to a secondary-coordination-sphere mechanism,
in which hydrogen-bonding interactions are the decisive sub-
strate interactions (see below). The creation of a free coor-
dination site via ring-slippage or NO-bending would lead to
the formation of [ReH
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
termediates. However, no such structures were traced as
local minima on the potential-energy hypersurface for the
CH2O complexes. This result was interpreted in terms of the
Figure 7. DFT-calculated free-energy diagram [kJmolꢀ1] of the secon-
dary-coordination-sphere double-H-transfer reaction at 298.15 K, starting
from [Re(H)(NO)
N
ACHTUNGTRENNUNG
ꢀ
formation of a too-weak Re O(CO) bond with the aldehyde,
hyde to yield [Re
EtOH.
N
N
E
which could not compensate for the breakage of the stron-
ꢀ
ger Re CACHTUNGTRENNUNG(C H OH) bond or the bending of the nitrosyl group.
5
4
ꢀ
In this respect, the Re NO system differed from the analo-
[7d]
ꢀ
gous Ru CO systems,
for which an associative primary-
the C5H4OH hydroxy group, which was accompanied by
a stabilization of ꢀ26.0 kJmolꢀ1 for the A···CH2O arrange-
ment. At this stage, the interaction between the C(CO) and
coordination-sphere mechanism that involved ring-slippage
could be made plausible by DFT calculations.[7d]
ꢀ
ꢀ
Thus, the secondary-coordination-sphere mechanism was
investigated as an alternative, for which the geometries of
the crucial species were optimized without constraints
(Figure 6). The calculated free energies (Figure 7) revealed
a low-energy reaction path that would be accessible under
real catalytic conditions if transposed to the transfer hydro-
genation reactions with compounds 5a/5b. Next, we focused
on the transfer-hydrogenation reaction of the CH2O/
CH3OH couple, because the transfer-hydrogenation reaction
of the CH3CHO/CH2CH2OH couple was calculated to be
very similar in terms of the geometric parameters of the
structures. Starting from structure A, the first step was the
formation of a hydrogen bond between the substrate and
the Re H atom was negligible (Re H···CH2O 3.264 ꢂ). In
the next step, a concerted double-H-transfer was anticipated
to occur from structure A to CH2O via the seven-membered
transition state TS5 (9.5 kJmolꢀ1), which was 35.5 kJmolꢀ1
higher in free energy than the A···CH2O adduct. In transi-
ꢀ
tion state TS5, the bond between the Re H and the CH2O
units was manifested in the shortening of the Re H···CH2O
distance to 1.338 ꢂ. At the same time, the C=O bond-length
in CH2O became stretched to 1.299 ꢂ, which was intermedi-
ꢀ
ꢀ
ate in length between the C O bond in E···CH3OH
(1.376 ꢂ) and the C=O bond in A···CH2O (1.218 ꢂ). Like-
wise, the C5H4OH bond-length in transition state TS5
(1.306 ꢂ) lay structurally in between those in A···CH2O
ꢀ
(1.345 ꢂ) and E···CH3OH (1.254 ꢂ), and the Re H distance
in transition state TS5 (1.813 ꢂ) lay in the middle of those
in A···CH2O (1.679 ꢂ) and E···CH3OH (1.975 ꢂ). Therefore,
we concluded that, in transition state TS5, the hydride
group was shared equally between the Re-complex and the
CH2O substrate. At the same time, the C5H4OH proton still
appeared to remain on the Re complex, as reflected by a rel-
ꢀ
atively short C5H4O H distance (1.146 ꢂ) and a longer, but
in absolute terms still quite short, C5H4OH···OCH2 hydro-
gen-bonding distance (1.280 ꢂ) when compared to
ꢀ
E···CH3OH (C5H4O···H 1.748 ꢂ, H OCH3 0.984 ꢂ). Thus,
the proton transfer seemed to lag somewhat behind the hy-
ꢀ
dride-transfer. Moreover the Re C1 bond (2.479 ꢂ) in tran-
sition state TS5 was relatively short compared to the expect-
ed 2.529 ꢂ (average of A···CH2O and E···CH3OH). Seem-
ingly, the geometric requirements for the double-H-transfer
ꢀ
outweighed the compression of the Re C1 bond in transi-
tion state TS5. In a real system, we could imagine a more-
concerted pathway that would involve protic functionalities
that would be capable of relieving some of this strain. Fol-
lowing transition state TS5, E···CH3OH was observed next
Figure 6. Drawings of transition state TS5 and adducts E···CH3OH and
A···CH2O with selected bond lengths [ꢂ]. The PMe3 methyl groups and
the protons on the C5H4OH/C5H4O rings are omitted.
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5709

H. Berke et al.
(+2.7 kJmolꢀ1), which was 23.3 kJmolꢀ1 higher in free
energy than A···CH2O. Consequently, the
Trapping the cyclopentadienone intermediate: To acquire
further support for the catalytic cycle, which we proposed to
a large extent on the basis of DFT calculations, we tried to
trap the spectroscopically untraceable 16eꢀ intermediates
9a/9b. Therefore, the transfer-hydrogenation reaction of sol-
utions of compounds 5a/5b in 2-propanol and an excess of
2-butanone at 808C was quenched by the addition of pyri-
dine. Overnight, complete conversion of compounds 5a/5b
into pyridine complexes 10a/10b was observed (Scheme 4).
A···CH2OÐE···CH3OH equilibrium lay towards the left
side, with only 0.008% of E···CH3OH present at 298.15 K.
In this structure, methanol was bound through a hydrogen
bond to the cyclopentadienone ligand (C5H4O···HOCH3
1.748 ꢂ) and through an agostic interaction between the Re
center and a H(Me) atom (Re···HCH2OH 1.975 ꢂ). To com-
plete the catalytic cycle, the formaldehyde in A···CH2O and
the methanol group in E···CH3OH needed to be exchanged.
The expected barrier for the carbonyl-exchange in
A···CH2O would be associated with the disruption of the H-
bond, which amounted to a stabilization of ꢀ26.0 kJmolꢀ1,
thereby indicating a much-faster process than the concerted
double-H-transfer (Figure 6). In contrast to this process, the
exchange of the alcohol molecule in E···CH3OH would for-
mally require the splitting of the hydrogen bond and of the
agostic interaction via the formation of structure E and free
MeOH (46.5 kJmolꢀ1), thereby leading to a free-energy
span[31] of 75.2 kJmolꢀ1 in the gas phase. However, this
energy only set an upper energy limit for the overall process,
because it was more likely that this ligand exchange would
proceed via several steps, each with lower barriers, and with-
out the formation of the completely unsupported structure
E. Such processes were difficult to model, because the in-
volvement of solvent molecules might considerably stabilize
the intermediates. For instance, a complex network of H-
bonds and agostic interactions between the Re center and
the PiPr3 or PCy3 ligands could be envisaged. Therefore, we
can safely state that the second coordination-sphere path-
way was kinetically much-more favorable than any inner-co-
ordination-sphere alternative. However, the overall free-
energy barrier for this process is still to be considered a best
guess, because the exchange of the alcohol in E···CH3OH is
also difficult to model with the methodology used.
In addition, we also modeled the H2-transfer from struc-
ture A to acetaldehyde by using the secondary-coordina-
tion-sphere approach. Structurally, species A···MeCHO,
TS5···EtOH, and E···EtOH resembled species A···CH2O,
TS5···MeOH, and E···CH3OH, respectively, and the differen-
ces in the bond-lengths were usually less than 1%, with
a few exceptions of up to 3%. However, energetically, the
acetaldehyde cycle differed significantly from the formalde-
hyde cycle (Figure 7), mainly owing to the lower hydrogena-
tion enthalpy of acetaldehyde (68.6 kJmolꢀ1) compared to
formaldehyde (92.4 kJmolꢀ1).[30] This result demonstrated
the often-observed close relationship between thermody-
namics and kinetics. The two states, A···RCHO and
E+RCH2OH, defined the kinetics (or rather, they define an
upper limit for the real free-energy span, see above). The
energies of species A···RCHO and E+RCH2OH could
roughly be divided into energetic contributions from struc-
tures A and E, which were identical for both systems, and
from the aldehydes and to the alcohols, which varied with
their respective hydrogenation enthalpies, thereby establish-
ing a tight connection between the thermodynamics and ki-
netics of the transfer-hydrogenation processes.
Scheme 4. Catalytic cycle for the reduction of acetophenone with com-
pounds 5a and 5b and trapping of intermediate 9 with pyridine to obtain
compounds 10a and 10b.
However, without the presence of either 2-butanone, pyri-
dine, or changing the solvent from 2-propanol to THF, com-
plexes 10a/10b could not be obtained. We concluded that
the transfer of H2 from the hydrogen donor to an acceptor
molecule was kinetically feasible, if potential ligands were
available to stabilize the [Re(NO)ACTHNUTRGNEUNG
(PR3)(h4-cyclopenta-
dione)] fragment. The fact that 2-propanol could not be re-
placed by THF indicated that the 2-propanol complex of
whatever structure was more stable than that with THF. Ap-
plying only pyridine without the presence of the 2-butanone
to a solution of compounds 5a/5b in 2-propanol at 808C re-
sulted in conversion into compounds 10a/10b in less than
10% yield after 15 h, which was in contrast to the full con-
version of compounds 5a/5b into compounds 10a/10b in the
presence of the H2-acceptor 2-butanone. Thus, the Re
system behaved markedly differently from the analogous Ru
and Fe systems. In these cases, catalytic as well as stoichio-
metric H2 transfers to acceptor substrates occurred concomi-
tantly with dimerization to the bridged [Ru] dimers[21] or
with PPh3- and pyridine-trapped intermediates.[4a,5d]
The pyridine–cyclopentadienone complexes [Re(NO)-
ACHUTNGREN(NUG PR3)ACHTUGNTRENN(UGN C5H4O)(py)] (10a (R=iPr), 10b (R=Cy)) were sug-
gested to be formed according to Scheme 4. These species
were fully characterized by NMR and IR spectroscopy, MS,
and elemental analysis. The 1H NMR spectra in
[D3]acetonitrile exhibited signals that corresponded to the
C5H4O ligand: multiplets at d=6.07, 4.47, 4.26, and
3.26 ppm for compound 10a and at d=6.00, 4.30, 4.22, and
3.23 ppm for compound 10b. The Cipso atoms were observed
at d=171.4 ppm (10a) and at d=170.6 ppm (10b), which
5710
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
were comparable to cyclopentadienone–dihydride com-
plexes 6a (d=175.0 ppm) and 6b (d=181.6 ppm). In the
31P NMR spectra, the signals of the phosphorus ligands shift-
ed to higher field, from d=47.8 ppm (5a) to d=21.8 ppm
(10a) and from d=36.5 ppm (5b) to d=12.6 ppm (10b).
The structure of compound 10b was established by single-
crystal X-ray diffraction, which showed a non-planar C5H4O
moiety (Figure 8). The C1 atom was located above the C2-
of transfer-hydrogenation reactions are the bifunctional
complexes 5a and 5b.
Conclusion
Bifunctional rhenium catalysts of the type [Re(H)(NO)(L)-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
which is atypical of bifunctional catalysts, with the apparent
ꢀ
formation of strong Re H bonds. These complexes were the
basis for deuterium exchange reactions with D2O and D2 at
the acidic HO and hydridic HRe atoms. DFT calculations sup-
ported the existence of stable trans-dihydride complexes and
of a HOR-associated transition state (TS3) in the hydrogen-
exchange reaction. Furthermore we demonstrated that com-
pounds 5a and 5b performed well in the catalytic transfer
hydrogenation reactions of ketones and imines, with some
very good TOFs using 2-propanol. The catalytic activity was
significantly influenced by the presence of primary alcohols,
as well as by water. When pyridine was present in the cata-
lytic solution, the 16eꢀ cyclopentadienone complexes were
Figure 8. Molecular structure of compound 10b. Thermal ellipsoids are
set at 50% probability. All hydrogen atoms and solvent molecules are
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: Re1–N1
1.7638(15), Re1–N2 2.1540(14), Re1–P1 2.4084(4), N1–O1 1.2064(19),
Re1–C1 2.5678(16), Re–C2 2.2914(16), Re–C3 2.2011(16), Re–C4
2.1796(16), Re–C5 2.2962(17), C1–O2 1.249(2), C1–C2 1.469(2), C2–C3
1.448(2), C3–C4 1.424(2), C4–C5 1.435(2), C5–C1 1.461(2); Re1-N1-O1
173.35(14), N1-Re1-P1 90.16(5), N1-Re1-N2 101.52(6), P1-Re1-N2
93.14(4).
trapped, thereby forming [Re(NO)ACTHNUTRGENN(UG PR3)ACHTUGNTRNE(NUGN C5H4O)(py)] com-
pounds (R=iPr (10a), Cy (10b)). The 16eꢀ intermediates
that were generated from the loss of pyridine from com-
pounds 10a and 10b (E) were unstable, which may be the
cause for the observed catalyst degradation, especially when
no donor ligands of any kind were available for stabilization.
Compounds 10a and 10b were used as pre-catalysts in the
transfer hydrogenation reactions, albeit with lower activities
than compounds 5a and 5b, which remains a challenge to
tune of the lability of this ligand. In summary, we have de-
veloped a new active Shvo-type transfer hydrogenation
system that is based on the non-platinum-metal rhenium
and contributes to the field of metal–ligand bifunctional cat-
alysis.
C3-C4-C5 plane and the Re–C1 bond length was elongated
(2.5678(16) ꢂ). The former O2–C1 single bond in compound
5a was converted into a double bond, with a distance of
1.249(2) ꢂ. The exchange of the hydride ligand in the type-3
structures for a pyridine ligand led to a widening of the
angle to the PCy3 ligand from 81(2)8 (3b) to 93.14(4)8 and
the angle to the nitrosyl ligand from 97.2(19)8 (3b) to
101.52(6)8, with a Re1–N2 bond length of 2.1540(14) ꢂ. The
isolated complexes (10a/10b) were stable in 2-propanol at
room temperature; however, on heating to 808C for 24 h,
the 1H NMR spectra indicated the regeneration of com-
pounds 5a and 5b, with spectroscopic yields of 60% for
compound 5a and 46% for compound 5b. We presumed
that compounds 10a and 10b could also operate as pre-cata-
lysts in transfer-hydrogenation reactions if the pyridine
ligand was labile enough to temporarily set this moiety free
for the catalytic cycle. Indeed, 6 mol% of a catalytic mixture
of compound 10a enabled the reduction of acetophenone at
778C in [D8]2-propanol, with a conversion of 95% within
2.5 h and a TOF value of 6.4 hꢀ1; this catalytic performance
was lower in comparison to the catalytic reduction of aceto-
phenone with compound 5a (1.2 mol%, 778C, TOF
133 hꢀ1). Compound 10b showed only a modest conversion
of 20% within 2.5 h (TOF 1.3 hꢀ1) and 76% within 23 h.
Therefore, the best initial rhenium species for the catalysis
Experimental Section
General: All operations were carried out by using Schlenk techniques or
in a glove box (M. Braun 150 G-B) under a nitrogen atmosphere. The
solvents were dried over sodium benzophenone (THF, Et2O, hydrocar-
bons). The deuterated solvents that were used for NMR experiments
were dried over sodium benzophenone (C6D6, [D8]toluene, [D8]THF)
and vacuum transferred for storage in Schlenk flasks that were fitted
with Teflon valves. 2-Propanol (dried over 3ꢂ molecular sieves) and
[D8]2-propanol were degassed three times and used without further pu-
rification. tert-Butyldimethylsilyl trifluoromethylsulfonate and cyclopen-
tenone (Acros Organics) were purchased, degassed, and used without
further purification. Substrates for catalysis were distilled before use
(Fluka, Aldrich). NMR experiments were carried out on
a Varian
Gemini 300 and on Bruker DRX 500, AV2 500, and AV2 400 machines
with 5 mm diameter NMR tubes that were equipped with Teflon valves,
which allowed for degassing and the further introduction of gases into
the
[Re(NO)(H)
ture procedures.[15b,32]
samples.
1-(tert-Butyldimethylsiloxy)
cyclopentadiene
and
AHCTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5711

H. Berke et al.
Preparation of the ligands and complexes: (C5H4OSiMe2tBu)Li (2):[33] To
solution of 1-(tert-butyldimethylsiloxy) cyclopentadiene (33 mg,
pound 4b was precipitated from Et2O and excess of pentane. The precipi-
tate was dried in vacuo to give an orange oil. Yield: 85%; 1H NMR
(400 MHz, C6D6): d=5.52, 4.83, 4.56, 3.25 (m, 1H; C5H4), 3.26 (m; N-
a
0.169 mmol) in pentane, was added a solution of nBuLi (0.075 mL,
0.169 mmol, 1.6m in n-hexane) and the solution was stored at ꢀ308C
overnight. The solvent was removed, the precipitate was washed with
pentane, and the solvent was evaporated in vacuo. The residual solid
gave compound 2 as a white powder in about 95% yield. 1H NMR
(200 MHz, [D8]THF): d=5.25 (m, 2H; C5H4), 5.14 (m, 2H; C5H4), 0.93
A
2ACHTUNGTNERN(NUG CH2)2CH3)4), 2.44–1.74 (m, 33H; Cy), 1.51, 1.38 (m; NACHTUNGTRENNUNG(CH2-
R
NACTHNGURTEN(NUG CH2ACHTUNGTRENNUNG
(CH2)2CH3)4), ꢀ8.11 ppm (d, 1H, 2J-
AHCTUNGTRENNUNG
(H,P)=22.8 Hz; ReH); 13C{1H} NMR (100 MHz, C6D6): d=167.2 (s;
CO), 67.9, 65.5, 64.7 63.9 (s; C5H4), 58.3 (s; CH
30.0, 28.1, 27.5 (s; Cy), 24.1, 19.9 (s; CH2A(NBu4)), 13.8 ppm (s; CH3-
(NBu4)); 31P{1H} NMR (162 MHz, C6D6): d=31.7 ppm (s; PCy3);
2ACHTNUGTREN(UNGN NBu4), 35.7, 35.5, 31.5,
CTHUNGTRENNUNG
(s, 9H; SiC
[D8]THF): d=140.1 (s; CipsoC5H4), 97.4, 92.1 (s; C5H4), 28.4 (s; SiC-
(CH3)3), 18.9 (s; SiC (CH3)2).
(CH3)3), ꢀ4.1 ppm (s; Si
[Re(H)(NO)(PiPr3)(C5H4OSiMe2tBu)] (3a): A mixture of compound 2
(55 mg, 1.75 equiv, 0.292 mmol) and [Re(H)(NO)(Br)(PiPr3)2] (103 mg,
A
N
ACHTUNGTRENNUNG
IR (ATR): n˜ =2958, 2853 (CH, N(Bu)4), 2925, 2872 (CH, Cy), 1958
(ReH), 1533 cmꢀ1 (NO); MS (ESIꢀ): m/z: 578.4 [M]ꢀ; elemental analysis
calcd (%) for C39H74N2O2PRe: C 57.11, H 9.09, N 3.42; found: C 57.04,
H 9.15, N 3.40.
A
R
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
0.167 mmol) in THF (5 mL) was stirred for 2 h at room temperature to
produce a dark orange color. The solvent was evaporated in vacuo and
the product was extracted with pentane until the extracts were colorless.
Yield of 3a: 92–94%; 1H NMR (400 MHz, C6D6): d=4.64, 4.70 (m, 2H;
[Re(H)(NO)ACHTUNGRTENNU(G PiPr3)ACHTUNGRTNE(NUNG C5H4OH)] (5a): An excess of ammonium bromide
was added to a solution of compound 4a (483 mg, 0.60 mmol) in THF
(15 mL) and the mixture was stirred overnight. The solvent was removed
in vacuo and the residue was dissolved in toluene and filtered over celite.
The solvent was removed in vacuo and the product was purified by
column chromatography on silica gel (Et2O/THF, 7:1) at ꢀ308C to obtain
compound 5a as a yellow solid. Yield: 51%; MS (ESIꢀ): m/z: 458.4
[M]ꢀ; elemental analysis calcd (%) for C14H27NO2PRe: C 36.67, H 5.93,
N 3.05; found: C 36.78, H 5.99, N 3.00. 1H NMR (400 MHz, [D8]THF):
d=8.03 (br s, 1H; OH), 4.80 (m, 1H; C5H4), 4.77 (m, 1H; C5H4), 4.66
C5H4), 2.02 (m, 3H; CH
9H; SiC(CH3)3), 0.13 (s, 6H; Si
28.1 Hz; ReH); 13C{1H} NMR (100 MHz, C6D6): d=136.9 (s; CipsoC5H4),
73.6, 72.8, 72.6, 72.1 (s; C5H4), 27.5, 27.3 (s; SiCH(CH3)2), 25.4 (s; SiC-
(CH3)3), 20.3, 19.4 (s; PCH(CH3)2), 18.0 (s; SiC
(CH3)), ꢀ5.0, ꢀ5.2 ppm
(s; SiACHTUNGTRENNUNG
(CH3)2); 31P{1H} NMR (162 MHz, C6D6): d=47.1 ppm (s; PiPr3);
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
IR (ATR): n˜ =2958, 2930, 2870 (CH), 1976 (ReH), 1629 cmꢀ1 (NO); MS
(ESI+): m/z: 574.2 [M+H]+; elemental analysis calcd (%) for
C20H41NO2PReSi: C 41.94, H 7.21, N 2.45; found: C 42.27, H 7.28, N 2.27.
(m, 1H; C5H4), 4.62 (m, 1H; C5H4), 2.13 (m, 3H; PCH
18H; PCH (H,P)=27.4 Hz; ReH);
(CH3)2), ꢀ9.42 ppm (d, 1H, 2J
13C{1H} NMR (100 MHz, [D8]THF): d=141.1 (s; CipsoC5H4), 72.1 (s, 2C;
C5H4), 71.7, 70.5 (s; C5H4), 28.1, 27.9 (s; PCH(CH3)2), 20.4, 19.5 ppm (s;
ACHTUGNTREN(UNNG CH3)2), 1.16 (m,
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[Re(H)(NO)
ACHTUNGTRENNUNG
PCHACHTNUTGRNEUNG
(CH3)2); 31P{1H} NMR (162 MHz, [D8]THF): d=47.8 ppm (s; PiPr3);
(231 mg, 1.14 mmol) and [Re(H)(NO)(Br)ACHTUNGTRENNNUG
1H NMR (500 MHz, C6D6,): d=9.20 (br s, 1H; OH), 5.26 (s, 1H; C5H4),
in THF (15 mL) was stirred for 2 h at room temperature to give a dark
orange solution. The solvent was evaporated in vacuo and the product
was extracted with pentane until the extracts were colorless. After drying
in vacuo, compound 3b was obtained in 86% yield, along with minor
amounts of free tricyclohexylphosphine. Crystallization from pentane at
ꢀ308C over several days provided pure compound 3b in 64% yield.
1H NMR (400 MHz, C6D6): d=4.83, 4.79, 4.64, 4.57 (m, 1H; C5H4), 2.12–
4.82 (s, 1H; C5H4), 4.62 (s, 1H; C5H4), 4.58 (s, 1H; C5H4), 2.00 (m, 3H;
PCH
(H,P)=27.2 Hz; ReH); 13C{1H} NMR (125.8 MHz, C6D6): d=142.0 (s;
CipsoC5H4), 73.1, 71.6, 70.9, 67.9 (s; C5H4), 28.1, 27.9 (s; PCH(CH3)2), 20.6,
19.7 ppm (s; PCH
(CH3)2); 31P{1H} NMR (202.5 MHz, C6D6): d=48.1 ppm
A
N
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(s; PiPr3); IR (ATR): n˜ =3118 (br; OH), 2960, 2922, 2909, 2868 (CH),
2009 (ReH), 1607 cmꢀ1 (NO).
1.16 (m, 33H; Cy), 0.90 (s, 9H; SiC
A
ACHTUGNRTNE(NUNG CH3)2),
ꢀ8.97 ppm (d, 1H, 2J
G
A
ACHTUNGTRENNUNG
C6D6): d=138.1 (s; CO), 74.7, 73.4, 72.9, 71.4 (s; C5H4), 38.2, 37.9, 31.3,
ACHTUNGTRENNUNG
30.6 (s; Cy), 28.3, 28.3, 28.1 (s; SiC
G
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
d=37.8 ppm (s; PCy3); IR (ATR): n˜ =2924, 2847 (CH), 1975 (ReH),
1620 cmꢀ1 (NO); MS (ESI+): m/z: 694.3 [M+H]+; elemental analysis
calcd (%) for C29H53NO2PSiRe: C 50.26, H 7.71, N 2.02; found: C 50.29,
H 7.90, N 2.02.
C5H4), 28.5, 28.2 (s; PCHACTHGNUTREN(NUG CH3)2), 19.2, 19.2 ppm (s; PCHACHTUNGTRENNUNG(CH3)2);
31P{1H} NMR (202.5 MHz, C6D6): d=52.3 ppm (s; PiPr3); IR (ATR): n˜ =
2960, 2922, 2909, 2868 (CH), 2218, 2079 (ReH), 1736, 1713 cmꢀ1 (NO/
CO).
[Re(H)(NO)
(PiPr3)
E
G
[Re(H)(NO)ACHTUNGERTNNU(G PCy3)ACHTUNGRTNE(NUNG C5H4OH)] (5b): An excess of ammonium bromide
(317 mg, 0.554 mmol) in THF (10 mL) was added TBAF (554 mL,
0.554 mmol, 1m in THF). After the mixture had been stirred for 2 h at
room temperature, the solvent was removed in vacuo and the residue
was re-dissolved in toluene and filtered through celite. The solvent was
removed in vacuo and compound 4a was precipitated from a toluene/
pentane mixture. After drying in vacuo, compound 4a was obtained as
an orange oil. Yield: 64%; 1H NMR (400 MHz, C6D6): d=5.56 (s, 1H;
C5H4), 4.89 (s, 1H; C5H4), 4.56 (s, 1H; C5H4), 3.27 (s, 1H; C5H4), 3.19
(425 mg, 4.34 mmol) was added to a solution of compound 4b (178 mg,
0.217 mmol) in THF (15 mL) and stirred overnight. The solvent was re-
moved in vacuo and the residue was dissolved in toluene and filtered
through celite. The solvent was removed in vacuo and the product was
purified by column chromatography on silica gel (Et2O/THF, 7:1) at
ꢀ308C and compound 5b was obtained as a yellow solid. Yield: 54%;
MS (ESIꢀ): m/z: 578.3 [M]ꢀ; elemental analysis calcd (%) for
C23H39NO2PRe: C 47.73, H 6.79, N 2.42; found: C 48.00, H 6.87, N 2.23.
1H NMR (400 MHz, [D8]THF): d=8.03 (s, 1H; OH), 4.88 (m, 2H;
C5H4), 4.65 (m, 1H; C5H4), 4.58 (m, 1H; C5H4), 1.98–1.26 (m; Cy),
(m; N
(CH
R
ACHTUNGTRENNUNG
(CH3)2 and NCH
N
R
N
N
CHTUNGTRENNUNG
ꢀ8.01 ppm (d, 1H, 2J
ꢀ9.48 ppm (d, 1H; 2J
(H,P)=27.4 Hz; ReH); 13C{1H} NMR (100 MHz,
ACHTUNGTRENNUNG
C6D6): d=167.5 (s; CO), 68.8, 66.1, 65.2, 64.7 (s; C5H4), 58.8 (s, CH2-
[D8]THF): d=140.4 (s; CO), 71.7, 71.7, 71.2, 70.2 (s; C5H4OH), 37.5,
37.2, 30.4, 29.8, 27.6, 26.7 ppm (s; Cy); 31P{1H} NMR (162 MHz,
[D8]THF): d=36.5 ppm (s; PCy3); 1H NMR (400 MHz, C6D6): d=6.71
(br s, 1H; OH), 4.96 (m, 1H; C5H4), 4.66 (m, 2H; C5H4), 4.34 (m, 1H;
A
R
2ACHTUNGTREN(UNGN NBu4)), 21.7 (s; CH2-
A
ACHTUNGTRENNUNG
31P{1H} NMR (162 MHz, C6D6): d=43.3 (s; PiPr3); IR (ATR): n˜ =2957,
2929, 2869 (CH), 1934 (ReH), 1533 cmꢀ1 (NO); MS (ESIꢀ): m/z: 458.4
[M]ꢀ; elemental analysis calcd (%) for C30H62N2O2PRe: C 51.47, H 8.93,
N 4.00; found: C 51.39, H 8.95, N 3.92.
C5H4), 2.18–1.15 (m; Cy), ꢀ8.82 ppm (d, 1H; 2J
ACHTUNGTREN(NUNG H,P)=26.9 Hz; ReH);
13C{1H} NMR (100 MHz, C6D6): d=141.2 (s; CipsoC5H4), 73.4, 72.4, 71.3,
68.2 (s; C5H4), 38.4, 38.2, 31.3, 30.6, 27.5, 26.8 ppm (s; Cy); 31P{1H} NMR
(162 MHz, C6D6): d=37.0 ppm (s; PCy3). IR (ATR): n˜ =3144 (OH),
2924, 2848 (CH), 2005 (ReH), 1620 cmꢀ1 (NO).
[Re(H)(NO)ACHTUNGTRENNUNG(PCy3)ACHTUNGTRENNUNG(C5H4O)]ACHTNUGRTEN[NUGN NBu4] (4b): To a solution of compound 3b
in THF (10 mL) containing free PCy3 (312 mg, 0.321 mmol) was added
TBAF (321 mL, 0.321 mmol, 1m in THF). After the mixture had been
stirred for 2 h at room temperature, the solvent was removed in vacuo.
The residue was washed with pentane to remove free PCy3. and com-
(PCy3)(H)2] (6b): 1H NMR (400 MHz, C6D6): d=5.14,
ACHUTNRGEN[NUG (C5H4O)Re(NO)ACHUTNGTRENNGUN
4.89 (m, 4H; C5H4), 2.18–1.15 (m; Cy), ꢀ3.14 ppm (d, 2H; 2J
ACHTUNGTRENNUNG(H,P)=
47.0 Hz; ReH); 13C{1H} NMR (100 MHz, C6D6): d=181.6 (s; CO), 82.6,
5712
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714

Bifunctional Rhenium Complexes for Transfer Hydrogenations
FULL PAPER
68.5 (s, 2C; C5H4), 38.7, 30.1, 28.3, 28.2, 27.8, 27.7 ppm (s; Cy);
31P{1H} NMR (162 MHz, C6D6): d=41.6 ppm (s; PCy3); IR (ATR): n˜ =
2923, 2850 (CH), 2205, 2021 (ReH), 1712 cmꢀ1 (NO).
lographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[Re(NO)
(PiPr3)
N
In the crystal structures of compounds 3a and 3b, artefactual electron
density was observed near the heavy-atom sites (less than 0.8 ꢂ). Differ-
ent types of absorption correction were tried but without significant im-
provements. In the crystal structure of compound 10b, THF solvent mol-
ecules co-crystallized with the metal–organic species in a ratio 2:1. One
of these solvent molecules was disordered over two sets of positions with
a site-occupancy factor of 0.5. The hydride atoms in compounds 3a, 3b,
and 5a, and the H atom of the hydroxy group in compound 5a were lo-
cated in difference Fourier maps and freely refined. All other hydrogen
positions were calculated after each cycle of refinement using a riding
(10 mg, 0.0218 mmol) was dissolved in 2-propanol (5 mL) and an excess
of pyridine (175.6 mL, 2.18 mmol) and the substrate (4.36 mmol) were
added. The solution was stirred overnight at 808C and the solvent was re-
moved in vacuo. The residue was washed several times with pentane.
Compound 10a was obtained as an orange-red solid in 56% yield.
1H NMR (400 MHz, CD3CN): d=8.67 (d, 2H; o-CH), 7.63 (t, 1H; p-
CH), 7.25 (t, 2H; m-CH), 6.07, 4.47, 4.26, 3.26 (m, 1H; C5H4O), 2.45–
2.36 (m, 3H; PCHACHTUNGTRENNUNG(CH3)2), 1.22–1.15 ppm (m, 18H; PCHACHTUGNTRENN(UNG CH3)2);
13C{1H} NMR (100 MHz, CD3CN): d=171.4 (s; CO), 156.6 (s, 2C; o-
CH), 137.3 (s; p-CH), 126.1 (s, 2C; m-CH), 77.6, 75.1 (s, 2C; C5H4), 28.6,
ꢀ
model, with C H=0.93 ꢂ and Uiso(H)=1.2Ueq(C) for aromatic H atoms,
(CH3)2); 31P{1H} NMR
28.3 (s; PCHACHTUNGTRENNUNG(CH3)2), 20.2, 19.6 ppm (s; PCHACHTUNGTRENNUGN
ꢀ
ꢀ
C H=0.97 ꢂ and Uiso(H)=1.2Ueq(C) for methylene H atoms, C H=
ꢀ
0.98 ꢂ and Uiso(H)=1.2Ueq(C) for methine H atoms, and C H=0.96 ꢂ
and Uiso(H)=1.5Ueq(C) for methyl H atoms.
(162 MHz, CD3CN): d=21.8 (s; PiPr3); IR (ATR): n˜ =2963, 2931, 2873
(CH), 1625, 1584 cmꢀ1 (NO/CO); MS (ESI): m/z: 537.2 [M+H]; elemen-
tal analysis calcd (%) for C19H30N2O2PRe: C 42.60, H 5.65, N 5.23;
found: C 42.23, H 5.56, N 5.16.
Kinetic studies: Kinetic studies were carried out in an NMR tube that
was fitted with a Young Teflon valve. The complex (0.00218 mmol) was
dissolved in [D8]2-propanol (0.4 mL) and acetophenone (0.1823 mmol) or
benzaldehyde (0.218 mmol) were added. The disappearance of acetophe-
none was monitored by 1H NMR spectroscopy as a function of time at
350 K.
[Re(NO)ACHTUNGTRENNUNG(PCy3)ACHTUNGTRENNUNG(pyridine)(cyclopentadienone)] (10b): Compound 5b
(10 mg, 0.0218 mmol) was dissolved in 2-propanol (5 mL) and an excess
of pyridine (175,6 mL, 2.18 mmol) and the substrate (4.36 mmol) were
added. The solution was stirred overnight at 808C and the solvent was re-
moved in vacuo. Compound 10b was precipitated from a THF and excess
of pentane solution and, after drying, compound 10b was obtained as
a yellow solid in 68% yield. 1H NMR (400 MHz, CD3CN): d=8.56 (d,
2H; o-CH), 7.58 (t, 1H; p-CH), 7.17 (t, 2H; m-CH), 6.00 (m, 1H;
C5H4O), 4.30 (m, 1H; C5H4O), 4.22 (m, 1H; C5H4O), 3.23 (m, 1H;
C5H4O), 2.08–1.16 ppm (m; Cy); 13C{1H} NMR (100 MHz, CD3CN): d=
170.6 (s; CO), 156.2 (s, 2C; o-CH), 136.7 (s; p-CH), 125.7 (s, 2C; m-CH),
76.9, 73.9 (s, 2C; C5H4), 37.9, 37.6, 30.3, 29.6, 27.8 ppm (s; Cy);
31P{1H} NMR (162 MHz, CD3CN): d=12.6 ppm (s; PCy3); IR (ATR): n˜ =
2928, 2847 (CH), 1629, 1568 cmꢀ1 (NO/CO); MS (ESI): m/z: 657.3
[M+H]; elemental analysis calcd (%) for C28H43N2O2PRe: C 51.20,
H 6.60, N 4.26; found: C 50.98, H 6.49, N 4.08.
Catalytic reduction: All catalysis was carried out in Schlenk flasks that
were fitted with Teflon valves. Compound 5a (0.00128 mmol) and sub-
strate (0.436 mmol) were mixed in 2-propanol (1 mL, 30 equiv) and the
mixture was heated at 1208C. After an appropriate reaction time, an ali-
quot of the reaction mixture was diluted in pentane and filtered through
celite to remove the catalyst. The products were identified and the yields
were determined by GCMS analysis (r.t.=retention time).
PhCO
N
ACHTUNGTRENNUNG
122; PhCO
N
ACHTUNGTRENNUNG
7.84 min, m/z 136; PhCOPh: r.t.=11.27 min, m/z 182; PhCHOHPh: r.t.=
11.20 min, m/z 184; p-FPhCOPh-pF: r.t.=5.52 min, m/z 218; p-
FPhCHOHPh-pF: r.t.=4.98 min, m/z 220; CH3CO(C
2.59 min, m/z 100; CH3CHOH(C(CH3)3): r.t.=3.10 min, m/z 102; C5H8O:
r.t.=4.29 min, m/z 84; C5H9OH: r.t.=4.14 min, m/z 86;
ACHTUGNTRNEN(UNG CH3)3): r.t.=
[Re(D)(NO)ACHTUNGTRENNUNG(PR3)ACHTUNGTRENNUNG(C5H4OD)]: Method A (D2O): Rhenium hydride 5a/
AHCTUNGTRENNUNG
5b was dissolved in [D8]THF (0.4 mL) in a Young NMR tube that was
fitted with a Teflon valve. An excess of D2O (100 equiv) was added and
the mixture was shaken over a period of 5 h at room temperature to
afford 94% H/D exchange for compound 5a and 92% for compound 5b.
CH3COPhCOCH3: r.t.=9.06 min, m/z 162; CH3CHOHPhCOCH3: r.t.=
9.56 min, m/z 164; CH3CHOHPhCHOHCH3: r.t.=9.21 min, m/z 166;
C6H10: r.t.=2.26 min, m/z 82; C6H12: r.t.=2.09 min, m/z 84; PhCHNPh:
r.t.=9.093, m/z 181; PhCH2NHPh: r.t.=9.355, m/z 183; p-ClPhCHNPh-
pCl: r.t.=9.779, m/z 249; p-ClPhCH2NHPh-pCl: r.t.=10.235, m/z 251;
PhCHN(a-naphtyl): r.t.=10.513, m/z 231; PhCH2NH(a-naphtyl): r.t.=
10.683, m/z 233.
Method B (D2): Rhenium hydride 5a/5b was dissolved in [D8]THF
(0.4 mL) in a Young NMR tube that was fitted with a Teflon valve. The
NMR tube was degassed by three freeze-pump-thaw cycles and filled
with D2 (1.5 bar). The disappearance of the hydride signal was monitored
by 1H NMR spectroscopy as a function of time. After 77 h, the HRe ex-
change was 85% for compound 5a and 55% for compound 5b.
Computational methods: DFT calculations on freely optimized PMe3-
substituted model compounds (A–E) and on the transition states (TS1–
TS5) were carried out by employing the B3LYP[22] functional and the 6–
Acknowledgements
31GACHTUNGTRENNUNG
(d,p)[23] basis set for H to P and the LANL2DZ[24] basis set in con-
We thank the Swiss National Science Foundation and the University
Zurich for financial support.
nection with the associated effective core potential for Re with the
GAMESS program package.[25] All free energies were calculated at
298.15 K and referenced to the energy of structure A1.
X-ray diffraction: Single-crystal X-ray diffraction data were collected at
183(2) K on an Xcalibur diffractometer (Agilent Technologies, Ruby
CCD detector) for all compounds by using a single-wavelength Enhance
X-ray source with MoKa radiation (l=0.71073 ꢂ).[34] Suitable single crys-
tals were mounted by using polybutene oil on the top of a glass fiber that
was fixed on a goniometer head and immediately transferred into the dif-
fractometer. Pre-experimental-, data-collection-, data-reduction-, and an-
alytical-[35] or semi-empirical absorption corrections were performed with
the program suite CrysAlisPro.[36] The crystal structures were solved with
SHELXS97[37] using direct methods. The structure refinements were per-
formed by full-matrix least-squares on F2 with SHELXL97.[37] All pro-
grams used during the crystal structure determination process are includ-
ed in the WINGX software.[38] PLATON[39] was used to check the result
of the X-ray analyses. CCDC-854668 (3a), CCDC-854669 (3b), CCDC-
854670 (5a), and CCDC-854671 (10b) contain the supplementary crystal-
[1] a) Y. Shvo, D. Czarkie, J. Organomet. Chem. 1986, 315, C25; b) Y.
Blum, D. Czarkie, Y. Rahamim, Y. Shvo, Organometallics 1985, 4,
1459; c) B. L. Conley, M. K. Pennington-Boggio, E. Boz, T. J. Wil-
liams, Chem. Rev. 2010, 110, 2294.
[2] Y. Shvo, I. Goldberg, D. Czerkie, D. Reshef, Z. Stein, Organometal-
lics 1997, 16, 133.
[3] a) J. S. M. Samec, J. E. Bꢀckvall, Chem. Eur. J. 2002, 8, 2955; b) C. P.
Casey, G. A. Bikzhanova, Q. Cui, I. A. Guzei, J. Am. Chem. Soc.
2005, 127, 14062.
[4] a) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana,
J. Am. Chem. Soc. 2001, 123, 1090; b) N. Menashe, Y. Shvo, Organo-
metallics 1991, 10, 3885; c) N. Menashe, E. Salant, Y. Shvo, J. Orga-
nomet. Chem. 1996, 514, 97.
[5] a) L. Koren-Selfridge, H. N. Londino, J. K. Vellucci, B. J. Simmons,
C. P. Casey, T. B. Clark, Organometallics 2009, 28, 2085; b) C. P.
Chem. Eur. J. 2012, 18, 5701 – 5714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5713

H. Berke et al.
Casey, T. E. Vos, G. A. Bikzhanova, Organometallics 2003, 22, 901;
c) C. P. Casey, N. A. Strotman, S. E. Beetner, J. B. Johnson, D. C.
Priebe, T. E. Vos, B. Khodavandi, I. A. Guzei, Organometallics 2006,
25, 1230; d) C. P. Casey, N. A. Strotman, S. E. Beetner, J. B. Johnson,
D. C. Priebe, I. A. Guzei, Organometallics 2006, 25, 1236.
[6] a) S. Hashiguchi, A. Fujii, J. Takehara, T. Ikariya, R. Noyori, J. Am.
Chem. Soc. 1995, 117, 7562; b) J. X. Gao, T. Ikariya, R. Noyori, Or-
ganometallics 1996, 15, 1087; c) T. Ikariya, K. Murata, R. Noyori,
Org. Biomol. Chem. 2006, 4, 393; d) R. Noyori, S. Hashiguchi, Acc.
Chem. Res. 1997, 30, 97.
[7] a) H. H. Zhang, D. Z. Chen, Y. H. Zhang, G. Q. Zhang, J. B. Liu,
Dalton Trans. 2010, 39, 1972; b) J. B. Johnson, J. E. Bꢀckvall, J. Org.
Chem. 2003, 68, 7681; c) C. P. Casey, H. R. Guan, J. Am. Chem. Soc.
2009, 131, 2499; d) A. Comas-Vives, G. Ujaque, A. Lledos, Organo-
metallics 2007, 26, 4135.
Messmer, H. Jacobsen, H. Berke, Chem. Eur. J. 1999, 5, 3341;
c) G. J. Kubas, Chem. Rev. 2007, 107, 4152.
[20] a) D. M. Heinekey, A. Lledos, J. M. Lluch, Chem. Soc. Rev. 2004, 33,
175; b) D. G. Hamilton, R. H. Crabtree, J. Am. Chem. Soc. 1988,
110, 4126.
[21] C. P. Casey, J. B. Johnson, S. W. Singer, Q. Cui, J. Am. Chem. Soc.
2005, 127, 3100.
[22] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) P. J. Stephens, F. J.
Devlin, C. F. Chabalowski, M. J. Frisch, J Phys Chem. Us. 1994, 98,
11623; c) C. T. Lee, W. T. Yang, R. G. Parr, Phys. Rev. B 1988, 37,
785.
[23] a) W. J. Hehre, Ditchfie. R, J. A. Pople, J. Chem. Phys. 1972, 56,
2257; b) J. D. Dill, J. A. Pople, J. Chem. Phys. 1975, 62, 2921;
c) M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S.
Gordon, D. J. Defrees, J. A. Pople, J. Chem. Phys. 1982, 77, 3654.
[24] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270.
[25] M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S.
Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J.
Su, T. L. Windus, M. Dupuis, J. A. Montgomery, J. Comput. Chem.
1993, 14, 1347.
[8] H. Berke, ChemPhysChem 2010, 11, 1837.
[9] a) X. F. Wu, D. Vinci, T. Ikariya, J. L. Xiao, Chem. Commun. 2005,
4447; b) K. Murata, T. Ikariya, J. Org. Chem. 1999, 64, 2186.
[10] a) X. W. Li, P. Chen, J. W. Faller, R. H. Crabtree, Organometallics
2005, 24, 4810; b) X. F. Wu, J. K. Liu, X. H. Li, A. Zanotti-Gerosa,
F. Hancock, D. Vinci, J. W. Ruan, J. L. Xiao, Angew. Chem. 2006,
118, 6870; Angew. Chem. Int. Ed. 2006, 45, 6718.
[26] T. A. Albright, J. K. Burdett, M. H. Whangbo, Orbital Interactions in
Chemistry, Wiley, New York, 1985.
[11] J. Washington, R. Mcdonald, J. Takats, N. Menashe, D. Reshef, Y.
Shvo, Organometallics 1995, 14, 3996.
[27] G. J. Kubas, in Metal dihydrogen and s-band complexes, 1st ed.,
Kluwer Academic, New York, 2000.
[12] a) C. P. Casey, H. R. Guan, J. Am. Chem. Soc. 2007, 129, 5816;
b) R. M. Bullock, Angew. Chem. 2007, 119, 7504; Angew. Chem. Int.
Ed. 2007, 46, 7360; c) M. G. Coleman, A. N. Brown, B. A. Bolton,
H. R. Guan, Adv. Synth. Catal. 2010, 352, 967.
[28] M. Ito, T. Ikariya, Chem. Commun. 2007, 5134.
[29] A. Berkessel, H. Grçger, Asymmetric Organocatalysis: from Bio-
mimetic Concepts to Applications in Asymmetric Synthesis, Wiley-
VCH, Weinheim, 2005.
[13] Z. L. Chen, I. Timokhin, H. W. Schmalle, T. Fox, O. Blacque, H.
Berke, Eur. J. Inorg. Chem. 2009, 4119.
[30] D. R. Lide, Handbook of Chemistry and Physics, 73th ed., CRC
Press, Boca Raton, FL, 1993.
[14] A. Choualeb, A. J. Lough, D. G. Gusev, Organometallics 2007, 26,
3509.
[31] S. Kozuch, S. Shaik, Acc. Chem. Res. 2011, 44, 101.
[32] a) H. Plenio, C. Aberle, Organometallics 1997, 16, 5950; b) H.
Plenio, A. Warnecke, Organometallics 1996, 15, 5066.
[33] WO 01/53362 A1; US 6, 815,514 B2.
[34] Agilent Technologies (formerly Oxford Diffraction), Yarnton, Eng-
land, 2011.
[35] R. C. Clark, J. S. Reid, Acta Crystallogr. Sect. A 1995, 51, 887.
[36] CrysAlisPro (Versions 1.171.33.34d/1.171.35.11), Agilent Technolo-
gies (formerly Oxford Diffraction), Yarnton, England, 2011.
[37] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112.
[38] L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837.
[39] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7.
[15] a) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H.
Berke, Organometallics 2008, 27, 3474; b) Y. Jiang, O. Blacque, T.
Fox, C. M. Frech, H. Berke, Chem. Eur. J. 2009, 15, 3039.
[16] a) Y. F. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Organome-
tallics 2009, 28, 5493; b) X. Y. Liu, K. Venkatesan, H. W. Schmalle,
H. Berke, Organometallics 2004, 23, 3153; c) B. Dudle, K. Rajesh,
O. Blacque, H. Berke, J. Am. Chem. Soc. 2011, 133, 8168.
[17] a) G. W. Stowell, R. R. Whittle, C. M. Whaley, D. P. White, Organo-
metallics 2001, 20, 1050; b) B. Machura, Coord. Chem. Rev. 2005,
249, 2277.
[18] C. A. Tolman, Chem. Rev. 1977, 77, 313.
[19] a) N. V. Belkova, E. S. Shubina, A. V. Ionidis, L. M. Epstein, H. Ja-
cobsen, A. Messmer, H. Berke, Inorg. Chem. 1997, 36, 1522; b) A.
Received: November 23, 2011
Published online: March 27, 2012
5714
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 5701 – 5714