Heterolytic H2 activation by rhodium thiolato complexes bearing the hydrotris(pyrazolyl)borato ligand and application to catalytic hydrogenation under mild conditions

Article abstract of DOI:10.1039/b923557d

Thiolato complexes of Rh(iii) bearing a hydrotris(3,5-dimethylpyrazolyl) borato ligand (Tp^Me2) have been prepared, and their reactivity toward H₂ has been investigated. The bis(thiolato) complex [Tp ^Me2Rh(SPh)₂(MeCN)] (1) reacted with 1 atm H₂ at 20 °C to produce the hydrido-thiolato complex [Tp^Me2RhH(SPh) (MeCN)] (2) and PhSH via heterolytic cleavage of H₂. This process is reversible and in equilibrium in THF and benzene. The bis(selenolato) complex [Tp^Me2Rh(SePh)₂(MeCN)] (4) was also converted to [Tp ^Me2RhH(SePh)(MeCN)] and PhSeH under 1 atm H₂, but the equilibrium largely shifted to 4. Reaction of the dithiolato complex [Tp ^Me2Rh(bdt)(MeCN)] (3; bdt = 1,2-C₆H₄S ₂) with H₂ occurred in the presence of amine, giving the anionic hydrido complex [Tp^Me2RhH(bdt)]^- and an equimolar amount of ammonium cations. Catalytic activity for hydrogenation has been examined under 1 atm H₂ at 20-50 °C. While 1, 2, and 4 slowly hydrogenated styrene at similar rates at 50 °C, activities for the hydrogenation of N-benzylideneaniline increased in the order, 2 < 1 < 4. Complex 3 was found to be the most active and selective catalyst for hydrogenation of imines, and thus a variety of imines were reduced at 20 °C under 1 atm H₂, with the CC and CO bonds in the substrate molecules completely preserved. An ionic mechanism was involved to explain such high chemoselectivity.

Full text of DOI:10.1039/b923557d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Heterolytic H2 activation by rhodium thiolato complexes bearing the
hydrotris(pyrazolyl)borato ligand and application to catalytic hydrogenation
under mild conditions†
Hidetake Seino,* Yoshiyuki Misumi, Yoshihiro Hojo and Yasushi Mizobe*
Received 10th November 2009, Accepted 7th January 2010
First published as an Advance Article on the web 1st February 2010
DOI: 10.1039/b923557d
Thiolato complexes of Rh(III) bearing a hydrotris(3,5-dimethylpyrazolyl)borato ligand (TpMe2) have
been prepared, and their reactivity toward H2 has been investigated. The bis(thiolato) complex
[TpMe2Rh(SPh)2(MeCN)] (1) reacted with 1 atm H2 at 20 ◦C to produce the hydrido-thiolato complex
[TpMe2RhH(SPh)(MeCN)] (2) and PhSH via heterolytic cleavage of H2. This process is reversible and in
equilibrium in THF and benzene. The bis(selenolato) complex [TpMe2Rh(SePh)2(MeCN)] (4) was also
converted to [TpMe2RhH(SePh)(MeCN)] and PhSeH under 1 atm H2, but the equilibrium largely shifted
to 4. Reaction of the dithiolato complex [TpMe2Rh(bdt)(MeCN)] (3; bdt = 1,2-C6H4S2) with H2
occurred in the presence of amine, giving the anionic hydrido complex [TpMe2RhH(bdt)]- and an
equimolar amount of ammonium cations. Catalytic activity for hydrogenation has been examined
under 1 atm H2 at 20–50 ◦C. While 1, 2, and 4 slowly hydrogenated styrene at similar rates at 50 ◦C,
activities for the hydrogenation of N-benzylideneaniline increased in the order, 2 < 1 < 4. Complex 3
was found to be the most active and selective catalyst for hydrogenation of imines, and thus a variety of
◦
=
=
imines were reduced at 20 C under 1 atm H2, with the C C and C O bonds in the substrate molecules
completely preserved. An ionic mechanism was involved to explain such high chemoselectivity.
bifunctional catalysts. However, such a role of thiolato ligand is
Introduction
uncommon, and only a few of complexes have been confirmed
to form M(H)–S(H)R species by the action of M–SR with H2.11
Consequently, reports of catalysis based on heterolytic activation
of H2 by thiolato complexes are still scarce.12
Heterolytic activation of H2 by transition metal complexes has
been widely utilized in catalytic hydrogenation of polar bonds.
Heterolysis of H2 typically proceeds in the manner that H+
is stripped off from the acidic H2 ligand to leave H- on the
metal center with the assistance of an external base or an
internal ancillary ligand.1 Dihydrogen or dihydride complexes
which operate by an ionic mechanism sequentially transfer H+
followed by H- to a substrate.2 In well established bifunctional
catalysis of chelate amido complexes, H2 is efficiently cleaved by
cooperative action of metal and amido nitrogen, forming hydride
and amine hydrogen, which are then added to a substrate as H-
and H+.3 Hydrogen metabolism mediated by hydrogenase enzymes
also involve heterolytic splitting of H2 at the initial step.4,5 Recent
studies on [FeFe]-hydrogenase propose that the bridging dithiolato
ligand in the diiron subunit contains a heteroatom (O or N) that
participates in the heterolytic reactivity of H2.6 In the mechanism
of H2 conversion by [NiFe]-hydrogenases, it is suggested that a
terminal S(Cys) moiety bound to Ni possibly accepts H+ from the
coordinated H2.7 Such a role of S(Cys) is also postulated in the
function of [Fe]-hydrogenase, which generates H+ and transfer H-
to an organic molecule at the monoiron site bound to cysteine
residue.8,9 As these examples in nature indicate, anionic S-donor
ligands, RS- and S2-, are expected to assist heterolysis of H2
as internal proton acceptors,10–13 similarly to amido ligand in
We have been studying transition metal thiolato complexes
to develop their use in activating small molecules.14,15 We have
recently reported that the Rh(III) bis(thiolate) complex hav-
ing a hydrotris(3,5-dimethylpyrazolyl)borato (TpMe2) co-ligand
[TpMe2Rh(SPh)2(MeCN)] (1) reacts reversibly with H2 to form
the hydrido–thiolato complex [TpMe2RhH(SPh)(MeCN)] (2) and
PhSH as shown in eqn (1).16 Based on this heterolysis
of H2, 1 showed moderate catalytic activity of hydrogena-
tion under mild conditions. An analogous dithiolato complex
2
[TpMe2Rh(bdt)(MeCN)] (3; bdt = 1,2-C6H4S2-k S,S¢) has turned
out to be more active under ambient temperature and pressure
and exclusively chemoselective to imines. We herein report a
detailed study of the heterolytic activation of H2 and catalytic
hydrogenation by 1, 3, and analogous complexes.
(1)
Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-
ku, Tokyo, 153-8505, Japan. E-mail: seino@iis.u-tokyo.ac.jp, ymizobe@
iis.u-tokyo.ac.jp; Fax: +81 (0)3 5452 6361; Tel: +81 (0)3 5452 6360
† CCDC reference numbers 759754, 753657–753659. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b923557d
3072 | Dalton Trans., 2010, 39, 3072–3082
This journal is
The Royal Society of Chemistry 2010
©

Results and discussion
Preparation and properties of bis(chalcogenolato) complexes and
related compounds
The synthetic routes of the chalcogenolato complexes in this study
are summarized in Scheme 1. We have previously reported that
bis(benzenechalcogenolato) complexes [TpMe2Rh(EPh)2(MeCN)]
(E = S (1), Se (4), Te (5)) are prepared in good yields by the
reactions of the Rh(I) complex [TpMe2Rh(cyclooctene)(MeCN)] (6)
with 1 equiv of diphenyl dichalocogenides PhEEPh.15a,b It has been
found that 1 is formed also from 6 and PhSH via the intermediate 2
with concomitant H2 evolution.15c Although 6 reacts more rapidly
with PhSH (60% yield of 1 after only 1 h at 20 ◦C) than PhSSPh
(85% yield after 27 h), conversion to 1 does not complete in a closed
reaction vessel due to the presence of the equilibrium shown in
eqn (1).
Fig.
1
ORTEP drawing of [TpMe2Rh(bdt)(MeCN)] (3). Solvating
˚
THF molecules are omitted for clarity. Selected bond distances (A)
and angles (deg): Rh–S(1), 2.3187(8); Rh–S(2), 2.3155(9); Rh–N(2),
2.132(2); Rh–N(4), 2.107(2); Rh–N(6), 2.057(2); Rh–N(7), 1.998(2);
S(1)–C(16), 1.765(3); S(2)–C(17), 1.765(3); S(1)–Rh–S(2), 88.16(3);
S(1)–Rh–N(2), 178.76(7); S(2)–Rh–N(4), 178.08(7); N(6)–Rh–N(7),
177.26(10); Rh–S(1)–C(16), 102.47(10); Rh–S(2)–C(17), 102.76(10).
measured in CD3CN was resolvable even at room temperature
and exhibited the signal of free MeCN. These observations
probably indicate that facile dissociation of the MeCN ligand
and subsequent pseudo-rotation of the coordination geometry are
occurring in solution. The color of the solution is orange-red in
MeCN and violet in THF at room temperature, while the latter
turned to orange-red at low temperature. These solvo- and thermo-
chromic properties of 3 also support the existence of a dissociation
equilibrium.
Scheme 1
In this study, reactions of 6 were further explored with
thiol compounds which could form bidentate thiolato ligands.
Treatment of 6 with 1 equiv of 1,2-benzenedithiol afforded the
benzenedithiolato complex [TpMe2Rh(bdt)(MeCN)] (3; bdt = 1,2-
Reaction of 6 with 4,5-bis(mercaptomethyl)-o-xylene in THF
at room temperature gave the hydrido-mercaptothiolato complex
2
[TpMe2RhH{SCH2(C6H2Me2)CH2SH-k S,S¢}] (7). The IR absorp-
2
C6H4S2-k S,S¢) in moderate yield. This reaction proceeded rapidly
tion at 2102 cm-1 indicates the existence of a terminal hydrido
at 0 ◦C with evolution of gas and, to achieve high yield, it was better
to be conducted with slow evacuation. In solution, 3 was much
more sensitive to O2 than 1, although crystals of 3 could be handled
under air for a short period. The molecular structure of 3·2THF
has been determined by single crystal X-ray analysis (Fig. 1). The
five-membered chelate ring constituted by the bdt ligand around
the octahedral Rh(III) center is slightly bent along the S–S vector
by 20◦ away from the TpMe2 ligand. This bending is considered
to be caused mainly by crystal packing, because the bent angle
observed for the independently analyzed 3·THF of the different
crystal system was only 9◦. The Rh–S distances at 2.3187(8) and
ligand. In the H NMR spectrum measured in C6D6 at 50 ◦C,
1
one hydrido signal appeared at d -15.62 as a doublet with 1H
intensity (JRhH = 8.0 Hz). Pyrazolyl groups were observed as
two sets of signals in a 2 : 1 ratio, and the signal pattern of the
mercaptothiolato ligand also indicated a pseudo-Cs symmetry
with respect to the plane bisecting the S–Rh–S angle. This is
probably due to a rapid shift of the mercapto hydrogen between
two S atoms. Although the SH group could not be confirmed by
the IR spectrum, its 1H signal appeared at d 3.28. By addition of
D2O, not only this signal but also the hydrido signal disappeared
at 50 ◦C, suggesting that the hydrido ligand was deuterated via
exchange with the mercapto deuterium. At lower temperature, the
1H NMR signals became broad and split again at -50 ◦C into
many peaks as shown in Fig. 2. Although these signals could
not be fully assigned, the presence of four major species was
estimated from the spectrum of the hydrido region. The numbers
of signals corresponding to TpMe2 and mercaptothiolato ligands
suggested that each species had a non-symmetric structure. This
can be explained by the presence of conformational isomers arising
from the geometry around S atoms as illustrated in Chart 1. 4,5-
Bis(mercaptomethyl)-o-xylene reacted with 6 more slowly than
1,2-benzenethiol, probably due to the lower acidity of alkyl-SH as
compared to aryl-SH, and for the same reasoning, elimination of
2.3155(9) A are slightly shorter than those in [TpMe2Rh(SC6H4Me-
˚
p)2(MeCN)] (1¢) at 2.338(1) and 2.3420(10) A. Other bonding
parameters around the Rh center in 3 are almost comparable to
those in 1¢. No significant difference is found in the C–S bond
distances between 3 and 1¢.
15a
˚
Although the 1H NMR spectra of 3 in C6D6 or THF-d8 exhibited
broad signals at around room temperature in contrast to 1 and
other analogues, well-resolved spectra consistent with the solid-
state structure were available at -20 ◦C in THF-d8. On raising the
temperature, the signals assignable to two inequivalent pyrazolyl
groups in a ratio of 2 : 1 broadened and finally merged to one set
1
of pyrazolyl signals. On the other hand, the H NMR spectrum
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3072–3082 | 3073
©

Fig. 2 VT 1H NMR spectra of 7 in THF-d8. Aromatic (left) and hydride
(right) regions are shown.
2
Fig. 3 ORTEP drawing of [TpMe2RhH(SC6H4-2-COOH-k S,O)]·THF
(8·THF). Hydrogen atoms in solvating THF molecules are omit-
˚
ted for clarity. Selected bond distances (A) and angles (deg): Rh–S,
2.2948(5); Rh–O(1), 2.044(1); Rh–N(2), 2.075(2); Rh–N(4), 2.014(2);
Rh–N(6), 30.187(2); S–C(16), 1.742(2); O(1)–C(22), 1.233(2); O(2)–C(22),
1.329(2); C(17)–C(22), 1.472(2); Rh–H(28), 1.58(2); O(2)–H(27), 0.79(2);
O(3) ◊ ◊ ◊ H(27), 1.83(2); S–Rh–O(1), 92.58(3); S–Rh–N(2) 178.64(4);
O(1)–Rh–N(4), 174.80(6); N(6)–Rh–H(28), 179.0(6); C(22)–O(2)–H(27),
112(2).
tautomeric o-C6H4(SH)(COO) ligand has been also reported.19
The distances of the Rh–H bond and the elongated Rh–N bond
at its trans position are not exceptional in comparison with
Chart 1
H2 from the hydrido and mercapto moieties did not proceed for
7.17 Reaction of 6 with 1,4-butanedithiol occurred at 50 ◦C in THF
to give a product probably analogous to 7, although this could not
be isolated. Alkanedithiols having shorter alkyl chains were found
to be more reactive, but no tractable complexes were obtained.
One equiv of thiosalicylic acid reacted with 6 at 0 ◦C to give
previously reported values for the TpMe2Rh hydrido complexes
16,20
˚
(Rh–H, 1.44–1.49; Rh–N, 2.19–2.26 A).
To compare the reactivity with chalcogenolato complexes, a
diiodo complex was prepared according to the method reported
for TpiPr2 analogue21 and [TpMe2RhI2(MeCN)] (9) was obtained in
high yield by treatment of 6 with an equimolar amount of I2. The
molecular structure was fully determined by X-ray crystallography
as depicted in Fig. 4. The Rh–N distances trans to the I atoms are
2
[TpMe2RhH(SC6H4-2-COOH-k S,O)] (8) in high yield. Complex 8
was formed quite cleanly, whereas the reaction of 6 with 1 equiv
of PhSH afforded a mixture of 1, 2, and unreacted 6. The H
1
NMR spectrum of 8 showed the signals due to three inequivalent
pyrazolyl groups, which were resolved well at 20 ◦C. Existence
of one hydrido ligand was confirmed by the signal at d -13.79 (d,
JRhH = 11.6 Hz). Although characteristic IR absorptions of neither
˚
shorter than those trans to the S atoms in 1¢ and 3 (2.107–2.136 A).
1
OH nor SH could be found, the broad H signal recorded at d
9.13 might be assigned to the OH group rather than SH because
of its highly deshielded chemical◦shift. This signal disappeared by
addition of D2O or Et3N at 20 C, while the hydrido signal was
=
preserved. Because a low frequency shift of the C O stretching
band to 1617 cm-1 indicates the coordination to the Rh center, it
is considered that the SC6H4-2-COOH ligand is bound also to the
2
carbonyl O atom in a k S,O coordination mode.
Such a structure forming a six-membered chelate-ring was
confirmed by single crystal X-ray analysis (Fig. 3). The geometry
of the Rh center is only slightly distorted from a regular
octahedron. Fourier map indicated the existence of an hydrogen
atom beside the O(2) atom, and this position was suitable to make a
hydrogen bonding with a solvating THF molecule. The apparently
shorter C(22)–O(1) distance than the C(22)–O(2) bond supports
Fig. 4 ORTEP drawing of [TpMe2RhI2(MeCN)] (9). Solvating THF
molecules are omitted for clarity. Selected bond distances (A)
and angles (deg): Rh–I(1), 2.6747(3); Rh–I(2), 2.6669(4); Rh–N(2),
2.079(3); Rh–N(4), 2.076(3); Rh–N(6), 2.055(4); Rh–N(7), 2.015(4);
I(1)–Rh–I(2), 91.31(1); I(1)–Rh–N(2), 177.1(1); I(2)–Rh–N(4), 177.28(9);
N(6)–Rh–N(7), 178.2(1).
=
that the COOH group is coordinated by the C O group. No
˚
crystallographic evidence for the SH hydrogen has been found,
and the Rh–S bond is even shorter than those in other TpMe2Rh
thiolato complexes, e.g. 1¢, 2, and 3. Such a coordination mode
of thiosalicylate monoanion has been rarely found,18 whilst the
3074 | Dalton Trans., 2010, 39, 3072–3082
This journal is
The Royal Society of Chemistry 2010
©

Reactivities toward H2
ion-pair formation between 11- and the protonated base molecule
may be suggested from the NMR chemical shifts of 11- varying
significantly as the change in the counter cation (the largest Dd =
0.15 between DMAN·H+ and Bu3NH+ salts). In contrast to these
reactions conducted in THF-d8 that complete within 30 min, those
in CD3CN proceeded much more slowly. Thus, a CD3CN solution
of 3 and DMAN (each 10 mM) converted to an equilibrium
mixture after 24 h, where the 3:11- ratio was 26 : 74. Increase in 11-
suggests that anionic 11- is more stabilized in polar CD3CN than
in THF-d8, and ion-pairing seems to be loose because the NMR
signals of 11- are not affected so much by the counter cations.
Reactions of the chalcogenolato complexes with H2 were investi-
gated using NMR. When a 10 mM solution of 1 was stirred under
H2 (1 atm) at 20 ◦C, the equilibrium mixtures shown in eqn (1) were
obtained smoothly. The ratios of 1:2:PhSH were 1 : 10 : 10 in C6D6
after 2 h and 1 : 14 : 14 in THF-d8 after 1 h. When these solutions
were left standing for more than 10 h, other minor species were
slowly formed. When they were degassed and then kept under N2,
the amounts of 2 and PhSH gradually decreased with regeneration
of 1.
The Se analogue 4 also reacted with H2 to give the hydrido–
selenolato complex [TpMe2RhH(SePh)(MeCN)] (10) and Ph-
SeH. The 1H NMR signal of the hydrido ligand in 10 appeared at
d -14.05 as a doublet (JRhH = 10.4 Hz) in C6D6, which was slightly
shielded in comparison with 2 (d -13.80, JRhH = 11.6 Hz). Other 1H
resonance patterns of 10 resembled those of 2. For the selenolato
complexes in equilibrium, however, bis(selenolato) complex 4
is predominant as demonstrated by the ratio 4:10:PhSeH of
1 : 0.60 : 0.47 in C6D6 after 2 h under the same conditions. It is
interesting to note that the equilibrium ratio 4:10:PhSeH shifts
to 1 : 0.20 : 0.13 in THF-d8 (after 2.5 h). Smaller amounts of
PhSeH than 10 might indicate the formation of other species,
but these could not be detected. Nevertheless, these mixtures
generated under H2 were converted back to 4 quantitatively under
N2 atmosphere after ~3 days at 20◦C. Isolation of 10 was hampered
because of its low concentration in equilibrium, although the
hydrido–thiolato complex 2 had been successfully crystallized
under H2.16 Reaction of the Te congener 5 with H2 did not proceed
analogously but gave a green precipitate insoluble in common
organic solvents.
The dithiolato complex 3 in solution did not show any
NMR-detectable changes under H2 atmosphere. However, in
the presence of Brønsted bases such as trialkylamines or 1,8-
bis(dimethylamino)naphthalene (DMAN), the anionic hydrido
complex [TpMe2RhH(bdt)]- (11-) was generated together with a
stoichiometric amount of the protonated base. The 1H NMR
signals of 11- contained those of two inequivalent pyrazolyl groups
in a 2 : 1 ratio, one symmetric bdt, and one hydrido ligand at d
-18.75 (d, JRhH ~ 15 Hz), indicating a structure as the one depicted
in eqn (2). Treatment of a THF-d8 solution containing 3 (10 mM)
and 1.2 equiv of DMAN with H2 (1 atm) gave an equilibrium
mixture of 3, 11-, DMAN·H+, and DMAN in a 65 : 35 : 33 : 87
ratio. When DMAN was replaced by 1.0 equiv of Bu3N, the ratio
of 3:11- changed to 41 : 59. These equilibrium ratios are considered
to be not only depending on the basicity of amines but also affected
by the stability of the ion pairs. It has been discussed that the cation
and anion formed from the reaction between a Brønsted acid and
a base are often pairing in THF solution.22 In this system, the
(2)
When Et3N was added to a THF solution of 3 un-
der H2, precipitation of an off-white solid formulated as
[Et3NH][TpMe2RhH(bdt)] ([Et3NH]11) occurred rapidly, which
exhibited the n(Rh–H) band at 2104 cm-1 together with the
n(N–H) band characteristic of a tertiary ammonium cation at
2670 cm-1 in its IR spectrum. By treating 1 with excess Et3N
in a similar manner, [Et3NH][TpMe2RhH(SPh)2] ([Et3NH]12) was
isolated as a yellow solid in moderate yield. While the n(Rh–
H) band of [Et3NH]12 appeared at 2084 cm-1, its hydrido signal
shifted to the lower field by ~3 ppm as compared to that of
[Et3NH]11. The analogous complex [TpMe2RhH(SePh)2]- (13-) was
also formed by treatment of 4 with a base under H2, but the
reaction did not proceed selectively. When a THF-d8 solution of 4
and DMAN (each 10 mM) was stirred under 1 atm H2 at 20 ◦C,
the product distribution converged after 9 h as shown in eqn (3).
Because PhSeH was observed at the initial stage (2 h) and finally
disappeared, [TpMe2Rh(SePh)3]- (14-) was formed presumably by
the reaction of PhSeH and DMAN with 4. Consumption of PhSeH
in this way probably increased the ratio of 10 in comparison with
the reaction without the base (vide supra).
These results have shown how the thiolato and selenolato
complexes studied here heterolytically split the H2 molecule into
a hydrido ligand and a protic hydrogen (Scheme 2). It is plausible
that these complexes interact with H2 at the site generated by disso-
ciation of MeCN, although the coordination equilibrium of either
MeCN or h -H2 is largely directed to the former. An intramolec-
ular shift of the H atom in h -H2 to the chalcogen atom forms a
hydrido and a thiol (or a selenol) ligands. The following substi-
tution of the chalcogenol ligand by MeCN leads to the hydrido–
chalcogenolato complexes 2 or 10, but this process hardly proceeds
on the bidentate bdt ligand. The formation of the hydrido–thiol
complex [Cp*IrH(PMe3)(HSDmp)][BArF ] (Cp* = h -C5Me5;
2
2
5
4
(3)
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3072–3082 | 3075
©

Table 1 Hydrogenation of styrene catalyzed by [TpMe2RhX1X2(MeCN)]
entry
catalyst (X1; X2)
solvent
T/◦C
time/h
yield (%)a
1
2
3
4
5
6
7
8
(SPh; SPh) (1)
(SPh; SPh) (1)
(SPh; SPh) (1)
(SPh; SPh) (1)
(SePh; SePh) (4)
(SePh; SePh) (4)
(o-S2C6H4) (3)
(o-S2C6H4) (3)
(H; SPh) (2)
THF
THF
benzene
EtOH
THF
benzene
THF
benzene
THF
THF
20
50
50
50
20
50
50
50
50
50
20
10
10
10
20
16
20
20
10
20
16
72
76
59
14
88
1
2
86
25
Scheme 2
9
10
(I; I) (9)
Dmp = 2,6-(mesityl)2C6H3; ArF = 3,5-(CF3)2C6H3) from the
Conditions: styrene (1.00 mmol), catalyst (10 mmol), solvent (5 cm3), and
reaction of the thiolato complex [Cp*Ir(PMe3)(SDmp)][BArF ]
4
H2 (1 atm).a Yields of ethylbenzene determined by GLC analyses.
with H2 has been reported.11c In our study, although the postulated
2
intermediates having the h -H2 ligand or the Rh(H)–E(H)R (E =
S, Se) moiety were not detected for any aromatic chalcogenolato
complexes, the latter structure could be seen in 7, in which the
thiolato ligands are more basic than the aliphatic ones. The other
path of H2 heterolysis is abstraction of a proton from the H2-
added complexes to leave a hydrido ligand on the metal. From
is hardly generated under these reaction conditions for 9 (entry
10). With respect to hydrogenation of styrene, heterolysis of H2 is
a necessary step in generating active species but not involved in
the catalytic cycle.
Hydrogenation of N-benzylideneaniline has been examined
under various conditions to evaluate the activity toward imines
and the results are summarized in Table 2. The activity of thiolato
complex 1 toward N-benzylideneaniline is not high and almost
similar to that toward styrene when reactions are conducted in
THF (entries 1, 2), whereas selenolato complex 4 shows high
activity even at 20 ◦C (entry 6). Interestingly, bdt complex 3
achieves hydrogenation much more rapidly at 20 ◦C (entry 8),
2
our experiments, it could not be clarified whether the h -H2
complex or the Rh(H)–E(H)R species is deprotonated. Since the
acidity of the H2 adduct is relatively low, an intermolecular action
of Brønsted bases is necessary. Trialkylamine and DMAN had
enough basicity for this process, but 2,6-lutidine and 2,4,6-colidine
were not effective at all.
To evaluate the effect of chalcogenolato ligands on cleavage of
H2, the reactivity of diiodo complex 9 toward H2 was examined,
and it turned out that no reactions occur for 9 under 1 atm H2 at
20 ◦C in the absence or presence of excess Et3N. Under the same
conditions, chalcogenolato complexes 1, 3, and 4 readily cleave
the H2 molecule. In the presence of exc◦ess Et3N, 9 was slowly
converted to [TpMe2RhHI(MeCN)] at 50 C in THF. It has been
reported that the reaction of [TpMe2RhCl2(MeOH)] with Et3N in
refluxing toluene under H2 yields [Et3NH][TpMe2RhHCl2].23 These
facts demonstrate that chalcogenolato ligands considerably reduce
the activation energy in breaking H–H bond heterolytically.
=
and hence is exhibiting high chemoselectivity toward the C N
=
bond over C C. When using 2 alone, the conversion of N-
benzylideneaniline decreases greatly (entry 12 vs. 2), indicating that
not only the Rh–H but also the S–H hydrogens are essential for
=
hydrogenating the C N bond. As predicted from the complicated
or poor reactivity to H2, 5 and 9 are not active (entries 7, 14).
In addition to a drastic change in activity associated with the
chalcogenolato ligands, a remarkable solvent effect has also been
found. Hydrogenation of N-benzylideneaniline catalyzed by 1 is
more accelerated in EtOH rather than in THF (entries 4, 5), but
Catalytic activity
Table 2 Hydrogenation of N-benzylideneaniline catalyzed by
[TpMe2RhX1X2(MeCN)]
It is well known that transition metal complexes that can heterolyze
H2 molecules often become effective catalysts for hydrogenation
entry catalyst (X1; X2)
solvent
T/◦C time/h yield (%)a
=
=
of especially polarized bonds such as C O and C N. Since some
chalcogenolato complexes investigated here can cleave H2 under
atmospheric pressure, their catalytic activities for hydrogenation
have been investigated under 1 atm H2 to probe the feature of
cleaved H atoms.
1
(SPh; SPh) (1)
(SPh; SPh) (1)
(SPh; SPh) (1)
(SPh; SPh) (1)
(SPh; SPh) (1)
(SePh; SePh) (4)
(TePh; TePh) (5)
(o-S2C6H4) (3)
(o-S2C6H4) (3)
(o-S2C6H4) (3)
(o-S2C6H4) (3)
(H; SPh) (2)
THF
THF
benzene
EtOH
EtOH
THF
THF
THF
benzene
THFd
MeCN
THF
20
50
50
20
50
20
50
20
20
20
50
50
20
50
20
10
20
20
10
6
16
1
2
21
63
5
2
3b
4
32
99
99
22
98
48c
45e
63
18
42
27
5
6
7
8
As shown in Table 1, bis(chalcogenolato) complexes 1 and 4
are found to hydrogenate styrene in moderate rates at 50 ◦C
(entries 1–6). In contrast, bdt complex 3 is essentially inactive
for hydrogenation of styrene under these conditions (entries 7, 8).
Because isolated 2 shows a slightly higher activity than 1 (entry
9
10
11b
12b
13
14b
2
10
10
20
10
(SPh; SPh; H) f(12)
(I; I) (9)
THF
THF
=
9), the active species in C C reduction is presumed to be the
hydrido complexes related to 2. The traditional mechanisms of
hydrogenation may be adopted here: styrene is coordinated to
the Rh center (probably at the site occupied by MeCN in 2) and
inserted to the Rh–H bond, and the following reaction with H2
produces ethylbenzene and the hydrido complex. The low activity
of 9 may be explained by the fact that the active hydrido complex
Conditions: N-benzylideneaniline (1.00 mmol), catalyst (10 mmol), solvent
(5 cm3), and H2 (1 atm).a Yields of N-benzylaniline determined by GLC
analyses. b No reaction at 20 ◦C. c Reached 80% after 20 h. d One mmol of
MeCN was added. e Reached 97% after 48 h. f A combination of [Et3NH]12
and an equimolar amount of [Et2OH][BF4] was used as catalyst.
3076 | Dalton Trans., 2010, 39, 3072–3082
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The Royal Society of Chemistry 2010
©

hardly proceeds in benzene (entry 3). A decrease of the reaction
rate in benzene is also observed in catalysis by 3 (entry 9). Such
solvent effect cannot be seen in hydrogenation of styrene (Table 1,
entries 2–4). The activity of 3 is also lowered by addition of
MeCN or by conducting the reaction in MeCN medium (entries
10, 11), probably because of difficulties in generating a vacant
site for activating H2. To achieve conditions without any MeCN
molecules, the preparation of a catalyst from [Et3NH]12 and an
equimolar amount of [Et2OH][BF4] has been attempted, and this
has resulted in higher conversion of N-benzylideneaniline than 1
as expected (entry 13 vs. 1).
is shifted to the imine (directly or mediated by any present
bases). The resulting anionic hydrido complex transfers H- to this
iminium cation to produce an amine. Such a reaction pathway
mediated by formation of active iminium has been proposed for
other catalysts.26,29 The lowest catalytic activity of 1 arises from the
finding that 1 is converted easily to 2, which is not included in the
catalytic cycle. Thus, when hydrogenation of N-benzylideneaniline
by 1 at 20 ◦C was monitored by NMR spectroscopy, hydrogenation
gradually slowed down as the amount of 2 increased and finally
stopped when 1 almost disappeared. Since such deactivation is
suppressed by the use of the bidentate bdt ligand, 3 is the superior
catalyst among them.30 The dependence of the catalytic rate on
solvents is in the order: benzene < THF < EtOH, probably because
a polar solvent assists in the formation of ionic intermediates. The
=
In contrast to the relatively high activity toward the C N
bond, none of the chalcogenolato complexes in this study hy-
=
drogenates the more polarized C O bonds in e.g. benzaldehyde
=
and acetophenone under 1 atm H2. Many molecular catalysts
H2-added complex is not acidic enough to protonate C O (vide
which heterolytically activate H2 have already been reported,
supra), and the anionic hydrido complex is presumably unable to
=
=
and these are generally effective in the hydrogenation of C O
reduce the non-activated C O bond.
=
=
bonds. Preferential hydrogenation of the C N bond over C O
has rarely been found regardless of the operating mechanisms.24,25
In the transfer hydrogenation using formic acid, NEt3, and the
bifunctional Ru catalyst developed by Noyori group, the imine is
>1000 times more reactive than the ketone.25a According to Casey
Catalytic activities of 7 and 8 toward various unsatu-
rated organic substrates were also examined at 1 atm, but
only the latter could hydrogenate a stoichiometric amount
of N-benzylideneaniline. The displacement of TpMe2 by other
isoelectronic co-ligands was attempted to some extent. The
hydrotris(pyrazolyl) borate (Tp) complex [TpRh(SC6H4Me-
p)2(MeCN)]15a showed a low activity for hydrogenation of N-
benzylideneaniline (1 atm, 50 ◦C, 50 h, TON = 16) and it was
and co-workers,26 the Shvo’s complex [(h -C5Ph4OH)Ru(CO)2H],
which is known to catalyze H2- and transfer-hydrogenation of
C O bonds, transfers H and H to PhCH NMe 26 times faster
than to PhCHO in a stoichiometric reaction.
Two representative mechanisms involving heterolytic activation
of H2 have been proposed for catalytic hydrogenation. One, which
is specific for metal–ligand bifunctional catalysts, is the concerted
5
27
+
-
=
=
=
=
found to be inert for the C C and C O reductions. The related
complex having a cyclopentadienyl ligand [Cp*Rh(bdt)],31 which
is known to exist in equilibrium with its dimer, has exhibited no
activity of hydrogenation. These Cp* complexes did not exhibit
any interaction with H2 under 1 atm, regardless of the amine
additives. Because the Cp* ligand is known to be more electron-
donating than the TpMe2 ligand in the case of Group 9 metal
complexes,15a,32 the Rh center of the non-ionic Cp* complex may
not be acidic enough to activate H2 heterolytically.
transfer of hydride and proton from the M(H)–E(H) moiety to
3
=
a polar C X bond (X = O, N). The monothiolate complexes
[Cp*M(PMe3)(SDmp)][BArF ] (M = Rh, Ir) recently reported by
4
Ohki, Tatsumi et al. are suggested to operate in this manner via
the active intermediates [Cp*MH(PMe3)(HSDmp)][BArF ] and
4
12b
=
=
hydrogenate both C O and C N bonds. The other route, in
which proton and hydride are transferred separately and step-
wise to the substrate, is called ionic mechanism.2,28 To explain
the features of catalysis, this mechanism seems plausible for
hydrogenation of imine by the chalcogenolato complexes in this
study. As in the cycle illustrated in Scheme 3, the chalcogenolato
complexes 1, 3, or 4 form an adduct with H2, from which H+
As a conclusion, it has been revealed that 3 is an excellent
catalyst, which can chemoselectively hydrogenate imines under
ambient temperature and pressure. As summarized in Table 3,
=
the C N bonds in various imines are efficiently hydrogenated
under ambient temperature and pressure. While the size of
the substituent on the N atom does not affect the reactivity
so much, bulkiness of the groups attached to the imine car-
bon retards the reaction. N-(2,2-dimethylpropylidene) and N-
(1-methylethylidene) derivatives were hydrogenated more slowly
than N-benzylideneamines (entries 6 and 7 vs. 1–4), but N-(1-
phenylethylidene)aniline could not be reduced even at 50 ◦C
(entry 8). The conversion of N-benzylidenemethylamine was
unexpectedly low, probably because the catalyst was deactivated
by forming stable adducts, whose presence was detected by NMR
(entry 5).
=
=
The C C or C O functions present in the substrate molecules
=
are preserved (entries 9–11). However, the C C bond conjugated
=
with the C N group was partially hydrogenated (entry 10), which
probably proceeds via 1,4-addition, because allylamine is not
susceptible to hydrogenation under these conditions. The amount
of the saturated amine became smaller when EtOH was used
=
instead of THF as a solvent. By virtue of inertness to C O bonds,
reductive amination33 can be performed by mixing an aldehyde
and a primary amine directly under hydrogenation conditions
(entry 12).
Scheme 3
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3072–3082 | 3077
©

Table 3 Hydrogenation of various imines catalyzed by 3
at dH 7.15 ppm, THF-d8 at dH 3.58, CD3CN at dH 1.93, and CDCl3
at dH 7.26 and dC 77.00). Sample solutions were made under
a nitrogen atmosphere and measured at 20 ◦C unless otherwise
specified. IR spectra were recorded on a JASCO FT/IR-420
spectrometer. Elemental analyses were done using a Perkin-Elmer
2400 series II CHN analyzer. GC and GC-MS analyses were
performed by using Shimadzu GC-14B gas chromatographs and
a GC-MS QP-5050 spectrometer equipped with a CBP 1 or CBP
10 fused silica capillary column (25 m ¥ 0.25 mm f).
entry
1
substrate
product
(yield/time)
(97%a/1 h)
2
(100%b/2 h)
(98%b/1 h)
(100%b/3 h)
(4%b,d/10 h)
(73%f/20 h)
(91%b/20 h)
3
Preparation of [TpMe2Rh(bdt)(MeCN)] (3). To a THF (50 cm3)
solution of 6 (646 mg, 1.17 mmol) was added 1,2-benzenedithiol
(135 mm3, 1.17 mmol) at 0 ◦C under a N2 atmosphere. The yellow
solution immediately turned to dark brown, and gas evolution
was observed. After stirring for 5 min, the solvent was slowly
evaporated under reduced pressure at 0 ◦C until the volume of the
solution was reduced to 10 cm3. Hexane (65 cm3) was layered on
the resulting brownish purpl◦e solution, and the mixture was left
standing for 2 weeks at -20 C. The liquid phase was discarded,
and a powdery material was removed from the deposited solid by
decantation and washing with hexane. The remaining claret crys-
tals were dried under vacuum (493 mg, 61% yield as 3·1.5THF).
The crystals thus obtained were polymorphic, and two crystal
systems containing one and two equivalents of THF molecules
were confirmed by X-ray analyses. The amount of THF solvate
molecules in bulk solid was determined by NMR measurement to
be ~1.5 equiv. Anal. Calcd for C29H41BN7O1.5RhS2: C, 50.51; H,
5.99; N, 14.22%. Found: C, 50.41; H, 6.14; N, 13.90%. 1H NMR
(C6D6): d 0.22 (s, 3H, MeCN), 2.07, 2.81 (br, 9H each, Me in
TpMe2), 5.34 (br, 1H, CH in TpMe2), 5.63 (br, 2H, CH in TpMe2),
6.96, 7.80 (dd, JHH = 6.0, 3.2 Hz, 2H each, C6H4), 1.40, 3.57 (m,
6H each, THF). 1H NMR (THF-d8, -20 ◦C): d 2.32, 2.34, 2.42 (s,
3H each, Me in Tp* and MeCN), 2.40, 2.68 (s, 6H each, Me in
TpMe2), 5.65 (s, 1H, CH in TpMe2), 5.85 (s, 2H, CH in TpMe2), 6.61,
7.05 (dd, JHH = 6.0, 3.2 Hz, 2H each, C6H4). 1H NMR (CD3CN):
d 2.20, 2.36 (br, 3H each, Me TpMe2), 2.40, 2.63 (br, 6H each, Me
TpMe2), 5.72 (s, 1H, CH in TpMe2), 5.93 (s, 2H, CH in TpMe2), 6.74,
7.11 (dd, JH–H = 5.9, 3.3 Hz, 2H each, C6H4). IR (KBr): 2530
(nB–H), 2323 (nC≡N) cm-1.
4
5c
6e
7e
8c
no reaction
9
(90%, a67%f/1 h)
10
(82%a/1 h)
(88%a/3 h)g
(15%a/1 h)
(4%a/3 h)g
11
(99%f/1 h)
(61%f/1 h)
12e
Conditions: su◦bstrate (1.00 mmol) and 3 (10 mmol) in THF (5 cm3) under
1 atm H2 at 20 C.a Determined by NMR. b Determined by GLC analyses.
c Conducted at 50 ◦C. d Imine was recovered in 90%. e Conducted with 20
mmol catalyst loading. f Isolated yield. g Ethanol was used instead of THF.
Preparation of [TpMe2RhH(SCH2C6H2Me2CH2SH-j2S,S¢)] (7).
A mixture of 6 (861 mg, 1.56 mmol), 4,5-bis(mercaptomethyl)-o-
xylene (310 mg, 1.56 mmol), and THF (10 cm3) was stirred at room
temperature under a N2 atmosphere for 17 h. A deposited yellow
solid was filtered off, washed with a small amount of THF, and
dried under vacuum to yield 7 (158 mg). Addition of hexane to
the above filtrate afforded the second crop as yellow microcrystals
(331 mg, totally 52% yield). Anal. Calcd for C25H36BN6RhS2: C,
50.18; H, 6.06; N, 14.04%. Found: C, 50.08; H, 6.17; N, 13.79%.
1H NMR (C6D6, 50 ◦C): d -15.62 (d, JRhH = 8.0 Hz, 1H, RhH),
2.05, 2.12 (s, 6H each, Me in TpMe2 or C6H2Me2), 2.25 (s, 3H, Me
in TpMe2), 2.38 (br, 6H, Me in TpMe2 or C6H2Me2), 2.82 (br, 3H,
Me in TpMe2), 3.28 (br, 1H, SH), 3.5–3.65 (m, 2H, SCH2), 3.88 (d,
JH–H = 12.4 Hz, 2H, SCH2), 5.51 (s, 2H, CH in TpMe2), 5.67 (s, 1H,
CH in TpMe2), 6.77 (br, 2H, C6H2). IR (KBr): 2512 (nB–H), 2102
(nRh–H) cm-1.
Experimental
General
All manipulations were performed using standard Schlenk
techniques under 1 atm of nitrogen or hydrogen atmosphere.
Solvents were dried by common procedures and distilled
under nitrogen before use. Complexes 1,15a 4, 5,15b and 621
were synthesized according to the literature methods. Imines
N-(benzylidene)butylamine,34
N-(2,2-dimethylpropylidene)-
aniline,35 N-(1-methylethylidene)aniline,36 and N-(1-pheyl-
ethylidene)aniline37 were prepared from the corresponding amines
and aldehydes or ketones. Some other organic compounds have
been previously reported in the preceding communication.16
Other reagents were commercially available and used as received.
NMR spectra (1H at 400 MHz and 13C at 100.5 MHz) were
recorded on a JEOL alpha-400 spectrometer, and chemical shifts
were referenced using those of residual solvent resonances (C6D6
Preparation of [TpMe2RhH(SC6H4-2-COOH-j2O,S)] (8).
Thiosalicylic acid (174 mg, 1.13 mmol) and 6 (620 mg, 1.12 mmol)
were dissolved together in THF (10 cm3) at 0 ◦C while stirring
3078 | Dalton Trans., 2010, 39, 3072–3082
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The Royal Society of Chemistry 2010
©

under N2. The resulting orange solution was filtered after 1 h,
and hexane was layered on the concentrated filtrate. After storing
at -20 ◦C for 1 week, an orange precipitate was filtered off,
washed with hexane, and dried under vacuum to give 8·THF
(556 mg). The second crop was obtained from the mother liquor
by crystallization from THF–hexane (39 mg, totally 84% yield).
Anal. Calcd for C26H36BN6O3RhS: C, 49.85; H, 5.79; N, 13.42%.
Found: C, 50.01; H, 5.94; N, 13.28%. 1H NMR (C6D6): d -13.79
(d, JRhH = 11.6 Hz, 1H, RhH), 2.08, 2.19, 2.26, 2.31, 2.46, 2.79
(s, 3H each, Me in TpMe2), 5.40, 5.61, 5.77 (s, 1H each, CH in
TpMe2), 6.60 (ddd, JHH = 8.2, 6.8, 1.2 Hz, 1H, 4- or 5-H in C6H4),
6.78 (ddd, JHH = 8.2, 6.8, 1.2 Hz, 1H, 4- or 5-H in C6H4), 7.96
saw cycles and stirred under a H2 atmosphere at 20 ◦C. A part
of the solution was periodically (30–60 min intervals) taken out
and analyzed by NMR spectroscopy until the conversion reached
a steady state. After 1 h, an equilibrium mixture of 1, 2, and PhSH
in a 1 : 14 : 14 ratio was observed. This solution was then degassed
by freeze-pump-saw cycles and kept under a N2 atmosphere for
40 h. NMR measurements of the resulting solution confirmed
that the ratio of 1, 2, and PhSH became 1 : 0.32 : 0.31, and it
changed to 1 : 0.21 : 0.21 after further 26 h. Data for the complexes
characterized by 1H NMR in these experiments are listed below.
[TpMe2RhH(SePh)(MeCN)] (10). d(C6D6) = -14.05 (d, JRhH
=
10.4 Hz, 1H, Rh–H), 0.39 (s, 3H, MeCN), 2.09, 2.15, 2.30, 2.32,
2.80, 2.86 (s, 3H each, Me in TpMe2), 5.44, 5.64, 5.86 (s, 1H each,
CH in TpMe2), 8.2–8.25 (m, 2H, o-H in Ph). d(THF-d8) = -14.70
(d, JRhH = 10.8 Hz, 1H, Rh–H), 2.319, 2.324, 2.33, 2.41, 2.60 (s,
3H each, Me in MeCN or TpMe2, other two Me signals could not
be assigned due to overlapping with those of other species), 5.62,
5.72, 5.76 (s, 1H each, CH in TpMe2), 6.8–6.9 (m, 3H, m-, and p-H
in Ph), 7.65–7.7 (m, 2H, o-H in Ph).
(dd, JHH = 8.2, 1.2 Hz, 1H, 3- or 6-H in C6H4), 8.01 (dd, JHH
=
8.2, 1.2 Hz, 1H, 3- or 6-H in C6H4), 9.13 (br, 1H, OH), 1.40, 3.57
(m, 4H each, THF). IR (KBr): 2523 (nB–H), 2057 (nRh–H) 1617
(nC=O) cm-1.
Preparation of [TpMe2RhI2(MeCN)] (9). A mixture of 6
(270 mg, 0.490 mmol) and I2 (125 mg, 0.492 mmol) in THF
(10 cm3) was stirred under a N2 atmosphere at room temperature
overnight. The resulting reddish brown solution was filtered and
concentrated to 8 cm3. Addition of hexane (40 cm3) yielded
brown crystals (three polymorphs were found), which were filtered
off and dried in vacuo (288 mg, 84% yield). Anal. Calcd for
C17H25BI2N7RhS2: C, 29.38; H, 3.63; N, 14.11%. Found: C, 29.72;
H, 3.62; N, 13.70%. 1H NMR (C6D6): d 0.60 (s, 3H, MeCN), 2.04,
2.73 (s, 6H each, Me in TpMe2), 2.05, 3.36 (s, 3H each, Me in TpMe2),
5.49 (s, 1H, CH in TpMe2), 5.58 (s, 2H, CH in TpMe2). IR (KBr):
2526 (nB–H), 2330 (nC≡N) cm-1.
[DMAN·H][TpMe2RhH(SePh)2]
([DMAN·H]13). d(THF-
d8) = -16.48 (d, JRhH = 8.8 Hz, 1H, Rh–H), 5.38 (s, 2H, CH in
TpMe2), 5.60 (s, 1H, CH in TpMe2), 6.45–6.55 (m, 6H, m-, and p-H
in Ph), 7.25 (d, JH–H = 7.6 Hz, 4H, o-H in Ph).
[DMAN·H][TpMe2Rh(SePh)3] ([DMAN·H]14). d(THF-d8) =
5.31 (s, 3H, CH in TpMe2), 6.35 (t, JHH = 7.6 Hz, 6H, m-H in
Ph), 6.48 (t, JHH = 7.6 Hz, 3H, p-H in Ph), 6.54 (d, JHH = 7.6 Hz,
6H, o-H in Ph).
Assignment of Me groups in above two complexes was
hampered by severe overlapping of both signals. Signals of
[DMAN·H]+ were as follows: d(THF-d8) = 3.25 (d, JHH = 2.8 Hz,
12H), 7.67 (t, JHH = 8.0 Hz, 2H), 8.00, 8.04 (d, JHH = 8.0 Hz, 2H
each).
Preparation of [TpMe2RhH(SPh)(MeCN)] (2). Complex 1
(69 mg, 0.11 mmol) was dissolved in THF (5 mL) under a N2
atmosphere, and the resulting solution was concentrated to 2 mL
under reduced pressure. Then, H2 gas was introduced, and the
solution was allowed to stand at room temperature for 5 h. During
this time, the color of the solution turned from red to reddish
orange. After layering hexane (18 mL), the mixture was kept at
-20 ◦C for 2 weeks. Deposited crystals were filtered off and dried
in a stream of H2. Lemon yellow prismatic crystals of 2·2THF
(47 mg, 64% yield) were manually separated from a small amount
of red microcrystals of 1. For use in the determination of the
catalytic activity, this crop was further purified by repeating the
above procedure another time. Anal. Calcd for C31H47BN7O2RhS:
C, 53.53; H, 6.81; N, 14.10. Found: C, 53.83; H, 6.74; N, 14.63%.
1H NMR (C6D6): d -13.80 (d, JRhH = 11.6 Hz, 1H, Rh–H), 0.35
(s, 3H, MeCN), 2.08, 2.15, 2.31, 2.32, 2.72, 2.84 (s, 3H each, Me in
[DMAN·H][TpMe2RhH(bdt)] ([DMAN·H]11). d(THF-d8) =
-18.75 (d, JRhH = 15.2 Hz, 1H, Rh–H), 2.12, 2.36 (s, 3H each,
Me in TpMe2), 2.31, 2.45 (s, 6H each, Me in TpMe2), 3.00 (br, 12H,
NMe2), 5.48 (s, 1H, CH in TpMe2), 5.57 (s, 2H, CH in TpMe2),
6.34, 7.04 (dd, JHH = 5.6, 3.2 Hz, 2H each, C6H4), 7.67 (t, JH–H
=
7.6 Hz, 2H, aromatic H in DMAN·H+) 7.9–8.0 (m, 4H, aromatic
H in DMAN·H+). d(CD3CN) = -18.89 (d, JRhH = 15.2 Hz, 1H,
Rh–H), 2.05, 2.38 (s, 3H each, Me in TpMe2), 2.33, 2.42 (s, 6H each,
Me in TpMe2), 5.60 (s, 1H, CH in TpMe2), 5.70 (s, 2H, CH in TpMe2),
6.45, 6.95 (dd, JHH = 6.0, 3.2 Hz, 2H each, C6H4).
[Bu3NH][TpMe2RhH(bdt)] ([Bu3NH]11). d(THF-d8) = -18.76
(d, JRhH = 14.4 Hz, 1H, Rh–H), 1.97, 2.37 (s, 3H each, Me in
TpMe2), 2.33, 2.43 (s, 6H each, Me in TpMe2), 5.56 (s, 1H, CH in
TpMe2), 5.65 (s, 2H, CH in TpMe2), 6.46, 7.04 (dd, JHH = 5.8, 3.4 Hz,
2H each, C6H4). Signals of Bu3NH+ were merged with those of free
Bu3N.
TpMe2), 5.42, 5.64, 5.86 (s, 1H each, CH in TpMe2), 6.95 (tt, JHH
=
7.2, 1.2 Hz, 1H, p-H in Ph), 7.15 (overlapping with C6HD5, m-H in
Ph), 8.15 (dd, JHH = 8.4, 1.2 Hz, 2H, o-H in Ph), 1.40, 3.57 (m, 8H
each, THF). 1H NMR (THF-d8): d -14.46 (d, JRhH = 10.8 Hz, 1H,
Rh–H), 2.20, 2.32, 2.33, 2.337, 2.342, 2.44, 2.56 (s, 3H each, Me
in TpMe2 and MeCN), 5.61, 5.73, 5.76 (s, 1H each, CH in TpMe2),
6.69 (m, 1H, p-H in Ph), 6.86 (t, JHH = 7.6 Hz, 2H, m-H in Ph),
7.55 (d, JHH = 7.6 Hz, 2H, o-H in Ph). IR (KBr): 2523 (nB–H), 2332
(nC≡N), 2067 (nRh–H) cm-1.
Preparation of [Et3NH][TpMe2RhH(bdt)] ([Et3NH]11)
To a solution of 3·1.5THF (62 mg, 0.090 mmol) in THF (5 cm3),
NEt3 (45 mL, 0.32 mmol) was added and the mixture was stirred
at room temperature under H2. The purple solution turned pale
yellow-brown within 5 min, and a off-white solid started to deposit
after 10 min. After stirring the mixture for 1 h, a solid was filtered
off, washed with a small amount of THF, and dried under vacuum
1H NMR studies under H2. A representative procedure is as
follows. Complex 1 (20 mg, 0.030 mmol) was dissolved in THF-
d8 (3.0 cm3) under a N2 atmosphere (if necessary, additives were
mixed at this point). This solution was degassed by freeze-pump-
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3072–3082 | 3079
©

(33 mg, 56% yield). Anal. Calcd for C27H43BN7RhS2: C, 50.39; H,
6.73; N, 15.24. Found: C, 50.66; H, 6.75; N, 15.23%. H NMR
program package.41 All unique data within the limits of 6◦ < 2q <
55◦ were used. The positions of the non-hydrogen atoms were
determined by Patterson methods (PATTY42) for 3 and 9 or direct
method (SHELXS9743) for 8 and subsequent Fourier synthesis
(DIRDIF9944), and they were refined with anisotropic thermal
parameters by full-matrix least-squares techniques. Hydrogen
atoms of the complex molecules in 3·2THF were found from
difference Fourier map and refined isotropically, while those in
3·THF and 9·THF were placed at the calculated positions. For
8·THF, all hydrogen atoms of complex and THF molecules were
positioned by means of difference Fourier map and isotropically
refined. Hydrogen atoms of other solvating THF molecules were
placed according to geometry except for those in the disordered
ones.
1
(CD3CN): d -18.90 (d, JRhH = 14.8 Hz, 1H, RhH), 1.16 (t, JHH
=
7.2 Hz, 9H, NCH2CH3), 2.04, 2.38 (s, 3H each, Me in TpMe2),
2.34, 2.42 (s, 6H each, Me in TpMe2), 2.98 (q, JHH = 7.2 Hz, 6H,
NCH2CH3), 5.61 (s, 1H, CH in TpMe2), 5.71 (s, 2H, CH in TpMe2),
6.47, 6.96 (dd, JHH = 5.7, 3.3 Hz, 2H each, C6H4). IR (KBr): 2670
(nN–H), 2508 (nB–H), 2104, 2072sh (nRh–H) cm-1.
Preparation of [Et3NH][TpMe2RhH(SPh)2] ([Et3NH]12)
Immediately (<5 min) after dissolving 1 (65 mg, 0.099 mmol)
in THF (4.5 cm3) under H2 at room temperature, NEt3 (42 mL,
0.30 mmol) was added to this solution. A pale orange solution was
obtained after stirring for 3 h, and it was dried up under reduced
pressure, leaving a yellow crystalline solid. This was washed with
benzene (5 cm3) and dried under vacuum (34 mg, 47% yield). Anal.
Calcd for C33H49BN7RhS2: C, 54.92; H, 6.84; N, 13.59. Found: C,
Crystal data of 3·2THF
C31H45BN7O2RhS2, M = 725.58, monoclinic, a = 10.180(2), b =
55.50; H, 6.99; N, 13.53%. 1H NMR (CD3CN): d -15.93 (d, JRhH
=
◦
3
˚
˚
22.607(4), c = 14.737(2) A, b = 91.2870(7) , U = 3390.8(9) A ,
T = 113 K, space group P21/c (no. 14), Z = 4, 35590 reflections
measured, 8065 unique (Rint = 0.029) which were used in all
calculations. The final wR(F2) was 0.123 (all data).
10.8 Hz, 1H, RhH), 1.18 (t, JHH = 7.2 Hz, 9H, NCH2CH3), 2.20,
2.35 (s, 6H each, Me in TpMe2), 2.47, 2.49 (s, 3H each, Me in TpMe2),
3.02 (q, JHH = 7.2 Hz, 6H, NCH2CH3), 5.54 (s, 2H, CH in TpMe2),
5.67 (s, 1H, CH in TpMe2), 6.50–6.55 (m, 2H, p-H in Ph), 6.61–6.66
(m, 4H, m-H in Ph), 7.01–7.04 (m, 4H, o-H in Ph). IR (KBr): 2656
(nN–H), 2519 (nB–H), 2084 (nRh–H) cm-1.
Crystal data of 3·THF
C27H37BN7ORhS2, M = 653.47, monoclinic, a = 10.248(2), b =
◦
3
˚
˚
Catalytic hydrogenation
19.925(4), c = 15.443(3) A, b = 101.7510(7) , U = 3087(1) A ,
T = 293 K, space group P21/n (no. 14), Z = 4, 24530 reflections
measured, 7041 unique (Rint = 0.023) which were used in all
calculations. The final wR(F2) was 0.111 (all data).
A standard method of the hydrogenation is as follows. The
substrate organic compound (1.00 mmol) was dissolved in THF
(5 cm3), and the solution was degassed by freeze-pump-saw cycles.
Hydrogen gas was introduced to the reaction vessel, and the
catalyst complex (0.010 mmol) was added. The solution was stirred
under 1 atm of H2, and other conditions were set as described
in Tables 1–3. The reaction mixtures were analyzed by GLC or
NMR methods after addition of internal standards or worked up
for isolation of the products. Compounds which have not been
reported in previous report16 and whose authentic samples are not
commercially available are listed below.
Crystal data of 8·THF
C26H36BN6O3RhS, M = 626.38, monoclinic, a = 10.137(2), b =
◦
3
˚
˚
18.086(3), c = 15.833(3) A, b = 102.7935(8) , U = 2831(1) A ,
T = 113 K, space group P21/c (no. 14), Z = 4, 22692 reflections
measured, 6749 unique (Rint = 0.030) which were used in all
calculations. The final wR(F2) was 0.053 (all data).
N-neopentylaniline38. A pale yellow oil was obtained in 74%
yieldby column chromatographyon silicageleluting withCH2Cl2–
MeOH (9/1). 1H NMR (CDCl3): d 0.99 (s, 9H, CH3), 2.89 (br, 2H,
NCH2), 3.62 (br, 1H, NH), 6.62 (d, JHH = 8.8 Hz, 2H, o-H in Ph),
6.66 (t, JHH = 7.4 Hz, 1H, p-H in Ph), 7.1–7.2 (m, 2H, m-H in Ph).
Crystal data of 9·THF
C21H33BI2N7ORh, M = 767.06, monoclinic, a = 7.978(2), b =
◦
3
˚
˚
13.431(3), c = 26.285(5) A, b = 97.916(3) , U = 2789.8(9) A ,
T = 113 K, space group P21/c (no. 14), Z = 4, 33333 reflections
measured, 6664 unique (Rint = 0.036) which were used in all
calculations. The final wR(F2) was 0.117 (all data).
1
13C{ H} NMR (CDCl3): d 27.7, 31.9, 55.8, 112.5, 116.8, 129.1,
149.0.
N-benzylallylamine. A colorless oil was isolated in 67% yield
by column chromatography on silica gel eluting with CH2Cl2–
MeOH (9/1). NMR data were in good agreement with the
literature values.39
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search (No. 18065005 on Priority Area “Chemistry of Concerto
Catalysis” and No. 21350033 (B)) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. We also thank
Professor Robert H. Morris for helpful discussion.
Crystallographic study of 3·2THF, 3·THF, 8·THF, and 9·THF
X-Ray diffraction studies were done on a Rigaku Mercury-
CCD diffractometer equipped with a graphite monochromatized
˚
Mo-Ka source (l = 0.71071 A) by using the CrystalClear
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