Selective synthesis of Rh5 carbonyl clusters within a polyamine dendrimer for chemoselective reduction of nitro aromatics

Article abstract of DOI:10.1039/c4cc00976b

The selective synthesis of the [Rh₅(CO)₁₅] ^- cluster within the PPI dendrimer was successfully demonstrated. The dendrimer-encapsulated [Rh₅(CO)₁₅]^- was resistant to decomposition under the catalytic reaction conditions and exhibited extremely high selectivity for the chemoselective reduction of nitro groups of various nitro aromatics with other reducible groups using CO/H₂O as a reductant. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00976b

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Selective synthesis of Rh5 carbonyl clusters within
a polyamine dendrimer for chemoselective
reduction of nitro aromatics†
Cite this: Chem. Commun., 2014,
50, 6526
Received 6th February 2014,
Accepted 23rd April 2014
Zen Maeno,a Takato Mitsudome,a Tomoo Mizugaki,a Koichiro Jitsukawaa and
Kiyotomi Kaneda*ab
DOI: 10.1039/c4cc00976b
www.rsc.org/chemcomm
The selective synthesis of the [Rh5(CO)15]ꢀ cluster within the PPI den- cluster is difficult and requires multiple steps.6 However, this
drimer was successfully demonstrated. The dendrimer-encapsulated PPI dendrimer easily encapsulated a Rh6(CO)16 precursor within
[Rh5(CO)15]ꢀ was resistant to decomposition under the catalytic reaction the internal nanovoid and selectively transformed it into the
conditions and exhibited extremely high selectivity for the chemo- [Rh5(CO)15]ꢀ cluster in one step. Furthermore, the dendrimer-
selective reduction of nitro groups of various nitro aromatics with encapsulated [Rh5(CO)15]ꢀ promoted chemoselective reduction
other reducible groups using CO/H2O as a reductant.
of aromatic nitro compounds having other reducible functional
groups with greater than 99% selectivity, leading to a straight-
Dendrimers are well-defined macromolecules with a highly forward synthesis of functionalized anilines, which are important
branched structure from the core to the surface. The physical and intermediates for dyes, pharmaceuticals, and rubber chemicals.7
chemical properties of dendrimer molecules, e.g. size, flexibility, and This is the first report of a selective synthesis of metal carbonyl
solubility, can be adjusted by changing the components of the core, clusters within dendrimers.8 The high stability of the [Rh5(CO)15]ꢀ
branch, and surface units of the dendrimer framework.1 One of the provided by the PPI dendrimer promotes selectivity for nitro groups
structural features of dendrimers is an internal nanovoid consisting that cannot be achieved by the Rh clusters reported previously.
of well-regulated branch units. Organic molecules and metal species
The PPI dendrimer possesses an internal nanovoid consisting
are easily accommodated in the internal nanovoids, which can of regularly arranged tertiary amino groups. A series of third-
allow the dendrimers to act as nanocarriers in drug delivery2a and generation PPI dendrimers (G3) with several functional groups on
bioimaging systems.2b The dendrimer nanovoids also serve as the surface were synthesized, such as a G3 with a C16 long alkyl
nanoreactors for the synthesis of unique compounds including chain (G3-C16), a G3 with a C5 short alkyl chain (G3-C5), and a G3
metal nanoclusters,3a,b metal complex assemblies,3c and quantum containing a dimethylamino surface group (G3-NMe2) (Fig. 1).11
dots.3d In addition, dendrimers encapsulating these metal species
When G3-C16 was added to toluene with H2O in the presence
provide catalytic nanoreactors with unique activity, such as the of Rh6(CO)16 at 353 K under 10 atm of CO pressure, Rh6(CO)16
condensation of substrates and size-selective activity based on the gradually dissolved and the color of the solution became
independent tunability of the core and surface units.4 In this context, reddish purple. Infrared (IR) spectrum of the solution revealed
our research group has been developing several dendritic nano- that the CO stretching peak near 2075 cmꢀ1 derived from
reactors encapsulating metal species for selective molecular Rh6(CO)16 disappeared and new peaks attributed to CO stretching
transformations, such as hydroformylations,5a allylic substitu-
tions,5b hydrogenations,5c and oxidative coupling reactions.5d
In the present study, a [Rh5(CO)15]ꢀ carbonyl cluster was
synthesized successfully within a poly(propylene imine) (PPI)
dendrimer. Conventionally, the selective synthesis of the [Rh5(CO)15]ꢀ
a Department of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan. E-mail: kaneda@cheng.es.osaka-u.ac.jp; Fax: +81 6-6850-6260;
Tel: +81 6-6850-6260
b Research Center for Solar Energy Chemistry, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Fig. 1 Structures of a series of surface-modified G3 poly(propylene imine)
(PPI) dendrimers.
† Electronic supplementary information (ESI) available: Experimental details, IR,
13C NMR, and ESI-MS spectra. See DOI: 10.1039/c4cc00976b
6526 | Chem. Commun., 2014, 50, 6526--6529
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Table 1 Reduction of o-nitrobenzaldehyde (1) using CO/H2O as a reduc-
tant catalyzed by various Rh carbonyl cluster catalystsa
Yieldb [%]
Rh carbonyl
Entry cluster catalyst
Time [h] Conv.b [%]
2
3
4
1
2
3
4
5
6
G3-C16–Rh5(CO)15
G3-C16–Rh5(CO)15 12
4
499
499
499
499
2
99
99
0
0
0
0
DMAPS–Rh
DMAPS–Rh
[PPN][Rh5(CO)15
Rh6(CO)16
4
12
4
77 10 12
60 15
2
—
0
0
—
]
0
—
4
0
a
Reaction conditions: Rh carbonyl cluster catalyst (Rh: 60 mmol), 1
Fig. 2 IR spectrum of the reaction mixture of Rh6(CO)16 and G3-C16
{[PPN][Rh5(CO)15], IR nCO: 2045, 2010, 1868, 1838, 1785 cmꢀ1; see ref. 6a}.
b
(1.0 mmol), toluene (2 mL), H2O (0.72 mL), CO (10 atm). Determined
by GC internal standard technique.
appeared at 2044, 2010, 1870, 1840, and 1785 cmꢀ1 (Fig. 2).12,13 control experiment using benzaldehyde (5) as a starting mate-
These peaks are consistent with a [Rh5(CO)15]ꢀ cluster6 (referred rial under the same reaction conditions, which resulted in the
to as G3-C16–Rh5(CO)15). Besides, this spectrum showed no complete recovery of 5 after the treatment.
peaks derived from Rh(I)–carbonyl complexes around 2100 and
To examine the catalytic activity of G3-C16–Rh5(CO)15, it was
1993 cm
.
ꢀ1 13 The negative ESI-MS spectrum of G3-C16–Rh5(CO)15 compared with that of other Rh clusters, such as dimethylami-
also contained a signal at m/z = 934, which corresponds nated polystyrene-bound [Rh6H(CO)15]ꢀ (DMAPS–Rh)18 and
to [Rh5(CO)15]ꢀ (Fig. S3, ESI†). The 13C NMR spectrum of [PPN][Rh5(CO)15] (PPN = (Ph3P)2N+),6a which were synthesized
G3-C16–Rh5(CO)15 contained a signal for the a-carbon atoms of according to previously reported procedures. Chemoselective
inner tertiary amino groups of G3-C16 upfield from that of free reduction of 1 to 2 was conducted using these Rh clusters. In
G3-C16 (Fig. S2, ESI†), suggesting protonation of the internal contrast to G3-C16–Rh5(CO)15, the use of DMAPS–Rh resulted in
tertiary amino groups of G3-C16.14 In addition, DLS measurements non-selective production of 2 with 77% selectivity along with
confirmed that G3-C16–Rh5(CO)15 was a unimolecular micelle the formation of 3 and 4 (entry 3). Extending the reaction time
with an average diameter of 6.2 ꢁ 1.2 nm.15 These combined further decreased selectivity of 2 (entry 4). The [PPN][Rh5(CO)15]
results strongly support the formation of the [Rh5(CO)15]ꢀ decomposed during the reduction and possessed extremely low
cluster within the nanovoid of G3-C16 and stabilization through catalytic activity (entry 5). In addition, Rh6(CO)16, which is a
electrostatic interactions between the [Rh5(CO)15]ꢀ anion and a precursor to G3-C16–Rh5(CO)15, did not exhibit any catalytic
quaternary ammonium cation of the branch units.
activity (entry 6). These results demonstrate that G3-C16–Rh5(CO)15
In contrast, synthesis attempted using G3-C5 with a short acts as an efficient catalytic nanoreactor for the selective trans-
alkyl chain or G3-NMe2 without an alkyl chain failed to yield the formation of 1 into 2.
[Rh5(CO)15]ꢀ cluster; instead, Rh carbonyl species such as
With the G3-C16–Rh5(CO)15 catalyst in hand, the chemo-
[Rh12(CO)30]2ꢀ and [Rh13H2(CO)24]3ꢀ were formed together with selective reduction of functionalized aromatic nitro compounds
unidentified carbonyl species (Fig. S1, ESI†). These phenomena was investigated extensively. The results are summarized in
demonstrate that G3-C16 plays a key role in selective synthesis of Table 2. The G3-C16–Rh5(CO)15 was applicable to a wide range
the [Rh5(CO)15]ꢀ cluster. Adding C16 alkyl chains to the surface of functionalized nitro aromatics. Aromatic nitro compounds
of the G3 dendrimer enables the internal nanovoid to act as a containing other carbonyl groups such as ketones, esters, and
suitable basic nanoreactor for the transformation of Rh6(CO)16 amides were converted to the corresponding anilines in excellent
into [Rh5(CO)15]ꢀ.13,16
yields (entries 2–4). The sulfonamide, methyl, halogen, cyano,
Next, the catalytic performance of G3-C16–Rh5(CO)15 was inves- and ether groups were resistant to reaction during the reduction
tigated in the chemoselective reduction of o-nitrobenzaldehyde (entries 5–10). 3-Nitrostyrene also was selectively reduced to
(1) to o-aminobenzaldehyde (2) using CO/H2O as a reductant 3-aminostyrene in 98% yield while the olefinic moiety remained
(Table 1).17 Interestingly, 1 was converted successfully to the intact (entry 11). Furthermore, G3-C16–Rh5(CO)15 was capable of
desired product 2 in 99% yield in 4 h without the occurrence of working under the gram-scale reactions of o-nitrobenzaldehyde
any side reactions, such as reduction of 2 to o-aminobenzyl (1) and 20-nitroacetophenone (Scheme 1). The excellent chemo-
alcohol (3) or reductive cyclization of 1 to 2,1-benzoxazole (4) selectivity for nitro groups over both formyl and olefinic groups
(entry 1). Even upon a prolonged reaction time after full indicated that the reduction path might involve a Rh-nitrene
conversion of 1, the formyl group of 2 was completely retained species formed through deoxygenation of nitro groups by a
(entry 2). These results demonstrate that G3-C16–Rh5(CO)15 Rh–CO species.19,20 The original [Rh5(CO)15]ꢀ remained unchanged
actively reduces the aromatic nitro group, but is inactive toward during the reductions, which was confirmed by IR spectroscopy,21
reduction of the formyl group. This fact is also supported by the and indicated that the [Rh5(CO)15]ꢀ cluster was strongly stabilized
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Table 2 Substrate scope of reduction of aromatic nitro compounds using
G3-C16–Rh5(CO)15
2 (a) S. Mignani, S. E. Kazzouli, M. Bousmina and J.-P. Majoral, Adv.
Drug Delivery Rev., 2013, 65, 1316; (b) Y. Tsai, C.-C. Hu, C.-C. Chu
and T. Imae, Biomacromolecules, 2011, 12, 4283.
a
3 (a) V. S. Myers, M. G. Weir, E. V. Carino, D. F. Yancey, S. Pande and
R. M. Crooks, Chem. Sci., 2011, 2, 1632; (b) A. Siani, O. S. Alexeev,
D. S. Deutsch, J. R. Monnier, P. T. Fanson, H. Hirata, S. Matsumoto,
C. T. Williams and M. D. Amiridis, J. Catal., 2009, 266, 331;
(c) K. Yamamoto and K. Takanashi, Polymer, 2008, 49, 4033;
(d) B. I. Lemon and R. M. Crooks, J. Am. Chem. Soc., 2000,
122, 12886.
4 (a) A. Ouali and A.-M. Caminade, in Dendrimers: towards catalytic,
material and biomedical uses, ed. A.-M. Caminade, C.-O. Turrin,
R. Laurent, A. Ouali and B. Delavaux-Nicot, John Wiley & Sons,
Chichester, 2011, ch. 4, pp. 183–195; (b) K. Kirkorian, A. Ellis and
L. J. Twyman, Chem. Soc. Rev., 2012, 41, 6138; (c) D. Wang and
D. Astruc, Coord. Chem. Rev., 2013, 257, 2317.
Entry
Substrate
Time [h]
Conv.b [%]
Yieldb [%]
1
2
2-CHO
4
6
10
10
8
6
4
4
16
6
499
499
499
499
98
499
499
499
499
499
98
99
99
99
99
98
99
99
99
99
99
98
2-COCH3
2-COOCH3
2-CONH2
2-SO2NH2
2-CH3
4-Cl
4-Br
4-CN
3-OCH3
3-CHQCH2
3
4c
5c
6
7
8
9
5 (a) T. Mizugaki, Y. Miyauchi, M. Murata, K. Ebitani and K. Kaneda,
Chem. Lett., 2005, 34, 286; (b) M. Ooe, M. Murata, T. Mizugaki,
K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2004, 126, 1604;
(c) M. Ooe, M. Murata, T. Mizugaki, K. Ebitani and K. Kaneda, Nano
Lett., 2002, 2, 999; (d) Z. Maeno, M. Okao, T. Mitsudome,
T. Mizugaki, K. ꢀJitsukawa and K. Kaneda, RSC Adv., 2013, 3, 9662.
10
11c
24
a
Reaction conditions: Rh6(CO)16 (10 mmol), G3-C16 (12 mmol), substrate
b
(1.0 mmol), toluene (2 mL), H2O (0.72 mL), 80 1C, CO (10 atm). Deter-
mined by GC using internal standard technique. Substrate (0.5 mmol).
c
6 The [Rh5(CO)15
] clusters were prepared through multi-step synth-
eses. See: (a) A. Fumagalli, T. F. Koetzle, F. Takusagawa, P. Chini,
S. Martinengo and B. T. Heaton, J. Am. Chem. Soc., 1980, 102, 1740;
(b) S. Kawi and B. C. Gates, Inorg. Chem., 1994, 33, 503.
7 (a) G. W. Kabalka and R. S. Verma, in: Comprehensive Organic
Synthesis, ed. B. M. Trost and I. Fleming, Oxford, Pergamon,
´
1st edn, 1992, vol. 8, p. 363; (b) M. Stratakis and H. Garcıa, Chem.
Rev., 2012, 112, 4469; (c) H.-U. Blaser, H. Steiner and M. Studer,
ChemCatChem, 2009, 1, 210.
8 Although studies of dendrimer-bound metal carbonyl clusters have
been reported,9,10 this is the first example of the synthesis of a metal
carbonyl cluster within the internal nanovoid of a dendrimer.
9 For metal carbonyl clusters incorporated into dendrimer frameworks,
see: (a) A. Albinati, P. Leoni, L. Marchetti and S. Rizzato, Angew.
Chem., Int. Ed., 2003, 42, 5990; (b) B. J. Lear and C. P. Kubiak, Inorg.
Chem., 2006, 45, 7041; (c) L. Viau, A. C. Willis and M. G. Humphrey,
J. Organomet. Chem., 2007, 692, 2086.
10 For metal carbonyl clusters highly dispersed in dendrimers, see:
(a) J. Geng, H. Li, W. T. S. Huck and B. F. G. Johnson, Chem.
Commun., 2004, 2122; (b) E. C. Constable, D. Gusmeroli,
C. E. Housecroft, M. Neuburger and S. Schaffner, Polyhedron,
2006, 25, 421; (c) J. R. Aranzaes, C. Belin and D. Astruc, Angew.
Chem., Int. Ed., 2006, 45, 132; (d) A. Nievas, M. Medel, E. Hernandez,
E. Delgado, A. Martin, C. M. Casado and B. Alonso, Organometallics,
2010, 29, 4291.
11 S. Stevelmans, J. C. M. van Hest, J. F. G. A. Jansen, D. A. F. J. van
Boxtel, E. M. M. de Brabander-van den Berg and E. W. Meijer, J. Am.
Chem. Soc., 1996, 118, 7398.
12 Details of IR spectral measurements are in the ESI†.
13 Rh6(CO)16 disproportionates in pyridine (py) solvent into [Rh5(CO)13(py)2]ꢀ
and cis-[Rh(CO)2(py)2]+. The resulting cis-[Rh(CO)2(py)2]+ is reduced
by CO/H2O into [(py)2H][Rh5(CO)13(py)2]. See: G. Fachinetti and
T. Funaioli, J. Organomet. Chem., 1993, 460, C34.
Scheme 1 Gram-scale reactions of G3-C16–Rh5(CO)15-catalyzed chemo-
selective reduction of 1 and 20-nitroacetophenone.
within G3-C16 through ionic interactions between the [Rh5(CO)15]ꢀ
cluster anion and the quaternary ammonium cation of the
branch units.
In conclusion, the selective synthesis of a [Rh5(CO)15]ꢀ cluster
within a PPI dendrimer was accomplished and this dendrimer-
encapsulated [Rh5(CO)15]ꢀ promoted highly chemoselective
reduction of nitro aromatics. The G3-C16 PPI dendrimer not only
acted as an efficient nanoreactor for the selective synthesis of the
[Rh5(CO)15]ꢀ cluster in one step, but also transformed into a catalytic
nanoreactor by encapsulation of the [Rh5(CO)15]ꢀ cluster within the
nanovoid. The G3-C16-encapsulated [Rh5(CO)15]ꢀ promoted chemo-
selective reduction of aromatic nitro groups containing reducible
functional groups. The G3-C16 can function as a basic nanoreactor
for direct transformation of Rh6(CO)16 to [Rh5(CO)15]ꢀ, a cation
stabilizer for the [Rh5(CO)15]ꢀ anion generated, and a macroligand
that suppresses decomposition of the [Rh5(CO)15]ꢀ carbonyl cluster
during catalytic reactions.
14 J. B. Stothers, in Carbon-13 NMR Spectroscopy, Organic Chemistry,
ed. A. L. Blomquist and H. Wasserman, Academic Press, London,
1972, ch. 5, vol. 24, p. 153.
15 Dynamic light scattering (DLS) measurements revealed that G3-C16
–
Rh5(CO)15 was a unimolecular micelle with an average diameter of
6.2 ꢁ 1.2 nm, which is similar to the diameter of the original G3-C16
in toluene (6.0 ꢁ 0.9 nm). In contrast, the diameter of the vesicle
consisting of G3-C16 was 33–200 nm. See: A. P. H. J. Schenning,
´
C. Elissen-Roman, J.-W. Weener, M. W. P. L. Baars, S. J. van der
Gaast and E. W. Meijer, J. Am. Chem. Soc., 1998, 120, 8199.
This study was supported by JSPS KAKENHI (25630368 and
24246129).
16 The internal nanovoids of G3-C5 and G3-NMe2 are less confined
compared to G3-C16, in which [Rh5(CO)15
resulting in the aggregation into higher nuclearity Rh clusters such
]
ꢀ might not be stabilized,
2ꢀ
3ꢀ
as [Rh12(CO)30
]
and [Rh13H2(CO)24] .
17 Only two catalytic systems have been reported for the quantitative
chemoselective reduction of 1 to 2 (495% yield). See: (a) S. Fu¨ldner,
P. Pohla, H. Bartling, S. Dankesreiter, R. Stadler, M. Gruber, A. Pfitzner
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6528 | Chem. Commun., 2014, 50, 6526--6529
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.
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