Solution-phase assembly of a rhodium complex via specific hydrogen bonding between barbituric acid and triaminopyrimidine moieties

Article abstract of DOI:10.1246/cl.2010.1166

Solution-phase assembly of a Rh complex using specific hydrogen bonding between BA and TAP moieties was achieved by tuning the substituent on TAP to control the solubility of the assembly.

Full text of DOI:10.1246/cl.2010.1166

doi:10.1246/cl.2010.1166
1166
Published on the web October 5, 2010
Solution-phase Assembly of a Rhodium Complex via Specific Hydrogen Bonding
between Barbituric Acid and Triaminopyrimidine Moieties
Masataka Kondo,1 Takuya Kochi,1 Mitsuo Sato,1 Aki Kitajima,2 and Fumitoshi Kakiuchi*1
1Department of Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522
2Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
(Received August 18, 2010; CL-100716; E-mail: kakiuchi@chem.keio.ac.jp)
R
N
R
N
H
H
N
H
N
H
N
Solution-phase assembly of a Rh complex using specific
N
=
Rh
H
H
N
H
N
H
N
hydrogen bonding between BA and TAP moieties was achieved
by tuning the substituent on TAP to control the solubility of the
assembly.
O
O
N
H
N
H
N
H
N
H
H
O
H
O
H
O
H
O
N
N
O
Rh
OC
PPh3
Rh
Rh
Great efforts have been devoted to developing novel
methods for the assembly of transition-metal complexes,
because of their intriguing electronic and photochemical proper-
ties1 as well as unique catalytic activities.2 Use of noncovalent
bonds offers convenient strategies to form the assembly
spontaneously by the components.3 Hydrogen bonding has
received particular attention, due to its relatively high strength
and directionality4 and has been applied for assembly of metal
complexes both in solid and solution states.
Figure 1. A possible structure of assembly of Rh complexes via
hydrogen bonding between BA and TAP.9
OH
O
O
PPh3
CO
Rh
Rh
N
N
N
OH
a-e
CO
CO
f
g
N
(CH2)5
(CH2)5
(CH2)5
O
O
O
O
O
O
Melamine (MEL) and 2,4,6-triaminopyrimidine (TAP) are
known to self-assemble with barbituric acid (BA) and cyanuric
acid (CA) in alternative fashion by specific hydrogen bonding to
form cyclic rosette or linear tape structures,5 and several studies
of self-assembly of metal complexes covalently linked to these
molecules have been reported. Solution-phase assembly of
complexes have been achieved by using MEL and BA to form
cyclic rosette.6 To the best of our knowledge, however, TAP has
not been used to assemble metal complexes, though the
properties of TAP can be easily tuned without affecting the
hydrogen-bonding scaffolds by introducing substituents at the 5-
position.7,8
Herein we report that the solution-phase assembly of a Rh
complex was achieved using specific hydrogen bonding between
BA and TAP moieties (Figure 1). A new type of TAP derivative,
which serves to solubilize a poorly-soluble assembly formed by
hydrogen bonding, was designed, synthesized, and used for the
assembly formation.
A Rh complex bearing a BA moiety was prepared as shown
in Scheme 1. 2-Methyl-8-hydroxyquinoline (1) was converted to
a ligand bearing a BA moiety (2) in five steps.9 Reaction of
ligand 2 with Rh(acac)(CO)2 provided dicarbonyl complex 3 in
95% yield.10 Because complex 3 has relatively low solubility in
most of the common organic solvents, one of the CO ligands of
3 was exchanged with PPh3 to form complex 4, which is soluble
in organic solvents.
Formation of solution-phase assembly of complex 4 with
TAP derivatives was then examined. When TAP derivatives with
a butyl (5) or decyl (6) group (Figure 2) was mixed with 4 in
nonpolar solvents, the assembly precipitated out from the
solution. Increase of the number of alkyl chains using a TAP
derivative bearing a dialkoxyphenyl group (7) led to the
formation of assembly with higher solubility in halogenated
solvents. However, precipitation in dichloroethane started at
1
HN
NH
HN
NH
HN
NH
O
O
O
2
3
4
Scheme 1. Preparation of Rh complexes bearing a BA moiety.
Reagents and conditions: (a) MOMCl, NaH, DMF, quant; (b) LDA,
Br(CH2)4Br, THF, 60%; (c) CH3CH(CO2Et)2, NaH, DMF, 97%; (d)
urea, NaH, DMF; (e) 0.9 M HCl in H2O/MeOH, 75% (2 steps); (f)
Rh(acac)(CO)2, CH2Cl2, 95%; (g) PPh3, CH2Cl2, 65%.
NH2
N
N
5: R = C4H10 7: R =
6: R = C10H22
R' = C6H13
OR'
H2N
NH2
R'O
R
Figure 2. TAP derivatives with various substituents.
30 mM and vapor pressure osmometer (VPO) measurement to
estimate the degree of assembly was difficult to perform.
To further increase the solubility of the BA/TAP assembly,
a TAP derivative with an extended alkyl tether (10) was prepared
(Scheme 2). Methyl 3,5-dihydroxybenzoate (8) was transformed
to aldehyde 9 in five steps.9 Condensation with malononitrile,
followed by reduction and reaction with guanidine hydro-
chloride provided TAP derivative 10.11 As expected, the
assembly of 4 and 10 showed high solubility in halogenated
solvents, and no precipitate was observed at 100 mM, which
enabled the VPO measurements12 to assess the molecular weight
of the assembly.13
VPO measurements of complex 4, TAP derivative 10, and
their equimolar mixture in dichloroethane were conducted at
various concentrations to calculate Mn. Solutions of 4 (FW 748)
and 10 (FW 488) showed gradual increase in Mn along with the
concentration, which is thought to be caused by self-association
of each compound (Mn = 1200 and 620 at 40 mM, respectively).
Chem. Lett. 2010, 39, 1166-1167
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1167
NH2
N
In conclusion, solution-phase assembly of a Rh complex
was achieved using specific hydrogen bonding between BA and
TAP moieties. By tuning the substituent on TAP, the solubility
of the formed assembly in organic solvents was greatly
enhanced. The VPO measurement revealed that ca. 2:2 assembly
of Rh complex 4 and TAP derivative 10 was formed in
dichloroethane. Application of the strategy to self-assemble
other transition-metal complexes and examination of their
catalytic activities are in progress.
CHO
(CH2)3
O
N
COOMe
H2N
NH2
a-e
f-h
(CH2)4
O
HO
OH
8
R'O
OR'
9
R' = C6H13
R'O
OR'
10
Scheme 2. Preparation of TAP derivative 10. Reagents and
conditions: (a) C6H13Br, K2CO3, CH3CN, 87%; (b) LiAlH4, THF,
93%; (c) CBr4, PPh3, CH2Cl2, quant; (d) 1,4-butanediol, Ag2O,
CH2Cl2, 69%; (e) SO3¢py, Et3N, DMSO, CH2Cl2, 79%; (f)
CH2(CN)2, basic Al2O3, toluene, 91%; (g) NaBH4, i-PrOH, 71%;
(h) guanidine hydrochloride, EtONa, EtOH, 94%.
F.K. gratefully acknowledges financial support by a Grant
for Private Education Institute Aid from MEXT.
References and Notes
1
a) F. Würthner, C.-C. You, C. R. Saha-Möller, Chem. Soc. Rev. 2004,
33, 133. b) M. Ruben, J. Rojo, F. J. Romero-Salguero, L. H.
Uppadine, J.-M. Lehn, Angew. Chem., Int. Ed. 2004, 43, 3644.
N. Madhavan, C. W. Jones, M. Weck, Acc. Chem. Res. 2008, 41,
1153, and references cited therein.
a) S. Leininger, B. Olenyuk, P. J. Stang, Chem. Rev. 2000, 100, 853.
b) G. F. Swiegers, T. J. Malefetse, Chem. Rev. 2000, 100, 3483. c)
Transition Metals in Supramolecular Chemistry, Perspectives in
Supramolecular Chemistry, ed. by J.-P. Sauvage, Wiley, Chichester,
1999, Vol. 5. d) D. S. Lawrence, T. Jiang, M. Levett, Chem. Rev.
1995, 95, 2229.
O
2
3
N
N
O
(CH2)5
O
Rh
OC
11
PPh3
Figure 3. A Rh complex bearing a dimethylglutarimide moiety.
4
5
a) L. J. Prins, D. N. Reinhoudt, P. Timmerman, Angew. Chem., Int.
Ed. 2001, 40, 2382. b) D. Braga, F. Grepioni, G. R. Desiraju, Chem.
Rev. 1998, 98, 1375.
a) J.-M. Lehn, M. Mascal, A. DeCian, J. Fischer, J. Chem. Soc.,
Chem. Commun. 1990, 479. b) G. M. Whitesides, E. E. Simanek, J. P.
Mathias, C. T. Seto, D. N. Chin, M. Mammen, D. M. Gordon, Acc.
Chem. Res. 1995, 28, 37, and references cited therein.
6
a) C. M. Drain, K. C. Russell, J.-M. Lehn, Chem. Commun. 1996,
337. b) W. T. S. Huck, R. Hulst, P. Timmerman, F. C. J. M.
van Veggel, D. N. Reinhoudt, Angew. Chem., Int. Ed. Engl. 1997, 36,
1006. c) M. C. Calama, P. Timmerman, D. N. Reinhoudt, Angew.
Chem., Int. Ed. 2000, 39, 755. d) H.-J. van Manen, V. Paraschiv, J. J.
García-López, H. Schönherr, S. Zapotoczny, G. J. Vancso, M. Crego-
Calama, D. N. Reinhoudt, Nano Lett. 2004, 4, 441. e) S. Vázquez-
Campos, M. Péter, M. Dong, S. Xu, W. Xu, H. Gersen, T. R.
Linderoth, H. Schönherr, F. Besenbacher, M. Crego-Calama, D. N.
Reinhoudt, Langmuir 2007, 23, 10294.
Figure 4. 1H NMR spectra of 1:1 mixture of complex 4 and TAP
derivative 10 at (a) 10, (b) 20, (c) 40, and (d) 67 mM.
7
8
Rh complexes linked to BA moieties have been used for catalysis in
solution, but the assemblies with TAP were formed as solids to
recycle the catalysts: a) J. H. Yoon, Y. J. Park, J. H. Lee, J. Yoo, C.-H.
Jun, Org. Lett. 2005, 7, 2889. b) D.-W. Kim, S.-G. Lim, C.-H. Jun,
Org. Lett. 2006, 8, 2937.
a) R. Ahuja, P.-L. Caruso, D. Möbius, W. Paulus, H. Ringsdorf, G.
Wildburg, Angew. Chem., Int. Ed. Engl. 1993, 32, 1033. b) K.
Hanabusa, T. Miki, Y. Taguchi, T. Koyama, H. Shirai, J. Chem. Soc.,
Chem. Commun. 1993, 1382. c) T. M. Bohanon, P.-L. Caruso, S.
Denzinger, R. Fink, D. Möbius, W. Paulus, J. A. Preece, H.
Ringsdorf, D. Schollmeyer, Langmuir 1999, 15, 174. d) F.
Rakotondradany, A. Palmer, V. Toader, B. Chen, M. A. Whitehead,
H. F. Sleiman, Chem. Commun. 2005, 5441.
On the other hand, an Mn of 2400 was observed for an equimolar
solution of 4 and 10 at 40 mM, indicating ca. 2:2 assembly
formation (FW 2470) in the dichloroethane solution.9
In order to examine the effect of the specific hydrogen
bonding in assembly formation, a Rh complex bearing a 3,3-
dimethylglutarimide moiety (11) was examined (Figure 3). VPO
measurements showed that the Mn of complex 11 (FW 747) and
the 1:1 mixture of 10 and 11 at 40 mM was 840 and 730,
respectively and suggested that the assembly formation of 4 and
10 was mediated by the BA moiety.
NMR studies of the 1:1 mixture of 4 and 10 in CD2Cl2 were
also performed at various concentrations (Figure 4). The chemi-
cal shift of the N-H protons of 10 was observed at 6.0 ppm at
10 mM, and the N-H signal shifted downfield as the concen-
tration of the mixture increased. This suggests that the TAP
moiety of 10 was involved in the assembly formation and
hydrogen bonding between the BA of 4 and the TAP of 10
mediated the self-assembly.
The ability of TAP 10 to dissolve the assembly was also
shown by dissolution of an equimolar mixture of 3 and 10 in
dichloroethane to give a 10 mM solution upon heating and then
cooling to rt, though without 10, a significant amount of 3
remained as a precipitate after the same protocol.
9
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
10 Rh complexes of 2 were prepared according to the reported method
for the syntheses of 8-quinolinolato Rh complexes: P. Lahuerta, M.
Sanau, L. A. Oro, D. Carmona, Synth. React. Inorg. Met.-Org. Chem.
1986, 16, 301.
11 P. B. Russell, G. H. Hitchings, J. Am. Chem. Soc. 1952, 74, 3443.
12 a) T. N. Solie, Methods Enzymol. 1972, 26, 50. b) E. E. Schrier,
J. Chem. Educ. 1968, 45, 176.
13 Determination of the degree of the assembly formed by BA and TAP
or related compounds: a) A. G. Bielejewska, C. E. Marjo, L. J. Prins,
P. Timmerman, F. de Jong, D. N. Reinhoudt, J. Am. Chem. Soc. 2001,
123, 7518. b) E. A. Archer, M. J. Krische, J. Am. Chem. Soc. 2002,
124, 5074. c) H. Gong, M. J. Krische, J. Am. Chem. Soc. 2005, 127,
1719. See also refs. 5b and 6a.
Chem. Lett. 2010, 39, 1166-1167
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/