Synthesis and characterization of NH-triazole-bound rhodium complexes: Substituted-group-controlled regioselective coordination

Article abstract of DOI:10.1021/om8011163

4,5-Disubstituted NH-1,2,3-triazoles (TRIA) were used to coordinate with Rh metal cation, forming a new class of triazole-bridged [Rh(COD)(TRIA)]2 complexes in near- quantitative yields. X-ray crystallography studies revealed that the C-4-substituted grou

Full text of DOI:10.1021/om8011163

2352
Organometallics 2009, 28, 2352–2355
Synthesis and Characterization of NH-triazole-Bound Rhodium(I)
Complexes: Substituted-Group-Controlled Regioselective
Coordination
Haifeng Duan, Sujata Sengupta, Jeffrey L. Petersen, and Xiaodong Shi*
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506
ReceiVed NoVember 23, 2008
referred to as “click chemistry”) allows the formation of 1,4-
substituted triazoles.8 Since then, the importance of these
compounds has been continually demonstrated in research fields
as diverse as material science,9 chemical biology10 and medicinal
chemistry.11 Surprisingly, to date, there are still very few studies
regarding the application of 1,2,3-triazole compounds in metal
coordination. One recent example is the NCN-pincer Pd and Pt
coordination with 1,4-disubstituted triazole complexes reported
by van Koten, Gebbink, and co-workers.12 Meanwhile, the
synthesis of triazole-based P-N ligands, the clickphines, was
recently reported and their Pd complexes were applied as
effective catalysts for allylic alkylation.13 Overall, among the
few catalytic systems reported in the literature, the N-substituted
triazoles were dominant, due to the well-developed click
chemistry in the synthesis of these ligands. The NH-triazoles,
which have been reported as good binding ligands even in
neutral form,14 on the other hand, have been much less studied.
To the best of our knowledge, NH-triazoles have not been
appliled to catalytic systems, nor has any crystal structures of
a NH-triazole-bound transition-metal complex been reported so
far. In this paper, we report the crystal structure of triazole-
Summary: 4,5-Disubstituted NH-1,2,3-triazoles (TRIA) were
used to coordinate with Rh(I) metal cation, forming a new class
of triazole-bridged [Rh(COD)(TRIA)]2 complexes in near-
quantitatiVe yields. X-ray crystallography studies reVealed that
the C-4-substituted groups effectiVely controlled the triazole
binding sites, resulting in different coordination patterns. In
addition, these air- and moisture-stable [Rh(COD)(TRIA)]2
complexes showed effectiVe catalytic properties in Pauson-Khand
reactions, with superior stability.
Advances in transition-metal catalysis, a primary approach
in modern synthesis, are strongly correlated with a fundamental
understanding of the ligand-metal coordination.1 In addition
to the unique chemical properties of individual metal elements,
binding ligands are crucial in determining the overall reactivity
of complexes.2 Numerous efforts have been put into the
development/discovery of novel metal binding ligands in
adjusting the chemical and stereo selectivity of the metal
complexes.3 Some successful examples, such as 2-(diphe-
nylphosphino)-1-(2-(diphenylphosphino)naphthalen-1-yl)naph-
thalene (BINAP)4 and salen,5 have been extensively applied in
academic research and industrial synthesis. It has been continu-
ously demonstrated in the literature that ligands with unique
functionality can significantly influence the reactivity of the
transition-metal complexes/catalysts. Therefore, effective new
catalytic systems are highly desirable.
(8) For reviews see: (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.;
Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599. (b) Kolb,
H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40,
2004–2021. (c) Bock, V. D.; Hiemstra, H.; Van Maarseveen, J. H. Eur. J.
Org. Chem. 2006, 51–68. (d) Moses, J. E.; Moorhouse, A. D. Chem. Soc.
ReV. 2007, 36, 1249–1262.
(9) (a) Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel,
A.; Voit, B.; Pyun, J.; Fre´chet, J. M. J.; Sharpless, K. B.; Fokin, V. V.
Angew. Chem., Int. Ed. 2004, 43, 3928–3932. (b) Aucagne, V.; Ha¨nni, K. D.;
Leigh, D. A.; Lusby, P. J.; Walker, D. B. J. Am. Chem. Soc. 2006, 128,
2186–2187. (c) Ye, C. F.; Gard, G. L.; Winter, R. W.; Syvret, R. G.;
Twamley, B.; Shreeve, J. M. Org. Lett. 2007, 9, 3841–3844. (d) Liu, Q. C.;
Zhao, P.; Chen, Y. M. J. Polym. Sci., Part A: Polym. Chem. 2007, 45,
3330–3341.
Having a strong dipole moment, the 1,2,3-triazoles are
considered as potential ligands in coordination with transition
metals.6 However, few studies have been reported regarding
triazole-metal binding before 2000 due to the limited avail-
ability of functional triazole derivatives.7 The discovery of Cu-
catalyzed azide alkyne 1,3-dipolar cycloaddition (CuAAC; also
* To whom correspondence should be addressed. E-mail: Xiaodong.Shi@
mail.wvu.edu.
(1) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; Wiley-Interscience: New York, 2005; (b) Hegedus, L. S.
Transition Metals in the Synthesis of Complex Organic Molecules, 2nd ed.;
University Science Books: Mill Valley, CA, 1999.
(2) Fu, G. C. Acc. Chem. Res. 2006, 39, 853–860.
(3) Shimizu, H.; Nagasaki, I.; Saito, T. Tetrahedron 2005, 61, 5405–
5432.
(4) For recent reviews: (a) Shimizu, H.; Nagasaki, I.; Matsumura, K.;
Sayo, N.; Saito, T. Acc. Chem. Res. 2007, 40, 1385–1393. (b) Berthod, M.;
Mignani, G.; Woodward, G.; Lemaire, M. Chem. ReV. 2005, 105, 1801–
1836.
(10) (a) Agard, N. J.; Prescher, J. A.; Bertozzi, C. R. J. Am. Chem. Soc.
2004, 126, 15046–15047. (b) Moorhouse, A. D.; Santos, A. M.; Gunaratnam,
M.; Moore, M.; Neidle, S.; Moses, J. E. J. Am. Chem. Soc. 2006, 128,
15972–15973. (c) Kumar, R.; El-Sagheer, A.; Tumpane, J.; Lincoln, P.;
Wilhelmsson, L. M.; Brown, T. J. Am. Chem. Soc. 2007, 129, 6859–6864.
(11) (a) Manetsch, R.; Krasiski, A.; Radi, Z.; Raushel, J.; Taylor, P.;
Sharpless, K. B.; Kolb, H. C. J. Am. Chem. Soc. 2004, 126, 12809–12818.
(b) Wang, J.; Sui, G.; Mocharla, V. P.; Lin, R. J.; Phelps, M. E.; Kolb,
H. C.; Tseng, H.-R. Angew. Chem., Int. Ed. 2006, 45, 5276–5281. (c)
Sugawara, A.; Sunazuka, T.; Hirose, T.; Nagai, K.; Yamaguchi, Y.; Hanaki,
H.; Sharpless, K. B.; Omura, S. Bioorg. Med. Chem. Lett. 2007, 17, 6340–
6344. (d) Chen, H.; Taylor, J. L.; Abrams, S. R. Bioorg. Med. Chem. Lett.
2007, 17, 1979–1983.
(5) (a) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421–431. (b) Katsuki,
T. Synlett 2003, 3, 281–297.
(12) Suijkerbuijk, B. M. J. M.; Aerts, B. N. H.; Dijkstra, H. P.; Lutz,
M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Dalton Trans.
2007, 1273–1276.
(13) Detz, R. J.; Arevalo Heras, S.; de Gelder, R.; van Leeuwen,
P. W. N. M.; Hiemstra, H.; Reek, J. N. H.; van Maarseveen, J. H. Org.
Lett. 2006, 8, 3227–3231.
(14) According to ref 12, the Ka value for NH-triazole to NCN-pincer
Pd complex is 4.16, while the value for pyridine is 19.5 and for aniline is
0.30.
(6) (a) Busetto, L.; Marchetti, F.; Zucchini, S.; Zanotti, V. Inorg. Chim.
Acta 2005, 358. (b) Bronisz, R. Inorg. Chem. 2005, 44, 4463. (c) Armstrong,
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Wheatley, A. E. H. Eur. J. Inorg. Chem. 2003, 3363–3366.
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10.1021/om8011163 CCC: $40.75
2009 American Chemical Society
Publication on Web 03/25/2009

Communications
Organometallics, Vol. 28, No. 8, 2009 2353
Scheme 1. Coordination Sites on 1,2,3-Triazoles
Scheme 2. Synthesis of 1,2,3-Triazoles and Selective N-2 Derivatization
bridged Rh(I) complexes along with their catalytic activity
toward the Pauson-Khand reaction.
and can even be purified by column chromatography.19 Single
crystals of triazole-Rh(I) complexes 1a,b were successfully
prepared, and their structures were determined by X-ray
diffraction (Scheme 3).
In comparison to N-substituted triazoles, the NH-triazole
possesses acidic N-H protons (pKa < 10)15 and can therefore
be applied as an anionic ligand with multiple binding sites. So
far, all reported efforts regarding triazole anion binding have
focused on benzotriazoles,16 which decreased the overall interest
in this class of complexes (limited functionality and difficulty
in derivatization), especially their unlikely potential in the
discovery of new chiral ligands. Moreover, the presence of
multiple nitrogen binding sites in the triazole anion can result
in complicated coordination systems, where two terminal
nitrogens possess higher electron density while the internal N-2
nitrogen is more exposed for coordination. Therefore, systems
with regioselective coordination of the triazole anion are highly
desirable.
As revealed by the crystal structures, in both complexes, two
triazole anions coordinate with the Rh(I) cation as bridging
ligands to link two metal cations, forming bis-metallic com-
plexes. The two Rh cations, with two COD ligands and two
nitrogen coordinations, adopt a 16-electron square-planar ge-
ometry. In these two complexes, both the N-2 and N-3 nitrogens
were bound to the metal, while the N-1 nitrogen was left open
with no coordination. The electron-withdrawing p-NO2 group
on the aromatic ring does not change the fundamental binding
pattern (complex 1b). The complexes pack in a chiral lattice,
where the two triazoles adopt a pseudo-C2 symmetry, omitting
the dynamic COD coordination.
To evaluate the chiral selectivity of these complexes, the
chiral triazole 1c was prepared to coordinate with Rh(I), and
the crystal structure is shown in Scheme 4.
Recently, our group reported a catalytic cascade three-
component condensation of nitroalkene, aryl aldehyde, and
NaN3, giving 4,5-disubstituted-NH-1,2,3-triazoles in one step
with good yields (Scheme 2A).17 Furthermore, with the ap-
plication of substrate conformational control, the N-2-substituted
triazoles were successfully prepared by a simple alkylation
reaction, as shown in Scheme 2B.18 This new synthetic
methodology and the success in regioselective triazole func-
tionalization spurred our interest in investigating the use of NH-
triazole as a new bridging ligand in the preparation of metal
complexes with good regioselectivity.
A similar triazole-bridged bis-Rh(I) complex was formed.
However, rather than the N-2 and N-3 binding as revealed in
complexes 1a,b, a different binding pattern was obtained in 1c,
in which the N-1 and N-2 nitrogens bind to the metal and the
N-3 is left uncoordinated. In both cases, the triazole anions bind
to two Rh cations. The N-2 nitrogen, although with a relatively
lower electron density than the N-1 and N-3 nitrogens, is a
favored σ-donor due to less steric hindrance. Meanwhile, since
the C-4- and C-5-substituted groups are bearing significant steric
repulsion, the conformations of these two groups influence each
other. As revealed in our previous N-2 alkylation report, the
C-5 aryl group is preferred to adopt a coplanar conformation
with the triazole ring to achieve the best π conjugation. This is
observed again in complexes 1a,b, where the C-4 styrenyl group
is perpendicular to the triazole ring. As a result, the C-5 group
blocks the N-1 binding site and leaves the N-3 nitrogen as the
preferred σ donor. In contrast, with the more bulky group on
the C-4 position (complex 1c), the C-5 aryl possesses a smaller
dihedral angle with the triazole ring (less planar). Therefore,
an increase of steric hindrance on the N-3 position in combina-
tion with less protection on the N-1 position leads to a different
coordination pattern, as observed in 1c.
With the 4,5-disubstituted NH-triazoles in hand, we examined
the binding ability of the triazole anion with Rh(I) complexes
by treating the NH-triazoles with [(COD)RhCl]2 under basic
conditions. As expected, the triazole anion effectively substituted
the Cl- anion and the triazole-Rh(I) complexes [Rh(COD)-
(TRIA)]2, were produced in nearly quantitative yields. These
new complexes showed great stability toward air and moisture
(15) Poznanski, J.; Najda, A.; Bretner, M.; Shugar, D. J. Phys. Chem.
A 2007, 111, 6501–6509. According to the paper, the pKa of benzyl triazole
is 8.3.
(16) (a) Katritzky, A. R.; Lan, X.; Yang, J. Z.; Denisko, O. V. Chem.
ReV. 1998, 98, 409. (b) Verma, A. K.; Singh, J.; Sankar, V. K.; Chaudhary,
R.; Chandra, R. Tetrahedron Lett. 2007, 48, 4207–4210.
(17) Sengupta, S.; Duan, H.; Lu, W.; Petersen, J. L.; Shi, X. Org. Lett.
2008, 10, 1493–1496.
(19) The complex is stable enough to be dissolved in methylene chloride
and purified by flash column chromatography (Rf ) 0.5, hexane-ethyl
acetate, 6/1 v/v).
(18) Chen, Y.; Liu, Y.; Petersen, J. L.; Shi, X. Chem. Commun. 2008,
3254–3256.

2354 Organometallics, Vol. 28, No. 8, 2009
Communications
Scheme 3. Synthesis and Crystal Structures of Triazole-Rh(I)
Scheme 5. Crystal Structure of 1d: Anionic Triazoles as
Better Coordination Ligands than Pyridine
Complexes
Table 1. N-Rh Bond Distances (Å) and Dihedral Angles (deg) of
Triazole-Rh Complexes
dihedral angle with triazole
N1-Rh
N2-Rh
N3-Rh
C-4 Ph
C-5 Ar
1a
1b
2.06 ( 0.01 2.12 ( 0.01
2.06 ( 0.01 2.12 ( 0.01
129 ( 6
131 ( 1
100 ( 4
99 ( 1
33 ( 1
34 ( 1
49 ( 1
44 ( 1
1c 2.10 ( 0.01 2.08 ( 0.01
1d 2.12 ( 0.01 2.06 ( 0.01
As indicated in the crystal structure, a similar N-1 binding
was observed in 1d, which strongly suggested that steric effects,
as discussed above, are dominant in triazole-metal coordination.
Notably, even with the presence of the strong σ donor pyridine,
the triazole anion is still the preferred binding site and the
complex is stable enough for column chromatography. A direct
comparison of all four complexes is given in Table 1.
Interestingly, enantiomeric self-sorting is observed in com-
plexes 1c,d. Similar to the case for 1a,b, complexes 1c,d are
chiral with pseudo-C2 symmetry between the two triazoles. With
a stereogenic center on the C-4 position, homochiral coordina-
tion (only R,R or S,S) was observed for [Rh(COD)(TRIA)]2 in
the solid state. This result is crucial, because it allows this new
class of bis-metal complexes to be potential chiral catalysts for
stereoselective functional group transformation. Solution NMR
studies gave complex spectra even at low temperature (-50
°C), due to the dynamic exchange of the COD ligands (see
spectra in the Supporting Information). However, considering
the superior stability of these complexes, it is likely that the
bimetal coordination should also exist in solution to maintain
the 16-electron configuration on the rhodium. With the least
steric hindrance, the N-2 nitrogen should be the preferred
binding site. It is possible that the N-1 and N-3 metal binding
is dynamic in solution. The dissociation of the bis-metal skeleton
is highly unlikely.
Scheme 4. Switching the N-Binding Site with a Different C-4
Group
Finally, to evaluate the catalytic reactivity of this new class
of Rh(I) complexes, catalytic Pauson-Khand reactions of enyne
2 were carried out to compare with those of the air- and
moisture-sensitive [Rh(COD)Cl]2. The results are shown in
Scheme 6.
Even with their greater stability toward air and moisture, all
four complexes showed effective catalytic reactivity. Develop-
ment of new chiral catalysts based on this new class of catalysts
toward asymmetric Pauson-Khand reactions is currently under
investigation and will be reported in due course.
Another factor that may be contributing to the triazole binding
site is electronic effects, which may provide different electron
densities on the N-1 and N-3 nitrogens. To investigate this effect,
the pyridine-triazole-Rh(I) complex 1d was synthesized and
the crystal structure is shown in Scheme 5.
In conclusion, we have reported the synthesis and structural
characterization of 4,5-disubstituted triazoles as organo-anionic

Communications
Organometallics, Vol. 28, No. 8, 2009 2355
Scheme 6. Triazole-Rh(I) Complex Catalyzed Pauson-Khand Reactions
ligands in coordination with Rh(I) complexes. The first examples
of X-ray structures of NH-triazole-bound transition-metal
complexes were reported. With the application of chiral triazoles,
enantiomeric self-sorting of the ligands was achieved, where
only homochiral complexes were observed. As a new class of
air- and moisture-stable compounds, these complexes showed
effective catalytic reactivity in Pauson-Khand reactions, further
illustrating the great potential of these complexes as new
catalysts in stereoselective reactions. Application of these
complexes to other metal-catalyzed reactions and further
development of triazole analogues for the formation of different
chiral derivatives are currently under investigation in our group.
Experimental Section. General Procedures. Unless other-
wise noted, all commercial reagents and solvents were obtained
from the commercial provider and used without further purifica-
tion. [RhCl(COD)]2 (crystalline solid) and DPPP were purchased
and stored under nitrogen. 1H NMR and 13C NMR spectra were
recorded on 270 and 600 MHz spectrometers. Chemical shifts
were reported relative to internal tetramethylsilane (δ 0.00 ppm)
the yellow solids were stable enough to be dissolved in
dichloromethane and purified by flash column chromatography
(Rf ) 0.5, hexane-EtOAc, 6/1 v/v).)
Rh-Triazole-Catalyzed Pauson-Khand Reaction. A typi-
cal procedure for the Rh-triazole-catalyzed Pauson-Khand
reaction is as follows. In a 20 mL dry sealable flask were added
the rhodium-triazole complex (0.025 mmol Rh), dppp (12.4
mg, 0.03 mmol), and xylene (3 mL). Then the mixture was
stirred for 10 min, after which the enyne (0.5 mmol) was added.
The flask was then charged with 5 atm of CO and heated to
130 °C for 20 h, after which the flask was cooled and the CO
was cautiously released in the hood. The crude product was
purified using flash column chromatography (hexane-EtOAc,
1
3/1 v/v) to afford 3 as a colorless oil. H NMR (600 MHz,
CDCl3): δ 7.52 (d, J ) 7.2 Hz, 2H), 7.41-7.38 (m, 2H),
7.35-7.32 (m, 1H), 4.92 (d, J ) 16.2 Hz, 1H), 4.57 (d, J )
16.2 Hz, 1H), 4.36 (t, J ) 8.4 Hz, 1H), 3.31-3.30 (m, 1H),
3.22 (dd, J ) 10.8 Hz, J ) 7.8 Hz, 1H), 2.83 (dd, J ) 18.0 Hz,
J ) 6.6 Hz, 1H), 2.32 (dd, J ) 18.0 Hz, J ) 3.6 Hz, 1H). 13C
NMR (150 MHz, CDCl3): δ 206.7, 177.3, 134.6, 130.5, 128.6,
128.5, 127.9, 71.3, 66.2, 43.2, 40.2.
1
or CDCl3 (δ 7.26 ppm) for H and CDCl3 (δ 77.0 ppm) for
13C. Flash column chromatography was performed on 230-430
mesh silica gel. Analytical thin-layer chromatography was
performed with precoated glass baked plates (250 µm) and
visualized by fluorescence. Melting points were determined on
a MelTemp apparatus and are uncorrected. Elemental analyses
were performed in the Department of Chemical Engineering,
College of Engineering and Mineral Resources, West Virginia
University.
Acknowledgment. We deeply appreciate the C. Eugene
Bennett Department of Chemistry and the Eberly College
of Arts and Science, WVNano Initiative, PsCoR program at
West Virginia University, Senate research award of West
Virginia State, and the donors of the Petroleum Researcb
Foundation (PRF-G), administered by the American Chemi-
cal Society, for financial support.
Synthesis of Rh-Triazole Complexes. A typical procedure
for the synthesis of Rh-triazole complexes is as follows. To a
suspension of [RhCl(COD)]2 (100 mg, 0.2 mmol) and triazole
(0.4 mmol) in 30 mL of methanol was added 10 mL of 0.04 M
KOH (22.4 mg, 0.4 mmol) solution in methanol, dropwise at
room temperature with constant stirring. After 8 h of stirring at
room temperature, a yellow powder of Rh-triazole was
produced and filtered using a folded filter paper (Whatman No.
2). The yellow powder was then washed with methanol and
dried by vacuum pump: yield ∼95%. (For further purification,
Supporting Information Available: Text, figures, tables, and
CIF files giving general experimental methods for the preparation
of the complexes and crystallization, NMR spectra, and crystal data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM8011163