2
I. Sinha et al. / Journal of Inorganic Biochemistry xxx (2015) xxx–xxx
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) on a
Bruker D8 Venture diffractometer. The structures were solved by direct
methods and were refined by full-matrix, least squares on F2 by using
the SHELXTL and SHELXL-97 programs [44]. Crystallographic data are
listed in Table 1.
2.2. Synthesis of 6-(3,5-dimethylpyrazol-1-yl)purine 1
6-Hydrazinylpurine (1.77 g, 11.8 mmol) was suspended in
acetylacetone (8.4 mL, 83 mmol), followed by the addition of CF3COOH
(8.8 μL, 0.12 mmol). The mixture was stirred overnight. The resulting
solid was filtered, washed with cold water and diethyl ether. It was
purified by silica gel column chromatography to produce the title com-
pound 1 as a white solid in 42% yield (1.07 g, 4.99 mmol). ESI-TOF m/z:
[M + H]+ 215.1040 (calcd. 215.1045), [M + Na]+ 237.0859 (calcd.
237.0865). Elemental analysis (%): found: C 56.3, H 4.7, N 39.5;
calcd. for C10H10N6: C 56.1, H 4.7, N 39.2. 1H NMR (400 MHz, DMSO-
d6), δ/ppm: 12.8 (s, 1H, NH), 8.76 (s, 1H, H2p), 8.61 (s, 1H, H8p), 6.25
(s, 1H, H4*), 2.75 (s, 3H, H6*), 2.33 (s, 3H, H7*). 13C NMR (101 MHz,
DMSO-d6), δ/ppm: 151.8 (C3*), 151.5 (C2p), 150.7 (C8p), 147.4 (C6p),
146.2 (C4p), 145.0 (C5p), 142.6 (C5*), 110.4 (C4*), 14.7 (C6*), 13.4
(C7*).
Fig. 1. Chemical structures of 3,5-dimethylpyrazolyl-substituted derivatives of purine,
adenine, and hypoxanthine (R = nucleic acid backbone) as previously reported by
Lönnberg et al. [32,33,35].
with the complexation behavior of the closely related adenine moiety
[32]. We report here a systematic structural study of the metal-
binding behavior of 9-methyl-6-(3,5-dimethylpyrazol-1-yl)purine 2,
acting as a model nucleobase. The use of alkylated ligands as model
nucleobases is well-known from nucleobase chemistry [38] and has
successfully been introduced to the study of metal-mediated base
pairs, too [19,27,39–41].
2.3. Synthesis of 9-methyl-6-(3,5-dimethylpyrazol-1-yl)purine 2 and
7-methyl-6-(3,5-dimethylpyrazol-1-yl)purine 3
Compound 1 (0.627 g, 2.93 mmol) was suspended in DMF (20 mL).
After the addition of NaH (0.141 g, 3.51 mmol) and stirring for 15 min at
50 °C, methyl iodide (0.219 mL, 0.495 g, 3.51 mmol) was added and
stirring continued for 6 h. The reaction mixture was allowed to reach
room temperature, diluted with water, and extracted with ethyl acetate
thrice. The combined organic phase was dried (MgSO4), and the crude
product mixture was purified by silica gel column chromatography to
produce a white solid in 26% yield (0.174 g, 0.762 mmol), containing
both N9 and N7 alkylated isomers 2 and 3 in a ratio of about 55:45.
The isomers could be unambiguously identified by NMR spectroscopy.
The mixture was used for further reactions without the necessity
for separation. ESI-TOF m/z: [M + H]+ 229.1196 (calcd. 229.1202),
[M + Na]+ 251.1016 (calcd. 251.1021). Elemental analysis (%): found:
C 55.2, H 5.5, N 34.7; calcd. for 2/3 · 0.67 H2O: C 55.0, H 5.6, N 35.0.
N9-alkylated isomer 2: 1H NMR (400 MHz, CDCl3), δ/ppm: 8.77 (s, 1H,
H2p), 8.14 (s, 1H, H8p), 6.07 (s, 1H, H4*), 3.92 (s, 3H, H9p), 2.69 (s,
3H, H7*), 2.38 (s, 3H, H6*). 13C NMR (101 MHz, CDCl3), δ/ppm: 154.1
(C4p), 152.8 (C3*), 151.3 (C2p), 149.8 (C6p), 145.1 (C8p), 143.0 (C5*),
123.8 (C5p), 110.2 (C4*), 30.0 (C9p), 14.6 (C7*), 14.1 (C6*). N7-
alkylated isomer 3: 1H NMR (400 MHz, CDCl3), δ/ppm: 8.91 (s, 1H,
H2p), 8.17 (s, 1H, H8p), 6.09 (s, 1H, H4*), 3.96 (s, 3H, H9p), 2.57 (s,
3H, H7*), 2.29 (s, 3H, H6*). 13C NMR (101 MHz, CDCl3), δ/ppm: 164.0
(C4p), 151.4 (C2p), 150.9 (C3*), 149.8 (C8p), 145.0 (C6p), 143.0 (C5*),
118.3 (C5p), 109.3 (C4*), 36.3 (C7p), 13.5 (C7*), 13.0 (C6*).
2. Experimental
2.1. General
All solvents were dried prior to their use. 6-Hydrazinylpurine and
6-pyrazol-1-yl-purine 6 were synthesized according to literature
procedures [42,43]. ESI-TOF measurements of compounds 1, 2, 3
and 7 were performed using the oa-TOF mass spectrometer MicrOTOF
(Bruker Daltonik GmbH, Bremen, Germany) equipped with a standard
ESI source. All mass spectra are quasi-internally mass calibrated by
the measurement of an infused calibrant (ammonium formate) prior
to the compound of interest. ESI-MS spectra of compounds 4, 5, 8 and
9 were measured on an LTQ Orbitrap XL (Thermo Scientific, Bremen,
Germany), equipped with the static nanospray probe (slightly modified
to use self-drawn glass nanospray capillaries). The elemental analyses
were performed on a Vario EL III CHNS analyzer. NMR spectra were re-
corded at 300 K on Bruker Avance (I) 400 and Avance (III) 400 spec-
trometers with an internal standard relative to tetramethylsilane
(δ = 0 ppm, CDCl3), trimethylsilyl propionate (δ = 0 ppm, D2O)
or the residual solvent peak (δ = 2.50, DMSO-d6). For the atom-
numbering scheme used in the following lists of NMR data, please
see Fig. 3. Single-crystal X-ray diffraction data were collected with
2.4. Synthesis of [Cu(2)(NO3)2] 4
Cu(NO3)2·3 H2O (0.158 g, 0.657 mmol) was added to a methanolic
solution of 2 and 3 (0.150 g, 0.657 mmol). The mixture was heated at
60 °C for 18 h. After one week at 4 °C, a blue crystalline compound
could be isolated. The crystals proved to be [Cu(2)(NO3)2] 4 according
to single crystal X-ray diffraction analysis. ESI-MS m/z: [Cu(2)(NO3)]+
353.0301 (calcd. 353.0298), [Cu(2)2(NO3)]+ 581.1432 (calcd.
581.1421).
2.5. Synthesis of [Cu(2)Cl2] 5
CuCl2·2 H2O (75 mg, 0.44 mmol) was added to a methanolic
solution of 2 and 3 (0.100 g, 0.438 mmol). The mixture was heated at
60 °C for 18 h. The solvent was evaporated, affording a green solid
Fig. 2. Ambident metal-binding behavior of 6-(3,5-dimethylpyrazol-1-yl)purine, resulting
from an unhindered rotation around the C–N bond (R = nucleic acid backbone).
Please cite this article as: I. Sinha, et al., Metal complexes of 6-pyrazolylpurine derivatives as models for metal-mediated base pairs, J. Inorg.