Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270108005957/gd3189sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270108005957/gd3189IIsup2.hkl |
CCDC reference: 686437
The title compound was obtained in 71% yield from the reaction of (I) with methyl iodide in acetonitrile at room temperature following the procedure described by Kowalska et al. (1993). Single crystals were grown from aqueous solution at room temperature.
H atoms in the cation were treated as riding atoms in geometrically idealized positions, with C—H distances of 0.95 Å (ring) or 0.98 Å (CH3), and with Uiso(H) = kUeq(C), where k = 1.5 for the methyl groups and k = 1.2 otherwise. The H atoms of the water component could not be located in difference maps, but their positions were calculated as described by Nardelli (1999), and the water molecule was then refined as a rigid body [O—H = 0.85 Å and Uiso(H) = 1.5Ueq(O)]. Even applying an analytical absorption correction, there was a large (> 6 e Å-3) peak in the Fourier electron density maps approx 0.5 Å from the iodide anion. Applying a disorder of the I- anion (4:1) gave better convergence of the refinement. The large thermal displacement parameter of the water molecule, when compared with the other atoms, may be explained by possible disorder-induced formation of hydrogen bonds to the disordered I- anion.
Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO and SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2008).
C9H13N4O2+·I−·0.5H2O | F(000) = 1352 |
Mr = 345.14 | Dx = 1.767 Mg m−3 |
Orthorhombic, Cmca | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2bc 2 | Cell parameters from 15312 reflections |
a = 6.9841 (2) Å | θ = 2.6–35.0° |
b = 13.0977 (4) Å | µ = 2.47 mm−1 |
c = 28.3695 (8) Å | T = 100 K |
V = 2595.12 (13) Å3 | Block, colourless |
Z = 8 | 0.21 × 0.15 × 0.08 mm |
Bruker KappaAPEXII diffractometer | 2983 independent reflections |
Radiation source: Enraf-Nonius FR590 | 2699 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.064 |
Detector resolution: 8.3 pixels mm-1 | θmax = 35.0°, θmin = 2.9° |
CCD rotation images scans | h = −11→11 |
Absorption correction: analytical (Alcock, 1970) | k = −21→16 |
Tmin = 0.641, Tmax = 0.828 | l = −45→45 |
15312 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.041 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.082 | H-atom parameters constrained |
S = 1.19 | w = 1/[σ2(Fo2) + (0.P)2 + 10.7633P] where P = (Fo2 + 2Fc2)/3 |
2983 reflections | (Δ/σ)max < 0.001 |
115 parameters | Δρmax = 0.74 e Å−3 |
0 restraints | Δρmin = −0.99 e Å−3 |
C9H13N4O2+·I−·0.5H2O | V = 2595.12 (13) Å3 |
Mr = 345.14 | Z = 8 |
Orthorhombic, Cmca | Mo Kα radiation |
a = 6.9841 (2) Å | µ = 2.47 mm−1 |
b = 13.0977 (4) Å | T = 100 K |
c = 28.3695 (8) Å | 0.21 × 0.15 × 0.08 mm |
Bruker KappaAPEXII diffractometer | 2983 independent reflections |
Absorption correction: analytical (Alcock, 1970) | 2699 reflections with I > 2σ(I) |
Tmin = 0.641, Tmax = 0.828 | Rint = 0.064 |
15312 measured reflections |
R[F2 > 2σ(F2)] = 0.041 | 0 restraints |
wR(F2) = 0.082 | H-atom parameters constrained |
S = 1.19 | w = 1/[σ2(Fo2) + (0.P)2 + 10.7633P] where P = (Fo2 + 2Fc2)/3 |
2983 reflections | Δρmax = 0.74 e Å−3 |
115 parameters | Δρmin = −0.99 e Å−3 |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
C2 | 0.0000 | 0.2494 (2) | 0.46713 (10) | 0.0220 (6) | |
C4 | 0.0000 | 0.2970 (2) | 0.39276 (10) | 0.0193 (5) | |
C5 | 0.0000 | 0.1980 (2) | 0.37747 (10) | 0.0178 (5) | |
C6 | 0.0000 | 0.1214 (2) | 0.41236 (10) | 0.0191 (5) | |
C8 | 0.0000 | 0.2955 (3) | 0.31546 (11) | 0.0253 (6) | |
H8 | 0.0000 | 0.3178 | 0.2836 | 0.030* | |
C21 | 0.0000 | 0.3702 (2) | 0.53004 (11) | 0.0244 (6) | |
H21A | −0.1061 | 0.4072 | 0.5152 | 0.037* | 0.50 |
H21B | 0.1215 | 0.4027 | 0.5214 | 0.037* | 0.50 |
H21C | −0.0154 | 0.3717 | 0.5644 | 0.037* | 0.50 |
C61 | 0.0000 | −0.0529 (2) | 0.43593 (11) | 0.0248 (6) | |
H61A | −0.0146 | −0.1206 | 0.4217 | 0.037* | 0.50 |
H61B | −0.1066 | −0.0403 | 0.4576 | 0.037* | 0.50 |
H61C | 0.1212 | −0.0500 | 0.4533 | 0.037* | 0.50 |
C71 | 0.0000 | 0.1102 (3) | 0.29729 (12) | 0.0391 (11) | |
H71A | 0.0892 | 0.0585 | 0.3092 | 0.059* | 0.50 |
H71B | 0.0400 | 0.1316 | 0.2657 | 0.059* | 0.50 |
H71C | −0.1292 | 0.0812 | 0.2958 | 0.059* | 0.50 |
C91 | 0.0000 | 0.4692 (3) | 0.35194 (13) | 0.0302 (8) | |
H91A | −0.1234 | 0.4947 | 0.3634 | 0.045* | 0.50 |
H91B | 0.0203 | 0.4926 | 0.3195 | 0.045* | 0.50 |
H91C | 0.1031 | 0.4951 | 0.3721 | 0.045* | 0.50 |
N1 | 0.0000 | 0.1489 (2) | 0.45736 (9) | 0.0203 (5) | |
N3 | 0.0000 | 0.32885 (19) | 0.43801 (9) | 0.0183 (4) | |
N7 | 0.0000 | 0.1986 (2) | 0.32894 (9) | 0.0221 (5) | |
N9 | 0.0000 | 0.3578 (2) | 0.35306 (9) | 0.0204 (5) | |
O2 | 0.0000 | 0.26578 (18) | 0.51398 (8) | 0.0252 (5) | |
O6 | 0.0000 | 0.02437 (17) | 0.39915 (8) | 0.0229 (5) | |
O1W | 0.2500 | 0.4344 (5) | 0.2500 | 0.0463 (16) | 0.50 |
H1W | 0.2900 | 0.3832 | 0.2657 | 0.069* | 0.25 |
H2W | 0.2135 | 0.4128 | 0.2232 | 0.069* | 0.25 |
I1A | 0.0000 | 0.26071 (6) | 0.17445 (4) | 0.02465 (15) | 0.80 |
I1B | 0.0000 | 0.2855 (2) | 0.17975 (14) | 0.0218 (5) | 0.20 |
U11 | U22 | U33 | U12 | U13 | U23 | |
C2 | 0.0341 (16) | 0.0177 (13) | 0.0141 (11) | 0.000 | 0.000 | −0.0017 (9) |
C4 | 0.0216 (13) | 0.0178 (12) | 0.0186 (12) | 0.000 | 0.000 | 0.0013 (9) |
C5 | 0.0186 (12) | 0.0210 (13) | 0.0140 (10) | 0.000 | 0.000 | 0.0007 (9) |
C6 | 0.0218 (13) | 0.0185 (12) | 0.0171 (11) | 0.000 | 0.000 | 0.0020 (9) |
C8 | 0.0384 (18) | 0.0228 (14) | 0.0149 (12) | 0.000 | 0.000 | 0.0021 (10) |
C21 | 0.0334 (17) | 0.0208 (14) | 0.0191 (13) | 0.000 | 0.000 | −0.0052 (10) |
C61 | 0.0395 (19) | 0.0172 (13) | 0.0176 (12) | 0.000 | 0.000 | 0.0027 (10) |
C71 | 0.080 (3) | 0.0215 (15) | 0.0162 (13) | 0.000 | 0.000 | −0.0025 (11) |
C91 | 0.048 (2) | 0.0168 (13) | 0.0253 (15) | 0.000 | 0.000 | 0.0027 (11) |
N1 | 0.0252 (13) | 0.0199 (11) | 0.0157 (10) | 0.000 | 0.000 | 0.0009 (8) |
N3 | 0.0207 (12) | 0.0178 (11) | 0.0165 (10) | 0.000 | 0.000 | 0.0006 (8) |
N7 | 0.0328 (14) | 0.0199 (11) | 0.0135 (10) | 0.000 | 0.000 | 0.0012 (8) |
N9 | 0.0286 (13) | 0.0167 (11) | 0.0160 (10) | 0.000 | 0.000 | 0.0022 (8) |
O2 | 0.0421 (15) | 0.0183 (10) | 0.0153 (9) | 0.000 | 0.000 | −0.0007 (7) |
O6 | 0.0366 (14) | 0.0160 (9) | 0.0162 (9) | 0.000 | 0.000 | 0.0005 (7) |
O1W | 0.056 (5) | 0.042 (3) | 0.041 (3) | 0.000 | 0.001 (3) | 0.000 |
I1A | 0.02620 (18) | 0.0259 (3) | 0.0218 (2) | 0.000 | 0.000 | 0.0048 (2) |
I1B | 0.0147 (5) | 0.0324 (15) | 0.0184 (9) | 0.000 | 0.000 | 0.0076 (9) |
C2—N3 | 1.329 (4) | C21—H21C | 0.9800 |
C2—N1 | 1.344 (4) | C61—O6 | 1.454 (4) |
C2—O2 | 1.346 (4) | C61—H61A | 0.9800 |
C4—N3 | 1.350 (4) | C61—H61B | 0.9800 |
C4—C5 | 1.368 (4) | C61—H61C | 0.9800 |
C4—N9 | 1.380 (4) | C71—N7 | 1.466 (4) |
C5—N7 | 1.377 (4) | C71—H71A | 0.9800 |
C5—C6 | 1.409 (4) | C71—H71B | 0.9800 |
C6—O6 | 1.325 (4) | C71—H71C | 0.9800 |
C6—N1 | 1.326 (4) | C91—N9 | 1.459 (4) |
C8—N7 | 1.326 (4) | C91—H91A | 0.9800 |
C8—N9 | 1.343 (4) | C91—H91B | 0.9800 |
C8—H8 | 0.9500 | C91—H91C | 0.9800 |
C21—O2 | 1.442 (4) | O1W—H1W | 0.85 |
C21—H21A | 0.9800 | O1W—H2W | 0.85 |
C21—H21B | 0.9800 | ||
N3—C2—N1 | 129.7 (3) | N7—C71—H71A | 109.5 |
N3—C2—O2 | 119.2 (3) | N7—C71—H71B | 109.5 |
N1—C2—O2 | 111.1 (3) | H71A—C71—H71B | 109.5 |
N3—C4—C5 | 126.5 (3) | N7—C71—H71C | 109.5 |
N3—C4—N9 | 126.7 (3) | H71A—C71—H71C | 109.5 |
C5—C4—N9 | 106.8 (3) | H71B—C71—H71C | 109.5 |
C4—C5—N7 | 108.1 (3) | N9—C91—H91A | 109.5 |
C4—C5—C6 | 116.9 (3) | N9—C91—H91B | 109.5 |
N7—C5—C6 | 135.0 (3) | H91A—C91—H91B | 109.5 |
O6—C6—N1 | 122.2 (3) | N9—C91—H91C | 109.5 |
O6—C6—C5 | 118.9 (3) | H91A—C91—H91C | 109.5 |
N1—C6—C5 | 118.9 (3) | H91B—C91—H91C | 109.5 |
N7—C8—N9 | 110.6 (3) | C6—N1—C2 | 117.7 (3) |
N7—C8—H8 | 124.7 | C2—N3—C4 | 110.4 (3) |
N9—C8—H8 | 124.7 | C8—N7—C5 | 107.1 (3) |
O2—C21—H21A | 109.5 | C8—N7—C71 | 125.5 (3) |
O2—C21—H21B | 109.5 | C5—N7—C71 | 127.4 (3) |
H21A—C21—H21B | 109.5 | C8—N9—C4 | 107.3 (3) |
O2—C21—H21C | 109.5 | C8—N9—C91 | 126.2 (3) |
H21A—C21—H21C | 109.5 | C4—N9—C91 | 126.5 (3) |
H21B—C21—H21C | 109.5 | C2—O2—C21 | 117.6 (2) |
O6—C61—H61A | 109.5 | C6—O6—C61 | 117.7 (2) |
O6—C61—H61B | 109.5 | H1W—O1W—H2W | 107.7 |
H61A—C61—H61B | 109.5 | I1A—O1W—I1Ai | 101.2 (2) |
O6—C61—H61C | 109.5 | I1B—O1W—I1Bi | 107.2 (2) |
H61A—C61—H61C | 109.5 | I1A—O1W—I1Bi | 104.2 (2) |
H61B—C61—H61C | 109.5 | ||
N3—C4—C5—N7 | 180.0 | N9—C8—N7—C5 | 0.0 |
N9—C4—C5—N7 | 0.0 | N9—C8—N7—C71 | 180.0 |
N3—C4—C5—C6 | 0.0 | C4—C5—N7—C8 | 0.0 |
N9—C4—C5—C6 | 180.0 | C6—C5—N7—C8 | 180.0 |
C4—C5—C6—O6 | 180.0 | C4—C5—N7—C71 | 180.0 |
N7—C5—C6—O6 | 0.0 | C6—C5—N7—C71 | 0.0 |
C4—C5—C6—N1 | 0.0 | N7—C8—N9—C4 | 0.0 |
N7—C5—C6—N1 | 180.0 | N7—C8—N9—C91 | 180.0 |
O6—C6—N1—C2 | 180.0 | N3—C4—N9—C8 | 180.0 |
C5—C6—N1—C2 | 0.0 | C5—C4—N9—C8 | 0.0 |
N3—C2—N1—C6 | 0.0 | N3—C4—N9—C91 | 0.0 |
O2—C2—N1—C6 | 180.0 | C5—C4—N9—C91 | 180.0 |
N1—C2—N3—C4 | 0.0 | N3—C2—O2—C21 | 0.0 |
O2—C2—N3—C4 | 180.0 | N1—C2—O2—C21 | 180.0 |
C5—C4—N3—C2 | 0.0 | N1—C6—O6—C61 | 0.0 |
N9—C4—N3—C2 | 180.0 | C5—C6—O6—C61 | 180.0 |
Symmetry code: (i) −x+1/2, y, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
C8—H8···O1W | 0.95 | 2.51 | 3.131 (5) | 123 |
C8—H8···I1B | 0.95 | 2.98 | 3.852 (5) | 154 |
O1W—H2W···I1A | 0.85 | 2.85 | 3.580 (4) | 146 |
O1W—H2W···I1B | 0.85 | 2.55 | 3.290 (5) | 145 |
O1W—H1W···I1Ai | 0.85 | 2.76 | 3.580 (4) | 163 |
O1W—H1W···I1Bi | 0.85 | 2.49 | 3.290 (5) | 157 |
Symmetry code: (i) −x+1/2, y, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C9H13N4O2+·I−·0.5H2O |
Mr | 345.14 |
Crystal system, space group | Orthorhombic, Cmca |
Temperature (K) | 100 |
a, b, c (Å) | 6.9841 (2), 13.0977 (4), 28.3695 (8) |
V (Å3) | 2595.12 (13) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 2.47 |
Crystal size (mm) | 0.21 × 0.15 × 0.08 |
Data collection | |
Diffractometer | Bruker KappaAPEXII diffractometer |
Absorption correction | Analytical (Alcock, 1970) |
Tmin, Tmax | 0.641, 0.828 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 15312, 2983, 2699 |
Rint | 0.064 |
(sin θ/λ)max (Å−1) | 0.807 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.041, 0.082, 1.19 |
No. of reflections | 2983 |
No. of parameters | 115 |
H-atom treatment | H-atom parameters constrained |
w = 1/[σ2(Fo2) + (0.P)2 + 10.7633P] where P = (Fo2 + 2Fc2)/3 | |
Δρmax, Δρmin (e Å−3) | 0.74, −0.99 |
Computer programs: COLLECT (Nonius, 1998), DENZO and SCALEPACK (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEPIII (Burnett & Johnson, 1996) and X-SEED (Barbour, 2001), publCIF (Westrip, 2008).
N3—C2—N1 | 129.7 (3) | C8—N9—C4 | 107.3 (3) |
N3—C2—O2 | 119.2 (3) | C8—N9—C91 | 126.2 (3) |
N1—C2—O2 | 111.1 (3) | C4—N9—C91 | 126.5 (3) |
O6—C6—C5 | 118.9 (3) | C2—O2—C21 | 117.6 (2) |
C6—N1—C2 | 117.7 (3) | C6—O6—C61 | 117.7 (2) |
C2—N3—C4 | 110.4 (3) | I1A—O1W—I1Ai | 101.2 (2) |
C8—N7—C5 | 107.1 (3) | I1B—O1W—I1Bi | 107.2 (2) |
C8—N7—C71 | 125.5 (3) | I1A—O1W—I1Bi | 104.2 (2) |
C5—N7—C71 | 127.4 (3) |
Symmetry code: (i) −x+1/2, y, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
C8—H8···O1W | 0.95 | 2.51 | 3.131 (5) | 123.3 |
C8—H8···I1B | 0.95 | 2.98 | 3.852 (5) | 153.9 |
O1W—H2W···I1A | 0.85 | 2.85 | 3.580 (4) | 145.5 |
O1W—H2W···I1B | 0.85 | 2.55 | 3.290 (5) | 145.4 |
O1W—H1W···I1Ai | 0.85 | 2.76 | 3.580 (4) | 162.5 |
O1W—H1W···I1Bi | 0.85 | 2.49 | 3.290 (5) | 157.4 |
Symmetry code: (i) −x+1/2, y, −z+1/2. |
Purinium salts exhibit interesting biological activity against adenocarcinoma and cytomegalovirus (Fujii & Itaya, 1999b) and affect pig brain ATPase (Fujii & Itaya, 1999a). Some substituted 6-methoxy-7,9-dimethylpurinium salts (isolated from Heterostemma brownii Hay – Asclepiadaceae) exhibit cytotoxic activity towards several tumor cell lines (Lin et al., 1996, 1997), and some 2,6-dimethoxy-7,9-dimethylpurinium salts exhibit cytotoxic activity against alga Chlorella vulgaris (Kowalska & Sochacka, 2003). Purinium salts also play an important role in the synthesis of purine compounds via the Hilbert–Johnson reaction (Pliml & Prystas, 1967) or transglycosylation (Boryski, 1996). We found that quaternization of 2,6-dialkoxy- and 2(6)-chloroalkoxy-7-methylpurines with alkyl halides in aprotic solvents led mostly to 2,6-disubstituted-7,9-dialkylpurinium halides and 6-alkoxyhypoxanthines instead of 1,3,7-trialkylxanthines (Kowalska et al., 1993; Kowalska & Maślankiewicz, 2001), although reaction of 2,6-dimethoxy-7-methylpurine, (I), with methyl iodide in a sealed tube at 373 K gave caffeine (Bergman & Heimbold, 1935). The absence of N1-alkylation was postulated to result from the conformations of the two alkoxy groups, causing steric hindrance at N1 (Kowalska et al., 1993).
Recently, the structures of a few 7,9-dialkylpurinium salts (where the anion is iodide, bromide or perchlorate) have been analyzed (Sigel et al., 2002; Nasiri et al., 2005; Hocek et al., 2005; Fu & Lam, 2005; Torii et al., 2006). We have now determined the crystal structure of 2,6-dimethoxy-7,9-dimethylpurinium iodide, (II), as its hemihydrate, to obtain information about the positions and conformations of the methoxy groups and the geometry of the imidazole ring in comparison with (I) (Kowalska et al., 1999), in an attempt to explain some results of the internal thermal O–N and N–N migrations of the alkyl groups (Kowalska & Maślankiewicz, 1997).
The non-H atoms of the cation of (II) (Fig. 1) all lie on a mirror plane in Cmca. For the O-methoxy groups, this permits complete overlap between the non-bonding p-type orbitals of the O atoms and π-orbitals of the pyrimidine ring. Alkylation at atom N9 does not influence the conformation of the O2/CH3 group as compared with that in (I). Whereas the bond lengths and bond angles in the pyrimidine ring of (II) are very close to those found in (I), the geometric details of the imidazole rings are different. The essential difference concerns the bonds and bond angles connected with atoms N7, C8 and N9. Whereas in (I) the N7—C8 bond is longer than the C8—N9 bond [1.349 (2) versus 1.312 (2) Å], in (II) the C8—N9 bond is longer [1.326 (4) versus 1.343 (4) Å]. There is also a change in the bond angles C5—N7—C8 [104.6 (1) versus 107.1 (3)° in compounds (I) and (II), respectively], N7—C8—N9 [115.1 (1) versus 110.6 (3)°] and C4—N9—C8 [103.6 (1) versus 107.3 (3)°]. The methyl atoms C71 and C91 are equally directed towards atom C8.
All these geometric details make this fragment of the imidazole ring more regular in (II) than in (I). The N7—C8 and N9—C8 bond lengths are also unequal in the recently reported 7,9-dimethylpurinium salts (Sigel et al., 2002; Nasiri et al., 2005; Hocek et al., 2005; Fu & Lam, 2005; Torii et al., 2006) represented by structures (III)–(V), respectively [it would be preferable to specify exactly which authors reported which compounds, since there are 3 structures and 5 references] (the differences in N—C bond lengths are in the range 0.016–0.025 Å). As the N7—C8 bond is shorter by 0.017 Å in salt (II), double-bond character can be assigned to this bond and the resonance structure is represented by formula (IIB). The presence of the second methyl group on the imidazole ring has some consequences in physicochemical properties. We found atom H8 to be much more acidic in the purinium salt (II) than in the purine (I). We observed the signal of atom H8 to be strongly shifted downfield by 2.89 p.p.m. in 1H NMR spectroscopy [7.75 p.p.m. and 10.64 p.p.m. in compounds (I) and (II), respectively; Kowalska et al., 1993; Kowalska, 2007). Both N-methyl groups have very close chemical shifts (4.03 p.p.m. for N7/CH3 and 4.07 p.p.m. for N9/CH3) and the chemical shift of N7/CH3 is different from that in (I) (3.93 p.p.m.).
There is a close contact of 3.101 (4) Å between the peri-substituents (O6···C71H3), which is less than the sum of their van der Waals radii (3.40 Å; Pauling, 1960), meaning that the methoxy group is directed towards atom N1. We have reported that 2,6-dialkoxy-7,9-dimethylpurinium salts undergo O–N and N–N thermal rearrangement to give all four possible 1,3-dialkylxanthines via inter- and intramolecular alkyl group migration (Kowalska, & Maślankiewicz, 1997). The very close intramomolecular contacts, i.e. C61···N1 of 2.714 (4) Å, C21···N3 of 2.668 (4) Å and C91···N3 of 3.055 (4) Å, well explain the observed O6–N1, O2–N3 and N9–N1 methyl-group migration as intramolecular rearrangement.
As the crystals of (II) were grown from aqueous solution, we found water molecules in the crystal structure, with the O atom located οn a twofold rotation axis with an occupancy of 0.5. There is a weak C—H···O hydrogen bond between the cation and the partial water molecule (Table 2). There are also hydrogen bonds between the disordered iodide anions and water molecules (Table 2). The cations and anions are arranged in layers separated by 3.492 (2) Å (Fig. 2), while the water molecules are located in discrete cavities between the layers (Fig. 3).