Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102000860/bm1478sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102000860/bm1478Isup2.hkl |
CCDC reference: 159566
The dipm ligand was synthesized using the literature method of Yao et al. (2000). Metal salts and solvents were sourced commercially and were used without further purification. Copper(II) tetrafluoroborate (?.?? mg, 1.3 mmol) and dipm (?.?? mg, 1.3 mmol) were each dissolved in ethanol/water (1:1, 10 ml) and the solutions mixed carefully together. A few drops of 2 M sodium hydroxide solution were then added gradually, so that no precipitate was formed and pH ≈ 5. The mixture was filtered to remove any solid particles and was then placed in a desiccator under a CO2 atmosphere generated from solid CO2 (van Albada, Mutikainen et al., 2000), and after a period of a few weeks to one month, long blue needles of (I) separated. The crystals were filtered off and washed with ethanol. Attempts to synthesize the compounds by the addition of sodium carbonate or potassium carbonate were unsuccessful. Satisfactory elemental analyses were obtained. A crystal was selected for the X-ray measurements and mounted on a glass fibre using the oil-drop method (Kottke & Stalke, 1993). Please give masses of reagents used above.
Please check these details Water H atoms were picked from a difference map and refined using restraints for the O—H and H···H distances of 0.83 (1) and 1.34 (1) Å, respectively. Other H atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms, at distances of 0.88 and 0.95 Å for N—H and C—H, respectively. For all H atoms, Uiso(H) = 1.2Ueq(carrier atom).
Data collection: CAD-4 Software (Nonius, 1998); cell refinement: CAD-4 Software; data reduction: XCAD4 (Harms & Wocadlo, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1995) and PLATON?; software used to prepare material for publication: SHELXL97.
[Cu(C8H7N5)(CO3)(H2O)]·2H2O | F(000) = 716 |
Mr = 350.78 | Dx = 1.838 Mg m−3 |
Monoclinic, P21/c | Cu Kα radiation, λ = 1.54178 Å |
a = 5.8520 (12) Å | Cell parameters from 25 reflections |
b = 12.897 (3) Å | θ = 12–27° |
c = 16.801 (3) Å | µ = 2.83 mm−1 |
β = 90.87 (3)° | T = 193 K |
V = 1267.9 (4) Å3 | Column, blue |
Z = 4 | 0.40 × 0.25 × 0.25 mm |
Enraf-Nonius CAD-4 diffractometer | 2079 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.041 |
Graphite monochromator | θmax = 65.0°, θmin = 4.3° |
ω/2θ scans | h = −6→6 |
Absorption correction: ψ-scan (North et al., 1968) | k = −15→15 |
Tmin = 0.400, Tmax = 0.493 | l = −19→19 |
2955 measured reflections | 3 standard reflections every 200 reflections |
2144 independent reflections | intensity decay: 0.0% |
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)] = 0.036 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.103 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.03 | w = 1/[σ2(Fo2) + (0.0754P)2 + 1.1584P] where P = (Fo2 + 2Fc2)/3 |
2144 reflections | (Δ/σ)max = 0.001 |
208 parameters | Δρmax = 0.52 e Å−3 |
0 restraints | Δρmin = −0.33 e Å−3 |
[Cu(C8H7N5)(CO3)(H2O)]·2H2O | V = 1267.9 (4) Å3 |
Mr = 350.78 | Z = 4 |
Monoclinic, P21/c | Cu Kα radiation |
a = 5.8520 (12) Å | µ = 2.83 mm−1 |
b = 12.897 (3) Å | T = 193 K |
c = 16.801 (3) Å | 0.40 × 0.25 × 0.25 mm |
β = 90.87 (3)° |
Enraf-Nonius CAD-4 diffractometer | 2079 reflections with I > 2σ(I) |
Absorption correction: ψ-scan (North et al., 1968) | Rint = 0.041 |
Tmin = 0.400, Tmax = 0.493 | 3 standard reflections every 200 reflections |
2955 measured reflections | intensity decay: 0.0% |
2144 independent reflections |
R[F2 > 2σ(F2)] = 0.036 | 0 restraints |
wR(F2) = 0.103 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.03 | Δρmax = 0.52 e Å−3 |
2144 reflections | Δρmin = −0.33 e Å−3 |
208 parameters |
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. The intensity data were corrected for Lorentz and polarization effects and for absorption. All nonhydrogen atoms were refined anisotropically. |
x | y | z | Uiso*/Ueq | ||
Cu1 | 0.35416 (5) | 0.38080 (2) | 0.171674 (18) | 0.01520 (17) | |
N111 | 0.3810 (3) | 0.32871 (15) | 0.06179 (11) | 0.0171 (4) | |
C112 | 0.5540 (4) | 0.35070 (18) | 0.01255 (13) | 0.0165 (5) | |
N113 | 0.5843 (4) | 0.30800 (16) | −0.05875 (12) | 0.0211 (4) | |
C114 | 0.4333 (4) | 0.2366 (2) | −0.08194 (15) | 0.0237 (5) | |
H11B | 0.4515 | 0.2043 | −0.1323 | 0.028* | |
C115 | 0.2501 (4) | 0.2076 (2) | −0.03531 (15) | 0.0240 (5) | |
H11C | 0.1441 | 0.1562 | −0.0526 | 0.029* | |
C116 | 0.2290 (4) | 0.25626 (19) | 0.03647 (15) | 0.0214 (5) | |
H11D | 0.1044 | 0.2388 | 0.0695 | 0.026* | |
N1 | 0.7196 (3) | 0.42252 (15) | 0.03229 (11) | 0.0181 (4) | |
H1 | 0.8218 | 0.4320 | −0.0049 | 0.022* | |
N121 | 0.6128 (3) | 0.47737 (14) | 0.16102 (11) | 0.0152 (4) | |
C122 | 0.7550 (4) | 0.48205 (17) | 0.09914 (13) | 0.0155 (5) | |
N123 | 0.9398 (3) | 0.54366 (15) | 0.09508 (11) | 0.0176 (4) | |
C124 | 0.9850 (4) | 0.60329 (18) | 0.15832 (15) | 0.0199 (5) | |
H12C | 1.1149 | 0.6474 | 0.1574 | 0.024* | |
C125 | 0.8504 (4) | 0.60323 (19) | 0.22492 (15) | 0.0208 (5) | |
H12D | 0.8853 | 0.6457 | 0.2697 | 0.025* | |
C126 | 0.6630 (4) | 0.53878 (18) | 0.22355 (13) | 0.0183 (5) | |
H12E | 0.5658 | 0.5376 | 0.2683 | 0.022* | |
C1 | 0.0550 (4) | 0.36856 (17) | 0.26314 (14) | 0.0174 (5) | |
O4 | 0.2387 (3) | 0.42356 (13) | 0.27722 (9) | 0.0196 (4) | |
O5 | 0.0527 (3) | 0.32572 (13) | 0.19326 (10) | 0.0191 (4) | |
O6 | −0.1036 (3) | 0.35672 (14) | 0.31162 (10) | 0.0235 (4) | |
O1 | 0.5747 (3) | 0.24475 (14) | 0.21689 (11) | 0.0253 (4) | |
H11 | 0.685 (3) | 0.275 (2) | 0.2393 (16) | 0.030* | |
H12 | 0.623 (5) | 0.210 (2) | 0.1792 (13) | 0.030* | |
O2 | 0.6791 (4) | 0.07989 (17) | 0.10284 (12) | 0.0367 (5) | |
H21 | 0.729 (6) | 0.076 (3) | 0.0569 (9) | 0.044* | |
H22 | 0.733 (6) | 0.0297 (19) | 0.1281 (16) | 0.044* | |
O3 | 0.7592 (4) | 0.0221 (2) | −0.06045 (14) | 0.0507 (6) | |
H32 | 0.803 (6) | 0.060 (3) | −0.0975 (16) | 0.061* | |
H31 | 0.643 (5) | −0.008 (3) | −0.077 (2) | 0.061* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cu1 | 0.0142 (2) | 0.0161 (2) | 0.0153 (2) | −0.00217 (11) | 0.00032 (15) | −0.00093 (11) |
N111 | 0.0173 (10) | 0.0168 (10) | 0.0173 (9) | −0.0025 (7) | −0.0021 (7) | 0.0000 (8) |
C112 | 0.0176 (11) | 0.0130 (10) | 0.0187 (11) | 0.0009 (9) | −0.0010 (9) | 0.0026 (9) |
N113 | 0.0246 (11) | 0.0206 (11) | 0.0180 (10) | −0.0042 (8) | 0.0007 (8) | −0.0036 (8) |
C114 | 0.0302 (13) | 0.0230 (13) | 0.0179 (12) | −0.0040 (10) | −0.0019 (10) | −0.0034 (10) |
C115 | 0.0275 (13) | 0.0221 (13) | 0.0224 (12) | −0.0084 (10) | −0.0044 (10) | −0.0032 (10) |
C116 | 0.0197 (12) | 0.0215 (12) | 0.0231 (12) | −0.0050 (9) | −0.0007 (9) | 0.0007 (10) |
N1 | 0.0190 (10) | 0.0197 (11) | 0.0156 (10) | −0.0057 (8) | 0.0040 (7) | −0.0038 (8) |
N121 | 0.0167 (9) | 0.0122 (9) | 0.0168 (9) | 0.0019 (7) | −0.0006 (7) | 0.0001 (7) |
C122 | 0.0163 (11) | 0.0117 (10) | 0.0185 (11) | 0.0019 (8) | −0.0014 (9) | 0.0000 (8) |
N123 | 0.0198 (10) | 0.0134 (9) | 0.0197 (10) | −0.0031 (8) | 0.0010 (8) | −0.0024 (8) |
C124 | 0.0219 (12) | 0.0162 (11) | 0.0216 (12) | −0.0027 (9) | −0.0010 (9) | −0.0012 (9) |
C125 | 0.0246 (13) | 0.0164 (12) | 0.0214 (13) | −0.0012 (9) | −0.0002 (10) | −0.0034 (9) |
C126 | 0.0211 (12) | 0.0165 (12) | 0.0173 (11) | 0.0021 (9) | 0.0021 (9) | −0.0010 (9) |
C1 | 0.0157 (12) | 0.0127 (11) | 0.0237 (12) | 0.0045 (8) | −0.0019 (9) | 0.0040 (9) |
O4 | 0.0172 (8) | 0.0219 (9) | 0.0196 (8) | −0.0010 (7) | 0.0015 (6) | −0.0014 (7) |
O5 | 0.0155 (8) | 0.0214 (9) | 0.0203 (8) | −0.0023 (6) | −0.0003 (6) | 0.0002 (7) |
O6 | 0.0200 (9) | 0.0261 (9) | 0.0245 (9) | 0.0010 (7) | 0.0049 (7) | 0.0039 (7) |
O1 | 0.0191 (9) | 0.0217 (9) | 0.0348 (11) | −0.0022 (7) | −0.0050 (7) | 0.0032 (7) |
O2 | 0.0438 (12) | 0.0353 (12) | 0.0311 (11) | 0.0082 (9) | 0.0018 (9) | 0.0030 (9) |
O3 | 0.0495 (14) | 0.0605 (16) | 0.0421 (13) | −0.0113 (12) | −0.0032 (10) | 0.0212 (11) |
Cu1—O1 | 2.3004 (18) | N121—C126 | 1.345 (3) |
Cu1—O4 | 1.9852 (17) | N121—C122 | 1.343 (3) |
Cu1—O5 | 1.9410 (16) | C122—N123 | 1.345 (3) |
Cu1—N111 | 1.973 (2) | N123—C124 | 1.335 (3) |
Cu1—N121 | 1.971 (2) | C124—C125 | 1.378 (4) |
Cu1—C1 | 2.352 (3) | C124—H12C | 0.9500 |
N111—C112 | 1.347 (3) | C125—C126 | 1.376 (4) |
N111—C116 | 1.354 (3) | C125—H12D | 0.9500 |
C112—N113 | 1.333 (3) | C126—H12E | 0.9500 |
C112—N1 | 1.377 (3) | C1—O6 | 1.253 (3) |
N113—C114 | 1.330 (3) | C1—O5 | 1.298 (3) |
C114—C115 | 1.389 (4) | C1—O4 | 1.307 (3) |
C114—H11B | 0.9500 | O1—H11 | 0.836 (10) |
C115—C116 | 1.367 (4) | O1—H12 | 0.832 (10) |
C115—H11C | 0.9500 | O2—H21 | 0.830 (10) |
C116—H11D | 0.9500 | O2—H22 | 0.833 (10) |
N1—C122 | 1.373 (3) | O3—H32 | 0.834 (10) |
N1—H1 | 0.8800 | O3—H31 | 0.831 (10) |
O1—Cu1—O4 | 96.53 (7) | C122—N1—H1 | 114.0 |
O1—Cu1—O5 | 99.56 (7) | C112—N1—H1 | 114.0 |
O1—Cu1—N111 | 89.84 (8) | C126—N121—C122 | 116.6 (2) |
O1—Cu1—N121 | 94.83 (7) | C126—N121—Cu1 | 117.40 (16) |
O4—Cu1—O5 | 67.19 (7) | C122—N121—Cu1 | 125.77 (16) |
O4—Cu1—N111 | 164.43 (7) | N121—C122—N123 | 125.0 (2) |
O4—Cu1—N121 | 100.25 (8) | N121—C122—N1 | 121.4 (2) |
O5—Cu1—N111 | 97.79 (8) | N123—C122—N1 | 113.6 (2) |
O5—Cu1—N121 | 161.79 (8) | C124—N123—C122 | 116.7 (2) |
N111—Cu1—N121 | 93.33 (8) | N123—C124—C125 | 122.5 (2) |
O5—Cu1—C1 | 33.47 (8) | N123—C124—H12C | 118.8 |
N121—Cu1—C1 | 133.02 (8) | C125—C124—H12C | 118.8 |
N111—Cu1—C1 | 131.22 (8) | C126—C125—C124 | 116.9 (2) |
O4—Cu1—C1 | 33.74 (8) | C126—C125—H12D | 121.6 |
O1—Cu1—C1 | 98.77 (7) | C124—C125—H12D | 121.6 |
C112—N111—C116 | 116.6 (2) | N121—C126—C125 | 122.2 (2) |
C112—N111—Cu1 | 125.03 (16) | N121—C126—H12E | 118.9 |
C116—N111—Cu1 | 117.89 (16) | C125—C126—H12E | 118.9 |
N113—C112—N111 | 125.2 (2) | O6—C1—O5 | 122.6 (2) |
N113—C112—N1 | 113.0 (2) | O6—C1—O4 | 124.3 (2) |
N111—C112—N1 | 121.7 (2) | O5—C1—O4 | 113.1 (2) |
C112—N113—C114 | 116.9 (2) | O6—C1—Cu1 | 176.85 (17) |
N113—C114—C115 | 122.4 (2) | O5—C1—Cu1 | 55.59 (11) |
N113—C114—H11B | 118.8 | O4—C1—Cu1 | 57.53 (11) |
C115—C114—H11B | 118.8 | C1—O4—Cu1 | 88.73 (14) |
C116—C115—C114 | 117.1 (2) | C1—O5—Cu1 | 90.94 (14) |
C116—C115—H11C | 121.5 | Cu1—O1—H11 | 103 (2) |
C114—C115—H11C | 121.5 | Cu1—O1—H12 | 111 (2) |
N111—C116—C115 | 121.8 (2) | H11—O1—H12 | 109 (2) |
N111—C116—H11D | 119.1 | H21—O2—H22 | 107 (2) |
C115—C116—H11D | 119.1 | H32—O3—H31 | 106 (2) |
C122—N1—C112 | 132.0 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···N123i | 0.88 | 2.10 | 2.979 (3) | 179 |
O1—H11···O5ii | 0.84 (3) | 2.39 (3) | 3.017 (3) | 132 (2) |
O1—H11···O6ii | 0.84 (3) | 2.02 (3) | 2.841 (3) | 167 (3) |
O1—H12···O2 | 0.83 (3) | 2.14 (3) | 2.933 (3) | 160 (3) |
O2—H21···O3 | 0.83 (3) | 2.10 (3) | 2.888 (3) | 159 (4) |
O2—H22···O4iii | 0.83 (3) | 2.10 (3) | 2.886 (3) | 156 (3) |
O3—H31···O2iv | 0.83 (3) | 2.14 (3) | 2.960 (3) | 171 (4) |
O3—H32···O6v | 0.83 (3) | 1.95 (4) | 2.785 (4) | 177 (4) |
Symmetry codes: (i) −x+2, −y+1, −z; (ii) x+1, y, z; (iii) −x+1, y−1/2, −z+1/2; (iv) −x+1, −y, −z; (v) x+1, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | [Cu(C8H7N5)(CO3)(H2O)]·2H2O |
Mr | 350.78 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 193 |
a, b, c (Å) | 5.8520 (12), 12.897 (3), 16.801 (3) |
β (°) | 90.87 (3) |
V (Å3) | 1267.9 (4) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 2.83 |
Crystal size (mm) | 0.40 × 0.25 × 0.25 |
Data collection | |
Diffractometer | Enraf-Nonius CAD-4 diffractometer |
Absorption correction | ψ-scan (North et al., 1968) |
Tmin, Tmax | 0.400, 0.493 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2955, 2144, 2079 |
Rint | 0.041 |
(sin θ/λ)max (Å−1) | 0.588 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.036, 0.103, 1.03 |
No. of reflections | 2144 |
No. of parameters | 208 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.52, −0.33 |
Computer programs: CAD-4 Software (Nonius, 1998), CAD-4 Software, XCAD4 (Harms & Wocadlo, 1996), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXTL/PC (Sheldrick, 1995) and PLATON?, SHELXL97.
Cu1—O1 | 2.3004 (18) | Cu1—N111 | 1.973 (2) |
Cu1—O4 | 1.9852 (17) | Cu1—N121 | 1.971 (2) |
Cu1—O5 | 1.9410 (16) | ||
O1—Cu1—O4 | 96.53 (7) | O4—Cu1—N111 | 164.43 (7) |
O1—Cu1—O5 | 99.56 (7) | O4—Cu1—N121 | 100.25 (8) |
O1—Cu1—N111 | 89.84 (8) | O5—Cu1—N111 | 97.79 (8) |
O1—Cu1—N121 | 94.83 (7) | O5—Cu1—N121 | 161.79 (8) |
O4—Cu1—O5 | 67.19 (7) | N111—Cu1—N121 | 93.33 (8) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···N123i | 0.88 | 2.10 | 2.979 (3) | 179 |
O1—H11···O5ii | 0.84 (3) | 2.39 (3) | 3.017 (3) | 132 (2) |
O1—H11···O6ii | 0.84 (3) | 2.02 (3) | 2.841 (3) | 167 (3) |
O1—H12···O2 | 0.83 (3) | 2.14 (3) | 2.933 (3) | 160 (3) |
O2—H21···O3 | 0.83 (3) | 2.10 (3) | 2.888 (3) | 159 (4) |
O2—H22···O4iii | 0.83 (3) | 2.10 (3) | 2.886 (3) | 156 (3) |
O3—H31···O2iv | 0.83 (3) | 2.14 (3) | 2.960 (3) | 171 (4) |
O3—H32···O6v | 0.83 (3) | 1.95 (4) | 2.785 (4) | 177 (4) |
Symmetry codes: (i) −x+2, −y+1, −z; (ii) x+1, y, z; (iii) −x+1, y−1/2, −z+1/2; (iv) −x+1, −y, −z; (v) x+1, −y+1/2, z−1/2. |
The title compound was obtained in our ongoing research into ligands which can act as a trap for CO2 by the insertion of atmospheric CO2 into coordination compounds. Insertion reactions of CO2 into metal-ligand bonds are not uncommon, and in the past few years many review papers have been published on the subject (Gibson, 1999; Walther et al., 1999; Yin & Moss, 1999). Several dinuclear copper(II) compounds have been synthesized by means of fixation of CO2 from air (Nishida et al., 1999; Kitajima et al., 1993; Kruger et al., 1995; Youngme et al., 2000). In order to build and control larger aggregates, we have started a research programme in which the ligands used also act as hydrogen-bond donors or acceptors.
The recently developed ligand bis(pyrimidin-2-yl)amine (abbreviated to dipm; Yao et al., 2000) is a prototype, as it may form Watson-Crick-type hydrogen bonds, as already shown in the literature with the first-generation ligand 2-aminopyrimidine (van Albada, Quiroz-Castro et al., 2000; van Albada, Smeets et al., 2000). Continuing this work, we present here the crystal stucture of the title complex, (I). \sch
Structural analysis of (I) shows a neutral [Cu(dipm)(CO3)(H2O)] unit and two non-coordinating water molecules. The CuII anion has a distorted square-pyramidal environment, with a basal plane formed by two N atoms of the dipm ligand and two O atoms of the carbonato group [basal Cu—N/O distances vary from 1.9410 (16) to 1.9852 (17) Å]. The apical position is occupied by the O atom of the coordinating water molecule at a distance of 2.3004 (18) Å.
The distortion from square-pyramidal geometry can be best expressed with the parameter τ (for five-coordinate complexes, τ describes the relative amount of trigonality; τ = 0 for square pyramidal and τ = 1 for trigonal bipyramidal; Addison et al., 1984), and in this case τ = 0.045, representing a very small distortion from square pyramidal and indicating a dx2-y2 orbital ground state.
The aromatic ring-ring stacking interactions between neighbouring bipyrimidine rings, which help to stabilize the lattice structure, are characterized by a centroid-centroid distance of 3.592(?)Å Please provide s.u. on this distance.
The Cu geometry in (I) is related to that in [Cu(dpyam)(CO3)(H2O)]·2H2O (dpyam is di-2-pyridylamine; Akhter et al., 1991). In the latter structure, the distortion is slightly larger (τ = 0.097).
Compound (I) has an additional hydrogen-bonding feature which generates dinuclear units: the amino N atom forms a Watson-Crick-type hydrogen bond with the neighbouring ring N atom [N1···N123 2.979 (3) Å] (Fig.2). Additional hydrogen-bonding contacts occur between the O atom of the axial water molecule and the non-coordinating O atoms of a neighbouring carbonato anion and a lattice water molecule [O1···O5 3.018 (3), O1···O6 2.841 (3) Å (a bifurcated hydrogen bond) and O1···O2 2.933 (3) Å]. The lattice water molecules form hydrogen bonds with neighbouring carbonato anions and uncoordinated water molecules, the O···O distances varying from 2.785 (3) to 2.960 (4) Å.
The IR spectrum of (I) clearly shows the out-of-plane mode of the CO3 dianion, with a medium to strong band at 807 cm-1, and this matches well with the bands observed in the literature (van Albada, Mutikainen et al., 2000; Youngme et al., 2000) for chelating carbonato ligands.