research communications
3]·H2O using non-spherical atomic form factors
of K[HgIaInstituto de Física, Benemérita Universidad Autónoma de Puebla, 72570 Puebla, Pue., Mexico
*Correspondence e-mail: sylvain_bernes@hotmail.com
The 3]·H2O, based on single-crystal data, was reported 50 years ago [Nyqvist & Johansson (1971). Acta Chem. Scand. 25, 1615–1629]. We have now redetermined this structure with X-ray diffraction data at 0.70 Å resolution collected at 153 K using Ag Kα radiation. Combined quantum mechanical methods (ORCA) and computation of non-spherical scattering form factors (NoSpherA2) allowed the of the shape of the water molecule with anisotropic H atoms, despite the presence of heavy elements in the crystal. The refined shape of the water molecule via this Hirshfeld is close to that determined for liquid water by neutron diffraction experiments. Moreover, the Laplacian of the electron density clearly shows how electron density accumulates along the O—H σ-valence bonds in the water molecule.
model for potassium triiodidomercurate(II) monohydrate, K[HgIKeywords: crystal structure; redetermination; atomic form factors; NoSpherA2.
CCDC reference: 2087087
1. Chemical context
It is well known that the `independent atom model' (IAM), universally implemented in mainstream X-ray crystallography software, has the drawback of affording insufficient ca 20% by X-rays. However, neutron diffraction facilities are scarce, and even non-existent in underdeveloped countries. As a matter of fact, only 0.2% of the structures currently deposited in the CSD originate from neutron diffraction studies (Groom et al., 2016).
models. Given that a spherical distribution of electron density around each atom is assumed, for example, by using the Cromer–Mann parameterization of the non-dispersive part of the form factors, any density involved in bonds, lone pairs and intermolecular charge transfer is completely ignored. In this context, satisfactory structure models can be obtained only on the basis of neutron diffraction data. An extreme case of discrepancy between results obtained with both radiations is the O—H bond length for the hydroxyl group in and water, which is underestimated byWithin many approaches available to overcome this issue, the `Hirshfeld atom et al., 2014) strategy is gaining popularity. After calculating a molecular wave function for a structural model (not necessarily limited to the asymmetric unit), the electronic density functions of the so-called Hirshfeld atoms are extracted through a partitioning process (Hirshfeld, 1977), and eventually Fourier transformed, to afford non-spherical scattering factors for each individual atom in the real space and each reflection in the More accurate structure factors can then be calculated during a least-squares and the full process can be iterated until convergence.
(HAR; CapelliA user-friendly implementation of HAR has been recently released with OLEX2 (version 1.3) and is fully interfaced with the olex2.refine least-squares engine (Kleemiss et al., 2021). This new tool, coined as NoSpherA2 (pronounced `Nosferatu'), is virtually universal since any element can be present in the structure. Moreover, the structure can be disordered, with atoms in special positions, squeezed with a solvent mask, or can include restrained parts. Twinned crystals can also be handled in the same way as single crystals, by computing a single wave function for each twin component. Finally, data resolution is not a concern, as long as atomic resolution is achieved [dmin = 0.84 Å, corresponding to (sin θ/λ)max = 0.6 Å−1]. At worst, a data set with no information at all about aspherical local densities would give a Hirshfeld close to that obtained with Cromer–Mann form factors.
So far, HAR has been used mainly for organic compounds, for at least two reasons. Many accurate orbital basis sets are available for light elements and, more significantly, this class of molecules is the most interesting one for such refinements: organic compounds include a large variety of chemical bonds (σ, π, aromatic, 2c–3e bonds, etc.) and heteroatoms frequently bear electron lone pairs. The structural model obtained via HAR is thus expected to be greatly improved compared to that derived from a traditional with spherical densities.
We used NoSpherA2 to refine the of a material including both heavy and light elements, with the aim of assessing whether a non-spherical is suitable and useful for such materials. The matter has been already studied for challenging compounds, namely transition-metal hydrides (Woińska et al., 2021; Kleemiss et al., 2021), and is now extended to an iodidomercurate hydrate, K[HgI3]·H2O.
2. Structural commentary
The 3]·H2O, was reported 50 years ago, using data collected on a Philips–Norelco PAILRED diffractometer, with monochromatized Mo K radiation (1542 reflections in the 0kl–10kl half-sphere; R = 0.081 for an anisotropic model omitting H atoms; Nyqvist & Johansson, 1971). The powder diffraction pattern is also deposited in the PDF-2 database, with reference PDF 00-027-0415 (Gates-Rector & Blanton, 2019). Using low-temperature data collected with Ag Kα radiation, we now obtained the same structure at 0.70 Å resolution in the same Pna21 (Fig. 1 and Table 1). The Hg atoms form distorted [HgI4] tetrahedra sharing one corner and giving a chain structure along the a-axis direction. Water molecules bridge K+ cations and are sandwiched between these chains, at normal distances, K—OH2 ≃ 2.75 Å. The cations are seven-coordinate, a common for K+, characterized by its large ionic radius. The three-dimensional structure is completed by K+ cations bridging [HgI4] tetrahedra in neighbouring chains. The water molecules are oriented in such a way that O—H⋯I hydrogen bonds are formed with two I atoms on the edge of an [HgI4] tetrahedron.
of potassium triiodidomercurate(II) monohydrate, K[HgIAlthough H atoms were visible in a difference-Fourier map, the IAM SHELXL (Sheldrick, 2015b) gave an odd shape for the water molecule. Hydroxyl O—H groups were then restrained to have the same bond lengths with an effective standard deviation of 0.04 Å. Rigid bond restraints with a standard deviation of 0.008 Å for 1,2 and 1,3 distances in the K/O1/H1a/H1b fragment were also applied. Both O—H bond lengths in the water molecule converged to 0.84 (11) Å, and the H—O—H angle was too acute, 87 (10)°. Moreover, isotropic displacement parameters for the H1a and H1b atoms were unbalanced, 0.06 (5) and 0.18 (9) Å2, respectively. For this preliminary hydrogen bonds were determined with large uncertainties for O—H⋯I angles, 160 (12) and 159 (26)°.
carried out withWith the hope of improving the shape of the water molecule, a non-spherical SHELXL model as a starting point. The wave functions were calculated using ORCA with the two-component relativistic basis set x2c-TZVPP and the generalized gradient approximation PBE functional (Neese, 2018). The least-squares refinements were then carried out with olex2.refine (Bourhis et al., 2015), while keeping the same restraints as for the SHELXL For the final calculation of non-spherical form factors with NoSpherA2, a neutral dimeric cluster [KHgI3·H2O]2 was used as a structure model, in order to take into account O—H⋯I hydrogen bonds. The final was done with olex2.refine (Table 1), and a comparison of the asymmetric units for the IAM and HAR refinements is given in Fig. 2.
was carried out using theThe heavy part of the structure is almost unchanged after HAR, as expected. When comparing bonds lengths and angles, the largest difference is observed for the K—O bonds, with a shift of 0.006 Å; for bond angles, the largest difference between the two refinements is 0.25° for the angle K1—O1—K1i [symmetry code: (i) x + , −y + , z]. Moreover, uncertainties for bond lengths and angles are systematically improved with HAR. Likewise, displacement parameters for Hg, I and K atoms are almost unaffected after using non-spherical form factors. In contrast, the water molecule clearly displays a more accurate shape. Bond lengths for the O—H groups are 1.07 (6) and 1.11 (7) Å for the HAR model, with an H—O—H angle of 107 (8)°. For liquid water, neutron diffraction experiments afforded O—H = 0.970 ± 0.005 Å and H—O—H = 106.1 ± 1.8° (Ichikawa et al., 1991; Milovanović et al., 2020). These dimensions are also consistent with the shape previously described for a water molecule bridging two K+ cations in a potassium aryloxide aggregate characterized by neutron diffraction at 100 K: O—H = 0.963 (16)–1.009 (16) Å and H—O—H = 108.0 (13)° (Morris et al., 2007). It was possible to refine anisotropic displacement parameters for the H atoms, although it was necessary to use rigid bond restraints for the K—OH2 group, in order to avoid non-positive definite H atoms. In the final model, displacement ellipsoids for H atoms are well balanced (Fig. 2).
The final residual map is featureless, but the deformation density map in the water molecule vicinity is insightful (Fig. 3). A positive density close to the O atom reflects the presence of electron lone pairs, while a negative density centred on the H-atom sites indicates the positively charged character of the H atoms, as a consequence of the difference of with the O atom. A diffuse positive density is even visible at the midpoint of the O—H bonds, related to the contribution of the covalent σ-bonds to non-spherical densities. Beyond features observed for the water molecule, the deformation map is flat, confirming that a Hirshfeld adds very little to the conventional IAM approximation in those parts. Finally, the Laplacian of the electron density, , also shows expected features. Electronic density is locally concentrated over the attractive covalent O—H σ-bonds in the water molecule (Fig. 4), while heavy atoms display isosurfaces with spherical symmetry.
3. Discussion and conclusions
Regarding the R1 and wR2 is marginal with a HAR compared to a IAM with SHELXL, at any resolution, since the structure-factor amplitudes are dominated by the contribution of heavy scatterers, Hg and I. However, in the present case, diffraction data contain information about the non-sphericity of the form factors for the O and H atoms, warranting a HAR. Given that computational cost associated with the calculation of the wave function increases drastically for large molecular systems or large clusters of molecules, HAR may prove challenging to implement as a day-to-day routine, as long as desktop computers are used for structure refinements. However, the reported here shows that an alternative would be to perform refinements through a hybrid IAM/HAR strategy, with structure factors including conventional spherical form factors for heavy atoms, and non-spherical form factors for light atoms. Obviously, this may not apply to large organic systems, like proteins, unless super-computing is involved (Capelli et al., 2014).
the drop for residuals4. Synthesis and crystallization
Caution!! Any mercury compound poses potential health risks; appropriate safety precautions and disposal procedures must be taken to handle the complexes here reported.
The compound under study was obtained as a by-product during the synthesis of Ag2[HgI4]. A procedure to obtain Ag2[HgI4] single crystals involves the near saturation of K2[HgI4] with HgI2 and AgI in an aqueous medium (Browall et al., 1974). Potassium tetraiodomercurate(II), commonly known as Nessler reagent, was obtained by dissolving 2.603 g of KI and 3.574 g of HgI2 in an aqueous medium, following the reaction: HgI2 + 2 KI → K2[HgI4]. The resulting solution was nearly saturated with HgI2 and subsequently with AgI. The solution was kept under constant stirring for 30 min at 323 K. After that, the solution was stored in 50 ml plastic tubes in complete darkness for one month.
The crystals obtained were washed with a 2 M solution of K2[HgI4] and distilled water. Since the process for the preparation of these compounds contains the precursors HgI2 and KI in an aqueous medium, this also favours the crystallization of K[HgI3]·H2O within a temperature range of 273–353 K (Sieskind et al., 1998). One small crystal of K[HgI3]·H2O recovered from such a crystallization was used for the present study.
5. details
Crystal data, data collection, and structure olex2.refine are summarized in Table 1. All atoms were refined anisotropically. In the water molecule, O—H bonds were restrained to have the same length, with a standard deviation of 0.04 Å. Rigid bond restraints with a standard deviation of 0.008 Å for 1,2 and 1,3 distances in the K—OH2 fragment were also applied.
details for the last least-squares cycle ofSupporting information
CCDC reference: 2087087
https://doi.org/10.1107/S2056989021005582/wm5609sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989021005582/wm5609Isup2.hkl
Data collection: X-AREA (Stoe & Cie, 2019); cell
X-AREA (Stoe & Cie, 2019); data reduction: X-AREA (Stoe & Cie, 2019); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: olex2.refine 1.3 (Bourhis et al., 2015); molecular graphics: Mercury (Macrae et al., 2020) and OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: publCIF (Westrip, 2010).K[HgI3]·H2O | Dx = 4.676 Mg m−3 |
Mr = 638.41 | Ag Kα radiation, λ = 0.56083 Å |
Orthorhombic, Pna21 | Cell parameters from 25947 reflections |
a = 8.5810 (2) Å | θ = 2.2–30.8° |
b = 9.2648 (3) Å | µ = 14.87 mm−1 |
c = 11.4073 (4) Å | T = 153 K |
V = 906.89 (5) Å3 | Block, colourless |
Z = 4 | 0.06 × 0.05 × 0.03 mm |
F(000) = 1072 |
Stoe Stadivari diffractometer | 2720 independent reflections |
Radiation source: Sealed X-ray tube, Axo Astix-f Microfocus source | 2179 reflections with I > 2σ(I) |
Graded multilayer mirror monochromator | Rint = 0.070 |
Detector resolution: 5.81 pixels mm-1 | θmax = 23.6°, θmin = 2.2° |
ω scans | h = −12→12 |
Absorption correction: multi-scan (X-AREA; Stoe & Cie, 2019) | k = −13→13 |
Tmin = 0.064, Tmax = 0.132 | l = −16→16 |
29699 measured reflections |
Refinement on F2 | Hydrogen site location: difference Fourier map |
Least-squares matrix: full | All H-atom parameters refined |
R[F2 > 2σ(F2)] = 0.021 | w = 1/[σ2(Fo2) + (0.0157P)2] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.038 | (Δ/σ)max = 0.0003 |
S = 0.87 | Δρmax = 1.17 e Å−3 |
2720 reflections | Δρmin = −1.25 e Å−3 |
74 parameters | Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
21 restraints | Extinction coefficient: 0.00015 (5) |
0 constraints | Absolute structure: Flack (1983) |
Primary atom site location: dual | Absolute structure parameter: 0.033 (11) |
Secondary atom site location: difference Fourier map |
Refinement. Refinement using NoSpherA2, an implementation of NOn-SPHERical Atom-form-factors in Olex2. Please cite: F. Kleemiss et al. DOI 10.1039/D0SC05526C - 2020 NoSpherA2 implementation of HAR makes use of tailor-made aspherical atomic form factors calculated on-the-fly from a Hirshfeld-partitioned electron density (ED) - not from spherical-atom form factors. The ED is calculated from a gaussian basis set single determinant SCF wavefunction - either Hartree-Fock or DFT using selected funtionals - for a fragment of the crystal. This fregment can be embedded in an electrostatic crystal field by employing cluster charges. The following options were used: SOFTWARE: ORCA PARTITIONING: NoSpherA2 INT ACCURACY: High METHOD: PBE BASIS SET: x2c-TZVPP CHARGE: 0 MULTIPLICITY: 1 RELATIVISTIC: DKH2 DATE: 2021-04-12_22-39-08 |
x | y | z | Uiso*/Ueq | ||
Hg1 | 0.25561 (3) | 0.70101 (2) | 0.500694 (18) | 0.02464 (6) | |
I1 | 0.25904 (7) | 0.42035 (5) | 0.56854 (3) | 0.02692 (9) | |
I2 | 0.49597 (5) | 0.77133 (3) | 0.33996 (3) | 0.01831 (7) | |
I3 | 0.23628 (7) | 0.92253 (5) | 0.65827 (3) | 0.02791 (10) | |
K1 | 0.44530 (17) | 0.15898 (17) | 0.3610 (2) | 0.0413 (4) | |
O1 | 0.6331 (5) | 0.4017 (5) | 0.3698 (5) | 0.0321 (11) | |
H1a | 0.634 (12) | 0.437 (11) | 0.459 (6) | 0.034 (16) | |
H1b | 0.607 (14) | 0.498 (9) | 0.316 (9) | 0.06 (2) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Hg1 | 0.02658 (10) | 0.02249 (10) | 0.02484 (11) | −0.00072 (11) | 0.00067 (13) | −0.00180 (11) |
I1 | 0.0331 (2) | 0.0247 (2) | 0.02295 (19) | −0.0019 (2) | −0.0010 (2) | 0.00594 (16) |
I2 | 0.01527 (15) | 0.02130 (15) | 0.01837 (17) | 0.00014 (15) | −0.00029 (16) | 0.0012 (2) |
I3 | 0.0337 (2) | 0.0264 (2) | 0.0236 (2) | 0.0034 (2) | −0.0026 (3) | −0.00697 (17) |
K1 | 0.0251 (7) | 0.0278 (7) | 0.0711 (13) | 0.0039 (5) | −0.0017 (8) | 0.0034 (9) |
O1 | 0.030 (2) | 0.020 (2) | 0.047 (3) | 0.0026 (18) | 0.002 (2) | −0.002 (2) |
H1a | 0.02 (4) | 0.04 (2) | 0.047 (10) | 0.007 (15) | 0.002 (7) | −0.005 (5) |
H1b | 0.10 (5) | 0.021 (16) | 0.05 (2) | 0.001 (11) | −0.017 (16) | −0.003 (8) |
Hg1—I1 | 2.7131 (5) | I3—K1iv | 3.659 (2) |
Hg1—I2 | 2.8356 (5) | I3—K1v | 3.7067 (19) |
Hg1—I2i | 2.8968 (5) | K1—O1ii | 2.739 (5) |
Hg1—I3 | 2.7333 (5) | K1—O1 | 2.769 (5) |
I1—K1ii | 3.6595 (19) | O1—H1a | 1.07 (6) |
I1—K1 | 3.745 (2) | O1—H1b | 1.11 (7) |
I2—K1iii | 3.6257 (16) | ||
I3—Hg1—I1 | 122.189 (16) | I2vii—K1—I1 | 137.19 (6) |
I2—Hg1—I1 | 113.375 (16) | I1viii—K1—I1 | 92.00 (5) |
I3—Hg1—I2 | 107.267 (15) | I3ix—K1—I1 | 148.42 (5) |
I2i—Hg1—I1 | 105.885 (15) | I3x—K1—I1 | 77.82 (3) |
I3—Hg1—I2i | 107.648 (15) | O1ii—K1—I2vii | 85.19 (10) |
I2—Hg1—I2i | 97.460 (14) | O1—K1—I2vii | 137.50 (10) |
K1ii—I1—Hg1 | 90.00 (3) | O1ii—K1—I1viii | 130.89 (15) |
K1—I1—Hg1 | 116.37 (3) | O1—K1—I1viii | 73.23 (12) |
K1iii—I2—Hg1 | 95.60 (3) | O1ii—K1—I3ix | 135.32 (15) |
K1iii—I2—Hg1vi | 87.86 (3) | O1—K1—I3ix | 75.86 (11) |
K1iv—I3—Hg1 | 102.44 (3) | O1ii—K1—I3x | 75.34 (12) |
K1v—I3—Hg1 | 86.63 (3) | O1—K1—I3x | 74.46 (12) |
K1—I1—K1ii | 77.01 (4) | O1ii—K1—I1 | 72.08 (12) |
K1iv—I3—K1v | 77.50 (4) | O1—K1—I1 | 72.56 (11) |
Hg1—I2—Hg1vi | 99.816 (14) | K1—O1—K1viii | 113.65 (15) |
I2vii—K1—I1viii | 75.86 (3) | O1ii—K1—O1 | 137.29 (12) |
I3ix—K1—I2vii | 70.37 (3) | H1a—O1—K1viii | 95 (5) |
I3ix—K1—I1viii | 79.52 (3) | H1a—O1—K1 | 106 (6) |
I3x—K1—I2vii | 131.42 (6) | H1b—O1—K1viii | 110 (6) |
I3x—K1—I1viii | 147.69 (5) | H1b—O1—K1 | 121 (6) |
I3ix—K1—I3x | 93.18 (5) | H1b—O1—H1a | 107 (8) |
Symmetry codes: (i) x−1/2, −y+3/2, z; (ii) x−1/2, −y+1/2, z; (iii) x, y+1, z; (iv) −x+1, −y+1, z+1/2; (v) −x+1/2, y+1/2, z+1/2; (vi) x+1/2, −y+3/2, z; (vii) x, y−1, z; (viii) x+1/2, −y+1/2, z; (ix) −x+1, −y+1, z−1/2; (x) −x+1/2, y−1/2, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1a···I3vi | 1.07 | 2.76 | 3.777 (6) | 158 |
O1—H1b···I2 | 1.11 | 2.72 | 3.637 (5) | 140 |
Symmetry code: (vi) x+1/2, −y+3/2, z. |
Acknowledgements
We are grateful to Dr Florian Kleemiss (Olexsys Ltd, UK) for helpful guidance during the NoSpherA2.
of the structure and for the continuous development ofFunding information
Funding for this research was provided by: Consejo Nacional de Ciencia y Tecnología (grant No. 268178; grant No. A1-S-10011).
References
Bourhis, L. J., Dolomanov, O. V., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2015). Acta Cryst. A71, 59–75. Web of Science CrossRef IUCr Journals Google Scholar
Browall, K. W., Kasper, J. S. & Wiedemeier, H. (1974). J. Solid State Chem. 10, 20–28. CrossRef ICSD CAS Web of Science Google Scholar
Capelli, S. C., Bürgi, H.-B., Dittrich, B., Grabowsky, S. & Jayatilaka, D. (2014). IUCrJ, 1, 361–379. Web of Science CSD CrossRef CAS PubMed IUCr Journals Google Scholar
Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339–341. Web of Science CrossRef CAS IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Gates-Rector, S. & Blanton, T. (2019). Powder Diffr. 34, 352–360. CAS Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CrossRef IUCr Journals Google Scholar
Hirshfeld, F. L. (1977). Theor. Chim. Acta, 44, 129–138. CrossRef CAS Web of Science Google Scholar
Ichikawa, K., Kameda, Y., Yamaguchi, T., Wakita, H. & Misawa, M. (1991). Mol. Phys. 73, 79–86. CrossRef CAS Web of Science Google Scholar
Kleemiss, F., Dolomanov, O. V., Bodensteiner, M., Peyerimhoff, N., Midgley, L., Bourhis, L. J., Genoni, A., Malaspina, L. A., Jayatilaka, D., Spencer, J. L., White, F., Grundkötter-Stock, B., Steinhauer, S., Lentz, D., Puschmann, H. & Grabowsky, S. (2021). Chem. Sci. 12, 1675–1692. Web of Science CSD CrossRef CAS Google Scholar
Macrae, C. F., Sovago, I., Cottrell, S. J., Galek, P. T. A., McCabe, P., Pidcock, E., Platings, M., Shields, G. P., Stevens, J. S., Towler, M. & Wood, P. A. (2020). J. Appl. Cryst. 53, 226–235. Web of Science CrossRef CAS IUCr Journals Google Scholar
Milovanović, M. R., Živković, J. M., Ninković, D. B., Stanković, I. M. & Zarić, S. D. (2020). Phys. Chem. Chem. Phys. 22, 4138–4143. Web of Science PubMed Google Scholar
Morris, J. J., Noll, B. C., Schultz, A. J., Piccoli, P. M. B. & Henderson, K. W. (2007). Inorg. Chem. 46, 10473–10475. Web of Science CSD CrossRef PubMed CAS Google Scholar
Neese, F. (2018). WIREs Comput. Mol. Sci. 8, e1327. Google Scholar
Nyqvist, L. & Johansson, G. (1971). Acta Chem. Scand. 25, 1615–1629. CrossRef ICSD CAS Web of Science Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sieskind, M., Amann, M. & Ponpon, J. P. (1998). Appl. Phys. A, 66, 655–658. Web of Science CrossRef CAS Google Scholar
Stoe & Cie (2019). X-AREA. Stoe & Cie, Darmstadt, Germany. Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
Woińska, M., Chodkiewicz, M. L. & Woźniak, K. (2021). Chem. Commun. 57, 3652–3655. Google Scholar
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