Formation of extended polyiodides at large cation templates

Analysis of the structures of [Pd2I2([18]aneN2S4)](I)2·(I2)5 and [H2([2.2.2]cryptand)](I3)(I)(I2)2.5·CH2Cl2 identify some of the factors responsible for the structural features of extended polyiodides.


Introduction
Among extended anionic inorganic frameworks, the formation of polyhalides (Sonnenberg et al., 2020;Aragoni et al., 2003Aragoni et al., , 2022) ) and, in particular, polyiodides represents a remarkable example of supramolecular self-assembly (Blake et al., 1998c;Svensson et al., 2003), and it continues to capture the interest of many researchers in the field (Savastano, 2021;Savastano et al., 2022;Horn et al., 2003a,b;Aragoni et al., 2004Aragoni et al., , 2023a) ) due to the richness of its unpredictable and puzzling structural chemistry, and interesting applicative possibilities (Paulsson et al., 2004;Yin et al., 2012;Fei et al., 2015).Iodine and iodides together tend to catenate (Arca et al., 1999;Garau et al., 2022) via the combination of (Lewis acidic) I 2 with (Lewis basic) I À / I 3 À building blocks (Ciancaleoni et al., 2016).This affords extended arrays exhibiting a range of topologies, and these are highly dependent on the size, shape and charge of the countercation acting as a template.Some polyiodides are present in the crystal structure as discrete aggregates, but frequently they form extended networks in which the identification of the basic repeat unit of general formula [I n (I 2 ) m ] nÀ or [I 2m+n ] nÀ (n, m > 0) can become arbitrary.Consequently, they are better described as aggregates of I 2 , I À and I 3 À , held together by I� � �I interactions of varying strengths, from rather strong (ca 3.3 A ˚) to fairly weak, up to the van der Waals contact distance (ca 4 A ˚).Our interest in this field has been mainly focused on the use of metal complexes of macrocyclic ligands (mainly thioether crowns) as templating cations for controlling the selfassembly of extended polyiodide arrays (Blake et al., 1996(Blake et al., , 1998a,b),b).These complex cations are relatively chemically inert and their shape, size and charge can be changed readily, thus providing cationic templates for different targeted polyiodide topologies.Furthermore, we have also been interested in the reactivity of macrocyclic ligands with I 2 and inter-halogens IX (X = Br and Cl) to better understand the structural nature of the resulting products (Blake et al., 1997).The formation of polyiodide networks featuring spirals, belts, ribbons, sheets and cages as their structural motifs has been achieved either by reacting the PF 6 À or BF 4 À salts of the complex cation templates with an excess of I 2 in a single phase, or by addition of an NaI/I 2 mixture in a single phase, the preferred polyiodide being formed via self-assembly.As a further example of the versatility of this synthetic approach to the formation of multidimensional polyiodide networks, we report here the use of the metal complex [Pd 2 Cl 2 ([18]aneN 2 S 4 )](PF 6 ) 2 ([18]aneN 2 S 4 is 1,4,10,13-tetrathia-7,16-diazacyclooctadecane; see Scheme 1) and the neutral [2.2.2]cryptand (4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane) (Scheme 1) as templates in the reaction with I 2 .

Materials and methods
All starting materials, including [18]aneN 2 S 4 and [2.2.2]cryptand, and solvents, were obtained from Aldrich or Merck and were used without further purification.[Pd 2 Cl 2 ([18]-aneN 2 S 4 )](PF 6 ) 2 was prepared according to the literature (Blake et al., 1990).Microanalytical data were obtained on a Fisons EA 1108 CHNS-O instrument operating at 1000 � C. FT-Raman spectra (resolution 4 cm À 1 ) were recorded on a Bruker RF100FTR spectrometer fitted with an indium-gallium-arsenide detector operating at room temperature with an excitation wavelength of 1064 nm (Nd:YAG laser).No sample decomposition was observed during the experiments at the power level of the laser source used between 20 and 40 mW.The values in parentheses next to the values represent the intensities of the peaks relative to the strongest, which is taken to be equal to 10.No precipitate formed upon mixing, but dark-brown prismatic crystals of title compound (I) (Scheme 2) formed after several days by slow evaporation of the solvent from the reaction mixture.These were isolated from the mother liquor and washed with diethyl ether (8.4 mg, 36.3% yield).Elemental analysis found [calculated (%) for C 6 H 13 I 7 NPdS 2 ]: C 6.28 (6.22), H 1.15 (1.13), N 1.24 (1.21), S 5.52 (5.54).FT-Raman (range 500-50 cm À 1 ): �(I-I) 169.7 (10).

Refinement of X-ray crystal structures
Crystal data, data collection and structure refinement details are summarized in Table 1.H atoms were placed geometrically and refined isotropically riding on their parent C atoms, with U iso (H) = 1.2U eq (C).For (II), H atoms bonded to quaternary N atoms could be located from the difference Fourier map and their positions were refined freely.OLEX2 (Dolomanov et al., 2009) was used both as the graphical interface for the structural investigation and for the preparation of the figures.

Figure 2
View of a chain of interacting [Pd 2 I 2 ([18]aneN 2 S 4 )] 2+ dications found in the crystal structure of (I).The dications are arranged into chains via I� � �I interactions of 3.545 (2) A ˚running along the b axis.[Symmetry codes: (i) View of the two symmetry-related I 12 2À anions in (I) formed by the interaction of the two components of the disordered I 2 molecules with two 'V-shaped' I 5 À moieties of the type Chains of [Pd 2 I 2 ([18]aneN 2 S 4 )] 2+ complex dications (Fig. 2) run parallel to the b axis crossing adjacent 1D polyiodide tubes through the pseudo-cubic cavities (Fig. 6).It is interesting to note that, as the I7 atom of the disordered I 2 molecule lies on a glide plane, the resulting ratio between the two components is imposed by symmetry and the maximum occupancy possible is 0.5.As a consequence, the ratio between the two types of tubes described above remains constant in the crystal structure and cannot vary between different crystals.
That said, a unique crystal packing is observed in the crystal structure of (I) featuring the two sets of tubes formed by fused pseudo-cubic boxes (see above) running parallel (green) and perpendicular (blue) to the To illustrate further the importance of the shape, charge and dimensions of the template cation in the polyiodide network assembly, we treated the macropolycyclic ligand [2.2.2]cryptand (Scheme 1) with I 2 in a 1:4 molar ratio in CH 2 Cl 2 .Upon 2À is the component depicted in green as in Fig. 3).[Symmetry codes: (iv) View of the two differently-oriented polyiodide 1D tubes of interacting I 12 2À units co-existing at 50% occupancy in the crystal structure of (I).Colours are consistent with those in Fig. 3 for the differently-oriented I 12 2À units generating the two 1D polyiodides tubes.for selected geometric parameters).In (II), all three diiodine molecules are slightly elongated with respect to the I-I distance found in the crystal structure of orthorhombic I 2 [2.715 (6) A ˚] (Blake et al., 1998b).Each I1 atom interacts with an asymmetric triiodide unit at the I2 atom to afford a 'Z-shaped' I 8 2À dianion [I1� � �I2 = = 3.4123 (9) A ˚] that can be regarded as an I 3 (Savastano et al., 2022).Additional longer contacts of 3.907 (1) A ˚, still within the sum of the van der Waals radii for iodine, between each I1 atom and the terminal iodine (I5) of a pentaiodide moiety, lead to an overall discrete 'grasshoppershaped' I 18  (Bigoli et al., 1998).

Figure 7
The crystal structure of (II), showing the numbering scheme adopted.Displacement ellipsoids are drawn at the 50% probability level.H atoms and the cocrystallized CH 2 Cl 2 molecules are not shown.[Symmetry code:

FT-Raman spectroscopy
Despite the high number of extended polyiodides that have been structurally characterized, and the associated crystal structure data available, the assignment of higher molecular polyiodides (higher than I 3 À ) with their own distinctive structural features is still a matter of debate (Savastano et al., 2022).The reductionist approach whereby higher polyiodides are considered as aggregates of I 2 , I À and I 3 À held together by I� � �I interactions of varying strengths, from rather strong (up to ca 3.3-3.4A ˚) (covalent interactions) to fairly weak (up to the van der Waals contact distance, ca 4 A ˚) (supramolecular interactions), is still the most reasonable and least arbitrary.On the basis of structural data, all known higher discrete polyiodides can be regarded, therefore, as weak or mediumweak adducts of the type ] nÀ (n, m > 0), whose geometrical and topological features can be very different and often unpredictable (Arca et al. 2006).This way of considering higher polyiodides from a structural point of view is strongly supported by spectroscopic evidence.In particular, FT-Raman spectroscopy confirms that extended polyiodides do not have distinctive vibrational properties other than those of perturbed (slightly elongated) I 2 molecules and symmetric/slightly asymmetric I 3 À .Perturbed I 2 molecules are characterized by only one strong band in the range 180-140 cm À 1 in the FT-Raman spectrum, the wavenumber depending on the extent of the I� � �I elongation; for linear and symmetric I 3 À , only the Raman-active symmetric stretch (� 1 ) occurs near 110 cm À 1 , while the antisymmetric stretch (� 3 ) and the bending deformation (� 2 ) are only IR-active (Aragoni et al., 2023b).The latter two modes also become Raman-active for slightly asymmetric I 3 À and they are found near 134 (� 3 ) and 80 cm À 1 (� 2 ), having medium and medium-weak intensities, respectively.Highly asymmetric I 3 À ions show only one band in their FT-Raman spectra in the range 180-140 cm À 1 , so that they should be regarded as weak (I À )•I 2 adducts.To date, FT-Raman spectra of polyiodides of the general formula [I 2m+n ] nÀ show peaks in the low wavenumber region with either one strong peak in the range 180-140 cm À 1 or the characteristic peaks due to both perturbed I 2 and symmetric/ slightly asymmetric I 3 À .They would therefore be better described as The polyiodides here described are no exception.The FT-Raman spectrum of (I) features only a strong and broad peak centred at 169 cm À 1 indicative of the presence of differently perturbed I 2 molecules (Fig. S1 in the supporting information).The FT-Raman spectrum of (II) is shown in Fig. 10.The two peaks at about 167 and 150 cm À 1 can be assigned to the stretching vibration of the two differently elongated I 2 molecules I5-I6/I6-I7  and I1-I1 i [symmetry code: (i) À x + 2, À y + 1, À z], respectively.These data correspond closely to the established linear correlation �(I-I)/cm À 1 versus d(I-I)/A ˚for weak or mediumweak adducts (Arca et al., 2006).The peak at 106 cm À 1 can be attributed to the symmetric stretch (� 1 ) of the I 3 À ion (I2-I3-I4), thus confirming the description of the I 18 4À polyiodide as an [(I À ) 2 •(I 3 À ) 2 •(I 2 ) 5 ] adduct.

Conclusions
In this article, we confirm the structural variety of extended polyiodides that can be generated by changing the shape, charge and dimension of the cation template, as well as the synthetic strategy adopted and the experimental conditions.Although it is still often difficult to characterize [I 2m+n ] nÀ polyiodides higher than I 3 À on the grounds of any distinctive structural parameters, such as I-I bond distances, FT-Raman spectroscopy appears to confirm their characterization as aggregates of I 2 , I À and (symmetric or slightly asymmetric) I 3 À building blocks held together by I� � �I interactions of varying strengths.On the other hand, FT-Raman spectroscopy cannot provide any information on the topological features of extended polyiodides.The two techniques should therefore be used together in the analysis of this kind of compound.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.Diffraction data were collected on Stoe STADI4 4-circle and APEXII CCD area detector diffractometers for [Pd 2 I 2 ([18]aneN 2 S 4 )](I) 2 .(I 2 ) 5 , (I)< and [H 2 ([2.2.2]cryptand)](I 3 )(I)(I 2 ) 2.5 .CH 2 Cl 2 , (II), respectively.The structures were solved by direct methods using SHELXS (Sheldrick, 1997) or SHELXT2018 (Sheldrick, 2015a) and developed by iterative cycles of least-squares refinement on F 2 using SHELXL2018 (Sheldrick, 2015b).OLEX2 (Dolomanov et al., 2009) was used both as the graphical interface for the structural investigation and for the preparation of the figures.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.Diffraction data were collected on Stoe STADI4 4-circle and APEXII CCD area detector diffractometers for [Pd 2 I 2 ([18]aneN 2 S 4 )](I) 2 .(I 2 ) 5 , (I)< and [H 2 ([2.2.2]cryptand)](I 3 )(I)(I 2 ) 2.5 .CH 2 Cl 2 , (II), respectively.The structures were solved by direct methods using SHELXS (Sheldrick, 1997) or SHELXT2018 (Sheldrick, 2015a) and developed by iterative cycles of least-squares refinement on F 2 using SHELXL2018 (Sheldrick, 2015b).OLEX2 (Dolomanov et al., 2009) was used both as the graphical interface for the structural investigation and for the preparation of the figures.

Figure 6 (
Figure 6 (a) View along the b axis of the crystal packing in (I), showing the relative positions between the 1D polyiodide tubes and the chains of [Pd 2 I 2 ([18]-aneN 2 S 4 )] 2+ complex dications.The polyiodide network is also portrayed in parts (b) and (c) as blue and green tubes according to Fig. 3, and cations are coloured according to the type of tubes they cross.

Figure 9
Figure 9Views along approximately (a) the a axis and (b) the [101] direction of the crystal packing in (II).The blue colour and the ball-and-stick representation have been used for one of the discrete I 18 4À polyiodide units to better highlight its atomic connectivity in the crystal packing.H atoms and the cocrystallized CH 2 Cl 2 molecules are not shown.

Figure 10 FT-
Figure 10FT-Raman spectrum of (II) in the low frequency region.

Table 1
Experimental details.For both structures: Z = 4. Experiments were carried out with Mo K� radiation.