Synthesis and Crystal Structures of Cadmium ( II ) Cyanide with Branched-Butoxyethanol

Two novel 3D cadmium(II) cyanide coordination polymers with branched-butoxyethanol compounds (iBucel = iso-butoxyethanol, tBucel = tert-butoxyethanol), [{Cd(CN)2 (iBucel)2}{Cd(CN)2(H2O)(iBucel)}2{Cd(CN)2}6·2(iBucel)]n I and [{Cd(CN)2(H2O)1.06(tBucel)0.94} {Cd(CN)2(tBucel)}2{Cd(CN)2}2·1.06(tBucel)]n II, were synthesized and characterized by structural determination. Complex I contains two distinct Cd(II) coordination geometries: octahedral and tetrahedral. In contrast, complex II contains three distinct Cd(II) coordination geometries: octahedral, square-pyramidal, and tetrahedral. In the two complexes, branched-butoxyethanol molecules behave as both a ligand and a guest in the Cd(CN)2 cavities. The framework in I contains octahedral and tetrahedral Cd(II) in a 3:6 ratio. In I, the coordination environments of octahedral Cd(II) are cis-O-Cd-O. The framework in II contains octahedral, square-pyramidal, and tetrahedral Cd(II) in a 1:2:2 ratio. In II, the coordination environment of octahedral Cd(II) is disordered trans-O-Cd-O and the axial oxygen ligand is either a water or tBucel molecule. In II, the square-pyramidal Cd(II) geometry is formed by one tBucel ligand and four cyanide ligands. The Cd(CN)2 frameworks of the two complexes exhibit different structures.


Single Crystal X-ray Diffraction
The structural characterization for I and II was determined by the single crystal X-ray diffraction using a Bruker ApexII Smart CCD area-detector diffractometer (Bruker, Madison, WI,

Single Crystal X-ray Diffraction
The structural characterization for I and II was determined by the single crystal X-ray diffraction using a Bruker ApexII Smart CCD area-detector diffractometer (Bruker, Madison, WI, USA) with monochromated Mo-Kα radiation (λ = 0.71073 Å) under a temperature controlled N 2 gas flow.The diffraction data were treated using APEX2 [23] and SAINT ver.7.23A [24].The absorption data were collected using SADABS ver.2014/5 [25].The structures were solved using the direct method, expanded using Fourier techniques, and refined using a full-matrix least-square refinement.H atoms of water molecules and of the hydroxyl group of alkoxyethanol behaving as a ligand were located in difference syntheses and refined isotropically.The remaining H atoms were placed at calculated positions and all H atoms were allowed to ride on a parent atom.

Crystal Structure
Single crystals of complexes I and II were prepared by a method similar to that found in the literature [4,22].Crystal data for I and II are listed in Table 1.Complex I crystallizes in the space group C2/c.The space group of complex II was transformed from C2/c to I2/a of the alternate setting of C2/c so that its monoclinic angle would be less than 120 degrees [28,29].In both complexes, all cyanide bridges between the two Cd(II) ions, Cd(CN) 2 , form a three-dimensional framework with large cavities (Figure 1, Figure S1, Figure 2 and Figure S2).We observed from the IR spectra that the peaks of C≡N stretching (ν CN ) were at approximately 2180 cm −1 .The ν CN values showed a blue shift compared with that observed for terminal C≡N of K 2 [Cd(CN) 2 ] (2145 cm −1 ).This observation supports the view that C≡N bridges two Cd(II) ions.The exact arrangements of cyanides (Cd-NC-Cd or Cd-CN-Cd) cannot be determined by single X-ray diffraction because these arrangements are disordered.Nishikiori et al. [13,30,31] showed that the arrangements of cyanides in a Cd(CN) 2 host are disordered, by 113 Cd-CP/MAS NMR.The crystal structure of complex I is shown in Figure 1 and Figure S1, and selected parameters are listed in Table 2. Complex I has five crystallographically independent Cd(II) sites denoted as Cd1, Cd2, Cd3, Cd4, and Cd5 (Figure 1a).Cd1 and Cd2 have octahedral six-coordination geometries, and the remaining sites have tetrahedral four-coordination geometries.In the unit cell, Cd2 is located on a special position, and the other Cd(II) sites are located on general positions.Therefore, the complex contains Cd OC and Cd T in a 3:6 ratio.The iBucel molecules exist as both a ligand and a guest in the Cd(CN) 2 cavities (Figure 1a and Figure S1c).Because all iBucel molecules are strongly disordered (Figure 1a), the R factor is relatively high (Table 1).Around Cd1, two oxygen atoms are located at the cis-positions (Figure 1a) and four cyanides are located at the remaining positions.At the cis-position, one oxygen (O2) atom is located in the hydroxyl group of the iBucel ligand and another oxygen (O1) is located in the water molecule.Around Cd2, two oxygen (O3) atoms of iBucel ligands are located at the cis-positions (Figure 1a) and four cyanides are located at the remaining positions.Atoms labeled N9/C9 or N10/C10 are hybrids because of the disordered arrangement of cyanide (Figure 1 [22] and other complexes with cis-O-Cd-O [14][15][16].The Cd(CN) 2 framework forms a NOQ net and these cavities' shapes are a distorted [4 2 .5 2 .6 2 .8 2 ] tile [32][33][34] (Figure 1c).The iBucel guest connects the water ligand through weak hydrogen bonds between etheric oxygen and water oxygen, and between hydroxyl oxygen and water oxygen.The iBucel ligands are connected with the neighboring iBucel ligands by hydrogen bonds between one ligand's hydroxyl oxygen atoms (O2 and O4) and the other's etheric oxygen atoms (O5 and O3) (Figure 1d and Table 2), and these iBucel ligands form (O-CH 2 -CH 2 -O) 2 eight-membered rings as in the case of complexes [Cd(CN) 2 (Etcel)] n and [{Cd(CN) 2 (Bucel)} 3 {Cd(CN) 2 }] n [22].
For complex II, the crystal structure is shown in Figure 2 and Figure S2, and selected parameters are listed in Table 3. Complex II has three crystallographically independent Cd(II) sites denoted as Cd1, Cd2, and Cd3 (Figure 2a).Cd1 has a four-coordination geometry, Cd2 has a five-coordination geometry, and Cd3 has a six-coordination geometry.In the unit cell, Cd1 and Cd2 are located on general positions, and Cd3 is located on a special position.Therefore, the complex contains Cd1, Cd2, and Cd3 in a 2:2:1 ratio.Cd1 is Cd T and is coordinated by only four cyanides.Around the Cd2, one hydroxyl oxygen atom O1 of tBucel ligand is located at one of the positions and four cyanides are located at the remaining positions.The five-coordination geometry is estimated by a simple distortion parameter τ proposed by Addison et al. [22,35,36].The τ value is simply defined by τ = (θ 1 − θ 2 )/60, where θ 1 is the largest basal angle and θ 2 is a second largest; a perfect square pyramid is characterized by τ = 0, while τ = 1 represents a perfect trigonal bipyramid [22,35,36].For the Cd2, τ is 0.46 from the selected bond angles, as shown in Table 3.Therefore, Cd2 has a distorted square pyramid five-coordination geometry (Cd SP ).The N2 atom of cyanide is located at the apex of the square pyramid.The N1 atom of cyanide is located at the trans-position of O1.The Cd2-N1 bond length is longer than the other Cd2-(CN) bonds (Table 3).The Cd2-O1 bond length is slightly longer than the Cd TB -O bond lengths of complexes [Cd(CN) 2 (Etcel)] n and [{Cd(CN) 2 (Bucel)} 3 {Cd(CN) 2 }] n [22].The tBucel ligand on Cd2 is connected with the neighboring tBucel on Cd2 by hydrogen bonds between one ligand's hydroxyl oxygen atom O1 and the other's etheric oxygen atom O2 (Figure 2c and Table 3), as in the case of complexes [Cd(CN) 2 (Etcel)] n , [{Cd(CN) 2 (Bucel)} 3 {Cd(CN) 2 }] n [22], and I. Around the Cd3, two oxygen (O3) atoms are located at the axial positions and four cyanides are located at the equatorial positions.The equatorial plane of Cd3 is almost parallel to the xz plane of the cell.The axial positions are disordered.The O3 atom is either the oxygen of the water ligand (Figure 2d) or the hydroxyl oxygen of the tBucel ligand (Figure 2e,f).Furthermore, the tBucel molecule over the axial position of Cd3 is either a guest or ligand.Specifically, if O3 is water, then the tBucel molecule on the axial position is a guest; otherwise, the tBucel is a ligand.The conformation of the tBucel ligand on Cd3 comprises two types (Figure 2e,f).However, C23 denotes that the center carbon atom of the tert-butyl group is ordered regardless of the above-mentioned disorder.The Cd(CN) 2 cavity around O3 is similar to Cd(CN) 2 •2/3H 2 O•t-BuOH (t-BuOH = tert-butanol) [18,19] (Figure 2d).We suggest that one of the factors affecting the topologies of Cd(CN) 2 cavities, as in Figure 2d, are variations in the tert-butyl groups.The occupancies of disordered parts are 0.531(3) (Figure 2e), 0.322(3) (Figure 2f), and 0.146(3) (Figure 2g).The TG and DTG curves (Figure S4 the cell.The axial positions are disordered.The O3 atom is either the oxygen of the water ligand (Figure 2d) or the hydroxyl oxygen of the tBucel ligand (Figure 2e,f).Furthermore, the tBucel molecule over the axial position of Cd3 is either a guest or ligand.Specifically, if O3 is water, then the tBucel molecule on the axial position is a guest; otherwise, the tBucel is a ligand.The conformation of the tBucel ligand on Cd3 comprises two types (Figure 2e,f).However, C23 denotes that the center carbon atom of the tert-butyl group is ordered regardless of the above-mentioned disorder.The Cd(CN)2 cavity around O3 is similar to Cd(CN)2•2/3H2O•t-BuOH (t-BuOH = tert-butanol) [18,19] (Figure 2d).We suggest that one of the factors affecting the topologies of Cd(CN)2 cavities, as in Figure 2d, are variations in the tert-butyl groups.The occupancies of disordered parts are 0.531(3) (Figure 2e), 0.322(3) (Figure 2f), and 0.146(3) (Figure 2g).The

Conclusions
We  1d and 2c) [22].The (O-CH 2 -CH 2 -O) 2 eight-membered rings may assist in the coordination of alkoxyethanol molecules to Cd(II) ions and in the stabilization of that coordination.The Cd(CN) 2 frameworks of complexes I and II exhibit different structures.Furthermore, these Cd(CN) 2 framework structures of the two complexes are different from the framework structure of complex [{Cd(CN) 2 (Bucel)} 3 {Cd(CN) 2 }] n containing a Bucel ligand and having a butyl group.This suggests that the Cd(CN) 2 framework and coordination environment of Cd(II) are strongly affected by not only the number of carbons of the alkyl group of the Rcel molecule but also by the shape of the alkyl group of the Rcel molecule.Because of the differences in their alkyl groups, the Cd(CN) 2 frameworks of cadmium(II) cyanide coordination polymers with alkoxyethanol [22] change more dramatically than the frameworks of complexes with either dialkyl-ether [16,17] or with alkyl-alcohol [8,13,14,18,19].The effect of the alkyl group of alkoxyethanol compounds was found to be stronger than the effect of either alkyl-alcohol or dialkyl-ether compounds.These new cadmium cyanide host-guest compounds also demonstrate potential advantages of the clathrate chemistry for molecular recognition, a theme with a long history of research on inclusion compounds [1].
) and because of the effects of the midpoints of the C≡N bonds on the symmetry of the lattice [2-13,22,30,31].Cd T are coordinated by only four cyanides.The TG and DTG curves (Figure S3) were observed in four steps of weight loss ranging from approximately 45 • C to 100 • C (first weight loss), from 100 • C to 160 • C (second and third weight losses), and from 160 • C to 230 • C (fourth weight loss).The first weight loss seems to have caused the desorption of iBucel guests from complex I.Because the second and third weight losses suggest that the elimination of the iBucel ligands occur near each other, we conclude that these two DTG peaks overlap.The fourth weight loss seems to involve the elimination of water ligands.The values of bond lengths around Cd(II) ions decrease in the order Cd OC -O ≈ Cd OC -(CN) ≈ Cd T -(CN).The cis-O-Cd-O angles are similar to those in complex ) were observed in three steps of weight loss ranging from approximately 80 • C to 145 • C, from 143 • C to 175 • C, and from 175 • C to 230 • C. The first weight loss seems to be due to the release of 1.06 tBucel guest molecules and 0.94 tBucel ligands coordinated to Cd3.The peak shoulder on the DTG curve in the first weight loss seems to be due to the release of the 1.06 tBucel guests.The second weight loss suggests that the tBucel ligands coordinated to Cd2 were removed from II.The third weight loss seems to involve the elimination of the 1.06 H 2 O ligand.The Cd3-O3 bond length is in the range of Cd OC -O bonds of complexes with trans-O-Cd OC -O [14,16-19].Crystals 2018, 8, x FOR PEER REVIEW 5 of 10

Figure 2 .
Figure 2. The crystal structure of II.Because arrangements of cyanides (Cd-NC-Cd or Cd-CN-Cd) are disordered, the cyanide atoms are labeled arbitrarily: (a) Asymmetric unit containing disordered parts.Displacement ellipsoids are drawn at the 50% probability level.H atoms are omitted for clarity; (b) The Cd(CN)2 network view along the b axis; (c) Hydrogen bonds between neighboring tBucel ligands on Cd2 in the cavities of cadmium cyanide network of II.Displacement ellipsoids are drawn at the 50% probability level.H atoms, except for OH hydrogens, are omitted for clarity; (d) Cd(CN)2 cavity around O3. Displacement ellipsoids are drawn at the 50% probability level.H atoms are omitted for clarity; (e-g).The disordered parts of the axial positions on the Cd3 and hydrogen bonds between the neighboring ligands.(e) Shows where water molecules coordinate to the Cd3.(f,g) show how the tBucel ligands coordinate to the Cd3.All hydrogen bonds might not be found due to the complexity of the observed disorder.(Symmetry codes: i = −x + 1/2, −y + 3/2, −z + 3/2; iii = −x, −y + 2, −z + 2; iv = x + 1/2, −y + 2, z; vii = −x + 1/2, −y + 1/2, −z + 3/2; viii = −x + 1/2, y, −z + 2).

Figure 2 .
Figure 2. The crystal structure of II.Because arrangements of cyanides (Cd-NC-Cd or Cd-CN-Cd) are disordered, the cyanide atoms are labeled arbitrarily: (a) Asymmetric unit containing disordered parts.Displacement ellipsoids are drawn at the 50% probability level.H atoms are omitted for clarity; (b) The Cd(CN) 2 network view along the b axis; (c) Hydrogen bonds between neighboring tBucel ligands on Cd2 in the cavities of cadmium cyanide network of II.Displacement ellipsoids are drawn at the 50% probability level.H atoms, except for OH hydrogens, are omitted for clarity; (d) Cd(CN) 2 cavity around O3. Displacement ellipsoids are drawn at the 50% probability level.H atoms are omitted for clarity; (e-g).The disordered parts of the axial positions on the Cd3 and hydrogen bonds between the neighboring ligands.(e) Shows where water molecules coordinate to the Cd3.(f,g) show how the tBucel ligands coordinate to the Cd3.All hydrogen bonds might not be found due to the complexity of the observed disorder.(Symmetry codes: i = −x + 1/2, −y + 3/2, −z + 3/2; iii = −x, −y + 2, −z + 2; iv = x + 1/2, −y + 2, z; vii = −x + 1/2, −y + 1/2, −z + 3/2; viii = −x + 1/2, y, −z + 2).
Figure S2: Cd(CN) 2 network structure of II; (a) along the c axis and (b) along the (101) direction.
Figure S3: TG and DTG plots of I.
Figure S4: TG and DTG plots of II.comp_I.cifand comp_II.cif:Crystal analysis data for complexes I and II, respectively.Author Contributions: Takafumi Kitazawa conceived and designed the experiments; Takeshi Kawasaki performed the experiments and analyzed data; Takeshi Kawasaki and Takafumi Kitazawa wrote the paper.

Table 1 .
The crystal data.