crystals Article Synthesis and Crystal Structures of Cadmium(II) Cyanide with Branched-Butoxyethanol Takeshi Kawasaki 1 and Takafumi Kitazawa 1,2,* ID 1 Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan; [email protected] 2 Research Centre for Materials with Integrated Properties, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan * Correspondence: [email protected]; Tel.: +81-47-472-5077; Fax: +81-47-475-5077 Received: 27 December 2017; Accepted: 15 May 2018; Published: 17 May 2018 Abstract: 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. Keywords: crystal engineering; cadmium(II) cyanide; coordination polymer; self-assembly; disorder; host-guest; inclusion compound; hydrogen bond 1. Introduction Because of their unique chemical properties, cadmium cyanide systems included with various guest species have been found to be one of the most important and mineralomimetic materials since the discovery of the clathrate complexes [1–22]. The discovery of the clathrate Cd(CN)2·CCl4 [4] is very important in the history of the simple, yet subtle, cadmium(II) cyanide clathrates and their related systems. Cadmium(II) cyanide is a 3D porous coordination polymer clathrating various guest molecules (for example, CCl4 [4–6], C6H6 [9] CH2ClCHCl2 [10], and Bu2O[10]) by van der Waals force in its cavity space. Interestingly, the Cd(CN)2 host structures change according to the guest [4–20], and the hosts form a mineralomimetic framework. The coordination geometry of Cd(II) in Cd(CN)2 is normally a tetrahedral four-coordination geometry (denoted as CdT). However depending on the influence of other ligands (for example, H2O[8,13–19]), the geometry might also be trigonal-bipyramidal five-coordination geometry (CdTB), or octahedral six-coordination geometry (CdOC). For Cd(CN)2 clathrates containing a lipophilic guest, the coordination geometries of Cd(II) were CdT and the Cd(CN)2 frameworks were cristobalite-like or tridymite-like structures [4–11,21]. In contrast, the Cd(CN)2 clathrates with the water molecule(s) coordinating to Cd(II) ion contain alcohol or short dialkyl-ether (alkyl group with a carbon number less than 3) as a guest, and Crystals 2018, 8, 221 ; doi:10.3390/cryst8050221 www.mdpi.com/journal/crystals Crystals 2018, 8, 221 2 of 10 Crystals 2018, 8, x FOR PEER REVIEW 2 of 10 theframeworks host frameworks function as function zeolite‐mimetic as zeolite-mimetic structures [8,13–19]. structures Incidentally, [8,13–19]. Incidentally, for Cd(CN)2, forthere Cd(CN) was no2, thereguest wasmolecule no guest found molecule to facilitate found a todoubly facilitate interpenetrated a doubly interpenetrated diamond‐like diamond-likeframework. Furthermore, framework. Furthermore,the coordination the geometry coordination of Cd(II) geometry was Cd ofT [3–7]. Cd(II) Thus, was we Cd surmisedT [3–7]. that Thus, the we hydrophilic surmised groups that the of hydrophilicguest molecules groups influence of guest the Cd(II) molecules coordination influence environment. the Cd(II) coordination We are interested environment. in the effect We of arethe interestedcoexistence in of the two effect kinds of theof coexistencehydrophilic ofgroups two kindson the of hydrophilicCd(II) coordination groups onenvironment. the Cd(II) coordinationAlkoxyethanol environment. (Rcel) has both Alkoxyethanol hydroxyl and (Rcel) etheric has groups both hydroxyl as shown and in etheric Scheme groups 1. We as recently shown inreported Scheme three1. Wecrystal recently structures reported of cadmium three crystal cyanide structures coordination of cadmium polymers cyanide with an alkoxyethanol coordination polymersof normal‐ withalkyl anchain alkoxyethanol structure; that of normalis, [Cd(CN)-alkyl2 chain(Etcel)] structure;n, [{Cd(CN) that2(Bucel)} is, [Cd(CN)3{Cd(CN)2(Etcel)]2}]n, andn, [{Cd(CN)2(H(Bucel)}2O)2}{Cd(CN)3{Cd(CN)2}32∙2(Hexcel)]}]n, andn (Etcel [{Cd(CN) = 2‐ethoxyethanol,2(H2O)2}{Cd(CN) Bucel2 }=3 ·22(Hexcel)]‐butoxyethanol,n (Etcel Hexcel == 2-ethoxyethanol,2‐hexyloxyethanol) Bucel [22]. = In 2-butoxyethanol, [Cd(CN)2(Etcel)] Hexceln and [{Cd(CN) = 2-hexyloxyethanol)2(Bucel)}3{Cd(CN) [22].2 In}]n, [Cd(CN) hydroxyl2(Etcel)] oxygenn andatoms [{Cd(CN) of Rcel 2ligands(Bucel)} coordinate3{Cd(CN)2 }]ton ,the hydroxyl Cd(II) oxygenions, and atoms the Cd(II) of Rcel ions ligands exhibit coordinate CdTB. In to contrast, the Cd(II) in ions,[{Cd(CN) and2(H the2O) Cd(II)2}{Cd(CN) ions2 exhibit}3∙2(Hexcel)] CdTBn., Hexcel In contrast, molecules in [{Cd(CN) do not coordinate2(H2O)2}{Cd(CN) to the Cd(II)2}3·2(Hexcel)] ions, andn, Hexceltwo water molecules molecules do notare coordinatelocated at tothe the cis‐ Cd(II)positions ions, of andCdOC two. The water three molecules Cd(CN)2 areframework located atstructures the cis-positions varied according of CdOC to. differences The three in Cd(CN)the normal2 framework‐alkyl chain structuresof the Rcel variedmolecules. according We report to differencesherein, the insynthesis the normal and-alkyl crystal chain structures of the Rcelof two molecules. novel cadmium(II) We report herein,cyanide the complexes synthesis with and crystalbranched structures‐butoxyethanol of two novel cadmium(II)compounds cyanide complexes withof branched-butoxyethanolformulae compounds[{Cd(CN)2(iBucel) of2}{Cd(CN) formulae2(H2 [{Cd(CN)O)(iBucel)}2(iBucel)2{Cd(CN)2}{Cd(CN)2}6∙2(iBucel)]2(Hn2 O)(iBucel)}2{Cd(CN)I 2}6·2(iBucel)]andn I[{Cd(CN)and [{Cd(CN)2(H2O)1.062(tBucel)(H2O)1.060.94(tBucel)}{Cd(CN)0.942(tBucel)}}{Cd(CN)2{Cd(CN)2(tBucel)}2}2∙1.06(tBucel)]2{Cd(CN)2}2n· 1.06(tBucel)]II (iBucel = ison ‐butoxyethanol,II (iBucel = isotBucel-butoxyethanol, = tert‐butoxyethanol tBucel = (Schemetert-butoxyethanol 1)). (Scheme1)). (a) (b) Scheme 1. The structural formulae of alkoxyethanol in thisthis work:work: (a)) isoiso-butoxyethanol‐butoxyethanol (iBucel); and ((b)) terttert-butoxyethanol‐butoxyethanol (tBucel).(tBucel). 2. Materials and Methods 2. Materials and Methods 2.1. Synthesis 2.1.1. Synthesis of [{Cd(CN)2(iBucel)(iBucel)2}{Cd(CN)}{Cd(CN)22(H(H22O)(iBucel)}O)(iBucel)}2{Cd(CN)2{Cd(CN)2}26∙}2(iBucel)]6·2(iBucel)]n, nI , I An aqueousaqueous solutionsolution (45(45 mL)mL) containingcontaining CdClCdCl22·∙2.5H2O (2 mmol), andand KK22[Cd(CN)4]] (4 (4 mmol), was stirredstirred forfor 3030 minmin atat roomroom temperature.temperature. After the solution was filteredfiltered through a membranemembrane filter,filter, colorlesscolorless crystals of I were obtained from the filtratefiltrate with 3 mL of iBucel kept in a sealed container at room temperature for two weeks. Elemental analysis found, found, II:: C; 28.44 H; 3.84, and N; −1 −1 11.21%.11.21%. Calculated Calculated for for C C5454HH8888NN18O1814OCd14Cd9: C;9: 29.15, C; 29.15, H; 3.99, H; 3.99, and and N; 11.33%. N; 11.33%. IR(nujol IR(nujol mull, mull, cm ): cm νOH ):= n3355(br),OH = 3355(br), νCN = 2180(s),nCN = 2180(s), νCOC = 1112(s).nCOC = 1112(s). 2.1.2. Synthesis of [{Cd(CN)2(H(H2O)O)1.061.06(tBucel)(tBucel)0.940.94}{Cd(CN)}{Cd(CN)2(tBucel)}2(tBucel)}2{Cd(CN)2{Cd(CN)2}2∙2(1.06tBucel)]}2·(1.06tBucel)]n, IIn , II An aqueousaqueous solutionsolution (45(45 mL)mL) containingcontaining CdClCdCl22·∙2.5H2O (2 mmol), andand KK22[Cd(CN)4]] (4 (4 mmol), was stirredstirred for for 30 30 min min at at room room temperature. temperature. After After the the solution solution was was filtered filtered through through a membrane a membrane filter, colorlessfilter, colorless crystals crystals of II were of II obtained were obtained from the from filtrate the withfiltrate 3 mL with of tBucel3 mL of kept tBucel in a sealedkept in container a sealed atcontainer room temperature at room temperature for a week. for Elemental a week. analysisElementalII :analysis C; 30.71 II H;: C; 4.11, 30.71 and H; N; 4.11, 10.70%. and CalculatedN; 10.70%. −1 forCalculated C34H58.12 forN C1034OH9.0658.12CdN105:O C;9.06Cd 31.08,5: C; H;31.08, 4.46, H; and 4.46, N; and 10.66%. N; 10.66%. IR(nujol IR(nujol mull, mull, cm cm):−1n):OH νOH= = 3319(br), 3319(br), nνCN == 2179(s), 2179(s), νnCOCCOC = 1089(s).= 1089(s). 2.2. 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, Crystals 2018, 8, 221 3 of 10 2.2.