International Journal of Molecular Sciences Article Intramolecular Spodium Bonds in Zn(II) Complexes: Insights from Theory and Experiment Mainak Karmakar 1, Antonio Frontera 2 , Shouvik Chattopadhyay 1,* , Tiddo J. Mooibroek 3,* and Antonio Bauzá 2,* 1 Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India; [email protected] 2 Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma (Baleares), Spain; [email protected] 3 van ‘t Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands * Correspondence: [email protected] (S.C.); [email protected] (T.J.M.); [email protected] (A.B.) Received: 30 August 2020; Accepted: 22 September 2020; Published: 25 September 2020 Abstract: Two new dinuclear zinc(II) complexes, [Zn (µ -OAc)(L1) ]I MeOH (1) and [Zn (µ - 2 1,3 2 · 2 1,3 OAc)(L2)(NCS)] (2), (where HL1 = 2-(((3-(dimethylamino)propyl)amino)methyl)-6-methoxy-phenol 2 and H2L = 2,20-[(1-Methyl-1,2-ethanediyl)bis(iminomethylene)]bis[6-ethoxyphenol]) have been synthesized and characterized by elemental and spectral analysis. Their X-ray solid state structures have been determined, revealing the existence of intramolecular Zn O spodium bonds in both ··· complexes due to the presence of methoxy (1) or ethoxy (2) substituents adjacent to the coordinated phenolic O-atom. These noncovalent interactions have been studied using density functional theory (DFT) calculations, the quantum theory of “atoms-in-molecules” and the noncovalent interaction plot. Moreover, a search in the Cambridge structure database (CSD) has been conducted in order to investigate the prevalence of intramolecular spodium bonds in Zn complexes. To our knowledge this is the first investigation dealing with intramolecular spodium bonds. Keywords: zinc complexes; spodium bonds; σ-hole interactions; CSD analysis; DFT calculations 1. Introduction Noncovalent interactions are very important in many fields of research, including molecular recognition, crystal engineering and catalysis [1–3]. Among the great deal of noncovalent forces, investigations on σ-hole interactions [4,5] are growing very fast and are definitively recognized by the scientific community as an alternative to the ubiquitous hydrogen bonding [6]. Very recently, the attractive interaction between elements of Group 12 of the Periodic Table and any electron rich ‘accepting’ atom (:A) [7,8] has been termed as a spodium bond (SpB) [9]. Therefore, the SpB has become a new member of the σ-hole family of interactions and it is adequate to differentiate the coordination bond (high covalent character) typical of transition metals from the noncovalent contact. SpBs are directional, the electron rich atom is located at distances that are longer than the sum of covalent radii and they are considerably weaker than coordination bonds. Moreover, the participation of the antibonding σ*(Sp–Y, where Y can be any atom) in the SpB bonding interaction (Y–Sp :A) has been ··· evidenced [9], as is common in σ-hole interactions [4,5]. This manuscript reports the synthesis and X-ray characterization of two new Zn(II) complexes that exhibit intramolecular SpBs. The utilization of two different tridentate and tetradendate ligands allows analyzing the interaction in two different Zn-coordination modes, square-pyramidal in (1) Int. J. Mol. Sci. 2020, 21, 7091; doi:10.3390/ijms21197091 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 7091 2 of 14 Int. J. Mol. Sci. 2020, 21, 7091 2 of 14 andpseudotetrahedral pseudotetrahedral in (2). in The (2). SpBs The in SpBs both incompounds both compounds have been have studied been using studied density using functional density functionaltheory (DFT) theory calculations (DFT) calculations and characterized and characterized by a combination by a combination of the quantum of the quantumtheory of theoryatoms ofin atomsmolecules in molecules (QTAIM) (QTAIM) [10] and [10 the] and noncovalent the noncovalent interaction interaction (NCI) (NCI) method method [11]. [11Moreover,]. Moreover, the theCambridge Cambridge Structural Structural Database Database (CSD) (CSD) [12] [12 has] has been been examined examined in in order order to to investigate investigate the prevalence ofof intramolecularintramolecular SpBsSpBs inin Zn(II)Zn(II) complexescomplexes andand theirtheir geometric geometric features. features. 2.2. Results and Discussion 2.1.2.1. Synthesis InIn this this work, work, two two acetate acetate bridged bridged dinuclear dinuclear zinc complexes zinc complexes have been have synthesized been synthesized using reduced using 1 2 1 reducedSchiff bases Schi ff HLbases1 and HL Hand2L2 Has2 L ligands.as ligands. HL1 HLwas wasprepared prepared by by1:1 1:1 condensation condensation of of 3-3‐ methoxysalicylaldehydemethoxysalicylaldehyde and andN, NN-dimethyl-1,3-diaminopropane,N‐dimethyl‐1,3‐diaminopropane in methanol in methanol and subsequent and subsequent reduction 2 usingreduction NaBH using4 following NaBH4 following a literature a literature method [ 13method–15]. In[13–15]. the same In the way, same H2 way,L was H2L prepared2 was prepared by 2:1 condensationby 2:1 condensation of 3-ethoxysalicylaldehyde of 3‐ethoxysalicylaldehyde and rac and-1,2-diaminopropane rac‐1,2‐diaminopropane in methanol in methanol followed followed by the 1 reductionby the reduction with NaBH with4. NaBH Methanol4. Methanol solution solution of HL on of reaction HL1 on with reaction zinc acetatewith zinc dihydrate acetate anddihydrate potassium and 2 iodidepotassium in a iodide 2:2:1 molar in a 2:2:1 ratio molar produced ratio complexproduced1 .complex On the other1. On hand,the other methanol hand, methanol solution of solution H2L on of reactionH2L2 on withreaction zinc with acetate zinc dihydrate acetate dihydrate and sodium and thiocyanate sodium thiocyanate in a 1:2:1 molar in a 1:2:1 ratio molar produced ratio complex produced2. Thecomplex synthetic 2. The routes synthetic to the routes complexes to the1 complexesand 2 are shown 1 and 2 in are Figure shown1. in Figure 1. FigureFigure 1.1. Synthetic routesroutes toto complexescomplexes 11 andand 22.. Int. J. Mol. Sci. 2020, 21, 7091 3 of 14 2.2. Description of Compounds 1 and 2 A perspective view of complexes 1 and 2 along with selective atom numbering scheme is depicted in Figure 2a,b respectively. 1 crystallizes in the orthorhombic space group P212121 and its asymmetric unit consists of an acetato bridged dinuclear zinc(II) L12 complex, one I– counter anion and a methanol solvent molecule. Each zinc is coordinated to the tridentate L1 via two amine N‐ atoms, one bridging acetate ligand and two bridging phenolate oxygen atoms (see Supplementary Figure S1). Both penta‐coordinated zinc(II) centers are in a distorted square pyramidal geometry (see SupplementaryInt. J. Mol. Sci. 2020 ,Figures21, 7091 S2 and S3). The asymmetric unit contains one lattice methanol molecule3 that of 14 is H‐bonded to the I− counter anion. The counter ion is also H‐bonded to N4–H4 of the reduced Schiff base ligand. 2.2. Description of Compounds 1 and 2 Compound 2 (Figure 2b and Supplementary Figure S4) crystallizes in the triclinic space group Pī andA perspectivedoes not contain view of additional complexes 1ionsand or2 alongsolvent with molecules selective atomin its numbering asymmetric scheme unit. isZn2 depicted is N‐ coordinatedin Figure2a,b to respectively. the thiocyanate,1 crystallizes connected in theto O6 orthorhombic of the bridging space acetate group andP21 2bound121 and to its two asymmetric bridging 1 – phenolateunit consists O‐ ofatoms an acetato to form bridged a distorted dinuclear tetrahedron zinc(II) (see L 2 complex, Supplementary one I counter Figure S5). anion Zn1 and is asituated methanol in asolvent distorted molecule. square Each pyramidal zinc is environment coordinated to where the tridentate the ligand L1 [O1,N1,N2,O3]via two amine N-atoms,donor atoms one form bridging the baseacetate of ligandthe pyramid and two and bridging O5 of the phenolate bridging oxygen acetate atomsis at the (see apex. Supplementary The coordination Figure distances S1). Both of penta-coordinatedcompounds 1 and 2 zinc(II) are given centers in Table are in1 and a distorted range from square 1.92 pyramidal to 2.21 Å. geometryThese distances (see Supplementary are similar or slightlyFigures S2longer and S3).that The the asymmetric sum of covalent unit contains radii ( oneΣRcov lattice = 1.93 methanol and 1.86 molecule Å for thatZn+N is H-bonded and Zn+O, to respectively)the I− counter and anion. can Thebe considered counter ion as is normal also H-bonded [16]. to N4–H4 of the reduced Schiff base ligand. Figure 2. Perspective ball ball and stick view of the complexes 1 (a) and 2 (b) along with selective atom numbering scheme (only the coordinating atoms are labeled forfor clarity).clarity). TableCompound 1 also 2gathers(Figure longer2b and Zn Supplementary∙∙∙O distances Figure (values S4) in crystallizesitalics) where in thethe triclinic O‐atom space belongs group to the P ¯ı methoxyand does not(in L contain1) or ethoxy additional (in L2 ions) substituent or solvent of molecules the aromatic in its ring. asymmetric These distances unit. Zn2 are is N-coordinated around 0.8 Å longerto the thiocyanate,than ΣRcov and, connected consequently, to O6 can of the be bridgingconsidered acetate as intramolecular and bound tospodium two bridging bonds, phenolateas further analyzedO-atoms to below. form These a distorted interactions tetrahedron are depicted (see Supplementary in Figure 3 as Figure black S5).dashed Zn1 lines. is situated In compound in a distorted 1 the SpBssquare are pyramidal located opposite environment to the Zn–O(acetate) where the ligand coordination [O1,N1,N2,O3] bonds donorat an angle atoms of form~170°. the In basecompound of the pyramid2, the ethoxyde and O5 O of‐atoms the bridging are located acetate opposite is at the the phenoxy apex. The O’s coordination at an angle of distances ~145°.