Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes – experimental and theoretical investigations

In this contribution we report for the first time fabrication, isolation, structural and theoretical characterization of the quasi-aromatic Möbius complexes [Zn(NCS)2LI] (1), [Zn2(μ1,1-N3)2(LI)2][ZnCl3(MeOH)]2·6MeOH (2) and [Zn(NCS)LII]2[Zn(NCS)4]·MeOH (3), constructed from 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (LI) or benzilbis(acetylpyridin-2-yl)methylidenehydrazone (LII), respectively, and ZnCl2 mixed with NH4NCS or NaN3. Structures 1–3 are dictated by both the bulkiness of the organic ligand and the nature of the inorganic counter ion. As evidenced from single crystal X-ray diffraction data species 1 has a neutral discrete heteroleptic mononuclear structure, whereas, complexes 2 and 3 exhibit a salt-like structure. Each structure contains a ZnII atom chelated by one tetradentate twisted ligand LI creating the unusual Möbius type topology. Theoretical investigations based on the EDDB method allowed us to determine that it constitutes the quasi-aromatic Möbius motif where a metal only induces the π-delocalization solely within the ligand part: 2.44|e| in 3, 3.14|e| in 2 and 3.44|e| in 1. It is found, that the degree of quasi-aromatic π-delocalization in the case of zinc species is significantly weaker (by ∼50%) than the corresponding estimations for cadmium systems – it is associated with the Zn–N bonds being more polar than the related Cd–N connections. The ETS-NOCV showed, that the monomers in 1 are bonded primarily through London dispersion forces, whereas long-range electrostatic stabilization is crucial in 2 and 3. A number of non-covalent interactions are additionally identified in the lattices of 1–3.


Introduction
Helical molecules are highly favoured by nature. 1 Such molecules are of great importance, which is also supported by the structure of deoxyribonucleic acid rst discovered in 1953. 2 On the other hand, zinc(II) (Zn II ) ions are found in all six main classes of metalloenzymes and are essential for living organisms. 3,4 Moreover, the dinuclear Zn II complex fabricated from doubly deprotonated octaethyl formylbiliverdine is the rst established helical doublestranded structure, which was reported in 1976. 5 Following this discovery, strategies towards helical structure as well as their self-assembly have been the focus of researchers. 6,7 Obviously, the most powerful strategy towards metal-based helical structures is the smart predesign of parent ligands. The other strategy, which, however, is less predictable and thus less efficient, is the choice of a metalcontaining precursor. The latter is much less investigated. [5][6][7] Some efforts have been focused on the design and preparation of helical metal complexes by applying chelating ligands with suitable donor sites. [7][8][9][10][11][12][13][14][15][16] The point is that the ligand should produce a helical topology upon binding to the metal ions. In some cases, the coordination features of the cations dictate the wrapping of non-helical chelating ligands around them in such a manner that they can be twisted and eventually form helical complexes. 7,[9][10][11]13,15 However, synthesis of organic ligands with a helical topology is more difficult than metallo-organic compounds and there are only a few reported synthetic helical organic molecules. 8,12,14,16,17 Researchers mainly focused on the design and construction of metal complexes with synthetic helical chelating ligands. [18][19][20] Recently, we have also directed our attention to Schiff bases comprising two pyridyl-imine functions obtained from benzyldihydrazone. [21][22][23][24][25][26][27] These ligands were found to be efficient for helical structures upon coordination to metal centers. Particularly our comprehensive efforts were directed to various Cd II salts as complexing agents. [23][24][25][26][27] Moreover, we were able to demonstrate for the rst time that the helical motif in the obtained complexes together with the chelate metalloring correspond to a quasi-aromatic Möbius object. 24,27 Herein, we report Zn(NCS) 2 -and Zn(N 3 ) 2 -derived structures with 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene) hydrazono)ethane (L I ) and benzilbis(acetylpyridin-2-yl) methylidenehydrazone (L II ). Using thiocyanate (NCS À ) and azide (N 3 À ) counterions is intriguing and of great interest since both anions are known to be ambidentate ligands, which can bind metal centers in different coordination modes. 28

Results and discussion
Interaction of ZnCl 2 mixed with NH 4 NCS or NaN 3 with L I or L II in MeOH at 60 C has allowed to isolate complexes 1-3 (Scheme 1 and ESI †). The elemental analysis data supports their compositions. Notably, the same one-pot reaction of L I or L II with Cd(NO 3 ) 2 in the presence of NH 4 NCS produced a dinuclear structure [Cd 2 (m 1,3 -NCS) 2 (NCS) 2 (L III ) 2 ]$4MeOH (4), where L III is formed upon hydrolysis of one of the 2-PyC(Ph) functions of L I , 27 and neutral mononuclear complex [Cd(NCS) 2 (L II )(MeOH)] (5), 24 respectively. The FTIR spectrum of 1 contains a characteristic intense band at 2073 cm À1 attributed to the CN stretching of NCS À . The same stretching mode in the IR spectrum of 3 is shown as two clearly dened bands at 2066 and 2112 cm À1 , corresponding to two different types of the NCS À ligands (Scheme 1). These bands are in the typical region for the N-linked terminal NCS À ions. 30 The N 3 À anions in the FTIR spectrum of 2 are shown as an intense band at 2054 cm À1 arising from the n asym stretching vibration, as well as a band at 1441 cm À1 corresponding to the n asym stretching vibration. The C]N stretching vibration is at a lower energy by 40 cm À1 for complexes 1 and 2 compared to the free ligand L I , whilst a similar 20 cm À1 difference is observed in this vibration between compound 3 and L II . 22 This rmly conrms the participation of the azomethine nitrogen atoms in chelate formation. The FTIR spectra of 2 and 3 further contain a broad band for the methanol at 3353 to 3443 cm À1 , respectively. Complex 1 crystallizes in the monoclinic space group P2 1 /n, while complexes 2 and 3 each crystallize in the triclinic space group P 1. It is worthy to note, that 1 is isostructural to its Mn II and Co II analogues. 22,31 Complex 1 has a neutral discrete heteroleptic mononuclear structure, where the Zn II metal center is coordinated by one ligand L I via its two pyridyl-imine chelate functions as well as two N-bound NCS À anions giving rise to the ZnN 6 chromophore with a distorted trigonal-prismatic coordination environment around the cation (Fig. 1, Table S1 in the ESI †), which has been proven by the SHAPE 2.1 soware. 32, 33 Complexes 2 and 3 each exhibit a salt-like structure (Scheme 1), as opposed to our previously studied Cd II based counterparts. [23][24][25][26][27] In 2, the cationic part exhibits a doubly charged centrosymmetric dinuclear structure, were two Zn II centers are interlinked via two m 1,1 -N 3 À anions and the coordination domain of each metal is lled by the tetracoordinated ligand L I (Fig. 2). Here a coordination geometry is best described as a distorted octahedron (Table S1 in the ESI †). 32, 33 The anionic part represents a discrete mononuclear structure of the composition [ZnCl 3 (MeOH)] À with a tetracoordinate Cl 3 O environment around the metal atom (Fig. 2). As evidenced from the so-called distortion index s 4 ¼ 0.9513, 34 the coordination core of [ZnCl 3 (MeOH)] À is almost a perfect tetrahedron. This is also supported by the SHAPE 2.1 soware (Table S1 in the ESI †). 32, 35 The anionic part of 2 via one of its chlorine atoms and the methanol OH hydrogen atom is engaged in intermolecular hydrogen bonds with the lattice MeOH molecules yielding a synthon of motif R 8 8 (20) of the ([ZnCl 3 (MeOH)] 2 ) 2À $6MeOH composition (Fig. 2, Table S2 in the ESI †).
The salt like structure of 3 is built from two [Zn(NCS)L II ] + cations, where the Zn II metal center is, similar to 1 and 2, chelated by two pyridyl-imine fragments of one parent ligand L II , and further bound by one N-linked NCS À anion, exhibiting a pentacoordinated geometry (Fig. 3). The distortion index s 5 34 is 0.693 and 0.653 for two [Zn(NCS)L II ] + cations. These values are best described as being about 31% and 35%, respectively, along the pathway of distortion from the ideal trigonal bipyramidal structure towards square pyramidal structure. The trigonal bipyramidal coordination environment is also evidenced from the SHAPE 2.1 soware (Table S1 in the ESI †). 32, 36 The anionic part of 3 exhibits a doubly charged [Zn(NCS) 4 ] 2À species, where four NCS À anions are bound via their N-atoms yielding a tetracoordinated coordination geometry around the metal atom (Fig. 3). The s 4 value of 0.9594 indicates almost a perfect tetrahedron, which has also been supported by the SHAPE 2.1 soware (Table S1 in the ESI †). 32, 35 Notably, the CS fragment of one of the NCS À anions is disordered over three positions with a ratio of 35% : 35% : 30% (Fig. 3).
The Zn-N bonds in 1-3, formed by four nitrogen atoms of the corresponding organic ligands, are in the range from 2.073(3) A to 2.2960(15) A, maintaining that Zn-N Py < Zn-N imine ( Table 1). It is worthy to note, that the Cd(L)-NCS bonds  Table 1). The Zn/Zn separation within the dinuclear molecule of 2 is 3.3729(15) A. Organic ligands in the structures of 1-3 each produce a twisted geometry of different extent. As a result of this conformation, the N-C(Ph)-C(Ph)-N fragments adopt a torsion angle of about 65 (Table 1), which is signicantly lower than in  the corresponding Cd II analogues. [23][24][25][26][27] This is also reected in the Zn-N-N-C(Ph), C(Ph)-N-N-C(Ph) in 1 and 2, and C(Me)-N-N-C(Ph) in 3 torsion angles. Particularly, a simultaneous inuence of a pentacoordination mode of the metal center together with the presence of less bulkier Me substituents in complex 3 leads to the C(Me)-N-N-C(Ph) torsion angles of close values ($138-153 ), while a hexacoordination mode of Zn II and the presence of the organic ligand, which is highly enriched by four phenyl fragments, induces signicantly different C(Ph)-N-N-C(Ph) torsion angles within single species (Table 1). The Zn-N-N-C(Ph) torsion angles are very similar in 1, while the same angles in 2 and 3 are signicantly different ( Table 1). The torsion angle between two pyridyl rings ranges from about 51 to 64 , increasing from 2 through 3 to 1 (Table 1).
Due to a twisted helical topology of organic ligands in the mononuclear structures of 1 and 3, enantiomers of the coordination species can be expected. Indeed, both structures exhibit molecules with D and L helicity. The overall structure of 1 and 3 is a racemic mixture. The molecule of 2, although also containing organic ligands with a twisted helical topology, is constructed from two chiral centres, resulting in the formation of the achiral meso-form.   The crystal packing of 1-3 is described by a network of faceto-face p/p stacking between the aromatic rings (Table S3 in the ESI †). The structures of 1 and 2 are also dictated by C-H/p interactions (Table S4 in the ESI †).
For more detailed analyses of non-covalent interactions in 1-3 the charge and energy decomposition scheme ETS-NOCV 37 is applied as available in the ADF program. 38 We have applied BLYP-D3/TZP since they provide reliable results for noncovalent interactions. 39 The X-ray models are considered.
In order to evaluate aromaticity in 1-3, we have applied the electron density of delocalized bonds (EDDB) method, which has been proposed to visualize and quantify aromaticity and chemical resonance in a wide range of chemical species. [57][58][59][60] Moreover, it has recently been shown that, in the case of organometallics, the EDDB method provides very useful data on the role of the transition metal d-orbitals in electron delocalization, 24,27,50,61 which is inaccessible by means of such popular and commonly used aromaticity descriptors as the nucleusindependent chemical shi (NICS) 62 or the anisotropy of the induced current density (ACID). 63 The global EDDB isocontours and the corresponding electron populations of 1-3 are collected in Fig. S4 in the ESI. † Here,  we focus our attention on the characteristic seven-membered quasi-aromatic motif (7-MR), encompassing the twisted 1,1 0 -(1,2-ethenediyl)bis-diazene (BDA) fragment and the metal atom (abbreviated as BDA-Zn). The BDA-based complexes with cadmium have recently been demonstrated to exhibit a unique type of transition-metal induced Möbius-like aromaticity in which the metal d-orbitals themselves do not contribute to the p-conjugation occurring at BDA. 24,27 Since the quantitative study of aromaticity/electron delocalization in large systems is very difficult in practice, we have decided to consider the simplied BDA-Zn models adopting the exact fragments geometries from crystals of 1-3 (Fig. 7). The calculated total EDDB contours and electron populations have been dissected (according to the orbital symmetry) to get the strict p-contributions to quasiaromaticity; natural atomic charges on the metal and the two closest nitrogen atoms have been added together with the average dihedral angles and the calculated electric dipole moments (EDM). It was found, that the number of p-electrons delocalized in the quasi-aromatic rings, particularly in 1 and 2 (on average $3.3|e|), resembles pretty much the values found for the previously studied BDA-Cd complexes, despite different congurations of the phenyl units and applying other ligands. 24,27 The most twisted 7-MR in 1 (containing the most bulky substituents) is at the same time the most stabilized by quasi-aromaticity (3.44|e|, i.e. $0.6|e| per a quasi-ring member, which is comparable to the corresponding value for pyrrole 60 ). Interestingly, it is found for the rst time, that the systematic increase of the Zn-N bond polarization when going from 1 to 3 reveals a strict correlation between EDM (i.e. indirectly the topology and the metal to BDA charge transfer) and p-electron delocalization: R ¼ À0.986. In other words, the more twisted is the 7-MR, the more quasi-aromatic character is observed (Fig. 7). It demonstrates the two-folded role of bulky substituents: they are not only dispersion donors, 40-56 but they also lead to amplication of the 7-MR twist (and enhanced quasiaromaticity). Previously only the former feature has been recognized. [23][24][25][26][27] Interestingly, the optimized BDA-Zn structure (without steric effects from the Ph units) has signicantly reduced quasi-aromaticity compared to 1 and 2 (2.43|e|, i.e. $0.4|e| per a quasi-ring member, which is comparable to the corresponding value for furan), 60 but at the same time, it is  Imagining of the EDDB(r) and EDDB p (r) functions with the corresponding electron populations (in |e|) for the isolated 7-MR model systems at geometries adopted from the corresponding crystals of 1-3, as well as the fully optimized units: BDA-Zn and BDA-Cd. 27 The natural atomic charges (colored bold numbers), average dihedral angles and the calculated electric dipole moments (EDM) have been added for comparison. almost twice less aromatically-stabilized than its optimized BDA-Cd analogue (4.74|e|, i.e. $0.8|e| per a quasi-ring member, which is exactly between the corresponding values for pyrrole and benzene). 60 Since both equilibrium structures have comparable average dihedral angles N-N-M-N, it is clearly the larger metal-nitrogen bond polarization (the charges q Zn ¼ +1.15, q N ¼ À0.9, EDM ¼ 1.26 D in BDA-Zn compared to q Zn ¼ +0.72, q N ¼ À0.7, EDM ¼ 0.68 D in BDA-Cd) that limits the pelectron delocalization in the 7-memberd quasi-aromatic unit (changes in the effectiveness of p-conjugation involving the 2pz orbitals of nitrogen atoms at close proximity of the metals are well marked in the EDDB p (r) isocontours, Fig. 7). Such interrelation between the nature of metal-ligand bonding and quasi-aromaticity of the ligand has not been known before. [23][24][25][26][27] Conclusions In summary, we successfully isolated and characterized the quasi-aromatic Möbius type zinc complexes [Zn(NCS) 2  Complex 1 has a neutral discrete heteroleptic mononuclear structure with the Zn II metal atom being chelated by one tetradentate ligand L I and two N-bound NCS À anions with the formation of a distorted trigonal-prismatic ZnN 6 coordination core. The [Zn(NCS) 2 L I ] monomers were found (due to the ETS-NOCV calculations) to be bonded to each other primarily through London dispersion forces exerted by the presence of bulky hydrophobic substituents. Contrary, complexes 2 and 3 exhibit a salt-like structure where the long-range electrostatic forces were found to be of prime importance additionally to more typical non-covalent interactions (O-H ] + units appeared to be repulsive. Finally, we have proven, by means of the EDDB 57-61 study, that the seven-membered rings in 1-3 constitute a quasi-aromatic Möbius-type motif, though the absolute magnitude of such p-delocalization is notably weaker than in the corresponding cadmium-based analogs. 24,27 Bulkiness of the ligands (L) are found not only to amplify London dispersion stabilization, 24,27,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] but also inuence the magnitude of quasi-p-delocalization (of Möbius-type) through modi-cation of the polarity of Zn-L bonding.

Materials
All chemicals and solvents were used from commercial sources without further purication. L I and L II were synthesized according to a literature method. 20 Physical measurements FTIR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer. Microanalyses were performed using a Ele-mentarVario EL III analyzer.

Synthesis
ZnCl 2 (0.068 g, 0.5 mmol), NH 4 NCS (0.076 g, 1 mmol) or NaN 3 (0.065 g, 1 mmol) and L I or L II (0.284 and 0.222 g, respectively; 0.5 mmol) were placed in the main arm of a branched tube. MeOH (15 mL) was carefully added to ll the arms. The tube was sealed and immersed in an oil bath at 60 C while the branched arm was kept at ambient temperature. X-ray suitable crystals were formed during the next days in the cooler arm and were ltered off.

ETS-NOCV charge and energy decomposition method
The Natural Orbitals for Chemical Valence (NOCV) j i constitute the canonical representation for a differential density matrix DP (it is formed by subtracting the appropriate molecular fragments density matrices from a density matrix of a molecule under consideration) in which DP adopts a diagonal form. It gives rise to the corresponding eigenvalues v i and the related vectors j i . NOCVs occur in pairs (j Àk ,j k ) related to |v k | and they decompose overall deformation density Dr into bonding components with different symmetries (Dr k ): Usually, a few k allow to recover a major shape of Dr. By combining NOCVs with ETS scheme in ETS-NOCV, one can obtain the related energetics, DE orb (k), in addition to qualitative picture emerging from Dr k . ETS originally divides the total bonding energy, between fragments, DE total , into four distinct components: The DE elstat is an energy of quasi-classical electrostatic interaction between fragments. The next term, DE Pauli , is responsible for repulsive Pauli interaction between occupied orbitals on the two fragments. The third component, DE orb , is stabilizing and shows formation of a chemical bond (including polarizations). In the ETS-NOCV scheme DE orb is expressed in terms of the eigenvalues v k and diagonal Fock energy matrix elements F TS i,i (transformed into NOCV representation) as: Finally, DE dispersion denotes the semiempirical Grimme dispersion correction (D3).

Single-crystal X-ray diffraction
The X-ray data were collected on a Bruker APEX-II CCD single crystal diffractometer using graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 A). The collected frames were integrated with the Saint 64 soware using a narrow-frame algorithm. Data were corrected for absorption effects using the multi-scan method in SADABS. 65 The space groups were assigned using XPREP of the Bruker ShelXTL 66 package, solved with ShelXT 66 and rened with ShelXL 66 and the graphical interface ShelXle. 67 All non-hydrogen atoms were rened anisotropically. Hydrogen atoms attached to carbon were positioned geometrically and constrained to ride on their parent atoms. Figures were generated using the program Mercury. 68 Crystal data for 1. C 40 H 28 N 8 S 2 Zn, M r ¼ 750.19 g mol À1 , T ¼ 296(2) K, monoclinic, space group P2 1 /n, a ¼ 12.8146 (10)

Contributions
Mariusz P. Mitoraj has planned and partially performed (ETS-NOCV) the theoretical calculations, written the manuscript text and analyzed the entire data. Farhad Akbari Ahami has primarily done the experimental part. Ghodrat Mahmaoudi has planned the experimental research. Ali Akbar Khandar has supported the work, whereas Atash V. Gurbanov has synthesized the compounds. Fedor I. Zubkov has also participated in the synthesis of the compounds. Rory Waterman is a crystallographer of compound 1-2, whereas Himanshu Sekhar Jena is a crystallographer of system 3. D W. Szczepanik has done the aromaticity calculations. Damir A. San and Maria G. Babashkina have analysed and discussed the results as well as have written the manuscript.

Conflicts of interest
There are no conicts to declare.