Quasi-aromatic MÃ¶bius Metal Chelates - Inorganic Chemistry (ACS

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Quasi-aromatic Mö bius Metal Chelates Ghodrat Mahmoudi,*,† Farhad A. Afkhami,‡ Alfonso Castiñeiras,§ Isabel García-Santos,§ Atash Gurbanov,∥,+ Fedor I. Zubkov,⊥ Mariusz P. Mitoraj,*,# Mercedes Kukułka,# Filip Sagan,# Dariusz W. Szczepanik,# Irina A. Konyaeva,¶,▽ and Damir A. Safin*,⊗ †

Department of Chemistry, Faculty of Science, University of Maragheh, 55181-83111 Maragheh, Iran Young Researchers and Elite Club, Tabriz Branch, Islamic Azad University, 51579-44533Tabriz, Iran § Departamento de Química Inorgánica, Facultad de Farmacia, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain ∥ Department of Chemistry, Baku State University, Z. Xalilov Str. 23, AZ1148, Baku, Azerbaijan + Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049−001 Lisboa, Portugal ⊥ Organic Chemistry Department, Faculty of Science, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Street, Moscow 117198, Russian Federation # Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R. Gronostajowa 2, 30-387 Cracow, Poland ¶ Limited Liability Company “NIOST”, Kuzovlevski trakt 2, 634067 Tomsk, Russian Federation ▽ Department of Technology of Organic Substances and Polymer Materials, National Research Tomsk Polytechnic University, 43 Lenin Avenue, 634050 Tomsk, Russian Federation ⊗ Institute of Chemistry, University of Tyumen, Perekopskaya Street 15a, 625003 Tyumen, Russian Federation ‡

S Supporting Information *

ABSTRACT: We report the design as well as structural and spectroscopic characterizations of two new coordination compounds obtained from Cd(NO3)2·4H2O and polydentate ligands, benzilbis(pyridin-2-yl)methylidenehydrazone (LI) and benzilbis(acetylpyridin-2-yl)methylidenehydrazone (LII), in a mixture with two equivalents of NH4NCS in MeOH, namely [Cd(SCN)(NCS)(LI)(MeOH)] (1) and [Cd(NCS)2(LII)(MeOH)] (2). Both LI and LII are bound via two pyridylimine units yielding a tetradentate coordination mode giving rise to the 12 π electron chelate ring. It has been determined for the first time (qualitatively and quantitatively), using the EDDB electron population-based method, the HOMA index, and the ETS-NOCV charge and energy decomposition scheme, that the chelate ring containing CdII can be classified as a quasiaromatic Möbius motif. Notably, using the methyl-containing ligand LII controls the exclusive presence of the NCS− connected with the CdII atom (structure 2), while applying LI allows us to simultaneously coordinate NCS− and SCN− ligands (structure 1). Both systems are stabilized mostly by hydrogen bonding, C−H···π interactions, aromatic π···π stacking, and dihydrogen C− H···H−C bonds. The optical properties have been investigated by diffused reflectance spectroscopy as well as molecular and periodic DFT/TD-DFT calculations. The DFT-based ETS-NOCV analysis as well as periodic calculations led us to conclude that the monomers which constitute the obtained chelates are extremely strongly bonded to each other, and the calculated interaction energies are found to be in the regime of strong covalent connections. Intramolecular van der Waals dispersion forces, due to the large size of LI and LII, appeared to significantly stabilize these systems as well as amplify the aromaticity phenomenon.

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INTRODUCTION Helical molecules belong to one of the most beautiful and intriguing classes of compounds.1 Deoxyribonucleic acid is the most well-known self-assembled helicate system.2 Metals can also be involved in formation of helicate structures, as proposed by Lehn in 1987.3 However, the first helical ZnII complex was developed earlier (in 1976).4 Since then, the self-assembly phenomenon of helicate systems has been of particular interest.5,6 The rational design of ligands involved in © XXXX American Chemical Society

coordination is the most powerful tool toward helicate metalcontaining complexes. The strategy based on the usage of metal-containing precursors to produce helicate species is, however, less explored.4−6 In order to obtain helical topologies, one can apply chelating ligands with donor sites which may produce a helical topology Received: January 8, 2018

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DOI: 10.1021/acs.inorgchem.8b00064 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry due to binding to metal centers.7−10 As an example, Schiff bases containing two pyridyl coordination sites fabricated from benzyldihydrazone are monohelical ligands with restrained rotation around the C−C bond.11−13 The conformational freedom of the two N−N bonds is also blocked upon coordination. Thus, the ligand can efficiently be locked in a twist conformation (Scheme 1).

several aromatic metal chelates and clathrochelates have been reported in the literature, in which metal-to-ligand charge transfer induces strong π-electron delocalization but only in the ligand part giving rise to the quasi-aromatic planar rings.28−32 In such metallacycles, the metal center itself is not stabilized by the conjugation effects, and its nature might be a powerful tool in designing quasi-aromatic structures with the targeted topology and properties.33 Accordingly, using the CdII atom for this purpose is of particular interest, since this metal cation exhibits a rich variety of coordination numbers and can form nonpredictable structures with properties of interest. Some of us have recently reported a solvent-driven synthesis of Cd(N3)2-based coordination polymers.34 In this contribution we have decided to direct our attention toward applying thiocyanate (NCS−) since it is known to be a highly versatile ambidentate ligand with a polarizable π system and different donor sites, which can be linked to metal atoms through either the sulfur (thiocyanate) or the nitrogen (isothiocyanate) atom and exhibit a variety of bridging modes.33,35 Thus, the thiocyanate ligand and CdII can generate various types of structures with variable degrees of π-electron delocalizations. Accordingly, we report herein the synthesis along with detailed structural and spectroscopic studies of the two Cd(NCS)2-based mononuclear helical complexes constructed from tetradentate ligands LI and LII, derived from benzildihydrazone and 2-pyridinecarboxaldehyde or 2-acetylpyridine, respectively. For the first time it is shown by various aromaticity descriptors (e.g., EDDB, ETS-NOCV, and HOMA methods) that these systems can be classified as quasi-aromatic Möbius objects. Accordingly, they bridge the gap between already known planar quasi-aromatic species and typical aromatic Möbius objects. Additionally, DFT and TD-DFT molecular and periodic calculations were performed to characterize these complexes.

Scheme 1. Synthesis of Complexes 1 and 2

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RESULTS AND DISCUSSION A one-pot reaction of Cd(NO3)2·4H2O with closely related organic ligands LI and LII in a mixture with 2 equiv of NH4NCS at 60 °C in MeOH in a branched tube yields mononuclear discrete complexes [Cd(SCN)(NCS)(LI)(MeOH)] (1) and [Cd(NCS)2(LII)(MeOH)] (2), respectively (Scheme 1). Notably, it was recently reported that an equimolar reaction of a methanol solution of Cd(ClO4)2·6H2O and LII with an aqueous solution of NH4NCS under slowly evaporating conditions at room temperature yielded colorless block-like crystals of the composition [Cd2(μ-1,3-NCS)2(NCS)2(LII)]· 2MeOH (3).36 Complexes 1 and 2 were synthesized in good yields. Subsequently, they have been comprehensively described by FTIR and diffuse reflectance spectroscopy, powder, and singlecrystal X-ray diffraction, and Hirshfeld surface study. Their compositions were supported by elemental analysis. As evidenced from single-crystal X-ray diffraction, the structures of 1 and 2 were best solved in the monoclinic space groups P21/c and P2/c, respectively. The asymmetric unit of both complexes contains a single molecule. Each CdII atom exhibits the coordination number of seven due to the two pyridyl-imine fragments of the corresponding tetradentate Schiff-base, one methanol oxygen atom and two donor atoms of two pseudohalido ligands (Figure 2). In 1 the two donor atoms are one nitrogen atom of one isothiocyanate and one sulfur atom of another thiocyanate ligands. This behavior is surprising since the CdII ion is a soft acidic metal

Helical nature of a system renders additionally the question of a possible role of the aromaticity (or resonance effects). Among various types of aromatic species which have been invented in recent years, some of the most fascinating from the mathematical and predominantly chemical points of view are the so-called Möbius objects (Figure 1).14−24 Heilbronner’s

Figure 1. (Left) A circular strip, (middle) a Möbius strip with N = 1, and (right) a Möbius strip with N = 2. The images adapted from ref 26.

rule states that, a conjugated system, including 4n π electrons, can be aromatic if it shows a Möbius topology.14 A strip of the Möbius type exhibits N number of half-twists resulting its order (Figure 1). The first Möbius example was reported by Herges and coworkers.15 Thereafter, numerous Möbius structures were proposed.16−22 A CdII-derived double-helical complex, containing two 13-membered chelate metallacycles with 12 π electrons, has been reported by Datta and co-workers.25 It was noted that the complex is aromatic and of Möbius type.26 However, aromatic stabilization in the metallacyclic units is not inextricably linked with a full π-conjugated pattern.27 In fact, B


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Inorganic Chemistry

Figure 2. (Top) Crystal structures of 1 (left) and 2 (right). Hydrogen atoms are omitted for clarity. (Bottom) Coordination polyhedron around the CdII atom in the crystal structures of 1 (left) and 2 (right). Color code: C = gold, N = blue, O = red, S = yellow, Cd = magenta.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2 complex 1 bond lengths Cd−NPy Cd−Nimine Cd−Nisothiocyanate Cd−Sthiocyanate Cd−Omethanol bond angles NPy−Cd−NPy NPy−Cd−Nimine NPy−Cd−Nisothiocyanate NPy−Cd−Sthiocyanate NPy−Cd−Omethanol Nimine−Cd−Nimine Nimine−Cd−Nisothiocyanate Nimine−Cd−Sthiocyanate Nimine−Cd−Omethanol Nisothiocyanate−Cd−Nisothiocyanate Nisothiocyanate−Cd−Sthiocyanate Nisothiocyanate−Cd−Omethanol Sthiocyanate−Cd−Omethanol torsion angles N−C(Ph)−C(Ph)−N C(Ph)−N−N−C Py···Py Py···Ph Ph···Ph

complex 2

2.389(2), 2.421(2) 2.507(2), 2.594(2) 2.285(2) 2.6872(7) 2.374(2)

2.352(2), 2.370(2) 2.471(2), 2.519(2) 2.272(2), 2.274(3) − 2.618(2)

168.75(6) 65.62(7), 66.30(6), 115.34(6), 122.31(6) 85.45(7), 88.61(7) 93.36(5), 96.54(5) 77.54(6), 92.30(6) 68.50(6) 142.77(7), 148.36(7) 71.85(5), 110.42(4) 74.88(6), 107.61(6) − 93.42(6) 83.87(7) 173.51(4)

167.01(8) 66.09(8), 67.14(8), 123.74(7), 125.77(8) 84.57(9), 88.22(9), 89.08(9), 92.58(9) − 81.51(8), 86.24(8) 73.38(7) 81.20(8), 82.71(9), 119.06(8), 120.65(9) − 140.07(7), 146.54(7) 154.21(10) − 74.36(9), 79.87(8) −

72.6(3) 94.5(3), 128.6(2) 51.55(12) 57.21(12), 65.93(12), 68.61(12), 83.17(12) 85.58(12)

−76.2(4) −125.6(3), − 131.9(3) 67.60(14) 56.74(14), 62.66(14), 65.20(14), 84.59(14) 87.47(14)

create hardness character at the CdII center; so it may be a major reason for the preference of N-donation upon S-

center and therefore, according to symbiosis logic of Jørgensen,37 the ligand LI containing four nitrogen atoms C


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Inorganic Chemistry Table 2. Classic Hydrogen Bond Lengths (Å) and Angles (deg) for 1 and 2 complex

D−H···A

d(D−H)

d(H···A)

d(D···A)

∠(DHA)

1a 2b

O(1)−H(1)···N(3) O(1)−H(1A)···S(2)

0.74 0.84

2.01 2.59

2.730(3) 3.352(2)

165.7 151.9

a Symmetry transformations used to generate equivalent atoms: 1 − x, −1/2 + y, 1/2 − z. bSymmetry transformations used to generate equivalent atoms: 1 − x, y, 1/2 − z.

Figure 3. Crystal packing of 1 (top) and 2 (bottom). CH hydrogen atoms are omitted for clarity. Color code: C = gold, N = blue, O = red, S = yellow, Cd = magenta.

in the above-mentioned order. On the contrary, in 2 the two N3 basal planes are almost parallel with a 6.2° dihedral angle, and the CdII ion is shifted by 1.53 and 1.58 Å. This difference is probably due to the fact that in 1 both basal planes include the O and S atoms (Figure 2). This is also reflected in the Cd−O distance, which is significantly longer in 2, where the oxygen atom is apical, than in 1 (Table 1), where it is basal and of the same order of magnitude as those found in other CdII complexes with methanol as a ligand.39 The Cd−S bond distance in 1 of 2.6872(7) Å, where the sulfur atom occupies a basal position, is shorter than in cadmium complexes with thiocyanate. Ligands LI and LII in both complexes adopt a twist helical conformation. However, there is some difference in conformation derived not only from the presence of pseudohalido ligands but also by the existence of the methyl groups as substituents in LII (Table 1). In 1 the conformation of LI yields a torsion angle of 72.6(3)° for the N−C(Ph)−C(Ph)−N fragment and the coordination results different torsion angles of

donation of the ambidentate thiocyanate by means of its electronegative hard center, as takes place in the structure of 2. The Cd−N bonds to the Schiff-base four nitrogen atoms (2.352(2)−2.594(2) Å) are ordinary Cd−N connections, maintaining that Cd−NPy < Cd−Nimine, as observed in both complexes (Table 1). It should be noted, that the bond lengths to the NCS− ligands (2.272(2)−2.285(2) Å) are much shorter than the bonds to the LI and LII nitrogen coordination sites. This suggests the presence of some π-electron donation.38 This contention is corroborated by the (S)C−N−Cd angle of av. 151.8°, showing that π-donation promotes a more linear binding mode for the isothiocyanate ligand. These ligands in 1 and 2 are almost linear with the valence N−C−S angle being av. 178.5°. The coordination polyhedron in each complex, with CdN5SO (1) and CdN6O (2) chromophores, is best described as a square face monocapped trigonal prism (Figure 2). In 1, the two basal planes N2O and N2S form a 17.0° angle and the CdII atom is shifted by 1.63 and 1.67 Å from these two planes D


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Inorganic Chemistry

agreement with the calculated ones (Figure 6). Thus, the bulk materials are free from phase impurities. The FTIR spectra of 1 and 2 each contain a wide band for the methanol ligand centered at 3440 and 3450 cm−1, respectively (Figure 7). In the FTIR spectrum of 1 a couple of intense bands at 2055 and 2105 cm−1 correspond to the (iso)thiocyanate CN stretching modes (Figure 7). The former band is in the region characteristic of the terminal N-bound isothiocyanate ions, while the latter band is in the region of the terminal S-bound thiocyanate ion.45 An intense band at 2050 cm−1, characteristic for the N-bound isothiocyanate CN stretching mode, was found in the FTIR spectrum of 2 (Figure 7). Unfortunately, bands for the CS vibrations of the (iso)thiocyanate anions are obscured by the presence of the bands characteristic of the parent ligands LI and LII and, thus, cannot be identified unequivocally. We have also applied the Hirshfeld surface analysis46 and associated 2D fingerprint plots,47 obtained using CrystalExplorer 3.1,48 to study the intermolecular interactions in the structures of 1 and 2. As evidenced from the Hirshfeld surface analysis, for 1 the intermolecular H···H and H···C contacts, being 32.9% and 31.3%, are two main contributors to the crystal packing (Table 5). On the surface of the molecule of 2 the former contacts are more and of 38.0%, while the latter contacts is less and of 28.1% (Table 5). The shortest H···H distances are revealed in the fingerprint plots of both molecules as a broad spike at de + di ≈ 2.0−2.4 Å (Figures S1 and S2 in the Supporting Information). The H···C contacts in fingerprint plots of 1 and 2 are observed in the form of “wings” (Figures S1 and S2 in the Supporting Information), with the shortest de + di ≈ 2.6 Å. These contacts are of the C−H···π nature.46 Notably, the fingerprint plot of 2 contains a remarkable number of points at large de and di, which are shown at the top right of the plot as tails (Table 5). These points correspond to regions on the molecular surface without any close distance to nuclei in neighboring molecules, that is typical similar for the fingerprint plots of benzene46 and phenyl-containing compounds.49−54 The molecular surfaces of both molecules are also characterized by H···S and H···N contacts, comprising 17.5−19.3% and 9.5− 13.3%, respectively (Table 5). The H···N contacts in the fingerprint plot of 1 are observed as a pair of sharp spikes at de + di ≈ 1.8 Å (Figure S1 in the Supporting Information) and originate from the O−H···N hydrogen bonds (Figure 3 and Table 5). The molecular surface of both 1 and 2 is further described by C···C and C···N contacts. These contacts, however, occupy a negligible proportion of the total surface area and of 2.5−3.3% and 1.2−1.3%, respectively (Table 5). These contacts are observed in the fingerprint plots of both molecules as the area on the

94.5(3) and 128.6(2)° around the C(Ph)−N−N−C bonds. In the structure of 2 the N−C(Ph)−C(Ph)−N torsion angle of the LII ligand is −76.2(4), which is of the same order of magnitude as that present in the structure of 3.27 In 2, the coordination by the two pyridyl-imine arms of LII gives rise to the similar C(Ph)−N−N−C torsion angles of −125.6(3) and −131.9(3)°. The torsion angle formed by two phenyl rings from twist ligands of about 85.6 and 87.5° in 1 and 2 (Table 1), respectively, is similar to that found in related complexes.40 Weak interactions such as hydrogen bonding, C−H···π-type and aromatic π···π stacking, among others, contribute significantly to the self-assembly and molecular recognition processes.41−44 Here, the crystal structures are stabilized through different intermolecular interactions. The packing of 1 is primarily driven by the MeOH ligand, since a strong classical hydrogen bond is observed between the methanol and thiocyanate nitrogen atom (Table 2). These interactions cause each complex molecule to bind to two neighboring molecules, resulting in a 1D zigzag chain along the b-axis (Figure 3). The molecules of 2 are interlinked with the formation of dimers (Figure 3) via a strong hydrogen bond between the methanol and the NCS− sulfur atom and vice versa (Table 2). These non-covalent interactions yield a homosynthon of motif R22(12) (Figure 4). The crystals of 1 and 2 are

Figure 4. Top (left) and side (right) views on a homosynthon of motif R22(12) formed by means of a hydrogen bond between the OH hydrogen atom of the methanol ligand and the sulfur atom of one of the isothiocyanate ligands and vice versa in the structure of 2. Color code: C = gold, N = blue, O = red, S = yellow, Cd = magenta.

additionally stabilized by face-to-face π···π stacking and C− H···π interactions (Tables 3 and 4). The former interactions in 1 are formed between one of the phenyl rings of adjacent molecules, while the same interactions in the structure of 2 are formed by the pyridyl rings (Figure 3). Since the topologies of both complexes are helical (Figure 5), enantiomers are expected. Moreover, 1 and 2 crystallize in the monoclinic space groups P21/c and P2/c, respectively. Therefore, each structure is a racemic mixture in the solid state (Figure 5). According to the X-ray powder diffraction analysis, the experimental powder patterns of both complexes are in full

Table 3. π···π Interaction Distances (Å) and Angles (deg) for 1 and 2a complex

Cg(I)

Cg(J)

d[Cg(I)−Cg(J)]

α

β

γ

slippage

1b 2c

Cg(4) Cg(1) Cg(2)

Cg(4)#1 Cg(1)#1 Cg(2)#2

3.816(2) 3.589(2) 3.760(2)

0.00(12) 0.38(15) 0.00(13)

28.2 5.9 22.3

28.2 5.9 22.3

1.801 0.370 1.425

d[Cg(I)−Cg(J)], distance between ring centroids; α, dihedral angle between planes Cg(I) and Cg(J); β, angle Cg(I)→Cg(J) vector and normal to plane I; γ, angle Cg(I)→Cg(J) vector and normal to plane J; slippage, distance between Cg(I) and perpendicular projection of Cg(J) on ring I. b Symmetry transformations used to generate equivalent atoms: #1, 1 − x, −y, 1 − z. Cg(4) = C(37)−C(38)−C(39)−C(40)−C(41)−C(42). c Symmetry transformations used to generate equivalent atoms: #1, 1 − x, y, 1/2 − z; #2, 1 − x, 1 − y, −z. Cg(1) = N(11)−C(12)−C(13)−C(14)− C(15)−C(16), Cg(2) = N(30)−C(25)−C(26)−C(27)−C(28)−C(29). a

E


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Inorganic Chemistry Table 4. C−H···π Interaction Distances (Å) and Angles (deg) for 1 and 2a complex

X−H(I)

Cg(J)

d[H(I)−Cg(J)]

d[X−Cg(J)]

∠(XHCg)

γ

1b

C(1)−H(1A) C(13)−H(13) C(15)−H(15)

Cg(1) Cg(2)#1 Cg(3)#1

2.98 2.79 2.84

3.771(3) 3.343(3) 3.557(3)

139 118 132

22.58 20.24 7.15

2c

C(14)−H(14) C(28)−H(28) C(34)−H(34) C(39)−H(39)

Cg(4)#1 Cg(3)#2 Cg(4)#3 Cg(1)#4

2.99 2.48 2.78 2.88

3.695(3) 3.277(3) 3.697(3) 3.768(3)

132 142 162 157

19.64 6.38 7.62 5.81

H(I)−Cg(J), distance of H to ring centroid; X−Cg(J), distance of X to ring centroid; ∠(XHCg), angle X−H−Cg; γ, angle H(I)→Cg(J) vector and normal to plane J. bSymmetry transformations used to generate equivalent atoms: #1, −1 + x, y, z. Cg(1) = N(11)−C(12)−C(13)−C(14)−C(15)− C(16), Cg(2) = N(30)−C(25)−C(26)−C(27)−C(28)−C(29), Cg(3) = C(31)−C(32)−C(33)−C(34)−C(35)−C(36). cSymmetry transformations used to generate equivalent atoms: #1, 1 − x, y, 1/2 − z; #2, 1 − x, 1 − y, −z; #3, −x, 2 − y, −z; #4, −1 + x, y, z. Cg(1) = N(11)− C(12)−C(13)−C(14)−C(15)−C(16), Cg(3) = C(31)−C(32)−C(33)−C(34)−C(35)−C(36), Cg(4) = C(37)−C(38)−C(39)−C(40)−C(41)− C(42). a

Figure 6. Calculated (black) and experimental (red) X-ray powder diffraction patterns of 1 and 2.

Figure 5. Space-filling models of the left-handed (left) and righthanded (right) helical nature of molecules in the crystal structures of 1 (top) and 2 (bottom). The phenyl, thiocyanate, and methanol groups and hydrogen atoms are omitted for clarity. Color code: C = gold, N = blue, Cd = magenta.

Figure 7. FTIR spectra of 1 (black) and 2 (red).

S in 2 (0.3% each) contacts (Table 5, Figures S1 and S2 in the Supporting Information). We have also calculated the enrichment ratios (E)55 of the intermolecular contacts to shed light on the susceptibility of two entities to be in contact. The H···C, H···N and H···S contacts are highly favored for both molecules since the corresponding EHH, EHN, and EHS ratios are larger than unity

diagonal at de = di ≈ 1.7−2.4 Å (Figures S1 and S2 in the Supporting Information). These contacts are characteristic for π···π interactions (Figure 3 and Table 3). Close analysis of the remaining intermolecular contacts in 1 and 2 also revealed a proportion of H···O (0.3−0.7%), N···N, and N···S in 1 and C··· F


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Inorganic Chemistry

Table 5. (Top) Relative Contributions of Intermolecular Contacts to the Hirshfeld Surface Area and 2D Fingerprint Plots of Observed Contacts for 1 and 2, and (Bottom) Hirshfeld Contact Surfaces and Derived “Random Contacts” and “Enrichment Ratios” for 1 and 2a

Values are obtained from CrystalExplorer 3.1.48 bThe enrichment ratios were not computed when the “random contacts” were lower than 0.9%, as they are not meaningful.55

a

remarkably higher proportion of random contacts RCN in comparison with a proportion of C···C contacts (Table 5). Finally, the structures of 1 and 2 are characterized by very impoverished N···S (ENS = 0.21) and C···S (ECS = 0.09) contacts, respectively. To further describe the synthesized systems, we have performed first periodic DFT calculations by the CP2K package.56−58 In the case of both structures the geometry optimizations have provided the minima on the potential energy surface, and the resulting lattice cell values are in qualitative accord with the experimental ones (Table S1 in the Supporting Information). For the purpose of describing quantitatively non-covalent interactions, which influence the topology of the isolated systems, the charge and energy decomposition scheme (ETS-

(1.17−1.54) (Table 5). This can be explained by a relatively higher fraction of these contacts on the molecular surface area over corresponding random contacts RHH, RHN, and RHS, respectively (Table 5). By contrast, the H···H contacts on the surface of 1 and 2 are less preferred (EHH = 0.80 and 0.86, respectively), which is due to a high amount of random contacts RHH, despite both species described by a similar high amount of SH (Table 5). The C···C contacts in 2 are enriched (ECC = 1.00), which is explained by a relatively high value of their proportion on the total surface area. Although the SC value and the random ratio RCC of 1 are very similar to those in 2, its molecule is characterized by significantly fewer C···C contacts (ECC = 0.71). This is due to a smaller number of C···C contacts in 1 (Table 5). The structures of both molecules are further described by impoverished C···N contacts. This caused by a G


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Inorganic Chemistry

Figure 8. (Top) Cluster models of 1 and 2 together with the fragmentation patterns applied in the ETS-NOCV analysis. (Bottom) Overall deformation density Δρorb with the corresponding orbital interaction energies ΔEorb.

NOCV)-based59 study has been performed by the ADF/DFT/ BLYP-D3/TZP including relativistic effects at ZORA level.60,61 The monomers in the crystals interact extremely strongly, and the calculated interaction energies in the selected clusters are ΔEint = −89.61 kcal/mol for 1 and ΔEint = −98.02 kcal/mol for 2 (Figure 8). In 1 the dispersion and electrostatic terms are both the leading forces (ΔEdispersion = −49.19 kcal/mol and ΔEelstat = −50.21 kcal/mol), whereas in 2 the dispersion contribution evidently outweighs (ΔEdispersion = −74.94 kcal/ mol) the other bonding components (ΔEelstat = −57.26 kcal/ mol and ΔEorb = −39.40 kcal/mol) (Figure 8). The dispersion (ΔEdispersion), ionic (ΔEelstat), and charge delocalization (ΔEorb) terms contribute to the overall stabilization of 1 as much as 35.5%, 36.2%, and 28.3%, respectively, whereas these values are 43.6%, 33.4%, and 23% for 2, thus supporting also quite strong covalent character of these systems. It is noteworthy that typical charge delocalization contributions Lp→σ*(OH) within the hydrogen bonds O−H···N and O−H···S are visible for 1 and 2, respectively (Figure 8). These results, highlighting a crucial role of the dispersion stabilization in bulky systems, conform to the recent findings summarized in comprehensive reviews.62,63 It must be noticed that the calculated interaction energies in the range ΔEint from −90 to −100 kcal/mol for non-covalent interactions constituting 1 and 2 are as significant as in many purely covalently bounded molecules.64 Accordingly, these systems are expected to be stable at higher temperatures or more demanding conditions. To evaluate aromaticity and resonance effects in both complexes we used the recently proposed method of

partitioning the one electron density (ED) into components representing different levels of electron delocalization,65−67 especially the ED component called electron density of delocalized bonds (EDDB).65−67 A brief description of the EDDB method is given in the Supporting Information. What should be mentioned here is that the electron population from the EDDB approach can straightforwardly be interpreted as the number of electrons delocalized through the system of conjugated bonds (i.e., delocalized/shared in a multicenter sense).65−67 Table 6 collects results of the EDDB analysis for 1 and 2, and their particular fragments with the full set of the atomic EDDB populations are presented in Tables S2 and S3 in the Supporting Information, while Figure 9 presents a schematic representation of the model planar and Möbius-like aromatic and quasi-aromatic 7-membered rings (7-MR) as well as the Table 6. EDDB Population Analysis (in |e|) for the Particular Fragments of 1 and 2

Cd NCS SCN MeOH LI (ring) LI (chain) LII (ring) LII (chain) H

complex 1

complex 2

0.14 2.33 1.42 0.33 23.80 3.94 − −

0.13 2.33 + 2.10 − 0.24 − − 23.99 4.28 DOI: 10.1021/acs.inorgchem.8b00064 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2. It is clear that in both cases there is no resonance connected with the Cd−N interactions. Apparently, in 1 and 2 the metal atom does not contribute to the aromatic stabilization, which is consistent with the predictions by the ETS-NOCV scheme (lack of metal participation in the π-channels of the Cd−L bonding) (Figure 10). It has been proven that ETS/ETS-NOCV, which is rooted in the MO model, is useful to quantify aromaticity in various systems.24,64,68−70 The popular NICS parameters have been shown to exhibit relatively small positive values (Figure S3 in the Supporting Information). However, they were shown to be rather unreliable for aromatic species containing transition metals since the local currents around metal centers are usually so strong that they mask any potential diatropic ring current.24,27,68,69 Moreover, for the quasi-aromatic rings there is no cyclic delocalization of electrons and hence the response of the π-system to the external magnetic field is less marked than in typical aromatic species (which is in fact a direct consequence of Ampère’s law). Accordingly, diatropic ring current in quasi-aromatic units is usually not observed,27 and the values of NICS presented herein only reaffirm this fact. To this end, we turn our attention predominantly to the EDDB method.65−67 Apart from some evident resonance effects on the (iso)thiocyanate ligands, more than 85% of the electrons delocalized through bond resonance are found on LI and LII. Particularly, about 24 |e| is predicted to be delocalized in molecular rings. This is in full agreement with expectations (12 fully delocalized π-bonds, three per six-membered ring). The resonance effects in the aliphatic parts of both ligands account for about 4 |e|, which equals half of the formal number of π-electrons in these

Figure 9. (Top) Schematic representation of the model planar and Möbius-like aromatic and quasi-aromatic 7-MRs. (Bottom) EDDB contours and populations for the twisted 7-MRs in 1 and 2.

results of the EDDB analysis (contours and electron populations) for the corresponding twisted 7-MRs of 1 and

Figure 10. (Top) Monomer of 1 together with the fragmentation applied in the ETS-NOCV analysis of the LI−Cd bonding. (Bottom) Most important NOCV-based deformation density channels Δρorb(i) with the corresponding orbital interaction energies ΔEorb(i). I


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Inorganic Chemistry

Figure 11. Normalized Kubelka−Munk spectra of LI, LII, 1, and 2 under ambient conditions. The experimental spectrum is shown in black, while the Gaussian deconvolution of the experimental spectrum is shown in blue and red.

fragments (4 π-bonds). Moreover, an overwhelming majority of this EDDB population is delocalized in the metallacyclic units with the characteristic twisted topology thus contributing toward the Möbius quasi-aromaticity (Figure 9). Admittedly, the effectiveness of π-bond delocalization in both 7-MRs seems to be much less marked in the overall picture of global aromaticity of both complexes than the fully aromatic sixmembered units (Figures S4 and S5 and additional files in the Supporting Information). However, one should realize that even the 40% effectiveness of the resonance stabilization is pretty much as for the quasi-aromatic ring,26−28 especially when compared to such well-known heteroaromatic species like pyrrole (ca. 55%), thiophene (ca. 40%) or furan (ca. 30%).65−67 It is important to stress that the planar quasi-aromatic rings have already been reported in different metal chelates and clathrochelates.30−32,65−67 However, we have identified herein for the first time the quasi-aromatic Möbius motif in the novel helical CdII complexes. We expect to find more such situations in other fascinating transition metal-based systems including, e.g., porphyrinoids71 or σ-, π- and δ-aromatic species.24,72 Since typical canonical molecular orbitals are well understood we have decided to depict them in Figure S6 in the Supporting Information. It is seen evidently a sign inversion what is characteristic for the molecular orbitals of a Möbius type. The same conclusion is valid from the contours of the EDDB function and HOMA index (Figure S6 in the Supporting

Information). It is important to comment that excluding of the metal leads to non-aromaticity of the system (Figure S6 the Supporting Information). Finally, we have estimated on the basis of ETS-NOCV that the total π-bonding in the helical ring LI is quite comparable to benzene (∼ −80 vs ∼ −108 kcal/mol, respectively) (Figure S7 in the Supporting Information). Such a relative trend has already been noted for aromatic metallabenzenes in an elegant papers by Fernández and Frenking.68,69 Due to the fact that the dispersion stabilization caused by the bulkiness of LI and LII appeared to be an important factor, we have decided to check whether considering smaller (non-bulky) models will influence the resonance phenomenon. We have determined that the replacement of the phenyl and pyridine units by the corresponding hydrogen atoms leads to a significant decrease in aromaticity, by ca. 12% as compared to the original system (Table S4 in the Supporting Information). At a first glance it might seem insignificant. However, note that the change in aromaticity by 4% (at the level of electron populations) as it is the case for anthracene vs phenanthrene leads to extreme differences in the stability and properties.73 Hence, the bulkiness of LI and LII, leading to the intramolecular dispersion stabilization caused by various non-covalent interactions, e.g., C−H···π, C−H···N, π···π, and C−H···H−C (Figure S8 in the Supporting Information), not only plays a J


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crucial role for the stability, but also influences the resonance phenomenon. To shed light on the optical properties the parent ligands LI and LII as well as complexes 1 and 2 were for the first time analyzed by diffuse reflectance spectroscopy. The Kubelka− Munk absorption coefficient (K/S) was calculated from the observed reflectance spectra by the formula K/S = (1 − R)2/ 2R, where K, S, and R represent the absorption coefficient, scattering coefficient, and reflectivity, respectively. The diffuse reflectance spectra of LII, 1 and 2 demonstrate bands in the UV region up to about 450 nm, while the spectrum of LI however exhibits bands up to about 500 nm (Figure 11). This bathochromic shift in the spectrum of LI is most likely due to the formation of dimeric units.22 The band gap energies of the LI, LII, 1, and 2 phosphors were estimated to be 2.52, 2.74, 2.86, and 2.83 eV, respectively. In order to obtain some hints on the optical properties of 1 and 2, TD-DFT calculations have been also conducted. For the monomeric form of 1, the two dominant absorption bands at 260 and 290 nm have been determined. These bands are mainly characterized by intramolecular charge transfers within the LI and NCS− ligands (Figure S9 in the Supporting Information). Qualitatively comparable spectrum is obtained for the monomer of 2 (Figure S10 in the Supporting Information). The absorption in the UV region is in accord with the experimental data (Figure 11). Notably, the metal does not participate in the transitions that is in line with the quasiaromatic Möbius features discussed above (Figures 9 and 10).

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Δρ(r ) =

k=1

M /2

ΔEorb =

∑ ΔEorb(k) = ∑ vk[−F −TSk , −k + FkTS,k] k

k=1

where FTS i,i are diagonal Kohn−Sham matrix elements defined over NOCVs. Finally, the last dispersion term ΔEdispersion within the overall interaction energy ΔEtotal is a semiempirical Grimme correction (D3). Synthesis of Complexes. In the main arm of a branched tube, a mixture of Cd(NO3)2·4H2O (0.154 g, 0.5 mmol), NH4NCS (0.076 g, 1 mmol), and LI or LII (0.208 and 0.222 g, respectively; 0.5 mmol) was placed, followed by addition of MeOH (15 mL). The main tube was then sealed and placed in an oil bath at 60 °C. The branched arm was kept at ambient temperature. Crystals suitable for X-ray diffraction were formed within the next few days in the cooler arm. Crystals were filtered off, washed with acetone and diethyl ether, and dried in air. 1. Colorless prism-like crystals. Yield: 0.291 g (86%). Anal. Calc. for C29H24CdN8OS2 (677.09) (%): C 51.44, H 3.57 and N 16.55; found: C 51.38, H 3.44 and N 16.66. 2. Colorless plate-like crystals. Yield: 0.321 g (91%). Anal. Calc. for C31H28CdN8OS2 (705.15) (%): C 52.80, H 4.00 and N 15.89; found: C 52.89, H 3.87 and N 15.97. X-ray Powder Diffraction. X-ray powder diffraction for bulk samples was performed by a Rigaku Ultima IV X-ray powder diffractometer. The Parallel Beam mode was set up to collect the results (λ = 1.54184 Å). Single-Crystal X-ray Diffraction. Agilent Technologies SuperNova-Atlas CCD diffractometer has been used to collect the data followed by the treatment with the Apex274 and CrysAlisPro75 programs and revised for absorption by SADABS.76 The structures were solved by direct methods,77 which discovered the positions of all non-hydrogen atoms, which were refined on F2 by a full-matrix leastsquares approach by means of anisotropic displacement factors.77 Hydrogen atoms, excluding those of hydroxyl groups, which were positioned from a difference Fourier map, were placed at calculated positions. All hydrogen atoms were incorporated as fixed components riding on attached atoms with isotropic thermal displacement factors 1.2 times those of the corresponding atom. The PLATON78 has been also applied to gather some geometry features. Figures were prepared using the program Mercury.79 Crystal Data for 1. C29H24CdN8OS2, Mr = 677.08 g mol−1, T = 150(2) K, monoclinic, space group P21/c, a = 9.1623(3), b = 14.4515(4), c = 22.1671(9) Å, β = 101.114(3)°, V = 2880.08(17) Å3, Z = 4, ρ = 1.562 g cm−3, μ(Mo Kα) = 0.942 mm−1, reflections: 15 557 collected, 8914 unique, Rint = 0.031, R1(all) = 0.0558, wR2(all) = 0.0880. Crystal Data for 2. C31H28CdN8OS2, Mr = 705.13 g mol−1, T = 100(2) K, monoclinic, space group P2/c, a = 10.7353(4), b = 9.6414(4), c = 30.839(1) Å, β = 93.811(2)°, V = 3184.9(2) Å3, Z = 4, ρ = 1.471 g cm−3, μ(Mo Kα) = 0.855 mm−1, reflections: 44 249 collected, 7070 unique, Rint = 0.069, R1(all) = 0.0595, wR2(all) = 0.0769. CCDC 1559651 and 1559652 contain the supplementary crystallographic data for this paper.

Materials. LI and LII were synthesized following the reported method.22 All other chemicals were purchased from commercially sources and used as received. Physical Measurements. Fourier-transform infrared spectroscopy (FTIR) spectra were obtained on a Bruker Tensor 27 FTIR spectrometer. Diffuse reflectance spectra were recorded with a Analytik Jena SPECORD 200 spectrometer. Polytetrafluoroethylene (PTFE) was used as a reference. Microanalyses were done by the ElementarVario EL III analyzer. DFT Calculations. We have completed calculations by means of the ADF/BLYP-D3/ZORA/TZP. 60,61 The charge and energy decomposition scheme ETS-NOCV59 has been used for bonding analyses. Additionally, periodic computations by the CP2K program56−58 have been performed (PBE-D3/DZP) to describe the newly obtained chelates. To quantify the aromaticity the recently proposed EDDB method (Electron Density of Delocalized Bonds)65−67 has predominantly been applied (for more details see the Supporting Information). It allows to analyze quantitatively the number of electrons which are delocalized between conjugated bonds.65−67 It has already been proven that it is very powerful and reliable approach to describe the aromaticity of various systems including transition metal systems.65−67,73 ETS-NOCV Bonding Analysis. The natural orbitals for chemical valence (NOCV)59 are computed by solving an eigenvalue equation: N

ψi =

k=1

where νk and M are the NOCV eigenvalues and the number of basis functions, respectively. In addition to Δρk, within the ETS-NOCV,59 one can obtain the corresponding energetic estimations, ΔEorb(k). In the ETS method the total bonding energy, ΔEint, between fragments can be split into four components: ΔEtotal = ΔEelstat + ΔEPauli + ΔEorb + ΔEdispersion. The ΔEelstat is due to classical electrostatic interaction between subunits (positions as in the final molecule). The next term, ΔEPauli, involves the repulsive Pauli interaction between occupied orbitals on the two fragments. The third component, ΔEorb, covers interactions between the occupied molecular orbitals of one fragment with the unoccupied molecular orbitals of the other fragment and intrafragment polarizations. In the ETS-NOCV scheme ΔEorb is further expressed in terms of vk as

EXPERIMENTAL SECTION

ΔPCi = vC i i

M /2

∑ vk[−ψ−2k(r) + ψk2(r)] = ∑ Δρk (r)

∑ Ci , jλj j

where Ci is a vector applied to express Ψi in the basis of fragment orbitals λj, N is a total number of fragment λj orbitals, and ΔP matrix denotes a difference between a density matrix of a molecule and the corresponding considered molecular fragments. Such eigenvectors which are in pairs (ψ−k,ψk) decompose the deformation density Δρ into different bonding contributions (Δρk): K


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Inorganic Chemistry

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Accession Codes

CONCLUSIONS In summary, we have designed and fully characterized two new coordination compounds, namely [Cd(SCN)(NCS)(LI)(MeOH)] (1) and [Cd(NCS)2(LII)(MeOH)] (2), synthesized from Cd(NO3)2·4H2O and closely related benzilbis(pyridin-2yl)methylidenehydrazone (LI) and benzilbis(acetylpyridin-2yl)methylidenehydrazone (LII ) in a mixture with two equivalents of NH4NCS in MeOH. Each CdII center is bound by the two pyridyl-imine fragments of the corresponding tetradentate Schiff base, one methanol oxygen atom, and two donor atoms of two pseudohalido ligands. Obviously, the choice of ligand plays a key role in the coordination behavior of cadmium when NCS− was the anionic ligand present. Particularly, the presence of the methyl groups in the ligand leads to the exclusive coordination of the isothiocyanate ligands as in the structure of 2, while the absence of the methyl groups allows to simultaneously coordinate thiocyanate and isothiocyanate ligands as in the structure of 1. Thus, the coordination polyhedra are CdN5SO in 1 and CdN6O in 2 forming a square face monocapped trigonal prism. Both complexes contain intermolecular hydrogen bonding, C−H···π interactions and aromatic π···π stacking. Hirshfeld surface analysis showed that the structures of both 1 and 2 are highly dominated by H···X (X = H, C, N and S) contacts. The Kubelka−Munk-based absorption studies have revealed that LII, 1 and 2 exhibit bands almost exclusively in the UV region up to about 450 nm, while the spectrum of LI shows bands up to about 500 nm. The TDDFT-based calculations have indicated solely intramolecular transitions within the LI and LII ligands. It has been determined for the first time by the EDDB method, HOMA index, and ETS-NOCV scheme that the helical motif in 1 and 2, including the chelate ring with the metal, can be classified as a quasi-aromatic Möbius object. The ETS-NOCV results have indicated that the obtained structures are very tightly packed, which results in extremely low values of the interactions energies (in the range typical for covalent bonds) between the monomers. The dispersion and electrostatic forces have been found to be dominant in both complexes, followed by charge delocalization contributions. The former factor is recently regaining considerable attention in various fields of chemistry.62,63 Furthermore, it has been determined for the first time that the bulkiness of LI and LII additionally enhanced the resonance phenomenon in 1 and 2.

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CCDC 1559651 and 1559652 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.M.). *E-mail: [email protected] (M.P.M.). *E-mail: damir.a.safi[email protected] (D.A.S.). ORCID

Fedor I. Zubkov: 0000-0002-0289-0831 Mariusz P. Mitoraj: 0000-0001-5359-9107 Filip Sagan: 0000-0001-5375-8868 Dariusz W. Szczepanik: 0000-0002-2013-0617 Damir A. Safin: 0000-0002-9080-7072 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University of Maragheh for the financial support of this research. This work was also supported by the Ministry of Education and Science of the Russian Federation (02.a03.21.0008), the National Science Centre, Poland (grant no. 2015/17/D/ST4/00558, D.W.S.), and Tomsk Polytechnic University within the framework of Tomsk Polytechnic University Competitiveness Enhancement Program. DFT calculations were partially performed using the PL-Grid Infrastructure and resources provided by the ACC Cyfronet AGH (Cracow, Poland). M.K. and F.S. acknowledge financial support from the Polish Ministry of Science and Education “Tsubsidy” for young researchers. The authors are grateful to Israel Fernández for his kind support in terms of the ETSNOCV analyses. The crystal structure of the compound 1 has been refined by Annalisa Guerri from the Università di Firenze.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00064. EDDB method and NICS calculations; 2D and decomposed 2D fingerprint plots of observed contacts for 1 and 2; NICS values for LI, LII, 1, and 2; visualization of the molecular structure and the EDDB(r) function for 1 and 2; visualization of EDDB(r) at different isosurface values for 1 and 2; experimental and DFT optimized cell parameters for 1 and 2; atomic coordinates and electron populations from the EDDB method for 1 and 2 (PDF) Animated visualization of the molecular structure for 1 (MPG) Animated visualization of the molecular structure for 2 (MPG) L


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Inorganic Chemistry

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Quasi-aromatic MÃ¶bius Metal Chelates - Inorganic Chemistry (ACS

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