1. Introduction
1,2-Bis(arylimino)acenaphthene (Ar-bian) is a well-studied class of aromatic acceptor diimines with versatile coordination and redox chemistry [1,2,3,4]. Ar-bian is composed of a central 1,4-diazabutadiene fragment, which is complemented by a naphthalene backbone. The combination of these elements gives rise to strong σ-donor and π-acceptor properties, thereby ensuring the stabilization of metal ions in both high and low oxidation states. Furthermore, the rigid naphthalene moiety stabilizes the anti-anti conformation of the α-diimine fragment, facilitating strong chelation to the metal center.
To date, metal complexes based on Ar-bian with almost all d-block metals have been described in the literature [3,5,6,7,8,9]. However, despite the large number of publications, such complexes have been the focus of research in a highly uneven manner. For example, an impressive number of publications are devoted to copper, nickel, and palladium complexes [10,11,12,13,14,15]. This is mainly due to their demand for various catalytic applications [16,17]. A similar trend continues for the metals of the 12th group of the Periodic Table. At present, approx. 60 structurally characterized zinc complexes are known, most of which are mononuclear complexes of the type [Zn(Ar-bian)Cl2]. They are formed as intermediates in the synthesis of free Ar-bian ligands, which determines their demand [18]. This contrasts with the complete absence of structurally characterized cadmium analogues. Meanwhile, mercury complexes based on Ar-bian are confined to only three examples. Ferreira et al. demonstrated the potential for utilizing HgX2 (X = Cl, SCN) (rather than ZnCl2) in the template synthesis of Ar-bian. Respectively, the reaction of HgX2 with acenaphthenequinone and 4-tert-butylaniline resulted in the formation of [Hg(2-tBu-bian)Cl2] or [Hg(2-tBu-bian)(SCN)2] [19]. In a similar manner, El Ayaan obtained the complex [Hg(dpp-bian)Cl2] [20].
In the present study, we report the synthesis and crystal structures of the first cadmium/BIAN complexes: dimeric [Cd2(tmp-bian)2Cl2(µ-Cl)2] (1), [Cd2(tmp-bian)2(H2O)4(µ-Cl)2](OTf)2 (2) and monomeric [Cd(tmp-bian)2(OTf)2] (3) (tmp-bian = 1,2-bis[(2,4,6-trimethylphenyl)imino]acenatephene).
2. Results and Discussion
2.1. Synthesis and Characterization
The reaction of CdCl2 with 1 equivalent of tmp-bian in ethanol afforded the binuclear complex [Cd2(tmp-bian)2Cl2(μ-Cl)2] (1) in high yield (90%). Evans et al. demonstrated the possibility of the formation of analogous dimeric zinc(II) complexes [Zn2(4-OCF3-C6H4-bian)2Cl2(μ-Cl)2] and [Zn2(4-CF3-C6H4-bian)2Cl2(μ-Cl)2] [21]. Treatment of complex 1 with four equivalents of AgOTf led to a mixture consisting of [Cd2(tmp-bian)2(H2O)4(μ-Cl)2](OTf)2 (2) and [Cd(tmp-bian)2(OTf)2] (3) in approximately equal proportions. Unfortunately, we were unable to isolate the complexes individually from this mixture, even using column chromatography. Alternatively, complex 3 was synthesized in 85% yield by the reaction of Cd(OTf)2 with tmp-bian in a 1:2 molar ratio. Attempts to obtain complex 2 in pure form by reacting complex 1 with two equivalents of AgOTf and tmp-bian were unsuccessful. The general synthetic scheme is shown in Figure 1. The purity of compounds 1 and 3 was confirmed by elemental analysis.
The IR spectrum of complex 1 showed typical ν(C-H) vibration bands in the region of 3078–2817 cm−1. ν(C=N) and ν(C-C) vibration bands were detected at 1660–1590 cm−1 and 1477 cm−1, respectively, which is consistent with the neutral state of tmp-bian. In the IR spectrum of complex 3, ν(C-H) vibration bands were observed in the region of 3070–2831 cm−1. ν(C=N) and ν(C-C) bands were detected at 1658–1585 cm−1 and 1477 cm−1, respectively. In addition, typical vibrations of the triflate group were observed at 1238, 1028, and 637 cm−1 (Figures S1 and S2).
The 1H NMR spectrum of complex 1 revealed characteristic signals of acenaphthene moiety of Ar-bian, appearing as two doublets at 8.10 and 6.55 ppm and one triplet at 7.55 ppm. The signals of the aryl rings appeared as a singlet at 7.05 ppm, as well as two upfield singlets at 2.25 and 2.45 ppm. Note that two tmp-bian ligands in 1 are equivalent to each other. The 1H NMR spectrum of complex 3 has a similar pattern. The signals of the acenaphthene fragment appeared as two doublets and one triplet at 8.14, 6.91, and 7.59 ppm. The signals of the aryl rings appeared as a singlet at 7.09, 2.12, and 2.09 ppm (Figures S3 and S4).
2.2. X-Ray Structure Description
Single crystals of 1•2(C2H5)2O and 3•1.2CH2Cl2 suitable for X-ray diffraction analysis, were obtained by slow diffusion of diethyl ether into a solution of 1 and 3 in CH2Cl2. Single crystals of 2, suitable for XRD, were obtained by slow evaporation of a mixture of complexes 2 and 3 formed after the corresponding reaction (see Figure 1).
The molecular structures of neutral complexes 1 and 3, as well as cationic complex 2, are shown in Figure 2. The values of selected geometric parameters are given in Table 1.
In complex 1, the cadmium atoms have a highly distorted square-pyramidal coordination environment (τ = 0.20), consisting of two nitrogen atoms of tmp-bian, located in the equatorial plane, a chlorine atom located at the apex of the pyramid, and two bridging chlorine atoms that connect the {Cd(tmp-bian)Cl} fragments into a binuclear structure. Each cadmium ion is located above the equatorial plane at a distance of about 0.8 Å. Tmp-bian is coordinated to the Cd(II) ion asymmetrically: the Cd–N1 and Cd–N2 distances are 2.354(2) Å and 2.427(2) Å, respectively. The C–N1 (1.280(3) Å), C–N2 (1.269(3) Å), and C-C (1.514(3) Å) bond lengths indicate the neutral state of the tmp-bian ligand. An asymmetric coordination of the bridging ligands is also observed: the Cd–(μ-Cl) distances are 2.5944(8) Å and 2.5100(7) Å, respectively. The Cd–Cl distance is 2.4211(8) Å.
It is noteworthy that in the literature, there are examples of dimeric centrosymmetric Ni(II) and Zn(II) complexes with Ar-bian ligands of the composition [{M(tmp-bian)(X)}2(μ-X)2] (X = Cl, Br, I). Two variants of the coordination environment of the metal ion are possible. In the first case, the metal ion has a distorted square-pyramidal environment, characteristic of nickel complexes [22,23,24,25,26,27]. In the second case, the coordination polyhedron is a distorted trigonal bipyramid, which is observed in both zinc and nickel complexes [21,28].
In complex 2, the cadmium atoms have a distorted octahedral coordination environment, consisting of two nitrogen atoms of tmp-bian, two oxygen atoms of water, and two bridging chlorine atoms that connect the {Cd(tmp-bian)(H2O)2} fragments into a binuclear structure. In the outer sphere, there are two triflate anions. Unlike complex 1, in complex 2, tmp-bian is coordinated almost symmetrically: the Cd–N1 and Cd–N2 bond lengths are 2.375(3) and 2.362(2) Å, respectively. Cd–O and Cd–Cl distances are 2.329(2), 2.349(3) Å and 2.559(1), 2.6000(9) Å, respectively. The C–N1 (1.278(4) Å), C–N2 (1.271(5) Å), and C-C (1.528(5) Å) distances indicate the neutral state of the tmp-bian ligand [29,30].
Complex 3 is a mononuclear Cd(II) complex in which the metal ion has an octahedral coordination geometry. In the equatorial plane, there are four nitrogen atoms of two tmp-bian ligands, and the axial positions are occupied by oxygen atoms of the coordinated triflate anion. The tmp-bian molecules are coordinated asymmetrically: the Cd–N distances are 2.314(2) and 2.391(3) Å. Cd–O distances are 2.381(2) Å. The C–N and C-C bond lengths are 1.275(4) Å, 1.288(4) Å and 1.515(5) Å, respectively. A similar octahedral bis-chelated Mn(II) complex [Mn(tmp-bian)2(ClO4)2] has been reported [31].
2.3. Electrochemical Properties
The electrochemical properties of complexes 1 and 3 were investigated using cyclic voltammetry (Figure 3). The electrochemical properties of complex 2 were not studied because it was not isolated in individual form. The CV of complex 1 in dichloromethane showed a cathodic peak at Ec = −1.01 V (vs. Ag/AgCl) with the corresponding anodic counter peak at Ea = −0.80 V. The large difference between these peaks (0.21 V) indicates the irreversibility of the redox process. For complex 3, two reduction peaks were detected at Ec = −0.56 V and Ec = −0.73 V (vs. Ag/AgCl); an anodic counter peak appeared at −0.57 V. Oxidative processes in the range of 0–2 V were not detected for either complex. The reduction processes found for 1 and 3 are in good agreement with the electron-accepting capacity of Ar-bian. Irreversible reduction at −1.69 V (vs. Ag/AgCl) was previously shown for the free tmp-bian ligand [15].
To confirm the ligand-centered character of redox processes, quantum chemical calculations were carried out within the framework of density functional theory. The X-ray diffraction data were utilized as the initial geometry.Optimized geometry coordinates of 1–3 in the gas phase are presented in Tables S2–S4. The identification of the minimum point on the potential energy surface was substantiated by the absence of imaginary vibrational frequencies.
The optimized geometries of 1, the cation of 2, and 3 are consistent with the X-ray diffraction data. However, in the case of complex 1, the structure of the {Cd2(μ-Cl)2} moiety differed from the initial geometry. In the optimized geometry, the angle between [μ-Cl1 Cd1 μ-Cl2] and [μ-Cl1 Cd2 μ-Cl2] planes is 40.2°, whereas, according to X-ray diffraction data, the Cd and μ-Cl atoms lie in the same plane. An attempt to rectify the inversion center during geometry optimization resulted in the appearance of an imaginary frequency.
The composition of frontier molecular orbitals for 1, the cation of 2, and 3 are presented in Table 2. The views of HOMOs and LUMOs of 1–3 are shown in Figure 4. Remarkably, HOMOs and LUMOs are almost completely localized on the tmp-bian ligand; however, the contribution of cadmium and chlorine atomic orbitals to the HOMO is approx. 19% for 1. It is worth noting that the LUMO is mainly localized on the acenaphthene and diimine fragments of tmp-bian, whereas the HOMO is localized on the trimethylphenyl rings. The ligand-centered nature of the LUMO indicates that the observed redox processes are most likely consistent with the reduction of tmp-bian to the anion radical. The presence of two closely located reduction peaks in the cyclic voltammogram of complex 3 correlates with the presence of two tmp-bian ligands in its structure. For complex 1, these peaks seem to merge into one broad cathodic peak. This can be explained by the structural differences between the two complexes. In complex 1, two tmp-bian molecules are coordinated to different cadmium ions. In contrast, in structure 3, tmp-bian ligands are bound to the same Cd(II) ion.
3. Experimental
3.1. Materials and Methods
1,2-Bis[(2,4,6-trimethylphenyl)imino]acenaphthene (tmp-bian) was synthesized according to the literature [32]. The remaining reagents were purchased from commercial sources: acenaphthenequinone (99%, Sigma Aldrich, St. Louis, MI, USA), 2,4,6-trimethylaniline (99%, Sigma Aldrich), CdCl2 (98%, Sigma Aldrich), and Cd(OTf)2 (Sigma Aldrich, 98%). All solvents used were purified according to standard procedures.
Elemental analysis of C, H, N, and S was conducted using a EuroEA3000 Eurovector analyzer. The IR spectra were recorded within the 4000–300 cm−1 range with a Perkin-Elmer System 2000 FTIR spectrometer, with the samples prepared as KBr pellets. The 1H NMR spectra (500 MHz) were acquired on a Bruker Avance-500 spectrometer equipped with a 5 mm PABBO-PLUS probe at ambient temperature. The chemical shifts were expressed in parts per million (ppm) from tetramethylsilane. The cyclic voltammograms (CV) were recorded with a 797 VA Computrace system (Metrohm, Herisau, Switzerland). All measurements were conducted utilizing a conventional three-electrode configuration, comprising a glassy carbon working electrode, a platinum auxiliary electrode, and Ag/AgCl/KCl reference electrodes. The solvent used in all experiments was dichloromethane, which was deoxygenated before use. Tetra-n-butylammonium hexafluorophosphate (0.1 M solution) was used as a supporting electrolyte. The concentration of the complexes was 10−3 M. The half-wave potential (E1/2) was calculated as the mean of the cathodic and anodic peak potentials. Ferrocene was utilized as an internal standard, with the Fc/Fc+ potential measuring 0.49 V.
3.2. X-Ray Diffraction Analysis
Single-crystal X-ray diffraction data were collected on a Bruker D8 Venture diffractometer, equipped with a CMOS PHOTON III detector and an IμS 3.0 microfocus source (collimating Montel mirrors). All experiments were conducted at a temperature of 150 K using Mo Kα (λ = 0.71073 Å) radiation. Absorption correction was applied using SADABS [33]. The structures were solved by a dual space algorithm with SHELXT-2018/2 [34] and refined by full-matrix least-squares treatment against |F|2 with SHELXL-2019/3 [35] using the ShelXle GUI [36]. The atomic displacement parameters for non-hydrogen atoms were refined anisotropically. For compound 3, the presence of highly disordered solvent molecules inside the molecular structure was discovered. Due to the complexity of the modeling, the electron density of disordered solvent molecules was excluded from consideration using the PLATON/SQUEEZE program [37,38]; 52e per formula unit was found, which corresponds to 1.2 CH2Cl2 molecules per formula unit (3·1.2 CH2Cl2). Crystallographic data and details of diffraction experiments for complexes 1•2(C2H5)2O), 2 and 3•1.2CH2Cl2 are presented in Table S1, which contain(s) the supplementary crystallographic data for this paper. Deposition numbers CCDC 2435400 (for 1•2(C2H5)2O), 2435401 (for 2), and 2435402 (for 3•1.2 CH2Cl2) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre
3.3. Quantum Chemical Calculations
Quantum chemical calculations were performed using the ORCA 6.0.0 software package [39,40]. The crystal structure data for 1–3 were utilized as the initial points for the geometry optimization process. Calculations were conducted within the framework of density functional theory (DFT), employing the non-empirical PBE functional [41] in conjunction with empirical corrections for dispersion interactions, D4 [42]. All calculations used the Karlsruhe family of Gaussian-type orbital basis sets [43]. To describe Cd atoms, the def2-TZVPP basis set was used, which is a triple-zeta basis set augmented with two sets of polarization functions. The effective nuclear potential (Def2-ECP) was used to describe the inner electrons (up to 3d) [44]. To describe H atoms, the double-zeta basis set was used, augmented with one set of polarization functions. To describe all other atoms, the def2-TZVP basis sets were used. All calculations were performed using the RI approximation (RI-J) [45] with an auxiliary def2/J basis set [46].
3.4. Synthesis of [Cd2(tmp-bian)2Cl2(μ-Cl)2] (1)
CdCl2 (50 mg, 0.272 mmol) and tmp-bian (111 mg, 0.272 mmol) were dissolved in 10 mL of ethanol. The resulting solution was refluxed for 24 h. An orange precipitate was filtered using a glass filter and washed with n-hexane. Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of diethyl ether into a solution of 1 in dichloromethane. Yield: 145 mg (90%).
Calculated for C60H56N4Cl4Cd2: С 60.8; H 3.40; N 4.7%. Found: C 61.1, H 3.63, N 4.8%.
IR (KBr, cm−1): 2966 (w), 2913 (w), 2850 (w), 1667 (m), 1634 (s), 1587 (s), 1478 (s), 1433 (m), 1420 (m), 1379 (w), 1289 (m), 1240 (s), 1225 (m), 1206 (w), 1159 (w), 1146 (w), 1115 (w), 1053 (w), 1032 (w), 961 (w), 937 (w), 893 (w), 853 (m), 847 (m), 831 (m), 806 (w), 795 (w), 779 (s), 656 (w), 574 (w), 561 (w), 554 (w), 517 (w), 449 (w).
1H NMR (CDCl3, δ ppm): 2.25 (s, 12H), 2.42 (s, 6H), 6.85 (d, 2H), 7.05 (s, 4H), 7.55 (t, 2H), 8.10 (d, 2H).
3.5. Synthesis of [Cd(tmp-bian)2(OTf)2] (3)
Cd(OTf)2 (50 mg, 0.12 mmol) and tmp-bian (99 mg, 0.024 mmol) were dissolved in 5 mL of CH2Cl2. The resulting solution was stirred at room temperature for 24 h, then evaporated under vacuum. The solid residue was washed with cold ethanol and n-hexane, and dried on a glass filter. Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of diethyl ether into a solution of 3 in dichloromethane. Yield: 136 mg (90%).
Calculated for C62H56F6N4O6Cd: С 59.8; H 4.5; N 4.50%. Found: C 60.0, H 4.67, N 4.7%.
IR (KBr, cm−1): 2991 (w), 2914 (w), 2858 (w), 1658 (m), 1624 (m), 1585 (m), 1477 (m), 1437 (w), 1421 (w), 1385 (w), 1300 (s), 1282 (m), 1238 (s), 1221 (m), 1163 (s), 1107 (m), 1028 (s), 852 (w), 837 (w), 783 (m), 637 (s), 578 (w), 559 (w), 518 (w), 511 (w).
1H NMR (CDCl3, δ ppm): 2.09 (s, 24H), 2.12 (s, 12H), 6.91 (d, 4H), 7.09 (s, 8H), 7.59 (t, 4H), 8.14 (d, 4H).
4. Conclusions
Despite numerous examples of zinc complexes bearing bis(imino)acenaphthenes (Ar-bians), cadmium analogues were unknown until this work. Herein, we describe the first representatives of a new family of cadmium complexes with Ar-bian. These are monomeric and dimeric Cd(II) complexes based on bis-(2,4,6-trimethylphenylimino)acenaphthene (tmp-bian), for which crystal structures were determined by X-ray diffraction analysis. The coordination number of the Cd(II) ion in these complexes varies from five (square-pyramidal) to six (octahedral). The complexes exhibit ligand-centered reduction, typical of metal complexes with redox-active Ar-bian ligands.
E.V.C., A.K.G., and N.O.S.—Investigation (synthesis, spectroscopy); I.V.B.—Investigation (SCXRD); V.I.K.—Investigation (DFT); S.A.—Funding acquisition, Resources, Visualization; B.D.—Investigation (electrochemistry); K.D.—Writing—review and editing, Funding acquisition; N.A.—Writing—review and editing, Conceptualization, Supervision, Funding acquisition; N.F.R.—Methodology, Project administration, Writing—original draft preparation; A.L.G.—Supervision, Writing—review and editing, Conceptualization. All authors have read and agreed to the published version of the manuscript.
The data supporting this article have been included as part of the
The authors declare no conflicts of interest.
Footnotes
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Figure 1 Synthetic routes for complexes 1–3.
Figure 2 Molecular structures of complex 1•2(C2H5)2O, the cation of 2, and complex 3•1.2CH2Cl2. Hydrogen atoms and solvent molecules are omitted for clarity.
Figure 3 Cyclic voltammograms of complexes 1 (left) and 3 (right) in dichloromethane.
Figure 4 View of the frontier molecular orbitals of 1, the cation of 2, and 3.
Selected geometric parameters for 1•2(C2H5)2O, 2, and 3•1.2CH2Cl2.
| 1•2(C2H5)2O | Distance, Å | 2 | Distance, Å | 3•1.2CH2Cl2 | Distance, Å |
|---|---|---|---|---|---|
| Cd–N | 2.354(2) | Cd–N | 2.375(3) | Cd–N | 2.314(2) |
| Cd–(μ-Cl) | 2.5944(8) | Cd–(μ-Cl) | 2.559(1) | Cd–O | 2.381(2) |
| Cd–Cl | 2.4211(8) | Cd–O | 2.329(2) | С–N | 1.275(4) |
| С–N | 1.280(3) | С–N | 1.278(4) | С–С | 1.515(5) |
| С–С | 1.514(3) | С–C | 1.528(5) |
Contributions of fragments (tmp-bian, Cd, Cl) to the frontier molecular orbitals of complexes 1–3.
| 1 | 2 | 3 | |
|---|---|---|---|
| HOMO | |||
| tmp-bian | 74.8% | 86.7% | 89.4% |
| Cd | 9.2% | 1.0% | 3.0% |
| Cl | 9.9% | 0.60% | - |
| LUMO | |||
| tmp-bian | 93.1% | 91.4% | 88.8% |
| Cd | 1.0% | 0.5% | 1.3% |
| Cl | 0.5% | 0.1% | - |
Supplementary Materials
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; Komlyagina, Veronika I 2 ; Appazova Saltanat 3
; Diyarova Banu 4
; Darmagambet Klara 3 ; Nurbol, Appazov 3
; Romashev, Nikolai F 2
; Gushchin, Artem L 2
1 Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia; [email protected] (E.V.C.); [email protected] (I.V.B.); [email protected] (A.K.G.); [email protected] (N.O.S.); [email protected] (V.I.K.); [email protected] (N.F.R.);, Department of Natural Sciences, Novosibirsk State University, Novosibirsk 630090, Russia
2 Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia; [email protected] (E.V.C.); [email protected] (I.V.B.); [email protected] (A.K.G.); [email protected] (N.O.S.); [email protected] (V.I.K.); [email protected] (N.F.R.);
3 Department of Engineering and Technology, Institute of Engineering and Technology, Korkyt Ata Kyzylorda University, Ayteke bi Street 29A, Kyzylorda 120014, Kazakhstan; [email protected] (S.A.); [email protected] (K.D.); [email protected] (N.A.)
4 Department of Chemistry, Institute of Natural Sciences, Kazakh National Women’s Teacher Training University, Gogol Street 114/8, Almaty 050000, Kazakhstan