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Article

Copper(II) Prevents the Saccarine-Dialkylcyanamide Coupling by Forming Mononuclear (Saccharinate)(Dialkylcyanamide)copper(II) Complexes

by
Yulia N. Toikka
,
Dar’ya V. Spiridonova
,
Alexander S. Novikov
and
Nadezhda A. Bokach
*
Institute of Chemistry, Saint Petersburg State University, 7/9 Universitetskaya Nab., 199034 Saint Petersburg, Russia
*
Author to whom correspondence should be addressed.
Inorganics 2021, 9(9), 69; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9090069
Submission received: 12 August 2021 / Revised: 1 September 2021 / Accepted: 7 September 2021 / Published: 8 September 2021
(This article belongs to the Section Coordination Chemistry)
Browse Figures
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Abstract

:
The reaction in the system CuII/sacNa(H)/NCNR2 (sacNa(H) = sodium saccharinate (saccharin); R = Me, Et) results in the formation of the complexes [Cu(sac)2(NCNR2)(H2O)2] (R = Me 1, Et 2) instead of the expected products derived from the saccharin–cyanamide coupling. Complexes 1, 2, and hydrate 1·2H2O were characterized by IR, AAS (Cu%), TGA, and also by single-crystal X-ray diffraction for 1 and 1·2H2O. An integrated computational study of model structure 1 in the gas phase demonstrates that the Cu–Ncyanamide and Cu–Nsac coordination bonds exhibited a single bond character, polarized toward the N atom and almost purely electrostatic, with the calculated vertical total energies for the Cu–Ncyanamide and Cu–Nsac of 43.6 and 156.4 kcal/mol, respectively. These data confirmed that the copper(II) completely blocks the nucleophilic centers of ligands via coordination, thus preventing the saccharin–cyanamide coupling.

Graphical Abstract

1. Introduction

Saccharin (sacH), which is a common and widely used artificial sweetener [1], also finds application in diverse areas of chemistry, particularly being applied as a catalyst of a number of organic reactions (leading to, e.g., compounds of biological importance [2,3,4,5]) and a functional group protector [2,6]. SacH is also applied in the preparation of drug-based co-crystals exhibiting an improved solubility and bioavailability compared to the poorly soluble parent drugs [7].
Our recent finding in the saccharin chemistry was the observation of unusual metal-free “two saccharin–one cyanamide” coupling between sacH and NCNR2 to grant guanidinated saccharin derivatives [8]. The distinctive feature of this transformation is its formal three-component character, while other known examples of reactivity of sacH toward multiple bond substrates includes the additions to amino alkynes [9,10], isocyanide [11], isocyanates [12], and carbodiimides [12,13]; all these examples include two-component 1:1 additions.
In light of the general interest in coordination chemistry of NCNR2 (R2 = Alk2, Ar2, H/Alk, H/Ar, H/CN, etc.) and their metal-mediated and metal-catalyzed reactions (for our and other group reviews on metal-involving reactivity and coordination chemistry of cyanamides see [14,15] and [16,17], respectively), we now studied the reaction between sacH and NCNR2 in the presence of a metal center, which as we assumed, should change its directionality. For these purposes, we addressed copper(II), as a kinetically labile metal center, which is commonly used in various catalytic reactions including coupling-based transformations. We observed that the introduction of copper(II) completely changed the directionality of the reaction by blocking the nucleophilic centers of both reactants by the ligation to give (sac)(NCNR2)CuII species instead of the guanidinated saccharin derivatives.

2. Results and Discussion

2.1. Synthesis and Characterization

For the study of the CuII-involving reactions between cyanamides and saccharin, we addressed the copper salts CuCl2·2H2O or CuBr2, N,N-disubstituted cyanamides NCNR2 (R = Me, Et), and sacH or its sodium salt (sacNa). These reactions were performed in a neat NCNR2 or NCNR2/solvent mixture (solvent = H2O, MeOH, EtOH) where the copper salts and saccharin(ate) are well soluble. In the systems, CuII/NCNR2/SacNa, [Cu(sac)2(NCNR2)(H2O)2] (R = Me 1, Et 2) complexes were obtained and isolated as crystalline solids. The results previously reported for metal-free process “two saccharin–one cyanamide” addition products were neither isolated, nor identified by HRESI+-MS in the reaction mixtures, although peaks from the 1:1 addition product were detected in the HRESI+-MS spectra (see ESI, Supplementary Materials Figures S1 and S2). Furthermore, we optimized the synthesis conditions in order to increase the yield of complexes 1 and 2.
Copper(II) saccharinate/dimethyl cyanamide complexes 1 and 1·2H2O (50–75%) were obtained by the dissolution of CuCl2·2H2O or CuBr2 in NCNR2 (R = Me, Et) at 60 °С followed by the addition of a solution of sacNa in MeOH, EtOH, or H2O (Scheme 1). Notably, the choice of solvent affects the release of either hydrated, or anhydrous form of the complex. When the reaction proceeds in dried EtOH, complex 1 was obtained, while from undried MeOH, EtOH, or in H2O, hydrate 1·2H2O was isolated. In this reaction, sacH can be used instead of sacNa, however, this treatment gives a lower yield of the product (up to 35% for 1·2H2O). The yield reduction can be explained by a lower reactivity of the protonated form (pKa of sacH = 1.6 [18] in H2O). The reason of the moderate yield of 1 and 1·2H2O is the formation of various unidentified by-products. IR monitoring of the filtrate from the reaction mixture verified broad bands at 1641s and 1620 m-s sh cm–1 and band 2251 cm–1 s, which were attributed to ν(C=O) from the saccharinate moiety of the 1:1 addition product, and Me2NC(O)NH2, and ν(CN)cyanamide from the coordinated NCNMe2, respectively.
Upon the extension of the complexation to the other cyanamides NCNR2 (R2 = Et2, C4H8, C5H10, C4H8O), we found that CuCl2·2H2O and CuBr2 were almost insoluble in most of these cyanamides (R2 = C4H8, C5H10, C4H8O). Therefore, we modified the reaction conditions, namely CuCl2·2H2O (or CuBr2) was dissolved in THF and then a solution of both sacNa and NCNR2 in MeOH (or EtOH) was added to a THF solution. Under these conditions, a lantern-like complex [Cu2(sac)4(THF)2]·2THF (3·2THF) was obtained. When we attempted to replace THF in this reaction with another solvent (MeOH, EtOH, or MeCN), we obtained the complex [Cu(sac)2(H2O)4]·2H2O (from the system CuCl2·2H2O/NCNC5H10/sacNa). The structure of this known [19] complex [Cu(sac)2(H2O)4]·2H2O was confirmed by X-ray diffractometry (XRD; see ESI). In the case of NCNEt2, in which CuCl2·2H2O and CuBr2 demonstrated limited solubility, we succeeded in obtaining complex 2 under conditions similar to those applied for the synthesis of 1·2H2O.
Complexes 1, 1·2H2O, and 3·2THF were characterized by IR, AAS (Cu%), and TGA methods and also by single-crystal X-ray diffraction. Complex 2 was characterized by IR and AAS (Cu%). The AAS (Cu%) data agree with the calculated values for the proposed formulas. In the IR spectra of 1, 1·2H2O, and 2, the ν(C≡N) of the NCNR2 ligand was observed in the range 2226–2245 cm–1; these values are comparable with those observed for the homoleptic complexes [Cu(NCNR2)4](BF4) (R = Me, Et; ca. 2240 cm–1) [20] and the mixed-ligand copper(I) complexes [Cu(tpm)(NCNR2)](BF4) (R = Me, Et; ca. 2250 cm–1) [21] and the clusters [Cu4X6O(NCNMe2)4] (2255–2261 cm–1) [22]. In the IR spectra of all complexes, the two strong bands in the ranges 1306–1330 and 1165–1177 cm–1 were attributed to νsym(SO2) and νasym(SO2) of the saccharinate ligands, respectively. We also attempted mass-spectrometric characterization of the obtained species, but the HRMS+ (ESI) spectra (in MeCN) did not display molecular ions for all complexes and only products of deep fragmentation of complexes (e.g., ions Cu(NCMe)2+) were observed due to the lability of copper(II) complexes in a solution.
Complexes 1 and 2 are stable until ca. 70 °C and then decompose with the loss of the NCNR2 ligand in the interval ca. 70–150 °C and two H2O ligands at ca. 150–250 °C; after 250 °C, the residue undergoes decomposition of sac– ligands forming yet unidentified species. Hydrate 1·2H2O somehow demonstrates greater stability and its decomposition starts at ca. 100 °C with loss of the crystallization water and the NCNMe2 ligand at 100–150 °C; after 250 °C, the decomposition to yet unidentified species occurs. Complex 3·2THF is stable until ca. 120 °C and then starts losing the solvated THF (125–260 °C), whereupon the coordinated THF (260–326 °C) is lost. After 326 °C, non-stochiometric decomposition of the sac– ligands occurs.

2.2. X-ray Diffraction Studies

Single-crystal XRD studies were performed for 1, 1·2H2O (Figure 1), and 3·2THF.
Complex 1 is crystallized in two forms, viz. anhydrous 1 (from dry EtOH) and dihydrate 1·2H2O, which was crystallized from H2O. In both structures, the copper(II) centers are surrounded by two saccharinate, two water, and one dimethylcyanamide ligands, thus forming the complexes exhibiting a distorted square-pyramidal geometry, where the NCNMe2 ligand occupies the apical position. The degree of the geometry distortion is greater for the dihydrate (geometry index [23] τ5 = 0.16 for 1 and 0.35 for 1·2H2O). Two saccharinate ligands are coordinated to the copper(II) center with different ∠N–Cu–N angles (160.24(14)° for 1 and 174.65(15)° for 1·2H2O), and for the {(H2O)2Cu} moieties ∠O–Cu–O are also different: 169.93(12)° for 1 and 153.39(19)° for 1·2H2O. The Cu–N distances are 2.020(2) (1) and 2.011(3) Å (1·2H2O), similar to those in some other known square-pyramidal (sac)2CuII complexes (e.g., 1.997(4)–2.046(4) Å for [Cu(sac)2(H2O)(nicotinamide)]n [24], [Cu(sac)2(H2O)(EtOH)(benzimidazole)] [25], and [Cu(sac)2(H2O)2(2-methylpyrazine)]) [26]). The Cu–O separations exhibited values (1.963(3) and 1.971(3) for 1 and 1.995(4) and 1.971(4) Å for 1·2H2O) comparable with the Cu–O distances in similar (H2O)(sac)2CuII complexes (1.959(4)–2.3145(14) Å) [24,25,26]. The Cu–Ncyanamide bond lengths (2.152(3) Å in 1 and 2.158(4) Å in 1·2H2O) were longer than the Cu–Ncyanamide bond lengths in the known copper(II) complexes featuring cyanamides, viz. the tetranuclear clusters Cu4Cl6O(NCNR2)4 (R = Me, [22] allyl [27]) and the solvates Cu4X6O(NCNMe2)4·4(arene) [22] (X = Cl, Br; arene = PhMe, PhCH=CH2; 1.903(10)–1.952(6) Å) as a consequence of different geometry and composition of the complexes. The C≡N and C–N bond lengths of the NCNMe2 ligands (1.153(5) and 1.312(5) Å for 1 and 1.156(7) and 1.306(7) Å for 1·2H2O, respectively) was quite similar. In contrast, the structures of 1 and 1·2H2O demonstrate ∠Cu–N–C (171.6(3) and 146.8(5)°, respectively), which differently deviate from the linearity because of a noticeable contribution of the heterocumulene mesomeric form N(–)=C=N(+)R2. The strong deviation from the linearity of the M–N–C fragment of the coordinated NCNR2 was observed by us for iridium(III) [28], copper(I) [20], and zinc(II) [29] complexes featuring disubstituted cyanamides. This strong deviation from linearity of the M–N–C fragment distinguishes NCNR2 ligands from NCR.
Various type hydrogen bonds were detected in the structures of 1 and 1·2H2O (Figure 2 and Figure 3). In 1, eight-membered hydrogen bond-based cycle {Ocarbonyl···H–O–H··· Ocarbonyl···H–O–H···} is formed by three molecules of 1. These cycles link the complexes into 1D-zigzag chains. The oxygen atoms of the SO2 groups from weak hydrogen bonds with H–Carene binding 1D-chains into a 3D-structure.
In the structure 1·2H2O, we identified several hydrogen bond-based patterns. One out of the two H2O ligands forms two hydrogen bonds with two H2O, which in turn form each hydrogen bond with the double bonded O atom of two sac– ligands, and these two saccarinate ligands are connected by another coordinated H2O. This 12-atom hydrogen bond-based cycle {Ocarbonyl···H–O–H···Ocarbonyl···H–O(hydr)···H–O–H···O–H(hydr)···} links three molecules of 1 and two molecules of H2O. In addition, metal-bound H2O of each molecule is linked with the O=S moiety of the sac– ligand.
Different systems of hydrogen bonding and types of molecular packing affect geometric parameters of complex 1 in the structures of 1 and 1·2H2O. Thus, 1 and 1·2H2O differ by ∠Cu–N–C (171.6(3) and 146.8(5)°, respectively) and the degree of the square pyramidal geometry distortion.
On one hand, complexes 1 and 1·2H2O represent the first examples of the structurally characterized mononuclear (NCNR2)CuII species. Structures of cyanamide copper(II) complexes were previously represented by the clusters Cu4Cl6O(NCNR2)4 (R = Me, allyl) [22,27,30], and the solvates Cu4X6O(NCNMe2)4·4(arene) (X = Cl, Br; arene = PhMe, PhCH=CH2) [22], and no single example of mononuclear (NCNR2)CuII was known. For copper(I), there are several known structures of mononuclear complexes [Cu(NCNR2)4](BF4) [20], [Cu(tpm)(NCNR2)](BF4) (tpm = tris-(1,3-dimethylpyrazolyl)methane) [21], and [Cu(NCNMe2)2(DPEphos)](BF4) (DPEphos = bis[2 -diphenylphosphino)phenyl]-ether) [31], dinulear [Cu2(μ2-Cl)2(dppm)2(NCNMe2)]·2NCNMe2, [Cu2(μ2-Cl)(dppm)2(NCNMe2)](Cl), [Cu2(μ2-X)(dppm)2(NCNMe2)2](X) (X = ClO4, NO3), [Cu2(dppm)2(NCNMe2)3](BF4)2 [32], and polymeric [CuX(NCNAllyl2)] (X = Cl, Br, NO3) species [33].
On the other hand, these complexes represent examples of the (N-sac)2CuII-type complexes. Metal complexes of saccharin(ate) exhibit versatile coordination chemistry due to the existence of different coordination sites [34]. For copper(II), several types of complexes are reported, demonstrating monodentate N-, or O-coordination, and N,O-bidentate coordination modes of the sac— ligand [34]. The (N-sac)2CuII moiety is rather typical for various saccharinate complexes. In relation to 1 and 1·2H2O, the structures of pentacoordinate complexes should be emphasized (e.g., [Cu(sac)2(H2O)2(2-methylpyrazine)] [26], [Cu(sac)2(H2O)(ethylnicotinate)2] [35], Cu(sac)2(H2O)(nicotinic acid)2] [36], [Cu(sac)2(H2O)(4-PrC5H4N)2] [37], [Cu(sac)2(H2O)(pyridazine)2] [38], [Cu(sac)2(H2O)(Py)2] [39], etc). Notably, no complexes featuring the (sac)(NCR)CuII (R = Alk, Ar, NR2) entity were reported prior to this study.
Complex 3·2THF features two THF per one [Cu2(THF)2(sac)4] (3) entity. Complex 3 exhibits a lantern-like structure, which is comprised by two copper(II) centers, four N,O-bridging saccharinate ligands, and two terminal THF ligands (Figure 4).
The Cu–N (2.0246(14) and 2.0335(15) Å) and Cu–O distances (1.9567(12) and 1.9633(12) Å) for the saccharinate ligands are comparable with those of the corresponding Cu–N (2.0243(14) and 2.0329(15) Å) and Cu–O (1.9562(12) and 1.9624(12) Å) bond values in the closely related lantern-like copper(II) species bearing N,O-bridging ligands, namely [Cu2(Me2NCHO)2(C7H5NS)4]·2(Me2NCHO) (C7H5NS-1,2-benzothiazol-3-ato) [40]. The latter complex exhibits the trans-N,O-configuration of the benzothiazolato ligands around the copper(II) center, while complex 3 exhibited the trans-N,N/trans-O,O geometry, thus preventing the repulsion of the SO2 groups and the THF ligand. In 3·2THF, the Cu–Cu distance (2.7519(5) Å) was slightly longer than those in Cu2(Me2NCHO)2(1,2-benzothiazol-3-ato)]·2(Me2NCHO) (2.6816(10) Å). The Cu–OTHF bond lengths (2.1089(17) and 2.1023(18) Å) were in the typical CuII–O bond range [41]. The bond length and angles for the saccharinate ligand were similar to those reported for the [Cr2L2(μ-sac)4] complexes (L = THF, Py) [42,43]. In addition, 3·2THF represents a rare reported example of saccarinate-based lantern-like complexes with a core {M2(Sac)4}. No significant H-bonding was detected in the structures of the complexes, while weak π–π interactions between aryl···aryl rings were identified.

2.3. Theoretical Calculations

In order to obtain additional indirect evidence supporting the blocking role of the copper(II) in the saccharine/cyanamide coupling, we studied the nature of Cu–Ncyanamide and Cu–Nsac coordination bonds in 1 and carried out an integrated computational study including the full geometry optimization of the model of structure 1 in the gas phase using the appropriate experimental XRD structure as a starting point, the topological analysis of the electron density distribution (QTAIM) [44], the natural bond orbital and charge decomposition analyses (NBO [45] and CDA [46]), and calculation of the vertical total energies for the Cu–Ncyanamide and Cu–Nsac coordination bond dissociations (see ESI for all details). This approach has already been successfully used by us upon studies of bonding properties in various similar transition metal complexes [20,47,48,49,50]. The results of our computational study revealed that (i) crystal-packing strongly affects the structural characteristics of 1 in the solid state; (ii) the dialkylcyanamide copper(II) complexes featuring noticeable contribution of the heterocumulene mesomeric form; (iii) Cu–Ncyanamide and Cu–Nsac coordination bonds in 1 exhibit a single bond character, clearly polarized toward the N atom and almost purely electrostatic; (iv) the {M}←L σ-donation substantially prevails over the {M}→L π-back-donation in both Cu–Ncyanamide and Cu–Nsac coordination bonds in 1; (v) the calculated vertical total energies (Ev) for the Cu–Ncyanamide and Cu–Nsac coordination bond dissociation in optimized equilibrium model structure 1 were 43.6 and 156.4 kcal/mol, respectively. Overall, one can conclude that the nature of Cu–N coordination bonds in 1 is similar, but the saccharinate is a stronger ligand toward the copper(II) center than the cyanamide.

3. Materials and Methods

All solvents and reactants were obtained from commercial sources and used as received. Sodium saccarinate was obtained according to the published method [51]. Atomic absorption spectrometry (AAS) was carried out on a Shimadzu AA-7000 spectrometer (Shimadzu, Japan) (spectral range 189–900 nm) using the flame emission spectroscopy method. Standard Cu samples for the calibration solutions were prepared by MERCK standard (Merck KGaA, Darmstadt, Germany) in 0.1 M HNO3; calibration solutions were 0.01–100.0 mg/L. Spectral analysis of the sample solutions was carried out with 100-fold dilution. Infrared spectra (4000–400 cm–1) were recorded using a Bruker FTIR TENSOR 27 (Bruker, Germany) instrument in Nujol. The thermogravimetry/differential thermal analysis was performed with a NETZSCH TG 209 F1 Libra thermoanalyzer (NETZSCH Group, Selb, Germany) and MnO2 powder was used as the standard. The initial weights of the samples were in the range 1.1–1.8 mg. The experiments were run in an open aluminum crucible in a stream of argon at a heating rate of 10 K/min. The final temperature was 530 °C. Processing of the thermal data was performed with Proteus analysis software [52].

3.1. Synthetic Work

Synthesis of [Cu(sac)2(NCNR2)(H2O)2] (1, 1·2H2O, 2). CuCl2·2H2O or CuBr2 (0.25 mmol) was dissolved in 5-fold excess of NCNR2 (R2 = Me2 1, Et2 2; 0.10 mL, 1.3 mmol) at room temperature (RT), whereupon a solution of sodium saccharinate (0.25 mmol) in certain solvents (2 mL, dried EtOH for 1, MeOH, EtOH, or H2O for 1·2H2O and MeOH or EtOH for 2) was added. The resulting mixture was left to stand for 3–5 days at RT without stirring and the bright greenish-blue prismatic crystals were precipitated. These crystals were filtered off, washed by methanol, and dried in air at RT. One- or two-fold reprecipitation from the mother liquid allowed the increased yield of 1·2H2O (75%). The total isolated yields were 50–75%. Alternatively, 1·2H2O was obtained from a CuX2/saccharin mixture with excess NCNMe2 and without solvents by keeping for 1 h at 60 °C and then at RT for 2–3 days, however, the reaction proceeds in lower yields (15–35%). A few crystals of pure 1–2 were mechanistically separated from the reaction mixture and yields were not calculated.
1. Cu-% (AAS) for C17H18N4CuO8S2 found (calcd.) = 11.4% (11.8%). IR in Nujol (selected bands, cm−1): 3331 m-s br ν(O–H), 2261 m ν(C≡N), 1646, 1618, and 1587 m ν(C=O) and δ(O–H), 1313 m-s νsym(S=O), 1170 m νasym(S=O).
1·2H2O. Cu-% (AAS) for C17H22N4CuO10S2 found (calcd.) = 10.7% (11.1%). IR in Nujol (selected bands, cm−1) 3334 m-s br ν(O–H), 2226 m ν(C≡N), 1646, 1618, and 1566 m ν(C=O) and δ(O–H), 1313 m-s νsym(S=O), 1170 m νasym(S=O).
2. Cu-% (AAS) for C21H22N4CuO8S2 found (calcd.) = 10.9% (11.3%). IR in Nujol (selected bands, cm−1) 3330 m-s br ν(O–H), 2245 m ν(C≡N), 1628 s br and 1584 m ν(C=O) and δ(O–H), 1309 m-s νsym(S=O), 1168 m-s νasym(S=O).
Synthesis of [Cu2(sac)4(THF)2]·2THF (3·2THF). CuCl2·2H2O or CuBr2 (0.25 mmol) were dissolved in THF (2 mL) at RT, whereupon a solution of sodium saccarinate (0.25 mmol) and NCNR2 (100 mkL, 1.3 mmol, R2 = C4H8, C5H10, C4H8O) in MeOH or EtOH (2 mL in each case) was added. The resulting mixture was left to stand for five days at RT without stirring until the bright greenish-blue prismatic crystals were released. The crystals were washed by THF (1–2 mL) and dried in air at RT. The isolated yields were 40–50%.
3·2THF. Cu-% (AAS) for C44H48N4Cu2S4O16 found (calcd.) = 11.3% (11.1%). IR in Nujol (selected bands, cm−1): 1640 m and 1568 m-s cm−1 ν(N–C=O in sac–), 1330 m-s νsym(S=O), 1177 m νasym(S=O).

3.2. X-ray Structure Determinations

X-ray diffraction studies were performed at 100 K on Rigaku XtaLAB Synergy-S diffractometer (Rigaku Oxford Diffraction) (HyPix-6000HE type detector) in the case of 1·2H2O, 3·2THF, and [Cu(sac)2(H2O)4]·2H2O and Rigaku XtaLAB SuperNova diffractometer (Agilent Technologies (Oxford Diffraction), Yarnton, Oxfordshire, UK) (HyPix-3000 type detector) in the case of 1 using Cu Kα (λ = 1.54184 Å) radiation. The structures were solved with the ShelXT [53] structure solution program using Intrinsic Phasing for 1, 1·2H2O, and 3·2THF, and the Superflip [54] structure solution program using Charge Flipping for [Cu(sac)2(H2O)4]·2H2O and refined with the ShelXL [55] refinement package incorporated in the OLEX2 program package [56] using least squares minimization. Empirical absorption correction was applied in the CrysAlisPro 1.171.40.67a (Rigaku Oxford Diffraction, 2019) (for 1 and 1∙2H2O) and 1.171.40.71a (Rigaku Oxford Diffraction, 2020) (for 3∙2THF and [Cu(sac)2(H2O)4]·2H2O)) [57] program complex using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The crystallographic data and structure refinement parameters are given in Table S1. All structures have been deposited at Cambridge Crystallographic Data Center (CCDC numbers 2062446 (1), 2062447 (1∙2H2O), 2062448 (3∙2THF), 2062451 ([Cu(sac)2(H2O)4]·2H2O)) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. (Accessed on 07 September 2021)

3.3. Computational Details

The full geometry optimization of model structure 1 was carried out at the DFT level of theory using the M06 functional [58] with the help of the Gaussian-09 program package [59]. No symmetry restrictions were applied during the geometry optimization procedure and appropriate experimental X-ray structure 1 was used as a starting point. The calculations were carried out using the multi electron fit fully relativistic energy-consistent pseudopotential MDF10 of the Stuttgart/Cologne group that described 10 core electrons and the appropriate contracted basis set for the copper atom [60]) and the 6-31G(d) basis sets for other atoms. The Hessian matrix was calculated analytically for the optimized model structure 1 in order to prove the location of correct minima on the potential energy surface (no imaginary frequencies). The topological analysis of the electron density distribution with the help of the “atoms in molecules” method developed by Bader (QTAIM) [44] and charge decomposition analysis developed by Dapprich and Frenking (CDA) [46] were carried out by using the Multiwfn program (version 3.7) [61]. The Cartesian atomic coordinates for optimized equilibrium model structure 1 are presented in Table S2, ESI, and as the attached xyz-file.

4. Conclusions

In this study, we demonstrate that in the system CuII/SacNa(H)/NCNR2, copper(II) forms complexes with saccharinate and disubstituted cyanamides. The directionality of the reaction is completely different from that observed for the metal-free reaction between saccharin and NCNR2; the latter results in the formation of guanidinated saccharins [8].
We succeeded in the isolation and characterization of the [Cu(sac)2(NCNR2)(H2O)2] (R = Me, Et) complexes, which represent the first example of structurally characterized mononuclear (cyanamide)CuII species. The experimental X-ray structure of 1 was used as a starting point for an integrated computational study of model structure 1 in the gas phase. As can be inferred from inspection of the obtained theoretical data, Cu–Ncyanamide and Cu–Nsac coordination bonds in 1 exhibited a single bond character, clearly polarized toward the N atom and predominately electrostatic, but the saccharinate was a better ligand toward the copper(II) center than the cyanamide (the calculated vertical total energies for the Cu–Ncyanamide and Cu–Nsac were 43.6 and 156.4 kcal/mol, respectively). All these data confirmed the role of the copper(II) in the change of the directionality of the reaction between saccharin and NCNR2. The copper(II) completely blocks the nucleophilic centers of ligands via coordination, thus preventing the saccharin–cyanamide coupling.

Supplementary Materials

The following are available online at https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/inorganics9090069/s1, crystal data and structure refinement, experimental spectra, and TG/dTG curves, other experimental data, and cartesian atomic coordinates. The CIF and the checkCIF output files.

Author Contributions

Conceptualization, N.A.B. and Y.N.T.; Methodology, N.A.B. and A.S.N.; Investigation, Y.N.T., A.S.N., and D.V.S.; Writing—original draft preparation, N.A.B. and Y.N.T.; Writing—review and editing, N.A.B.; Visualization, A.S.N. and N.A.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Foundation for Basic Research (synthetic studies: project 20-33-90240) and the Russian Science Foundation (crystallographic and theoretical studies: project 19-13-00013).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article and Supplementary materials. CIFs are available on www.ccdc.cam.ac.uk/data_request/cif (accessed on 7 September 2021).

Acknowledgments

The authors are grateful to the Center for X-ray Diffraction Studies, Center for Chemical Analysis and Materials Research, and Thermogravimetric and Calorimetric Research Center (all belonging to Saint Petersburg State University) for the physicochemical studies.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 09 00069 sch001 550
Scheme 1. Generation of 1, 1·2H2O (R = Me), and 2 (R = Et).
Scheme 1. Generation of 1, 1·2H2O (R = Me), and 2 (R = Et).
Inorganics 09 00069 sch001
Inorganics 09 00069 g001 550
Figure 1. (a) Molecular structure of 1 with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. (b) Molecular structure of 1·2H2O with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. The H2O of hydration was omitted for clarity.
Figure 1. (a) Molecular structure of 1 with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. (b) Molecular structure of 1·2H2O with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. The H2O of hydration was omitted for clarity.
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Inorganics 09 00069 g002 550
Figure 2. A fragment of crystal packing of 1 with the inter-ligand hydrogen bonds shown in dotted lines.
Figure 2. A fragment of crystal packing of 1 with the inter-ligand hydrogen bonds shown in dotted lines.
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Inorganics 09 00069 g003 550
Figure 3. A fragment of crystal packing of 1·2H2O with the interligand hydrogen bonds shown in dotted lines.
Figure 3. A fragment of crystal packing of 1·2H2O with the interligand hydrogen bonds shown in dotted lines.
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Inorganics 09 00069 g004 550
Figure 4. The molecular structure of 3·2THF with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. Solvated THF molecules were omitted for clarity.
Figure 4. The molecular structure of 3·2THF with the atomic numbering. Thermal ellipsoids are given at the 50% probability level. Solvated THF molecules were omitted for clarity.
Inorganics 09 00069 g004
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Toikka, Y.N.; Spiridonova, D.V.; Novikov, A.S.; Bokach, N.A. Copper(II) Prevents the Saccarine-Dialkylcyanamide Coupling by Forming Mononuclear (Saccharinate)(Dialkylcyanamide)copper(II) Complexes. Inorganics 2021, 9, 69. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9090069

AMA Style

Toikka YN, Spiridonova DV, Novikov AS, Bokach NA. Copper(II) Prevents the Saccarine-Dialkylcyanamide Coupling by Forming Mononuclear (Saccharinate)(Dialkylcyanamide)copper(II) Complexes. Inorganics. 2021; 9(9):69. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9090069

Chicago/Turabian Style

Toikka, Yulia N., Dar’ya V. Spiridonova, Alexander S. Novikov, and Nadezhda A. Bokach. 2021. "Copper(II) Prevents the Saccarine-Dialkylcyanamide Coupling by Forming Mononuclear (Saccharinate)(Dialkylcyanamide)copper(II) Complexes" Inorganics 9, no. 9: 69. https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9090069

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