Introduction

Sarcosine (N-methylglycine) (Fig. 1) is an α-amino acid occurring in various living organisms as an intermediate in amino acid metabolism [1] and as a component of peptides, e.g., actinomycines [2]. Additionally, it can potentially serve as a liposome cryoprotectant and as a drug in the treatment for schizophrenia [3, 4].

Fig. 1
figure 1

Scheme of the hydrolysis of 1-methylhydantoin, which results in a sarcosine as the final products

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In recent years, various sarcosine adducts, salts, and metal complexes have been prepared. The latter have been studied in order to understand the mechanism of interactions between amino acids and metal ions found in biological systems, i.e., Ca, Zn, Cu [5–7]. They can serve as potential laser materials [8] as well as anticancer drugs [9]. In all cases, they have been prepared using sarcosine as a starting reagent.

Sarcosine can also be a product of acid or basic hydrolysis of 1-methylhydantoin (Fig. 1). Hydantoin and its derivatives at moderate temperature and pH form complexes with metal ions [10–13]. However, when elevated temperature and extreme pH are applied, hydantoins hydrolyze, forming in the most cases, amino acids [14]. Such in situ formed amino acids could subsequently coordinate to metal ions [15].

In the present work, the complex [Ni(sar)2(H2O)2] (1) has been obtained as a result of the reaction between 1-methylhydantoin and nickel(II) salt. At elevated temperature and pH, 1-methylhydantoin hydrolyzed resulting in a sarcosine, ammonia, and carbon dioxide as the final products (Fig. 1) [14]. Then, in situ produced ligand was coordinated by metal ion forming (1). Incomplete structure of (1) was known earlier [16]. Here, it has been characterized in detail using both experimental (X-ray diffraction, IR, Raman, and electronic spectroscopy) and theoretical (DFT) methods. To the best of our knowledge, this is the first example of a sarcosine complex synthesized from an organic substrate other than sarcosine itself. It was found that crystals of (1) obtained by this “indirect” method are of better quality than those created from sarcosine as a starting reagent. For example, they do not show tendency to twinning.

Experimental

Preparation

1-Methylhydantoin was purchased from Sigma-Aldrich, while nickel(II) chloride (anhyd.) and ammonia solution (25 %) were from POCH S.A. and used without further purification. Nickel(II) chloride (0.5 mmol) and 1-methylhydantoin (1 mmol) were dissolved in 10 cm3 of ammonia solution and then heated in the solvothermal conditions at 383 K for 15 h. After cooling down, the resultant solution was left to slow evaporation in the air. After 1 month, blue single crystals of (1) were obtained.

Elemental analysis

Elemental analysis was carried out using elemental analyzer Vario El III (Elementar). Anal. Calc. for C6H16N2NiO6 (%): C, 26.60; N, 10.34; H, 5.95. Found: C, 26.60; N, 10.31; H, 5.98.

X-ray diffraction

X-ray diffraction data were collected on a KUMA Diffraction KM-4 four-circle single crystal diffractometer equipped with a CCD detector using graphite-monochromatized MoK α radiation (α = 0.71073 Å). Experiment was carried out at 295 K. The raw data were treated with the CrysAlis Data Reduction Program (version 1.172.33.42) taking into account an absorption correction. The intensities of the reflection were corrected for Lorentz and polarization effects. The crystal structure was solved by direct methods [17] and refined by full-matrix least-squares method using SHELXL-2013 and ShelXle programs [17, 18] (Table 1). Non-hydrogen atoms were refined using anisotropic displacement parameters. H-atoms of the sarcosine anion were visible on the Fourier difference maps, but placed by geometry and allowed to refine “riding on” the parent atom. The positions of H-atoms of the water molecule and NH group were refined without constraints, but Uiso(H) = 1.5Ueq(O) and Uiso(H) = 1.2Ueq(N).

Table 1 Crystal data and structure refinement for the [Ni(sar)2(H2O)2]
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NIR–UV–Vis electronic, IR, and Raman spectroscopy

Electronic spectra of solid state (reflectance) and water solution (absorbance) (c = 6.94 10−3 M) were recorded on Cary 500 Scan (Varian) UV–Vis–NIR spectrophotometer in the range of 7500–35,000 cm−1, with resolution of 10 cm−1. To obtain accurate band positions, the spectra were analyzed using variable digital filter method [19, 20] with the following parameters: a real number determining the degree of resolution enhancement, α = 200; the integer number determining the filter width, N = 10; the increment between points (step) = 100 cm−1. Crystal field parameters were calculated based on the known equations for 3d8 configuration with Oh and D4h symmetry of the metal surrounding [21, 22].

The FT-IR spectrum was measured on Bruker IFS 113 V spectrometer in the region of 4000–400 cm−1 with resolution of 2 cm−1 using KBr pellets. The far-infrared spectrum, in the range of 600–50 cm−1, was recorded on FT-IR Bruker IFS 66/S spectrometer with resolution of 2 cm−1 using Nujol mull technique. The FT-Raman spectrum, in the range of 4000–50 cm−1, was measured on Bruker MultiRAM spectrometer equipped with Nd:YAG laser, emitting radiation at a wavelength of 1064 nm and liquid nitrogen-cooled germanium detector. The spectrum was recorded with resolution of 2 cm−1.

Theoretical study

The molecular structure of (1) has been fully optimized with unrestricted density functional one-parameter hybrid protocol PBE0 [23–25]. The combined basis sets were used in calculation: the polarized valence double-ζ basis set D95v(d,p) for nonmetal atoms and for nickel the LanL2DZ effective core potential with conjunct valence basis set [26]. The examined value of total spin, Ŝ2, was equal to 2.000, which corresponds to a triplet ground-state wave function with no spin contamination [27]. All calculations have been performed using the Gaussian 09 set of programs [28]. Natural charges were obtained by the natural bond orbital (NBO) analysis using version 5.0 of the program [29, 30].

For clear-cut vibrational assignment of the experimental spectra of (1), a normal coordinate analysis was applied. The potential energy distribution (PED) was calculated with the procedure as described earlier [31, 32]. A non-redundant set of 87 internal coordinates has been constructed, as suggested by Fogarasi et al. [33]. PED calculations were performed using the Balga program [34]. The theoretical frequencies above 1510 cm−1 have been scaled.

The theoretical Raman intensities IR were calculated according to the following formula [35–37]:

$$I_{i}^{R} = C\left( {\upsilon_{0} - \upsilon_{i} } \right)^{4} \upsilon_{i}^{ - 1} B_{i}^{ - 1} S_{i}$$

where C is a constant equal to 10−12, B i is the temperature factor represented by the Boltzmann distribution (in this work assumed to be 1 as discussed earlier [37]), υ 0 is the wave number of the laser radiation (in this work, υ 0 = 9398.5 cm−1, which corresponds to a wavelength of the 1064-nm line of a Nd:YAG laser), υ i is the wave number of the normal mode (cm−1), and Si is the computed Raman scattering activity of normal mode Q i . The calculated Raman intensities I R presented in Table 6 are given in arbitrary units.

Results and discussion

Examination of Cambridge Structural Database (CSD) (version 5.34) revealed that the sarcosine moiety may occur in various forms in complexes: an anion, a zwitterion, or a cation and its various forms coordinate in different ways. When anionic form is present, sarcosine forms a bidentate ligand where N and one of O atoms are involved in coordination (Fig. 2a) [9, 38–40]. In the case of zwitterion and cationic forms, it acts as mono- [7], bidentate [8] or bridging ligand [5, 41–43], but only carboxylate group binds to metal (Fig. 2b, c). Among metal complexes with sarcosine, cationic form of ligand is the least common. Anionic and zwitterion forms are much more abundant, especially as bidentate or bridging ligands.

Fig. 2
figure 2

Forms of sarcosine ligand and types of the coordination in sarcosine metal complexes: a anionic bidentate ligand coordinating by the N and one of the O atoms, b zwitterion mono- or bidentate, sometimes bridging, ligand, c cationic monodentate ligand

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Crystal structure

The structural parameters of (1) were published a long time ago, but the positions of the hydrogen atoms were not reported [16]. Therefore, the crystal structure of this compound is presented de novo in this paper. The complex crystallizes in centrosymmetric space group P-1 of the triclinic symmetry (Table 1). Two sarcosine anions bind through amino and carboxylate groups, and with two water molecules create a distorted octahedral arrangement around the nickel ion (Fig. 3). Half of the (1) molecule lies in the asymmetric part of the unit cell because the nickel ion lies on the inversion center and therefore (1) possesses C i point group symmetry. Table 2 shows that Ni–N1 and Ni–O1 bond lengths are significantly shorter than Ni–O1W distances resulting in tetragonal elongation of the nickel(II) octahedron along two Ni–O1W bonds.

Fig. 3
figure 3

a Crystal structure of [Ni(sar)2(H2O)2]. b A view of [Ni(sar)2(H2O)2] molecules arranged in layers parallel to ab plane

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Table 2 Selected geometric parameters (Å, deg) for [Ni(sar)2(H2O)2]
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In the crystal structure of (1), N–H···O and O–H···O hydrogen bonds join adjacent molecules arranged in a two-dimensional network of hydrogen bonds (Fig. 3b, Table 3). The network results in an intersection of the chain patterns which propagate along the main crystallographic directions, a and b (Fig. 4a, b). Each and every chain contains only one type of hydrogen bond, and therefore, these chain patterns described by the unitary graph-set descriptors are the most important one in the crystal structure of (1) [44]. Additionally, ring patterns are also present, two R 22 (8) and one R 22 (12). Although two rings R 22 (8) are described by the same descriptor, they arise from different summation of the elementary graph-set descriptors because different atomic pathways are related to each pattern [45]. The first one results from summation E 02 (5)HNNiOH + E 20 (3)OCO = R 22 (8) and the second one from 2·E 11 (4)HONiO = R 22 (8) (Fig. 4a). Since two chains C(6) intersect each other at the nickel ion (Fig. 4b), the ring pattern R 22 (12) is created. It is connected to the relation 2·E 11 (6)HONiOCO = R 22 (12). It is worth noting that R 22 (12) ring and the second R 22 (8) ring result from doubling E 11 (6)HONiOCO and E 11 (4)HONiO pathways, respectively, due to inversion center located in both ring patterns.

Table 3 Selected hydrogen bond parameters (Å, deg) for [Ni(sar)2(H2O)2]
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Fig. 4
figure 4

a Chain and ring patterns of hydrogen bonds along (a) a axis and b b direction

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Electronic spectra

The structural analysis of (1) shows a large angular distortion from the ideal octahedron. It was also confirmed with electronic spectra, measured in the solid state (diffuse reflectance) and water solution (absorbance).

Generally, hexacoordinate octahedral Ni(II) complexes show three broadbands in the NIR, visible and UV regions at 7000–13,000, 11,000–20,000, and 19,000–27,000 cm−1 assigned to the spin-allowed transitions: 3A2g → 3T2g (υ 1), 3A2g → 3T1g (F) (υ 2), and 3A2g → 3T1g (P) (υ 3), respectively [46]. These transitions are observed in the reflectance spectrum of (1) at 10,450 cm−1 (υ 1), 16,550 cm−1 (υ 2), and 25,850 cm−1 (υ 3). However, splitting of the highest energy d–d band (Fig. 5) indicates distortion of octahedral geometry. The digital filtration analysis of the spectrum revealed that the positions of the bands are typical for complexes of tetragonal (D4h) symmetry (Table 4; Fig. 6b). Therefore, further analysis of the solid-state spectrum was carried out assuming tetragonal geometry (vide infra).

Fig. 5
figure 5

Electronic spectra of [Ni(sar)2(H2O)2]: reflectance (solid line) and absorbance in water solution (dashed line) as received and upon digital filtration (inset)

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Table 4 Assigned transitions (in cm−1) on diffuse reflectance and absorbance in water solution (C = 6.94 10−3 M) spectra of [Ni(sar)2(H2O)2], ε coefficient (M−1 cm−1), crystal field (Dq, Dt, and Ds) and Racah B parameters
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Fig. 6
figure 6

a Correlation diagram (d8 configuration) for Oh and D4h symmetries and b the effect of filtration of the solid-state spectrum of [Ni(sar)2(H2O)2]

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In contrast to the reflectance spectrum, band splitting was observed neither in the water solution spectrum nor in its filtered form (Fig. 5). The digital filtration revealed only bands of spin-forbidden transitions (3A2g → 1Eg = 13,400 cm−1, 3A2g → 1T2g = 21,400 cm−1). This indicates higher, i.e., pseudooctahedral, symmetry of (1) in solution [11].

On the basis of the band positions, crystal field and Racah B parameters for solid state and solution were calculated (Table 4). Obtained parameter values: Dq = 1016 cm−1 and B = 795 cm−1 of complex (1) in solution were close to the values found for pseudooctahedral Ni(II) complexes with [NiN2O4] chromophore [10, 11, 47, 48]. Crystal field Dq parameter in D4h symmetry has a slightly different meaning than that in Oh symmetry, because CF splitting in the former is described by three parameters: Dq, Ds, and Dt [49]. Thus, nephelauxetic parameter β, which means the ratio of the B value of complex to the B value of free ion (1041 cm−1 [46]), is the only one to be compared. A much higher value for the solid state (β = 0.94) was found than for solution (β = 0.76). This means that ionicity of M–L bond in solid state is significantly higher than in solution, which was observed earlier for other Ni(II) complexes [11].

On the basis of 10Dq (Δ) value obtained from solution spectra, sarcosine was located in the spectrochemical series of ligands for octahedral [NiL6] or pseudooctahedral [Ni(L–L)3] complexes. This series is constructed based on the crystal field parameter Δ for Oh symmetry and provides information about the strength of a ligand in the crystal field, i.e., its ability to split metal d levels in the electrostatic environment [46]. “Average environmental rule” [49] was applied in order to obtain the Δ value for hypothetical monoligand [Ni(sar)6]4− complex assuming exclusively monodentate N–Ni coordination. Δ was calculated from the equation:

$$\Delta \left[ {{\text{Ni}}\left( {{\text{H}}_{2} {\text{O}}} \right)_{2} {\text{L}}_{4} } \right] \, = \, 1/6\{ 2\Delta \left[ {{\text{Ni}}\left( {{\text{H}}_{2} {\text{O}}} \right)_{6} } \right]^{2 + } + \, 4\Delta \left[ {{\text{NiL}}_{6} } \right]^{4 - } \}$$

Assuming Δ[Ni(H2O)6]2+ = 8500 cm−1 [46] and Δ[Ni(H2O)2L2] = 10,160 cm−1 (this work, Table 4), a hypothetical Δ[Ni(sar)6]4− was found to be 10,990 cm−1. This value locates sarcosine ligand in the spectrochemical series as follows: H2O (8500) < py (10,150) < NH3 (10,750) < 1-mhyd (10,750) < sar (10,990) < en (11,700) < bpy (12,650).

Analysis of known crystal structures of sarcosine metal complexes (see Sect. 3, Fig. 2) indicates that bidentate coordination [Ni(sar)3]− is more likely to occur. Moreover, sarcosine ligand in (1) is coordinated to Ni(II) ion with not only nitrogen atom (as it was assumed) but also with oxygen atom. Thus, the position of sarcosine in this row should be treated tentatively.

Geometry and charge distribution

The studied complex is an open-shell system, d8 electron configuration of Ni(II) cation with an pseudooctahedral environment of ligand donor atoms, which required the use of the unrestricted methods for the calculations of an electronic structure. The molecular parameters predicted for (1) are in good agreement with the X-ray diffraction results (Table 2). The calculated metal–ligand distances are equal to 2.002 (Ni1–O1), 2.224 Å (Ni1–O1 W), and 2.081 Å (Ni1–N1).

According to the NBO results, the electronic configuration of Ni is [core]4s(0.25)3d(8.32)4p(0.02): 18 core electrons, 8.59 valence electrons (on 4s and 3d atomic orbitals), and 0.02 Rydberg electrons, mainly on the 4p orbital. This gives the total number of 26.59 electrons, which is consistent with the calculated natural charge (+1.41) on the Ni atom in (1). The calculated natural charges on the atoms are collected in Table 5.

Table 5 Natural charge (e) on selected atoms of [Ni(sar)2(H2O)2] calculated at the DFT/PBE0 level
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Vibrational spectra

The selected experimental and theoretical frequencies, IR intensities, and Raman intensities are shown in Table 6. All the experimental and calculated frequencies are listed in Table S1 of the Supporting Information. Figure 7 shows the experimental Raman and IR vibrational spectra of (1) in the range 4000–500 cm−1.

Table 6 Selected experimental frequencies in the IR and Raman spectra and the theoretical wavenumbers, ν (cm−1), IR intensities, IIR (km mol−1), IR Raman intensities (arbitrary units) of [Ni(sar)2(H2O)2] calculated at the PBE0 density functional
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Fig. 7
figure 7

Experimental FT-IR and FT-Raman spectra in the range of 4000–500 cm−1

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In the experimental Raman spectrum, the strong band is observed at 3247 cm−1. According to the PED calculation, this band is due to stretching vibration of NH group of sarcosine ligands. The subsequent Raman bands of strong intensities were assigned to the CH stretching of methyl and methylene groups of organic ligands. The characteristic band of symmetric CH stretching vibrations of N–CH3 group is observed at 2814 cm−1 in experimental Raman and IR spectra.

The characteristic band due to C=O stretching is observed at 1607 cm−1 in the experimental IR spectrum. This assignment is supported by the predicted large IR intensity of the mode 18.

According to the PED calculation, the strong band at 1397 cm−1 is associated with C–O stretching vibration. The separation of the bands due to CO stretching of carboxylic groups denoted as v as(COO−) and v s(COO−) over 200 cm−1 is consistent with monodentate manner of the coordination of sarcosine carboxylic group to the nickel ion.

The next strong IR band at 1320 cm−1 was assigned to the mode 33 with predominant contribution (66 %) from bending deformation of the CH2 groups.

The experimental Raman and IR spectra in the range of 500–100 cm−1 are presented in Fig. 8. As revealed by the PED calculation, the nickel–ligand vibrations are mixed with each other and also with different bending deformation of coordination rings of two sarcosine ligands. According to the calculated theoretical wave numbers, IR and Raman intensities, the bands due to vibrations with predominant contribution from v(Ni–O) and v(Ni–N) stretchings are observed at 445 cm−1 (IR) and 442 cm−1 (Raman).

Fig. 8
figure 8

Experimental FT-IR and FT-Raman spectra in the range of 500–100 cm−1

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Summary and conclusions

Sarcosine complex [Ni(sar)2(H2O)2] was obtained by a novel route—as an unexpected solid product of the system: [Ni(II)–1-methylhydantoin]. Sarcosine was generated through basic hydrolysis of 1-methylhydantoin and was coordinated in situ to the metal ion. To the best of our knowledge, this is the first example of the sarcosine complex synthesis, which starts from a substrate other than sarcosine itself. Moreover, the crystals of the complex are of better quality than those obtained with the traditional method. Although the compound was known earlier, the hydrogen bond analysis and spectroscopic (IR, Raman, and NIR–UV–Vis) studies combined with DFT calculations presented here allowed for its better characterization. Single crystal X-ray diffraction revealed that adjacent molecules are bound by H-bonds. It leads to the formation of a 2D network, created by two chain patterns along the main crystallographic directions a and b. Solid-state reflectance electronic spectrum strengthened by digital filtration proved that geometry of the nickel(II) surrounding can be described as tetragonal. However, in water solution, the complex exhibits higher, pseudooctahedral symmetry. Crystal field and Racah B parameters were calculated for both tetragonal and pseudooctahedral geometries. Comparison of nephelauxetic parameters calculated for (1) in solid state and solution shows that M–L bonds have a more ionic nature in the solid state than in the water solution. By application of the “average environment rule,” sarcosine was tentatively ranked in the spectrochemical series of ligands. The obtained 10Dq value of 10,990 cm−1 located sarcosine between ammonia and 1-methylhydantoin. It means that this amino acid has moderately strong splitting ability.

The PED analysis revealed that carboxylic groups of sarcosine ligands show a monodentate mode of coordination to the nickel atom. The presence of the Ni1–N1 and Ni1–O1 bands in the vibrational spectra of the studied complex is consistent with the presence of the chelating rings formed during coordination of nickel by sarcosine ligands.