l-Valinate hydrates of nickel, copper and zinc – a structural study

1 Introduction

As building blocks of proteins amino acids contribute to central processes of life. Enzymes as nature’s catalysts enable an enormous variety of reactions at physiological conditions. In addition to a peptide backbone, several enzymes feature at least one transition metal atom coordinated by the same [1], for example Ni in urease [2], Cu in amine oxidase [3] and superoxide dismutase [4] as well as Zn in carbonic anhydrase [5] and dehydrogenase [6]. The discovery of the natural appearance of peptide-supported transition metals has inspired the study of related compounds, e.g. the binary compounds consisting of a metal ion and anions of simple amino acids obtained from the natural pool or derivatives thereof. Hence, numerous examples have already been reported in the literature [7, 8].

For educational purposes we have chosen the l-valinato complexes of nickel (1), copper (2) and zinc (3) as an entry into inorganic biochemistry making use of synthesis procedures published with the X-ray crystallographic characterization of these complexes [9–11]. In this context we noticed unresolved questions regarding the published structures, and we now report on a reinvestigation of the crystallography of hydrates of 1, 2 and 3 (Scheme 1) which resulted in significantly enhanced precision of the data for 1 · 2 H₂O and 2 · H₂O, refinement of the latter in space group P1 as a perfectly ordered structure (whereas the initial report was a disordered structure in space group C2) [10] and the structure of a dihydrate 3 · 2 H₂O, which complements the literature known monohydrate 3 · H₂O [11].

Scheme 1:

Ni, Cu and Zn complexes of this study.

2 Results and discussion

Compounds 1 · 2 H₂O, 2 · H₂O and 3 · 2 H₂O were obtained following a procedure that involves a salt metathesis reaction in aqueous solution [12, 13]. These compounds are readily isolated as crystalline material suitable for X-ray diffractometry. Liang et al. [9] published the structure of diaqua-bis(l-valinato)nickel(II) 1 · 2 H₂O and mentioned that this compound appears as green block-like crystals, whereas we now obtained this compound as pale blue shiny leaflets. Driven by this color contradiction we characterized our product 1 · 2 H₂O by single-crystal X-ray diffraction. The data set collected matched the previously reported structure in space group C2 (Tables 1 and 2). The resulting molecular structure is given in Fig. 1. The crystals were twinned according to the twin law (1̅ 0 0 0 1̅ 0 1 0 1) with almost perfect overlap of the diffraction data, thus enabling straightforward refinement with an HKLF4 format data set. Consideration of the twin law afforded a structure of enhanced bond precision (with respect to the structure reported by Liang et al. [9]) and a twin population ratio of 0.509(1):0.491(1). Furthermore, the water molecules were refined in two positions with site occupancies restrained to 0.5. The coordination geometry around Ni may be described as a distorted octahedral arrangement of the donor atoms with the nitrogen atoms in trans positions. Selected geometry data of 1 · 2 H₂O are given in Table 1 (for comparison together with the corresponding data for 2 · H₂O and 3 · 2 H₂O).

Fig. 1:

Left: molecular structure of 1 · 2 H₂O in the crystal. Displacement ellipsoids are shown at the 50 % probability level; C and N bound hydrogen atoms are omitted for clarity, and only one of the two alternative water positions is shown. Right: photograph of the crystalline material of 1 · 2 H₂O.

Whereas for compound 3 a crystal structure of a monohydrate 3 · H₂O with pentacoordinate Zn atom had been published [11], we found a dihydrate 3 · 2 H₂O (Fig. 2) which appeared to be isomorphous with 1 · 2 H₂O (Table 2). Most geometric features within these structures are comparable (Table 1). Interestingly, significant differences can be found with the oxygen metal separations. Whereas in 1 · 2 H₂O the Ni–O bonds are equal within the limits of standard deviation, in 3 · 2 H₂O the Zn–O separation to the water molecules is significantly shorter than the Zn–O separation to the carboxylate oxygen donor atom. According to the different ionic radii of Ni²⁺ and Zn²⁺ (r_Ni (coord. numb. = 6) = 0.830 Å, r_Zn (coord. numb. = 6) = 0.880 Å [14]), one would expect an increase of the interatomic separations for the Zn coordination sphere by 0.05 Å, but for the coordinated water molecules, the Zn–O bonds remain unexpectedly short, being only 0.02 Å longer than the Ni–O bonds. Rombach et al. reported similar Zn–O bond length characteristics for the isoleucinato zinc complex [Zn(Ile)₂(H₂O)₂], i.e. Zn–O = 2.139(5) and 2.070(5) Å for the bonds to the amino acid and water, respectively [15].

Fig. 2:

Molecular structure of 3 · 2 H₂O in the crystal. Displacement ellipsoids are shown at the 50 % probability level; C and N bound hydrogen atoms are omitted for clarity, and only one of the two alternative water positions is shown.

The structure of the pentacoordinate copper complex 2 (as the monohydrate 2 · H₂O) has previously been reported in space group C2 only [10, 16–18], and these structures were reported with disorder. Now we collected an X-ray diffraction data set of this crystalline compound at 150 K in a unit cell setting which corresponds to the same reduced unit cell as for the previously reported structures but solved the structure in space group P1 with two crystallographically independent molecules in the asymmetric unit (refinement in space group C2 also produced a disordered structure model with the site occupancies of the disordered isopropyl groups refining to 0.5, thus hinting at an ordered superstructure). The structure was refined to the satisfactory R factor of 0.037 (despite using an HKLF5 data set obtained from a twinned crystal) and appeared to be perfectly ordered (Fig. 3). Selected structural features of 2 · H₂O are summarized in Table 1.

Fig. 3:

Molecular structures of the two crystallographically independent molecules of 2 · H₂O in the asymmetric unit in the crystal. Displacement ellipsoids are shown at the 50 % probability level; C and N bound hydrogen atoms are omitted for clarity.

To testify the color characteristics of the reported compounds 1 · 2 H₂O (pale blue, see Fig. 1, in accordance with [19]), 2 · H₂O (deep blue) and 3 · 2 H₂O (colorless) in an unequivocal manner, UV/Vis spectra were recorded using solid-state samples, i.e. 10 % (w/w) powder dispersions in BaSO₄ (Fig. 4). Absorption maxima for 1 · 2 H₂O are found at 389 nm and 613 nm, respectively. We assign these absorption bands to the d–d transitions ³T_1g(P) ← ³A_2g and ³T_1g(F) ← ³A_2g, respectively, as the slight increase of absorbance around 800 nm indicates that the third band (³T_2g ← ³A_2g), which is expected for d–d transitions in an octahedral d⁸ system, is located in the infrared. An absorption minimum for 1 · 2 H₂O is seen at 477 nm causing its pale blue appearance. For compound 2 · H₂O also two absorption maxima are observed, one of which is shifted to the UV region at 250 nm while the other one is found at 645 nm. Whereas we assign the latter to a set of energetically similar d–d transitions of the d⁹ system, we assign the former to a charge transfer from ligand to metal. Here, an absorption minimum is found at 393 nm. For compound 3 · 2 H₂O and l-valine literally no distinct absorbance is observed over the whole visible region as expected for these colorless compounds. As the d¹⁰ system of 3 · 2 H₂O does not allow for d–d transitions, we attribute the absorption around 200 nm to π–π* transitions of the amino acid (or its anion) for both l-valine and 3 · 2 H₂O.

Fig. 4:

UV/Vis spectra (uncorrected) of 1 · 2 H₂O, 2 · H₂O, 3 · 2 H₂O and l-valine in the solid state measured as 10 % (w/w) powder dispersions with BaSO₄ using an integrating sphere.

3 Conclusion

l-Valinates of Ni, Cu and Zn are easily accessible as crystalline compounds and are interesting representatives of amino acid metal complexes. Their simple syntheses enable these candidates to be used for education either in coordination or bioinorganic chemistry. Our crystallographic study of these complexes has delivered significantly improved structural models for the nickel and copper l-valinates 1 · 2 H₂O and 2 · H₂O, respectively, and has revealed the accessibility of a zinc l-valinate dihydrate 3 · 2 H₂O.

4 Experimental section

Chemicals were used as received without further purification. Complexes were synthesized by a method described by Nakamoto et al. [12] and Bernard et al. [13]. Single crystals suitable for X-ray diffraction were grown by slow evaporation of aqueous solutions of 1, 2 and 3 at room temperature. Elemental analyses were performed using an Elementar vario MICRO cube. UV/Vis spectra were recorded on a Jasco V-650 spectrophotometer.

l-Valine (5.86 g, 0.050 mol) was dissolved in 0.5 m NaOH (100 mL). Upon the addition of a solution of NiCl₂ hexahydrate (5.96 g, 0.025 mol) in water (250 mL) a light blue solution resulted. Evaporative concentration of the reaction mixture led to the formation of a pale blue crystalline material of 1 · 2 H₂O within 1 week. It was separated from the mother liquor by vacuum filtration, washed with ice water (3 × 2 mL) and dried in air at room temperature. Yield 3.47 g (10.6 mmol, 42 %). – C₁₀H₂₄N₂NiO₆ (327.02): calcd. C 36.73, H 7.40, N 8.57; found C 36.43, H 7.35, N 8.49.

The corresponding complexes of copper and zinc were synthesized similarly utilizing CuCl₂ dihydrate and ZnSO₄ heptahydrate, respectively. Compound 2 · H₂O was obtained as royal blue crystals. Yield 5.90 g (18.8 mmol, 75 %). – C₁₀H₂₂N₂CuO₅ (313.84): calcd. C 38.27, H 7.07, N 8.93; found C 37.99, H 6.97, N 8.86. Compound 3 · 2 H₂O was obtained as colorless crystals. Yield 4.29 g (12.9 mmol, 52 %). – C₁₀H₂₄N₂O₆Zn (333.68): calcd. C 35.99, H 7.25, N 8.39; found C 36.03, H 7.23, N 8.52.