Hexakis(μ-3-aminopropanethiolato-1κ⁶N,S:2κ³S;3κ⁶N,S:2κ³S)cadmium(II)dirhodium(III) Dibromide Tetrahydrate

Abstract

Cadmium(II) complexes with thiolate ligands have received considerable attention because of their intriguing structural features and relevance to metalloproteins. In this study, a new cadmium(II)–rhodium(III) trinuclear complex, [Cd{Rh(apt)₃}₂]Br₂·4H₂O (1, apt = 3-aminopropanethiolate), was synthesized by the reaction of fac-[Rh(apt)₃] with cadmium bromide. Compound 1 was characterized using elemental analysis, X-ray fluorescence and IR spectroscopies, and powder X-ray diffraction study. Single-crystal X-ray analysis revealed that the cadmium(II) center in 1 was surrounded by six thiolato S atoms from two fac-[Rh(apt)₃] units.

1. Introduction

During the past decades, cadmium(II) compounds with thiolate ligands have attracted attention due to their structural versatility and structural similarity with the active center of metallothioneins [1,2]. Organic thiolates are often used for constructing homometallic thiolato cadmium(II) complexes. On the other hand, heterometallic polynuclear complexes have been rationally synthesized from metal complexes containing aminothiolate ligands in combination with cadmium(II) ions [3,4]. For example, it has been reported that the reaction of fac-[Rh(aet)₃] (aet = 2-aminoethanethiolate) with Cd²⁺ forms a T-cage-type octanuclear complex, [Cd₄O{Rh(aet)₃}₄]⁶⁺, in which four fac-[Rh(aet)₃] units surround a tetrahedral {Cd^II₄O}⁶⁺ core in a bridging coordination mode [4]. However, the number of this class of polynuclear cadmium(II) complexes is still limited.

In the course of our development of new metalloligands with thiolate donors, we recently found that an octahedral rhodium(III) metalloligand, fac-[Rh(apt)₃] (apt = 3-aminopropanethiolate), which is analogous to fac-[Rh(aet)₃] but has larger N,S-chelate rings, tend to act as a chelating metalloligand toward Pd^II, Ni^II, Cu^II, and Zn^II [5,6,7]. In this work, we report that the reaction of fac-[Rh(apt)₃] with cadmium(II) bromide gives the title compound [Cd{Rh(apt)₃}₂]Br₂·4H₂O (1), in which two fac-[Rh(apt)₃] units coordinate to a Cd²⁺ center in a chelating mode. Although several cadmium(II) complexes with a ‘CdS₆’ octahedral motif have been prepared from dithiocarbamate [8,9,10,11,12,13,14], sulfide [15,16], thioether [17,18,19,20], thiocyanate [21,22,23,24,25], and thiourea [26,27] ligands, 1 is the first cadmium(II) complex surrounded by six thiolate S atoms. Complex 1 was structurally determined using single-crystal X-ray analysis and was characterized using powder X-ray analysis, fluorescence X-ray spectrum, elemental analysis, and IR spectrum.

2. Results and Discussion

Bulk sample of the title compound 1 was obtained as a yellow powder by the reaction of fac-[Rh(apt)₃] with cadmium bromide in a 1:2 ratio (Figure 1). The elemental analysis data matched well with the formula of 1. The X-ray fluorescence analysis revealed the presence of Rh and Cd as the metallic elements in a 2:1 ratio (Figure S2). The IR spectrum of 1 was very similar to that of the trinuclear Rh^III₂Zn^II complex with apt, [Zn{Rh(apt)₃}₂]Br₂ [5], showing ν_H2O and δ_H2O band due to H₂O and the ν_N–H and ν_C–H bands due to fac-[Rh(apt)₃] (Figure S1). Bulk purity of the yellow powder was confirmed using powder X-ray diffraction study, which was well matched with the simulated pattern of the single-crystal X-ray diffraction data (Figure S3).

X-ray quality single-crystals of 1 were obtained from the filtrate after standing at room temperature for 1 month. Single-crystal X-ray diffraction analysis of 1 was performed using a synchrotron X-ray radiation at 100 K (Table S1). The asymmetric unit of 1 contains one-sixth of complex cation and one-third of bromide ion, and two-thirds of water molecules. The symmetry expansion operation generated the S-bridged trinuclear Cd^IIRh^III₂ structure, which is illustrated in Figure 2. The overall molecular structure of 1 was reminiscent of that of [Zn{Rh(apt)₃}₂]Br₂, which has κ³-chelate coordination of fac-[Rh(apt)₃] units [5]. In the complex cation of 1, an octahedral cadmium ion was surrounded by six S atoms from two fac-[Rh(apt)₃] units. The Δ and Λ isomers of fac-[Rh(apt)₃] were disordered in a 1:1 ratio (Figure S4). Thus, it is difficult to assign whether 1 contains the meso-(ΔΛ) and/or racemic-(ΔΔ/ΛΛ) isomers. The Cd–S bond distance (2.6804(12) Å) in 1 was longer than the Zn–S bond distance in [Zn{Rh(apt)₃}₂]²⁺ (2.5250(12) Å) by ca. 0.15 Å due to the larger ionic radii of Cd²⁺ compared with Zn²⁺ (Table S2). The Cd–Rh separation was 3.2730(5) Å. The bromide ions formed N–H···Br hydrogen bonds (3.24 Å).

3. Materials and Methods

3.1. Physical Measurements

The IR spectrum was recorded with a JASCO FT/IR-4100 infrared spectrophotometer using KBr pellets at room temperature. X-ray fluorescence analyses were performed on a SHIMADZU EDX-900 spectrometer. Elemental analyses (C, H, N) were performed at Osaka University using a Yanaco CHN Corder MT-5. High-quality powder X-ray diffraction (PXRD) was performed at room temperature in transmission mode (synchrotron radiation, λ = 1.0 Å; 2θ range = 2–78°; step width = 0.01°; data collection time 1 min) on a diffractometer equipped with a MYTHEN microstrip X-ray detector (Dectris Ltd., Baden-Daettwil, Switzerland) at the SPring-8 BL02B2 beamline [28].

3.2. X-ray Crystal Structure Determination

Diffraction data for 1 was recorded on a Rayonix MX225HS CCD area detector with synchrotron radiation (λ = 0.6300 Å) at the 2D beamline at the Pohang Accelerator Laboratory (PAL). The intensity data were processed using the HKL3000 program and collected by using the ω-scan technique. The structures were solved by direct methods using SHELXS-2014 [29]. The apt ligands were positionally disordered over two positions, whose occupancy factors were fixed to 0.5. The Br and O atoms were positionally disordered over three positions and their occupancy was fixed to 0.167. The Br1 and O1 occupy the same position, so their coordinates and thermal ellipsoid parameters were refined using EXYZ and EADP commands. An ISOR restraint was applied for O2.

3.3. Synthesis of [Cd{Rh(apt)₃}₂]Br₂·4H₂O

fac-[Rh(apt)₃] was prepared by the reported method [5]. To a yellow suspension containing 30 mg of fac-[Rh(apt)₃] (80 μmol) in water (20 mL) was added 46 mg of cadmium bromide (169 μmol). The smoky yellow mixture was stirred at 50 °C overnight. After cooling to room temperature, the resulting yellow precipitate was collected by filtration. Yield: 29 mg (66%). IR spectrum, ν, cm⁻¹: 3456 (ν_H2O), 3435 (ν_H2O), 3231 (ν_N–H), 3209 (ν_N–H), 3189 (ν_N–H), 3129 (ν_N–H), 2950 (ν_C–H), 2919 (ν_C–H), 2893 (ν_C–H), 1620 (δ_H2O), 1597 (δ_NH2), 1459, 1419, 1408, 1357, 1335, 1300, 1263, 1241, 1218, 1186, 1120, 1079, 1025, 995, 907, 849, and 827. Anal. Calcd. for [Cd{Rh(apt)₃}₂]Br₂·4H₂O = C₁₈H₅₆Br₂Cd₁N₆O₄Rh₂S₆ (Mw = 1091.11): C, 19.81%; H, 5.17%; and N, 7.70%. Found: C, 20.07%; H, 4.98%; and N, 7.48%.

Yellow block crystals suitable for single-crystal X-ray diffraction analysis were obtained from the yellow filtrate after standing at room temperature for 1 month.

4. Conclusions

In conclusion, a new trinuclear Cd^IIRh^III₂ complex 1 was obtained by the reaction of fac-[Rh(apt)₃] with CdBr₂ in water. Single-crystal X-ray analysis revealed that the cadmium center in 1 was coordinated by six S atoms from two fac-[Rh(apt)₃] units. The overall molecular structure of the complex cation of 1 was quite similar to that of [Zn{Rh(apt)₃}₂]²⁺. The present study revealed that fac-[Rh(apt)₃] prefers to take a κ³-chelate coordination mode toward group 12 metal ions.