Atmospheric Carbon Dioxide Capture as Carbonate into a Luminescent Trinuclear Cd(II) Complex with Tris(2-aminoethyl)amine Tripodal Ligand

Abstract

Spontaneous atmospheric CO₂ capture as carbonate anion occurred in the synthesis of a trinuclear Cd(II) complex with tris(2-aminoethyl)amine ligand. In reaction two types of compounds were obtained and structurally characterized by X-ray diffraction on a single crystal: initially [{Cd(tren)}₃(tren)](ClO₄)₆·2H₂O (1) and subsequently [{Cd(tren)}₃(tren)][{Cd(tren)}₃(µ₃-ηCO₃)](ClO₄)₁₀ (2). The carbonate anion replaces partially the bridging tren molecule and coordinates in a µ₃ fashion. The luminescent properties of the compounds were investigated.

1. Introduction

Chemical fixation of CO₂ by metal complexes, especially as carbonate anion, attracted particular attention in terms of fundamental research but also of potential applicability for reducing concentration of this major greenhouse gas. Carbonate anion is a versatile bridging ligand, able to link multiple metal ions in various coordination modes [1,2,3]. Among these numerous coordination modes of the carbonate anion, several types of µ₃ bridges were reported: syn-syn µ₃ [4], syn-anti µ₃ [5,6,7,8], µ₃−k²:k²:k² [7,9,10,11,12], µ₃−k²:k²:η¹ [13,14], and µ₃−k²: η¹:η¹ [7,15,16] (Scheme 1).

Most of the carbonate metal complexes are obtained by the addition of carbonate or bicarbonate salts to the reaction mixtures. Atmospheric CO₂ capture is less frequent and it is usually helped by the presence of basic reagents. In this approach, we intended synthesis of trinuclear Cd(II) complexes with tris(2-aminoethyl)amine ligand (tren), in which the tren ligand must play a double role: to act as chelating tripodal ligand capping the metal ions and, also, as bridging ligand connecting three capped Cd(II) ions. This dual behavior of the tren ligand was already reported in the presence of Cu(II) ions, generating trinuclear complexes [17,18,19], and Cd(II) ions, leading to heptanuclear complexes [20].

In the trinuclear complexes, the bridging tren ligand is replaceable by carbonate anion because it pre-organizes the metal ions in a trigonal manner favorable to a µ₃-k²:k²:k² coordination mode, and the basic free ligand enhances the atmospheric CO₂ capture. As potential mechanisms for carbonate formation, we can mention here hydration of the CO₂ followed by the deprotonation of the carbonic acid in basic condition [2], or generation of tren carbamates followed by hydrolysis and deprotonation [21]. Despite their toxicity, the interest in cadmium complexes is justified by their potential luminescent properties, in the attempt to combine the CO₂ capture ability with sensing application.

2. Materials and Methods

2.1. Synthesis

The chemicals and the solvents were acquired from commercial sources and used as received.

Syntheses of [{Cd(tren)}₃(tren)](ClO₄)₆·2H₂O (1) and [{Cd(tren)}₃(tren)][{Cd(tren)}₃(µ₃-CO₃)](ClO₄)₁₀ (2).

An amount of 0.8 mmol of tris(2-aminoethyl)amine and 0.2 mmol of Cd(ClO₄)₂·6H₂O were dissolved in 30 mL of water and 15 mL ethanol. The mixture was stirred for 30 min and then filtered. The solution was left for slow evaporation at room temperature. Colorless crystals of [{Cd(tren)}₃(tren)](ClO₄)₆·2H₂O (1) appeared after one to two days on the wall of the beaker, while the colorless crystals of [{Cd(tren)}₃(tren)][{Cd(tren)}₃(µ₃-CO₃)](ClO₄)₁₀ (2) were obtained after one week, preponderantly on the bottom of the beaker. The crystals were separated mechanically (yields: 1 about 20%, 2 about 45%). Compound 2 was obtained as the sole crystalline product by slow evaporation of the solution containing tren and cadmium perchlorate in a 4:1 molar ratio which resulted after bubbling CO₂ in solution for 10 min. The bubbling of CO₂ in solution was used to increase the amount of CO₂ physically dissolved in the water-ethanol mixture. The crystals were collected prior to total evaporation of the solvent in order to avoid contamination (the yield was 60%).

FT-IR (cm⁻¹) 1: 3603 br, 3348 s, 3295 s, 3174 w, 2956 m, 2905 m, 1590 m, 1465 w, 1316 w, 1055 vs, 977 s, 880 m, 614 s. FT-IR (cm⁻¹) 2: 3556 w, 3352 s, 3294 s, 3173 w, 2958 w, 2855 w, 1590 m, 1464 m, 1316 w, 1061vs, 983 s, 614 s.

Elemental analysis: (1) Calculated: C, 18.54; H, 4.93; N, 14.41%. Found: C, 18.91; H, 4.52; N, 14.07%; (2) Calculated: C, 18.76; H, 4.61; N, 14.25%. Found: C, 18.96; H, 4.37; N, 13.86%.

2.2. Physical Measurements

2.2.1. X-ray Structure Determination

X-ray single crystal diffraction measurements were made on a Rigaku XtaLAB Synergy-S diffractometer operating with a Mo-Kα (λ = 0.71073 Å) micro-focus sealed X-ray tube. The structures were solved by direct methods and refined by full-matrix least squares techniques based on F². The non-H atoms were refined with anisotropic displacement parameters. Calculations were performed using the SHELX-2018 crystallographic software package. A summary of the crystallographic data and the structure refinement for crystals 1 and 2 are presented in

Table 1. Crystallographic data, details of data collection and structure refinement parameters for compounds 1 and 2.

Compound	1	2
Chemical formula	C24H76Cd3Cl6N16O26	C43H126Cd6Cl10N28O43
M (g mol−1)	1554.90	2752.61
Temperature, (K)	293 (2)	293 (2)
Wavelength, (Å)	0.71073	0.71073
Crystal system	Triclinic	Monoclinic
Space group	P-1	P21
a (Å)	14.8621 (7)	17.4973 (6)
b (Å)	15.3643 (6)	12.2107 (4)
c (Å)	15.5852 (8)	23.5653 (8)
α (°)	99.667 (4)	90
β (°)	96.885 (4)	100.059 (3)
γ (°)	116.107 (4)	90
V (Å3)	3073.9 (3)	4957.4 (3)
Z	2	2
Dc (g cm−3)	1.680	1.844
μ (mm−1)	1.372	1.629
F (000)	1572	2764
Goodness-of-fit on F2	1.023	1.046
Final R1, wR2 [I > 2σ(I)]	0.0806, 0.2320	0.0523, 0.1404
R1, wR2 (all data)	0.0960, 0.2529	0.0598, 0.1473

. The CCDC reference numbers are: 2119345 and 2119346.

2.2.2. Spectroscopy

IR spectra were collected on an FT-IR Bruker Vertex 70 spectrometer (Billerica, MA, USA) in the 4500–400 cm⁻¹ range using the ATR technique. The abbreviations used are: w = weak, m = medium, s = strong, v = very, br = broad. Absorption spectra on powder (diffuse reflectance technique) were measured with a JASCO V-670 (Oklahoma, OK, USA) spectrophotometer. The fluorescence spectra were recorded on powder using a JASCO FP-6500 (Oklahoma, OK, USA) spectrofluorometer.

3. Results

By reacting tris(2-aminoethyl) amine with cadmium perchlorate in 4:1 molar ratio, two different compounds were obtained: initially [{Cd(tren)}₃(tren)](ClO₄)₆·2H₂O (1) and subsequently [{Cd(tren)}₃(tren)][{Cd(tren)}₃(µ₃-CO₃)](ClO₄)₁₀ (2). Both compounds contain trinuclear Cd (II) complexes.

3.1. Description of the Crystal Structures

Compound 1 crystallizes in the triclinic P-1 space group and the asymmetric unit consists of trinuclear [{Cd(tren)}₃(tren)]⁶⁺ cations, six perchlorate anions and two crystallization water molecules (Figure 1a). The trinuclear cationic complexes present a tripodal shape (Figure 1b). The Cd(II) ions are pentacoordinated with a trigonal bipyramidal stereochemistry. The equatorial positions and one axial position are occupied by a tren molecule coordinating as a tetradentate chelating ligand. Three such units are bridged by the fourth tren molecule coordinating through the primary amine groups in the free axial position of the Cd(II) ions. One perchlorate ion is hosted between the arms of the tripodal cation.

The equatorial Cd-N bond lengths are: Cd1-N1 = 2.299(7), Cd1-N2 = 2.294(7), Cd1-N3 = 2.277(8), Cd2-N5 = 2.258(8), Cd2-N6 = 2.300(8), Cd2-N7 = 2.310(9), Cd3-N9 = 2.267(10), Cd3-N10 = 2.288(13) and Cd3-N11 = 2.264(14) Å, while the axial Cd-N bond lengths are: Cd1-N4 = 2.430(6), Cd1-N13 = 2.252(6), Cd2-N8 = 2.459(7), Cd2-N14 = 2.282(6), Cd3-N12 = 2.418(8) and Cd3-N15 = 2.282(8) Å. The Cd···Cd distances within the trinuclear complex are: Cd1···Cd2 = 6.67, Cd2···Cd3 = 7.83 and Cd3···Cd1 = 6.77 Å.

The trinuclear cations [{Cd(tren)}₃(tren)]⁶⁺ are organized in layers in the crystallographic ab plane and within one layer the arms of the tripodal cations are orientated in the same direction (Figure 2).

The exposure of the reaction mixture for longer time to atmospheric condition allows the partial replacement of the bridging tren ligands with carbonate anions resulted from the capture of atmospheric CO₂. Compound 2, [{Cd(tren)}₃(tren)][{Cd(tren)}₃(µ₃-CO₃)](ClO₄)₁₀, crystallizes after more than one week and its structure consists in two types of trinuclear cations, [{Cd(tren)}₃(tren)]⁶⁺ and [{Cd(tren)}₃(µ₃-CO₃)]⁴⁺ (Figure 3), and perchlorate anions.

The cation [{Cd(tren)}₃(tren)]⁶⁺ preserves its general features from the compound 1 (Figure 3a). The Cd(II) ions from this cation are also pentacoordinated with a trigonal bipyramidal stereochemistry. The equatorial Cd-N bond lengths are: Cd1-N1 = 2.296(19), Cd1-N2 = 2.283(14), Cd1-N3 = 2.258(14), Cd2-N5 = 2.43(2), Cd2-N6 = 2.24(2), Cd2-N7 = 2.21(2), Cd3-N9 = 2.289(16), Cd3-N10 = 2.215(12) and Cd3-N11 = 2.236(15) Å, while the axial Cd-N bond lengths are: Cd1-N4 = 2.402(11), Cd1-N13 = 2.265(13), Cd2-N8 = 2.354(13), Cd2-N14 = 2.305(17), Cd3-N12 = 2.397(13) and Cd3-N15 = 2.299(12) Å. The Cd···Cd distances within the trinuclear complex are: Cd1···Cd2 = 6.68, Cd2···Cd3 = 7.04 and Cd3···Cd1 = 7.80 Å.

In the second trinuclear cationic complex, [{Cd(tren)}₃(µ₃-CO₃)]⁴⁺, the carbonate anion coordinates in the µ₃-k²:k²:k² bridging tris-chelating fashion (Figure 3b). The coordination number for the three Cd(II) ions is six, the axial amino group of the bridging tren molecule being replaced by two chelating oxygen atoms of the carbonate anion. In the cation [{Cd(tren)}₃(µ₃-CO₃)]⁴⁺ the Cd-N bond lengths are: Cd4-N17 = 2.278(14), Cd4-N18 = 2.307(14), Cd4-N19 = 2.276(11), Cd4-N20 = 2.471(9), Cd5-N21 = 2.292(13), Cd5-N22 = 2.260(12), Cd5-N23 = 2.261(12), Cd5-N24 = 2.453(10); Cd6-N25 = 2.290(12), Cd6-N26 = 2.279(11), Cd6-N27 = 2.281(10) and Cd6-N28 = 2.434(10) Å, while the Cd-O bond lengths are: Cd4-O1 = 2.429(8), Cd4-O2 = 2.430(8), Cd5-O2 = 2.327(8), Cd5-O3 = 2.629(9), Cd6-O1 = 2.347(8) and Cd6-O3 = 2.556(9) Å. The Cd···Cd distances within the tetracationic trinuclear complex are shorter: Cd4···Cd5 = 4.66, Cd5···Cd6 = 5.11 and Cd6···Cd4 = 4.68 Å.

The two types of trinuclear cations, [{Cd(tren)}₃(tren)]⁶⁺ and [{Cd(tren)}₃(µ₃-CO₃)]⁴⁺, are also arranged in layers, but in the crystallographic bc plane (Figure 4). In these layers, each type of trinuclear cation forms columns running along the crystallographic b axis with an alternating distribution of the two types of columns within the layer. Similarly to compound 1, the arms of the tripodal cations [{Cd(tren)}₃(tren)]⁶⁺ are orientated on the same direction within one layer and in opposite directions in the neighboring layers.

3.2. Spectral Properties

The absorption spectra of compounds 1 and 2 were acquired in the wavelength range 200–1000 nm on solid samples (using the diffuse reflectance technique) and both compounds present absorption maxima around 370 nm.

The room temperature photoluminescence of compounds 1 and 2 was explored using excitation wavelengths from the 350–400 nm range. The emission spectra displayed asymmetric bands with maxima at 470 nm for both compounds. The highest intensity of these bands was obtained when λ_ex = 350 nm. Compound 2 also presented a maximum at 450 nm and a “shoulder” at around 500 nm. In the emission spectrum of 1 there are two “shoulders” at 450 and 500 nm. The presence of the “shoulder” at 450 nm in the emission spectrum of 1 may suggest the presence of impurities of compound 2. The compounds were separated mechanically and we cannot exclude the presence of impurities because both compounds are colourless. The corresponding excitation spectra (λ_em = 470 nm) reveals complex bands within the 250–450 nm domain with maxima at 410 nm for 1 and 400 nm for 2 (Figure 5).

4. Conclusions

The cationic trinuclear Cd(II) complex with the tripodal tren ligand, [{Cd(tren)}₃(tren)]⁶⁺, proved to be a good sequestering agent for atmospheric CO₂. The bridging tren ligand pre-organized the metal ions in a trigonal manner, favorable to a µ₃-k²:k²:k² coordination mode of the carbonate anion and this basic ligand is easy replaceable by the carbonate anion. The luminescent properties of the Cd(II) complexes may combine the sequestering abilities of such complexes with a sensing application.