2D Coordination Polymer [Fe(piv)₂(dab)₂]_n

Abstract

The interaction of preorganized iron(II) pivalate complexes with aromatic N-donor ligand (pyridine (py) or 2,2′-bipyridine (bpy)) and 1,4-diaminobutane (dab, putrescine) in anhydrous acetonitrile yielded a new 2D coordination polymer [Fe(piv)₂(dab)₂]_n (1, piv = Me₃CCO₂^–). The molecular structure of 1 in crystal was determined by single-crystal X-ray diffraction analysis and ATR-FTIR spectroscopy.

1. Introduction

The study of the structure and properties of coordination polymers, which are compounds of metals with organic linkers, attracts the attention of researchers [1,2]. Coordination polymers are used in almost all areas of human activities, for example, catalysis [3,4,5,6], luminescence [7], magnetism [8], new types of energy-storing compounds [9,10] and selective gas sorption [11], and even exhibit biological activity, being objects of research in bioinorganic chemistry [12,13,14,15,16,17]. Of particular interest are coordination polymers based on transition metals such as iron, which are used in the creation of switchable materials [8,18], energy storage and conversion [19], or in the sorption of small molecules [20,21].

Carboxylate ligands, in particular pivalate, are used as the building blocks for the formation of coordination polymers due to their ability to coordinate with metal ions in various modes, forming bridged or chelate structures [22]. Work on the synthesis and study of iron pivalate complexes is being carried out nowadays, since these objects can exhibit catalytic properties [23,24], and can be used as single blocks for coordination polymer construction [25,26,27] and nanoparticles synthesis [28,29,30].

Diamines such as 1,6-diaminohexane or 1,4-diaminobutane are an important class of organic ligands capable of forming stable complexes with metal ions [25]. Metal complexes with 1,6-diaminohexane are used to create layered organic−inorganic perovskites with tunable optoelectronic properties [31] and to create ultraviolet or visible photodetection materials [32]. It was also found that compounds of cadmium chloride and zinc bromide with hexamethylenediamine exhibit reversible phase changes under the influence of external stimuli [33,34]. In contrast to the longer homologue, 1,4-diaminobutane can act as a chelating ligand [35,36] as well as a linker to form coordination polymers [37,38]. The combination of carboxylates and diamines as ligands provides the possibility of synthesizing coordination polymers with interesting structural and functional characteristics, including the formation of switchable materials. For example, we have previously shown that the interaction of cobalt(II) pivalate with 1,6-diaminohexane led to the formation of a polymer product capable of reversibly changing its structure with a change in temperature, forming a magnetization hysteresis [39]. We have also shown that a mixed-valence iron(II/III) complex can be obtained using 1,6-diaminohexane [25].

The aim of this work was primarily to develop a method for synthesizing a coordination polymer of iron(II) with 1,4-diaminobutane as a bridging ligand, to grow crystals suitable for X-ray diffraction, and to determine the structure of the substance in the crystal. This work is a continuation of our studies on the synthesis of coordination polymers of cobalt(II) and iron(II/III) with diamines.

2. Results and Discussion

2.1. Synthesis and Characterization

The reaction (Scheme 1, method a) of in situ-produced iron(II) pivalate with one equivalent of pyridine in anhydrous acetonitrile leads to the formation of the previously described binuclear paddlewheel complex [Fe₂(piv)₄(py)₂] [40]. This complex is highly soluble, and the solution has a bright yellow color. The condensation of two equivalents of 1,4-diaminobutane to this solution led to the formation of white crystals of the product [Fe(piv)₂(dab)₂]_n (1).

A similar reaction (Scheme 1, method b) of iron(II) pivalate with one equivalent of 2,2′-bipyridine also resulted in complete dissolution to form a purple solution. CSD (version 5.45, November 2023) does not contain any iron(II) trimethylacetate complex with 2,2′-bipyridine, but a search for similar structures for cobalt shows that different stoichiometric ratios are possible [41]. The condensation of two equivalents of 1,4-diaminobutane to this solution led the formation of white crystals of the product [Fe(piv)₂(dab)₂]_n (1).

The need to obtain an intermediate complex with an aromatic N-donor ligand is due to the polymer structure and poor solubility of iron pivalate in common organic solvents. This approach was first tested by us in the preparation of coordination compounds with N-heterocyclic carbenes [42]. We then used this method to synthesize a mixed-valent coordination polymer of iron trimethylacetate with 1,6-diaminohexane [25].

2.2. Crystal Structure

In structure 1, the iron atom is in an octahedral environment (FeN₄O₂), which is formed by two oxygen atoms of two monodentate carboxylate groups in axial positions (Fe-O 2.0618(18), 2.1031(18) Å) and four nitrogen atoms of four dab molecules in equatorial positions (Fe-N 2.221(2)-2.272(2) Å) (Figure 1a). The dab molecules act as bridges, linking the iron atom to neighboring atoms (Fe…Fe 7.168, 8.206 Å; Figure 1a,b) to form a structure with hcb topology (Figure 1c). The shortest Fe…Fe interatomic distance (6.704 Å) corresponds to the intralayer distance. The layered structure provides additional stabilization of the N-H…O interactions (

Table 1. Parameters of H-bonds in the crystal 1 (the analysis was performed using PLATON software [43]).

H-Bond	Symmetry Equivalent	D-H, Å	H…A, Å	D…A, Å	D-H…A, deg.
N1-H1A…O4	1 − x, −y, 1 − z	0.91	2.21	3.027(3)	149
N2-H2A…O2	3/2 − x, 1/2 + y, 3/2 − z	0.91	2.36	3.080(3)	135
N2-H2B…O2	x, 1 + y, z	0.91	2.29	3.194(3)	172
N3-H3B…O4	1 − x, 1 − y, 1 − z	0.91	2.15	3.056(3)	172
N4-H4A…O2	1 − x, −y, 1 − z	0.91	2.42	3.049(3)	126

). The tert-butyl substituents on the carboxylate groups form the outer shell of the layers, which interact with each other via van der Waals contacts.

2.3. FTIR Spectroscopy

IR spectra were taken in air, immediately after opening the ampoule with the crystallic sample. The spectra of the samples obtained by different methods coincided, which also confirms their identity (Figure S1).

In the region 3400–3200 cm⁻¹, there are three weak absorption bands at 3335, 3299 and 3269 cm⁻¹. These bands are caused by the vibration (ν_as and ν_s) of the N-H bonds of the dab ligand. Free diaminobutane at room conditions (24 °C) in this region of the IR spectrum has two vibration bands—3360 and 3281 cm⁻¹ (Figure S2) [44].

In the region 3000–2800 cm⁻¹, there are three absorption bands at 2955 (m), 2915 (w) and 2867 (w) cm⁻¹. These bands are caused by the vibration of the C–H bonds of the piv ligand. Free cobalt(II) pivalate, similar in structure to iron(II) pivalate, has 2964 (m), 2927 (w) and 2868 (w) cm⁻¹ absorption bands in the IR spectrum (Figure S3).

In addition, IR spectroscopy proves the completeness of ligand substitution. In the sample obtained by method a, there are no characteristic bands of C–H bond vibration (1200 (s), 1000 (s) cm⁻¹) or ring vibrations (1400–1600 (m) cm⁻¹) of the py ligand (Figure S4). In the sample obtained by method b, there are no characteristic bands of C–H bond vibration (1448 (s) and 1416 (s) cm⁻¹) or ring vibrations (1553 (m) and 1579 (s) cm⁻¹) of the bpy ligand (Figure S5) [45,46].

3. Materials and Methods

3.1. General Remarks

The synthesis of compound 1 was carried out using a modified Schlenk technique. Acetonitrile CH₃CN (Khimmed (Moscow, Russia) reagent grade) was dried over phosphorus(V) oxide, distilled, and stored in a vacuumed glass flask with activated molecular sieves (3 Å), from where it was taken by condensation into a chemical reactor (glass ampule with a Teflon stopcock) prior to use; ethanol EtOH (Ferein (Emmen, The Netherlands) 95%) was degassed and taken by condensation into the chemical reactor. Iron sulfate heptahydrate FeSO₄·7H₂O (Khimmed, reagent grade), pyridine (Khimmed, analytical grade), 2,2′-bipyridine bpy (Khimmed, analytical grade), and 1,4-diaminobutane dab (Macklin (Shanghai, China) 98%) were used without additional purification.

Potassium pivalate (Kpiv) was obtained according to a known synthetic procedure. Before reacting with iron(II) sulfate, potassium pivalate was heated at 140 °C in an oil bath for 24 h in a dynamic vacuum. Iron(II) pivalate [Fe(piv)₂]_n was obtained by an exchange reaction from FeSO₄·7H₂O and Kpiv. Iron pivalate is poorly soluble in ethanol and acetonitrile; it forms a gel-like mass. Adding an equimolar amount of pyridine, 2,2′-bipyridine or an 1,10-phenanthroline [25] converts iron(II) pivalate into soluble complexes, which allows filtration from insoluble potassium sulfate.

IR spectra of the compounds were recorded in the range of 400–4000 cm^–1 on a Perkin Elmer Spectrum 65 spectrophotometer (Waltham, MA, USA) equipped with a Quest ATR Accessory (Specac, Orpington, UK) using the attenuated total internal reflection (ATR) method.

The X-ray diffraction data were collected on a Bruker Apex II (Mo-Kα, λ = 0.71073 Å) diffractometer equipped with a CCD detector and a monochromatic radiation source. Semi-empirical absorption correction was applied [47]. The structure was solved by direct methods and refined by the full-matrix least squares in the anisotropic approximation for non-hydrogen atoms. Hydrogen atoms of the carbon-containing ligands were geometrically generated and refined in the riding model. The calculations were carried out by SHELXL-2018/3 program package [48] using Olex2 [49]. The crystallographic parameters and experimental X-ray diffraction data at 150 K are as follows: colorless, parallelepiped-like crystals, C₁₈H₄₈FeN₄O₂, M_w = 434.40 g·mol^–1, monoclinic space group P2₁/n, a = 14.387(3), b = 9.7896(14), c = 16.3898(17) Å, β = 91.228(7)°, V = 2307.9(6) ų, Z = 4, ρ_calc = 1.250 g·cm^–3, μ = 0.681 mm^–1, 1.90° ≤ θ ≤ 28.28°, sphere segment –17 ≤ h ≤ 19, –11 ≤ k ≤ 13, –21 ≤ l ≤ 19, T_min/T_max = 0.5920/0.7465, R_int = 0.0606, 14,940 measured reflexes, 5702 independent reflexes, 4235 reflexes c I > 2.0σ(I), GooF = 1.044, R₁(I > 2σ(I)) = 0.0526, wR₂(I > 2σ(I)) = 0.0990, R₁(all data) = 0.0798, wR₂ (all data) = 0.1139, Δρ_min/Δρ_max, e Ǻ^–3 = –0.489/0.342. The crystallographic data reported in this paper have been deposited with the Cambridge Crystallographic Data Center (CCDC number 2426862).

The topology analysis was performed with the topcryst.com [50].

3.2. Synthesis of [Fe(piv)₂(dab)₂]_n (1), Method a

To the iron(II) pivalate prepared in situ from FeSO₄·7H₂O (0.278 g, 1.0 mmol) and Kpiv (0.280 g, 2.0 mmol), pyridine (0.087 g, 1.1 mmol) was condensed to 10 mL of acetonitrile and added. The yellow-colored reaction mixture was filtered to remove potassium sulfate. Adding a 1,4-diaminobutane (0.192 g, 2.2 mmol) solution into 2 mL of acetonitrile led to white precipitate. The reaction mixture was sealed in a glass ampoule and heated in an oil bath at 100 °C for 5 h until a clear solution was formed (caution! high pressure). Cooling to room temperature (24 °C) led to the formation of colorless crystals of 1.

IR (ATR, ν, cm^–1): 3335 (w), 3299 (w), 3269 (w), 2955 (m), 2915 (w), 2867 (w), 1608 (m), 1542 (s), 1476 (m), 1399 (s), 1351 (s), 1219 (m), 1173 (w), 1057 (w), 997 (m), 965 (w), 932 (w), 878 (m), 848 (w), 789 (m), 740 (w), 623 (w), 591 (m), 538 (m), 418 (w).

3.3. Synthesis of [Fe(piv)₂(dab)₂]_n (1), Method b

To the iron(II) pivalate prepared in situ from FeSO₄·7H₂O (0.278 g, 1.0 mmol) and Kpiv (0.280 g, 2.0 mmol), 10 mL of acetonitrile was condensed and added 2,2′-bipyridine (0.156 g, 1.0 mmol). The purple-colored reaction mixture was filtered to remove potassium sulfate. Adding a 1,4-diaminobutane (0.192 g, 2.2 mmol) solution into 2 mL of acetonitrile led to the white precipitate. The reaction mixture was sealed in a glass ampoule and heated in an oil bath at 100 °C for 5 h until a clear solution was formed (caution! high pressure). Cooling to room temperature (24 °C) led to the formation of colorless crystals of 1.

IR (ATR, ν, cm^–1): 3335 (w), 3299 (w), 3269 (w), 2955 (m), 2915 (w), 2867 (w), 1607 (m), 1541 (s), 1477 (m), 1399 (s), 1351 (s), 1219 (m), 1176 (w), 1058 (w), 998 (m), 964 (w), 934 (w), 880 (m), 848 (w), 790 (m), 739 (w), 622 (w), 591 (m), 537 (m), 455 (w).

4. Conclusions

Thus, we have shown that the interaction of iron(II) pivalate, 2,2′-bipyridine and 1,4-diaminobutane in anhydrous acetonitrile led to the formation of a new 2D coordination polymer [Fe(piv)₂(dab)₂]_n (1) sensitive to moisture and atmospheric oxygen. The layered structure with hcb topology consists of {Fe(piv)₂} fragments linked by bridging dab molecules and additionally stabilized by intralayer H-bonds between amino groups and oxygen atoms of carboxylate groups.