Synthesis and crystal structure of the two-dimensional coordination compound potassium oxo-bis(hydroxylamido)-pyridine-2, 3-dicarboxylato-vanadate(V)

Structure description

In the molecular structure of the compound, the vanadium atom is seven-coordinated in a pentagonal bipyramidal geometry by two bidentate hydroxylamide ligands, one oxo ligand and one nitrogen and one oxygen atom from a 2,3-dipicolinate ligand (Fig. 1; Tables 1–3). The hydroxylamide ligands coordinate in a side-on manner, as observed in related structures [4–7]. The 2,3-dipicolinate behaves as a chelating ligand, leading to the formation of a five-membered chelate ring. The centroids of the two hydroxylamide ligands and the atom N1 of the dipicolinate define the equatorial plane perpendicular to the V=O bond. The coordinating carboxylate O atom, O1, is in an axial position trans to the oxide ligand, with an axial O1–V–O5 angle of 164.56°. The terminal V=O5 bond length is 1.6078(13) Å, leading to the expected trans lengthening of the V1–O1 bond to 2.1365(12) Å, which is longer than the V–O bonds to the two hydroxylamide ligands (Table 2). The O–N, V–O, and V–N distances and the O–V–N angles involving the hydroxylamide ligands are comparable with related vanadium hydroxylamide complexes reported in the literature (Table 4). When comparing the bond lengths and angles of the compound with those of the vanadium(V) hydroxylamide complexes reported previously with other ligands (Table 4), the following points should be noted. The first is the geometry and coordination modes, which are different from those in [VO(NH₂O)(2,6-dipic)(H₂O)] [6], although both have the same type of ligand. The 2,6-dipicolinate therein is a tridentate ligand, and the equatorial plane of the seven-coordinate vanadium(V) is composed of one hydroxylamide ligand (N,O) and one dipicolinate ligand (O,N,O), and the water and the oxo ligand are in an axial position. The second point is that the geometry and coordination modes of this complex are very much like those of the vanadium(V) hydroxylamide complexes reported previously with amino acid ligands [8] and malonic acid [14, 15]. A majority of the metric parameters in the compounds are very similar (Table 2), specifically the O–N, V–O, and V–N distances and the O–V–N angles involving the hydroxylamide ligands. The third point is the V–N_py distance of 2.1428(15) Å and V–O_carb of 2.1365(12) Å, which are significantly longer than those in Cs[VO(NH₂O)(2,6-dipic)(H₂O)] (Table 4), the former perhaps reflecting the crowding effect of two hydroxylamide ligands in the equatorial plane, and the latter being apparently due to the trans influence of V=O [6]. The coordination environment around the K ion can be described as a distorted capped square antiprism. The vertices are occupied by nine O atoms, of which five belong to dipicolinate ligands (K–O = 2.715–3.027 Å), with the uncoordinating O(O2) of the coordinating carboxylate at the cap position. The other three belong to the hydroxylamide ligands (K–O = 2.786–2.925 Å), and O8 belongs to the H₂O ligand.

Fig. 1:

The mode of coordination of V and K for K₂[VO(C₇H₃O₄N)(NH₂O)₂(H₂O)]₂.

In the crystal structure, adjacent K polyhedra are linked, sharing an edge at one side and a plane on the other, to form an infinite chain. These chains are connected by dipicolinate bridges to form layers.

The polyhedra are grafted onto this layer via dipicolinate ligands, which are distributed on both sides of the layers owing to the alternating orientation of the N and O atoms in the dipicolinate ligands. This arrangement favors the minimization of steric hindrance and boosts the stability of the crystal structure.

The remarkable organization of the crystal structure of the compound can be recognized in a view along the a axis, which shows the layers parallel to the (001) plane. An extensive hydrogen-bonding network (Table 3) links the layers through functional groups, such as water O and hydroxylamide N, or water O and carboxylate O. This leads to the formation of a stable three-dimensional supramolecular structure.

UV/Vis spectrum and IR spectrum

The UV/Vis spectrum is shown in Fig. 2. Two bands were expected for it, one low-intensity band at 263 nm and one high-intensity band at 214 nm. The higher energy peak is also expected to contain the π–π^* transition of the 2,3-dipicolinic acid and the n–d, π^*–d transition from the 2,3-dipicolinic acid to the metal ion. The shoulders at 263 nm are due to the transition from the hydroxylamido ligands to the metal [5].

Fig. 2:

The UV/Vis curve of K₂[VO(C₇H₃O₄N)(NH₂O)₂(H₂O)]₂.

The IR spectra of 2,3-dipicolinic acid and K₂[V(C₇H₃O₄N)(NH₂O)₂O(H₂O)]₂ were recorded in the region from 4000 to 400 cm^–1. The assignments of the absorption bands are based on literature data of related compounds [13, 16, 17]. The band at 3452 cm^–1 can be assigned to the asymmetric stretching vibration ν(O–H) of H₂O, whereas the bands at 3265, 3230, and 3121 cm^–1 can be assigned to ν_as(NH₂) and ν_s(NH₂) of hydroxylamine. The strong band with a maximum around 3014 cm^–1 is attributable to the C–H at the pyridine ring. The other spectral range of special interest is around 1600 cm^–1. In the range of 1300–1700 cm^–1, the ligand and the complex all have a few strong bands with only little difference. The band at 1609 and 1367 cm^–1 (1602 and 1366 cm^–1 in the ligand) can be assigned to the ν_as(COO^–) and ν_s(COO^–). The strong band with a maximum around 1475 cm^–1 of the protonated ligand disappeared upon coordination. Like α-amino acids, 2,3-dipicolinic acid exists as zwitterion in the crystalline state; thus, we assigned the band at 1475 cm^–1 to the protonated pyridine nitrogen. The V=O stretching vibration was observed at 962 cm^–1, in agreement with those observed in other hydroxylamido/amino acid oxovanadium complexes [10, 11, 17].

Thermal analysis

To investigate the pyrolysis behavior of K₂[VO(C₇H₃O₄N)(NH₂O)₂ (H₂O)]₂, a TG analysis was performed (Fig. 3). Surface water was lost in the low-temperature region before 150 °C. The 19.0 wt% weight loss from 150 °C to 220 °C was due to the loss of hydroxylamine [14]. The 21.0 wt% weight loss from 220 °C to 500 °C was due to the decomposition of the dipicolinic acid ligands and loss of water of crystallization. When the sample was further heated above 800 °C, a continued weight loss was observed because of the sublimation of vanadium oxides in the decomposition of the complex [14].

Fig. 3:

The TG curve of K₂[VO(C₇H₃O₄N)(NH₂O)₂(H₂O)]₂.

In summary, the complex K₂[VO(C₇H₃O₄N)(NH₂O)₂(H₂O)]₂ has a new two-dimensional framework worth considering for further studies.

General methods and materials

All chemicals were of reagent grade and used without further purification. KOH, NH2OH·HCl and NNH4VO3 (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), 2,3-dipicolinic acid (Sigma Aldrich, Shanghai, China) were used as supplied. The infrared spectrum was recorded from a KBr pellet on a Nicolet nexus 670 FI-IR spectrometer (Thermo company, USA) in the range 4000–400 cm^-1. Thermogravimetric analysis was performed on a NETZSCH STA 409 PC/PG instrument (NETZSCH group, Germany) in N₂ with a heating rate of 10 K/min.

Synthesis

NH₄VO₃ (1.0 mmol), KOH (3.19 mmol), and 2,3-dipicolinic acid (1.03 mmol) were dissolved in H₂O (15 mL) at room temperature. In an ice bath, NH₂OH·HCl (2.34 mmol) was added gradually to the above solution with constant stirring for 0.5–1.0 h. KCl was added to the resulting yellow solution (pH = 6.7). Pale yellow crystals of the compound suitable for single-crystal X-ray diffraction were obtained by slow evaporation of a mixture of the filtrate and anhydrous ethanol at 277 K over 10 days. The isolated yield was 0.1516 g (42.8 %). – Selected IR data [KBr, υ_max(cm^–1)]: 3265(s), 1616(s), 1394(s), 1368(s), 962(s), 626(s), 3121(m), 1588(m), 1658(m), 1119(m), 840(m), 684(m), 626(m), 787(w), 937(w), 587(w), 1266(w). – Anal. for C₇H₁₈O₄N₆ O₁₆V₂K₂: calcd. C 23.79, H 2.55, N 11.89, V 14.44; found: C 23.72, H 2.866, N 11.57, V 14.47.

Crystal structure determination

The intensity data were collected using graphite-monochromated MoK_α radiation (λ = 0.71073 Å) using a Bruker Smart CCD diffractometer (Bruker company, Germany). Accurate unit cell parameters and the orientation matrix were obtained from a least-squares refinement using the programs smart and saint, and the data were integrated using saint. The structure was solved by direct methods using the shelxs-97 software package [18] and refined by full-matrix least-squares techniques on F² using the same software [19, 20]. The water H atoms bonded to O8 were located in a difference Fourier map, and their positions were refined with geometric restraints of O–H = 0.85(3) Å and H···H = 1.38(1) Å. Other H atoms were placed in calculated positions, with N–H = 0.90 Å for amino H atoms and C–H = 0.93 Å for pyridine H atoms.

Additional details of data collection and structure refinement are listed in Table 1, and selected bond lengths and angles are listed in Tables 2 and 3.

CCDC 1024427 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.