Transition-metal complexes with oxidoborates. Synthesis and XRD characterization of [(H₃NCH₂CH₂NH₂)Zn{κ³O,O′,O′′-B₁₂O₁₈(OH)₆-κ¹O′′′}Zn(en)(NH₂CH₂CH₂NH₃)]·8H₂O (en=1,2-diaminoethane): a neutral bimetallic zwiterionic polyborate system containing the ‘isolated’ dodecaborate(6−) anion

Synthesis and characterization

The title compound, [(H₃NCH₂CH₂NH₂)Zn{κ³O,O′,O′′-B₁₂O₁₈(OH)₆-κ¹O′′′}Zn(en)(NH₂CH₂CH₂NH₃)]·8H₂O (1), was prepared in moderate yield (40%) as a colorless crystalline material by a self-assembly process involving a prolonged (30 days) recrystallization period of an aqueous methanolic reaction mixture originally containing B(OH)₃ and [Zn(en)₃](OH)₂ (eq. 1). The [Zn(en)₃](OH)₂ was obtained from [Zn(en)₃]Cl₂ by use of an ion exchange resin.

Compound 1 was characterized primarily by single-crystal XRD study (see below) and its structure is fully consistent with other characterization data that we were able to obtain. Compound 1 is formulated as a neutral molecule although it is zwitterionic with several atoms possessing formal charges and it contains some interesting features (see below). The bulk sample gave satisfactory elemental analysis and its powder XRD pattern was consistent with that derived from the single-crystal XRD analysis, confirming homogeneity. Compound 1 is diamagnetic with a molar magnetic susceptibility of −414.2×10⁻⁶ cm³ mol⁻¹ and its IR spectrum shows several strong absorptions in the B–O stretching region which are further assigned as follows: 1355 cm⁻¹ (asymmetric B_trig–O stretch), 1163 cm⁻¹ and 1041 cm⁻¹ (asymmetric B_tet–O stretch), 951 cm⁻¹ and 900 cm⁻¹ (symmetric B_trig–O stretch), and 856 cm⁻¹ (symmetric B_tet–O) [31]. Compound 1 is decomposed by dissolution in aqueous solution with (¹H and ¹³C) NMR consistent with the ethylenediamine species and ¹¹B NMR showing a single peak (+16.1 ppm) indicative of one signal arising from rapid B(OH)₃/OH⁻ exchange equilibria [21], [22], [23], [24], [25].

Solid-state structure

Crystallographic data for compound 1 are given in Table 1, and the structure of 1, together with associated atomic numbering scheme is given in Fig. 1. Its structure is best represented as comprised of a [B₁₂O₁₈(OH)₆]⁶⁻ ligand coordinated to, and bridging two zinc(II) centers.

Fig. 1:

Diagram showing atomic numbering scheme for [(H₃NCH₂CH₂NH₂)Zn{κ³O,O′,O′′-B₁₂O₁₈(OH)₆-κ¹O′′′}Zn(en)(NH₂CH₂CH₂NH₃)] (1).

Compound 1 contains a dodecaborate(6−) anion. This anion was first reported in 1990 in Ag₆[B₁₂O₁₈(OH)₆]·3H₂O [32] and was described as an ‘insular’ polyborate anion. The anion is comprised of six boroxole rings linked together in such a way as to produce a larger central 12-membererd {B₆O₆} ring (Fig. 2), with each boron atom within this ring being 4-coordinate and carrying a formal negative charge. These six 4-coordinate boron centers (B1–B6) have B–O bond lengths ranging from 1.435(5) to 1.499(6) Å and the O–B–O angles ranging from 106.3(3) to 112.6(3)°. The other six boron centers in the anion, B7–B12, are 3-coordinate and have significantly shorter B–O bonds {1.349(6)–1.389(6) Å} and larger O–B–O angles which range from 116.1(4) to 123.9(4)°. These distances and angles are very similar to distances and angles observed in the previously reported Ag₆[B₁₂O₁₈(OH)₆]·3H₂O [32] and in acyclic polyboroxole ‘chain’ species, [B₅O₆(OH)₄]⁻ [33], [34], [35], [36], [37], [38], [B₇O₉(OH)₅]²⁻ [39], [40], [41], [42], [43], [B₉O₁₂(OH)₆]³⁻ [44], [45], [46], and related boroxole systems [47], [48], [49] containing both 3- and 4-coordinate boron centers bound to oxygen. The dodecaborate(6−) in 1 is closely related to deprotonated structures Na₈[B₁₂O₂₀(OH)₄] [50] and Zn₆[B₁₂O₂₄] [51] and is an alternative (but not isomeric) to the hydrated dodecaborate found in K₄[B₁₂O₁₆(OH)₈] [52] which has four 4-coordinate boron atoms and carries a charge of 4–.

Fig. 2:

The dodecaborate(6−) ligand as found in 1. The central {B₆O₆} ring as shown on the left is non-planar with oxygen atoms (red) alternating ‘up’ and ‘down’ as shown in the side-on view (right). Boron atoms are pale pink.

The dodecaborate(6−) anion in 1 utilizes its ‘inner’ ring oxygen atoms to coordinate to the Zn(II) centers and due to their stereochemistry the dodecaborate(6−) anion is ideally set up to bridge metal centers. The dodecaborate(6−) anion has been previously observed coordinated tridentate to metal centers in the following compounds: Na₂Cs₄Ba₂[B₁₂O₁₈(OH)₆](OH)₄ [53], K₇[(BO₃)Mn{B₁₂O₁₈(OH)₆}]·H₂O [54], and K₇[(BO₃)Zn{B₁₂O₁₈(OH)₆}]·H₂O [55] with the latter also containing a zinc(II) ion. The coordination modes of the dodecaborate(6−) ligand towards the Zn centers are separated out in Fig. 3. Both Zn atoms in 1 are 4-coordinate and Zn1 is coordinated by the dodecaborate(6−) ligand in a tridentate fashion through oxygen donors O1, O3 and O5 with bond lengths to Zn1 of 1.971(3), 1.976(3) and 1.937(3) Å, respectively. The Zn2 atom is coordinated by the dodecaborate(6−) anion in a mondentate manner solely through O2 with as Zn2–O2 distance of 1.931(3) Å. The coordination number of four is completed on Zn2 by a bidentate en (ethylenediamine) ligand and a monodentate [enH]⁺ group, whist Zn1 just has one additional monodentate [enH]⁺ group. The Zn–N distances range from 2.018(5) to 2.043(4) Å, whilst borate O–Zn atom distances (listed above) are significantly shorter and range from 1.937(3) to 1.976(3) Å, and are comparable to those observed elsewhere related zinc-borate species [56], [57], [58], [59], [60], [61], [62], [63] which typically range from 1.919(2) to 2.0099(14) Å. The Zn–N distances are comparable to those observed in related zinc borate complexes with organoamino ligands [56], [57], [58], [59], [60], [61], [62], [63] and ethylenediamine complexes such as [Zn(en)₃]Cl₂·2H₂O [64].

Fig. 3:

Diagrams showing the two coordination modes of the dodecaborate(6−) ligand. The 4-coordinate Zn1 atom has a tridentate dodecaborate(6−) ligand and a monodentate protonated en ligand (left). The 4-coordinate Zn2 atom a monodentate dodecaborate(6−) ligand, a bidentate en ligand, and a monodentate protonated en ligand coordinated to it.

The hydroxyl hydrogen atoms of the [B₁₂O₁₈(OH)₆]⁶⁻ anion, the amino hydrogen atoms of the en ligands and protonated en ligands, and the waters of crystallization form multiple intramolecular H-bond donor interactions which are presumably responsible for the remarkable self-assembly of 1 from its component parts. In particular, all six hydroxyl groups in the polyborate anion of 1 are involved in energetically favorable R₂²(8) interactions (Etter nomenclature [65]) with six neighboring molecules of 1, as shown in Fig. 4. Details of these H-bond interactions can be found in the caption to Fig. 4. In addition, the protonated ends of the mondentate H₂NCH₂CH₂NH₃ ligands further H-bond to two more molecules of 1 forming a supramolecular chain (Fig. 5) giving an overall ‘coordination number’ of eight for each molecule of 1. The amino hydrogens of the en ligand on Zn2 are involved in an intramolecular H-bond (Fig. 5) and H-bond donation to water molecules, and the waters of crystallization H-bond to each other and dodecaborate anions to further glue the structure together.

Fig. 4:

The [B₁₂O₁₈(OH)₆]⁶⁻ anion in 1 forms six H-bond donor interactions with six neighboring dodecaborate(6−) units. The interactions involve 8-membered rings R₂²(8) with D···A oxygen atom distances as follows: O19H19···O14*, 2.779(4) Å; O20H20···O16*, 2.634(4) Å; O21H21···O17*, 2.701(4) Å; O22H22···O8*, 2.899(5) Å; O23H23···O10*, 2.695(4) Å; O24H24···O11*, 2.680(4) Å, where * indicates an acceptor oxygen atom of a neighboring unit. H₂O molecules omitted for clarity.

Fig. 5:

Intermolecular H-bond interactions involving the amino hydrogen atoms link the dodecaborates(6−) anions into an extended chain. The D···A oxygen atom distances as follows: N2H2C···O7*, 2.914(5) Å; N2H2C···O18*, 3.068(5) Å; N11H11B···O13*, 3.053(5) Å; N12H12A···O4*, 2.903(5) Å, where * indicates an acceptor oxygen atom of a neighboring unit. The intramolecular H-bond interaction is also shown: N21H21B···O4, 2.871(5) Å. H₂O molecules omitted for clarity.

General

NMR spectra were obtained on a Bruker Avance-400 spectrometer in D₂O Solution at 128, 400 or 101 MHz for ¹¹B, ¹H, and ¹³C, respectively, and referenced to either BF₃·OEt₂ (¹¹B) or TMS (¹H and ¹³C). FTIR spectra were obtained as KBr pellets on a Perkin Elmer 100FTIR spectrometer. Powder X-ray diffraction was carried out on a Phillips X’Pert 2040/60 XRD diffractometer with spectra obtained using the Phillips X’Pert Data Collector software. Single-crystal X-ray diffraction was performed at the EPSRC National Crystallographic Service at the University of Southampton. Magnetic susceptibility measurements were performed on a Johnson-Matthey magnetic susceptibility balance. CHN analyses were obtained from OEA Laboratories (Callingham, Cornwall).

Preparation of [Zn(en)₃]Cl₂·2H₂O

Tris(ethylenediamine)zinc(II) chloride dihydrate was prepared as described in the literature [64]. A slight excess of the ethylenediamine (en) (2.10 g, 70%, 24.45 mmol) was added to an aqueous solution of zinc(II) chloride (3.0 g, 22.0 mmol) in distilled water (10 mL). The solution was concentrated by gentle evaporation n a warm water bath before being cooled in an ice bath to yield colorless crystals of tris(ethylenediamine)zinc(II) chloride dihydrate (5.4 g, 70%). M.p.=185–187°C. χ_m=−211.5×10⁻⁶ cm³ mol⁻¹. ¹H/ppm: 2.7 (m, 12H, CH₂ of en), 4.8 (s, 16 H, NH₂, H₂O). ¹³C{¹H}/ppm: 39.1. IR (KBr/cm⁻¹): 3383(s), 3261(s), 3154(s), 2949(s), 2933(s), 2885(s), 1651(m), 1587(s), 1458(s), 1331(s), 1275(s), 1147(m), 1023(s), 998(s), 976(m), 645(s), 511(s). [Lit. 3385, 3275, 3150, 2940, 2882, 1653, 1595, 1460, 1335, 1272, 1144, 1022, 996, 977, 644, 515] [66].

Preparation of [Zn₂(en)(enH)₂{B₁₂O₁₈(OH)₆}]·8H₂O (1)

A solution of tris(ethylenediamine)zinc(II) chloride dihydrate (1.0 g, 2.8 mmol) in water (10 mL) was added to an aqueous suspension solution of excess activated Dowex 550A (50 g) monosphere anion exchange resin in water (40 mL). The resulting mixture was stirred at room temperature for 24 h. The Dowex 550A resin was separated by filtration with a Buchner funnel and washed four times with a distilled water (4×5 mL). The solution of [Zn(en)₃](OH)₂ was reduced to a volume of 15 mL using a rotary evaporator. Methanol (15 mL) was added to the concentrated solution, followed by boric acid (2.6 g, 42 mmol). The reaction mixture was gently warmed for 3 h. The solution volume was reduced to 5 mL using a rotary evaporator. The concentrated solution was left for 30 days in NMR tubes for crystallization to yield colorless crystals of [Zn₂(en)(enH)₂{B₁₂O₁₈(OH)₆}]·8H₂O (0.55 g, 40%). M.p.300°C. χ_m=−414.2×10⁻⁶ cm³ mol⁻¹. C₆H₄₈B₁₂N₆O₃₂Zn₂. Anal. Calc.: C=7.4%, H=5.0%, N=8.6%. Found: C=6.9%, H=4.7%, N=8.3%. ¹H/ppm: 2.75 (m, 12H, CH₂ of en), 4.8 (s, 34 H, NH₂, H₂O, OH). ¹³C{¹H}/ppm: 39.4. ¹¹B/ppm: 16.1. IR (KBr/cm⁻¹): 3433(s), 3373(s), 3268(s), 2986(m), 1637(w), 1550(w), 1355(s), 1163(sh), 1041(s), 951(m), 900(s), 856(m), 758(w), 692(w), 646(w), 616(w), 519(w). p-XRD d-spacing/Å (% rel. int.): 9.40 (68), 8.05 (39), 7.59 (100), 6.78 (75), 4.21 (22), 3.82 (22), 3.14 (28), 2.91 (45).

X-ray crystallography

Crystallographic data for compound 1 is given in Table 1. A suitable crystal (0.10×0.09×0.04) mm³ was selected and mounted on a MITIGEN holder in oil on a Rigaku FRE+ equipped with HF Varimax confocal mirrors and an AFC12 goniometer and HG Saturn 724+ detector diffractometer. The crystal was kept at T=100(2) K during data collection. The data collection was carried out using CrystalClear [67] and cell determination and data reduction was carried out using CrysAlisPro [68]. Using Olex2 [69], the structure was solved and Superflip [70] structure solution program, using the Charge Flipping solution method. The model was refined with version 2014/7 of ShelXL [71] using Least Squares minimisation.