A hexaniobate expanded by six [Hg(cyclam)]²⁺ complexes via Hg–O bonds yields a positively charged polyoxoniobate cluster

1 Introduction

Polyoxometalates (POMs) of group 5 and group 6 cations are intensively investigated due to their pronounced structural variety, partially unusual properties and potential applications in several scientific and technological areas [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Polyoxoniobates (PONbs) are a sub-class of POMs featuring mainly the Lindqvist {Nb₆O₁₉} [12], [13], [14], [15] or Keggin clusters {XNb₁₂O₄₀} (X=Si, Ge) [16], [17], [18], [19]. Using K₈{Nb₆O₁₉}·16 H₂O and K₇{HNb₆O₁₉}·13 H₂O [12], [20] as precursors large PONb clusters were prepared, like, e.g. {Nb₁₂O₄₀} [21], [22], {Nb₁₆O₅₆} [23], {Nb₁₈O₅₄} [19], [24], {Nb₂₄O₇₂} [25], [26], {Nb₂₇O₇₆} [27], {Nb₃₁O₉₃(CO₃)} [27], {Nb₃₂O₉₆} [26], {Nb₅₂O₁₅₀} [28], {Nb₁₁₄O₃₁₆} [28] and {Nb₂₈₈O₇₆₈(OH)₄₈(CO₃)₁₂} [29]. In addition, the PONbs could be chemically modified as observed in [Cu(en)₂]₂{[Cu(bipy)][Cu(bipy)(H₂O)]Nb₆O₁₉}·9 H₂O [30] (bipy=2,2′-bipyridine), K₁₀[(Nb₆O₁₉)Cr(H₂O)₂]₂·28 H₂O [31], K₅[H₂AgNb₆O₁₉]·11 H₂O [32], K₅[Cu(H₂O)₂(cyclam)]_1.5{[Cu-(cyclam)][Cu(H₂O)(cyclam)]₂HSiNb₁₈O₅₄}(NO₃)·30 H₂O [33], {[Ni(cyclam)]₄ [Ti₂Nb₉O₂₈]}_n·~28n H₂O [34] or [Cu(cyclam)(H₂O)]{[Cu(cyclam)]₂[HTiNb₉O₂₈]}·26 H₂O [35].

To further chemically modify PONb clusters, transition metal complexes offering free binding sites are attractive components offering functionalities for bond formation to oxygen atoms of the cluster anion. Hence, we hypothesized that complexes with the cyclam ligand are suitable candidates because the axial positions are either unoccupied or often contain labile ligands, that can easily be replaced by other ligands. In a preliminary study we selected [Hg(cyclam)]²⁺ considering its large stability constant (log β=23.0) [36]. Reacting Hg(NO₃)₂·H₂O, cyclam and K₈{Nb₆O₁₉}·16 H₂O in a H₂O-DMSO mixed solvent afforded crystallization of {[Hg(cyclam)]₆Nb₆O₁₉}(NO₃)₄·14 H₂O which consists of a rarely encountered positively charged PONb cluster. Here we report the synthesis and crystal structure of this compound.

2 Results and discussion

The compound {[Hg(cyclam)]₆Nb₆O₁₉}(NO₃)₄·14 H₂O crystallizes in the triclinic space group P1̅ with all atoms except one O atom located on a general position. The unique Nb⁵⁺ cations are surrounded by six O²⁻ anions to form NbO₆ octahedra, which share common edges yielding the well-known Lindqvist cluster anion (Fig. 1). Three types of Nb–O bonds can be distinguished: terminal Nb=O_t bonds between 1.755(6) and 1.779(7) Å, Nb–μ₂-O bridging bonds from 1.953(6) to 2.039(6) Å and Nb–μ₆-O bridging bonds ranging from 2.3686(8) to 2.3835(7) Å (Table S1; Supporting information available online). The bond valence sum analysis confirms an oxidation state 5+ with values between 5.01 and 5.09 (average: 5.06) for the niobium atom (Table S2). The three crystallographically independent Hg²⁺ cations are pentacoordinated by four N atoms of the cyclam ligand and one μ₂-O atom of the heptaniobate anion (Fig. 2). The resulting polyhedra are severely distorted rectangular pyramids with the Hg²⁺ cations residing above the plane formed by the four N atoms of the cyclam ligand. This distortion is not unexpected, since the optimal ionic radius to coordinate a cation in a square-planar fashion for this ligand lies between 0.65 and 0.7 Å, which is exceeded by Hg²⁺ (rHg2+ for CN=4: 1.10 Å) [37], [38].

Fig. 1:

View of the crystal structure of the cluster {[Hg(cyclam)]₆Nb₆O₁₉}⁴⁺. H atoms are not shown for clarity.

Fig. 2:

Coordination environments of the niobium and mercury atoms; the Hg–O bond lengths of the peripheral complex units are given; hydrogen atoms are omitted for clarity and only selected atoms are labelled.

The Hg–N bonds range from 2.294(8) to 2.391(9) Å with corresponding N–Hg–N cis-angles between 77.2(3) and 95.3(3)° and N–Hg–N trans-angles ranging from 126.8(3) to 155.5(3)° (Table S3) matching the values for other [Hg(cyclam)]²⁺ complexes [38], [39]. The Hg–O bond lengths are between 2.235(6) and 2.259(6) Å (Table S3). The corresponding N–Hg–O angles are between 98.6(3) and 131.4(3)° (Table S3).

Using the minimal bonding ellipsoid approach [40] the calculated volumes of the polyhedra are 52.07 ų (Hg1), 50.56 ų (Hg2), and 50.61 ų (Hg3). The shape parameter S indicating deviations from a sphere are 0.006 (Hg1), 0.078 (Hg2), and 0.027 (Hg3). According to these values all polyhedra are axially stretched, which is most pronounced for the polyhedron containing Hg2. The {Nb₆O₁₉}⁸⁻ anion is expanded by the six Hg²⁺ centred complexes yielding a {[Hg(cyclam)]₆Nb₆O₁₉}⁴⁺ moiety (Figs. 1 and 2). The positive charge is compensated by NO₃⁻ anions. The {[Hg(cyclam)]₆Nb₆O₁₉}⁴⁺ molecules are arranged in a layer like fashion with the stacking sequence ···ABAB··· (Fig. 3, top).

Fig. 3:

Top: Crystal structure of the title compound as viewed along the crystallographic c axis. Bottom: Hydrogen bonding network with labelling of selected atoms. Intermolecular N–H···O and O–H···O hydrogen bonding shown as black and intermolecular O···O distances as red dashed lines. Please note that not all O–H hydrogen atoms were located. Symmetry code for the generation of equivalent atoms: *−x, −y+1, −z+1.

The clusters are linked to the water molecules by intermolecular N–H···O hydrogen bonds between the amino hydrogen atoms of the cyclam ligands and the water oxygen atoms that act as acceptors (Fig. 3: bottom and Table S4). These H₂O molecules are further connected via additional water molecules to the NO₃⁻ anions by intermolecular N–O···H–O hydrogen bonding (Fig. 3: bottom). The O–H···O angles are close to linearity indicative for strong interactions (Table S4). For some of the water molecules not all hydrogen atoms could be located but if O···O distances below 3.04 Å are considered (Table S5), water clusters can be identified that can be described as D6 according to the nomenclature given in Refs. [41], [42], [43]. There are additional intermolecular C–H···O interactions between C–H hydrogen atoms and water molecules but also intramolecular C–H···O and C–H···N hydrogen bonds within the clusters that might stabilize the structure (Table S4).

3 Experimental section