Crystal structures and hydrogen-bonding analysis of a series of solvated ammonium salts of molybdenum(II) chloride clusters

The crystal structures of four different ammonium salts of the molybdenum halide cluster anion, [Mo6Cl8Cl6]2−, are reported. They display varying degrees of hydrogen bonding between ammonium ions and the respective solvent molecules.


Chemical context
The unique photochemistry of the molybdenum and tungsten halide clusters [M 6 X 8 Y 6 ] 2À (M = Mo, W; X, Y = Cl, Br, I) has been known for over 30 years (Maverick et al., 1983) and researchers continue to explore the tunabilty of the redox potentials, crystal structures and photochemical properties of cluster-containing compounds via variation of the bridging and terminal ligands and the counter-ion (Mikhailov et al., 2016;Saito et al., 2017;Akagi et al., 2018). Metal clusters, such as molybdenum halides, consist of an inner [Mo 6 X 8 ] 4+ core surrounded by six axial ligands which are more labile than the core ligands, making the preparation of mixed-ligand cluster complexes relatively straightforward.
Charge-assisted hydrogen bonds (CAHBs) are particularly strong among hydrogen bonds (Gilli & Gilli, 2009) and can be a significant factor in the design and formation of supramolecular complexes. CAHBs have been exploited in the formation of supramolecular organic-inorganic uranyl materials (de Groot et al., 2014), noncovalent macrocycles and catenanes (Pop et al., 2016), molecular switches (Gurbanov et al., 2017), and CAHB networks (Ward, 2009). Protonated diamines are a common motif found in hydrogen-bonded materials (Brozdowska & Chojnacki, 2017;Zick & Geiger, 2018). Examination of the nature and range of hydrogen bonding for solvates can provide information about the stability and physical properties of molecular solids (Brychczynska et al., 2012).
We have prepared a series of ammonium salts of the [Mo 6 Cl 8 Cl 6 ] 2À complex anion, each containing cations 'solvated' by either dimethylformamide or acetone through strong CAHBs.

Structural commentary
The asymmetric unit of dianilinium salt (I) (Fig. 1) contains half a cluster unit, one anilinium cation, and two independent N,N-dimethylformamide (DMF) molecules. The structure with the atom-numbering scheme is shown in Fig. 2. The [Mo 6 Cl 8 Cl 6 ] 2À cluster unit resides on a crystallographic inversion center, as it does in all four structures. In compound (II), the asymmetric unit contains half a cluster unit, half a p-phenylenediammonium cation, and three independent DMF molecules. The p-phenylenediammonium cation is disordered 1706 Johnston and Agho (C 6  The structures of (I)-(IV).

Figure 2
Displacement ellipsoid plot and atom-numbering scheme for (I), with ellipsoids drawn at the 50% probability level.

Figure 3
Displacement ellipsoid plot and atom-numbering scheme for (II), with ellipsoids drawn at the 50% probability level. The minor component of the disordered p-phenylenediammonium cation is not shown for clarity. over two positions (rotation of 70.6 about the N-N axis), with a refined occupancy of 0.918 (4) for the primary orientation. The structure with the atom-numbering scheme is shown in Fig. 3.
The asymmetric unit of Schiff base salt (III) contains half a cluster unit, half a Schiff base cation, and two independent acetone molecules. The structure with the atom-numbering scheme is shown in Fig. 4. One acetone molecule is disordered over an inversion center. The Schiff base cation, presumably formed from the reaction between a p-phenylenediammonium cation and two acetone molecules, shows strong similarities to the cation found in the bismuthate structure reported by Shestimerova et al. (2018).

Figure 4
Displacement ellipsoid plot and atom-numbering scheme for (III), with ellipsoids drawn at the 50% probability level.

Figure 5
Displacement ellipsoid plot and atom-numbering scheme for (IV), with ellipsoids drawn at the 50% probability level.

Figure 6
The cationic hydrogen-bonded dimer formed by anilinium cations and DMF molecules in (I).

Hydrogen-bonding analysis
In compound (I), the anilinium cation and DMF molecules form a cyclic R 2 4 (8) hydrogen-bonded motif centered on a crystallographic inversion center, with an additional DMF forming a D(2) interaction, as illustrated in Fig. 6. Although similar to some motifs discussed by Loehlin & Okasako (2007), the hydrogen-bonding network in (I) does not represent an example of saturated hydrogen bonding, as one DMF molecule has an additional lone pair that is not involved in hydrogen bonding (Table 1). The DMF molecules in compound (II) form three unique D(2) interactions with the three N-H bonds on each end of the p-phenylenediammonium cations, as shown in Fig. 7 (Table 2). In compound (III), one acetone molecule forms a hydrogen-bonding interaction with the N-H group of the Schiff base, as shown in Fig. 8 (Table 3).
In spite of the lack of conventional hydrogen bonding in compound (IV), the methyl viologen cation forms several C-HÁ Á ÁO contacts, with the O atoms of the two independent DMF molecules forming close contacts with the H atoms of the aromatic ring (OÁ Á ÁH = 2.23 Å ) and the methyl group (OÁ Á ÁH = 2.31 Å ) ( Table 4).
Analysis of the hydrogen bonding and close contacts via Hirshfeld surfaces and fingerprint plots was conducted using CrystalExplorer (Spackman & Jayatilaka, 2009) and the results are shown in Fig. 9. Compound (II) has the strongest hydrogen-bonding interactions, with similar, but slightly weaker, interactions for (I) and (III). All four compounds show very similar H(cation)Á Á ÁCl(cluster anion) interactions.  Hydrogen bonding in the DMF-solvated p-phenylenediammonium dication in (II). The minor component of the disordered p-phenylenediammonium cation is not shown for clarity.

Figure 8
Hydrogen bonding in the acetone-solvated Schiff base dication in (III).

Figure 9
Fingerprint plots and Hirshfeld surfaces for (I)-(IV). For (II), only the major component of the disordered p-phenylenediammonium cation was included in the generation of the fingerprint plot.
The C-HÁ Á ÁO contacts in (IV), especially with the aromatic C-H group of the methyl viologen, can be clearly identified on the Hirshfeld surface.

Database survey
Interest in molybdenum(II) halide clusters and related compounds have led to numerous structural studies, with 45 entries in the Cambridge Structural Database (CSD, Version 5.40; Groom et al., 2016) (2003) has some similarities to (I). In that structure, the three N-H bonds of the anilinium cation serve as hydrogen-bond donors to one water molecule (hydrate) and two terminal Cl atoms on two discrete cluster anions. The N-HÁ Á ÁCl interactions create C 4 4 (15) chains. The water molecules create R 4 4 (14) rings involving two water molecules and two cluster units, as well as C 2 2 (8) and C 2 2 (7) chains. While DMF-solvated ammonium salts appear to be relatively uncommon, a series of molybdenum halide cluster salts have been prepared with dimethylformamide-coordinated metal cations serving as the counter-cation (Khutornoi et al., 2002;Kozhomuratova et al., 2007;Liu et al., 2006) Perruchas et al. (2002) and Yarovoi et al. (2006).
A separate search of the CSD for structures with similar hydrogen-bonded networks containing anilinium and p-phenylenediammonium cations yielded a large number of hits due to their propensity for forming significant hydrogenbonding networks. In the structure of anilinium dihydrogen phosphate (Kaman et al., 2012), each of the three independent   (Mrad et al., 2006a) and p-phenylenediammonium dihydrogen diphosphate (Mrad et al., 2006b). While less closely related to the current report, the structure of p-phenylenediammonium tetrachloridozincate(II) (Bringley & Rajeswaran, 2006) also displays alternating organic and inorganic layers and strong hydrogen bonding between the tetrachloridozinc(II) anions and the p-phenylenediammonium cations. A dimethyl sulfoxide (DMSO)-solvated p-phenylenediammonium salt of an iodidobismuthate reported by Shestimerova et al. (2018) displays strong structural similarities to (II) in the way the DMSO solvates the p-phenylenediammonium cation. Three unique DMSO molecules also form D(2) interactions with each end of the p-phenylenediammonium. One of the three DMSO molecules simultaneously coordinates to one of the Bi atoms.

Synthesis and crystallization
All reagents were used as received from the manufacturer.

Cluster synthesis, metathesis, and crystallization of (I), (II), and (IV)
The hydronium salt of the [Mo 6 Cl 8 Cl 6 ] 2À anion was prepared by the method of Hay et al. (2004) and then metathesized to the appropriate ammonium salt by combining an ethanolic solution of (H 3 O) 2 [Mo 6 Cl 8 Cl 6 ]Á6H 2 O with a slight stoichiometric excess ($2.5 times) of the respective ammonium chloride salt (anilinium chloride, p-phenylenediamine hydrochloride, and methyl viologen dichloride). The brightyellow precipitate that formed was isolated by filtration and the product was recrystallized by vapor diffusion of diethyl ether into a dimethylformamide solution of the cluster salt.

Synthesis and crystallization of Schiff base salt (III)
The cluster in compound (III) was prepared and metathesized to the diammonium salt via the same procedure as above using the p-phenylenediammonium chloride to isolate a yellow precipitate. The salt was then redissolved in acetone and allowed to evaporate. The acetone inadvertently formed a Schiff base dication in a reaction with the p-phenylenediammonium cation (Kolb & Bahadir, 1994).

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 5. All H atoms were located in a difference map. All carbon-bonded H atoms were placed in idealized positions using a riding model, with aromatic and amide C-H = 0.95 Å and methyl C-H = 0.98 Å , and with U iso (H) = 1.2U eq (C) (aromatic and amide) or U iso (H) = 1.5U eq (C) (methyl). The positions of all H atoms bonded to N atoms were refined with N-H distances restrained to 0.91 (2) (NH 3 ) or 0.88 (2) Å (Schiff base), and with U iso (H) = 1.5U eq (N).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.