Coordination compounds containing bis-dithiolene-chelated molybdenum(IV) and oxalate: comparison of terminal with bridging oxalate

[Mo(tfd)2(ox)]2− as tetra-n-butylammonium salt [co-crystal with oxalic acid and chloroform; tfd is S2C2(CF3)2 and ox is C2O4] and [(tfd)2Mo(μ-ox)Mo(tfd)2]2− as tetra-n-butylammonium salt.


Chemical context
The oxalate (ox 2À , C 2 O 4 2À ) ion is a very useful ligand in transition metal chemistry. Its usefulness stems in part from its ability to act as a chelate ligand toward a metal cation while retaining two more O atoms with the ability to donate to another metal cation. Thus, while coordination compounds containing terminal oxalate are known, oxalates can easily act as bridging ligands to allow for the synthesis of dimetallic and multimetallic molecular compounds, as well as extended ISSN 2056-9890 coordination polymers (Clemente-Leó n et al., 2011;Gruselle et al., 2006). Most of the work has involved V, Cr, Mn, Fe, Co, Ni and Cu, as well as Ru and Rh. Compounds where oxalate coordinates to molybdenum are rare, although some examples have been synthesized, mostly in the context of nitrogenase models, where oxalate was deemed a model for homocitrate (Demadis & Coucouvanis, 1995). Stimulated by our previous results on the molybdenum(IV) dithiolene fragment Mo(tfd) 2 [tfd 2À = S 2 C 2 (CF 3 ) 2 2À ] with a labile 'cap' (Harrison et al., 2007;Nguyen et al., 2010), we added oxalate to the Mo(tfd) 2 fragment, as described in the 'Synthesis and crystallization' section (x5). The [Mo(tfd) 2 (ox)] 2À (1 2À ) and [(tfd) 2 Mo(-ox)Mo-(tfd) 2 ] 2À (2 2À ) anions were indeed obtained, offering an opportunity for a structural comparison.

Structural commentary
The counter-cation for both complex molybdate anions was tetra-n-butylammonium. 1 2À was obtained as (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 , while 2 2À was obtained as (N n Bu 4 ) 2 -2. The molecular structure of 1 2À is shown in Fig. 1, where N n Bu 4 + counter-ions and co-crystallized oxalic acid, as well as chloroform solvent molecules, are not shown. Only one orientiation is shown for the disordered trifluoromethyl groups involving atoms C7 and C8. The charge on the molybdenum-containing moiety, which is identified as 1 2À , is unambiguous, due to the tetra-n-butylammonium cations. While tfd can be redox-non-innocent (Hosking et al., 2009), it is redox-innocent here. The C-C bond lengths in the two tfd ligand backbones [1.349 (8)  A view of the molecular structure of the oxalic acid (oxH 2 ) molecule in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 . Anisotropic displacement ellipsoids are shown at the 30% probability level.

Figure 1
A view of the molecular structure of 1 2À in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 . Anisotropic displacement ellipsoids are shown at the 30% probability level.

Figure 3
A view showing the 2 2À anion and the (disordered) N n Bu 4 + cation in (N n Bu 4 ) 2 -2. Anisotropic displacement ellipsoids are shown at the 30% probability level. The minor component of disorder is shown with dashed bonds. Unlabelled atoms are related by a crystallographic inversion centre (symmetry code: Àx + 2, Ày, Àz + 1). C5-C6] are a clear indication of fully reduced (dianionic) ene-dithiolate (tfd 2À ), such that the oxidation state of the metal is +IV. The Mo-S bond lengths, ranging from 2.3265 (14) to 2.3390 (15) Å , are as expected for tfd complexes of Mo IV (Nguyen et al., 2010) (6) Å (average of two values), for a difference of 0.044 (12) Å . While it may be tempting to describe the longer C-O bond as a single bond and the shorter C-O bond as a double bond, such a description would not be fully accurate since the bond-length alternation is only partial and less pronounced than for oxalic acid. The oxalic acid (oxH 2 ) molecule found in the structure of (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 is shown in Fig. 2. This oxalic acid molecule exhibits stronger bond-length alternation: a difference (for C O versus C-OH) of 0.117 (14) Å is observed. For further comparison, the structure of 2 2À , in (N n Bu 4 ) 2 -2, is valuable. Both 2 2À and the (disordered) tetra-n-butylammonium ion in the structure of (N n Bu 4 ) 2 -2 are shown in Fig. 3. For the bridging oxalate ligand in 2 2À , bond-length equalization is observed, within the error margin of one bondlength determination (0.006 Å ). The details of the oxalate substructure are shown in Fig. 4, where Fig. 4(a) highlights the bond-length changes on going from a terminal oxalate in 1 2À to a bridging oxalate in 2 2À , where parameters related to chemically equivalent bonds are averaged for clarity, and Fig. 4(b) shows all data before averaging. Fig. 4(c) shows the bond lengths in the free oxalic acid molecule in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 . Fig. 4(d) summarizes the findings: oxalic acid contains a localized -system in its carboxylic acid groups, the bridging oxalate in 2 2À contains a delocalized -system and terminal oxalate in 1 2À contains a partially localized -system. While only marginally significant (ca 1), an effect involving the C-C bonds of oxalate can be seen: upon becoming bridging, the oxalate C-C bond shortens from 1.528 (7) Å to 1.51 (1) Å (Figs. 4a and 4b). While this bond shortening may initially be surprising, it is actually theoretically expected: the -system in a localized butadiene-like system is antibonding with respect to the central C-C bond. When oxalate becomes bridging, due to delocalization in the -system, the electronic structure is no longer butadiene-like but rather resembles two allyl anions linked at the central C atom, where the -overlap at the central C atoms is not antibonding but just nonbonding. Apart from the specifics of the oxalate substructure in 2 2À , there are no dramatic changes in the coordination sphere of molybdenum on going from 1 2À to 2 2À . The points made above for 1 2À related to Mo-S bond lengths (normal) and C-C bond lengths in the tfd ligand (double bond) typically apply also to 2 2À . Also, both metal centres much more closely resemble a trigonal prismatic structure than an octahedral structure, as is expected for d 2 tris-chelates involving dithiolenes. Using the X-M-X trans criterion (Beswick et al., 2004;Nguyen et al., 2010), the geometry around molybdenum in 1 2À is 88% trigonal-prismatic. Using the same method, the geometry around molybdenum in 2 2À analyzes as 99% trigonal-prismatic.

Supramolecular features
The oxalic acid solvent molecule and the metal-coordinating oxalate ligand in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 form a hydrogenbonded network (  Bond-length changes on going from terminal to bridging oxalate, summarized (a) and in detail (b), as well as bond lengths in the oxalic acid molecule observed (c) and concluding resonance description (d).
HOOC-COOH-1 2À -, etc' are formed. The (N n Bu 4 ) 2 + cations (one of them containing disorder) are packed around the 1 2À anion, along with a CHCl 3 solvent molecule that forms part of the structure. A plot showing anisotropic displacement ellipsoids for all non-H atoms (including disordered ones) in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 is shown in Fig. 5. In contrast, there are no hydrogen bonds or notable close contacts in the structure of (N n Bu 4 ) 2 -2, which consists of a packing of 2 2À anions and N n Bu 4 + cations, both of which are shown in Fig. 3.

Database survey
Relevant coordination compounds containing dithiolenes are discussed above, where review articles for coordinating oxalate are also referenced. A search of the Cambridge Structural Database (Version 5.38, including updates up to May 2017; Groom et al., 2016) reveals no reports of molybdenum dithiolene complexes that contain oxalate.

Figure 5
Anisotropic displacement plot (30% probability level) showing all non-H atoms (including disordered ones and those of chloroform solvent) in (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 . The minor component of disorder is shown with dashed bonds. Atom N2 is disordered over two sites and the major component is obscured by the minor component.

5.2.
Synthesis of (N n Bu 4 ) 2 -1.CHCl 3 ÁoxH 2 We were unable to obtain 1 2À as the only molybdenum product produced in a reaction. Attempts always led to significant decomposition to form a blue material, almost certainly molybdenum that is reduced below the oxidation state +IV due to the reducing power of oxalate. However, 1 2À can be obtained as crystals (co-crystals with oxalic acid and chloroform) in the form of brown blocks. 2 mg of Mo(tfd) 2 (bdt) (2.9 mmol) were dissolved in a small amount of chloroform in a glass vial. In a second glass vial, 16.7 mg (29 mmol) of tetra-n-butylammonium oxalate were dissolved in the amount of chloroform needed to create a clear solution.
The contents of the two vials were mixed and 3.3 ml (14.6 mmol) of bis(trimethylsilyl)acetylene, needed to labilize the bdt fragment (Nguyen et al., 2010), were added via microlitre syringe. The initially dark (blue-green) solution became lighter, and small brown particles began to form. After 72 h, the solvent was reduced under vacuum, and orange-brown crystals grew. Blue-green needles (not of X-ray quality) of a different (likely reduced) molybdenum product were also growing. The orange-brown blocks were manually separated and chosen for X-ray crystallography.
5.3. Synthesis of (N n Bu 4 ) 2 -2 2 mg (2.8 mmol) of Mo(tfd) 2 (tht) 2 were dissolved in a minimal amount of chloroform. A solution of 16 mg (28 mmol) of tetra-n-butylammonium oxalate in 2 ml of chloroform was added. The solution turned red and, after 2 h, thin pink rectangular crystals had formed. The liquid was decanted and the crystals were washed twice with chloroform and dried under vacuum. X-ray-quality crystals were grown using vapour diffusion. In a small vial, the product was dissolved in dichloromethane. The small vial was placed uncapped into a larger vial with chloroform. The larger vial was capped, and over a period of 2 d, the dichloromethane solvent had evaporated from the small vial and dissolved in the chloroform in the larger vial, leaving pink crystals in the smaller vial. The crystals were found to be very air-sensitive, and exposure to air leads to decomposition to form a liquid that colours the surface of the crystals initially green and later blue.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In (N n Bu 4 ) 2 -1ÁCHCl 3 ÁoxH 2 , H atoms bonded to C atoms were placed in calculated positions and included in a riding-motion approximation, while H atoms bonded to O atoms were refined independently with isotropic displacement parameters. In the anion 1 2À , atoms F7/F8/F9 were included as disordered over two sets of sites, with refined occupancies of 0.58 (2) and 0.42 (2). Atoms F10/F11/F12 were included as disordered, with refined occupancies of 0.502 (10) and 0.498 (10). The C-F bond lengths and FÁ Á ÁF distances were restrained using the SADI command in SHELXL (Sheldrick, 2015) and the anisotropic displacement parameters of the disordered F atoms and bonded C atoms were restrained using the SIMU command. In addition, the N and 8 C atoms (C29-C36) of one of the independent N n Bu 4 + cations were refined as disordered over two sets of sites, with refined occupancies of 0.676 (9) and 0.324 (9). The SAME command in SHELXL was used to restrain the geometry of the disordered C-atom chains to those of the ordered N n Bu 4 + cation and the SIMU command was used to restrain anisotropic displacement parameters of the disordered atoms. In (N n Bu 4 ) 2 -2, all H atoms were placed in calculated positions and refined in a riding-motion approximation. During the refinement of the structure of (N n Bu 4 ) 2 -2, electron-density peaks were located that were believed to be highly disordered solvent molecules (crystallization solvents were CH 2 Cl 2 / CHCl 3 ). Attempts made to model the solvent molecule were not successful. The SQUEEZE (Spek, 2015) option in PLATON (Spek, 2009) indicated that there was a large solvent cavity of 156 Å . In the final cycles of refinement, this contribution of 62.6 electrons to the electron density was removed from the observed data. The density, the F(000) value, the molecular weight and the formula are given without taking into account the results obtained with the SQUEEZE option. Similar treatments of disordered solvent molecules were carried out by Stä hler et al. (2001), Cox et al. (2003), Mohamed et al. (2003) and Athimoolam et al. (2005). Also in (N n Bu 4 ) 2 -2, the whole molecule of the unique N n Bu 4 + cation was included as disordered over two sets of sites, with refined occupancies of 0.589 (6) and 0.411 (6). The same command in SHELXL was used to restrain the geometry of the minor component of disorder to that of the major component and the SIMU command was used to restrain all anisotropic diplacement parameters of the disordered atoms.  et al., 1994); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 (Farrugia, 2012) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Data collection
Nonius KappaCCD diffractometer Radiation source: fine-focus sealed tube Detector resolution: 9 pixels mm -1 φ scans and ω scans with κ offsets Absorption correction: multi-scan SORTAV (Blessing, 1995) T min = 0.759, T max = 0.869 40314 measured reflections 13841 independent reflections 9858 reflections with I > 2σ(I)  (18) 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.