(η5-Carboxycyclopentadienyl)(η7-cycloheptatrienyl)manganese(I) hexafluoridophosphate

The ‘tromancenium-8-carboxylic acid’ entity with a hexafluoridophosphate counter-ion represents a rare case of a cationic carboxylic acid.


Structure description
Cobaltocenium carboxylic acid hexafluoridophosphate (Vanicek et al., 2014) is a key compound for other monofunctionalized cobaltocenium salts and was synthesized starting from cobaltocenium by nucleophilic attack using (H 3 C) 3 SiC CLi, followed by hydride abstraction, silicon dissociation using NaF and oxidation to the desired carboxylic acid using KMnO 4 . As a result of the instability against nucleophiles of the parent compound tromancenium hexafluoridophosphate (Basse et al., 2021), the related title compound was synthesized by bypassing the use of carbon nucleophiles, whereby the carboxylic acid functionality was introduced as a masked methyl ester on its cymantrene precursor level. Photolysis of all three carbonyl ligands in presence of cycloheptatrienyl, followed by oxidation with tritylium led to 8-carbomethoxy tromancenium, the masked carboxylic acid (Basse et al., 2021). Approaches for hydrolysis using aqueous NaOH led data reports to complete decomposition, but interestingly the weaker base Na 2 CO 3 led to hydrolysis without decomposition of the complex.
The molecular entities of the title compound are shown in Fig. 1. Positional disorder of the cycloheptatrienyl ligand as well as of the PF 6 À counter-ion was observed. The tromancenium carboxylic acid exists as a centrosymmetric dimer linked by mutual Osp 2 Á Á ÁH-Osp 3 /Osp 3 -HÁ Á ÁOsp 2 hydrogen bonds of the carboxylic acid moiety (Table 1), with tromanceniumyl in an anti-conformation to each other. The average Mn-C Cp bond length of 2.09 Å is slightly longer than the average Mn-C Cht bond length of 2.06 Å resulting from geometric reasons. The C12-C13 bond length of 1.482 (8) Å is typical for a carbon-carbon single bond. The C13-O1 bond length of 1.205 (10) Å is shorter than the C13-O2 bond length of 1.303 (10) Å , which is coherent with the expectations.
The comparable organometallic compound cobaltocenium carboxylic acid hexafluoridophosphate (Vanicek et al., 2014) shows an average Co-C(unsubstituted Cp) bond length of 2.02 Å and an average Co-C(substituted Cp) average bond length of 2.04 Å , which are slightly shorter than the average Mn-C Cht/Cp bond lengths in the title compound. The C O bond in cobaltocenium carboxylic acid is of the same length as the C-O bond, due to disorder.
We find typical bond lengths within the carboxylic acid moiety in the tromancenium system comparable to common organic carboxylic acids, but because of the cationic charge there are two counter-ions (PF 6 À ), which fill the space within the packing of the dimers (Fig. 1). The packing along the crystallographic b axis displays alternating layers of tromancenium carboxylic acid dimers and hexafluoridophosphate counter-ions. (Fig. 2).

Synthesis and crystallization
A round-bottom flask was charged with 0.0563 g of 8-carbomethoxytromancenium hexafluoridophosphate (Basse et al., 2021) (0.1359 mmol, 1 equiv) and dissolved in 10 ml of THF/ water (1:1) before 0.266 ml of a saturated sodium carbonate solution were added. The mixture was stirred for 4 h and cooled to 273 K before 0.090 ml of an aqueous solution of HCl (37% wt ) were added. The solvents were removed on a rotary evaporator and the crude material dried in vacuo. The product was dissolved in acetonitrile and filtered through a folded paper filter. Acetonitrile was removed on a rotary evaporator and the product was dried in vacuo giving pure 8-tromancenium carboxylic acid hexafluoridophosphate in 92% yield (0.050 g, 0.1249 mmol). Single crystals were obtained by diffusion crystallization in acetonitrile out of diethyl ether at room temperature.

Figure 2
The packing along the crystallographic b axis, displaying alternating layers of tromancenium carboxylic acid dimers and hexafluoridophosphate counter-ions.

Figure 1
The molecular entities of the title compound, with displacement ellipsoids drawn at the 50% probability level. The left cation and the anion at the bottom are related to their counterparts by inversion symmetry (symmetry operation Àx + 1, Ày + 1, Àz + 1). For clarity, only one of the two positionally disordered parts of the cycloheptatrienyl rings is shown. Likewise, for the disordered PF 6 À anion, only the part with the higher occupancy is displayed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All probed crystals showed twinning by non-merohedry by rotation of 180 around the real axis [110]. The hydrogen atom attached to O2 was found from a difference-Fourier map and was refined isotropically with a distance restraint (d = 0.83 Å ). A positional disorder in a ratio of 1:1 for the carbon atoms and attached hydrogen atoms of the seven-membered ring: C1-C7: C1A-C7A was considered; the corresponding carbon atoms were refined with isotropic displacement parameters. A further positional disorder of all fluorine atoms of the PF 6 À anion was refined in ratio 45:55 for F1-F6:F1A-F6A with anisotropic displacement parameters. In an alternative model, the crystal structure was also refined in the non-centrosymmetric space group P1 with a new data set, for which TWINABS (Bruker, 2013) was used for absorption correction without merging Friedel pairs. This led to an ordered arrangement of two cycloheptatrienyl rings and two PF 6 À anions but unrealistic interactomic distances. The resulting Flack x parameter of 0.37 (8) in the P1 model and several remaining electron-density peaks between the carbon atoms of the two seven-membered rings clearly show that the disorder will be retained in the non-centrosymmetric space group. Hence, the latter was discarded and the centrosymmetric model used for final processing. Computer programs: APEX3 and SAINT (Bruker, 2013), SHELXT (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009).

data-1
IUCrData ( 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. Refinement. Refined as a 2-component twin by rotation of 180 degrees around [1-10]. Hydrogen at O2 was found and refined isotropically with bond restraint (d = 83pm). A positional disorder in ratio of 1:1 for the carbon atoms of the 7mebered ring: C1-C7 : C1A-C7A were refined with isotropic displacement parameters for the carbon atoms. A further positional disorder of all flourine atoms of the PF6-anion was refined in ratio 45:55 F1-6:F1A-6A) with anisotropic displacement parameters. The structure was also refined in the non-centrosymmetric space group P1 with a new data set, using for absorption correction TWINABS without merging Friedel pairs. This led for the ordered structure to a Flack x parameter of 0.37 (8) and several rest-electron density peaks between the carbon atoms of the two 7-membered rings, clearly showing that the disorder will be retained in the non-centrosymmetric space group.