[Bis(2,6-diisopropylphenyl) phosphato-κO]pentakis(methanol-κO)manganese bis(2,6-diisopropylphenyl) phosphate methanol trisolvate

The crystal structure of the complex [Mn{OOP(O-2,6-iPr2C6H3)2}(CH3OH)5]+[OOP(O-2,6-iPr2C6H3)2]−·(CH3OH)3 exhibits O—H⋯O bonds between the cations, anions and non-coordinating methanol molecules, forming infinite one-dimensional associates. The complex demonstrates inhibition of thermal oxidation of polydimethylsiloxane.

The title compound, [Mn(C 24 H 34 O 4 P)(CH 3 OH) 5 ](C 24 H 34 O 4 P)Á3CH 3 OH, was formed in the reaction between a hydrate of a manganese(II) salt [either Mn(NO 3

Chemical context
Polydimethylsiloxane (PDMS) liquids are widely applied in many devices as shock-absorbing, hydraulic and damping liquids, as bases for greases and as heat-transfer agents for many industrial processes carried out at elevated temperatures. Various lipophilic derivatives of metals with variable valency, such as Mn, Fe, Ni, Ce, etc., are used for the inhibition of thermo-oxidative decomposition of polyorganosiloxane heat carriers (Swihart & Jones, 1985;Nielsen, 1961;Halm, 1980;Kobzova et al., 1966;Kishimoto et al., 1976;Rozanova et al., 1995;Minyaev et al., 2018a) in order to increase their operating time and temperature (usually up to ca 550 K). As manganese-based inhibitors, cymantrene and its derivatives have shown promising results (Sobolevskiy et al., 1970). However, these Mn compounds are not available on an industrial scale. Easily accessible disubstituted organophosphate ligands are usually regarded as being lipophilic. For ISSN 2056-9890 example, rare-earth complexes with such disubstituted organophosphate ligands are highly soluble in hydrocarbon media (Nifant'ev et al., 2013(Nifant'ev et al., , 2014. Therefore, the obtained manganese derivative with the organophosphate ligand might be a readily available alternative to cymantrene and to its derivatives. Herein we report on the crystal structure of the Mn organophosphate complex [Mn{OOP(O-2,6-i Pr 2 C 6 H 3 ) 2 }-(CH 3 OH) 5 ] + [OOP(O-2,6-i Pr 2 C 6 H 3 ) 2 ] À Á3CH 3 OH, which contains a lipophilic diaryl-substituted organophosphate ligand, and on its properties regarding inhibition of the thermal oxidation of polydimethylsiloxane.

Analysis of thermal decomposition inhibition properties
We tested the title Mn compound as a possible inhibitor for the thermal decomposition of the heat-transfer agent PDMS in air at a temperature of 573 K, and compared the obtained results with control experiments and with experiments, where the Ce complex [Ce{O 2 P(O-2,6-i Pr 2 C 6 H 3 ) 2 } 2 (CH 3 OH) 5 ]Á-CH 3 OH bearing the same ligand was used (Minyaev et al., 2018a). All experiments were carried out under the same conditions (Table 1).
The results indicate that the manganese derivative inhibits the thermal decomposition of the silicone heat carrier, although to a much lesser extent than the cerium derivative at the same loads (each 0.1% by mass, entries 2 and 4). Moreover, the PDMS liquid containing 0.1% of the Mn complex The structures of the [Mn{OOP(O-2,6-i Pr 2 C 6 H 3 ) 2 }(CH 3 OH) 5 ] + cation (left) and [OOP(O-2,6-i Pr 2 C 6 H 3 ) 2 ] À anion (right). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.   became solidified at the end of the experiment. However, with an increase of the manganese derivative load of up to 0.5% (entry 3), the PDMS decomposition decreases to the level displayed by the cerium additive at 0.1%. Thus, the lipophilic manganese derivative may be used as an accessible alternative to cerium and organometallic manganese derivatives.
All of these facts point not only to an approximately equal negative charge redistribution on atoms O9, O10 and O13, O14, but also to more pronounced double-bond character for the corresponding P-O bonds compared to the P-O C bonds. These results are in good agreement with data obtained for rare-earth phosphates bearing the same ligand: [Ln{O 2 P(O-2,6-i Pr 2 C 6 H 3 ) 2 } 2 Cl(CH 3 OH) 4 ]Á2CH 3 OH (Ln = Nd, Lu, Y; Minyaev et al., 2017) (Minyaev et al., 2018b).
The presence of two separate ions in the crystal lattice can be explained by the relatively large solvation energy obtained from the formation of many O-HÁ Á ÁO bonds within a onedimensional hydrogen-bond network. This might be one of the driving forces for crystal formation.  (Sathiyendiran & Murugavel, 2002), ODEWOK (Rafizadeh et al., 2007), SAMNEA/SAMNEA01 (Pothiraja et al., 2004(Pothiraja et al., , 2005, TEKQOR and TEKQUX (Dey et al., 2013) and WENSUE (Rafizadeh et al., 2006)]. All of the above are heteroleptic complexes containing the following di-substituted organophosphate ligands: PO 2 (OPh) 2 , PO 2 (OC 6 H 4 -4-NO 2 ) 2 , PO 2 (OMe) 2 , PO 2 (O t Bu) 2 and PO 2 (OCMe 2 CMe 2 O). The ligands mainly display a 2 -1 O: 1 O 0 bridging coordination mode, and occasionally a 1 O terminal mode. The Mn complexes, especially mononuclear ones, with other disubstituted organophosphate anions are yet to be synthesized. It is worth mentioning that the tile complex is mononuclear, incorporates a novel organophosphate ligand, and is the first Mn-phosphate complex with a phosphate anion separated from the Mn complex cation in the crystal lattice.

Synthesis and crystallization of the complex
A solution of Mn(NO 3 ) 2 (H 2 O) 6 (159 mg, 0.55 mmol) in 5 ml of methanol was carefully added to a solution of [Li{OOP(O-2,6-i Pr 2 C 6 H 3 ) 2 }(CH 3 OH) 3 ]ÁCH 3 OH (580 mg, 1.05 mmol) in 5 ml of methanol at room temperature. The mixture was stirred for 10 s. Crystals started to precipitate out after 20 min.. The following day, some crystals were taken from the mother liquor for X-ray studies. The remaining crystals were filtered off, washed with methanol (2 Â 10 ml) and dried briefly under dynamic vacuum [yield 485 mg (0.42 mmol, 81%) as colourless prismatic crystals. Analysis found (calculated for C 56 H 100 Mn-O 16 P 2 ): C 58.75 (58.68), H 8.72% (8.79%). The same compound was prepared in 80% yield from MnCl 2 (H 2 O) 4 under similar reaction conditions. The crystal shapes varied from needles to blocks, depending on the synthesis and crystal growth conditions. The formed high-spin Mn complex cannot be studied by NMR techniques because of its paramagnetic behaviour.

Thermal oxidation of polydimethylsiloxane
A mixture (2.000 g) of the Mn complex (either 2 mg or 10 mg) and PDMS was placed in a glass beaker. No additive was used in the control experiments. The beaker was placed into a muffle furnace with a preset temperature of 573 K. The beaker was periodically taken out from the furnace and weighed to determine the weight loss.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The positions of most hydrogen atoms were found from the difference electron-density map, but they were positioned geometrically (C-H = 0.95 Å for aromatic, 0.98 Å for methyl and 0.99 Å for methylene H atoms) and refined as riding atoms with relative isotropic displacement parameters U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) otherwise. The positions of the hydroxy H atoms were refined with restrained O-H distances of 0.85 (2) Å with U iso (H)= 1.2U eq (O). A rotating group model was applied for methyl groups. Two reflections (2 0 0 and 2 0 0) were affected by the beam stop, and were therefore omitted from the refinement. Two reflections (8 2 10 and 4 0 4) were also omitted from the final cycles of the refinement as their (I obs À I calcd )/(w) values were over 10.
One of the isopropyl groups is disordered over two sets of sites with an occupancy ratio of 0.57 (4):0.43 (4) for atoms C40A/C41A and C40B/C41B, respectively. Four HC-CH 3 distances in the disordered fragment were restrained to be equal within an estimated standard deviation of 0.01 Å . Similarity restraints for thermal displacement ellipsoids were also applied. The crystal studied was refined as an inversion twin with a domain ratio of 0.47 (3):0.53 (3). The final crystallographic model exhibits some problems, including two relatively high remaining Q peaks of residual electron density, which could not be reasonably handled, and a rather high Á max /Á min ratio.
The problems might have been caused by (1) incomplete substitution of NO 3 À in crystals initially made from Mn(NO 3 ) 2 (H 2 O) 6 , (2) some content of other metal impurities, (3) crystal decomposition during data collection, (4) twinning or (5) disorder. Several attempts to prepare crystal batches were made, starting from Mn(NO 3 ) 2 (H 2 O) 6 and from MnCl 2 (H 2 O) 4 by varying the crystal-growth conditions slightly. Several attempts to reestablish the crystal structure were made using different diffractometers and software (see Table S1 in the supporting information for details). Crystallographic models of the studied crystals demonstrated the same problems regardless of differences in the preparation and the instrument used. Modelling disorder and applying various twinning laws (using CELL_NOW) were unsuccessful. The X-ray fluorescence (XRF) analysis demonstrated the presence of only the elements P and Mn and the absence of a noticeable quantity of any other heavy element (heavier than Ne). Several C/H analyses undertaken immediately after the crystal preparation showed very similar results that were nearly identical to calculated values.  Interesting results were obtained by using the powder X-ray diffraction (pXRD) method (see the supporting information). After several days without being in the solvent, the sample became non-single-phased. Moreover, the sample demonstrated dramatic changes in its phase composition during the pXRD measurements (see Figs. S2-S5). Such a phase change might be attributed to the facile loss of non-coordinating methanol molecules.
Therefore, the inherent problems of the presented crystallographic model can only be the result of slow crystal decomposition during the X-ray measurements or/and, more likely, from some subtle unrevealed twinning. Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008), publCIF (Westrip, 2010) and Mercury (Macrae et al.,2006). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 3.11 e Å −3 Δρ min = −0.74 e Å −3 Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.47 (2) 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Refined as a 2-component inversion twin.

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