Crystal structure of a seven-coordinate manganese(II) complex with tris(pyridin-2-ylmethyl)amine (TMPA)

The crystal structure of [Mn(TMPA)(Ac)(CH3OH)]BPh4 (TMPA = tris(pyridin-2-ylmethyl)amine, Ac = acetate, BPh4 = tetraphenylborate) has been determined. The structure reveals a seven-coordinate MnII center with distorted, pentagonal bipyramidal geometry.

Structural analysis of (acetato-2 O,O 0 )(methanol-O)[tris(pyridin-2-ylmethyl)amine-4 N,N 0 ,N 00 ,N 000 ]manganese(II) tetraphenylborate, [Mn(C 2 H 3 O 2 )-(C 18 H 18 N 4 )(CH 3 OH)](C 24 H 20 B) or [Mn(TMPA)(Ac)(CH 3 OH)]BPh 4 [TMPA = tris(pyridin-2-ylmethyl)amine, Ac = acetate, BPh 4 = tetraphenylborate] by single-crystal X-ray diffraction reveals a complex cation with tetradentate coordination of the tripodal TMPA ligand, bidentate coordination of the Ac ligand and monodentate coordination of the methanol ligand to a single Mn II center, balanced in charge by the presence of a tetraphenylborate anion. The Mn II complex has a distorted pentagonal-bipyramidal geometry, in which the central amine nitrogen and two pyridyl N atoms of the TMPA ligand, and two oxygen atoms of the acetate ligand occupy positions in the pentagonal plane, while the third pyridyl nitrogen of TMPA and the oxygen from the methanol ligand occupy the axial positions. Within the complex, the acetate O atoms participate in weak C-HÁ Á ÁO hydrogen-bonding interactions with neighboring pyridyl moieties. In the crystal, complexes form dimers by pairs of O-HÁ Á ÁO hydrogen bonds between the coordinated methanol of one complex and an acetate oxygen of the other, and weak -stacking interactions between pyridine rings. Separate dimers then undergo additional -stacking interactions between the pyridine rings of one moiety and either the pyridine or phenyl rings of another moiety that further stabilize the crystal.

Chemical context
A variety of manganese(II/III) complexes have been studied as structural and functional mimics of superoxide dismutase (SOD) enzymes (Batinić-Haberle et al., 2010Iranzo, 2011;Bani & Bencini, 2012;Miriyala et al., 2012;Policar, 2016). The efficacy of these mimics is reliant on their stability in aqueous solution, retention of open or substitutional coordination sites on the manganese ion, and Mn III /Mn II redox potential lying in the narrow range of 0.2-0.4 V versus a normal hydrogen electrode (Iranzo, 2011;Policar, 2016). These factors are directly related to the nature of the ligands employed, their coordinating atoms, and the geometry of the coordination sphere (Policar, 2016).
One family of manganese(II) complexes that has been studied incorporates N-centered, tripodal, tetradentate ligands (Policar et al., 2001;Durot et al., 2005;Ribeiro et al., 2015). These ligands can be readily synthesized to provide a variety of N and O donors that give rise to the structural diversity of their metal complexes (Policar et al., 2001). With that in mind, we have begun to examine manganese(II) complexes with tripodal ligands containing either pyridine or quinoline groups. Herein, we report the synthesis and structural characterization of [Mn(TMPA)(Ac)(CH 3 OH)]BPh 4 [TMPA = tris(pyridin-2-ylmethyl)amine, Ac = acetate, BPh 4 = tetraphenylborate]. This compound is prepared by a two-step process (see reaction scheme) in which manganese(II) acetate is reacted with TMPA in a methanol solution, followed by anion exchange with sodium tetraphenylborate. The resulting monomeric complex exhibits notable characteristics including a high coordination number of seven, a distorted pentagonalbipyramidyl geometry, asymmetric bidentate coordination of the acetate ligand, and coordination by a methanol ligand.

Structural commentary
The title compound (Fig. 1), which consists of the [Mn(TMPA)(Ac)(CH 3 OH)] + monocation and tetraphenylborate counter-anion, crystallizes in the triclinic space group P1. The manganese(II) ion is heptacoordinate with a geometry that is best described as a distorted pentagonal bipyramid. While this is a high coordination number for a first row transition metal ion, seven-coordinate manganese(II) complexes with N-donor ligands have been described previously (Deroche et al.., 1996;Policar et al., 2001;Lessa et al., 2007;Dees et al., 2007;Wu et al., 2010;Lieb et al., 2013). The TMPA ligand is tetradentate, with its central N2 and two pyridyl nitrogen atoms (N1 and N3) in the pentagonal plane, and the third pyridyl nitrogen (N4) occupying an axial position. The remaining two positions in the pentagonal plane are completed by the bidentate coordination of the acetate ligand (O2 and O3), while the final axial position is occupied by O1 of the methanol ligand. Distortion of the pentagonal-bipyr-amidal geometry of the coordination sphere is produced by the bite angles of the TMPA and acetate chelate rings. For example, the N2-  ] of the five-membered metallacycle spanning an equatorial and axial position, is significantly reduced from 90 (Table 1). This results in a trans O1-Mn1-N4 angle of 166.95 (5) . Likewise, the O2-  ] that results from bidentate coordination of the acetate ligand is significantly reduced from the ideal 72 bond angle within the pentagonal plane. The O2-Mn1-O3 plane is also twisted outside of the the pentagonal plane by approximately 10 as a result of weak intramolecular C-HÁ Á ÁO hydrogen-bonding interactions with neighboring pyridyl rings (Table 2). What is perhaps most remarkable about the bidentate coordination of the acetate ligand is how asymmetric it is. The Mn1-O2 and Mn1-O3 bond lengths differ from each other by 0.3005 Å . This does not appear to result from steric hindrance, but may be due to an intermolecular hydrogen-bonding interaction between the O2 acetate oxygen of one complex and the hydroxyl hydrogen of the coordinated methanol of another, having the effect of lengthening the Mn1-O2 bond. The bond between the manganese(II) ion and the central TMPA nitrogen, Mn1-N2 is also considerably long at 2.4092 (13) Å . This elongation has been observed in other manganese(II) complexes with tripodal, tetradentate ligands (Deroche et al., 1996;Wu et al., 2010). The other Mn-O and Mn-N bonds fall into the range 2.2-2.3 Å , which is typical of manganese(II) complexes (Deroche et al., 1996;Policar et al., 2001;Lessa et al., 2007;Dees et al., 2007;Wu et al., 2010;Lieb et al., 2012).

Synthesis and crystallization
All chemicals were obtained from commercial sources and used without further preparation. The water used was deionized. The 1 H NMR spectrum was recorded with a JEOL ECX-300 NMR spectrometer and referenced against the 1 H peak of the chloroform solvent. IR spectra were recorded with a Perkin Elmer Spectrum 100 FT-IR. Tris(pyridin-2-ylmethyl)amine (TMPA). In a 250 mL round-bottom flask, 10 g (61 mmol) picolyl chloride hydrochloride was dissolved in 20 mL H 2 O and cooled to 273 K in an ice bath. A solution of 5.0 g (120 mmol) NaOH in 20 mL H 2 O was added dropwise under stirring. Following this, a solution of 2-methylaminopyridine (3.3 g, 31 mmol) in CH 2 Cl 2 (40 mL) was added. The reaction mixture was then removed from the ice bath, capped, and allowed to stir vigorously for five days. The CH 2 Cl 2 layer was then separated, washed twice with brine, and dried over anhydrous sodium sulfate. The solution was filtered and concentrated on a rotary evaporator producing 6.5 g of a red-brown oil that solidified upon cooling. The crude product was chromatographed on alumina (chromatographic grade, 80-200 mesh) eluting with 20:1 ethyl acetate/methanol, producing 4.9 g (55%) of a pure, golden oil that solidified upon standing. 1 H NMR (CDCl 3 , 300 MHz) 3.88 (s, 6H), 7.15 (t, 3H), 7.57-7.69 (m, 6H), 8.53 (d, 3H).
[Mn(TMPA)(Ac)(CH 3 OH)]BPh 4 . In a 100 mL roundbottom flask, 0.41 g (1.4 mmol) TMPA was dissolved in 10 mL of methanol. To this solution, 0.35 g (1.4 mmol) of manganese(II) acetate tetrahydrate was added, and the solution was brought to reflux for 20 minutes. A solution of 0.48 g (1.4 mmol) of sodium tetraphenylborate in 10 mL of methanol was then added dropwise to the warm reaction mixture. A precipitate formed during this addition. The reaction mixture was cooled to room temperature and filtered to produce tan microcrystals that were washed twice with cold methanol and air dried to give 0.75 g (74%) of product. The filtrate was then capped and placed in the refrigerator to promote further crystallization. After several days, crystals suitable for X-ray diffraction formed, which gave an IR spectrum identical to the original product. IR (ATR, cm À1 ) 3000-3053 (aromatic C-H, w), 1589 (C-O, s), 1425 (C-O, s), 731 (BPh 4 À , s), 701 (BPh 4 À , s).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydroxy H atom was located in a difference-Fourier map and refined with the distance restraint O1-H1 = 0.85 AE 0.01 and with U iso (H) = 1.2U eq (O).

Figure 2
A view along the b axis of the crystal packing of the title compound. The intramolecular O-HÁ Á ÁO and intermolecular C-HÁ Á ÁO hydrogen bonds (Table 2) are shown as dashed lines.

Funding information
Funding for this research was provided by: NSF-MRI (grant No. CHE-1039027 to Jerry P. Jasinski).
Acta Cryst. (2018). E74, 1075-1078 research communications   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.