Geometrical variations of two manganese(II) complexes with closely related quinoline-based tripodal ligands

The crystal structures of two manganese(II) complexes have been determined. The manganese(II) centers of each structure are six-coordinate with a distorted octahedral geometry. Although the bis(quinolin-2-ylmethyl)ethanamine ligands differ only by a methyl group, the structure of one complex is dimeric with bridging acetate ligands and exhibits a trans coordination and coplanarity of the quinolyl moieties, while the second complex is monomeric with a cis coordination of the quinolyl groups.

BPh 4 ÁCH 3 OH, by single-crystal X-ray diffraction reveal distinct differences in the geometry of coordination of the tripodal DQEA and DQMEA ligands to Mn II ions. In the asymmetric unit, compound [1](BPh 4 ) 2 Á(CH 2 Cl 2 ) 1.45 crystallizes as a dimer in which each manganese(II) center is coordinated by the central amine nitrogen, the nitrogen atom of each quinoline group, and the methoxy-oxygen of the tetradentate DQMEA ligand, and two bridging-acetate oxygen atoms. The symmetric Mn II centers have a distorted, octahedral geometry in which the quinoline nitrogen atoms are trans to each other resulting in co-planarity of the quinoline rings. For each Mn II center, a coordinated acetate oxygen participates in C-HÁ Á ÁO hydrogen-bonding interactions with the two quinolyl moieties, further stabilizing the trans structure. Within the crystal, weakstacking interactions and intermolecular cation-anion interactions stabilize the crystal packing. In the asymmetric unit, compound [2]BPh 4 ÁCH 3 OH crystallizes as a monomer in which the manganese(II) ion is coordinated to the central nitrogen, the nitrogen atom of each quinoline group, and the alcohol oxygen of the tetradentate DQEA ligand, an oxygen atom of OAc, and the oxygen atom of a methanol ligand. The geometry of the Mn II center in [2]BPh 4 ÁCH 3 OH is also a distorted octahedron, but the quinoline nitrogen atoms are cis to each other in this structure. Hydrogen bonding between the acetate oxygen atoms and hydroxyl (O-HÁ Á ÁO) and quinolyl (C-HÁ Á ÁO and N-HÁ Á ÁO) moieties of the DQEA ligand stabilize the complex in this cis configuration. Within the crystal, dimerization of complexes occurs by the formation of a pair of intermolecular O3-H3Á Á ÁO2 hydrogen bonds between the coordinated hydroxyl oxygen of the DQEA ligand of one complex and an acetate oxygen of another. Additional hydrogen-bonding and intermolecular cation-anion interactions contribute to the crystal packing.   (Icsel et al., 2020;Prihantono et al., 2020;Liu et al., 2015;Wang et al., 2014;Zhou et al., 2011), antibacterial (Saha et al., 2020;Maurya et al., 2011, Dong et al., 2017, optoelectronic (Qin et al., 2020), catalytic (Sarma et al., 2019), and MRI enhancement (Wang et al., 2018, Boros et al., 2015 properties. Manganese(II) tends to be less toxic than other metal ions (Iranzo, 2011;Bani & Bencini, 2012), can often reversibly access the Mn III oxidation state, and exhibits luminescence in some instances (Qin et al., 2020). The ability to form stable, efficacious Mn II compounds for these applications is dependent upon the nature of the ligands employed, their coordinating atoms, and other groups that can alter the geometry, bulkiness, and/or optical properties of the compound (Signorella et al., 2018, Policar, 2016, Qin et al., 2020.

Chemical context
We have recently begun to study Mn II compounds with tetradentate, tripodal ligands (Frey, Li et al., 2018;Frey, Ramirez et al., 2018). These ligands are readily synthesized to provide a variety of N and O donors and other groups that can potentially alter the structural and/or electronic properties of the Mn II center. Quinoline groups, for example, provide bulkiness that can lead to distorted coordination geometries, potentially altering the coordination number, redox potential, substrate specificity, and/or photophysical properties of a complex. Quinoline ring systems are also the basis for a number of biologically active molecules, suggesting that their presence might lead to medicinally-relevant compounds (Kakoulidou et al., 2021). We report here the synthesis and structural characterization of [Mn(DQMEA)(-OAc) 2 -Mn(DQMEA)](BPh 4 ) 2 Á(CH 2 Cl 2 ) 1.45 , [1](BPh 4 ) 2 Á1.45CH 2 Cl 2 where DQMEA = 2-methoxy-N,N-bis(quinolin-2-ylmethyl)ethanamine, OAc = acetate, BPh 4 = tetraphenylborate and [Mn(DQEA)(OAc)(CH 3 OH)]BPh 4 ÁCH 3 OH, [2]BPh 4 Á-CH 3 OH where DQEA = 2-hydroxy-N,N-bis(quinolin-2-ylmethyl)ethanamine). These compounds are prepared in a twostep reaction (see reaction scheme) in which manganese(II) acetate is reacted with either DQMEA or DQEA in methanol, followed by anion exchange with sodium tetraphenylborate. The resulting complexes demonstrate how minor alterations in ligand structure can result in significant differences in the complex structure.
The compound [2]BPh 4 ÁCH 3 OH crystallizes in the monoclinic space group P2 1 /c. The structure of this compound consists of the [Mn(DQEA)(OAc)(CH 3 OH)] + monocation, [2], tetraphenyl borate counter-ion, and a methanol solvent molecule (Fig. 3). The Mn II ion is hexacoordinate with a distorted octahedral geometry. As with [1], the bulky quinoline groups likely prevent a seven-coordinate species from forming. The DQEA ligand is tetradentate, but the quinolyl nitrogen atoms in this structure, N2 and N3, are cis to each other, and the rings are therefore not co-planar. The central nitrogen of DQEA, N1 and the quinolyl nitrogens occupy an octahedral face, while the alcohol oxygen, O3 is trans to the quinolyl nitrogen N3. In addition to the DQEA ligand, a monodentate acetate oxygen, O1 is trans to the central nitrogen of DQEA, while a methanol oxygen, O4 occupies a position trans to the quinolyl nitrogen, N2. Like DQMEA in [1], binding constraints of the DQEA ligand in [2] result in significant distortions of the octahedral geometry of the coordination sphere. Bond angles involving the central nitrogen of DQEA and quinolyl nitrogens, N1-Mn1-N2 and N1-Mn1-N3 are 75.63 (5) and 73.81 (5) , respectively ( Table 2). The alcohol oxygen and quinolyl nitrogen that are trans to each other, form a bond angle with manganese, O3-  . The remaining trans bond angles, O1-Mn1-N1 and N2-Mn1-O4 are 175.54 (6) and 161.38 (6) , respectively.
The cis coordination of DQEA to Mn(II) in [2] may result from a hydrogen-bonding network involving the alcohol and The title compound [2](BPh 4 )ÁCH 3 OH with displacement ellipsoids drawn at the 30% probability level. (Only the major disorder components for the hydroxyethyl fragment are shown.)
Within the crystal of [2]BPh 4 ÁCH 3 OH, dimerization of complexes occurs by the formation of a pair of intermolecular O3-H3Á Á ÁO2 hydrogen bonds (Table 4) between the coordinated hydroxyl oxygen of DQEA ligand of one complex and an acetate oxygen of another (Fig. 5), forming an R 2 2 (12) ringmotif interaction. In addition, the methanol solvent molecule forms strong O-HÁ Á ÁO hydrogen bonds (Table 4) with the coordinated methanol and acetate ligands of the cationic complex, forming an R 4 4 (16) ring motif influencing the crystal packing. Weak C11-H11AÁ Á ÁCg12 (X-H, = 58 ; where Cg12 is the centroid of the C13A-C18A ring) intermolecular cation-anion interactions (Table 4) are also present and contribute additionally to the crystal packing.

Database survey
To the best of our knowledge, structures of the manganese(II) compounds described herein have not been reported previously. We have previously reported the structure of a mononuclear copper(II) complex with DQMEA (Frey, Ramirez et al., 2018). In this structure, the DQMEA ligand is tetradentate with a tris configuration of the quinoline groups as observed in [1]. A search of the Cambridge Crystallographic Database (updated in May 2021; Groom et al., 2016) revealed a related manganese(II) complex with a pentadentate, tripodal ligand containing two methyl quinolyl groups and an imine thiolate group (Coggins & Kovacs, 2011). This ligand binds the Mn II ion in a trigonal-bipyramidal geometry with the quinoline rings cis to each other in the equatorial plane, similar to [2].

Synthesis and crystallization
All chemicals were obtained from commercial sources and used without further purification. The water used was deion-Jerry P. Jasinski tribute Acta Cryst.     (Table 4) are shown as dashed lines. Solvate molecules were omitted for clarity. Symmetry codes: (i) Àx; Ày þ 1; Àz þ 1; (ii) x þ 1; y; z.
2-Hydroxy-N,N-bis(quinolin-2-ylmethyl)ethanamine (DQEA). In a 100 ml round-bottom flask, 2.5 g (12 mmol) of 2-chlormethylquinoline hydrochloride was dissolved in 10 ml of H 2 O and cooled to 273 K in an ice bath. A solution of 0.95 g (24 mmol) of NaOH in 10 ml of H 2 O was added dropwise with stirring. Following this, a solution of 0.36 g (6.0 mmol) of ethanolamine in 10 ml of CH 2 Cl 2 was added. The reaction mixture was then removed from the ice bath, and brought to reflux for 7 days. The mixture was then cooled to room temperature, and the CH 2 Cl 2 layer was separated, washed twice with brine, and dried over anhydrous sodium sulfate. The solution was then filtered, and the filtrate was chromatographed on alumina (chromatographic grade, 80-200 mesh) eluting with 100:1 CH 2 Cl 2 /methanol. Fractions were collected that produced a single spot by TLC on alumina plates (eluting with 100:1, CH 2 Cl 2 /methanol) with an R F value of 0.33. Rotary evaporation of these fractions gave 0.70 g (20%) of a lightyellow solid. 1 H NMR (CDCl 3 , 400 MHz) 3.02 (t, 2H), 3.54 (t, 2H), 4.17 (s, 4H), 7.51 (m, 4H), 7.74 (m, 4H), 8.07 (m, 4H).
[Mn(DQMEA)(l-OAc) 2 Mn(DQMEA)](BPh 4 ) 2 . In a 100 ml round-bottom flask, 0.20 g (0.56 mmol) of DQEA was dissolved in 10 ml of methanol. To this solution, 0.14 g (0.58 mmol) of manganese(II) acetate tetrahydrate was added, and the solution was brought to reflux for 30 minutes. A solution of 0.19 g (0.56 mmol) of sodium tetraphenylborate in 10 ml of methanol was then added dropwise to the warm reaction mixture. The solution was then cooled in a refrigerator to promote crystallization of the compound. After several hours, the reaction mixture was filtered to produce light-yellow microcrystals that were washed twice with cold methanol and air dried to give 0.36 g (82%) of product. Recrystallization of 20 mg of this product in a mixture of dichloromethane and methanol gave crystals suitable for X-ray diffraction. These crystals had an IR spectrum identical to the original product. IR (ATR, cm À1 ) 2800-3200 (aromatic C-H, w), 1600 (C-O, s), 1425 (C-O, s), 731 (BPh 4 , s), 704 (BPh 4 , s).
[Mn(DQEA)(OAc)(CH 3 OH)]BPh 4 ÁCH 3 OH. In a 100 ml round-bottom flask, 0.20 g (0.58 mmol) of DQEA was dissolved in 10 ml of methanol. To this solution, 0.14 g (0.58 mmol) of manganese(II) acetate tetrahydrate was added, and the solution was brought to reflux for 30 minutes. A solution of 0.20 g (0.58 mmol) of sodium tetraphenylborate in 10 ml of methanol was then added dropwise to the warm reaction mixture. The solution was then cooled in a refrigerator to promote crystallization of the compound. After several hours, the reaction mixture was filtered to produce light yellow microcrystals that were washed twice with cold methanol and air dried to give 0.31 g (69%) of product. Recrystallization of 20 mg of this product in a mixture of dichloromethane and methanol gave crystals suitable for X-ray diffraction. These crystals had an IR spectrum identical to the original product. IR (ATR, cm À1 ) 2800-3200 (aromatic C-H, w), 1578 (C-O, s), 1427 (C-O, s), 736 (BPh 4 , s), 700 (BPh 4 , s).

Refinement
Crystal data, data collection and structure refinement details for [1](BPh 4 ) 2 Á(CH 2 Cl 2 ) 1.45 and [2]BPh 4 ÁCH 3 OH are summarized in Table 5. For [1](BPh 4 ) 2 Á(CH 2 Cl 2 ) 1.45 , all H atoms were positioned geometrically and refined using a riding model: C-H = 0.93-0.99 Å , with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). Idealized methyl groups were refined as rotating groups. A solvate methylene chloride molecule was refined as threefold disordered. All C-Cl bond distances were restrained to be the same within a standard deviation of 0.02 Å . U ij components of ADPs were restrained to be similar to each other (SIMU command, esd = 0.01 Å 2 ). Occupancies were not constrained to unity and refined to 0.401 (3), 0.234 (4) and 0.090 (4). In [2]BPh 4 ÁCH 3 OH, the ethanol group of C21, C22 and O3 was found to be disordered. Bond distances and angles of major and minor moiety were restrained to be similar to each other (SAME and SADI commands, esd = 0.02 Å ). U ij components of ADPs were restrained to be similar to each other (SIMU command, esd = 0.01 Å 2 ). The hydroxy H atoms (O3-H3, O3B-H3B, O4-H4) were located in a difference-Fourier map and refined with the distance restraint O-H = 0.8 (2) Å and with U iso (H) = 1.5U eq (O). C-bound H atoms were positioned geometrically and refined as riding: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). Idealized methyl groups were refined as rotating groups. An idealized tetrahedral OH group was also refined as a rotating group: O1S(H1S). generous with his time and willing to share his expertise and guidance. He will be missed. program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).