Two New Mononuclear Manganese ( III ) Salen Complexes

Two new Mn(III)-salen complexes with 4-aminopyridine (4-apy) have been prepared and structurally characterized. [Mn(ac)(4-apy)(sal)], 1, exhibits mononuclear molecular structure with an octahedral environment around the Mn center. The salen ligand occupies all four basal positions, while the axial sites are occupied by endocyclic nitrogen atom of the 4-apy ligand and acetato oxygen atom. [Mn(4-apy)2(sal)]PF6, 2, is an ionic compound in which the coordination cations, [Mn(4-apy)2(sal)] , exhibit octahedral geometry. Both axial sites are occupied by endocyclic nitrogen atoms of the 4-aminopyridine ligands. Cations are connected into zig-zag chains via H-bonds. The positive charge is compensated by PF6 – anions located between the chains.


Introduction
6] Among them, complexes with "salen type" ligands have received much attention.Of the many manganese salen (N,N'-ethylenebis(salicylimine), sal) complexes reported as biomimetic models [Mn(sal)Cl] has been recognized as an efficient reactive oxygen species scavenger in vivo prolonging the life of C. elegans and preventing the DNA damage in human spermatozoa. 7,8][11][12] All these observations prompted us to prepare new manganese(III) complexes which besides salen ligand (sal) contain additional ligands.4-aminopyridine (4-apy) was chosen as a neutral nitrogen donor ligand and acetate (ac) as an oxygen donor ligand.Acetate has often been used to model the coordination environment at the enzyme active sites with carboxylate rich amino acids (aspartic or glutamic) participating in the coordination to the metal

1. General Remarks
Chemicals were purchased from Sigma-Aldrich and used as received.The infrared spectra were recorded with a Perkin Elmer 2000 Fourier Transform infrared spectrometer with an ATR sampling accessory.IR spectra are contained in the supplementary data file.Microanalyses were performed by the Chemistry Department service at the University of Ljubljana.

Synthesis of [ [Mn(ac)(4-apy)(sal)] ], 1
N,N'-ethylenebis(salicylimine) (0.268 g, 1.00 mmol) and manganese(II) acetate tetrahydrate (0.502 g, 2.05 mmol) in absolute ethanol (25 mL) were refluxed for 2 hours.The reaction mixture was evaporated under reduced pressure to dryness.A brown-red product was dissolved in hot water (25 mL) and the resulting solution was filtered.4-aminopyridine (0.193 g, 2.05 mmol) was added to a filtrate and the mixture was stirred for 2 hours at room temperature.Dark brown prismatic crystals of 1 were isolated from the obtained solution after slow evaporation at room temperature.Analysis calculated for C 23  N,N'-ethylenebis(salicylimine) (0.268 g, 1.00 mmol) and manganese(II) acetate tetrahydrate (0.502 g, 2.05 mmol) in absolute ethanol (25 mL) were refluxed for 2 hours.The reaction mixture was evaporated under reduced pressure to dryness.A brown-red product was dissolved in hot water (25 mL) and the solution was filtered.4-aminopyridine (0.193 g, 2.05 mmol) was added to the filtrate and the mixture was stirred for 2 hours at room temperature.To the resulting brown solution KPF 6 (660 mg, 3.59 mmol) was added and the solution was kept at 4 °C for 2 hours.A brown precipitate was filtered off and dissolved in methanol (5 mL).Dark brown prismatic crystals of 2 were isolated from the obtained solution after slow evaporation at room temperature.Analysis calculated for C 26 H 26 MnN 6 O 2 PF 6 : C, 47.72; H, 4.00; N, 12.84%; found: C, 47.90; H, 4.12; N, 12.75%.

X-ray crystallography
X-ray data collections for compounds 1 and 2 were carried out on an Agilent SuperNova diffractometer at 150 K using Cu Kα radiation (λ = 1.5484Å).CrysAlis PRO software 13 was used for the data collection, cell refinement and data reduction.The structures were solved by direct methods implemented in SIR 14 and refined on F 2 using SHELX-97 15 .Hydrogen atoms were placed in geometrically idealized positions and refined as riding on their parent atoms.All the non-hydrogen atoms of the coordination cation in compound 2 were successfully refined with anisotropic displacement parameters.The remaining considerable electron density was evidently due to the presence of a disordered PF 6 -anion.The disorder couldn't be modelled satisfactorily.Therefore, the initial data set was treated using the SQUEEZE routine 16 to remove the contribution of the disordered PF 6 -anion.A summary of the crystallographic data and structure refinements is given in Table 1.The relevant geometric parameters are listed in Table 2 and Table 3.

Results and Discussion
Reach Mn(III) chemistry with salen type ligands results in a large number of complexes with a high level of structural diversity.The Cambridge Structural Database 17 contains 17 examples of structures with salen or its derivative and acetate being coordinated to a manganese center.3][24][25][26] Acetate acts as a bridging ligand in four of them, while in one of the dimers phenolate oxygen of a substituted salen ligand serves as bridge between the two Mn(III) centers and the acetate is coordinated as a terminal ligand.Seven complexes possess mononuclear structure.8][29] Acetate is coordinated as a monodentate ligand and occupies one axial site.The re-maining site is occupied by a H 2 O molecule in all six complexes.One example is reported with square pyramidal coordination around a manganese center. 30Its structure is similar to the octahedral complexes with a lack of an additional ligand on the axial site.A search through CSD revealed no examples of a mononuclear Mn(III)-salenacetate complex with an octahedral geometry in which the second axial site would be occupied by a neutral nitrogen donor ligand as observed in the case of [Mn(ac)(4-apy) (sal)], 1, reported herein.
Octahedral Mn(III) species with a [Mn(sal)N 2 ] + cation, where N 2 denotes two neutral nitrogen donor ligands at the axial sites, are less frequent.There is only one example with such stoichiometry reported so far for the compound with imidazole as a neutral N-donor ligand. 31he positive charge is compensated with the PF 6 -anion, similar to the ionic compound with 4-aminopyridine [Mn(4-apy) 2 (sal)]PF 6 , 2, presented here.

Crystal structure of [ [Mn(ac)(4-apy)(sal)] ], 1
Compound 1 crystallizes in the monoclinic space group P2 1 /c with one Mn(ac)(4-apy)(sal) molecule in the asymmetric unit (Figure 1).The manganese ion is six coordinated and possesses MnN 3 O 3 distorted octahedral geometry.The equatorial positions are occupied by salen donor atoms (N 2 O 2 ), while the axial sites are occupied by endocyclic N-atom of the 4-apy ligand and one of acetate O-atoms.By inspection of Table 2 it can be seen that the equatorial metal-ligand bond lengths are similar and span the range of 1.8899(13)-1.9990(14) Å, while the axial bond lengths to 4-apy and acetate moieties are longer: 2.3290(15) and 2.1854( 14) Å, respectively.The significant difference in axial bond lengths should be pointed out.Particularly, the Mn1-N11 distance is rather long and is considered to be one of the longest Mn-N distances observed in Mn(III) compounds.The amino group on the para position to the coordinated pyridine N-atom is involved in hydrogen bond with the non-coordinated oxygen atom of the acetate ligand from the adjacent molecule.The result is the formation of linear chains (Figure 2) propagated along the ac diagonal.The arrangement of four symmetry related chains in the space is depicted in Figure 3. showing 30% probability ellipsoids and the labeling scheme for non-hydrogen atoms.Hydrogen atoms are omitted for clarity.5).The packing diagram (Figure 6) shows the arrangement of the chains resulting in the formation of channels in which the disordered PF 6 -anions are situated.Symmetry codes: i) x + 1, -y + 0.5, z + 0.5; ii) -x + 1, y -0.5, -z + 0.5   are listed in Table 3. Pyridine rings of the 4-aminopyridine ligands are twisted with respect to each other.The angle between the planes of the two crystallographically different 4-apy ligands is 60.00(7)°.In addition, the pyridine ring N11-C16 is tilted towards the equatorial plane of the salen ligand by 64.15(5)°, while the pyridine ring N21-C26 is almost perpendicular to the salen plane (88.78(5)°).The hydrogen bonding network cannot be described properly due to the problems with the exact location of the disordered PF 6 -anions.Both amino groups are able to participate in the formation of H-bonded structure and are probably involved as donors to fluorine atoms of the PF 6 -anions.However, we can reliably report only the geometry of one N-H•••O type contact between the amino group and the oxygen atom of the salen ligand from the neighboring coordination species (Table 4).The result is the formation of zig-zag chains of coordination

Conclusions
In summary, two new mononuclear manganese(III) coordination species have been prepared and structurally characterized.Their structures contribute to the richness of Mn(III) chemistry and indicate possible catalytic activity in dismutase reactions.A study of their SOD mimic activity is in progress.

Acknowledgements
The financial support of the Slovenian Research Agency through the grants P1-0175 and P1-0134 is grate-fully acknowledged.The EN-FIST Centre of Excellence is acknowledged for the use of the SuperNova diffractometer.

Supplementary Material
CCDC-1017716 (1) and CCDC-1017717 (2) contain the supplementary crystallographic data for this paper.The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated to this article can be found in the online version.

Figure 3 .
Figure 3. Packing view of compound 1 showing four symmetry related infinite chains colored red, blue, green and grey.

Figure 4 .
Figure 4.An ORTEP view of a cation in compound 2, [Mn(4apy) 2 (sal)]PF 6 , showing the 30% probability ellipsoids and the labeling scheme for non-hydrogen atoms.Hydrogen atoms are omitted for clarity.

Figure 5 .
Figure 5.View of a zig-zag infinite chain in compound 2.

Figure 6 .
Figure 6.A packing diagram of compound 2 viewed along a axis.Note the formation of channels which are filled by disordered PF 6 - . Herein we report the synthesis and crystal structures of two new mononuclear coordination species: an ionic compound [Mn(4-apy) 2 (sal)]PF 6 and molecular compound [Mn(ac)(4-apy)(sal)].

Table 3 .
Selected geometric parameters for compound 2.