Elsevier

Polyhedron

Volume 155, 15 November 2018, Pages 291-301
Polyhedron

Synthesis and electron paramagnetic resonance studies of seven coordinated Mn(II) complexes with tridentate N-donor ligands

This work is dedicated to our dearest friend, colleague Prof. Spyros P. Perlepes on the occasion of his 65th birthday.
https://doi.org/10.1016/j.poly.2018.08.014Get rights and content

Abstract

The reactions of Mn(NO3)2·4H2O with the heterocyclic aromatic ligands 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz), 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) and 2,6-bis(pyrazol-1-yl)pyridine (bppy) in MeOH gave the cationic complexes [Mn(L)(NO3)(MeOH)2](NO3) (L = tppz (1), tptz (3), bppy (5)) and the neutral complexes [Mn(L)(NO3)2(S)] (L = tppz, S = H2O (2); L = tptz, S = MeOH (4); L = bppy, S = MeOH (6)), all of which contain the MnII ion in the rare seven-coordinate pentagonal bipyramidal geometry (the crystal structure of 6 contains also a MnII complex showing the rare eight-coordinate triangular dodecahedron geometry). The reactions of Mn(ClO4)2·6H2O with tptz and bppy in MeCN gave [Mn(tptz)(H2O)2(MeCN)](ClO4)2 (7) and [Mn(bppy)2](ClO4)2 (8), in which the MnII ion exhibits six-coordination. Electron Paramagnetic Resonance spectroscopy at X- and Q-band on powder samples was used in order to determine the zero field splitting (ZFS) parameters for 14 and 7, 8. All complexes exhibit ZFS parameter, |D| in the range 0.08–0.11 cm−1. The experimentally determined values are in agreement with theoretical calculations which reveal the contribution of each mechanism responsible for the ZFS interaction. The present results add further evidence that for seven-coordinated MnII complexes comprising N/O ligands ZFS is comparable to this found in six-coordinated MnII complexes.

Graphical abstract

The synthesis and crystal structure of seven-, six- and eight-coordinated MnII complexes is reported. Electron Paramagnetic Resonance Spectroscopy in combination with Density Functional Theory calculations is performed in order to determine the Zero Field Splitting (ZFS) parameters. We verify that the ZFS parameters in seven- and six-coordinated complexes are comparable.

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Introduction

The coordination of Mn in its various oxidation states constitutes a critical parameter determining the properties and performance of Mn containing compounds in areas such as biological/biomimetic catalysis, biomedical applications and magnetism. In order to elucidate the coordination environment of Mn ions in cases where the structural characterization is not feasible specific spectroscopic quantities are used as probes. For instance this is the case in Mn containing enzymes in the presence or absence of substrates and inhibitors or are trapped in several steps of the enzymatic cycle. An appropriate spectroscopic technique is Electron Paramagnetic Resonance (EPR) which has been used to characterize Mn centers either in synthetic complexes or in more complex cases including biological [1] or catalytic systems [2]. In the present work we focus on MnII (S = 5/2) complexes. For ions with S > 1/2 the key spectroscopic parameter which can be determined by EPR is the zero field splitting (ZFS) term through the interaction [3]:H^ZFS=D[Sz2-S(S+1)/3]+E(Sx2-Sy2)+I·A·S+βS·g·Bwhere D and E are the axial and rhombic terms of the ZFS interaction respectively, A is the hyperfine interaction tensor (I = 5/2, 55Mn) and g the Zeeman interaction tensor.

For d5 ions such as MnII, |D| is less than 1.5 cm−1 [4]. Systematic experimental and theoretical studies of mononuclear MnII complexes have revealed that |D| depends upon the coordination number as well as the type of ligands [5]. For N/O ligands a systematic difference between six-coordinated and five-coordinated MnII ions has been established [6]. For six-coordinated ions, |D| is in the range of 0.010–0.137 cm−1 whereas for five-coordinated ions |D| > 0.25 cm−1. Therefore the two cases can be distinguished via determination of |D| by EPR spectroscopy. Less studied are the cases where the MnII ion is seven-coordinated [7], [8], [9], [10], [11]. From the few examples found in the literature [8], [9], [11], in cases with N/O ligands |D| is in the range of 0.068–0.137 cm−1. Therefore, the differentiation in D between seven- and six coordinated MnII is not pronounced as it is in the case between six- and five-coordinated MnII.

In order to explore the afore-mentioned trend, more examples needed to be studied. In the present work, we present the synthesis and structural characterization of four seven-coordinated and two six-coordinated MnII complexes by using the ligands 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz); 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz); and 2,6-bis(pyrazol-1-yl)pyridine (bppy) (scheme 1).

EPR spectroscopy at dual mode X-band and perpendicular mode Q-band with powder samples is applied in order to determine the ZFS parameters of the complexes. In order to rationalize the spectroscopic parameters theoretical calculations using Density Functional Theory approach are performed. The combined experimental and theoretical studies indicate that the ZFS parameters for the seven-coordinated complexes are within the range previously reported for other complexes with the same coordination. Therefore the values of the ZFS parameters are not ideal to distinct between six- and seven-coordination in MnII ions.

Section snippets

General and spectroscopic measurements

All manipulations were performed under aerobic conditions using materials as received (Aldrich Co). 2,6-Bis(pyrazol-1-yl)pyridine (bppy) was prepared as previously described [12]. All chemicals and solvents were of reagent grade. Elemental analysis for carbon, hydrogen, and nitrogen was performed on a Perkin Elmer 2400/II automatic analyzer. Infrared spectra were recorded as KBr pellets in the range 4000–400 cm−1 on a Bruker Equinox 55/S FT-IR spectrophotometer. EPR spectra were recorded at

Synthesis and infrared spectroscopic characterization

Compounds 16 were isolated from equimolar reactions between Mn(NO3)2·4H2O and the respective ligand in MeOH. The presence of the nitrate anions and the crystallization method are critical to the isolation of two different structural types. Compounds 1, 3 and 5 are cationic complexes with general formula [Mn(L)(NO3)(MeOH)2](NO3) (L = tppz (1), tptz (3), bppy (5)) and were isolated from closed vials (1, 3) or layering with Et2O/n-hexane (5). Compounds 2, 4 and 6 are neutral complexes with

Concluding comments

In this work we present the synthesis, crystal structure and EPR spectroscopic characterization of mononuclear clusters in a N/O coordination environment. Four of those exhibit a seven-coordination ligation of the type N3O4. These complexes are added to a rather short list with similar coordination [7], [8], [9], [10], [11]. By EPR spectroscopy performed at room temperature at X- and Q-band the ZFS parameters have been determined. It is found that the D magnitude for all of these complexes are

Acknowledgment

We acknowledge support from the E. U. COST action CM1305 (ECOST-Bio).

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