A Mononuclear Mn(II) Pseudoclathrochelate Complex Studied by Multi

Letter pubs.acs.org/JPCL

A Mononuclear Mn(II) Pseudoclathrochelate Complex Studied by Multi-Frequency Electron-Paramagnetic-Resonance Spectroscopy Mykhailo Azarkh,*,† Larysa V. Penkova,‡ Svitlana V. Kats,‡ Oleg A. Varzatskii,§ Yan Z. Voloshin,∥ and Edgar J. J. Groenen*,† †

Huygens-Kamerlingh Onnes Laboratory, Department of Physics, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands ‡ Department of Chemistry, Kyiv National Taras Shevchenko University, 01601 Kyiv, Ukraine § Vernadskii Institute of General and Inorganic Chemistry of the National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine ∥ Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, 119991 Moscow, Russian Federation S Supporting Information *

ABSTRACT: Knowledge of the correlation between structural and spectroscopic properties of transition-metal complexes is essential to deepen the understanding of their role in catalysis, molecular magnetism, and biological inorganic chemistry. It provides topological and, sometimes, functional insight with respect to the active site properties of metalloproteins. The electronic structure of a high-spin mononuclear Mn(II) pseudoclathrochelate complex has been investigated by electron-paramagnetic-resonance (EPR) spectroscopy at 9.5 and 275.7 GHz. A substantial, virtually axial zero-field splitting with D = −9.7 GHz (−0.32 cm−1) is found, which is the largest one reported to date for a Mn(II) complex with six nitrogen atoms in the first coordination sphere. SECTION: Spectroscopy, Photochemistry, and Excited States

C

where g-, A-, and D-tensors represent the interactions. The tensor D is taken traceless (ΣDi = 0), and its principal values can be represented by two parameters D and E:

lathrochelates are prominent representatives of the family of complexes carrying encapsulated metal ions, i.e., cage complexes.1 Currently, these systems attract particular interest as molecular scaffolds for medicinal diagnostics and radiotherapy as well as MRI shift reagents.2,3 Most of the clathrochelates are formed by polyazomethine caging ligands, which determine the geometry of the metal coordination by means of their high chemical stability and mechanical rigidity.4 A coordination geometry corresponds to a particular electronic structure and splitting of the energy levels of the metal ion. Consequently, specific magnetic properties can be realized in such coordination compounds with high-spin metal ions (MnII, MnIII, FeII, FeIII, CoII, NiII).5,6 Despite the interest in the magnetic properties of clathrochelate complexes, no detailed investigation of their electronic structure has been performed so far. We focus on the mononuclear Mn(II) pseudoclathrochelate complex with the pyrazoloximate (PzOx) ligand [Mn(PzOx)3(BC6H5)]Cl, designated as 1. We report on continuous wave (cw) EPR studies at X-band (9.5 GHz) and at J-band (275.7 GHz). The EPR spectra of the high-spin Mn(II) ion (S = 5/2, I = 5/ 2) can be well described by the spin Hamiltonian that comprises an electron Zeeman (EZ), a nuclear hyperfine (HF), and a zero-field splitting (ZFS) term: / = /EZ + /HF + /ZFS = μB SgB + IAS + SDS © 2014 American Chemical Society

D=

3 Dz , 2

E=

1 (Dx − Dy) 2

(2)

The ratio λ = E/D describes the rhombicity of the D-tensor. The ground-state term of the free Mn(II) ion is spherically symmetric 6S, which entails no orbital angular momentum. Consequently, the first-order contributions from the spin−orbit coupling to the EZ interaction and the anisotropy of the HF interaction are expected to be small for the Mn(II) ion in a complex. The second-order contributions from the spin−orbit coupling give rise to the ZFS.7 For Mn(II) complexes, the magnitude of the ZFS depends on the coordination topology and nature of the ligands and commonly does not exceed 30 GHz (1 cm−1).8−12 The only exception from this range (43.8 GHz) has been reported by Pichon et al.13 For values of the ZFS of the order of the microwave quantum (9.5 GHz at Xband), the EPR spectrum is complicated and often not easy to interpret. Received: January 15, 2014 Accepted: February 17, 2014 Published: February 19, 2014

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According to Dowsing et al., the X-band EPR spectra allow one to draw qualitative conclusions on the relationship between stereochemistry of Mn(II) complexes with a tetrahedral or (tetragonally distorted) octahedral coordination and the corresponding magnitudes of D.14 So far, EPR data for trigonally prismatic Mn(II) complexes are limited to one report, and discussions of the ZFS parameters of such complexes are based on estimated values as yet.15 Precise determination of the spin Hamiltonian parameters can be achieved by high-frequency/high-field EPR.12,16 The EPR spectrum is considerably simplified at high magnetic fields, if the corresponding microwave frequency is large enough to reach |D| ≪ hν (high-field limit). The microwave quantum at Jband (275 GHz) is appreciably larger than characteristic values of the ZFS in Mn(II) complexes and the high-field limit can be reached. In this case, the ZFS in eq 1 can be treated as a perturbation, and the spin Hamiltonian parameters can be estimated directly from the experimental spectrum. The synthesis and structure of 1 have been reported elsewhere.17 Briefly, the ‘cage’ is formed by three pyrazoloximate moieties covalently bound to a boron atom (Figure 1a). At the opposite end to the boron atom, the ‘cage’ is closed by a hydrogen-bonded chloride. The Mn(II) ion is surrounded by six nitrogen atoms in a trigonally prismatic arrangement, in contrast to the commonly observed (tetragonally distorted) octahedral coordination.15 The cw X-band EPR spectrum of a 1 mM solution of 1 in CHCl3 features the major peak at g = 4.74 (143 mT) and several minor peaks at g ≈ 2 and lower (Figure 1b). Such a frozen-solution spectrum does not show manganese hyperfine structure. This structure becomes resolved by diamagnetic dilution of the complex in a suitable host.18,19 The X-band spectrum of a powder sample containing 1% (mol) of 1 in the iso-structural Zn(II) host, denoted as 1@[Zn], is shown in Figure 2. The six hyperfine lines (I = 5/2) are resolved on top of both dominant and minor signals. The magnitude of the hyperfine splitting was determined from the spectrum, Aiso = 239 MHz, and falls within the range of reported values for Mn(II)N6-systems, 210 MHz < |A| < 240 MHz.11 The position of the major peak in the X-band spectrum, being at a value of g much higher than 2, indicates that the magnitude of the ZFS is significant as compared to the X-band microwave quantum (9.5 GHz). The condition of the high-field limit is not fulfilled at the X-band. The J-band EPR spectra have been recorded on a home-built spectrometer with a cavity specially designed for cw measurements.20,21 The spectrum of 1@[Zn] at 10 K shows a strong signal in the g = 2 region and less intense signals at lower and

Figure 2. Experimental (black) cw X-band EPR spectrum of the diamagnetically diluted 1@[Zn] with the corresponding simulation (red), T = 40 K. Simulation parameters are given in the text.

higher magnetic fields, which extend over 3 T (Figure 3a). The hyperfine structure is clearly seen for all peaks and is best resolved on top of the central one (Figure 3b). In the analysis of the spectrum we assume that the principal axes of D and g are collinear. This assumption is justified because g is expected to be close to isotropic. In this case, the field separation between transitions for each principal direction (i = x, y, z) is determined by the magnitude of the corresponding principal value of the D-tensor:22 ΔBi [T] =

3hDi[Hz] gi μ B

(3)

where h is Planck’s constant and μB is the Bohr magneton.

Figure 3. Experimental (black) and simulated (red) cw J-band EPR spectra of the diamagnetically diluted 1@[Zn], T = 10 K. Simulation parameters are given in the text. (a) Entire field range. Roman numerals mark transitions arising from different principal directions of D and g. (b) Detail of the spectrum in the g = 2 region. Stars mark impurity signals from rhombic Mn(II) of unknown origin (see Supporting Information).

Figure 1. (a) Molecular structure of 1. (b) cw X-band EPR spectrum of a 1 mM solution of 1 in CHCl3, T = 40 K. 887

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The magnitude of D = −9.7 GHz exceeds the range of reported values for six-coordinated Mn(II) with N- and/or Odonor atoms, 0.3 GHz < |D| < 5.3 GHz.11 It is the largest value known for coordination to six nitrogen atoms and approaches the values known for five-coordinated Mn(II).8−11 The unusually high D-value for 1 is attributed to the trigonal deformation of the Mn(II) center in this complex. Commonly, large ZFS for Mn(II) complexes are observed with coordination number 5 and/or halide ligands.24−28 The proper simulation of the 275 GHz EPR spectra of complex 1 required a distribution of D and E parameters. For a complete description we even had to go beyond single Gaussian distributions for both parameters. Individual features of spectra at 9.5 and 275.7 GHz give a clue to using two strain distributions with significantly different widths. Broad strain is required to simulate an appropriate overall line shape with respect to experimental spectra at both frequencies (see Figure S1). At the same time, narrow distribution is necessary to reproduce sharp hyperfine lines at 200 mT and 350 mT in Xband and around g = 2 in J-band (see Figure S1). Also for the Mn(II) center of superoxide dismutase a simple Gaussian distribution was not sufficient.29 We interpret the distribution in D and E in terms of a structural variation, i.e., a slight variation of the bond distances and/or bond angles around the manganese, because it is known that subtle structural differences cause significant and detectable changes of the ZFS parameters.30 These structural differences are visible only in EPR; X-ray studies showed no structural polymorphism.17 The obtained distribution is broad with D-values between −12 and −8 GHz and a subensemble with a narrow distribution around −9.7 GHz. A theoretical analysis of the ZFS of complex 1 as a function of the metal−ligand bond lengths and angles would allow the translation of this observation into structural knowledge. With present day ab initio quantum chemical methods, such an analysis is within reach.

In the J-band spectrum of 1@[Zn], three groups of transitions arising from the three principal directions are clearly visible and marked with roman numerals (Figure 3a). The field separations between transitions within each group are 0.69, 0.33, and 0.36 T. From these values, the corresponding magnitudes of the principal values of the D-tensor were calculated using eq 3 and the principal axes were chosen according to |Dx| < |Dy| < |Dz|. Consequently, |Dx| = 3.1 GHz, | Dy| = 3.3 GHz, and |Dz| = 6.4 GHz. At 10 K, only the low-lying energy levels are populated, which allows determination of the sign of the principal values of D. The transitions arising from the z principal direction are found at low fields (i.e., g > 2) and are indicative of a negative Dz. As the D-tensor is traceless, Dx and Dy are positive, which leads to estimated values of the ZFS parameters (eq 2): D = −9.6 GHz and E = −0.1 GHz. For refinement, the EasySpin simulation software has been used, and the computed spectra were fitted to the experimental ones.23 The spin Hamiltonian parameters estimated above were taken as starting values. The spectral simulation implied numerical diagonalization of the spin Hamiltonian. In order to reproduce the lineshapes in the J-band spectrum, strain in D and E had to be taken into account.23 Modeling the strain in terms of a Gaussian distribution in D and E did not yield a fully satisfactory description of the experimental spectrum (see Figure S1). A proper description of the experimental spectra required a linear combination of two Gaussian distributions (i.e., Strain = c1*{Strain1} + c2*{Strain2}, where c1 + c2 = 1 and {Strain} refers to the fwhm of the Gaussian distribution), as shown in Figure 4. The optimum simulation of the spectra at both X- and J-band was obtained with the following set of parameters: g = [2.0003 2.0003 1.9998], Aiso = 239 MHz, D = −9.7 GHz, E = −0.42 GHz (λ = 0.0434), D-strain = 0.65*{3.2 GHz} + 0.35*{0.1 GHz}, E-strain = 0.65*{0.15 GHz} + 0.35*{0.03 GHz}. Experimental and simulated spectra are virtually identical after addition of 1% of a rhombic Mn(II) impurity (see Figure S4). The ZFS of the Mn(II) complex reflects its structure. The symmetry of the trigonally prismatic coordination of the transition metal center in 1 is significantly lower than octahedral. For the exact trigonally prismatic Mn(II) coordination the symmetry is C3v, which would correspond to an axial D-tensor (λ = 0). The observed λ-value of 0.0434 points to a slight distortion of this trigonally prismatic coordination, in agreement with the reported structure.17

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, spectral simulations with different strains (Figure S1), temperature-dependent J-band measurements (Figures S2−S4), diagrams of splitting of energy levels (Figures S5, S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +31(0)71 527 5907; Fax: +31(0)71 527 5936; E-mail: [email protected] (M.A.). *E-mail: [email protected] (E.J.J.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported with financial aid by The Netherlands Organization for Scientific Research (NWO), Department of Chemical Sciences (CW).

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REFERENCES

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Figure 4. Schematic representation of the distribution of D around −9.7 GHz: Gaussians with fwhm 3.2 GHz (red) and 0.1 GHz (black) and their linear combination, 35% of 0.1 GHz and 65% of 3.2 GHz, (blue). All distributions are normalized to their maxima. 888

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