Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy

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Letter

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates Valentin Vladimirov Novikov, Alexander A. Pavlov, Alexander S. Belov, Anna V. Vologzhanina, Anton Savitsky, and Yan Z Voloshin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz502011z • Publication Date (Web): 16 Oct 2014 Downloaded from http://pubs.acs.org on October 17, 2014

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The Journal of Physical Chemistry Letters

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates Valentin V. Novikov1*, Alexander A. Pavlov1, Alexander S. Belov1, Anna V. Vologzhanina1, Anton Savitsky2, Yan Z. Voloshin1 1

Nesmeyanov Institute of Organoelement Compounds, RAS, 119991, Moscow, Russia

2

Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany

* [email protected]

1 ACS Paragon Plus Environment


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Transition metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10-32 m3 at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way towards the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

TOC GRAPHICS

KEYWORDS Paramagnetic tags, clathrochelates, transition metal complexes, magnetic anisotropy, EPR


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Paramagnetic tags are gaining popularity as probes for NMR spectroscopy to provide valuable information on structure and dynamics of proteins and their complexes1. Their possible application includes NMR structure determination of proteins2 and their complexes3 in solution, solid state NMR crystallography4 and characterization of otherwise inaccessible low-populated conformational states5. Lanthanide ion complexes6 account for most of the paramagnetic shift tags available to date owing to their unprecedentedly large magnetic anisotropy, reasonably high chemical stability and tunable (by the choice of a specific lanthanide ion) paramagnetic behavior. Magnetic properties of the central ion govern the changes in chemical shifts, relaxation rates and weak alignment of a biomolecule in a magnetic field; all of those resulting in restrains useful for structure calculations7. Their further advancement is, however, complicated by high coordination number of lanthanide ions, large size of chelating probes, formation of several diastereomers8 or conformational isomers9 with different magnetic properties and net positive charge of probe’s moiety10. Despite 3d transition metal complexes were the first shown to have residual dipolar couplings11, they are rarely considered as shift agents, as they usually have significantly lower magnetic anisotropy resulting in smaller paramagnetic pseudocontact shifts and lower alignment ability. Additionally, these complexes are often not stable enough, owing to the oxidation of a central metal ion or its undesirable coordination, and the delocalization of unpaired electrons over the ligand leads to unwanted contact contribution to paramagnetic shifts that makes the interpretation of the experimental results even more difficult. Thus, the use of transition metal ions with few notable exceptions12-13 is mostly limited to assessing natural metal-binding sites14 or, in a chelated form, to their use as relaxation agents15. The magnetic susceptibility anisotropy of complexes is determined by the g-tensor anisotropy and (for high-spin metal ions) by the value of zero-field splitting (ZFS), which in the case of transition metal complexes strongly depends on the features of the ligands and thus is amendable by the molecular design. Indeed, the recent progress in the field of single molecular magnets 3 ACS Paragon Plus Environment


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resulted in transition-metal complexes with high magnetic anisotropy16. In particular, cobalt(II) compounds with a trigonal prismatic geometry17 were predicted to have a large ZFS18.

H O 2+

M

Cl

N

Cl

N

+ 3

Cl

R + 2 HO

B

OH

O H

CH3NO2 reflux argon

Cl

R Cl B O O O N N N M2+ N

N

N

O Cl

O B

Cl Cl

O

R 1

M2+ = Co2+

3

M2+ = Fe2+ 2+

CF3 R= CF3

2+

2

M

4

M2+ = Fe2+

5

M2+ = Co2+

= Co

R = n-C16H33 R=F

Scheme 1 The trigonal prismatic coordination, rather unusual for cobalt(II) compounds, is characteristic of cage complexes, so-called clathrochelates19. In these complexes, the paramagnetic ion is encapsulated by a three-dimensional caging ligand that shields the metal ion from any unwanted coordination20, compensates for its charge and provides a rigid macrobicyclic framework for its subsequent functionalization21. Conjugates of clathrochelates with several proteins were recently reported22-23, and it was shown that they were stable towards oxidation or hydrolysis in biological conditions24-25. Despite these favorable properties and straightforward synthesis from available and inexpensive initial reagents, the cobalt(II) clathrochelates were never considered as precursors of paramagnetic tags due to most of them being either low spin21 or undergoing temperature-induced

spin

crossover26.

We

demonstrated27,

however,

that

cobalt(II)

hexachloroclathrochelates are entirely high-spin in solution at temperatures above 200 K. Here we report paramagnetic cobalt(II) cage complexes with a trigonal prismatic geometry that demonstrate high anisotropy of magnetic susceptibility and are therefore perfectly suitable for probing both the static and dynamic molecular structures in solution. The complexes 1 and 2 as 4 ACS Paragon Plus Environment

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well as their isostructural iron(II)-containing diamagnetic analogues 3 and 4, Scheme 1, were obtained by direct template condensation of three dichloroglyoxime molecules with boronic acids on the metal ions (see SI for synthetic details). As expected, the crystalline cobalt(II) complexes undergo temperature-induced spin-transition27 (Fig. S1 in SI) but are entirely highspin in solution at temperatures above 190 K as demonstrated by the Evans method28 (the detailed description of their magnetic behavior in the solid state will be published elsewhere). Given the D3-symmetry of these complexes, the orientation of the main axis of the magnetic susceptibility tensor was chosen along the B…B axis. As the conformational dynamics of 1 is limited to the rotation around the B–C bond ensuring the axiality of the magnetic anisotropy tensor and the rotation of the CF3 group, the values of NMR paramagnetic shifts of 1H and

19

F

nuclei (Table S1) were used directly (after correction for contact contribution, see SI) to obtain the magnetic susceptibility anisotropy of ∆χax = 12.6×10-32 m3 from its molecular geometry. Note that this value is appreciably higher compared to that of hexa- and tetra-coordinated cobalt(II) complexes29 (7×10-32 and 3×10-32 m3, respectively) and approach those of lanthanide-based shift reagents (See Table S2 for comparison).

Fig. 1. 1H (a) and fragment of 1H–13C HSQC (b) NMR spectra (600 MHz, 298 K, 5 mM solution in CDCl3) of the complex 2. A sharp signal at 7.25 ppm corresponds to the residual signal of the solvent.



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The small number of nuclei in the capping fragment of 1 and the non-zero contact contribution reduce the precision of ∆χax determination. In contrast, the complex 2 demonstrates the paramagnetic shift of all the nuclei of its hexadecyl aliphatic fragment (Fig. 1) as compared to the diamagnetic compound 4. As the contact contribution to the paramagnetic shifts is negligible for the nuclei that are more than five single bonds away from the paramagnetic center1, and given the σ-character of the bonds in hexadecyl fragments, all these shifts can be considered as pseudocontact and should correlate with those calculated from molecular geometry for a given value of ∆χax (equation S1 in SI). The nuclei of the first methylene group of the hexadecyl fragment were excluded from the further analysis due to them being too close to the paramagnetic ion to neglect a possible contact contribution. The aliphatic chain of 2 and 4 is highly flexible, so the conformational mobility significantly influences the observed pseudocontact shifts (PCS)30. Indeed, the fit of paramagnetic shifts to the molecular structure of

2 resulted in a poor agreement between the experimental and calculated PCS (Fig. 2a). A standard method for calculating the library of all possible rotamers31-32 was not applicable owing to a very large number of rotable bonds and associated conformational explosion33. Therefore, a simplified model that assumes a highly limited number of conformations for each C–C bond was used (see SI for detailed description and justification of the model). Accounting for the conformational mobility of the complex 2 results in significant improvement between calculated and experimental PCS for ∆χax = 10.0×10-32 m3 (Fig. 2b) and suggests a very small influence of the apical group on the magnetic properties of the encapsulated ion.


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Fig. 2. Correlation plot of (a) 1H and (b)

13

C paramagnetic shifts of the complex 2 obtained

experimentally and calculated using static geometry of the hexadecyl fragment (■) and a dynamically averaged model (▲). The difference between the magnetic anisotropy of the complexes 1 and 2 can be also caused by the breakdown of the point-dipole approximation34 or underestimation of the contact shifts in the case of the compound 1. From the calculation, maps characterizing the probability density distribution of the aliphatic chain were built (Fig. S2). They show that the magnetic anisotropy of the cobalt(II) cage complexes is high enough to allow not only the study of a static structure but also investigation of a highly complicated dynamic behavior of aliphatic chains.



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PSC are usually not the only source of structural constraints in paramagnetic biomolecular NMR1; however, relaxation properties of the reported compounds are hard to evaluate correctly, as the separation of the contributions of individual relaxation mechanisms is extremely difficult for compounds with significant unpaired electron delocalization (the complex 1) or flexibility (the complex 2). Owing to very short electronic relaxation times of high-spin cobalt(II) ion,35 we do not expect the reported compounds to be effective for relaxation-based experiments15 and propose the use of clathrochelates as shift agents owing to the reported remarkable magnetic anisotropy. To our knowledge, the compounds 1 and 2 demonstrate the largest reported ∆χax values for cobalt(II) complexes, with the possible source of this high magnetic anisotropy being a large negative ZFS value. In general, the Van-Vleck equation36 can be employed to determine the components of the g-tensor and ZFS37 by fitting the temperature dependence of the magnetic susceptibility. In our case, however, this approach is not applicable as the low-temperature magnetic behavior of the complexes 1 and 2 is masked by spin-crossover. Unfortunately, EPR spectroscopy is also not suitable in the case of these complexes, as the signals of a high-spin cobalt(II) ion are very difficult to detect in the presence of a predominantly populated low-spin state, owing to the sign and the value of zero-field splitting energy as well as short electronic spin-lattice relaxation time. To study the EPR and SQUID properties of the high-spin cobalt(II) ion in a cage ligand, we have therefore used the isostructural complex 5 that was previously shown to be high-spin at all temperatures26. High-spin complexes with a very large negative ZFS value are usually EPR silent38 for the following reasons: (i) the inter-Kramers transitions cannot be induced by typical microwave frequencies, (ii) the transition between the ms = ±3/2 levels of the lowest Kramers doublet is formally forbidden and (iii) the ms = ±½ doublet is thermally depopulated at low temperatures required for the detection of the fast-relaxing high-spin cobalt(II) ions (Fig. S3)35. The sample of 5 was indeed EPR silent in the X-band (9.5 GHz) within a broad temperature 8 ACS Paragon Plus Environment

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range (5 – 290 K). In the Q-band (34 GHz), a very broad EPR signal assigned to the ms = ±½ transition (Fig. 2a) was detected. The magnetic field position of this signal (~700 mT at 34.1 GHz) corresponds to a system with total spin S = 3/2: it has a large negative ZFS with the perpendicular component of the g-tensor equal to about 1.9 and the parallel component being invisible owing to a very large linewidth. The temperature dependence of the EPR spectral intensity also agrees well with the large negative value of ZFS. The signals observed above 30K became more pronounced with increase in the temperature up to 100K, implying together with the decrease in the EPR intensity at higher temperatures a slow population of a higher-lying Kramers doublet followed by a gradual decrease in the population difference between its components (see Fig. S4 for theoretical temperature dependence of the spectrum’s intensity at different values of ZFS). To exclude the decrease in the intensity owing to the broadening, we simulated the experimental spectra and integrated the obtained best-fit results, including the regions outside of the experimental magnetic field range. Analysis of the obtained temperature dependence of the EPR intensity yields the estimate for the value of ZFS of D ≈ −40 cm-1. This value is in a good agreement with D ≈ −63 cm−1 obtained from the analysis of the temperature-dependent magnetic susceptibility for the solid sample 5 (Fig. S5), given the uncertainty of both methods and the relative simplicity of the model used for SQUID data interpretation (Eqs. S6–8). These D values were used as initial parameters for the analysis of the temperature-dependent paramagnetic shifts for the complexes 1 and 2 (Fig. S6) resulting in D ≈ −65 cm−1. Thus, the possibility to modify cobalt(II) clathrochelates with functionalizing groups (with linker abilities, if necessary) without sacrificing the magnetic anisotropy is demonstrated.



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Fig. 3. (a) Experimental Q-band cw EPR spectra of the polycrystalline sample of the complex 5 acquired at the temperatures indicated. (b) Temperature dependence of EPR spectral intensity (double integral of the EPR spectrum, see SI). The theoretical curve (solid line) was calculated using a ZFS value of −40 cm−1. Our results show that the molecular design of a caging ligand allows obtaining transition metal complexes with high magnetic anisotropy and chemical stability matching those of lanthanidebased paramagnetic tags. These compounds can be covalently bounded to a protein using either apical group with the tagging substituent for cysteine labeling39 or direct nucleophilic substitution of the ribbed chlorine atom by the cysteine thiol moiety in basic conditions21. Both these approaches allow for single- and two-point attachments, depending on the nature of the studied protein. A new EPR-based approach for estimating the ZFS value for an “EPR-silent” 10 ACS Paragon Plus Environment

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sample allows ascribing the origin of the high magnetic anisotropy in cage complexes to the large negative zero-field splitting energy. The large magnetic anisotropy property is also of extreme importance in the field of molecular magnetism37. Thus, the fine-tuning of the caging ligand may be the way to create new chemically stable mononuclear single-molecule magnets. Taking into account that both the successful paramagnetic shifting agents and single-molecular magnets share the same prerequisite (large magnetic anisotropy), the clathrochelates are expected to emerge as a link between these two fields. This study was financially supported by the Russian Science Foundation (grant 14-13-00724) and Max Planck Society.

Supporting Information. Supplementary methods for synthetic procedures, NMR and EPR spectroscopy, DFT calculations, SQUID magnetometry and X-ray crystallography. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interests. REFERENCES (1) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of Paramagnetic Molecules: Applications to Metallobiomolecules and Models. Elsevier: 2001. (2) Yagi, H.; Pilla, K. B.; Maleckis, A.; Graham, B.; Huber, T.; Otting, G. ThreeDimensional Protein Fold Determination from Backbone Amide Pseudocontact Shifts Generated by Lanthanide Tags at Multiple Sites. Structure 2013, 21, 883-890. (3) Hass, M. A.; Ubbink, M. Structure Determination of Protein–Protein Complexes with Long-Range Anisotropic Paramagnetic NMR Restraints. Curr. Opin. Struct. Biol. 2014, 24, 4553. (4) Luchinat, C.; Parigi, G.; Ravera, E.; Rinaldelli, M. Solid-State NMR Crystallography through Paramagnetic Restraints. J. Am. Chem. Soc. 2012, 134, 5006-5009. 11 ACS Paragon Plus Environment


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(5) Iwahara, J.; Clore, G. M. Detecting Transient Intermediates in Macromolecular Binding by Paramagnetic NMR. Nature 2006, 440, 1227-1230. (6) Liu, W.-M.; Overhand, M.; Ubbink, M. The Application of Paramagnetic Lanthanoid Ions in NMR Spectroscopy on Proteins. Coord. Chem. Rev. 2014, 273–274, 2-12. (7) Otting, G. Protein NMR Using Paramagnetic Ions. Annu. Rev. Biophys. 2010, 39, 387405. (8) Ikegami, T.; Verdier, L.; Sakhaii, P.; Grimme, S.; Pescatore, B.; Saxena, K.; Fiebig, K. M.; Griesinger, C. Novel Techniques for Weak Alignment of Proteins in Solution Using Chemical Tags Coordinating Lanthanide Ions. J. Biomol. NMR 2004, 29, 339-349. (9) Prudencio, M.; Rohovec, J.; Peters, J. A.; Tocheva, E.; Boulanger, M. J.; Murphy, M. E. P.; Hupkes, H. J.; Kosters, W.; Impagliazzo, A.; Ubbink, M. A Caged Lanthanide Complex as a Paramagnetic Shift Agent for Protein NMR. Chem. Eur. J. 2004, 10, 3252-3260. (10) Keizers, P. H. J.; Desreux, J. F.; Overhand, M.; Ubbink, M. Increased Paramagnetic Effect of a Lanthanide Protein Probe by Two-Point Attachment. J. Am. Chem. Soc. 2007, 129, 9292-9293. (11) Bothner-By, A. A.; Domaille, P. J.; Gayathri, C. Ultra-High Field NMR Spectroscopy: Observation of Proton-Proton Dipolar Coupling in Paramagnetic Bis [Tolyltris (Pyrazolyl) Borato] Cobalt (II). J. Am. Chem. Soc. 1981, 103, 5602-5603. (12) Bertini, I.; Jonsson, B.-H.; Luchinat, C.; Pierattelli, R.; Vila, A. J. Strategies of Signal Assignments in Paramagnetic Metalloproteins. An NMR Investigation of the Thiocyanate Adduct of the Cobalt (II)-Substituted Human Carbonic Anhydrase II. Journal of Magnetic Resonance, Series B 1994, 104, 230-239. (13) Banci, L.; Bertini, I.; Eltis, L. D.; Felli, I. C.; Kastrau, D. H.; Luchinat, C.; Piccioli, M.; Pierattelli, R.; Smith, M. The Three‐Dimensional Structure in Solution of the Paramagnetic High‐Potential Iron‐Sulfur Protein I from Ectothiorhodospira Halophila through Nuclear Magnetic Resonance. Eur. J. Biochem. 1994, 225, 715-725. (14) Piccioli, M.; Turano, P. Transient Iron Coordination Sites in Proteins: Exploiting the Dual Nature of Paramagnetic NMR. Coord. Chem. Rev. 2014. (15) Lauffer, R. B. Paramagnetic Metal-Complexes as Water Proton Relaxation Agents for NMR Imaging - Theory and Design. Chem. Rev. 1987, 87, 901-927. (16) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.; Long, J. R. Magnetic Blocking in a Linear Iron (I) Complex. Nature chemistry 2013, 5, 577-581. (17) Chandrasekhar, V.; Azhakar, R.; Pandian, B. M.; Boomishankar, R.; Steiner, A. A Phosphorus-Supported Multisite Coordination Ligand Containing Three Imidazolyl Arms and Its Metalation Behaviour. An Unprecedented Co-Existence of Mononuclear and Macrocyclic Dinuclear Zn(II) Complexes in the Same Unit Cell of a Crystalline Lattice. Dalton Trans. 2008, 5962-5969. (18) Gomez-Coca, S.; Cremades, E.; Aliaga-Alcalde, N.; Ruiz, E. Mononuclear SingleMolecule Magnets: Tailoring the Magnetic Anisotropy of First-Row Transition-Metal Complexes. J. Am. Chem. Soc. 2013, 135, 7010-7018. (19) Voloshin, Y. Z.; Kostromina, N. A. e.; Krämer, R. Clathrochelates: Synthesis, Structure and Properties. Elsevier: 2002. (20) Voloshin, Y. Z.; Varzatskii, O. A.; Vorontsov, I. I.; Antipin, M. Y. Tuning a Metal's Oxidation State: The Potential of Clathrochelate Systems. Angew. Chem. Int. Ed. 2005, 44, 3400-3402. (21) Voloshin, Y. Z.; Varzatskii, O. A.; Belov, A. S.; Starikova, Z. A.; Dolganov, A. V.; Novikov, V. V.; Bubnov, Y. N. Synthesis, Structural and Electrochemical Features of Alicyclic and Aromatic Alpha,Alpha '-N-2- and-S-2-Dioximate Macrobicyclic Cobalt(II,III) and Ruthenium(II) Tris-Complexes. Inorg. Chim. Acta 2011, 370, 322-332. (22) Belov, A. S.; Vologzhanina, A. V.; Novikov, V. V.; Negrutska, V. V.; Dubey, I. Y.; Mikhailova, Z. A.; Lebed, E. G.; Voloshin, Y. Z. Synthesis of the First Morpholine-Containing 12 ACS Paragon Plus Environment

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Iron(II) Clathrochelates: A New Class of Efficient Functionalized Transcription Inhibitors. Inorg. Chim. Acta 2014, 421, 300-306. (23) Novikov, V. V.; Varzatskii, O. A.; Negrutska, V. V.; Bubnov, Y. N.; Palchykovska, L. G.; Dubey, I. Y.; Voloshin, Y. Z. Size Matters, So Does Shape: Inhibition of Transcription of T7 Rna Polymerase by Iron(II) Clathrochelates. J. Inorg. Biochem. 2013, 124, 42-45. (24) Kovalska, V. B.; Losytskyy, M. Y.; Varzatskii, O. A.; Cherepanov, V. V.; Voloshin, Y. Z.; Mokhir, A. A.; Yarmoluk, S. M.; Volkov, S. V. Study of Anti-Fibrillogenic Activity of Iron(II) Clathrochelates. Biorg. Med. Chem. 2014, 22, 1883-1888. (25) Losytskyy, M. Y.; Kovalska, V. B.; Varzatskii, O. A.; Sergeev, A. M.; Yarmoluk, S. M.; Voloshin, Y. Z. Interaction of the Iron(II) Cage Complexes with Proteins: Protein Fluorescence Quenching Study. J Fluoresc 2013, 23, 889-895. (26) Voloshin, Y. Z.; Varzatskii, O. A.; Novikov, V. V.; Strizhakova, N. G.; Vorontsov, I. I.; Vologzhanina, A. V.; Lyssenko, K. A.; Romanenko, G. V.; Fedin, M. V.; Ovcharenko, V. I., et al. Tris-Dioximate Cobalt(I,II,III) Clathrochelates: Stabilization of Different Oxidation and Spin States of an Encapsulated Metal Ion by Ribbed Functionalization. Eur. J. Inorg. Chem. 2010, 5401-5415. (27) Novikov, V. V.; Ananyev, I. V.; Pavlov, A. A.; Fedin, M. V.; Lyssenko, K. A.; Voloshin, Y. Z. Spin-Crossover Anticooperativity Induced by Weak Intermolecular Interactions. J. Phys. Chem. Lett. 2014, 5, 496-500. (28) Evans, D. F. The Determination of the Paramagnetic Susceptibility of Substances in Solution by Nuclear Magnetic Resonance J. Chem. Soc. 1959, 2003-2005. (29) Bertini, I.; Luchinat, C.; Parigi, G.; Pierattelli, R. NMR Spectroscopy of Paramagnetic Metalloproteins. ChemBioChem 2005, 6, 1536-1549. (30) Shishmarev, D.; Otting, G. How Reliable Are Pseudocontact Shifts Induced in Proteins and Ligands by Mobile Paramagnetic Metal Tags? A Modelling Study. J. Biomol. NMR 2013, 56, 203-216. (31) Peterson, R. W.; Dutton, P. L.; Wand, A. J. Improved Side-Chain Prediction Accuracy Using an Ab Initio Potential Energy Function and a Very Large Rotamer Library. Protein Sci. 2004, 13, 735-751. (32) Polyhach, Y.; Bordignon, E.; Jeschke, G. Rotamer Libraries of Spin Labelled Cysteines for Protein Studies. PCCP 2011, 13, 2356-2366. (33) Patrick, G. L. An Introduction to Medicinal Chemistry. Oxford university press: 2013. (34) Wilkens, S. J.; Xia, B.; Volkman, B. F.; Weinhold, F.; Markley, J. L.; Westler, W. M. Inadequacies of the Point-Dipole Approximation for Describing Electron-Nuclear Interactions in Paramagnetic Proteins: Hybrid Density Functional Calculations and the Analysis of NMR Relaxation of High-Spin Iron(III) Rubredoxin. J. Phys. Chem. B 1998, 102, 8300-8305. (35) Pilbrow, J. Transition Ion Electron Paramagnetic Resonance. Clarendon Press Oxford: 1990. (36) Van Vleck, J. H. The Theory of Electric and Magnetic Susceptibilities. Oxford University Press London: 1965. (37) Kahn, O. Molecular Magnetism. VCH Publishers, Inc.: 1993. (38) Krzystek, J.; Ozarowski, A.; Telser, J. Multi-Frequency, High-Field EPR as a Powerful Tool to Accurately Determine Zero-Field Splitting in High-Spin Transition Metal Coordination Complexes. Coord. Chem. Rev. 2006, 250, 2308-2324. (39) Berliner, L. J. Spin Labeling I. Theory and Application. Academic Press Inc.: New York., 1976.



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H O Cl

+ 2 Cl

Cl

R

N

M2+ + 3 N

HO

B

OH

O H

CH3NO2 reflux argon

Cl

R Cl B O O O N N N M2+ N O Cl

N

N O B

Cl Cl

O

R 1

M2+ = Co2+

3

M2+ = Fe 2+

2

M2+ = Co2+

4

M2+ = Fe 2+

5

M2+ = Co2+

CF3 R= CF3 R = n-C16H33 R=F

ACS Paragon Plus Environment

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a


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EPR intensity, a.u.

a 1 25 2 3 150 K 4 20 5 132 K 6 114 K 7 15 100 K 8 95 K 9 10 86 K 10 11 78 K 12 67 K 13 14 5 57 K 15 47 K 16 38 K 17 0 18 19 20 -5 21 0 200 22 23 24 b 25 3.5 26 27 3 28 29 30 2.5 31 32 33 2 34 35 36 1.5 37 38 39 1 40 41 42 0.5 43 44 0 45 0 20 40 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 800 1000 Magnetic field, mT

1200

1400

Intensity, a.u.

400

60

80 100 120 Temperature, K

140

160

180

200


Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy

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