The „ metal-radical approach “

1. Eva Rentschler Universität Mainz The „metal-radical approach“ toward magnetic materials workshop synthetic strategies towards .... Kaiserslautern, 23.-25.10.2005

2. “Toward Molecular Magnets: The metal-radical approach“ Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22, 392 (1989). • The strategy is simple: • Strong direct metal-ligand magneticexchange interactions are achieved from the coordination of stable free radicals to paramagnetictransition-metal ions, • and if these interactions are extended in one, two, or three spatial directions, cooperativemagnetic behavior is obtainable in these molecule-basedsystems. Lemaire, Pure Appl. Chem., Vol. 76, No. 2, pp. 277–293, 2004.

3. “Toward Molecular Magnets: The metal-radical approach“ Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22, 392 (1989). • - Since (and prior to) 1989,literally hundreds of metal-radical complexes have been reported, including a number of magneticallyordered materials. • A wealth of knowledge about the structure and magnetic properties of coordinationcomplexes containing stable radical ligands has been unearthed, and as a result, the metal-radicalapproach is recognized as one of the more fruitful efforts toward molecular magnetic materials. Lemaire, Pure Appl. Chem., Vol. 76, No. 2, pp. 277–293, 2004.

4. The families of radicals to be discussed are limited to stable, isolable free-radical species i.e., radicals that can be prepared and storedunder ambient conditions.

5. 1901 Gomberg: triphenylmethyl radical

6. 1901 Gomberg: triphenylmethyl radical

7. phenalenyl radical

8. phenalenyl radical • 2-azaphenalenyl radical • 2,5-di- and • 2,5,8-triaza derivatives

11. stable free radicals

12. Charge Transfer Salts [FeCp*2]+.[TCNQ] -. Bis(ethylenedithio)tetrathiafulvalene [FeCp*2]+.[TCNE] -. TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNE = tetracyanoethene

14. Nitronyl Nitroxide

15. N1 – O1 1.285(1) Å N4 – O4 1.287(1) Å N – O 1.143 Å N = O 1.316 Å C5 – N 1.353(1) Å C – N 1.438 Å C = N 1.260 Å

19. O N  d d1 N O r purely organic: including metal ions:

20. g = 2.0119(1),J = -143.1(1) cm-1, TIP = 2.27 * 10-4 emu mol-1. Cu(tfac)2NITMe

21. Typical Values of the Magnetic Coupling Constant J forMetal-Nitroside Complexes metal ion type of coupling J,acm-1 copper(II)b AF  500copper(II)cF -10 to 70nickel(II) AF  500 cobalt(II) AF  300manganese(II)AF 150-300 aPositive J means antiferromagnetic coupling. The energy separation between singlet and triplet is J. b The nitroxide is anequatorial site. cThe nitroxide in an axial site. Note: H = J S1.S2

22. rel. weak interaction between the magnetic orbitals  themetal-radical overlap is small, energyseparation betweenthe two orbitals is large  molecular orbitals 1 and2mainly localized onmetal and on the radical fragment,respectively.  Since (2-2)1/2S JAF(2-2)1/2S JAFisdetermined by the variation of the squaredoverlap between themagnetic orbitals. J = 2k + 4S J  2 J  S2

23. For  = 0°,Cu-O-N angle = 180°, * anddx2-y2 orbitals , irrespective of  and . • afm-contribution = 0, a moderate ferromagnetic coupling can be developed. • (the shorter the copper-oxygen distance,the larger the coupling.) •  0  and angles becomeimportant. When  = 0, an increase in  from 0 to 90°, causesan increase of the overlap. The effect is much morepronounced at  = 90° than at smaller angles.

24. Structural and Magnetic Parameters for DiamagneticEquatorially Coordinated Copper(I1)-Nitroxide Complexes compd R    (a) Square-Planar or Square-Pyramidal Complexes Cu(hfac)2NITPh1.955 88.4 59.056.5 Cu(hfac)2TEMPO 1.920 84.7 56.2 63.7 CuCl2(NITPh)21.980 64.1 56.3 67.5 (b) Trigonal-Pyramidal Complexes Cu(hfac)2NITPh1.94880.2 59.8 41.9 Cu(tcact)2TEMPO1.942 81.5 56.5 7.0 Cu(tcact)2TEMPO 1.950 85.8 56.2 1.9 Cu(tcact)2PROXYL 1.970 79.4 47.4 75.8 Cu(tcact)2PROXYL 1.961 85.2 54.0 11.7 • For  = 0°,Cu-O-N angle = 180°, * anddx2-y2 orbitals , irrespective of  and . • afm-contribution = 0, a moderate ferromagnetic coupling can be developed. • (the shorter the copper-oxygen distance,the larger the coupling.) •  0  and angles becomeimportant. When  = 0, an increase in  from 0 to 90°, causesan increase of the overlap. The effect is much morepronounced at  = 90° than at smaller angles.

25. Geometrical, Magnetic, and Molecular OrbitalParameters for Mn(hfac)2(radical)2 Complexes r   JS Trans Adducts M(hfac)2(TEMPO)2.127(4) 38.6 12.8 25.5 1584.8 Mn(hfac)2(PROXYL)2.150 (4) 79.6 34.7 14.0 210 19.7 Mn(hfac)2(NITPh)2 2.144 (5) 77.2 49.5 29.8 1809.8 2.154 (5) 81.4 47.1 27.9 Cis Adduct Mn(hfac)2 (NITMe)2 2.122 (5) 86.6 52.0 80.7 187 13.2 2.127 (5) 83.0 48.9 7.2

26. bridging Nitronyl Nitroxide radicals

27. non-bridging: -1,3 bridging: -1,1 bridging: -1,1 and -3,3 bridging:

28. Cambridge Structural Database non-bridging: -1,3 bridging: 180 40 -1,1 bridging: -1,1 and -3,3 bridging: - 3

29. non-bridging: 180

30. -1,3 bridging: 40

31. [Cu(hfac)2,(NITEt) • vs. T follows theCurie law with C = 0.4639 S = 1/2 withg = 2.225. J1 J2 J2 J1 Cu2------R------Cu1------R------Cu2 • nitroxide occupiesan • equatorial position in the coordination environment of copper(II) •  strongly coupled  • axial position •  a weak-to-moderate  coupling.

34. Spin transitions in non-classical systems „head to tail“:

35. „Change in the Jahn-Teller axis of the Cu bipyramids“:

37. 1 D 2D / 3D ?

38. charge distribution

39. Cu2(NIT-PhCOO)4(DMSO)2

40.  = + 0.50 K  = - 0.85 K

41. NIT phenolates as ligands 2D / 3D network high spin density

42. 4-hydroxo phenolates and their metal complexes

44. magnetic dilution of a nitronyl nitroxide poly-vinylchloride matrix microcrystalline film

50. syntheses of molecular building blocks • electronic structures • magnetic dilution • sign of the magnetic interaction •  construction of polynuclear compounds