EXCHANGE INTERACTION IN METAL RADICAL SYSTEMS - CHLORO(MESO-TETRAPHENYLPORPHYRINATO)CHROMIUM(III)S AND (HEXAFLUOROACETYLACETONATO)MANGANESE(II) LIGATED WITH 3-(N-OXY-N-TERT- AND 4-(N-OXY-N-TERT-BUTYLAMINO)PYRIDINES

Ligands 3- and 4-(N-oxy-N-tert-butylamino)pyridines (3NOPy and 4NOPy, respectively) bearing unpaired electrons were prepared and allowed to react with chloro(meso-tetraphenyl- and meso-tetrakis(p-methoxyphenyl)-porphyrinato)chromium(III)'s {Cr(TPP)Cl and Cr(TAP)Cl, respectively} and (hexafluoroacetylacetonato)-manganese(II) {Mn(hfac)2} to give the complexes Cr(TPP)(3- and 4NOPy)Cl, Cr(TAP)(3- and 4NOPy)Cl, Mn(3- and 4NOPy)(2)(hfac)(2), and [Mn(4NOPy)(hfac)(2)](n), (n = 2 for complex a and n > 2 for b). The complex Mn-(4NOPy)(2)(hfac)(2) crystallizes in the triclinic space group P (1) over bar, with a = 11.592(2) Angstrom, b = 12.826(2) Angstrom, c = 6.5853-(2) Angstrom, alpha = 98.20(1)degrees, beta = 92.28(1)degrees, gamma 64.01(1)degrees, and Z = 1, and has two 4NOPy's coordinated with the manganese ion not at the nitroxide radical but at the pyridine nitrogens in a trans configuration. The complex a, [Mn(4NOPy)(hfac)(2)](2), monoclinic space group P2(1)/n with a 9.324(3) Angstrom, b = 20.055(4) Angstrom, 13.763(3) Angstrom, beta = 100.10(2)degrees, and Z = 4, has a cyclic dimer structure in which either manganese ion is coordinated with the pyridine nitrogen of one ligand 4NOPy and the oxygen atom of the nitroxide of the other ligand in a cis configuration. EPR spectroscopy, SQUID susceptometry, and Faraday-balance magnetometry revealed that the chromium ions in Cr(TPP)(3- and 4NOPy)Cl and Cr(TAP)(3- and 4NOPy)Cl interact ferro- and antiferromagnetically with the unpaired electrons of 3- and 4NOPy to produce quintet and triplet ground states, respectively. In Mn(4NOPy)(2)(hfac)(2), the exchange coupling parameter Jlk(B) between the S = (5)/(2) manganese(II) ion and the S = (1)/(2) nitroxide radicals has been estimated to be -12.4 +/- 0.1 K by fitting a linear three-spin model to the observed mu(eff) vs T plot. In Mn(3ONOPy)(2)(hfac)(2), the mu(eff) value of 5.74 mu(B) at 290 K was rather insensitive to temperature. The manganese ions in 1:1 complex a interact with the directly attached nitroxide radical strongly in an antiferromagnetic fashion and the effective exchange coupling between the two residual spins S = (4)/(2) has been evaluated to be ferromagnetic. The mu(eff) value for the complex b surpassed a:theoreticaI value of 8.94 mu(B) for the dimer with S = (4)/(2) at 16 K, suggesting a linear chain structure for complex b.