Manganese(II) and Copper(II) Hexafluoroacetylacetonate 1:1 Complexes with 5-(4-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine: Regiochemical Parity Analysis for Exchange Behavior of Complexes between Radicals and Paramagnetic Cations Lora M. Field, † Paul M. Lahti,* ,† Fernando Palacio, ‡ and Armando Paduan-Filho § Contribution from the Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003, Instituto de Ciencia de Materiales de Arago ´ n, CSIC-UniVersidad de Zaragoza, 50009 Zaragoza, Spain, and Instituto de Fisica, UniVersidade de Sa ˜ o Paulo, Sa ˜ o Paulo, Brazil Received January 28, 2003; E-mail: lahti@chemistry.umass.edu Abstract: Mn(hfac)2 and Cu(hfac)2 form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)- pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)2)2 and (Cu[PyrimPh-NIT](hfac)2)2, 1 and 2, respectively, are cyclic M2L2 dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication (M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing model with J/k )-0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing between S ) 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected paramagnetic susceptibility (T) to an AFM spin pairing model at 200 Oe yields J/k ) (-)3.8 K, quite similar to earlier measurements at 1000 Oe yielding J/k ) (-)5.0 K. At 1.40 K, the magnetization of 2 does not approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization curve shows steps characteristic of field-induced crossover between the S ) 0 ground state and excited spin states. From the steps in the 0.55 K data, we estimate J/k ) (-)3.8-4.0 K for 2, in good agreement with the analysis of (T). Introduction One very promising strategy for the design and synthesis of molecular magnetic materials is to combine paramagnetic cations with organic open-shell molecules to make hybrid systems. Nitronylnitroxides, nitroxides, and verdazyls have all been coordinated with paramagnetic ions to make hybrid materials with varying types of magnetic behavior. 1 Various qualitative models for understanding spin density distribution in organic radicals and polyradicals have been combined with overlap models incorporating the magnetic spin-orbitals of transition metals, as part of efforts to predict the magnetic behavior of the hybrid molecular magnetic materials. Such materials com- bine the increased magnetic moment of transition metals with the structural control of organic chemistry. A particularly useful feature of this strategy is that even antiferromagnetic (AFM) exchange coupling between transition metal and organic building block can lead to a material with a net magnetic moment, so long as the transition metal has a higher spin moment than the coordinated organic fragment. In this article, we describe the crystallography and magnetic behavior of two coordination complexes formed between manganese(II) hexafluoroacetylacetonate/copper(II) hexa- fluoroacetylacetonate and the conjugated radical 5-(4-[N-tert- butyl-N-aminoxyl]phenyl)pyrimidine (PyrimPh-NIT): 1:1 cy- clic, dimeric complexes[Mn(PyrimPh-NIT)(hfac) 2 ] and [Cu- (PyrimPh-NIT)(hfac) 2 ], 1 and 2, respectively. We shall refer to these complexes as 1:1 complexes because of the ion-to-radical ratio, although they are structurally M 2 L 2 type complexes. A preliminary account of some of these results has been published elsewhere. 2 Results Figure 1 summarizes the syntheses of complexes 1 and 2. Silyl-protected hydroxylamine 3 was converted to boronic acid 4, subjected to palladium-catalyzed coupling with 5-bromopy- rimidine to give 5, and deprotected to hydroxylamine 6. The hydroxylamine was oxidized with lead dioxide to give large, † University of Massachusetts. ‡ CSIC-Universidad de Zaragoza. § Universidade de Sa ˜o Paulo. (1) For general references, see the following: (a) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (b) Iwamura, H.; Inoue, K.; Hayamizu, T. Pure Appl. Chem. 1996, 68, 243. (c) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989, 22, 392. (d) Caneschi, A.; Gatteschi, D.; Sessoli, R. In Magnetic Molecular Materials; Gatteschi, D., Kahn, O., Miller, J. S., Palacio, F., Eds.; Kluwer: Dordrecht, The Netherlands, 1991; p 215. (e) Gatteschi, D.; Rey, P. In Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999. (f) Caneschi A.; Gatteschi, D.; Rey, P. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New York, 1991; Vol 39, p 331. (2) Field, L. M.; Lahti, P. M.; Palacio, F. Chem. Commun. 2002, 636. Published on Web 07/29/2003 10110 9 J. AM. CHEM. SOC. 2003, 125, 10110-10118 10.1021/ja0343813 CCC: $25.00 © 2003 American Chemical Society