Experimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex Presenting Interheteromolecular Hydrogen Bonds Yves Pontillon, ² Takeyuki Akita, ‡ Andre Grand, § Keiji Kobayashi, ‡ Eddy Lelievre-Berna, | Jacques Pe ´ caut, § Eric Ressouche, ² and Jacques Schweizer* ,² Contribution from the Commissariat a ` l’Energie Atomique, MDN/SPSMS/DRFMC, CEN-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, Department of Chemistry, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-Ku, Tokyo 153-8902, Japan, Institut Laue-LangeVin, AV. des Martyrs, BP 156, 38042 Grenoble Cedex 9, France, and Commissariat a ` l’Energie Atomique, SCIB/DRFMC, CEN-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9 ReceiVed April 1, 1999. ReVised Manuscript ReceiVed August 18, 1999 Abstract: The association of phenylboronic acid (no unpaired electron, compound 1) with the free radical phenyl nitronyl nitroxide (PNN, S ) 1 / 2 , compound 2) constitutes an interheteromolecular hydrogen bonding system displaying ferromagnetic intermolecular interactions. We have investigated its spin density distribution to visualize the pathway of these magnetic interactions. This complex crystallizes at room temperature in the monoclinic space group P2 1 /n. The unit cell includes one pair (1 + 2). The molecule (1) bridges two radicals (2) by hydrogen bonds OH‚‚‚ON: the two different hydrogen bond lengths are quite similar (1.95 and 1.92 Å). Infinite chains of this run along the b-axis. In this structure the methyl groups of the PNN are randomly distributed in two different configurations. Below T ) 220 K the compound undergoes a crystallographic phase transition due to the ordering of these methyl groups. We have determined the low-temperature structure using both X-ray and neutron diffraction. The new space group is P1 h. The global structure is preserved and infinite chains still run along the b-axis, but the unit cell now comprises two different pairs (1 + 2) instead of one, with four different hydrogen bond OH‚‚‚ON distances: 1.96 and 1.84 Å for the first pair, 1.96 and 1.91 Å for the second pair. The spin density of this complex was measured at T ) 1.8 K (H ) 4.6 T) by polarized neutron diffraction. The data were treated using both maximum entropy approach and wave function modeling. As in the isolated PNN, the main part of the spin density is located on the O-N-C-N-O fragment of each radical in the unit cell. However, compared to the isolated case, a significant difference exists: a large unbalance is observed between the two oxygen atoms of each radical. Moreover, a positive contribution is found on the two hydrogen atoms involved on the OH‚‚‚ON hydrogen bonds of each phenylboronic acid molecule. The stronger contribution corresponds to the longer hydrogen bonds. On the radical the stronger reduction is observed on the oxygen atoms involved in the shorter hydrogen bonds. The experimental results are compared to those obtained by density functional theory (DFT) calculations: on the whole, the experimental effects have been reproduced. However, if there is a good qualitative agreement, from the quantitative point of view, the DFT results are still very far from the experimental ones. Introduction Ullman’s nitronyl nitroxide radicals 1 (2-substituted 4,4,5,5- tetramethyl-4,5-dihydro-1H-imidazolyl-1oxyl-3-oxide, Figure 1), first synthesized as possible spin labels, have been widely used in the preparation of metal complexes displaying ferromagnetic properties. 2 Later, it was found that these kinds of materials could by themselves present ferromagnetic order or ferromag- netic behavior at low temperature. 3 Since then, only a few compounds exhibiting these behaviors have been synthesized, 4 and some theoretical modeling works were reported. 5-7 The macroscopic physical properties of molecular crystals are defined by intermolecular electronic interactions present in ² MDN/SPSMS/DRFMC. Present address for Y. Pontillon: University of Florence. ‡ University of Tokyo. § SCIB/DRFMC. | Institut Laue-Langevin. (1) Ullman, E. F.; Osieki, J. H.; Boocock, D. G. B.; Darcy, R. J. Am. Chem. Soc. 1972, 94, 7049. (2) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. 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