Experimental and theoretical spin density in a ferromagnetic molecular complex presenting interheteromolecular hydrogen bonds

The association of phenylboronic acid (no unpaired electron, compound 1) wi th the free radical phenyl nitronyl nitroxide (PNN, S = 1/2, compound 2) co nstitutes an interheteromolecular hydrogen bonding system displaying ferrom agnetic 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 lengt hs are quite similar (1.95 and 1.92 Angstrom). Infinite chains of this run along the b-axis. In this structure the methyl groups of the PNN are random ly distributed in two different configurations. Below T = 220 K the compoun d undergoes a crystallographic phase transition due to the ordering of thes e methyl groups. We have determined the low-temperature structure using bot h X-ray and neutron diffraction. The new space group is pi. The global stru cture 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 fo ur different hydrogen bond OH ON distances: 1.96 and 1.84 Angstrom for the first pair, 1.96 and 1.91 A for the second pair. The spin density of this c omplex was measured at T = 1.8 K (H = 4.6 T) by polarized neutron diffracti on. The data were treated using both maximum entropy approach and wave func tion 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. Howeve r, compared to the isolated case, a significant difference exists: a large unbalance is observed between the two oxygen atoms of each radical. Moreove r, a positive contribution is found on the two hydrogen atoms involved on t he OH ON hydrogen bonds of each phenylboronic acid molecule. The stronger c ontribution corresponds to the longer hydrogen bonds. On the radical the st ronger reduction is observed on the oxygen atoms involved in the shorter hy drogen bonds. The experimental results are compared to those obtained by de nsity functional theory (DFT) calculations: on the whole, the experimental effects have been reproduced. However, if there is a good qualitative agree ment, from the quantitative point of view, the DFT results are still: very far from the experimental ones.