A bis(radical) derivative based on a triphenylphosphane core substituted by two nitronyl nitroxide units is reported. This compound was isolated in two forms. One consists of an H 2 O adduct where H 2 O is hydrogen-bonded to the radical units, whereas the second lacks this bridging unit. Magnetic studPurely organic magnets are desirable materials and the design of open-shell supramolecular architectures is currently attracting an increasing interest. [1,2] The approach involving molecular crystals of aminoxyl radicals is certainly the most investigated. It has quickly become evident that to reach the objective it is essential to gain control over the construction of the supramolecular network as well as over the exchange pathways between the spin carriers. Indeed, the bulk magnetic properties will depend on the dimensionality of the system. The basic tools of organic supramolecular chemistry provide solutions for a directed material synthesis. [3,4] Among them, the hydrogen-bonding strategy attracts special interest because it has been suggested from both theoretical [5Ϫ7] and experimental [8Ϫ11] results that a hydrogen bond might mediate the intermolecular magnetic communication.An increasing number of supramolecular frameworks consisting of H-bonded paramagnetic subunits bearing either OH, [12Ϫ14] NH, [15Ϫ17] or acetylenic CH [18] groups as H-donor moieties were found to exhibit long-range magnetic correlation. However, whereas the chemical dimensionality of the solid-state structure and the intermolecular links between the subunits can be controlled by the means of the molecular synthon, it remains difficult to anticipate the final properties of the supramolecular material. In these homomolecular architectures, the exchange interactions between the paramagnetic units proceed through the supramolecular links and the core of the molecules. The resulting dominant exchange will depend on several parameters re- [a] lated both to the building block (such as the chemical nature of the molecule itself, the substitution pattern i.e. aminoxyl unit versus H-donor or -acceptor group positions, and the main exchange pathway among several intermolecular connections) and to the crystal lattice organization. Consequently, it is difficult to manage the dominant exchange between the magnetic centers.An alternative approach is to link the paramagnetic centers by an independent molecule which allows for ferromagnetic exchange. Obtaining materials with a remnant magnetization by a homo-spin strategy requires a ferromagnetic interaction between the molecular units. A prominent example of this approach involves the use of phenylboronic acid. [19,20] In this report, we show that a simple hydrogenbonded water molecule may be the driving force behind ferromagnetic interaction between aminoxyl radical units.
The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.
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