To whom correspondence should be addressed. Journal of Solid State Chemistry 159, 440 } 450 (2001) doi:10.1006/jssc.2001.9175, available online at http://www.idealibrary.com on Chirality of -Nitronyl Nitroxide Radicals in the Solid State Maria Minguet,* David B. Amabilino,* Klaus Wurst,- and Jaume Veciana* *Institut de Cie % ncia de Materials de Barcelona (CSIC), Campus Universitari, 08193 Bellaterra, Spain; and -Institut fu ( r Allgemeine Anorganische und Theoretische Chemie, Universita ( t Innsbru ( ck, A-6020, Innrain 52a, Austria E-mail vecianaj@icmab.es, amabilino@icmab.es Received March 20, 2001; accepted March 21, 2001 IN DEDICATION TO THE LATE PROFESSOR OLIVIER KAHN FOR HIS PIONEERING CONTRIBUTIONS TO THE FIELD OF MOLECULAR MAGNETISM The way in which chirality is manifested in the solid state in a variety of phenyl -nitronyl nitroxides is discussed in the context of the preferred conformations of this family of com- pounds. The preparation of chiral phenyl -nitronyl nitroxides apparently guarantees the formation of just one conformational diastereomer in the crystals, even if prediction of which is pres- ently unfeasible: of four chiral radicals with the same stereogenic center all four gross conformers are represented. Circular dichro- ism spectra give useful information regarding the interplay be- tween molecular conformation and optical activity. A variety of both strong and weak hydrogen bonds are at play, leading to the formation of chains and sheets of molecules. The magnetic exchange interactions between the radicals is always weak and antiferromagnetic, and can be attributed to a small face-to-face overlap of the SOMOs.  2001 Academic Press Key Words: chirality; circular dichroism; conformational di- astereoisomerism; hydrogen bonds; molecular magnets; nitrox- ide radicals. INTRODUCTION Perhaps one of the most appealing challenges for contem- porary molecular magnetism (1}3) is the combination and synergy of magnetism and other material properties in the framework of organic and coordination chemistries, a quest actively promoted and pursued by Olivier Kahn (4}12). Amongst these combinations, one of the most intriguing is, perhaps, that of magnetism and chirality, which amongst other things can result in magneto-optical e!ects (13}16). Chirality in magnetic materials has up to now been ad- dressed elegantly in the framework of coordination com- pounds (17}21), but while many chiral radicals have been reported (22}33), the consequences for their magnetic and other properties have not been pursued in detail. Recently, Olivier Kahn's group reported the structure and magnetic properties of an enantiomerically pure chiral -nitronyl nitroxide derived from a triazole moiety (8), the aim being to apply this ligand to complex paramagnetic metal ions and control the supramolecular organization of the resulting coordination compounds. Also, a &&chiral molecular-based metamagnet'' was synthesized by coordination of manga- nese(II)hexa#uoroacetylacetonate with a triplet organic radical (34). Concurrently, we started a research program targeting chiral phenyl -nitronyl nitroxide radicals of the type 1 (Fig. 1) for the preparation of molecular magnetic materials that might display magneto-optical e!ects. The solid state structures and optical and magnetic properties are the subject of this discussion. CONFORMATIONS OF PHENYL -NITRONYL NITROXIDES IN THE SOLID STATE In order to optimize optical activity of these compounds, and thus the relation between the molar extinction coe$c- ient and the molar elipticity (critical for the observation of the desired e!ects), both constitutional and conformational stereochemistry must be considered. It will be shown that the latter has an important in#uence on chiroptical proper- ties of the solids. The phenyl -nitronyl nitroxide radicals have two principle angles that govern their global confor- mation: (i) the angle formed by the C4}C5 bond and the plane formed by the NCN unit in the imidazolyl ring, ¹  , and (ii) that produced by the twist of the NCN unit of the imidazolyl ring and the plane of the phenyl ring, A  . Each of these angles has an associated helicity*de"ned by the descriptors minus (M) or plus (P) *which imply four gross conformational diastereoisomers (35), MM, PP, MP, and PM, composed of two enantiomeric pairs (Fig. 1). In the solid state, a structural database study showed that the enantiomeric MP and PM pseudo-eclipsed conformers are favored over the less planar pseudo-anti enantiomers MM and PP (36). A graphic illustration of this situation is pre- sented in Fig. 2, which is a plot of the calculated probability of the molecules in terms of ¹  and A  . 440 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.