Paramagnetic pyrazolylborates Tp2M and Tp*2M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp2M] + and [Tp*2M] +(M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The 1 H signal shift ranges vary from ~30 ppm [Cu(II) and V(III)] to ~220 ppm [Co(II)] and the 13 C signal shift ranges from ~180 ppm [Fe(III)] to ~1150 ppm [Cr(II)]. The 11 B and 14 N shifts are ~360 ppm and ~730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved 13 C, 1 H couplings. All chemical shifts have been calculated from first-principles calculations on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splittingand g-tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology. magnets. 10,11,12 Other promising application fields of pyrazolylborate complexes are catalysis 13,14,15,16,17 and the incorporation into polymers, 18 dendrimers, 19 molecular machines, 20 and metal-organic frameworks. 21 From the very first, these compounds have been studied by using paramagnetic NMR spectroscopy (pNMR) 1,22,23,24 and the initial results have been compiled. 25 Note that in the early literature the signs of pNMR signal shifts are inverted. Subsequent work, which shall be referred to below, is based on those findings and usually limited to proton NMR of Fe(II), Co(II), and Ni(II) derivatives. For three Co(II) derivatives Rouf, Mareš, and Vaara have tested theoretical and computational methods of calculating molecular properties, e.g., 1 H pNMR parameters. 26 Clearly, pyrazolylborate derivatives of many other paramagnetic ions exist or are conceivable and, given the diversity of this chemistry, some general questions from a pNMR point of view emerge: Which TpM-type compounds are worth investigating? What are the most useful nuclei? At what theoretical level can the spectra be reproduced? Do the calculated signal shifts support the signal assignment and thus the determination of the molecular and electronic structure? How are the signal shifts composed? Along these lines we have studied the parent compounds (Figure 1a) Tp2M (M = Cu, Ni, Co, Fe, Mn, Cr, and V), their monocations [Tp2M] + (M = Fe, Cr, and V), the methylated analogues Tp*2M (Tp* = hydrotris(3,5-dimethylpyrazole)borate, M = Cu, Ni, Co, Fe, Mn, Cr, and V), and [Tp*2M] + (M = Fe, Cr, and V). The present paper reports the 1 H, 13 C, 11 B, and 14 N NMR spectral data and their theoretical interpretation. To this end, first-principles calculations according to the recently redressed version 27,28 of the Kurland-McGarvey theory 29 have been carried out. These computations provide the respective overall pNMR signal shifts and their tensorial components, as well as co...

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and bInstitut fur Allgemeine Anorg. und Theoret. Chemie. Universitat Innsbruck. A-6020, Innrain 52a, AustriaWe have synthesized and chemically as well as structurally characterized a new a-nitronyl nitroxide. The spin density distribution in this uracil-substituted nitronyl nitroxide Ur-NN was examined as well as its correlation with the molecular structure, together with a study of the formation of aggregates of Ur-NN in solution.

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